The synergistic trigger of the reductive dissolution of Schwertmannite-As(III) and the release of arsenic from citric acid and UV irradiation

Chemical Geology 520 (2019) 11–20

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The synergistic trigger of the reductive dissolution of Schwertmannite-As (III) and the release of arsenic from citric acid and UV irradiation ⁎

Jian Zhanga, Yuxin Lia, Wei Lib,c, Lixiang Zhoub, Yeqing Lana, , Jing Guoa,

T

⁎

a

College of Sciences, Nanjing Agricultural University, Nanjing 210095, PR China College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, PR China c Nanjing Cigarette Factory, Jiangsu Tobacco Industrial Limited Company, Nanjing 210095, China b

A R T I C LE I N FO

A B S T R A C T

Editor: Karen Johannesson

This paper focused on understanding the effect of citric acid (CA) on the reductive dissolution of Schwertmannite loaded with arsenite (SCH-As(III)) under UV irradiation and the release of arsenic (As). Results demonstrated that CA could significantly promote the reductive dissolution of SCH and SCH-As(III) with the assistance of ultraviolet light. Initially, the dissolved total iron (Fe) and As exhibited a rapid increase (0 to 240 min), followed by a quick decline due to the depletion of CA. It was found that the dissolved total Fe and As were present mainly as the species of Fe(II) and As(V), respectively. This can be attributed to the electronic transfer from ligand to metal in the complex of Fe(III)-(CA)n occurring under ultraviolet light, leading to the production of Fe(II) and organic radicals. Moreover, As(V) was further generated via the interaction of As(III) with %OH from a series of radical reactions. In the dark, CA could also enhance the total Fe and As released from the SCH-As(III). The lower the initial pH level, the greater the amount of total dissolved Fe and As. The presence of Al3+ clearly suppressed the reductive dissolution of SCH-As(III) triggered by CA and ultraviolet light. SEM spectra indicated that the mineralogical phases of SCH-As(III) before and after the reaction were discrepant. Thus, this study provides an insight into the stability of SCH-As(III) and the migration of As in the natural environment.

Keywords: Schwertmannite Reductive dissolution As(III) Citric acid Irradiation

1. Introduction Over the recent years, an increasing amount of attention has been paid to arsenic pollution due to its high toxicity, even at very low concentrations (Coussy et al., 2012). The high concentration of arsenic (As) in mining environments mainly results from the oxidation of Asbearing-sulfide minerals, such as arsenopyrite (FeAsS) and arsenianpyrite (Antelo et al., 2013; Burton et al., 2009). It is well known that Asbearing minerals are usually abandoned after mining and mineral processing due to their low economic value (Coussy et al., 2012). The tailings from the irrational disposal react with oxygen and water in atmosphere, leading to the release of sulfates, metals and As (Smedley and Kinniburgh, 2002). Thus, the concentration of As (mostly in the form of H2AsO4−) in acidic mine wastewater can reach hundreds of mg L−1 (Burton et al., 2009). As(III), a low valence state arsenic, exhibits a higher degree of fluidity in the natural environment and a higher toxicity than As(V). Thus, As(III) seriously threatens the ecological environment (Meharg and Hartley-Whitaker, 2002). Arsenic migration in mining environments is usually affected by the distribution and abundance of iron oxide. Schwertmannite (SCH), a

⁎

poorly crystalline Fe(III)-oxyhydroxylsulfate mineral, commonly forms in acidic mine wastewater. The structure of SCH (Fig. S1) is similar to that of akaganéite, in which SO42− replaces Cl− and occupies the parallel square tunnels (Cornell and Schwertmann, 2003). Schwertmannite has a unit cell of FeO6 octahedra forming double chains. Moreover, SO42− ions are present in SCH as both structural and adsorbed sulfate (Bigham et al., 1990; Waychunas et al., 1995; Zhu et al., 2012; Wang et al., 2015). It has been reported that the extremely high content of arsenic in acidic mine wastewater can be reduced to the background level by adding SCH downstream to the sewage outlet (Fukushi et al., 2003). This demonstrates the strong adsorption ability of SCH to arsenic. Nevertheless, the stability of SCH is heavily impacted by several chemico-physical factors, including pH, temperature, redox status, coexisting ions, and organic substances (Fukushi et al., 2003; Regenspurg and Peiffer, 2005). Dissolved organic matter (DOM), produced by plant corruption, is widely distributed in water environments. DOM can promote the reductive dissolution of iron oxides (Panias et al., 1996), which undergoes the following three stages: 1) adsorption of organic ligands on the surface of iron oxides, 2) non-reductive dissolution, and 3) reductive

