Browsing by Author "Van Leeuwen, H. P."
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- Cd(II) speciation in alginate gelsPublication . Davis, T. A.; Kalis, E. J. J.; Pinheiro, J. P.; Town, R. M.; Van Leeuwen, H. P.Polysaccharides, such as those occurring in cell walls and biofilms, play an important role in metal speciation in natural aqueous systems. This work describes the speciation of Cd(II) in alginate gels chosen as a model system for biogels. The gels are formed by bridging calcium ions at junction zones present along adjacent homopolymeric guluronic acid chain sequences. The free Cd2+ concentration in the gel phase is measured by a novel in situ microelectrode voltammetric technique that monitors the electroactive probe cation Cd2+ by its reduction at a Au-amalgam microelectrode. In situ voltammetric measurement, coupled with total Ca(II) and Cd(II) determinations, as well as potentiometric titration, permits the full reconstruction of the charging environment and the cation binding for the gel phase. Three independent combinations of measuring and modeling the charged gel layer thereby permit accurate prediction of the Donnan potential, ΨD, and the Donnan enrichment coefficient, ΠD. At an ionic strength of 10 mM, Donnan potentials in the gel ranged from approximately −10 to −20 mV, corresponding to an enhancement in the level of free Cd2+ ions in the gel phase relative to the bulk solution by a factor of approximately 3. In contrast, the total level of Cd(II) was found to be enhanced by a factor of approximately 60, resulting predominantly from the specific binding of the Cd by the uronic acids of the alginate gel. These results emphasize that large differences in Cd(II) speciation can arise due to the combination of specific and electrostatic modes of binding. The results of this speciation analysis, for charged biological gels, have important consequences for mechanistic interpretation of metal biouptake processes involved in complex media.
- Dynamics and heterogeneity of Pb(II) binding by SiO 2 nanoparticles in an aqueous dispersionPublication . Goveia, D.; Pinheiro, J. P.; Milkova, V.; Rosa, A. H.; Van Leeuwen, H. P.ABSTRACT: Pb(II) binding by SiO2 nanoparticles in an aqueous dispersion was investigated under conditions where the concentrations of Pb2þ ions and nanoparticles are of similar magnitude. Conditional stability constants (log K) obtained at different values of pH and ionic strength varied from 4.4 at pH 5.5 and I = 0.1Mto 6.4 at pH 6.5 and I = 0.0015 M. In the range of metal to nanoparticle ratios from 1.6 to 0.3, log K strongly increases, which is shown to be due to heterogeneity in Pb(II) binding. For an ionic strength of 0.1 M the Pb2þ/SiO2 nanoparticle system is labile, whereas for lower ionic strengths there is loss of lability with increasing pH and decreasing ionic strength. Theoretical calculations on the basis of Eigen-type complex formation kinetics seem to support the loss of lability. This is related to the nanoparticulate nature of the system, where complexation rate constants become increasingly diffusion controlled. The ion binding heterogeneity and chemodynamics of oxidic nanoparticles clearly need further detailed research.
- Measurement of Donnan potentials in gels by in situ microelectrode voltammetryPublication . Davis, T. A.; Yezek, L. P.; Pinheiro, J. P.; Van Leeuwen, H. P.This work describes the electrochemical methodology for the determination of the Donnan potential from diffusion-limited steady-state voltammograms of acrylamide gels. The technique is based upon the measurement of gel–sol systems that have reached Donnan equilibrium and contain Cd2+ as a probe ion. Au-amalgam microelectrodes are used to measure the Cd concentration in the gel phase relative to the solution phase, thus permitting comparison of the Cd voltammograms obtained in both phases. This approach yields two independent measures of the Donnan potential resulting from (i) the potential shift relative to the reference electrode, and (ii) the enhancement of the Cd2+ wave. Two suites of acrylamide gels containing 0.2% and 0.5% Na-acrylate were studied as a function of ionic strength by varying [NaNO3] and maintaining a constant concentration of the electroactive probe ion, [Cd2+] = 1 · 10 5 mol/L in the equilibrating solutions. Independent model predictions of the Donnan potential as a function of ionic strength that consider the effects of differential swelling on the charge density, the influence of a mixed electrolyte on the potential developed in the gel at the limit of low ionic strength and the effects of incomplete dissociation of the carboxylic functional groups were in agreement with the Donnan potentials independently measured by the twofold steady-state voltammetric approach.
- Metal speciation by DGT/DET in colloidal complex systemsPublication . Van Der Veeken, P. L. R.; Pinheiro, J. P.; Van Leeuwen, H. P.Gel-layer-based sensors are increasingly employed for dynamic trace metal speciation analysis in aquatic media. In DGT (diffusive gradients in a thin film), two different types of polyacrylamide hydrogels, designated as “open pore” and “restricted”, are commonly used for the diffusive gel layer. While both gels are known to be fully permeable to metal ions and small complexes, colloidal particles with radii from tens to hundreds of nanometers have generally been assumed to be excluded from the gel. Here we show, however, that for dispersions with Pb(II) as the probe metal and monodisperse latex particles as metal-binding agents, relatively large particles do enter the gel to a significant extent. By complementing DGT flux analysis with diffusive equilibration in a thin film accumulation data for the colloidal complex systems, it is demonstrated that with radii up to 130 nm permeation for particles is substantial. The consequences for interpretation of dynamic speciation data furnished by gel-based sensors are discussed.
- Metal speciation dynamics in colloidal ligand dispersionsPublication . Pinheiro, J. P.; Minor, M.; Van Leeuwen, H. P.In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.
