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

Abstract

Black carbons (BCs), including soots, chars, activated carbons, and engineered nanocarbons, have different surface properties, but we do not know to what extent these affect their sorbent properties. To evaluate this for an environmentally ubiquitous form of BC, biomass char, we probed the surface of a well-studied wood char using 14 sorbates exhibiting diverse functional groups and then fit the data with a polyparameter linear free energy relationship (ppLFER) 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 Kd(L/kg) =

[(4.03 ± 0.14) + (-0.15 ± 0.04) log ai)] V + [(-0.28 ± 0.04) log ai)] S + (-5.20 ± 0.21) B

where ai 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 generally observed for activated carbon, the sorbate’s size encouraged sorption from water to the char, while its electron donation/proton acceptance discouraged sorption from water. However, the magnitude and saturation dependence differed significantly from what has been seen for activated carbons, presumably reflecting the unique surface chemistries of these two BC materials and suggesting BC-specific sorption coefficients will yield more accurate assessments of contaminant mobility and bioavailability and evaluation of a site's response to remediation.