Chemosphere 150 (2016) 71e78

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Evidence for the generation of reactive oxygen species from hydroquinone and benzoquinone: Roles in arsenite oxidation Wenxiu Qin a, b, Yujun Wang a, **, Guodong Fang a, Tongliang Wu a, b, Cun Liu a, Dongmei Zhou a, * a b

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China University of Chinese Academy of Sciences, Beijing 100049, China

h i g h l i g h t s
As(III) was effectively oxidized to As(V) in the presence of HQ or BQ.
ESR technique was used to identify the ROS for As(III) oxidation.
Pathways for ROS generated from HQ and BQ solutions were proposed.
The As(III) oxidation rate constant (kobs) was calculated at different pH.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 November 2015 Received in revised form 19 January 2016 Accepted 29 January 2016 Available online 15 February 2016

Natural organic matter (NOM) significantly affects the fate, bioavailability, and toxicity of arsenic in the environment. In the present study, we investigated the oxidation of As(III) in the presence of hydroquinone (HQ) and benzoquinone (BQ), which were selected as model quinone moieties for NOM. It was found that As(III) was oxidized to As(V) in the presence of HQ or BQ at neutral conditions, and the oxidation efficiency of As(III) increased from 33% to 92% in HQ solutions and from 0 to 80% in BQ solutions with pH increasing from 6.5 to 8.5. The oxidation mechanism was further explored with electron spin resonance (ESR) technique. The results showed that semiquinone radicals (SQ
) were generated from the comproportionation reaction between BQ and HQ, which mediated the formation of superoxide anion (O2
), hydrogen peroxide (H2O2) and hydroxyl radical (
OH). Both the SQ
, H2O2 and
OH contributed to the oxidation of As(III). The increase of pH favored the formation of SQ
, and thus promoted the generation of reactive oxygen species (ROS) as well as As(III) oxidation. Increasing concentrations of HQ and BQ from 0.1 to 1.0 mM enhanced As(III) oxidation from 65% to 94% and from 10% to 53%, respectively. The findings of this study facilitate our understanding of the fate and transformation of As(III) in organic-rich aquatic environments and highlight quinone moieties as the potential oxidants for As(III) in the remediation of arsenic contaminated sites. © 2016 Elsevier Ltd. All rights reserved.

Handling Editor: X. Cao Keywords: Semiquinone radical Reactive oxygen species ESR Arsenite oxidation

1. Introduction Arsenic is considered as one of the most significant environmental causes of cancer throughout the world, which has contributed to a number of environmental and human health problems (Nordstrom, 2002; Zhao et al., 2010). Nowadays, the environmental fate and behavior of arsenic has caught global

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (D. Zhou). http://dx.doi.org/10.1016/j.chemosphere.2016.01.119 0045-6535/© 2016 Elsevier Ltd. All rights reserved.

attention due to its extensive contamination in ground, air, and drinking waters (Christen, 2001). The behavior of arsenic in the environment and in water treatment processes is strongly dependent on its speciation. Arsenic exists in natural environments primarily as oxyacids arsenite (As(III); pK1 ¼ 9.2; nonionic H3AsO3 at neutral pH) and arsenate (As(V); pK1 ¼ 2.2; anions H2AsO 4 and HAsO2 4 at neutral pH) (Ilgen et al., 2012). As(III) is more mobile in natural environments and is more difficult to be removed by adsorption or coagulation processes than As(V) (Hug and Leupin, 2003; Ilgen et al., 2012; Yürüm et al., 2014). Additionally, the toxicity of As(III) is much higher than As(V). Therefore, it is advisable to preoxidize As(III) to As(V) in both natural and technical

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W. Qin et al. / Chemosphere 150 (2016) 71e78

