|Title||Electrochemical metal speciation in colloidal dispersions|
|Source||Agricultural University. Promotor(en): J. Lyklema; H.P. van Leeuwen. - S.l. : Wonders - ISBN 9789054854708 - 88|
|Department(s)||Physical Chemistry and Colloid Science|
|Publication type||Dissertation, internally prepared|
|Keyword(s)||chemische speciatie - zware metalen - colloïden - elektrochemie - chemical speciation - heavy metals - colloids - electrochemistry|
|Categories||Colloid and Surface Chemistry / Electrochemistry|
The term "heavy metals" is connected with toxicity. They form strong complexes with enzymes, other proteins and DNA in living organisms, which causes dysfunctioning and hence poisoning. In combination with the uptake mechanism of the organism, speciation of heavy metal determines the bio-availability of heavy metals. In the environment, heavy metals are complexed by soil particles or molecules of organic and inorganic origin. This thesis deals with the speciation and the binding characteristics of heavy metals. Since complexation of heavy metals with soil particles is far too complex because of the wide range of different particles, this investigation is restricted to binding to a model system. The model system consists of polystyrene latices with and without a hydrophilic polymer shell. The surfaces of these latices contain negatively charged surface (shell) groups which can act as metaI-complexing agents. The binding can be investigated using various types of voltammetric techniques (Chapter I). To study metal binding, we first determined the amounts and types of surface groups present on the latices using potentiometry and conductometry (Chapter II). The polystyrene latex without shell showed a very high density of mainly weak, carboxylic groups on the surface. The surfaces (and shell) of the core-shell latices consist of a fraction strong acids (sulphonics) and a fraction of weak acids (carboxylics). Their shells and surfaces contain a lower total amount of groups than the polystyrene latex without shell. All conductometric results are qualitatively in agreement with those obtained by potentiometry, although the conductometric data appear to be more accurate. Potentiometry using potassium hydroxide, followed by a titration using nitric acid, was performed on one core-shell latex, indicating reversibility. During the titration with KOH, surface groups in the shell migrate to the surface. This effect is reversible. For one core-shell latex, potentiometric studies were carried out at different concentrations of supporting electrolyte (potassium nitrate). As expected, the pH increases more the lower the ionic strength during a titration. The total amount of titratable surface groups increases with higher concentration of supporting salt.
As a following step, the metal complexes formed were characterized (Chapter III). voltammetric experiments, such as Cottrell type experiments, with all core-shell latices studied, show the formation of labile zinc(II) and cadmium(II) ion complexes at very low metal-to-site ratios in the time scale of pulse voltammetry. This means that the residence time of the metal ion in the complex form is very small compared to the pulse time. The application of the voltammetric model of de Jong et al. for dissolved complexes is succesfully used for the analysis of the binding of metal ions by colloidal particles. At a decreased metal to ligand ratio, the complexes formed were still labile, but their stability constants were slightly higher. Perhaps there is a minority of strong complexing surface groups, due to clustering or impurities in the shell, resulting in different affinities for metal ions. The metal/carboxylate surface complexes of the highly charged latex lose lability at high degree of dissociation. Also, stability constants obtained from the normalized current diverged from those obtained from the potential shift, with higher stability constants for the latter one. Some aspects of this discrepancy are discussed. The calculation of the kinetics of the lead-carboxylate complexes using the lability criterion of de Jong et al . shows that these complexes are marginally labile.
Chapter IV deals with the characterization of surface groups by voltammetric titration, which is more complex than often assumed. This chapter tackles some of the methodological pitfalls which can be easily overlooked. Further, we estimated the amount of cadmium-complexing surface groups of some latices. The (complete) titration curves for all latices are regularly shaped. At the very onset of the titration curves complexes with larger binding constants were formed. This is probably due to the heterogeneity in surface groups described above. A procedure in which a regression line is computed using the diffusion coefficient of the latex metal complexes, can be used in the analysis. This procedure also provides one of the checks whether or not a metal complex is labile. The cadmium(II)-complexing capacity of the latices increases parallel to the fraction of carboxylic groups. Assuming a 1:2 binding ratio, roughly 30% of the sulphonate groups and 80% of the carboxylate groups bind cadmium(II) It seems that charge compensation plays a major role. Since the complexes formed by the polystyrene latex with a very high density of carboxylic groups only are not labile, the data for this latex were treated as if its surface sites would form inert complexes. An impression about the error of this treatment can be given; it seems rather small, just a few percent, due to the low diffusion coefficient of the latex particles.
On the basis of potentiometric titrations at varied supporting electrolyte concentrations, we applied Donnan and Donnan-derived models by Ohshima and Kondo to describe the proton binding using the potential in the shell of a latex in Chapter V. In addition, the cadmium-binding properties of a core-shell type of latex were determined using differential pulse polarography. The assumptions in the shell potential model used are: the shell has a constant thickness independent of the ionic strength, the relative dielectric permittivity coefficient is 80, the degree of dissociation is constant over the shell and the site distribution is homogeneous. These assumptions did not affect the description of proton binding to a core-shell latex. Donnan's approach describes reasonably well the proton binding on the surface groups of the core-shell latex coded AOY5. Ohshima's model refines this description, by taking a Poisson-Boltzmann distribution of ions near and in the shell into account. This is an improvement. It seems that the potential correction based on the (indifferent) salt concentration is a major parameter for the binding of protons. The logarithm of the intrinsic cadmium binding constant (extrapolated to a shell charge of zero) is 1.0-1.2 for the carboxylic groups, comparable to corresponding bulk values for various organic cadmium-carboxyl complexes.