Mn₃O₄ Nanozyme Coating Accelerates Nitrate Reduction and Decreases N₂O Emission during Photoelectrotrophic Denitrification by <i>Thiobacillus denitrificans</i>-CdS

Abstract

Biosemiconductors are highly efficient systems for converting solar energy into chemical energy. However, the inevitable presence of reactive oxygen species (ROS) seriously deteriorates the biosemiconductor performance. This work successfully constructed a Mn₃O₄ nanozyme-coated biosemiconductor, <i>Thiobacillus denitrificans</i>-cadmium sulfide (<i>T. denitrificans</i>-CdS@Mn₃O₄), <i>via</i> a simple, fast, and economic method. After Mn₃O₄ coating, the ROS were greatly eliminated; the concentrations of hydroxyl radicals, superoxide radicals, and hydrogen peroxide were reduced by 90%, 77.6%, and 26%, respectively, during photoelectrotrophic denitrification (PEDeN). <i>T. denitrificans</i>-CdS@Mn₃O₄ showed a 28% higher rate of nitrate reduction and 78% lower emission of nitrous oxide (at 68 h) than that of <i>T. denitrificans</i>-CdS. Moreover, the Mn₃O₄ coating effectively maintained the microbial viability and photochemical activity of CdS in the biosemiconductor. Importantly, no lag period was observed during PEDeN, suggesting that the Mn₃O₄ coating does not affect the metabolism of <i>T. denitrificans</i>-CdS. Immediate decomposition and physical separation are the two possible ways to protect a biosemiconductor from ROS damage by Mn₃O₄. This study provides a simple method for protecting biosemiconductors from the toxicity of inevitably generated ROS and will help develop more stable and efficient biosemiconductors in the future.