The synthesis of calcium arsenate@iron arsenate coating materials and their application for arsenic-containing wastewater treatment

The current method of treating arsenic-containing wastewater is mainly to use a calcium method to synthesize stable calcium arsenate. It is easy to cause reverse dissolution by rain or other erosion, releasing arsenic into the natural world and polluting soil and groundwater. So, calcium arsenate is not an ideal material for removing and immobilizing arsenic. Iron arsenate (FeAsO4) is much better than calcium arsenate because of its stability and acid resistance. In this study, calcium arsenate@iron arsenate coating materials were synthesized. From the results of the XRD and SEM analyses, it was shown that calcium arsenate was coated by an iron arsenate shell which consisted of nanoparticles. The stability of the coating materials was determined using the Toxicity Characteristic Leaching Procedure (TCLP). The results showed that the concentrations of As for CaHAsO4 and Ca3(AsO4)2 were 744 mg L-1 and 302.2 mg L-1, respectively. Arsenic was not detected through the TCLP tests for CaHAsO4@FeAsO4 and Ca3(AsO4)2@FeAsO4 coating materials, and the best coating condition was confirmed to be an Fe/As molar ratio of 4 : 1, pH of 4, and temperature of 50 °C. The stability of the materials showed a significant improvement. The results indicated that calcium arsenate materials could be converted to coating materials by using ferric salts. The coating materials had excellent stability in an aqueous solution. Thus, the coating was suitable for the removal and immobilization of arsenic in industrial applications. This work provided a new way to treat arsenic-containing wastewater, which was simple and economical. This method has potential for use in the field of wastewater treatment containing arsenic.