Kinetics and Mechanism of Photopromoted Oxidative Dissolution of

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Kinetics and Mechanism of Photopromoted Oxidative Dissolution of Antimony Trioxide Xingyun Hu, Linghao Kong, and Mengchang He* State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, P.R. China S Supporting Information *

ABSTRACT: Light (sunlight, ultraviolet, simulated sunlight) irradiation was used to initiate the dissolution of antimony trioxide (Sb2O3). Dissolution rate of Sb2O3 was accelerated and dissolved trivalent antimony (Sb(III)) was oxidized in the irradiation of light. The photopromoted oxidative dissolution mechanism of Sb2O3 was studied through experiments investigating the effects of pH, free radicals scavengers, dissolved oxygen removal and Sb2O3 dosage on the release rate of antimony from Sb2O3 under simulated sunlight irradiation. The key oxidative components were hydroxyl free radicals, photogenerated holes and superoxide free radicals; their contribution ratios were roughly estimated. In addition, a conceptual model of the photocatalytic oxidation dissolution of Sb2O3 was proposed. The overall pH-dependent dissolution rate of Sb2O3 and the oxidation of Sb(III) under light irradiation were expressed by r = 0.08·[OH−]0.63 and rox = 0.10·[OH−]0.79. The present study on the mechanism of the photooxidation dissolution of Sb2O3 could help clarify the geochemical cycle and fate of Sb in the environment.

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INTRODUCTION Antimony (Sb), like Pb, As, and Hg, is a toxic element of global concern and has been classified as a priority pollutant by the United States Environmental Protection Agency and the European Union.1,2Sb is a strong chalcophile elementand primarily occurs in nature as Sb2S3 (stibnite) and Sb2O3 (senarmontite, valentinite), which is a transformation product of stibnite.3 Most Sb is emitted as Sb2O3 into air, water and soil from manufactures, formulations, processing, coal combustion, refuse incineration, nonferrous metal production, road traffic,4,5 and from the use and disposal of Sb products (e.g., flame retardants and catalysts in PET-production, among others).6 The dissolution of stibnite (Sb2S3) and Sb2O3 and the release of Sb ions into the environment affect their mobility and bioavailability; in addition, the Sb ion can cause environmental and health risks. Under laboratory conditions, Sb2S3 and Sb2O3 are characterized by extremely low solubility in aqueous solutions: 5.2 × 10−7 mol·L−1 dissolved Sb2S3 concentration in pure water with 80 mmol·L−1 Sb2S3,7 0.812 and 1.54 mg·L−1 dissolved Sb concentration in reconstituted standard water (ISO 6341) with100 mg·L−1 Sb2O3 at pH 7.16 and 7.86.8 Recently, the kinetics of the mobilization of Sb from Sb2O3 under different pH levels have been studied.9−11 Some studies showed the importance of light on the oxidation of Sb(III) on goethite12 and in the presence of humic acid.13 Sb2O3 is a semiconducting metal oxide mineral,14 and it is possible for the dissolution to be affected by photochemical reactions induced by sunlight. Thus, we © 2014 American Chemical Society

hypothesized that under light irradiation, Sb2O3 can act as an oxidative photocatalyst on dissolved Sb(III), converting it to Sb(V) and promoting the dissolution reaction of Sb2O3.15,16 The purpose of the present study was to analyze how light affects the kinetics of dissolution of Sb2O3 and to clarify its mechanism of photoinduced dissolution. First, a preliminary experiment was conducted on the irradiation of solar light to verify the light-prompted oxidation of Sb(III), the dissolution of Sb2O3 and to determine a working optical wavelength range. Subsequently, the mechanism of photoprompted oxidative dissolution of Sb2O3 was investigated through experiments of pH, additive free radical scavengers, dissolved oxygen removal and dosage of Sb2O3 on dissolution of Sb2O3 under simulated sunlight.

