Modeling the role of humic substances in the distribution of trace metals in selected ecosystems in Ghana

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The complexity of semi-natural ecosystems in terms of the composition and chemical properties of the solid and solution phases has cause researchers to develop intelligent and innovative ways of modeling and using the models to describe the chemistry of metal distribution in natural environments. But a comprehensive model that adequately describes the role humic substances play in trace metal distribution in semi-natural ecosystems is absent. The colloidal properties and aggregation properties at cloud point have not been effectively correlated with its cation binding properties. In view of these the role of humic substances in trace metal distribution at the mining communities at Talensi-Nabdam District of Upper East Region of Ghana were modeled in this study. Development of the models was based on the novel idea that there is a relationship between the fraction of metal associated with humic matter, intrinsic properties of the humic matter and metal partition constant. A quantitative account of the role of humic substance in metal distribution was studied using component factor analysis approach to model the speciation and distribution phenomena of trace metals at the selected ecosystem for the study. The properties of humic substance and the interactions of trace metals were correlated and used to develop mathematical models for the description of the role of humic substance in metal partition. The conceptual and mathematical models were proved to have the potential to explain the humic-impacted-partition of trace metals when they were fitted to data obtained from field and simulation experiments. The distribution model explained 97.99% of the factors which influence trace metal distribution in the terrestrial environment out of which humic substances accounts for more than 30 % of all the factors. The speciation model also showed that humic substances account for VI 13.06% of the variables which determine trace metal speciation at the ecosystem studied. It was concluded that humic substances play a significant role in trace metal distribution in terrestrial environment. The mathematical models gave good correlations and R2 values when fitted to data from simulation experiments. The models described the trace metal partition well in terms of humic acid (HA) complexation and partition. The overall impression of the models is that HA influences Cu distribution in soil solution system to an extent determined by the relative proportion of other cations which compete with the metal for both the humic acid and the soil mineral. The models also indicated that modification of the solid phase by HA introduces two major heterogeneous binding sites in the solid phase (i.e., the organic and inorganic binding sites) and that changes in the relative moles of the site types are determined by the cation binding and hydrophobic properties of HA. The models indicated that the properties of humic substance which makes it capable of influencing trace metal distribution are hydrophobicity, detergent properties and metal complexing ability.
A thesis submitted to the Department of Chemistry, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY.