Environ. Sci. Technol. 2008, 42, 113–118
Highly Charged Swelling Mica-Type Clays for Selective Cu Exchange RAMESH RAVELLA, SRIDHAR KOMARNENI,* AND CARMEN ENID MARTINEZ Department of Crop and Soil Sciences, The Pennsylvania State University, University Park, Pennsylvania 16802
Received April 11, 2007. Revised manuscript received October 8, 2007. Accepted October 26, 2007.
There is a need to develop highly Cu2+ selective materials which can potentially remediate copper contaminated soils and water. Here we show that several highly charged synthetic swelling mica-type clays are highly selective for copper exchange. The synthetic micas have cation exchange capacities (CECs), which are close to their theoretical values. Both Nasaturated and Mg-saturated micas were investigated for Cu ion exchange selectivity. Ion exchange isotherms and Kielland plots were constructed using the equilibrated solution analyses. From these studies it was found that Na-4-mica and Na-3mica could selectively exchange copper at lower concentrations from solution, whereas Na-2-mica sample performed better by showing Cu ion exchange selectively to almost its capacity. The EPR spectra of Cu-exchanged micas coincide with the mica’s charge characteristics that predict increased binding strength of exchangeable Cu in Na-4-mica and Na-3-mica than in Na-2-mica.
Introduction Mining activities and mineral processing have a significant impact in deteriorating water and soil quality with heavy metal contamination of soils and water posing health risks to humans, animals, and plants. Copper, with its multitude of uses, is a very extensively mined and processed metal. Although it is an essential micronutrient to plants, it can cause toxic effects if present in excess quantities. In the Earth’s crust copper occurs at an average concentration of 50 ppm (1). The U.S. is the second largest producer of copper (after Chile) (2), and the continuous consumption of copper in various industries (1) will drive copper mining, and subsequently, copper may be released into the environment. Accumulation of copper can have a toxic effect on plants and microorganisms (3). Several remediation and removal techniques for copper contamination are in practice. Chelant extraction using ethylene diamine tetraacetic acid (EDTA), citric acid, and nitrilotriacetic acid (NTA) was effective for copper in Aberdeen Proving Ground’s J-field (4) and s-carboxymethylcysteine (SCMC) chelating agent removed copper from soil in batch experiments (5). Fly ash (6) and a clay mineral, palygorskite (7) were tested as adsorbents for copper. Other clay minerals such as Na-bentonite and Ca-bentonite (at 2%) were also found effective for adsorbing copper in a soil (Stuttgart, SW Germany) contaminated with heavy metals due to sewage sludge disposal (8). Another study conducted * Corresponding author phone: 814-865-1542; fax: 814-865-2326; e-mail:
[email protected]. 10.1021/es070854k CCC: $40.75
Published on Web 12/01/2007
2008 American Chemical Society
with Ca-bentonite and Na-bentonite for adsorbing heavy metals from a water solution showed that modified form of bentonite (Na-bentonite) was more effective in removing copper along with other cations (9). Smectite was found to be superior to zeolite and halloysite (10) for copper uptake. Modified montmorillonite products such as Al-montmorillonite and Al-13-montmorillonite specifically adsorbed copper and also incorporated copper in to their interlayer space, thereby reducing the mobility of copper (11). Copper was effectively removed from water by modified montmorillonite with surfactant sodiumdodecylsulphate (SDS) (12). Acid (HCl) treated and NaOH neutralized clay minerals can also be used for removal of copper from wastewater (13). Thus many types of minerals and other adsorbents such as ion exchange resins (14) anatase under UV radiation (15) were proposed for Cu2+ uptake. In this study synthetic sodium fluorophlogopite micas of different cation exchange capacities (CECs) were tested for their ability to selectively exchange Cu2+ from solutions. A special swelling-type of sodium fluorophlogopite mica, Na4Si4Al4Mg6O20F4.xH2O was identified in 1972 (16), named as Na-4-mica (17) as it has four interlayer Na ions per unit cell balancing the charge developed due to the isomorphous substitution in the tetrahedral sheet with a high CEC of 468 meq/100 g. The Na-4-mica has been synthesized by various investigators (18–24). Na-3-mica, Na3Si5Al3Mg6O20F4.xH2O with three interlayer Na ions per unit cell (25–27) and Na2-mica, Na2Si6Al2Mg6O20F4.xH2O with two interlayer Na ions per unit cell (28–30) were also synthesized previously. Both the high cation exchange capacity and high selectivity for some cations (20–23) displayed by Na-4, Na-3, and Na-2micas makes these mica-type clays potential agents for remediation of soils and water. Previous experiments (18, 20, 22, 31, 32) with these synthetic micas have led to partial but very substantial (about half-the CEC) selective exchange of some incoming cations such as Cu, Ba, Sr, etc. apparently due to difficulty of diffusion into the narrow interlayer spacing of about 2.8 Å or due to collapse of interlayers at the edges. Hence we wanted to verify the hypothesis that restricted interlayer spacing leads to lower selective exchange by using Mg exchanged clays with initially expanded interlayers. Accessing the interlayer space completely is one of the keys to realize the full potential of these materials. In this study, we tested both the Na and Mg forms of synthetic clays (Na-4-, Na-3-, and Na-2-micas) for selective uptake of Cu for potential remediation purposes.
