Hydrogel adsorbents of poly(N-isopropylacrylamide-co-methacryloyloxymethyl-12-crown-4) for Li⁺ recovery prepared by droplet microfluidics

This article describes the synthesis and application of hydrogel adsorbents for lithium ion (Li⁺) recovery from seawater. The monodisperse hydrogel adsorbent particles of crosslinked poly(N-isopropylacrylamide-co-methacryloyloxymethyl-12-crown-4) [P(NIPAAm-co-M12C4)] were prepared using droplet microfluidics. For selective binding of Li⁺, methacryloyloxymethyl-12-crown-4 (M12C4) was synthesized from 2-hydroxymethyl-12-crown-4 and methacryloyl chloride. M12C4 was then used as a comonomer in the photo-polymerization of aqueous droplets containing N-isopropylacrylamide (NIPAAm) and N,N′-methylenebisacrylamide (MBA) in the presence of 2,2-diethoxyacetophenone (DEAP) under 365 nm UV irradiation. The chemical composition and morphology of P(NIPAAm-co-M12C4) hydrogel adsorbents were analyzed by IR, NMR, optical microscopy, and SEM techniques. Inductively coupled plasma mass spectrometry (ICP-MS) and optical emission spectrometry (ICP-OES) analyses revealed 89% Li⁺ adsorption efficiency with high selectivity among several different metal ions from artificial seawater and a maximum adsorption of 1.4 mg Li⁺/g hydrogel adsorbent achieved within 20 min.