Mechanistic insights and performance of Mn redox cycling in a dual-bacteria bioreactor for ammonium and Cr(VI) removal.

The co-contamination of hexavalent chromium (Cr(VI)) and ammonium (NH4+-N) in industrial wastewater has attracted considerable attention due to its serious threats to both ecological systems and public health. Manganese(IV) (Mn(IV))-driven NH4+-N oxidation (Mnammox) coupled with Mn(II)-mediated denitrification (MnOD), built on the Mn redox cycle, is a promising nitrogen removal process, where Mn(II) and NOx--N generated during Mnammox were effectively controlled by MnOD. Herein, a bioreactor integrating Mnammox and MnOD for NH4+-N and Cr(VI) removal was constructed utilizing core-shell gel beads embedded with two core strains and δ-MnO2. When the C/N was 1.5, pH was 6.5, and HRT was 20 h, the removal efficiencies for Cr(VI) and NH4+-N reached 96.3 and 91.3 %, respectively. Cr(VI) can be bioreduced to Cr(III) in bioreactors. Additionally, the microbial activity and electron transfer properties in the Mn redox system were studied under varying Cr(VI) concentrations. High-throughput data revealed that high Cr(VI) concentrations significantly impacted microbial community diversity, while Aromatoleu and Zoogloea consistently remaining the dominant species in the bioreactor. KEGG database analysis showed that appropriately increasing C/N promoted the expression of genes related to nitrification and Mn redox cycling. This study provides novel perspectives on the application of the Mnammox coupled MnOD process driven by the Mn redox cycle for treating NH4+-N and Cr(VI) co-contaminated industrial wastewater.