Selective lithium and magnesium adsorption by phosphonate metal-organic framework-incorporated alginate hydrogel inspired from lithium adsorption characteristics of brown algae

Abstract

With increasing global demands for lithium resources, the separation of lithium from seawater-based resources has been widely explored. Herein, we propose phosphonate metal organic framework (MOF)-incorporated alginate hydrogels inspired by brown algae. The analysis of Li+ adsorption characteristics of brown algae revealed that carboxylic acid and phosphonate groups were the dominant functional groups for Li+ adsorption. Biomimetic phosphonate MOF-incorporated alginate hydrogels (pMOF@Alg) were fabricated on the basis of the Li+ adsorption characteristics of brown algae. Their adsorption behaviors indirectly evaluated by conductivities showed that the growth of phosphonate MOF in Cu2+-based alginate hydrogel enhances Li+ adsorption but inhibits Mg2+ adsorption with a small difference, which might be attributed to size confinement effect by heterogeneously distributed phosphonate MOF. By contrast, the growth of phosphonate MOF homogeneously distributed in Al3+-based alginate hydrogel greatly enhances Mg2+ adsorption while rejecting Li+ adsorption. This reverse adsorption behavior of pMOF@Alg(Al) seems to be attributed to partial dehydration, depending on the dehydration energy of hydrated metal ions. The adsorbed ions are easily removable by using ethanol/DI water because of the weak electrostatic interaction with negatively charged groups. This study would provide novel selective adsorption methodology for the separation of Li+ and Mg2+ and an effective lithium recovery technology.