Metal Sequestration Is Influenced By Biochar Properties and Changes in pH

Abstract:

Addition of biochars to impounded metal mine waste may improve the physical and chemical properties, and the biological activity and diversity of these contaminated sites. However, understanding how biochar addition influences metal(loid) mobility is necessary. Here, the sequestration of 5 metal(loid)s in suspensions of biochars adjusted to pH 4.5 or 6.0 were characterized. Solutions of the oxyanion As and the cations Ni, Cu, Zn, Cd were added at a single rate of 3 mmol kg^-1 biochar. Six biochars were obtained by large-scale pyrolysis of softwood, hardwood, grass, and poultry litter under different conditions. Biochars were characterized using N₂-BET (surface area), proximate analysis, elemental analysis, X-ray diffraction and Fourier transform infrared spectroscopy. In this study, metal(loid) sequestration depended on the suspension pH, metal(loid), and biochar characteristics. In most cases, a significantly (α=0.05) greater proportion of cation was sequestered in biochar suspensions adjusted to pH 6.0 compared to pH 4.5. In contrast, pH had no significant effect on the sequestration of As. The magnitude of the increase in sequestration at pH 6.0 compared to pH 4.5 can be attributed to specific biochar characteristics. Enhancement of Ni, Zn, and Cd sequestration at pH 6.0 compared to pH 4.5 was correlated (R²>0.50) to inorganic C content and neutralization potential of biochars. Furthermore, increased Cu sequestration at pH 6.0 compared to pH 4.5 was correlated (R²>0.50) to % Fixed matter and organic C content of biochars. This data suggests that at pH 6.0, sequestration of Ni, Zn, and Cd as carbonates and interactions between organic C and Cu were more favourable compared to pH 4.5. For Ni, Cu, and Cd, differences in sequestration at the two pH were also related (R²>0.50) to the relative distribution of functional groups. This study emphasizes the need for a more holistic understanding of how biochar properties influence metal sequestration.