Electrochemical metal speciation in natural and model polyelectrolyte systems

Abstract

The purpose of the research described in this thesis was to examine the applicability of electro-analytical techniques in obtaining information on the speciation of metals, i.e. their distribution over different physico-chemical forms, in aquatic systems containing charged macromolecules. In chapter 1 a general introduction is given to (i) metal speciation in aquatic systems, (ii) (bio)polyelectrolytes and their counterion distributions and (iii) electrochemical methods emphasizing their apllication to metal speciation.Chapter 2 deals with the conductometric: measurement of counterion association with macromolecules. First, we have surveyed theoretical developments concerning ion association for purely electrostatic interaction and as reflected in the conductivities of polyelectrolyte solutions. It will be shown that for the salt free case, the distribution of monovalent counterions can be obtained from plots of the molar conductivity of the polyelectrolyte solution versus the molar conductivity of the monovalent counterion, so-called Eisenberg plots. Experimental results for various alkali polymethacrylate concentrations show that the fraction of conductometrically- free monovalent counterions is in close agreement with theoretical predictions, which are based on a two-state appoach. Furthermore, for linear polyelectrolytes a recently proposed model for the case of counterion condensation in systems with ionic mixtures is presented. Finally, the treatment of conductometric data for polyelectrolyte solutions with either one type of counterion or mixtures of two types of counterions in terms of free and bound fractions is discussed.In chapter 3 we describe a voltammetric methodology for the analysis of labile homogeneous heavy metal-ligand complexes in terms of a stability K . The method takes into account the difference between the diffusion coefficients of the free and bound metal. Since the relationship between voltammetric current and mass transport properties under stripping voltammetric conditions is not yet well esthablished, we propose a relationship between the experimentally obtained current and the mean diffusion coefficient of the metal-complex system. A sensitivity analysis of this expression for different parameters, such as the stability and the ratio of the diffusion coefficients of the bound and free metal is performed.Natural complexing agents are often heterogeneous with regard to their affinity to metal ions. Therefore, we will discuss the evaluation of the heterogeneity of these complexes from voltammetric data for various metal-to-ligand ratios. For the case of a large excess of ligand over the metal atom concentration, the stability of the metal-complex system may be obtained independently from the potential shift. For this an equation is given similar to the classical one derived by DeFord and Hume. Finally, we present an experimental procedure based on adding ligand to the solution of the metal and measuring its voltammetric characteristics. The procedure takes into account (i) possible adsorption of metal ions to elements of the equipement and (ii) measuring all protolysis of the polyacids involved.The characteristic features of applying the two electrochemical techniques conductometry and voltammetry to the study of ion binding by polyelectrolytes are discussed and compared in chapter 4. Analysis of data on K +/Zn(II)/polyacrylate and K +/Zn(II)/polymethacrylate systems illustrates a certain complementary of the two methodologies. Conductometry primarily measures the Zn(II)/K +exchange ratio. Voltammetry measures the Zn(II)/polyion binding strength; its dependence on the (excess) K +concentration also yields information on the Zn(II)/K +exchange ratio. The different results seem to be fairly coherent.Experimental conductometric and voltammetric speciation data of metal-synthetic polyacid systems are presented and discussed in chapter 5. The competitive binding of monovalent and divalent counterions has been studied by the conductometric procedure described in chapter 2 for aqueous solutions of alkali metal polymethacrylates in the presence of Ca(NO 3 ) 2 and Mg(NO 3 ) 2 . The experimentally obtained fractions of conductometrically free counterions are compared with theoretical values computed according to a new thermodynamic model described in the same chapter. For the systems studied, the fractions of free monovalent and divalent counterions can be fairly well described by the theory. In fact, the results support the assumption that under the present conditions the conductometrically obtained distribution parameters f 1 and f 2 approximate the equilibrium fractions of free monovalent and divalent counterions. The experimentally obtained M +/M 2+exchange ratios agree well with the theoretical ones. Similar experiments have been performed for the Zn(II)/polyacrylate and Zn(II)/polymethacrylate systems. It seems that, compared to Ca 2+and Mg 2+ions, the Zn(II)-ions are bound more strongly. This could be due to some specific binding of Zn(II)-ions. Since the theoretical model does not incorporate this mechanisme, the experimental results do not agree well with the theoretical ones.Furthermore, chapter 5 collects the results of a systematic study of the stripping voltammetric behaviour of Zn(II)- and Cd(II)-ions in polyacrylate and polymethacrylate solutions. All metal- ligand complexes involved apprear to be voltammetrically labile over the whole range of metal-to- ligand ratios under the various experimental conditions employed. Hence, the voltammetric data could be analyzed in terms of a stability K according to the methodology presented in chapter 3. The first set of experiments is concerned with the influence of the molar mass of the polyacrylate anion on the stability. Analysis of the data in terms of a mean diffusion coefficient, which decreases with increasing molecular mass, yields a consistent picture with molar mass- independent complex stabilities. The speciation of Zn(II) in such a polyelectrolyte system varies with the concentration of carboxylate groups, but it is invariant with the polyionic molar mass. Secondly, the competition between monovalent (K +) and divalent (Zn(II) and Cd(II)) counterions has been investigated by varying the concentration of electroinactive supporting electrolyte. The results show that the stability of the heavy metal/polymethacrylate complex decreases with increasing KNO 3 concentration. This effect is largely due to the reduction of the electrostatic component of the metal/polyanion interaction, which is generally the case for polyelectrolytes with high charge densities. For the Zn(II)/polymethacrylate system, a comparison with conductometric data representing the competitive behaviour of monovalent and divalent counterions has been made in chapter 4. The influence of the polyelectrolyte charge density of the polymethacrylic acid on the stability K has been studied by varying the degree of neutralization of the polyanion. For the Zn(II)/PMA complexes, the stability increases approximately linearly with increasing degree of neutralization, i.e. with increasing polyionic charge density. This is in accordance with the general polyelectrolytic feature that counterion binding is stronger with higher polyionic charge density. Finally, for later comparison with natural complexing agents, the chemical homogeneity of the macromolecules involved has been verified by varying the total metal ion concentration for a given polyelectrolyte concentration. The results indeed confirm that the Zn(II)/polymethacrylate and Cd(II)/polymethacrylate complexes have a homogeneous energy distribution. This is in line with expectation, since these macromolecules consist of only one repeating chemical binding site, i.e. the carboxylate group.Chapter 6 deals with the pretreatment and characterization of humic material. The pretreatment procedure is used to purify the humic material in such a way that (i) the molecules are soluble under the experimental conditions employed in chapter 7, (ii) the amount of impurities is minimized and (iii) the resulting humic material is transferred into the acid form. Furthermore, a fractionation method based on the solubility of the humic substances is described. The humic material is characterized in terms of (i) the amount of chargeable groups by means of conductometric titration and (ii) molar mass distribution by flow field-flow-fractionation. It will be shown that although the fractionation by varying pH results in samples with different molar masses, the separation is far from ideal.As was done with the synthetic polyacids, experiments have been performed for natural occuring polyelectrolytes. Conductometric and voltammetric results forvarious metal humic acid systems are presented in chapter 7. Solutions of humic acids were conductometrically titrated with potassium, sodium, lithium, calcium and barium hydroxide solutions. The results have been analyzed in terms of fractions of free and bound metal. The conductance properties of humic acids are basically different from those of a linear polyelectrolyte such as polymethacrylate. A marked difference was observed between the shapes of the curves for alkali metal hydroxides and those for alkaline earth metal hydroxides. It appears that monovalent cations are hardly bound by the humate polyion, whereas divalent counterions show a strong interaction. The latter feature may be fruitfully utilized in quantitative analysis.The association of the heavy metals zinc(II) and cadmium(II) with humic acid samples has been furthere studied by differential pulse anodic stripping voltammetry (i) for various concentrations of supporting electrolyte (KNO 3 and Ca(NO 3 ) 2 ), (ii) for different degrees of neutralization of the humate polyion, (iii) for different metal-to-ligand ratios and (iv) for different fractions of the humic acid. Under the experimental conditions employed, all heavy metal/humate complexes have been found to be voltammetrically labile over the whole range of metal-to-ligand ratios. Hence, the stability (K) of the complex could be computed taking into account the difference between the diffusion coefficients of the free and bound metal. The dependence of K on the concentration of 1-1 electrolyte (KNO 3 ) is of comparable extent for various metal-humate complexes, but significantly smaller than in the case of the highly charged linear polyelectrolyte polymethacrylic acid. For the humic acid systems, it has been concluded that the relatively weak dependency of K on the salt concentration mainly reflects screening effects. The influence of the concentration of 2-1 electrolyte (Ca(NO 3 ) 2 ) on the stability of the heavy metal/humate complex is more pronounced than for the corresponding case of 1-1 electrolyte. By taking into account the association of calcium with the humate polyion, the stability of the heavy metal/humate complex was found to be more or less constant over the range of Ca(NO 3 ) 2 concentrations studied and comparable to the stability of the corresponding complex in the absence of calcium.The stability of the heavy metal/humate complex has been found to increase with increasing degree of neutralization, i.e. with increasing charge density of the humate polyion. It seemed that the increase of K is less pronounced for higher values of α n . This observation could not be interpreted from an electrostatic point of view, and is in fact a further indication that the binding of heavy metals with the humate polyion is mainly governed by the chemical characteristics of the humic acid. The chemical heterogeneity of the humic acids was investigated by varying the metal-to-ligand ratio for different total concentrations of the heavy metals but in a range of comparable ligand concentrations. The results show that the stability K of the heavy metal/humate complex decreases with increasing total metal ion concentration, reflecting a certain chemical heterogeneity of the humic acid. For various heavy metal/fractionated humate complexes, the stability K was found to be comparable to the K value for the corresponding unfractionated humic acid system. This means that the distribution of functional groups is more or less the same for different molar masses of the humic acid.For the present metal/humate complexes, the general conclusion is that the distribution of counterions over the free and bound states is mainly governed by the chemical heterogeneity of the humate polyion