Enhancing the performance of lithium recovery in seawater with membrane distillation and manganese oxide metal-organic framework nanoparticles

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Recovering lithium (Li) from natural sources such as seawater is a sustainable alternative to meet its high demands. In this work, the potential of enhancing Li recovery from seawater by acid-treated manganese oxide ion sieve (HMO) is evaluated by increasing Li concentration in seawater using direct contact membrane distillation (DCMD) and reducing competitive ions. DCMD achieved enhanced water recovery upon pre-treatment with oxalic acid (88–91%) compared to caustic soda ash (65–68%) and without pre-treatment (47–51%). Caustic soda ash required Na addition in alkaline condition for Ca removal, while oxalic acid removed Ca in acidic condition without any inorganic ion addition. The low ion concentration in acidic condition upon oxalic acid pre-treatment enabled DCMD to concentrate seawater to high levels, increasing Li concentration by 7 times. In Li solution, HMO achieved a maximum adsorptive capacity (Langmuir Qmax) of 17.8 mg/g in alkaline condition. Multiple cycles of desorption and regeneration of HMO showed only a 7–11% decline of Li uptake and minimal Mn dissolution, which established HMO’s reuse capacity. Selective Li mechanism is attributed to H/Li exchange as well as a high negative surface charge of HMO. In seawater, Li uptake by HMO reduced by 44–46% due to Mg. Seawater with minimal Mg was favourable for enhancing Li uptake by HMO. Seawater treatment in stages – divalent pre-treatment and concentrating seawater, followed by HMO, provided a favourable scenario for attaining high-quality water, selective Li recovery, and other resources – Ca and Mg. The drawback of this method is HMO can only recover Li at high pH (11-12). Therefore, in this study, a new HMO with the metal organic framework -ZIF-8@MOF was synthesised and The ZIF-8@MOF showed higher Li+ adsorption capacity compared to HMO. More importantly, ZIF-8@MOF can selectively extract Li+ in seawater at its original pH (7.5-8.0). This is favourable in attaining selectively Li+ recovery from seawater without the need for pH adjustment to 11 with chemical addition (NaOH).
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