This chapter is intended for a batch adsorption study for methylene blue (MB) dye using silica gel prepared from different fly ashes. Silica gel was prepared from fly ashes, which include coal fly ash (CFA), bagasse fly ash (BFA), palm oil mill fly ash (POMFA), wheat straw fly ash (WSFA), and rice straw fly ash (RSFA). These fly ashes and their respective silica underwent an X-ray fluorescence spectrometer (XRF) to evaluate the percent silica contained in each sample, which revealed a good recovery of silica gel. The mechanism of adsorption was evaluated by four adsorption models, i.e., Freundlich, Langmuir, Dubinin–Radushkevich, and Temkin isotherm. The kinetic models, i.e., pseudo-first-, pseudo-second-order, intra-particle, and Elovich equation, are utilized to study the rate of the adsorption process. Maximum Langmuir adsorption capacity of MB was achieved up to 1512.2, 1452.2, 1487.2, 1502.3, and 1482.9 mg/g using silica gel derived from CFA, BFA, POMFA, WSFA, and RSFA, respectively. Adsorption of MB follows the Freundlich isotherm and pseudo-first-order, derived from values of the correlation coefficient. The thermodynamic parameters, i.e., free energy of adsorption (∆G°), enthalpy change (∆H°), and entropy change (∆S°), were accessed. A comparison of the adsorption capacity of methylene blue with various fly ashes from available literature and their respective silica is also represented.