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Release of Pb(II) from Monochloramine-Mediated Reduction of Lead Oxide (PbO₂)

Yi-Pin Lin^*^†

and

Richard L. Valentine^*

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Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242-1527

* Corresponding author phone: (65) 6516-4729 (Y.P.L.), (319) 335-5653 (R.L.V.); e-mail: [email protected] (Y.P.L.), [email protected] (R.L.V.).

†Current Address: Division of Environmental Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576

Cite this: Environ. Sci. Technol. 2008, 42, 24, 9137–9143

Publication Date (Web):November 11, 2008

https://doi.org/10.1021/es801037n

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Abstract

A contributing factor causing the sudden release of excessive lead into drinking water is believed to involve the change in redox conditions occurring when monochloramine (NH₂Cl) replaces free chlorine as a disinfectant. Studies suggest that NH₂Cl cannot effectively oxidize Pb(II) to form PbO₂, a Pb(IV) mineral scale formed from the oxidation of metallic lead and Pb(II) species by free chlorine. Unexpectedly, we observed that NH₂Cl is actually capable of reducing PbO₂ to form Pb(II). We systematically investigated this reaction by varying important water chemistry factors such as solution pH, total carbonate concentration, and the Cl/N molar ratio to control chloramine speciation and its rate of decomposition via a complex set of autodecomposition reactions. The amount of Pb(II) formed was found to be proportional to the amount of NH₂Cl that autodecomposed regardless of the rate of this reaction. This implies that the rate of Pb(II) release is proportional to the absolute rate of NH₂Cl decomposition. It is proposed that the species responsible for the reduction of PbO₂ is likely a reactive intermediate produced during the decay of NH₂Cl. This finding is the first to report that NH₂Cl can act as a reductant.

Synopsis

Monochloramine, a disinfectant generally presumed an oxidant, can act as a reductant via a mechanism hypothesized to involve an intermediate formed from its decomposition.

Introduction

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The change of disinfectant from free chlorine to monochloramine has been a strategy adopted by water utilities with difficulties to meet the standards of disinfection byproduct (DBPs) and maintain a sufficient disinfectant residual in the distribution system (1).

Recent public concern on this practice aroused from the abrupt rise of lead levels in Washington, DC, in 2003 (2, 3). Lead concentration as high as 4800 μg/L, about 300 times higher than the 15 μg/L action level of the 1991 Lead and Copper Rule set by the U.S. EPA (4), was found in the customer residence after the water utility adopted NH₂Cl for secondary disinfection (3). Free chlorine is a strong oxidant and has been demonstrated to be able to oxidize metallic Pb((0)) and Pb(II) ion or solid phases to form PbO₂, a Pb(IV) solid phase (2, 3, 5-9). PbO₂ is a strong oxidant and almost insoluble (5). Its formation on inside pipes may help to reduce soluble Pb concentration to an acceptable level in systems that historically use free chlorine to maintain a disinfectant residual in the distribution system. It is believed that the switch to NH₂Cl, a weaker oxidant than free chlorine, creates an environment with a lower oxidation potential that may limit the formation of PbO₂ or alter the stability of PbO₂(2, 10). The mechanisms and processes involving this release of lead are not yet well understood. However, recent work has shown that PbO₂ is not stable in water and is slowly reduced, releasing Pb(II) into water (11). It is hypothesized that this is caused by the oxidation of water itself. In addition, natural organic matter is also capable of reducing PbO₂(7, 11).

The initial motivation of this work was to investigate the influence of NH₂Cl on the stability of PbO₂ in water. The initial hypothesis was that the presence of NH₂Cl would not significantly influence the rate of PbO₂ reduction and subsequent Pb(II) formation. If anything, a minor reduction in the rate of Pb(II) release was expected because of the minor oxidation of Pb(II) resulting from the water-induced PbO₂ reduction by the extremely low free chlorine concentration that exists in the NH₂Cl equilibrium (Table S1, Supporting Information). This hypothesis was based on a thermodynamic argument that NH₂Cl is slightly lower in redox potential than PbO₂ around neutral pH so that NH₂Cl can not directly oxidize Pb(II) to PbO₂(10). Quite unexpectedly, we observed that the presence of NH₂Cl greatly increased the rate of PbO₂ reduction.

NH₂Cl is not stable in water and autodecomposes primarily to nitrogen by a complex set of reactions (12, 13). While a comprehensive reaction model incorporating these reactions has been developed to successfully describe the autodecomposition of NH₂Cl, some of these reactions have only been hypothesized to involve unidentified intermediates that must be redox active (12, 14, 15). Identification of these intermediates has been shown challenging (16-18), and only a few, such as amidogen radical (^•NH₂) and hydrazine (N₂H₄), have been proposed (18). Both oxidation and reduction of substances are therefore theoretically possible in a system involving multiple oxidants and reductants. For example, both oxidations and reductions of organic substances have been observed in solutions of Fenton’s reagent, typically used as a source of the powerful oxidant hydroxyl radical (19-24).

It is hypothesized that the autodecomposition of NH₂Cl, which results in its slow loss in solutions, can also cause the reduction of PbO₂, presumably via the reduction by reactive intermediates. Furthermore, consideration of the NH₂Cl autodecomposition mechanism also supports the idea that these intermediates should exist at pseudo-steady-state at exceedingly low concentrations in proportion to the rate of NH₂Cl decay. If the rate of PbO₂ reduction by an intermediate is first order in the concentration of intermediate, then the overall rate of PbO₂ reduction should occur in proportion to the rate of NH₂Cl decay resulting in a simple apparent stoichiometry between the amount of NH₂Cl decomposed and the amount of Pb(II) produced for a given concentration of PbO₂. The variables that increase the decay of NH₂Cl should therefore also increase the rate of PbO₂ reduction and Pb(II) formation.

The detailed reactions of NH₂Cl autodecomposition is provided in the Supporting Information (Table S1). The overall rate of NH₂Cl decay has been approximated by a second-order relationship (25)

(1) where k_vcsc is a water quality-dependent coefficient which can be described as

(2) where C_T and [NH₃]_T are total carbonate and free ammonia concentrations, respectively; α₀ and α₁ are the ionization constants for carbonate system, and α_0,N is the ionization constant for the neutral ammonia species, which are all pH dependent; k_H⁺, k_H_₂CO₃, and k_HCO₃ are rate constants that account for the general acid catalysis of NH₂Cl loss; k₃ is a rate constant characterizing the reaction between NH₂Cl and hypochlorous acid to form dichloramine; K_e is an equilibrium constant describing dichloramine and hypochlorous acid equilibrium.

This approximation can be derived from the more comprehensive mechanism by assuming that the rate limiting reactions involve the formation and subsequent loss of dichloramine, which is maintained at pseudo-steady state (25-27). This assumption is reasonable only at pH 8 or above at NH₂Cl concentrations generally less than about 0.1−0.2 mM. It is presented only as a way of easily seeing the influence of several water quality parameters on NH₂Cl decay, not as an exact predictor. Integration of this relationship yields only an approximate relationship between NH₂Cl concentration and time under all reaction conditions because free ammonia concentration increases as NH₂Cl decomposes (25-27). However, this relationship is still useful in understanding monochloramine decay kinetics. This relationship indicates that the absolute rate of NH₂Cl decay increases with increasing initial NH₂Cl concentration, carbonate concentration, which acts as a general acid catalyst (28), and Cl/N molar ratio which influences the rate of NH₂Cl hydrolysis. The decay of NH₂Cl also increases with decreasing pH because of the acid-catalyzed dichloramine formation.

The objective of this study was to investigate how the reduction of PbO₂ as measured by the subsequent formation of Pb(II) depends on the rate and extent of NH₂Cl decay under conditions where its overall loss was governed by autodecomposition. Systematic studies were conducted over a range of water quality parameters including initial NH₂Cl concentration, pH, total carbonate concentration, and Cl/N molar ratio that can greatly influence the rate of NH₂Cl decay and encompass the conditions typically encountered in drinking water supplies subjected to chloramination.

Material and Methods

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Chemicals

Reagent grade ammonia chloride and sodium bicarbonate (Fisher Scientific) were used as the source ammonia and total carbonate. Deionized water obtained from a Barnstead ULTRO pure water system (Barnstead-Thermolyne Corp.) was used to prepare working solutions. Stock NH₂Cl (∼3.95 mM or 280 mg/L as Cl₂) was freshly prepared by the addition of concentrated NaOCl solution (∼1 M, Fisher Scientific) to a carbonate-buffered ammonium chloride solution according to the published procedures (14). This NH₂Cl stock solution was calibrated by the DPD-FAS method before use. Ammonia concentration was varied to prepare NH₂Cl solutions with different Cl/N molar ratios.

Lead Oxide

PbO₂ particles were prepared in our laboratory as follows: To 250 mL of 0.1 M Pb(NO₃)₂ solution, the desired amount concentrated NaOCl solution (∼1M, Fisher Scientific) was added to make a solution with a NaOCl/Pb²⁺ molar ratio of 1.1. A white-yellow solid phase quickly precipitated and turned into brown-reddish color as the reaction proceeded. After 24 h, the brown-reddish slurry was transferred to several 50 mL polypropylene tubes and centrifuged at 6000 rpm for 6 min. The supernatant was decanted and replaced with deionized water. The new slurry was centrifuged again. The above procedure was repeated several times to remove Pb²⁺ as much as possible. The solid phase was then transferred to a dialysis membrane tube (MWCO = 6−8000, Spectra/Por) submerged in a DI water bath with regular change of the DI water for three days to remove residual Pb²⁺. The solid phase was collected in polypropylene tubes followed by centrifuge and decanting the supernatant. Dry particles were obtained by freeze-drying the solid phase. Final solid phase weighted 3.34 g, approximately 55.8% recovery of the added Pb²⁺. Most of the unrecovered lead was due to the attachment of particles on the container walls and loss during the solid transfer. A dialysis membrane was used to remove residual Pb(II) because PbO₂ is reduced to Pb(II) at low pH (11).

Scanning electron microscope (SEM) images of the PbO₂ particles is shown in Figure S1 in the Supporting Information. The size of individual particles was less than 100 nm. However, the primary particles appeared to be composed of small particles that had aggregated. The specific surface area determined by 7-point BET method was 24.29 m²/g. X-ray diffraction (XRD) analysis of the particles is shown in Figure S2 in the Supporting Information. The diffraction pattern indicated that the laboratory-prepared PbO₂ is plattnerite, a tetragonal polymorph of PbO₂ that has been found in distribution systems (6, 29)

PbO₂ Reduction Studies

Experiments were conducted headspace-free using 125 mL polypropylene bottles covered by aluminum foil at 25 °C. All experiments utilized 10 mg/L of PbO₂, an amount which did not significantly influence the rate of NH₂Cl autodecomposition. Solution pH values were adjusted by 1 N HNO₃ and NaOH. Experimental variables investigated including initial NH₂Cl concentration (14.1, 42.3, and 70.4 μM or 1, 3, and 5 mg/L as Cl₂), solution pH (6.0, 7.0, and 8.0), total carbonate concentration from added bicarbonate (C_T = 2, 5 and 10 mM) and Cl/N molar ratio (0.2, 0.5 and 0.7). After they were filled with the working solution, the polypropylene bottles were placed on a shaking table rotating at 200 rpm. The Pb(II) concentration, NH₂Cl residual, and solution pH value were periodically measured over a course of 7 days. The variations of pH values during the course of the experiments were within target pH ±0.2.

Analytical Methods

NH₂Cl concentration was measured by a modified iodometric method. Because iodide can react with PbO₂ to produce a positive interference in this method (30), 5 mL of solution was filtered by a nylon syringe filter with a 0.2 μm pore size (Fisher Scientific) to remove PbO₂ particle aggregates before NH₂Cl measurement; 50 μL of acetate buffer was added to the filtrate to adjust the pH to about 4. Sufficient potassium iodide was then added to the filtrate to allow the oxidation of iodide by NH₂Cl. The triiodate (I₃⁻) liberated from the oxidation was determined by its UV absorbance at 351 nm using a predetermined molar absorbance of 23 325 cm⁻¹ M⁻¹. This method showed comparable results with the widely used DPD-FAS method (less than 10% difference) with the advantage that a much smaller sample volume was required.

A Nano-Band Explorer II (TraceDetect, Seattle) anodic stripping voltammetry (ASV) was used for total Pb(II) measurements (11). The pH value of unfiltered sample taken from the well mixed solution was adjusted to 4 by addition 0.1 M acetate buffer. At this pH value, colloidal and Pb(II) carbonate solid phases quickly dissolve to soluble Pb²⁺ ion (31, 32). Thus, total Pb(II) including soluble, colloidal, and solid Pb(II) is measured by the ASV. A Hitachi S-4000 scanning electron microscope (SEM) was used for acquiring the images of PbO₂ particles. The Xay-ray diffraction (XRD) pattern was determined by a MiniFlex II desktop X-ray diffractometer (Rigaku Americas). The solution pH value was measured by an Accument pH meter (A15, Fisher Scientific) coupled with a calomel combination pH electrode pre-equilibrated by standard buffers.

