Manganese(II)-Catalyzed and Clay-Minerals-Mediated Reduction of

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Manganese(II)-Catalyzed and Clay-Minerals-Mediated Reduction of Chromium(VI) by Citrate Binoy Sarkar, Ravi Naidu,* Gummuluru SR Krishnamurti,† and Mallavarapu Megharaj CERARCentre for Environmental Risk Assessment and Remediation, Building X, University of South Australia, University Boulevard, Mawson Lakes, SA 5095, Australia CRC CARECooperative Research Centre for Contamination Assessment and Remediation of the Environment, P.O. Box 486, Salisbury, SA 5106, Australia S Supporting Information *

ABSTRACT: Unlike lower valent iron (Fe), the potential role of lower valent manganese (Mn) in the reduction of hexavalent chromium (Cr(VI)) in soil is poorly documented. In this study, we report that citrate along with Mn(II) and clay minerals (montmorillonite and kaolinite) reduce Cr(VI) both in aqueous phase and in the presence of dissolved organic carbon (SDOC) extracted from a forest soil. The reduction was favorable at acidic pH (up to pH 5) and followed the pseudo-first-order kinetic model. The citrate (10 mM) + Mn(II) (182.02 μM) + clay minerals (3% w/v) system in SDOC accounted for complete reduction of Cr(VI) (192.32 μM) in about 72 h at pH 4.9. In this system, citrate was the reductant, Mn(II) was a catalyst, and the clay minerals acted as an accelerator for both the reductant and catalyst. The clay minerals also serve as a sink for Cr(III). This study reveals the underlying mechanism of the Mn(II)induced reduction of Cr(VI) by organic ligand in the presence of clay minerals under certain environmental conditions.

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(Fe(II)).5−9 The Cr(VI) reduction process by naturally occurring organic compounds is usually very slow. However, the synergistic activities of Fe(II), organic molecules, and suitable clay mineral surfaces can accelerate the reduction reaction in the soil environment.5,6,9 Low molecular weight organic ligands such as citrate, oxalate, salicylate, and tartrate are abundant in the natural soil and water environment as a result of microbial decomposition of organic matter and plant root exudates. These organic ligands are very effective in reducing Cr(VI) into Cr(III).6,10 Other organic ligands that are effective in Cr(VI) reduction include ascorbic acid,11 lactic acid,12 and bacterial exopolymeric substances.13 Although a number of studies report reduction of Cr(VI) in systems using divalent iron5,6,9,10,14 or zerovalent iron15−18 as the catalyst and/or reductant, only a few reports are available using other metals with variable oxidizing states. For example, Li et al.19 reported catalytic action of Mn(II) on Cr(VI) reduction using citrate, which was attributed to the formation of Mn(II)-complexes. However, the concentration of citrate in their experiment was excessive when compared to Cr(VI).19 In another study, Kabir-ud-Din et al.20 reported the acceleration of Cr(VI) reduction by oxalic acid in the presence of Mn(II).

INTRODUCTION Chromium (Cr), the 21st most abundant element in the earth’s crust, has widespread industrial and commercial applications. Many industrial activities such as electroplating and the manufacturing of dyes, leather tanning, and manufacturing of alloy and steel release chromium-containing effluents into the environment. Depending on the redox conditions and pH, Cr most frequently exists in two major states in the environment, namely hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)). While Cr(III) is essential in trace quantities for sugar and lipid metabolism in humans, Cr(VI) is extremely toxic when present even at very low concentrations. It is a known carcinogen and mutagen and can readily convert into forms that can adduct with genetic material of the cell causing permanent damage and cancer.1 Being an anion, Cr(VI) is highly mobile through the soil profile (pH > 6) and thus poses a serious threat to groundwater quality. For these reasons, Cr(VI) is considered to be very harmful to the environment and attracts serious attention. The most common approach to remediate Cr(VI) is through its reduction into chemically stable and relatively nontoxic Cr(III), followed by precipitation or adsorption of the cationic species.2 Other popular technologies include adsorption, extraction, and membrane separation.3,4 However, many of these remediation methods are expensive. In natural systems, some organic molecules are known to reduce Cr(VI) in the presence of electron acceptors such as zero or divalent iron © 2013 American Chemical Society

Received: Revised: Accepted: Published: 13629

April 11, 2012 October 29, 2013 November 6, 2013 November 7, 2013 dx.doi.org/10.1021/es401568k | Environ. Sci. Technol. 2013, 47, 13629−13636

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an inductively coupled plasma mass spectroscopy (ICP-MS) analysis. The soil extract was stored in a cold room at 4 °C for subsequent use. Kinetics of Cr(VI) Reduction. The Cr(VI) reduction was carried out in a 40 mL reaction mixture placed in a 50 mL glass beaker. The possibility of photoreduction of Cr(VI) was eliminated by wrapping the reaction beakers with aluminum foil during the experiment. The reaction mixtures containing Cr(VI), Mn(II), citrate, and clay minerals were magnetically stirred over a predetermined time period. The clay mineral concentration was set at 3 g L−1. In a typical experiment, 0.12 g of clay mineral was suspended in 40 mL of either water or SDOC, and the required volumes of Mn(II) and citrate stock solutions were added to create a predetermined concentration of Mn(II) and citrate in the reaction mixture. Then, the pH of the reaction mixture, which ranged from 7.0 to 8.0, was adjusted to a predetermined value using 2 N HCl. Finally, the required volume of Cr(VI) stock solution was added, and this resulted in an initial concentration of 192 μM Cr(VI) in the reaction mixture. Keeping other conditions the same, the control experiments constituted treatments in the absence of citrate, Mn(II), and clay minerals as appropriate to specific reactions. All the reaction treatments were carried out in triplicate. At various time intervals, 1 mL of solution was pipetted from the reaction mixture and centrifuged at 10 000 rpm for 5 min (Biofuge pico, Hevaeus, Kendo Laboratory Products, Germany) to obtain clay-free supernatant. The Cr(VI) concentration in the supernatant was determined spectrophotometrically (dilution was made before analysis if required). At every stage of sampling, the pH of the reaction mixture was recorded using a pH meter (Orion 3, Thermo Electron Corporation, U.S.A.). X-ray Photoelectron Spectroscopy (XPS) Analysis. The oxidation states of Cr and Mn deposited on the mineral surface after the reduction reaction were confirmed by XPS analysis. The metal laden montmorillonite sediment collected after a reduction reaction conducted at pH 3 was washed thrice with water. The supernatant was discarded following centrifugation. Prior to XPS mounting, the sample was freeze-dried in a vacuum freeze-drier for 24 h. The XPS analysis was performed on a Kratos AXIS Ultra DLD with a monochromatic Al X-ray source at 225 W with a characteristic energy of 1486.6 eV. The XPS analysis conditions are described in the Supporting Information. Analytical Method. Hexavalent chromium (Cr(VI)) in the supernatant was estimated using spectrophotometry as suggested by Sethunathan et al.24 In a typical procedure, 2 mL of the aliquot (after proper dilution) was reacted with 2 mL of 1N H2SO4 and 0.8 mL of the color developing reagent (4.0 g of phthalic acid and 0.25 g of DPC in 100 mL of 95% ethanol). The color intensity was measured at 540 nm wavelength by an Agilent 8453 UV−vis spectrophotometer (Agilent Technologies, Japan). Total aqueous Cr and Mn in selected supernatant samples was determined by inductively coupled plasma−mass spectrometry (ICP-MS) (model 7500c, Agilent Technologies, Japan) after proper dilution.

