## 3 Results and Discussion
### Experimental Performance of EC
#### 3.2.2 Response Surface Plots  
It is well documented in the literature that initial pH is an essential operating parameter that strongly affects the EC process performance. The pH effect on the removal efficiencies of metal ions after EC treatment was validated in the experimental observations. Maximum removal efficiency for Zn (99.46%), Ni (98.14%), and Cu (99.96%) has been observed at pH 6. Figure 4a-c demonstrates an increase in the removal efficiency with an increase in pH. This indicates that metal ion elimination decreases in an acidic medium [50]. As reported, in an intensely acidic medium, the protons in the solution get reduced to H\({}_{2}\) gas at the cathode, and a sufficient number of hydroxyl ions are not generated. The pH of the initial solution affects the EC process by changing the solution's physico-chemical properties, such as solubility of metal hydroxides, electric conductivity, and size of colloidal particles of iron (III) complexes, which are most reactive agents for metal ions [51]. A slight reduction in the removal efficiency with the rise in the initial concentration of the metals in solution, as shown in Figure 4, is attributed to the fact that the amount of dissolved iron from the electrode may not have been enough to treat the metal ions present in the wastewater. The higher initial concentration in the feed solution was also reportedly found to significantly affect the EC process [52].  
It was observed (Figure 4d-f) that increasing the constant current substantially reduces metal ions. The constant current emerged as a crucial parameter in improving metal ion removal, which may have contributed to the direct current field, and potential electrolysis, resulting in more release of ferric ions and generating more iron hydroxides, further forming coagulants for metal removal.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Source** & **Sum of Squares** & **Degree of Freedom** & **Mean Square** & \(F\)**-Value** & \(p\)**-Value** & \(R^{2}\) & \(R^{2}_{\text{adj}}\) & **Resposes** \\ \hline Model & 10,750.36 & 14 & 767.88 & 6.03 & 0.0007 & 0.8492 & 0.7085 & Zn Removal\% \\ \hline Model & 13,024.85 & 14 & 930.35 & 9.39 & \textless{}0.0001 & 0.8976 & 0.8020 & Ni Removal\% \\ \hline Model & 940.48 & 14 & 67.18 & 2.78 & 0.0293 & 0.7220 & 0.4625 & Cu Removal\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: ANOVA analysis for the EC process.  
Electrolysis time period plays a vital role in metal ion removal studies along with the constant current, pH and concentration. The concentration of Zn, Ni, and Cu has been observed to decrease with an increase in the electrolysis time. The complete reduction of metal ions was possible at a lower constant current by extending the electrolysis time. It may be stated that higher metal ion concentration consumes the adsorption ability of flocs formed, with fewer flocs being accessible for adsorption. Moreover, removal was limited by the formation rate of flocs of iron hydroxide complexes at the anode surface. It has been shown (Figure 4) that the minimum reduction was observed at lower electrolysis reaction times. The present study highlights that both the current and reaction time play a vital role in the removal efficiency of the EC process. Table 6 reports the results of this study vis-a-vis other studies reported in the literature on metal removal through EC processes.  
Figure 4: Electrocoagulation RSM graphs: effects of concentration and pH on removal% efficiency of metal ions: (**a**) Zn ions, (**b**) Ni ions and (**c**) Cu ions. Effect of current and reaction time on removal% efficiency of metal ions: (**d**) Zn ions, (**e**) Ni ions and (**f**) Cu ions.