Separating the effects of organic matter–mineral interactions and organic matter chemistry on the sorption of diuron and phenanthrene

Even though it is well established that soil C content is the primary determinant of the sorption affinity of soils for non-ionic compounds, it is also clear that organic carbon-normalized sorption coefficients (KOC) vary considerably between soils. Two factors that may contribute to KOC variability are variations in organic matter chemistry between soils and interactions between organic matter and soil minerals. Here, we quantify these effects for two non-ionic sorbates—diuron and phenanthrene. The effect of organic matter–mineral interactions were evaluated by comparing KOC for demineralized (HF-treated) soils, with KOC for the corresponding whole soils. For diuron and phenanthrene, average ratios of KOC of the HF-treated soils to KOC of the whole soils were 2.5 and 2.3, respectively, indicating a substantial depression of KOC due to the presence of minerals in the whole soils. The effect of organic matter chemistry was determined by correlating KOC against distributions of C types determined using solid-state 13C NMR spectroscopy. For diuron, KOC was positively correlated with aryl C and negatively correlated with O-alkyl C, for both whole and HF-treated soils, whereas for phenanthrene, these correlations were only present for the HF-treated soils. We suggest that the lack of a clear effect of organic matter chemistry on whole soil KOC for phenanthrene is due to an over-riding influence of organic matter–mineral interactions in this case. This hypothesis is supported by a correlation between the increase in KOC on HF-treatment and the soil clay content for phenanthrene, but not for diuron.