The reduction of Cr(VI) by low valency cations is very much pH dependent. Buerge and Hug9 reported that the rate of the Cr(VI) reduction reaction by Fe(II) was minimal at a pH value as low as 4.0. The authors9 studied the reaction kinetics at environmentally relevant pH conditions (pH = 4.0 to 7.2) taking a micromolar concentration of Cr(VI) in the system and found that the reaction rate increased when pH decreased. These results are in agreement with several other reports.21,22 It can be assumed that the reduction reaction carried out by organic ligand in the presence of Mn(II) is also greatly influenced by the system pH and the presence of other materials having catalytically active surfaces, such as clay minerals. However, the kinetics of Cr(VI) reduction by organic ligands in the presence or absence of Mn(II) is not extensively reported, and the mechanism is not clearly known. In the present study, an attempt was made to determine the kinetics of Cr(VI) reduction by the Mn(II)−citrate system as catalyzed by clay minerals at pH values less than 5.0. The reduction reaction was conducted in water as well as in natural organic matter extracted from a forest soil. In contrast to most of the previously published reports where Cr(VI) reduction was conducted involving a low micromolar concentration of Cr(VI) in the system, all reactions in this study were done using a higher initial concentration of Cr(VI) (192 μM). This study provides insights about the process and its underlying mechanism by which the Mn(II)-catalyzed reduction of Cr(VI) by organic ligand takes place in the presence of clay minerals under certain environmental conditions. This might encourage a new potential strategy for Cr(VI) remediation in contaminated soil and wastewater.

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MATERIALS AND METHODS Materials. The two clay minerals used in this study were montmorillonite and kaolinite, which are abundant in Queensland, Australia. These clay minerals were further purified according to methods suggested by Jackson23 to obtain less than 2 μm size fractions. An energy dispersive x-ray spectroscopy (EDAX) analysis revealed that this size fraction of clay minerals contained a negligible amount of Fe, Cr, and Mn. All of the experiments were carried out in 2% HNO3 and Milli-Q (18.2 Ω) water-rinsed glassware. Chemicals. Potassium dichromate (K2Cr2O7), manganese sulfate (MnSO4.4H2O), trisodium citrate (Na3C6H5O7.4H20), hydrochloric acid (HCl), 1,5-diphenyl carbazide (DPC), phthalic acid, ethanol, and sulphuric acid (H2SO4) were of AR grade and purchased from Sigma-Aldrich (St. Louis, MO). The reagent stock solutions, 19.2 mM Cr(VI), 18.2 mM Mn2+, and 1000 mM citrate, were prepared in Milli-Q (18.2 Ω) water. Soil Extract. The soil used to extract dissolved organic carbon (SDOC) was collected from the organic horizon of a forest soil in the Adelaide Hills region in South Australia. The 1:1 soil−water suspension placed into a 50 mL polypropylene centrifuge tube was shaken for 24 h in an end-over-end shaker. The soil suspension was then centrifuged at 4000 rpm for 30 min (Multifuge 3 S-R, Hevaeus, Kendo Laboratory Products, Germany). Following centrifugation, the supernatant was further filtered through Whatman No. 1 filter paper to remove the floating plant residue materials. The DOC concentration in the soil extract (SDOC) was 71.30 mg L−1, as measured by an OI Analytical 1010 total organic carbon (TOC) analyzer (OI Corporation, College Station, TX). The contents of Fe, Cr, and Mn in aqua-regia digested SDOC was below detection limit in

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RESULTS AND DISCUSSION Use of Citrate for Cr(VI) Reduction. It is reported extensively that Cr(VI) could be reduced to less toxic Cr(III) by organic acids in combination with low valent metal species 13630

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or alone.6,10,19,20,25 Figure 1 shows the reduction of Cr(VI) by 1, 5, and 10 mM citrate in the presence of 182.02 μM Mn(II)

