Industrial Selenium Pollution: Wastewaters and Physical – Chemical Treatment Technologies

Industrial activities including mining, metal and petrol refining, coal-fired power generation, and intensive agriculture may generate wastes and effluents loaded with selenium (Se). Currently, even if Se discharge is strictly regulated in North America, the large volumes of selenium-laden effluents produced represent a critical problem for both industry and environmental agencies. Numerous treatment technologies for Se removal based on physical and chemical processes have been tested at bench scale, pilot scale, and full scale, and several are commercially available. Physical treatment (e.g., membrane filtration and evaporative systems) can be effective in bringing the Se levels below the discharge standards, but it entails prohibitive operational costs due to high-energy consumption. Adsorption is a less financially demanding alternative treatment option; however, the competition with other anions present in real industrial effluents severely limits the Se removal performance. Chemical treatment has been tested for selenium removal, being mainly founded on the redox change of this element. Various reducing agents (e.g., zero-valent iron and sodium dithionite) have been tested mostly for synthetic wastewaters. pH plays a critical role in the effectiveness of the chemical treatment and, similarly to adsorption, the competition with other anions limits its performance. Several studies have coupled physical and chemical treatment systems in order to attain higher Se removal efficiencies. Residual products of Se treatment are generated by all physical–chemical treatment systems, incurring additional treatment and disposal costs.