The dissolution characteristics of CaS in the presence of CO2 has been investigated by monitoring sulfide speciation, solution conductivity and pH during dissolution. The sulfide speciation associated with CaS dissolution was utilized for metal precipitation from acidic wastewater effluents. The mechanism involved in the dissolution process was observed to be pH-dependent, characterized by increased solution conductivity as the HS− species becomes dominant in solution in the form of the Ca(HS)2 complex. The replacement of HS− by CO32− in the Ca(HS)2 complex triggered CaCO3 precipitation and H2S stripping and this was characterized by decreased solution pH and conductivity. The sulfide to total metal molar ratio was observed to have an effect on the pH and therefore sulfide speciation as well as extent of metal removal. The utilization of CaS in the treatment of acidic wastewater effluents demonstrated complete metal removal, with the potential of a pH-controlled selective metal removal and recovery.
Keywords: calcium sulfide, dissolution, precipitation, wastewater effluent