Effect of speciation and composition on the kinetics and precipitation of arsenic sulfide from industrial metallurgical wastewater

Precipitation of arsenic as As₂S₃ produces little waste sludge, has the potential for low chemical consumption and for selective metal(loid) removal. In this study, arsenic removal from acidic (pH 2), metallurgical wastewater was tested in industrially relevant conditions. Sulfides added at a S:As molar ratio of 2.5 and 5 resulted in removal of 99% and 84% of As(III) and As(V). Precipitation of As₂S₃ from the As(III) and industrial wastewater containing 17% As(V) was nearly instantaneous. For the synthetic As(V) solution, reduction to As(III) was the rate limiting step. At a S:As ratio of 20 and an observed removal rate (k₂ = 4.8 (mol L^-1) h^-1), two hours were required to remove of 93% of arsenic from a 1 g As L^-1 solution. In the case of As(V) in industrial samples this time lag was not observed, showing that components in the industrial wastewater affected the removal and reduction of arsenate. Speciation also affected flocculation and coagulation characteristics of As₂S₃ particles: As(V) reduction resulted in poor coagulation and flocculation. Selective precipitation of arsenic was possible, but depended on speciation, S:As ratio and other metals present.