Photoredox of Cr(III)–Malate Complex and Its ImpactingFactors

Feng Yang & Hui Li & Jing Zhang & Yeqing Lan

Received: 18 October 2013 /Accepted: 15 January 2014 /Published online: 12 March 2014# Springer International Publishing Switzerland 2014

Abstract The transformation of less toxic Cr(III) spe-cies to harmful Cr(VI) is worth concerning. Comparedwith free Cr(III), however, the photo-oxidation ofCr(III)–organic acid complexes is seldom reported. Inthis study, Cr(III)–malate complex was synthesized andpurified, and its photo-oxidation was investigated toreveal the potential conversion pathway of Cr(III) toCr(VI). The results indicated that Cr(III)–malate com-plex could be gradually photo-oxidized to Cr(VI)through a ligand–metal charge transfer path. HigherpH and stronger light intensity promoted the conversionprocess. A 50-μM Cr(III)–malate complex was almostcompletely oxidized to Cr(VI) within 420-min irradia-tion of 500Wmedium-pressure mercury lamp at pH 12.The introduction of H2O2, considered as a direct sourceof hydroxyl radicals (·OH) in the presence of Cr(II),markedly enhanced the yield of Cr(VI), and a completeoxidation of Cr(III)–malate complex (50 μM ) wasrealized within 20 min. Under a weak acidic condition,the production of Cr(VI) was coupled with the reductionof Cr(VI) by malic acid and its free radical generatedfrom Cr(III)–malate complex, leading a gradual de-crease in Cr(VI) concentration with the reaction.

Keywords Cr(III)–malate complex . Photo-oxidation .

Cr(VI) . Hydroxyl radical

1 Introduction

Chromium (Cr) exists in numerous oxidation states, butonly Cr(III) and Cr(VI) are stable in natural environ-ment. Cr(VI) exerts toxic effects on biological systems,whereas Cr(III) is considered as a trace essential elementfor proper functioning of living organisms (Kavanaugh1994). In addition, Cr(VI) is usually highly soluble andmobile as compared with the sparingly soluble speciesof Cr(III) (Kota’s and Stasicka 2000). Hence, manyresearches on the remediation of the sites contaminatedby chromium focus on the conversion of Cr(VI) toCr(III) by reductants (Fendorf and Li 1996; Melitaset al. 2001; Lan et al. 2005, 2006, 2008; Li et al.2007). Organic acids containing α-OH groups, such ascitric acid and tartaric acid, are very efficient in thereduction of Cr(VI) to Cr(III), especially in the presenceof Mn(II) or Fe(III) under light irradiation (Kabir-ud-Din et al. 2000; Sun et al. 2009; Tian et al. 2010).

However, another concern is focusing on the possibletransformation of immobilized, less toxic Cr(III) speciesto harmful Cr(VI) when they are exposed to someoxidants. Manganese oxides have been considered tobe the important inorganic oxidants capable of oxidizingCr(III) to Cr(VI) in surface environments (Eary and Rai1987; Lan et al. 2005, 2006 ). Palmer and Wittbrodt(1991) observed a significant increase in Cr(VI) con-centration in the presence of manganese oxides, but the

Water Air Soil Pollut (2014) 225:1875DOI 10.1007/s11270-014-1875-3

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11270-014-1875-3) contains supplementarymaterial, which is available to authorized users.

F. Yang :H. Li : J. Zhang :Y. Lan (*)College of Sciences, Nanjing Agricultural University,Nanjing 210095, People’s Republic of Chinae-mail: lanyq102@njau.edu.cn

efficiency is often limited by many factors including thesurface characteristics of oxides, solution acidic degree,competitive ions, and the availability of dissolvedCr(III) on the manganese oxide surface (Bartlett andJames 1979; Apte et al. 2006). For instance, Bartlett(1991) found that even under favorable conditions, alarge portion of Cr(III) in soils was not converted toCr(VI) by manganese oxides owing to the shortage ofdissolved Cr(III). This was further confirmed by Daiet al. (2009), who reported that some insoluble formsof Cr(III), such as Cr(OH)3, CrFe(OH)6, and CrPO4,were very difficult to be oxidized by manganese oxides.

Dissolved oxygen is considered to be another possibleoxidant for Cr(III) oxidation in natural environment, butan exiguous contribution of dissolved oxygen to thetransformation of Cr(III) to Cr(VI) was observed(Schroeder and Lee 1975). It has been also reported thatdissolved oxygen contributed trivially to the direct oxi-dation of Cr(III) owing to its slow kinetics (Eary and Rai1987). Nevertheless, an investigation by Ciesla et al.(2004) demonstrated that dissolved oxygen plays animportant role in the photo-oxidation of Cr(III). Apartfrom directly serving as an oxidant for Cr(II) (a precursorof Cr(III) oxidization to Cr(VI)), oxygen is also neces-sary to engender hydroxyl radicals (·OH), a strongeroxidant in the reaction system (Dai et al. 2010, 2011).

