Biogeochemistry of Chelating Agents - ACS Publications - American

Chapter 6

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Analysis of Metal-Chelating Agent Complexes by Capillary Electrophoresis Scott C. Brooks Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS 6038, Oak Ridge, TN 37831-6038 ([email protected])

Direct measurement of metal-ligand ( M L ) complexes is essential for understanding metal speciation and ML reactivity in natural and experimental settings. Capillary electrophoresis (CE) is highly efficient separation technique that can provide direct measurement of ML species revealing slow reaction kinetics, subtle changes in coordination, and oxidation-reduction reactions involving the metal and ligand. Applications of this technique to understanding ML reactivity in multicomponent systems are discussed.

Introduction The land based codisposal of radionuclides with synthetic organic chelating agents has created a legacy of vast amounts of contaminated soils and groundwater within the U.S. Department of Energy complex (1). A survey of E P A Superfund and other agency sites suggests that similar mixed waste contamination problems exist across the United States (2). Shallow land burial of mixed wastes was considered acceptable, in part, because of the large sorption capacity of subsoils and aquifer solids for many metals and radionuclides. Nevertheless, the soluble metal-chelate (ML) complexes often exhibit sorption behavior that may promote the undesirable movement of contaminant metals and radionuclides away from primary disposal areas.

© 2005 American Chemical Society Nowack and VanBriesen; Biogeochemistry of Chelating Agents ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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122 During transport through subsoils and aquifers, the fate of metals/ radionuclides and chelating agents is inescapably linked. As demonstrated through prior research on M L complexes in contact with synthetic and natural geologic materials, aqueous complexation, geochemical dissociation (competition among metals for ligand, competition between surfaces and aqueous ligands for metals), and geochemical oxidation of the metal, organic ligand, or both are all important reactions to consider when evaluating the coupled fate of chelated metals. To improve our understanding of the coupled fate and transport of metalchelate complexes in the environment, the development and use of analytical techniques that allow direct measurement of chelate speciation is essential. The influence of chelating agents on the fate and transport of metals and radionuclides has been studied for years. Due to a lack of adequate techniques, most of these studies have relied on the separate analysis of total metal (e.g., A A S , ICP-MS) and chelate (e.g., G C , H P L C ) concentrations followed by equilibrium speciation calculations to predict metal speciation. As good as these analytical methods are they do not provide direct analysis of the M L species and therefore may lead to an incomplete or incorrect understanding of the underlying mechanisms governing M L behavior. For example, slow exchange kinetics may invalidate the equilibrium assumption implicit in the use of equilibrium speciation models. Measurement of total metal concentration will miss changes in oxidation state of the metal (e.g., Co(II) versus Co(III)). Radiometric assays using C labeled organic compounds can miss changes to the organic chelate (e.g., oxidation of nitrilotriacetate to iminodiacetate). 1 4

Over the past ten years analytical techniques using standard chromatography equipment have been reported that allow the direct determination of some metal-EDTA (M-EDTA) complexes. Taylor and Jardine (3) described an ion chromatography (IC) method for the direct determination of C o E D T A and C o E D T A . The method was later extended to include C d E D T A (4). Nowack et al. (5) have described a high performance liquid chromatography (HPLC) method for quantifying E D T A following precolumn conversion to its Fe(III) complex. The method has also been used to quantify Co(III), Cr(III), and Bi(III) complexes of E D T A without conversion to the Fe(III) form (6). The purpose of this chapter is to introduce the reader to capillary electrophoresis (CE), an analytical technique that has proved useful for the direct determination of organic chelates and their metal complexes. A brief overview of C E is followed by a summary of C E methods that have been used for M L analysis and finally examples of applications of those methods for understanding M L reactions. Results of C E analyses conducted by this author n

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Nowack and VanBriesen; Biogeochemistry of Chelating Agents ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

123 (viz. Figures 3, 4, 5, 6, and 7) were collected using a Waters Quanta 4000E instrument (Waters Corp, Milford, M A ) .

Fundamentals of Capillary Electrophoresis Capillary electrophoresis is a highly efficient separations technique that has been used for the analysis of inorganic and organic ions. The separation mechanism is based on the differential mobility of charged solutes under an applied electric field. Several texts and reviews are available for details on the different modes capillary electrophoresis and their applications (7-9). The following serves as a cursory introduction to some of the basic fundamentals.

