Development of Adsorption Based Technology for Removal of Selenium from Industrial Wastewater

Abstract

The presence of inorganic selenium compounds in wastewater has been a concern for many industrial sectors, including mining, coal-fired power generation, oil and gas refineries. Technologies based on biological treatment process have been widely adopted by these industries to meet the low discharge limits set by regulatory agencies. While these treatment systems are complicated by themselves, recent studies have shown that microbiological metabolism of inorganic selenium, can result in formation of organic-selenium compounds, such as selenomethionine (SeMet) and selenocysteine (SeCys). These organoselenium compounds exhibit higher bioavailability and toxicity, leading to bioaccumulation in organisms in the receiving streams, including fishes. This finding has led to the decision of the United States Environmental Protection Agency (USEPA) to set environmental selenium thresholds based on biotic tissue-based concentration rather than the traditional aqueous concentration limits. Meeting these regulatory limits are challenging as the currently used technology which is based on biological processes lead to the formation of highly soluble organoselenium compounds. To this end, there is a need for treatment technologies that can remove organoselenium compounds from effluents after biological treatment processes. This research presents a detailed investigation into the feasibility of using adsorption based-technology for the removal of organoselenium from wastewaters. Firstly, the adsorption capacities of commercially applied adsorbent nano-scale zerovalent iron (nZVI), granular activated carbon (GAC), and magnetite (Fe3O4) to remove organoselenium compounds, selenite (Se(IV) and selenate (VI)) were evaluated through batch experiment. Ferrous co-precipitation technology was developed for the treatment of inorganic selenium contaminated water. Kinetics studies were conducted for all adsorbent candidates tested and the results were compared. The study further explored the removal of SeMet and SeCys with nZVI/GAC composites technology. The primary objectives were: a) to prepare nZVI/GAC composites and test their performance for the removal of SeMet and SeCys contaminants from wastewaters, b) investigate the removal mechanisms of organoselenium by nZVI/GAC based on operating conditions, including solid adsorbent loading, pH influence, adsorbate initial concentrations, adsorption kinetics and isotherms and, c) evaluate the influence of coexisting ions in water during organoselenium remediation. To the best of our knowledge, this is the first research to investigate the use of nZVI/GAC composites to treat organoselenium present in wastewater. With over 99.9% removal efficiency, the results show that nZVI/GAC composites can be considered an effective and promising technology for removing organoselenium from wastewater.