Factors influencing the distribution and retention of heavy metals in two Ghanaian soils

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Two Ghanaian soils, Oda and Amantin were characterized to determine their pbysicochemical properties. The pHs were found to have mean values of 4.03 and 6.42 respectively. The organic matter content were found to be 1.09 x 106 mg/kg and 0.86 x 106 mg/kg respectively. While the cation exchange capacity of Oda was 93.30 milliequivalent per 100g of soil, that of Amantin was 27.33 milliequivalent per 100g of soil. The particle size distribution analyses revealed that the clay content of Oda and Amantin were 35.00 % and 6.46 % respectively. The silt content were 42.30 % and 68.60 %.The sand content also had mean values of 25.00 % and 22.52 % respectively. The retention of Zinc and Cadmium by Amantin and Oda soil series with varying parameters of soil properties were investigated. Batch adsorption was carried out at the actual (unadjusted) pH of the soil to determine equilibration time for the experiments. An equilibration time of 18 hours was measured. The pH values of 4.03 and 6.42 of the soil solutions were adjusted by the addition of acid or base and the adsorbed values were determined for Cadmium and Zinc at equilibrium. The amount of Zinc and Cadmium sorbed ranged from 15.90 mg/kg to 82.95 mg/kg at pH of 1.9 and 12.30 respectively. The adsorption depended significantly on the pH. The linear regression coefficients, R2 at 0 O1M ionic strength were 0 996 and 0 934 for oda and Amautin respectively for Zinc. However, at 0.IM ionic strength the value were Q9255 and 0.9932 respectively. For Cadmium, the regression coefficients were 0.8819 and 0.7505 respectively for Oda and Amantin at ionic strength of 0.OIM, whereas at 0.1M the R2 values were 0.8659 and 0.9346 respectively. The presence of a larger percentage of clay in Oda affected its adsorption behaviour. Tests on different solid / solution ratio indicated the dependency of adsorption on solid / solution ratio. The adsorption isotherms obtained were fitted by the linear Freundlich equation: Log Cs = log K + (n-1) log Ce, Ce is the concentration of solution after equilibrium is attained, Cs is the amount of metal adsorbed in mg/kg of soil, K and n are constants. K and n are experimental parameters that depended on the system of adsorbent and adsorbate.
A thesis submitted to the Department of Chemistry in partial fulfilment of the requirements for the degree of Master of Science in Analytical Chemistry, 2005