An electrochemical flow cell was introduced into the electrochemical Fenton-type process using a Cu(I)/HOCl system. The effects of the current density and the initial cupric ion (Cu2+) concentration on the process performance were discussed. The current efficiency of the process improved from 6.1% for an electrolytic tank system to 33% for the electrochemical flow cell system at a current density of 5.0 mA/cm2 and an initial Cu2+ concentration of 1.0 mM. The current efficiency increased to 58% for Cu2+ concentrations of 2.0 mM and beyond. The cathodic reduction of Cu2+ to the cuprous ion (Cu+) emerged as the rate-determining step in comparison to the anodic production of free chlorine. The introduction of the electrochemical flow cell enhanced the cathodic production of Cu+ by reinforcing the mass transfer of the Cu2+ to the cathode, and the detachment of micro bubbles generated electrochemically at the cathode surface. A decrease in the current density and an increase in the initial Cu2+ concentration also improved the current efficiency by promoting the cathodic production of Cu+. This involved the prevention of the cathodic reduction of protons to hydrogen gas and the elevation of the electrode potential of the cathodic reaction from Cu2+ to Cu+.
Advanced oxidation processes (AOPs) are chemical oxidation processes utilizing reactive radical species (RRSs) as oxidants for water purification. Existing AOPs include the Fenton-based, the ozone-based, and the ultraviolet radiation (UV)-based processes, where the hydroxyl radical (HȮ), the sulfate radical (SO4̇), the chlorine radical (Cl̇), the dichloride radical (Cl2̇−), and others ensure water purification (Stefan 2017). The Fenton-based processes are traditional AOPs and various derivatives have been proposed such as photochemical, electrochemical, and sonochemical Fenton processes (Brillas et al. 2009; Chakma & Moholkar 2013; Rahim Pouran et al. 2015). Among them, electrochemical Fenton processes are attractive, because they successfully eliminate the drawbacks of Fenton-based processes, namely the accumulation of iron sludge, the high chemical costs, and the risks associated with the transportation and storage of H2O2 (Oturan & Aaron 2014), through electrochemical generation of Fenton reagents (Tomat & Vecchi 1971; Qiang et al. 2003). However, the competitive electrochemical reduction of the ferric ion (Fe3+) and oxygen (O2) on the cathode remains a disadvantage.
Substitutes reported for Fenton's reagents include the trivalent cerium (Ce3+), the trivalent chromium (Cr3+), the tetravalent chromium (Cr4+), the monovalent copper (Cu+) for Fe2+, and hypochlorous acid (HOCl) for H2O2 (Candeias et al. 1994; Bokare & Choi 2014). The use of substitute reagents coupled with an electrolytic technique offers an attractive option for Fenton-based AOPs. In fact, an electrochemical Fenton-type process using an Fe2+/HOCl system was demonstrated with the recycling of iron sludge and the chloride ion (Cl−) in wastewater (Kishimoto & Sugimura 2010; Kishimoto et al. 2013).
MATERIALS AND METHODS
Cupric sulfate pentahydrate (CuSO4·5H2O) and sodium chloride (NaCl) served as sources for Cu2+ and Cl−. The current efficiency of anodic production of free chlorine is known to depend on the Cl− concentration and reaches around 80% over 50 mM (Kishimoto et al. 2018). Therefore, the initial Cl− concentration was set to 70 mM for safety. 1,4-Dioxane (stabilizer free, FUJIFILM Wako Pure Chemical, Osaka) was used as a chemical probe for RRSs. The 1,4-dioxane dose was set to 20 mM for almost complete trapping of RRSs (Kishimoto et al. 2017). Sulfuric acid and sodium hydroxide facilitated the adjustment of the pH. All chemicals except 1,4-dioxane were of analytical grade and purchased from Nacalai Tesque (Kyoto).
