Voltametric study of stability constants and speciation of lead in a tropical freshwater environment (lake Baringo)
Ndinya, Elizabeth Betty
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The study was undertaken to determine the stability constants of mononuclear complexes of lead with the major inorganic ligands that naturally exist in lake Baringo and from such constants assess the probable speciation and species distribution in the lake water. Lake Baringo is a very unique ecosystem, being an intermediate between fresh and saline lakes, hence the choice of the study area. First, a detailed qualitative and quantitative chemical analysis was carried out on the water, suspended silt and sediment samples from the lake. Sodium and potassium ions were found to be the major cations in the lakewater with sodium concentration at 3.35 x 10-4 mol/1. The major anions were found to be chloride and fluoride with concentrations, 2.56 x 10-3 mol/1 and 5.62 x 10-4 mol/1 respectively. From the analysis of silt and sediment samples, the concentration of aluminium ions and iron ions were found to the higher than those for other metals. Aluminium ions had a percentage concentration of 21.6% and iron,, 18.58%. Sodium ions measured 0.75% whereas potassium ions were found to be 1.99%. Heavy metal concentration (total ion concentration) in sediment samples were obtained as follows; total lead ions 6.27 x 10-5 mol/1, copper ions 3.08 x 10-4mol/1 and zinc ions 1.33 x 10-3 mol/1. The speciation of lead chloro, fluoro, carbonato and bicarbonato complexes was determined in lake Baringo aqueous media and in synthetic solutions of the same ionic strength as the lake water (I=0.140M). The ionic strength of the synthetic solutions was kept constant using potassium nitrate. The stability constants were determined by measuring the shift in polarographic peak potentials of lead using differential pulse anodic stripping voltammetry (DPASV) in conjunction with the Deford and Hume method of mathematical analysis. The shifts were measured at a lead concentration of 1.0 x 10-7M upon complexation. The study revealed that lead forms four stable cholo-complexes in aqueous media of low ionic strength. There are the mono-, bis-, tris- and tetrakis-, co-ordinated as PbCl+, PbCl2o, PbCl3- and PbCl42- respectively. The stability constants obtained were, Log K1 = 1.58, Log K2 =2.10, Log K3 = 2.74 and Log K4 = 2.88 respectively. Two fluoro- complexes, the mono- and bis-, co-ordinated as PbF+ and PbF2o were obtained with stability constants; Log K1 = 1.88 and Log K2 = 2.76 respectively. The study also showed that lead forms four stable carbonato and bicarbonato complexes, the mono-, bis-, tris-, and tetrakis- in each case. The carbonato complexes are co-ordinated as Pb (CO3)o, Pb (CO3)22-, Pb (CO3)34- and Pb (CO3)46- respectively. Their stability constants are; Log K1=6.16, Log K2=8.49, Log K3=10.66 and Log K4=12.42 respectively. The bicarbonato complexes obtained were; Pb (HCO)3)+, Pb (HCO3)2o, Pb (HCO3)3- and Pb (HCO3)42- with stability constants Log K1=2.99, Log K2=5.16, Log K3=7.43 and Log K4=10.15 respectively. From these stability constants, the species distribution (speciation) was calculated using the Incenzy method. The results revealed that for the halide complexes, the free metal fraction and the Mono- complexes appear as the dominant species at the predetermined ligand concentrations of the lakewater. The predominant species at the predetermined carbonate concentrations of the lakewater were mono- and bis- complexes, while for the bicarbonato system the predominant species were the tris- and tetrakis- complexes. However, at the natural ligand concentrations found in the lakewater it is concluded that lead would exist predominantly as the precipitates of the carbonato and bicarbonato species, with the halide complexes being of only minor importance to the complexation of lead in the lake.