In-Depth Study on the Effects of Impurity Ions in Saline Wastewater Electrolysis

Concentration followed by electrolysis is one of the most promising ways for saline wastewater treatment, since it could produce H₂, Cl_2, and an alkaline solution with deacidification potential. However, due to the diversity and difference of wastewater, knowledge on the suitable salt concentration for wastewater electrolysis and the effects of mixed ions are still lacking. In this work, electrolysis experiments of mixed saline water were conducted. The salt concentration for stable dechlorination was explored, with in-depth discussions on the effects of typical ions such as K⁺, Ca²⁺, Mg²⁺, and SO₄^2-. Results showed that K⁺ had a positive effect on the H₂/Cl₂ production of saline wastewater through accelerating the mass transfer efficiency in the electrolyte. However, the existence of Ca²⁺ and Mg²⁺ had negative effects on the electrolysis performance by forming precipitates, which would adhere to the membrane, reduce the membrane permeability, occupy the active sites on the cathode surface, and also increase the transport resistance of the electrons in the electrolyte. Compared to Mg²⁺, the damaging effect of Ca²⁺ on the membrane was even worse. Additionally, the existence of SO₄^2- reduced the current density of the salt solution by affecting the anodic reaction while having less of an effect on the membrane. Overall, Ca²⁺ ≤ 0.01 mol/L, Mg²⁺ ≤ 0.1 mol/L and SO₄^2- ≤ 0.01 mol/L were allowable to ensure the continuous and stable dechlorination electrolysis of saline wastewater.