Sorption and kinetics of imazaquin in surface soils
Abstract
Recent trends toward conservation tillage as a soil saving method have forced an increased reliance on new types of chemicals for weed control. In order to meet these challenges, organic herbicides such as imazaquin were developed. Imazaquin, the common name for 2- (4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl) -3-quinoline-carboxilic acid, was the first imidazolirrone developed by American Cyanamid Company and is now widely accepted for its broad spectrum weed control in soybean (Glycine max (L.) Merr.). Since pesticides are toxic by design, it is necessary to predict and understand their behavior in the environment. This study quantifies sorption of imazaquin as a function of pH and organic carbon content and evaluate the effects of solution ionic strength and ionic composition on sorption with different soil types. The reaction kinetics were also determined so that we could properly understand the dynamic interactions of this pesticide with soils and sediments. Real sorption/desorption equilibrium conditions were not attained during the experimental execution time of 126 d. Imazaquin persistence, aging, increased with increasing residence time in all soils. Sorption of imazaquin in to a high organic matter, acidic soil (Tracy) seemed to be dominated by the partitioning of the molecular forms of imazaquin into organic matter. Sorption in to the lower organic matter and more neutral soils (Clermont and Drummer) seemed to require orientation of herbicide molecules with colloid surface allowing nonpolar portions of each to associate, while preventing interaction between the negatively charged portion of the molecule and the colloid surface. Imazaquin desorption appeared to occur in a two-step process. The initial phase was rapid while the second phase could be as much as four orders of magnitude slower. Positively charged microsites of the Fe and Al oxyhydroxides could have acted as major contributor of sorption sites to anionic species of imazaquin. Electrolyte composition and ionic strength of the soil solution also affected sorption of imazaquin. The distinct reactivity of the inorganic ions with both the imazaquin molecule and the soil surface will dictate sorption changes. In general, Al$\sp{3+}$ increased and $\rm H\sb2PO\sb4\sp-$ decreased sorption of imazaquin, mainly in highly weathered soils.
Degree
Ph.D.
Advisors
Turco, Purdue University.
Subject Area
Agronomy
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