sulfite reaction system: Involvement of reactive oxygen species and intramolecular electron transfer

Journal of Hazardous Materials 304 (2016) 457–466

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The roles of polycarboxylates in Cr(VI)/sulfite reaction system: Involvement of reactive oxygen species and intramolecular electron transfer Bo Jiang a,b , Xianli Wang a , Yukun Liu a , Zhaohui Wang c,d , Jingtang Zheng a,∗ , Mingbo Wu a,∗ a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, PR China School of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266033, PR China c College of Environmental Science and Engineering, Donghua University, Shanghai 201620, PR China d Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia b

h i g h l i g h t s • • • •

• • The formations of SO4 − and OH , involve in Cr(VI) reduction induced by S(IV). Affinity of polycarboxylate to Cr(VI) accelerates Cr(VI) reduction rate. Polycarboxylates can act as electron donors for Cr(VI) reduction retrenching S(IV). • • Only oxalate can enhance the formations of SO4 − and OH in Cr(VI)/S(IV) system.

a r t i c l e

i n f o

Article history: Received 8 August 2015 Received in revised form 20 October 2015 Accepted 8 November 2015 Available online 14 November 2015 Keywords: Cr(VI) Reduction Sulfite Reactive oxygen species Polycarboxylates

a b s t r a c t In this study, the effects of polycarboxylates on both Cr(VI) reduction and S(IV) consumption in Cr(VI)/S(IV) system was investigated in acidic solution. Under aerobic condition, the productions of reactive oxygen species (ROS), i.e., SO4 • − and OH• , have been confirmed in S(IV) reducing Cr(VI) process by using electron spin resonance and fluorescence spectrum techniques, leading to the excess consumption of S(IV). However, when polycarboxylates (oxalic, citric, malic and tartaric acid) were present in Cr(VI)/S(IV) system, the affinity of polycarboxylates to CrSO6 2− can greatly promote the reduction of Cr(VI) via expanding the coordination of Cr(VI) species from tetrahedron to hexahedron. Besides, as alternatives to S(IV), these polycarboxylates can also act as electron donors for Cr(VI) reduction via intramolecular electron transfer reaction, which is dependent on the energies of the highest occupied molecular orbital of these polycarboxylates. Notably, the variant electron donating capacity of these polycarboxylates resulted in different yield of ROS and therefore the oxidation efficiencies of other pollutants, e.g., rhodamine B and As(III). Generally, this study does not only shed light on the mechanism of S(IV) reducing Cr(VI) process mediated by polycarboxylates, but also provides an escalated, cost-effective and green strategy for the remediation of Cr(VI) using sulfite as a reductant. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The widespread use of chromium in industrial applications such as leather tanning, metallurgy and electroplating has caused serious chromium contamination in surface waters, ground waters and atmospheric waters [1,2]. Two primary oxidation states, Cr(VI) and

∗ Corresponding authors. E-mail addresses: [email protected] (B. Jiang), [email protected] (J. Zheng), [email protected] (M. Wu). http://dx.doi.org/10.1016/j.jhazmat.2015.11.011 0304-3894/© 2015 Elsevier B.V. All rights reserved.

Cr(III) species, are present in aqueous media. The former, listed among the top 20 contaminants on Superfund Priority List of Hazardous Substances, is known to be highly toxic and carcinogenic to human and animals, while the latter is generally non-toxic [3]. Consequently, the reduction of Cr(VI)–Cr(III) is considered as the most common process to minimize the toxicity of chromium pollution [4]. Sulfite (S(IV))(E0 (SO4 2− /SO2 ) = 0.158 VNHE ), one of the strongest reductants, can effectively reduce Cr(VI) (E0 (HCrO4 − /Cr3+ ) = 1.35 VNHE ) to Cr(III) in the acidic solution, which is a common process for