The C E System A C E system consists of a run buffer (electrolyte), high voltage power supply, capillary, and detector (Figure 1). Two electrolyte reservoir vials are connected by the capillary, typically 50 μπι to75 μπι i.d., that is also filled with the electrolyte. Each reservoir also contains an electrode connected to the power supply. The electrolyte has some p H buffering capacity to minimize variability between runs and to offset H production at the anode and OH" at the cathode. Samples are usually introduced into the capillary using either hydrodynamic injection, in which the capillary is placed in the sample vial and a positive pressure is applied to the sample vial, or hydrostatic injection in which the capillary is placed in the sample vial and the vial is raised a preset distance above the collection reservoir causing sample to siphon into the capillary. +

Solute Separation Following sample introduction, the capillary end is returned to the electrolyte reservoir and when an electric field is applied across the liquid, ions in the fluid will migrate towards the electrode of opposite polarity. The observed solute mobility results from the vector sum of the electroosmotic flow (EOF) and the electrophoretic velocity. E O F is the bulk movement of electrolyte as a result of a charged inner capillary wall and the applied potential. At p H > ~3 silanol groups of fused silica dissociate imparting a net negative charge at the surface. Cations in the buffer are attracted to the silanoate (Si-O) groups forming a diffuse double layer: an inner layer held tightly by the Si-O" groups and an outer mobile layer. When an electric field is applied the outer later of cations, along with their hydration shells, move toward the cathode resulting in the bulk movement of buffer creating the EOF.


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Figure 1. Schematic diagram of capillary electrophoresis. As depicted, the instrument is set up for routine anion analysis with injection at the cathodic end of the capillary and detection at the anodic end. The inner surface of the capillary has been treated with electroosmoticflow(EOF) modifier reversing the surface charge of the fused silica capillary resulting in EOF towards the anode. Smaller, more highly charged ions have higher electrophoretic mobility (VEP) and migrate more rapidly toward their respective electrodes than larger less highly charged ions. Neutral solutes are not separated and move with the bulk EOF. Depending on the relative magnitude of the EOF to the μερ of the cations, the cations may also reach the detector after neutrals and water.


125 The magnitude and direction of the E O F can be manipulated by changing the run voltage and polarity or by manipulating the electrolyte chemistry such as pH, ionic strength, viscosity, or the addition of surfactants to modify the charge of the capillary wall (EOF modifier). Reversal of the E O F toward the anode is commonly achieved by using the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) or its hydroxide form (TTAOH). In general, i f the charge on the inner capillary wall is negative, E O F is towards the cathode, if it is positive, E O F is towards the anode. There is no E O F i f the inner wall of the capillary is neutral. The electrophoretic velocity for a solute depends on the applied electric field and the electrophoretic mobility. In general, ions are separated based on their charge to size ratio. Smaller more highly charged ions have a higher electrophoretic mobility than larger less highly charged ions.

The Capillary Typically fused silica capillaries are used in C E . Before their first use and routinely during analytical runs, the capillaries are conditioned by rinsing with some combination of dilute acid, base, deionized water, and electrolyte. Periodic conditioning of the capillary surface minimizes variability in performance. The.inner surface of the capillary can be dynamically coated (adding surfactant to run buffer) or permanently coated by a variety of methods of varying complexity (10-14). Capillary coatings are used to modify the E O F (see above) or to minimize ion interaction with the inner surface of the capillary.

Detection Several on-column detectors with zero dead volume have been developed for C E analysis including fluorescence and conductivity detectors. For environmental analyses U V / V i s detectors are most commonly used in either the direct or indirect detection mode. Direct photometric detection relies on the light absorbing characteristics of the solutes of interest. Indirect photometric detection is used when the solutes of interest do not absorb well at the available wavelengths. A light absorbing compound (a chromophore, e.g. chromate or paminobenzoic acid) which absorbs strongly at a specific wavelength is added to the run buffer creating a high background absorbance. When nonabsorbing solutes pass through the detector a decrease in absorbance is recorded. In this case the detector polarity can be reversed so the peaks take on their conventional appearance.


