Coexisting nickel ions enhance tungstate removal by calcium-aluminum layered double hydroxide: Characteristics and synergistic mechanism

Tungsten (W) and nickel (Ni) have divergent chemical properties and coexist in some industrial wastewater, making their simultaneous removal a significant challenge. In this study, a nitrate-intercalated CaAl-layered double hydroxide (CaAl-LDH) with high purity and crystallinity was synthesized using the microwave-assisted method for the removal of tungstate (WO 4 2− ) and nickel (Ni 2+ ) ions in single and/or binary systems. In single systems, low concentrations of WO 4 2− (20–100 mg W/L) resulted in low removal efficiency and rapid dissolution of the LDH, forming CaWO 4 colloids, while high concentrations of WO 4 2− (1000 mg/L) led to the formation of large and easily filterable CaWO 4 precipitates. Such a dissolution-precipitation process is responsible for the high removal amount of ∼1000 mg/g. In binary systems, the presence of Ni 2+ significantly enhanced the removal of WO 4 2− ; for instance, 100 mg/L Ni 2+ enabled the complete removal of WO 4 2− , increasing the amount of W removed from 40 to ∼210 mg/g. Similarly, WO 4 2− promoted Ni 2+ removal (increasing the removal capacity from 350 to 406 mg/g), though this effect was itself concentration-dependent. Oxyanions inhibited WO 4 2− removal by competing for Ca 2+ ; however, in ternary systems containing Ni 2+ , they unexpectedly promoted WO 4 2− removal. Solid characterization confirmed that WO 4 2− was mainly removed by forming CaWO 4 precipitates, while Ni 2+ was removed by forming NiAl-LDH. When WO 4 2− and Ni 2+ coexisted, the in-situ formation of NiAl-LDH boosted the interfacial precipitation of WO 4 2− , demonstrating a visible synergistic effect. This study elucidates the simultaneous removal mechanism and concentration-dependent synergies of CaAl-LDH for WO 4 2− and Ni 2+ , providing a cost-effective alternative and theoretical basis for treating complex contaminated environmental media.