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Cr(VI) detoxification throughout the world [5–8]. Previous literatures illustrated that S(IV) is widely present in natural waters owing to biochemical processes as well as in hydrothermal waters due to geochemical process [9,10]. In addition, S(IV), an industrial available waste, is abundant in many relevant wastewaters and exhaust gas [11]. Thus, Cr(VI) reduction by S(IV) plays an important role not only in the remediation of Cr(VI)-contaminating effluents or sites, but also in surface waters and in the atmospheric transport of chromium [1,5–8]. Similar to sulfite, some polycarboxylates (e.g., citric acid, malic acid and oxalic acid) have been also demonstrated as effective sacrificial agents for Cr(VI) reduction when being subject to the catalysis by surface-bound or dissolved metals with or without light irradiation [12–14]. For example, Chen et al. [13] reported that Cr(VI) reduction by citric acid at pH 4.5 was greatly enhanced with the presence of Mn(II) as a catalyst and the rate constant increased by 45 times in comparison with that of the controlled case. In these cases, polycarboxylates exhibit a high affinity for surface-bound or dissolved metals, and their complex was mostly proposed as the prerequisite process for Cr(VI) reduction. Besides, it is widely known that polycarboxylates are ubiquitous in natural environments and many industrial processes (e.g., metallurgy, decontamination of nuclear plants and boilers, wood preservation and textile industries) [15–17]. However, until now, it is still unclear whether and how accumulated polycarboxylates in wastewaters or contaminated sites affect the detoxification of Cr(VI) induced by S(IV). In this study, we evaluated the effect of polycarboxylates on the reaction process of S(IV) reducing Cr(VI) in aquatic matrices at micromolar level. The influences of reagent concentrations, pH and gas atmosphere on the redox conversions of Cr(VI) and S(IV) have been examined. To clarify the underlying mechanisms, fluorescence spectrometry, electron spin resonance (ESR) and density functional theory (DFT) calculation were applied to identify the reactive oxygen species (ROS), the chromium intermediates formed in Cr(VI) reduction process and to evaluate the electron donating capacity of polycarboxylates, respectively. The present study may provide a new insight into the reaction mechanism of Cr(VI)/S(IV) system mediated by polycarboxylates, which is of significant interest for engineered systems concerned with the remediation of Cr(VI). 2. Materials and methods 2.1. Materials Sodium arsenite (NaAsO2 , 97%), rhodamine B (RhB, 96%), anhydrous sodium sulfite (Na2 SO3 , >97%), potassium chromate (K2 Cr2 O7 , >99%), diphenylcarbazide (98%), ammonium molybdate tetrahydrate ((NH4 )6 Mo7 O24 ·4H2 O, >99%), ethanol (EtOH, 73.0–75.0%), 5,5 -Dithiobis (2-nitrobenzoic acid) (DTNB, >99%), ethylenediaminetetraacetic acid (EDTA, >99.5%), antimony potassium tartrate (K(SbO)C4 H4 O6 ·0.5H2 O, >99%), l-ascorbic acid (C6 H8 O6 , >99.7%), coumarin (C9 H6 O2 ), tert-butyl alcohol (TBA, >98%), H2 SO4 (73.0–75.0%), H3 PO4 (>98.5%), NaOH (>96%), HCl (36–38%), oxalic acid (Ox, ≥99.6%), citric acid (AR), malic acid (99%), tartaric acid (AR) and malonic acid (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. All chemical reagents were used without further purification. Ultra pure water (18.2 M cm) was used for all experiments.

ture of 20 ± 1 ◦ C in a circulating water jacket. Stock solutions of 20 mM Cr(VI) was prepared by dissolving analytical grade K2 Cr2 O7 in deionized water. All working solutions were freshly prepared by diluting the stock solution with pure water, and the pH was adjusted to the desired values with concentrated H2 SO4 or NaOH solution. The addition of freshly prepared sulfite was used to initiate an experiment. For the experiments using a volumetric flask (250 mL) as the reaction vessel in the presence of argon or oxygen (gas velocity, 0.6 L min−1 ), the solution was purged with the corresponding gas for 10 min prior to initiating the reaction. The samples were withdrawn at various time intervals, and followed by immediate measurements. 2.3. Analytical methods The pH of the solution was determined by pH meter (PHS3C). The concentration of Cr(VI) remaining in the solution was measured by a diphenylcarbazide method. The concentrated acids (H3 PO4 :H2 SO4 :H2 O = 1:1:2, v:v:v) should be premixed with diphenylcarbazide reagent to minimize the interference of acidification with Cr(VI) determination. The absorbance of sample solutions was detected at 540 nm after full color development (>15 min) [18,19]. The concentration of S(IV) was determined using a modified colorimetric procedure with DTNB [20]. Briefly, 1 mL of sample was added into a cuvette containing a mixture with 1 mL of EDTA (1 mM), 2 mL of DTNB (1 mM), and 5 mL of Na2 HPO4 /KH2 PO4 buffer (pH 7). The color was developed after 15 min and the sample solutions were detected at 412 nm with an UV–vis spectrophotometer. As(V) concentration was determined using modified molybdenum-blue method [21]. Briefly, for each 2.0 mL quenched aliquot (1 mL of sample + 1 mL of methanol), 0.5 mL of the 2% HCl acidifying solution and 0.3 mL of the coloring reagent were mixed sequentially. The concentration of RhB was monitored at 557 nm using a UV–vis spectrophotometer. The chemical of 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) was used as a spin-trapping agent when utilizing electron spin resonance (ESR) for the detection of active radicals [22]. The chemical solutions of DMPO, and Cr(VI) were mixed for 15 s and transferred into a 100 ␮L capillary tube, which was then inserted into the cavity of the ESR spectrometer (Bruker A200). To detect the radicals in argon atmosphere, the mixed solution was previously deoxygenized for 10 min. Sulfite solution was in situ added in above mixed solution in the cavity. The ESR experiments were performed on the ESR spectrometer under the following conditions: center field, 323.3 mT; sweep width, 10.0 mT; microwave frequency, 9.056 GHz; microwave power, 0.998 mW; temperature, 295.0 K. Coumarin (1 mM) was employed as a chemical probe for • OH. 7-Hydroxycoumarin (Reaction (1)) was measured using a spectrofluorometer (F97PRO, Lengguang Tech.) [23]. •

