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.

Incubation Time Effects on Imazaquin Desorption as Determined by Nonequilibrium Thin‐Soil Disc Flow

“…2). The weak retention observed on the Demopolis silt loam agrees with results reported by Smith et al (2003) Imazaquin is moderately sorbed in the nondissociated form but weakly sorbed in the anionic form (Mangels, 1991). Imazaquin sorption is negatively correlated with soil pH (Goetz et al, 1986; Basham et al, 1987; Renner et al, 1988; Loux et al, 1989; Loux and Reese, 1992).…”

Sorption behaviour of oxadiazon in tropical rice soils

“…Under the saturated conditions, the herbicide may interact with the soil matrix within a time period of seconds to minutes. Additionally, there are micropores and macropores in natural conditions which are eliminated in the conditions imposed in using the batch method (Smith et al, 2003). The experiment was also carried out to check the sorption capacity of the experimental soils for oxadiazon by repeated application of oxadiazon.…”

Fate and transport of chlormequat in subsurface environments

“…These structureless soil samples represent a simplified and relatively homogenous model system reflecting a subset of the soil matrixes existing under field conditions, but do not take into account important characteristics such as soil aggregates, micropores and macropores that are present under actual field conditions (Smith et al 2003). The differences in sorption and desorption reactions in steadystate and dynamic systems have been discussed in several papers, moreover, and it has been argued that the use of simple steady-state K d studies in modelling is problematic (Reardon 1981;Bethke and Brady 2000;Smith et al 2003). Despite these drawbacks, the laboratory methods are generally accepted as an approach for setting a range for the parameters needed for modelling.…”