Corresponding authors. E-mail addresses: [email protected] (Y. Lan), [email protected] (J. Guo).

https://doi.org/10.1016/j.chemgeo.2019.05.004 Received 10 March 2019; Received in revised form 25 April 2019; Accepted 1 May 2019 Available online 03 May 2019 0009-2541/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (a) The SEM of SCH, (b) SCH-As(III), and (c) SCH-As(III) after 9 h of UV irradiation in the presence of CA.

dissolution. It has been reported that up to 25.0 mg L−1 of dissolved Fe (II) is generated after ultraviolet (UV) light irradiation for 5 min at a pH level of 4 in the system of SCH (0.2 g L−1) and oxalate (2 mM) (Wu et al., 2012). Oxalic acid can cause more Fe dissolution from SCH-As (V) compared to that in the absence of oxalic acid. In addition, UV light irradiation favors the retention of As (Ren et al., 2018). Xie et al. (2017) found that the dissolution of total Cr from SCH-Cr (VI) reached maximum levels in the presence of L-tryptophan (5 mM) at pH 6.5. The dissolved Cr (VI) can be further reduced to Cr (III) at pH 3, whose toxicity is much lower than Cr(VI). However, studies on the stability of SCH-As(III) are limited. The valence changes of Fe and As dissolved from SCH-As (III), and the main impacting factors on the dissolution of SCH-As(III) triggered by UV and CA, including the pH and coexisting ions, remain to be fully understood. In this study, we investigated the stability of SCH-As(III) within the pH range of 3 to 5 and citric acid concentrations between 1 mM to 2.5 mM, under the irradiation of UV light. The analyses of Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Photoelectron Spectroscopy (XPS) were conducted in order to reveal the mineralogical phase variation of SCH-As(III) and the transformation of Fe and As species. Fluorescence spectra of coumarin were analyzed to prove the generation of %OH during the reaction. Finally, using the experimental results, a possible mechanism for the SCH-As(III) reductive dissolution and the As(III) oxidation induced by CA and UV was proposed.

2.2. Preparation and characterization of SCH and SCH-As(III) The synthesis of SCH and SCH-As(III) followed the method reported by Zhang et al. (2019). Briefly, 11.12 g FeSO4·7H2O was dissolved into a 500 mL flask with an adequate amount of deionized water. Following this, 5 mL suspension of the freshly prepared A. ferrooxidans LX5 cell was introduced into the flask. The mixture solution was adjusted to pH 2.5 with dilute sulfuric acid, with a final volume of 250 mL. The flask was sealed with 8 layers of sterile gauze and incubated in a shaker at 180 rpm at 28 °C for 3 days. SCH was collected by centrifugation and washed with acidified deionized water and deionized water, respectively. Finally, SCH was dried using a vacuum oven at 60 °C for 6 h prior to use. SCH-As(III) was prepared as follows. First, 0.01 g SCH was introduced into 100 mL solution containing 50 mg L−1 As(III). The initial pH of the suspension was adjusted to 10 (the optimal pH for As(III) adsorption by SCH) with a diluted NaOH solution. The suspension was then placed on a reciprocating shaker and shaken at a speed of 180 rpm at 25 °C for 24 h. The suspension was centrifuged at 5000 rpm for 5 min, and the solid was harvested and washed three times with deionized water. Finally, SCH-As(III) was dried using a vacuum oven at 60 °C. The amount of As(III) loaded on the SCH was calculated to be 94.1 mg g−1. The solid samples after the reaction were collected by centrifugation, washed three times with deionized water and dried using the vacuum oven at 60 °C prior to characterization. FTIR spectra of the mineral samples were recorded using a Bruker Transor 27 FTIR spectrometer before and after the reaction. The specimens were mixed with KBr at a fixed proportion, ground to powder and squeezed into sheets using the pressed-disk technique. Transmission measurements were made within the wavelengths of 400 and 4000 cm−1, with a 4 cm−1 spectral resolution. The valence states of Fe and As before and after the reaction were characterized using X-ray photoelectron spectrometer (XPS, ESCALAB250i, Thermo Fisher Scientific, USA) and a monochromatic Al Ka X-ray source. The resolution function of the instrument was 0.43 eV, with an energy range of 5000–0 eV, a pass energy of 30 eV and a step size of 0.05 eV. The mineral phases of SCH and SCH-As(III) before and after the

