Comparison of adsorption properties of commercial silica and rice husk ash (RHA) silica: A study by NIR spectroscopy

Adsorption properties of Silica gel (commercial silica), rice husk ash (RHA) silica, and their hydrothermally treated products towards water molecules were studied in this work. RHA silica was prepared by heating rice husk to 650 degrees C for 4 h after pretreatment with hydrochloric acid. Portions of the samples of silica were evacuated at 200. and then allowed to adsorb water molecules from the surrounding air at either 42 or 50% humidity. The near infrared spectra of these samples were measured at different time intervals during their adsorption of water molecules. The evolved near infrared spectra were analyzed using second derivative techniques. Adsorption properties of these samples were further studied by gravimetric analysis. Surface functionalities and adsorption evolution ofwatermolecules on silica samples studied showed that the adsorption of watermolecules on commercial silica gel sample and RHA silica Samples takes place readily with hydrogen-bonded silanol groups on the surfaces of the samples. This adsorption arising from the OH stretching and bending vibrations of these water molecules gives rise to a band at 5,270 cm(-1) in the near infrared region. The adsorbed water molecules then continue at the free silanol groups which is indicated by the absorption around 5,314 cm(-1). The results clearly showthat the silica made from rice husk adsorb water in the same manner as the commercial silica, but the quantity of water molecules adsorbed is less compared to commercial silica used in this experiment. However, the study clearly shows that the rice husk silica can replace the commercial silica as a substitute for desiccant purposes in industry.