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

Abstract

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 ( K OC) vary considerably between soils. Two factors that may contribute to K OC 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 K OC for demineralized (HF-treated) soils, with K OC for the corresponding whole soils. For diuron and phenanthrene, average ratios of K OC of the HF-treated soils to K OC of the whole soils were 2.5 and 2.3, respectively, indicating a substantial depression of K OC due to the presence of minerals in the whole soils. The effect of organic matter chemistry was determined by correlating K OC against distributions of C types determined using solid-state 13C NMR spectroscopy. For diuron, K OC 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 K OC 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 K OC on HF-treatment and the soil clay content for phenanthrene, but not for diuron.