OH + coumarin → 7 − hydroxycoumarin

k = 2.0 × 109 M−1 s−1 (1)

2.4. DFT calculation The energies of the highest occupied molecular orbital (EHOMO ) of tested organic acid were calculated by using the density functional theory (DFT), using the B3LYP functionals and the basis set of 6-311G [24]. 3. Results and discussion 3.1. Effects of polycarboxylates

2.2. Reaction procedures All experiments were conducted in an open, 150 mL, cylindrical glass tube, magnetically stirred and maintained at a tempera-

In this study, oxalic acid (Ox) and citric acid (Cit) were selected as the model polycarboxylates due to their ubiquities in natural environment. As shown in Fig. 1, in Cr(VI)/S(IV) reac-

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Fig. 1. The reduction of Cr(VI) (a) and consumption of S(IV) (b) in various reaction systems ([Cr(VI)]0 = 50 ␮M, [S(IV)]0 = 300 ␮M, pHini 3.5, [Ox]0 = 500 ␮M, [Cit]0 = 500 ␮M).

Fig. 2. Effect of initial solution pH on Cr(VI) reduction in various reaction systems ([Cr(VI)]0 = 50 ␮M, [S(IV)]0 = 300 ␮M, [Ox]0 = 500 ␮M, [Cit]0 = 500 ␮M). Table 1 Effects of various polycarboxylates on Cr(VI) reduction, S(IV) consumption and [S(IV)]consumption /[Cr(VI)]reduction with or without the presence of 100 mM ethanol ([Cr(VI)]0 = 50 ␮M, pHini = 3.5, [polycarboxylate]0 = 500 ␮M, [S(IV)]0 = 300 ␮M, time 20 min). Cr(VI)/S(IV)/polycarboxylate

Cr(VI)/S(IV)/polycarboxylate/100 mM ethanol

System

Cr(VI) reduction (␮M)

S(IV) consumption (␮M)

[S(IV)]consumption / [Cr(VI)]reduction

Cr(VI) reduction (␮M)

S(IV) consumption (␮M)

[S(IV)]consumption / [Cr(VI)]reduction

No addition Acetic acid Ox Malonic acid Malic acid Tartaric acid Cit

18.8 20.4 28.9 31.0 211.9 199.2 199.2

292.6 281.8 300 245.5 4.5 4.1 4.4

15.6 13.8 10.4 7.9 3.0 2.9 2.5

38.6 40.8 49.9 40.6 47.8 47.7 47.9

152.8 164.9 183 165.2 143.4 138.0 114.5

4.0 4.0 3.7 4.1 3.0 2.9 2.5

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Fig. 3. The ESR signals of SO3 • − (a), SO4 • − and • OH (b) in Cr(VI)/S(IV) system ([Cr(VI)]0 = 300 ␮M, [S(IV)]0 = 1800 ␮M, pHini 3.5, [Ox]0 = [Cit]0 = 3000 ␮M; For (a) reaction time 20 min; For (b) the reaction was initiated in oxygen-free condition and then proceeded with adding DMPO under oxygen bubbling condition, • represents DMPO-• OH adduct and • represents DMPO-SO4 • − adduct).

tion system, only 18.8 ␮M Cr(VI) was reduced at the cost of 300 ␮MS(IV) with [S(IV)]consumption /[Cr(VI)]reduction value of 15.6. However, Cr(VI) reduction was markedly enhanced and accelerated when Ox or Cit was separately added into Cr(VI)/S(IV) reaction system. In Cr(VI)/S(IV)/Ox system, approximately 28.8 ␮M Cr(VI) was quickly reduced with the rapid depletion of S(IV) within 5 min. As compared with the case mediated by Ox, adding Cit into Cr(VI)/S(IV) system was observed to be superior for Cr(VI) reduction at the cost of less S(IV), approximately 202.7 ␮M, leading to less [S(IV)]consumption /[Cr(VI)]reduction value of 4.4. As shown Fig. 1, in sulfite-free reaction system, the presence of Ox or Cit led to negligible reduction of Cr(VI). Thus, the possibility of direct reaction between the tested polycarboxylates and Cr(VI) for Cr(VI) reduction can be excluded in Cr(VI)/S(IV)/polycarboxylate reaction systems.