systems. Previous studies demonstrated that the toxicity and mobility of arsenic in the environment were controlled by the adsorption, desorption and redox transformation occurred on the surface of minerals, particularly for iron (hydr)oxides (Bauer and Blodau, 2006; Prucek et al., 2013). For example, enrichment factors of As within Fe crusts revealed concentrations up to 14 and 58 times higher than the background in the sediments of Lake Baikal (Müller et al., 2002). Numerous studies focused on the adsorption of arsenic by different iron compounds (e.g. red muds, zero-valent iron, Fe3O4, iron-loaded activated carbons) in water treatment (Castaldi et al., 2010; Lodeiro et al., 2013; Liu et al., 2015). Some complex redox transformation of the adsorbed arsenic on mineral surfaces has also been revealed. Amstaetter et al. (2009) reported the rapid oxidation of As(III) to As(V) in Fe(II)-goethite systems. Manganese oxides minerals were observed to act as potent sorbents and oxidants for the removal of As(III) under natural conditions (Wu et al., 2012, 2015). However, limited attention has been paid to the transformation of arsenic mediated by natural organic matter (NOM). NOM is a ubiquitous component of natural water with concentrations up to 100 mg L1 and plays a critical role in the biogeochemical cycling of trace metals and the mobility of colloidal particles in aquatic environments (Chen et al., 2015; Pradhan et al., 2015). Previous studies showed that NOM dramatically diminished the extent of sorption of both As(III) and As(V) on hematite (Redman et al., 2002). Saada et al. (2003) reported that the coated humic acids on kaolinite facilitated As(V) adsorption at pH 7.0. Besides the complexation speciation of arsenic, NOM also significantly influenced the oxidation of arsenic on the surface of minerals (Redman et al., 2002). It is well known that NOM contains large numbers of carboxyl, methoxyl, enolic-OH, and phenolic-OH groups (Klüpfel et al., 2014), and quinones or quinone-like moieties embedded in NOM molecules are identified as the predominant redox-active center in NOM as confirmed by NMR, fluorescence spectroscopy, and electrochemical methods (MacAlady and Walton-Day, 2011; Jiang et al., 2015). The complex and variability in NOM structure and chemistry make quantitative studies of its redox chemistry particularly difficult, and thus simple quinone couples are commonly used as the redox surrogate of NOM (Orsetti et al., 2013). Simple quinone compounds are not very stable under ambient conditions, and they undergo polymerization reaction in the presence of light or alkaline pH. However, such aromatic condensation reactions generally do not eliminate its reactivity of the quinone group (Sadykh-Zade et al., 1972), and the higher conjugated and substituted environment of the polymers might stabilize the semiquinone radicals which result in higher redox reactivity than the monomers. Similarly, NOM has been found to have similar reaction behavior but more effective than simple quinones in the iron redox reaction (Jiang et al., 2015). It has been reported that semiquinone radicals (SQ
) produced from microbial or chemical reduction of 9,10-anthraquinone-2,6disulfonic acid (AQDS) could induce As(III) oxidation (Jiang et al., 2009). Dissolved organic matter (DOM) extracted from biochar also enhanced the oxidation of As(III), which was attributed to SQ
 (Dong et al., 2014). However, the generation of reactive oxygen species (ROS) by quinone moieties for As(III) were not reported. In our previous study, SQ
 and ROS were found to be produced by low molecular weight organic phenolic acids, which played important roles in the oxidation of As(III) (Qin et al., 2016). Unfortunately, the mechanisms of ROS formation in the quinone moieties of NOM and the speciation of quinone moieties in the transformation of As(III) in natural environment were not clearly illustrated. Furthermore, our previous studies were conducted at alkaline pH (e.g., 9.0 and 10.0), which were rare in the realistic environment and thus greatly hindered its environmental

implication. Hence, in order to better understand the underlying interaction between As(III) and NOM in the natural environment, hydroquinone (HQ) and benzoquinone (BQ) were selected as the model reduced and oxidized quinone moieties for NOM, which were used to oxidize As(III) in neutral conditions. Electron spin resonance (ESR) technique was used to identify the major free radical species in quinone solution. Furthermore, the corresponding quinone moieties were quantified during the oxidation of As(III). The oxidation pathway of As(III) was proposed on the basis of the ROS identification. In addition, the effects of pH and the concentrations of HQ and BQ on the oxidation of As(III) were assessed. The results of this study would improve our understanding about arsenic transformation in natural environment and give implications on using quinone moieties for the natural attenuation and remediation of arsenic contaminated sites.

