Electrochemical disinfection is an efficient method used for treatment of drinking water. It has great environmental compatibility as compared to conventional disinfection methods. In this study, the effects of the electrode materials and working conditions were investigated. The experimental results show that the type 1 (iridium oxide-coated anode and cathode) electrode system generated a high concentration of free available chlorines (FACs) because iridium has higher electrocatalytic activity than ruthenium. When the applied voltage increases, the acceleration of oxidation reactions in the electrochemical cell resulted in the increased generation of FACs. The solution inflow rate is approximately inversely proportional to the residence time of the salt solution in the electrochemical cell. A long residence time can induce a higher FAC generation. In addition, the production of FACs is increased with the decreasing electrode open ratio (a/A). With a/A > 0 and a lower inflow rate, the FAC concentration tends to approach a maximum value because of by-product generation. The ozone species generated in the electrochemical cell were determined by the maximum voltage. The electrode open ratio affected the ozone generation rate due to the mixing effect of cathode products.
Water pollution has become a critical problem due to the rapid development of industries in past decades. The scarcity of clean and safe drinking water has reduced life expectancy and increased infant mortality rates, particularly in developing countries (Gadgil 1998). The most common water pollution is of biological origin (Crittenden et al. 2005; Tang et al. 2014), which includes the mixing of various micro-organisms (e.g. viruses, pathogenic bacteria, protozoa) and parasites in water. To prevent waterborne diseases, both physical and chemical water disinfection methods are frequently used (Gadgil 1998; Vasudevan & Oturan 2014). The physical methods of water disinfection include boiling, filtration, and UV disinfection. Boiling and filtration are traditional and very effective methods for water disinfection, however they require a lot of energy. The UV disinfection method is also very effective for various types of pathogenic bacteria and viruses, yet it requires periodic maintenance, e.g. UV lamp replacement. The chemical water disinfection technologies usually involve the addition of oxidation species into raw water for oxidization of any harmful micro-organisms present. The major oxidation species used for water disinfection are ozone and chlorine. Ozone (O3) is a potent oxidation species that can be decomposed into an oxygen molecule and a nascent oxygen atom. Although ozone can exhibit a strong disinfection effect, it cannot provide residual protection against recontamination in a water distribution system (Gadgil 1998; Crittenden et al. 2005; Loo et al. 2012; Amin et al. 2013; Tang et al. 2014).
Chlorine is currently the most widely used water disinfectant. Traditionally, liquid chlorine is injected into raw water, producing HOCl (hypochlorous acid), OCl− (hypochlorite ion), H+, and Cl−. The hypochlorous acid is a stronger bacterial disinfectant than the hypochlorite ion among these species (Gadgil 1998; Crittenden et al. 2005). Although the liquid chlorine method provides a greater residual disinfection concentration than other chemical methods (e.g. ozone), it has a critical disadvantage in that liquid chlorine needs an appropriate supply chain and there are associated safety issues. It also shows a relatively low residual concentration compared to the other chlorine-based disinfection method, i.e. an electrochemical disinfection process (Gadgil 1998; Diao et al. 2004).
The electrochemical disinfection process is based on electrical decomposition of highly concentrated sodium chloride solution into a variety of oxidants in the electrochemical cell (Patermarakis & Fountoukidis 1990; Gadgil 1998). The major products, Cl2, HOCl, and OCl−, from this process are called free available chlorines (FACs). Recently, advancements have been made in the electrochemical disinfection method to generate mixed oxidants including ozone (Jeong et al. 2006; Jung et al. 2006). Researchers at Los Alamos Technical Associates, Inc. (Gram et al. 1988; Baker & Bradford 1994) have demonstrated a mixed oxidant electrochemical cell and have tested mixed oxidants (FAC and ozone species) production performance. They found that the mixed oxidant solution can result in strong inactivation of specific micro-organisms. The possible applications of electrochemical disinfection in water treatment include small water treatment utilities, remote communities, emergency and disaster areas (Venczel et al. 1997). The mixed oxidants can be generated on-site in the field and it requires little time and cost to maintain the system. Moreover, this kind of system merely needs water and salt or seawater to generate mixed oxidants without special chemicals. Jeong et al. (2006) studied the role of reactive oxygen species (e.g. O3, H2O2) compared with FACs in the electrochemical inactivation of micro-organisms, and their results showed that the reactive oxygen species have a higher disinfecting activity than FAC species. Although a number of studies have been conducted to investigate the exact mechanism of mixed oxidant production, there is still no clear mechanism due to the limitation of detection techniques for the mixed species (Gordon et al. 1998, 2002). The following mechanism is widely accepted for mixed oxidant production (Martínez-Huitle & Brillas 2008; Jeong et al. 2009).
In this study, the effects of electrode type and working conditions of the electrochemical cell on the generation of mixed oxidant disinfectants have been investigated. The different combinations of anode and cathode materials in the mixed oxidant electrochemical cell consisting of two electrodes have been examined. With regard to electrochemical cell working conditions, voltage-current characteristics, effect of the electrode open ratio, and the rate of salt solution inflows have been investigated in detail.
