The ubiquitous presence of cyclic volatile methylsiloxanes (cVMS) in the global atmosphere has recently raised environmental concern. In order to assess the persistence and long-range transport potential of cVMS, their second-order rate constants (k) for reactions with hydroxyl radical (•OH) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of •OH oxidation of a series of cVMS, hexamethylcyclotrisiloxane (D3), octamethycyclotetrasiloxane (D4), and decamethycyclopentasiloxane (D5). Experimentally, we measured k values for D3, D4, and D5 with •OH in a gas-phase reaction chamber. The Arrhenius activation energies for these reactions in the temperature range from 313 to 353 K were small (−2.92 to 0.79 kcal·mol–1), indicating a weak temperature dependence. We also calculated the thermodynamic and kinetic behaviors for reactions at the M06-2X/6-311++G**//M06-2X/6-31+G** level of theory over a wider temperature range of 238–358 K that encompasses temperature...

Wastewater (WW) reuse is expected to be increasingly indispensable in future water management to mitigate water scarcity. However this increases the risk of antibiotic resistance (AR) dissemination via irrigation. Herein a conventional (chlorination) and an advanced oxidation process (heterogeneous photocatalysis (HPC)) were used to disinfect urban WW to the same target of Escherichia coli <10 CFU/100 mL and used to irrigate lettuce plants (Lactuca sativa) set up in 4 groups, each receiving one of four water types, secondary WW (positive control), fresh water (negative control), chlorinated WW and HPC WW. Four genes were monitored in water and soil, 16S rRNA as an indicator of total bacterial load, intI1 as a gene commonly associated with anthropogenic activity and AR and two AR genes blaOXA-10, and qnrS. Irrigation with secondary WW resulted in higher dry soil levels of intI1 (from 1.4x104 copies per gram before irrigation to 3.3x105 copies per gram after). HPC treated wastewater showed higher copy numbers of intI1 in the irrigated soil than chlorination, but the opposite was true for blaOXA 10. The results indicate that current treatment is insufficient to prevent dissemination of AR markers and that HPC does not offer a clear advantage over chlorination.

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.

Heterogeneous photocatalysis (HPC) is a subset of Advanced Oxidation Processes (AOPs) with potential future applications in water disinfection. Herein, a zinc oxide photocatalyst was doped with cerium at various atomic ratios ranging from 0 to 0.1 Ce : Zn. Keeping in mind that the application of HPC is often limited by its cost of use, a simple and easy to upscale method, that is the hydroxide induced hydrolysis of zinc nitrate in the presence of Ce³⁺ followed by calcination at 300 °C, was used to synthesise the catalysts. The catalysts have been characterized by different techniques such as X-ray diffraction (XRD), UV-vis diffuse reflectance (UV-vis DRS) and Raman spectroscopy. XRD results showed that Ce³⁺ ions were successfully incorporated into the ZnO lattice. UV-vis DRS spectra evidenced that Ce–ZnO samples present band-gap values of about 2.97 eV, lower than those of undoped ZnO (3.21 eV). These various photocatalysts, at 0.1 g L⁻¹ in saline 0.85%, were used to inactivate Escherichia coli previously isolated from an urban wastewater treatment plant. Higher atomic ratios of Ce in the ZnO lattice, as confirmed by XRD and Raman spectroscopy, showed significant improvements to the inactivation rate; the resulting recommended optimum cerium loading of 0.04 : 1 Ce : Zn gave multiple orders of magnitude higher rate of inactivation after 60 min of treatment when compared to un-doped ZnO. This optimum loading of cerium was faster than the de facto literature standard TiO₂-P25 tested under identical conditions.