Synthesis of MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid and its use as a novel adsorbent for adsorptive removal of organic pollutants from wastewaters
Authors: Tourani S. , Behvandi A. 
Published: March 2022

Abstract
Metal-organic frameworks (MOFs) are considered strong adsorbents to the removal of organic pollutants due to their unique characteristics. In this work, a new type of metal-organic porous material MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid successfully synthesized by a hydrothermal approach for the first time. The synthesized Cr-TA@SSZ and MIL-101(Cr) adsorbents were applied for adsorption of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA). The Cr-TA@SSZ and MIL-101(Cr) were characterized by the general tests including X-ray diffraction (XRD), Fourier transform infrared (FTIR), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), Scanning electron microscope(SEM), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), zeta potential, and Elemental analysis (EDX). Based on the above analyses, it was concluded that Cr-TA@SSZ has a different composition and network structure to the MIL-101(Cr). The formula for new MOF (Cr-TA@SSZ) proposed as: Cr3F (H2O)2O[C6H4(CO2)2][C6H3N(OH)(CO2)], 2.5H2. The experiments for evaluating the effect of the different parameters such as pH, initial concentration, contact time, and temperature on the removal of the terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) were carried out in batch mode. The isotherm, kinetic and thermodynamic models were also analyzed for the adsorption of TA, p-tol, and BA. Equilibrium adsorption was evaluated employing Langmuir, Freundlich, Temkin, and Redlich–Peterson equations, in which Langmuir and Redlich–Peterson models were in good agreement with the experimental results. Maximum adsorption capacity (q0) of the Cr-TA@SSZ for terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) were obtained 2208.4 mg.g− 1, 1241.2 mg.g− 1, and 1009.5 mg.g− 1, respectively while for MIL-101(Cr) were obtained 1692.0 mg.g− 1, 952.4 mg.g− 1, and 769.2 mg.g− 1 respectively. The Cr-TA@SSZ was found to be more efficient in the removal of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) from water than the MIL-101(Cr). Also, the results showed that a pseudo-second-order kinetic model with a higher correlation coefficient (R2 > 0.99) matched well for the adsorption of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) onto MIL-101(Cr) and Cr-TA@SSZ. The thermodynamic parameters such as a change in Gibbs free energy (ΔG), enthalpy (∆H), and entropy (ΔS) were determined and the negative values of ΔG indicated that the process of removal was spontaneous at all values of temperatures. Further, the values of ∆H indicated the exothermic nature of the removal process. Moreover, adsorption experiments using industrial wastewater from a TA production plant showed that Cr-TA@SSZ‌ can be used as a promising adsorbent in the adsorptive removal of organic pollutants from wastewaters. This MOF was able to remove 40% COD from the concentrated phase (equivalent to 13000 ppm) and remove 77.3% COD from the diluted phase (equivalent to 4250 ppm) wastewater.