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

Microbial reduction of perchlorate (ClO₄^-) is emerging as a cost-effective strategy for groundwater remediation. However, the effectiveness of perchlorate reduction can be suppressed by the common co-contamination of nitrate (NO₃^-). We propose a means to overcome the limitation of ClO₄^- reduction: depositing palladium nanoparticles (Pd⁰NPs) within the matrix of a hydrogenotrophic biofilm. Two H₂-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 Pd⁰NPs in the biofilm matrix (bioPd-MBfR). For long-term co-reduction, bioPd-MBfR had a distinct advantage of oxyanion reduction fluxes, and it particularly alleviated the competitive advantage of NO₃^- reduction over ClO₄^- reduction. Batch tests also demonstrated that bioPd-MBfR gave more rapid reduction rates for ClO₄^- and ClO₃^- 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 H₂ to NADH to the reductases were supplanted by extracellular electron flow with the addition of Pd⁰NPs.