The solubility product of cadmium sulfide is 3.6 X 10~29. Its solubility is 8.6 X 10~10 mg/l. As cadmium electroplating is performed in cyanide baths, the drag-out is alkaline. Therefore, alkaline carbonates and sulfides can remove cadmium as an insoluble salt. The hydroxide is too soluble, resulting in cadmium concentrations of 5 to 10 mg/l. If carbon dioxide is subsequently absorbed before neutralization, additional cadmium will be removed. Cadmium, even when present in trace concentrations, is strongly coprecipitated with calcium carbonate.
Removal to concentration levels around 0.01 mg/l requires the removal of particulate carbonates or sulfides, since residual soluble cadimum can be expected to be within limits. The particulates are very small and settle very slowly, requiring digestion to increase particle size followed by settling or filtration to remove the fines.
A treatment solution containing NaOH, Na2CO3, Na2S and CaO will effect satisfactory treatment, but sulfide release may result in disagreeable odors when final effluent pH is reduced to low values, if the sulfide is not destroyed (e.g., by sodium hypochlorite, which is used for cyanide destruction).
Ion exchange, reverse osmosis, electrodialysis, distillation, and flotation processes can all remove cadmium from wastewaters.
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