Grantee Research Project Results

2003 Progress Report: Speciation of chromium in environmental media using capillary electrophoresis with multiple wavlength UV/visible detection

EPA Grant Number: R828771C005
Subproject: this is subproject number 005 , 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: Center for the Study of Childhood Asthma in the Urban Environment
Center Director: Hansel, Nadia
Title: Speciation of chromium in environmental media using capillary electrophoresis with multiple wavlength UV/visible detection
Investigators: Stone, Alan T. , OMelia, Charles R.
Institution: The Johns Hopkins University
EPA Project Officer: Aja, Hayley
Project Period: October 1, 2001 through September 30, 2002
Project Period Covered by this Report: October 1, 2002 through September 30, 2003
Project Amount: Refer to main center abstract for funding details.
RFA: Hazardous Substance Research Centers - HSRC (2001) Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management

Objective:

Appraising extents of contamination, predicting future contaminant migration behavior, and devising mitigation schemes all rely heavily upon the quality of chemical analysis information. In addition to total elemental concentrations, it often is worthwhile to obtain speciation information. With chromium (Cr), for example, it is widelyaccepted that oxidation state determinations are important. Although the diphenylcarbazide (DPC) colorimetric test can distinguish Cr(III) from Cr(VI), it is subject to interferences and may yield misleading results. Information beyond oxidation state (e.g., the number and identity of ligands coordinated to the central metal ion) also is important. Neutral Cr(III) complexes, for example, partition and adsorb to soils in ways quite distinct from cationic and anionic complexes.

Capillary electrophoresis (CE) is a promising new analytical method that offers the opportunity of obtaining speciation information that is more reliable (e.g., oxidation state determination) and more complete (coordination information) in comparison to methods currently employed by environmental professionals. CE separates analyte molecules based on differences in the charge and hydrodynamic radii. Multiwavelength ultraviolet (UV)-visible detection, in turn, is inexpensive and easy to maintain. Our objective is to develop the analytical methodology to obtain chromium speciation information from aqueous samples relevant to hazardous waste sites using this new technique.

Progress Summary:

We began work with a new Beckman-Coulter MDQ capillary electrophoresis system in January 2002. As noted in the previous progress report, the Cr^VI species HCrO4^- and CrO4^2- are easily distinguished from a wide range of Cr^III-containing low molecular-weight complexes. Diode-array multiwavelength detection (from 190 to 600 nm) allows us to use the wavelength of maximum absorbance (max) for each chromium species in generating calibration curves. If a discernable CE peak appears, from our experience a detection limit of at least 50 ppb (1 µM) can be obtained.

We began with the premise that an unknown peak appears in an electropherogram. Because full spectra are recorded by our UV-visible diode-array detector, Cr^III and Cr^VI are readily distinguished owing to their different values of max (wavelength of maximum absorbance). What additional information can we obtain from the time required for electromigration to the detector?

Citric acid, secreted by plant roots into soils, is a common and naturally occurring chelating agent. The potential exists for the solubilization of sorbed or precipitated Cr^III at hazardous waste sites. To illustrate this point, citric acid has been brought into contact with amorphous Cr^III (hydr)oxide solid (see Figure 1). A single, new peak appears in the electropherogram. The observed electrophoretic mobility (µobs) reflects contributions from the effective electrophoretic mobility (µeff) and the electroosmotic flow (µeof):

µobs = µeff + µeof	µeff = Ld • Lt - µeof
	tm    V

Ld is the capillary length to the detector, Lt is the total capillary length, tm is the time for the peak to reach the detector, and V is the applied electric field (in volts). µeof is known from the time required for the analyte to reach the detector.

Figure 1. Characterization of a Dissolved Cr^III Complex With Citrate by Capillary Electrophoresis. 100 µM was added to 1 mM Cr^III(OH)3 x H2O at pH 4.02. The peak at 4.2 minutes grew over a period of 12 days, and was attributed to a Cr^III complex with citrate. The electrophoretic mobility of this peak was -1.78 x 10^-4 cm²/V-s, consistent with a monovalent anionic metal complex. Experimental conditions: 10 mM NaCl, 5 mM acetate.

Figure 2 represents our efforts to "calibrate" electrophoretic mobility in terms of the molecular charge of the analyte, using complexes of Cr^III, Co^III, Co^II, and Ni^II. A clear linear relationship exists; as the charge of the complexes becomes more anionic, the electrophoretic mobility becomes increasingly more negative. Based on this graph, we can quite justifiably conclude that the Cr^III-citrate complex possesses a charge of -1. This is the expected charge, based on Cr^III coordination of one alcoholate and two carboxylate groups of the citrate molecules.

Figure 2. Effective Mobility Versus Charge for Monomeric Co^II, Ni^II, Cr^III, and Co^III Aminocarboxylate Complexes. The average molecular charge of each complex was computed at the pH of the CE buffer (7.10) using pKa values reported in the literature. The identity of each data point is as follows: 1. [Co^II(EDTA)]^2- 2. [Ni^II(EDTA)]^2-, [Ni^II(TMDTA)]^2- 3. [Ni^II(CDTA)]^2- 4. [Ni^II(H2O)(HEDTA)]^- 5. [Cr^III(EDTA)]^- 6. [Cr^III(OH)(HEDTA)]^- 7. [Cr^III(OH2)(OH)(NTA)]^- 8. [Cr^III(CDTA)]^- 9. [Cr^III(TMDTA)]^- 10. u-fac-[Cr^III(IDA)2]^- 11. [Co^III(NTA)2]^3- 12. [Co^III(OH2)(OH)(NTA)]^- 13. [Co^III(CDTA)]^- 14. u-fac-[Co^III(IDA)2]^- 15. [Co^III(EDTA)]^-, s-fac-[Co^III(IDA)2]^-.