3.2. Effects of reagent concentration and solution pH The effects of polycarboxylates and S(IV) concentrations on Cr(VI) reduction and S(IV) consumption were further investigated. Fig. S1(a and b) shows that the reduction of Cr(VI) was evidently enhanced with the increase of polycarboxylates concentration. Compared with Ox, equivalent addition of Cit led to more reduction of Cr(VI) and less [S(IV)]consumption /[Cr(VI)]reduction value. For example, the amount of Cr(VI) reduction was increased from 35.4 ␮M to 48.7 ␮M with increasing Cit from 100 ␮M to 800 ␮M, while only from 23.9 ␮M to 30.9 ␮M for Ox. However, it is evidently noted that the presence of Ox accelerated both of Cr(VI) reduction and S(IV) depletion much faster than those of the Cit mediating cases. Similar phenomena can be also observed for various concentration of S(IV) in Fig. S2. In addition, increasing the concentration of S(IV) resulted in much more reduction of Cr(VI) in the cases with or without the presence of polycarboxylates.

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Fig. 4. Effect of gas atmosphere on Cr(VI) reduction, S(IV) consumption and [S(IV)]consumption /[Cr(VI)]reduction value in Cr(VI)/S(IV) reaction system ([Cr(VI)]0 = 50 ␮M, [S(IV)]0 = 300 ␮M, pHini 3.5).

The pH is a potentially critical variable for Cr(VI) reduction, because of its effect on the speciation and redox potentials of Cr(VI) and sulfite. Fig. S3 shows that SO2 ·H2 O and HSO3 − were dominant species at lower pH values (pH < 7.0). Fig. S4 depicts that the major Cr(VI) species at pH < 6.0 is HCrO4 − , while CrO4 2− is the major species at pH > 6.0. Since CrO4 2− is a much weaker oxidant than HCrO4 − [E0 (CrO4 2− /Cr2 O3 ) = 0.56 VNHE at pH 7.0 vs E0 (HCrO4 − /Cr3+ ) = 0.94 VNHE at pH 3.0], CrO4 2− is less reactive than HCrO4 − for the Cr(VI)-induced oxidation of S(IV) [25]. Therefore, in various Cr(VI)/S(IV) reaction system, faster reduction of Cr(VI) was expected in stronger acidic solution, as shown in Fig. 2. In addition, it is noteworthy that adding polycarboxylates, especially Cit, can significantly enhance the reduction of Cr(VI) in the examined pH range of 2.5–4.5. 3.3. Identification of reactive oxygen species and chromium intermediates The reduction of Cr(VI) induced by S(IV) in the pH range 2.0–5.0 can be described by the general reactions in Reactions (2) and (3) with the excess of S(IV) or Cr(VI), respectively [26,27]. However, in present study, we find that the actual values of [S(IV)]consumption /[Cr(VI)]reduction were much higher, especially in the case with the absence of polycarboxylates in Table 1. Thus, there must be active intermediate species in Cr(VI)/S(IV) reaction system responsible for the deviant stoichiometric values of [S(IV)]consumption /[Cr(VI)]reduction . − 3+ 2− + 2HCrO− + 2SO2− 4 + 4HSO3 + 6H → 2Cr 4 + S2 O6 + 6H2 O

(2)

− 3+ + 2HCrO− + 3SO2− 4 + 3HSO3 + 5H → 2Cr 4 + 5H2 O

(3)

Here, to cast more light on the reaction mechanism, ESR technique using 100 mM DMPO as a spin trap was applied for identifying the primary radicals in argon atmosphere. In Fig. 3(a), the characteristic ESR spectrum of the DMPO/• SO3 − adduct can be observed in Cr(VI)/S(IV) reaction system [28]. The addition of Ox or Cit resulted in stronger intensity of DMPO/SO3 •− adduct signal, suggesting that much more production of SO3 •− . As shown in Fig. 4, when Cr(VI) reduction proceeds in argon atmosphere, it can be found that approximately 41.5 ␮M Cr(VI) was reduced, accompanying with the oxidation of 164.7 ␮M S(IV), in which [S(IV)]consumption /[Cr(VI)]reduction value of 3.3 was much less than

