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Hydrolysis and cyanide speciation of some heavy metals relevant to the fate of cyanide in the environment

Perera, Nimal (2001) Hydrolysis and cyanide speciation of some heavy metals relevant to the fate of cyanide in the environment. PhD thesis, Murdoch University.

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Quantitative understanding of the fate of cyanide in the environment and its role in industrial processes requires quantitative characterisation of metal ion–cyanide complex formation and also the corresponding metal ion hydrolysis reactions that frequently compete with them. This thesis presents quantitative data for complex formation constants obtained using a range of techniques for a number of metal ion–hydroxide and metal ion–cyanide systems of environmental and hydrometallurgical importance that have been found difficult to study in the past. A specially–constructed combined spectrophotometric /potentiometric cell has enabled a variety of systems to be studied by UV–Visible spectroscopy at very low metal ion concentrations. This has circumvented problems associated with precipitation and/or polynuclear metal complex formation, which frequently predominate in these systems at higher concentrations.

Using this approach it has been possible to determine the formation constants and spectra of the mononuclear hydroxide complexes of Pb(II), Cu(II), Fe(III), Ag(I) and for the cyanide complexes of Pb(II), Ag(I), Ni(II) and Fe(III). It has also been possible to estimate the solubility products of Pb(CN)2(s), AgOH(s), AgCN(s) and NaFeFe(CN)6(s). For some of these species (namely, Pb(OH)42–, the higher order hydroxo–complexes of Cu(II) and Fe(III), Pb(CN)+, and the lower cyano–complexes of Fe(III)), the present results are the first quantitative estimates of their formation constants. These complexes have long been assumed to exist on theoretical grounds but have proven difficult to quantify experimentally. The main reasons for this difficulty are the sparing solubility of the neutral hydroxides and /or cyanides and the tendency of CN– to form either very weak or extremely strong complexes.

Where possible, attempts were made to confirm the spectrophotometric results using other techniques such as polarography, NMR spectroscopy (for Pb(II)), Raman spectroscopy and, for Cu(II), ESR spectroscopy. However, although these techniques were sometimes able to provide useful insights into the nature of the species formed, in general they were not sufficiently sensitive, or suffered from other constraints that meant that they yielded little quantitative information.

The formation constants measured in this work were combined with literature data to model the chemical behaviour of hypothetical cyanide–infiltrated soil. This modelling indicates that under typical contaminated soil conditions (i.e. soil containing Fe(OH)3(s) and [CN–] = 0.1mM), cyanide will be present mainly as the various forms of Prussian blue. However, strong competition between cyanide and hydroxide ions for Fe(III), points to possible conditions for the chemical degradation of the Prussian blue.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Chemical and Mathematical Science
Supervisor: May, Peter and Hefter, Glenn
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