Long-Term Continuous Test of H₂-Induced Denitrification Catalyzed by Palladium Nanoparticles in a Biofilm Matrix

Pd⁰ catalysis and microbially catalyzed reduction of nitrate (NO₃^--N) were combined as a strategy to increase the kinetics of NO₃^- reduction and control selectivity to N₂ gas versus ammonium (NH₄⁺). Two H₂-based membrane biofilm reactors (MBfRs) were tested in continuous mode: one with a biofilm alone (H₂-MBfR) and the other with biogenic Pd⁰ nanoparticles (Pd⁰NPs) deposited in the biofilm (Pd-H₂-MBfR). Solid-state characterizations of Pd⁰NPs in Pd-H₂-MBfR documented that the Pd⁰NPs were uniformly located along the outer surfaces of the bacteria in the biofilm. Pd-H₂-MBfR had a higher rate of NO₃^- reduction compared to H₂-MBfR, especially when the influent NO₃^- concentration was high (28 mg-N/L versus 14 mg-N/L). Pd-H₂-MBfR enriched denitrifiers of Dechloromonas, Azospira, Pseudomonas, and Stenotrophomonas in the microbial community and also increased abundances of genes affiliated with NO₃^--N reductases, which reflected that the denitrifying bacteria could channel their respiratory electron flow to NO₃^- reduction to NO₂^-. N₂ selectivity in Pd-H₂-MBfR was regulated by the H₂/NO₃^- flux ratio: 100% selectivity to N₂ was achieved when the ratio was less than 1.3 e^- equiv of H₂/e^- equiv N, while the selectivity toward NH₄⁺ occurred with larger H₂/NO₃^- flux ratios. Thus, the results with Pd-H₂-MBfR revealed two advantages of it over the H₂-MBfR: faster kinetics for NO₃^- removal and controllable selectivity toward N₂ versus NH₄⁺. By being able to regulate the H₂/NO₃^- flux ratio, Pd-H₂-MBfR has significant implications for improving the efficiency and effectiveness of the NO₃^- reduction processes, ultimately leading to more environmentally benign wastewater treatment.