A facile, effective, and eco-friendly process was developed for the preparation of chemically expanded graphite (CEG) under ambient conditions using natural flake graphite as raw material, potassium permanganate (KMnO4) as an oxidative intercalating agent, and hydrogen peroxide (H2O2) as the reactive species. The results showed that the CEG had an interconnected and highly porous structure, and some oxygen-containing groups were grafted on the graphite layer by the oxidation-intercalation process. The absence of the graphite diffraction peak at 26o in the XRD pattern of expanded graphite (EG) indicates that the intercalation and expansion processes were complete, and most of the starting graphite layers were converted into the graphene sheets. The sulfuric acid concentration was the most effective parameter on the expansion, and the maximum expansion occurred at a sulfuric acid concentration of 77.5%. The other optimum preparation conditions were obtained at 1.5 g of KMnO4 and 30mL of H2O2 30%. Under the optimal condition, the developed room-temperature liquid-phase intercalation and expansion processes led to an expansion volume of up to 250 times. The potential application of the as-prepared CEG in environmental clean-up was evaluated by adsorptive removal of methylene blue (MB) from the aqueous solution. The kinetic studies exhibited that the MB adsorption onto the CEG followed a pseudo-second-order kinetic model. Equilibrium data were fitted well with the Langmuir model with a maximum adsorption capacity of 399.08mg g-1. The findings indicate that the CEG would be potentially applicable in water purification.