Efficient removal and immobilization of arsenic from smelting wastewater through the formation of amorphous ferric arsenate minerals under ambient-pressure hydrothermal conditions

Arsenic removal from waste acid using ferric oxide as iron source. • Amorphous ferric arsenate was formed under ambient-pressure hydrothermal conditions. • Prospective process for the effective treatment of arsenic-containing wastewater. Owing to its high toxicity and environmental mobility, high‑arsenic smelting wastewater (HSW) poses a persistent risk to ecological stability and human health. In this study, an effective method was proposed to remove and immobilize arsenic from HSW by using ferric oxide as a solid iron source to form amorphous ferric arsenate (AFA). Initially, ferric oxide was pre-dissolved in HSW to release Fe 3+ ions, while simultaneously removing 35.5% of arsenic. During the ambient-pressure hydrothermal precipitation process, the dissolved Fe 3+ reacted with arsenic to form AFA precipitate. Notably, additional H + ions generated during this reaction promoted the continuous dissolution of ferric oxide, establishing a self-sustaining dissolution-precipitation cycle. Under optimized conditions, including a pre-dissolution time of 300 min, a Fe/As molar ratio of 4, an initial pH of 1.5, a reaction temperature of 90 °C, and a reaction time of 24 h, the arsenic concentration decreased from an initial 8276 mg/L to 585 mg/L, corresponding to a removal efficiency of 92.9%. A subsequent treatment process, involving adsorption onto fresh ferric oxide followed by lime neutralization of the post-precipitation supernatant, successfully reduced the final arsenic concentration to below 0.08 mg/L and ensured that all other heavy metal ions met industrial wastewater discharge standards. Moreover, compared with direct one-step neutralization, the proposed method first removed most arsenic as an AFA precipitate under acidic conditions, thereby reducing the amount and hazard potential of the final neutralization sludge. This study demonstrates that the hydrothermal method holds significant potential for the large-scale treatment of HSW. Download: Download high-res image (273KB) Download: Download full-size image