Removal of cadmium ions from industrial wastewater by cementation

Abstract

Electroplating waste solutions constitute a major source for environmental pollution with heavy metal ions such as Cu 2+ , Cd 2+ , Pb 2+ , and Cr 6+ . These ions are lethal to man and aquatic life as well, therefore should be removed from wastewater before disposal of waste. The present work is concerned with the removal of Cd 2+ from waste solutions by cementation on zinc in a batch stirred tank reactor lined with a cylindrical zinc sheet. The rate of Cd 2+ removal was studied under different conditions of impeller rotation speed, initial Cd 2+ concentration, temperature, and time of cementation. It was found that the rate of Cd 2+ removal increases with increasing impeller rotation speed, temperature, initial concentration and residence time. Mechanistic study of the reaction through activation energy (35.98 kJ/mol) determination showed that cementation of Cd 2+ on zinc is a diffusion controlled reaction. Visual observations showed that cadmium deposits on zinc are in the form of powder. Cadmium powder is a valuable salable product which can improve the economy of the process. Rates of cementation were expressed in terms of the rate of mass transfer. The experimental data for the conditions: 8444<Re<56768 and 707<Sc<1567 fit the following equation: Sh=0.394 Re 0.438 .Sc 0.33 -17.208. Addition of polyox WSR-301 drag reducing polymer was found to reduce the rate of cementation by an amount depending on polymer concentration.