variation in pH was observed in all the systems. The pH of the reaction mixtures throughout 24 h duration were 2.28 ± 0.05 for the citrate + Mn + clay system, 2.26 ± 0.04 for the citrate + clay system, 2.25 ± 0.02 for the citrate + Mn system, and 2.26 ± 0.02 for the citrate alone system. The data clearly demonstrate synergistic effects of citrate, Mn(II), and clay mineral on the reduction of Cr(VI). In the absence of Mn(II) and clay, a small amount of Cr(VI) reduction (up to 12%) was observed, the maximum portion of which occurred instantaneously within the first 3 to 4 h of reaction. After that, the reaction rate was very slow. However, up to 52 and 84% reduction was achieved in combined systems including citrate + Mn(II) and citrate + clay mineral, respectively, in 24 h reaction duration. Although the initial reaction rate for the citrate + Mn(II) system was slightly higher than the citrate + clay mineral system, the latter system showed faster reaction rates 4 h onward. In the present study, the citrate + Mn(II) + clay mineral reaction mixture was observed as the most efficient reductant system which accounted for almost 100% reduction of Cr(VI) within just 6 h of reaction. Li et al.19 also found that Mn(II) combined with citrate could carry out the reduction of Cr(VI) much faster than citrate alone, and the reaction rate was enhanced with increasing Mn(II) concentration in the reaction system. Effect of Initial pH on Cr(VI) Reduction by Citrate in the Presence of Mn(II) and Clay Mineral. One of the primary objectives of the current study is to carry out Cr(VI) reduction at comparatively higher pH values relevant to what is frequently encountered in the terrestrial environment. It is well documented in the literature that Fe(II) /Fe(0) is capable of reducing Cr(VI) at pH ≤ 5.0.9,21,22,26 However, the pH dependence of Cr(VI) reduction by organic ligand catalyzed by Mn(II) is poorly documented. Figure 3 depicts the reduction of

Figure 1. Effect of citrate concentrations on the reduction of Cr(VI) in the presence of 182.02 μM Mn(II) and 3% (w/v) montmorillonite at initial pH = 2.0; initial Cr(VI) concentration (Ci) 192.32 μM; Ct is the Cr(VI) concentration at time t; data represent mean of three replicates.

and 3% (w/v) montmorillonite. The reaction mixtures had an initial pH of approximately 7.5. The preliminary experiment revealed that at pH > 7.5, the reduction of Cr(VI) was negligible (data not presented) because the higher oxidation state species of Cr and Mn predominated at higher pH values. However, following adjustment of the initial pH to 2.0, the reduction of Cr(VI) was faster with 10 mM citrate (100% reduction in 6 h) compared to 5 mM citrate (90% reduction in 24 h) and 1 mM citrate (38% reduction in 24 h). A negligible amount of Cr(VI) (only around 3%) was reduced in the absence of citrate at the end of 24 h (Figure 1). No buffer was used in the present experiment. However, there was only a little variation in the pH values (2.16 ± 0.06) throughout the experiment. All subsequent sets of experiments were conducted at a 10 mM citrate concentration because it provided 100% reduction of Cr(VI) in just 6 h duration. Effect of Mn(II) and Clay Minerals on Cr(VI) Reduction. Figure 2 shows the reduction of Cr(VI) by 10 mM citrate in the presence or absence of 182.02 μM Mn(II) and 3% (w/v) montmorillonite at initial pH of 2.25. The experiment was conducted in an unbuffered system with an initial Cr(VI) concentration (Ci) of 192.32 μM. However, no significant

Figure 3. Effect of initial pH on the reduction of Cr(VI) by 10 mM citrate in the presence of 182.02 μM Mn(II) and 3% (w/v) montmorillonite; initial Cr(VI) concentration (Ci) 192.32 μM; Ct is the Cr(VI) concentration at time t; data represent mean of three replicates.

Cr(VI) by citrate (10 mM) in the presence of 182.02 μM Mn(II) and 3% (w/v) montmorillonite at initial pH values of 2.23, 2.92, 3.85, and 4.86. As illustrated by the slope of the graphs, the rate of reduction decreased with increasing initial pH. The reduction reaction was continued for 24 h. Approximately 95% reduction was observed after 6, 7, and 8 h of reaction at pH 2.23, 2.92, and 3.85, respectively. The reduction reaction had been the slowest when it was carried out at initial pH 4.85. During the 24 h reaction time, only about 80% reductions were achieved at this initial pH value. Lan et al.10 also reported that montmorillonite and illite greatly

Figure 2. Reduction of Cr(VI) by 10 mM citrate in the presence or absence of 182.02 μM Mn(II) and 3% (w/v) montmorillonite clay mineral at initial pH = 2.25; initial Cr(VI) concentration (Ci) 192.32 μM; Ct is the Cr(VI) concentration at time t; data represent mean of three replicates. 13631

