Modification of cyclodextrin-based microgels with 2-hydroxymethyl-12-crown ether-4 for higher and selective Li+ ion adsorption from aqueous medium

The cyclodextrin (CD) oligosaccharides derived materials, e.g., p(alpha-CD), p(beta-CD), and p(gamma-CD) microgels were used in Li + ion adsorption studies, which revealed 55.4 +/- 3.9, 85.1 +/- 3.8, and 117.8 +/- 4.9 mg Li+/g, respectively, in 12 h from 500 mg/L 100 mL aqueous solutions. The Li+ ion adsorption by p(CD)-based microgels is best described by pseudo-first-order adsorption kinetics and Freundlich adsorption isotherms, with higher R-2 values. There is no significant selectivity observed for any microgels to Li + ions in the presence of Na+, K+, and Ca2+ ions. However, the relative selectivity (kl) calculation among the adsorbents revealed that p(beta-CD) microgels afforded higher selectivity than p(alpha-CD) and p(gamma-CD) microgels, with kl values greater than 1. Therefore, to further increase selectivity, p(beta-CD) microgels were modified (M-p(beta-CD)) with 2-hydroxymethyl-12-crown ether-4, known for its specificity for Li + ion. The adsorbed amount of Li+ ions by M-p(beta-CD) microgels was determined as 91.9 +/- 1.9 mg Li+/g, a slight increase with respect to the unmodified microgel upon 12 h contact time. Interestingly, the selectivity of M-p(beta-CD) microgels toward Li+ ion for Li+/Na+, Li+/K+, and Li+/Ca2+ was measured as 3.1, 4.5, and 5.1-fold higher, respectively, than bare p(beta-CD) microgels. Also, M-p(beta-CD) microgels retained >80% adsorption capacity of Li+ ions after 5 consecutive uses. Microgels employing cyclodextrin, particularly beta-CD systems demonstrated an effective and reusable adsorption capability for Li+, and the selectivity is markedly enhanced amid competing ions via the functionalization with crown ethers. [GRAPHICS]