Efficient removal of a pharmaceutical compound on organoclay: batch experiment, DFT calculation, statistical physics, and modeling

Abstract

This study was performed to evaluate the adsorptive properties of bentonite. Natural bentonite was found to adsorb considerable amounts of Salicylic acid (SA) after modification by hexadecyltrimethylammonium bromide (HDTMA). Batch studies investigated the adsorptive properties of the modified natural bentonite (organobentonite). The characterization results show the increase in the d001 spacing from 1.13 to 1.53 nm for raw bentonite and organobentonite, respectively. The FTIR analysis indicates the asymmetric vs. (CH2) and symmetric vs. (CH2) stretching modes for clay loaded with surfactant molecules are 2919 and 2845 cm−1, respectively. We investigate parameters such as contact time, pH, temperature, and initial concentration. Desorption and regeneration were also studied. The kinetic analysis shows that the adsorption equilibrium of salicylic acid onto organoclay was reached after 120 min and fits well to a pseudo-second-order kinetic model with the contribution of intraparticular diffusion. In addition, the adjustment of the isothermal models indicated that the Langmuir-Freundlich model best fit the isotherms data for unmodified bentonite and the Langmuir model for modified bentonite. The maximum adsorption capacity of organobentonite is 128 mg g−1 at 25 °C. According to the statistical physics model, the SA could be adsorbed on the surface of organoclay in a non-parallel orientation. The desorption-regeneration investigation revealed that the material could be successfully reused for three cycles of SA adsorption. Mechanistic studies of SA sorption on organobentonite have highlighted several mechanisms: hydrogen bonding, electrostatic attraction, and hydrophobic interaction.