Date of Award

12-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Toxicology

Advisor

Carraway, Elizabeth R

Committee Member

Hains , John J

Committee Member

Johnson , Alan R

Committee Member

Schlautman , Mark A

Committee Member

Yang , Yanru

Abstract

Global industrialization, coupled with a large biogeochemical cycle that moves mercury through the environment, has resulted in higher mercury concentrations being detected in many environmental compartments. Although the presence of mercury in each compartment has the potential to elicit toxicity, mercury found in natural waters provides the greatest risk, due to the various reactions it can undergo, resulting in a plethora of different species.
Due to the fact that mercury contamination in aquatic systems has the potential to bioaccumulate and biomagnify up the food chain and cause toxicity, understanding the behavior of mercury in these systems is imperative in the protection of both organisms and humans. Although much research has focused on the human health effects of mercury contamination, little research has examined the environmental implications of mercury contamination. This research attempts to bridge the gap in understanding of mercury toxicity in the presence of natural organic matter (NOM) to aquatic organisms in natural waters. Whole water samples were collected from seven sites along the length of the Ogeechee River in southeastern Georgia. Physiochemical parameters of each water sample, including hardness, alkalinity, pH, conductivity, and ion concentrations, were measured to obtain a more comprehensive understanding of the sites. Due to the fact that organic matter collected from waters can be varied in both concentration and nature, measurements were also performed in an effort to construct profiles for the dissolved organic matter (DOM) in each water sample. These profiles were later used in the quantification of accurate metal concentrations for toxicity experiments. The results from the physiochemical analysis of the Ogeechee River water samples show that while the DOM is relatively similar with regards to both molecular weight and size distribution, there are differences observed in the molecular weight polydispersity values as well as the concentration of reduced sulfur associated with the DOM at each site.
The Ogeechee River site water was used in mercury toxicity experiments to assess the role that DOM complexation plays in the amelioration of metal toxicity. The original whole water and a variety of manipulations to the original water were used to quantify the reduction in mercury toxicity. Results indicated that although a reduction in mercury toxicity was observed in the presence of Ogeechee River DOM, the observed reduction in toxicity was independent of DOM concentration in all experiments. To confirm the mercury toxicity results using the Ogeechee River waters, Suwannee River DOM was obtained from the International Humic Substances Society and used in similar experiments. In addition to mercury, copper toxicity experiments were also conducted to compare results. These experiments showed that although copper toxicity was systematically decreased with higher concentrations of organic matter, mercury toxicity was again independent of DOM concentration. The toxicity results observed between the two metals was hypothesized to be due to the differences in binding strengths between the metals and various ligands present in DOMs. Due to the fact that such different toxicity reductions were observed between copper and mercury using the same organic matter source, the kinetic lability of various metal complexes was quantified using competitive ligand-exchange reactions. The complexation strengths of each metal were assessed using a variety of simple organic ligands. These ligands were used separately to determine rate constants for individual metal-simple ligand complexes. Results show that, although both copper and mercury share similar rate constants to simple ligands, the metal's dependence on the ligand concentration is equally important. In the copper simple ligand experiments, an independence is observed, minimizing the importance of the disjunctive mechanistic pathway. Mercury, on the other hand, shows a dependence on ligand concentration, resulting in the disjunctive pathway playing a larger role. The simple ligand relationships were then used as a model to predict the expected binding strength of each metal when complexed to dissolved organic matter, a much more complicated and complex ligand. Results show that copper is much more kinetically stable when complexed to DOM, which is a result of its stronger complexation to both oxygen and nitrogen ligands, which are predominantly found on DOM. Mercury complexed to DOM results in a much more labile complex, due to the fact that it does not strongly bind to wither oxygen or nitrogen ligands. Therefore, mercury bound to DOM is much more likely to be bioavailable for organismal uptake in the water column, supporting the hypothesis of kinetic lability to explain the toxicity results obtained using both metals.

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