Materials and Methods Na-4-mica (RR159) was prepared using kaolinite, MgCl2, MgF2, and NaCl by stoichiometric fluorine content method, Na-3-mica (RR156) was synthesized using kaolinite, silica gel, MgCl2, and NaF, and Na-2-mica (RR162) was synthesized using kaolinite, silica gel, MgF2, and NaCl (33). A poorly crystallized kaolinite of composition (34) 43.9% SiO2, 38.5% Al2O3, 2.08% TiO2, 0.98% Fe2O3, 0.15% FeO, and 0.03% MgO (Clay Mineral Repository, University of Missouri, Columbia MO 65201) was used for synthesis. The precursors were weighed in stoichiometric proportions for the individual micas and mixed gently but thoroughly using a mortar and pestle. The mixture was then kept in platinum crucibles and heated in a programmable furnace with a ramp temperature of 2 °C per minute at 850 °C/10 h (RR159), and at 900 °C/10 h (RR156, RR162), then cooling the mixtures down to 70 at 2 °C per minute ramp down temperature. The micas were then washed with deionized water (DI) four times, each time centrifuging and decanting the supernatant. Mica samples VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. X-ray Diffraction Data of Synthetic Micas and Their Mg saturated Analogues and Theoretical and Determined CEC values of Na and Mg Saturated Expandable Micas Na-saturated
Mg-sat. (exchange)
theoretical CECa
d(001)Å
CECa
d(001)Å
CECa
d(001)Å
CECa
Na-4-mica Na-3-mica Na-2-mica
468 351 234
12.2 12.2 12.2
450 286 184
13.6 13.7 13.7
314 219 146
13.8 13.8 14.3
473 268 167
a
CEC- Cation Exchange Capacity, meq/100 g.
washed free of excess salts were dried at 60 °C overnight or until completely dry. The micas were characterized by powder X-ray diffraction (XRD) for phase confirmation. The three mica samples were characterized by XRD using a Scintag diffractometer with Cu KR radiation and a Ge solid state detector. Diffraction patterns were collected with a step size of 0.020° 2θ and with a speed of 4° 2θ per minute. Selectivity of the as-prepared Na saturated micas for Cu was determined, i.e., 2Na+ f Cu2+ exchange. For Mg2+ f Cu2+ exchange the three micas were saturated with Mg by two methods. First method is called a melt method where in the samples were mixed with Mg(NO3)2 at a molar ratio of 20:1 Mg-salt to mica, heated in an oven at 90 °C for 24 h and washed five times with DI water to remove excess Mg salt. Washed samples were dried at 70 °C for 1 day. The second method of saturation is an exchange process in which the samples were kept in 2 M Mg(NO3)2 solution for 3 days, and solutions were changed every 24 h period. The saturated samples were washed with DI water five times and dried at 70 °C for 1 day. The cation exchange capacities (CEC) of the micas were determined as follows: Samples (50 mg) of each mica were weighed in triplicate into clean centrifuge tubes. The samples were then equilibrated with 1.0 M KCl for 24 h. After equilibration, the samples were centrifuged, and the supernatant 1.0 M KCl solution was carefully decanted without loss of solid. This saturation step was performed three times to ensure complete exchange with K+ ions. The samples were then washed five times with 0.01 M KCl, and the supernatant liquid was discarded. After this, the tubes, samples, and entrained liquids were weighed to the nearest 0.0001 g. The K+ ions from the micas were replaced by washing the samples five times with 0.5 M NaCl (equilibrating 24 h each time) followed by centrifugation, and the supernatants were collected in 100 mL volumetric flasks. The volumetric flasks were brought to volume with 0.5 M NaCl. The concentration of K in the 0.5 M NaCl, 0.01 M KCl, blanks, and standards were determined by atomic emission spectroscopy using a SpectraMetric SpectraSpan III. The cation exchange capacities were calculated based on the amount of K released from the exchange positions and expressed as meq/100 g. Copper selectivity of all the three synthetic samples was determined using cation