Polyparameter linear free energy relationship for wood char-water sorption coefficients of organic sorbates

Black carbons, including soots, chars, activated carbons, and engineered nanocarbons, have different surface properties, but the extent to which these affect their sorbent properties is not known. To evaluate this for an environmentally ubiquitous form of black carbon, biomass char, the surface of a well-studied wood char was probed using 14 sorbates exhibiting diverse functional groups, and the data were fit with a polyparameter linear free energy relationship to assess the importance of the various possible sorbate-char surface interactions. Sorption from water to water-wet char evolved with the sorbate's degree of surface saturation and depended on only a few sorbate parameters: log K(d)L/kg) = [(4.03 ± 0.14) + (-0.15 ± 0.04) log a(i)] V + [(-0.28 ± 0.04) log a(i)] S + (-5.20 ± 0.21) B, where a(i) is the aqueous saturation of the sorbate i, V is McGowan's characteristic volume, S reflects polarity, and B represents the electron-donation basicity. As is generally observed for activated carbon, the sorbate's size encouraged sorption from water to the char, whereas its electron donation and proton acceptance discouraged sorption from water. The magnitude and saturation dependence differed significantly from what has been seen for activated carbons, presumably reflecting the unique surface chemistries of these 2 black carbon materials and suggesting that black carbon-specific sorption coefficients will yield more accurate assessments of contaminant mobility and bioavailability, as well as evaluation of a site's response to remediation.