RETENTION OF METAMITRON BY MODEL AND NATURAL PARTICULATE MATTER

Adsorption isotherms of metamitron on model soil colloidal components: kaolinite, illite, montmorillonite, iron oxide and humic acid, and th eir binary associations were obtained using a batch equilibration proc edure. Sorption parameters, K-f and n(p) were calculated by fitting th e sorption data to the Freundlich equation and results obtained for bi nary associations were compared with those obtained for the individual model components. The sorption efficiency of the humic acids and thei r binary associations was measured as Koc. The adsorption behaviour of the < 2 mu m fraction of two soils from Southern Spain was also studi ed as natural particulate matter. Montmorillonite and humic acids were found to be the most important components responsible for metamitron retention by the model. adsorbents studied. On the contrary, metamitro n showed little interaction with kaolinite, illite or iron oxide. Thes e individual adsorption behaviours were reproduced in the montmorillon ite-iron oxide-humic acid binary systems, but with differences suggest ing changes on the surface properties upon association. Differences in Koc values of isolated humic acids and their associations indicate th at the interaction transforms the humic acid surfaces and suggest diff erent types of bonding between colloids and metamitron. The results ob tained with model adsorbents and their associations were in agreement with the highest adsorption of metamitron found for the natural clay f raction of two soils which displayed the largest adsorption in that wi th the highest content in montmorillonite and organic carbon. The impo rtance of organic matter and montmorillonite in metamitron adsorption by colloidal components was also shown by the decrease in K-f and the increase in Koc observed after removal of organic matter from the soil clay fraction with the highest organic carbon content.