Electrochemical disinfection of bacteria-laden water using antimony-doped tin-tungsten-oxide electrodes

Highlights

• Sb-doped Sn_80%-W_20%-oxide anodes exhibit strong bactericidal activity.

• Nearly instantaneous inactivation of 10⁷ CFU/mL E. coli observed at 6 mA/cm².

• Reactive oxygen species were main disinfectants at high current densities.

• Reactive chlorine species were main disinfectants at low current densities.

• Anode material is effective for inactivation of three bacterial strains.

Abstract

Electrochemical disinfection has been shown to be an efficient method with a shortrequired contact time for treatment of drinking water supplies, industrial raw water supplies, liquid foodstuffs, and wastewater effluents. In the present work, the electrochemical disinfection of saline water contaminated with bacteria was investigated in chloride-containing solutions using Sb-doped Sn_80%-W_20%-oxide anodes. The influence of current density, bacterial load, initial chloride concentration, solution pH, and the type of bacteria (E. coli D21, E. coli O157:H7, and E. faecalis) on disinfection efficacy was systematically examined. The impact of natural organic matter and a radical scavenger on the disinfection process was also examined. The electrochemical system was highly effective in bacterial inactivation for a 0.1 M NaCl solution contaminated with ∼10⁷ CFU/mL bacteria by applying a current density ≥1 mA/cm² through the cell.100% inactivation of E. coli D21 was achieved with a contact time of less than 60 s and power consumption of 48 Wh/m³, by applying a current density of 6 mA/cm² in a 0.1 M NaCl solution contaminated with ∼10⁷ CFU/mL. Reactive chlorine species as well as reactive oxygen species (e.g. hydroxyl radicals) generated in situ during the electrochemical process were determined to be responsible for inactivation of bacteria.

Introduction

The elimination of pathogenic microorganisms, suspended or as attached biofilms, is a primary step in the treatment of water from many different sources, such as raw water supply (Trussell, 1998), ballast water (Nanayakkara, 2010), drinking water reservoirs and water distribution systems (Flemming, 2002), process wash water in food processing plants (López-Gálvez et al., 2012), brackish or industrial briny waters for use in food industry, swimming pools and drinking water (Feng et al., 2004, Martínez-Huitle and Brillas, 2008), amongst many others. This is usually carried out by adding chemicals such as chlorine, chlorine dioxide and ozone (Harper et al., 2001, Rahmawati et al., 2010). The most common method, chlorination, is associated with some problems such as transport and storage of chlorine (Kraft et al., 1999, Rajeshwar and Ibanez, 1997), generation of hazardous by-products such as trihalomethanes (THMs), haloacetic acids (HAAs), and N-nitrosodimethylamine (NDMA) (Lawton and Robertson, 1999, Gusmão et al., 2010, Mitch et al., 2003), and the resistance of some pathogens including pathogenic Escherichia coli (E. coli) and Campylobacter jejuni bacteria, enteric viruses such as rotavirus and calicivirus, and the parasites Cryptosporidium and Giardia lamblia (Gusmão et al., 2010, Szewzyk et al., 2000).

Electrochemical disinfection has gained attention as a potential alternative to conventional chlorination (Jeong et al., 2007, López-Gálvez et al., 2012) due to its environmental compatibility, easy installation and operation, and effectiveness for inactivation of a wide variety of microorganisms from bacteria to viruses and algae under mild pressures and temperature (Diao et al., 2004, Drees et al., 2003, Long et al., 2015, Mascia et al., 2013). It has been demonstrated that electrochemical technology can provide high disinfection efficiency for drinking water (Matsunaga et al., 1992), raw water supply (Patermarakis and Fountoukidis, 1990), liquid foodstuff (Grahl and Märkl, 1996), and industrial and domestic wastewater effluents (Anglada et al., 2009, Schmalz et al., 2009). The practice of cleaning water by passing an electric current had been reported as early as the nineteenth century; however, only recently has this technology come into long-term practical use (Kraft, 2008).

Electrochemical water treatment is a green and powerful technology with a two-stage mechanism of action: (1) direct oxidation at the electrode surface, which is characterized by the instantaneous killing of microbial cells (Jeong et al., 2007), and (2) indirect oxidation in the bulk solution by disinfecting species produced from water oxidation, such as hydroxyl radical (OH), atomic oxygen (O), hydrogen peroxide (H₂O₂), and ozone (O₃) (Eqs. (1), (2), (3), (4), (5)) (Liang et al., 2005, Martínez-Huitle and Brillas, 2008, Panizza and Cerisola, 2005) classified as reactive oxygen species (ROS), or by oxidants produced from the substances dissolved in the water, e. g., chloride is oxidized to free chlorine, according to Eq. (6) (Anglada et al., 2009, Kraft, 2008). Dissolved chlorine is subsequently hydrolyzed to hypochlorous acid/hypochlorite ion and hydrochloric acid, in main side reactions of anodic production of chlorine, as given in Eqs. (7), (8). In water treatment, the overall concentration of dissolved chlorine after the chlorination process is termed active chlorine, and is given by the summation of three species: free chlorine (Cl₂), hypochlorous acid (HClO) and hypochlorite ion (ClO¯). The mass distribution of these three main reactive chlorine species (RCS) depends on the pH of the medium (Bergmann and Koparal, 2005).