126 In C E the applied electric field provides the driving force generating a flat electroosmotic flow profile across the capillary. The solutes being separated experience the same velocity regardless of their cross-sectional position in the capillary and migrate in narrow zones generating narrow peaks, and thus the high efficiency of C E . The parabolic velocity profile generated in liquid chromatography tends to generate broader peaks (Figure 2). Images of solute migration under electrokinetically and pressure driven flow regimes verify this phenomenon (15,16).

Electroosmotic flow

Detector response

Pumped flow

Figure 2. Electroosmoticflowhas a relativelyflatvelocity profile resulting in narrow peaks and high efficiency of capillary electrophoresis.

C E offers a number of advantages relative to more traditional chromatographic techniques. The most widely recognized advantage is the high efficiency of C E with greater than 10 theoretical plates routinely achieved compared to ~10 for gas chromatography (GC) or 10 for H P L C . C E is compatible with a wide range of sample and separation chemistries. It can be applied over a wide range of p H with or without organic modifiers (e.g., methanol, acetonitrile) for the determination of solutes with a range of hydrophobicities. Solutes ranging in size from simple ions (Cl", Br", NO3") to 5

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127 high molecular weight compounds such as proteins can be determined by C E . In addition, much smaller sample and reagent volume is required. Usually, only a few nanoliters of sample are injected and an entire day's analyses use less than 100 m L of electrolyte. Perhaps the primary disadvantage to C E is the lower sensitivity due to the shorter optical path length. This limitation can be partially compensated by the use of "Z-cells" or bubble cells but these can also lead to peak broadening compromising resolution. In some cases the lower sensitivity can be overcome by using electrokinetic injection in which sample is introduced by applying a voltage to the sample causing sample ions to migrate into the capillary due to electroosmosis and electrophoretic mobility. Dahlen et al. (17) improved detection limits for low molecular weight organic acids by two orders of magnitude using electrokinetic injection (from ~0.1 mg/L to ~0.001 mg/L). Finally, the community of C E users for environmental applications is relatively small compared to other forms of chromatography and fewer "standard" methods for these applications are available.

Metal-Chelate Speciation by Capillary Electrophoresis Successful metal-chelate separation and quantification by C E depends on the kinetic and thermodynamic stability of the complex under the separation conditions employed. Complexes that undergo rapid ligand exchange dissociate and reassociate more often than complexes with slow exhange rates. Uncomplexed metal and ligand migrate towards opposite electrodes during separation hampering reliable analysis. M L complexes with large stability constants are suitable for C E analysis, regardless of their rate of ligand exchange. Finally, the interaction of the complex with the silica capillary or with constituents of the electrolyte will influence M L separation and quantification. For example, Burgisser and Stone (18) obtained linear calibration curves for C o E D T A " (log K tion = 18.16) with a reported detection limit of 2 χ 10" M , but C o % T A (log Κ = 11.66) gave nonlinear calibration curves with an approximate detection limit of 1 χ 10* M . The authors attributed this to ligand exchange reactions in which phosphate in the electrolyte exchanged with N T A . The fractional amount of exchange increased as the total C o N T A in the sample decreased. This problem can be resolved using a different buffer composition. For example, using a run buffer of 25 m M N a B 0 , 0.5 m M T T A O H , p H 9.3, we are able to achieve linear calibration curves with C o N T A detection limits of 8.7 χ ΙΟ" M (Figure 3) n

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[Co"NTA] (μΜ) Figure 3. Co NTA calibration plot for standards analyzed using phosphate (A) or borate (B) buffer. Separation conditions: for both panels 75 μτη χ 60 cm (52 cm to detector) capillary, 30s hydrostatic sampling, direct detection at 185 nm. Buffer composition (A) 25 mM phosphate, 0.5 mM TTAOH, pH 7.2; (B) 25 mM Na B 0 , 0.5 mM TTAOH, pH 9.3. 2

