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2001 Progress Report: Tailoring Activated Carbon Surfaces for Water, Wastewater and Hazardous Waste Treatment Operations
EPA Grant Number: R828157Title: Tailoring Activated Carbon Surfaces for Water, Wastewater and Hazardous Waste Treatment Operations
Investigators: Karanfil, Tanju , Kilduff, James E.
Institution: Clemson University
Current Institution: Clemson University , Rensselaer Polytechnic Institute
EPA Project Officer: Krishnan, Bala S.
Project Period: June 1, 2000 through May 31, 2002 (Extended to July 31, 2004)
Project Period Covered by this Report: June 1, 2000 through May 31, 2001
Project Amount: $223,978
RFA: Exploratory Research - Environmental Engineering (1999)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:Granular activated carbon (GAC) treatment has been proven to be an excellent option for removal of a broad range of synthetic organic compounds (SOCs) from drinking water sources and industrial wastewaters. The primary objective of the proposed research is to develop a fundamental understanding of how activated carbon surface chemistry influences adsorption of priority pollutants from complex solution matrices. Specifically, our goals are to: (1) elucidate how carbon surface chemistry influences adsorption mechanisms (e.g., dispersive, electrostatic, hydrophobic interactions) of SOCs and organic macromolecules; (2) investigate how sorbate molecular properties impact adsorption by activated carbons with different surface chemistry; (3) examine the role of carbon surface chemistry on competitive (i.e., simultaneous adsorption and preloading) adsorption interactions between target pollutants and organic macromolecules in the background water matrices; and (4) provide a rational basis for selecting or preparing activated carbons for removal (and in some cases regeneration) of SOCs and organic macromolecules from water and wastewaters.
The hypothesis guiding this study is that carbon surface chemistry plays a key role in the adsorption of target compounds, especially for small molecular weight organic priority pollutants and, in some cases, may overwhelm the effects of pore structure. Preliminary results suggest that a good understanding of the interactions between carbon surfaces and priority pollutants is a viable route to producing novel sorbents for effective and economic application of adsorption processes in meeting our increasingly stringent water quality standards.
Progress Summary:We have begun to prepare modified activated carbons and carbon fibers as outlined in Phase I of the research plan. To date, we have focused on heat treatment in an inert (N2) atmosphere to remove oxygen functionality from the carbon surface (as CO and CO2) and liquid phase oxidation (using NO3). We have investigated the effects of reaction time (2-16 hours) and temperature (400-1,000?C) on the surface properties of an acid-activated, wood-based carbon using a new reactor system that is part of an Autosorb 1 Chemisorption Analyzer (Quantachrome Corporation). The uptake of trichloroethyelene by the surface treated carbons was found to be a strong function of treatment temperature, increasing with treatment temperature. However, no significant dependence on time over the range investigated was observed. Based on this finding, subsequent experiments will employ a 2-hour reaction time. Currently, we are characterizing the treated carbons using FTIR, water vapor adsorption, and acid-base characteristics. We are developing an automated titration technique for measuring acid-base properties of the surface. In addition, we are preparing surface modification experiments using hydrogen and ammonia in the gas phase.
We have completed a series of experiments to probe the catalytic properties of carbon surfaces as outlined in Phase II of the research plan. The reactivity of carbon surfaces may be a combined function of its heterogeneous and porous characteristics. We have completed experiments using graphite as a potential model carbon adsorbent for investigating phenol adsorption and recovery. Graphite is a more homogeneous sorbent than activated carbons (and fibers) and is useful to evaluate the effects of the carbon surface without confounding effects of porosity. Nitrogen BET surface area and pore size distribution (PSD) of the graphite were determined to verify the absence of significant microporosity. The stability of phenol solutions with respect to time also was tested to demonstrate that phenol does not undergo significant oxidative coupling in the absence of a catalyst. Both rate and isotherm studies were performed to measure total phenol uptake with time, as well as the evolution of reversible and irreversible uptake and the formation of oxidative coupling products (hydroxy biphenyls). Isotherms at fixed contact time and different pH were measured to determine the effect of phenol in solution and surface coverage on total, reversible, and irreversible adsorption and on reaction product formation. Total uptake of phenol on graphite increases with time, reaches an apparent equilibrium plateau, and then increases further with time. The initial plateau is evidence that mass transfer is not a rate-limiting phenomenon. The increase after reaching an initial plateau suggests that sorption is controlled by phenomena other than adsorption at long equilibration times.
