Elsevier

Water Research

Volume 184, 1 October 2020, 116157
Water Research

Hierarchical Bi2O2CO3 wrapped with modified graphene oxide for adsorption-enhanced photocatalytic inactivation of antibiotic resistant bacteria and resistance genes

https://doi.org/10.1016/j.watres.2020.116157Get rights and content

Highlights

  • Nitrogen doping enhanced electron transfer and bacterial adsorption to RGO.

  • This improved ROS generation and ARB inactivation near photocatalytic sites.

  • Surface-bound oxidizing species were main contributors to ARB inactivation.

  • eARGs released near catalytic sites were also efficiently degraded by NGWM.

Abstract

There is growing pressure for wastewater treatment plants to mitigate the discharge of antibiotic resistant bacteria (ARB) and extracellular resistance genes (eARGs), which requires technological innovation. Here, hierarchical Bi2O2CO3 microspheres were wrapped with nitrogen-doped, reduced graphene oxide (NRGO) for enhanced inactivation of multidrug-resistant E. coli NDM-1 and degradation of the plasmid-encoded ARG (blaNDM-1) in secondary effluent. The NRGO shell enhanced reactive oxygen species (ROS) generation (•OH and H2O2) by about three-fold, which was ascribed to broadened light absorption region (red-shifted up to 459 nm) and decreased electron-transfer time (from 55.3 to 19.8 ns). Wrapping enhanced E. coli adsorption near photocatalytic sites to minimize ROS scavenging by background constituents, which contributed to the NRGO-wrapped microspheres significantly outperforming commercial TiO2 photocatalyst. ROS scavenger tests indicated that wrapping also changed the primary inactivation pathway, with photogenerated electron holes and surface-attached hydroxyl radicals becoming the predominant oxidizing species with wrapped microspheres, versus free ROS (e.g., •OH, H2O2 and •O2) for bare microspheres. Formation of resistance plasmid-composited microsphere complexes, primary due to the π-π stacking and hydrogen bonding between the shell and nucleotides, also minimized ROS scavenging and kept free plasmid concentrations below 102 copies/mL. As proof-of-concept, this work offers promising insight into the utilization of NRGO-wrapped microspheres for mitigating antibiotic resistance propagation in the environment.

Introduction

The propagation of antibiotic resistant bacteria (ARB) poses a growing threat to public health (Davies and Davies, 2010; O’Neill, 2016), underscoring the need to consider the occurrence and fate of ARB in water infrastructure. Some wastewater treatment plants (WWTPs) may serve as breeding grounds and point sources for ARB and extracellular antibiotic resistance genes (eARGs) that contribute to the environmental resistome (Ju et al., 2016; Luo et al., 2014). Furthermore, conventional secondary effluent disinfection approaches (e.g., chlorination and ultraviolet (UV) radiation) exhibit relatively low efficiency to mitigate the discharge of ARB and eARGs (He et al., 2019; Lorenz and Wackernagel, 1994; Luo et al., 2011), and residual disinfectants may enhance ARG horizontal transfer, contributing to resistance propagation (Dotson et al., 2010; Li et al., 2016; Zhang et al., 2017). Thus, there is a critical need for technological innovation to develop robust and sustainable approaches to mitigate antibiotic resistance dissemination through efficient ARB inactivation and associated eARGs degradation.

Photocatalysis has received significant attention as a potential eco-friendly disinfection process for eliminating waterborne microbial contaminants, including ARB (Giannakis et al., 2018; Pelaez et al., 2012; Zhao et al., 2014a). The process involves production of reactive oxygen species (ROS) such as hydroxyl radicals (•OH) and superoxide (O2•-), as well as electron holes (h+), upon UV irradiation (Table S1). These oxidizing species can efficiently inactivate bacteria by attacking essential biomolecules (e.g., lipids, proteins, and DNA) (Cho et al., 2004; Ren et al., 2018; Xia et al., 2015). Three-dimensional (3D) hierarchical microspheres composed of 2D photoactive nanosheets exhibit high specific surface area to facilitate light harvesting and photocatalytic performance (Liang et al., 2017; Zhang et al., 2013), while their relatively large size enables cost-effective separation and recovery (Zhang et al., 2018). We previously showed that nanosheet-assembled Bi2O2CO3 microspheres can be easily composited with electron-shuttling reduced graphene oxide (RGO) to promote electron transfer and increase ROS production (Zhang et al., 2014). However, while enhanced ROS generation may improve disinfection (Moreira et al., 2018), a higher rate of cells lysis increases the release of eARGs. Due to ROS dilution and scavenging by background constituents (e.g., carbonates, soluble microbial products (SMPs), and natural organic matter), eARGs released in bulk water away from photocatalytic sites may escape treatment (Dunlop et al., 2015; Ribeiro et al., 2019). Accordingly, previous work showed that RGO-enhanced photocatalytic disinfection of ARB did not efficiently degrade the associated eARGs in wastewater (Karaolia et al., 2018). Thus, it is important to enhance bacterial and eARG adsorption near photocatalytic sites for more efficient ROS utilization (i.e., trap-and-zap strategy) (Zhang et al., 2018).