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MATERIALS AND METHODS Chemicals. Antimony trioxide (Sb2O3) (99.999%) was obtained from Alfa Aesar (Johnson Matthey), which is characterized as valentinite by X-ray diffraction (XRD) (Supporting Information (SI) Figure S1). Its specific surface area is 0.873 m2·g−1 and the minimum and average particle size is 0.376 and 13.0 μm, respectively. Perchloric acid (G.R.), hydrochloric acid (G.R.) and isopropanol (G.R.), sodium Received: Revised: Accepted: Published: 14266

July 4, 2014 November 10, 2014 November 14, 2014 November 14, 2014 dx.doi.org/10.1021/es503245v | Environ. Sci. Technol. 2014, 48, 14266−14272

Environmental Science & Technology

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curves were always better than 0.9990. The relative standard deviation was 2%, which was obtained in 10 measurements performed each day. The limit of detection was 0.01 μg·L−1. Aliquots from the samples were diluted using 6 mol·L−1 hydrochloric acid to fit the working range. Electron Spin Resonance (ESR) Measurements. To verify the existence of hydroxyl free radical in solutions in the irradiation of light, the ESR measurements were performed on a Bruker EleXsys E500 EPR spectrometer (Germany) at room temperature (25 °C ± 1) using 50 μL capillary tubes. Typical instrument settings were: sweep width 100.0 G, power attenuation 13.0 dB, modulation amplitude of 2 G, sweep time 40 s. The ESR signals were recorded from the samples irradiated for 10 min at pH3.0, pH 6.6 and pH 9.0 using 500 W high-pressure mercury lamp. The sample at pH 9.0 in the darkness for 10 min was as a control experiment. Hydrogen Peroxide (H2O2) Measurement. Concentration of H2O2 was measured by an UV Power UV−vis spectrophotometer (LabTech, Tampa, FL) at 400 nm with the help of the yellow complex formed with potassium titanyl oxalate dihydrate (K2TiO(C2O4)2·2H2O) and H2O2 in glass cuvettes with a 1 cm path length. The method has a 0.1 mg·L−1 detection limit. Estimation of Dissolution Rate of Sb 2 O 3 and Oxidation Rate of Sb(III). The dissolution reaction of Sb2O3 in the absence of light may be formally expressed by eq 1:

carbonate (A.R.), sodium bicarbonate (A.R.), thiourea (G.R.), ascorbic acid (G.R.), potassium borohydride (G.R.), potassium hydroxide (G.R.), oxalic acid dihydrate (A.R.) and hydrogen peroxide (30%, G.R.) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All aqueous solutions were prepared using deionized water treated with a Milli-Q water purification device. Photo Reaction System. The experiments were conducted in a 500 mL beaker with 0.2 g·L−1 Sb2O3 powder and 250 mL of aqueous solution. A 500 W long-arc xenon lamp (35 × 1 cm) (Shanghai Jiguang Special Lighting Appliance Factory, China) with a lamp cover used to irradiate visible light (λmax = 400− 700 nm) was placed approximately 50 cm above the beaker. Because the lamp was calorigenic, the reactions require the use of cooling water to keep it at room temperature. Aqueous samples (0.5 mL) were taken and filtered through 0.22 μm cellulose acetate membrane filters (Membrana, Germany) for analysis at selected time intervals. The visible (λ ≥ 400 nm) and ultraviolet light (λmax = 365 nm) was emitted from a 500 W long-arc xenon lamp withan ultraviolet cut off filter and a 500 W high-pressure mercury lamp (Shanghai Jiguang Special Lighting Appliance Factory, China), respectively. Natural sunlight (9560 ± 100 Lux determined by a TES1330 lx meter) was also used for samples placed on the balcony of the laboratory equipped with cooling water. Each experiment was conducted in triplicate. To test the effect of pH and dissolved oxygen on the dissolution of Sb2O3, acidic feed (approximate pH 3.0, 4.4 and 6.0) and basic feed solutions (pH 9.0 and 10.0) were prepared using dilute perchloric acid and 5 mmol·L−1 carbonatebicarbonate buffers with ultrapure water, respectively; anoxic environment was created using a gas purging tube to exclude oxygen (O2) from the solutions using high purity nitrogen (N2, 99.99%) for approximately 5 h before the reaction and sustained insufflations in the irradiation of light. The pH of the solutions was measured at the beginning and at the end of the experiments. No drift in pH was observed within experimental error; thus, only the initial pH value was reported for each experiment. Analytical Methods. Characterization of Sb2O3. Solid phase Sb2O3 was analyzed using X-ray diffraction (XRD). The XRD patterns were recorded on a PANalyticalX’Pert Pro diffractometer (conditions: Cu Kλ radiation, 40 kV and 40 mA). Its specific surface area and particle size were determined using a Quantachrome Autosorb-iQ surface area measuring apparatus (Boynton Beach, FL) and Microtrac S3500 laser particle size analyzer (Montgomeryville, PA), respectively. The UV−vis absorption spectrum of Sb2O3 was determined by a SPECORD 200 spectrophotometer (Analytik Jena AG, Germany). Analysis of Sbtot and Sb(III). For the determination of total antimony (Sbtot) by hydride generation-atomic fluorescence spectrometry (Titan Instrument Co. Ltd., Beijing, China), 1% (m/V) thiourea and ascorbic acid were used to reduce Sb(V) in aqueous solutions to Sb(III); the conditions for the hydride generation system were as follows: carrier solution 5% (V/V) HCl; 2% (m/V) potassium borohydride and 0.5% (m/V) potassium hydroxide. The concentration of Sb(III) was measured after the addition of 10% oxalic acid was used to mask the Sb(V) signal with the same hydride generation conditions for Sbtot. The Sb(V) concentration was calculated as the difference between the concentration of Sbtot and Sb(III). The calibration curves were obtained from 0 to 50 μg·L−1 Sbtot. The correlation coefficients for the calibration

