Cr(III

Aug 29, 2008 - The roles of chromium species on photochemical cycling of iron and mineralization of polycarboxylates are examined in the presence of C...
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Environ. Sci. Technol. 2008, 42, 7260–7266

Photochemical Coupling Reactions between Fe(III)/Fe(II), Cr(VI)/Cr(III), and Polycarboxylates: Inhibitory Effect of Cr Species ZHAOHUI WANG, WANHONG MA, CHUNCHENG CHEN, AND JINCAI ZHAO* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China

Received May 19, 2008. Revised manuscript received June 25, 2008. Accepted July 18, 2008.

The roles of chromium species on photochemical cycling of iron and mineralization of polycarboxylates are examined in the presence of Cr(VI) or Cr(III) at pH 2.2-4.0. Under UV irradiation, Cr(III) altered the redox equilibrium of iron species, leading to the shift of the photosteady state toward Fe(II). After a longer time of illumination, total organic carbon (TOC) approached a steady state in the presence of Cr(III) or Cr(VI), whereas oxalate was thoroughly mineralized in the absence of Cr species. The TOC of steady state was closely related to the kind of polycarboxylates, Cr species dosages, pH and O2 atmosphere, but hardly affected by more addition of Fe(III). ESI-MS data indicates that several Cr-oxalate complexes formed in the photochemical reactions, which are responsible for protecting oxalate against further oxidation. A mechanism is proposed for the inhibitory effect of Cr species on oxidation of oxalate and Fe(II). The present study may provide a new insight into the dual environmental effects induced by Cr contaminants especially at heavily chromium-contaminated and dissolved organic matter (DOM)-rich sites.

Introduction Environmental photochemistry of iron species has received considerable attention because it plays a significant role in the numerous iron-dependent geochemical cycles, such as phytoplankton blooms (1, 2), carbon cycle (3), and redox cycling of other transition metal complexes (4). Fe speciation in environmental systems is particularly relevant with naturally occurring organic ligands, most of which are capable of controlling the photochemical activities of the iron species. The carboxylate group RC(O)O- is one of the most common functional groups in dissolved organic matters (DOM) present in natural waters (5-8). The photoredox behavior has been observed for some simple Fe(III)-polycarboxylates (e.g., oxalate, malonate, and citrate) complexes in atmospheric and surface waters. Under sunlight irradiation, Fe(III) undergoes rapid photoreduction to Fe(II) on a time scale of minutes, accompanied by oxidative degradation of polycarboxylic ligands to carbon dioxide in the aerated systems (6-8). Photochemically generated Fe(II) is readily reoxidized to Fe(III), which is achieved mostly by molecular oxygen, but also by other strongly oxidizing species like hexachromium (9). * Corresponding author fax: +86-10-8261-6495; e-mail: jczhao@ iccas.ac.cn. 7260