exchange equilibria with Cu and Na in exchanging solutions. Solutions of Cu/Na, and Cu/Mg with different ratios were prepared. All the starting solutions of different Cu/Na and Cu/Mg ratios were prepared approximately at the same pH i.e., pH values ranging between 4.47 and 4.55. Overall normality of the solutions was kept at 0.00468, 0.00351, and 0.00234 for Na-4, Na-3, and Na-2-micas, respectively. Twenty-five mg of each sample was weighed into centrifuge tubes in triplicate. Mica samples were equilibrated with 25 mL portions of the solutions for 4 days. The pH after exchange was elevated by a value ranging between 0.4 and 0.5. The solution and solid phase were separated by centrifugation and the supernatants were transferred to small bottles followed by chemical analysis of the solutions. The results obtained were used to construct the isotherms and Kielland plots to determine the selectivity. Plotting the logarithm of the corrected selectivity coefficient, 114
Mg-sat. (Melt)
synthetic mica
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M versus XM (a Kielland plot) is often useful for log KNa M determining ion selectivity. When the log KNa is larger than M n+ zero, the ion exchanger is selective for M . When the log KNa + is less than zero the ion exchanger is selective for Na . When M the log KNa ) 0, no preference for either ion exists. Cation exchange theory was given in our previous papers (25, 26, 30) and will not be repeated here. EPR Analyses of Cu-Exchanged Mica-Type Clays. EPR studies were conducted to determine the bonding nature of exchanged Cu on to the mica-type clays. The electron paramagnetic resonance (EPR) spectra of Cu exchanged mica-type clays (powder samples) were collected using a Bruker ESP 300 E (X-band) spectrometer. The Cu exchanged mica-type clays were weighed (15 mg) into quartz EPR tubes and their spectra (10 coadded scans) recorded at room temperature using 50 mW of microwave power and a frequency of ∼9.8 GHz.
Results and Discussion The three synthetic clays of nominal compositions such as Na4Si4Al4Mg6O20F4.xH2O (Na-4-mica), Na3Si5Al3Mg6O20F4.xH2O (Na-3-mica), and Na2Si6Al2Mg6O20F4.xH2O (Na-2mica) were characterized by X-ray diffraction for phase formation and purity (see Supporting Information (SI) Figure S1), particle sizes were measured by SEM (see SI Figure S2), and surface areas were measured by BET N2 sorption. The particle size of these micas is in the range of 2–4 µm (see SI Figure S2) and their edge surface area is expected to be very low because the BET N2 surface areas are very low. The measured BET surface areas of Na-4-mica, Na-3-mica, and Na-2-mica are 9.4, 4.1, and 4.4 m2/g, respectively. The BET surface areas measure both edge and external planar particle surface areas. The theoretical internal surface areas of these materials are about 800 m2/g (34) and the interlayer surfaces overwhelm the external planar and edge sites in the cation exchange process. Characterization of the as-prepared synthetic Na-4, Na-3, and Na-2-micas and Mg-exchanged micas confirmed the formation of expandable mica phases (Table 1). The Mg-saturated mica samples show expanded d(001) spacing compared to the Na-saturated samples (Table 1). Mg saturation causes increase in the d spacing of (001) reflection as was previously reported (22) (23) because of the higher hydrated size of Mg. CEC values were determined for the original Na saturated micas and Mg-saturated micas (Table 1). The CECs of synthetic Na-saturated samples are close to theoretical CECs (based on Al substitution for Si) confirming the chemical composition of the micas indirectly. Others have reported similar values for Na-4-mica (19, 23). Although the XRD patterns showed expanded phases in all the three mica types, the Na-3 and Na-2-mica samples showed lower than theoretical CEC values probably because of lower Al substitution in the tetrahedral sheets than expected based on theoretical compositions. Ion exchange selectivity properties of Na-4-mica. Na4-mica in its original form and two Mg saturated forms were used as solid phase ion exchangers in this study. The ion exchange isotherm for 2 Na+ f Cu2+ of the Na-4-mica is
