Impact of soil organic matter heterogeneity and soil aggregation on the sorption of herbicides by soils.

摘要

Experiments were performed in pursuit of understanding of interactions between herbicides and soils, focusing on the effects of heterogeneity within soil organic matter (SOM), and the aggregation of SOM with mineral matter in soils, on equilibrium sorption and sorption rates of herbicides. For this purpose, sorption of three herbicides---atrazine, metolachlor and napropamide---was studied on a bulk soil, a bulk peat and three fractions that were chemically isolated from it. Studies were also performed using the well studied polynuclear aromatic hydrocarbon (PAH) phenanthrene to provide a comparison for the herbicides. The sorbents extracted from the soil and peat---Base extracted fraction (BE), humic acids fraction (HA) and kerogen and black carbon fraction (KB)---were characterized with elemental analysis, scanning electron microscopy and surface area measurements and subjected to sorption and desorption equilibrium studies and sorption rate investigations.;The herbicides were found to exhibit nonlinear sorption isotherms on all the sorbents; they exhibited least nonlinear isotherms and fastest sorption rates for HA among all the sorbents. HA fraction also showed the least dependence of equilibrium or time dependent organic carbon normalized sorption capacity on initial aqueous solute concentration. This result was in accordance with the amorphous nature of the HA material and similar to that observed for PAHs. Herbicide atrazine was found to react with HA and was transformed to hydroxyatrazine in its presence. High sorption capacity of KB dominated the sorption for all the herbicides, with this capacity being diminished by the aggregation structure of the soil. Significant sorption hysteresis was not observed for the sorption of herbicides on the KB fraction, unlike that observed for phenanthrene, indicating large herbicide molecules do not penetrate nanopores of KB. Hysteresis observed for the bulk soil and BE for atrazine was attributed to chemical interactions between the sorbents and the herbicide. Overall this dissertation found that the herbicides exhibit hydrophobic interactions with soils, but owing to their large molecular sizes and polarities may exhibit site specific interactions and lack of hysteresis that call for changes in existing fate and transport models and further microscopic understanding.