2. Materials and methods 2.1. Chemicals NaAsO2 (> 98%) was obtained from Sigma-Aldrich. Citric acid (CA) (AR) was provided by Shanghai Chemical Reagent Co., Ltd. Methanol (HPLC grade) was purchased from the Tedia Company, Inc. Coumarin was obtained from the Aladdin Industrial Corporation. More details of the remaining chemicals used in this study were reported in Zhang et al. (2019).

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Fig. 2. The XPS of SCH-As(III) for Fe and As before and after the reaction.

reaction were characterized using a field emission scanning electron microscope (FESEM, S4800, Hitachi, Japan) following the coating treatment. The maximum magnification was set at 80,0000 times, with a resolution ratio of 1.0 nm.

to guarantee stable irradiation during the reactions. Following this, 0.01 g SCH or SCH-As(III) was added to the photo-reaction tube containing 40 mL CA solution. CA concentrations were set at 1, 1.75 and 2.5 mM, and the initial pH was adjusted with diluted H2SO4 and diluted NaOH solutions to 3, 4 and 5. Concentrations of coexisting ions, including K+, Ca2+, Al3+ and CO32−, were given as 500 mg L−1. At a specific time interval, a 2.0 mL sample was obtained and filtered with 0.45 μm filter membrane to determine the concentrations of As, Fe and CA. Experiments without light were also carried under the same conditions. All the experiments in this section were performed in triplicate and the results were represented using average values with error bars.

2.3. Photochemical experiments Photoreductive dissolution of SCH and SCH-As(III) was performed in the XPA-7 photochemical reactor (Xujiang electromechanical plant, Nanjing, China). The temperature was maintained at 25 ± 1 °C using a temperature-controlling device. A 300 W Hg lamp with a maximum light intensity output at 365 nm was used as the light source. Before the photochemical experiments, the lamp was turned on for 5 min in order 13

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Fig. 3. Dissolution experiments of SCH and SCH-As(III) in the presence and absence of CA (1 mM) under the UV irradiation. (a) Dissolved total Fe and Fe(II) and (b) dissolved total As, (c) the variation of CA concentration and (d) pH during the reaction.

spectra of SCH and SCH-As (III) before and after the reaction are shown in Fig. 1. Aggregates of spheres with a mulberry shape were observed on the surface of the synthetic SCH, and a large specific surface area can be noted (Fig. 1a). This is in agreement with the observation of Paikaray et al. (2017). A slight different morphology of the SCH-As(III) (Fig. 1b), with a new substance covering its surface, may indicate the presence of arsenic. However, the mineral phase of the SCH-As(III) following 9 h of UV irradiation in the presence of CA changed significantly, and uneven particulate matter appearing on its surface (Fig. 1c). The FTIR spectra of SCH and SCH-As(III) before and after the reaction are depicted in Fig. S2. According to Regenspurg et al. (2004), the wavenumber 3097 cm−1 corresponds to the telescopic vibration of the hydroxyl group, and the peak around 1557 cm−1 is assigned to the deformation of water molecules. The peaks at 1114 and 695 cm−1 are attributed to the SO42− stretching vibration. It has been reported that the band located at 1114 cm−1 can be split into 1 to 3 peaks, depending on the nature of the surface complexes, e.g. inner or outer-sphere complexes (Hug, 1997). Thus, a slight difference is noted by comparing the FTIR spectra of SCH with those of SCH-As(III) before and after the reaction. Along with the strong peak representing the SO42− stretching vibration at 1114 cm−1, two additional weaker peaks were also present at 1195 and 1038 cm−1 in the FTIR spectra of SCH. However, this phenomenon is not observed in the spectra of SCH-As(III). The disappearance of the peaks at 1038 and 1195 cm−1 may be attributed to the replacement of the partial SO42− by As ions. An extra adsorbance peak at 841 cm−1 is observed following 9 h of UV irradiation in the presence of CA. This is attributed to the AseO stretching vibration of As–O–Fe coordination (Jia et al., 2007). The XPS spectra of the SCH-As(III) before and after the reaction are illustrated in Fig. 2. Due to the coupling of spin orbits, the Fe 2p core orbit of SCH-As(III) was cracked into 2p3/2 and 2p1/2 orbits, with bonding energies located at 711.3 eV and 724.9 eV, respectively. All binding energies of Fe 2p3/2 lines were higher than 710.5 eV, which strongly suggests that iron present in the form of Fe(III) species (Gan

