Date of Award

12-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Engineering and Earth Science

Advisor

Lee, Cindy M

Committee Member

Elzerman , Alan W

Committee Member

Freedman , David L

Committee Member

Garrison , Arthur W

Abstract

Chiral pesticides are prevalent in the environment today and are known to react selectively with chiral environmental components such as microbes, enzymes, and other naturally occurring chiral materials. In addition, chiral sorption is a process that has been occasionally investigated in the study of homochirality (the exclusive presence of one enantiomer in living organisms), but almost overlooked in environmental science. For chiral sorption to occur, the sorbent and sorbate must be chiral entities. In the environment, there are abundant natural surfaces that are chiral, including clay minerals and organic matter present in soil, sediment and aqueous solution. A knowledge of isomerization, including the special case of enantiomerization, of chiral pesticides is also crucial in understanding the fate of these chiral pollutants, especially for pesticides marketed as a 'chiral switch':, i.e. formulations containing only or mostly the active enantiomer. The goal of this research was to provide more insight into sorption and enantiomerization of chiral pesticides in the presence of mineral surfaces.
Sorption experiments with malathion and metalaxyl and selected minerals, along with their individual enantiomers, were conducted and results analyzed by achiral and chiral high performance liquid chromatography (HPLC). Investigation with racemic malathion resulted in significant sorption to all sorbents, with the exception of the negligible sorption to kaolinite. Sorption was fit to the Freundlich model, with n2hr > 1 for bentonite, calcite, and montmorillonite while the biotic solids diatomaceous earth and seashells had n2hr < 1. KF,2hr values indicated decreasing sorption capacity in the order montmorillonite, bentonite, calcite, seashells, and diatomaceous earth. Chiral analysis indicated nonenantioselective sorption for malathion, which exhibited racemic enantiomeric fractions (EFs) for all sorbents.
Sorption of the individual enantiomers of malathion, R-(+)- and S-(-)-malathion, resulted in enantiomerization to racemic EFs after contact with the mineral sorbents. Adding malathion enantiomers to minerals and analyzing the aqueous phase after 15 min of centrifugation resulted in enantiomerization to racemic EF values for R-(+)-malathion, but only partial enantiomerization of S-(-)-malathion. Malathion enantiomers also decreased or increased in EF for controls containing either only water or â-cyclodextrin, but enantiomerization to the racemate was not complete after two hours. It was hypothesized that malathion is undergoing proton exchange with the mineral surfaces causing enantiomerization. Construction of sorption isotherms revealed that the racemate and enantiomers have different isotherm shapes, possibly indicating different sorption mechanisms. Sorption was fit to the Freundlich model, with n2hr being statistically different for the racemate and S-(-)-malathion and R-(+)- and S-(-)-malathion with bentonite, but not for calcite and montmorillonite. KF,2hr values were not significantly different for the racemate and enantiomers for bentonite, calcite, or montmorillonite, suggesting that the minerals had the same sorption capacity for the racemate or enantiomers.
Racemic metalaxyl and metalaxyl-M (consisting of 97% R-(-)-metalaxyl and 3% S-(+)-metalaxyl) were found to sorb to bentonite and montmorillonite, but sorption to calcite and kaolinite was not apparent. Sorption was fit to the Freundlich model with n > 1 and KF indicating that bentonite had a higher sorption capacity than montmorillonite. Sorption of R-(-)-metalaxyl from metalaxyl-M was greater than racemic metalaxyl for both sorbents, but S-(+)-metalaxyl sorption was the greatest overall after consideration of the small amount of S-(+)-metalaxyl present in metalaxyl-M. Chiral analysis indicated that sorption of racemic metalaxyl was nonenantioselective with racemic EFs. However, sorption of metalaxyl-M resulted in an increase of EF with bentonite and montmorillonite indicating the possibility of enantioselective sorption. Solution EFs also increased for calcite, and at one concentration level EF decreased for kaolinite, suggesting that enantiomerization is occurring for metalaxyl-M when in contact with these sorbents, since sorption on calcite and kaolinite was not observed.
Although enantiomerization of chiral pesticides has been observed during degradation and in aprotic solvents, this research presents the first evidence of enantiomerization during sorption for two current-use chiral pesticides. The rapid enantiomerization of malathion and the enantiomerization and possible enantioselective sorption of metalaxyl to mineral surfaces questions the efficacy of using a chiral switch for these two pesticides. This research substantiates the need for enantiomer specific sorption analysis for chiral pesticides to understand their fate and effects in the environment.

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