FIGURE 1. Ion exchange isotherms for (a) 2Na+ f Cu2+ Na-4-mica (b) Mg2+ f Cu2+ with Mg (Melt) saturated Na-4-mica (c) Mg2+ f Cu2+ with Mg (Exchange) saturated Na-4- mica and Kielland plots for (d) 2Na+ f Cu2+ Na-4-mica (e) Mg2+ f Cu2+ with Mg (Melt) saturated Na-4-mica (f) Mg2+ f Cu2+ with Mg (Exchange) saturated Na-4-mica. plotted in Figure 1a. Isotherms for Mg2+ f Cu2+ exchange of the Na-4-mica samples saturated with Mg by the two methods (melt and exchange) are given in Figures 1b and 1c. The 2 Na+ f Cu2+ isotherm shows that the exchange at lower concentrations of Cu2+ is substantial with little or no Cu2+ in solution (Figure 1a). Cu was exchanged from the solution until the equivalent fraction in the solid was about 0.57. This result is supported by those of Pidugu (35), who reported similar values. The Cu exchange isotherm shows that the Mg saturated (melt method) sample (Figure 1b) takes up almost all the Cu2+ at the lowest concentration but reaches a plateau at an equivalent fraction on solid at about 0.40. For the Mg saturated (exchange method) sample the isotherm reached a maximum at an equivalent fraction on solid at 0.18 (Figure 1(c)) and decreased there after. Thus, the latter sample has a smaller capacity for Cu. Kielland plots were developed for all of the ion exchange reactions reported above. Figure 1d shows the kielland plot for Cu2+ exchange on to Na saturated Na-4-mica. Cu2+ ions were selectively taken up till XCu is 0.23. The corrected selectivity coefficient, however dropped drastically from 101 – 10-1. Kielland plots for Cu2+ ions exchanged with Mg saturated (Melt method) (Figure 1(e)) and Mg saturated (Exchange method) (Figure 1(f)) Na-4-mica show the selectivity styles of the two samples. Both samples were found to be selective for Cu2+ at low concentrations. Mg saturation of Na-4-mica by solution exchange seems to give a good sample which was unaffected by this process where as exchange by Melt method appears to degrade the sample as indicated by lower CEC (Table 1). Ion exchange properties of Na-3-mica. Na-3-mica sample synthesized by stoichiometric fluorine content method (33) and Mg saturated samples by the two methods of saturation as described above are the ion exchanging solid phases in this study. The ion exchange isotherm for 2 Na+ f Cu2+ of the Na-3-mica is plotted in Figure 2a. The isotherm rose steeply at lower concentrations of Cu2+ and then kept on increasing very gently. The equivalent fraction on the solid was about 0.40 (Figure 2a). Shimizu et al. (27), reported similar
TABLE 2. Cu retained (% by weight) by Mg-saturated mica-type clays at three Cu:Mg ratios Mg-saturated synthetic mica-type clays
Cu:Mg ratio
Na-4-mica
Na-3-mica
Na-2-mica
1:0 0.6:0.4 0.1:0.9
3.4 2.6 1.5
2.2 1.6 0.95
5.7 3.9 0.70
values for the Na-3-mica exchange with divalent cations. Selectivity of Na-3-mica was tested for Sr by Kodama et al. (25) and they reported a higher CEC for Sr than with Na4-mica. Later on CEC of Na-3-mica was determined for alkaline metals Ca, Mg, and Ba as 338, 322, and 246 meq/100 g respectively (26). Selectivity order of Na-3-mica for alkaline earth metal cations was given by Shimizu et al. (27), as follows: Ca2+ < Mg2+ < Sr2+ < Ba2+. Cu2+ exchange isotherms of the Na-3-mica samples saturated with Mg by Melt and exchange procedures are given in Figures 2b and 2c respectively. Both Mg-saturated samples showed selectivity only at lower concentrations of copper. Kielland plot of Na saturated Na-3-mica (Figure 2d) shows selectivity at low values of Cu equivalent fraction in solid. j Cu value of 0.25 there is no selectivity for the cation. After X Mg (Melt) saturated sample’s Kielland plot (Figure 2e) showed that at low Cu concentration in the solid fraction (0.1–0.2), the mica is selective, but after 0.2 the mica is not selective. Similar result can be seen in case of the Mg (Exchange) saturated sample (Figure 2f). The partial replacement of Mg by Cu is preventing further selectivity for Cu probably because of structural changes (33). Ion exchange properties of Na-2-mica. Ion exchange studies were done on Na-2-mica synthesized by NaCl melt method (36) and on its Mg saturated phases. The ion exchange isotherm for 2 Na+ f Cu2+ of the Na-2-mica is plotted in Figure 3a. Na-2-mica displayed a high selectivity at lower Cu concentrations as indicated by little or no Cu VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Ion exchange isotherms for (a) 2Na+ f Cu2+ Na-3-mica, (b) Mg2+ f Cu2+ with Mg (Melt) saturated Na-3-mica, (c) Mg2+ f Cu2+ with Mg (Exchange) saturated Na-3- mica and Kielland plots for, (d) 2Na+ f Cu2+ Na-3-mica, and (e) Mg2+ f Cu2+ with Mg (Melt) saturated Na-3-mica (f) Mg2+ f Cu2+ with Mg (exchange) saturated Na-3-mica.
FIGURE 3. Ion exchange isotherms for (a) 2Na+ f Cu2+ Na-2-mica, (b) Mg2+ f Cu2+ with Mg (melt) saturated Na-2-mica, (c) Mg2+ f Cu2+ with Mg (exchange) saturated Na-2- mica, and Kielland plots for (d) 2Na+ f Cu2+ Na-2-mica (e) Mg2+ f Cu2+ with Mg (Melt) saturated Na-2-mica (f) Mg2+ f Cu2+ with Mg (exchange) saturated Na-2-mica. in solution at those concentrations. The exchange isotherms of Na-2-mica samples saturated with Mg by Melt and Exchange procedures are given in Figures 3b and 3c, respectively. Similar results were obtained in case of the Mg saturated samples as that of Na-2-mica. Na-2-mica synthesized by Kodama et al. (29) demonstrated a high selectivity for Sr compared to other cations like Ca, K, Cs, and Li and has a higher CEC for Sr (199 meq/100 g) than that of Na-4-mica. High Cu exchange occurred at low concentrations of Cu in the liquid phase which confirms 116
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that the Na-2-mica is highly selective for Cu at low concentrations. The Kielland plot of Na saturated Na-2Mg at low values mica (Figure 3d) shows an increase in logKCu j Cu value of about of Cu equivalent fraction in solid. After X 0.40 there is no selectivity for the Cu2+cation. The Kielland plot of Mg (melt) saturated sample (Figure 3e) showed Mg values. Another sample, Mg selectivity at all log KCu (Exchange) saturated (Figure 3f) also exhibited high Mg values. selectivity and gave almost steady log KCu
FIGURE 4. EPR spectra of Cu-exchanged Mg-saturated synthetic mica-type clays. Cu exchange was performed using Cu solutions that yielded Cu:Mg ratios of 1:0, 0.6:0.4, and 0.1:0.9. Dotted lines indicate the gΠ (2.40) and g⊥ (2.08) rigid-limit Cu-EPR parameters. Variable ordinate scales are used in the plots. The peaks at a g value of ca. 1.95 from the Na-3-mica and to a lesser extent Na-4-mica and Na-2-mica (0.1:0.9 Cu:Mg ratio) result from a paramagnetic ion (Fe3+) present either in the silicate framework or as an impurity (see SI Figure S4). Copper-EPR Spectra of Cu-Exchanged Mg-Saturated Mica-Type Clays. Electron paramagnetic resonance (EPR) spectroscopy was used to determine the type of coordination for Cu exchanged into Mg-saturated synthetic mica-type clays under acidic (∼pH 4.5-5) conditions and varying Cu loadings (see Table 2). The Cu-EPR spectra of three synthetic micas (with varying charge characteristics) with four levels of Cu for Mg exchange were collected. An anisotropic (rigid-limit) EPR signal was observed in the Na-4-mica and Na-3-mica specimens after Cu exchange using Cu:Mg ratios of 1:0, 0.6:0.4, and 0.1:0.9 (Figure 4). The rigid-limit signal indicates reduced movement of the Cu2+ ion due to attraction forces between Cu(H2O)62+ and permanent charge sites and hydrogen bonding with basal oxygens of the silicate lattice ( (37), (38)). The EPR parameters of the rigid-limit Cu were