Sorption of organic compounds to two diesel soot black carbons in water evaluated by liquid chromatography and polyparameter linear solvation energy relationship

Abstract

Substantial variability in sorption capacity of black carbon (BC) has been a major challenge for accurate fate and risk assessment of organic pollutants in soils and sediments. 16 model organic sorbates (logK_OW = 0.38-4.21) encompassing diverse chemical functionalities were used to probe the sorption capacity of two diesel soot samples representative of graphitic BC (BC1, specific surface area (SSA) = 87 m²/g) and amorphous, oxygenated BC (BC2; SSA = 3.6 m²/g). The BC-water sorption coefficients (logK_BC) of the model sorbates were determined using reversed-phase liquid chromatography (RP-LC) on soot-filled columns. It was found that mass-based logK_BC's of BC1 (1.64-3.66 L/kg_BC) exceeded those of BC2 (0.68-3.48 L/kg_BC) consistently for all model sorbates. However, area-normalized logK_BC's of BC2 were larger than those of BC1, suggesting that the overall sorption was more favored on the oxygenated sorbent per area basis. Linear solvation energy relationships (LSERs) for sorption onto BC1 and BC2 were found to be logK_BC = (2.49 ± 0.65)E + (-2.71 ± 0.88)S + (1.17 ± 0.46)A + (2.52 ± 0.34)V and logK_BC = (1.12 ± 0.39)E + (-1.68 ± 0.32)S + (-3.70 ± 0.57)B + (4.37 ± 0.38)V + (-1.51 ± 0.22), respectively. The LSERs indicated that sorption onto soot was generally enhanced with increasing non-specific van der Waals and decreasing cavitation cost (i.e., eE, sS, and vV terms). The logK_BC difference between BC1 and BC2, ΔlogK_BC, appeared to be correlated with the H-bonding capacity of the sorbates but not logK_OW. Analysis of literature and experimental logK_BC's revealed that logK_BC and logSSA across different types of BC (i.e., soot, char, charcoal, activated carbon) were linearly correlated for benzene and toluene (r² = 0.88-0.91). This work illustrates the utility of RP-LC in determining the sorption coefficients of high-capacity sorbents and suggests the possibility of a unified sorption model for the continuum of black carbon.

Keywords: Black carbon; Linear solvation energy relationship; Organic contaminants; Reversed-phase liquid chromatography; Sorption coefficient; Surface area.