OMPARATIVE ADSORPTION STUDY OF BLEACHED OILS FROM FOUR SELECTED SEEDS USING LOCALLY SOURCED AND MODIFIED MATERIALS

Abstract

In this study the potentials of acid and alkaline activated oyster shell, periwinkle shell, and locally sourced kaolin clay for the bleaching of palm, kernel, groundnuts, and cottonseed oils had been investigated. The adsorbent materials were activated using various concentrations of H2SO4, HCl, HNO3, NaOH, and NH4OH. The raw and modified samples were characterized using Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS), Fourier Transform Infrared (FT-IR), and X-ray Diffraction (XRD) spectroscopy. The bleaching experiments were carried out under different variables (effect of dosage; 1.0, 1.5, 2.0, 2.5, 3.0 g, adsorbate/adsorbent contact time; 15, 45, 75, 105, 135 min and temperature; 35, 55, 75, 95, 115 oC). The bleaching efficiency showed that the adsorbents compete favourably with the activated commercial bentonite powder. The adsorption data were tested by four different isotherms namely Langmuir, Freundlich, Temkin, and Dubinin-Redushkevich (D-R) isotherms. It was observed in the results obtained that Dubinin-Redushkevic model was most obeyed for the bleaching of groundnut and cotton seed oils based on the overall correlation coefficient R2, closely followed by Langmuir isotherm model. In the bleaching of palm oil, Temkin isotherm model was best obeyed, while in the bleaching of palm kernel oil, Langmuir isotherm was best obeyed among the four isotherms used. The kinetics of the process was analyzed using pseudo-first, pseudo-second, Bangham and Elovich models: Elovich model was best obeyed for the bleaching of groundnut oil, closely followed by pseudo-second order model. Pseudo-second order model was most obeyed for the bleaching process of cottonseed oil, followed by Elovich model based on the high correlation coefficient R2; in the bleaching of palm oil, pseudo-second order was best obeyed followed by Bangham kinetic model due to high R2. The values of the heat of adsorption (EkJ/mol) revealed that adsorption process in palm kernel oil and cottonseed oil was physisorption as the values were less than 8 kJ/mol while palm oil showed strong chemical adsorption as the values are greater than 16 kJ/mol. However, in cottonseed oil the adsorption was ion exchange (between 8 to 16 kJ/mol) except in kaolin powder modified with 5 M NaOH (4.083 kJ/mol), oyster shell powder and periwinkle shell powder modified with 5 M NH4OH having heat adsorption values of 7.14 and 0.50 kJ/mol respectively suggesting physisorption. Furthermore, the evaluation of the adsorption thermodynamic parameters revealed that the adsorption process in groundnut oil was exothermic (negative values of ΔH) and non-spontaneous (positive values of ΔGo). In cottonseed oil, the adsorption was endothermic (positive ΔHo), non-spontaneous (positive ΔGo). In palm oil the ΔHo and ΔSo values were negative except in commercial bentonite powder and kaolin powder modified with 5 M NaOH indicating exothermicity and low degree of randomness, while ΔGo values were positive except in commercial bentonite powder and kaolin powder modified with NaOH and 5 M NH4OH. However, the adsorption was endothermic in palm kernel oil (positive ΔHo), and non-spontaneous (positive of ΔGo) but could be feasible if agitated or heated