Remediation of surface water contaminated by pathogenic microorganisms using calcium peroxide: Matrix effect, micro-mechanisms and morphological-physiological changes
Graphical abstract
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.
Section snippets
Materials
Details of all chemicals and target pathogen indicators used in this study are provided in Supporting Information (SI) Text S1. All bacteria and bacteriophages used in this study were inactivated by being autoclaved at 121 °C for 15 min following all experiments. Natural surface water (SW) was collected from a local river (Dasha River, Shenzhen, China (22.6076° N, 113.9987° E)) at noon on a sunny day in the summer of 2020, filtered by 0.45 μm filters, sterilized at 121 °C for 15 min, and stored
Effect of CaO2 dosage
CaO2 effectively inactivated E. coli and VCSM13 (Fig. 1(A, B)). The most suitable CaO2 dosage for inactivating E. coli and VCSM13 were 1.0 and 4.0 mM, respectively, which inactivated 76% of E. coli and 41% of VCSM13 at the shortest exposure time (i.e., 2 min), and subsequently reached to ∼100% at 120 min, respectively in both PW and SW groups. A sharp decrease in the normalized concentrations of E. coli and VCSM13 (C/C0) upon CaO2 addition was most likely due to the production of HRS from CaO2,
Conclusions
CaO2 could effectively inactivate E. coli, S. aureus, and VCSM13 in SW with 100% efficiencies under 1, 1, and 4 mM of CaO2 in 120 min. For E. coli, and VCSM13, environmental prevalent ions such as CO32−, HCO3−, Cl−, SO42−, and Fe3+ had insignificant influence on their inactivation except that Cu2+ substantially promoted the inactivation. Besides, HA and FA induced a negative effect on E. coli inactivation but little effect on VCSM13 inactivation. Furthermore, the inactivation of pathogen
Declaration of Competing Interest
The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (Grant No. 2021YFA1202500), China Postdoctoral Science Foundation (No. 2020M671067), Guangdong Basic and Applied Basic Research Foundation (2020A1515110591), Shenzhen Science and Technology Innovation Committee (KCXFZ20201221173410029, JCYJ20190809164201686), National Natural Science Foundation of China (Grant No. 42077223), State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater
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