Based on this finding, we can conclude that the molecular charge of unknown peaks is readily obtained by CE.

Aqueous samples collected from the field sometimes must be stored for hours or for days before analysis can be performed. What changes accompany sample storage?

Figure 3 shows data obtained from the dissolution of Cr^III (hydr)oxide solids by nitrilotriacetic acid (NTA) ethylenediaminetetraacetic acid (EDTA) and iminodiacetic acid (IDA). Under the conditions employed, concentrations of Cr^III complexes with inorganic anions are negligible. Concentrations of discrete Cr^III-containing species are denoted by open circles. Total dissolved chromium concentrations determined by atomic absorption spectrophotometry (AAS) are denoted by filled circles. All the samples were collected, filtered, and then stored until analysis at the end of the experiment. In the experiments with NTA and EDTA, 80 percent or more of total dissolved chromium can be attributed to discrete CE peaks. In other words, the complexes were not adversely affected by up to 10 days of storage. IDA is a substantially weaker complexing agent, and the effects of sample storage are more severe. Our interpretation of these findings is that the acetate buffer employed in these experiments gradually replaced IDA within the inner coordination sphere of Cr^III. As a consequence, speciation information in the presence of this weaker complexing agent was lost during storage.

Figure 3. Characterization of Dissolved Cr^III Complexes With IDA, NTA, and EDTA by Capillary Electrophoresis. Measurements of total dissolved chromium, CrT (closed symbols), were compared with measurements of individual Cr^III complexes by CE (open symbols). The left panel shows results for kinetic experiments with NTA at pH 5.72 and 6.50. The top right panel shows results for experiments performed at pH 5.97 with EDTA; the bottom right panel shows results of an experiment at pH 4.65 with IDA. 1 mM Cr^III(OH)3 xH2O, 100 ìM Ltot, 10 mM NaCl, 5 mM buffer (Acetate or MES).

Future Activities:

During the next 6 months, we will focus on natural organic matter (NOM)-rich surface waters. "Raw" water samples will contain a wide variety of organic molecules, including molecules that sorb strongly onto silica (SiO2) and other mineral surfaces. We also will pre-contact NOM-rich surface waters with high surface area silica. This "pre-contacted" NOM will have less interaction with the quartz silica wall of our CE capillary. An important issue under investigation will be the extent to which interaction with this quartz silica wall affects what we are able to discern with CE.

We will spike both raw and precontacted NOM with: (1) Cr^III complexes with relatively weak chelating agents (e.g., oxalate); and (2) Cr^VI (chromate ion). The first type of experiments provides insight into slow rates of metal ion and ligand exchange reactions, and the subsequent speciation. The second type of experiments provides information regarding the redox incorporation of chromium into NOM.

Journal Articles:

No journal articles submitted with this report: View all 5 publications for this subproject

Supplemental Keywords:

toxics, inorganics, trace analysis, oxidation state, complexation, health, physical aspects, waste, water, analytical chemistry, chemical engineering, chemistry and materials science, ecological risk assessment, ecology, and ecosystems, engineering, chemistry, environmental chemistry, environmental engineering, geochemistry, hazardous, health risk assessment, hydrology, physical processes, risk assessments, adsorption, analytical measurement methods, aquatic ecosystem, capillary electrophoresis, capillary electrophoresis, chemical composition, chemical detection techniques, chemical exposure, chemical kinetics, chemical releases, chromium speciation, contaminant dynamics, contaminant transport, ecotoxicological effects, electrochemical technology, environmental risks, exposure, fate and transport, fate and transport, groundwater, groundwater contamination, hazardous substance contamination, hazardous waste treatment, human exposure, human health risk., Health, RFA, Scientific Discipline, PHYSICAL ASPECTS, Water, Waste, Hazardous, Chemical Engineering, Ecological Risk Assessment, Health Risk Assessment, Physical Processes, Risk Assessments, Environmental Chemistry, Engineering, Chemistry, & Physics, Analytical Chemistry, Chemistry and Materials Science, Hazardous Waste, Geochemistry, Hydrology, Ecology and Ecosystems, Environmental Engineering, contaminant transport, adsorption, chromium speciation, hazardous waste treatment, aquatic ecosystem, environmental risks, groundwater contamination, fate and transport , human exposure, analytical measurement methods, chemical composition, chemical detection techniques, electrochemical technology, chemical exposure, fate and transport, groundwater, ecotoxicological effects, contaminant dynamics, exposure, chemical kinetics, hazardous substance contamination, capillary elecrophoresis, human health risk, capillary electrophoresis, hydrodynamics

Relevant Websites:

http://www.jhu.edu/hsrc Exit

Progress and Final Reports:

Original Abstract

Final

Main Center Abstract and Reports:

R828771    Center for the Study of Childhood Asthma in the Urban Environment

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