Reclamation of sulfate-laden wastewater via a hybrid selective nanofiltration and electrodialysis metathesis process

Abstract

Electrodialysis metathesis (EDM) could be considered as a sustainable method for reclamation of SO42– from wastewater. However, its feasibility is limited by the coexisting Cl–. In this study, for the first time, an integrated process of selective nanofiltration and EDM (SNF/EDM) was proposed to prepare K2SO4 from sulfate-laden wastewater. In SNF unit, concentration of Na2SO4 accompanied by the efficient separation of Cl– and SO42– were realized in a diafiltration mode. A Na2SO4 concentrate stream (53.0 g/L) of 96.7% purity was obtained employing 4/2 of preconcentration factor and 2.0 MPa of operating pressure. Then, the stream was further treated by EDM process to continuously convert Na2SO4 into K2SO4 using KCl as metathesis solution. Under the relatively optimized operating parameters of 0.5 V repeating unit voltage and 1.25 L/h replenishment flowrate, K2SO4 solution (104.5 g/L) with purity of 92.6% was obtained during two-stage EDM with the energy consumption of 0.59 kWh/(kg·K2SO4). Finally, K2SO4 powder was obtained by evaporation process and the product purity met the standard of premium grade agricultural K2SO4 based on XRD and composition analysis. This study thus identified an effective way to make high-valued reclamation of sulfate-laden wastewater which contained considerable amounts of Cl–.