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, CaCl 2, and MgCl 2, 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.