![[logo] US EPA](https://webarchive.library.unt.edu/eot2008/20081106042359im_/http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
2005 Progress Report: Solubilization of Particulate-Bound Ni(II) and Zn(II)
EPA Grant Number: R828771C012Subproject: this is subproject number 012 , established and managed by the Center Director under grant R828771
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: HSRC (2001) - Center for Hazardous Substances in Urban Environments
Center Director: Bouwer, Edward J.
Title: Solubilization of Particulate-Bound Ni(II) and Zn(II)
Investigators: Stone, Alan T.
Institution: Johns Hopkins University
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 2001 through September 30, 2007
Project Period Covered by this Report: October 1, 2004 through September 30, 2005
RFA: Hazardous Substance Research Centers - HSRC (2001)
Research Category: Hazardous Waste/Remediation
Description:
Objective:Models currently used to manage metal-contaminated sites have three principal shortcomings: (1) toxic metal speciation in many instances is under kinetic control rather than thermodynamic control; (2) host solids are often reworked by precipitation/dissolution reactions such that toxic metal ions are “buried” and hence physically inaccessible; and (3) subsurface organic constituents are difficult to characterize and hence difficult to account for. Our objective is to perform carefully designed laboratory experiments to gain specific, quantitative information about these three phenomena. Our work focuses on the dissolution of Ni(OH)2(s) and the desorption of Ni(II) and Zn(II) from FeOOH(goethite) and Fe(OH)3(ferrihydrite) surfaces. We are aided in this research by capillary electrophoresis (CE), which allows us to monitor processes taking place in aqueous solutions, and by high resolution transmission electron microscopy (HRTEM), which allows us to see surface structural changes.
Progress Summary:Our work during the past year has focused on two tasks: (1) the synthesis and characterization of solid phases that possess the same qualities as metal-sorbing phases in the subsurface; and (2) refining our use of CE-based techniques for discerning the aqueous speciation of Ni(II) and Fe(III).
Synthesis and Characterization of Solid Phases
For all of our synthesis work, reagent-grade water employed in synthesis was sparged with argon, boiled for 15 minutes, then covered during cooling. Synthetic Ni(OH)2(s) was prepared by dropwise adding 200 mLs of sparged ~0.1 M NH4OH solution to a 500 mL solution of sparged 0.2 M NiCl2·6H2O, yielding a final pH of 11. The light blue-green precipitate was collected on a 0.2 micron pore-diameter etch-track polycarbonate filter (Nuclepore, Whatman), then rinsed several times with reagent-grade water. The suspension was stored at 6 < pH < 7. A high resolution transmission electron micrograph of this preparation is shown in Figure 1. Careful inspection of this micrograph reveals the presence of parallel striations called lattice fringes, which are caused by positive and negative interference generated through the interactions of the electron beam with crystalline material. After recovering the particles by filtration, total dissolved Ni in the supernatant solution was found to be 4.0x10-4 M. Hence, Ni(OH)2(s) is too soluble and dissolves too rapidly for use in our dissolution studies.
Figure 1. High Resolution Transmission Electron Micrograph of Ni(OH)2(s) Synthesized in Our Laboratory
The FeOOH (goethite) preparation shown in Figure 2 was prepared using a modification of the synthesis procedure of Varanda, et al. (2002). Twenty-four hours after preparation of a 0.10 M Fe2(SO4)3, 0.62 M H2SO4 stock solution, it was filtered using a 0.1 micron pore-diameter etchtrack polycarbonate filter (Nuclepore, Whatman). To this solution, 34.5 mL of 1 M NaOH was added dropwise over a 30-minute period. (The temperature was set at 21°C using a constant temperature bath and constant stirring was employed.) After 3 hours, 31 mLs of 1 M NaOH were added, again over a 30-minute period. Four hours later, the dark brown suspension had a pH of 2.69. Then, 12.5 mLs of 1 M NaOH were added (yielding a pH of 10.295), and the temperature in the constant temperature bath was raised to 40°C for 45 hours. Next, the temperature was raised to 60°C for 72 hours. After this treatment, the color of the suspension was a lighter brown. The suspension was allowed to return to room temperature and stirring was no longer performed. After particles had settled, the supernatant solution was removed and the volume made up by reagent-grade water addition. This process was repeated until particles no longer settled. For the remaining rinse steps, centrifugation at 5000 rpm for 30 minutes was used.
