Quantitative Prediction of Atrazine Sorption in a Manitoba Soil Using Conventional Chemical Kinetics Instead of Empirical Parameters

Abstract

How to predict the persistence and leaching of organic chemicals in soils is a long-standing environmental issue. The injection of soil slurries into an HPLC together with separate solution-phase analysis permits the resolution of total sorption into intraparticle-diffused and labile-sorbed fractions. Sorption site stoichiometry was revealed, and two-step sorption was confirmed. This permitted conventional chemical kinetics including Laidler's integral rate law for second-order kinetics to be adapted to the natural mixture of irregular sorption sites. Quantitative predictions were successfully tested, and the effects of site saturation on the kinetics were demonstrated. Other authors have independently published evidence related to sorption site stoichiometry, by using scanning tunnelling and fluorescence microscopy for sorption onto idealized crystals. This presents opportunities for research advances in two directions. HPLC and microscopy methods could be used together for sorption mechanisms in both environmental and pure crystal systems.

Propanil in a Manitoba soil: an interactive spreadsheet model based on conventional chemical kinetics

“…The experimentally observed labile sorption limit is evidence of hydrophobic sorption in the interface region. As noted previously, 36 the water layers within the solid-solution interface are more densely structured than bulk water is. The sorption of a hydrophobic organic chemical molecule disturbs the surface water structure around it.…”

“…As a consequence, the intraparticle diffusion showed first order kinetics, instead of the pseudo zero order kinetics previously found for an example in which site saturation was maintained. 36 The decrease in the calibration curve for intraparticle diffusion after 3 days should be noted, on Sheet 1 of Model #5.xlsx. This decrease is not physically reasonable because diffusion out from particle interiors would be too slow.…”

“…The on line HPLC m extraction method for sorption-reaction kinetics in soil slurries has been previously described. 11,15,[19][20][21][23][24][25]36 It is the only experimental method available for resolving total extractable chemical into dissolved and labile sorbed fractions. Sorption-reaction experimental samples are prepared with slurries of pre-wetted soils in aqueous pesticide solutions.…”

“…In this project the previously described Osborne clay soil was used. 36 Its analysis is reproduced here in Table 1. In one experiment with Hg(II) catalyst, 51 data points were obtained.…”

Online Sequential-Injection Chromatography with Stepwise Gradient Elution: A Tool for Studying the Simultaneous Adsorption of Herbicides on Soil and Soil Components

“…The adsorption of triazines is characterized by a fast initial step (few minutes) followed by slow intraparticle diffusion that takes from several hours to several days to reach equilibrium. − Characterization of the initial steps of adsorption would be valuable to evaluate the potential runoff of herbicides during floods or heavy rains. High-performance liquid chromatography separates the herbicides from the soluble soil-matrix components and resolves the total sorption into intraparticle-diffused and labile-sorbed fractions . Sorption kinetics of herbicides and their metabolites on soils are usually investigated for individual compounds to avoid the effects of competition by the adsorption sites .…”