δ-MnO2-modified peanut shell biochar and humic acid promoted Mn(IV) reduction-driven ammonium oxidation with Cr(VI) removal.

Ammonium nitrogen (NH4+-N) and chromium(VI) (Cr(VI)) are common wastewater pollutants that threaten aquatic ecosystems. This study explored the performance of the strain Aromatoleum evansii MAY27 with NH4+-N oxidation dependent on manganese(IV) (Mn(IV)) reduction (Mnammox) capability in removing Cr(VI) and NH4+-N. To enhance Mnammox efficiency, a novel Mn(IV)-modified peanut shell biochar (PSB@δ-MnO2) was developed and combined with humic acid (HA) as electron shuttles. Following kinetic optimization (pH 6.5, C/N ratio 0.5, and HA and PSB@δ-MnO2 concentrations of 40 and 1100 mg L-1, respectively), the NH4+-N removal efficiency reached 97.03 %, with Mn(II) produced at 18.93 mg L-1. HA-PSB@δ-MnO2 enhanced microbial activity, energy production, and electron transfer capacity. At initial Cr(VI) concentrations below 1 mg L-1, the removal efficiencies of Cr(VI) and NH4+-N exceeded 98 % and 93 %, respectively. The changes in the surface valence states of the precipitates and the formation of Mn(III/II) and Cr(III) precipitates confirmed the occurrence of Mn(IV) and Cr(VI) reduction. Transcriptional analysis revealed that microbial energy, central carbon, and amino acid metabolism were enhanced in HA-PSB@δ-MnO2 system, with upregulation of genes for Mn(IV) reduction. This study elucidated the performance and mechanism of HA-PSB@δ-MnO2 in enhancing Mnammox, providing a theoretical foundation for its sustainable application in NH4+-N and Cr(VI) contaminated wastewater.