estimated to be gΠ ) 2.40 and g⊥ ) 2.08 in Cu-exchanged Na-4-mica and Na-3-mica, and they were similar regardless of the level of Cu for Mg exchange. The hyperfine lines of the gΠ component, however, were more prominent at the highest Cu:Mg ratios (1:0 and 0.6:0.4) for both micas (Figure 4). Hyperfine lines of the gΠ component indicate that Cu occupies magnetically isolated (well dispersed) sites and that Cu has the ability to coordinate to adsorbed water. Moreover, broadening of the rigid-limit Cu-EPR signal was observed for Na-4-mica at all Cu:Mg ratios (Figure 4) thus suggesting shorter Cu-Cu distances and magnetic interactions due to clustering of Cu atoms that might be occurring in this highly charged mica. Yet, the persistence of the hyperfine lines in the rigid-limit spectra indicates that a fraction of the Cu ions exists in discrete sites, perhaps attributed to the preferred association of one Cu2+ ion with two adjacent negative charge sites within the silicate structure (37). Signal broadening was greatly reduced in the Na-3-mica, whereas it was not observed in the Na-2-mica. Reduction or absence of signal broadening in these synthetic micas with lower layer charge suggests that the Cu ions were distant enough to prevent magnetic interactions. The coordination environment of the Cu ion in Cuexchanged Na-4-mica is comparable to the one obtained for highly charged 2:1 layer silicates such as vermiculite (37). On
the other hand, Cu-exchanged hectorite (37) and Cuexchanged synthetic hydroxyhectorite (38), with permanent charge from octahedral substitutions, yielded Cu-EPR spectra similar to the spectra of Cu-exchanged Na-3-mica. The CuEPR spectra for Na-2-mica seemed to transition from an anisotropic to an isotropic signal with increasing Cu:Mg ratios (Figure 4). At low loadings (0.1:0.9 Cu:Mg ratio), the Cu2+ ion is likely to occupy sites that are in close proximity to permanent charge sites of the clay thus forming hydrogen bonds with surface oxygens of the silicate structure along the z axis while retaining its 4 coordination waters in the xy plane (37). As the concentration of Cu in the interlamellar space of this relatively low-charge mica increases, the proportion of freely tumbling Cu ions, which generates an isotropic EPR signal (see below), is expected to increase. The anisotropic EPR signal of all three mica-type clays was lost after Cu2+ was fully exchanged for Mg2+ (SI Figure S3). These spectra exhibit an isotropic signal with g ) 2.15. The appearance of an isotropic signal is a consequence of rapid tumbling or Jahn–Teller distortion (rapid changes in axial elongation among the three equivalent H2O-Cu-H2O axes) of the Cu(H20)62+ complex in the fully exchanged (and presumably expanded) interlayers that averages the anisotropy. These spectra are analogous to those observed for Cu2+ in water and for Cu-exchanged montmorillonite (37–39). Changes in the Cu-EPR spectra at Cu:Mg ratios of 1:0, 0.6:0.4, and 0.1:0.9 coincide with charge characteristics of the synthetic mica-type clays that predict stronger retention by Na-4-mica than by Na-2-mica. Copper (Cu2+) has the ability to coordinate to adsorbed water in Na-4-mica and in Na-3-mica but is mostly present as a more mobile (rapidly tumbling) Cu2+ species in Na-2-mica.
Acknowledgments This study was supported by Interfacial, Transport, and Separation Program, Chemical and Transport Systems, Division of the National Science Foundation under Grant no. CTS-0242285 and the College of Agricultural Sciences under Station Research Project no. PEN03963. We thank Dr. John H. Golbeck and Dr. Rama Balasubramanian in the VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Department of Biochemistry and Molecular Biology at our university for letting us use the EPR spectrometer.
Supporting Information Available Additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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