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Cr(VI) and Cr(III) are the two primary oxidation states of chromium in aquatic environment (10). Cr(VI) species are known acutely toxic, carcinogenic and are of environmental concern (10). In China, there is a continual and abundant influx of chromium contaminants (for example, 103.12 tons in 2003 (11)) into the environment, thus posing a severe environmental problem. Since Fe(II) is the dominant environmental reductant of Cr(VI) at acid and nearly neutral pH (12, 13), iron-catalyzed photochemical reduction of Cr(VI) contaminants is a vital and naturally occurring process in natural waters (14, 15). Hug et al. investigated the photocatalytic reduction of Cr(VI) by iron(III)-organic complexes (9, 16). It was found that reduction rates of Cr(VI) varied widely and were strongly dependent upon the nature of DOM substrates. Fe(II), HO•2, and O•2 species were speculated as the most likely reductant candidates of Cr(VI). Moreover, under similar conditions, it was reported that light-induced oxidation of Cr(III) in the presence of Fe(III) would be one potential pathway for regenerating Cr(VI) in atmospheric waters (17, 18), in which •OH from photolysis of Fe(OH)2+ was considered as a dominant oxidant to oxidize Cr(III). Therefore, Cr(III) should be regarded as a latent pollutant, although it is generally regarded as benign and even essential for human and animal (10). To date, much attention has been paid to redox reactions of Cr(VI)/Cr(III) induced by the photocatalytic reactions of Fe(III)-polycarboxylates (9, 16-18), however, little work has been done in investigating the effect of Cr contaminants on either redox cycling of iron or the fate of organic ligands. Our recent work (19) has investigated the effect of periodic influx of DOM on photoredox cycling of iron under UV irradiation. Surprisingly, photoinduced oscillation of Fe(II)/Fe(total) ratio was observed due to the disturbance induced by DOM for the equilibrium between Fe(III) photoreduction and Fe(II) photooxidation. However, it is still unclear whether Cr pollutants affect the photoredox cycling of iron in the similar manner. The objectives of the present study were to (i) determine whether and how the concurrence of Cr pollutants and polycarboxylates influences the steady-state equilibrium of Fe(III)/Fe(II), and (ii) elucidate the mechanism details of acceleration or inhibition effect of Cr contaminants on the photochemical degradation of DOM. The present study may provide a new insight into the environmental processes induced by the abundant influx of Cr contaminants and give a better understanding on the interaction among dissolved iron pool, Cr species and DOM under solar irradiation.

Experimental Section Chemicals. Iron(III) perchlorate hydrate and perchloric acid were purchased from Aldrich. Oxalic acid, formic acid, succinic acid, phthalic acid, citric acid, tartaric acid, 1,10phenanthroline, diphenylcarbazide, potassium dichromate (K2Cr2O7), chromium nitrate nonahydrate (Cr(NO3)3 · 9H2O), sodium perchlorate (NaClO4), and sodium hydroxide (NaOH) were of reagent grade and used as supplied. Barnstead UltraPure water (18.3 MΩ) was used for all experiments. Experimental Procedures. The photochemical reactions were performed using a 100 W Hg lamp (Toshiba SHL100UVQ-2) as the irradiation source. The irradiation setup and protocol were identical as described elsewhere (19). The reaction solutions were always freshly prepared by dilution of stock solutions of 0.01 M organic acids, 5 mM Fe(III) at pH < 2 (HClO4) and 4 mM Cr(VI)/Cr(III). Additional electrolytes were avoided to prevent the iron(III) complexation with other inorganic anions. Unless otherwise noted, 10.1021/es801379j CCC: $40.75

 2008 American Chemical Society

Published on Web 08/29/2008

all reactions were carried out under exposure to air. For deaerated experiments, the solutions in the cap-sealed Pyrex bottles were purged with high-purity Ar (O2 e 0.001%) for at least 30 min prior to illumination and continuously bubbled during the experiment. Methods and Analysis. The concentration of Fe(II) was determined by 1,10-phenanthroline method as previously described (19, 20). Aqueous Cr(VI) concentration was measured by a diphenylcarbazide method (21). The color was fully developed after 5 min and the sample solutions were detected at 540 nm with a Hitachi U-3100 spectrophotometer. TOC was determined on a Tekmar Dohrmann Apllo 9000 TOC analyzer. Each sample was measured in duplicate and relative error was less than 2%. For the quantitive measurement of oxalate concentration, ion chromatograph (IC) (DX120, Dionex Co.) was applied with conductivity detection. A Dionex Ionpac AS 11 column (4 mm) and a Dionex anion micromembrane suppressor were employed. 10 mM NaOH solution was chosen as eluent. Electron spin resonance (ESR) spectra of Cr(V) were recorded at room temperature on a Bruker EPR ELEXSYS 500 spectrometer equipped with an in situ irradiation source (a Quanta-Ray ND:YAG laser system λ)355 nm), and the same quartz capillary was used for all the measurements to minimize errors. Typical parameters for acquisition of Cr(V) signals were as follows: center field, 3540 G; sweep width, 100 G; resolution, 1024 pts; microwave frequency, ∼9.78 GHz; microwave power, 10 mW; modulation frequency, 100 kHz; time constant, 40.96 ms; receiver gain, 50. For all ESR measurements, ionic strength was maintained constant (I ) 1.0 M; HClO4+NaClO4).