2.4. Analytical methods The As(V) concentration in the solution was analyzed via a colorimetric method of molybdenum blue (Dhar et al., 2004). The principle of the method is based on the reaction between As(V) and the color developer, forming an arsenic, antimony and molybdenum ternary complex with a maximum absorption at 890 nm. In order to measure As(III), As(III) in the solution was first oxidized to As(V) with 0.03 mmol/L KMnO4 solution. The concentration of total As, including the As(V) generated from photoreductive dissolution of SCH-As(III) and the newly oxidized As(V) by KMnO4, was subsequently determined according to the analysis of As(V). Finally, the concentration of As(III) was calculated on the basis of the difference between total As and As(V). The concentration of Fe(II) was directly determined using the ferrozine method at the wavelength of 510 nm with an Alpha-1502 Spectrophotometer (Shanghai Puyuan Instrument Co. Ltd., China). After reducing Fe(III) to Fe(II) with excessive HONH3Cl, the dissolved total Fe concentration was monitored with the same method as that of Fe(II). The concentration of CA was determined via HPLC (LC-16, Shimadzu, Japan) according to the method reported by Dai et al. (2011). H2O2 produced during the reaction was detected using a photometric method (Zhang et al., 2017). In order to measure the %OH generated during the reaction, coumarin was introduced into the reaction system to capture the hydroxyl radical, thus forming 7-hydroxycoumarin. Following this, the 7-hydroxycoumarin was measured using a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Japan) (Liu et al., 2009).

3. Results and discussion 3.1. Characterization of SCH and SCH-As(III) before and after the reaction Using XRD analysis, our previous work has confirmed that SCH and SCH-As(III) were successfully prepared (Zhang et al., 2019). The SEM 14

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Fig. 4. (a) Dissolved total Fe and Fe(II) and (b) dissolved total As in the presence and absent of CA (1 mM) at initial pH 3 in the dark.

19.9 mg L−1, respectively. The dissolved total Fe from SCH-As(III) decreased by 21% compared to that from SCH, suggesting the enhanced stability of SCH loaded with As(III). In the absence of CA, however, the concentration of the dissolved total Fe gradually increased during the reaction (Fig. 3a), with the maximum dissolution value at 5.8 mg L−1. This is far less than that in the presence of CA. Note the similar concentrations of the dissolved total Fe and the dissolved Fe(II) in the systems, demonstrating that the iron in the solution existed almost in the species of the dissolved Fe(II). It has been reported that dicarboxylic acid and polycarboxylic acid possess a strong affinity with Fe(III) in forming a complex with light activity that can absorb UV and blue visible light (200–450 nm) to produce Fe(II) and organic acid free radicals (Borer and Hug, 2014). Similarly, Fe(II) is generated from Fe(III)CAn via ligand-to-metal charge transfer (LMCT). Furthermore, in the absence of CA (Fig. 3a), the dissolved total Fe and the dissolved Fe(II)

et al., 2015). The fitting lines of Fe2p3/2 orbit demonstrated the appearance of Fe-SO4, FeeS, and FeOOH at the structure and the adsorption sites (Gan et al., 2015). Based on Zhang et al. (2010), the bonding energies of As(III) and As(V) 3d orbits appeared at 44.3 eV, 45.4 eV, respectively. This suggests that the As(III) adsorbed by SCH was mostly converted into As(V) following the reaction. 3.2. Effect of CA and UV on the stability of SCH and SCH-As(III) The variation of the dissolved total Fe and total As from SCH and SCH-As(III) was investigated, with the results demonstrated in Fig. 3. It is noted from Fig. 3a that the concentration of the dissolved total Fe increased at the initial stage of the reaction (0–240 min) and subsequently decreased (240–540 min). The maximum concentrations of the dissolved total Fe from SCH and SCH-As(III) were 25.2 mg L−1 and 15

