# Simultaneous Electrochemical Removal of Selenium and Strontium from Aqueous Solution
## 4 Discussion
### pH Control Stage  
In the electrochemical pH reactor, water oxidation occurs at the anode (connected to the positive terminal of the power supply), producing protons H\({}^{+}\) (H\({}_{3}\)O\({}^{+}\)) and oxygen gas O\({}_{2}\). The generated H\({}^{+}\) ions increase the acidity of the anode side (reactor compartment where the anodes are). At the cathode (electrode connected to the negative terminal of the power supply), water reduction occurs, consuming protons (H\({}^{+}\)) and producing hydroxide ions (OH\({}^{-}\)) and hydrogen gas (H\({}_{2}\)). The produced OH\({}^{-}\) ions make the cathode side alkaline, causing the pH to rise. The proton exchange membrane (PEM) allows only protons to pass through from the acidic anode side to the alkaline cathode side, preventing other ions like OH\({}^{-}\) or larger cations from crossing. As protons leave the anode side, the acidity increases. Simultaneously, protons arrive at the cathode side, neutralizing some OH\({}^{-}\), but the continual production of OH\({}^{-}\) keeps the cathode side alkaline.  
During the pH control stage of the treatment, samples of the treated solution were taken and compared with the initial values of Se and Sr concentrations. A slight reduction in  
Figure 12: Change in the Se and Sr concentrations in each phase of the process (bars), relative efficiency trends (lines), and cumulative efficiency trends (dashed lines) for run 51 (15 A, 4 Al electrodes, 25 min, no pH adjustment).  
Se concentration on the cathode side and an increase in the Sr concentration were observed. The observed distribution of Sr and Se arose from the pH gradient created by the selective transport of protons through the PEM and the electrochemical reactions at the electrodes; the solubility changed due to the pH value and selective ion transport. These factors combined to influence how the different ions segregated under the influence of an applied electric field. The PEM allowed only cations to cross, while the other ions were confined to their compartments, which influenced the local solubility equilibria.  
Depending on its chemical state in water, Se is an anion. Anions are repelled from the cathode due to the negative charge and tend to accumulate at or move toward the anode. However, since the PEM may block their passage, their concentration might decrease on the alkaline (cathode) side primarily because they are not being replenished from the anode side. The high concentration of H\({}^{+}\) on the anode side stabilizes H\({}_{2}\)SeO\({}_{3}\) and HSeO\({}_{3}\)\({}^{-}\), reducing the mobility of Se species. On the cathode side, SeO\({}_{3}\)\({}^{2-}\) ions are more mobile in the alkaline environment and may even precipitate with Sr\({}^{2+}\) to form strontium selenite (SrSeO\({}_{3}\)), which is sparingly soluble in water. This precipitation, if it does occur, reduces the overall Se concentration in the cathode-side solution.  
As a cation, Sr is attracted to the cathode. The accumulation of Sr on the cathode side could be due to both the migration of Sr ions toward the negatively charged cathode and the possible passage through the PEM if it is not completely selective against divalent cations. The higher pH on the cathode side can also increase the Sr solubility or differentially affect its speciation. If any carbonate ions are present, they may also react with Sr\({}^{2+}\), forming SrCO\({}_{3}\) precipitates in some areas.  
Various other ions, like Na\({}^{+}\), Cl\({}^{-}\), Ca\({}^{2+}\), and CO\({}_{3}\)\({}^{2-}\), likely present in the solution, can also participate in similar migration and electrochemical reactions. These ions can compete with or facilitate the movement of Se and Sr ions, affecting their overall distribution in the cell.  
In summary, the observed reduction in Se concentration on the cathode side and the increase in Sr concentration can be attributed to the electrochemical reactions at the electrodes, selective ion migration through the PEM, and the effects of the changing pH across the cell.