We postulate that nitrogen doping of the RGO shell can improve bacterial adhesion to microspheres due to a less negative zeta potential than undoped RGO (Hasan et al., 2012; Smith et al., 2019), which decreases electrostatic repulsion and thus increase the probability of catalyst-bacteria collision (Xue et al., 2018; Zhao et al., 2014b). Subsequently, bacteria can be adsorbed by the catalyst due to hydrophobic interaction between hydrophobic amino acids on bacterial surfaces and the photocatalyst surface (e.g., graphitic basal planes). Bacterial adsorption near photocatalytic sites is not only conducive to more efficient disinfection, but also may enhance the immediate capture and degradation of eARGs that are released due to bacterial cell lysis. Furthermore, nitrogen doping may accelerate electron transfer and enhance ROS generation by decreasing defects within the NRGO plane, and improving the interfacial contact between NRGO and the photocatalyst (Mou et al., 2014; Xu et al., 2016).

Here, we report the synthesis of novel NRGO-wrapped Bi2O2CO3 microspheres (NGWM), and demonstrate their efficacy for simultaneous photocatalytic ARB inactivation and eARGs degradation in a secondary effluent polishing context. Enhanced bacterial adsorption and inactivation, as well as eARG degradation, were confirmed by direct comparisons versus treatment with bare Bi2O2CO3 microspheres or undoped RGO-wrapped Bi2O2CO3 microspheres (GWM). The photocatalytic mechanism was investigated through ROS scavenging tests, and oxidative stress to the ARB was assessed by quantifying cell membrane integrity and antioxidant enzymatic activity. We discerned the interfacial mechanisms for enhanced eARGs adsorption and degradation relative to uncoated catalyst. Benchmarking experiments were also performed using a commercial TiO2 photocatalyst to demonstrate the applicability of NGWM for municipal wastewater treatment effluent disinfection and associated mitigation of antibiotic resistance propagation.

Section snippets

Chemicals, reagents and bacterial strains

Bismuth citrate (≥99%), urea (≥97%), ethanol (≥99.5%), (3-aminopropyl)trimethoxysilane (APTMS, ≥97%), p-chlorobenzoic acid (pCBA, ≥[X]%), hydrazine monohydrate (N2H4, ≥98%), potassium iodide (KI, ≥99%), isopropanol (IPA, ≥99%), superoxide dismutase (SOD), sodium pyruvate (SP, ≥99%), protein carbonyl content assay kit, and 2,7-dichlorodihydrofluorescein diacetate (DCFH-DA, ≥97%) were purchased from Sigma-Aldrich and used as received. Single layer graphene oxide power (GO, ≥99%) was purchased

Bi2O2CO3 microspheres were successfully wrapped with NRGO nanosheets

NGWM microspheres (1.5 ± 0.5 μm in diameter) exhibited a core-shell structure with a rose-like core of Bi2O2CO3 nanosheets, which was completely wrapped by a thin layer of NRGO (Fig. 1A–C). XPS spectra confirm the chemical bonds that constitute Bi2O2CO3 and NRGO composite materials (Fig. 1D–F & Figs. S2A–B). Specifically, the C-Bi bond observed at 164.2 eV (Bi 4f spectrum) and 281.2 eV (C 1s spectrum) suggests formation of chemical bond at the interface of Bi2O2CO3 core and NRGO shell (Fig. 1D

Conclusions

Wrapping of hierarchical Bi2O2CO3 microspheres with NRGO enhanced bacterial adsorption and photocatalytic ARB inactivation in the secondary effluent (which contains abundant ROS scavengers). Photogenerated electron holes (h+) and surface-attached hydroxyl radicals (•OHads) were the predominant oxidizing species responsible for ARB inactivation with wrapped microspheres, versus free ROS (e.g., •OH, H2O2 and •O2) for bare microspheres. Due to the short distance between the adsorbed bacteria and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by National Natural Science Foundation of China [No. 51625804], the National Key R&D Program of China (2018YFD1100500), NSF ERC on Nanotechnology-Enabled Water Treatment (EEC-1449500), and NSF PIRE grant (OISE-1545756). We thank Huaqiang Chu, Jiabin Chen, Libin Yang, Shaoze Xiao, Danning Zhang and Ling-Li Li for their help on catalyst characterization.

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