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Figure 4. (a) Effect of clay mineral types (3% (w/v)) and (b) clay mineral concentrations (at pH 2.9) on the reduction of Cr(VI) in aqueous medium by 10 mM citrate in the presence of 182.02 μM Mn(II); initial Cr(VI) concentration (Ci) 192.32 μM; Ct is the Cr(VI) concentration at time t; data represent mean of three replicates.

accelerated the reduction of Cr(VI) at pH 4.0 and 4.5, but their effects were dramatically reduced at pH 5.0. In all cases, the pH was practically constant during the 24 h reaction period (Supporting Information). Similar to previous reports on the reduction of Cr(VI) by citrate in the presence of Fe,21,22 this study also showed pH dependence of this reduction reaction in presence of Mn. Effect of Different Clay Minerals on Cr(VI) Reduction by Citrate in the Presence of Mn(II). Two clay minerals, kaolinite and montmorillonite, were examined to study their effect on the reduction of Cr(VI) by citrate in the presence of Mn(II). The effect of different clay minerals on the reduction reaction was studied at pH 2.9 and 4.9. The reduction rates at both these pH levels were faster when montmorillonite was used in the reduction reaction (Figure 4a). At pH 4.9, the rate of reaction was significantly slower in comparison to that at pH 2.9. However, the montmorillonite-mediated reaction showed faster reduction of Cr(VI) as compared to the kaolinitemediated reaction at pH 4.9. Buerge and Hug5 reported greater acceleration of Cr(VI) reduction by montmorillonite than kaolinite, which was attributed to the high reactivity of adsorbed Fe(II) on the clay mineral surface. Montmorillonite could adsorb more molar fraction of Fe(II) in the reaction mixture, and hence, the result was a faster reduction of Cr(VI).5 Similar results were observed in the present study for reduction of Cr(VI) carried out by citrate in the presence of Mn(II) and clay minerals. The cation exchange capacity (CEC) of montmorillonite and kaolinite used here were 66.7 and 9.8 cmol (p+) kg−1, respectively, as measured by the ammonia electrode method.27 However, the ICP-MS analysis of total Mn in the solution revealed only around 5% adsorption to the montmorillonite surface, which indicated the existence of Mn− citrate complexes mostly in the solution. The same mineral surface rather adsorbed about 25% chromium as Cr(III) following the reduction of Cr(VI) (97%) at pH 3, which was further supported by the XPS analysis of the sediment (Figure 5). Lan et al.10 noted that the amount of Mn(II) adsorbed on the surface of illite, montmorillonite, and kaolinite had a positive correlation with the rate of Cr(VI) reduction at pH 4.0 and 4.5 in the presence of citric acid. The authors, however, reported no significant effect of the specific surface area of clay minerals in the reaction.10 Therefore, montmorillonite in our study might have demonstrated faster reduction than kaolinite by adsorbing more molar fraction of Cr(III) from the reaction mixture (the adsorption of Mn(II) is small; only 5% of the total Mn(II) added). The direct response of the reduction reaction

Figure 5. High-resolution XPS spectrum collected from the Cr 2p core regions of the montmorillonite sediment after Cr(VI) reduction experiment conducted at pH 3; Mn(II) concentration 182.02 μM; initial Cr(VI) concentration (Ci) 192.32 μM; montmorillonite application dose 3% (w/v); reaction time 24 h.

