Date of Award

5-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Engineering and Earth Sciences

Committee Member

Dr. Cindy Lee, Committee Chair

Committee Member

Dr. Kevin Finneran

Committee Member

Dr. Lindsay Shuller-Nickles

Abstract

A meta-analysis was performed to investigate nonracemic enantiomer fraction (EF) levels of chiral polychlorinated biphenyls (PCBs) found in various media across the world. Nonracemic EF levels are those determined to be statistically different than 0.5. The existing body of knowledge from the literature provided the necessary data to determine via meta-analysis a true average EF for five selected congeners, 91, 95, 132, 136, and 149. A thorough literature search was conducted to find relevant articles that contained sampling data from lab and field studies for chiral PCBs. A database was constructed to organize the sampling data obtained from the literature. SAS statistical software was used to perform a meta-analysis of the dataset. True average EF values for each of the five select congeners were determined in each of four media: air, soil, sediment, and tissue. The p values were determined to distinguish any difference between the calculated EF values and a racemic value. All statistical tests were conducted at the 0.05 significance level. Nonracemic EF levels indicate that an enantioselective process occurred at some point. Several hypotheses were proposed and tested through the meta-analysis. The first possibility was enantioselective bioaccumulation. Many organisms possess the capability to bioaccumulate one enantiomer of a PCB over the other. All organisms bioaccumulating one preferred enantiomer may be the reason why nonracemic EF levels are observed in soil or sediment. The hypothesis was evaluated by statistically determining the EF levels of chiral PCBs in the tissue of these organisms. The EF levels of the chiral PCBs in tissue were then reconciled with EF levels in soils and sediments to reveal any significant impact of enantioselective bioaccumulation. The second possibility that might result in nonracemic EF levels comes from parent/product congener relationships. If a higher chlorinated, chiral PCB is dechlorinated enantioselectively to another chiral PCB, then the lower chlorinated, chiral PCB will show an enrichment of one enantiomer. Currently, the body of data did not allow for any significant analysis of these relationships. Too much remains unknown concerning the elution order of each chiral congener as well as which product enantiomer is produced by the dechlorination of the parent enantiomer. Enantioselective dechlorination is the last possibility considered. Microorganisms can dechlorinate chiral contaminants and show a preference for to dechlorination of one enantiomer of the contaminant over the other. Lab and field samples from around the world indicated nonracemic levels of chiral PCBs in soil and sediment. Results from the meta-analysis confirmed enantioselective dechlorination has occurred. The five chiral congeners analyzed in this study, PCBs 91, 95, 132, 136, and 149, all contain a biphenyl ring with two ortho-chlorines and one meta-chlorine, a 236 ring. The known dechlorination process N removes the meta-chlorine on a 236 ring. PCBs 91 and 95 are known to only dechlorinate through process N while the others have the potential to dechlorinate through process N. Results from the meta-analysis provided sufficient evidence that dechlorination process N functions enantioselectively. Overall this study provided significant evidence for understanding the enantioselective processes that may affect chiral PCBs. Enantioselective bioaccumulation was observed throughout much of the literature. Organisms typically did not show a trend for selectively bioaccumulating favoring one enantiomer over the other. Parent/product relationships between chiral PCBs were determined to be inconsequential for many of the chiral PCBs in this study. Results indicate that enantioselective dechlorination of the chiral PCBs had occurred and was the most reasonable source of nonracemic EF levels observed in worldwide samples.

Share

COinS