Influence of pyrolysis temperature on sludge biochar: the ecological risk assessment of heavy metals and the adsorption of Cd(II)

Pyrolysis of sludge to biochar can not only reduce the sludge volume, toxic organic compound, and pathogens, but also be applied as effective adsorbents. However, the immobilization of heavy metals in the sludge and the properties of the biochar greatly rely on the pyrolysis temperature. In this paper, municipal sludge biochar (SBC) was prepared from 400 to 1000 °C. Pyrolysis immobilized heavy metals in sludge and the potential ecological risk of heavy metals significantly decreased to low level at temperature above 500 °C. At 700 °C, the adsorption capacity of Cd(II) reached a maximum (120.24 mg·g −1 ). The Cd(II) adsorption fitted the Pseudo-second-order model, indicating the existence of chemical adsorption. The adsorption capacity increased along with the initial pH and slowed down after pH reached 5.5. The existence of coexisting cations (Ca 2+ and Na + ) and anions (SO 4 2− and NO 3 − ) displayed different degree of inhibitory action on Cd(II) adsorption. The SEM, XRD, FTIR, and XPS analysis of sludge biochar before and after adsorption revealed that there were CdCO 3 , CdSO 4 , Cd 2 SiO 4 , Cd 3 (PO 4 ) 2 , and Cd 9 (PO 4 ) 6 appearing on the surface of sludge biochar, suggesting that the adsorption of Cd(II) by SBC included co-precipitation, ion exchange, coordination with π electrons, and complexation. It was confirmed that different properties formed by pyrolysis temperature made a difference in adsorption mechanism of sludge biochar.