3.5. Adsorption properties
Fig. 9 shows the adsorption–desorption isotherms of the silica
obtained. The result resembles a type V isotherm which indicates weak
interaction between adsorbate and absorbent. Hysteresis is observed in
isotherms which caused by the capillary condensation in macropores
(N50 nm) and mesopores (2–50 nm). The pore size distribution of the silica
has high proportions of mesopores which centred around 10 nm. In
addition, considerable amount of macropores in which the size ranged
from 50 to 120nmwas also observed. As a result, the silica has an average
pore size of 15.66 nm. Adsorption data show that silica from cogon grass
has a low BET surface area, i.e., 28.92 m2 g−1 with a corresponding total
pore volume of 0.0983 cm3 g−1. The primary particle size determined in
SAXS is very small, and such low specific surface area indicates that the
obtained bio-silica is highly agglomerated. This result is further confirmed
by the Density Functional Theory (DFT) calculations, which predicted an
average particle size close to 0.102 μm. Thus, using silica obtained from
cogon grass as an adsorption material or dispersion may require pretreatment
steps to de-agglomerate the particles.