Carboxyl-rich carbon microspheres (CSp) were synthesized through hydrothermal carbonization of hemicellulose-derived by-products of pulp refining with no other additives. Characterization of the products revealed that CSp rich in oxygen-containing groups with good dispersion and thermal stability can be obtained. Because of the abundant surface oxygen-containing functional groups, the prepared CSp exhibited excellent adsorption capacities for Pb(II) and Cd(II) from aqueous solution, with highest adsorption capacities of 380.1 and 100.8 mg/g for Pb(II) and Cd(II), respectively. Following metal ion adsorption, the content of carboxyl groups decreased from 39.1% to 6.8% for CSp-Pb and to 9.8% for CSp-Cd. Freundlich and Langmuir model isotherms were used to determine the equilibrium uptakes of the two heavy metal ions, revealing that both Pb(II) and Cd(II) were chemically adsorbed as multilayers on CSp. (C) 2014 Elsevier Ltd. All rights reserved.