Polymer microcapsules containing TBP–P507 prepared through in situ cross-linking for highly stable and selective extraction of Li+ from high-Mg2+/Li+-ratio brines
The efficient separation and extraction of Li + from salt lake brines with a high Mg 2+ /Li + ratio remains challenging. Herein, poly(vinyl alcohol-co-ethylene) microcapsules encapsulating tributyl phosphate (TBP) and mono-2-ethylhexyl 2-ethylhexyl phosphate (P507) as carriers were successfully fabricated via one-step phase inversion followed by in situ cross-linking with boric acid. Correspondingly, the obtnined microcapsules possess a fine three-dimensional cross-linked network structure, which can effectively stabilize the extractant carriers and significantly enhance the encapsulation stability. These microcapsules were employed for the stable and selective extraction of Li + from high-Mg 2+ /Li + -ratio brines. The surface morphology, composition, structure, and adsorption characteristics of the TBP–P507 microcapsules were thoroughly investigated. The microcapsules comprising 30 wt% (m/m) polymer matrix and 70 wt% (m/m) TBP–P507 as the extractant (denoted as TBP–P507/P-3) exhibited the highest separation efficiency for Li + and Mg 2+ . The separation factor of Li + over Mg 2+ was up to 427, and the equilibrium adsorption capacity of the TBP–P507/P-3 microcapsules for Li + reached 0.83 mg/g in synthetic brine with a low Li + concentration and a high Mg 2+ /Li + ratio. These results illustrated that Li + could be separated with high selectivity over Mg 2+ . Adsorption kinetics and isotherms investigations indicated that Li + adsorption by TBP–P507/P-3 conformed to the pseudo-second-order kinetic model and was consistent with the Langmuir–Freundlich model, with the mechanism classified as chemisorption. Accordingly, the maximum amount of Li + adsorbed by the TBP–P507/P-3 microcapsules calculated using the Langmuir–Freundlich model was 4.4 mg/g. After 10 adsorption–elution cycles with pure water as the eluent, 96 % of the maximum equilibrium adsorption capacity was retained. The results clearly illustrate the construction of a finely-tuned three-dimensional cross-linked network structure is pivotal to enhancing stability of the microcapsules, efficiently decreasing the risk of carrier leakage from the involved polymer matrixes. Overall, this research presents a valuable microcapsule technology for separating and extracting Li + from high-Mg 2+ /Li + -ratio salt lake brines.
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