The Measurement of the Complexation of Heavy Metals and Radionuclides with Natural Humic Substances

[摘要] Ombrotrophic (rain-water fed) peat bogs have been used to study the contents and distributions of heavy metals and radionuclides. If these systems are dated, a chronology of atmospheric deposition can be assigned to the profiles. Humic substances are an integral, characteristic and substantial component of organic soils such as peats, and complexation of metals with these substances plays an integral role in the dynamics of their interactions in peat soils. Studies on the complexation of metals to humic substances have centred around monitoring the interaction of metals with humic or fulvic acids. These are operationally-defined fractions which are extracted in order to reduce the heterogeneity of humic substances, and remove contaminants such as silicates or polysaccharide materials. The main problem with such an approach is the question of whether isolation of humic acid leads to chemical or structural changes which may invalidate any results from complexation studies. This question was addressed in this thesis by carrying out all analyses on both humic acid and unextracted peat. The results indicated that isolation of humic acid led to a change in the functional group content and reactivity as well as an increased capacity to bind with metals, as a result of chemical and structural changes. The effect of isolation of humic acid on the stability of interaction depended on the metal being studied, but still demonstrated a difference between unextracted peat and humic acid. The binding of a range of elements with humic substances was studied, with particular emphasis on the difference in reactions between Cu2+, Pb2+ and Cs+. Qualitative studies on the nature of binding were carried out using infra-red analysis and highlighted how Cs+ binds through predominantly ionic-type linkages, whilst Pb2+ and Cu2+ show evidence of more covalent linkages. The stability of these interactions was investigated using three different methods: the Base Titration method, Scatchard analysis of titration data and based on this, an Incremental Addition Method. Each had its own set of limitations and advantages, and analysis of how accurately these methods quantify metal binding was carried out. These studies on the nature and stability of metal binding to humic substances are laboratory based, questioning whether or not they reflect the interactions of metals in the environment. Field-based studies were also carried out, looking at the distributions and contents of metals and radionuclides in undisturbed peat cores under differing conditions. Interpretation of these data was carried out with the knowledge gained from the laboratory-based studies. Peat cores were collected from a site to the south west of Glasgow on the Fenwick Moor. This site was chosen since it was affected by the Chernobyl accident, and previous studies had recorded significant levels of radiocaesium on the site. Accordingly, six cores were collected from the site, at different aspects and altitudes in order to investigate more fully the contamination of the site by Chernobyl-derived caesium. In addition, the effects of changes in geochemistry, water content and table, peat type, altitude and aspect on the distribution and content of radiocaesium, 210Pb, Cu, Br, Pb, Mn, Fe, Ta, La, Hf, Sm, Sc, Se, Ce, Th, Hg, Sb, As & Co were investigated. Where no redeposition of 210Pb was observed the 210Pb profiles were used to provide a chronology for the core, using the constant initial concentration model. Consequently, if no redeposition of the pollutant metals was observed, changes in the fluxes of these metals to the profile could then be dated. In addition, analysis of the changes in the 206Pb/207Pb) ratios over time was carried out in order to attribute different sources to the total content of Pb within each core. The results produced from the different studies all demonstrated the importance of humic substances on the behaviour of heavy metals and radionuclides in the environment, as well as the complexity of their interactions.

[发布机构] University:University of Glasgow

[关键词] Environmental science, Soil sciences, Geochemistry, Nuclear chemistry