Sorption of Cd(II) and Ni(II) on biochars produced in nitrogen and air-limitation environments with various pyrolysis temperatures: Comparison in mechanism and performance

N2 and air-limitation environments are two common pyrolysis atmospheres to produce biochars. However, their influence on the sorption mechanism and performance of biochar toward heavy metal ions (HMs) remains unknown. Hence, biochars produced in N2 and air-limitation environments at 300–750 °C were applied to sorb Cd(II) and Ni(II) in this study. The results show that the maximum adsorption quantities (Qm) of Cd(II) and Ni(II) on biochars of air-limitation pyrolysis were 1.08–1.84 folds and 1.66–3.11 folds, respectively, of those on biochars of N2 pyrolysis. The Qm of both studied HMs on biochars produced in the two studied atmospheres both generally improved with increasing pyrolysis temperature due to the increased contents of inorganic elements such as P, S, and Si at higher temperatures. Besides, the Qm of both studied HMs on biochars of N2 pyrolysis improved with the increasing graphitic C content. The results indicate the sorption mechanism of HMs on biochars of air-limitation pyrolysis was dominated by the mineral anions complexation, while the sorption mechanism of HMs on biochars of N2 pyrolysis was controlled by both the mineral anions complexation and cation-π interaction. Additionally, the protonated oxygen-containing groups (e.g., hydroxyl groups) linked to C fraction possibly formed hydrogen-bonds with mineral anions and thus suppressed the sorption of HMs on biochars. A pyrolysis condition of the maximum adsorption efficiency for HMs is the N2 pyrolysis at 750 °C. The results of this study are beneficial for selecting suitable pyrolysis methods to produce biochars and raising their efficiency in HMs sorption.