High-value conversion of waste Na2SO4 by a bipolar membrane electrodialysis metathesis system

Na2SO4 is generated during zero-liquid discharge treatment of high-salinity wastewater and is of limited industrial use. In this study, a five-compartment bipolar membrane electrodialysis metathesis (BMED-5) system was established to produce (NH4)2SO4, HCl, and NaOH simultaneously from waste Na2SO4 by adding an equivalent amount of NH4Cl. The conditions for optimizing the performance of the BMED-5 system in terms of its energy efficiency and conversion ratio were investigated. The inflection point of the stack voltage occurred at a final Na2SO4 concentration of 0.03 mol/L, corresponding to a conductivity of approximately 6 mS/cm in the Na2SO4 compartment, which could be considered the end point of the reaction. In comparison with the widely used three-compartment bipolar membrane electrodialysis (BMED) system, which could convert the waste salts into an acid and a base, the BMED-5 system in study only consumed a little higher energy to produce three products. The highest (NH4)2SO4 (0.44 mol/L), HCl (1.03 mol/L), and NaOH (1.35 mol/L) concentrations were obtained with an initial Na2SO4 concentration of 0.8 mol/L and a current density of 40 mA/cm2, yielding a conversion ratio of up to 96% and energy consumption of 2.961 kWh/kg Na2SO4. The gross profit and payback period of BMED-5 were estimated to be 178.4 $/t Na2SO4 and 2.3 years, respectively.