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Fig. 5. Effect of CA concentration on (a) the stability of SCH-As(III) under UV irradiation at initial pH 3. Dissolved total Fe and Fe(II) and (b) dissolved total As.

the observed trend of the dissolved total Fe. This indicates that UV irradiance favors the retention of As on SCH, which is supported by the observations of Ren et al. (2018). In order to understand the variation tendencies of the dissolved total Fe and the dissolved total As during the reaction, the concentration of CA and the pH levels were further monitored (Fig. 3c and d). It is observed that CA (1 mM) was fully consumed within 240 min, while the pH increased from 3 at the initial stage to 3.8 at 240 min for the SCH/ CA and SCH-As(III)/CA systems. Following this, the pH began to decline to 3.5 up to the end of the reaction. The decomposition of CA was accompanied by a pH increase, implying that the increase of the solution pH was related to the consumption of CA. In addition, pH variations corresponded to those of the dissolved total Fe and the dissolve total As, suggesting that the increase in pH promoted the precipitation between the dissolved total Fe and the dissolved total As. This is discussed

were also similar, implying that under UV irradiation, LMCT also occurs in the complex of Fe-OHn on the surface of SCH. The variation of the dissolved total As in the systems of SCH-As(III)/ CA and SCH-As(III) under UV irradiation are presented in Fig. 3b. Similar to the dissolved total Fe, the dissolved total As rapidly increased at the initial stage of the reaction (0–240 min), followed by an abrupt drop from 240 to 540 min in the SCH-As(III)/CA system. The dissolved total As concentration reached a maximum (6.1 mg L−1) at 240 min. Note that 60–70% of the dissolved total As was present in the form of As (V) species, suggesting that most of the As(III) was converted into As(V) during the reaction. This is in accordance with the analysis of the XPS spectra of SCH-As(III). Almost no dissolved total As was detected in the solution by the end of the reaction. In the absence of CA, the concentration of the dissolved total As gradually decreased from 1.7 mg L−1 at 30 min to zero at the end of the reaction, differing from 16

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Fig. 6. Effect of pH on the stability of SCH-As(III) in the present of CA (1 mM) under UV irradiation. (a) Dissolved total Fe and Fe(II) and (b) dissolved total As.

end of the reaction, the dissolved total Fe in the presence and absence of CA was observed as 3.1 mg L−1 and 1.6 mg L−1, respectively. The concentrations of the dissolved total As in the presence and absence of CA was given as 4.3 mg L−1 and 3.5 mg L−1, respectively (Fig. 4b). These values are higher than those detected under UV irradiation at the end of the reaction; this observation is subsequently discussed in Section 3.3. This again proves that UV irradiation can improve the immobilization of As on SCH. The results obtained in the dark are in accordance with those reported by Redman et al. (2002), who found that the introduction of DOM to systems of hematite loaded with As(III) and As(V) led to the re-dissolution of As. Furthermore, it is noted from Fig. 4a that the dissolved total Fe was much lower than that with UV light, and almost no Fe(II) was detected in the solution regardless whether the CA present or not (in this case, no

further in Section 3.3. In the absence of CA, however, the variation of pH was small for the SCH-As(III)/UV system and the solution pH remained at approximately 3. In this case, the precipitate of arsenic and iron cannot occur because of the low pH and the low dissolved total Fe. Thus, the decline of the dissolved total As is assigned to the adsorption of As(V), as the As(III) was converted into As(V) by the %OH resulting from the LMCT in Fe(III)-OH under UV irradiation. The presence of As (V) as negative ions in a weak acidic solution instead of As(III) as neutral molecules is conducive to the arsenic re-adsorption by SCH. This implies that UV irradiation can improve the immobilization of As (III). In the dark, CA also promoted the dissolution of total Fe and total As from SCH-As(III). As shown in Fig. 4a, the concentration of the dissolved total Fe in the solution increased with the reaction time. At the 17

