Abstract
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.
Highlights
The efficacy of antibiotics in treating bacterial infections is dwindling due to an increase in antibiotic resistance (AR)
Photocatalyst and Characterisation outlinedImmobilisation procedure was successful in producing a coating that did not flake off when submerged in waterprocedure and produced discs thatinwere reusable any did recovery steps
A simple and low-cost method was used to synthesise cerium-doped zinc oxide (Ce-ZnO) and immobilise it on ordinary 304 stainless steel discs by aerosol spraying of the hydroxide precursor suspension on the pre-treated metal
Summary
The efficacy of antibiotics in treating bacterial infections is dwindling due to an increase in antibiotic resistance (AR). The concern is that the environmental resistome acts as a source of AR to the clinical resistome [6], and a higher prevalence of resistance in the environment, which is due to anthropogenic action, implies a higher risk of transfer to the clinical settings. This concern is further compounded by the projected increase in water stress due to climate change and population growth [7]. Since AR mitigation is not a direct target of wastewater treatment, there is a possibility that treatment while promoting the reduction of AR does not guarantee the adequate reduction of the relative abundance of AR [10]
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