The electrochemical flow cell system comprised a single-chamber electrochemical flow cell, a 1000-mL glass beaker serving as a reservoir, a magnetic stirrer (CT-1AT, As One, Tokyo) for mixing, a peristaltic pump (RP-1000, EYELA, Tokyo) for circulation, a DC power supply (AD-8735D, AND, Tokyo), and a pH meter (FD-02, TGK, Tokyo) (Figure 1). The electrochemical flow cell contained a plate DSE® anode (JP-330, De Nora Permelec, Fujisawa) and a titanium plate cathode, with an inter-electrode gap of 0.20 cm. Both electrodes occupied a width of 1.0 cm and a length of 21 cm, yielding an effective area of 21.0 cm2. The electrode materials were the same as in Ito et al. (2018), but the previous study involved an electrolytic tank instead of an electrochemical flow cell.
The synthetic solution used contained 20 mM of 1,4-dioxane, 70 mM of NaCl, and 1.0–5.0 mM of CuSO4·5H2O. The pH of the solution was set to 3.0 for all experimental runs. A 1000-mL volume of the solution was poured into the reservoir, followed by the start of the magnetic stirrer and the peristaltic pump. The circulation rate of the solution between the reservoir and the electrochemical flow cell was set to 0.36 L/min (linear velocity of 30 cm/s) because the linear velocity over 20 cm/s is preferable to the lower one for decreasing electrolytic voltage (Nishio et al. 1986). Following the equalization of the solution, switching the DC power supply on started the run. The DC power supply, operated in a galvanostatic mode, provided electrolytic currents ranging from 0.105 to 0.315 A (current density of 5.0–15.0 mA/cm2). Sampling for the chemical analysis of the solution in the reservoir employed a 20 mL sample and occurred every 10 min.
The characteristics of the anodic production of free chlorine were explored using the electrochemical flow cell in a throughflow mode without circulation. The synthetic solution without 1,4-dioxane and CuSO4·5H2O was continuously injected into the electrochemical flow cell at a flow rate of 0.36 L/min. The DC power supply was then switched on and operated in a galvanostatic mode. After keeping the electrolytic current constant for more than 30 s, sampling of the electrochemical flow cell's effluent occurred, accompanied by the measurement of the free chlorine concentration. The electrolytic current changed in stages from 0.053 to 0.315 A.
The pH meter ensured the monitoring of the pH of the solution. The free chlorine concentration was measured with a residual chlorine analyzer (HI95711, Hanna Instruments Chiba, Japan) shortly after sampling. The sample for free chorine determination was diluted with ultrapure water when necessary. For chemical analysis of 1,4-dioxane, chlorate, and perchlorate, the free chorine was quenched using an adequate amount of sodium sulfite powder (Na2SO3) followed by centrifugation for 10 min at 1000 rpm. High-performance liquid chromatography allowed for the determination of the concentration of 1,4-dioxane according to Ito et al. (2018). An ion chromatography system (ICS-1100, Thermo Fisher Scientific, Tokyo) permitted the determination of the concentrations of chlorate and perchlorate. The analysis utilized a Dionex IonPac AS22 (4 × 250 mm) column, a Dionex ACRS 500 suppressor, an aqueous solution with 4.5 mM Na2CO3 and 1.4 mM NaHCO3 as the mobile phase, a flow rate of 1.20 mL/min, an injection volume of 50 µL, and an oven temperature of 30 °C.
RESULTS AND DISCUSSION
1,4-Dioxane degradation kinetics
Figure 2 shows changes in the cumulative amounts of 1,4-dioxane degraded during the electrochemical Fenton-type process at a current density of 5.0 mA/cm2. Since the 1,4-dioxane degradation approximately followed zero-order kinetics, Equation (8) was suitable for the estimation of η.
Effects of current density
Figure 3 represents a summary of the dependence of η on the current density. The highest η of 58% was obtained at a current density of 5.0 mA/cm2 and initial Cu2+ concentrations of 2.0 and 5.0 mM. The η drastically decreased with an increase in the current density regardless of the initial Cu2+ concentration. The standard electrode potential (E°) for the anodic oxidation of Cl− (reaction (2)) is 1.396 V with a normal hydrogen electrode (NHE), which is higher than a competitive anodic evolution of water to oxygen (2H2O = O2 + 4H+ + 4e−, E° = 1.2291 V vs. NHE; Bratsch 1989). A higher current density favors an anodic reaction with a higher electrode potential because a higher current density causes a higher anodic overvoltage. In fact, Figure 4 reveals that a higher current density enhanced the current efficiency for free chlorine production. Consequently, the inhibition of the anodic production of free chlorine does not account for the decrease in η at a higher current density. Contrary to the anodic reaction, a higher current density favors a cathodic reaction with a lower electrode potential. At the cathode, the cathodic reduction of Cu2+ (reaction (4)) competes with a cathodic production of hydrogen (2H+ + 2e− = H2). The E° of the former is 0.161 V vs. NHE, which is higher than that of the latter (E° = 0 V vs. NHE) (Bratsch 1989). Accordingly, the cathodic production of Cu+ experienced stronger inhibition at a higher current density, explaining the decrease in η shown in Figure 3.