Results and Discussion

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PbO₂ Reduction Dependence on Initial NH₂Cl Concentration and NH₂Cl Decay

Figure 1 graphically shows the loss of NH₂Cl concentration, formation of total Pb(II) with time, and the amount of total Pb(II) produced as a function of the change in NH₂Cl concentration for experiments conducted using initial NH₂Cl concentration of 14.1, 42.3, and 70.4 μM (1.0, 3.0, and 5.0 mg/L as Cl₂). The decay of NH₂Cl approximately followed the second-order kinetics described in eq 1 and the decay rate was faster at the higher initial NH₂Cl concentration (see Figure S2 in Supporting Information for the second-order kinetics plot). It should be pointed out that the presence of 10 mg/L PbO₂ only slightly influenced the rate of NH₂Cl decay (Figure 1a). The rate of total Pb(II) formation increased with increasing initial NH₂Cl concentration (Figure 1b), which was concurrent with the trend of NH₂Cl decay rate. A linear relationship between total Pb(II) formation and loss in NH₂Cl was observed (Figure 1c), which is consistent with the hypothesis that PbO₂ reduction is caused by a reaction with an intermediate that is linearly related to the decay of NH₂Cl. In a control experiment in the absence of NH₂Cl, Pb(II) was formed but at a greatly reduced rate, with a concentration of 0.31 μM (65 μg/L) formed after 7 days (Figure 1b). This has been attributed to a water-induced PbO₂ reduction (11). We hypothesize that the water reaction may occur in parallel to the NH₂Cl-induced PbO₂ reduction, although it may not be significant when a high concentration of NH₂Cl is present. The SEM image of the solid phase after the experiment with 70.4 μM NH₂Cl is shown in Figure 2. A secondary solid phase was formed discretely. The identity of this solid was not certain because of the limited amount of sample that could not be analyzed by XRD. However, based on the solution composition and solubility consideration, this solid phase should be either cerrusite (PbCO₃) or hydrocerrusite (Pb₃(CO₃)₂(OH)₂) (33). The discrete solid phase did not form a passivating layer to physically protect PbO₂ from being reduced to form Pb(II) in NH₂Cl solutions. However, it can provide a sink to regulate soluble Pb(II) concentration (i.e., Pb²⁺ and Pb(II)−carbonate complex).

Figure 1. Effect of NH₂Cl concentration on the formation of total Pb(II) and NH₂Cl decay: (a) NH₂Cl decay as a function of time, (b) total Pb(II) formation as a functiom of time, and (c) total Pb(II) formation vs NH₂Cl decay (data of duplicate experiments were shown by separate points). Fixed experimental parameters: PbO₂ = 10 mg/L, pH 7.0, C_T = 5 mM, Cl/N = 0.7. In control experiment for NH₂Cl decay, no PbO₂ was added.

Figure 2. SEM image of the solid phase after 7 day experiment. NH₂Cl = 70.4 μM, PbO₂ = 10 mg/L, pH 7.0, C_T = 5 mM, Cl/N = 0.7.

Influence of Solution pH

Figure 3 summarizes the results for experiments conducted at pH 6.0, 7.0, and 8.0 at a constant initial NH₂Cl concentration, carbonate concentration, and Cl/N ratio. The Pb(II) formation from control experiments in the absence of NH₂Cl is shown in the Supporting Information. The decay of NH₂Cl is controlled by the rate-determining step that is believed to be the formation of dichloramine (NHCl₂), a product of NH₂Cl hydrolysis and disproportionation that quickly decomposes upon its formation (12, 34). Dichloramine formation is enhanced at lower pH and, in turn, leads to the faster decay of monochloramine at lower pH as observed in our experiments (Figure 3a). For the control experiment without PbO₂ conducted at pH 7.0, the rate of NH₂Cl decay was only slightly slower than that in the presence of PbO_2. The rates of total Pb(II) formation increased with decreasing solution pH value. At pH 6.0, the rate of total Pb(II) formation sharply decreased after 2 days (Figure 3b), which was accompanied by a decrease in NH₂Cl decomposition rate. The exhaustion of the most reactive sites on the PbO₂ surface may also contribute to the decrease in total Pb(II) formation. Figure 3c again shows that total Pb(II) formation is linearly related to the amount of NH₂Cl decomposed.

Figure 3. Effect of solution pH on the formation of total Pb(II) and NH₂Cl decay: (a) NH₂Cl decay as a function of time, (b) total Pb(II) formation as a function of time, and (c) total Pb(II) formation vs NH₂Cl decay vs total Pb(II) formation (data of duplicate experiments were shown by separate points). PbO₂ = 10 mg/L, NH₂Cl = 42.3 μM, C_T = 5 mM, Cl/N = 0.7. In control experiment for NH₂Cl decay, no PbO₂ was added.

Influence of Total Carbonate Concentration, C_T

The rates of NH₂Cl decay and total Pb(II) formation increased with increasing total carbonate concentration at a constant pH and Cl/N molar ratio (Figure 4a and b). The increasing rate of NH₂Cl decay with increasing C_T can be attributed to the general-acid catalyzed nature of the NH₂Cl disproportionation (14, 28). Valentine and Jafvert (28) experimentally determined the general-acid catalyzed NH₂Cl disproportionation by sulfate and phosphate, and proposed that carbonate may act at the same fashion based on a linear free energy relationship (LFER), which was later experimentally demonstrated by Vikesland et. al (14). Our results are consistent with this nature of NH₂Cl decay. In the control experiment without PbO₂ conducted at 10 mM, C_T, no statistically significant difference was observed for the NH₂Cl decay when comparing to that in the presence of PbO₂. Figure 4c shows that the amount of lead formed is linearly related to NH₂Cl decay.

Figure 4. Effect of total carbonate concentration on the formation of total Pb(II) and NH₂Cl decay: (a) total Pb(II) formation as a functiom of time, (b) NH₂Cl decay as a function of time, and (c) NH₂Cl decay vs total Pb(II) formation (data of duplicate experiments were shown by separate points). PbO₂ = 10 mg/L, NH₂Cl = 42.3 μM, pH 7.0, Cl/N = 0.7. In control experiment for NH₂Cl decay, no PbO₂ was added.

Influence of Cl/N Molar Ratio

The rates of NH₂Cl decay and total Pb(II) formation increased with increasing Cl/N molar ratio (Figure 5a and b). The free ammonia content of the working solution increased as the Cl/N ratio decreased because a fixed free chlorine concentration was used to prepare these NH₂Cl solutions. Ammonia is the product of the NH₂Cl hydrolysis and disporportionation, the reactions that lead to the decay of NH₂Cl (12). The stability of NH₂Cl increases with increasing ammonia concentration (or decreasing Cl/N molar ratio) because of the greater backward rates of NH₂Cl hydrolysis and disporportionation (14). The slower decay rate of NH₂Cl at a smaller Cl/N molar ratio was consistent with this reaction scheme of NH₂Cl decomposition. Figure 5(c) shows a similar linear relationship between Pb(II) formation and NH₂Cl decay.

Figure 5. Effect of Cl/N molar ratio on the formation of total Pb(II) and NH₂Cl decay: (a) total Pb(II) formation as a functiom of time, (b) NH₂Cl decay as a function of time, and (c) NH₂Cl decay vs total Pb(II) formation (data of duplicate experiments were shown by separate points). PbO₂ = 10 mg/L, NH₂Cl = 42.3 μM, pH 7.0, C_T = 5 mM. In control experiment for NH₂Cl decay, no PbO₂ was added.

Comparison of Correlations between PbO₂ Reduction and NH₂Cl Autodecomposition

The formation of total Pb(II) was related to NH₂Cl loss by a single linear relationship that was independent of how the rate of NH₂Cl autodecomposition was controlled (Figure 6). The overall slope of 0.204(±0.011) is statistically identical to the slopes of the linear relationships obtained by varying one parameter at a time (Table 1).

Figure 6. Summary plot of total Pb(II) formation vs NH₂Cl decay.

Table 1. Equations of Least-Square Linear Regression between Total Pb(II) Formation and ΔNH₂Cl for Each Water Parameter with a 95% Confidence Level in the Slopea

water parameter	least-square linear regression equation
NH₂Cl	y = 0.169(±0.015)x − 0.039, r² = 0.938
pH value	y = 0.210(±0.018)x − 0.046, r² = 0.947
total carbonate	y = 0.206(±0.020)x − 0.119, r² = 0.931
Cl/N molar ratio	y = 0.173(±0.022)x − 0.038, r² = 0.908

y: total Pb(II) formation (μM). x: ΔNH₂Cl (μM).

The linear relationship supports the hypothesis that the autodecomposition of NH₂Cl leads to the formation of total Pb(II) (or the reduction of PbO₂) and that the product(s) from the NH₂Cl decomposition should be responsible for this instead of a direct reaction with NH₂Cl. On the basis of this linear relationship, the rate of total Pb(II) formation in NH₂Cl solutions can be approximated by the following equation:

(3) where α is a constant approximately equal to 0.2 in the present study. While it is tempting to interpret the constant as a measure of a true stoichiometry of approximately 1 μM Pb(II) formed to 5 μM of NH₂Cl decomposed, this is not likely because the value of α is expected to depend on the type, size, and concentration of PbO₂(30) and can only be determined by experiments. Increasing the amount of PbO₂ should increase the amount of total Pb(II) formed in the presence of independently autodecomposing monochloramine.

The nature of the reaction causing the formation of Pb(II) in the presence of NH₂Cl is hypothesized to involve a reaction of a reactive intermediate produced during the relatively slow NH₂Cl autodecomposition. The reduction of PbO₂ by free ammonia can be ruled out because we observed a slower rate of total Pb(II) formation at lower Cl/N ratio (or higher free ammonia concentration). It should be noted that the rate of NH₂Cl decomposition under our experimental conditions was approximately second-order with respect to NH₂Cl concentration (see Figures S3−S6 in Supporting Information); therefore, the formation of total Pb(II) should also be similarly approximately second-order with respect to NH₂Cl concentration as shown in eq 3.

Environmental Significance

Monochloramine has been widely considered as an alternative disinfectant by water utilities to combat the formation of DBPs when an unacceptable level occurs. However, field evidence has suggested that NH₂Cl may alter the stability of lead-containing scale, primarily PbO₂ and result in a hazardous level of lead in drinking water (2, 3, 35). In this study, we revealed that total Pb(II) formation from the reduction of PbO₂ is attributed to the decomposition of NH₂Cl, presumably by an unidentified intermediate formed in the autodecomposition reactions. The water chemistry that enhances the decomposition, including a higher initial NH₂Cl concentration, lower solution pH, higher total carbonate concentration, and higher Cl/N molar ratio, can in turn enhance the formation of total Pb(II). These water parameters need to be carefully evaluated for lead release when a water utility attempts to use monochloramine for secondary disinfection. Additional studies focused on the identification of the intermediate that leads to PbO₂ reduction are warranted because they may help to develop a strategy to eliminate the undesired lead release while still take the advantage of monochloramine in water treatments.

Supporting Information

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Additional table of NH₂Cl autodecomposition reactions and figures of PbO₂ SEM images, and second-order plots of monochloramine decay. This material is available free of charge via the Internet at http://pubs.acs.org.

es801037n_si_001.pdf (613.62 kb)

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Author Information

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Corresponding Authors
- Yi-Pin Lin - Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242-1527; Email: [email protected] [email protected]
- Richard L. Valentine - Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242-1527; Email: [email protected] [email protected]

Acknowledgment

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This work was supported by a grant from the American Water Works Association Research Foundation (Project No. 3172). The assistance of ASV measurements from TraceDetect was greatly appreciated.

