Enhanced Cr(VI) sorption capacity of the mechanochemically synthesized defective UiO-66 and UiO-66-NH2

Sorption of hexavalent chromium has become a growing global concern due to its high aqueous solubility and good mobility in underground environments. Among other Metal-Organic Frameworks (MOFs), water-stable Zr-MOFs have been recently investigated for the efficient decontamination of aqueous solution from Cr(VI). Herein, Zr-benzene dicarboxylate (UiO-66) and its amino derivative, Zr-NH2-benzene dicarboxylate (UiO-66-NH2), were prepared via a facile, benign mechanochemical method (Liquid Assisted Grinding, LAG) and for the first time applied to sorption studies for Cr(VI) removal. Through the comparative examination of the mechanochemically isolated UiO-66 (LAG) and UiO-66-NH2 (LAG) with the same Zr-MOFs made via conventional solvothermal synthesis (solvothermal, SOL) UiO-66 (SOL) and UiO-66-NH2 (SOL), we demonstrate better performance of the former towards Cr(VI) sorption. The Cr(VI) sorption kinetics for all compounds were well described by the pseudo second-order kinetic model, while the calculated sorption capacities of UiO-66 (LAG) and UiO-66-NH2 (LAG) for Cr(VI) (at pH 3) by the Langmuir model were 28.8 and 36.6 mg/g, respectively, overperforming those of UiO-66 (SOL) and UiO-66-NH2 (SOL), which were found to be 24.9 mg/g and 26.1 mg/g, respectively. Our findings point out that the increased missing-linker defects, generated through milling, boost Cr(VI) sorption. In addition, this work opens the perspective for the implementation of mechanochemical synthesis in order to examine the performance of other known MOF sorbents towards water remediation applications.