Fe(OH)3(ferrihydrite) prepared in the presence of dissolved silicate reportedly has distinctive properties (Schwertmann, et al., 2004). Because dissolved silicate is present in subsurface waters, we wanted to include this material in our investigations. We employed a synthesis procedure that is a modification of the method of Schwertmann, et al. (2004). Solution A contained 0.1 M Fe(NO3)3 and 0.1 M HNO3. Solution B contained 0.33 M NH4OH, 0.10 M sodium silicate, and 7.8x10-2 M NaOH. To prepare the particles, Solution B was added drop wise over an 8-hour period to Solution A under constant stirring, yielding a final pH of 8.35. Supernatant solution was removed after a period of quiescent settling, with the volume made up by reagent-grade water addition. This process was repeated until particles no longer settled. For the remaining rinse steps, centrifugation at 5000 rpm for 30 minutes was used. A high resolution transmission electron micrograph of this phase is shown in Figure 3.
 |
 |
(A) |
(B) |
Figure 2. FeOOH(goethite) Synthesized For Use in Dissolution Experiments. (A) High resolution transmission electron microscope image. (B) Single crystal X-ray diffraction pattern.
CE Techniques for Ni(II) and Fe(III) Complexes
We need to refine our CE techniques to the point where laboratory experiments in well-characterized media can be monitored, and to the point where worthwhile speciation information can be obtained from real-world samples. Chelating agents span a wide range of properties (e.g. number of ligand donor groups, molecular structure, molecular charge), and therefore are helpful in refining CE techniques.
The diode-array spectrophotometer used as CE detector allows us to collect spectra in the 200 to 300 nm range for individual CE peaks. Based upon these spectra, we can distinguish Ni(II)-containing species, Fe(III)-containing species, and “free” chelating agent (bound to protons or to UV-transparent metal ions such as Ca(II)). Electromigration times are determined by overall species charge and hydrodynamic cross sectional area. We would like to “work backwards” from electromigration times and learn as much as we can about the properties of metal ion-chelating agent complexes.
Figure 3. High Resolution Transmission Electron Micrograph of Synthetic Fe(OH)3 (ferrihydrite) Prepared in the Presence of Dissolved Silicate For Use in Dissolution Experiments
Electromigration times are a function of inherent analyte properties and of attributes of the CE setup itself, i.e. the applied voltage, the electroosmotic flow, the entire column length, the column length to the detector. Attributes of the CE setup can be corrected for (Carbonaro and Stone, 2005), yielding effective electrophoretic mobilities (meff, in units of cm2V-1s-1) that are specific for each analyte.
We have previously explored the relationship between overall species charge and meff using complexes with the substitution-inert metal ions Cr(III) and Co(III) (Carbonaro and Stone, 2005). The disadvantage of working with Cr(III) and Co(III) complexes is that each must be separately synthesized and purified. Once the complex is synthesized, however, entry or exit of chelating agent to/from the inner coordination sphere will not take place. Ni(II) and Fe(III) complexes are moderately substitution-labile, and hence a different approach is warranted. System constituents (fixed quantities of stock metal ion solution, stock chelating agent solution, and pH buffer) are mixed together and stored for at least 24 hours to ensure that equilibrium can be attained. Computer-based equilibrium speciation calculations are then performed. If the system has indeed reached equilibrium and if the complex formation constants that serve as the basis for our calculations are correct, then the calculations provide us with the identities and concentrations of predominant species in our samples.
Let us suppose that one metal ion and one chelating agent have been selected. Depending upon their relative concentrations and the system pH, several stoichiometries are possible. In the chemical reactions below, we show the formation of the 1:1 and 1:2 complexes between Ni(II) and the tridentate chelating agent IDA2-.
| [NiII(H2O)6(aq)]2+ + IDA2- = [NiII(IDA)1(H2O)3(aq)]0 + 3H2O | (1) |
| [NiII(H2O)6(aq)]2+ + 2IDA2- = [NiII(IDA)2(aq)]2- + 6H2O | (2) |
At equilibrium, the concentration of the 1:1 complex [NiII(IDA)1(H2O)3(aq)]0 is directly proportional to [IDA2-], while the concentration of the 1:2 complex [NiII(IDA)2(aq)]2- is proportional to the square of [IDA2-]. Hence, the 1:1 complex is predominant when the chelating agent concentration is low, while the 1:2 complex increases in significance as the chelating agent concentration is increased.
