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

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.