# Simultaneous Electrochemical Removal of Selenium and Strontium from Aqueous Solution
## 2 Materials and Methods  
### Experimental Setup  
Experiments were conducted in a three-stage, two-phase recirculating batch reactor shown in Figure 1. It consisted of a pH value control reactor, 2 electrocoagulation (EC) reactors, and 2 settling tanks. The electrochemical pH control reactor, constructed from Plexiglass, was divided into 3 sections by 2 cation-exchanging membranes (Nafion 117, DuPont, Wilmington, DE, USA), 10 cm \(\times\) 20 cm, placed in Plexiglass frames, as pictured in Figure 2. Graphite plate electrodes (U20) with a total active surface area of 168 were positioned on either side of the membranes and connected to the laboratory power supply (CSP-3000-120 from Mean Well, Taiwan, China) in an anode/cathode cathode/anode configuration. Two rectangular EC reactors, 4 L each, were also made of Plexiglass (Figure 3). Each reactor was equipped with an overhead mixer set to 150 rpm and electrode stands that accommodated up to 8 electrodes with a 1 cm inter-electrode distance. The electrodes were connected to another 3000 W laboratory power supply (CSP-3000-120 from Mean Well, Taiwan, China) in monopolar parallel mode. High-purity perforated aluminum (1050-H24)or iron (ARMCO G2) plate electrodes, each with an active surface area of 86.73 cm\({}^{2}\), were used during the EC stage of the process. A two-channel function generator (JT-IDS6600 from Joy It, Neukirchen-Vluyn, Germany) was used to control both power supplies. System cooling was achieved using thermoelectric cooling blocks (Peltier modules) paired with passive aluminum heatsinks on all reactor sides. Additionally, 3 heat pipes were inserted directly into each reactor, including the pH control reactor. For better heat dissipation, a fan (12 V, 1.1 W) was mounted on each heatsink, heat pipe, and thermoelectric cooling block with an additional mounted passive heatsink. Figure 1: A photo and a schematic overview of the electrochemical water treatment system. Figure 2: pH value control reactor. Two cone-shaped settlers, each with a 4 L volume, were used for sedimentation after the EC treatment. Each settler featured two valves. The side valves were connected to the two plastic pumps (Jebao DCP-4000, Jebao, Zhongshan, China) and the bottom valves were used to discard the sludge. The process efficiency was analyzed using synthetic wastewater (model solution) containing 10 mg/L of Se and Sr. For each experimental run, 7 L of this model solution was used (3.5 L for each EC reactor). Each experimental run utilized 7 L of the model solution (3.5 L per EC reactor). Initially, the solution entered the pH control reactor, where the pH adjustment occurred. Acidic conditions were established in the outer sections, while the middle section was alkaline. The solution from the acidic sections was directed to one EC reactor, and the solution from the alkaline section to the other. After the electrochemical treatment, the treated solution flowed into the settlers for sedimentation. The sludge was separated, and the supernatant was recycled back into the pH control reactor--but into the opposite section--to alternate the solution's exposure to acidic and alkaline conditions in the second phase. This alternating approach enhanced the Se and Sr removal. The model solution had an average initial pH of 7.04, 316 mg/L of total dissolved solids, 1.9 FNU turbidity, 651 \(\upmu\)S/cm conductivity, and a temperature of 21.9 \({}^{\circ}\)C. The physicochemical properties, including pH, were measured using an HI9829 multimeter (Hanna Instruments, Smithfield, VA, USA). A total of 102 experiments were conducted based on a statistically designed experimental matrix. The variables included the electrode material (Al or Fe), number of electrodes (4, 6, or 8 per EC reactor), treatment time (5, 15, or 25 min), and applied current (5, 15, or 25 A). Comparisons were made between experiments with and without the electrochemical pH adjustment.