The determination of stability constants of cadmium complexes with selected ligands and modelling the likely speciation of cadmium in lake Bogoria, Kenya
This thesis is a report of a speciation study of the complexes formed by cadmium with selected ligands found to be dominant in an inland salt lake, Lake Bogoria. Water from Lake Bogoria was analysed using standard analytical techniques to obtain the stoichiometric concentrations of the major ions. Differential pulse anodic stripping voltammetry (DPASV) and hanging drop amalgam voltammetry (HDA V) were then used to measure oxidation peak potential shifts of cadmium in presence of the inorganic ligands carbonate, bicarbonate, chloride, fluoride and hydroxide. The peak potential shifts were then used to calculate the stability constants of cadmium complexes at the ionic strength of the lake using DeFord-Hume graphical method. The determined stability constants were then used to model the speciation of the lake using Incenzy method. Three carbonato species CdC03°, Cd(C03)22- and Cd(C03)34- were identified in both the KN03 aqueous and lake water media using data from DP ASV while only two species, "CdC03 ° and Cd(C03)l- were identified with data from HDA V. With DP ASV data, four bicarbonato complexes, CdHCO/, Cd(HC03)20 , Cd(HC03)3- and Cd(HC03)/- were shown to exist in both the KN03 aqueous and lake water media while three bicarbonato species; CdHCO/, Cd(HC03)20 Cd(HC03)3- were obtained using data from HDA V. DPASV data showed existence of six hydroxo complexes, CdOW, Cd(OH)2°, Cd(OH)3-, Cd(OH)/-, Cd(OH)s3- and Cd(OH)64- in the lake water medium and four complexes vi , . CdOW, Cd(OH)2° , Cd(OH)3- and Cd(OH)/-in the KN03 aqueous medium. With HDAV, five complexes were identified. These were; Cd(OH)+, Cd(OH)20 , Cd(OH)J, Cd(OH)/- and Cd(OH)s3-. Five chloro species were found to exist in the KN03 aqueous medium using data from both the DPASV and HDAV. These were cacr, CdCho, CdCl,", CdCI/- and CdCls2-. Three fluoro complexes CdF+, CdF20 and CdF3- were found using data from both techniques. The study has shown that the stabilities, relative proportions and distribution of the various species vary quite significantly with the composition of the matrix mixture.