SORPTION OF THIAZAFLURON BY IRON-COATED AND HUMIC ACID-COATED MONTMORILLONITE

The intimate association between the different soil constituents may d etermine the sorptive behavior of soil aggregates. This study was cond ucted to study the influence of interassociation on the sorption capac ity of montmorillonite for the polar uncharged herbicide thiazafluron l-1-(5-trifluormethyl-1,3,4-thiadiazol-2-yl)urea). Calcium Wyoming mon tmorillonite was coated with iron species and humic macromolecules and sorption of thiazafluron determined and compared with that for the un coated clay. The presence of precipitated Fe(III)-OH-species on montmo rillonite gave rise to an increase in the specific surface area measur ed by N adsorption (from 18-45 m(2) g(-1)) and to a decrease in the so rption of thiazafluron by the clay (from 156.9-110.8 mmol kg(-1)). Bot h commercial and soil humic acid also decreased the sorption of thiaza fluron by montmorillonite, but the decrease was higher in the case of soil humic acid (from 100.3-77.0 mmol kg(-1)), which was more associat ed with the clay. Iron and organic matter removal from the clay surfac e increased again the sorption of the herbicide thiazafluron by montmo rillonite. The results of this work show that mineral and organic coat ings on clays alter the nature of the surface exposed for sorption of organics, thus affecting the sorption behavior of colloidal soil parti cles.