15.6 in air atmosphere and 21.0 in oxygen atmosphere. The formation of mild species, SO3 •− (E0 (SO3 •− /SO3 2− ) = 0.76 VNHE ) cannot account for the excess S(IV) consumption in Cr(VI) reduction process. Thus, it can be rationally assumed that many secondary reactive species were readily produced from SO3 •− in oxygen-rich condition but hardly produced in the oxygen-depleted systems. As presented in Fig. 3(b), when Cr(VI)/S(IV) reaction system proceeded under oxygen atmosphere with DMPO concentration of 25 mM, typical signals of DMPO−• OH adducts and DMPO−SO4 •− adducts can be confirmed based on the standard spectra of • OH (AN = 14.9 G and AH = 14.9 G) and SO4 •− (AN = 13.51 G, AH = 9.93 G, A␥1 H = 1.34 G, and A␥2 H = 0.88 G) simulated by Winsim program [29–31]. This indicates that SO4 •− and • OH can be generated in S(IV) reducing Cr(VI) process. However, the chain reactions for ROS formation can be interrupted in the anaerobic environment, which can be partially validated by the production of 7-hydroxycoumarin in Fig. 5. Besides, it should be noted that the presence of Ox can enhance the generation of • OH according to the fluorescence intensity of 7-hydroxycoumarin, whereas it was retarded by the Cit. To evaluate the oxidation capacity of various Cr(VI)/S(IV) reaction system, RhB (10 mg L−1 ) and As(III) (50 ␮M) were selected as two model pollutants. Fig. 6 shows that approximately 90% RhB was bleached within 2 min and 24.6 ␮M As(III) was oxidized to As(V). Notably, the oxidation efficiencies of RhB and As(III) were improved with the presence of Ox while inhibited by adding Cit into Cr(VI)/S(IV) system. As shown in Fig. S5, the oxidation of As(III) undoubtedly consumed a part of ROS and thus prevented the excess consumption of S(IV), leading to the enhanced reduction of Cr(VI). However, the oxidation of these pollutants were interrupted in argon atmosphere (see Fig. 6), which were consistent with ROS results in Fig. 5. 3.4. Effects of various polycarboxylates in Cr(VI) reduction Table 1 compares the influences of various organics on the reduction of Cr(VI) in Cr(VI)/S(IV) reaction system. It can be observed that adding various organics into Cr(VI)/S(IV) reaction system can improve the reduction of Cr(VI) with the order of none < acetic acid < oxalic acid < malonic acid < ethanol < citric acid < malic acid < tartaric acid. In addition, the same variation tendency with Cr(VI) reduction can be also observed for [S(IV)]consumption /[Cr(VI)]reduction values in Table 1. Ethanol,

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Fig. 5. The comparisons of • OH formation using fluorescence spectrum under air or argon atmosphere in various reaction systems ([Cr(VI)]0 = 50 ␮M, [S(IV)]0 = 300 ␮M, pHini 3.5, [Ox]0 = 500 ␮M, [Cit]0 = 500 ␮M, time 5 min). Table 2 The chemical properties of various tested polycarboxylates [32,33]. Carboxylates Structural formula kOH (M−1 s−1 ) kSO• − (M−1 s−1 ) EHOMO (a.u.) 4

–

–

–

Acetic acid

1.6 × 107

8.8 × 104

−0.28675

Ethanol

1.9 × 109

1.6 × 107

−0.26936

Oxalic acid

1.4 × 106

1.1 × 106

−0.31573

Malonic acid

2.0 × 107

5.5 × 106

−0.29275

Malic acid

7.3 × 108

7.7 × 106

−0.28208

Tartaric acid

7.0 × 108

105 − 107

−0.28247

Citric acid

5.0 × 107

2.7 × 106

−0.27715

None

–

as a scavenger of • OH and SO4 •− , can effectively scavenge the produced ROS (k• OH = (1.2 − 2.8) × 109 M−1 s−1 , kSO• − = (1.6 − 7.7) × 107 M−1 s−1 ), and therefore interrupted the 4

oxidation of S(IV) induced by ROS [19,32]. Consequently, much more S(IV) was available for Cr(VI) reduction, e.g., 237.5 ␮M S(IV) accounting for 34.5 ␮M Cr(VI) reduction. As for polycarboxylates in Table 2, the rate constants (104 − 109 M−1 s−1 ) of polycarboxylates reacting with ROS imply that similar with ethanol, the added polycarboxylates scavenging ROS is probably in part responsible for the enhanced reduction of Cr(VI) [32–34]. However, the rate constant of ethanol scavenging ROS is approximately 10–103 faster than those of polycarboxylates, which is evidently inconsistent with the above order of Cr(VI) reduction in various organics-containing cases. Besides, of various reaction systems in Fig. S6(a), the fastest

