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INTERACTIONS OF PHOSPHONATE HERBICIDES WITH QUARTZ, MONTMORILLONITE, AND QUARTZ-ENRICHED AGRICULTURAL SOIL: MOLECULAR MODELING SIMULATIONS AND ADSORPTION-DESORPTION EXPERIMENTS

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Abstract

Glyphosate was conventionally considered relatively immobile in soils due to strong complexation with metal oxides, but detection in agricultural effluents and surface waters points to glyphosate mobilization and transport. Glyphosate adsorption has been well-documented, but both release from soils and the underlying mechanisms have not been fully elucidated, especially with respect to quartz-enriched soils. Here we employed theoretical and experimental approaches to gain new insights on the mobility of glyphosate and two other phosphonate herbicides, glufosinate and fosamine, on runoff-prone soils. Using an annealing Monte Carlo molecular modeling approach, we obtained optimized adsorbate complexes of phosphonate herbicides with quartz and montmorillonite in the presence of Na+ or Ca2+ counterions. With quartz, the phosphonate compounds complexed with Na+ remained suspended in the diffuse layer away from the mineral surface, whereas the presence of Ca2+facilitated inner-sphere complexes with glyphosate and glufosinate, and an outer sphere complex with fosamine due to Ca-mediated water-bridged outer-sphere interactions that did not occur with Na+. With montmorillonite in the presence of Na+, we observed interactions between the terminal amino groups of glufosinate and fosamine and the negatively-charged montmorillonite surface, whereas the centrally-located amino group of glyphosate was not involved. Similar to quartz, metal complexation and water-bridging contributed to inner- and outer-sphere complexes of all three phosphonate herbicides on montmorillonite in the presence of Ca2+. In sum, our theoretical findings highlighted Ca-enhanced interactions for all three herbicides on quartz and montmorillonite, and more favorable interactions for glufosinate and fosamine with Na-montmorillonite than glyphosate. Na-complexed phosphonate herbicides dis not interact strongly with quartz. We conducted adsorption and desorption experiments using glyphosate and quartz-enriched agricultural soils to evaluate the mobility of bound glyphosate. Compared to CaCl2, adsorption conducted in the presence of NaCl resulted in reduced adsorption and retention of glyphosate, consistent with our theoretical findings. Our desorption experiments indicated that Ca2+ presence in solution during the desorption process resulted in enhanced mobility by 15.5-18.6% relative to desorption in the presence of Na+. Under our experimental conditions, glyphosate release from quartz-enriched agricultural soils ranged from 29.6% to 89.4% of the initially bound glyphosate, whereby soil samples of higher field relative wetness were more prone to glyphosate mobilization than soil samples of lower relative wetness. Glyphosate is known to adsorb strongly on variable-charge soils; this study emphasizes instead the role of soil relative wetness and salt presence in the field on the mobility of glyphosate and similar phosphonate herbicides.

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60 pages

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2021-08

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Adsorption; Desorption; Glyphosate; Mechanism; Mobility; Molecular Modeling

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Aristilde, Ludmilla

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Richards, Brian K.
Steenhuis, Tamme S.

Degree Discipline

Civil and Environmental Engineering

Degree Name

M.S., Civil and Environmental Engineering

Degree Level

Master of Science

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Government Document

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dissertation or thesis

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