Adsorption and desorption of benzoic and salicylic acids by using natural and modified bentonites in supercritical carbon dioxide medium

In this research, adsorption and desorption of benzoic and salicylic acids, which were thought as model drugs, in SCO2 medium were carried out. Natural bentonite and organo-clay (HDTMA-bentonite) obtained by the modification of natural bentonite obtained from Çanakkale region with hexadecyltrimethylammonium (HDTMA) bromide were used as adsorbents. The characterization studies by using different methods (BET, XRF, XRD, SEM, zeta potential measurement, FT-IR, TG and elemental analysis) were also carried out to identify the modification of natural bentonite with HDTMA-Br and its adsorption behavior. In this manner, the effects of pressure, temperature and contact time in SCO2 medium were investigated for the adsorption of benzoic or salicylic acids onto HDTMA-bentonite. Adsorption kinetic, isotherm and thermodynamic parameters were deduced by using experimental data. Lagergren-first-order, pseudo-second-order and Elovich kinetic and intraparticle diffusion models and Langmuir and Freundlich isotherm models were applied to the experimental data to obtain adsorption kinetics and adsorption equilibrium, respectively. Thermodynamic parameters such as the changes in standard free energy, enthalpy and entropy were also determined for the adsorption process. The desorption studies were carried out to obtain information about the reusability of the adsorbents and the effects of pressure, temperature, flow rate of CO2 and modifier on desorption in SCO2 medium were also examined.