Adsorption properties of stearic acid onto untreated kaolinite

The focus of the study is to investigate adsorption property and determine thermodynamic parameters for the adsorption of stearic acid onto untreated kaolinite at the temperatures of 25, 35 and 45 °C. The equilibrium adsorption isotherms were analyzed by linear Langmuir and Freundlich models. Adsorption experiments indicated that the sorption capacity of kaolinite decreased with increasing temperature. The calculated Langmuir equilibrium parameter, R_L indicated that the kaolinite was a good material for the sorption process of the stearic acid. The free energy change of adsorption, ΔG°_ads, was found between - 19.98 and -20.50 kJ/mol at examined temperatures and enthalpy of adsorption, ΔG°_ads, and entropy of adsorption, ΔS°_ads were found as -12.30 kJ/mol and 0.0259 kJ/(mol.K), respectively. The calculated thermodynamic parameters (ΔH°_ads and ΔG°_ads) showed that the adsorption process of stearic acid onto kaolinite was spontaneous and exothermic in nature. Furthermore, Fourier Transform infrared (FT-IR) spectroscopy was used to indicate the possible interaction between the stearic acid molecules and the surface groups of adsorbent.