Simple approach to carboxyl-rich materials through low-temperature heat treatment of hydrothermal carbon in air

[Display omitted] ► Carboxyl-rich hydrothermal carbon can be obtained by heat-treatment in air. ► Content of carboxyl groups on the heat-treated carbon can reach up to 3.70 mmol/g. ► Much higher sorption capacity for Pb and Cr is observed after heating the matrix. ► Mechanism for newly generating carboxyl groups during heat-treatment is supposed. 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 °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 Pb 2+ and Cd 2+ ions.