Different responses of gram-negative and gram-positive bacteria to photocatalytic disinfection using solar-light-driven magnetic TiO2-based material, and disinfection of real sewage

Abstract

A solar-light-driven magnetic photocatalyst, reduced-graphene-oxide/Fe,N-TiO₂/Fe₃O₄@SiO₂ (RGOFeNTFS), was developed for the photocatalytic disinfection of different strains of bacteria: gram-negative Escherichia coli (E. coli) and Salmonella typhimurium (S. typhimurium), and gram-positive Enterococcus faecalis (E. faecalis). The different responses of the bacteria during the reaction were investigated. Gram-positive E. faecalis was found to be more susceptible to photocatalytic disinfection and exhibited a higher leakage of intracellular components than the two gram-negative bacteria. The interactions between the bacteria and RGOFeNTFS were analyzed for Zeta potential, hydrophilicity and SEM. Under the experimental conditions, the opposite surface charges of the bacteria (negative Zeta potential) and RGOFeNTFS (positive Zeta potential) contribute to their interactions. With a more negative Zeta potential (than E. coli and E. faecalis), S. typhimurium interacts more strongly with RGOFeNTFS and is mainly attacked by •OH near the photocatalyst surface. E. coli and E. faecalis (with less negative Zeta potentials) interact less strongly with RGOFeNTFS, and compete for the dominant reactive species (•O₂^-) in the bulk solution. Therefore, the co-existence of bacteria significantly inhibits the photocatalytic disinfection of E. coli and E. faecalis, but insignificantly for S. typhimurium. Moreover, photocatalytic disinfection using RGOFeNTFS show potential for treating real sewage, which meets the local discharge standard (of E. coli) after a 60-min reaction. In real sewage, different bacteria are disinfected simultaneously.

Keywords: E. coli; E. faecalis; Interaction; Real sewage; S. typhimurium.