Preparation of sludge biochar rich in carboxyl/hydroxyl groups by quenching process and its excellent adsorption performance for Cr(VI)

A facile pyrolysis-quenching-reroasting process was developed to prepare a sludge-based biochar adsorbent, and its adsorption performance for Cr(VI) ions was investigated. The unquenched biochar (U-BC) and quenched biochar (Q-BC) were systematically compared and characterized. Fourier transform infrared spectroscopy (FTIR) results showed that more carbon and oxygen functional groups such as -COOH and -OH were formed on the surface of Q-BC. These functional groups could be used as active sites during the adsorption process and help to improve the adsorption performance of the material. The results of Brunauer-Emmett-Teller (BET) analysis showed that the specific surface area of U-BC biochar was 523.36 m2/g, while the specific surface area of Q-BC biochar after quenching treatment increased to 785.3 m2/g. The adsorption performance of Q-BC biochar was studied, and the effects of pH, contact time and temperature on the adsorption performance of the material were explored. The pseudo-second-order model and Langmuir isotherm model indicated that the removal of Cr(VI) by Q-BC biochar material was a chemical adsorption-based adsorption process. At a temperature of 298 K and a pH of 1, the maximum Cr(VI) adsorption capacity of the quenched Q-BC biochar is as high as 291.54 mg/g, which was much higher than the maximum adsorption capacity of U-BC biochar (91.46 mg/g). This pyrolysis-quenching-reroasting process to prepare modified biochar provides a new, economical and effective way for the preparation of high-performance adsorption materials from municipal sludge.