References

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Recent research showed that Pb(IV) oxides play a significant geochem. role in drinking water distribution systems. However, most of the guidance for lead control in drinking water is based on the presumption that Pb(II) solids control lead soly. Therefore, a better understanding of the chem. of Pb(IV) in water is needed. Long-term lead pptn. expts. were conducted in chlorinated water (1-3 mg/L Cl2) at pH 6.5, 8, and 10, with and without sulfate. Two Pb(IV) dioxide polymorphs-plattnerite (β-PbO2) and scrutinyite (α-PbO2)-formed over time, as long as a high suspension redox potential was maintained with free chlorine. Neither mineral formed spontaneously, and the rate of formation increased with increasing pH. Hydrocerussite and/or cerussite initially pptd. out and overtime either disappeared or coexisted with PbO2. Water pH dictated mineralogical presence. High pH favored hydrocerrusite and scrutinyite; low pH favored cerussite and plattnerite. Along with a transformation of Pb(II) to Pb(IV) came a change in particle color from white to a dark shade of red to dark gray (differing with pH) and a decrease in lead soly. If free chlorine was permitted to dissipate, the aging processes (i.e., mineralogy, color, and soly.) were reversible.
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6
Schock, M. R.. New Insights into lead corrosion control and treatment change impacts (with some considerations towards Cu). AWWA Meeting Section Emerging Issues in Water Treatment, Michigan, 2007;
Available at http://www.epa.gov/nrmrl/wswrd/cr/corr_pubs_lead.html accessed Jan. 15, 2008.
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7
Dryer, D. J.; Korshin, G. V. Investigation of the reduction of lead dioxide by natural organic matter Environ. Sci. Technol. 2007, 41, 5510– 5514
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7
Investigation of the Reduction of Lead Dioxide by Natural Organic Matter
Dryer, Deborah J.; Korshin, Gregory V.
Environmental Science & Technology (2007), 41 (15), 5510-5514CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Expts. with immobilized lead dioxide showed that this solid was reduced by natural org. matter (NOM) isolated from Potomac River water. Kinetically, the process was slow and occurred throughout many weeks of exposure. The amt. of mobilized lead was affected by the concn. of NOM and exposure time but not significantly influenced by the type of NOM used in the expts. The interactions of NOM with PbO2 were quantified using differential absorbance spectroscopy. It showed that the oxidn. of chromophoric groups in NOM was strongly correlated with lead release. Because lead release yields were higher that those predicted based on the depletion of the arom. groups, it is hypothesized that NOM moieties other than arom. functionalities are engaged in the redn. of PbO2 by NOM and/or lead mobilization involves the formation of mixed Pb(II)/Pb(IV) sol. and colloidal species.
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Schock, M. R.; Scheckel, K. G.; DeSantis, M.; Gerke, T. L. Mode of occurrence, treatment and monitoring significance of tetravalent lead. Presented at the AWWA Water Quality Technology Conference, Quebec, Canada, 2005.
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9
Schock, M. R.; Harmon, S. M.; Swertfeger, J.; Lohmann, R. Tetravalent lead: A hitherto unrecognized control of tap water lead contamination. Presented at the AWWA Water Quality Technology Conference, Nashville, TN, 2001.
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10
Rajasekharan, V. V.; Clark, B. N.; Boonsalee, S.; Switzer, J. A. Electrochemistry of free chlorine and monochloramine and its relevance to the presence of Pb in drinking water Environ. Sci. Technol. 2007, 41, 4252– 4257
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10
Electrochemistry of Free Chlorine and Monochloramine and its Relevance to the Presence of Pb in Drinking Water
Rajasekharan, Vishnu V.; Clark, Brandi N.; Boonsalee, Sansanee; Switzer, Jay A.
Environmental Science & Technology (2007), 41 (12), 4252-4257CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The commonly used disinfectants in drinking water are free chlorine (in the form of HOCl/OCl-) and monochloramine (NH2Cl). While free chlorine reacts with natural org. matter in water to produce chlorinated hydrocarbon byproducts, there is also concern that NH2Cl may react with Pb to produce sol. Pb(II) products-leading to elevated Pb levels in drinking water. In this study, electrochem. methods are used to compare the thermodn. and kinetics of the redn. of these two disinfectants. The std. redn. potential for NH2Cl/Cl- was estd. to be +1.45 V in acidic media and +0.74 V in alk. media vs. NHE using thermodn. cycles. The kinetics of electroredn. of the two disinfectants was studied using an Au rotating disk electrode. The exchange current densities estd. from Koutecky-Levich plots were 8.2 × 10-5 and 4.1 × 10-5 A/cm2, and by low overpotential expts. were 7.5 ± 0.3 × 10-5 and 3.7 ± 0.4 × 10-5 A/cm2 for free chlorine and NH2Cl, resp. The rate const. for the electrochem. redn. of free chlorine at equil. is approx. twice as large as that for the redn. of NH2Cl. Equil. potential measurements show that free chlorine will oxidize Pb to PbO2 above pH 1.7, whereas NH2Cl will oxidize Pb to PbO2 only above about pH 9.5, if the total dissolved inorg. carbon (DIC) is 18 ppm. Hence, NH2Cl is not capable of producing a passivating PbO2 layer on Pb, and could lead to elevated levels of dissolved Pb in drinking water.
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11
Lin, Y. P.; Valentine, R. L. The release of lead from the reduction of lead oxide (PbO₂) by natural organic matter Environ. Sci. Technol. 2008, 42, 760– 765
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11
The Release of Lead from the Reduction of Lead Oxide (PbO2) by Natural Organic Matter
Lin, Yi-Pin; Valentine, Richard L.
Environmental Science & Technology (2008), 42 (3), 760-765CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
PbO2 has been identified as an important scale in some distribution systems that historically use lead service lines and free chlorine for maintaining a disinfectant residual. The stability of this highly insol. scale with respect to its reductive dissoln. may play an important role in lead release into drinking water. In this study, we investigated the release of lead from a com. available PbO2 in the presence of natural org. matter (NOM) using a hydrophobic acid extd. from the Iowa River. Expts. were conducted using synthetic solns. with different NOM concns., soln. pH, and NOM samples with different levels of prechlorination. It was found that release of lead from PbO2 occurred both in solns. with and without NOM, and the extent of lead release increased with increasing NOM concn. and decreasing pH value. Furthermore, the released lead was Pb(II) and not particulate PbO2 conclusively showing that reductive dissoln. occurred. Prechlorination of NOM reduced the rate of lead release. Our results indicate that PbO2 can be reduced both by water and NOM. Characterization of final solid phases by SEM and XPS are also presented.
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12
Jafvert, C. T.; Valentine, R. L. Reaction scheme for the chlorination of ammoniacal water Environ. Sci. Technol. 1992, 26, 577– 586
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12
Reaction scheme for the chlorination of ammoniacal water
Jafvert, Chad T.; Valentine, Richard L.
Environmental Science and Technology (1992), 26 (3), 577-86CODEN: ESTHAG; ISSN:0013-936X.
A kinetic model of the reacting aq. Cl-NH3 system is proposed that describes equally well the rapid breakpoint oxidn. of NH3, where the applied Cl dose (Cl2) to NH3-N molar ratio (Cl/N) is ⪆1.6; the slow oxidn. of NH3 in aq. NH2Cl solns. (Cl/N <1); and the transition region of 1 < Cl/N < 1.6, where rapid initial decay results in chloramine species residuals. Calcd. time-dependent concns. of the Cl species, detd. by numerical soln. of the rate expressions, compare favorably to measured values, detd. during expts. performed at initial pH (6-8) and Cl/N (0.25-2.0) conditions. The exptl. measured species include free Cl (HClO + ClO-), NH2Cl, and NHCl2. The model appropriately considers the catalysis of certain key reactions by several commonly encountered inorgs., such as HCO3- and phosphate species.
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Ozekin, K.; Valentine, R. L.; Vikesland, P. J. Modeling the decomposition of disinfecting residuals of chloramine. InWater Disinfection and Natural Organic Matter; ACS Symposium Series 649; American Chemical Society: Washington, DC, 1996; pp 115− 125.
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14
Vikesland, P. J.; Ozekin, K.; Valentine, R. L. Monochloramine decay in model and distribution system waters Water Res. 2001, 35, 1766– 1776
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14
Monochloramine Decay in Model and Distribution System Waters
Vikesland, P. J.; Ozekin, K.; Valentine, R. L.
Water Research (2001), 35 (7), 1766-1776CODEN: WATRAG; ISSN:0043-1354. (Elsevier Science Ltd.)
Chloramines have long been used to provide a disinfecting residual in distribution systems where it is difficult to maintain a free Cl residual or where disinfection byproduct (DBP) formation is of concern. While chloramines are generally considered less reactive than free Cl, they are inherently unstable even in the absence of reactive substances. These reactions, often referred to as auto-decompn., always occur and hence define the max. stability of monochloramine in water. The effect of addnl. reactive material must be measured relative to this basic loss process. A thorough understanding of the auto-decompn. reactions is fundamental to the development of mechanisms that account for reactions with addnl. substances and to the ultimate formation of DBPs. A kinetic model describing auto-decompn. was recently developed. This model is based on studies of isolated individual reactions and on observations of the reactive ammonia-Cl system as a whole. This work validates and extends this model for use in waters typical of those encountered in distribution systems and under realistic chloramination conditions. The effect of carbonate and temp. on auto-decompn. is discussed. The influence of bromide and nitrite at representative monochloramine concns. is also examd., and addnl. reactions to account for their influence on monochloramine decay are presented to demonstrate the ability of the model to incorporate inorg. demand pathways that occur parallel to auto-decompn.
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Duirk, S. E.; Gombert, B.; Croue, J. P.; Valentine, R. L. Modeling monochloramine loss in the presence of natural organic matter Water Res. 2005, 39, 3418– 3431
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15
Modeling monochloramine loss in the presence of natural organic matter
Duirk, Stephen E.; Gombert, Bertrand; Croue, Jean-Philippe; Valentine, Richard L.
Water Research (2005), 39 (14), 3418-3431CODEN: WATRAG; ISSN:0043-1354. (Elsevier B.V.)
A comprehensive model describing monochloramine loss in the presence of natural org. matter (NOM) is presented. The model incorporates simultaneous monochloramine autodecompn. and reaction pathways resulting in NOM oxidn. These competing pathways were resolved numerically using an iterative process evaluating hypothesized reactions describing NOM oxidn. by monochloramine under various exptl. conditions. The reaction of monochloramine with NOM was described as biphasic using four NOM specific reaction parameters. NOM pathway 1 involves a direct reaction of monochloramine with NOM (kdoc1=1.05 × 104-3.45 × 104/M-h). NOM pathway 2 is slower in terms of monochloramine loss and attributable to free Cl (HOCl) derived from monochloramine hydrolysis (kdoc2=5.72 × 105-6.98 × 105/M-h), which accounted for the majority of monochloramine loss. The free Cl reactive site fraction in the NOM structure was found to correlate to specific UV absorbance at 280 nm (SUVA280). Modeling monochloramine loss allowed for insight into disinfectant reaction pathways involving NOM oxidn. This knowledge is of value in assessing monochloramine stability in distribution systems and reaction pathways leading to disinfection byproduct formation.
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16
Valentine, R. L.; Brandt, K. I.; Jafvert, C. T. A spectrophotometric study of the formation of an unidentified monochloramine decomposition product Water Res. 1986, 20, 1067– 1074
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16
A spectrophotometric study of the formation of an unidentified monochloramine decomposition product
Valentine, Richard L.; Brandt, Kirk I.; Jafvert, Chad T.
Water Research (1986), 20 (8), 1067-74CODEN: WATRAG; ISSN:0043-1354.
The formation of an unidentified product(s) of the slow decompn. of NH2Cl in org.-free aq. solns. at pH values and Cl-N ratios of importance in the chloramination disinfection of drinking water was studied. NH2Cl and total oxidant concns. detd. spectrophotometrically in these solns. became significantly higher with time than those detd. by a titrimetric methods due to the absorbance of the unidentified product(s). The UV spectra of the product(s) were calcd. from the difference between measured and predicted spectra and were similar to those obtained on NH2Cl-free soln. resulting from the rapid decompn. of dichloramine at high pH. No spectrophotometric evidence could be found for the formation of significant concns. of NO2- and/or NO3-. Relative concn. changes of the unidentified product(s) as measured by its calcd. absorbance at 243 nm showed that the product(s) accumulates with time and, therefore, is not an intermediate in the formation of N gas. Both increased pH and PO43- buffer increased its formation rate. A formation mechanism involving the decompn. of NHCl2 is suggested. The age-history of NH2Cl solns. could be an important variable in toxicol. studies of chloramines and their reaction products depending on the health effects of the unidentified product(s).