Results and Discussion Photosteady State Equilibrium of Fe(III)/Fe(II) in the Presence of Cr Species. Figure 1a depicts the changes of the photoproduced Fe(II) concentration in iron(III)-ox (ox ) oxalate) solution in the absence and presence of Cr(VI)/ Cr(III). Upon UV irradiation, whether in the presence of Cr species or not, the concentration of Fe(II) sharply increased within the initial 5 min and then followed by a rapid decay. Similar fluctuations induced by DOM influx were ever observed in the case of salicylic acid and benzoic acid (19). The rapid generation of Fe(II) should be attributed to ligandto-metal charge transfer (LMCT) process of iron(III)-oxalate complex under irradiation (6-8) (eq 1). The subsequent decay of Fe(II) was due to its reoxidation • by reactive oxygen species (ROS) (i.e., O•2 , HO2, H2O2, and •OH), which were generated from reduction of oxygen by CO•2 radicals and subsequent disproportionation of superoxide and Fenton reaction (eq 2-5). hv

[Fe(C2O4)n]3-2n f [FeII(C2O4)n-1]4-2n + CO2 + CO•2

(1)

•CO•2 + O2 f CO2 + O2

(2)

+ O•2 +H

f

HO•2

(3)

2HO•2 f H2O2 + O2 •

Fe(II) + H2O2 f Fe(III) + OH + OH

(4) -

(5)

After 30 min of irradiation, Fe(II) slowly increased and finally a photosteady state of Fe(II)/Fe(total) approached respectively in the three systems, implying the balance between Fe(III) photoreduction and aerobic Fe(II) photooxidation. Interestingly, in the presence of Cr(III) or Cr(VI), the ultimate photosteady of Fe(II) concentration was, respectively, 40 and 25% higher than that in the control experiment. In our previous study (19), it was observed that different DOM may contribute to the fluctuation of Fe(II), but the ultimate photosteady state was constant and independent of the kinds and concentrations of DOM under irradiation at constant

