Bi-based photocatalysts for bacterial inactivation in water: Inactivation mechanisms, challenges, and strategies to improve the photocatalytic activity

Bi-based photocatalysts have been considered suitable materials for water disinfection under natural solar light due to their outstanding optical and electronic properties. However, until now, there are not extensive reviews about the development of Bi-based materials and their application in bacterial inactivation in aqueous solutions. For this reason, this work has focused on summarizing the state of the art related to the inactivation of Gram- and Gram + pathogenic bacteria under visible light irradiation using different Bi-based micro and nano structures. In this sense, the photocatalytic bacterial inactivation mechanisms are analyzed, considering several modifications. The factors that can affect the photocatalytic performance of these materials in real conditions and at a large scale (e.g., water characteristics, pH, light intensity, photocatalyst dosage, and bacteria level) have been studied. Furthermore, current alternatives for improving the photocatalytic antibacterial activity and reuse of Bi-based materials (e.g., surface engineering, crystal facet engineering, doping, noble metal coupling, heterojunctions, Z-scheme junctions, coupling with graphene derivatives, magnetic composites, immobilization) have been explored. According to several reports, inactivation rate values higher than 90% can be achieved by using the modified Bi-based micro/nano structures, which become them excellent candidates for photocatalytic water disinfection. However, these innovative photocatalytic materials bring a variety of future difficulties and opportunities in water disinfection.

Development of novel BiOBr0.75I0.25 nanostructures with remarkably High dark phase bactericidal activities

Semiconductor materials with promising photocatalytic activities are being developed for numerous applications including their use in the development of antibacterial products. However, the light may not be available everywhere, which restrict the use of semiconductor photocatalytic materials in real applications. In this area, we report a novel nanostructure of BiOBr_0.75I_0.25 to show enormously high bactericidal activities even at dark. We used a solution based single step method at room temperature to produce highly porous and crystalline BiOBr_xI_1-x (x = 0-1) nanostructures. Next, the developed materials were thoroughly characterized by different analytical techniques, such as FESEM, XRD, XPS, etc. To evaluate the bactericidal activities Escherichia coli (gram-negative bacteria) and Bacillus subtilis (gram-positive bacteria) were selected. Interestingly we found that the solid solutions exhibited high potential towards both the bacteria and among them, BiOBr_0.75I_0.25 showed extremely high efficiencies even at dark. Due to their semiconductor behavior, the materials have shown higher activities in the presence of any light source. The knowledge about the behavior of these unique materials revels a new area of research and would certainly help to find out the solution for ever-increasing environmental issues.

Insights into the Photocatalytic Bacterial Inactivation by Flower-Like Bi2WO6 under Solar or Visible Light, Through in Situ Monitoring and Determination of Reactive Oxygen Species (ROS)

This study addresses the visible light-induced bacterial inactivation kinetics over a Bi2WO6 synthesized catalyst. The systematic investigation was undertaken with Bi2WO6 prepared by the complexation of Bi with acetic acid (carboxylate) leading to a flower-like morphology. The characterization of the as-prepared Bi2WO6 was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), specific surface area (SSA), and photoluminescence (PL). Under low intensity solar light (<48 mW/cm2), complete bacterial inactivation was achieved within two hours in the presence of the flower-like Bi2WO6, while under visible light, the synthesized catalyst performed better than commercial TiO2. The in situ interfacial charge transfer and local pH changes between Bi2WO6 and bacteria were monitored during the bacterial inactivation. Furthermore, the reactive oxygen species (ROS) were identified during Escherichia coli inactivation mediated by appropriate scavengers. The ROS tests alongside the morphological characteristics allowed the proposition of the mechanism for bacterial inactivation. Finally, recycling of the catalyst confirmed the stable nature of the catalyst presented in this study.

