Innovative uses from carbon adsorption of heavy metals from plating wastewaters: I. Activated carbon polishing treatment

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Abstract
Activated carbon treatment as a polishing step following either metal hydroxide or metal sulfide precipitation has been shown to be effective for removal of residual zinc, cadmium, and excess sulfide from plating wastewaters. When hydroxide precipitation is employed, the use of activated carbon allows significantly lower effuent metal concentrations to be achieved; the residual zinc and cadmium concentrations were reduced in excess of 70% with the use of activated carbon following hydroxide precipitation. In the absence of chelating agents and other interferences, removal of zinc exceeded 70% and sulfide removal exceeded 50% over the pH range of 7 to 10 by employing the activated carbon polishing step. With 100 mg/l of ammonia or cyanide (acting as interferences) present, zinc removal following sulfide precipitation was typically reduced 50%. For waters high in sulfides, removals exceeded 74%. The activated carbon polishing step shows great promise as a treatment alternative to minimize the potential for H2S gas evolution and the concern for sulfide toxicity associated with sulfide precipitation of heavy metals.