Chlorothalonil and its 4-hydroxy derivative in simple quartz sand soils: Acomparison of sorption processes

Quartz sandy soils from Simcoe, Ontario, Canada and North Carolina had sorp tion properties for chlorothalonil that were nearly the same. For labile su rface sorption kinetics, the Simcoe soil gave a pseudo first-order rate con stant of k(S1) = (7.4 +/- 0.7) x 10(-2) days(-1). At equilibrium, the labil e surface sorption capacity theta (C) of Simcoe soil for chlorothalonil was 23.8 x 10(-6) (mol/g). The sorption properties of the 4-hydroxy derivative of chlorothalonil were different in two important respects. They were larg er by a? order of magnitude, and they were substantially different for the two soils. Sorption by the Simcoe soil was too fast for kinetics measuremen ts by the on-line HPLC micro extraction method, but for the North Carolina soil k(S1) = (1.15 +/- 0.01) days(-1) was recorded. For the Simcoe and Nort h Carolina soils, respectively, Be > 200 (mu mol/g) and theta (C) approxima te to 113 (mu mol/ g). Two conclusions can be drawn. First, the replacement of the Cl by OH on the 4 position of chlorothalonil makes the sorption eff ects much greater. Second, the stronger interactions are associated with a greater sensitivity to small differences in the chemical compositions of th e soils. Subtle soil properties causing significant effects might include s mall amounts and physical structures of organic matter and metal oxides. Th is implies that, for predictive computer models, mechanism parameters will have to be correlated in two dimensions: chemical structure, and the compos ition and amounts of chemical materials in soils.