The effects of organic matter–mineral interactions and organic matter chemistry on diuron sorption across a diverse range of soils

• Causes of variation in diuron KOC for a diverse set of soils were investigated.
• HF-treatment increased KOC for all but three soils, on average by a factor of 2.4.
• This showed organic matter–mineral interactions reduced KOC of whole soils.
• There was no significant correlation between KOC for the whole and HF-treated soils.
• KOC of HF-treated soils increased with aryl C and decreased with O-alkyl C, alkyl C.

Sorption of non-ionic organic compounds to soil is usually expressed as the carbon-normalized partition coefficient (KOC), because it is assumed that the main factor that influences the amount sorbed is the organic carbon content of the soil. However, KOC can vary by a factor of at least ten across a range of soils. We investigated two potential causes of variation in diuron KOC – organic matter–mineral interactions and organic matter chemistry – for a diverse set of 34 soils from Sri Lanka, representing a wide range of soil types. Treatment with hydrofluoric acid (HF-treatment) was used to concentrate soil organic matter. HF-treatment increased KOC for the majority of soils (average factor 2.4). We attribute this increase to the blocking of organic matter sorption sites in the whole soils by minerals. There was no significant correlation between KOC for the whole soils and KOC for the HF-treated soils, indicating that the importance of organic matter–mineral interactions varied greatly amongst these soils. There was as much variation in KOC across the HF-treated soils as there was across the whole soils, indicating that the nature of soil organic matter is also an important contributor to KOC variability. Organic matter chemistry, determined by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, was correlated with KOC for the HF-treated soils. In particular, KOC increased with the aromatic C content (R = 0.64, p = 1 × 10−6), and decreased with O-alkyl C (R = −0.32, p = 0.03) and alkyl C (R = −0.41, p = 0.004) content.