# Simultaneous Electrochemical Removal of Selenium and Strontium from Aqueous Solution
## 4 Discussion
### EC Stage  
EC relies on the dissolution of electrodes to release metal ions that form hydroxides or oxyhydroxides, which adsorb onto and co-precipitate the contaminants. The metal cations Al\({}^{3+}\) or Fe\({}^{2+/3+}\) released from the anode hydrolyze to form metal hydroxides Al(OH)\({}_{3}\) and Fe(OH)\({}_{3}\). Se is adsorbed onto the surfaces of metal hydroxide particles and/or precipitated as metal selenates. Sr can co-precipitate with carbonate or hydroxide ions under alkaline conditions or adsorb onto the hydroxides. The colloidal particles formed aggregate into flocs that settle, removing contaminants from the water. At a neutral pH, Al(OH)\({}_{3}\) and Fe(OH)\({}_{3}\) flocs are optimally formed, providing high adsorption capacity. Se species are efficiently removed by adsorption and co-precipitation. Sr removal is moderate due to lower precipitation of carbonates or hydroxides. At very low pH values, Al\({}^{3+}\) and Fe\({}^{3+}\) ions remain highly soluble, reducing the formation of flocs. Sr removal is poor, as the precipitation of Sr compounds is minimal in acidic conditions. In alkaline conditions, Al(OH)\({}_{3}\) and Fe(OH)\({}_{3}\) flocs may dissolve to form soluble Al(OH)\({}_{4}\)\({}^{-}\) and Fe(OH)\({}_{4}\)\({}^{-}\), reducing their adsorption capacity. However, Sr removal improves because of increased precipitation with carbonate or hydroxide. On the other hand, Se removal is lower due to competition between hydroxide ions and selenate for adsorption sites. It can be concluded that optimal pH for removal of Se is neutral-to-slightly acidic pH (-5-7) where hydroxide flocs are stable, and Se species are effectively adsorbed. Slightly alkaline pH (-8-9) is better for Srremoval, favoring precipitation of its compounds. The best-performing experiment (run 48) confirmed this, as the middle range of pH value was used. pH governs the solubility and formation of hydroxide flocs, directly influencing the adsorption and co-precipitation efficiency of Se and Sr during electrocoagulation. Fine-tuning the pH ensures optimal contaminant removal, balancing the behavior of aluminum or iron species in the system. The applied current directly affects the rate of electrode reactions, which is the core driver of electrocoagulation. According to [PERSON]'s law, the mass of ions released from the electrode (Al3+ or Fe3+) is proportional to the applied current. A higher current means more ions are generated per unit time, increasing the rate of hydroxide precipitation, as well as the adsorption and co-precipitation of contaminants, like Se and Sr. In addition, an increased current induces more vigorous gas bubbling (H2 evolution at the cathode and O2 at the anode), enhancing the mixing and dispersion of ions present in the solution. Basically, the current controls the rate of reactions and determines the availability of metal ions and hydroxide for contaminant removal. An insufficient current can lead to smaller ion production, while an excessive current might cause energy inefficiency and overheating without proportional gains in the removal efficiency. Adding more electrodes does not inherently improve the process unless other parameters, such as the current distribution and spacing, are also optimized. Increasing the number of electrodes increases the total surface area available for reactions. However, if the applied current is fixed, the current density (current per unit surface area) decreases, which can slow the reaction rates at each electrode. The electrical field created by the electrodes drives ionic movement and reactions. Simply adding more electrodes without changing the current or configuration may not significantly enhance the process if the spacing and arrangement do not optimize the current pathway. Once sufficient Al3+ or Fe3+ ions are generated to remove the available Se and Sr ions, adding more electrodes does not increase the removal efficiency.