Studies have demonstrated that photoexcitation ofFe(III)–organic complexes can lead to the generationof·OH and Fe(II) through a ligand-to-metal chargetransfer path, and then·OH causes the decompositionof organic compounds as well as the conversion ofFe(II) to Fe(III) (Hug et al. 1997; Zhang and Bartlett1999; Liu et al. 2007; Zou and Hoigne 2009). Cr(III),structurally similar to Fe(III), is capable of chelatingwith organic ligands, such as EDTA, tartaric acid, andcitric acid. Thus, it is speculated that the oxidation ofCr(III) in organic acid complexes to Cr(VI) can be alsorealized under an irradiation. It has been found thatphoto-oxidation results in a complete and quantitativeconversion of Cr(III) to Cr(VI) and the removal ofligands (Rodman et al. 2006; Li et al. 2006).

In this study, Cr(III)–malate complex was synthesizedand purified and its photo-oxidation under different con-ditions was investigated in batch reaction systems toreveal the potential oxidation pathway of Cr(III) toCr(VI) and its possible mechanism. Malic acid is selectedsince it is usually found in soil and water environments(Tian et al. 2010). Additionally, malic acid is utilized toreduce Cr(VI) and Cr(III)–malate complex forms

subsequently (Puzon et al. 2008). The species of Cr(III)–malate complex were also analyzed by high-performanceliquid chromatography (HPLC) to demonstrate the rela-tionship between the species and their photochemicalactivities. To the best of our knowledge, such a detailedinvestigation on the photoredox of Cr(III)–malate com-plex has not been reported. Therefore, it is expected thatthe present study will make a contribution to the compre-hensive understanding of Cr(III) re-oxidation in naturalenvironment.

2 Materials and Methods

2.1 Materials

The stock solution of 1, 5-diphenylcarbazid (DPC),purchased from Sigma Aldrich Company, was preparedby dissolving 0.2 g DPC in 100 mL acetone, and then itwas stored in a brown bottle and kept in a fridge prior touse. The stock solution of Cr(III) (4 mM) was obtainedby dissolving CrCl3·6H2O(s) (Sinopharm ChemicalRegent Co., Ltd) in diluted HCl solution (pH<2 in caseof hydrolysis). The solution of malic acid (2.5g L−1) wasprepared by dissolving DL-malic acid (Chengdu KelongChemical Co., Ltd.) in distilled water. Cr(III)–malatecomplex was synthesized and purified according to themethod introduced in “Section 2.2”. The solutions ofCr(III)–malate complex (50 μM) with a pH range of 6–12 were obtained by diluting 2 mM Cr(III)–malatecomplex with distilled water and then adjusting themto the desired pH with diluted NaOH and HCl. It wasimportant tomaintain pH stability for 2 days to allow thesystem to reach equilibrium prior to measurement. Allthe other chemicals used in this study were at least ofreagent grade.

2.2 Preparation of Cr(III)–malate complex

Cr(III)–malate complex was synthesized according to themethod reported by Wu et al. (2011). In brief, 7.5 mmolmalic acid and 5 mmol CrC13·6H2O were mixed in aflask and completely dissolvedwith 20mL distilled water.The mixed solution was adjusted to pH 4.5 with 2 MNaOH and then was heated to 80°C for 6–8 h on amagnetic stirrer equipped with a digital intelligent temper-ature control (SZCL-3A). During the reaction process, thesolution volume was maintained by adding distilled waterat regular intervals. After that, the mixed solution was

1875, Page 2 of 9 Water Air Soil Pollut (2014) 225:1875

centrifuged at 4,000 rpm for 10 min and the supernatantwas collected. Then, approximately 100 mL C2H5OH(100%) was added into the supernatant, and the mixedsolution was stirred with a glass rod and then centrifuged.The precipitates were collected. To purify Cr(III)–malatecomplex, the collected precipitates were washed and cen-trifuged with the same amount of C2H5OH (100%) fortwo to three times. The purified Cr(III)–malate complexwas analyzed with thin-layer chromatography, and no freemalic acid was detected. The constituent of Cr(III)–malatecomplex was analyzed by an atom absorption spectro-scope (AAS, HITACHI 180-80) for Cr and by elementalanalyzer (EA, Vario EL III, Elementar Co., Ltd, German)for C. Finally, the purified crystals were kept in a vacuum-dried box prior to use.