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Given that the first commercially available C E apparatus was introduced in the early 1980's the analysis of metal-chelate complexes by capillary electrophoresis has a fairly rich history. However, many of the reported methods have the goal of separating and analyzing mixtures of alkali, alkaline earth, and transition metals (19-28). Optimum methods reported generally require the addition of excess chelate to the sample and/or the electrolyte making these methods not directly applicable to M L speciation studies in water samples. Schaffer et al. (29) analyzed N T A and its breakdown products in water from a desulfuration process. The mixture components were separated but


129 excess E D T A was added to samples to remove interference from Fe(III). A l l components could not be analyzed under a single set of run conditions. Buchberger and Mulleder (30) developed a method for analysis of D T P A and its Fe(ffl), Bi(ffl), Cu(II), Pb(II), and Ni(II) complexes using 10 m M borate buffer, p H 9.3, with 0.2 m M T T A B as E O F modifier and direct detection at 254 nm. Reported detection limits ranged from 5 χ 10' M for F e D T P A to 3.5 χ 10" M for B i E D T A . The method was successfully applied to the analysis of waste water from a paper mill. Harvey (31) described a method for the separation of a mixture of aminopolycarboxylates (EDTA, N T A , ED3A, D T P A , H E D T A ) by on-column formation of their Cu(II) complexes. Using a variation of C E , micellar electrokinetic chromatography ( M E K C ) , the method uses differential affinity for surfactant micelles in the run buffer to assist in the separation of solutes. The performance of several buffer compositions and separation conditions is reported. While the method is not directly applicable to the study of metal-chelate complexes in water it does illustrate the rapid (< 7.5 minutes) separation of complexes with very similar electrophoretic mobility. Ballou et al. (32) also described a method for separation of a mixture of aminopolycarboxylates (EDDA, H E D T A , E D T A , N T A , DTPA) as their Cu(II) complexes using 10 m M phosphate buffer, p H 11.5, and direct detection at 254 nm. The authors demonstrated the successful application of the method to the analysis of a high level radioactive waste tank simulant. Method detection limits for standards prepared in the waste simulant were on the order of 10" M . 7

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Few studies have been published that focus on the application of C E for the purpose of quantifying free chelates and their metal complexes in aqueous samples. Burgisser and Stone (18) described a method for analyzing the Co(II) and Co(III) complexes of E D T A , N T A , and 13 other aminocarboxylate ligands using 25 m M phosphate buffer, p H 7, as an electrolyte with 0.5 m M T T A B as E O F modifier and direct detection at 185 nm. The fifteen chelates were separated without pre-column or on-column complexation. Changes in metal oxidation state, subtle changes in coordination and stereochemistry were readily determined. The Co(II) and Co(III) complexes with E D T A (as C o E D T A " and C o E D T A " , respectively) were separated and quantified. Co(III) complexes with N T A were synthesized and analyzed. Acidification of solutions of aC o N T A yielded a new peak with slower migration time confirming the formation of a new Co(III) complex. The C E results were consistent with a dimer-monomer transition upon acidification that was first proposed by Smith and Sawyer (33) based on proton N M R spectra. Hexadentate C o E D T A " can be separated from the pentadentate C o E D T A " that forms under acidic conditions (Stone, personal communication). Diastereomers of Co IDA2 (IDA = iminodiaacetic acid) were also separated by C E despite having the same molecular weight and ionic charge. The authors hypothesized that the greater degree of symmetry for s-fac-Co JDA " may result in a slightly smaller size and H

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consequently higher charge to size ratio than the «-^c-Co IDA2" diastereomer enabling their separation by C E (Figure 4).

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Figure 4. Separation ofCo (IDA) diastereomers by capillary electrophoresis. Despite having the same molecular weight and charge, the two compounds are separated by CE. Separation conditions: 25 mM ΉαιΒ^, 0.5 mM TTAOH, pH 9.3; -i5 kV, 30s hydrostatic sampling. Capillary 75 pmld. x 60 cm (52 cm to detector); direct detection at 254 nm. 100 μΜ each compound. 2

