Transformation behavior of the morphology, structure and toxicity of amorphous As2S3 during hydrothermal process
Graphical abstract
Introduction
Arsenic is a trace element with high toxicity founded in the earth crust (Bowell et al., 2014; Drahota and Filippi, 2009; Nazari et al., 2017; Wang et al., 2021). According to the study of anthropogenic arsenic cycles, China is the largest producer and consumer of arsenic-containing ores today, resulting in a large amount of arsenic-containing wastes every year (Chen et al., 2016; Shi et al., 2017). Arsenic sulfide sludge (ASS), the main arsenic-containing hazardous waste in nonferrous smelting industry, is formed in the treatment of acid wastewater and waste electrolyte by sulfide precipitation (Guo et al., 2015; Peng et al., 2018; Peng et al., 2012; Zheng et al., 2013). The main component of ASS is amorphous As2S3, which is susceptible to light, pH, temperature and coexisting substance (Lu et al., 2019). In the long-term storage process of untreated ASS, arsenic is prone to slow release, reverse dissolution and transformation, which would cause adverse effects to the surrounding environment and human health (Yao et al., 2019). Therefore, the safe and stable disposal of ASS has become an urgent problem that should be tackled as soon as possible.
At present, stabilization/solidification (S/S) is the most common approach to treat arsenic-containing hazardous wastes (Desogus et al., 2013; Li et al., 2016). In this process, the arsenic-containing waste is mixed with cement, fly ash, polymeric materials or other agents to constitute a cementitious binder system, which could effectively prevent arsenic from entering the environment (Leist et al., 2000; Sullivan et al., 2010). For the case of ASS, it is proven that there is a remarkable decrease in the leaching concentration of arsenic after stabilization/solidification (Xiao and Jing-Yu, 2014). Nevertheless, this method requires a large addition of chemicals, which leads to a significant increase in the volume and weight of final products and in disposal costs. For example, in the study of Lu et al. (2017), the Ca/As molar ratio should be up to 8:1 in order to use CaO to control the As leaching concentration of ASS. Hence, there is an urgent need to develop new technologies for the treatment of ASS due to the adverse effects brought by the existing technology.
Hydrothermal treatment is an effective method to synthesize minerals (Berre et al., 2007; Zhao et al., 2010). Under the hydrothermal conditions, the duration of mineralization process can be shortened markedly from millions of years to only few hours. Recent researches have shown that hydrothermal treatment is widely applied in the disposal of arsenic-containing hazardous wastes by forming stable arsenic-containing minerals (Borg et al., 2014; Itakura et al., 2007; Liu et al., 2017). For instance, Chai et al. (2018) found that the well-crystallized tooeleite (Fe6(AsO3)4SO4(OH)4·4H2O) synthesized by hydrothermal treatment exhibited favorable stability and had a significant effect on the fixation of arsenite. Viñals et al. (Vinals et al., 2010) reported that the arsenical natroalunite (~(Na,Ca)(Al,Fe)3((S,As,P)O4)2(OH)6) precipitated during the hydrothermal stabilization process of calcium arsenate wasted could be effective for the long-term storage. In our previous works, hydrothermal treatment was utilized to dispose ASS, which showed dramatical effects on the volume reduction, dehydration, and detoxication of final products (Xu et al., 2020; Yao et al., 2018). Specifically, the moisture content declined from 62.59% to 6.50%, the volume reduction ratio reached 91.67%, and the leaching concentration of arsenic decreased from 702.0 mg/L to only 0.67 mg/L. It was also demonstrated that the morphology and structure of ASS changed remarkably after hydrothermal treatment. Therefore, we suspect that the leaching toxicity of ASS is closely related to its micromorphology and phase structure. Therefore, it is necessary to systematically study the transformation behavior of the morphology, structure and toxicity of amorphous As2S3 (the main component of ASS) during hydrothermal process.
In this study, amorphous As2S3 precipitated by the reaction of NaAsO2 and Na2S·9H2O in acidic aqueous was regarded as the research object. The effects of hydrothermal temperature, initial pH, and solid-to-liquid (S/L) ratio on the transformation of amorphous As2S3 were investigated. Meanwhile, the evolution behavior of morphology and structure of amorphous As2S3 and the variation of the As leaching concentration were explored. This study assists in the understanding of the connection between the morphologic structure and stability of arsenic sulfide, so as to further explain the hydrothermal stabilization mechanism of arsenic sulfide.
Section snippets
Chemicals
Sodium arsenite (NaAsO2), sodium sulfide (Na2S·9H2O), sulfuric acid (H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH) are of analytical regent (AR) grade. Sodium arsenite was purchased from Aike Reagent Co., Ltd. (China). All the other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd. (China).
Preparation of amorphous As2S3
The experiment was performed in a 1000 mL conical flask as shown in Fig. 1a. 3.25 g NaAsO2 and 9.00 g Na2S·9H2O were successively added into the conical flask containing 500 mL
Characterization of arsenic sulfide precipitate
In the sulfidation process, As (III) and S (-II) precipitated in the form of amorphous arsenic sulfide (As2S3), which was identified by the XRD pattern (Fig. 2a) that no peaks indicative of crystalline phases was found. Raman spectrum analysis further confirmed the presence of amorphous As2S3. As can be seen from Raman spectrum (Fig. 2b), there was a broad band centered around 341 cm−1, which was corresponded to the symmetric stretching vibration of AsS3/2 structural units of amorphous As2S3 (
Conclusions
In this work, the transformation behavior of amorphous As2S3 in the hydrothermal environment was investigated. The results showed that amorphous As2S3 could be transformed to orpiment in a wide temperature range from 180 °C to 270 °C. Especially, 210 °C was the most appropriate temperature for the mineralization of orpiment. When the hydrothermal temperature reached 270 °C, As4S4 crystals appeared and coexisted with orpiment. Initial pH had little effect on the transformation of amorphous As2S3
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (51904354 and 51634010), the National Science Fund for Distinguished Young Scholars (51825403), the National Key R&D Program of China (2018YFC1900301 and 2019YFC1907405), and the Key R&D Program of Hunan Province (2019SK2291).
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