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Abstract
Different types of silica were prepared from rice hulls under various treatments. The surface and textural properties of the silicas obtained were elucidated using the different techniques as elemental analysis, moisture content, XRD, FT-IR, TGA/DTA analysis, zeta potential, surface area and Lewis acid and Lewis base sites of silica surface.
The silica obtained was tested towards the adsorption of Cu2+ and Mg2+. Silica S4 showed higher uptake towards Mg2+ while silica S5 showed better adsorption for Cu2+ than Mg2+. The free energy of adsorption of Cu2+ and Mg2+ on silica S5 were calculated to be -25.83 and -22.63 KJ/mol, respectively. The negative value of ?G0 indicates a spontaneous nature of the adsorption process. The values of ?G0 obtained for the adsorption of Mg2+ and Cu2+ ions on silica is comparable to ?G0 for fatty acids adsorption on silica which indicates that metal ions and fatty acid interact with silica via hydrogen bonding. The higher negative value of ?G0 of Cu2+ relative to that of Mg2+ indicates stronger binding of Cu2+ to silica surface compared to Mg2+ analogue.
The values of binding constant (Kads) of the adsorption of Mg2+ and oleic acid on silica S4 revealed that Mg2+ ions bind strongly with silica compared to oleic acid. The values of ?G0 for the adsorption of Mg2+ and oleic acid on S4 indicate that the adsorption of Mg2+ on silica S4 proceeds more spontaneously compared to oleic acid on the same silica.