to the concentrations of the montmorillonite (Figure 4b) further illustrated the effect of clay minerals in enhancing the reaction rates. The montmorillonite clay mineral at 1% concentration resulted in 100% removal of Cr(VI) in 25 h, whereas it achieved the same in just 6 h at 3% concentration (Figure 4b). At 6% montmorillonite concentration, the duration for reaction completion was almost similar to that at 3% concentration, except that the initial rate of reaction was a bit faster at higher clay mineral concentration. X-ray Photoelectron Spectroscopy (XPS) Analysis. The oxidation states of Cr and Mn adsorbed on the montmorillonite surface following a reduction reaction conducted at pH 3 were confirmed by XPS analysis. Low concentration of Cr and Mn was detected in the solid phase at high-resolution scanning. The Cr 2p3/2 orbitals are assigned at 577.2 eV (CrCl3) and 576.2− 576.5 eV (Cr2O3) for Cr(III) compounds, while Cr(VI) forms are characterized by higher binding energies such as 578.1 eV (CrO3) or 579.2 eV (K2Cr2O7).28,29 The spectrum in Figure 5 confirms the presence of Cr on the montmorillonite surface only as Cr(III). Mn was also detected on the clay mineral surface at high-resolution scan. But the amount of Mn(II) on the montmorillonite surface (only 5% of the total Mn(II) was adsorbed) was not substantial. Also, the evidence for Mn(III) and Mn(IV) formation was weak due to very low concentration of Mn (Supporting Information). Reduction of Cr(VI) by Citrate in the Presence of Mn(II) and Clay Minerals in SDOC. In order to simulate Cr(VI) reduction by citrate in the presence of Mn(II) and clay minerals under natural terrestrial environmental conditions, experiments were carried out in SDOC instead of water. The 13632

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DOC content in the soil extract was measured to be 71.30 mg L−1, and it had a pH value of 7.01. In the first set of experiments, Cr(VI) reduction was carried out in SDOC without adjusting the pH (i.e., at pH 7.01). The experiment was also done in SDOC after adjusting the pH of the system to 4.9 and 4.0. The experiments were initially executed up to 25 h in the presence of montmorillonite. The Cr(VI) reduction patterns in SDOC in 25 h at all three pH values are shown in Figure 6. Similar to the results presented in the previous

Figure 7. Effect of clay minerals on the reduction of Cr(VI) in SDOC or SDOC with 10 mM citrate, in the presence of 182.02 μM Mn(II); initial Cr(VI) concentration (Ci) 192.32 μM; Ct is the Cr(VI) concentration at time t; clay minerals application dose 3% (w/v) kaolinite or montmorillonite; pH = 4.9; data represent mean of three replicates.

better than kaolinite in retaining the pH value of the reaction system closer to the starting pH (4.86) and thus exhibited slightly greater reaction rate initially (Figure 7). Both the clay minerals showed an almost steady reaction rate throughout the reaction period. In the absence of citrate (SDOC alone), the reduction occurred slowly (26 and 28% in 72 h for kaolinite and montmorillonite, respectively) due to the nature and lower contents of ligands present in SDOC (Figure 7). Kinetic Modeling of Cr(VI) Reduction Reaction. Many previous reports demonstrated that the reduction of Cr(VI) in aqueous solution in the presence or absence of a solid catalytic surface, such as clay minerals and metal ions, could be explained by a pseudo-first-order kinetic model.5,31,32 When fitted to various mechanistic models, it appeared in this study that the Cr(VI) reduction reactions under environmental conditions (in SDOC and at pH∼5.0) were best fit to the pseudo-first-order kinetic model (Supporting Information) with R2 > 0.990 and P < 0.0001. The reaction mixture containing montmorillonite in SDOC had a rate coefficient of 0.027 h−1 compared to kaolinite, which was 0.028 h−1. Complete reduction of Cr(VI) was observed within 72 h. Although the initial rate was greater in the case of montmorillonite than the kaolinite-mediated reaction, the overall reaction rate over the 72 h reaction period did not significantly vary between these two systems. Reaction Mechanism. The direct oxidation of Mn(II) by Cr(VI) is thermodynamically unfavorable.20 In the presence of citrate, however, Mn(II) formed a complex in the solution that facilitated Cr(VI) reduction (Figure 8). In Table 1, it is shown that the adsorption affinity (Kf value calculated from the Freundlich isotherm model) of Mn(II) on the montmorillonite surface in the presence of citrate was significantly higher at pH 5 than that at pH 2.1. The coexistence of citrate and Cr(III) in the system at pH 2.1 caused little increase in the Mn(II) Kf values (Table 1). Therefore, Mn(II) showed a preference to remain in the solution at pH 2.1, corresponding with a higher percentage of Cr(VI) reduction. It has been reported10 that the formation of Mn(II)−citrate complexes is a prerequisite for carrying out Cr(VI) reduction by citrate in the presence of Mn(II) and clay minerals. A mass balance calculation revealed that only around 5% of the total Mn(II) added in the system adsorbed to the montmorillonite surface despite the fact that the adsorption was thermodynamically favorable (n < 1) (Table