Sb2 O3(s) + 3H 2O(l) → 2Sb(OH)3 (aq)

(1)

After the dissolution step, the Sb(III) will undergo oxidation under the photoreaction system; thus, the overall dissolution reaction of Sb2O3 under irradiation can be expressed by eq 2: hν

Sb2 O3(s) + 3H 2O(1) → 2Sbtot (SbIII + SbV )(aq)

(2)

Actual rates of dissolution of Sb2O3(r) can be calculated from the rate of appearance of Sbtot in the solution.11 r=−

d(Sb2 O3) d[Sbtot ] = dt 2dt

(3)

In addition, we also monitored the increase in the Sb(V) concentration over time; thus, the oxidation rate equation of Sb(III) (rox) for the dissolution reaction alone was derived from our results. rox = −

d{Sb(III)}oxidation d[Sb(V)] = dt dt

(4)

−2

The release amount (mg·m ) of Sbtot and Sb(V) in all experiments were normalized with respect to specific surface area of Sb2O3.

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RESULTS AND DISCUSSION Dissolution of Sb2O3 under Sunlight Irradiation. To determine the effect of sunlight irradiation on the dissolution of Sb2O3, a preliminary experiment was conducted under natural sunlight, and the control experiment was performedin the dark for 60 min. The results showed that natural sunlight irradiation accelerated the release of total Sb from Sb2O3 and induced the oxidation of Sb(III) to Sb(V) (Figure 1). However, almost no Sb(V) was found in the aqueous solution in the dark. The results supported our hypothesis. The wavelength range that induced the oxidation of Sb(III) and the mechanism by which 14267

dx.doi.org/10.1021/es503245v | Environ. Sci. Technol. 2014, 48, 14266−14272


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= 0.992) was approximately 20 and 25 times higher compared to the irradiation of simulated sunlight (rox = 6.3 × 10−2 mg· m−2·min−1, R2 = 0.929) and visible light (rox = 5.2 × 10−2 mg· m−2·min−1, R2 = 0.894), respectively. Almost no Sb(V) in the aqueous solution was found in the dark. These results indicated that ultraviolet light was the primary working wavelength range that initiated the dissolution of Sb2O3 and oxidation of Sb(III), which was determined by the absorption spectrum of Sb2O3 (SI Figure S2). Sb2O3 exhibited better absorption at the ultraviolet region (250−400 nm) compared to the visible light region (400−700 nm). The results further confirmed that the oxidation of Sb(III) to Sb(V) accelerated the dissolution of Sb2O3 with light irradiation. Mechanism of Oxidation Dissolution of Sb2O3 under Simulated Sunlight. Hypothetical Mechanism of Photocatalytic Oxidation Dissolution of Sb 2O3. Combining preliminary experimental results with the principle of heterogeneous photocatalysis of semiconductors,14,17 a postulated photocatalysis oxidation dissolution mechanism was proposed as follows: First, a fraction of Sb2O3 was dissolved in aqueous solution releasing Sb(III) (eq 1). Simultaneously, Sb2O3 received light of sufficient energy irradiation (hv > Eg), and an electron (ecb−) from its valence band jumped to the conduction band with the concomitant generation of a hole (hvb+) in the valence band (eq 5).