Cr(VI) reduction rate but least Cr(VI) reduction efficiency can be simultaneously observed in Ox mediated reaction system. These imply that these polycarboxylates acted not only as ROS scavengers but also some other roles influencing Cr(VI) reduction. To further inspect the roles of polycarboxylates, ROS scavenging experiments using 100 mM ethanol were carried out to eliminate the influences of polycarboxylates/S(IV) scavenging ROS on Cr(VI) reduction [35,36]. Table 1 and Fig. S6(b) show the simultaneous enhancement of Cr(VI) reduction and retrenchment of S(IV) consumption can be induced by adding 100 mM ethanol into various Cr(VI)/S(IV)/polycarboxylate systems. The optimal amount of Cr(VI) reduction (approximately 50 ␮M) can be obtained in Cr(VI)/S(IV)/Ox reaction system. However, as presented in Table 1, [S(IV)]consumption /[Cr(VI)]reduction value in the presence of 100 mM ethanol increased in the order of no addition (4.0), acetic acid (4.0), malonic acid (4.1) < Ox (3.7) < malic acid (3.0), tartaric acid (2.9) < Cit (2.5). Thus, in spite of 100 mM ethanol present in various Cr(VI)/S(IV)/polycarboxylate systems, the four tested polycarboxylates, i.e., malic acid, tartaric acid, Cit and Ox, probably still acted as electron donors for Cr(VI) reduction, in which the electrons probably transfer from the highest occupied molecular orbital (HOMO) of polycarboxylates to the lowest occupied molecular orbital (LUMO) of high valent chromium species. To reveal the dependence of Cr(VI) reduction on the electron donating capacity of polycarboxylates, an important property of polycarboxylates associated with energies of the highest occupied molecular orbital(EHOMO ) was further studied using DFT calculation. As shown in Table 1, the calculated EHOMO is increased in the order of Ox < tartaric acid ≈ malic acid < citric acid, which is generally accordant with the variation tendency of [S(IV)]consumption /[Cr(VI)]reduction values with the presence of 100 mM ethanol. In Fig. 7, it is found that there is a proximate linear correlation between the [S(IV)]consumption /[Cr(VI)]reduction and EHOMO of polycarboxylates with the regression equation of  = −26.7 EHOMO – 4.7 (R2 = 0.85). As the value of EHOMO increases, the energy gap between HOMO of polycarboxylates and LUMO of high valent chromium species is enlarged, enhancing the driving force of electron transfer between chromium and poly-

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[S(IV)]consumption /[Cr(VI)]reduction , i.e., 3/2 and 2/1 (see Reactions (2) and (3)), which is sensitive to the initial reagent concentration. [CrSO2− 6 ] 2− + H O K = HCrO4 − + HSO− 2 − = 36 3 ↔ CrSO6 [HCrO− 4 ][HSO3 ] (4) CrSO6 2− + SO2 ↔ CrO2 (SO3 )2 2− (ratedeterminingstep) +

CrO2 (SO3 )2 2− + 4H2 O + 2H+ ↔ SO4 Cr(H2 O)5 + SO3

(5)

◦−

(6)

CrSO6 2− + H+ + HCrO4 − ↔ O3 CrOCrO3 SO3 2− + H2 O O3 CrOCrO3 SO3 SO3

•−

+ SO3

•−

Cr (VI) + SO3

•−

2−

→ 2Cr(V) + SO4

(7)

2−

(8)

↔ S2 O6

(9)

↔ Cr (V) + SO4

2−

(10)

Cr(V) + S(IV) → Cr(III) + S(VI)

(11) •−

Accompanying with the reduction of Cr(VI), SO3 was produced in Reaction (6), which can be confirmed by the detection of DMPO-SO3 •− using spin trapping technique in Fig. 3(a). Under aerobic atmosphere, the formed SO3 •− can react with molecular oxygen to form a peroxyl radical, SO5 •− in Reaction (12) [28,35,37]. The subsequent reactions of SO5 •− with S(IV) was proposed for the formations of ROS, i.e., SO4 •− and OH• (see Reactions (13) and (14)), validated by the formations of DMPO-SO4 •− adduct and DMPO-OH• adduct in Fig. 3(b) [38,39]. SO3

•−

+ O2 → O3 SOO

HSO3 2− + O3 SOO

•−

•−

k = 2.5 × 109 M−1 s−1

→ SO4

•−

+ HSO4 2−

(12)

k = 1.3 × 107 M−1 s−1 (13)

HSO3 2− + O3 SOO

• − H2 O

•

→ 2SO4 2− + OH + 2H+

(14) •− and

Fig. 6. The oxidation of RhB (a) and As(III) (b) in various reaction systems ((a): [Cr(VI)]0 = 100 ␮M, [S(IV)]0 = 600 ␮M, pHini 3.5, [Ox]0 = 500 ␮M, [Cit]0 = 500 ␮M, [RhB]0 = 10 mg L−1 ; (b): [Cr(VI)]0 = 50 ␮M, [S(IV)]0 = 300 ␮M, pHini 3.5, [Ox]0 = 500 ␮M, [Cit]0 = 500 ␮M, [As(III)]0 = 50 ␮M).

carboxylates. Thus, instead of S(IV), the tested polycarboxylate with a high EHOMO can be as an effective electron donator for Cr(VI) reduction and thereby decreased the value of [S(IV)]consumption /[Cr(VI)]reduction . Besides, based on the relationship between EHOMO and [S(IV)]consumption /[Cr(VI)]reduction (see Fig. 7), we can infer that acetic acid and malonic acid behaved dissimilarly with other four polycarboxylates, i.e., malic acid, tartaric acid, Cit and Ox, in the reduction reaction of Cr(VI).