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Diyamandoglu, V.; Selleck, R. E. Reactions and products of chloramination Environ. Sci. Technol. 1992, 26, 808– 814
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17
Reactions and products of chloramination
Diyamandoglu, Vasil; Selleck, Robert E.
Environmental Science and Technology (1992), 26 (4), 808-14CODEN: ESTHAG; ISSN:0013-936X.
The stoichiometry of the reaction of Cl with excess NH3 in dil. aq. soln. was studied at near-neutral pH and at pH 9.3-9.6. A significant portion of Cl was incorporated into unrecognized reaction products at near-neutral pH, and essentially none at the elevated pH. The absence of detectable NO2- did not prohibit the prodn. of significant amts. of NO3- at the pH levels studied. The relative significance of the NO3- prodn. increased upon diln. of the reactants at near-neutral pH, with NO3- becoming the primary end product of N oxidn. under conditions approaching those encountered in the disinfection of drinking water. The decompn. of H2NNH2 appeared to be responsible for the formation of N gas at the elevated pH. No explanation can be offered for the significant quantities of NO3- produced at that pH.
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Vikesland, P. J.; Valentine, R. L. Reaction pathways involved in the reduction of monochloramine by ferrous iron Environ. Sci. Technol. 2000, 34, 83– 90
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18
Reaction Pathways Involved in the Reduction of Monochloramine by Ferrous Iron
Vikesland, Peter J.; Valentine, Richard L.
Environmental Science and Technology (2000), 34 (1), 83-90CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The maintenance of disinfectants in distribution systems is crit. to ensure the safety of drinking water. Reduced iron is one type of reactive constituent believed to play a central role in disinfectant loss as the water travels through the pipe. Specifically, reactions involving Fe(II), both in soln. and at the pipe-water interface, are felt to be important processes that account for this loss. This work shows that the rate-limiting reactions responsible for the disappearance of monochloramine involve a direct reaction between mol. monochloramine and Fe(II), leading to the formation of the amidogen radical (•NH2) as a reactive intermediate. In addn., the mechanism was found to be autocatalytic with the ferric oxide ppt. acting to accelerate the overall redn. of monochloramine. A variable stoichiometry was obsd. for this system, and this was rationalized by accounting for radical scavenging in the reaction scheme.
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Corbin, J. F.; Teel, A. L.; Allen-King, R. M.; Watts, R. J. Reactive oxygen species responsible for the enhanced desorption of dodecane in modified Fenton’s systems Water Environ. Res. 2007, 79, 37– 42
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19
Reactive oxygen species responsible for the enhanced desorption of dodecane in modified Fenton's systems
Corbin, Joseph F., III; Teel, Amy L.; Allen-King, Richelle M.; Watts, Richard J.
Water Environment Research (2007), 79 (1), 37-42CODEN: WAERED; ISSN:1061-4303. (Water Environment Federation)
The enhanced treatment of sorbed contaminants has been documented in modified Fenton's reactions; contaminants are desorbed and degraded more rapidly than they desorb by fill-and-draw or gas-purge desorption. The reactive species responsible for this process was studied using dodecane as a model sorbent. Hydroxyl radical, hydroperoxide anion, and superoxide radical anion were generated sep. to evaluate their roles in enhanced dodecane desorption. Dodecane desorption from silica sand over 180 min was negligible in gas-purge systems and in the hydroxyl radical and hydroperoxide anion systems. In contrast, enhanced desorption of dodecane occurred in superoxide systems, with >80% desorption over 180 min. Scavenging of superoxide eliminated the enhanced desorption of dodecane in both superoxide and modified Fenton's systems, confirming that superoxide is the desorbing agent in modified Fenton's reactions. Conditions that promote superoxide generation in Fenton's reactions may enhance their effectiveness for in situ subsurface remediation of sorbed hydrophobic contaminants.
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Smith, B. A.; Teel, A. L.; Watts, R. J. Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton’s reagent J. Contam. Hydrol. 2006, 85, 229– 246
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20
Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton's reagent
Smith, Brant A.; Teel, Amy L.; Watts, Richard J.
Journal of Contaminant Hydrology (2006), 85 (3-4), 229-246CODEN: JCOHE6; ISSN:0169-7722. (Elsevier B.V.)
The destruction of CCl4 and chloroform was studied in reactors contg. 0.5 mL DNAPL and a soln. of modified Fenton's reagent (2M H2O2 and 5mM Fe(III)-chelate). CCl4 and chloroform masses were followed in the DNAPLs, the aq. phases, and the off gasses. The rate of DNAPL destruction was compared to the rate of gas-purge dissoln. CCl4 DNAPLs were rapidly destroyed by modified Fenton's reagent at 6.5 times the rate of gas purge dissoln., with 74% of the DNAPL destroyed within 24 h. Use of reactions in which a single reactive O species (hydroxyl radical, hydroperoxide anion, or superoxide radical anion) was generated showed that superoxide is the reactive species in modified Fenton's reagent responsible for CCl4 destruction. Chloroform was also destroyed by modified Fenton's reagent, but at a rate slower than the rate of gas purge dissoln. Reactions generating a single reactive O species demonstrated that chloroform destruction was the result of both superoxide and hydroxyl radical activity. Such a mechanism of chloroform destruction is in agreement with the slow but relatively equal reactivity of chloroform with both superoxide and hydroxyl radical. The results of this research demonstrate that modified Fenton's reagent can rapidly and effectively destroy DNAPLs of contaminants characterized by minimal reactivity with hydroxyl radical, and should receive more consideration as a DNAPL cleanup technol.
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Watts, R. J.; Sarasa, J.; Loge, F. J.; Teel, A. L. Oxidative and reductive pathways in manganese-catalyzed Fenton’s reactions J. Environ. Eng. 2005, 131, 158– 164
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21
Oxidative and Reductive Pathways in Manganese-Catalyzed Fenton's Reactions
Watts, Richard J.; Sarasa, Judith; Loge, Frank J.; Teel, Amy L.
Journal of Environmental Engineering (Reston, VA, United States) (2005), 131 (1), 158-164CODEN: JOEEDU; ISSN:0733-9372. (American Society of Civil Engineers)
Sol. Mn(II) and amorphous and cryst. Mn(IV) oxides were studied as catalysts for the Fenton-like decompn. of H2O2 into oxidants and reductants. 1-Hexanol was used as a hydroxyl radical probe and CCl4 was used as a reductant probe. Sol. Mn(II)-catalyzed reactions at acidic pH resulted in >99% degrdn. of 1-hexanol and no measurable transformation of CCl4, indicating that hydroxyl radicals were generated but reductants were not. However, when these reactions were conducted at near-neutral pH, an amorphous Mn oxide ppt. formed and 89% of the CCl4 degraded in 60 min, while 1-hexanol degrdn. was negligible. Using an amorphous Mn oxide synthesized in a sep. reactor, CCl4 was rapidly degraded while 1-hexanol oxidn. was undetectable. Reactions catalyzed by the cryst. Mn oxide pyrolusite(β-MnO2) at near-neutral pH also resulted in significant CCl4 degrdn., indicating that reductants are generated by both the cryst. and amorphous Mn oxide-catalyzed decompn. of H2O2. The presence of Mn oxides in the subsurface and their ability to catalyze the generation of reductants in modified Fenton's reactions has important implications for H2O2 stability and contaminant transformation pathways during the in situ Fenton's treatment of contaminated soils and groundwater.
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22
Smith, B. A.; Teel, A. L.; Watts, R. J. Identification of the reactive oxygen species responsible for carbon tetrachloride degradation in modified Fenton’s systems Environ. Sci. Technol. 2004, 38, 5465– 5469
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22
Identification of the Reactive Oxygen Species Responsible for Carbon Tetrachloride Degradation in Modified Fenton's Systems
Smith, Brant A.; Teel, Amy L.; Watts, Richard J.
Environmental Science and Technology (2004), 38 (20), 5465-5469CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The reactive oxygen species responsible for the transformation of carbon tetrachloride (CT) by a modified Fenton's reagent using hydrogen peroxide concns. >0.1M was investigated. The addn. of the hydroxyl radical scavenger 2-propanol to modified Fenton's reactions did not significantly lower CT transformation rates. Scavenging by 2-propanol not only confirmed that hydroxyl radicals are not responsible for CT destruction, but also suggested that a major product of an iron (III)-driven initiation reaction, superoxide radical anion (O2•-), is the species responsible for CT transformation. To investigate this hypothesis, CT degrdn. was studied in aq. KO2 reactions. Minimal CT degrdn. was found in CT-KO2 reactions; however, when H2O2 was added to the KO2 reactions at concns. similar to those in the modified Fenton's reactions (0.1, 0.5, and 1M), CT degrdn. increased significantly. Similar results were obtained when 1M concns. of other solvents were added to aq. KO2 reactions, and the obsd. first-order rate const. for CT degrdn. correlated strongly (R2 = 0.986) with the empirical solvent polarity (ETN) of the added solvents. The results indicate that even dil. concns. of solvents, including H2O2, can increase the reactivity of O2•- in water, probably by changing its solvation sphere. The higher reactivity of O2•- generated in modified Fenton's reagent, which has a less polar nature due to the presence of H2O2, may result in a wider range of contaminant degrdn. than previously thought possible.
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23
Teel, A. L.; Watts, R. J. Degradation of carbon tetrachloride by modified Fenton’s reagent J. Hazard. Mater. 2002, 94, 179– 189
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23
Degradation of carbon tetrachloride by modified Fenton's reagent
Teel, Amy L.; Watts, Richard J.
Journal of Hazardous Materials (2002), 94 (2), 179-189CODEN: JHMAD9; ISSN:0304-3894. (Elsevier Science B.V.)
The degrdn. of tetrachloromethane (carbon tetrachloride-CT) by modified Fenton's reagent (catalyzed H2O2) was investigated using a range of H2O2 concns. and 1 mM Fe(III) catalyst. The documented reactive species in modified Fenton's reactions, hydroxyl radical (OḢ), is not reactive with CT, yet CT degrdn. was obsd. in the Fenton's reactions and was confirmed by chloride generation. Because CT is not reactive with OḢ, a reductive mechanism which may involve superoxide radical anion is proposed for CT degrdn. in modified Fenton's systems. Scavenging of reductants by excess chloroform prevented CT degrdn., confirming a reductive mechanism. Similar to CT, 3 other oxidized aliph. compds., hexachloroethane, bromotrichloromethane, and tetranitromethane, were also degraded by modified Fenton's reagent. Modified Fenton's reactions act through a reductive mechanism to degrade compds. that are not reactive with OḢ, which broadens the scope of this process for hazardous waste treatment and remediation.
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Watts, R. J.; Bottenberg, B. C.; Hess, T. F.; Jensen, M. D.; Teel, A. L. Role of reductants in the enhanced desorption and transformation of chloroaliphatic compounds by modified Fenton’s reactions Environ. Sci. Technol. 1999, 33, 3432– 3437
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24
Role of Reductants in the Enhanced Desorption and Transformation of Chloroaliphatic Compounds by Modified Fenton's Reactions
Watts, Richard J.; Bottenberg, Brett C.; Hess, Thomas F.; Jensen, Mark D.; Teel, Amy L.
Environmental Science and Technology (1999), 33 (19), 3432-3437CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The mechanism for enhanced desorption of chloroaliph. compds. from a silty loam soil by modified Fenton's reagent was studied using a series of probe compds. of varying hydrophobicities. Hexachloroethane, which has negligible reactivity with hydroxyl radicals, was transformed more rapidly in modified Fenton's reactions (≥0.3M H2O2) than it was lost by gas-purge desorption, suggesting the existence of a non-hydroxyl radical mechanism. The addn. of excess 2-propanol to scavenge hydroxyl radicals slowed, but did not stop, the desorption and degrdn. of hexachloroethane. In the presence of the reductant scavenger chloroform, hexachloroethane did not desorb and was not degraded, indicating that a reductive pathway in vigorous Fenton-like reactions is responsible for enhanced contaminant desorption. Fenton-like degrdn. of hexachloroethane yielded the reduced product pentachloroethane, confirming the presence of a reductive mechanism. In the presence of excess 2-propanol, toluene, which has negligible reactivity with reductants, was displaced from the soil but not degraded. The results are consistent with enhanced contaminant desorption by reductants, followed by oxidn. and redn. in the aq. phase. Vigorous Fenton-like reactions in which reductants and hydroxyl radicals are generated may provide a universal treatment matrix in which contaminants are desorbed and then oxidized and reduced in a single system.
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Evaluation of a chloramine decomposition model incorporating general acid catalysis
Valentine, Richard L.; Jafvert, Chad; Leung, Solomon W.
Water Research (1988), 22 (9), 1147-53CODEN: WATRAG; ISSN:0043-1354.
