ZIF-90: PSM Assisted Acid Resistance, Accelerated Sequestration and Selective Sensing of Actinides

This manuscript reports synthesis of Zn(II) based zeolitic imidazolate framework, ZIF-90, and its PSM (via pendant carbonyl moiety) to three new ZIF-90 analogues with amine/sulphur/phosphorous moieties. PSM ZIFs were characterized by NMR, FTIR, powder XRD, BET surface area, SEM, EDX and TGA confirming successful modification of C=O, with intact parent framework. ZIFs were explored for their potential applications in extraction, recovery and sensing of U(VI)/Th(IV) from acidic solutions. The influence of solution pH, sorption time and recyclability for U(VI)/Th(IV sorption onto ZIFs was studied, which displayed simultaneously rapid and efficient sorption (90-99.9%) at acidic pH (1-4), following pseudo-second-order kinetics model. ZIF-91-DPPC grafted with phosphinic amide is not only acid resistant (stable at pH 1), but also along with amine grafted ZIF-90-ED showed rapid U(VI)/Th(IV) extraction (pH 1 and 3) within 5 minutes of contact time. PSM offered enhancement of ZIF-90 properties enabling acidic solutions stability and superior coordination capabilities of P=O and -NH2 groups towards U(VI)/Th(IV) capture thereby leading to efficient sequestration. ZIFs showed facile desorption by lowering the pH and recyclability for two cycles. Additionally, ZIF-90-ED displayed sensing of U(VI) (pH 3) via fluorescence quenching (∼45%) based on “turn-off” mechanism with a detection limit ∼1.5 mg L−1. Besides, ZIF-90-ED showed desirable selectivity towards U(VI) over competing metal ions, La(III), Ce(III), Nd(III), Sm(III), Eu(III), Gd(III) and Th(IV), even if latter ions are present in five folds concentration. These results prove the merit of ZIF-90-ED as a “turn-off” sensory probe detecting selectively U(VI) from radioactive wastes. It is of prime importance to develop MOF materials which can withstand acidic media in order to serve as sorbent materials and this work not only reports efficient sorption of U(VI) at pH 1, but also presents very first and excellent Th(IV) extraction using ZIFs. Henceforth, paving way for facile approach for construction of acid-resistant MOFs via PSM for highly efficient extraction accompanied with sensing (selective) ability for radionuclides from acidic solutions for potential application in nuclear waste management.