Sorption of thiazafluron by iron- and humic acid-coated montmorillonite

The intimate association between the different soil constituents may determine the sorptive behavior of soil aggregates. This study was conducted to study the influence of interassociation on the sorption capacity of montmorillonite for the polar uncharged herbicide thiazafluron (1,3-dimethyl-1-(5-trifluormethyl-1,3,4-thiadiazol-2-yl)urea). Calcium Wyoming montmorillonite was coated with iron species and humic macromolecules and sorption of thiazafluron determined and compared with that for the uncoated clay. The presence of precipitated Fe(III)-OH-species on montmorillonite gave rise to an increase in the specific surface area measured by N adsorption (from 18-45 m(2) g(-1)) and to a decrease in the sorption of thiazafluron by the clay (from 156.9-110.8 mmol kg(-1)). Both commercial and soil humic acid also decreased the sorption of thiazafluron 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 associated with the clay. Iron and organic matter removal from the clay surface increased again the sorption of the herbicide thiazafluron by montmorillonite. The results of this work show that mineral and organic coatings on clays alter the nature of the surface exposed for sorption of organics, thus affecting the sorption behavior of colloidal soil particles.