Green recovery of lithium from seawater by hydrophobic interaction of poly($N$-isopropylacrylamide)/alginate composite

The demand of lithium, which is known as a crucial strategic element, has been largely increased in various applications, including lithium ion batteries. Herein, we propose the technique in a green recovery of Li$^{\mathrm{+}}$ ions from seawater. Poly($N$-isopropylacrylamide)(PNIPAAm)/alginate(Alg) composite crosslinked with Al$^{\mathrm{3+}}$ ions selectively adsorbs Li$^{\mathrm{+}}$ ions from seawater. Strong repulsion force by Al$^{\mathrm{3+}}$ ions rejects cations with a high adsorption affinity, while less rejecting Li$^{\mathrm{+}}$ ions with a low adsorption affinity. Structural characteristics of PNIPAAm/Alg composite were analyzed by using \textit{in situ} TEM and \textit{in situ} FTIR techniques. PNIPAAm/Alg composite is rapidly hydrated within 60 min, which enables rapid adsorption of Li$^{\mathrm{+}}$ ions. In addition, the incorporation of thermoresponsive PNIPAAm polymer induces a green recovery of Li$^{\mathrm{+}}$ ions by a hydrophobic interaction with applying a small thermal energy without the acidic treatment. 7.3{\%} of Li$^{\mathrm{+}}$ ions can be recovered from Li$^{\mathrm{+}}$-spiked seawater containing an extremely high concentration of impurities. The present study will provide an efficient Li$^{\mathrm{+}}$ recovery method with a novel interaction between ions and polymeric network.