Highly efficient adsorptive removal of tannic acid from aqueous solutions by nitro-functionalized MIL-101(Cr)

In the present work, functionalized MIL-101(Cr)-X (X = –Br, –CH 3 , –HSO 3 , –NO 2 ) materials with mesoporous pores were synthesized and used for tannic acid (TA) adsorption from aqueous solutions. In comparison with MIL-101(Cr), NO 2 -substituted MIL-101(Cr) was found to have positive surface charge and only slight reduction in its specific surface area, resulting in higher TA adsorption capacity than the parent MIL-101(Cr) and the others. Effect of operating parameters such as adsorption time, temperature, pH and ionic strength on the adsorption performance of MIL-101(Cr)-NO 2 was investigated. The TA adsorption capacity remarkably increases with increasing ionic strength adjusted by CaCl 2 . MIL-101(Cr)-NO 2 exhibits an adsorption capacity of 1619 mg/g, higher than that of the reported adsorbents, probably due to electrostatic interaction, π–π stacking interaction and Cr–O complexation interaction between MOF framework and TA. Adsorption isotherms and kinetic results are in accordance with Freundlich model and pseudo-second-order model, respectively. Moreover, the thermodynamics of TA adsorption on MIL-101(Cr)-NO 2 suggests an endothermic process.