High-purity amorphous silica originated in rice husks via carboxylic acid leaching process

The environmentally benign, harmless to human, and economically effective process to produce high-purity amorphous SiO2 materials from rice husks has been established by using a carboxylic acid leaching, not the conventional strong acids. TG-DTA measurement and GCMS analysis indicated that the leaching was effective for the hydrolysis of celluloses and hemicelluloses contained in rice husks at 473-873 K, which produced the same results in using the conventional sulfuric acid. In particular, the formation of furfurals and levoglucosans occurred via the hydrolysis at 473 K and 673 K, respectively, when using rice husks leached by citric acid solutions. The metallic impurities could be also removed from the husks via a chelate reaction between carboxyl groups (-COOH) and the metal elements. Concerning the burning conditions of rice husks after the acid leaching, it was necessary to supply a suitable amount of air to completely combust organics; for example, it required air supplement of 50 mL/min or more. High-purity amorphous silica materials with 99 wt% or more were prepared from rice husks by applying the citric acid leaching treatment and burning process at 1073 K in air.