In the current study, calcination and thermo-chemical methods were applied in treatment of the processed diatomite fraction (<45 mu m), which containing nearly 82.6 wt.% of the raw Egyptian diatomaceous earth. The untreated and modified diatomite fractions were characterized by optical microscopy (OM), X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Thermo-chemical purification produced the highest concentration of diatom frustules (>92% SiO2) without blocking impurities and created -Si-O-Si- active sites. These fractions were tested for Methylene blue (MB) adsorption at different pH solutions (2.0-10.0). The purified diatomite via thermo-chemical treatment (PD) gave the greatest adsorption capacity for MB compared to the untreated (UD) and calcinated (CUD) diatomite fractions. Effects of experimental parameters such as MB concentration (60-200 mg L-1), contact time (5-480 min), adsorbent mass (50-250 mg) and temperature (30-55 degrees C) on MB uptake were investigated. Linear and non-linear forms of Langmuir, Freundlich and Dubinin-Radushkevich (D-R) models indicated that Langmuir model with a maximum adsorption capacity(q(max) = 105.03 mg g(-1)) fitted well the adsorption data. The chemical nature of MB uptake was revealed by the values of mean free energy (E = 8.655 kJ/mol)and correlation coefficient of the pseudo-second-order model (R-2 = 0.9997). The calculated thermodynamic parameters (Delta H-0, Delta G(0) and Delta S-0) indicated that the removal of MB is spontaneous and endothermic. (C) 2018 Elsevier Inc. All rights reserved.