SORPTION OF 2 POLAR HERBICIDES IN SOILS AND SOIL CLAYS SUSPENSIONS

Adsorption of the polar herbicides thiazafluron ethyl-1-(5-trifluorome thyl-1,3,4-diazol-2-yl)urea) and metamitron 4,5-dihydro-3-methyl-6-phe nyl-1,2,4-triazin-5-one) in the aqueous suspension of five soils of so uthern Spain, their respective clay fractions (with diverse organic ca rbon content and clay mineralogy) and model pure clay minerals has bee n monitored as an integrated study to assess the role of the diverse c olloidal soil components and their solid/solution ratio, as relevant t o the transport of contaminants by particulate matter in water. Adsorp tion isotherms obtained were analysed and fitted to the logarithmic fo rm of the Freundlich equation and adsorption coefficients Kr calculate d. Thiazafluron adsorbs on soils, soil clays and model mineral sorbent s to a higher extent than the herbicide metamitron due to their differ ent molecular structure. The sorption of both herbicides in clay fract ions increases with decreasing solid/solution ratio. The highest Kr va lue al high solid/solution ratio for both herbicides is found in a sal ine soil with its clay fraction predominantly composed of an altered i llite mineral which behaves as a montmorillonite. Thiazafluron and met amitron also show the highest adsorption capacity (at low solid/soluti on ratio) on a predominantly montmorillonitic clay fraction of low cat ion exchange capacity (CEC), whereas low adsorption is found on a mont morillonitic clay fraction of high CEC. The negative influence of the clay CEC is confirmed in adsorption studies on pure clay minerals susp ensions. The sorption of both herbicides by soil clays after removing organic matter (OM), shows that contribution of the colloidal OM is ve ry low for thiazafluon, although rather important for metamitron. The influence of the different nature of the OM associated to the clay fra ctions of diverse soils is suggested. The mineral components of the so il clays, especially expandable layer silicates such as montmorillonit e and a type of altered illite, are revealed to be responsible for the adsorption and hence the transport of these polar herbicides by water s in contact with soils or fine-size soil separates. Not only the rela tive amounts of the organic and inorganic components are important, bu t also the surface properties and the accessibility of the functional active groups of the herbicide molecule to those surfaces. (C) 1997 El sevier Science Ltd.