A model describing the slow decompn. of chloramines in the presence of excess NH4+ was modified to incorporate general acid catalysis of NH2Cl disproportionation, a key reaction in detg. the rate of chloramine loss. Expts. were conducted at various reaction conditions in the presence of varying concns. of PO43- which was used as an analog for other naturally occurring H+ donors. Results were used to obtain an est. of the specific rate const. characterizing the effect of H2PO4- and to demonstrate the validity of the overall model formulation. Measured chloramine concns. compared favorably with predicted values indicating that inclusion of the catalytic effect on monochloramine disproportionation alone appears justified in assessing the effect of phosphate. By analogy, similar inorg. H+ donors such as CO32- and silicate should affect only NH2Cl disproportionation and the modified model should more capably assess NH2Cl decay in natural waters.
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Dichloramine decomposition in the presence of excess ammonia
Jafvert, Chad T.; Valentine, Richard L.
Water Research (1987), 21 (8), 967-73CODEN: WATRAG; ISSN:0043-1354.
The decompn. of aq. NHCl2 in the presence of excess NH3 was studied to verify a proposed decompn. mechanism. Expts. were conducted under conditions in which 3 significant reactions were isolated from others that could potentially complicate interpretation of results. The decompn. of NHCl2, producing primarily N gas and NH2Cl, was initiated by mixing solns. of NHCl2 and NH2Cl under varying exptl. conditions of pH, PO43- concn., and initial NHCl2 and NH2Cl concn. The chloramine concns. were then monitored titrimetrically with time. Rate consts. characterizing the reactions were detd. using nonlinear least squares regression anal. and the reaction stoichiometry detd. by comparing NHCl2 loss to NH2Cl formed. PO43- did not catalyze the decompn. which suggests that the mechanism does not involve general base catalysis. A mechanism including a direct reaction of NH2Cl was indicated based on both kinetic and stoichiometric considerations. The exptl. results obtained at a high initial ratio of NHCl2 to NH2Cl could be reasonably predicted with the proposed mechanism and a set of rate consts. However, the consts. were somewhat dependent on the initial NHCl2 ratio. This discrepancy may be due to the existence of another reaction(s) not included in the proposed mechanism.
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General acid catalysis of monochloramine disproportionation
Valentine, Richard L.; Jafvert, Chad T.
Environmental Science and Technology (1988), 22 (6), 691-6CODEN: ESTHAG; ISSN:0013-936X.
Expts. showed that NH2Cl disproportionation, which results in the formation of NHCl2, involves a general acid-catalyzed reaction pathway. Rate consts. characterizing the effect of H+, PO43-, and SO42- were detd. by measuring the rate of NH2Cl disappearance under pH conditions that simplified interpretation of results. These rate consts. were used to develop a linear free energy relationship that was used to predict the effect of carbonate and silicate. The predictions indicate that carbonate, and possibly silicate, may significantly increase the rate of acid-catalyzed disproportionation at concns. and pH values typical of many drinking waters. Since this reaction may govern the overall rate of oxidant loss, appropriate consideration must be given to the presence of potential proton donors when predictions relating to NH2Cl speciation and fate are made on the basis of reaction models or when the results of studies with NH2Cl solns. are evaluated.
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Lin, Y. P.; Washburn, M.; Valentine, R. L. Reduction of lead oxide (PbO₂) by iodide and formation of iodoform in the PbO₂/I⁻/NOM system Environ. Sci. Technol. 2008, 42, 2919– 2924
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Reduction of Lead Oxide (PbO2) by Iodide and Formation of Iodoform in the PbO2/I-/NOM System
Lin, Yi-Pin; Washburn, Michael P.; Valentine, Richard L.
Environmental Science & Technology (2008), 42 (8), 2919-2924CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
PbO2 can be an important form of Pb mineral scale occurring in some water distribution systems. It is believed to be formed by the oxidn. of Pb-contg. plumbing materials by free Cl. Its reactivity in water, however, has not been well studied. Iodide is also found in source drinking waters, albeit at low concns. Consideration of thermodn. suggests that I- can be oxidized by PbO2. I- was used as a probe compd. to study the redn. of PbO2 and the formation of iodoform, which has been predicted to be a carcinogen, in the presence of natural org. matter (NOM). The redn. of PbO2 by I- can be expressed as PbO2 + 3I- + 4H+ → Pb2+ + I3- + 2H2O, and the reaction kinetics was detd. In the presence of NOM, I3- reacts with NOM to form iodoform and its concn. is proportional to the NOM concn. Our results indicate that PbO2 is a very powerful oxidant and can possibly serve as an oxidant reservoir for the formation of iodinated disinfection byproduct through a novel reaction pathway.
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Zhang, P. C.; Ryan, J. A. Transformation of Pb(II) from cerrusite to chloropyromorphite in the presence of hydroxyapatite under varying conditions of pH Environ. Sci. Technol. 1999, 33, 625– 630
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Transformation of Pb(II) from Cerrusite [Cerussite] to Chloropyromorphite in the Presence of Hydroxyapatite under Varying Conditions of pH
Zhang, Pengchu; Ryan, James A.
Environmental Science and Technology (1999), 33 (4), 625-630CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The sol. Pb concn. and formation of chloropyromorphite [Pb5(PO4)3Cl] were monitored during the reaction of cerussite (PbCO3), a highly bioavailable soil Pb species, and hydroxyapatite [Ca5(PO4)3OH] at various P/Pb molar ratios under const. and dynamic pH conditions. Under pH-const. systems at pH 4 and below, the dissoln. rates of both cerrusite and apatite were rapid, and complete conversion of cerussite to chloropyromorphite occurred within 60 min when the amt. of phosphate added via apatite was stoichiometrically equal to that needed to transform all added Pb into chloropyromorphite. The concn. of sol. Pb depended upon the soly. of chloropyromorphite. The dissoln. rates of apatite and cerussite decreased with increasing pH, and the transformation was incomplete at pH 5 and above in the 60-min reaction period. The sol. Pb level, therefore, was detd. by the soly. of cerrusite. In the pH-dynamic system, which simulated the gastrointestinal tract (GI tract), a complete transformation of Pb from cerrusite to chloropyromorphite was achieved due to the complete dissoln. of apatite and cerrusite at the initial low pH. In both the const. and dynamic pH systems XRD anal. indicated that chloropyromorphite was the exclusive reaction product. The differences in transformation rate and the Pb soly. between the const. and dynamic pH systems indicate the significance of kinetics in controlling the bioavailability of Pb and the potential for the reaction to occur during ingestion.
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In Vitro Soil Pb Solubility in the Presence of Hydroxyapatite
Zhang, Pengchu; Ryan, James A.; Yang, John
Environmental Science and Technology (1998), 32 (18), 2763-2768CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
The transformation of lead (Pb) in contaminated soils to pyromorphite, by the addn. of phosphate minerals, may be an economic in-situ immobilization strategy that results in a redn. of bioavailable Pb. To test this hypothesis, two sets of soil-soln. expts. under const. (i.e., fixed) or dynamic (i.e., variable) pH conditions, were conducted as a function of time. In both sets of expts., Pb-contaminated soil was reacted with synthetic hydroxyapatite in order to det. the transformation rate of soil Pb to pyromorphite and the sol. Pb level during the reaction period. In the const. pH system, the sol. Pb concn. decreased with the addn. of apatite at pH 4 and above. However, the transformation was pH-dependent and incomplete at relatively high pH (≥6). The soly. of cerrusite (PbCO3), the major Pb mineral in this soil, still exhibited a strong influence on the soly. of soil Pb. In the dynamic pH expts., which simulated gastric pH conditions (i.e., pH variation from 2 to 7 within 25 or 45 min), both cerrusite and added apatite were dissolved at low pH values (pH 2 and pH 3), and chloropyromorphite was rapidly pptd. from dissolved Pb and PO4 when the suspension pH was increased. Complete transformation of soil Pb to chloropyromorphite occurred in the pH dynamic expts. within 25 min, indicating rapid reaction kinetics of the formation of chloropyromorphite. Chloropyromorphite soly. controls the sol. Pb concn. during the entire duration of the pH dynamic expts. Thus, specific site conditions, such as pH, are important when considering evaluation of soil Pb bioavailability and in-situ immobilization of Pb in Pb-contaminated soils using phosphate amendment. Furthermore, this study demonstrates that the kinetics of conversion of soil Pb to chloropyromorphite in the presence of apatite is fast enough to occur during ingestion and that gastric pH conditions would favor the formation of chloropyromorphite, thus rendering ingested soil Pb nonbioavailable.
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Vikesland, P. J.; Ozekin, K.; Valentine, R. L. Effect of natural organic matter on monochloramine decomposition: Pathway elucidation through the use of mass and redox balances Environ. Sci. Technol. 1998, 32, 1409– 1416
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Effect of Natural Organic Matter on Monochloramine Decomposition: Pathway Elucidation through the Use of Mass and Redox Balances
Vikesland, Peter J.; Ozekin, Kenan; Valentine, Richard L.
Environmental Science and Technology (1998), 32 (10), 1409-1416CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Monochloramine is often employed as a drinking water disinfectant for systems where free chlorine residuals are difficult to maintain or where disinfection byproduct formation is significant. However, monochloramine is unstable and decomps., leading to nitrogen oxidn. and chlorine redn. (auto-decompn.). The role of natural org. matter (NOM) in monochloramine loss is unclear. NOM could catalyze monochloramine auto-decompn., or it could act as an external reductant. This study elucidates the decay pathways of monochloramine in the presence and absence of NOM. When monochloramine decomps. in the absence of NOM, ammonia and nitrogen gas are the primary nitrogen decay products. When NOM is present, the product speciation changes such that little nitrogen gas prodn. occurs, yet prodn. of ammonia and nitrate increases. This product speciation shift indicates that under these conditions, NOM acts primarily as a reductant and not as a catalyst. This conclusion was verified using a redox balance which compares oxidized product, N2, and NO3- prodn. to monochloramine loss. The no. of electrons accounted for by oxidized product prodn. correlates well with monochloramine loss in the absence of NOM (60-100% recovery). However, there is a deficit in the presence of NOM (25-60% recovery). Clearly, much of the oxidizing capacity of monochloramine goes to NOM oxidn.
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Chloramines again linked to lead in drinking water
Renner, Rebecca
Environmental Science and Technology (2005), 39 (15), 314ACODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
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Figure 1
Figure 1. Effect of NH₂Cl concentration on the formation of total Pb(II) and NH₂Cl decay: (a) NH₂Cl decay as a function of time, (b) total Pb(II) formation as a functiom of time, and (c) total Pb(II) formation vs NH₂Cl decay (data of duplicate experiments were shown by separate points). Fixed experimental parameters: PbO₂ = 10 mg/L, pH 7.0, C_T = 5 mM, Cl/N = 0.7. In control experiment for NH₂Cl decay, no PbO₂ was added.
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Figure 2
Figure 2. SEM image of the solid phase after 7 day experiment. NH₂Cl = 70.4 μM, PbO₂ = 10 mg/L, pH 7.0, C_T = 5 mM, Cl/N = 0.7.
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Figure 3
Figure 3. Effect of solution pH on the formation of total Pb(II) and NH₂Cl decay: (a) NH₂Cl decay as a function of time, (b) total Pb(II) formation as a function of time, and (c) total Pb(II) formation vs NH₂Cl decay vs total Pb(II) formation (data of duplicate experiments were shown by separate points). PbO₂ = 10 mg/L, NH₂Cl = 42.3 μM, C_T = 5 mM, Cl/N = 0.7. In control experiment for NH₂Cl decay, no PbO₂ was added.
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Figure 4
Figure 4. Effect of total carbonate concentration on the formation of total Pb(II) and NH₂Cl decay: (a) total Pb(II) formation as a functiom of time, (b) NH₂Cl decay as a function of time, and (c) NH₂Cl decay vs total Pb(II) formation (data of duplicate experiments were shown by separate points). PbO₂ = 10 mg/L, NH₂Cl = 42.3 μM, pH 7.0, Cl/N = 0.7. In control experiment for NH₂Cl decay, no PbO₂ was added.
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Figure 5
Figure 5. Effect of Cl/N molar ratio on the formation of total Pb(II) and NH₂Cl decay: (a) total Pb(II) formation as a functiom of time, (b) NH₂Cl decay as a function of time, and (c) NH₂Cl decay vs total Pb(II) formation (data of duplicate experiments were shown by separate points). PbO₂ = 10 mg/L, NH₂Cl = 42.3 μM, pH 7.0, C_T = 5 mM. In control experiment for NH₂Cl decay, no PbO₂ was added.
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Figure 6
Figure 6. Summary plot of total Pb(II) formation vs NH₂Cl decay.
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  A switch from free Cl to chloramine disinfectant triggered problems with excessive Pb in Washington, D.C., drinking water. High levels of Pb originated in the service lines, but excessive Pb was also derived from solder or brass plumbing materials. In many cases, the highest Pb concns. emerged from the tap after ∼1 min of flushing, a troublesome outcome, given that routine public notification recommended that consumers flush for ∼1 min to minimize Pb exposure. Bench-scale testing found that Cl reacts with sol. Pb2+ to rapidly ppt. a red-brown-colored Pb solid that was insol. even at pH 1.9 for 12 wk; this solid did not form in the presence of chloramine. Expts. indicated that chloramines sometimes dramatically worsened Pb leaching from brass relative to free Cl, whereas new Pb pipe was not strongly affected.
