Synthesis of ettringite from desulfurization wastewater via the co-precipitation method and its application for Zn2+ removal

Lime softening is commonly used to treat flue gas desulfurization (FGD) wastewater, but the high solubility of calcium sulfate (2.1 g/L at 25 °C) results in considerable residual Ca 2+ and SO 4 2- , thereby increasing the risk of scaling in subsequent processes. This study proposes a practical co-precipitation strategy employing sodium aluminate (NaAlO 2 ) and sodium hydroxide (NaOH) to simultaneously remove Ca 2+ and SO 4 2- from softened FGD wastewater. The results demonstrated that increasing NaAlO 2 and NaOH dosages enhanced Ca 2+ removal, whereas SO 4 2- removal exhibited a rise-then- fall trend. Under optimal conditions ([Al]/[Ca] = 1:2, NaOH = 30 mM), the removal efficiencies of Ca 2+ and SO 4 2- reached 95.9% and 79.7%, respectively. X-ray diffraction (XRD) analysis revealed that the precipitate was mainly ettringite (87.9%) with some calcite (12.1%). This ettringite-rich solid was subsequently evaluated as an adsorbent for Zn 2+ , achieving a maximum adsorption capacity of 103.08 ± 4.17 mg/g. The adsorption kinetics followed a pseudo-second-order model and reached equilibrium within 30 minutes. Isotherm analysis matched the Langmuir model, indicating monolayer chemisorption. This integrated approach effectively removes residual Ca 2+ and SO 4 2- from softened FGD wastewater and generates a value-added product for heavy metal adsorption, promoting resource recovery and offering a sustainable, efficient strategy for FGD wastewater management. • Practical method proposed for Ca 2+ and SO 4 2- removal from softened FGD wastewater • High Ca 2+ and SO 4 2- removal efficiencies of 95.9% and 79.7% were achieved • The precipitate byproducts (PBPs) comprised 87.9% ettringite and 12.1% calcite • PBPs could efficiently adsorbed Zn 2+ with a maximum capacity of 103.08 ± 4.17 mg/g