Mn3O4 nanozyme coating accelerates nitrate reduction and decreases N2O emission during photoelectrotrophic denitrification by Thiobacillus denitrificans-CdS.

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, Thiobacillus denitrificans-cadmium sulfide (T. denitrificans-CdS@Mn3O4), via 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). T. denitrificans-CdS@Mn3O4 showed a 28% higher rate of nitrate reduction and 78% lower emission of nitrous oxide (at 68 h) than that of T. denitrificans-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 T. denitrificans-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.