Figure 6. Reduction of Cr(VI) in SDOC by 10 mM citrate in the presence of 182.02 μM Mn(II); initial Cr(VI) concentration (Ci) 192.32 μM; Ct is the Cr(VI) concentration at time t; clay mineral application dose 3% (w/v) montmorillonite; data represent mean of three replicates.

sections, the reduction of Cr(VI) by citrate (10 mM) in the presence of Mn(II) and montmorillonite in SDOC was also highly pH dependent (Figure 6). The reaction was extremely slow at pH 7.01. The reduction of Cr(VI) by SDOC alone (without citrate) was also slow (Figure 7). Previous reports showed that complexation of Fe(II) with organic ligands present in SDOC enhanced the Cr(VI) reduction.6,9 In contrast, Agrawal et al.25 reported that Fe(II) in the presence of dissolved organic matter (DOM) reduced Cr(VI) even slower than the Fe(II)-only system due to the pHdependent formation of a highly reactive Fe(OH)2+ species; this species was a stronger reductant than Fe(II)−DOM complexes. Iron humate, a waste material produced during industrial manufacturing of humic substances, was also shown to reduce Cr(VI) effectively and facilitate the subsequent adsorption of Cr(III).30 However, the aqueous chemistry of Fe and Mn are different. Despite the fact that Fe(II) is a direct reductant of Cr(VI) while Mn(II) would appear as the catalyst of the reaction, further studies are required to determine the species of Mn and Mn(II)−DOM complexes formed in the course of Cr(VI) reaction in presence of SDOC and citrate. It should be noted that about only 54% reduction of Cr(VI) (from initial 192.32 μM to 88.03 μM) was obtained in SDOC in 25 h reaction duration at pH 4.9, whereas the system attained 99% reduction at pH 4.0. We conducted additional experiments to study the reduction of Cr(VI) in SDOC in the presence of Mn(II) and clay minerals over an extended period of time (72 h). The objective was to achieve complete reduction of Cr(VI) at pH∼5.0, which is closer to relevant soil environmental pH values. Two clay mineralskaolinite and montmorillonitewere tested. In the presence of citrate, 99.2% and 99.3% reduction of Cr(VI), respectively, was observed for kaolinite and montmorillonite in nearly 72 h (Figure 7). Montmorillonite performed slightly 13633

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Figure 8. Proposed reaction scheme for the conversion of Cr(VI) to Cr(III) by the Mn(II)−citrate complex.

Table 1. Freundlich Parameters for the Adsorption of Mn(II) and Cr(III) on Montmorillonite under Different Conditions metal Mn(II)

Cr(III)

a

Kf

adsorption conditions in in in in in in in in

water at pH 5.0 citrate at pH 5.0 citrate at pH 2.1 citrate and 192.32 μM initial Cr(VI) at pH 2.1 water at pH 5.0 citrate at pH 5.0 citrate at pH 2.1 citrate and 182.02 μM Mn(II) pH 2.1

44.9 39.6 22.7 25.5 39.1 39.1 42.3 40.3

± ± ± ± ± ± ± ±

R2a

N 2.14b 1.37 2.18 1.19 3.39 2.79 1.64 1.75

0.777 0.761 0.860 0.883 1.155 1.157 1.152 1.153

± ± ± ± ± ± ± ±

0.034 0.054 0.049 0.037 0.133 0.152 0.098 0.173

0.9948 0.9994 0.9869 0.9849 0.9971 0.9966 0.9937 0.9895

All R2 values are calculated at p < 0.001 bStandard error at 95% confidence level, n = 3.

