Behavioral analysis of simultaneous photo-electro-catalytic degradation of antibiotic resistant E. coli and antibiotic via ZnO/CuI: a kinetic and mechanistic study

Abstract

Visible light responsive semiconductor-based photocatalysis is known to be an efficient method for the disinfection of bacterial cells. Here, we address the issue of aqueous contamination by persistent pollutants such as antibiotics and antibiotic resistant bacteria (ARB) from an innovative angle. Simultaneous degradation of an antibiotic (chloramphenicol) and antibiotic resistant bacteria (chloramphenicol resistant E. coli) is performed to observe the effect of the presence of antibiotic in the reaction system when it is required for survival of the bacteria. A p-n junction-based ZnO/CuI composite is shown to demonstrate drastic enhancement in photocatalytic activity due to the inbuilt potential barrier suppressing charge carrier recombination. Moreover, an additional driving force for the suppression of recombination was provided by using a potential bias. Hydrothermally grown ZnO/CuI electrode films were characterized to assess optical, electrochemical, physicochemical and structural properties of the composite. Electrochemical impedance spectroscopy and diffuse reflectance spectroscopy were performed to obtain insights into the band bending, band edge potential, band gap and transmittance of the semiconductors. X-ray-based spectroscopic methods and zeta potential measurement demonstrated the surface properties and surface charges of the moieties in the reaction system, allowing us to deduce justifiable conclusions. A model based on the interaction of photogenerated radicals with the bacteria was developed and rate expressions were used to obtain the rate constants for the experimental results. Photoelectrocatalysis and photocatalysis followed first order rate kinetics; however, due to the unavailability of direct hole attack in photolysis, the electrolysis and electrocatalysis followed Langmuir-Hinshelwood kinetics. Bacterial disinfection was confirmed by K⁺ ion leaching and by structural changes in the membrane observed by FTIR of the cells after the reaction. We also addressed the issue of bacterial adhesion on the films restricting the mobility of radicals to interact with the bacteria, affecting the reusability of the catalyst films. The present work opens a wide avenue to discuss and address the improvement of the reusability of nanomaterial films for bacterial applications by controlling bacterial adhesion.

Fig. 1. XRD patterns of FTO, ZnO/FTO, CuI/FTO and ZnO/CuI/FTO.

Fig. 2. (a) Transmittance spectra of ZnO, CuI and ZnO/CuI.

Fig. 3. Photoluminescence spectra of ZnO nanorods, CuI and ZnO/CuI.

Fig. 4. Mott–Schottky plot of ZnO/CuI.

Fig. 5. XPS spectra of (a) C-1s, (b) O-1s, (c) I-3d, (d) Cu-2p.

Fig. 6. (a) SEM of ZnO nanorods (0.025 M), (b) CuI (30 cycles), (c) cross section of ZnO grown on FTO, (d) cross section of ZnO/CuI on FTO, (e) heterojunction of ZnO and CuI in the composite.

Fig. 7. Photolytic inactivation and kinetic profiles of bacteria (initial concentration ≈ 10⁷ CFU mL⁻¹), inset: model fit for photolysis.

Fig. 8. SEM of electrodes with catalyst loading 0.05 M.

Fig. 9. (a) Photocatalytic inactivation and (b) kinetic profiles of bacteria (initial concentration ≈ 10⁷ CFU mL⁻¹) with catalyst loading 0.01 M, 0.025 M, 0.05 M and 0.1 M.

Fig. 10. Inactivation and kinetic plots of E. coli (initial concentration ≈ 10⁷ CFU mL⁻¹) with ZnO, CuI, ZnO/CuI composite: (a) & (b) electrocatalysis, (c) & (d) photocatalysis and (e) & (f) photoelectrocatalysis.

Fig. 11. Calibration curve for concentration of chloramphenicol using HPLC.

Fig. 12. Degradation and kinetic plots of chloramphenicol degradation with ZnO, CuI, ZnO/CuI composite (a) & (b) electrocatalysis, (c) & (d) photocatalysis and (e) & (f) photoelectrocatalysis. (g) Schematic of photoelectrocatalytic degradation of chloramphenicol.

Fig. 13. (a) Simultaneous inactivation of NARB in NARB + CHM (40 ppm) and (b) kinetic plots of CHM in NARB + CHM.

Fig. 14. (a) Simultaneous inactivation of ARB in ARB + CHM and (b) kinetic plots of CHM in ARB + CHM.

Fig. 15. Scavenger studies of photoelectrocatalytic inactivation of E. coli in presence of ZnO/CuI.

Fig. 16. (a) Reusability of the catalyst ZnO/CuI for 8 cycles, (b) XRD of the catalyst before and after 8 h of reaction.

Fig. 17. Schematic of (a) downward bending in n-type semiconductor, (b) upward bending in p-type semiconductor, (c) upward bending in n-type semiconductor, (d) downward bending in p-type semiconductor, (e) band bending in p–n junction, (f) electron transport in a photoelectrocatalysis setup and (g) electron hole dynamics in the photoelectrocatalyst in case of bacteria (composite).

Fig. 18. K⁺ ion leaching for photocatalytic inhibition of bacteria by photolysis, ZnO, CuI, ZnO/CuI.

Fig. 19. Reusability plots after washing with DMSO.