Sustainable treatment of desulfurization wastewater by ion exchange and bipolar membrane electrodialysis hybrid technology

Abstract

Residual Ca2+ and Mg2+ in chemically softened desulfurization wastewater may form hydroxides which block the pores of cationic exchange membranes (CEM) during bipolar membrane electrodialysis (BMED) used for acid and base recovery. In this study, ion exchange was investigated as a polishing treatment prior to BMED for sustainable treatment of desulfurization wastewater. Adsorption of Ca2+ and Mg2+ from desulfurization wastewater was investigated using three chelating resins (Amberlite IRC 747, Lewatit TP 208 and Lewatit TP 260) in batch adsorption studies as a function of contact time (0–720 min), solution pH (6.0–11.0), salinity (35–140 g L−1) and temperature (298–318 K). Langmuir isotherm fitted the experimental data better than Freundlich isotherm, and the pseudo second-order kinetics describe the adsorption behavior well. Evaluation of thermodynamic parameters (G°, free energy; S°, enthalpy change; and H°, entropy change) demonstrated that ion exchange of Ca2+ and Mg2+ from desulfurization wastewater was feasible, favorable and endothermic in nature. The activation energies for Ca2+ and Mg2+ adsorption using TP 208 were 58.5 kJ mol−1 and 73.6 kJ mol−1 respectively >TP 260 of 38.6 kJ mol−1 and 47.9 kJ mol−1 > IRC 747 of 36.9 kJ mol−1 and 39.5 kJ mol−1. Column studies showed that the breakthrough points of Ca2+ and Mg2+ by IRC 747 both advanced with increasing bed height and that the Thomas model can adequately describe the breakthrough curves. The effluent of resin columns was injected into a BMED stack with high-purity acid (99.3%) and base (99.0%) produced. The product acid and base was reutilized to regenerate and transform the saturated chelating resins, respectively, and proved to be effective with duplicate cycles.