It was found that a large number of oxygen-containing functional groups (OFGs) could be created on the surface of hydrothermal carbon (HTC) by simply heating at lower temperature in air during the course of our preliminary experiments which focused on oxidation pre-treatment of pristine HTC for the purpose of grafting functionalization. Especially carboxyl groups on HTC would increase significantly, from 0.53 to 3.70 mmol/g after heat treatment at 300 degrees C. So, effects of heat-treatment on the OFGs on the carbon microsphere were deeply studied to confirm and explain the findings. Experiments involving different materials (HTC, activated carbon and glucose) were performed under varying conditions (heating temperature and time, in air or in Ar atmosphere). A reaction mechanism for newly generating carboxyl groups on HTC surface during heat-treatment process was supposed based on the results from the sample characterization using Boehm titrations, infrared spectra, X-ray photoelectron spectroscopy, energy dispersive spectrometry and elemental analysis. In addition, the as heat-treated product has excellent sorption capability for Pb2+ and Cd2+ ions. (C) 2011 Elsevier B.V. All rights reserved.