Equilibrium, Thermodynamic, and Density Functional Theory Modeling Studies for the Removal of Dichromate Ions from Wastewater Using Calix[4]arene Modified Silica Resin

Water is contaminated by different toxicants such as anions, cations, and organic waste products. In this regard, this study reveals the removal of Cr2O7 ions from water using calix[4]arene modified silica (CMS) resin under the optimized conditions of concentration, pH, resin dosage, and effects of temperature. During the experiments, it was noticed that the adsorption of Cr2O7 ions onto CMS resin is highly pH-dependent and has high adsorption capacity using only 25 mg of resin. The adsorption equilibrium data have been subjected to different isotherm models such as Langmuir, Freundlich, and Dubinin–Radushkevich (D-R). From the results, it has been observed that the Freundlich model is the best fit for an adsorption phenomenon as compared to the Langmuir model, while the D-R model demonstrates that the adsorption is an ion exchange type according to the values of E (kJ/mol). The thermodynamic study reveals that the values of ΔH, ΔG, and ΔS show that the adsorption process is endothermic and spontaneous. Dichromic acid (H2Cr2O7) and its conjugated base (Cr2O72– anion) were optimized at the B3LYP/LANL2DZ/6-311++G(d,p) level by using G09W software to analyze the adsorbate-adsorbent interaction mechanism. The efficiency and adsorption capacity of CMS resin have also been checked on real wastewater collected samples.