Reversibility was assessed by solvent extraction (methanol), Soxlet extraction with acetone, extraction in 1 M sodium hydroxide or extraction in methanol aided by a phenolic displacer (4-chloro-3-methylphenol). It was found that phenol adsorbs irreversibly to graphite, demonstrating that irreversibility observed in activated carbons has components related at least in part to the graphene surface itself (i.e., absent pore effects). Irreversible adsorption on graphite is rate controlled and correlates well with total uptake. Aqueous solutions of phenol were stable at both low and high pH; phenol did not undergo oxidative coupling in the absence of a catalyst. In contrast, graphite was found to catalyze oxidative coupling of phenol in aqueous solutions at basic conditions. Products of the reaction were quantified by high performance liquid chromatography and confirmed by GC/MS; 2,2'-dihydroxy biphenyl is the major multimer found, and significant quantities were measured at pH 9 and above. Irreversible adsorption did not correlate with oxidative coupling product formation. Both rate and isothermal studies showed that irreversible adsorption occurred at low pH in the absence of significant oxidative coupling, demonstrating that oxidative coupling per se cannot explain irreversible adsorption of phenol on graphite. However, at high pH, significant irreversible adsorption and oxidative coupling both occur.
We have completed a series of experiments designed to identify the reactive components in solutions containing natural organic matter isolated from surface water sources. This was done in an effort to better understand the interactions between these compounds and carbon surfaces, and the competitive interactions between natural (macromolecular) organic matter and low molecular weight SOCs, as outlined in Phases III and IV of the research plan. To identify the reactive components of NOM, we have combined physical separation processes (fractionation by ultrafiltration and adsorptive separation using methyl methacrylate resins) and mathematical modeling using the Ideal Adsorbed Solution Theory and the fictive component methodology. From a modeling point of view, it is necessary to represent NOM properties mathematically; however, because the exact composition of NOM is not known, it is necessary to simplify how NOM properties are represented. Physical separation processes are promising because NOM is a source-specific heterogeneous mixture of complex organic materials encompassing a large range of molecular weights. The approach we have taken is to fractionate NOM into classes of compounds that have similar characteristics or reactivities. These include hydrophobic (HPO) and hydrophilic (HPL) fractions (operationally defined by adsorption to methyl methacrylate resin) and molecular weight fractions (operationally defined using cross-flow ultrafiltration). We have focused our attention on the low molecular weight fraction (passing a 1 kDa UF membrane, identified as the "1K" fraction) because previous research has identified the low molecular weight components (that can access large micropores and small mesopores) as the most reactive in a NOM mixture.
To evaluate whether the operationally defined component classes constituted reactive components of the NOM, first equilibrium adsorption isotherm experiments were conducted for: (1) NOM uptake by GAC (fractions and whole water); (2) TCE single-solute uptake by GAC; and (3) TCE uptake by GAC preloaded with NOM fractions and whole water. Adsorption isotherms data were fitted with the Freundlich isotherm. Isotherm parameters and a nonadsorbing fraction of the NOM were estimated using experimental NOM uptake data. It was found that the ability of a single Freundlich isotherm to describe NOM uptake generally improved as fractions become more narrowly defined (i.e., the HPO 1K fraction displays better fit than the 1K fraction, which in turn is better than the whole water). The NOM isotherms also were analyzed using a fictive component approach, which was used as a comparative tool. The fictive component method empirically simplifies the inherent heterogeneity of NOM by considering the overall observed NOM uptake isotherm to be the result of competitive adsorption of several "fictive components" of the whole mixture. Each fictive component (FC) is assumed to posses a narrow enough range of properties that its adsorption equilibria can be characterized by a unique isotherm, and that competition between components can be described by applying the IAST model. Using this approach and performing a statistical search for up to four FCs, it was found that the number of FCs required to describe NOM uptake correlated with the heterogeneity of the NOM fraction. Most significantly, for several NOM fractions, a single FC was identified, with parameters in close agreement with those determined experimentally, providing evidence that these fractions warrant treatment as single "reactive components" in the context of a competitive adsorption model.
Further analysis using the IAST to predict the uptake of mixtures of NOM
fractions was performed. For example, HPO1K and HPL1K isotherm parameters and
initial concentrations were used to predict uptake of the 1K fractions. This
prediction was compared to a prediction of the 1K fraction uptake using fictive
components determined from the experimental HPO1K and HPL1K isotherms. It was
found that predictions made using experimentally determined parameters for
physically isolated fractions were as good or better than those using fictive
components. This was true even when more than one fictive component was
identified for a given NOM fraction isotherm.
