A cationic Zr-based metal organic framework with enhanced acidic resistance for selective and efficient removal of CrO42−

Abstract

Water pollution caused by CrO₄²⁻ has become a global concern due to its high aqueous solubility and good mobility in the underground environment. Recently, water stable zirconium based metal–organic frameworks (Zr-MOFs) have been tested as adsorbents to remove pollutants from contaminated water. However, the low adsorption capacity of Zr-MOFs towards ionic pollutants is unsatisfactory due to their neutral nature. Herein, a cationic Zr-MOF (Zr-C-MOF) was obtained by directly introducing a pyridinium salt as a ligand with an impressive ion exchange capability. Zr-C-MOF demonstrated enhanced acidic resistance and selectively high adsorption of CrO₄²⁻. The characteristics of the obtained Zr-C-MOF were confirmed via various techniques including PXRD, FT-IR spectroscopy, ¹H-NMR spectroscopy, TGA, SEM, EDX and XPS. Batch adsorption studies have been conducted to gain a deep understanding of the kinetics mechanism, pH effects, adsorption isotherm and the effects of other competing ions. The kinetics and adsorption isotherm of CrO₄²⁻ adsorption onto Zr-C-MOF were found to well fit the pseudo-second-order rate equation and the Langmuir model, respectively. The comparison of FT-IR, PXRD, SEM, EDX and XPS results of the samples before and after CrO₄²⁻ adsorption revealed the adsorption mechanism of the anion exchange process.