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LMCT occurs). This further confirms that CA and UV light synergistically improve the reductive dissolution of SCH and SCH-As(III). 3.3. Effect of CA concentration and initial pH on the stability of SCH-As (III) The dissolution of the dissolved total Fe and the total As from SCHAs(III) at different CA concentrations (1, 1.75 mM and 2.5 mM) were further investigated. As shown in Fig. 5a and b, a higher concentration of CA resulted in more total Fe and total As dissolved out from SCH-As (III). At 2.5 mM CA, the maximum values of the dissolved total Fe and the dissolved total As were observed as 56.1 mg L−1 and 17.0 mg L−1, respectively. These are much greater than those corresponding to 1 mM CA. It is also observed from Fig. 5a that the turning point of the dissolution curve of the dissolved total Fe was greatly postponed at 2.5 mM CA. It is possible higher concentrations of CA take a longer time to be depleted. The effect of the initial pH (3, 4, 5) on the stability of SCH-As(III) was also investigated. It is observed that the release of the total Fe and the total As was related to the initial pH (Fig. 6a and b). At the initial pH 3, the maximum concentration of the dissolved total Fe was 20.0 mg L−1, while the concentration of the dissolved total Fe was just 6.6 mg L−1 at the initial pH 5. Moreover, the initial pH 5, the dissolution of the total As rose from 3.2 mg L−1 at 30 min to 5.7 mg L−1 120 min. The concentration of the dissolved total As in the solution then stabilized to an almost constant level. The variation tendency of the dissolved total As at the initial pH 5 was much different from that at the initial pH 3, in which the dissolved total As abruptly dropped at 240 min. It has been reported that Ksp(FeAsO4·xH2O) is 5.01 × 10−24 (Nordstrom et al., 2014), which implies that Fe(III) and AsO43− easily form stable precipitates. Thus, the decline of the dissolve total As at an initial pH 3 is assigned as the cause of the precipitation between Fe(III) and AsO43− from the high dissolved total Fe concentrations. However, a low concentration of the dissolved total Fe at the initial pH 5 is not conducive to forming a precipitate of FeAsO4. In addition, the distribution curves of the Fe(III) species at different pH levels was obtained by Medusa software (Fig. S3) indicates that Fe (III) exists almost completely as Fe(OH)3 at pH > 4. This further suggests that it is difficult to form the precipitate of FeAsO4 at pH 5. 3.4. Effect of coexistence ions on the stability of SCH-As(III) Considering the special environment of the formation of SCH, it is necessary to determine the effect of coexistence ions, such as K+, Ca2+, Al3+, and CO32−, on the stability of the mineral. As shown in Fig. 7a and b, in comparison with the control, the presence of CO32−, K+, and Ca2+ slowed down the decline of the dissolved total Fe in the solution, but promoted the decline of the dissolved total As. Note that the variation trend of the dissolution of the total Fe and the total As from SCHAs(III) in the presence of Al3+ was completely different. Al3+ was able to lead to a slow increase of the total Fe and the total As in the solution during the whole reaction. Chen et al. (2013) reported that Al3+ was able to form a stable complex with tartaric acid. Similarly, CA may react with Al3+ to produce a complex. Thus, the variation trend of the total Fe and the total As in the solution containing Al3+ can be explained by the following two factors. On the one hand, it is considered that LMCT may be inhibited in the Al-(CA)n complex due to the stability of Al3+, which postponed the consumption of CA and the increase of the pH in the solution. To prove this, we monitored the concentration of CA and pH during the whole reaction. As shown in Fig. 7c, the degradation of CA in the solution was significantly delayed compared with that in the absence of Al3+ (Fig. 3c), with the solution pH maintained < 3.1. On the other hand, the Al3+ concentration (500 mg L−1) was much higher than that of CA (1 mM), which led to a low free CA due to the interaction of CA with Al3+. Consequently, the reductive dissolution of SCHAs(III) was weakened.

Fig. 7. The effect of coexistence ions on the stability of SCH-As(III) in the present of CA (1 mM) during UV irradiation at initial pH 3. (a) Dissolved total Fe and Fe(II) and (b) dissolved total As. (c) The variation of CA concentration in the present of Al3+ (500 mg L−1).

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Fig. 8. The photoreductive dissolution mechanism of SCH-As(III) in the presence of CA (1 mM).