light intensity. Thereby, DOM and inorganic Cr species should exhibit different effect on the photosteady state of iron cycle: the former is irreversibly degradated to carbon dioxide and no longer has an affect on the photosteady state, whereas the latter may be always active due to its redox cycling. Therefore it was the Cr species that alter the redox equilibrium of iron species, leading to the shift of the photosteady state toward Fe(II). It is noteworthy that Cr species of different valences lead to different ultimate Fe(II) concentration, indicating their distinct roles in the photoredox of iron. Inhibitory effect of Cr Species on Mineralization of Oxalate. The photolysis of Fe(III)-polycarboxylates complex is always coupled with the degradation of organic ligands (6). In the present study, to eliminate the interference of the different Fe (or Cr) carboxylato complexes, the effect of Cr species on mineralization of oxalate was evaluated by TOC analysis instead of direct IC measurement. Figure 1c shows the time profile of mineralization of oxalate in 6 h time scale. At the beginning of the photoreaction (∼25 min), in parallel to the fluctuation of Fe(II), oxalate was mineralized completely in the absence of Cr species via reactions of eqs 1-2. Unexpectedly, TOC held in a constant level after 25 min when Cr(VI) was present, demonstrating that a part of oxalate species were resistant to degradation. We eliminated the possibility that Cr inorganic ions would interfere with TOC measurement by blank experiments. The ion chromatograph, together with FTIR and XPS experiments (Supporting Information Figures S1, S2, and S7), indicated that the residual TOC derived from oxalate complexes instead of other organic compounds (see discussion below). To manifest the stagnation of mineralization in the presence of Cr species, we performed the following experiments at higher concentrations of Cr(VI)/Cr(III). As expected, the inhibitory influence of Cr species on the mineralization of oxalic acid was also observed in the experiments as shown in Figure 2. After 25 min irradiation in the presence of Fe(III), the mineralization of oxalate in both Cr(VI) and Cr(III) systems nearly ceased, whereas oxalate in the absence of Cr species was mineralized completely after 40 min of photoreaction. In contrast, oxalate was hardly mineralized either in the dark (column A, B) or in the absence of Fe(III) (column C, D). It is also clarified by control experiments (data not shown) that the contribution of thermal (or dark) reactions to our photochemical reactions is negligible. For comparison, other polycarboxylates, such as citrate, succinate, and tartarate, were also selected as the ligands of Fe(III) at the same concentration (Table 1). The results prove that not only oxalate, but also other polycarboxylates remained recalcitrant to further oxidation in the presence of Cr species. However, this inhibitory effect was not obviously observed in the cases of formic acid and o-phthalic acid, accompanying the poor Cr(VI) removal efficiency. To verify the reproducibility of these experiments and explore which is responsible for this sort of inhibitory effect, after the TOC of the solutions remained constant, Fe(III), oxalate and Cr species were added again, respectively (Supporting Information Figure S3). It was observed that more Fe(III) failed to result in further decay of TOC, whereas more Cr species could keep more oxalate against oxidation when Cr(VI) (or Cr(III)) and oxalate were added simultaneously in the second run. The results revealed that Cr species dosage may alter the final steady level of TOC. Experiments with different levels of Cr(VI) or Cr(III) dosage were performed to investigate their effect on Fe(II) production and oxalate oxidation (Figure 3). Fe(II) could not be detected until Cr(VI) with lower initial dosages was totally reduced. However when the initial Cr(VI) concentration increased up to 320 or 640 µM to ensure the excess of Cr(VI), unexpectedly, Fe(II) could also be detected after 25 min irradiation although sufficient Cr(VI) was still present in the system. Figure 3c VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Time profile of Fe(II) production (a), Cr(VI)-Cr(III) interconversion (b) and oxalate mineralization (c) during UV irradiation. Inset in Figure 1a: Fe(II) production within the range of 0-60 min; Inset in Figure 1b: Cr(VI) reduction at the beginning of the reaction (0-5 min). Fe(III), 150 µM; Cr(VI) or Cr(III), 40 µM; oxalic acid, 200 µM; pH 3.0.

FIGURE 2. Mineralization of oxalate during UV irradiation in the presence of Cr species. A: Fe(III)/ox/ Cr(III), dark; B: Fe(III)/ox/ Cr(VI), dark; C: Cr(III)/ox; D: Cr(VI)/ox; E: Fe(III)/ox; F: Fe(III)/ox/ Cr(VI); G: Fe(III)/ox/Cr(III). Fe(III), 100 µM; oxalic acid, 500 µM; Cr(VI)/Cr(III), 80 µM; pH 3.0. All solutions were exposed to UV irradiation except A and B. Note that all TOC/TOC0 at 0 min are normalized to 1. shows the dependence of TOC of steady state (TOCss) (see Supporting Information Figure S4) on Cr species dosage. These results confirmed the contribution of Cr species to the inhibition of oxidation of oxalate. It is noticeable that Cr(VI) was more efficient in blocking the oxidation of oxalate than Cr(III) at the same concentration, indicating the diversity in reaction mechanism of different Cr species. Effect of Molecular Oxygen and pH. The mineralization of oxalate initiates from the photolysis of Fe(III)-oxalate •complexes and ends up when resulting C2O•4 or CO2 donates another electron with production of CO2, which easily occurs due to its low reduction potential, E0(CO2/CO•2 ) ) -1.8 V (NHE) (22). First, we excluded two possible pathways for oxalate radical back to oxalate thus evading the oxidation by O2: (i) oxalate radical generated from the Fe(III)-ox complexes undergoes a reversible reduction back to oxalate, but this process is equal to deactivation of the photoexcitation of Fe(III) complexes by UV illumination, as a result, Fe(II) should not be continuously generated by the photochemical reaction, which is not consistent with the results in Figure 3A; (ii) it is negligible at pH 3.0 for two CO2•- radicals to recombine to yield oxalate, though is indeed likely in neutral and alkaline aqueous solutions (23). Accordingly, the fate of CO•2 radical should be highly associated with those electron accepters, such as O2, Cr(VI), and Fe(III) in our systems. In 7262

