Mitigating chromate toxicity through concurrent denitrification in the H₂-based membrane biofilm reactor

High concentrations of hexavalent chromium (Cr(VI)) in industrial wastewaters pose significant environmental and health hazards. Biotranformation is a viable means to lower Cr(VI) toxicity, but research to date has focused on wastewaters with low concentrations (e.g., 2-5 mg/L Cr(VI)). This study evaluated the dynamics of biosorption and biotransformation of higher-concentration Cr(VI) by biofilms in the H₂-based membrane biofilm reactor (MBfR). While the biofilm in an MBfR receiving Cr(VI) alone had limited capacity to remove Cr(VI) and Cr(VI) removal ceased in 30 days, an autotrophic denitrifying biofilms achieved 99 % reduction of over 20 mg/L Cr(VI) to less-toxic trivalent chromium (Cr(III)) in continuous long-term operation system over 4 months. Increasing the H₂ pressure from 3 psig to 10 psig improved Cr(VI) removal from 87 % to 99 %, which occurred in parallel with over 95 % NO₃^- reduction to N₂. Metagenomic analyses revealed the mechanisms of Cr(VI) bioreduction and highlighted the beneficial role of nitrate (NO₃^-) as the primary electron acceptor. For example, nitrite reductase NrfA could reduce Cr(VI), which lowered Cr(VI) caused oxidative stress. This research demonstrates the MBfR's effectiveness in reducing elevated levels of Cr(VI) and provides mechanistic understanding of the roles of denitrification in accelerating Cr(VI) reduction and detoxification.