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Adsorption and transport of pyrithiobac in soils
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The fate of agrochemicals in soils is governed by various transport, retention, and transformation processes. The herbicide pyrithiobac (sodium 2-chloro-6(4,6 dimethoxypyrimidin-2-ylthio) benzoate), developed by DuPont, has shown excellent promise in controlling several obstinate broadleaf weed species in cotton. However, there is a paucity of information available concerning pyrithiobac activity and behavior in soils. This research was undertaken to investigate the behavior of pyrithiobac on reference adsorbents (Gonzales bentonite, Georgia kaolinite, goethite, and Michigan peat) and four soils (Houston Black c, Hidalgo sl, Orelia scl, and Ships sic) having a wide range of physical and chemical properties. Adsorption isotherms were developed for pyrithiobac adsorption on the reference adsorbents and the soils using the batch-equilibration technique. For the soils, pyrithiobac adsorption ranged from 3.1 to 12.8%, and in the reference adsorbents, from 2.1 to 63.5%. The empirical Freundlich equation adequately described pyrithiobac adsorption on the Michigan peat (C-type isotherm) and Gonzales bentonite (L-type isotherm), but poorly fit the curvilinear (S-type) adsorption isotherms which were observed in the soils and two reference adsorbents (Georgia kaolinite and goethite). However, irrespective of calculation method, the retention of pyrithiobac on the Michigan peat was always greatest, followed in order by Gonzales bentonite, Georgia kaolinite, goethite, and the soils. The affinity of soils for pyrithiobac was low compared to other herbicides based on K and K values. Differences in adsorption capacity of the four soils varied on the types and amounts of inorganic and organic constituents. Stepwise multiple-regression analyses indicated that free iron oxide content accounted for 45% of the variability (p < 0. 000 1) in pyrithiobac adsorption by the soils. Inclusion of organic carbon content, clay content, cation exchange capacity, and pH into the analyses did not significantly improve the correlations (W=0.48). Transport of pyrithiobac was studied on four soils using intact or undisturbed columns. Based on breakthrough-curves and associated parameters, preferential flow through macropores controlled the movement of pyrithiobac, particularly in the more structured soils (Ships, Houston Black, Orelia).
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Matocha, Christopher John (1996). Adsorption and transport of pyrithiobac in soils. Master's thesis, Texas A&M University. Available electronically from
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