$$2C{l}^{-}\to C{l}_2+2e^{-}$$

$$C{l}_2+H_{2}O\to C{l}^{-}+HClO+H^{+}$$

$$HClO\to Cl{O}^{-}+H^{+}$$

Other advantages of electrochemical disinfection are on-site generation of disinfectants with controllable dose (Jeong et al., 2007) and relatively low energy requirement that allows the use of green energy sources such as solar cells or fuel cells (Drogui et al., 2001, Ghernaout et al., 2008). Overall, the high efficacy of electrochemical disinfection is attributed to the synergistic effects of direct oxidation on the electrode surface (Grahl and Märkl, 1996, Matsunaga et al., 2000), generation of the reactive intermediate species such as ROS or RCS with strong bactericidal activity (Diao et al., 2004, Feng et al., 2004, Liang et al., 2005), and/or the electric field effect (Butterfield et al., 1997, Grahl and Märkl, 1996).

The selection of an appropriate anode material is a key factor in electrocatalytic processes, as it influences not only the efficiency of the process, but also the electrode selectivity (Feng et al., 2016). Common anode materials used in studies of electrochemical water disinfection are titanium with active coatings based on metal oxides, which are known as Dimensionally Stable Anodes (DSA^®s) (Gusmão et al., 2010), platinum (Jeong et al., 2007) and boron-doped diamond (BDD) electrodes (Lacasa et al., 2013, Long et al., 2015). DSA^® type electrode materials include IrO₂-RuO₂ (Bergmann and Koparal, 2005), TiO₂-RuO₂ (Gusmão et al., 2010), SnO₂ (Watts et al., 2008), and IrO₂-Sb₂O₅-SnO₂ (Fang et al., 2006). These electrodes have shown higher efficiencies in the production of free chlorine compared to Pt and diamond electrodes, which is of primary importance in environmental applications of electrochemical technology in the presence of chloride salts (Bergmann et al., 2008, Kraft, 2008). In addition, diamond electrodes may further oxidize hypochlorite to chlorate and perchlorate (Palmas et al., 2007) which are permissible in potable water only at very low concentrations (Kraft, 2008). Therefore, DSA^® type electrodes are more applicable for the electrochemical disinfection of water due to their higher efficiency in the production of oxidizing species.

The mechanism of electrochemical microbial inactivation is not completely understood. Fundamentally, the vital physiological functions of bacteria are based on the cell membrane, cytoplasm, and nucleic acids (DNA and RNA). Thus, damages to any of these subcellular constituents of bacteria could potentially lead to the inactivation of bacteria. Diao et al., (2004) observed leakage of substantial intracellular materials from E. coli cells after electrochemical treatment by DSA^®s under scanning electron microscope. In another study, Jeong et al., (2006) observed changes in the inner content and cell walls of E. coli as a result of BDD disinfection. The work of Tanaka et al., (2013) revealed lipid peroxidation in the cell membranes of electrochemically disinfected bacteria in seawater using a Pt anode. Long et al., (2015) investigated the subcellular mechanisms of E. coli inactivation during BDD electrochemical disinfection in three electrolytes: in chloride solution, E. coli inactivation was attributed to damage to the intracellular enzymatic systems; in sulfate solution, the elimination of certain essential membrane proteins such as K⁺ ion transport systems mainly induced cell inactivation; and in phosphate solution, mineralization of their organic intracellular components was responsible for cell inactivation.

To the best of our knowledge, the performance of mixed metal oxide (MMO) electrodes consisting of antimony-tin-tungsten-oxides in electrochemical inactivation of microorganisms has not been investigated. In this context, the main objective of the present work was to examine the potential of electrochemical disinfection of saline water employing antimony-doped tin-tungsten-oxide electrode coatings (Sb-doped Sn_80%-W_20%-oxide) formed on a titanium substrate via a thermal deposition method. This electrode was selected as it was found to have high intrinsic electrocatalytic activity for oxidation of phenol red and the drug carbamazepine in our previous studies (Ghasemian et al., 2017, Ghasemian and Omanovic, 2017). A non-pathogenic strain of E. coli was selected as a model bacterium, while pathogenic E. coli and Enterococcus faecalis (E. faecalis) were used to further validate the inactivation efficiency of the process. To examine the disinfection potential of the system, the electrochemical production of free chlorine as well as process parameters such as current density, bacterial cell density, pH, and the presence of natural organic matter (NOM) and methanol as a free radical scavenger were systematically investigated in chloride- and phosphate-containing solutions. Moreover, the energy requirement to reach a specific level of bacterial inactivation (3.5-log and 7.4-log reductions) at each current density was calculated.