Synthesis of amino-functionalized MIL-101(Cr) MOF for hexavalent chromium adsorption from aqueous solutions

Hexavalent chromium [Cr(VI)] is a very toxic pollutant, and its removal from wastewater is an absolute priority. Among the different approaches proposed to this aim, adsorption allows to reach very low concentrations with relative ease. In consideration of the promising performances of metal-organic frameworks as adsorbents, in this paper amino-functionalized MIL-101(Cr), or AFMIL, was tested for the adsorption of Cr(VI) from aqueous solutions. AFMIL was synthesized and was characterized by X-Ray diffraction, FTIR, SEM and microporosimetry by N2 adsorption/desorption at 77 K. Batch kinetic data for K2Cr2O7 removal from water using AFMIL indicated that the adsorption process is rather fast, and can be described by assuming that the limiting step is intraparticle diffusion, with a weak dependence of diffusivity on temperature. Adsorption isotherms, evaluated at three different temperatures in the range of 25–60 °C, were well fitted by the Dubinin–Radushkevich equation and indicated an adsorbent–adsorbate interaction energy in the order of 21 kJ mol−1, suggesting a markedly chemical phenomenon. The highest observed adsorption capacity of AFMIL toward Cr(VI) was 44 mg g−1 for a liquid phase concentration of 140 mg L−1 at T=60 °C, a value which compares well with others published in the literature. Central composite design (CCD) with response surface methodology (RSM) was used, and this approach indicated that the optimal pH for Cr(VI) adsorption was 3.51. Overall, the experimental results indicate that AFMIL has very interesting potential applications for Cr(VI) removal from water.