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  Recent research showed that Pb(IV) oxides play a significant geochem. role in drinking water distribution systems. However, most of the guidance for lead control in drinking water is based on the presumption that Pb(II) solids control lead soly. Therefore, a better understanding of the chem. of Pb(IV) in water is needed. Long-term lead pptn. expts. were conducted in chlorinated water (1-3 mg/L Cl2) at pH 6.5, 8, and 10, with and without sulfate. Two Pb(IV) dioxide polymorphs-plattnerite (β-PbO2) and scrutinyite (α-PbO2)-formed over time, as long as a high suspension redox potential was maintained with free chlorine. Neither mineral formed spontaneously, and the rate of formation increased with increasing pH. Hydrocerussite and/or cerussite initially pptd. out and overtime either disappeared or coexisted with PbO2. Water pH dictated mineralogical presence. High pH favored hydrocerrusite and scrutinyite; low pH favored cerussite and plattnerite. Along with a transformation of Pb(II) to Pb(IV) came a change in particle color from white to a dark shade of red to dark gray (differing with pH) and a decrease in lead soly. If free chlorine was permitted to dissipate, the aging processes (i.e., mineralogy, color, and soly.) were reversible.
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  Investigation of the Reduction of Lead Dioxide by Natural Organic Matter
  Dryer, Deborah J.; Korshin, Gregory V.
  Environmental Science & Technology (2007), 41 (15), 5510-5514CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Expts. with immobilized lead dioxide showed that this solid was reduced by natural org. matter (NOM) isolated from Potomac River water. Kinetically, the process was slow and occurred throughout many weeks of exposure. The amt. of mobilized lead was affected by the concn. of NOM and exposure time but not significantly influenced by the type of NOM used in the expts. The interactions of NOM with PbO2 were quantified using differential absorbance spectroscopy. It showed that the oxidn. of chromophoric groups in NOM was strongly correlated with lead release. Because lead release yields were higher that those predicted based on the depletion of the arom. groups, it is hypothesized that NOM moieties other than arom. functionalities are engaged in the redn. of PbO2 by NOM and/or lead mobilization involves the formation of mixed Pb(II)/Pb(IV) sol. and colloidal species.
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  Rajasekharan, V. V.; Clark, B. N.; Boonsalee, S.; Switzer, J. A. Electrochemistry of free chlorine and monochloramine and its relevance to the presence of Pb in drinking water Environ. Sci. Technol. 2007, 41, 4252– 4257
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  Electrochemistry of Free Chlorine and Monochloramine and its Relevance to the Presence of Pb in Drinking Water
  Rajasekharan, Vishnu V.; Clark, Brandi N.; Boonsalee, Sansanee; Switzer, Jay A.
  Environmental Science & Technology (2007), 41 (12), 4252-4257CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The commonly used disinfectants in drinking water are free chlorine (in the form of HOCl/OCl-) and monochloramine (NH2Cl). While free chlorine reacts with natural org. matter in water to produce chlorinated hydrocarbon byproducts, there is also concern that NH2Cl may react with Pb to produce sol. Pb(II) products-leading to elevated Pb levels in drinking water. In this study, electrochem. methods are used to compare the thermodn. and kinetics of the redn. of these two disinfectants. The std. redn. potential for NH2Cl/Cl- was estd. to be +1.45 V in acidic media and +0.74 V in alk. media vs. NHE using thermodn. cycles. The kinetics of electroredn. of the two disinfectants was studied using an Au rotating disk electrode. The exchange current densities estd. from Koutecky-Levich plots were 8.2 × 10-5 and 4.1 × 10-5 A/cm2, and by low overpotential expts. were 7.5 ± 0.3 × 10-5 and 3.7 ± 0.4 × 10-5 A/cm2 for free chlorine and NH2Cl, resp. The rate const. for the electrochem. redn. of free chlorine at equil. is approx. twice as large as that for the redn. of NH2Cl. Equil. potential measurements show that free chlorine will oxidize Pb to PbO2 above pH 1.7, whereas NH2Cl will oxidize Pb to PbO2 only above about pH 9.5, if the total dissolved inorg. carbon (DIC) is 18 ppm. Hence, NH2Cl is not capable of producing a passivating PbO2 layer on Pb, and could lead to elevated levels of dissolved Pb in drinking water.
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  Lin, Y. P.; Valentine, R. L. The release of lead from the reduction of lead oxide (PbO₂) by natural organic matter Environ. Sci. Technol. 2008, 42, 760– 765
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  The Release of Lead from the Reduction of Lead Oxide (PbO2) by Natural Organic Matter
  Lin, Yi-Pin; Valentine, Richard L.
  Environmental Science & Technology (2008), 42 (3), 760-765CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  PbO2 has been identified as an important scale in some distribution systems that historically use lead service lines and free chlorine for maintaining a disinfectant residual. The stability of this highly insol. scale with respect to its reductive dissoln. may play an important role in lead release into drinking water. In this study, we investigated the release of lead from a com. available PbO2 in the presence of natural org. matter (NOM) using a hydrophobic acid extd. from the Iowa River. Expts. were conducted using synthetic solns. with different NOM concns., soln. pH, and NOM samples with different levels of prechlorination. It was found that release of lead from PbO2 occurred both in solns. with and without NOM, and the extent of lead release increased with increasing NOM concn. and decreasing pH value. Furthermore, the released lead was Pb(II) and not particulate PbO2 conclusively showing that reductive dissoln. occurred. Prechlorination of NOM reduced the rate of lead release. Our results indicate that PbO2 can be reduced both by water and NOM. Characterization of final solid phases by SEM and XPS are also presented.
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  Jafvert, C. T.; Valentine, R. L. Reaction scheme for the chlorination of ammoniacal water Environ. Sci. Technol. 1992, 26, 577– 586
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  Reaction scheme for the chlorination of ammoniacal water
  Jafvert, Chad T.; Valentine, Richard L.
  Environmental Science and Technology (1992), 26 (3), 577-86CODEN: ESTHAG; ISSN:0013-936X.
  A kinetic model of the reacting aq. Cl-NH3 system is proposed that describes equally well the rapid breakpoint oxidn. of NH3, where the applied Cl dose (Cl2) to NH3-N molar ratio (Cl/N) is ⪆1.6; the slow oxidn. of NH3 in aq. NH2Cl solns. (Cl/N <1); and the transition region of 1 < Cl/N < 1.6, where rapid initial decay results in chloramine species residuals. Calcd. time-dependent concns. of the Cl species, detd. by numerical soln. of the rate expressions, compare favorably to measured values, detd. during expts. performed at initial pH (6-8) and Cl/N (0.25-2.0) conditions. The exptl. measured species include free Cl (HClO + ClO-), NH2Cl, and NHCl2. The model appropriately considers the catalysis of certain key reactions by several commonly encountered inorgs., such as HCO3- and phosphate species.
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13. 13
  Ozekin, K.; Valentine, R. L.; Vikesland, P. J. Modeling the decomposition of disinfecting residuals of chloramine. InWater Disinfection and Natural Organic Matter; ACS Symposium Series 649; American Chemical Society: Washington, DC, 1996; pp 115− 125.
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  Vikesland, P. J.; Ozekin, K.; Valentine, R. L. Monochloramine decay in model and distribution system waters Water Res. 2001, 35, 1766– 1776
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  Monochloramine Decay in Model and Distribution System Waters
  Vikesland, P. J.; Ozekin, K.; Valentine, R. L.
  Water Research (2001), 35 (7), 1766-1776CODEN: WATRAG; ISSN:0043-1354. (Elsevier Science Ltd.)
  Chloramines have long been used to provide a disinfecting residual in distribution systems where it is difficult to maintain a free Cl residual or where disinfection byproduct (DBP) formation is of concern. While chloramines are generally considered less reactive than free Cl, they are inherently unstable even in the absence of reactive substances. These reactions, often referred to as auto-decompn., always occur and hence define the max. stability of monochloramine in water. The effect of addnl. reactive material must be measured relative to this basic loss process. A thorough understanding of the auto-decompn. reactions is fundamental to the development of mechanisms that account for reactions with addnl. substances and to the ultimate formation of DBPs. A kinetic model describing auto-decompn. was recently developed. This model is based on studies of isolated individual reactions and on observations of the reactive ammonia-Cl system as a whole. This work validates and extends this model for use in waters typical of those encountered in distribution systems and under realistic chloramination conditions. The effect of carbonate and temp. on auto-decompn. is discussed. The influence of bromide and nitrite at representative monochloramine concns. is also examd., and addnl. reactions to account for their influence on monochloramine decay are presented to demonstrate the ability of the model to incorporate inorg. demand pathways that occur parallel to auto-decompn.
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15. 15
  Duirk, S. E.; Gombert, B.; Croue, J. P.; Valentine, R. L. Modeling monochloramine loss in the presence of natural organic matter Water Res. 2005, 39, 3418– 3431
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  Modeling monochloramine loss in the presence of natural organic matter
  Duirk, Stephen E.; Gombert, Bertrand; Croue, Jean-Philippe; Valentine, Richard L.
  Water Research (2005), 39 (14), 3418-3431CODEN: WATRAG; ISSN:0043-1354. (Elsevier B.V.)
  A comprehensive model describing monochloramine loss in the presence of natural org. matter (NOM) is presented. The model incorporates simultaneous monochloramine autodecompn. and reaction pathways resulting in NOM oxidn. These competing pathways were resolved numerically using an iterative process evaluating hypothesized reactions describing NOM oxidn. by monochloramine under various exptl. conditions. The reaction of monochloramine with NOM was described as biphasic using four NOM specific reaction parameters. NOM pathway 1 involves a direct reaction of monochloramine with NOM (kdoc1=1.05 × 104-3.45 × 104/M-h). NOM pathway 2 is slower in terms of monochloramine loss and attributable to free Cl (HOCl) derived from monochloramine hydrolysis (kdoc2=5.72 × 105-6.98 × 105/M-h), which accounted for the majority of monochloramine loss. The free Cl reactive site fraction in the NOM structure was found to correlate to specific UV absorbance at 280 nm (SUVA280). Modeling monochloramine loss allowed for insight into disinfectant reaction pathways involving NOM oxidn. This knowledge is of value in assessing monochloramine stability in distribution systems and reaction pathways leading to disinfection byproduct formation.
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  Valentine, R. L.; Brandt, K. I.; Jafvert, C. T. A spectrophotometric study of the formation of an unidentified monochloramine decomposition product Water Res. 1986, 20, 1067– 1074
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  Water Research (1986), 20 (8), 1067-74CODEN: WATRAG; ISSN:0043-1354.
  The formation of an unidentified product(s) of the slow decompn. of NH2Cl in org.-free aq. solns. at pH values and Cl-N ratios of importance in the chloramination disinfection of drinking water was studied. NH2Cl and total oxidant concns. detd. spectrophotometrically in these solns. became significantly higher with time than those detd. by a titrimetric methods due to the absorbance of the unidentified product(s). The UV spectra of the product(s) were calcd. from the difference between measured and predicted spectra and were similar to those obtained on NH2Cl-free soln. resulting from the rapid decompn. of dichloramine at high pH. No spectrophotometric evidence could be found for the formation of significant concns. of NO2- and/or NO3-. Relative concn. changes of the unidentified product(s) as measured by its calcd. absorbance at 243 nm showed that the product(s) accumulates with time and, therefore, is not an intermediate in the formation of N gas. Both increased pH and PO43- buffer increased its formation rate. A formation mechanism involving the decompn. of NHCl2 is suggested. The age-history of NH2Cl solns. could be an important variable in toxicol. studies of chloramines and their reaction products depending on the health effects of the unidentified product(s).
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  Diyamandoglu, V.; Selleck, R. E. Reactions and products of chloramination Environ. Sci. Technol. 1992, 26, 808– 814
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  Reactions and products of chloramination
  Diyamandoglu, Vasil; Selleck, Robert E.
  Environmental Science and Technology (1992), 26 (4), 808-14CODEN: ESTHAG; ISSN:0013-936X.
  The stoichiometry of the reaction of Cl with excess NH3 in dil. aq. soln. was studied at near-neutral pH and at pH 9.3-9.6. A significant portion of Cl was incorporated into unrecognized reaction products at near-neutral pH, and essentially none at the elevated pH. The absence of detectable NO2- did not prohibit the prodn. of significant amts. of NO3- at the pH levels studied. The relative significance of the NO3- prodn. increased upon diln. of the reactants at near-neutral pH, with NO3- becoming the primary end product of N oxidn. under conditions approaching those encountered in the disinfection of drinking water. The decompn. of H2NNH2 appeared to be responsible for the formation of N gas at the elevated pH. No explanation can be offered for the significant quantities of NO3- produced at that pH.
