# Simultaneous Electrochemical Removal of Selenium and Strontium from Aqueous Solution
## 4 Discussion  
Experimental run 48 (25 A, 15 min, four Fe electrodes, middle pH range) exhibited the best performance for the simultaneous removal of Sr and Se. The changes in Se and Sr concentrations during the process, as well as the relative and cumulative efficiency of each stage, are graphically presented in Figure 11. For each stage, the relative efficiency was calculated using the final concentration from the previous stage as the initial concentration. On the other hand, the cumulative efficiency considered only the initial concentration at the start of the experiment. The Se and Sr removal proceeded through two streams. After the pH stage, two streams were created: alkaline and acidic. The alkaline stream went to the alkaline EC reactor and then to the corresponding settler. After the sludge settled, the solution above the sludge was directed to the acidic part of the pH reactor. Following the pH treatment in phase 2, it became acidic and moved to the acidic EC reactor, and finally to another settler. The second stream followed the opposite pathway. The average efficiencies took both streams into account. In the first stage, the treatment in the pH control reactor during the first phase slightly reduced Se and Sr concentrations, with average relative and cumulative efficiencies of 23.90% and 36.12%, respectively. The EC stage in the first phase resulted in the highest removal rates--95.96% of Se and 92.85% of Sr (average relative efficiency). After the sludge settling, on average, 96.93% of Se and 95.43% of Sr were cumulatively removed at the end of the first phase. The second phase resulted in averages of 98.72% and 54.55% relative removal efficiencies for Se and Sr, respectively. Considering the total average cumulative efficiency of 99.96% for Se and 97.92% for Sr, it can be concluded that most of the contaminants were removed during the first phase of the treatment.  
Figure 11: Changes in the Se and Sr concentrations in each stage of the process (bars), relative efficiency trends (lines), and cumulative efficiency trends (dashed lines) for run 48 (25 A, 4 Fe electrodes, 15 min, middle pH range).  
In the pH stage of the second phase, an increase in the Sr concentration was observed, which resulted in negative efficiencies that are represented as 0 in Figure 11. This issue likely arose from incomplete sludge removal. If a small amount of sludge remained in the solution before it progressed to the second phase of treatment, Sr could leach back into the solution when the pH value changed. Additionally, it is possible that some Sr deposited on the electrodes during the first phase could be released back into the solution due to the pH change. In situations where the complete or near-complete removal of Se and Sr is not necessary, the first phase of the process alone might suffice. However, the second phase is crucial to ensure that the quality of the treated water meets stringent standards, such as those required for drinking water.  
When the pH control stage was skipped, the process was much simpler; however, the efficiencies, especially for Sr removal, were also lower. For comparison, the Se and Sr concentrations and cumulative and relative efficiencies for the best-performing experimental run without a pH adjustment (15 A, four Al electrodes, 25 min) for the simultaneous removal of Se and Sr are shown in Figure 12. In terms of the relative efficiencies, 91.21% of Se and 79.49% of Sr were removed after the first EC treatment, and 90.12% of Se and 75.12% of Sr in the second EC treatment. Cumulatively, removal efficiencies of 99.13% for Se and 94.90% for Sr were achieved.