Magnesium recovery from desalination reject brine as pretreatment for membraneless electrolysis

Abstract

Alkali-earth metals pose a significant challenge to water treatment technologies and electrochemical processes due to their propensity to precipitate as metal hydroxides, which can deposit on membranes and/or electrodes and reduce their efficiencies. Membraneless electrolyzers can partially overcome this issue because they lack membranes or diaphragms, however their electrodes are still susceptible to fouling. To overcome this issue, electrolyzers can be incorporated into a recirculating electrolyte scheme where magnesium is removed in the form of Mg(OH)2 before reaching the electrolyzer. Motivated by this system, the first part of this study focuses on the optimization of magnesium recovery from desalination brine by adding NaOH using response surface methodology. Brine salinity, NaOH dose, and temperature were optimized with complete magnesium removal set as the target response using central composite design. Results showed that 98.8% of magnesium can be removed at brine salinity, NaOH dose, and temperature of 73.5 g/L, 8.22 g/L and 45.5 �C, respectively. The second part of this study investigated the influence of Mg2+ concentration on the performance of a cathode within a membraneless electrolyzer. Experimental demonstrations show that Mg2+ concentrations below 5 mM can be used as a feed stream without any noticeable build-up of Mg(OH)2 deposits on the cathode surface over 3 h during electrolysis at 50 mA/cm2. Finally, an analysis is presented to predict how long a cathode can operate in Mg2+-containing electrolyte as a function of current density and superficial velocity of brine solution before the electrode reached its maximum tolerance of Mg(OH)2 deposits.