2. Materials and methods 2.1. Materials and chemicals Hydroquinone (HQ, 97%), 1,4-benzoquinone (BQ, 99.5%), NaOH, Na2HPO4, NaH2PO4 and HCl were purchased from J&K Scientific Ltd, China; NaAsO2 (99%) and 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO, 97%) were obtained from SigmaeAldrich, Inc. HPLC grade methyl alcohol and ethyl alcohol were supplied from TEDIA Company (USA). Other chemical reagents were of analytical grade or better. Solutions were prepared with deionized water (Milli-Q, Millipore, with resistivity of 18.2 MU cm1 at 25  C). All chemicals were used without any further purification. The stock solutions of HQ and BQ were kept in a refrigerator, protected from light with aluminum foil and used within 7 d. The structures of HQ and BQ were shown as:

2.2. Oxidation experiments Batch experiments were conducted in conical flasks with stopper at 25  C, and the total volume of reaction solution was 100 mL, buffered with phosphate solution (PBS) with ionic strength 2 ([H2PO 4 ] þ [HPO4 ] ¼ 50 mM). Firstly, 5.0 mL of 1.0 mM NaAsO2 stock solution was mixed with 185 mL PBS solution (pH adjusted to 7.5 with NaH2PO4 and Na2HPO4). Subsequently, 10 mL of 5.0 mM HQ or BQ solution was quickly added to initiate the reaction. The final As(III) concentration was 50 mM, and the HQ or BQ concentration was 0.5 mM. The mixtures were kept horizontally shaking at 200 rpm at 25  C. Control experiments without HQ or BQ were conducted under the same reaction conditions. All experiments were performed in triplicates. For the analysis of As(III), total arsenic, HQ and BQ, 2.0 mL samples were collected at a given time point (4, 10, 24, 48, 72, 96 h) and quenched by the addition of 0.1 mL 3.0 M HCl solutions, since the oxidation of As(III) would be completely stopped at acidic conditions, and the consumption of hydroquinone compounds was vanished (Strli
c et al., 2002).

W. Qin et al. / Chemosphere 150 (2016) 71e78

2.3. Analytical methods The concentrations of As(III) and total arsenic were selectively analyzed using a hydride generation atomic fluorescence spectrometer (HG-AFS, AFS-230E, Beijing Haiguang Instrumental Company, Beijing, China), following the procedure as described in previous studies (Maity et al., 2004; Xu et al., 2011). Briefly, aliquot of sample solution was pretreated with 5% HCl (v/v) and a reducing agent containing 5% (w/v) thiourea and 5% (w/v) ascorbic acid, which reduced As(V) to As(III) prior to hydride generation. The conditions for the hydride generation system were as follows: 5% HCl (v/v) solution and 2% KBH4 in 0.5% NaOH solution (w/v) were conducted as carrier solution. To quantify the concentrations of As(III) sensitively, the working solution was prepared in pH 4.5 sodium citrate buffer (0.4 M) for masking the As(V) signal, and 0.1 M citric acid acted as carrier solution instead of HCl solution (5%, v/v) (Xu et al., 2011). The concentration of As(V) was calculated by the difference between the total arsenic and As(III) concentrations. The detection limit of this method was 0.01 mg L1, and the mass balance analysis revealed that HQ did not interfere with arsenic analysis (date not shown). The oxidation efficiency of As(III) could be expressed as the following equation (Eq. (1)):

Oxidation efficiencyð%Þ ¼ ðC0  Ct Þ=Ct  100%

(1)

where C0 and Ct are the concentrations of As(III) before and after oxidation, respectively. The concentrations of HQ and BQ were analyzed using HPLC (Aglient1260, USA) at a UV wavelength of 226 nm. The separation was performed using an isocratic mobile phase composed of MilliQ water and methanol at a volume ratio of 30:70 and a flow rate of 1.0 mL min1, and the detection limits for HQ and BQ were 0.01 mM. 2.4. Electron spin resonance (ESR) measurements To verify the existence of free radicals in HQ and BQ solutions, the ESR spectra were collected at room temperature (25 ± 1  C) using a Bruker EMX/plus electron spin resonance spectrometer. The ESR conditions were as follows: resonance frequency, 9.85 GHz; microwave power, 20.49 mW; modulation frequency, 100 kHz; modulation amplitude, 1.0 G; sweep width, 200 G; time constant, 40.96 ms; sweep time, 81.92 s; receiver gain, 2.0  102. The ESR signals were recorded from 0.5 mM HQ or BQ solution in 50 mM PBS at various pH values (6.5e8.5), while DMPO-OH signal was acquired by adding 0.1 M spin trapping agent DMPO (Fang et al., 2013a). 3. Results and discussion 3.1. As(III) oxidative transformation in the presence of HQ Solution pH plays a vital factor in controlling the oxidation of As(III), therefore, the oxidation of As(III) in the presence of HQ at different pH (pH 6.5, 7.5 and 8.5) was examined. As shown in Fig. 1a, As(III) was oxidized by HQ effectively at neutral conditions. And the oxidation efficiency of As(III) after 96 h increased markedly from 33% to 92% with the increase of pH values from 6.5 to 8.5. In contrast, As(III) concentration changed insignificantly (p < 0.05) in the absence of HQ at pH 7.5. The kinetic data showed that the oxidation of As(III) followed the pseudo-first-order equations: Ct ¼ C0 ekobs t , where Ct (mM) is the concentration of As(III) at time t, C0 (mM) is the concentration of As(III) at time 0, and kobs is the pseudo-first-order reaction rate

