title: Using nonequilibrium thin-disc and batch equilibrium techniques to evaluate herbicide sorption.
abstract: Nonequilibrium disc-flow techniques may better reproduce dynamic soil-pesticide interactions than traditional batch sorption studies. Batch kinetic and equilibrium experiments and dual-label thin-disc flow experiments were conducted with atrazine (6-chloro-N-ethyl-N'-isopropyl-1,3,5-triazine-2,4-diamine) and imazaquin [2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl)-3-quinolinecarboxylic acid] using a Demopolis silt loam (loamy-skeletal, carbonatic, thermic, shallow Typic Udorthent; 8% clay, 62 g kg(-1) organic matter, 7.6 pH). Batch kinetic studies with both herbicides revealed an almost instantaneous rapid phase and a much slower gradual phase. The rapid phase was complete after 5 min and equilibrium was reached at 24 h. The rapid phase accounted for 74% and 12 to 30% of the total amounts adsorbed for atrazine and imazaquin, respectively. The sorption of both the rapid and 24-h isotherms for each herbicide best fit the Freundlich equation. The rapid and 24-h K(f) values of atrazine were 1.38 and 2.41, respectively, and the N value of both phases was approximately 0.93. For imazaquin, the rapid and 24-h K(f) values were 0.056 and 035, respectively, and the N value for the rapid phase of imazaquin was 0.71, compared with 0.86 for the 24-h isotherm. In the dual-label thin-disc flow experiments, the average partition coefficient for atrazine at the peak soil concentration point was 1.54. This value closely agreed with the observed rapid-phase K(f) value of 1.38. In contrast, the thin-disc flow experiments failed to detect any imazaquin retention. The thin-disc flow method can allow for a greater resolution of rapid sorption kinetics, which is impractical with batch studies. Along with dynamic partitioning data, the thin-disc flow method may provide kinetics data that may better complement environmental models than coefficients generated with batch techniques.
authors.affiliation: Department of Plant and Soil Sciences, Box 9555, 117 Dorman Hall, Mississippi State University, Mississippi State, MS 39762, USA. csmith@pss.msstate.edu
authors.forename: M Cade
authors.initials: MC
authors.lastname: Smith
authors.forename: David R
authors.initials: DR
authors.lastname: Shaw
authors.forename: Joseph H
authors.initials: JH
authors.lastname: Massey
authors.forename: Michele
authors.initials: M
authors.lastname: Boyette
authors.forename: William
authors.initials: W
authors.lastname: Kingery
mesh_terms: D000327:Adsorption; D001280:Atrazine / Q000737:chemistry*; D004784:Environmental Monitoring / Q000379:methods*; D006540:Herbicides / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D008962:Models, Theoretical; D011804:Quinolines / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis*; D013696:Temperature
chemical_list: D006540:Herbicides; D007093:Imidazoles; D011804:Quinolines; D012989:Soil Pollutants; C057728:imazaquin; D001280:Atrazine