Selenium recovery from wastewater based on exclusively extracellular selenium nanoparticles production

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Selenium (Se) is critical element to the US economy and national security. It is also a contaminant in wastewater that must be removed to meet strict discharge limits to the environment. Bacteria have been used previously to remove Se from wastewater by converting dissolved Se to Se nanoparticles that can be separated from the water. However, this process is not efficient for Se recovery because the Se nanoparticles are typically inside the microbial cells and thus difficult to recover. The goal of this project is to enable both Se removal and recovery from wastewater using novel biocathode reactors. In these systems, specific bacteria grow on electrodes and produce Se nanoparticles outside microbial cells for efficient Se recovery. Successful completion of this research will improve our understanding of the mechanisms of Se nanoparticle production in biocathode reactors. Additional benefits to the environment and society result from improving Se recovery from wastewater. The potential economic benefits of this process are significant, as Se recovery just from coal-fired power plant waste streams alone could meet all the US demand for this element. Broader impacts to society will result from an education and outreach plan focused on the role that bacteria play in the process. This will improve the scientific literacy of the Nation by correcting misperceptions that all bacteria are human pathogens.The goal of this project is to develop a novel biocathode reactor system to recover selenium (Se) from wastewater at coal-fired power plants and other Se-rich waste streams. This research is driven by new Environmental Protection Agency regulations greatly reducing the Se effluent limit from coal-fired power plant wastewater. Additionally, increased demand for Se in many different products and industries makes recovery and re-use a more sustainable means of meeting this increased demand. Microbial Se reactors exploit the microbial conversion of dissolved selenate to elemental Se nanoparticles. Previous research on Se bioreactors has not been able to demonstrate efficient recovery of Se because the elemental Se nanoparticles are produced intracellularly and thereby difficult to separate. Preliminary experiments in our laboratory using novel biocathode reactors have demonstrated the ability of specially enriched mixed cultures to produce extracellular Se nanoparticles on the biocathode regardless of inoculum. The first objective of this project is to determine the mechanisms of extracellular Se nanoparticle production on the biocathode. The effect of potential interferences like nitrate and sulfate that are commonly present in Se-laden wastewater will also be investigated. Techniques to prevent the negative effects of nitrate and sulfate on selenate reduction will be developed through integrated mathematical modeling with experiments on microbial community structure, function, and kinetics. Successful completion of this research has potential to be expanded for the recovery of other minerals of US national interests such as Cr, Pd, Sb, and U. Broader educational impacts to society result from an education and outreach plan focused on the production of an educational video of beneficial versus harmful bacteria, outreach efforts to attract students from underrepresented groups in STEM research and education, and a competition-based design project to accelerate knowledge transfer to engineers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.