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.