OH•

By virtue of their great reactivity, SO4 accounted for the excess consumption of S(IV) (k• OH = 4.5 × 109 M−1 s−1 and kSO• − > 2.0 × 109 M−1 s−1 ) and the inhibitive reduction of Cr(VI). 4

When various tested polycarboxylates were added into Cr(VI)/S(IV) system, polycarboxylates scavenging ROS may partially prevent the excess consumption of S(IV). However, Cr(VI) reduction and ROS productions appeared to be diversely affected by adding different polycarboxylates. Therefore, the resolution of the overall reaction pathways requires a thorough consideration of interactions among Cr(VI), S(IV) and various polycarboxylates. CrSO6 2− + polycarboxylate ↔ polycarboxylate-CrSO6 polycarboxylate-CrSO6 ↔ polycarboxylate-Cr(V) + SO3 polycarboxylate-Cr(V) ↔ organic products + Cr(III)

(15) •−

(16) (17)

polycarboxylate-Cr(V) + S(IV) ↔ polycarboxylate + Cr(III) + S(VI) 3.5. Mechanism discussion According to previous study [27], Cr(VI) reduction induced by sulfite (S(IV)) was initiated with the formation of CrSO6 2− via the condensation of Cr(VI) with S(IV) (Reaction (4)) in Scheme 1. Further condensations of CrSO6 2− with another S(IV) or Cr(VI) can lead to the formations of activated complexes, i.e., [CrO2 (SO3 )2 ]2− and [O3 CrOCrO3 SO3 ]2− (Reactions (5) and (7)). These two activated complexes can easily undergo the spontaneous disproportionation reactions leading to the reduction of Cr(VI) to Cr(III) and Cr(V) in Reactions (6) and (8), respectively. The formed Cr(V) can be subsequently reduced to Cr(III) by reductive species in Reactions (10) and (11). These two fates of CrSO6 2− in Reactions (5) and (7) account for the variation in theoretically stoichiometric ratio of

(18) Although some polycarboxylates (e.g., Ox and Cit) are effective organic ligands for chromium species, they cannot directly lead to the reduction of Cr(VI) unless S(IV) coexists in above reaction system, as shown in Fig. 1(a) [40,41]. Considering the enhanced formation of SO3 •− in Fig. 3(a) and the complexing constants of S(IV) and polycarboxylates with Cr(VI), we can infer that polycarboxylates probably form complexes with CrSO6 2− (see Reaction (15)), which thereby interrupts the Reaction (7) [42]. The affinity of polycarboxylates to CrSO6 2− can expand the coordination of Cr(VI) species from tetrahedron to hexahedron, which is favorable for the formation of octahedral Cr(III) species [43–45]. Thus, a

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Fig. 7. Relationships between the [S(IV)]consumption /[Cr(VI)]reduction and EHOMO of the tested polycarboxylates ([Cr(VI)]0 = 50 ␮M, pHini 3.5, [polycarboxylates]0 = 500 ␮M, [S(IV)]0 = 300 ␮M, [ethanol]0 = 100 mM).

Scheme 1. Proposed pathways for the reduction of Cr(VI) in the Cr(VI)/sulfite system mediated by polycarboxylates.

rate determining step in Reaction (5) and inhibited SO3 •− production step in Reaction (6) were replaced by Reactions (15) and (16), which can be partially validated by the formation Cr(V)–Cit complex in Fig. S7 [14]. Accordingly, the rate of Cr(VI) reduction was evidently accelerated by the presence of polycarboxylates in the order of tartaric acid ≈ malic acid ≈ citric acid < Ox as shown in Fig. S6. This acceleration is probably dependent on the complex ability of polycarboxylates with Cr(VI). Thus, the fastest Cr(VI) reduction in Cr(VI)/S(IV)/Ox reaction system can be ascribed to the strongest complexing ability of Ox among these tested polycarboxylates. Based on the values of [S(IV)]consumption /[Cr(VI)]reduction in 100 mM ethanol containing reaction systems in Table 1, when forming complex with chromium species, the added polycarboxylates probably also act as electron donors, instead of S(IV), for Cr(VI) reduction via intramolecular electron transfer reaction (Reaction (17)). Besides, S(IV) can also simultaneously reduce Cr(V) species in Reaction (18). The priority of Reactions (17) and (18) is greatly dependent on the values of EHOMO , which can reasonably account for the variant values of [S(IV)]consumption /[Cr(VI)]reduction in various Cr(VI)/S(IV)/polycarboxylate reaction systems. However, in spite of owning relatively high EHOMO values, other organics (i.e., malonic acid, acetic acid and ethanol) exhibit insignificant complex ability for Cr(VI), leading to negligible influence on Cr(VI) reduction [46].