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  Vikesland, P. J.; Valentine, R. L. Reaction pathways involved in the reduction of monochloramine by ferrous iron Environ. Sci. Technol. 2000, 34, 83– 90
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  Reaction Pathways Involved in the Reduction of Monochloramine by Ferrous Iron
  Vikesland, Peter J.; Valentine, Richard L.
  Environmental Science and Technology (2000), 34 (1), 83-90CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The maintenance of disinfectants in distribution systems is crit. to ensure the safety of drinking water. Reduced iron is one type of reactive constituent believed to play a central role in disinfectant loss as the water travels through the pipe. Specifically, reactions involving Fe(II), both in soln. and at the pipe-water interface, are felt to be important processes that account for this loss. This work shows that the rate-limiting reactions responsible for the disappearance of monochloramine involve a direct reaction between mol. monochloramine and Fe(II), leading to the formation of the amidogen radical (•NH2) as a reactive intermediate. In addn., the mechanism was found to be autocatalytic with the ferric oxide ppt. acting to accelerate the overall redn. of monochloramine. A variable stoichiometry was obsd. for this system, and this was rationalized by accounting for radical scavenging in the reaction scheme.
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  Corbin, J. F.; Teel, A. L.; Allen-King, R. M.; Watts, R. J. Reactive oxygen species responsible for the enhanced desorption of dodecane in modified Fenton’s systems Water Environ. Res. 2007, 79, 37– 42
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  Reactive oxygen species responsible for the enhanced desorption of dodecane in modified Fenton's systems
  Corbin, Joseph F., III; Teel, Amy L.; Allen-King, Richelle M.; Watts, Richard J.
  Water Environment Research (2007), 79 (1), 37-42CODEN: WAERED; ISSN:1061-4303. (Water Environment Federation)
  The enhanced treatment of sorbed contaminants has been documented in modified Fenton's reactions; contaminants are desorbed and degraded more rapidly than they desorb by fill-and-draw or gas-purge desorption. The reactive species responsible for this process was studied using dodecane as a model sorbent. Hydroxyl radical, hydroperoxide anion, and superoxide radical anion were generated sep. to evaluate their roles in enhanced dodecane desorption. Dodecane desorption from silica sand over 180 min was negligible in gas-purge systems and in the hydroxyl radical and hydroperoxide anion systems. In contrast, enhanced desorption of dodecane occurred in superoxide systems, with >80% desorption over 180 min. Scavenging of superoxide eliminated the enhanced desorption of dodecane in both superoxide and modified Fenton's systems, confirming that superoxide is the desorbing agent in modified Fenton's reactions. Conditions that promote superoxide generation in Fenton's reactions may enhance their effectiveness for in situ subsurface remediation of sorbed hydrophobic contaminants.
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20. 20
  Smith, B. A.; Teel, A. L.; Watts, R. J. Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton’s reagent J. Contam. Hydrol. 2006, 85, 229– 246
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  Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton's reagent
  Smith, Brant A.; Teel, Amy L.; Watts, Richard J.
  Journal of Contaminant Hydrology (2006), 85 (3-4), 229-246CODEN: JCOHE6; ISSN:0169-7722. (Elsevier B.V.)
  The destruction of CCl4 and chloroform was studied in reactors contg. 0.5 mL DNAPL and a soln. of modified Fenton's reagent (2M H2O2 and 5mM Fe(III)-chelate). CCl4 and chloroform masses were followed in the DNAPLs, the aq. phases, and the off gasses. The rate of DNAPL destruction was compared to the rate of gas-purge dissoln. CCl4 DNAPLs were rapidly destroyed by modified Fenton's reagent at 6.5 times the rate of gas purge dissoln., with 74% of the DNAPL destroyed within 24 h. Use of reactions in which a single reactive O species (hydroxyl radical, hydroperoxide anion, or superoxide radical anion) was generated showed that superoxide is the reactive species in modified Fenton's reagent responsible for CCl4 destruction. Chloroform was also destroyed by modified Fenton's reagent, but at a rate slower than the rate of gas purge dissoln. Reactions generating a single reactive O species demonstrated that chloroform destruction was the result of both superoxide and hydroxyl radical activity. Such a mechanism of chloroform destruction is in agreement with the slow but relatively equal reactivity of chloroform with both superoxide and hydroxyl radical. The results of this research demonstrate that modified Fenton's reagent can rapidly and effectively destroy DNAPLs of contaminants characterized by minimal reactivity with hydroxyl radical, and should receive more consideration as a DNAPL cleanup technol.
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  Watts, R. J.; Sarasa, J.; Loge, F. J.; Teel, A. L. Oxidative and reductive pathways in manganese-catalyzed Fenton’s reactions J. Environ. Eng. 2005, 131, 158– 164
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  Oxidative and Reductive Pathways in Manganese-Catalyzed Fenton's Reactions
  Watts, Richard J.; Sarasa, Judith; Loge, Frank J.; Teel, Amy L.
  Journal of Environmental Engineering (Reston, VA, United States) (2005), 131 (1), 158-164CODEN: JOEEDU; ISSN:0733-9372. (American Society of Civil Engineers)
  Sol. Mn(II) and amorphous and cryst. Mn(IV) oxides were studied as catalysts for the Fenton-like decompn. of H2O2 into oxidants and reductants. 1-Hexanol was used as a hydroxyl radical probe and CCl4 was used as a reductant probe. Sol. Mn(II)-catalyzed reactions at acidic pH resulted in >99% degrdn. of 1-hexanol and no measurable transformation of CCl4, indicating that hydroxyl radicals were generated but reductants were not. However, when these reactions were conducted at near-neutral pH, an amorphous Mn oxide ppt. formed and 89% of the CCl4 degraded in 60 min, while 1-hexanol degrdn. was negligible. Using an amorphous Mn oxide synthesized in a sep. reactor, CCl4 was rapidly degraded while 1-hexanol oxidn. was undetectable. Reactions catalyzed by the cryst. Mn oxide pyrolusite(β-MnO2) at near-neutral pH also resulted in significant CCl4 degrdn., indicating that reductants are generated by both the cryst. and amorphous Mn oxide-catalyzed decompn. of H2O2. The presence of Mn oxides in the subsurface and their ability to catalyze the generation of reductants in modified Fenton's reactions has important implications for H2O2 stability and contaminant transformation pathways during the in situ Fenton's treatment of contaminated soils and groundwater.
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22. 22
  Smith, B. A.; Teel, A. L.; Watts, R. J. Identification of the reactive oxygen species responsible for carbon tetrachloride degradation in modified Fenton’s systems Environ. Sci. Technol. 2004, 38, 5465– 5469
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  22
  Identification of the Reactive Oxygen Species Responsible for Carbon Tetrachloride Degradation in Modified Fenton's Systems
  Smith, Brant A.; Teel, Amy L.; Watts, Richard J.
  Environmental Science and Technology (2004), 38 (20), 5465-5469CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The reactive oxygen species responsible for the transformation of carbon tetrachloride (CT) by a modified Fenton's reagent using hydrogen peroxide concns. >0.1M was investigated. The addn. of the hydroxyl radical scavenger 2-propanol to modified Fenton's reactions did not significantly lower CT transformation rates. Scavenging by 2-propanol not only confirmed that hydroxyl radicals are not responsible for CT destruction, but also suggested that a major product of an iron (III)-driven initiation reaction, superoxide radical anion (O2•-), is the species responsible for CT transformation. To investigate this hypothesis, CT degrdn. was studied in aq. KO2 reactions. Minimal CT degrdn. was found in CT-KO2 reactions; however, when H2O2 was added to the KO2 reactions at concns. similar to those in the modified Fenton's reactions (0.1, 0.5, and 1M), CT degrdn. increased significantly. Similar results were obtained when 1M concns. of other solvents were added to aq. KO2 reactions, and the obsd. first-order rate const. for CT degrdn. correlated strongly (R2 = 0.986) with the empirical solvent polarity (ETN) of the added solvents. The results indicate that even dil. concns. of solvents, including H2O2, can increase the reactivity of O2•- in water, probably by changing its solvation sphere. The higher reactivity of O2•- generated in modified Fenton's reagent, which has a less polar nature due to the presence of H2O2, may result in a wider range of contaminant degrdn. than previously thought possible.
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23. 23
  Teel, A. L.; Watts, R. J. Degradation of carbon tetrachloride by modified Fenton’s reagent J. Hazard. Mater. 2002, 94, 179– 189
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  Degradation of carbon tetrachloride by modified Fenton's reagent
  Teel, Amy L.; Watts, Richard J.
  Journal of Hazardous Materials (2002), 94 (2), 179-189CODEN: JHMAD9; ISSN:0304-3894. (Elsevier Science B.V.)
  The degrdn. of tetrachloromethane (carbon tetrachloride-CT) by modified Fenton's reagent (catalyzed H2O2) was investigated using a range of H2O2 concns. and 1 mM Fe(III) catalyst. The documented reactive species in modified Fenton's reactions, hydroxyl radical (OḢ), is not reactive with CT, yet CT degrdn. was obsd. in the Fenton's reactions and was confirmed by chloride generation. Because CT is not reactive with OḢ, a reductive mechanism which may involve superoxide radical anion is proposed for CT degrdn. in modified Fenton's systems. Scavenging of reductants by excess chloroform prevented CT degrdn., confirming a reductive mechanism. Similar to CT, 3 other oxidized aliph. compds., hexachloroethane, bromotrichloromethane, and tetranitromethane, were also degraded by modified Fenton's reagent. Modified Fenton's reactions act through a reductive mechanism to degrade compds. that are not reactive with OḢ, which broadens the scope of this process for hazardous waste treatment and remediation.
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24. 24
  Watts, R. J.; Bottenberg, B. C.; Hess, T. F.; Jensen, M. D.; Teel, A. L. Role of reductants in the enhanced desorption and transformation of chloroaliphatic compounds by modified Fenton’s reactions Environ. Sci. Technol. 1999, 33, 3432– 3437
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  Role of Reductants in the Enhanced Desorption and Transformation of Chloroaliphatic Compounds by Modified Fenton's Reactions
  Watts, Richard J.; Bottenberg, Brett C.; Hess, Thomas F.; Jensen, Mark D.; Teel, Amy L.
  Environmental Science and Technology (1999), 33 (19), 3432-3437CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The mechanism for enhanced desorption of chloroaliph. compds. from a silty loam soil by modified Fenton's reagent was studied using a series of probe compds. of varying hydrophobicities. Hexachloroethane, which has negligible reactivity with hydroxyl radicals, was transformed more rapidly in modified Fenton's reactions (≥0.3M H2O2) than it was lost by gas-purge desorption, suggesting the existence of a non-hydroxyl radical mechanism. The addn. of excess 2-propanol to scavenge hydroxyl radicals slowed, but did not stop, the desorption and degrdn. of hexachloroethane. In the presence of the reductant scavenger chloroform, hexachloroethane did not desorb and was not degraded, indicating that a reductive pathway in vigorous Fenton-like reactions is responsible for enhanced contaminant desorption. Fenton-like degrdn. of hexachloroethane yielded the reduced product pentachloroethane, confirming the presence of a reductive mechanism. In the presence of excess 2-propanol, toluene, which has negligible reactivity with reductants, was displaced from the soil but not degraded. The results are consistent with enhanced contaminant desorption by reductants, followed by oxidn. and redn. in the aq. phase. Vigorous Fenton-like reactions in which reductants and hydroxyl radicals are generated may provide a universal treatment matrix in which contaminants are desorbed and then oxidized and reduced in a single system.
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  Valentine, R. L.; Ozekin, K.; Vikesland, P. J. Chloramine Decomposition in Distribution System and Model Waters; American Water Works Association Research Foundation: Denver, CO, 1998.
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  Valentine, R. L.; Jafvert, C. T.; Leung, S. W. Evaluation of a chloramine decomposition model incorporating general acid catalysis Water Res. 1988, 22, 1147– 1153
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  Evaluation of a chloramine decomposition model incorporating general acid catalysis
  Valentine, Richard L.; Jafvert, Chad; Leung, Solomon W.
  Water Research (1988), 22 (9), 1147-53CODEN: WATRAG; ISSN:0043-1354.
  A model describing the slow decompn. of chloramines in the presence of excess NH4+ was modified to incorporate general acid catalysis of NH2Cl disproportionation, a key reaction in detg. the rate of chloramine loss. Expts. were conducted at various reaction conditions in the presence of varying concns. of PO43- which was used as an analog for other naturally occurring H+ donors. Results were used to obtain an est. of the specific rate const. characterizing the effect of H2PO4- and to demonstrate the validity of the overall model formulation. Measured chloramine concns. compared favorably with predicted values indicating that inclusion of the catalytic effect on monochloramine disproportionation alone appears justified in assessing the effect of phosphate. By analogy, similar inorg. H+ donors such as CO32- and silicate should affect only NH2Cl disproportionation and the modified model should more capably assess NH2Cl decay in natural waters.
