Coupling of palladium nanoparticles with biofilm improved co-reduction of chromium(VI) and nitrate

Chromium (Cr) contamination in water poses significant health risks, yet advanced remediation methods remain limited. Cr(VI) reduction catalyzed by palladium nanoparticles (PdNPs) on hydrogen-transfer membranes has shown potential but requires further optimization. This study investigated the simultaneous microbial-driven and Pd-catalyzed Cr(VI) reduction, focusing on reduction efficiency and optimal conditions. Two hydrogen-based membrane reactors were compared: a Pd-biofilm reactor incorporating PdNPs associated with a biofilm, and a control biofilm reactor. Continuous experiments demonstrated the superior performance of the Pd-biofilm reactor, achieving immediate Cr(VI) reduction and effluent Cr(III) concentrations below 0.040 mg·L⁻¹, compared to 0.3 mg·L⁻¹ in the control biofilm reactor. High-throughput sequencing identified Dechloromonas as the dominant microbial species within Pd-biofilm, which plays a critical role in metal ion reduction. The Pd-biofilm reactor maintained high Cr(VI) reduction flux across varying conditions. When the influent Cr(VI) loading reached up to 10 mg·L⁻¹, where the control biofilm reactor experienced inhibition, the Pd-biofilm reactor achieved a Cr removal of 99%. Increased nitrate loading and hydrogen pressure further enhanced Pd-biofilm reactor performance without compromising Cr(VI) reduction since Cr(VI) is the preferential electron acceptor, whereas the biofilm reactor required hydrogen pressures ≥15 psig (1 psig = 6.895 kPa) for similar results. The optimal pH range for Cr(VI) reduction was 5.0–8.0 in the Pd-biofilm reactor and 7.0 in the biofilm reactor, with alkaline conditions being more inhibitory than acidic ones in both systems. The Pd-biofilm reactor effectively reduced Cr(VI) concentrations from 1 to 10 mg·L⁻¹ to below the maximum contaminant level of 0.1 mg·L⁻¹, thus appearing as an efficient technique to treat Cr-contaminated waters.