2.3 Photo-oxidation Experiments

The photo-oxidation of Cr(III)–malate complex was con-ducted in a XPA-7 photo-reactor (Xuj iangElectromechanical Plant, Nanjing, China), which isshown in Fig. 1. Medium pressure mercury lamps of100, 300, and 500Wand a 500-W xenon lamp were usedas light sources, with light intensities at quartz tube posi-tions of 12.7, 16.8, and 20.1 mW cm−2 (measured using aUV-A irradiation meter, Beijing Normal University,China) and 26,500 lx (measured using a ST-80C illumi-nation meter, Beijing Normal University, China), respec-tively. The distributions of wavelength and its relativeenergy of the light sources are listed in Table S1 andFig. S1 of the “Electronic supplementary material”. A1-mL aliquot of sample was drawn out at certain intervals(the intervals depended on the reaction rates) with apipet te to determine Cr(VI) concentra t ion.Approximately 2-mL sample was drawn out with a plastic

syringe and filtered through a 0.22-μm membrane filterinto a clean and dried glass tube. Then, the filtrate wasdrawn out with a pipette for determining the concentra-tions of·OH andmalic acid yielded in the reaction process(see “Section 2.4” for detail), respectively. All experi-ments in this study were performed in triplicate.

2.4 Analytical Methods

·OH in the reaction system was determined according tothe method reported by Liu et al. (2004) and Dai et al.(2010). In brief, 1,000 μM benzene was added into theCr(III)–malate complex solutions to capture·OH andused to indirectly determine the quantum yield of·OHin the solution under an irradiation of 500-W medium-pressure mercury lamp at pH 7 and 10. Phenol, a prod-uct of benzene capturing·OH, was determined onHPLC(Waters 2487) with LiChrospher C18 (5 μm, 250×4.6mm) serving as a separating column and a mobile phase(60% (v/v) MeOH) at the detection wavelength of280 nm at a rate of 0.8 mL min−1.

Cr(VI) concentration was measured by DPC colori-metric method (Dai et al. 2009). Sulfuric acid, diluted 20times by distilled water (v/v), was used for assistance tocontrol pH for color development. The absorbance of eachsample was determined by 1-cm cell at 540 nm on UV-9100 Spectrophotometer (Beijing Ruili Co., Ltd). Thismethod, however, was not suitable for the reaction systemto investigate the effect of additional H2O2 on photo-oxidation of Cr(III)–malate complex since H2O2 inter-fered with the measurement of absorbance. Instead wedetermined Cr(VI) directly by monitoring its absorbanceat 374 nm. The concentration of Cr(III) was measured byAAS. An Orion 868 pH meter, after three-point calibra-tion, was used to determine pH values.

Fig. 1 Schematic diagram of thephotochemical reactor

Water Air Soil Pollut (2014) 225:1875 Page 3 of 9, 1875

The chromatographs of the species of Cr(III)–malatecomplex in a pH range of 6 to 12 and the concentrationof malic acid were conducted on HPLC. The mobilephase consisted of 20 mM KH2PO4 with pH 2.75, andthe separation column was the same as mentioned pre-viously. The components in the effluent were detected at210 nm at a rate of 0.6 mL min−1.

3 Results and Discussion

3.1 Analysis of the Constituent and the Speciesof Cr(III)–Malate Complex

The synthesized Cr(III)–malate complex was analyzedby AAS for Cr and by EA for C, and the results indi-cated that Cr and C accounted for 16.67 and 22.6% ofCr(III)–malate complex in weight, respectively, indicat-ing that the molar ratio of Cr to C in Cr(III)–malatecomplex was 2:12. The finding was well in greementwith an investigation by Wu et al. (2011), who reportedthat the molecular formula of Cr(III)–malate complexwas Cr2(C4H4O5)3⋅5H2O.

The pH-dependent species of Cr(III)–malate com-plex were analyzed by HPLC, and the results are illus-trated in Fig. 2. Based on the results, it is speculated thatpeak 1 and peak 2 represented [Cr2(III)-mal3] and[Cr2(III)-mal3-OH]

−, respectively, appearing at all thetested pHs. At pH 6, the main species of Cr(III)–malatecomplex was [Cr2(III)-mal3], accompanied with a little[Cr2(III)-mal3-OH]

−, but it is noted that the areas orheights of peaks dramatically dropped when pH roseto 8 or higher. This implies that the concentration ofCr(III)–malate complex decreased greatly. It is hypoth-esized that the decrease of Cr(III)–malate concentrationdetected by HPLC may be from the formation of

