Tuning alkyl side chains in cyclohexyl-functionalized diglycolamides for efficient strontium separation: An experimental and theoretical study

Cyclohexyl-functionalized diglycolamides with tailored alkyl side chains demonstrate full compatibility with alkane diluents. • Cyclohexyl-functionalized diglycolamides exhibit high extraction efficiency and selectivity for Sr 2+ . • Extraction and complexation studies confirmed a neutral complexation model for Sr 2+ extraction. • Simulation experiments demonstrate a promising application in separating Sr 2+ from high level liquid waste. To overcome the limitations of poor lipophilicity in alkane diluents associated with tetracyclohexyl-substituted diglycolamide (TCHDGA), four asymmetric diglycolamide ligands with two cyclohexyl and varying alkyl chains were synthesized: N,N -dibutyl- N′,N′ -dicyclohexyl diglycolamide ( L I ), N,N -dihexyl- N′,N′ -dicyclohexyl diglycolamide ( L II ), N,N -dioctyl- N′,N′ -dicyclohexyl diglycolamide ( L III ), and N,N -didodecyl- N′,N′ -dicyclohexyl diglycolamide ( L IV ). Using n -dodecane as the diluent, their extraction behavior toward Sr 2+ in HNO 3 solution was investigated. Although L I and L II are soluble in n -dodecane, both formed a third phase upon contact with HNO 3 solution at concentrations exceeding 1.0 mol/L. Ligands L III and L IV exhibited significantly stronger Sr 2+ extraction than N,N,N′,N′ -tetraoctyl diglycolamide (TODGA). The D Sr values of L III and L IV reached 231 and 181 at the acidity of 2.5 and 3.0 mol/L, respectively—approximately 98 and 56 times higher than that of TODGA under identical conditions. Slope analysis indicated the formation of a 1:3 (metal-to-ligand) complex during extraction, which was further corroborated by ESI-MS, FT-IR, and ITC analyses. At 25 °C, Sr 2+ extraction by these ligands was found to be an exothermic, entropy-decreasing, and spontaneous process, following a neutral complexation mechanism. Quantitative recovery of Sr 2+ was achieved via a two-stage stripping process using 0.01 mol/L HNO 3 . Extraction tests with simulated high-level liquid waste further confirmed that L III enables efficient and selective separation of Sr 2+ from coexisting ions. Furthermore, density functional theory calculations revealed that, compared to L IV , L III possesses a more negative electrostatic potential, higher bond orders, and more negative binding energy, indicating a stronger tendency to form stable complexes with Sr 2+ , in good agreement with the extraction experimental results. Download: Download high-res image (192KB) Download: Download full-size image