Photochemical Coupling Reactionsbetween Fe(III)/Fe(II), Cr(VI)/Cr(III),and Polycarboxylates: InhibitoryEffect of Cr SpeciesZ H A O H U I W A N G , W A N H O N G M A ,C H U N C H E N G C H E N , A N D J I N C A I Z H A O *

Beijing National Laboratory for Molecular Sciences, KeyLaboratory of Photochemistry, Institute of Chemistry, TheChinese Academy of Sciences, Beijing 100190, China

Received May 19, 2008. Revised manuscript received June25, 2008. Accepted July 18, 2008.

The roles of chromium species on photochemical cycling ofiron and mineralization of polycarboxylates are examined in thepresence of Cr(VI) or Cr(III) at pH 2.2-4.0. Under UV irradiation,Cr(III) altered the redox equilibrium of iron species, leadingto the shift of the photosteady state toward Fe(II). After a longertime of illumination, total organic carbon (TOC) approached asteady state in the presence of Cr(III) or Cr(VI), whereas oxalatewas thoroughly mineralized in the absence of Cr species.The TOC of steady state was closely related to the kind ofpolycarboxylates, Cr species dosages, pH and O2 atmosphere,but hardly affected by more addition of Fe(III). ESI-MS dataindicates that several Cr-oxalate complexes formed inthe photochemical reactions, which are responsible forprotecting oxalate against further oxidation. A mechanism isproposed for the inhibitory effect of Cr species on oxidation ofoxalate and Fe(II). The present study may provide a newinsight into the dual environmental effects induced by Crcontaminants especially at heavily chromium-contaminated anddissolved organic matter (DOM)-rich sites.

IntroductionEnvironmental photochemistry of iron species has receivedconsiderable attention because it plays a significant role inthe numerous iron-dependent geochemical cycles, such asphytoplankton blooms (1, 2), carbon cycle (3), and redoxcycling of other transition metal complexes (4). Fe speciationin environmental systems is particularly relevant withnaturally occurring organic ligands, most of which are capableof controlling the photochemical activities of the iron species.The carboxylate group RC(O)O- is one of the most commonfunctional groups in dissolved organic matters (DOM) presentin natural waters (5-8). The photoredox behavior has beenobserved for some simple Fe(III)-polycarboxylates (e.g.,oxalate, malonate, and citrate) complexes in atmosphericand surface waters. Under sunlight irradiation, Fe(III)undergoes rapid photoreduction to Fe(II) on a time scale ofminutes, accompanied by oxidative degradation of poly-carboxylic ligands to carbon dioxide in the aerated systems(6-8). Photochemically generated Fe(II) is readily reoxidizedto Fe(III), which is achieved mostly by molecular oxygen,but also by other strongly oxidizing species like hexachro-mium (9).

Cr(VI) and Cr(III) are the two primary oxidation states ofchromium in aquatic environment (10). Cr(VI) species areknown acutely toxic, carcinogenic and are of environmentalconcern (10). In China, there is a continual and abundantinflux of chromium contaminants (for example, 103.12 tonsin 2003 (11)) into the environment, thus posing a severeenvironmental problem. Since Fe(II) is the dominant envi-ronmental 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 innatural waters (14, 15). Hug et al. investigated the photo-catalytic reduction of Cr(VI) by iron(III)-organic complexes(9, 16). It was found that reduction rates of Cr(VI) variedwidely and were strongly dependent upon the nature of DOMsubstrates. Fe(II), HO2

•, and O2•- species were speculated as

the most likely reductant candidates of Cr(VI). Moreover,under similar conditions, it was reported that light-inducedoxidation of Cr(III) in the presence of Fe(III) would be onepotential pathway for regenerating Cr(VI) in atmosphericwaters (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 essentialfor human and animal (10).

