Metal Interactions at the Biochar-Water Interface: Energetics and Structure-Sorption Relationships Elucidated by Flow Adsorption Microcalorimetry

摘要

Plant-derived biochars exhibit large physicochemical heterogeneity due to variations in biomass chemistry and combustion conditions. However, the influence of biochar heterogeneity on biochar-metal interaction mechanisms has not been systematically described. We used flow adsorption microcalorimetry to study structure-sorption relationships between twelve plant-derived biochars and two metals (K+ and Cd2+) of different Lewis acidity. Irrespective of the biochar structure, sorption of K+ (a hard Lewis acid) occurred predominantly on deprotonated functional groups via ion exchange with molar heats of adsorption (AH,^) of -4 kj moPl to -8 kj moP1. By comparison, although ion exchange could not be completely ruled out, our data pointed to Cd (a soft Lewis acid) sorption occurring predominantly via two distinct cation-^: bonding mechanisms, each with AHads of +17 kj moP1. The first, evident in low charge-low carbonized biochars, suggested Cd2+-JT bonding to soft ligands such as -C=O; while the second, evident in low charge-highly carbonized biochars, pointed to Cd +-Jt bonding with electron-rich domains on aromatic structures. Quantitative contributions of these mechanisms to Cd2+ sorp- tion can exceed 3 times that expected for ion exchange and therefore could have significant implications for the biogeochemical cycling of metals in fire-impacted or biochar-amended systems.