73

constant (h1). Table 1 listed the kobs, half-lives (t1/2), and correlation coefficient (R2) for the oxidation kinetics of As(III). The kobs of As(III) oxidation at pH 8.5 was calculated to be 0.15 h1, which was 6 times of that at pH 7.5 (3.68  103 h1) and about 41 times of that at pH 6.5 (2.66  102 h1), which suggested that the oxidation of As(III) in the presence of HQ was more quickly at relatively high pH values. Furthermore, the concentration of HQ and BQ was also monitored during the As(III) oxidation. As shown in Fig. 1b, the disappearance of HQ and the formation of BQ were observed in the HQ/ As(III) system at different pH values. HQ was relatively stable at pH 6.5. The loss of HQ was about 10%, and the concentration of BQ was accumulated to 40 mM after 96 h reaction. But at pH 7.5, the decay of HQ was about 93% after 96 h, and the concentration of BQ achieved maximum at 0.13 mM within 10 h and then decreased as the reaction proceeded. With pH increased to 8.5, HQ was rapidly consumed and was undetectable after 24 h reaction. Meanwhile, BQ was also not detected at pH 8.5 in the entire reaction time. This was because BQ was rapidly generated from the autoxidation of HQ, and then became the polymers of quinone moieties at high pH conditions. Previous studies have shown that quinones and polyhydric phenols were prone to be polymerized to produce humic acid-like substances at alkaline conditions, which had a quinoid structure and electron-exchange capacity (Sadykh-Zade et al., 1972). It suggested that the consumption of HQ was facilitated significantly with increasing pH, which was coincident with the oxidation efficiency of As(III) in response to the change of pH values. 3.2. The oxidation mechanism of As(III) 3.2.1. Determination of SQ
 in HQ solution The ESR technique was applied to probe ROS generation from HQ solution for As(III) oxidation. The autoxidation of HQ to corresponding quinones has been extensively studied for over a century (Uchimiya and Stone, 2009). When the fully oxidized quinone and reduced hydroquinone are present in solution, these quinones exchange electrons to form semiquinone radicals (SQ
) (Eq. (2)) (Song and Buettner, 2010): Q þ QH2 4 2Q
 þ 2Hþ

(2)

As shown in Fig. 2a, without any trapping agents, a typical fiveline with an intensity ratio of 1:4:6:4:1 and hyperfine splitting constants of aH ¼ 2.36 G, g ¼ 2.0045 was observed in HQ solution, which was characteristic of SQ
 (Fang et al., 2013b). The peak intensity of SQ
 increased rapidly from undetectable to the high peak intensity of 5.5  105 a.u at 4 h with pH increasing from 6.5 to 8.5, which indicated that increasing pH would produce more SQ
. The results were consistent with that the oxidation efficiency of As(III) in HQ solutions increased with pH increase. The redox potential for As(V)/As(III) was 0.40 to  0.2 V at pH 3.0e9.0 (Lee and Choi, 2002), whereas the redox potentials for semiquinone/hydroquinone couple was 0.48 V for HQ at pH 7.0 (Miliukiene_ et al., 2014). The simple comparison of the redox potentials indicated that SQ
 was capable of oxidizing As(III) to As(V) directly, which were consistent with previous study (Jiang et al., 2009). To test the stability of SQ
, the peak intensities of SQ
 in HQ solution at pH 7.5 were detected at different reaction time (Fig. 2b). The peak intensity of SQ
 reached the maximum at 4 h and then decreased from 7.5  104 to 8.5  103 a.u with reaction time prolonged to 72 h. Compared to highly reactive free radicals such as the hydroxyl radical (
OH), SQ
 was a relatively stable free radical, which could exist stably in the neutral environmental conditions for several days. As aforementioned, the decay of HQ was detected during the

