Advances in photocatalytic disinfection of bacteria:Development of photocatalysts and mechanisms

Photocatalysis has attracted worldwide attention due to its potential in solar energy conversion.As a ＂green＂ advanced oxidation technology, it has been extensively used for water disinfection and wastewater treatment. This article provides a review of the recent progress in solar energy-induced photocatalytic disinfection of bacteria, focusing on the development of highly efficient photocatalysts and their underlying mechanisms in bacterial inactivation. The photocatalysts are classified into Ti O2-based and non-Ti O2-based systems, as Ti O2 is the most investigated photocatalyst. The synthesis methods, modification strategies, bacterial disinfection activities and mechanisms of different types of photocatalysts are reviewed in detail.Emphasis is given to the modified Ti O2, including noble metal deposition, non-metal doping,dye sensitization and composite Ti O2, along with typical non-Ti O2-based photocatalysts for bacterial disinfection, including metal oxides, sulfides, bismuth metallates, graphene-based photocatalysts, carbon nitride-based photocatalysts and natural photocatalysts. A simple and versatile methodology by using a partition system combined with scavenging study is introduced to study the photocatalytic disinfection mechanisms in different photocatalytic systems. This review summarizes the current state of the work on photocatalytic disinfection of bacteria, and is expected to offer useful insights for the future development in the field.

Intensified inactivation of model and environmental bacteria by an atmospheric-pressure air-liquid discharge plasma compared with chlorination

Water-borne pathogenic bacteria are always the top priority to be removed through disinfection process in water treatment due to their threat to human health. It was necessary to develop novel disinfection methods since the conventional chlorine disinfection was inefficient in inactivating chlorine-resistant bacteria, inducing the viable but non-culturable(VBNC) bacteria and forming disinfection by-products(DBPs). In this study, the inactivation of four model strains including Gram-negative(G), Gram-positive(G) and environmental samples by atmospheric-pressure air-liquid discharge plasma(ALDP) was assessed systematically. The results showed that ALDP was superior in inactivating all of the samples compared with chlorination. During 10 min ALDP treatment, the Gbacteria were completely inactivated, and the Gone was inactivated by more than 4.61 logs. The inactivation of bacteria from a campus lake and a wastewater treatment plant effluent exceeded 99.82% and 97.78%, respectively. For G-bacteria, ALDP resulted in a much lower(10~2~10~3 times) levels of VBNC cells than chlorination. ALDP could effectively remove the chlorine-resistant bacteria. More than 96.41% of the intracellular DNA and 99.99% of the extracellular DNA were removed, whereas it was only 56.35% and 12.82% for chlorination. ALDP had a stronger ability to destroy cell structure than chlorination, presumably due to the existence of ROS( ·OH, ~1Oand O). GC-MS analysis showed that ALDP produced less DBPs than chlorination. These findings provided new insights for the application of discharge plasma in water disinfection, which could be complemental or alternative to the conventional disinfection methods.

The chlorination transformation characteristics of benzophenone-4 in the presence of iodide ions

Benzophenone-type UV filters are a group of compounds widely used to protect human skin from damage of UV irradiation. Benzophenone-4（BP-4） was targeted to explore its transformation behaviors during chlorination disinfection treatment in the presence of iodide ions. With the help of ultra performance liquid phase chromatograph and high-resolution quadrupole time-of-flight mass spectrometer, totally fifteen halogenated products were identified, and five out of them were iodinated products. The transformation mechanisms of BP-4 involved electrophilic substitution generating mono-or di-halogenated products,which would be oxidized into esters and further hydrolyzed into phenolic derivatives. The desulfonation and decarboxylation were observed in chlorination system either. Obeying the transformation pathways, five iodinated products formed. The p H conditions of chlorination system determined the reaction types of transformation and corresponding species of products. The more important was that, the acute toxicity had significant increase after chlorination treatment on BP-4, especially in the presence of iodide ions. When the chlorination treatment was performed on ambient water spiked with BP-4 and iodide ions,iodinated by-products could be detected.

Hybrid TiO_(2)/Ag NPs/g-C_(3)N_(4) nanocomposite coatings on TC4 titanium alloy for enhanced synergistic antibacterial effect under full spectrum light

The hybrid TiO_(2)/AgNPs/g-C_(3) N_(4) nanocomposite coatings were constructed on TC4 alloy by a hydrothermal and calcining method.TiO_(2)/AgNPs/g-C_(3) N_(4) nanocomposite coatings demonstrated excellent biocompatibility and osteogenesis compared to those of titanium alloy.The existence of trace AgNPs on the surface and interface of the heterojunction could further enhance the transfer and separation of photogenerated electron/hole pairs,which greatly improved the antibacterial performance under full spectrum light.Holes at the valence band of TiO_(2) and g-C_(3) N_(4) reacted with adsorbed H_(2) O to generate·OH,killing bacteria through photocatalytic redox reaction under light irradiation,while released AgNPs exhibited bacteriostatic efficacy with or without light.This study provides a pathway of coating modification for further improving the antibacterial properties of heterojunction coatings and maintaining the biocompatibility of matrix materials.