Cr(OH)3 due to hydrolysis of Cr(III)–malate complexunder high pH conditions. To prove the hypothesis,Cr(III) concentration in an initial 50 μM Cr(III)–malatecomplex solution with a pH range of 5 to 12 wasmeasured after the solutions were filtered through0.22-μm member to remove possible precipitate ofCr(OH)3. It is found that Cr(III) concentration basicallymaintained constant at a pH range of 5 to 7 and began toobviously drop since pH≥8 (data not shown). The con-centration of Cr(III) reduced by approximately by 40%at pH 12. The results demonstrated that Cr(OH)3 formedowing to partial hydrolysis of Cr(III)–malate complex athigh pH. On the other hand, higher pH results in theformation of Cr(OH)4

− (Spiccia 1988; Torapava et al.2009), enhancing Cr(III) concentration in the filtrate.Nevertheless, the partial hydrolysis of Cr(III)–malatecomplex cannot satisfactorily explain the dramaticweakness in peak height from pH 6 to 8. So, a furtherstudy is needed to explicate the change. Such a phenom-enon has not been observed by Dai et al. (2010, 2011),who investigated the species of Cr(III)–citrate complexand Cr(III)–tartrate complex with the samemethod. Thisimplies that the complexes of Cr(III)–citrate and Cr(III)–tartrate are more stable than Cr(III)–malate complex.Apart from peaks 1 and 2, peak 3 with a weak signalappeared at pH 8, 10, and 12. It is noted that peak 2 washigher than peak 1 in the cases, suggesting that [Cr2(III)-mal3-OH]

− probably became the main species ofCr(III)–malate complex. Peak 3 is speculated to standfor the species of [Cr2(III)-mal3-OH2]

2−. When pH roseto 10, peak 4 appeared and peak 2 declined obviously.Peak 4 is considered to represent the species of [Cr2(III)-mal3-OH3]

3−.These changes of species are assumed to be involved

in an addition or a loss of H+ and OH− as described inEq. (1):

Cr2 IIIð Þ‐mal3½ � ⇌OH‐

HþCr2 IIIð Þ‐mal3‐OH½ �‐ ⇌OH

‐

HþCr2 IIIð Þ‐mal3‐OH2½ �2‐ ⇌OH

‐

HþCr2 IIIð Þ‐mal3‐OH3½ �3‐ ð1Þ

3.2 Photo-oxidation of Cr(III)–Malate Complex

3.2.1 Effect of Light Intensity

The photo-oxidation of Cr(III)–malate complex underdifferent light sources at pH 10 and 25°C was

investigated, and the results are illustrated in Fig. 3. It isobserved from Fig. 3 that the production of Cr(VI) wasweak under the irradiation of mimicking solar light(500 W xenon lamp). Only approximately 2.6 μMCr(VI) was detected within 420 min. However, thephoto-oxidation extent and rate of Cr(III)–malate

1875, Page 4 of 9 Water Air Soil Pollut (2014) 225:1875

complex were markedly enhanced under the irradiationof 100-, 300-, and 500-W medium pressure mercury

lamps. Approximately 37.9, 44.4, and 58.8% of theinitial Cr(III) in Cr(III)–malate complex were converted

Fig. 2 HPLC chromatogram ofCr(III)-mal species. 1 [Cr2(III)-mal3], 2 [Cr2(III)-mal3-OH]

−, 3[Cr2(III)-mal3-OH2]

2−, 4[Cr2(III)-mal3-OH3]

3−.Conditions: LiChrospher, C18 (5μm, 250×4.6 mm), 20 mMKH2PO4 with 3% (v/v) MeOH atpH 2.75, flow rate at 0.6 mLmin−1, detection at 210 nm

Water Air Soil Pollut (2014) 225:1875 Page 5 of 9, 1875

into Cr(VI), with an average rate of 0.09, 0.11, and0.14 μM min−1, respectively. This trend of Cr(III)oxidation was consistent with the investigations onthe photo-oxidation of Cr(III)–cit and Cr(III)–tar (Daiet al. 2010, 2011). Therefore, it can be concludedthat higher irradiation intensity can accelerate thetransformation of Cr(III) to Cr(VI). Nevertheless, itis worthy indicating that Cr(VI) may be accumulatedwhen Cr(III)–organic acid complex is exposed tosolar light in spite of relatively slow rate. Thus, itis a potential pathway for the re-oxidation of Cr(III)in natural environment, especially under alkalinecondition.

3.2.2 Effect of Initial Concentration of Cr(III)–MalateComplex

The photo-oxidation of Cr(III)–malate complex with aninitial concentration of 50 to 200 μM under full light ofa 500-W medium pressure mercury lamp was

investigated at pH 10 and 25°C, and the results areillustrated in Fig. 4. It is observed that Cr(III) oxidationat the initial concentrations of 50, 100, 150, and 200 μMwas realized by approximately 65, 59, 53, and 47%,respectively, implying that the extent of Cr(III) oxida-tion increased with a decrease of the initial concentrationof Cr(III)–malate complex.

