Selective Removal of Heavy Metals from Acidic Wastewater

Acidic wastewater containing heavy metals and arsenic is produced by industrial processes such as metal smelting or mineral processing. This strongly contaminated water poses a severe environmental risk and requires extensive treatment.

One established procedure to treat this type of wastewater is based on neutralizing the acid with lime and storing the contaminants in solid form. Another option is the precipitation of the heavy metals and arsenic in the form of sulfides using H₂S generated from sulfide salts. However, both methods have disadvantages. Neutralization leads to large amounts of solid waste, while sulfides hydrolyze quickly in strongly acidic media and toxic H₂S is released from the solution.

Xianjia Peng, University of the Chinese Academy of Sciences, Beijing, and colleagues have developed a process to remove arsenic and heavy metals from strongly acidic wastewater via the selective generation of H₂S from thiosulfate under UV radiation. The practical applicability of the process was tested on wastewater from a zinc smelting factory. In the dark, about one-third of the contaminants were removed. Almost complete removal was achieved under UV light, because UV irradiation promotes the formation of H₂S via a radical mechanism. To avoid a negative effect of precipitated sulfide particles on UV transmission, the wastewater was filtered.

Without arsenic or heavy metals, no release of H₂S was observed because H₂S formed during thiosulfate decomposition is consumed by a reaction with sulfite or by photolysis. In the presence of these contaminants, the formation of heavy metal or arsenic sulfide precipitates is the preferred reaction path, leading to a highly selective system.

Overall, the method does not cause H₂S pollution while efficiently removing arsenic and heavy metal species from acidic wastewater—without neutralizing the acid, which remains available for further use. The estimated costs are $0.68 per m³ of wastewater, making the process suitable for industrial application.