The objective of this study was to investigate the mechanisms of cadmium (Cd) sorption on biochars produced at different temperature (300-600°C) and their quantitative contribution. The sorption isotherms and kinetics of Cd(2+) sorption on biochars were determined and fitted to different models. The Cd(2+) sorption data could be well described by a simple Langmuir model, and the pseudo second order kinetic model best fitted the kinetic data. The maximum sorption capacity (Qm) obtained from the Langmuir model for CIB500 was 188.8mgg(-1), which was greater than that of biochars produced at other temperature. Precipitation with minerals, ion exchange, complexation with surface oxygen-containing functional groups, and coordination with π electrons were the possible mechanisms of Cd(2+) sorption on the biochars. The contribution of each mechanism varied with the pyrolysis temperature. With increasing pyrolysis temperature, the contribution of surface complexation and metal ion exchange decreased from 24.5% and 43.3% to 0.7% and 4.7%, while the contribution of precipitation and Cd(2+)-π interaction significantly increased from 29.7% and 2.5% to 89.5% and 5.1%, respectively. Overall, the precipitation with minerals and metal ion exchange dominated Cd(2+) sorption on the biochars (accounted for 73.0-94.1%), and precipitation with minerals was the primary mechanism of Cd(2+) sorption on the high-temperature biochars (≥500°C) (accounted for 86.1-89.5%).
Keywords: Biochar; Heavy metal; Mechanisms; Pyrolysis temperature; Sorption characteristics.
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