High mesopore area obtained by improved chemical activation with ZnCl 2. Hydrothermal treatment of biomass with H 2 O 2 enhanced the chemical activation. Up to 100% increase in mesopore area is achieved by employing H 2 O 2 pre-treatment. OFG in biomass/precursor and mesopore area in the carbon are strongly correlated. High dye uptake capacity of 714 mg/g is achieved. g r a p h i c a l a b s t r a c t a b s t r a c t High-surface area mesoporous activated carbons were prepared from biomass by incorporating H 2 O 2 as an oxidizing agent during hydrothermal pre-treatment of the raw material (coconut shell). Use of H 2 O 2 led to enhanced formation of OFGs in the hydrochar precursor and resulted in activated carbons possessing high mesopore and BET surface areas. A strong interdependence is observed between the extent of formation of oxygenated functional groups (OFGs) in the hydrochars obtained from the hydrothermal pre-treatment and mesopore area in corresponding carbons. The mesoporous carbons showed high adsorption capacity for Rhodamine B up to 714 mg/g.