The role of mineral and organic components in phenanthrene and dibenzofuran sorption by soil

Improved predictions of sorption of hydrophobic organic compounds (HOCs) in soil require a better knowledge of the relative contribution of inorganic and organic soil constituents to the sorption process. In this paper, sorption of a three-ring polycyclic aromatic hydrocarbon (phenanthrene) and a three-ring heterocyclic-aromatic compound (dibenzofuran) by six agricultural soils, their clay-size fractions, and a series of single, binary, and ternary model sorbents was evaluated to elucidate the relative role of soil mineral and organic components in the retention of these two model HOCs. The sorption coefficients for phenanthrene and dibenzofuran on purified soil organic materials (Kd = 821-9080 litre kg⁻¹) were two orders of magnitude greater than those measured on mineral model sorbents (Kd = 0-114 litre kg⁻¹). This, along with the strong correlation between sorption and the organic C content of the soil clay fractions (r = 0.99, P < 0.01), indicated a primary role of soil organic matter in the retention of both compounds. However, weak relationships between phenanthrene and dibenzofuran sorption coefficients and the organic C content of the bulk soils and variability of Koc values among soils, clay fractions, and model sorbents (1340-21020 litre kg⁻¹ C for phenanthrene and 1685-7620 litre kg⁻¹ C for dibenzofuran) showed that sorption was not predictable exclusively from the organic C content of the materials. Organic matter heterogeneity and domain blockage arising from organic matter-clay interactions and associated pH shifts were identified as the most likely causes of the different organic C-normalized sorption capacities of the soils. A direct contribution from minerals to the sorption of phenanthrene and dibenzofuran by the soils studied was likely to be small. Our results suggested that suitable descriptors for the extent of organic matter-mineral interactions would help to improve current Koc-based sorption predictions and subsequently the assessment of risk associated with the presence of HOCs in soil.