Anthraquinone-2-Sulfonate as a Microbial Photosensitizer and Capacitor Drives Solar-to-N2O Production with a Quantum Efficiency of Almost Unity.

Semiartificial photosynthesis shows great potential in solar energy conversion and environmental application. However, the rate-limiting step of photoelectron transfer at the biomaterial interface results in an unsatisfactory quantum yield (QY, typically lower than 3%). Here, an anthraquinone molecule, which has dual roles of microbial photosensitizer and capacitor, was demonstrated to negotiate the interface photoelectron transfer via decoupling the photochemical reaction with a microbial dark reaction. In a model system, anthraquinone-2-sulfonate (AQS)-photosensitized Thiobacillus denitrificans, a maximum QY of solar-to-nitrous oxide (N2O) of 96.2% was achieved, which is the highest among the semiartificial photosynthesis systems. Moreover, the conversion of nitrate into N2O was almost 100%, indicating the excellent selectivity in nitrate reduction. The capacitive property of AQS resulted in 82-89% of photoelectrons released at dark and enhanced 5.6-9.4 times the conversion of solar-to-N2O. Kinetics investigation revealed a zero-order- and first-order- reaction kinetics of N2O production in the dark (reductive AQS-mediated electron transfer) and under light (direct photoelectron transfer), respectively. This work is the first study to demonstrate the role of AQS in photosensitizing a microorganism and provides a simple and highly selective approach to produce N2O from nitrate-polluted wastewater and a strategy for the efficient conversion of solar-to-chemical by a semiartificial photosynthesis system.

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.

[Mechanisms of Fe-cyclam/H2O2 System Catalyzing the Degradation of Rhodamine B].

Fenton reaction is a traditional method for the treatment of dye-containing wastewater. However, this process should be performed in a narrow pH range and requires large amounts of ferrous salt input, limiting its application. In this work, a robust iron complex bearing a cross-bridge cyclam ligand (Fe-cyclam) was successfully prepared. This complex could effectively activate H2O2 to degrade rhodamine B at a pH range of 2-7. The Fe-cyclam/H2O2 system was more effective in the degradation of rhodamine B than the Fenton reaction, when the input [Fe] was lower than 50 µmol·L-1. Moreover, in addition to rhodamine B, the Fe-cyclam/H2O2 system was also capable of degrading dyes such as acid red 88, acid orange II, reactive red 24, and neutral red. This system was more efficient in the degradation of azo dyes than that of triphenylmethane dyes. The removal of rhodamine B remained higher than 90% in three cycle experiments, indicating the excellent stability of Fe-cyclam. The quenching experiments proved that the degradation of rhodamine B by Fe-cyclam/H2O2 was a free-radical-control process. Meanwhile, the electron paramagnetic resonance captured the signals of high valent FeV-oxo species, indicating that FeV-oxo possibly mediated the degradation of rhodamine B in the Fe-cyclam/H2O2 system. This work proves the potential application of Fe-cyclam/H2O2 in the degradation of dyes in a practical environment.

Mechanisms of nitrous oxide emission during photoelectrotrophic denitrification by self-photosensitized Thiobacillus denitrificans.

Photoelectrotrophic denitrification (PEDeN) using bio-hybrids has the potential to remove nitrate (NO3-) from wastewater in an economical and sustainable way. As a gas of global concern, the mechanisms of nitrous oxide (N2O) emissions during this novel process remain unclear. Herein, a self-photosensitized bio-hybrid, i. e., Thiobacillus denitrificans-cadmium sulfide, was constructed and the factors affecting N2O emissions during PEDeN by the bio-hybrids were investigated. The system was sensitive to the input NO3--N and NO2--N, resulting in changes in the N2O/(N2+N2O) ratio from 1% to 95%. In addition to free nitrous acid (FNA), reactive oxidative species (ROS) were a unique factor affecting N2O emission during PEDeN. Importantly, the N2O reduction step exhibited greater susceptibility to the ROS than nitrate reduction step. The contributions of hydrogen peroxide (H2O2), superoxides (O2-•), hydroxyl radicals (•OH) and FNA to the inhibition of N2O reduction were >15.0%, >5.4%, 1.3%, and <70.2%, respectively for a reduction of 13.5 mg/L NO3--N. A significant down-regulation of the relative transcription of the gene nosZ demonstrated that the inhibition of N2O reductase occurred at the gene level. This finding has important implications not only for mitigating N2O emissions during the PEDeN process but also for encouraging a reexamination process of N2O emissions in nature, particularly in systems in which ROS are present during the denitrification process.