3.4. Element analysis of rice husk samples thermally treated at different temperatures
Petroleum adsorption by thermally treated rice husks is seemingly dependent on their chemical composition. Figs. 5 and 6 summarize the results of microanalyses of TRH700 using SEM/EDAX. In Fig. 5 (a) and (b), the variation of weight percent of four elements (Si, C, Al, K) in the heated rice husks is given for different temperatures. While the weight percentage of silicon and potassium increased with increasing heating temperature, the amount of aluminum and carbon decreased. The results of XRD analyses show that thermally treated rice husk consist mainly of amorphous silica (Fig. 7). Thereby, the element silicon in the thermally treated rice husk most probably existing as silica. In detail, the weight percentage of silicon in rice husk increased from 19.4% at 300 °C to 40.1% at 700 °C, and the carbon content decreased from 32.6% at 300 °C to 9.0% at 600 °C but increased again to 12.3% at 800 °C. This can be explained by the fact that higher temperatures cause the thermal decomposition of organic substances in the rice husks and hence the relative silica content increases [35]. Increasing weight percent of silica positively influences the petroleum sorption capacity of rice husk due to the good adsorbent properties of silica (SiO2). Fig. 6 shows that the principal element in TRH700 is silica.