A two-step electrochemical method for separating Mg(OH)2 and CaCO3: Application to RO reject and polluted groundwater

Abstract

The process of removing Ca²⁺ and Mg²⁺ ions typically results in the co-precipitation of Ca²⁺ and Mg²⁺ along with other salt waste. To improve water treatment efficiency towards a zero-waste goal, it is crucial to separate Ca²⁺ and Mg²⁺, and recover them in their purified form. This study proposes a two-step electrochemical approach that separately recovers Ca²⁺ as CaCO₃ and Mg²⁺ as Mg(OH)₂. The first step uses an undivided cell with 3D electrodes and controlled flow directions to selectively precipitate CaCO₃ on the electrode, keeping the cell removal efficiency. The second step employs a two-compartment cell with a cationic exchange membrane to recover Mg(OH)₂. This approach was evaluated on RO reject water with high Ca²⁺ to Mg²⁺ ratio and industrial effluent-polluted groundwater with a low ratio. Treatment of domestic RO reject water using undivided cell specifically recovered 64% of CaCO₃, although the low conductivity of the RO reject water limited further Mg²⁺ recovery. Conversely, treating industrial effluent-polluted groundwater with this two-step process successfully recovered 80% of CaCO₃ and 94% of Mg(OH)₂. SEM, EDAX, and XRD analysis confirmed the quality of the recovered products.

Keywords: Calcium; Groundwater; Hardness; Magnesium; Pollution; Salt waste.