The capacity of activated kaolins to remove colour pigments from rice bran oil: the effects of acid concentration and pre-heating prior to activation

This study focuses on the effects of both thermal treatment (between 80 and 700 degrees C) and chemical activation (concentration of sulfuric acid between 0.3 and 2 M) of natural Ranong kaolins (ground or not) from Thailand to remove the undesirable colour of rice bran oil. The mineralogical, physical and physicochemical properties of the initial and activated kaolins are discussed in relation with the bleaching effectiveness of the activated sample investigated. Generally, the greater the temperature used before the activation step and the concentration of sulfuric acid used during activation, the greater the structural degradation of the kaolinite; Al is removed from the octahedral sheet of kaolinite and amorphous SiO2 dominates the samples. The measured maximum bleaching capacity is not necessarily obtained when using the activated kaolin having the highest specific surface area and pore volume; rather, the bleaching capacity is dependent on both alumina contents and proportion of kaolinite in the samples. Indeed, the partial preservation of the kaolinite structure is crucial to obtain a good bleaching capacity of kaolin in relation to the preservation of the aluminol sites which are likely to be involved in the adsorption of unsaturated molecules present in the rice bran oil. Moreover, as previously demonstrated, a partial leaching of Al from octahedral sheets of kaolin is also an important factor in order to obtain good bleaching capacities. Finally, the optimal preheating temperature and concentration of sulfuric acid which permit the best bleaching capacity of kaolin are reported.