Structural behavior of rice husk silica in pressurized hot-water treatment processes

This paper deals with X-ray fluorescence (XRF), X-ray diffraction (XRD), and Si-29 nuclear magnetic resonance (29Si NMR) measurements of rice husk silica treated under hydrothermal conditions in autoclave and steam-explosion processes. An understanding of the structural behavior of rice husk silica provides insight into its prospective opportunities for industrial applications. The results show that the hydrothermal reaction hardly affected the local silicate structures within the temperature range tested, < 240 degreesC. Regarding the Si local structure, not only Q4 [*Si(OSi)(4)] but also Q2 [(OH)(2)*Si(OSi)(2)] and Q3 [(OH)*Si(OSi)(3)] units were observed in the hot-water-treatment residues, whereas it is known that rice husk ash consists of only Q4 units. This suggests a possible utilization of the hot-water-treatment residue different from that of rice husk ash. Incidentally, some portion of the silica dissolved in the aqueous phase, affecting the apparent XRD crystallinity but sustaining amorphous structures. The metallic impurity level of the solid phase was remarkably lowered because of dissolution of the coexisting metals.