Bio-Coated Graphitic Carbon Nitrides for Enhanced Nitrobenzene Degradation: Roles of Extracellular Electron Transfer

Abstract

Graphitic carbon nitrides (g-C3N4) and microorganisms could collaboratively enhance photocatalytic properties or facilitate environmental depollution through coupled photocatalytic and biological reactions, which prevented the destruction of photocatalytic stresses to ecological systems and resulted in a sustainable technology for water remediation in rivers and lakes. However, the roles of bio-substances as well as electronic interactions between inorganic and organic systems were still unclear. Herein, g-C3N4, nitrogen-deficient g-C3N4 (ND-g-C3N4), and fluorinated g-C3N4 (F-g-C3N4) were coated with representative bacteria, i.e., Escherichia coli MG 1655, and characterized using integrated spectroscopic techniques. Photocatalytic activities were then evaluated through nitrobenzene degradation performance in an aqueous solution under visible light illumination. Nano-photocatalysts were observed to be adsorbed onto bio-aggregates, and surface hydrophilicity was convinced to be determined in the toxicity of photocatalysts in dark environments. Layered structures of ND-g-C3N4 and F-g-C3N4 were revealed in XRD spectra, and surface coverage of the Luria–Bertani medium was eliminated during E. coli cultivation. Hetero-junctions between photocatalysts and bio-substances were indicated in XPS results. Red-shifts for g-C3N4 and F-g-C3N4 materials as well as a slight blue-shift for ND-g-C3N4 were demonstrated in UV-vis spectra, which might be attributed to the destruction of nitrogen defects on ND-g-C3N4. Owing to the attached bio-substances, nitrobenzene removal could reach twice that with pristine photocatalysts, and ROS quantitative analysis confirmed that hydroxyl radicals were the determined reactive species degrading nitrobenzene in the water solution. The observation of more OH species generation indicated that extracellular electron transfer of E. coli reduced electron–hole recombination and provided reduction sites during photocatalytic degradation of nitrobenzene. This work proved additional electron-transfer paths and reaction mechanisms in hybridized photocatalytic and biological processes, which indicated that bio-activities could be a great promoter of material modification and the incorporation between inorganic and organic systems successfully showed an eco-friendly and sustainable pathway to utilize photocatalysts in natural water.