Electric Field-stimulated Anaerobic Reduction Of Perchlorate And Nitrate

In recent years, the finding that microorganism can respire anaerobically with an electrodeserving as the electron donor has attracted widespread attention from researchers in the field ofmicrobial electrochemistry. This metabolism ability allows microbe to grow without the needof traditional chemical electron donors such as acetate and lactate. Thus, the bioelectrochemicaltechnology is considered to be a promising method for application in pollutant remediation dueto the less requirements of chemicals.Perchlorate is an emerging water pollutant and usually persists in the aqueous environmenttogether with nitrate, which is another important environmental contaminant. Our first effortwas made to understand how perchlorate reduction behaves when nitrate is present in thebioelectrochemical system. It was found that electrons derived from the electrode can bedirectly transferred to the bacteria with perchlorate or nitrate as the sole electron acceptor. Thepresence of nitrate, even at the0.07mM level, can slow reduction of perchlorate (0.70mM) asa poised potential of-0.50V (vs. SCE) was applied to the inoculated cathode. Increasing theconcentration of nitrate resulted in a noticeable inhibitory effect on perchlorate reduction. Whenthe nitrate concentration was2.10mM, reduction of0.70mM perchlorate was totally inhibited.Bacterial community analyses based on16S rDNA gene analysis with denaturing gradient gelelectrophoresis (DGGE) revealed that most of the bacteria newly enriched on the nitrate and/orperchlorate biocathodes were the known electrochemically active denitrifiers, which possiblyprefer to reduce nitrate over perchlorate. These results show that nitrate is a more favorableelectron acceptor than perchlorate in the bioelectrochemical system where the cathode directlyserves as the electron donor.Bioelectrochemical reduction of pollutants; however, suffers from the problem of beingapplied for treating real wastewater, because the cathodic bacteria are susceptible to poisoningby other components in the wastewater. Our second effort was made to demonstrate the conceptof an ion-exchange membrane bioelectrochemical reactor (IEMBER) that integrates thetransport of nitrate from a wastewater chamber through an anion-exchange membrane with itsbioelectrochemical reduction in a separate cathode chamber. Successful denitrification ofnitrate in the biological effluent from a coking wastewater treatment plant was achieved, ingood agreement with the obvious cathodic current observed in the IEMBER with its cathodepoised at a potential of-0.50V (vs. SCE). The IEMBER enabled81.7%of nitrate removedfrom the coking wastewater with a72mg·L-1NO3--N after the biological treatment, incomparison to14.73%obtained from the BER into which the real coking wastewater was directly fed. The anion-exchange membrane plays a crucial role in preventing the residuals inthe wastewater chamber to enter the cathode chamber, thus avoiding their potential toxicity tothe denitrifying bacteria.