Green Chemistry Preparation and Characterization of Rice Husk Derived Silica Gel in Kenya

Rice is a grass seed from Oryza glaberrima species also known as the African rice. In Kenya, rice is mostly grown in Central (Mwea) and Nyanza (Ahero, West Kano, Migori and Kuria) areas. Milling rice produces rice husks as by-products which can be sources of valuable chemical products (silica gel, sodium silicate). In trials to produce silica gel from rice husks, rice husks were charred in a combustion chamber (30 min) then ashed in a Muffle furnace (Advantec KL-420) at different temperatures. The ashes were then leached with distilled water/acids to remove metal oxides. Sixty grams (60 g) of the leached RHA (Rice Husk Ash) was mixed with 300 mL of 3 M NaOH solution in a Pyrex 500 mL beaker and boiled at 100 °C (1 h). The silica gel samples were characterized using several methods. Elemental analysis was done using TXRF (Total X-Ray Fluorescence), while FTIR (Fourier-Transform Infrared Spectroscopy) was used to obtain an infrared spectrum of absorption of the silica sample. Results of the analysis conform to local and international quality standards. The rice husks had an average moisture content of 7.07% and 1.00-2.00 mm diameter. And 1.74% of the rice husk had pore sizes of about 0.710 mm. The average ash content was 22.65%. At 600 °C ,leaching with water yielded 98.2% silica compared with 99.1% (H₂SO₄) and 96.9% (HCl). At 500 °C, leaching with HCl/H₂SO₄ causes a decrease. At 500 °C, the availability of SiO₂ is more for water leached samples. At 400 °C, water leaching gave 98.49% silica while HCl leaching was 97.85% silica and H₂SO₄ was 99.41%. Silica is a precursor of silica gel. Statistical analyses imply water leaching RHA instead of acid leaching at 500 °C will produce a significant amount of silica gel. The open burn samples produced equal or better SiO₂ (silica gel precursor) yields compared with the incineration samples. FTIR analysis of the silica gel sample compared well with adsorption peaks of silica gel in literature. XRD (X-Ray Diffraction) analysis produced a pattern consistent with other XRD patterns of silica gel published by other researchers.