Solar Photocatalytic Degradation of Trace Organic Pollutants in Water by Bi(0)-Doped Bismuth Oxyhalide Thin Films

Herein, we demonstrate the fabrication of Bi(0)-doped bismuth oxyhalide solid solution films for the removal of trace organic pollutants (TrOPs) in water. With the advantage of a viscous AlOOH sol, very high loadings (75 wt %) of bismuth oxyhalides were embedded within the thin films and calcined at 500 °C to develop porous alumina composite coatings. Various concentrations of Bi(0) doping were tested for their photocatalytic activity. Seven TrOPs including iopromide (IPRM), iohexol (IHX), iopamidol (IPMD), sulfamethoxazole (SMX), carbamazepine, venlafaxine, and bezafibrate (BZF) were selected for this study based on their occurrence and detection in effluents and surface waters worldwide. In all tests, with the exception of IPRM, 3% Bi(0)-doped BiOCl_0.875Br_0.125 showed highest activity, which can be attributed to its unique, highly organized, and compact morphology besides its well-matched energy band positions. Although IPMD, IHX, IPRM, and SMX are susceptible to photolysis, still the photocatalytic activity significantly augmented the removal of all tested compounds. In addition, analysis of the surface charge excluded electrostatic interactions and confirmed the ion-exchange adsorption mechanism for the high degradation rate of BZF in the presence of bismuth oxyhalides.

Doped ZnO nanoparticles impregnated on Kaolinite (Clay): A reusable nanocomposite for photocatalytic disinfection of multidrug resistant Enterobacter sp. under visible light

Water contamination by multidrug resistant (MDR) enteric bacteria can be considered as the foremost cause of gastrointestinal infections and poses a threat to global public health. Therefore, there is an urgent need to pursue unorthodox techniques with potential of community scale applications for purging of water borne pathogenic bacteria. We communicate visible-light assisted photocatalytic disinfection (PCD) of an enteric MDR bacterium; Enterobacter sp. using Fe-doped ZnO nanoparticles impregnated on Kaolinite (Clay) (ZnO/K). ZnO/K was synthesized by co-precipitation technique and was found to be more effective than Fe-doped ZnO (ZnO) and Kaolinite for PCD process. Analysis from fluorescence microscopy and electron microscopy (FESEM) proposed complete bacterial cell death via PCD due to damage of bacterial cell membrane. Experimental evidences indicated that O₂^- could be acting as the most significant component in disinfection of MDR Enterobacter sp. in visible-light assisted PCD process in presence of ZnO/K. Considering the experimental data of Resazurin assay, it is proposed that reactive oxygen species (ROS) generated during PCD might have impeded the oxido-reductase enzyme system of the bacteria and hence trammeling its metabolic activity. Crystal structure and particle size of ZnO/K was found to be unaltered during the photocatalytic process indicating its potential for reusability. When ZnO/K was exposed to HCT-116 Human Colorectal Carcinoma cell lines, about 79% cell survivability was noticed. The synthesized material was successful in completely disinfecting the target microorganism in Zebra Fish model, without producing any adverse effects on the Fish itself, further reinforcing its biocompatibility factor. High effectiveness of PCD process using ZnO/K under visible light in disinfecting enteric MDR bacteria, might have promising outcome as an alternative water disinfection technology to prevent the spread of infectious and resistant bacteria without producing any adverse effect on non-specific flora and fauna.

Rhodamine B in dissolved and nano-bound forms: Indicators for light-based advanced oxidation processes

Rhodamine B (RhB) is a water-soluble fluorescent dye that is often used to determine flux and flow direction in biotechnological and environmental applications. In the current research, RhB in soluble (termed free) and virus-bound (termed nano-bound) forms was used as an efficiency indicator for three environmental processes. The degradation of free and nano-bound RhB by (i) direct UV photolysis and (ii) UV/H₂O₂ advanced oxidation process (AOP) was studied in a collimated beam apparatus equipped with medium-pressure mercury vapor lamp. The degradation by (iii) solar light-induced photocatalysis was studied in a solar simulator with titanium dioxide and bismuth photocatalysts. Results showed negligible RhB degradation by direct UV and solar light, and its nearly linear degradation by UV/H₂O₂ and photocatalysis/photosensitization in the presence of a solid catalyst. Considerable adsorption of free RhB on bismuth-based catalyst vs. no adsorption of nano-bound RhB on this catalyst or of any form of the dye on titanium dioxide produced two important conclusions. First, the better degradation of free RhB by the bismuth catalyst suggests that close proximity of a catalyst hole and the decomposing molecule significantly influences degradation. Second, the soluble form of the dye might not be the best option for its use as an indicator. Nano-bound RhB showed high potential as an AOP indicator, featuring possible separation from water after the analysis.