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

Pd 0 catalysis and microbially catalyzed reduction of nitrate (NO 3 - -N) were combined as a strategy to increase the kinetics of NO 3 - reduction and control selectivity to N 2 gas versus ammonium (NH 4 + ). Two H 2 -based membrane biofilm reactors (MBfRs) were tested in continuous mode: one with a biofilm alone (H 2 -MBfR) and the other with biogenic Pd 0 nanoparticles (Pd 0 NPs) deposited in the biofilm (Pd-H 2 -MBfR). Solid-state characterizations of Pd 0 NPs in Pd-H 2 -MBfR documented that the Pd 0 NPs were uniformly located along the outer surfaces of the bacteria in the biofilm. Pd-H 2 -MBfR had a higher rate of NO 3 - reduction compared to H 2 -MBfR, especially when the influent NO 3 - concentration was high (28 mg-N/L versus 14 mg-N/L). Pd-H 2 -MBfR enriched denitrifiers of Dechloromonas , Azospira , Pseudomonas , and Stenotrophomonas in the microbial community and also increased abundances of genes affiliated with NO 3 - -N reductases, which reflected that the denitrifying bacteria could channel their respiratory electron flow to NO 3 - reduction to NO 2 - . N 2 selectivity in Pd-H 2 -MBfR was regulated by the H 2 /NO 3 - flux ratio: 100% selectivity to N 2 was achieved when the ratio was less than 1.3 e - equiv of H 2 /e - equiv N, while the selectivity toward NH 4 + occurred with larger H 2 /NO 3 - flux ratios. Thus, the results with Pd-H 2 -MBfR revealed two advantages of it over the H 2 -MBfR: faster kinetics for NO 3 - removal and controllable selectivity toward N 2 versus NH 4 + . By being able to regulate the H 2 /NO 3 - flux ratio, Pd-H 2 -MBfR has significant implications for improving the efficiency and effectiveness of the NO 3 - reduction processes, ultimately leading to more environmentally benign wastewater treatment.