An Electrochemical Process for Simultaneous Treatment of Metal and Cyanide Bearing Wastewater

Heavy metal and cyanide contamination in electroplating wastewater can pose a serious environmental problem if not removed prior to discharge (Rajeswari and Sailaja 2014). Electrocoagulation has been shown to effectively remove many of these contaminants separately using batch treatment systems (Adhoum et al. 2004). The present study seeks to test the technologic efficacy of mixed electrocoagulation and hydroxide precipitation to simultaneously treat metals common to the electroplating industry (Ni+2, Cu+2, Sn+2), and cyanide (CN-). The ultimate goal of the study is to offer a process which has the potential for use in continuous treatment. Cyanide ion (CN-) begins to form volatile and dangerous hydrogen cyanide (HCN) in increasing concentrations as the pH drops below 11. At the same time, literature shows a drop-off in metal removal efficiencies using electrocoagulation at pH’s above 10 (Kobya et al. 2010). To overcome this drop, process variables such as current density, agitation rate, and volumetric flow rate of air were optimized using removal speed and final concentration as the chosen metrics. The data followed first and pseudo-first order kinetics which allowed the rate constants to gauge the relative rates of removal. The present work was able to reduce Ni+2, Cu+2, Sn+2 and CN- concentrations below the discharge limits set forth by the United States Environmental Protection Agency (EPA). The electrocoagulation cell conditions were optimized at a current density of 50 ASF, spin speed of 0 RPM, and volumetric flowrate of air of 0.0022 ft3/min. This, along with a treatment time of 112 minutes, yielded an electricity consumption of 0.0206 kwh/L and an iron consumption of 3.83g/L. The study showed aeration to be necessary in achieving EPA discharge limits for complex cyanide removal. The use of aeration decreased the final concentration of complex cyanide from 4.1 to 0.91 mg/L after 2 hours of treatment. Testing also revealed direct reduction of metallic contaminants to the cathode to be another method of removal. Overall, the study shows electrocoagulation to be technologically and economically viable when treating high speed electroplating effluent. Due to the chosen conditions, the process also has the possibility of being extended to continuous treatment in the future