Effect of Alumina Content and Surface Area of Acid-Activated Kaolin on Bleaching of Rice Bran Oil

This study investigated the effect of kaolin acid activation on alumina losses, surface area changes and oil bleaching performance. Ground kaolin was treated with hydrochloric or citric acid, and bleaching tests were performed on rice bran oil. The adsorption studies showed that the optimal bleaching of similar to 83 or similar to 81 % were achieved by activation with 0.5 M hydrochloric or citric acid, respectively, whereas bleaching with a commercial clay was similar to 82 %. The highest bleaching value was not associated with the maximum clay surface area or porosity. X-ray fluorescence showed that alumina contents of 31-34 % were suggestive for optimum bleaching depending on the different acid used. Treating ground kaolin with the same hydrochloric acid strength by varying the acid concentration and clay/acid ratio also confirmed that the best Al3+ content was similar to 32 %, and a value lower than one indicated the extensively destruction of kaolinite proportions leading to a decrease in its bleaching capacity, even though it had the maximum surface area. The decrease in capacity was due to the reduction of alumina content, and the parallel formation of high amorphous silica was favorable for the adsorption of anionic pigments, such as chlorophyll-a.