If interconversion of one species into another is negligible within the timescale required for CE analysis, then each species will yield its own distinct CE peak. If interconversion is fast, on the other hand, it is necessary to treat the electromigrating analyte as an ensemble average of several distinct species. CRITICAL (Martell, et al., 2004) lists paKas for IDA (1.85, 2.84, and 9.79) and for the corresponding phosphonate IDMP (0.5, 1.4, 5.51, 6.74, and 11.64). Let us suppose that the pH of our CE electrolyte is 7.2. As the bars below indicate, HLˉ completely dominates the speciation of “free” IDA at this pH, and hence the effective molecular charge is -1.0.
With IDMP, on the other hand, paKa4 = 6.74 is relatively close to the pH of the CE electrolyte. To calculate the effective molecular charge, we start by calculating the concentration scale pKa for the ionic strength of our CE electrolyte (I ≈ 1.0x10-2 M). (pcKa4 = paKa4 + log(γ+ γ3-/ γ2-) = 6.74 - 0.264 = 6.48). The relative amounts of H2L2- and HL3- are found using the following two equations:
| [H2L2-] ≈ | [H+]IDAT = 0.16.IDAT | [HL3-] ≈ | cKa4IDAT = 0.84.IDAT | (3) | |
| [H+] + cKa4 | [H+] + cKa4 |
The effective molecular charge (Zeff) therefore corresponds to (0.16)(-2) + (0.84)(-3), which equals -2.84.
Our plot of electrophoretic mobility versus effective molecular charge (Figure 5) has entries for free chelating agent, for Fe(III)-chelating agent complexes, and for Ni(II)-chelating agent complexes. The vertical spread we see for species with the same charge (e.g., for Zeff = -2.0) is reasonable. Although all these species share the same charge, their hydrodynamic radii differ significantly. (Indeed, the ability to distinguish between like-charged species is analytically desirable.)
To illustrate the utility of Figure 5, let us imagine what we would do if we didn't know anything about the chelating agent IDA or about its complexes with Ni(II). Free IDA exhibits an effective electrophoretic mobility of -2.80x10-4 cm2V-1sec-1, from which we could conclude that it is a monoanion. Addition of Ni(II) and subsequent equilibration and injection yields a peak with an effective electrophoretic mobility of -3.63x10-4 cm2V-1sec-1, corresponding to a dianion. Because the complex with Ni(II) is more negative than the free chelating agent, we can conclude that the there are at least two chelating agent molecules coordinated to each Ni(II).
Future Activities:In last year’s progress report, we discussed preliminary Ni(II) desorption and Fe(III) (hydr)oxide dissolution experiments conducted in our laboratory. The FeOOH (goethite) and Fe(OH)3 (ferrihydrite) phases that we have synthesized and characterized will considerably improve what we can achieve in these experiments. In addition, we have much more experience with the analytes likely to be generated in these experiments, as reflected by the numerous species included in our new effective electrophoretic mobility-effective molecular charge diagram (Figure 4).
Figure 4. Electrophoretic mobilities (µeff) as a function of effective molecular charge (Zeff) for free chelating agents (circles), Fe(III)-chelating agent complexes (squares) and Ni(II)-chelating agent complexes (diamonds). Entries for metal-ion chelating agent complexes refer to 1:1 complexes, except those marked with an asterisk (*), which possess a 1:2 stoichiometry. Sample solutions contained 100 µM metal ion (where applicable), 500 µM chelating agent, 5 mM MOPS buffer (pH 7.1), and 10 mM NaNO3. The CE capillary had a total length of 40 cm, a length to the detector of 27.8 cm, and an inner diameter of 75 microns. The CE electrolyte consisted of 25 mM phosphate (pH 7.2) and 0.5 mM TTAB. An applied voltage of 25 kV was used.
