A novel selective lithium separation using amorphous pMOF/alginate interfaces

Anomalous opposite ion behaviors of Li$^{\mathrm{+}}$ and Mg$^{\mathrm{2+}}$ ions are experimentally observed from amorphous interfaces of phosphonate metal$-$organic framework (pMOF)/alginate composites. The amorphous structures of pMOF/alginate composites are significantly varied according to reaction temperature and intertwinement degree of alginate networks. As a proof-of-concept application, Li$^{\mathrm{+}}$ and Mg$^{\mathrm{2+}}$ ions are separated using pMOF/alginate composite depending on the degree of pMOF growth. Initially, lots of Li$^{\mathrm{+}}$ ions are effectively attracted compared to Mg$^{\mathrm{2+}}$ ions due to the strong repulsion force of Al$^{\mathrm{3+}}$ ions in amorphous alginate interfaces on multi valent metal ions. However, amorphous pMOF/alginate interfaces induce effective rejection of Li$^{\mathrm{+}}$ ions with low hydration energy through dehydration due to the significant interaction with water molecules, while Mg$^{\mathrm{2+}}$ ions with high hydration energy are significantly attracted by negatively charged phosphonate groups. The present results of amorphous MOF/alginate interfaces would provide a variety of benefiting separation opportunities with unique property in seawater desalination and rare metal recovery.