Effect of Ca on As release from Ferric and ALUM Residuals

Dewatering of residuals containing arsenic (As) has attracted little research but will become increasingly important as As is removed from water by sorption to ferric or aluminum hydroxides. Lime conditioning proves to be an effective method for immobilizing As even at high pH. In contrast, increasing pH with addition of caustic soda or soda ash leaches As from residuals. Soluble calcium appears to neutralize the negative surface charge on the hydroxide solids to enhance As sorption. In many cases, As‐containing residuals are stored in lagoons for months or even years. Although many parameters contribute to soluble As mobilization, the presence of calcium appears to assist in keeping As immobilized.