Toxic and precious metals removal from wastewater using freeze concentration and electrodeionization

Abstract

Industrial wastewater may contain metals such as chromium (VI), nickel, cobalt and palladium. These metals are toxic to aquatic organisms and also have an adverse impact on human health and environment. Due to limited availability and high demand, the value of the precious metals such as palladium has increased significantly. There has long been an interest in recovering toxic and precious metals from industrial effluents for sustainable use of resources and compensate the cost of wastewater treatment, in addition, to reduce environmental pollution. However, the low concentrations of precious metals in wastewater make recycle and recovery difficult. Some removal and recovery processes have been developed in recent years, but the costs of treatments are usually high and some processes also generate large volumes of secondary wastes. There is a need for a cost-effective treatment method for precious and toxic metal removal/recovery from wastewater. This research was carried out to investigate the potential of freeze concentration, electrodeionization and combination of the two processes for removal and recovery of wide concentration range of chromium (VI), nickel, cobalt and palladium in synthetic wastewater. The feedwater with metal concentrations starting from 20 mg/L was frozen at -15°C. By freezing only 80% (volume) of the feedwater, about 99% reduction of metal concentration in ice was achieved in single stage freeze concentration. The concentrated unfrozen liquid was then used as feedwater for the electrodeionization system. Multi-stage (two and three stages) freeze concentration were also used for further treatment of the unfrozen liquid with high concentrations of metals. By freezing only 80% (volume) of the feedwater and with the washing of the ice, about 99% reduction of metal content in ice and 99% volume reduction of feedwater were achieved by multi-stage freeze concentration process. Palladium concentrations of 160-fold increment of the initial feedwater concentration could be achieved using multi-stage freezing. Overall, it was found that freeze concentration is an effective method for chromium (VI), nickel, cobalt and palladium removal from synthetic wastewater. In the electrodeionization approach, using commercially available resins, the removal efficiencies for chromium (VI), nickel, cobalt and palladium using the unfrozen liquids from freeze concentration process as feedwater were investigated. The feedwater concentrations in the range of 120 - 650 mg/L examined using 15 mA and 50 mA current. Removal efficiencies were 97% for 120 mg/L of chromium (VI), 65% for 500 mg/L of cobalt and 90% for 250 mg/L of palladium. Promising results were shown by combination of freeze concentration and electrodeionization approach for the removal and recovery of toxic and precious metals from industrial wastewater. Results of freeze concentration and electrodeionization showed that using 80% freezing of the initial feedwater volume, very high removal efficiencies were achieved for freeze concentration process for separation of chromium (VI), nickel, cobalt and palladium while similar results were only possible in chromium (VI) and palladium using EDI. Overall, freeze concentration process was not so sensitive to changes in influent metal concentration and chemical natures of the metals as EDI.