title: Metal interactions at the biochar-water interface: energetics and structure-sorption relationships elucidated by flow adsorption microcalorimetry.
abstract: 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 Cd(2+)) 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 (ΔH(ads)) of -4 kJ mol(-1) to -8 kJ mol(-1). By comparison, although ion exchange could not be completely ruled out, our data pointed to Cd(2+) (a soft Lewis acid) sorption occurring predominantly via two distinct cation-π bonding mechanisms, each with ΔH(ads) of +17 kJ mol(-1). The first, evident in low charge-low carbonized biochars, suggested Cd(2+)-π bonding to soft ligands such as -C ═ O; while the second, evident in low charge-highly carbonized biochars, pointed to Cd(2+)-π bonding with electron-rich domains on aromatic structures. Quantitative contributions of these mechanisms to Cd(2+) sorption 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.
authors.affiliation: Water Management and Hydrologic Sciences, Texas A & M University, College Station, Texas 77843, United States. Omar.Harvey@pnnl.gov
authors.forename: Omar R
authors.initials: OR
authors.lastname: Harvey
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Herbert
authors.forename: Roy D
authors.initials: RD
authors.lastname: Rhue
authors.forename: Li-Jung
authors.initials: LJ
authors.lastname: Kuo
mesh_terms: D000327:Adsorption; D002104:Cadmium / Q000737:chemistry*; D002151:Calorimetry / Q000379:methods; D002606:Charcoal / Q000737:chemistry*; D011188:Potassium / Q000737:chemistry*; D013329:Structure-Activity Relationship; D014867:Water / Q000737:chemistry*
chemical_list: C540010:biochar; D002104:Cadmium; D014867:Water; D002606:Charcoal; D011188:Potassium