Adsorption mechanisms of pigments and free fatty acids in the discoloration of shea butter and palm oil by an acid-activated Cameroonian smectite

Abstract

Discoloration mechanism of shea butter and palm oil was studied by the adsorption of pigments and free fatty acids onto a Cameroonian clay. Adsorbent characterization was performed using ICP-AES, FTIR, SEM, N2 gas adsorption and CEC. The adsorbent is a microporous swelling clay of smectite type. Discoloration depends on temperature and the equilibrium time decreases with increase in temperature. Among pseudo-first, pseudo-second order and intra-particle diffusion kinetics models studies, pseudo-second order was the best applicable to describe pigments and free fatty acids adsorption, suggesting that discoloration of the two oils occurs in two steps: the transfer of pigments and free fatty acids from the oil to the clay surface, followed by the interaction between adsorbates and the surface. The lower values of activation energies and validation of the Temkin isotherm model indicate a physisorption process influenced by mass transfer. The equilibrium adsorption data were analyzed by Langmuir, Freundlich and Temkin isotherms models. Amongst them, Freundlich was the best model to describe adsorption in both cases, indicating the existence of heterogeneous adsorption sites on clay surface. The most appropriate model for pigments adsorption was Freundlich while Temkin model was the most appropriate for the adsorption of free fatty acids suggesting that pigments and free fatty acids are not adsorbed on the same type of sites.