Ionization and sorption of fomesafen and atrazine by soils and soil constituents

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Abstract
Fomesafen exhibited weakly acidic properties and was titrated spectrophotometrically and determined to have a pKAof 3.0. [14C]fomesafen and [14C]atrazine, as references, were used in slurry‐type sorption studies with H+‐ and Ca2+‐saturated soil organic matter, Ca2+‐saturated montmorillonite clay, Norfolk sandy loan (Typic Paleudult) and Drummer silt loam (Typic Haplaquoll) at suspension pH values of 2.0 to 6.3. Sorption of atrazine was greater than that of fomesafen for the organic matter sorbents but similar for montmorillonite clay and the two soils. Sorption of both herbicides on the Drummer soil was much greater than on the Norfolk soil, due to the higher organic matter content of the former. Decreasing the suspension pH greatly increased sorption of both herbicides by all sorbents. Atrazine sorption was attributed to ionic bonding at low pH and physical bonding at neutral pH. Fomesafen sorption occurred by way of physical forces at near neutral pH and by way of hydrophobic bonding and/or precipitation at low pH. Mobility and bioavailability of the herbicides in soils is expected to be lower at low pH than at high or neutral pH.