Treatment of Plating Wastewaters by Ferrous Reduction, Sulfide Precipitation, Coagulation & Upflow Filtration

摘要

Current practice in treatment of mixed metal wastewaters containing hexavalent chromium consists of acidic (pH approx. 3) chromium reduction followed by alkaline, metal-hydroxide precipitation; a two-stage process. A single-stage process could reduce capital and operation costs. Synthetic chromium, cadmium, and nickel wastewaters were successfully treated by ferrous sulfate and sodium sulfide in a single cell reactor at alkaline pH (7-10), thus clarifying conflicting reports in the literature. Batch tests were performed to study this process. Solids were removed by membrane filtration. Metal residuals were analyzed by atomic absorption spectrophotometry. Chromium was rapidly and stoichiometrically reduced by ferrous sulfate. Sodium sulfide by itself negligibly reduced chromium, and was no better than hydroxide in precipitating cadmium and nickel. However, in combination with ferrous sulfate, sulfide improved chromium reduction and metal removal. Alkalinity, EDTA and cyanide interfered with chromium reduction and metal removal; calcium hardness counteracted these interferences. (Author)