Synthesis of hierarchical porous carbon sphere via crosslinking of tannic acid with Zn2+ for efficient adsorption of methylene blue

•Hierarchical porous carbon spheres prepared via crosslinking of tannic with Zn2+.•Zn2+ plays dual role as crosslinker and pore-forming agent to produce micropores and mesopores.•A maximum adsorption capacity up to 697 mg/g for methyl blue was achieved.•The hierarchical porous structure is beneficial to the adsorption of methylene blue.•The performance of PCS is stable in a wide range of pH and in recycling.