Characterization and performance analysis of nanosilica powders from rice hull ash as moisture sorbents

Pure and ethylene glycol (EG)-impregnated nanosilica powders were synthesized from rice hull ash (RHA) and their sorption properties for water vapor were investigated. The morphology, elemental compositions, surface area and pore structures of nano powders, were elucidated using scanning electron microscopy (SEM) and nitrogen adsorption-desorption. SEM results showed that the particles were agglomerated form with dimensions of 62 to 84 nm and 41 to 65 nm for pure and EG-impregnated nanosilica powders, respectively. Energy dispersive x-ray spectroscopy indicated that silicon was the most abundant element present in both samples. The nitrogen sorption isotherms are categorized as type 4 with H3 hysteresis loops by IUPAC classification. Pure nanosilica powder surface area was 48.11 sq m/g, which was slightly more than that of EG-impregnated nanosilica powder with surface area of 38.65 sq m/g. Pore size distribution curves demonstrate the mesoporous nature of both samples. Water sorption isotherms of nanopowders at 5, 30 and 60 deg C relative humidity ranging from 10.95 to 88.61% were determined using the static gravimetric method. The equilibrium moisture content (dry basis) of pure nanosilica powder ranged from 0.051 to 0.857, 0.024 to 0.565, and 0.016 to 0.283 at 5, 30 and 60 deg C, respectively. The EG-impregnated nano silica powder the EMC ranged from 0.060 to 0.967, 0.027 to 0.388, and 0.026 to 0.209 at 50, 30 and 60 deg C, respectively. The correlation of experimental data to some chosen theoretical models shows that the Guggenheim-Anderson and de Boer (GAB) model is the most adequate to describe moisture sorption isotherms on nanosilica powders for various temperatures and relative humidities studied. The significant result in the moisture sorption characteristics of powders is that both nanosilica powders reveal potential to be utilized as moisture sorbents at air relative humidity levels greater than 70% at all temperatures studied. Since nanosilica is widely used in many researches and with applications in different fields as sorbents and catalyst, these results on water sorption characteristics of nanosilica powders will be beneficial for the processing and end-use applications of the powders.