Remediation of surface water contaminated by pathogenic microorganisms using calcium peroxide: Matrix effect, micro-mechanisms and morphological-physiological changes

Highlights

• Low levels of CaO₂ effectively inactivated pathogenic bacteria and virus.

• HRS (OH⁻, •OH, H₂O₂, and O₂^•−) accounted for pathogens inactivation.

• [HRS]_SS and their reaction rate constants was determined for micromechanism model.

• Environment-level ions and DOM may influence pathogens inactivation.

• Bactria and virus underwent morphological and physiological changes.

Abstract

Calcium peroxide (CaO₂), a common solid peroxide, has been increasingly used in contaminated site remediation due to its ability to release oxygen (O₂) and hydrogen peroxide (H₂O₂) and its environmental friendliness. Our present study is first to explore micromechnisms of CaO₂ to efficaciously inactivate pathogen indicators including gram-negative bacterium of Escherichia coli (E. coli), gram-positive bacterium of Staphylococcus aureus (S. aureus), and virus of Escherichia coli-specific M13 bacteriophage (VCSM13) under low concentration (≤ 4 mmol L⁻¹ (mM)). The inactivation mechanisms of E. coli, S. aureus (1 mmol L⁻¹ CaO₂) and VCSM13 (4 mmol L⁻¹) were mainly attributed to OH⁻ (32∼58%) and •OH (34∼42%), followed by H₂O₂ (13∼20%) and O₂^•− (10∼12%) generated from CaO₂, with the observed morphological and physiological-associated damages. Also, average steady-state concentrations of (OH⁻, •OH, H₂O₂, and O₂^•−) and their reaction rate constants with E. coli and VCSM13 were determined. Accordingly, the micro-mechanism model of inactivation was established and validated, and the inactivation efficiency of the same order of magnitude of pathogen was predicted. Furthermore, during the common environmental factors, the copper ions was found to be promote CaO₂ inactivation of pathogens, and dissolved organic matter (DOM) fractions had a negative effect on CaO₂ inactivation. The present study explored the mechanisms of CaO₂ inactivation of pathogens in real surface water, laying the foundation for its potential use in the inactivation of water-borne microbial pathogens.

Introduction

The transmission of pathogenic microorganisms from domestic drinking water remains the leading cause of waterborne diseases and associated deaths in many developing countries (Song et al., 2016; Wang et al., 2020). Between the propagation of waterborne microorganisms and limited resources available for microbial control, an alarming 2.2 million people are killed from diarrheal symptoms annually, especially for children under five years old (Li et al., 2008). As such, disinfecting water to reduce microbial pathogens is essential to maintaining safe water for domestic use (World Health Organization, 2004, 2017). Moreover, within the last 10 years, disease outbreaks of enveloped viruses such as Coronaviruses (Seong et al., 2016), Ebola virus (Chippaux, 2014), Hantavirus (Nunez et al., 2014), and Lassa virus (Prescott et al., 2017) placed increasing demands for the implementation of water safety management practices to extinguish the infectious viruses in waters (Prescott et al., 2017). Presently, this is of pertinent concern due to a big outbreak of novel coronavirus pneumonia (COVID-19) pandemic (Coronavirus Resource Center, accessed on January 18, 2022), which is capable of entering the aquatic environment through human excretion, potentially leading to a greater number of infections (Mao et al., 2020). The development of cost-effective and widely available disinfection technologies for control of pathogenic microorganisms, including bacteria and viruses, is increasingly urgent, though solutions to these concerns remains challenging.

Current disinfection methods for water treatment, including ultraviolet (UV) disinfection, and chlorine, chloramines or ozone chemical disinfection (Kheyrandish et al., 2017; Li et al., 2008, 2019) can control microbial pathogens. However, there may exist dilemmas between effective disinfection and high-energy consumption, the formation of harmful disinfection byproducts (DBPs) or pathogenic recolonization (Dalrymple et al., 2010; Krasner et al., 2006; Wang et al., 2020). Despite good disinfection, UV-based processes required additional power consumption (Wang et al., 2020). Furthermore, the resistance of some pathogenic microorganisms to decontaminants makes them difficult to disinfect except at high doses and intensity, posing greater challenges to traditional antivirus techniques (Yates et al., 2006). Meanwhile, a majority of DBPs with biotoxicity have been reported, with some byproducts shown to be more toxic than their parent compounds (Krasner et al., 2006; Richardson et al., 2008). Additionally, very few traditional disinfection techniques have been evaluated for their effectiveness against viruses during water treatment. Therefore, the effectiveness of novel disinfection techniques is urgently needed in the wake of the COVID-19 outbreak and for mitigating effects from additional viral outbreaks.

