Coupling of Pd nanoparticles and denitrifying biofilm promotes H₂-based nitrate removal with greater selectivity towards N₂

The concept of simultaneous microbial-driven and Pd-catalyzed nitrate (NO₃ ⁻) reduction was evaluated in terms of NO₃ ⁻-removal efficiency and reduction-product selectivity. Experiments were conducted in three identical H₂-based membrane biofilm reactors (MBfR) operated in parallel: biogenic Pd nanoparticles (PdNPs) associated with biofilm (“Pd-biofilm”), biofilm alone (“Biofilm”), and abiotic PdNPs alone (“Pd-film”). Solid-state characterizations confirmed that the PdNPs in Pd-biofilm were dominated by Pd⁰ nanocrystallites similar to those in Pd-film, and molecular microbiological analyses confirm that the microbial community in Pd-biofilm were dominated by β-proteobacteria with denitrifying activity similar to Biofilm. Pd-biofilm accelerated NO₃ ⁻ reduction to NO₂ ⁻ mainly through enzymatic activity and accelerated subsequent NO₂ ⁻ reduction mainly through PdNP catalysis. When H₂ could be delivered at a rate approximately equal to the total demand to reduce NO₃ ⁻ to N₂, active biofilm reduced NO₃ ⁻/NO₂ ⁻ exclusively to N₂, and it also attenuated NH₄ ⁺ formation; as a result, the overall selectivity towards N₂ in Pd-biofilm was nearly 100% and higher than in Pd-film. Thus, coupling PdNP catalysis and microbial denitrification promoted H₂-based NO₃ ⁻ reduction and led to greater selectivity towards N₂ as long as H₂ delivery was controlled. From a practical perspective, delivering H₂ by diffusion through bubbleless membranes enabled accurate control of N selectivity.