Influence of pre-treatment processes on the purity and characteristics of silica extracted from rice husk.

Rice husk being the major agro-waste could serve as the renewable source of silica. This silica could be a replacement for industrial and pharmaceutical applications. Industrial grade silica require purity ranging from 80 to 100% necessitating the elimination of organic and metallic impurities from biomass. This study compares the ability of pre-treatment process in improving the purity of silica. Silica with difference in purity is extracted by employing simple (conventional alkaline treatment—CAT) and rigorous (soaking/acid/alkaline—SAA) pretreatment on husk. The alkali earth metal removal percentage are 41.71%, 81.69%, and 40.37% for Na, K, and Mg respectively by adopting SAA method. In addition, the significance of impurity elimination is reflected in the textural properties where the SAA silica exhibited an 80% increase in BET surface area compared to CAT silica. SAA silica was purely amorphous whereas CAT silica showed crystalline phase in XRD analysis. Ultrasonication effect on size reduction was more pronounced. The binding nature of elements from XPS analysis indicated the presence of silica as silicon oxide and silicon dioxide. The elemental composition of SAA silica revealed drastic decrease in the contents of Fe₂O₃, K₂O, and Al₂O₃ compared to CAT silica. The difference in silica arises from the soaking process imparting hydrophilic nature to the husk increasing the hydroxyl group on its surface. The interaction between the hydroxyl group and the acid during pre-treatment aided in impurity elimination. This suggests the SAA pretreatment as suitable process for efficient extraction of silica through impurity elimination. [ABSTRACT FROM AUTHOR]