The key component of the mixed oxidant electrochemical cell is two electrodes. The anode part of the cell is called a dimensionally stable anode (DSA). The DSA (Samsung DSA Co., Korea) is made of titanium and various metal coatings on it (e.g. platinum, ruthenium oxide, iridium oxide) (Lacasa et al. 2013; Pan et al. 2014; Rajab et al. 2015). Figure 2(b) shows the cross section scanning electron microscope (SEM) image of the ruthenium oxide-coated DSA. In this study, two kinds of DSA and two kinds of cathodes are used. Table 1 lists the combination of these types along with the specification of the anode (iridium oxide and ruthenium oxide-coated anode) and the cathode (iridium oxide-coated cathode and SUS304 (stainless steel type 1.4301, C: Cr: Ni = 0.08%: 18.5%: 9%)).
|Type 1||Type 2||Type 3|
|Coating thickness||2 μm||2 μm||2 μm||–||2 μm||2 μm|
|Type 1||Type 2||Type 3|
|Coating thickness||2 μm||2 μm||2 μm||–||2 μm||2 μm|
Electrode dimension: 70 × 70 mm.
Distance between anode and cathode: 6 mm.
The concentration of FAC in the mixed oxidant solution was measured by the N,N-diethyl-p-phenylenediamine (DPD) colorimetric method. Upon mixing of DPD FAC powder with a sample solution, a red-violet color appeared. This mixture was treated with a colorimeter (Pocket Colorimeter II, HACH Co., Loveland, USA) to measure the FAC concentration. Because the measurement range of the colorimeter device is from 0.1 to 8 mg/L FAC (usual range of the DPD method), the original sample was diluted with deionized water with a 1:100 volume ratio (Vsample: VDI = 0.1 mL: 10 mL). The value of the diluted sample was analyzed by the colorimeter and the original FAC concentration was calculated considering the dilution factor. For measuring reactive oxygen species present in the mixed oxidant solution, the ozone species were detected by the 4-aminoantiphrine color comparison method using an enzyme (WAK-O3, Kyoritsu, Japan). The mixed oxidant sample solution was put in a test tube and allowed to react with the reagent for 1 minute. After the reaction, the solution color was compared with a standard color chart. All ozone measurement experiments were conducted three times at room temperature.
RESULTS AND DISCUSSION
Characteristics of iridium oxide-coated electrode cell
In characterizing the working conditions, iridium oxide-coated anodes and cathodes (type 1 in Table 1) were used for the mixed oxidant electrochemical cell. The working conditions such as the applied voltage, open ratio of the electrode (a/A), and inflow rate of the salt solution were considered as variable parameters. As important performance indicators, the total current generated through the electrochemical cell and the FAC concentration of the mixed oxidant solution were also measured.
Dependence on electrode materials
The experimental results show that the type 1 electrode system, which has iridium oxide as a coating material for both anode and cathode, generates a greater FAC concentration compared to the others. Two basic requirements of electrode materials for efficient mixed oxidant generation are high oxygen over-potential and higher electrocatalytic activity. Iridium has a higher electrocatalytic activity than ruthenium which resulted in higher FAC generation efficiency in the experiments (Kraft et al. 1999; Jeong et al. 2009).
Characteristics of ozone species generation
An inactivation study for a specific micro-organism such as Cryptosporidium is needed (Venczel et al. 1997). Recently, Venczel et al. (1997) reported that the mixture oxidants exhibit a disinfection efficacy on Cryptosporidium. They however did not explain the inactivation mechanism clearly. For the ozone species disinfection method, Finch et al. (1993) demonstrated that a relatively high concentration of ozone species can inactivate Cryptosporidium. The inactivation study may strongly confirm the existence of reactive oxygen species such as ozone.
This experimental study was conducted on the effect of electrode materials and some key working conditions on efficient generation of mixed oxidant disinfectants. Three types of electrode combinations were examined while varying working conditions such as applied voltages, salt solution inflow rates, and electrode open ratio. The experimental results showed that the type 1 (iridium oxide-coated anode and cathode) electrode system generates more FACs as compared to others under similar working conditions. This was ascribed to the higher electrocatalytic activity of iridium. The effect of the operating conditions on generation of mixed oxidant disinfectants are summarized as follows: when the applied voltage increases, the acceleration of oxidation reactions in the electrochemical cell can result in increased generation of FACs. The solution inflow rate relates to the residence time of the salt solution in the electrochemical cell, and a long residence time could result in high FAC generation. In addition the production of FACs is increased with decreasing electrode open ratio (a/A). When a/A > 0 and the inflow rate is low, the FACs concentration tends to approach a maximum value because of the generation of by-products. The ozone species generation in the electrochemical cell was determined by the maximum voltage that can produce ozone species. The electrode open ratio affects the ozone generation due to the mixing effect with cathode side products. These all important operating conditions can be utilized in designing an optimal mixed oxidant disinfectant generator. For future work, studies on sodium chloride solution concentration and inactivation of micro-organisms (e.g. Cryptosporidium) are underway.
This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. NRF-2014R1A2A2A01003618).