It is also noted that the concentration of Cr(III)–malate complex at first decreased relatively quicklyand then dropped gently after a turning point (approxi-mately 60 min), suggesting that the oxidation of Cr(III)–malate complex probably followed a complicated kinet-ics process. The tendency was similar with the investi-gation on photo-oxidation of Cr(III)–tartrate complex(Dai et al. 2011) but different from that of Cr(III)–citratecomplex (Dai et al 2010). The reaction process observedin this study can be obviously divided into two stages(as shown in Fig. 4). At the first stage, −lnct/c0 (c0 and ctrepresent the concentrations of Cr(III)–malate complexat the reaction time=0 and t, respectively) versus reac-tion time (t) was plotted, and good linearity with acorrelation coefficient (R2)>0.95 was obtained in theall cases (see Table 1), suggesting that the reaction atthis stage followed first-order kinetics. The rate con-stants, calculated by the slope of the curves, ranged from0.0039 to 0.0065 min−1. In the later stage, a character-istic of zero-order kinetics was observed. The slopes,representing reaction rate constant, for the initial con-centrations of 50 to 200 mM ranged from 0.0673 to0.0914 μM L−1 min−1.

3.2.3 Effect of Temperature

The effect of temperature on the photo-oxidation ofCr(III)–malate complex was conducted with the fulllight of a 500-W medium pressure mercury lamp atpH 10. It is observed from Table 2 that an increasingtemperature slightly facilitated the transformation ofCr(III) to Cr(VI). The production of Cr(VI) in freeCr(III) was improved from 11.97 to 17.1 μM, with thetemperature increasing from 15 to 35°C within 420-minirradiation. As compared with free Cr(III), the photo-oxidation extent and rate of Cr(III)–malate complexwere significantly enhanced. The yield of Cr(VI) at thetemperature of 15 to 35°C ranged from 53.01 to 61.64μM. Based on the results, it is understood that temper-atures exert a weak influence on the photo-oxidation ofboth free Cr(III) and Cr(III)–malate complex.

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350 400 450time/min

c (C

r(V

I))/

µM

Fig. 3 The effect of irradiation intensity on the photo-oxidation ofCr(III)–malate complex with an initial concentration of 50 μM atpH 10 and 25°C. Filled diamond 500-W medium-pressure mer-cury lamp, filled square 300-W medium-pressure mercury lamp,filled triangle 100-Wmedium-pressure mercury lamp, open trian-gle 500-W xenon lamp

0

50

100

150

200

250

0 100 200 300 400 500time/min

c (C

r(II

I)-m

al)/

µM

Fig. 4 The change of Cr(III)–malate complex with the reactiontime under full light of a 500-Wmedium-pressure mercury lamp atpH 10 and 25°C. Initial Cr(III)–malate complex concentration:filled diamond 50 mM, filled square 100 mM, filled triangle 150mM, open triangle 200 mM

1875, Page 6 of 9 Water Air Soil Pollut (2014) 225:1875

3.2.4 Effect of pH

The effect of pH on the photo-oxidation of Cr(III)–malate complex was investigated in a pH range of 5 to12. As shown in Fig. 5, Cr(VI) concentration increasedquickly at first and then rose gently at all the tested pHsexcept 6, at which Cr(VI) concentration accumulated upat the beginning stage of the reaction and then descendedobviously. In addition, Cr(VI) accumulation reached acertain stable level at pH of 7–9 since 150 min. A similarphenomenon was observed by Dai et al. (2011) on thephoto-oxidation of Cr(III)–tartrate complex. It seemslikely that the photo-oxidation of Cr(III)–malate com-plex was accompanied by the reduction of Cr(VI) byorganic acids and its free radicals produced from thereaction under the weak acidic condition. When Cr(VI)reduction was stronger than its production at a low-pHsolution such as 6, Cr(VI) concentration tended to dropdown in the later stage of the reaction. Cr(VI) concen-tration in the reaction system leveled off when Cr(VI)generationwas balanced by its reduction in a pH range of7 to 9. With a higher pH, the photo-oxidation of Cr(III)–malate complex dominated, and Cr(VI) concentrationrapidly increased with the reaction process.

In order to confirm the production of free organic acidin the reaction process, which affected the accumulationof Cr(VI), malic acid was determined byHPLC during thereaction at pH 7 and 10. As shown in Fig. 6, within 4 h of

irradiation at pH 7 and 10, malic acid increased with thereaction process and came to approximately 9 and 25μM,respectively. Although the malic acid determined couldnot stand for the actual concentration due to a portionconsumption by Cr(VI) and irradiation, it well supportedour hypothesis. A higher concentration of malic acid wasobserved at pH 10 than pH 7 (Fig. 6), but this could notimpede the increase of Cr(VI) concentration at pH 10because high pH is not conducive to Cr(VI) reduction.