The remaining year of our project will be dedicated to desorption and dissolution experiments. An important goal is to monitor the extent of chelating agent adsorption throughout the duration of our experiments. Measurements of extent of adsorption at different (hydr)oxide loadings, Ni(II) concentrations, chelating agent concentrations, and pH will help provide details about governing surface chemical reactions. We also are interested in how other system constituents (e.g., Ca(II), natural organic matter) affect desorption and dissolution.
References:
Schwertmann U, Friedl J, Kyek A. Formation and properties of a continuous crystallinity series of synthetic ferrihydrites (2- to 6-line) and their relation to FeOOH forms. Clays and Clay Minerals 2004;52:221-226.
Varanda LC, Morales MP, Jafelicci M, Serna CJ. Monodispersed spindle-type goethite nanoparticles from FeIII solutions. Journal of Materials Chemical 2002;12:3649-3653.
Carbonaro RF, Stone AT. Speciation of chromium(III) and cobalt(III) (Amino)carboxylate complexes using capillary electrophoresis. Analytical Chemistry 2005;77(1):155-164.
Martell AE, Smith RM, Motekaitis RJ. NIST critically selected stability constants of metal complexes database. Version 8.0. Presented to U.S. Department of Commerce, National Institute of Standards and Technology, Gaithersburg, MD, 2004.
Journal Articles:No journal articles submitted with this report: View all 1 publications for this subproject
Supplemental Keywords:nickel, zinc, metal ions, capillary electrophoresis, toxics, exposure, hazardous substances, assessment, cleanup, risk communication, international cooperation, pollutants/toxics, waste, chemicals, ecological risk assessment, environmental chemistry, environmental engineering, hazardous, hazardous waste, zinc, capillary electrophoresis, chemical releases, contaminated waste sites, hazardous waste characterization, hazardous waste disposal, hazardous waste management, hazardous waste treatment, heavy metals,
,
POLLUTANTS/TOXICS, INTERNATIONAL COOPERATION, Scientific Discipline, Waste, RFA, Ecological Risk Assessment, Chemicals, Hazardous Waste, Environmental Engineering, Environmental Chemistry, Hazardous, heavy metals, contaminated waste sites, capillary electrophoresis, nickel oxide, hazardous waste disposal, hazardous waste management, Zinc, hazardous waste characterization, hazardous waste treatment
Relevant Websites:
Progress and Final Reports:
2004 Progress Report
Original Abstract
Final Report
Main Center Abstract and Reports:
R828771 HSRC (2001) - Center for Hazardous Substances in Urban Environments
Subprojects under this Center:
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R828771C001 Co-Contaminant Effects on Risk Assessment and Remediation Activities Involving Urban Sediments and Soils: Phase II
R828771C002 The Fate and Potential Bioavailability of Airborne Urban
Contaminants
R828771C003 Geochemistry, Biochemistry, and Surface/Groundwater Interactions
for As, Cr, Ni, Zn, and Cd with Applications to Contaminated Waterfronts
R828771C004 Large Eddy Simulation of Dispersion in Urban Areas
R828771C005 Speciation of chromium in environmental media using capillary
electrophoresis with multiple wavlength UV/visible detection
R828771C006 Zero-Valent Metal Treatment of Halogenated Vapor-Phase Contaminants in SVE Offgas
R828771C007 The Center for Hazardous Substances in Urban Environments (CHSUE) Outreach Program
R828771C008 New Jersey Institute of Technology Outreach Program for EPA Region II
R828771C009 Urban Environmental Issues: Hartford Technology Transfer and Outreach
R828771C010 University of Maryland Outreach Component
R828771C011 Environmental Assessment and GIS System Development of Brownfield Sites in Baltimore
R828771C012 Solubilization of Particulate-Bound Ni(II) and Zn(II)
R828771C013 Seasonal Controls of Arsenic Transport Across the Groundwater-Surface Water Interface at a Closed Landfill Site
R828771C014 Research Needs in the EPA Regions Covered by the Center for Hazardous Substances in Urban Environments
R828771C015 Transport of Hazardous Substances Between Brownfields and the Surrounding Urban Atmosphere