out the reduction of Cr(VI). The Mn(II) adsorption and XPS results show that the reduction reaction can take place when the Mn(II)−citrate complex stays in the solution. In this system, citrate acts as the reductant (the reaction is extremely slow in the absence of citrate, Figure 7), Mn(II) acts as a catalyst (Mn(II) accelerates the reduction reaction, Figure 2), and the clay minerals act as an accelerator for the reductant (the citrate alone system is again very slow, Figure 2). The clay minerals also serve as a sink for Cr(III) (Figure 5). Whether Mn(II) simultaneously coordinated to both citrate and the clay surface could become a reductant of Cr(VI) warrants further investigation. Environmental Significance. Many researchers have reported the redox reactions between Mn and Cr in the soil environment.30,32 It is well-known that oxidized species of Mn such as Mn(III) and Mn(IV) cause oxidation of Cr(III) to the more toxic Cr(VI), which is unwanted from an environmental point of view. On the other hand, the Mn(II)-induced reduction of Cr(VI) by organic ligand in the presence of clay minerals described in this study is most relevant to specific conditions such as highly acidic pH (up to 5) and systems containing substantial aqueous Mn(II) and sufficient organic ligands such as citrate (about 5 to 10 mM). This study gives an understanding of such complex processes and their mechanisms which are important under certain environmental conditions.

1). Thus, the major contributor toward the reduction reaction appears to be the surface chemical reaction involving clay mineral via Cr(III) adsorption from the reaction mixture. The Kf values for Cr(III) adsorption to the montmorillonite were less affected by the pH and presence of citrate in the system than that of Mn(II) adsorption (Table 1). More importantly, the presence of Mn(II) in the solution also did not significantly affect Cr(III) adsorption (Table 1). A mass balance calculation found as high as 25% chromium adsorbed to the clay mineral surface as Cr(III) following reduction of Cr(VI) in the system. The XPS analyses of the sediments also support this result (Figure 5). The Mn(II)−citrate complex combining with Cr(VI) might have formed an ester-like species in a reversible reaction, and with decomposition of this ester (irreversible reaction), Cr(VI) was converted to Cr(III), as shown in Figure 8.10,20,34,35 From the reaction scheme in Figure 8, the rate equation can be derived (using the law of mass action)20,35 as: k = (k1kes1ka1[H+][A])/(1 + ka1[H+] + kes1ka1[H+] (eq 5)

or, 1/k = 1/k1 + 1/(k1kes1[A]) + 1/(k1kes1ka1[H+][A]) (eq 6)

Therefore, at a constant concentration of citrate−Mn(II) complex ([A] in eq 6), a decrease in pH value (i.e., an increase in [H+]) causes an increase in the reaction rate (k), as shown in the current study. Also, from a thermodynamic point of view, the lower valent species of Cr predominates at highly acidic pH.36 When the highly reactive α−OH in the ring structure is formed between the clay mineral surface, Mn(II) and citrate facilitates the complex formation with Cr(VI) and the subsequent redox reaction.10,19,34,37 However, the current study suggests that adsorption of the Mn−citrate complex to the clay mineral surface is not necessarily required for carrying

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ASSOCIATED CONTENT

S Supporting Information *

Variation of pH during the course of reduction reaction (Figure S1), XPS analysis conditions, high-resolution XPS spectrum of Mn 2p core regions (Figure S2), and pseudo-first order kinetic model of Cr(VI) reduction reaction (Figure S3) are given as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 13634

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Fax: +61-8-8302 3124. Tel.: +61-8-8302 5041. Present Address

† 313−855, West 16th Street, North Vancouver, BC V7P1R2, Canada

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial and infrastructural support from, firstly, the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE) and, secondly, the Centre for Environmental Risk Assessment and Remediation (CERAR), University of South Australia. The authors also acknowledge the help by Ms. Jeanette Tan with ICP-MS analysis at CERAR and the anonymous reviewers for their valuable comments to improve the quality of the manuscript.

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Environmental Science & Technology

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(37) Sun, J.; Mao, J. D.; Gong, H.; Lan, Y. Fe(III) photocatalytic reduction of Cr(VI) by low-molecular-weight organic acids with [alpha]-OH. J. Hazard. Mater. 2009, 168 (2−3), 1569−1574.

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