As mentioned previously, Cr(III)–malate complexexisted mainly in the species of both [Cr2(III)-mal3]and [Cr2(III)-mal3-OH]− at all the tested pHs.Although the concentrations of [Cr2(III)-mal3-OH2]

2−

and [Cr2(III)-mal3-OH3]3− were observed to be low at

corresponding pHs, they may make an important con-tribution to photo-oxidation of Cr(III)–malate complex.Based on the fact that more Cr(VI) was gendered withpH increasing, it is concluded that the photochemicalactivity is in the order: [Cr2(III)-mal3-OH3]

3−>[Cr2(III)-mal3-OH2]

2−>[Cr2(III)-mal3-OH]−>[Cr2(III)-mal3].

The assumption is supported by Ciesla et al. (2004),who proposed that the dangling COO− group in[Cr(III)–EDTA–OH]2− ion was more susceptible to ox-idation than the group coordinated to Cr(III) such as thatin [Cr(III)–EDTA]− complex ion. This was further con-firmed by the determination of·OH, an intermediate

Table 2 Influence of temperatures on the photo-production ofCr(VI) in the solutions of Cr(III) (50 μM) and Cr(III)–malatecomplex (50 μM) under full light of a 500-W medium pressuremercury lamp at pH 10

15°C 25°C 35°C

Cr(III) (μM) 11.97 14.15 17.1

Rate (μM h−1) 1.71 2.02 2.44

Cr(III)–mal (μM) 53.01 58.8 61.64

Rate (μM h−1) 7.57 8.4 8.81

Table 1 The fitted equations and correlation coefficients (R2) of photo-oxidation of Cr(III)–malate complex

Concentration/μM First-stage equations R2 Second-stage equations R2

50 −ln(ct/c0)=0.0065t+0.0309 0.9897 ct=−0.0673t+31.36 0.9589

Cr(III)-mal 100 −ln(ct/c0)=0.0059t+0.026 0.977 ct=−0.0775t+71.909 0.9687

150 −ln(ct/c0)=0.0049t+0.0187 0.9819 ct=−0.0846t+116.26 0.9898

200 −ln(ct/c0)=0.0039t+0.0223 0.9552 ct=−0.0914t+164.56 0.9984

0102030405060708090

100

0 50 100 150 200 250 300 350 400 450reaction time/min

c (C

r(V

I))/

µM

Fig. 5 Effect of pH on the photo-oxidation of Cr(III)–malatecomplex with an initial concentration of 50 μM under full lightof a 500-Wmedium-pressure mercury lamp at 25°C.Open squarepH 6, open triangle pH 7, multiplication symbol pH 8, filleddiamond pH 9, filled square pH 10, filled triangle pH 11, crosspH 12

Water Air Soil Pollut (2014) 225:1875 Page 7 of 9, 1875

reflecting photo-chemical activities in the reactionprocess at pH 7 and 10. The concentration of ·OHobserved at pH 10 was higher than at pH 7 (datanot shown since they have been reported in ourprevious paper (Dai et al. 2010)).

3.3 Possible Pathways of the Photo-oxidation of Cr(III)–Malate Complex

As discussed previously, O2 plays an important role inthe photo-oxidation of Cr(III) (Schroeder and Lee 1975;Eary and Rai 1987; Ciesla et al. 2004). A dramaticdifference in the photo-oxidation of Cr(III)–organic acidwas observed in the presence and in the absence ofdissolved O2 (Dai et al. 2010 and 2011).

Based on the mechanisms of the photo-oxidation ofCr(III)–organic acid complex proposed by Ciesla et al.(2004) and Dai et al. (2010, 2011) as well as taking intoaccount the fact that photo-oxidation of Cr(III)–malatecomplex occurred rapidly at pH ≥10.0 (see Fig. 5 ), thepossible pathways of the photo-oxidation of Cr(III)–malate complex are suggested as follows:

