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Thesis (Ph.D.)--Boston University / The most common polagraphic electrode in use at the present time is the dropping mercury electrode (DME). There are, however, certain well-recognized disadvantages attendant on the use of the DME: (1) oxidation analysis is limited to 0.4 v.; (2) there are rather large charging currents associated with the growing drop; (3) the area of the drops, and hence the sensitivity can not be appreciably increased; (4) because of drop growth, a a spiky sort of current is obtained which introduces a reading error into the results obtained, especially at low concentrations. It can be seen that these disadvantages are largely a function either of the electrode shape and size, or of the method of electrode propagation. [TRUNCATED]

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[From Introduction]. In 1955, Dry and Gledhill, both formerly of this Department, published their paper on the Solubility of Mercurous Chloride in Water at 25°0, and it was originally intended that this thesis should be an extension of the study to cover the temperature range from 5 t o 55°0. A preliminary investigation at 25°0, however, failed to yield results which were consistent with their findings, even though their apparatus and experimental technique were used. In an effort to resolve these difficulties their method of saturating the calomel in the conductance cell was thoroughly investigated, and as this procedure was found to be responsible, it was necessary to evolve an entirely new approach. The technique finally adopted was to saturate the mercurous chloride solutions by mechanical stirring in siliconed vessels and to carry out the conductance, pH, and total mercury concentration measurements on the filter ed solution. This method gave good results, and was free from the many extrapolations prevalent in the original procedure.

A study has been undertaken to investigate the loss of ductility and modified work hardening characteristics of zinc single crystals coated with mercury. Important results of tensile tests performed under fixed experimental conditions are summarized to be: (1) increase in critical resolved shear stress, and increase of work hardening slope in stage A and stage B (2) decrease in transition strain from stage A to stage B (3) decrease in fracture stress and fracture strain. The results have been interpreted in the context of the present understanding of deformation theory of C.P.Hex. metals. In addition, relevant mechanisms for crack initiation have been studied with the aid- of microscopic observations of deformed crystals. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Methylmercury (MeHg) is a potent neurotoxin capable of crossing the blood-brain barrier causing a profound negative impact on the central nervous system. After its release, Hg may be transported worldwide and eventually deposited in colder Arctic regions. Exposure of Aboriginal communities to MeHg occurs primarily through the consumption of traditional food. Dietary exposure studies are conducted using the total concentration of mercury in the food multiplied by the food consumption rate. This method does not take into account the oral bioavailability of Hg. Therefore, this study determines the bioavailability of Hg in four key traditional foods to provide a better estimation of Hg exposure and to improve the characterization of overall Hg risk to human health. We found that Hg concentrations significantly decreased for all foods after undergoing an in-vitro digestion process. Hg bioaccessibility percentage of ringed seal liver was 32.3%, ringed seal muscle was 69.0%, lake trout muscle was 28.8%, and lastly air-dried beluga muscle was 34.0%. Furthermore, no relationship was observed between bioaccessible Hg concentrations and original Hg concentrations in the raw food. The concentration of MeHg in the bioaccessible fraction was also examined and found to be significantly higher in muscle tissues than in the liver. Bioavailability of the foods was determined using Caco-2 cells. Hg bioavailability percentages were found to be 0.42% for RSL 5.24% for RSM, 7.30% for ADB, and finally, 12.70% for LT. Correlations were found between increased Hg uptake and higher percentages of bioaccessible MeHg as well as lower concentrations of bioaccessible selenium. Lastly, a significant decrease in MeHg uptake after 24 hours was observed when co-incubating with selenium. These results suggest that risk assessments should incorporate bioaccessibility and bioavailability when estimating mercury exposure. Additionally, nutrients such as selenium in traditional food may play a role in reducing mercury uptake in the gut.

Background: Recent research showing how dietary interventions substantially influence the potential presence of widespread and stable bacterial core phyla in the human colon has garnered a considerable amount of attention. Because the human gut can play a major role in host health, there is currently some interest in observing how diet influences human gut microbial composition and how changes in diet affect the potential for gut microbiota to transform mercury. This study aims to discover how different kinds of diet affect the nature and magnitude of microbial Hg transformations in the human gut environment. Methods: Fecal samples have been collected from 5 human male individuals at University of Ottawa and stored at -80ºC for further investigation. Using high throughput DNA amplicon sequencing targeting the 16s rRNA V4 region, we investigated the microbial community structure of the gut in 5 healthy male. Mercury biotransformations in the pooled fecal sample have been carried out using stable isotopes of mercury (198HgCl2 and Me199HgCl) and analysis was conducted by using inductively coupled plasma mass spectrometry (ICP-MS). Results and conclusions: We were not able to detect any significant Hg methylation or MeHg demethylation. We suspect this is due to Enterobacteria dominating the microbial community structure after 96h; Although Enterobacteria are part of the typical microbiota of a healthy individual, they do not possess genes required for Hg methylation. As such, our microbial data support our chemical analyses. We were not able to identify whether a change in diet affected Hg transformations in the human gut environment.

This work explores the metabolism of mercury in liver and spleen tissue of fish from a methylmercury contaminated wetland. Wild-caught bass, catfish, bowfin and gar were collected. Macrophage centers, which are both reactive and primary germinal centers in various fish tissues, were hypothesized to be the cause of demethylation of methylmercury in fish tissue. Macrophage centers are differentially expressed in fish tissue based on phylogenetic lineage, and are found primarily in the livers of preteleostean fish and in the spleen of teleostean fish. Histology of liver and spleen was examined in both control and wild-caught fish for pathology, size and number of macrophage centers, and for localization of mercury. Total mercury was estimated in the muscle tissue of all fish by direct mercury analysis. Selenium and mercury concentrations were examined in the livers of wild-caught fish by liquid introduction inductively coupled plasma mass spectrometry (ICP-MS). Total mercury was localized in histologic sections by laser ablation ICP-MS (LA-ICP-MS). Mercury speciation was determined for inorganic and methylmercury in liver and spleen of fish by bas chromatography-cold vapor atomic fluorescence spectroscopy (GC-CVAFS). Macrophage center tissue distribution was found to be consistent with the literature, with a predominance of centers in preteleostean liver and in spleens of teleostean fish. Little evidence histopathology was found in the livers or spleens of fish examined, but differences in morphology of macrophage centers and liver tissue across species are noted. the sole sign of liver pathology noted was increased hepatic hemosiderosis in fish with high proportions of liver inorganic mercury. Inorganic mercury was found to predominate in the livers of all fish but bass. Organic mercury was found to predominate in the spleens of all fish. Mercury was found to accumulate in macrophage centers, but concentrations of mercury in this compartment were found to vary less in relation to total mercury than hepatocyte mercury. No association was found between selenium content and inorganic mercury proportions. Overall, findings from this study to not support a primary role for macrophage centers in the demethylation of methylmercury in fish tissues.