# Simultaneous Electrochemical Removal of Selenium and Strontium from Aqueous Solution
## 3 Results
### Operational Costs Estimation  
The cost estimates, including the electricity and material used, for the top-performing experiment (25 A, four Fe electrodes, 15 min, and middle pH range) could be calculated using Equation (2) [27; 38; 39]:  
\[Operational\ costs=a\cdot C_{el}+b\cdot C_{EC}+c\cdot C_{p}+d\cdot C_{m} \tag{2}\]  
where \(C_{el}\) represents the electricity used per cubic meter of treated water (kWh m\({}^{-3}\)), encompassing the energy use by the electrodes in the pH control reactor, both EC reactors, and the pumps. \(C_{EC}\) (kg m\({}^{-3}\)) denotes the amount of electrode material per cubic meter of treated water, and \(C_{p}\) (kg m\({}^{-3}\)) indicates the pump material wear per cubic meter in a service cycle (1 year). \(C_{m}\) refers to the membrane area used to process (change the pH value) the total volume of model solution over its lifespan (0.04 m\({}^{2}\) for 9.5 m\({}^{3}\)). The coefficients \(a\), \(b\), \(c\), and \(d\) represent the costs associated with the electricity (0.16 EUR kWh\({}^{-1}\)), the price of Fe electrodes (36 EUR kg\({}^{-1}\)), maintenance materials for the pumps (estimated as 70 EUR kg\({}^{-1}\) per service cycle), and membrane costs per m\({}^{2}\) used in this research (177 EUR for 2 membranes 10 cm \(\times\) 20 cm), respectively. The lifespan of a Nafion 117 membrane depends on various factors, such as the operating conditions, chemical environment, mechanical stress, and maintenance practices. Under optimal conditions and with proper handling, a Nafion 117 membrane can last several years. However, in typical applications, they can last from 1000 to 10,000 h of operation, depending on the specific conditions. In 68 experiments in which the pH value was changed, a total of 952 L of model solution was processed. Ultimately, no notable effectiveness in the pH rate of change was observed, indicating the absence of significant membrane fouling. Therefore, it was assumed that the membranes used in this research can process at least a total of 10 times the amount of processed solution, that is, about 9.5 m\({}^{3}\). The power consumption of graphite electrodes and Fe electrodes for both phases, as well as the pumps, were measured with an EMOS P5822 power meter as 0.079, 0.426, and 0.003 kWh, per phase, respectively, resulting in a total electrical energy consumption of 145.14 kWh m\({}^{-3}\). The electrode material release (kg m\({}^{-3}\)) was calculated according to [PERSON]'s law [40; 41], expressed in Equation (3):  
\[C_{EC}=\frac{(I\cdot t\cdot M_{up})}{(z\cdot F\cdot V)}, \tag{3}\]  
\(M_{up}\) represents the molar mass (55.9 g mol\({}^{-1}\) for Fe), \(z\) is the number of electrons transferred (2 or 3), \(F\) is the Faraday constant (96 487 C mol\({}^{-1}\)), \(t\) is the treatment time (s), and \(V\) is the volume of treated water (m\({}^{3}\)). Thus, the released Fe electrode material was 1.34 kg m\({}^{-3}\). The released electrode material from the graphite electrodes was not included in the calculations because for carbon electrodes used in electrochemical processes, such as electrocoagulation or pH adjustment through water electrolysis, there typically is not any release of electrode material into the solution. Graphite electrodes are generally used because they are inert, meaning they do not easily participate in chemical reactions and do not dissolve into the solution under typical conditions. While the carbon material itself does not dissolve or release, surface reactions can occur where reactants or products may adhere to or interact with the surface of the carbon. This interaction is more about surface chemistry and does not involve the breakdown or release of the carbon material itself. The average operating costs for the top-performing experiment were calculated as 90.23 EUR m\({}^{-3}\). In addition, the use of photovoltaic systems can reduce costs, making theoperating expenses considerably lower.