Decolorization of vegetable oils: Adsorption mechanism of β-carotene on acid-activated sepiolite

The adsorption behaviour of acid-activated sepiolite (AAS) for the removal of β-carotene (C₄₀H₅₆) from rapeseed oil was studied as a function of different sepiolite dosages and bleaching temperatures. The β-carotene level in the oil was reduced from 42.3 to 1.60 mg/kg under the optimum conditions of 100 °C bleaching temperature and 1.5% sepiolite dosage. A thermodynamic analysis involving the Langmuir and Freundlich models was applied to identify the mode and extent of β-carotene adsorption. While the negative values of free energy of adsorption (Δ G_ads^o) indicate the spontaneity of adsorption, much larger values of the change in standard entropy (Δ S_ads^o) T compared to the enthalpy of adsorption (Δ H_ads^o) reveal that the reaction is entropically driven and physical in nature. Acid-activated sepiolite exhibits weak acid, low acidity and absolute low zeta potential values, all of which suggest the dominance of Lewis acid centres on the AAS surface. The mechanism of incorporation of β-carotene molecules was examined by calculating its packing area through molecular considerations. It is suggested that β-carotene molecules adsorb both directly on the meso-external surface of AAS and penetrate intra-crystalline channels and tunnels. The pore size distribution of sepiolite confirms this assertion.