Concomitant inactivation of Acanthamoeba spp. and Escherichia coli using suspended and immobilized TiO2

Comparison of Titanium Dioxide and Zinc Oxide Photocatalysts for the Inactivation of Escherichia coli in Water Using Slurry and Rotating-Disk Photocatalytic Reactors

The application of photocatalysis for the disinfection of water has been extensively reported over the past 30 years. Titanium dioxide (TiO2) has been the most widely and successfully used photocatalyst to date; however, it is not without its limitations. Frequently observed long lag times, sometimes up to 60 min, before bacterial inactivation begins and the presence of residual microorganisms, for example, up to 104 colony forming units, remaining after treatment are ongoing challenges with this particular photocatalyst. It is therefore important to find alternative photocatalysts that can address these issues. In this study, we compared the disinfection capacity of TiO2 with that of zinc oxide (ZnO) using Escherichia coli as a model organism in both a suspended and immobilized catalyst system. Our results showed that ZnO was superior to TiO2 in a number of areas. Not only were bacterial rates of destruction much quicker with ZnO, but no lag time was observed prior to inactivation in suspended systems. Furthermore, complete bacterial destruction was observed within the treatment times under investigation. The greater efficiency of ZnO is believed to be due to the decomposition of the bacterial cell wall being driven by hydrogen peroxide as opposed to hydroxyl radicals. The results reported in this paper show that ZnO is a more efficient and cost-effective photocatalyst than TiO2 and that it represents a viable alternative photocatalyst for water disinfection processes.

Photo-Fenton and TiO2 Photocatalytic Inactivation of Model Microorganisms under UV-A; Comparative Efficacy and Optimization

Photocatalytic inactivation of pathogens in aqueous waste is gaining increasing attention. Several homogeneous and heterogeneous photocatalytic protocols exist using the Fenton's reagent and TiO₂, respectively. A comprehensive study of homogeneous and heterogeneous photocatalysis on a range of microorganisms will significantly establish the most efficient method. Here, we report a comparative study of TiO₂- and Fe⁺³-based photocatalytic inactivation under UV-A of diverse microorganisms, including Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, bacterial spores (Bacillus stearothermophilus spores) and viruses (MS2). We also present data on the optimization of TiO₂ photocatalysis, including optimal catalyst concentration and H₂O₂ supplementation. Our results indicate that both photo-Fenton and TiO₂ could be successfully applied for the management of microbial loads in liquids. Efficient microorganism inactivation is achieved with homogeneous photocatalysis (7 mg/L Fe⁺³, 100 mg/L H₂O₂, UV-A) in a shorter processing time compared to heterogeneous photocatalysis (0.5 g/L TiO₂, UV-A), whereas similar or shorter processing is required when heterogenous photocatalysis is performed using microorganism-specific optimized TiO₂ concentrations and H₂O₂ supplementation (100 mg/L); higher H₂O₂ concentrations further enhance the heterogenous photocatalytic inactivation efficiency. Our study provides a template protocol for the design and further application for large-scale photocatalytic approaches to inactivate pathogens in liquid biomedical waste.

Floating TiO2 photocatalyst for efficient inactivation of E. coli and decomposition of methylene blue solution

The anatase phase TiO₂ films with nanocrystalline structure were successfully deposited on a water-floating non-expanded polystyrene (PS) beads via magnetron sputtering. The combination of UVB light and PS beads with TiO₂ film was used for decomposition of methylene blue as well as inactivation tests for intact and EDTA-treated Escherichia coli bacteria. Crystal structure, elemental composition, elemental mapping, surface morphology and chemical bonds of TiO₂ film were investigated. E. coli inactivation experiments showed that such floating photocatalyst could destroy >90% bacteria in 45 min under UVB irradiation. Results demonstrated that combination of TiO₂ and UVB light leads to disruption of the outer membrane which causes effective inactivation of E. coli bacteria.