Calcium peroxide (CaO₂), one of the most frequently-used solid inorganic peroxy compounds, has been widely used in agriculture, aquaculture, and medicine (Lu et al., 2017), and is considered as a stable and safe “solid form” of H₂O₂ (Zhang et al., 2015). CaO₂ may produce hydrolysates and reactive species (HRS) such as calcium hydroxide (Ca(OH)₂), oxygen (O₂), hydrogen peroxide (H₂O₂), hydroxyl radical (•OH) and superoxide anion radical (O₂^•−) in moist media, exhibiting its great potential for inactivating pathogenic microorganisms. Firstly, Ca(OH)₂ may exert antibacterial effects probably due to the release of hydroxyl ions (OH^‒), via damage to the bacterial cytoplasmic membrane, proteins, or the DNA (Siqueira Jr and Lopes, 1999). However, the bactericidal effect of Ca(OH)₂ from CaO₂ remains controversial in natural surface water (SW), e.g., Enterococcus faecalis, due to the influence of natural water matrix or buffering, CaO₂ dosage, and pathogens with different pH tolerance (Siqueira Jr and Lopes, 1999). Secondly, O₂ H₂O₂, and O₂^•− may exacerbate oxidative stress to some bacterial cells (Nelson et al., 2018), but the efficacy of these HRS produced from CaO₂ on target pathogens was unknown due to uncertainties in HRS yields and HRS susceptibility to pathogens, and matrix effect. Still, CaO₂ can be used as eco-friendly amendment to supply external O₂ in agriculture, horticulture, silviculture (Lu et al., 2017; Thani et al., 2016), and remediate black-odor water (Wang et al., 2019). Furthermore, exogenous oxidative inactivation is one of the most important ways. Capability of oxidation makes CaO₂ be used for degrading contaminants including MTBE-contaminated groundwater (Liu et al., 2006), and toxic materials in waste streams (Madan and Wasewar, 2018), and also shows the possibility of inactivating pathogens (Nelson et al., 2018; Zhang et al., 2020). H₂O₂, O₂^•−, and •OH can induce damage to macromolecules such as DNA and lipids, resulting in pathogen inactivation, but e.g., bacteria have several active enzymes such as superoxide dismutase and catalase that may protect the cells from oxidative damage of H₂O₂, and O₂^•− (Nelson et al., 2018; Raffellini et al., 2008; Rincon and Pulgarin, 2004). Additionally, compared with direct addition of H₂O₂, CaO₂ takes one more step to react with water to release HRS, allowing more extended periods to oxidize pollutants or inactivate pathogens (Lu et al., 2017). With respect to the continuous, strong reactivity of multi-HRS produced from CaO₂ and its safe application in many industrial processes (Lu et al., 2017), we hypothesize that CaO₂ could be readily implemented as a successful water disinfectant for pathogenic microorganisms. However, CaO₂-induced HRS yields and their comprehensive synergistic effect, efficacy of pathogen inactivation, and corresponding mechanism are vacant, which greatly hindered its practical application in natural water treatment and urgently need to be explored.

Therefore, the Escherichia coli, Staphylococcus aureus and E. coli-specific M13 bacteriophage, representative microorganisms of gram-negative bacteria, gram-positive bacteria, and viruses, respectively, were selected to evaluated disinfection of CaO₂ in natural water media. The micromechanisms and model-related reaction rate constants for CaO₂ inactivation of pathogens were investigated. Influences of several abiotic factors, such as ions, natural dissolved organic matter (DOM), and sunlight present in natural treatment systems were explored. This study will evaluate whether CaO₂ can be a promising water disinfection oxidant for both bacteria and viruses.