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the Ar-purged systems, oxalate decomposition was largely inhibited, indicating the significant role of oxygen in the oxalate mineralization (Figure 4). About 20% of oxalate was consumed in the anaerobic Fe(III)/ox/Cr(VI) solution partially because Cr(VI) acts as a substitute for oxygen to capture electrons from CO2•- radicals, although this rate is 2 orders of magnitude lower than that of oxygen (9). We also conducted the experiments at different pH values to explore whether the inhibition phenomenon is such an occasional and pH-dependent event. As illustrated in Figure 4, similar tendency was observed, although oxalate decomposition was more slowly at pH 4.0. It is in agreement with the previous Abrahamson’s finding that quantum yield of Fe(II) production (ΦFe(II)) at 366 nm from Fe(III)-oxalate complexes was approximately halved when the pH was increased from 2.7 to 4.0 (24). The Cr Intermediates Evolved in the Photochemical Systems. It is of great interest that which valent state of Cr (+3∼6) is responsible for the ultimate photosteady state. Above all, Cr(VI) or Cr(III) may be primarily excluded for several reasons: (i) at low initial Cr(VI) concentrations (20-100 µM), the inhibitory effect appeared only after Cr(VI) disappearance (Figure 3). It gives direct evidence that Cr(VI) itself is impossible to participate directly in the inhibition; (ii) if Cr(III) was the principal contributor (e.g., [Cr(C2O4)3]3- (25),), when Cr(III) was added (Figure 2), this inhibition for oxalate oxidation should have been more efficient than in the case of Cr(VI); (iii) according to the ultimate concentrations of Cr(VI)/Cr(III), Fe(III)/Fe(II) and residual oxalate, we prepared the reaction solution to mimic the ultimate state after 40 min of UV irradiation assuming that Cr species exist in Cr(VI) and Cr(III). The results (Supporting Information Figure S5) showed that the oxalate was subjected to degrade rather than hold in the simulated ultimate steady state of TOC. This result together with XPS analysis (Supporting Information Figure S6) further proved that there should be some Cr intermediates involved in the inhibitory effect, which actually form in photochemical reactions. Therefore, it seems that Cr(IV) or Cr(V) may be the potential candidate which plays a decisive role in inhibiting the mineralization of oxalate. We applied the ESR technique to obtain more information of Cr intermediates. Figure 5a shows typical ESR spectra recorded under continuous laser irradiation (λ ) 355 nm). The ESR spectra consists of four major signals (1-4), which have been assigned to four kinds of Cr(V)-ox complexes with different structures characterized by various g values (26).

TABLE 1. Effect of Cr Species on Mineralization of Different Carboxylates

a

Fe(III), 100 µM; carboxylic acid, 500 µM; Cr(VI) (or Cr(III)), 80 µM; pH 3.0. Irradiation time 40 min.

