Sorption of Inorganic Mercury on Soils from Ankobra Basin in the South - Western Part of Ghana

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SEPTEMBER, 2010
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Abstract
Understanding the adsorption of mercury on soil from aqueous solution is necessary for tracing the fate of mercury in the environment. The fate of mercury in the soil is important not only to assess surface and groundwater quality, but also to understand mercury fluxes at the soil/air and water/air interfaces. The most important chemical retention mechanisms are sorption processes and precipitation. This study was undertaken to predict the fate of mercury emitted from artisanal gold mining activities within the Ankobra basin. Total mercury (THg) concentrations were determined in soils close to River Ankobra and its major tributaries in the south-western part of Ghana by Cold Vapour Atomic Absorption Spectrophotometry. Some soil characteristic such as pH, particle size distribution and organic matter were also determined. THg content of the soils ranged from 0.051µg/g to 0.366µg/g with an average of 0.15µg/g, which were below the permissible level of THg for soils (200µg/g) set by US EPA. THg correlated positively (r = 0.57) with the organic matter content of the soil. This indicates increasing Hg concentration with increasing organic matter content. Sorption of Hg (II) onto seven (7) different soils from the A-horizon indicated that the soils had an L - type isotherm and the distribution coefficient (Kd) ranged from 41.4 µg/g to 182.2 µg/g. The Freundlich model showed a better fit followed by Langmuir model and Linear model because most of the regression coefficients were close to one. The best fitness of Freundlich model to the other models could be attributed to the heterogeneous surface energies and exponential distribution of active sites which are characteristic of such soils. All the soils had adsorption intensity (n) lower than one, indicating that the soils could be good sorbents for Hg (II). This suggests that the fate of Hg and its availability in soil for runoff and leaching could be low for soils with such characteristics.
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A Thesis Presented to the Department of Chemistry, College of Science, Kwame Nkrumah University of Science and Technology, in Partial Fulfilment of the Requirements for the Award of the Degree of Master of Science (Analytical Chemistry).
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