Fundamental research on selective arsenic removal from high-salinity alkaline wastewater

The alkaline leaching process of arsenic-containing solid waste discharged during nonferrous metal smelting affords typical high-salinity alkaline arsenic-containing wastewater (HSAW). In this study, for the first time, Me (Ca²⁺ and Mg²⁺)-AsO₄^3--OH^--H₂O and Me (Ca²⁺ and Mg²⁺)-AsO₄^3--CO₃^2--H₂O systems are studied based on a thermodynamic equilibrium diagram and an arsenic removal experiment, proving that the removal of arsenic using single metal ions in the presence of CO₃^2- is infeasible because of carbonate coprecipitation. Based on this observation, a new method that uses magnesium ammonium complex salts (MACSs) for HSAW treatment is proposed. Based on the thermodynamic calculations of the Mg²⁺-AsO₄^3--NH₄⁺-CO₃^2--H₂O system and the arsenic removal experiment, carbonate and arsenate can be selectively separated by the formation of magnesium ammonium arsenate (NH₄MgAsO₄·6H₂O). In an arsenic solution containing 150-g/L Na₂CO₃, the arsenic removal rate and the arsenic grade of the precipitation product reach 90.16% and 27.13%, respectively, when the molar ratios of Mg²⁺/NH₄⁺:As(V) are 1.8:1 and 2:1, respectively. The proposed method is successfully employed for treating a leaching solution of alkaline arsenic slag discharged during antimony smelting. The findings of this study will broaden the basic theory of HSAW treatment and lay a foundation for the resource treatment of arsenic-containing solid waste.