Silica xerogels from rice hull ash: structure, density and mechanical strength as affected by gelation pH and silica concentration

Abstract

Silica xerogels were produced from rice hull ash (RHA), and the structure, density and mechanical strength of the gels were investigated. Silica was extracted as sodium silicate from RHA using 1M NaOH. This silicate solution was concentrated by volume reduction and used to obtain silica concentrations of 0.04, 0.06, 0.08, 0.10, and 0.12 g cm−3. The pH values of these silicate solutions were adjusted to 9.0, 10.0, 10.5, or 11.0 to produce silica gels. The silica gels produced were then dried at 80 °C for 24 h. X-ray diffraction demonstrated the amorphous nature of silica gels. Diffuse reflectance Fourier Transform Infrared (FTIR) spectroscopy was used to investigate the effect of gelation pH, and silica concentration on the chemical structure of the xerogel and concomitant effect on density and mechanical strength of the xerogel. The FTIR spectra demonstrated that at higher pH of gelation, siloxane bonding was the primary network in the xerogel. As the pH of gelation decreased, the structural silica gel network became the major interaction in the silica xerogel. At each gelation pH, the silica gel network increased with increase in silica concentration. The higher pH led to condensed glassy solids, while higher silica concentration produced highly porous silica xerogel. Hence, gelation pH and silica concentration of gel-forming solution had significant effects on the density and the mechanical strength of xerogels produced from rice hull silica. © 2000 Society of Chemical Industry