Surprisingly, for three waters
studied, the HPO1K and HPL1K NOM fractions exhibited similar uptake. This
suggests the possibility of significant specific interactions between HPL
functional groups and the carbon surface, which may be comparable to the
entropic forces governing more hydrophobic compounds. Neither fraction can be
neglected when considering competition with TCE; both exerted significant
reductions in TCE uptake. Based on this finding, the ability to predict TCE
uptake by GAC preloaded with whole NOM was evaluated using both bi-solute and
multi-solute competitive adsorption frameworks. The competitive effect of the
whole NOM was modeled using either: (1) parameters for the 1K fraction as a
single reactive component; or, (2) parameters from the HPO1K and HPL1K NOM
fractions as two reactive components. Two different parameter sets were
compared: experimentally measured parameters, and those determined using the
fictive component approach. First, it was found that predictions made using a
bi-solute approach were similar to those made using a multi-solute approach;
that is, HPO1K and HPL1K NOM fractions could be combined as a single low
molecular weight "reactive fraction." Second, predictions made by using measured
and fictive component parameter sets were nearly identical. Currently, we are
developing ways to modify the IAST model to account for accessible surface area
and pore blockage to improve the accuracy of model predictions.
We have completed an analysis of our previously published data to provide insight into interactions between NOM compounds and carbon surface chemistry, and the subsequent competitive interactions between NOM and SOCs. These interactions were manipulated using a novel approach that focused on changing solution ionic strength, divalent ion concentration, and dissolved oxygen concentration. TCE adsorption by activated carbon preloaded with humic substances was found to decrease with increasing concentrations of monovalent ions (NaCl), calcium (until solubility was exceeded), or dissolved oxygen in the preloading solution. For a given percentage of organic carbon removal during humic acid loading, greater reductions in TCE adsorption occurred with increasing monovalent ion concentration and calcium concentration at constant ionic strength. However, this effect was related primarily to the amount of humic material adsorbed?the reduction in TCE adsorption was independent of the ionic composition of the preloading solution when compared at similar humic acid loading. Experiments were performed that showed that calcium ions can associate with humic material after the humic has been adsorbed, which subsequently reduces TCE uptake, but this effect does not dominate when calcium is present during humic loading. At sufficiently high calcium concentrations (approaching solubility), aggregation or coprecipitation of humic acid mitigated the effects of preloading. In contrast to the effects of ionic composition, the presence of dissolved oxygen did fundamentally change the mechanism by which organic macromolecules compete with TCE. TCE uptake was lower when preloading by poly (maleic acid) occurred in the presence of dissolved oxygen, even when the amount loaded was the same. One explanation invokes a coupling mechanism promoted by the carbon surface, which results in either additional blockage of TCE sorption sites, additional site competition, or both. In all experiments, the effects of preloading were consistent with those reported previously, which have been interpreted as a loss of high-energy sites available to TCE, causing a significant reduction in the site-energy heterogeneity and reduced uptake in the low concentration region.
Future Activities:Future activities will involve further modification and characterization of activated carbons and carbon fibers. These adsorbents will be used to examine the role of carbon surface chemistry on the adsorption mechanisms of aromatic and aliphatic compounds, as outlined in Phase II of the research plan. Furthermore, the effects of surface chemistry on adsorption and recovery of phenol will be probed. Based on the data supporting the reactivity of graphite surfaces, further experiments are planned to probe the effects of carbon pore structure on the adsorption and recovery of phenolic compounds. The uptake of whole NOM and NOM fractions by modified carbons and the subsequent effects on target compound adsorption will be further investigated as outlined in Phases II and III of the research plan.
Journal Articles on this Report: 1 Displayed | Download in RIS Format
| Other project views: | All 25 publications | 8 publications in selected types | All 8 journal articles |
| Type | Citation | ||
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Kilduff JE, Karanfil T. Trichloroethylene adsorption by activated carbon preloaded with humic substances: effects of solution chemistry. Water Research, Volume 36, Issue 7, April 2002, Pages 1685-1698. |
R828157 (2001) R828157 (2002) R828157 (Final) |
not available |
environmental chemistry, analytical measurements, adsorption, toxics, solvents. , Ecosystem Protection/Environmental Exposure & Risk, Water, Scientific Discipline, Waste, RFA, Drinking Water, Chemistry, Hazardous Waste, Fate & Transport, Environmental Chemistry, Hazardous, Engineering, industrial waste, treatment, water quality, wastewater treatment, other - risk management, fate and transport, monitoring, granular activated carbon
Progress and Final Reports:
Original Abstract
2002 Progress Report
Final Report