Graded synthesis of high performance adsorbent from coal gasification fine slag and its application in the removal of methyl blue

•Hierarchical activated carbon and mesoporous silica microspheres were prepared by CGFS.•Both porous materials were surface-functionalized for effective remove MB.•The adsorption capacities of T-AC4-900 and A-MSM15 reached 1708.01 and 1249.46 mg/g.•The adsorption of MB by T-AC4-900 is coexistence of various adsorption mechanisms.•Electrostatic interactions play a dominant role in the adsorption process of A-MSM15.