The C E method developed by Burgisser and Stone (18) was applied to study the reactions of aminocarboxylate chelates with the minerals heterogenite (CoOOH) and manganite (γ-ΜηΟΟΗ) (34). Suspensions of C o O O H in E D T A containing solution generated products resulting from ligand assisted dissolution ( C o E D T A ) as well as reductive dissolution of the solid (e.g., C o E D T A , ED3A, Co ED3A) reflecting the reduction of Co(III) and the oxidation of the organic ligand. Similarly, when N T A was substituted for E D T A , C o N T A , ( C o N T A ) , C o N T A , and I D A were identified in solution. In contrast, dissolution of C o O O H by I D A proceeded via ligand promoted dissolution only with the formation of isomers of Co IDA2; there was no evidence for a reductive dissolution mechanism in the presence of IDA. Reaction of E D T A with manganite resulted in the production of M n E D T A , ED3A, and E D D A . It ln

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131 was unclear whether the failure to detect M n E D T A was due to the predominance of the reductive dissolution pathway or the rapid reduction of aqueous M n E D T A once formed via ligand promoted dissolution. m

Owens et al. (35) analyzed E D T A and N T A speciation in the presence of a mixture of metals (Ca, Co(II), Cu(II), Cd(II), Zn(II), Pb(II), and Fe(III)) using 50 m M phosphate buffer, 0.5 m M T T A B , p H 6.86, and direct detection at 185 nm. Excess chelating agent was not needed in either the samples or the electrolyte for the analysis. The measured M L concentrations agreed well with concentrations predicted using an equilibrium speciation model. Detection limits for the M - E D T A and M - N T A complexes were on the order of ΙΟ" M and 10" M , respectively. However, not all the M - E D T A complexes could be resolved under the same run conditions due to similarities in their electrophoretic mobilities. The authors applied the method to the analysis of a hydroponic solution for N T A , Z n N T A , CaEDTA, and F e E D T A . As observed by Burgisser and Stone (18) C o N T A exhibited very high detection limits due to ligand exchange reactions with phosphate (cf. Figure 3); the problem was exacerbated due to the higher phosphate concentration. Similarly, Pb- and C d N T A were not stable under the separation conditions and injection of pure standards produced no peaks in the electropherograms. Although the stability constants for Pb- and C d - N T A are similar to that for C o N T A , the larger ionic radius of Pb and C d relative to Co may make their complexes with N T A more kinetically labile to ligand exchange reactions under the separation conditions. 6

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Kinetics of M L Speciation The slow kinetics of M L reactions can make metal-ligand mixtures slow to attain their predicted equilibrium state, making direct analysis of M L complexes a valuable asset in understanding metal speciation. A 100 μ Μ solution of F e E D T A in 10 m M NaN03, 10 m M 3-[N-morpholino]propanesulfonic acid (MOPS), p H 7 was spiked with Ni(II) at a final concentration of 100 μΜ. Equilibrium speciation of the M - E D T A system was predicted using the measured total concentrations and the equilibrium speciation code P H R E E Q C with the included M I N T E Q database (36). P H R E E Q C is a geochemical model with a broad range of functions including aqueous speciation and saturation index calculations. The model is a publicly available freeware program (wwwbrr.cr.usgs.gov/projects/GWC__coupled/phreeqc/index.html). The initial solution is undersaturated with respect to Ni(II) solids but oversaturated with respect to Fe(OH) . If precipitation is not allowed in the model, predicted UI

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equilibrium concentrations for F e E D T A and N i E D T A are 28 μ Μ and 72 μΜ, respectively. If F e ( O H ) precipitation is allowed, virtually all the Fe(III) precipitates and the predicted N i E D T A concentration is 100 μ Μ (ignoring sorption onto precipitated Fe(III) solids for simplicity). These model predictions are compared to direct measurement of M L speciation over time. 3fam

Dynamics of the M L system were monitored over time with direct measurement of M - E D T A species using C E for comparison to the predicted equilibrium speciation (Brooks, unpublished results). Total aqueous Ni(II) remained constant over 864 hours (36 days; Figure 5). The concentration of F e E D T A decreased slowly over the first 264 hours with corresponding increases in the concentration of N i E D T A . Total E D T A (sum of Fe E D T A and N i E D T A ) remained constant over the same interval. Between 264 and 456 hours an orange tint developed in the solution and C E analysis indicated that the loss of F e E D T A exceeded the increase in N i E D T A , consistent with precipitation of Fe(III) solids. Total E D T A in solution decreased, presumably by sorption onto the Fe(III) precipitate. The measured concentration of N i E D T A remained well below the predicted equilibrium concentration (adjusted for measured concentrations of Fe(III), Ni(II), and E D T A ) over 36 days. Constant concentrations of total Ni(II) demonstrated that neither Ni(II) nor N i E D T A sorbed onto the Fe(III) precipitate. The results illustrate slow Fe(III) precipitation in the presence of E D T A and how slow exchange kinetics can lead to an incorrect interpretation of metal speciation in the absence of direct M L measurement. ni