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W. Qin et al. / Chemosphere 150 (2016) 71e78

(a)

80

without HQ pH 6.5 pH 7.5 pH 8.5

60 40 20

(b)

0.5

Concentration (mM)

Oxidation efficiency (%)

100

pH 6.5 HQ pH 7.5 HQ pH 8.5 HQ pH 6.5 BQ pH 7.5 BQ pH 8.5 BQ

0.4 0.3 0.2 0.1 0.0

0 0

20

40

60

time (h)

80

100

0

20

40

60

time (h)

80

100

Fig. 1. Oxidation kinetics of As(III) in the presence of HQ at different pH (pH 6.5, 7.5, 8.5) (a); The consumption of HQ and formation of BQ in the HQ/As(III) system at different pH (pH 6.5, 7.5, 8.5) (b). Reaction conditions: [As(III)]0 ¼ 50 mM, [HQ]0 ¼ 0.5 mM; PBS ¼ 50 mM and 25  C. Error bars represent the standard deviation based on triplicate experiments.

Table 1 Pseudo-first-order rate coefficients (kobs) and half-lives (t1/2) for the oxidation of As(III) in the presence of HQ and BQ as a function of pH. Oxidant species HQ HQ HQ BQ BQ BQ

pH 6.5 7.5 8.5 6.5 7.5 8.5

kobs (h1) (3.68 (2.66 (1.52 e (3.04 (2.51

t1/2 (h) 3

± 0.28)  10 ± 0.04)  102 ± 0.05)  101 ± 0.11)  103 ± 0.01)  102

189.1 26.1 4.5 e 228.2 27.6

± 14.2 ± 0.4 ± 0.2 ± 8.5 ± 0.1

R2 0.99 0.99 0.83 e 0.93 0.97

3.2.2. Generation of
OH from HQ solution in the presence of O2 ESR technique coupled with DMPO as a spin-trapping agent was used to identify the formation of
OH in HQ solution. When spin trapping agent DMPO (0.1 M) was added to the HQ solution at pH 7.5, besides the signals of SQ
, a four-line spectrum with the intensity ratio of 1:2:2:1, hyperfine splitting constants of aH ¼ aN ¼ 14.89 G and g-factor of 2.0067 (determined by WinESR SimFonia software) was observed (Fig. 2c). This was a representative pattern of DMPO-OH. The results indicated that
OH radical was produced in HQ solution at neutral conditions. Previous studies

Fig. 2. Detection of free radicals in HQ solution with ESR: (a) ESR spectra of SQ
 in HQ solution after incubation for 4 h at different pH (pH 6.5, 7.5, 8.5); (b) Changes in the peak intensity of SQ
 at pH 7.5 as a function of reaction time; (c) ESR spectra of HQ solution at pH 7.5 in the presence of DMPO. Reaction conditions: [HQ]0 ¼ 0.5 mM, [DMPO]0 ¼ 0.1 M, PBS ¼ 50 mM and 25  C. SQ
: A, DMPO-OH: +, DMPO-O2: △.

oxidation of As(III), which resulted in the decrease of concentration of SQ
. These results revealed the vital roles of SQ
 in the oxidation of As(III).

have shown that semiquinone radicals could transfer electron to oxygen and form superoxide anion (O2
) and hydrogen peroxide (H2O2), which resulted in the generation of
OH through the

W. Qin et al. / Chemosphere 150 (2016) 71e78

Fenton-like reactions (Zhu et al., 2002; Page et al., 2012; Qin et al., 2016). These results indicated that in-situ generation of H2O2 and
OH was generally considered as O2
-mediated (Fujii et al., 2008). Meanwhile, the hyperfine splitting constants of aN ¼ 14.3 G, aH ¼ 11.5 G and aH ¼ 1.3 G were observed in the mixture of 0.1 M DMPO and 0.5 mM HQ at pH 7.5 (Fig. 2c). These were representative of O2
 radicals added to DMPO (DMPO-O2) (Fang et al., 2013a), which confirmed that SQ
 could transfer electron to oxygen to form O2
. Based on the previous studies and experimental results observed in our study, the possible mechanism regarding the generation of
OH in the HQ solution was described in Eqs. (3)e(6):

75

As(III) þ
OH / As(IV) þ OH

(8)

As(III) þ H2O2 / As(V) þ 2OH

(9)

2As(IV) / As(III) þ As(V)

(10)

