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

Bio-semiconductors are highly efficient systems for converting solar energy into chemical energy. However, the inevitable presence of reactive oxygen species (ROS) seriously deteriorates the bio-semiconductor performance. This work successfully constructed a Mn3O4 nanoenzyme-coated bio-semiconductor, 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 T. denitrificans-CdS. Moreover, the Mn3O4 coating effectively maintained the microbial viability and photochemical activity of CdS in the bio-semiconductor. 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 bio-semiconductor from ROS damage by Mn3O4. This study provides a simple method for protecting bio-semiconductors from the toxicity of inevitably generated ROS and will help develop more stable and efficient bio-semiconductors in the future.