Cr2 IIIð Þ−mal3½ � →hv

2Cr IIð Þ aqð Þ þmalþ 2mal⋅Cr2 IIIð Þ−mal3−OHn½ �n− n ¼ 1; 2; 3ð Þ →

hvð2Þ

2Cr IIð Þ aqð Þ þ 2malþmal⋅þ n−1ð ÞOH− þ ⋅OH ð3Þ

mal →hv

mal⋅ ð4Þ

mal⋅þ O2 → productsþ CO2 þ O2⋅− ð5Þ

Cr IIð Þ aqð Þ þ O2→Cr IIIð Þ þ O2⋅− ð6Þ

Hþ þ O2⋅− → HO2⋅ ð7Þ2HO2⋅→H2O2 þ O2 ð8Þ

H2O2 →hv; <300nm

2⋅OH ð9Þ

Cr IIð Þ aqð Þ þ 4⋅OHþ 4OH− → CrO42− þ 4H2O ð10Þ

According to the possible mechanism proposed ear-lier, ·OH should be the key oxidant for the photo-oxidation of Cr(III)–malate complex. In order to furtherprove the assumption, H2O2, a direct source of·OH inthe presence of Cr(II) (Dai et al. 2010), was introducedinto the reaction system. Consequently, a complete ox-idation of Cr(III)–malate complex was observed within20 min of irradiation when 10mMH2O2 was added intothe reaction system. However, without irradiation, onlyapproximately 30% Cr(III)–malate complex was oxi-dized by H2O2 under the same condition. Therefore, thisstrongly confirms that·OH was the key oxidant forCr(III) photo-oxidation.

4 Conclusions

The results obtained in this study demonstrated thatCr(III)–malate complex, through a ligand–metalcharge-transfer path, could be gradually oxidized toCr(VI) with an irradiation at an appropriate pH. Thekinetics study indicated that the photo-oxidation ofCr(III)–malate complex obeyed first-order at the initialstage and then zero-order was followed. Cr(II), an inter-mediate, was approved to be the precursor of Cr(III)oxidation to Cr(VI). It is concluded that photo-inducedoxidation of Cr(III)–organic acid complex forms harm-ful Cr(VI) again. Therefore, this study is helpful tocomprehensively understand the potential oxidation be-haviors of Cr(III) in environments.

Acknowledgments This study was supported by the NationalNatural Science Foundation of China (Grant No. 21377056) andthe Fundamental Research Funds for the Central Universities(Grant No. KYZ201124).

References

Apte, A. D., Tare, V., & Bose, P. (2006). Extent of oxidation ofCr(III) to Cr(VI) under various conditions pertaining to nat-ural environment. Journal of Hazardous Materials, 128(B),164–174.

0

5

10

15

20

25

30

0 1 2 3 4 5time/h

c (m

alic

aci

d)/µ

M

Fig. 6 Photo-production trend of malic acid in Cr(III)–malatecomplex (50 μM) under full light of a 500-W medium-pressuremercury lamp at 25°C. Filled square pH 7, filled diamond pH 10

1875, Page 8 of 9 Water Air Soil Pollut (2014) 225:1875

Bartlett, R. J., & James, B. R. (1979). Behavior of chromium insoils. III. Oxidation. Journal of Environmental Quality, 8,31–35.

Bartlett, R. J. (1991). Chromium cycling in soils and water: links,gaps and methods. Environmental Health Perspectives, 92,17–24.

Ciesla, P., Karocki, A., & Stasicka, Z. (2004). Photoredox behav-iour of the Cr-EDTA complex and its environmental aspects.Journal of Photochemistry and Photobiology, A: Chemistry,162, 537–544.

Dai, R., Liu, J., Yu, C., Sun, R., Lan, Y., & Mao, J. (2009). Acomparative study of oxidation of Cr(III) in aqueous ions,complex ions and insoluble compounds by manganese-bearing mineral (birnessite). Chemosphere, 76, 536–541.

Dai, R., Yu, C., Liu, J., Lan, Y., & Deng, B. (2010). Photo-oxidation of Cr(III)–citrate complexes forms harmfulCr(VI). Environmental Science and Technology, 44, 6959–6964.

Dai, R., Yu, C., Gou, J., Lan, Y., & Mao, J. (2011). Photoredoxpathways of Cr(III)–tartrate complexes and their impactingfactors. Journal of Hazardous Materials, 186, 2110–2116.

Eary, L. E., & Rai, D. (1987). Kinetics of chromium(III) oxidationto chromium(VI) by reaction with manganese dioxide.Environmental Science and Technology, 21, 1187–1193.

Fendorf, S. E., & Li, G. C. (1996). Kinetics of chromate reductionby ferrous iron. Environmental Science and Technology, 30,1614–1617.

Hug, S. J., Laubscher, H. U., & James, B. R. (1997). Iron(III)catalyzed photochemical reduction of chromium(VI) by ox-alate and citrate in aqueous solutions. Environmental Scienceand Technology, 31, 160–170.

Kabir-ud-Din, Hartani, K., & Khan, Z. (2000). Unusual rate inhi-bition of manganese (II) assisted oxidation of citric acid bychromium (VI) in the presence of ionic micelles. TransitionMetal Chemistry, 25, 478–484.

