Spiropyran-Decorated Nanoporous Metal–Organic Frameworks for Reversible Light-Controlled Adsorption of Multiple Salts from Water: Implications for Water Purification

Herein, we report a kind of light-responsive metal–organic framework (MOF) adsorption material which was fabricated by covalently immobilizing photoactive spiropyran (SP) molecules onto a chromium(III)-based amino-tagged MIL-101 framework through amide linkages. The photoisomerization of SP moieties endows these materials with the unique feature of reversible binding to metal ions. In the dark, the SP-modified MOFs were capable of adsorbing multiple monovalent and divalent salts from saline water and effectively released the adsorbed ions into water upon exposure to simulated sunlight. The SP-modified MOFs showed excellent salt adsorption capacity, of which the MOF with an SP modification ratio of 39% had an ion uptake of 2.27, 1.72, 1.0, 0.8, and 0.39 mmol g–1 for the monocomponent solutions of LiCl, NaCl, KCl, CaCl2, and MgCl2, respectively. After 10 adsorption–desorption cycles, the regenerated adsorbent still exhibited a higher ion adsorption of 1.27 mmol g–1 for the NaCl solution. This work provides a synthetic strategy for salt adsorption materials with nonchemical regeneration ability, which are promising for water desalination and purification.