To date, much attention has been paid to redox reactionsof Cr(VI)/Cr(III) induced by the photocatalytic reactions ofFe(III)-polycarboxylates (9, 16-18), however, little work hasbeen done in investigating the effect of Cr contaminants oneither redox cycling of iron or the fate of organic ligands. Ourrecent work (19) has investigated the effect of periodic influxof DOM on photoredox cycling of iron under UV irradiation.Surprisingly, photoinduced oscillation of Fe(II)/Fe(total) ratiowas observed due to the disturbance induced by DOM forthe equilibrium between Fe(III) photoreduction and Fe(II)photooxidation. However, it is still unclear whether Crpollutants affect the photoredox cycling of iron in the similarmanner.

The objectives of the present study were to (i) determinewhether and how the concurrence of Cr pollutants andpolycarboxylates influences the steady-state equilibrium ofFe(III)/Fe(II), and (ii) elucidate the mechanism details ofacceleration or inhibition effect of Cr contaminants on thephotochemical degradation of DOM. The present study mayprovide a new insight into the environmental processesinduced by the abundant influx of Cr contaminants and givea better understanding on the interaction among dissolvediron pool, Cr species and DOM under solar irradiation.

Experimental SectionChemicals. Iron(III) perchlorate hydrate and perchloric acidwere purchased from Aldrich. Oxalic acid, formic acid,succinic acid, phthalic acid, citric acid, tartaric acid, 1,10-phenanthroline, 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. BarnsteadUltraPure water (18.3 MΩ) was used for all experiments.

Experimental Procedures. The photochemical reactionswere performed using a 100 W Hg lamp (Toshiba SHL-100UVQ-2) as the irradiation source. The irradiation setupand protocol were identical as described elsewhere (19). Thereaction solutions were always freshly prepared by dilutionof stock solutions of 0.01 M organic acids, 5 mM Fe(III) atpH < 2 (HClO4) and 4 mM Cr(VI)/Cr(III). Additionalelectrolytes were avoided to prevent the iron(III) complex-ation with other inorganic anions. Unless otherwise noted,

* Corresponding author fax: +86-10-8261-6495; e-mail: jczhao@iccas.ac.cn.

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all reactions were carried out under exposure to air. Fordeaerated experiments, the solutions in the cap-sealed Pyrexbottles were purged with high-purity Ar (O2e 0.001%) for atleast 30 min prior to illumination and continuously bubbledduring the experiment.

Methods and Analysis. The concentration of Fe(II) wasdetermined by 1,10-phenanthroline method as previouslydescribed (19, 20). Aqueous Cr(VI) concentration was mea-sured by a diphenylcarbazide method (21). The color wasfully developed after 5 min and the sample solutions weredetected at 540 nm with a Hitachi U-3100 spectrophotometer.TOC was determined on a Tekmar Dohrmann Apllo 9000TOC analyzer. Each sample was measured in duplicate andrelative error was less than 2%. For the quantitive measure-ment of oxalate concentration, ion chromatograph (IC) (DX-120, Dionex Co.) was applied with conductivity detection. ADionex Ionpac AS 11 column (4 mm) and a Dionex anionmicromembrane suppressor were employed. 10 mM NaOHsolution was chosen as eluent. Electron spin resonance (ESR)spectra of Cr(V) were recorded at room temperature on aBruker EPR ELEXSYS 500 spectrometer equipped with an insitu irradiation source (a Quanta-Ray ND:YAG laser systemλ)355 nm), and the same quartz capillary was used for allthe measurements to minimize errors. Typical parametersfor acquisition of Cr(V) signals were as follows: center field,3540 G; sweep width, 100 G; resolution, 1024 pts; microwavefrequency, ∼9.78 GHz; microwave power, 10 mW; modulationfrequency, 100 kHz; time constant, 40.96 ms; receiver gain,50. For all ESR measurements, ionic strength was maintainedconstant (I ) 1.0 M; HClO4+NaClO4).

Results and DiscussionPhotosteady State Equilibrium of Fe(III)/Fe(II) in thePresence of Cr Species. Figure 1a depicts the changes of thephotoproduced 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 Crspecies or not, the concentration of Fe(II) sharply increasedwithin the initial 5 min and then followed by a rapid decay.Similar fluctuations induced by DOM influx were everobserved in the case of salicylic acid and benzoic acid (19).The rapid generation of Fe(II) should be attributed to ligand-to-metal charge transfer (LMCT) process of iron(III)-oxalatecomplex under irradiation (6-8) (eq 1).