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  Jafvert, C. T.; Valentine, R. L. Dichloramine decomposition in the presence of excess ammonia Water Res. 1987, 21, 967– 973
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  Dichloramine decomposition in the presence of excess ammonia
  Jafvert, Chad T.; Valentine, Richard L.
  Water Research (1987), 21 (8), 967-73CODEN: WATRAG; ISSN:0043-1354.
  The decompn. of aq. NHCl2 in the presence of excess NH3 was studied to verify a proposed decompn. mechanism. Expts. were conducted under conditions in which 3 significant reactions were isolated from others that could potentially complicate interpretation of results. The decompn. of NHCl2, producing primarily N gas and NH2Cl, was initiated by mixing solns. of NHCl2 and NH2Cl under varying exptl. conditions of pH, PO43- concn., and initial NHCl2 and NH2Cl concn. The chloramine concns. were then monitored titrimetrically with time. Rate consts. characterizing the reactions were detd. using nonlinear least squares regression anal. and the reaction stoichiometry detd. by comparing NHCl2 loss to NH2Cl formed. PO43- did not catalyze the decompn. which suggests that the mechanism does not involve general base catalysis. A mechanism including a direct reaction of NH2Cl was indicated based on both kinetic and stoichiometric considerations. The exptl. results obtained at a high initial ratio of NHCl2 to NH2Cl could be reasonably predicted with the proposed mechanism and a set of rate consts. However, the consts. were somewhat dependent on the initial NHCl2 ratio. This discrepancy may be due to the existence of another reaction(s) not included in the proposed mechanism.
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  Valentine, R. L.; Jafvert, C. T. General acid catalysis of monochloramine disproportionation Environ. Sci. Technol. 1988, 22, 691– 696
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  General acid catalysis of monochloramine disproportionation
  Valentine, Richard L.; Jafvert, Chad T.
  Environmental Science and Technology (1988), 22 (6), 691-6CODEN: ESTHAG; ISSN:0013-936X.
  Expts. showed that NH2Cl disproportionation, which results in the formation of NHCl2, involves a general acid-catalyzed reaction pathway. Rate consts. characterizing the effect of H+, PO43-, and SO42- were detd. by measuring the rate of NH2Cl disappearance under pH conditions that simplified interpretation of results. These rate consts. were used to develop a linear free energy relationship that was used to predict the effect of carbonate and silicate. The predictions indicate that carbonate, and possibly silicate, may significantly increase the rate of acid-catalyzed disproportionation at concns. and pH values typical of many drinking waters. Since this reaction may govern the overall rate of oxidant loss, appropriate consideration must be given to the presence of potential proton donors when predictions relating to NH2Cl speciation and fate are made on the basis of reaction models or when the results of studies with NH2Cl solns. are evaluated.
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29. 29
  Schock, M. R. Hot research topics in lead control. Presented at the EPA Inorganic Contaminant Issues Workshop, Cincinnati, Ohio, 2007;
  Available at http://www.epa.gov/nrmrl/wswrd/cr/pubs/schock_io_08212007. pdf accessed Jan. 15, 2008.
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30. 30
  Lin, Y. P.; Washburn, M.; Valentine, R. L. Reduction of lead oxide (PbO₂) by iodide and formation of iodoform in the PbO₂/I⁻/NOM system Environ. Sci. Technol. 2008, 42, 2919– 2924
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  30
  Reduction of Lead Oxide (PbO2) by Iodide and Formation of Iodoform in the PbO2/I-/NOM System
  Lin, Yi-Pin; Washburn, Michael P.; Valentine, Richard L.
  Environmental Science & Technology (2008), 42 (8), 2919-2924CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  PbO2 can be an important form of Pb mineral scale occurring in some water distribution systems. It is believed to be formed by the oxidn. of Pb-contg. plumbing materials by free Cl. Its reactivity in water, however, has not been well studied. Iodide is also found in source drinking waters, albeit at low concns. Consideration of thermodn. suggests that I- can be oxidized by PbO2. I- was used as a probe compd. to study the redn. of PbO2 and the formation of iodoform, which has been predicted to be a carcinogen, in the presence of natural org. matter (NOM). The redn. of PbO2 by I- can be expressed as PbO2 + 3I- + 4H+ → Pb2+ + I3- + 2H2O, and the reaction kinetics was detd. In the presence of NOM, I3- reacts with NOM to form iodoform and its concn. is proportional to the NOM concn. Our results indicate that PbO2 is a very powerful oxidant and can possibly serve as an oxidant reservoir for the formation of iodinated disinfection byproduct through a novel reaction pathway.
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31. 31
  Zhang, P. C.; Ryan, J. A. Transformation of Pb(II) from cerrusite to chloropyromorphite in the presence of hydroxyapatite under varying conditions of pH Environ. Sci. Technol. 1999, 33, 625– 630
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  31
  Transformation of Pb(II) from Cerrusite [Cerussite] to Chloropyromorphite in the Presence of Hydroxyapatite under Varying Conditions of pH
  Zhang, Pengchu; Ryan, James A.
  Environmental Science and Technology (1999), 33 (4), 625-630CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The sol. Pb concn. and formation of chloropyromorphite [Pb5(PO4)3Cl] were monitored during the reaction of cerussite (PbCO3), a highly bioavailable soil Pb species, and hydroxyapatite [Ca5(PO4)3OH] at various P/Pb molar ratios under const. and dynamic pH conditions. Under pH-const. systems at pH 4 and below, the dissoln. rates of both cerrusite and apatite were rapid, and complete conversion of cerussite to chloropyromorphite occurred within 60 min when the amt. of phosphate added via apatite was stoichiometrically equal to that needed to transform all added Pb into chloropyromorphite. The concn. of sol. Pb depended upon the soly. of chloropyromorphite. The dissoln. rates of apatite and cerussite decreased with increasing pH, and the transformation was incomplete at pH 5 and above in the 60-min reaction period. The sol. Pb level, therefore, was detd. by the soly. of cerrusite. In the pH-dynamic system, which simulated the gastrointestinal tract (GI tract), a complete transformation of Pb from cerrusite to chloropyromorphite was achieved due to the complete dissoln. of apatite and cerrusite at the initial low pH. In both the const. and dynamic pH systems XRD anal. indicated that chloropyromorphite was the exclusive reaction product. The differences in transformation rate and the Pb soly. between the const. and dynamic pH systems indicate the significance of kinetics in controlling the bioavailability of Pb and the potential for the reaction to occur during ingestion.
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32. 32
  Zhang, P. C.; Ryan, J. A.; Yang, J. In vitro soil Pb solubility in the presence of hydroxyapatite Environ. Sci. Technol. 1998, 32, 2763– 2768
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  32
  In Vitro Soil Pb Solubility in the Presence of Hydroxyapatite
  Zhang, Pengchu; Ryan, James A.; Yang, John
  Environmental Science and Technology (1998), 32 (18), 2763-2768CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  The transformation of lead (Pb) in contaminated soils to pyromorphite, by the addn. of phosphate minerals, may be an economic in-situ immobilization strategy that results in a redn. of bioavailable Pb. To test this hypothesis, two sets of soil-soln. expts. under const. (i.e., fixed) or dynamic (i.e., variable) pH conditions, were conducted as a function of time. In both sets of expts., Pb-contaminated soil was reacted with synthetic hydroxyapatite in order to det. the transformation rate of soil Pb to pyromorphite and the sol. Pb level during the reaction period. In the const. pH system, the sol. Pb concn. decreased with the addn. of apatite at pH 4 and above. However, the transformation was pH-dependent and incomplete at relatively high pH (≥6). The soly. of cerrusite (PbCO3), the major Pb mineral in this soil, still exhibited a strong influence on the soly. of soil Pb. In the dynamic pH expts., which simulated gastric pH conditions (i.e., pH variation from 2 to 7 within 25 or 45 min), both cerrusite and added apatite were dissolved at low pH values (pH 2 and pH 3), and chloropyromorphite was rapidly pptd. from dissolved Pb and PO4 when the suspension pH was increased. Complete transformation of soil Pb to chloropyromorphite occurred in the pH dynamic expts. within 25 min, indicating rapid reaction kinetics of the formation of chloropyromorphite. Chloropyromorphite soly. controls the sol. Pb concn. during the entire duration of the pH dynamic expts. Thus, specific site conditions, such as pH, are important when considering evaluation of soil Pb bioavailability and in-situ immobilization of Pb in Pb-contaminated soils using phosphate amendment. Furthermore, this study demonstrates that the kinetics of conversion of soil Pb to chloropyromorphite in the presence of apatite is fast enough to occur during ingestion and that gastric pH conditions would favor the formation of chloropyromorphite, thus rendering ingested soil Pb nonbioavailable.
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  Schock, M. R.; Wagner, I.; Oliphant, R. J. Corrosion and solubility of lead in drinking water. In Internal Corrosion of Water Distribution Systems; AWWA Research Foundation and American Water Works Association :Denver, CO, 1996.
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  Vikesland, P. J.; Ozekin, K.; Valentine, R. L. Effect of natural organic matter on monochloramine decomposition: Pathway elucidation through the use of mass and redox balances Environ. Sci. Technol. 1998, 32, 1409– 1416
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  34
  Effect of Natural Organic Matter on Monochloramine Decomposition: Pathway Elucidation through the Use of Mass and Redox Balances
  Vikesland, Peter J.; Ozekin, Kenan; Valentine, Richard L.
  Environmental Science and Technology (1998), 32 (10), 1409-1416CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Monochloramine is often employed as a drinking water disinfectant for systems where free chlorine residuals are difficult to maintain or where disinfection byproduct formation is significant. However, monochloramine is unstable and decomps., leading to nitrogen oxidn. and chlorine redn. (auto-decompn.). The role of natural org. matter (NOM) in monochloramine loss is unclear. NOM could catalyze monochloramine auto-decompn., or it could act as an external reductant. This study elucidates the decay pathways of monochloramine in the presence and absence of NOM. When monochloramine decomps. in the absence of NOM, ammonia and nitrogen gas are the primary nitrogen decay products. When NOM is present, the product speciation changes such that little nitrogen gas prodn. occurs, yet prodn. of ammonia and nitrate increases. This product speciation shift indicates that under these conditions, NOM acts primarily as a reductant and not as a catalyst. This conclusion was verified using a redox balance which compares oxidized product, N2, and NO3- prodn. to monochloramine loss. The no. of electrons accounted for by oxidized product prodn. correlates well with monochloramine loss in the absence of NOM (60-100% recovery). However, there is a deficit in the presence of NOM (25-60% recovery). Clearly, much of the oxidizing capacity of monochloramine goes to NOM oxidn.
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35. 35
  Renner, R. Chloramines again linked to lead in drinking water Environ. Sci. Technol. 2005, 39, 314A– 314A
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  Chloramines again linked to lead in drinking water
  Renner, Rebecca
  Environmental Science and Technology (2005), 39 (15), 314ACODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  There is no expanded citation for this reference.
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Additional table of NH₂Cl autodecomposition reactions and figures of PbO₂ SEM images, and second-order plots of monochloramine decay. This material is available free of charge via the Internet at http://pubs.acs.org.
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Release of Pb(II) from Monochloramine-Mediated Reduction of Lead Oxide (PbO2)

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Abstract

Synopsis

Introduction

Material and Methods

Chemicals

Lead Oxide

PbO2 Reduction Studies

Analytical Methods

Results and Discussion

PbO2 Reduction Dependence on Initial NH2Cl Concentration and NH2Cl Decay

Figure 1

Figure 2

Influence of Solution pH

Figure 3

Influence of Total Carbonate Concentration, CT

Figure 4

Influence of Cl/N Molar Ratio

Figure 5

Comparison of Correlations between PbO2 Reduction and NH2Cl Autodecomposition

Figure 6

Environmental Significance

Supporting Information

Terms & Conditions

Author Information

Acknowledgment

References

Cited By

Figure 1

Figure 2

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Figure 4

Figure 5

Figure 6

References

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Supporting Information

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Release of Pb(II) from Monochloramine-Mediated Reduction of Lead Oxide (PbO₂)

PbO₂ Reduction Studies

PbO₂ Reduction Dependence on Initial NH₂Cl Concentration and NH₂Cl Decay

Influence of Total Carbonate Concentration, C_T

Comparison of Correlations between PbO₂ Reduction and NH₂Cl Autodecomposition