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

•PdNPs with/without biofilm supporting were dominated by about 6×5 nm Pd0 crystallites.•Microbes controlled NO3− reduction toNO2− while PdNPs controlled NO2− reduction in Pd-biofilm.•The overall selectivity towards N2 in Pd-biofilm was nearly 100% and higher than in Pd-film.•Active biofilm reduced NO3−/NO2− exclusively to N2, and attenuated NH4+ formation.•Delivering H2 by diffusion through nonporous membrane enabled accurate control of N selectivity.

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.

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