Predatory bacteria in combination with solar disinfection and solar photocatalysis for the treatment of rainwater

The predatory bacterium, Bdellovibrio bacteriovorus, was applied as a biological pre-treatment to solar disinfection and solar photocatalytic disinfection for rainwater treatment. The photocatalyst used was immobilised titanium-dioxide reduced graphene oxide. The pre-treatment followed by solar photocatalysis for 120 min under natural sunlight reduced the viable counts of Klebsiella pneumoniae from 2.00 × 10⁹ colony forming units (CFU)/mL to below the detection limit (BDL) (<1 CFU/100 μL). Correspondingly, ethidium monoazide bromide quantitative PCR analysis indicated a high total log reduction in K. pneumoniae gene copies (GC)/mL (5.85 logs after solar photocatalysis for 240 min). In contrast, solar disinfection and solar photocatalysis without the biological pre-treatment were more effective for Enterococcus faecium disinfection as the viable counts of E. faecium were reduced by 8.00 logs (from 1.00 × 10⁸ CFU/mL to BDL) and the gene copies were reduced by ∼3.39 logs (from 2.09 × 10⁶ GC/mL to ∼9.00 × 10² GC/mL) after 240 min of treatment. Predatory bacteria can be applied as a pre-treatment to solar disinfection and solar photocatalytic treatment to enhance the removal efficiency of Gram-negative bacteria, which is crucial for the development of a targeted water treatment approach.

Kinetic study and performance comparison of TiO2-mediated visible-light-responsive photocatalysts for the inactivation of Aspergillus niger

Fungi are highly survived with exceptional resistance to environmental stress. Conventional fungicides are quite efficient, but the increase in use raises severe environmental problems. In this study, environmentally friendly TiO₂-mediated visible-light-responsive photocatalysts, namely N-TiO₂, N-T-TiO_2, C-TiO₂, and Pd-C-TiO₂, were used to compare the performance of disinfecting a mold fungi Aspergillus niger. Key parameters, including photocatalyst dosage, the initial fungal concentration, and visible-light intensity, affecting the disinfecting process, was investigated. A new developed Light-responsive Modified Hom's (LMH) kinetic model incorporating visible-light intensity and photocatalyst light-absorption coefficient was firstly used to predict such photocatalytic process in fungal inactivation. Among the photocatalysts, Pd-C-TiO₂ showed the highest inactivation performance against fungi, followed by C-TiO₂, N-T-TiO_2, and N-TiO₂. In general, inactivation increased with increasing photocatalyst dosage and light intensity while decreased with increasing initial fungal concentration. For kinetic modeling, the LMH model supports the hypothesis that photocatalyst performance toward visible-light-driven fungal inactivation primarily depends on the light-absorption capacity of the photocatalyst. In conclusion, mold fungi Aspergillus niger are effectively disinfected by TiO₂-mediated visible-light-responsive photocatalysts, and such fungal inactivation process could be predicted by LMH kinetic model.

Immobilised Cerium-Doped Zinc Oxide as a Photocatalyst for the Degradation of Antibiotics and the Inactivation of Antibiotic-Resistant Bacteria

The threat of antibiotic resistance to the wellbeing of societies is well established. Urban wastewater treatment plants (UWTPs) are recognised sources for antibiotic resistance dissemination in the environment. Herein a novel cerium-doped zinc oxide (Ce-ZnO) photocatalyst is compared to ZnO and the benchmark TiO2-P25 in the immobilised form on a metallic support, to evaluate a photocatalytic process as a possible tertiary treatment in UWTPs. The catalysts were compared for the removal of two antibiotics, trimethoprim (TMP) and sulfamethoxazole (SMX), and for the inactivation of Escherichia coli (E. coli) strain DH5-Alpha in isotonic sodium chloride solution and of autochthonous bacteria in real secondary wastewater. In real wastewater, E. coli and other coliforms were monitored, as well as the respective fractions resistant to ofloxacin and azithromycin. In parallel, Pseudomonas aeruginosa and the respective sub-population resistant to ofloxacin or ciprofloxacin were also monitored. Photocatalysis with both ZnO and Ce-ZnO was faster than using TiO2-P25 at degrading the antibiotics, with Ce-ZnO the fastest against SMX but slower than undoped ZnO in the removal of TMP. Ce-ZnO catalyst reuse in the immobilised form produced somewhat slower kinetics maintained >50% of the initial activity, even after five cycles of use. Approximately 3 log10 inactivation of E. coli in isotonic sodium chloride water was recorded with reproducible results. In the removal of autochthonous bacteria in real wastewater, Ce-ZnO performed better (more than 2 log values higher) than TiO2-P25. In all cases, E. coli and other coliforms, including their resistant subpopulations, were inactivated at a higher rate than P. aeruginosa. With short reaction times no evidence for enrichment of resistance was observed, yet with extended reaction times low levels of bacterial loads were not further inactivated. Overall, Ce-ZnO is an easy and cheap photocatalyst to produce and immobilise and the one that showed higher activity than the industry standard TiO2-P25 against the tested antibiotics and bacteria, including antibiotic-resistant bacteria.