(453a) Removal of Selenite with Microbial Fuel Cells Utilizing Shewanella Oneidensis MR-1

Selenium is a trace element with important physiological roles in immune function and oxidative stress; however, at elevated levels it is known to be toxic to all forms of living organisms. Apart from naturally occurring selenium in rocks, selenium compounds are widely used in electronics, oil production and agricultural applications, which has resulted in the contamination of ground water. Selenite (+4 oxidation state) and selenate (+6 oxidation state) are the oxidized forms of selenium. Both are soluble, mobile and toxic in soil and water systems. Even at low levels of contamination these elements have deleterious effects on fauna, where bio-magnification plays a significant role in inducing toxicity. The EPA has set a limit of .05 parts per million (ppm) in drinking water. Hence remediation is important and a variety of methods have been proposed, including activated alumina, coagulation/filtration, reverse osmosis and bioremediation. Recent investigations have shown that bioremediation, which utilizes indigenous or engineered bacteria for the removal of selenium compounds from the soil and/or water systems could be a viable option.

Microbial fuel cell (MFC) systems utilize the catalytic ability of electrogenic bacteria, unique organisms that can convert energy from organic compounds to electricity through the respiration of solid electrodes. There has been much research into the applications of MFCs, including recent investigations that suggest that the cathode of a MFC can be used for bioremediation and the extraction of valuable metals.  Bioremediation results from the ability of the bacteria to convert toxic compounds into more manageable, nontoxic forms. In a MFC, transformation of toxic compounds occurs when used as electron acceptors or oxidants in the cathode compartment. Researchers have shown the applicability of MFC bioremediation technology for the removal of various metals including copper, chromium, selenium and arsenic. In order to fully exploit the potential of this technology it is important to understand the electron transfer mechanism involved in this reduction/ remediation process.

Shewanella oneidensis MR-1 is a facultative anaerobic bacterium known for its versatility in using a variety of soluble and insoluble electron acceptors. This versatility stems from an array of multi-heme cytochromes which makes them an interesting candidate for bioremediation of toxic heavy metals such as uranium, selenium and chromium.  Understanding respiratory electron transfer pathways will help in the design of engineered organisms for better removal of toxic contaminants from water. Using a MFC, we have identified the key cytochromes involved in the reduction of selenium under fuel cell conditions. We have also optimized the maximum concentrations for optimal removal rates without affecting the performance of the MFC. Using various electrochemical techniques, the operating parameters, such as internal resistance and maximum power output, have been determined. Our results indicate this technology could satisfy the stringent EPA water quality standards for removal of selenium.