The subsequent decay of Fe(II) was due to its reoxidationby reactive oxygen species (ROS) (i.e., O2

•-, HO2•, H2O2, and

•OH), which were generated from reduction of oxygen byCO2

•- radicals and subsequent disproportionation of super-oxide and Fenton reaction (eq 2-5).

[Fe(C2O4)n]3-2n fhv

[FeII(C2O4)n-1]4-2n +CO2 +CO2•- (1)

CO2•- +O2 f CO2 +O2

•- (2)

O2•- +H+ f HO2

• (3)

2HO2• f H2O2 +O2 (4)

Fe(II)+H2O2 f Fe(III)+ •OH+OH- (5)

After 30 min of irradiation, Fe(II) slowly increased and finallya photosteady state of Fe(II)/Fe(total) approached respec-tively in the three systems, implying the balance betweenFe(III) photoreduction and aerobic Fe(II) photooxidation.Interestingly, in the presence of Cr(III) or Cr(VI), the ultimatephotosteady of Fe(II) concentration was, respectively, 40 and25% higher than that in the control experiment. In ourprevious study (19), it was observed that different DOM maycontribute to the fluctuation of Fe(II), but the ultimatephotosteady state was constant and independent of the kindsand concentrations of DOM under irradiation at constant

light intensity. Thereby, DOM and inorganic Cr species shouldexhibit different effect on the photosteady state of iron cycle:the former is irreversibly degradated to carbon dioxide andno longer has an affect on the photosteady state, whereasthe latter may be always active due to its redox cycling.Therefore it was the Cr species that alter the redox equilibriumof iron species, leading to the shift of the photosteady statetoward Fe(II). It is noteworthy that Cr species of differentvalences lead to different ultimate Fe(II) concentration,indicating their distinct roles in the photoredox of iron.

Inhibitory effect of Cr Species on Mineralization ofOxalate. The photolysis of Fe(III)-polycarboxylates complexis always coupled with the degradation of organic ligands(6). In the present study, to eliminate the interference of thedifferent Fe (or Cr) carboxylato complexes, the effect of Crspecies on mineralization of oxalate was evaluated by TOCanalysis instead of direct IC measurement. Figure 1c showsthe time profile of mineralization of oxalate in 6 h time scale.At the beginning of the photoreaction (∼25 min), in parallelto the fluctuation of Fe(II), oxalate was mineralized com-pletely in the absence of Cr species via reactions of eqs 1-2.Unexpectedly, TOC held in a constant level after 25 min whenCr(VI) was present, demonstrating that a part of oxalatespecies were resistant to degradation. We eliminated thepossibility that Cr inorganic ions would interfere with TOCmeasurement by blank experiments. The ion chromatograph,together with FTIR and XPS experiments (Supporting In-formation Figures S1, S2, and S7), indicated that the residualTOC derived from oxalate complexes instead of other organiccompounds (see discussion below).

To manifest the stagnation of mineralization in thepresence of Cr species, we performed the following experi-ments at higher concentrations of Cr(VI)/Cr(III). As expected,the inhibitory influence of Cr species on the mineralizationof oxalic acid was also observed in the experiments as shownin Figure 2. After 25 min irradiation in the presence of Fe(III),the mineralization of oxalate in both Cr(VI) and Cr(III) systemsnearly ceased, whereas oxalate in the absence of Cr specieswas 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). Itis also clarified by control experiments (data not shown) thatthe contribution of thermal (or dark) reactions to ourphotochemical reactions is negligible.

For comparison, other polycarboxylates, such as citrate,succinate, and tartarate, were also selected as the ligands ofFe(III) at the same concentration (Table 1). The results provethat not only oxalate, but also other polycarboxylatesremained recalcitrant to further oxidation in the presence ofCr species. However, this inhibitory effect was not obviouslyobserved in the cases of formic acid and o-phthalic acid,accompanying the poor Cr(VI) removal efficiency.