Kavanaugh, M. C. (1994). Alternative for groundwater cleanup.Washington, DC: National Academy.

Kota’s, J., & Stasicka, Z. (2000). Chromium occurrence in theenvironment and methods of its speciation. EnvironmentalPollution, 3, 263–283.

Lan, Y., Deng, B., Kim, C., Thornton, E. C., & Xu, H. (2005).Catalysis of elemental sulfur nanoparticles on chromium(VI)reduction by sulfide under anaerobic conditions.Environmental Science and Technology, 39, 2087–2094.

Lan, Y., Yang, J., & Deng, B. (2006). Catalysis of dissolved andadsorbed iron in soil suspension on the reduction of Cr(VI)by sulfide. Pedosphere, 16, 572–578.

Lan, Y., Li, C., Mao, J., & Sun, J. (2008). Influence of clayminerals on the reduction of Cr6+ by citric acid.Chemosphere, 71, 781–787.

Li, C., Lan, Y., & Deng, B. (2007). Catalysis of Mn2+ on thereduction of Cr6+ by citrate. Pedosphere, 17, 318–323.

Li, Y., Kristie, C. A., Royce, N. D., Nathan, A. C., James, Q. C., &Xue, Z. (2006). Quantitative analysis of trace chromium in

blood samples: combination of the advanced oxidation pro-cess with catalytic adsorptive stripping voltammetry.Analytical Chemistry, 789, 7582–7587.

Liu, X., Wu, F., & Deng, N. (2004). Photoproduction of hydroxylradicals in aqueous solution with algae under high-pressuremercury lamp. Environmental Science and Technology, 38,296–299.

Liu, Y. X., Deng, L., Chen, Y., Wu, F., & Deng, N. S. (2007).Simultaneous photocatalytic reduction of Cr(VI) and oxida-tion of bisphenol A induced by Fe(III)–OH complexes inwater. Journal of Hazardous Materials, 139, 399–402.

Melitas, N., Moscoso, O. C., & Farrell, J. (2001). Kinetics ofsoluble chromium removal from contaminated water byzerovalent iron media: corrosion inhibition and passive oxideeffects. Environmental Science and Technology, 35, 3948–3953.

Palmer, C. D., & Wittbrodt, P. R. (1991). Processes affecting theremediation of chromium-contaminated sites. EnvironmentalHealth Perspectives, 92, 25–40.

Puzon, G. J., Tokala, R. K., Zhang, H., David, Y., Peyton, B.M., &Xun, L. Y. (2008). Mobility and recalcitrance of organo-chromium(III) complexes. Chemosphere, 70, 2054–2059.

Rodman, D. L., Carrington, N. A., & Xue, Z. (2006). Conversionof chromium(III) propionate to chromium(VI) by the ad-vanced oxidation process pretreatment of a biomimetic com-plex for metal analysis. Talanta, 70, 668–675.

Schroeder, D. C., & Lee, G. F. (1975). Potential transformations ofchromium in natural waters.Water, Air, and Soil Pollution, 4,355–365.

Spiccia, L. (1988). Solubility of chromium(III) hydroxides.Inorganic Chemistry, 27, 432–434.

Sun, J., Mao, J.-D., Gong, H., & Lan, Y. (2009). Fe(III) photocat-alytic reduction of Cr(VI) by low-molecular weight organicacids with α-OH. Journal of Hazardous Materials, 168,1569–1574.

Tian, X., Gao, X., Yang, F., Lan, Y., Mao, J.-D., & Zhou, L.(2010). Catalytic role of soils in the transformation ofCr(VI) to Cr(III) in the presence of organic acids containingα-OH groups. Geoderma, 159, 270–275.

Torapava, N., Radkevich, A., Davydov, D., Titov, A., & Persson, I.(2009). Composition and structure of polynuclearchromium(III) hydroxo complexes. Inorganic Chemistry,48, 10383–10388.

Wu, X. Y., Fang, L., Xu, W. D., Zhao, J. L., & Yang, L. Q. (2011).Anti-hyperglycemic activity of chromim(III) malate complexin alloxan-induced diabetic rats. Biological Trace ElementResearch, 43, 1031–1043.

Zhang, H., & Bartlett, R. J. (1999). Light-induced oxidation ofaqueous chromium(III) in the presence of iron(III).Environmental Science and Technology, 33, 588–594.

Zou, Y., & Hoigne, J. (2009). Formation of hydrogen perodxideand depletion of oxalic acid in atmospheric water by photol-ysis of iron–oxalate complexes. Environmental Science andTechnology, 26, 1014–1022.

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