Brilliant Blue sorption characteristics of clay-organic aggregate coatings from Bt horizons

•Coatings are essential for sorption behaviour of preferential flow domains.•Domain-specific sorption parameters for improving the transport and mass transfer descriptions.•Diverging effects of the organic matter components of coatings on sorption characteristics. In structured soils, water and reactive solutes can move preferentially through larger inter-aggregate macropores (biopores and cracks) and smaller intra-aggregate pores. Especially clay-organic coating material is of major importance for the exchange of water and solutes between macro- and micropores and the soil matrix by affecting the reactive transport in a yet largely unknown way. The objective of this study was to compare the adsorption and desorption behaviour of clay-organic coatings from samples of till- (T-Bt) and loess- (L-Bt) derived Bt horizons of Haplic Luvisols with those of the soil matrix and a hillslope loam-derived Bsh (HL-Bsh) horizon of a forest Cambisol-Podzol (CM-PZ) without coatings. These coatings are characterized by qm-values of up to 1100 μmol cm−3 for clay contents of up to 330 g kg-1. The values are significantly higher than those of mixed samples without coatings (qm of 180 μmol cm−3 for clay content of 115 g kg-1). The results indicate two different adsorption mechanisms, i) sorption on siloxane surfaces of the alumosilicates (clay minerals) and ii) adsorption controlled by hydrophobic interaction with soil organic material which is possibly attached to clay-organic complexes. The great difference in sorption properties between coatings and matrix suggests that mean values obtained from analyzing mixed samples cannot be used to describe the retardation of dissolved reactive substances on the surfaces of biopores and larger cracks during preferential flow events.