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

Electrodialysis metathesis (EDM) could be considered as a sustainable method for reclamation of SO 4 2– 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 K 2 SO 4 from sulfate-laden wastewater. In SNF unit, concentration of Na 2 SO 4 accompanied by the efficient separation of Cl – and SO 4 2– were realized in a diafiltration mode. A Na 2 SO 4 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 Na 2 SO 4 into K 2 SO 4 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, K 2 SO 4 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·K 2 SO 4 ). Finally, K 2 SO 4 powder was obtained by evaporation process and the product purity met the standard of premium grade agricultural K 2 SO 4 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 – . • SNF/EDM integrated system for recycling SO 4 2– from wastewater was proposed and examined. • Concentration of Na 2 SO 4 accompanied by separation of SO 4 2– and Cl – were realized via SNF diafiltration. • Two-stage EDM were conducted to convert Na 2 SO 4 to K 2 SO 4 and promote closed cycle of residual brine. • K 2 SO 4 solid with purity of 97.4% was obtained by evaporation and crystallization process.