H+ + O2·− → HO2·−

3.5. Photoreductive dissolution of SCH-As(III) and As release mechanism

2 HO2

Based on the discussion above and the work of previous studies (Panias et al., 1996; Borer and Hug, 2014; Ren et al., 2018; Zhang et al., 2019), a possible mechanism of the photoreductive dissolution of SCHAs(III) and the release of As induced by CA was proposed, and summarized in Fig. 8. First, partial Fe(III) and As(III) is dissolved out from SCH-As(III) in an acidic solution, and the dissolved Fe(III) is further coordinated with CA to form Fe(III)-CAn (Eqs. (1)–(2)). Meanwhile, CA can also form a (CA)n-SCH-As(III) complex with Fe(III) on the surface of SCH-As(III) (Eq. (3)). Under UV irradiation, Fe(II) or CA radicals are generated via LMCT in Fe(III)-CAn, (CA)n-SCH-As(III) and ≡Fe(III)-OH (Eqs. (4)–(6)). H2O2 is generated via a series of free radical reactions triggered by the CA free radicals and dissolved oxygen (Eqs. (7)–(9)). Subsequently, %OH is produced via irradiation (Eq. (10)) and the Fenton reaction between Fe(II) and H2O2 (Eq. (11)). As(III) dissolved out from SCH-As(III) is further oxidized to As(V) by the generated %OH (Eq. (12)) and the produced As(V) then co-precipitates with Fe(III), or is re-adsorbed by SCH (Eq. (13)).

H2 O2

˙−

→ H2 O2 + O2 hv

→

< 300nm

2·OH

(8) (9) (10)

H2 O2 + Fe(II) → Fe(III) + OH– + ˙OH

(11)

2˙OH + AsO33 − → AsO43 − + H2 O

(12)

Fe(III) + AsO43 − → FeAsO4 (precipitation)

(13)

Fe(III) + nCA → [Fe(III) − (CA)n]

(2)

In order to prove the generation of H2O2 and %OH during the reaction, measurements of H2O2 and %OH were conducted, and the results are illustrated in Figs. S4 and S5, respectively. It can be seen from Fig. S4 that, in the presence of CA, H2O2 concentration clearly increased with reaction time. However, almost no H2O2 was detected in the absence of CA. Note that the fluorescence intensity of 7-Hydroxy-coumarin was much higher than that from N2 degassing (Fig. S5), suggesting that the dissolved oxygen was decisive in the production of %OH (Eq. (7)). Furthermore, the chemical and XPS spectral analysis confirmed the presence of As(V) after the reaction. Thus, the proposed mechanism for the photoreductive dissolution of SCH-As(III) and the refixation of dissolved As in the existence of CA may be reasonable.

SCH − As(III) + nCA → (CA)n − SCH − As(III)

(3)

4. Conclusion

[Fe(III) − (CA)n] + hv → Fe(II) + ˙CA

(4)

(CA)n − SCH − As(III) + hv → Fe(II) + ˙CA + As(III)

(5)

≡ Fe(III) − OH + hv → Fe(II) + ˙OH

(6)

SCH‐As(III)

dissolution

→

Fe(III) + As(III)

˙CA + O2 → CO2 + O2

˙ −+products

(1)

In this paper, the effect of the CA concentration, the initial pH, the coexisting ions including CO32−, K+, Ca2+, and Al3+ on the stability of SCH-As(III) and the release of As were all investigated. Results demonstrated that CA accelerated the dissolution of total Fe and total As from SCH-As(III), with or without UV light. The pH solution levels increased with the consumption of CA. Moreover, the concentration of

(7) 19

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the dissolved total Fe and the dissolved total As declined from the coprecipitation between Fe(III) and As(V), or from the re-adsorption when CA was depleted. Compared to SCH, the reductive dissolution of SCHAs(III) was suppressed. The presence of the CO32−, K+, and Ca2+ ions slowed down the decline of the dissolved total Fe, yet slightly improved the decline of the dissolved total As at the later stage of the reaction. However, Al3+ alleviated the dissolution of Fe and As from SCH-As(III) at the initial stage of the reaction, due to the formation of Al(III)-(CA)n. In this case, the concentrations of the dissolved total Fe and the total As slowly increased during the whole reaction. The Fe and As released into the solution were present mainly in the forms of Fe(II) and As(V), respectively, produced by LMCT in Fe(III)-(CA)n and a series of radical reactions. Thus, this study allows us to gain a deeper understanding into the stability of SCH-As(III) in a natural environment.

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