Modification of fly ash using acids and alkali by hydrothermal method and its application as adsorbents material for phosphate adsorption in aquatic system

In this research, fly ash was modified with acids, such as HCl, H2SO4 and mixture of both and alkali with hydrothermal method. XRF analysis showed a decrease of CaO and Fe2O3 contents in HMFA, SMFA and HSMFA while AMFAWH has higher CaO content and produced the new zeolite phase (hydroxy-sodalite) in 2 θ = 24.232 from XRD test. FTIR analysis showed that the fly ash gave absorption band of O-H group, H-O-H bend group, and Si-O stretching group. SEM images showed that acid-modified fly ash indicated a deposition of gehlenite crystals while AMFAWH showed flat rectangular shaped small particle known as hydroxy-sodalite. The BET method analysis indicated that all adsorbents were classified as mesopores. The experimental data showed that adsorption efficiency and capacity of fly ash was improved significantly after modification. The unmodified fly ash (UFA) has the highest phosphate adsorption efficiency and capacity in 0.16 g was 60.07 % and 0.485 mg P-PO4/g while modified fly ash (HSMFA) was 85.62 % and 0.705 mg P-PO4/g. The optimum contact time were obtained at 240 min for UFA, HMFA, SMFA and 480 min for HSMFA and AMFAWH. The optimum of pH were obtained at pH 5 and pH 7 for the others. The fifth adsorbents follow the Freundlich isotherm and pseudo-second-order kinetic model in phosphate adsorption.