To verify the reproducibility of these experiments andexplore 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 thatmore Fe(III) failed to result in further decay of TOC, whereasmore Cr species could keep more oxalate against oxidationwhen Cr(VI) (or Cr(III)) and oxalate were added simulta-neously in the second run. The results revealed that Cr speciesdosage may alter the final steady level of TOC.

Experiments with different levels of Cr(VI) or Cr(III) dosagewere performed to investigate their effect on Fe(II) productionand oxalate oxidation (Figure 3). Fe(II) could not be detecteduntil Cr(VI) with lower initial dosages was totally reduced.However when the initial Cr(VI) concentration increased upto 320 or 640 µM to ensure the excess of Cr(VI), unexpectedly,Fe(II) could also be detected after 25 min irradiation althoughsufficient Cr(VI) was still present in the system. Figure 3c

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shows the dependence of TOC of steady state (TOCss) (seeSupporting Information Figure S4) on Cr species dosage.These results confirmed the contribution of Cr species to theinhibition of oxidation of oxalate. It is noticeable that Cr(VI)was more efficient in blocking the oxidation of oxalate thanCr(III) at the same concentration, indicating the diversity inreaction mechanism of different Cr species.

Effect of Molecular Oxygen and pH. The mineralizationof oxalate initiates from the photolysis of Fe(III)-oxalatecomplexes and ends up when resulting C2O4

•- or CO2•- donates

another electron with production of CO2, which easily occursdue to its low reduction potential, E0(CO2/CO2

•-) ) -1.8 V(NHE) (22). First, we excluded two possible pathways foroxalate radical back to oxalate thus evading the oxidation byO2: (i) oxalate radical generated from the Fe(III)-ox com-plexes undergoes a reversible reduction back to oxalate, butthis process is equal to deactivation of the photoexcitationof Fe(III) complexes by UV illumination, as a result, Fe(II)should not be continuously generated by the photochemicalreaction, which is not consistent with the results in Figure3A; (ii) it is negligible at pH 3.0 for two CO2

•- radicals torecombine to yield oxalate, though is indeed likely in neutraland alkaline aqueous solutions (23). Accordingly, the fate ofCO2

•- radical should be highly associated with those electronaccepters, such as O2, Cr(VI), and Fe(III) in our systems. In

the Ar-purged systems, oxalate decomposition was largelyinhibited, indicating the significant role of oxygen in theoxalate mineralization (Figure 4). About 20% of oxalate wasconsumed in the anaerobic Fe(III)/ox/Cr(VI) solution partiallybecause Cr(VI) acts as a substitute for oxygen to captureelectrons from CO2

•- radicals, although this rate is 2 ordersof magnitude lower than that of oxygen (9).

We also conducted the experiments at different pH valuesto explore whether the inhibition phenomenon is such anoccasional and pH-dependent event. As illustrated in Figure4, similar tendency was observed, although oxalate decom-position was more slowly at pH 4.0. It is in agreement withthe previous Abrahamson’s finding that quantum yield ofFe(II) production (ΦFe(II)) at 366 nm from Fe(III)-oxalatecomplexes was approximately halved when the pH wasincreased from 2.7 to 4.0 (24).

The Cr Intermediates Evolved in the PhotochemicalSystems. 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 forseveral reasons: (i) at low initial Cr(VI) concentrations (20-100µM), the inhibitory effect appeared only after Cr(VI) disap-pearance (Figure 3). It gives direct evidence that Cr(VI) itselfis impossible to participate directly in the inhibition; (ii) ifCr(III) was the principal contributor (e.g., [Cr(C2O4)3]3- (25),),when Cr(III) was added (Figure 2), this inhibition for oxalateoxidation should have been more efficient than in the caseof Cr(VI); (iii) according to the ultimate concentrations ofCr(VI)/Cr(III), Fe(III)/Fe(II) and residual oxalate, we preparedthe reaction solution to mimic the ultimate state after 40min 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 thanhold in the simulated ultimate steady state of TOC. This resulttogether with XPS analysis (Supporting Information FigureS6) further proved that there should be some Cr intermediatesinvolved in the inhibitory effect, which actually form inphotochemical reactions.