Figure 1. Dissolution of Sb2O3 in the absence/presence of sunlight irradiation in super pure water.

Sb(III) was oxidized to Sb(V) under light irradiation are discussed in the subsequent sections. Determination of Working Wavelength of Light Region. The dissolution rate of Sb2O3 based on the linear fit of the data from Figure 2a under ultraviolet light irradiation (r

hν

Sb2 O3 → Sb2 O3(ecb− + h vb+)

(5)

Second, hvb+ at the valence bands can take part in two types of reactions.14 In the first type of reaction, hvb+ can be captured on the catalyst (Sb2O3) surface undergoing a charge transfer with adsorbed water molecules (H2O) or with surface-bound hydroxide species (OH−) to generate active hydroxyl free radicals (·OH) as shown in eqs 6 and 7: Sb2 O3(h vb+) + H 2O → Sb2 O3 + ·OH + H+

(6)

Sb2 O3(h vb+) + OH− → Sb2 O3 + ·OH

(7)

As strong oxidizer, ·OH can oxidize Sb(III) to Sb(V) (eq 8): Sb(OH)30 + 2 ·OH + H 2O → Sb(OH)6− + H+

(8)

Conversely, if the redox potential of the aqueous Sb(V)/Sb(III) is higher in energy than the valence band edge, electrons will be directly transferred from the aqueous Sb(III) to fill the holes in the valence band, thereby oxidizing the aqueous Sb(III) (eq 9). Sb(OH)30 + 2Sb2O3(h vb+) + 3H 2O → Sb(OH)6− + 2Sb2 O3 + 3H+

Third, in aerated aqueous suspensions, dissolved SbIII (OH)3 will be oxidize by dissolved oxygen (eq 10):

Figure 2. Effect of different light wavelength regions on the dissolution of Sb2O3 (a) and the oxidation of Sb(III) (b) in super pure water. −2

−1

(9)

Sb(OH)30 + 1/2O2 + 2H 2O → Sb(OH)6− + H+

= 0.74 mg·m ·min , R = 0.998) was approximately 6, 10, and 26 times higher compared to the irradiation of simulated sunlight (r = 0.13 mg·m−2·min−1, R2 = 0.984), in visible light (r =7.4 × 10−2 mg·m−2·min−1, R2 = 0.973) and in the absence of light (r = 2.9 × 10−2mg·m−2·min−1, R2 = 0.808). The oxidation rate of Sb(III) based on the linear fit to the data from Figure 2b under ultraviolet light irradiation (rox = 1.32 mg·m−2·min−1, R2 2

(10)

In addition, the photogenerated electrons may promote the reduction of dissolved oxygen (O2), producing superoxide free radicals (·O2−) (eq 11),18−21 which oxidize Sb(III) to Sb(V) (eq 12): Sb2 O3(ecb−) + O2 → Sb2 O3 + ·O2− 14268

(11)



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Figure 3. PH-dependent dissolution of Sb2O3 under the simulated sunlight irradiation: (a) release rate of total Sb; (b) oxidation rate of Sb(III); (c) Electron spin resonance signals intensity for hydroxyl free radical under different conditions; (d) Logarithm of rate vs logarithm of hydroxide ion concentration.

OH increased with the increase of pH in the photosystem. The ESR result verified the inference (Figure 3c). Thus, more Sb released from the dissolution of Sb2O3 due to the oxidation of Sb(III) at higher pH conditions. Therefore, pH-dependent dissolution rate of Sb2O3 and oxidation of Sb(III) under light irradiation is dependent on the content of ·OH formed on the Sb2O3 surface and is ultimately dependent on the OH− content.

