Repeated photocatalytic inactivation of E. coli by UV + Ni foam@TiO2: Performance and photocatalyst deactivation

Abstract

Photocatalysis (PC) has exhibited a bright prospect in the inactivation of waterborne pathogens. However, the deactivation of photocatalysts during PC bacterial inactivation has not been systematically studied so far. In this study, we investigated the performance of a PC system nickel foam-loaded TiO2 (NF@TiO2) during a 70-cycle of bacterial inactivation focusing on the photocatalyst deactivation mechanism, where E. coli DH5α was employed as the model photocatalyst and bacteria, respectively. Our results showed that the UV + NF@TiO2 process could effectively inactivate E. coli. Approximately 1 × 108 cfu mL−1 of E. coli could be completely inactivated within 4 h using a fresh NF@TiO2 photocatalyst. However, the inactivation efficiency substantially declined with the increase in cycle number, and NF@TiO2 was almost poisoned after 70 cycles. Through the characterization of scanning electron microscope, atomic force microscope, UV–vis diffuse reflectance spectroscopy, and electron paramagnetic resonance, as well as other experiments, a deactivation mechanism of NF@TiO2 was proposed: The PC-generated reactive oxygen species (ROS) fragmented E. coli to excrete a myriad of biomolecules that subsequently adhered to the surface of NF@TiO2. This accelerated the deterioration of PC inactivation by impeding light harvest and reactive species utilization. Moreover, biomolecules were found to play a pivotal role in photocatalyst deactivation, following the order of lipopolysaccharide ≈ DNA > proteins. This study could provide a fundamental view of the deactivation of photocatalysts during bacterial inactivation.