As(IV) þ O2 / As(V) þ O2


(11)

As(IV) þ
OH / As(V) þ OH

(12)

SQ
 þ O2 / BQ þ O2


(3)

3.3. As(III) oxidative transformation in the presence of BQ

O2
 þ O2
 þ 2Hþ / H2O2 þ O2

(4)

SQ
 þ O2
 þ 2Hþ / H2O2 þ BQ

(5)

SQ
 þ H2O2 / BQ þ
OH þ OH

(6)

BQ was the oxidized product of HQ, thus, the oxidation of As(III) in the presence BQ was also investigated. As shown in Fig. 3a, similar to HQ, As(III) was oxidized efficiently to As(V) in the presence of BQ under neutral conditions. And the oxidation efficiency of As(III) after 96 h increased markedly from 0 to 80% with an increase of pH values from 6.5 to 8.5. Since the As(III) oxidation was not detected in BQ solutions at pH 6.5, we concluded that As(III) would not be oxidized by BQ directly at natural conditions. As shown in Table 1, the kobs of As(III) oxidation in the presence of BQ at pH 8.5 was calculated to be 2.51  102 h1, which was 8 times of that at pH 7.5 (3.04  103 h1), indicating that higher pH facilitated the oxidation of As(III) in the presence of BQ. The kobs (0.15 h1) of As(III) oxidation in HQ/As(III) system at pH 8.5 was about 6 times of that in BQ/As(III) system (2.51  102 h1), revealing that HQ exhibited higher oxidative reactivity toward As(III) than BQ. The concentrations of HQ and BQ were detected during the oxidation of As(III) in BQ solutions. As shown in Fig. 3b, the reduction of BQ to form HQ was observed in the BQ/As(III) system at different pH values. At pH 6.5, the loss of BQ was about 40% after 96 h reaction, while the concentration of HQ was accumulated to 0.12 mM. At pH 7.5, BQ was totally disappeared after 96 h, and the concentration of HQ achieved maximum at 0.15 mM within 24 h and then decreased as the reaction progress. When pH increased to 8.5, BQ was totally consumed after 4 h reaction, and the concentration of HQ was accumulated to the maximum of 0.21 mM. These results indicated that higher pH facilitated the redox transformation of BQ and HQ, which resulted in the generation of more SQ
. Thus, the peak intensity of SQ
 increased rapidly from undetectable to the high intensity of 2.3  105 a.u after 4 h with pH increasing from 6.5 to 8.5 in BQ solution (Fig. 3c). These results further confirmed the important role of SQ
 in the oxidation of As(III). When the trapping agent DMPO was added to BQ solution, O2
 and
OH were also detected by ESR technique (Fig. 3d). Thus, the processes of ROS generation from BQ solution was similar to that in HQ solution, and the As(III) oxidation was also SQ
mediated.

According to above reactions, the generation of
OH in HQ solutions was a metal-independent process: SQ
 transferred electron to O2 to form O2
 (Eq. (3)), and then, O2
 reacted with SQ
 or dismutated to produce H2O2 (Eqs. (4) and (5)), which consequently induced the formation of
OH through the Fenton-like reactions (Eq. (6)) (Lee and Choi, 2002; Zhu et al., 2002; Page et al., 2012). These results confirmed that
OH, H2O2 and O2
 were produced from HQ solutions, and participated in the oxidation of As(III).