Sb(OH)30 + 2·O2− + 3H 2O + H+ → Sb(OH)6− + 2H 2O2

(12)

Furthermore, the superoxide (·O2−) undergo a protonationreduction- protonation sequence, generating hydrogen peroxide (H2O2), which can be decomposed on the photocatalyst surface to form hydroxyl radical (·OH).18−21 The corresponding eqs 13-16 are as follows:

d(Sb2 O3(S))

d[Sb(aq)]

Protonation of superoxide: ·O2− + H+ → HOO·

(13)

Reduction: HOO·+ecb− → HOO−

(14)

Protonation: HOO− + H+ → H 2O2

(15)

Decomposition of H2O2 H 2O2 : + hv → 2 ·OH

(16)

(18)

The photo-oxidants described above may be in the form of · OH,hvb+, O2, ·O2− or H2O2. Subsequently, each possibility could be verified through effects of pH, free radicals scavengers, dissolved oxygen removal, H2O2 and dosage of Sb2O3 on dissolution rate of Sb2O3 and oxidation rates of Sb(III) under simulated sunlight irradiation. Evaluation of Various Photo-Oxidants. (1). Hydroxyl Radicals. The results of the Sb2O3 dissolution and Sb(III) oxidation at different pH levels are shown in Figure 3a and 3b. In darkness, the pH dependence is not clearly visible and all release amounts of Sbtot and Sb(V) are less than those under light irradiation. However, the dissolution rate of Sb2O3 (Figure 3a) and oxidation rate of Sb(III) (Figure 3b) increased with the increase of pH under light irradiation. According to the generated pathways of ·OH (eq 6 and 7), the concentration of ·

Equations 17 and 18 describe a pseudozero-order dissolution reaction, where ∝ represents a positive correlation, and rox are in agreement with the linear Sbtot and Sb(V) release rates. Based on linear fit to the data from Figures 3a and 3b, values for r were assigned as 5.7 × 10−2, 8.6 × 10−2, 1.3 × 10−1, 1.8 × 10−1 and 2.5 × 10−1 mg·m−2·min−1 and for rox: 4.6 × 10−2, 5.7 × 10−2, 6.3 × 10−2, 1.9 × 10−1 and 2.6 × 10−1 mg·m−2·min−1; these values were assigned to pH3.0, pH 4.4, pH 6.0, pH 9.0, and pH 10.0, respectively. Higher Sb2O3 dissolution rates and Sb(III) oxidation rate resulted at higher pH levels; to model the rates dependent on [OH−], the total dissolution rate and oxidation rate could be expressed by eqs 19 and 20:

r=−

dt

=

2dt

∝ [·OH] ∝ [OH−] (17)

rox = −

14269

d{Sb(III)} d[Sb(V)] = ∝ [·OH] ∝ [OH−] dt dt

r = k·[OH−]m

(19)

rox = k′·[OH−]n

(20)



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where k and k′ are rate constants for the Sb2O3 dissolution rate and oxidation of Sb(III), respectively; m and n are exponent with an expected value 0−1. Fitting eq 13 and 14 to the rate data (Figure 3d), the values of k and k′ were equal to 0.08 and 0.10 min−1 with m and n equal to 0.63 and 0.79 in mg·m−2· min−1. Thus, the two rates are given by eqs 21 and 22: r = 0.08 ·[OH−]0.63

(21)

rox = 0.10 ·[OH−]0.79

(22)

Table 1. Oxidation Rate of Sb(III) and Contribution Ratio of Photo-Oxidants oxidation Rate (mg·m‑2·min−1)

Thus, when pH > 5, the oxidation rate of Sb(III) was only slightly larger than the total dissolution rate of Sb2O3. However, the oxidation rate of Sb(III) controlled the total dissolution rate of Sb2O3. To further verify the importance of ·OH radicals, experiments at pH3.0, 6.6, and 9.0 in the presence and absence of the ·OH scavenger isopropyl alcohol (IPA) (0.5 mol·L−1) were performed; at this concentration, IPA can scavenge 99% of the · OH radicals.22,23 Almost no effect was observed for the dissolution of Sb2O3 in the dark. The results are shown in SI Figure S3. The presence of a scavenger slowed the oxidation rate of Sb(III) by rox = 1.9 × 10−1, 8.6 × 10−2 and 4.6 × 10−2 mg·m−2·min−1 at pH 9.0, 6.6 and 3.0, respectively, compared to rox =1.2 × 10−1, 5.2 × 10−2 and 2.9 × 10−2 mg·m−2·min−1 when ·OH were scavenged. These results also suggested that ·OH participation played an important role in Sb(III) oxidation. (2). Oxygen and Superoxide Free Radical. In aerobic and darkness conditions, no Sb(V) was found in the aqueous solution, which indicated that dissolved oxygen did not participate in the oxidation of Sb(III) (Figure 3b). To further verify the effect of superoxide free radical, experiments were performed under anoxic conditions in the absence and presence of IPA at pH 3.0, 6.6, and 9.0 (Figure 4). The oxidation rates at

