In situ-generated palladium nanoparticles promoted co-reduction of bromate and nitrate in hydrogenotrophic biofilms

Bromate, a carcinogenic disinfection byproduct, threatens water safety due to its persistence and health risks. Although microbial reduction of bromate is a sustainable remediation approach, its efficiency is often hampered by the common co-contamination of nitrate. We addressed this issue by generating palladium nanoparticles (PdNPs) in situ within the biofilm matrix of a membrane biofilm reactor (MBfR), creating a Pd-MBfR. Co-reduction of bromate and nitrate by the Pd-MBfR was investigated in terms of bromate removal, reduction kinetics, and microbial functions. To evaluate the impact of catalytic hydrogenation, a non-palladized MBfR was operated in parallel. Continuous operation over 125 days demonstrated that the Pd-MBfR reduced bromate concentration (4 mg·L^-1) up to 70 %, versus 30 % for the conventional MBfR under competitive nitrate or nitrite conditions. Kinetic modeling revealed that extracellular Pd-catalyzed bromate reduction diverted approximately18 % of bromate flux from intracellular NADH-dependent pathways, while biogenic PdNPs rapidly scavenged nitrite via catalytic hydrogenation, diverting 39 % of intracellular electron flux from denitrification to extracellular catalytic reduction. Under nitrate stress, biofilms in the Pd-MBfR maintained syntrophic interactions between bromate-reducing bacteria Dechloromonas and homoacetogens Acetobacterium, whereas the conventional MBfR favored autotrophic denitrifiers Hydrogenophaga and Rhodoblastus that prioritized nitrate reduction. Functional-gene profiling confirms that the intracellular electron flow from hydrogen to NADH-dependent denitrification reductases was displaced by extracellular Pd-catalyzed hydrogenation. This diversion of electron flow enhanced bromate reduction in biofilms coupled with PdNPs by minimizing competition for intracellular NADH.