As shown in Table 2, Ox owns the least value of EHOMO among these tested polycarboxylates. Therefore, in Cr(VI)/S(IV)/Ox reaction system, Reaction (18) was dominant over Reaction (17), which thereby consumed much more S(IV) in comparison with other reaction systems. According to Fig. 3(a), the presence of Ox enhanced the production of SO3 •− , which resulted in the enhancement of ROS (see Fig. 5). Meanwhile, relatively large consumption of S(IV) for Cr(VI) reduction may relieve the decay of SO4 •− and OH• induced by S(IV), leading to the highlighted capacity for other pollutants oxidation (e.g., As(III) and RhB) in Fig. 6. However, as for other polycarboxylates, such as tartaric acid, malic acid and citric acid, Reaction (17) is dominant due to their preferable electron donating capacity for Cr(VI) reduction, in which the consumption of S(IV) was economized. Thus, in spite of improving SO3 •− formation, a large number of highly reductive reagent S(IV) residual in above reaction systems retarded the oxidation efficiency of other inorganic/organic pollutant in Fig. 6 and Fig. S8 via scavenging the produced SO5 •− in Reaction (19) and ROS (k• OH = 4.5 × 109 M−1 s−1 and kSO• − > 2.0 × 109 M−1 s−1 ) [42]. 4

HSO3

2−

+ O3 SOO

•−

→ SO3

•−

+ HSO5 2−

k = 3.0 × 107 M−1 s−1 (19)

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3.6. Environmental importance

References

Although plenty of new approaches have been developed for Cr(VI) detoxification, S(IV) reducing Cr(VI) process is still of significant interest for engineered systems concerned with the removal of Cr(VI) from wastewaters. In this process, the working pH for Cr(VI) reduction is recommended at highly acidic condition (usually pH 2–3) and then alkali is necessary for chromium precipitation in subsequent stage [5]. Furthermore, as revealed in this study, excess sulfite (ca. $650/t) overranging the stoichiometric ratio is required for Cr(VI) reduction due to the production of ROS. Thus, the demand of massive chemical reagent would limit its environmental compatibility. This study reported the beneficial influence of polycarboxylates on Cr(VI) reduction in slightly acidic solution and less requirement of sulfite. Polycarboxylates, especially oxalic acid, are the most notorious compounds in the aquatic environment due to its high production from a variety of industries such as textile, pulp and paper, metal treatment, and wood bleaching [47]. Thus, it is economical and practical that introducing polycarboxylatesbearing industrial wastewater to Cr(VI) contaminated discharges can accelerate Cr(VI) detoxification at a broad pH range with less consumption of S(IV). At the same time, polycarboxylates or other pollutants in industrial wastewaters can be oxidized without the external energy or oxidants (e.g., H2 O2 and O3 ) addition. Moreover, the proposed mechanism of S(IV) reducing Cr(VI) involving ROS and intramolecular electron transfer reaction contributes significantly to the knowledge of transport and fate of chromium in natural environments.

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4. Conclusion With considering the widespread presence of polycarboxylates in industrial processes and aquatic environments, the present work investigated the roles of polycarboxylates in the process of sulfite reducing Cr(VI). It was reported that the presence of polycarboxylates such as oxalic, tartaric, citric and malic acids can greatly accelerate the reduction rate of Cr(VI) with less values of [S(IV)]consumption /[Cr(VI)]reduction in comparison with the control experiment. The fastest rate of Cr(VI) reduction and largest yield of ROS can be achieved with the presence of oxalic acid. The added polycarboxylates can act as electron donors for Cr(VI) reduction resulting in the decrease of [S(IV)]consumption /[Cr(VI)]reduction , which is significantly dependent on the energies of the highest occupied molecular orbital and chemical structures of polycarboxylates. Overall, these results throws an enlightenment to understand the roles of polycarboxylates in Cr(VI) reduction induced by sulfite, and introducing of polycarboxylates into Cr(VI)/S(IV) reaction system was proposed to be a cost-effective and green strategy in wastewater treatment.

Acknowledgements This work is financially supported by National Natural Science Foundation of China (Nos. 21376268, 21176260, 41273108, 51372277, 21302224), the Taishan Scholar Foundation (No. ts20130929) and DHU Distinguished Young Professor Program.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2015.11. 011.

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