Biogenic Palladium Improved Perchlorate Reduction during Nitrate Co-Reduction by Diverting Electron Flow in a Hydrogenotrophic Biofilm

Abstract

Microbial reduction of perchlorate (ClO 4 − ) is emerging as a cost-effective strategy for groundwater remediation. However, the effectiveness of perchlorate reduction can be suppressed by the common cocontamination of nitrate (NO 3 − ). We propose a means to overcome the limitation of ClO 4 − reduction: depositing palladium nanoparticles (Pd 0 NPs) within the matrix of a hydrogenotrophic biofilm. Two H 2 -based membrane biofilm reactors (MBfRs) were operated in parallel in long-term continuous and batch modes: one system had only a biofilm (bio-MBfR), while the other incorporated biogenic Pd 0 NPs in the biofilm matrix (bioPd-MBfR). For longterm co-reduction, bioPd-MBfR had a distinct advantage of oxyanion reduction fluxes, and it particularly alleviated the competitive advantage of NO 3 − reduction over ClO 4 − reduction. Batch tests also demonstrated that bioPd-MBfR gave more rapid reduction rates for ClO 4 − and ClO 3 − compared to those of bio-MBfR. Both biofilm communities were dominated by bacteria known to be perchlorate and nitrate reducers. Functional-gene abundances reflecting the intracellular electron flow from H 2 to NADH to the reductases were supplanted by extracellular electron flow with the addition of Pd 0 NPs.