3.2.3. The oxidation pathway of As(III) in HQ solution Since
OH and H2O2 exhibited highly oxidative ability with oxidation potential of E(
OH) ¼ 2.8 VNHE, E(H2O2) ¼ 1.77 VNHE, and were capable of oxidizing a wide range of contaminants (Wang et al., 2014; Kong et al., 2015), they undoubtedly accounted for the oxidation of As(III). However, O2
 was a moderate oxidant, and its contribution to the oxidation of As(III) still remained unclear. Ryu and Choi (2006) claimed that superoxide anion was the main oxidant in TiO2 photocatalytic oxidation of As(III). However, this claim was disputed against by other researchers (Xu et al., 2005; Yoon et al., 2009). Yoon et al. (2009) pointed out that superoxide anion radical was inherently a negligible oxidant of As(III) compared with
OH in various advanced oxidation processes. Xu et al. (2005) have also argued that superoxide anion had little or no role in the oxidative conversion of As(III) during ultrasonic irradiation. Furthermore, superoxide anion was short-lived (halflife < 1 s) and underwent rapid disproportionation, the oxidation ability of superoxide anion in aqueous solution was limited (Sawyer and Valentine, 1981; Buxton et al., 1988). We, therefore, concluded that the direct oxidation of As(III) by O2
 was negligible in HQ/ As(III) system. Thus, the oxidation As(III) to As(V) was caused by SQ
,
OH and H2O2. Briefly, As(III) was initially oxidized to As(IV) by SQ
 and
OH (Eqs. (7) and (8)), and then As(IV) was immediately converted to As(V) by oxygen,
OH or disproportionation (Eqs. (10) and (12)) (Lee and Choi, 2002; Dutta et al., 2005). The transient species As(IV) was unstable and rapidly disproportionated into As(III) and As(V), almost at a rate close to diffusion limit (Eq. (10)) (Yoon et al., 2009; Fei et al., 2011). Moreover, As(IV) could be oxidized to As(V) by O2 with a high rate constant (k ¼ 1.1  109 M1 s1) (Eq. (11)) (Lee and Choi, 2002), which suggested that As(IV) would not exist stable in solution. And As(III) could be oxidized to As(V) by H2O2 by two one-electron charge transfer reactions(Eq. (9)) (Kim et al., 2006). SQ
 þ As(III) / As(IV) þ HQ

(7)

3.4. Effects of HQ or BQ concentrations on As(III) oxidation The oxidation kinetics of As(III) as a function of HQ and BQ concentrations (0, 0.1, 0.2, 0.5, 1.0 mM) at pH 7.5 were studied. As shown in Fig. 4a, the oxidation ratio of As(III) after 96 h increased markedly from 65% to 94% with an increase of HQ concentration from 0.1 to 1.0 mM. Furthermore, the oxidation kinetic curves were well described with the pseudo-first-order equations. The kobs for the As(III) oxidation in different HQ concentrations increased linearly from 9.3  103 to 3.6  102 min1 with the increase of HQ concentration from 0.1 to 1.0 mM (R2 ¼ 0.91) (Fig. 4b). As for in BQ solutions, the oxidation efficiency of As(III) was 10%, 18%, 48% and 53% after 96 h reaction with the addition of 0.1, 0.2, 0.5 and 1.0 mM BQ, respectively, which suggested that increasing BQ concentrations

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W. Qin et al. / Chemosphere 150 (2016) 71e78

Fig. 3. Oxidation kinetics of As(III) in the presence of BQ at different pH (pH 6.5, 7.5, 8.5) (a); The consumption of BQ and formation of HQ in the BQ/As(III) system at different pH (pH 6.5, 7.5, 8.5) (b); ESR spectra of SQ
 in BQ solution after incubation for 4 h at different pH (pH 6.5, 7.5, 8.5) (c); ESR spectra of BQ solution at pH 7.5 in the presence of DMPO (d). Reaction conditions: [As(III)]0 ¼ 50 mM, [BQ]0 ¼ 0.5 mM; [DMPO]0 ¼ 0.1 M; PBS ¼ 50 mM and 25  C. Error bars represent the standard deviation based on triplicate experiments. SQ
: A, DMPO-OH: +, DMPO-O2: △.

0 mM 0.1 mM 0.2 mM 0.5 mM 1 mM

80 60

(a)

0.04

kobs = 0.028×C(HQ) + 0.0096 0.03

(b)

R2 = 0.91

kobs (h-1)

Oxidation efficiency (%)

100

40

0.02

20 0.01

0 0

20

40

60

80

100

Oxidation efficiency (%)

60

0 mM 0.1 mM 0.2 mM 0.5 mM 1 mM

40

0.0

0.2

0.4

0.6

0.8

1.0

Concentration (HQ, mM)

time (h) (c)

20

0 0

20

40

60

80

100

time (h) Fig. 4. Effect of HQ concentrations on the oxidation of As(III) (a); kobs values for As(III) oxidation as a function of HQ concentrations (b); Effect of BQ concentration on the oxidation of As(III) (c). Reaction conditions: [As(III)]0 ¼ 50 mM, [HQ]0 ¼ [BQ]0 ¼ 0e1.0 mM; pH 7.5 (50 mM PBS) and 25  C. Error bars represent the standard deviation based on triplicate experiments.

