Optimizing silica extraction from coal fly ash: removal of methylene blue, adsorption process analysis and modeling Thapelo Manyepedza 1 , Emmanuel Gaolefufa 1 , James Darkwa 4 , Isaac N.Beas 2,3 , M. Tlhabologo Kabomo 1 1 Department of Chemistry, Botswana University, Gaborone, Botswana, 2 Department of Natural Resources and Materials, Botswana Institute for Technology Research and Innovation, Gaborone, Botswana, 3 Department of Chemical Engineering, University of South Africa, Johannesburg, South Africa, 4 Department of Chemistry, University of Johannesburg, South Africa This study successfully synthesized high-purity mesoporous silica from Botswana coal fly ash in sequential steps, pre-treatment of coal fly ash and the synthesis of silica. In the pre-treatment, coal fly ash was mixed with Na2CO3 and then calcined in the muffle furnace. Then it was cooled to room temperature, washed, and dried overnight. Acid refluxing method produced silica from the Na2CO3-CFA thermally activated coal fly ash. The residues were characterised by XRF, XRD, FTIR and BET. The silica content from the acid reflux was found to be 99.1%. The synthesized silica (BET surface area of 793.505 m2/g, pore-size 3.9 nm) FTIR spectra mapped very well with the Sigma Aldrich mesoporous silica (300 – 400 m2/g and pore-size 4 nm). Using response surface methodology, the effects of independent variables such as initial MB concentration (50-250 mg L—1), initial pH of the solution (3-11), adsorbent dosage (0.1-6.1 g L—1), and contact time (10-90 min) on MB adsorption were optimized. When the pH, MB, and adsorbent dosage were 7, 250 mg L—1 and 3.1 g L—1, respectively, the highest MB removal efficiency was 99%. The Langmuir and Freundlich isotherm models were used to test the experimental data. The obtained data were fully fitted with the Langmuir model (R2 = 0.9881, Qmax = 32.76 mg/g, KL = 0.059 L/mg), indicating monolayer adsorption and strong adsorbate adsorbent interaction. The pseudo-second-order model with R2 of 0.99 was used to predict the adsorption kinetics. This synthesized silica can be used widely for water and wastewater treatment as a low-cost and environmentally friendly adsorbent. References 1. el Alouani, M., Alehyen, S., el Achouri, M. and Taibi, M., 2018. R emoval of Cationic Dye – Methylene Blue - from Aque ous Solution by Adsorption on Fly Ash - based Geopolymer. Journal of Materials and Environmental Sciences, 9(1), pp.32–46. https://doi.org/10.26872/jmes.2018.9.1.5. 2. Amin, N., Gul, S., Sultana, S. and Alam, S., 2021. Preparation and Characterization of Mesoporous Silica from Bagasse Bottom Ash from the Sugar Industry. Crystals, 11(8), p.938. https://doi.org/10.3390/cryst11080938. 3. An, D., Guo, Y., Zhu, Y. and Wang, Z., 2010. A green route to preparation of silica powders with rice husk ash and waste gas. Chemical Engineering Journal, 162(2), pp.509–514. https://doi.org/10.1016/j.cej.2010.05.052. 4. Botswana Power Corporation, 2011. The Preparatory Survey on the Project for Morupule “A” Power Station Rehabilitation and Pollution Abatement in Republic of Botswana. 5. Cao, J.S., Lin, J.X., Fang, F., Zhang, M.T. and Hu, Z.R., 2014. A new absorbent by modifying walnut shell for the removal of anionic dye: Kinetic and thermodynamic studies. Bioresource Technology, 163, pp.199–205. https://doi.org/10.1016/J. BIORTECH.2014.04.046.
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