contribution Ratio (%)

pH

aerobic

aerobic + IPA

anoxic

anoxic + IPA

hvb+

·OH

·O2−

pH 3.0 pH 6.0 pH 9.0

0.046

0.029

0.026

0.018

39.1

17.4

43.5

0.086

0.052

0.060

0.023

26.74

43.03

30.23

0.19

0.12

0.17

0.069

36.3

53.2

10.5

(3). Hydrogen Peroxide. Previous studies suggested that Sb(III) can be oxidized by H2O2 with high concentrations (50− 500 μmol·L−1) under alkaline pH values.25 According to abovementioned assumption, H2O2 generated in the irradiation of light through protonation-reduction-protonation sequence of the superoxide (·O2−) (eq 12 and 15). But in our study, no H2O2 was detected (detection limit 6.0 and exceed −1.82 V when the pH< 6.0.27 That is, the redox potentials of Sb(V)/Sb(III) were higher than the valence band edge at pH 9.0. Therefore, according to the photoinduced electron transfer at the semiconductor interface,14 electrons can directly transfer from the aqueous Sb(III) to fill the hole in the valence band, that is, hvb+ can oxidize Sb(III) directly. To further prove the direct oxidation of Sb(III) by hvb+, the dissolution of different dosages of Sb2O3 (0.2, 0.4, and 1.0 g· L−1) at pH 9.0 was conducted in the dark and with light irradiation. The dissolution of Sb2O3 was independent of its dosage in the dark but increased with dosages of Sb2O3 under light irradiation (Figure 5a) because more holes were generated in the higher dosage of Sb2O3. These holes took part in two reactions: indirect oxidation of Sb(III) through ·OH formation and direct oxidation of Sb(III). However, the dissolution rate of Sb2O3 still increased with the Sb2O3 dosage in the presence of scavengers (Figure 5b). This also proved that hvb+ can oxidize Sb(III) directly. Conceptual Model of the Photocatalytic Oxidation Dissolution of Sb2O3. According to the above results, a postulated photocatalysis oxidation mechanism for promoting Sb2O3 dissolution was proposed in Figure 6. The first step was the dissolution of Sb2O3:

Figure 4. Effect of dissolved oxygen on the oxidation rate of Sb(III) under the simulated sunlight irradiation.

different conditions listed in Table 1. When under the aerobic and IPA condition, ·O2− and hvb+ acted as oxidants, while under anoxic and IPA condition, only hvb+ acted as oxidant. Thus, according to difference values of oxidation rates (0.011, 0.029, and 0.051 at pH 3.0, 6.0 and 9.0, respectively) between “aerobic and IPA” and “anoxic and IPA”, we found that photosuperoxide free radicals (·O2−) played a role in the oxidation of Sb(III). Previous study had also indicated that ·O2− is directly involved in the photo-oxidation process.24

Sb2 O3(s) + 3H 2O(l) → 2Sb(OH)3 (aq)

Simultaneously, Sb2O3 was excited by the light (black lines): 14270



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HOO·+ecb− → HOO−

HOO− + H+ → H 2O2

H2O2 can be decomposed instantaneously on the photocatalyst surface to form ·OH. H 2O2 + h v → 2·OH