Therefore, it seems that Cr(IV) or Cr(V) may be thepotential candidate which plays a decisive role in inhibitingthe mineralization of oxalate. We applied the ESR techniqueto obtain more information of Cr intermediates. Figure 5ashows typical ESR spectra recorded under continuous laserirradiation (λ ) 355 nm). The ESR spectra consists of fourmajor signals (1-4), which have been assigned to four kindsof Cr(V)-ox complexes with different structures characterizedby various g values (26).

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 thepresence 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 UVirradiation except A and B. Note that all TOC/TOC0 at 0 min arenormalized to 1.

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Figure 5b shows Cr(V) signal intensity (signal 3) as afunction of irradiation time in the absence and presence ofiron(III). In the presence of iron(III), the rates of formationand decay of Cr(V) under steady irradiation, even afterpausing laser illumination, were much greater than those inthe absence of iron. However, this result suggests thatCr(V)-ox complexes may not be involved in protectingoxalate against oxidation in the ultimate steady state due totheir poor stability in the dark, although it was indeedcontinuously evolved through one-electron-transfer pho-tochemical process. Our conclusion was further confirmedby another ESR result (data not shown) that there was noCr(V) ESR signal observed after TOC hold in a constant levelwith a longer time of irradiation for Fe(III)-ox-Cr(VI) andFe(III)-ox-Cr(III) systems.

Tong and King (27) reported that the rearrangement ofthe coordination shell from tetrahedral complex for Cr(V) tooctahedral one for Cr(IV) may be involved in the intercon-version between Cr(VI) and Cr(III). So it should undergo aninduction period to form Cr(IV), which would be graduallyaccumulated 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) complexfunctions as a inhibitor for oxalate mineralization, althoughwe could not observe any ESR signal of Cr(IV) species becauseboth Cr(IV) and Cr(IV)-ox are silent to ESR detection (30).We applied ESI-MS technique to identify the ionizedcompounds after the TOC kept constant (Supporting Infor-mation Figure S8). Several new peaks (m/z > 300) appearedand were assigned to different Cr-oxalate complexes (Sup-porting 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 accordingto ESI-MS and ESR results. The analogous oxo-bridgeddinuclear complexes have been reported during the reactionsof Cr(II) with Cr(III) and Fe(II) with Fe(IV) (31). Cr(III)-Cr(III)oxalato-bridged dimer also has also been structurally andmagnetically characterized by Triki et al. (32). The Cr(IV)-oxcomplexes should be responsible for the inhibitory effect onmineralization of oxalate, although the more details remainelusive.

Mechanism of Cr Species for Inhibition of Oxidation ofOxalate and Fe(II). To elucidate the mechanism of Cr speciesfor inhibition of oxidation of Fe(II) in detail, an unexpectedphenomenon should be first explained. Fe(II) abruptlyemerged after 25 min of irradiation while both Cr(VI) andoxalate were present at larger concentrations (500 and 200µM, respectively) (Figure 3A and B). It is obviously incon-sistent with the previous conclusion that the reaction betweenCr(VI) and Fe(II) can be greatly accelerated in the presenceof oxalate (k ) 1.2 ((0.3) × 107 M-1s-1 (9)). The reasonableexplanation on this phenomenon for coexistence of Cr(VI),Fe(II) and oxalate is that oxalate sequestered in Cr-oxcomplexes is inert and can not be decomposed in thefollowing reactions. Without oxalate ligand, the kineticconstant of reduction of Cr(VI) by Fe(II) at pH 3.0 wasestimated 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 experi-mental result that the slow reduction of Cr(VI) and ac-cumulation of Fe(II) (curve a) were observed in the absenceof oxalate as shown in Figure 6. Therefore, after TOC heldin 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) con-centration in the Cr(III) system was the highest among threesystems in Figure 1a. Similarly, in Figure 6B, the Fe(II)concentration in Cr(III)-present system exceeded the controlexperiment after 10 min, whereas Cr(VI) gradually appeared(Figure 6A, curve b). As supposed, Cr(VI) was indeedmeasured with a low concentration in Figure 3B. Thisphenomenon has been previously reported, namely, Cr(III)scavenges strongly oxidizing hydroxyl radical generated byphotolysis of Fe(OH)2+and is consequently converted toCr(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 irreversiblydegraded to carbon dioxide in the presence of oxygen,concomitantly with the generation of Fe(II), which rapidlyreduces Cr(VI) to Cr intermediates (i.e., Cr(IV) or Cr(V)) evento Cr(III) via process I. However, with the depletion of free