W. Qin et al. / Chemosphere 150 (2016) 71e78

(0.1e0.5 mM) facilitated the oxidation of As(III), while the As(III) oxidation efficiency increased slightly with further increasing BQ up to 1.0 mM. These results indicated that increasing the concentrations of HQ or BQ favored As(III) oxidation. 4. Conclusion This study clearly demonstrated that As(III) was efficiently oxidized to As(V) in HQ or BQ solutions at neutral conditions. And the oxidation rate constant (kobs) at pH 8.5 was about 6 times of that at pH 7.5 and about 41 times of that at pH 6.5 in HQ solutions, which was attributed to the significantly increased generation of SQ
 with pH increasing. SQ
 was produced from the comproportionation reaction between HQ and BQ, which mediated the formation of ROS such as O2
, H2O2 and
OH. The process of ROS generation from BQ solution was similar to that from HQ solution, and was also SQ
-mediated, since SQ
, O2
 and
OH were observed in BQ solution. The O2
 radical was detected in HQ and BQ solutions, which confirmed the
OH and H2O2 produced by quinone moieties was O2
 mediated. Increasing pH facilitated the transformation of HQ and BQ, thus promoted the generation of ROS as well as As(III) oxidation. Increasing BQ and HQ concentrations improved the efficiency of As(III) oxidation, and the kobs of As(III) oxidation increased linearly to HQ concentrations. The reduced quinone moiety e HQ had the stronger ability to generate ROS and to induce As(III) oxidation than the oxidized quinone moiety e BQ. The results of this study would provide novel insights into the mechanism of interactions between NOM and contaminants (e.g., arsenic), since quinone moieties are important components of NOM. However, further studies are needed to evaluate the role of NOM in the oxidation transformation of As(III) in the natural environment. Acknowledgments This work was supported by the National Natural Science Foundation of China (21537002; 41422105; 41171189; 41125007) and the Natural Science Foundation of Jiangsu Province (BK20130050). References Amstaetter, K., Borch, T., Larese-Casanova, P., Kappler, A., 2009. Redox transformation of arsenic by Fe(II)-activated goethite (a-FeOOH). Environ. Sci. Technol. 44, 102e108. Bauer, M., Blodau, C., 2006. Mobilization of arsenic by dissolved organic matter from iron oxides, soils and sediments. Sci. Total Environ. 354, 179e190. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (,OH/,O in aqueous solution. J. Phys. Chem. Ref. Data 17, 513e886. Castaldi, P., Silvetti, M., Enzo, S., Melis, P., 2010. Study of sorption processes and FTIR analysis of arsenate sorbed onto red muds (a bauxite ore processing waste). J. Hazard. Mater 175, 172e178. Chen, C., Kukkadapu, R., Sparks, D.L., 2015. Influence of coprecipitated organic matter on Fe2þ(aq)-catalyzed transformation of ferrihydrite: implications for carbon dynamics. Environ. Sci. Technol. 49, 10927e10936. Christen, K., 2001. The arsenic threat worsens. Environ. Sci. Technol. 35, 286Ae291A. Dong, X.L., Ma, L.Q., Gress, J., Harris, W., Li, Y.C., 2014. Enhanced Cr (VI) reduction and As (III) oxidation in ice phase: important role of dissolved organic matter from biochar. J. Hazard. Mater 267, 62e70. Dutta, P.K., Pehkonen, S., Sharma, V.K., Ray, A.K., 2005. Photocatalytic oxidation of arsenic (III): evidence of hydroxyl radicals. Environ. Sci. Technol. 39, 1827e1834. Fang, G.D., Dionysiou, D.D., Al-Abed, S.R., Zhou, D.M., 2013a. Superoxide radical driving the activation of persulfate by magnetite nanoparticles: implications for the degradation of PCBs. Appl. Catal. B Environ. 129, 325e332. Fang, G.D., Gao, J., Dionysiou, D.D., Liu, C., Zhou, D.M., 2013b. Activation of persulfate by quinones: free radical reactions and implication for the degradation of PCBs. Environ. Sci. Technol. 47, 4605e4611. Fei, H., Leng, W.H., Li, X., Cheng, X.F., Xu, Y.M., Zhang, J.Q., Cao, C., 2011. Photocatalytic oxidation of arsenite over TiO2: is superoxide the main oxidant in normal air-saturated aqueous solutions? Environ. Sci. Technol. 45, 4532e4539. Fujii, M., Ito, H., Rose, A.L., Waite, T.D., Omura, T., 2008. Superoxide-mediated Fe (II)

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