Potential mechanisms of cadmium removal from aqueous solution by Canna indica derived biochar

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%).