Sorption of two polar herbicides in soils and soil clays suspensions

Abstract

Adsorption of the polar herbicides thiazafluron (1,3-dimethyl-1-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)urea) and metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one) in the aqueous suspension of five soils of southern Spain, their respective clay fractions (with diverse organic carbon content and clay mineralogy) and model pure clay minerals has been monitored as an integrated study to assess the role of the diverse colloidal soil components and their solid/solution ratio, as relevant to the transport of contaminants by particulate matter in water. Adsorption isotherms obtained were analysed and fitted to the logarithmic form of the Freundlich equation and adsorption coefficients K(f) calculated. Thiazafluron adsorbs on soils, soil clays and model mineral sorbents to a higher extent than the herbicide metamitron due to their different molecular structure. The sorption of both herbicides in clay fractions increases with decreasing solid/solution ratio. The highest K(f) value at high solid/solution ratio for both herbicides is found in a saline soil with its clay fraction predominantly composed of an altered illite mineral which behaves as a montmorillonite. Thiazafluron and metamitron also show the highest adsorption capacity (at low solid/solution ratio) on a predominantly montmorillonitic clay fraction of low cation exchange capacity (CEC), whereas low adsorption is found on a montmorillonitic clay fraction of high CEC. The negative influence of the clay CEC is confirmed in adsorption studies on pure clay minerals suspensions. The sorption of both herbicides by soil clays after removing organic matter (OM), shows that contribution of the colloidal OM is very low for thiazafluon, although rather important for metamitron. The influence of the different nature of the OM associated to the clay fractions of diverse soils is suggested. The mineral components of the soil clays, especially expandable layer silicates such as montmorillonite and a type of altered illite, are revealed to be responsible for the adsorption and hence the transport of these polar herbicides by waters in contact with soils or fine-size soil separates. Not only the relative amounts of the organic and inorganic components are important, but also the surface properties and the accessibility of the functional active groups of the herbicide molecule to those surfaces.