Assuming the establishment of equilibrium in reactions (5)–(7), the estimates of RRSs at a pH of 3.0 and a Cl− concentration of 70 mM are 0.00052% for ̇OH, 99.99% for Cl2̇−, and 0.0075% for Cl̇. Thus, the contribution of radical scavenging by HOCl to the vain consumption was inferred to be limited.
The vain consumption yielded a low value at a current density of 5.0 mA/cm2. Therefore, the selectivity of reaction (1) against all reactions for free chlorine consumption exceeded 90%. This indicates the consumption of free chlorine in reaction (1) at the current density of 5.0 mA/cm2, probably due to a better balance between the production rate of free chlorine and that of Cu+.
Effect of the initial Cu2+ concentration
Comparison with the electrolytic tank system
The electrochemical flow cell system enhanced the η to 33% compared to 6.1% for the electrolytic tank system (Figure 3). The linear velocity of the synthetic solution on the electrode surface also differs between the electrochemical flow cell and the electrolytic tank. The electrochemical flow cell generated a steady water flow with a linear velocity of 30 cm/s in this study. Although it is difficult to attain steady water flow on the electrode surfaces in an electrolytic tank system, Ito et al. (2018) estimated a linear velocity of 19 cm/s for the tank. According to Kishimoto et al. (2005), the δ near a plate electrode is inversely proportional to the square root of the linear velocity. Therefore, the higher linear velocity in the electrochemical flow cell contributed to elevating the η through enhanced mass transport from the bulk solution to the electrode surface according to Equation (12). In addition, the higher linear velocity reinforces the shearing stress, promoting the detachment of the electrochemically generated micro bubbles on the electrode surface. The covering of the cathode surface by micro bubbles decreases the effective cathode area and thereby increases the effective current density of the cathode. Since an increase in the current density negatively impacts the η, the introduction of the electrochemical flow cell contributed to the improvement of the η through promoting the detachment of the micro bubbles on the cathode surface too.
An electrochemical flow cell was introduced into the electrochemical Fenton-type process using a Cu+/HOCl system. The effects of the current density and the initial Cu2+ concentration on the process performance were clarified using a radical probe of 1,4-dioxane.
The current density negatively affected the current efficiency of the process by inhibiting the cathodic production of Cu+ from Cu2+. The current efficiency of the process decreased from 58% at a current density of 5.0 mA/cm2 and an initial Cu2+ concentration of 2.0 or 5.0 mM to below 22% at 15 mA/cm2.
The current efficiencies at an initial Cu2+ concentration of 5.0 mM exceeded those at 1.0 mM by factors of 1.8–3.0. The increase in the Cu2+ concentration promoted the cathodic reduction of Cu2+ to Cu+, through the rise in the corresponding electrode potential given by the Nernst equation.
At a current density of 5.0 mA/cm2 and an initial Cu2+ concentration of 1.0 mM, the electrochemical flow cell system enhanced the current efficiency to 33% compared to 6.1% for the electrolytic tank system. The enhancement effect was attributed to a steady water flow on the surfaces of the electrodes, promoting mass transfer from the bulk solution to the electrodes and the detachment of electrochemically generated micro bubbles on the electrode surfaces.
The current efficiency depended on the balance of free chlorine and Cu+ production rates. The latter tended to be a rate-limiting factor in the electrochemical Fenton-type process using a Cu+/HOCl system. The introduction of an electrochemical flow cell, a decrease in current density, and an increase in Cu2+ concentration contributed to promoting the cathodic Cu+ production and thereby improving the current efficiency.
This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP17K00603. We would like to thank Enago (www.enago.jp) for English language editing.