Efficient extraction performance and mechanisms of Cd²⁺ and Pb²⁺ in water by novel dicationic ionic liquids

Ten novel hydrophobic dicationic ionic liquids (DILs) were synthesized and applied for the extraction of heavy metals in aqueous solutions. Their physicochemical properties were measured at ambient temperature, and the leaching behaviors of the as-prepared DILs in water were assessed by TOC analysis. Metal extraction experiments were carried out to evaluate the extraction performances of the DILs. It was found that the extraction rates of up to 0.45 and 0.53 mg·(g·min)^-1 were achieved with 100 mg DILs for 5 mL of 5 mg/L Cd²⁺ and Pb²⁺ solutions. Besides, the extraction efficiencies of Cd²⁺ and Pb²⁺ were respectively up to 95.48% and 98.46%, when the volumes of the simulated wastewater were expanded by a factor of 20 at a constant extraction phase ratio (1000 mg DILs for 50 mL of 5 mg/L Cd²⁺ or Pb²⁺ solutions). The reusability of the novel DILs was successfully proved by the back-extraction experiments with 0.5 M HNO₃. Finally, taking Cd²⁺ extraction as an example, the extraction mechanism based on FTIR analysis and quantum chemical calculations showed that both S and O atoms in the anions of DILs had physical and quasi-chemical interactions with Cd²⁺, which were stronger than the electrostatic attraction.