The hvb+ itself was another important photo-oxidizer. Finally, all of the above photo-oxidants (·OH, ·O2− and hvb+) participated the oxidation of dissolved Sb(III). We also wanted to investigate the participation ratio of these photo-oxidants. In the photoreaction system, the concentration of Sb(III) increased with time; thus, an accurate participation ratio could not be obtained. Herein, we based on the measured oxidation rate (rox) data under each condition to roughly estimate the participation ratio of each photo-oxidant. According to oxidation rates under different conditions (Table 1), we estimated corresponding contribution ratio (Table 1). According to above calculation, it is easy to notice the following trends: (A) The contribution ratio for hvb+ is not significantly affected by pH (36.3% vs 26.74% vs 39.1%). (B) The contribution ratio of ·OH decreases as pH decreases (53.2% vs 43.03% vs 17.4%), as was predicted by eqs 6 and 7 (formation of ·OH is favored by increase of [OH−] and decrease of [H+]). (C) The contribution ratio of ·O2− increases as pH decreases (10.5% vs 30.23% vs 43.5%), as was predicted by eq 12 (oxidation of Sb(III) to Sb(V) is favored by an increase in [H+]). Environmental Implications. Sb2O3 constitutes the largest fraction of antimony used (60% of total use of antimony).28 Global Sb2O3 production in 2005 was 120 000 tonnes,29 increasing from 112 600 tonnes in 2002.30 It is primarily used as a flame retardant, as catalyst in polyethylene terephthalate (PET)-production, as a finisher in glass, and in pigments, based on information from different industries.30 Sb2O3 in these products dissolves and thereby generates antimony ions. The solubility of Sb2O3 is extremely low in pure water (approximately 0.812−1.54 mg·L−1 of dissolved Sb at pH 7.16 and 7.86);8 when the samples are irradiated with light, the concentration of dissolved Sb reaches 5−12 mg·L−1 from pH 4.4 to pH10.0 under simulated sunlight after 2h and continues to increase. Under ultraviolet irradiation, the concentration of dissolved Sb reached approximately 9 mg·L−1 in pure water after 30 min and increased linearly with a rate of 0.13 mg·L−1· min−1. In addition, the oxidation of Sb(III) accompanied the dissolution of Sb2O3. The concentration of dissolved Sb(V) reached 1.5−6 mg·L−1 from pH 4.4 to pH10.0 after 2 h of simulated sunlight irradiation. The value increased to approximately 7.5 mg·L−1 in pure water after 30 min of ultraviolet irradiation. SbV(OH)6− is the most important oxidation species because it confirms the relatively high mobility of Sb.3,31 In conclusion, light greatly accelerated the dissolution of Sb2O3 and the oxidation of Sb(III), which shortened the service life of Sb products and increased the environmental risks of Sb. In addition, Sb2O3, as a semiconductor photocatalyst, can photo-oxidize Sb(III) and a wide range of compounds and some geochemically and environmentally important reactions, including the photooxidation of phenol, polychlorinated biphenyls, sulfur-containing organic compounds, and nitro-

Figure 5. (a) Effect of Sb2O3 dosages on the dissolution of Sb2O3; (b) Effect of Sb2O3 dosages and free radical scavenger on the oxidation rate of Sb(III) at pH 9.0 under the simulated sunlight irradiation.

Figure 6. Conceptual model of the photocatalytic oxidation dissolution of Sb2O3. hν

Sb2 O3 → Sb2 O3(ecb− + hvb+)

Then, the secondary products ·OH (blue lines) and ·O2− (red lines) were generated: Sb2 O3(hvb+) + H 2O → Sb2 O3 + ·OH + H+

Sb2 O3(hvb+) + OH− → Sb2 O3 + ·OH Sb2 O3(ecb−) + O2 → Sb2 O3 + ·O2−

The superoxide (·O2−) undergo a protonation-reductionprotonation sequence, generating H2O2 (green lines),

·O2− + H+ → HOO· 14271



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gen-containing organic compounds.14,23,24 Sb2O3 also affects the geochemical cycle processes of other important elements, such as sulfur and nitrogen. In addition, further research is needed.

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AUTHOR INFORMATION

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S Supporting Information *

Three figures relate to XRD pattern and optical absorption curve of Sb2O3 and effect of free radical scavengers on the oxidation rate of Sb(III). This material is available free of charge via the Internet at http://pubs.acs.org/. Corresponding Author

*Phone: +86-10-5880 7172; fax: +86-10-5880 7172; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (41273105, 21177011), the National Science Foundation for Innovative Research Group (Grant No. 51121003) and the Nonprofit Environment Protection Specific Project (201509080). We are grateful to the editors and the anonymous reviewers for their numerous valuable comments and suggestions on our paper.

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