Figure 5b shows Cr(V) signal intensity (signal 3) as a function of irradiation time in the absence and presence of iron(III). In the presence of iron(III), the rates of formation and decay of Cr(V) under steady irradiation, even after pausing laser illumination, were much greater than those in the absence of iron. However, this result suggests that Cr(V)-ox complexes may not be involved in protecting oxalate against oxidation in the ultimate steady state due to their poor stability in the dark, although it was indeed continuously evolved through one-electron-transfer photochemical process. Our conclusion was further confirmed by another ESR result (data not shown) that there was no Cr(V) ESR signal observed after TOC hold in a constant level with a longer time of irradiation for Fe(III)-ox-Cr(VI) and Fe(III)-ox-Cr(III) systems. Tong and King (27) reported that the rearrangement of the coordination shell from tetrahedral complex for Cr(V) to octahedral one for Cr(IV) may be involved in the interconversion between Cr(VI) and Cr(III). So it should undergo an induction period to form Cr(IV), which would be gradually accumulated and stabilized by carboxylato ligands if Cr(VI) complexing with ligands is prior to the reduction process (28, 29). Therefore, it is more likely that Cr(IV) complex functions as a inhibitor for oxalate mineralization, although we could not observe any ESR signal of Cr(IV) species because both Cr(IV) and Cr(IV)-ox are silent to ESR detection (30). We applied ESI-MS technique to identify the ionized compounds after the TOC kept constant (Supporting Information Figure S8). Several new peaks (m/z > 300) appeared and were assigned to different Cr-oxalate complexes (Supporting Information Table S1). For example, A Cr(IV)-Cr(III) dinuclear complex (m/z ) 330.8) with two possible structures (Suppporting Information Scheme S1) is proposed according to ESI-MS and ESR results. The analogous oxo-bridged dinuclear complexes have been reported during the reactions of Cr(II) with Cr(III) and Fe(II) with Fe(IV) (31). Cr(III)-Cr(III) oxalato-bridged dimer also has also been structurally and magnetically characterized by Triki et al. (32). The Cr(IV)-ox complexes should be responsible for the inhibitory effect on mineralization of oxalate, although the more details remain elusive.

Mechanism of Cr Species for Inhibition of Oxidation of Oxalate and Fe(II). To elucidate the mechanism of Cr species for inhibition of oxidation of Fe(II) in detail, an unexpected phenomenon should be first explained. Fe(II) abruptly emerged after 25 min of irradiation while both Cr(VI) and oxalate were present at larger concentrations (500 and 200 µM, respectively) (Figure 3A and B). It is obviously inconsistent with the previous conclusion that the reaction between Cr(VI) and Fe(II) can be greatly accelerated in the presence of oxalate (k ) 1.2 ((0.3) × 107 M-1s-1 (9)). The reasonable explanation on this phenomenon for coexistence of Cr(VI), Fe(II) and oxalate is that oxalate sequestered in Cr-ox complexes is inert and can not be decomposed in the following reactions. Without oxalate ligand, the kinetic constant of reduction of Cr(VI) by Fe(II) at pH 3.0 was estimated as only 4.7 M-1s-1 (k ) 4.4 × 103[H+]+3.0 × 105[H+]2 (12)). This explanation is supported by the further experimental result that the slow reduction of Cr(VI) and accumulation of Fe(II) (curve a) were observed in the absence of oxalate as shown in Figure 6. Therefore, after TOC held in a constant value, the change tendencies of Fe(II) and Cr(VI) can be readily understood according to control experiments. For example, The ultimate photosteady of the Fe(II) concentration in the Cr(III) system was the highest among three systems in Figure 1a. Similarly, in Figure 6B, the Fe(II) concentration in Cr(III)-present system exceeded the control experiment after 10 min, whereas Cr(VI) gradually appeared (Figure 6A, curve b). As supposed, Cr(VI) was indeed measured with a low concentration in Figure 3B. This phenomenon has been previously reported, namely, Cr(III) scavenges strongly oxidizing hydroxyl radical generated by photolysis of Fe(OH)2+and is consequently converted to Cr(VI) (17, 18), partially avoiding the Fe(II) reoxidation by •OH. A possible reaction mechanism is proposed in Scheme 1. In the photochemical coupling reactions between Fe(III)/ Fe(II), Cr(VI)/Cr(III), and oxalate, oxalate is irreversibly degraded to carbon dioxide in the presence of oxygen, concomitantly with the generation of Fe(II), which rapidly reduces Cr(VI) to Cr intermediates (i.e., Cr(IV) or Cr(V)) even to Cr(III) via process I. However, with the depletion of free VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Effect of oxygen and pH on mineralization of oxalic acid. Fe(III), 100 µM; Cr(VI) or Cr(III), 160 µM; oxalic acid, 1000 µM; irradiation time, 40 min.

