Using Nonequilibrium Thin‐Disc and Batch Equilibrium Techniques to Evaluate Herbicide Sorption

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‐1 H ‐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 0.35, 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.