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Reactions of C o N T A with Natural Mineral Assemblages Measurement of total concentrations can mask oxidation/ reduction reactions. Solids from the vadose zone of Melton Branch Watershed within the proposed solid waste storage area 7 on the Oak Ridge Reservation in eastern Tennessee were mixed with a 1 m M solution of C o N T A in 5 m M C a C l , p H 7.6 (natural p H of the solids). Total concentrations of Co (atomic absorption spectrometry), N T A (total organic carbon (TOC) and C E ) , and aqueous CoN T A species were monitored over time (Figure 6). Total N T A in solution remained constant reflecting lack of sorption of this constituent; the results from T O C and C E analyses agreed within 5%. B y contrast, total Co in solution decreased over 21 days suggesting that mineral surfaces in the solids had a higher affinity for Co than did the aqueous N T A . Assuming all the Co to be Co(II), C o N T A is predicted to account for all the aqueous Co using equilibrium n

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Figure 6. Reactions of Co NTA in contact with natural subsurface media. Measurement of total NTA and Co only mask important changes in the oxidation state of Co. Separation conditions: 25 mM phosphate, 1 mM TTAB, pH 7.8, -55 μΑ constant current, 30s hydrostatic sampling. Capillary 75 pm id. x 60 cm (52 cm to detector); direct detection at 185 nm plus 254 nm for Co(III) species.


135 thermodynamic calculations. Co(II)NTA determined by C E revealed lower than expected Co(II)NTA concentrations. Sample analysis by C E , coupled with comparison to a reference standard, revealed the formation of the Co(III) species C o ( H N T A ) . Formation of this oxidation product likely is the result of interaction with mineral surfaces as this species was not seen in controls without solids. Total Co determined by A A S was in excellent agreement with total Co determined from summing the identified species from the C E analysis. After 21 days, C o ( H N T A ) accounted for 12% of the aqueous C o and 16% of the aqueous N T A . n

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The formation of Co(III) complexes with N T A in contact with natural media has important implications for the fate and transport of this metal. Co(III) complexes are kinetically and thermodynamically stable, exhibit lower sorption, and are more resistant to biodégradation than their analogous Co(II) complexes (37,38). These characteristics may combine to facilitate the undesirable movement of Co in the subsurface. Laboratory experiments have investigated the fate and transport of C o N T A through intact cores of the same material used in Figure 6. The concentration history (breakthrough curve) for total C o exhibited two plateaus which is indicative of multispecies transport (Figure 7) (39-41). C E analysis of samples revealed formation of the Co(III) species C o ( H N T A ) , C o n N T A , and s-/ac-Co (IDA) reflecting the oxidation of both Co and N T A . The Co(III) species were transported through the column more rapidly than C o N T A and accounted for 44% of the recovered Co. n

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Capillary electrophoresis is a useful and relatively easy technique for the direct determination of M L species. Application of this analytical method can reveal slow exchange kinetics, subtle changes in coordination, and oxidation/ reduction reactions involving both the metal and ligand.

Acknowledgement This research was funded by the U.S. Department of Energy's Office of Science Biological and Environmental Research, Natural and Accelerated Bioremediation Research (NABIR) program. This work benefited greatly form conversations with Alan Stone, Johns Hopkins University. Oak Ridge National Laboratory is managed by UT-Battelle, L L C , for the U.S. Dept. of Energy under contract DE-AC05-00OR22725.


136 Cumulative Volume Eluted (L) Ο

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Figure 7. Transport ofCo NTA through intact cores of weathered saprolite. Formation ofCo(IH) species resulted in more rapid transport of Co through the column and accounted for 44% of the total recovered Co (adapted from (37)). Separation conditions same as for Figure 6.


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