FIGURE 5. (a) ESR spectra of the Cr(V) intermediates formed in Fe(III)-ox-Cr(VI) solution and (b) time-course of Cr(V) intermediate (signal 3 in (a)) formation and decay in Fe(III)ox-Cr(VI) or Cr(VI)-ox solution during irradiation (b). Fe(III), 500 µM; oxalic acid, 20 mM; Cr(VI), 2 mM; pH 1.4, I ) 1.0 M. Signals 1-4 denote four different Cr(V) intermediates, respectively.

FIGURE 3. Effect of Cr species concentration on Fe(II) production (A), Cr(VI)-Cr(III) interconversion (B) and oxalate mineralization (C) during UV irradiation. Fe(III), 100 µM; oxalic acid, 500 µM; Cr(VI)/Cr(III), 0-640 µM; pH 3.0. TOCss denotes TOC values of steady state. oxalate anion, Cr(IV)-containing complexes with oxalate serve as an antioxidative shield to prevent the rest of oxalate from coordination with Fe(III), thus preserving this part of oxalate against photocarboxylation. Due to lack of free oxalate ligand, the photochemical pathway of Fe(III) complex is switched from oxalate chelate state reaction to hydroxo state one (right to left in Scheme 1 correspondingly), which is supported by the fact that the broad absorption band (250-400 nm) of Fe(III)-ox components disappeared after 60 min of irradiation (see Supporting Information Figure S10). For the latter system, Cr(III) behaves as a good scavenger for hydroxyl radical and is gradually oxidized to Cr(IV), Cr(V) and Cr(VI) via process II. Cr(III) oxidation, coupled with photolysis of 7264

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FIGURE 6. Time profile of Cr(VI)-Cr(III) interconversion (A) and Fe(II) production (B) during UV irradiation. Initial conditions: Fe(III), 100 µM; pH 3.0; 80 µM Cr(VI) (a), 80 µM Cr(III) (b). Cr(t) denotes total Cr concentration added. Fe(OH)2+, alters redox equilibrium of iron species and results in the shift of the steady state toward Fe(II). Environmental Implications. Cr(VI) species are of great environmental concern due to their acute toxicity, carcinogenicity and high mobility in environmentally relevant media (10). The incidence and severity of global chromium pollution

SCHEME 1. Proposed Mechanism for the Inhibitory Effect of Cr Species on Oxalate Oxidation, Cr(int.) denotes Cr(IV) or Cr(V) Intermediates

has been investigated in many countries. The Cr contents in some heavily Cr-contaminated sites were even far beyond that we chose in this study (0.02-0.64 mM) (see Supporting Information Table S2). In this study, we simulated the photochemical coupling reactions between iron, chromium and oxalate in acidic solutions under UVA irradiation. The current research has shown that Cr pollutants, regardless of addition of Cr(VI) or Cr(III), may alter the photosteady state of Fe(III)/Fe(II) toward Fe(II). The enhancement of Fe(II) concentration is of great environmental concern, since Fe(II) may participate in the redox cycling of other trace metal in natural waters, such as Cu and Mn (4), and the decay of H2O2 thus resulting in a series of oxidizing species (7). In addition, Cr species can preserve some organic carbon in the form of Cr-organic complexes. It seems environmentally favorable because formation of soluble Cr complexes not only prevents the depletion of the organic compounds and molecular oxygen in natural waters, but also further avoids more production of greenhouse gas, CO2. However, these soluble Cr intermediates are quite mobile and unstable in alkaline solutions, thus greatly enhancing the environmental risk of Cr(VI) release in the process of their transport. Hence, these soluble Cr intermediates should be carefully taken into account in environmental remediation of heavily Crcontaminated wastewaters and sediments in the presence of Fe(III) and polycarboxylates. On all accounts, the present results may benefit our understanding on the interaction among iron, chromium, and oxalate in the sunlit aquatic environment.

Acknowledgments This work was financially supported by 973 project (No. 2007CB613306), NSFC (Nos. 20537010, 20677062, and 20777076), and CAS.

Supporting Information Available Additional experimental evidence. This material is available free of charge via the Internet at http://pubs.acs.org.

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