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.

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oxalate anion, Cr(IV)-containing complexes with oxalate serveas an antioxidative shield to prevent the rest of oxalate fromcoordination with Fe(III), thus preserving this part of oxalateagainst photocarboxylation. Due to lack of free oxalate ligand,the photochemical pathway of Fe(III) complex is switchedfrom oxalate chelate state reaction to hydroxo state one (rightto left in Scheme 1 correspondingly), which is supported bythe fact that the broad absorption band (250-400 nm) ofFe(III)-ox components disappeared after 60 min of irradia-tion (see Supporting Information Figure S10). For the lattersystem, Cr(III) behaves as a good scavenger for hydroxylradical and is gradually oxidized to Cr(IV), Cr(V) and Cr(VI)via process II. Cr(III) oxidation, coupled with photolysis of

Fe(OH)2+, alters redox equilibrium of iron species and resultsin the shift of the steady state toward Fe(II).

Environmental Implications. Cr(VI) species are of greatenvironmental concern due to their acute toxicity, carcino-genicity and high mobility in environmentally relevant media(10). The incidence and severity of global chromium pollution

FIGURE 3. Effect of Cr species concentration on Fe(II) pro-duction (A), Cr(VI)-Cr(III) interconversion (B) and oxalatemineralization (C) during UV irradiation. Fe(III), 100 µM; oxalicacid, 500 µM; Cr(VI)/Cr(III), 0-640 µM; pH 3.0. TOCss denotesTOC values of steady state.

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

FIGURE 5. (a) ESR spectra of the Cr(V) intermediates formed inFe(III)-ox-Cr(VI) solution and (b) time-course of Cr(V) inter-mediate (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, res-pectively.

FIGURE 6. Time profile of Cr(VI)-Cr(III) interconversion (A) andFe(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) de-notes total Cr concentration added.

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has been investigated in many countries. The Cr contents insome heavily Cr-contaminated sites were even far beyondthat we chose in this study (0.02-0.64 mM) (see SupportingInformation Table S2). In this study, we simulated thephotochemical coupling reactions between iron, chromiumand oxalate in acidic solutions under UVA irradiation. Thecurrent research has shown that Cr pollutants, regardless ofaddition of Cr(VI) or Cr(III), may alter the photosteady stateof 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 innatural 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 ofCr-organic complexes. It seems environmentally favorablebecause formation of soluble Cr complexes not only preventsthe depletion of the organic compounds and molecularoxygen in natural waters, but also further avoids moreproduction of greenhouse gas, CO2. However, these solubleCr intermediates are quite mobile and unstable in alkalinesolutions, thus greatly enhancing the environmental risk ofCr(VI) release in the process of their transport. Hence, thesesoluble Cr intermediates should be carefully taken intoaccount in environmental remediation of heavily Cr-contaminated wastewaters and sediments in the presenceof Fe(III) and polycarboxylates. On all accounts, the presentresults may benefit our understanding on the interactionamong iron, chromium, and oxalate in the sunlit aquaticenvironment.

AcknowledgmentsThis work was financially supported by 973 project (No.2007CB613306), NSFC (Nos. 20537010, 20677062, and20777076), and CAS.

Supporting Information AvailableAdditional experimental evidence. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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

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