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

Abstract

Metal–organic frameworks (MOFs) are currently considered as strong adsorbents to remove organic pollutants due to their unique characteristics. The main aim of this study is to synthesize the MIL-101(Cr)/Sulfasalazine (Cr-TA@SSZ) hybrid by a hydrothermal approach for the first time. Due to the presence of different functional groups such as amino, hydroxyl, and carboxyl in the structure of Sulfasalazine and its good activity, this compound can be a suitable auxiliary ligand to improve the surface of the MIL-101(Cr). The synthesized Cr-TA@SSZ was applied for the adsorption of terephthalic acid (TA), para-toluic acid (p-tol), and benzoic acid (BA) also the results were compared with MIL-101(Cr). 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 energy dispersive X-ray analysis (EDX). Based on the above analyses, it was concluded that Cr-TA@SSZ has a different composition and network structure than the MIL-101(Cr). The formula for the new MOF (Cr-TA@SSZ) is proposed as: $${\text{Cr}}_{{3}} {\text{F }}\left( {{\text{H}}_{{2}} {\text{O}}} \right)_{{2}} {\text{O}}\left[ {{\text{C}}_{{6}} {\text{H}}_{{4}} \left( {{\text{CO}}_{{2}} } \right)_{{2}} } \right]\left[ {{\text{C}}_{{6}} {\text{H}}_{{3}} {\text{N}}\left( {{\text{OH}}} \right)\left( {{\text{CO}}_{{2}} } \right)} \right],{ 2}.{\text{5H}}_{{2}}.$$ 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 results show that equilibrium was achieved after that 1440 min. As results, TA and p-tol at pH 5 and BA at pH 2–5 have the highest adsorption rate for the Cr-TA@SSZ, while the highest uptake for MIL-101(Cr) was obtained for all three solutes at pH 5. Removal percent was found to be dependent on the initial concentration of solutes. The Cr-TA@SSZ has the highest percentage removal of benzoic acid, terephthalic acid (TA), and p-toluic acid (p-tol) at an initial concentration of 100 ppm (81%, 90%, and 87% respectively), while the highest percentage removal for the MIL-101(Cr) are 70%, 84%, and 72% respectively. The isotherm, kinetic and thermodynamic models were also analyzed for TA, p-tol, and BA adsorption. Equilibrium adsorption was evaluated employing Langmuir, Freundlich, Temkin, Redlich–Peterson (R–P), and Dubinin–Radushkevich (D–R) equations, in which Langmuir and Redlich–Peterson models were in good agreement with the experimental results. The maximum adsorption capacities (q0) derived from the Langmuir model for the Cr-TA@SSZ were determined to be 2208.4 mg/g, 1241.2 mg/g, and 1009.5 mg/g for TA, p-tol, and BA, respectively while for MIL-101(Cr) the maximum adsorption capacities were determined to be 1692.0 mg/g, 952.4 mg/g, and 769.2 mg/g 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. The negative values of ΔG indicated that the removal process was spontaneous at all 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 could be used as a promising adsorbent in the adsorptive removal of organic pollutants from wastewaters. This MOF removed 40% COD from the concentrated phase (equivalent to 13,000 ppm) and 77.3% COD from the diluted phase (equivalent to 4250 ppm) wastewater. Reusability of the Cr-TA@SSZ in six cycles indicated the adsorbent's appropriate performance for industrial applications.