pH and electrolyte concentration
pH of a solution determines the dominance of H + and OH- ions in a sample. Often, the presence of H + and OH- ions in a solution determines the mechanism and oxidation efficiency of an electrochemical treatment system [20, 21]. In this context, COD analysis was conducted to find out the effect of variable pH in the degradation of ESO. Figure 2 (a) shows the COD removal efficiency achieved by electrochemical oxidation at different pH with 115 mA.cm− 2 current density, 60 mM sodium sulfate electrolyte concentration and 300 min treatment time. pH 3 was observed to be the optimal condition for maximum COD removal from ESO. The presence of sodium sulfate along with acidic condition might have created sulfate radicals that degraded ESO [20, 21]. The mechanism involved is given from Eq. 1 to Eq. 4
Na2SO4− → Na+ + 2 SO4− …Eq. 1
SO4− + C/Ti(e-) → SO4∙− … Eq. 2
SO4∙− + Organics → SO4− + Oxidation byproducts …Eq. 3
SO4∙− + Oxidation byproducts → SO4− + CO2 + H2O + NO2 …Eq. 4
Electrolytes such as sodium sulfate are imperative for inducing conductivity and producing oxidation radicals during electrochemical oxidation for the degradation of pollutants. The reason behind choosing sodium sulfate instead of sodium phosphate or sodium nitrate is the fact that the latter two are inert electrolytes that induces conductivity without producing oxidation radicals. Although sodium chloride is an active electrolyte capable of producing OCl−, HClO and Cl2 for oxidation, it is known to produce chlorinated byproducts that often end up more toxic than parent compounds [22, 23, 24, 25, 26, 18]. The concentration of sodium sulfate was varied during the study to determine the effect of the electrolyte quantity for electrochemical oxidation. Figure 2 (b) shows the COD removal achieved at various concentrations of sodium sulfate with 115 mA.cm− 2 current density, pH 3 and 300 min treatment time. It was observed that an increase in sodium sulfate increased the COD removal efficiency corroborating with the idea that sulfate radicals were also behind the oxidation of ESO along with direct electron transfer. However, further increase from 60 mM, to 80 mM sodium sulfate concentration didn’t exert much influence on the oxidation process. Hence, 60 mM was chosen as the optimal electrolyte concentration for the study.
The process of electrochemical oxidation of a pollutant is initiated by the passage of current from the AC/DC converter to the electrodes. The ions and the pollutants present in a sample come into contact with the electrons passing through the anode. The result is the oxidation of the pollutant either directly by the electron transfer from the anode or indirectly by the secondary oxidants produced by the interaction of the anode with the ions present in the solution. Whatever the case, all these processes take time to take place [27, 22, 23, 24]. Hence, treatment time of a process is often a crucial variable for the degradation of a pollutant. Present study used COD removal efficiency to determine the effect of treatment time on oxidation of ESO. Figure 2 (c) shows the effect of variable treatment time on the COD removal efficiency from 10 mg L− 1 ESO with pH 3, 60 mM electrolyte concentration and 115 mA.cm− 2 current density. The COD removal efficiency increased with treatment time but after reaching 93% at 300 min, further increase was not considerable. Hence, 300 min was chosen as the optimal treatment time.
In electrochemical oxidation, passing of electricity through the electrodes initiates the oxidation process. The current applied per unit area of the electrode (mA.cm− 2) often determines the efficiency and cost of EO [27, 22, 23, 24]. Therefore, it is important to determine the optimal current density involved in degrading a pollutant to understand whether a process is effective in terms of degradation and cost. COD removal and TOC removal at various current densities were utilized to determine the efficiency of EO at C/Ti anode for degrading 10 mg L− 1 ESO using pH 3, 60 mM electrolyte concentration and 300 min treatment time. 115 mA.cm− 2 was the maximum current density that could be applied with the available resources for the study. Although, the maximum current density applied for the study fetched 93% COD removal, TOC removal was only 44%. Optimal mineralization of ESO was rather difficult to achieve with EO at C/Ti anode and high current densities were required even for 44% TOC removal which will raise the energy consumption of the process. The anode material was robust and stable showing negligible decrease in its oxidation efficiency even after 20 times reuse. However, reusability of anode material alone is not enough to bring down the cost of the process if current application is high [27, 22, 23, 24].