Herbicide sorption by immersed soils: stoichiometry and the law of mass action in support of predictive kinetics.

The stoichiometry of labile herbicide sorption on immersed soils has been determined for a few herbicides and a number of soils (Gamble, D. S.; Khan, S. U. Atrazine in organic soil: Chemical speciation during heterogeneous catalysis. J. Agric. Food Chem. 1990, 38, 297-308; Gamble, D. S.; Ismaily, L. A. Atrazine in mineral soil: The analytical chemistry of speciation. Can. J. Chem. 1992, 70, 1590-1596; Gamble, D. S.; Khan, S. U. Atrazine in mineral soil: Chemical species and catalysed hydrolysis. Can. J. Chem. 1992, 70, 1597-1603; Gilchrist, G. F. R.; Gamble, D. S.; Khan, S. U. Atrazine interactions with clay minerals: Kinetics and equilibria of sorption. J. Agric. Food Chem. 1993, 41, 1748-1755; Gamble, D. S. Physical chemistry parameters that control pesticide persistence and leaching in watershed soils. Final report submitted to the Great Lakes Water Quality Program Committee, Guelph, Ontario, June, 1994; Li, J.; Langford, C. H.; Gamble, D. S. Atrazine sorption by a mineral soil: Processes of labile and nonlabile uptake. J. Agric. Food Chem. 1996, 44, 3672-3679; Li, J.; Langford, C. H.; Gamble, D. S. Atrazine sorption by a mineral soil: The effects of size fractions and temperature. J. Agric. Food Chem. 1996, 44, 3680-3684; Gamble, D. S. Pesticide-soil research for the behaviour of chlorothalonil and its metabolite SD-3701 in soil. Final report submitted to Ricerca Inc., Sept 15, 1998; Gamble, D. S. Atrazine sorption kinetics in a characterized soil: Predictive calculations. Environ. Sci. Technol. 2008, 42, 1537-1541). This was done by using equilibrium titrations forthe measurement of labile sorption capacities thetaC. The titrations were made possible by resolving total sorption into its labile and unrecovered fractions. But equilibrium was not also necessary for unrecovered fractions. The site saturation at titration plateaus defined thetaC. The first purpose of determining the stoichiometry is to permitthe use of second-order kinetics instead of unpredictive pseudo-first-order kinetics for sorption modeling. Another purpose is to replace empirical distribution coefficients such as K(D) with the law of mass action for describing equilibria as limiting states. Temperature trends and a comparison with EGME vapor deposition data from the literature indicate a control of herbicide sorption by sorbed water. A preliminary examination of limited data from different sources suggests that future research should investigate some additional correlations. ThetaC and equilibrium functions might both be influenced by soil organic matter carboxyls and carboxylate anions, as well as inorganic materials. Some disadvantages of K(D) are noted.