Removal of Antibiotic Resistant Bacteria and Genes by UV-Assisted Electrochemical Oxidation on Degenerative TiO2 Nanotube Arrays

Abstract

Antibiotic resistance has become a global crisis in recent years, while wastewater treatment plants (WWTPs) have been identified as a significant source of both antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs). However, commonly used disinfectants have been shown to be ineffective for the elimination of ARGs. With the goal of upgrading the conventional UV disinfection unit with stronger capability to combat ARB and ARGs, we developed a UV-assisted electrochemical oxidation (UV-EO) process that employs blue TiO₂ nanotube arrays (BNTAs) as photoanodes. Inactivation of tetracycline- and sulfamethoxazole-resistant E. coli along with degradation of the corresponding plasmid coded genes (tetA and sul1) is measured by plate counting on selective agar and qPCR, respectively. In comparison with UV₂₅₄ irradiation alone, enhanced ARB inactivation and ARG degradation is achieved by UV-EO. Chloride significantly promotes the inactivation efficiency due to the electrochemical production of free chlorine and the subsequent UV/chlorine photoreactions. The fluence-based first-order kinetic rate coefficients of UV-EO in Cl^- are larger than those of UV₂₅₄ irradiation alone by a factor of 2.1-2.3 and 1.3-1.8 for the long and short target genes, respectively. The mechanism of plasmid DNA damage by different radical species is further explored using gel electrophoresis and computational kinetic modeling. The process can effectively eliminate ARB and ARGs in latrine wastewater, though the kinetics were retarded.

Keywords: Antibiotic resistance genes; Antibiotic resistant bacteria; Blue TiO2 nanotube arrays (BNTAs); UV-assisted electrochemical oxidation (UV-EO); Wastewater treatment.

Figure 1.

Experimental scheme for treatment of ARB and ARGs by UV-assisted electrochemical oxidation with BNTAs as the anode.

Figure 2.

Schematic illustration of the position of the conduction band (CB), valence band (VB), and Fermi energy level (E_F) at an anodic potential for (a) NTAs, (b) BNTAs, and (c) BNTAs under UV irradiation. The band structure, E_F position, and electron tunneling mechanism demonstrated in (a) and (b) were determined in our previous study.

Figure 3.

BA degradation by BNTAs at 10 mA/cm² in the absence (EO) and presence (UV-EO) of UV₂₅₄ irradiation. All the tests were performed in 30 mM NaClO₄ except that “EO w/Cl⁻” and “UV-EO w/Cl⁻” were conducted in 30 mM NaCl. Dots and dash lines represent experimental data and the results of the kinetic model simulation, respectively.

Figure 4.

Inactivation of antibiotic resistant E. coli and degradation of tetA and sul1 genes with UV₂₅₄ irradiation alone or UV-EO treatment at the optimized current density of 10 mA/cm² by the BNTA anode. The experiments were conducted in 30 mM NaClO₄ (“ClO₄⁻”) or 30 mM NaCl (“Cl⁻”). The error bars represent standard deviations from triplicate experiments for ARB and i-ARGs and duplicate experiments for e-ARGs.

Figure 5.

DNA electrophoresis gel of extracellular pEB1-sul1 as a function of UV₂₅₄ dose (mJ/cm²) and time (s), with different treatments including (a) UV₂₅₄ in ClO₄⁻ and UV-EO with BNTAs at 30 mA (b) in ClO₄⁻ and (c) in Cl⁻. All the tests were carried out with an initial concentration of ~10 ng/μL plasmids in 30 mM NaClO₄ labeled as “ClO₄⁻” or 30 mM NaCl labeled as “Cl⁻”. The UV intensity was 5 mW/cm² at 254 nm. The first lane “L” of each image shows the standard 1 kb plus DNA ladder. All the DNA samples are presented without (w/o) any enzyme treatment (lanes 1–5) and with (w/) restriction by SbfI enzyme at 37 °C for 15 min (lanes 6–10).

Figure 6.

Bacteria inactivation and ARG degradation by UV-EO treatment in latrine wastewater. Native bacteria and ARGs present in latrine wastewater were measured by plate counting and qPCR, respectively. The error bars represent standard deviations from triplicate experiments.