Visible-light photocatalysis accelerates As(III) release and oxidation from arsenic-containing sludge
Graphical abstract
Introduction
Arsenic is ubiquitous in the Earth’s crust, with a mean concentration of 0.5–2.5 mg/kg – about 0.00005% of the Earth’s crust [1]. It often coexists with the ores containing precious and non-ferrous metals, or iron [2]. During metal smelting, mineral processing, and sulfuric acid production from pyrite ores, a large amount of arsenic-containing acid wastewater and tailings are produced [3]. For example, the arsenic concentration in the wastewater from a sulfuric acid plant can range from several to tens of thousands mg/L [4]. One of the most popular techniques for treatment of the arsenic wastewater in industry is sulfide precipitation, where sulfide is employed to transform arsenic ions into arsenic sulfide precipitate [5]. This technique has many advantages, such as low solubility of arsenic sulfide at a low pH, high sediment rate and efficiency, less sludge volume and water content [6,7]. As a result, large quantities of arsenic sulfide slag is discharged into the environment. For example, more than half a million tons of arsenic sludge are produced annually in China.
Arsenic sulfide sludge deposited in the environment is susceptible to temperature, pH, coexisting organics and inorganics (e.g. sulfides) [8,9]. The weathered and dissolved residues promote the release of arsenic ions into the surroundings, which can result in the transport and transformation of chemical species (e.g. arsenic and sulfur) in natural waters and so leading to environmental contamination. Previous studies have demonstrated that when pH is higher than 9, the dissolution of artificial As2S3 particles is significantly enhanced, owing to the enhanced activity of the hydroxylated surface species [10]. On the other hand, different types of sulfur species can influence the dissolution rate of arsenic sulfide. For instance, the added sulfide ions can react with arsenic sulfide and produce arsenic-sulfide complex (H2As2S6¯), according to eq 1, which will accelerate the dissolution of the solid [11,12].
Recently, the effect of light on the dissolution of minerals containing heavy metals has been studied to elucidate the mechanism of photocorrosion reactions on the release of metal ions, such as antimony and vanadium, from their parental minerals or the synthesized substitutes (e.g. senarmontite (Sb2O3) [13], stibnite (Sb2S3) [14], and vanadium titano-magnetite [15]. It has been demonstrated that simulated sunlight or UV irradiation can promote the dissolution of minerals and thus release heavy metal ions. Up to this time, no work has been reported on the effect of light-induced photochemical reactions on the fate of actual arsenic sludge from industry. As sunlight is one of the most important climate factors for ecosystems, it inevitably affects the fate of heavy metal slag deposited in the environment. On the other hand, arsenic sulfide is a semiconductor with a band gap (˜2.34 eV) in the range of visible light spectrum. It has been reported that photocorrosion of artificial As2S3 colloids could occur by light irradiation [16]. Therefore, it is expected that the actual sludge, which mainly contains arsenic sulfide, is photo-responsive under visible light conditions.
The objective of the investigations described in this paper was to study the dissolution and transformation mechanisms of actual arsenic-containing sludge under visible light conditions. The release and oxidation kinetics of arsenic and sulfur from the sludge, as well as the structure and state of the solid phase, were examined at different pHs under visible light. The intermediate sulfur species arising during the photoreactions were quantified by ion chromatography. The photo-generated active oxygen and sulfur species were identified by EPR and free radical quenching experiments, and their specific contributions to the transformation of the sludge are discussed. The findings of the present investigation assist in the understanding of the fate and transforming process of arsenic sulfide sludge in the environment.
Section snippets
Chemicals and materials
The details of all reagents used are provided in the Supporting Information. The arsenic sulfide sludge, a product of acid wastewater treatment, was sampled from a smelting plant in Fujian province, China.
Photo reaction system
All the photo reactions were performed in a 250 mL beaker by mixing 0.15 g of the solid sludge with 225 g of H2O. The concentration of ASS was fixed at 0.67 g/L. The initial pH of the suspension was adjusted with HCl or NaOH solution. A 500-W Xe arc lamp (Shanghai Jiguang Special Lighting
Analysis of raw ASS
The XRD pattern of ASS revealed that the sludge mainly contained nanosized and amorphous As2S3 particles plus some undefined impurities (Fig. 1a). The SEM image in Fig. 1b also shows that the sludge compromised of small particles with the size less than 100 nm. HRTEM image (Figs. 1b and S1) further confirmed that the particles are amorphous structure. EDS mapping images revealed that the two components, As and S, overlapped each other (Fig. 1d). The XRF and ICP-OES results (Table S2) confirm
Conclusion
In the environment, a larger amount of arsenic sludge is discharged and deposited, especially during the treatment of acid mining and ore smelting wastewater. The sludge poses a major environmental threat, due to the potential release of arsenic ions. When the sludge is exposed to solar light irradiation, not only the release rate of As(III), but also the oxidation rate of As(III) to As(V) can be markedly accelerated, which will further increase the environmental risk of the discharged sludge.
Acknowledgements
The authors acknowledge the financial support from National Key Research and Development Program of China (2017YFA0207204 and 2016YFA0203101), the National Natural Science Foundation of China (Grant No. 21876190 and 21836002), the Key Research and Development Program of Ningxia (2017BY064), and the “One Hundred Talents Program” in Chinese Academy of Sciences.
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