The efficacies of degrading antibiotic resistance genes (ARGs) by applying UV light emitting diodes (UV-LEDs) based advanced oxidation processes (AOPs)

Abstract

• Simultaneous use of 285 nm and Cl 2 had better synergy to remove extracellular ARG. • Simultaneous use of 265 nm and Cl 2 had higher removal for extracellular ARG. • For UV-LED/Persulfate, 265 nm outperformed 285 nm to remove extracellular ARG. • Radical contributions in AOPs varied by wavelengths for extracellular ARG removal. • Cellular radical scavenging diminished AOP synergy for intracellular ARG removal. Widespread dissemination of antibiotic resistance genes (ARGs) in the aquatic environment has become a concern for public health. This study evaluated the performance of UV light emitting diodes (UV-LEDs) based advanced oxidation processes (AOPs) such as the simultaneous application of UV-LEDs (265 and 285 nm) and oxidants (chlorine and persulfate) to degrade ARGs. Persulfate (PS)-based treatment systems showed lower log-removals than chlorine (Cl 2 ) to degrade extracellular ARGs (e-ARGs), with the molar absorption coefficients (ɛ) for PS being 13.66 and 66.4 times lower than those for chlorine at 265 nm and 285 nm, respectively. While 285/Cl 2 exhibited stronger synergistic effects achieving an optimal synergy value of 4.02 log, 265/Cl 2 displayed better degradation rates with the maximum degradation rate of 0.117 cm 2 /mJ. Degradation rates induced by 265/PS were 1.2 to 2.2 times higher than 285/PS across all applied concentrations of oxidants. 265/PS also demonstrated a more pronounced synergistic effect than 285/PS with an optimal synergy value of 2.56. Quantum yields (Φ) at 265 nm are ∼1.1 times higher than at 285 nm for both oxidants. Cl 2 has ∼1.7 times higher ɛ-value at 285 nm than at 265 nm, while persulfate's ɛ-value is ∼2.93 times higher at 265 nm than at 285 nm. Thus, the better ɛ-value of Cl 2 at 285 nm improved the performance of 285/Cl 2 over 285/PS than 265 nm-based AOPs. Radical roles were investigated using scavenger studies with nitrobenzene (NB) and ethanol (EtOH) as quenchers. EtOH reacts quickly with hydroxyl radical (HO·), reactive chlorine species (RCS), and sulfate radical (SO 4 ·‾), while NB primarily reacts with HO· and shows minimal reactivity with other radicals. The involvement of radicals in different AOPs varied depending on the wavelength. For 265/Cl 2 and 285/PS, HO· was the primary contributor, with minimal contributions from other radicals. Significant contributions from RCS and SO 4 ·‾ radicals were observed for 285/Cl 2 and 265/PS, respectively, alongside HO·. Plasmid linearization was observed when the plasmid was subjected to AOPs, confirming the role of radicals in initiating the process of plasmid linearization through their interaction with the sugar-phosphate backbone. Scavenging of radicals by cellular components diminished the synergistic impact of AOPs on intracellular ARGs (i-ARGs) degradation. While AOPs demonstrated a notable degradation of extracellular polymeric substances (EPS), the absence of EPS didn't enhance the degradation of i-ARGs. The overall concentration of free ARGs (f-ARGs) was influenced by the interplay of two factors: the extent of membrane damage and the efficacy of e-ARG degradation. This study offers detailed insights into the effectiveness and mechanisms of UV-LED based AOPs for inactivating various forms of ARGs, as well as the associated challenges. Understanding the relevant mechanisms and challenges will assist in developing a sustainable and efficient UV-LED based AOP technology for removing ARGs from water and wastewater.

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