Long-Term Continuous Test of H 2 -Induced Denitrification Catalyzed by Palladium Nanoparticles in a Biofilm Matrix.

Pd catalysis and microbially catalyzed reduction of nitrate (NO-N) were combined as a strategy to increase the kinetics of NO reduction and control selectivity to N gas versus ammonium (NH). Two H-based membrane biofilm reactors (MBfRs) were tested in continuous mode: one with a biofilm alone (H-MBfR) and the other with biogenic Pd nanoparticles (PdNPs) deposited in the biofilm (Pd-H-MBfR). Solid-state characterizations of PdNPs in Pd-H-MBfR documented that the PdNPs were uniformly located along the outer surfaces of the bacteria in the biofilm. Pd-H-MBfR had a higher rate of NO reduction compared to H-MBfR, especially when the influent NO concentration was high (28 mg-N/L versus 14 mg-N/L). Pd-H-MBfR enriched denitrifiers of , , , and in the microbial community and also increased abundances of genes affiliated with NO-N reductases, which reflected that the denitrifying bacteria could channel their respiratory electron flow to NO reduction to NO. N selectivity in Pd-H-MBfR was regulated by the H/NO flux ratio: 100% selectivity to N was achieved when the ratio was less than 1.3 e equiv of H/e equiv N, while the selectivity toward NH occurred with larger H/NO flux ratios. Thus, the results with Pd-H-MBfR revealed two advantages of it over the H-MBfR: faster kinetics for NO removal and controllable selectivity toward N versus NH. By being able to regulate the H/NO flux ratio, Pd-H-MBfR has significant implications for improving the efficiency and effectiveness of the NO reduction processes, ultimately leading to more environmentally benign wastewater treatment.