- Metal speciation dynamics in colloidal ligand dispersions. Part 2: electrochemical labilityPublication . Pinheiro, J. P.; Minor, M.; Van Leeuwen, H. P.We investigate the dynamic nature of metal speciation in colloidal dispersions using a recently proposed theory [J.P. Pinheiro, M. Minor, H.P. Van Leeuwen, Langmuir, 21 (2005) 8635] for complexing ligands that are situated on the surface of the particles. The new approach effectively modifies the finite rates of association/dissociation of the colloidal metal complexes, thus invoking consideration of the two basic dynamic criteria: the association/dissociation kinetics of the volume complexation reaction (the ‘‘dynamic’’ criterion), and the interfacial flux of free metal to a macroscopic surface due to dissociation of complex species (the ‘‘lability’’ criterion). We demonstrate that the conventional approach for homogeneous systems that assume a smeared-out ligand distribution, overestimates both the dynamics and the lability of metal complexes when applied to colloidal ligands. It is also shown that the increase of lability with increasing particle radius, as expected for a homogeneous solution, is moderated for spherical microelectrodes and practically eliminated for planar electrodes.
- Metal speciation dynamics in colloidal ligand dispersions. Part 3: Lability features of steady-state systemsPublication . Pinheiro, J. P.; Domingos, R. F.; Minor, M.; Van Leeuwen, H. P.A lability criterion is developed for dynamic metal binding by colloidal ligands with convective diffusion as the dominant mode of mass transport. Scanned stripping chronopotentiometric measurements of Pb(II) and Cd(II) binding by carboxylated latex core-shell particles were in good agreement with the predicted values. The dynamic features of metal ion binding by these particles illustrate that the conventional approach of assuming a smeared-out homogeneous ligand distribution overestimates the lability of a colloidal ligand system. Due to the nature of the spatial distribution of the binding sites, the change in lability of a metal species with changing ligand concentration depends on whether the ligand concentration is varied via manipulation of the pH (degree of protonation) or via the particle concentration. In the former case the local ligand density varies, whereas in the latter case it is constant. This feature provides a useful diagnostic tool for the presence of geometrically constrained binding sites.
- Metal speciation dynamics in monodisperse soft colloidal ligand suspensionsPublication . Duval, J. F. L.; Pinheiro, J. P.; Van Leeuwen, H. P.A comprehensive theory is presented for the dynamics of metal speciation in monodisperse suspensions of soft spherical particles characterized by a hard core and an ion-permeable shell layer where ligands L are localized. The heterogeneity in the binding site distribution leads to complex formation/dissociation rate constants (denoted as ka / and kd /, respectively) that may substantially differ from their homogeneous solution counterparts (ka and kd). The peculiarities of metal speciation dynamics in soft colloidal ligand dispersions result from the coupling between diffusive transport of free-metal ions M within and around the soft surface layer and the kinetics of ML complex formation/dissociation within the shell component of the particle. The relationship between ka,d / and ka,d is derived from the numerical evaluation of the spatial, time-dependent distributions of free and bound metal. For that purpose, the corresponding diffusion equations corrected by the appropriate chemical source term are solved in spherical geometry using a Kuwabara-cell-type representation where the intercellular distance is determined by the volume fraction of soft particles. The numerical study is supported by analytical approaches valid in the short time domain. For dilute dispersions of soft ligand particles, it is shown that the balance between free-metal diffusion within and outside of the shell and the kinetic conversion of M into ML within the particular soft surface layer rapidly establishes a quasi-steadystate regime. For sufficiently long time, chemical equilibrium between the free and bound metal is reached within the reactive particle layer, which corresponds to the true steady-state regime for the system investigated. The analysis reported covers the limiting cases of rigid particles where binding sites are located at the very surface of the particle core (e.g., functionalized latex colloids) and polymeric particles that are devoid of a hard core (e.g., polysaccharide macromolecules, gel particles). For both the transient and quasi-steady-state regimes, the dependence of ka,d / on the thickness of the soft surface layer, the radius of the hard core of the particle, and the kinetic rate constants ka,d for homogeneous ligand solutions is thoroughly discussed within the context of dynamic features for colloidal complex systems.
- Metal specification dynamics and bioavailability: bulk depletion effectsPublication . Pinheiro, J. P.; Galceran, J.; Van Leeuwen, H. P.Under conditions of bulk depletion, the speciation and bioavailability of trace metals must be considered at two different time scales: (i) the time scale of the biouptake flux, as determined by diffusion of the bioactive free metal, dissociation of the bioinactive complex species, and the internalization rate; and (ii) the time scale of depletion of the bulk medium. The implications of these two time scales for the speciation dynamics are discussed in terms of experimental conditions. The geometry of the system is taken into account via a spherical cellular model. It considers a spherical organism depleting a spherical volume in a nonstirred medium and assumes linear adsorption of the metal at the biointerface and first-order internalization kinetics. In cases where the rate of biouptake is fully controlled by the internalization step, concentration gradients in the medium are insignificant. Then the biouptake becomes independent of the geometry of the system, and the model has a much simpler solution. Examples of trace metal uptake by microorganisms are analyzed: (i) cobalt uptake by Prochlorococcus in the presence of NTA, under conditions where bulk depletion is the controlling process due to the large number of organisms and high internalization rates, (ii) silver uptake by Chlamydomonas reinhardtii with significant effects of bulk depletion, due to the high internalization rate; (iii) lead uptake by Chlorella vulgaris with pratically negligible bulk depletion due to the low internalization rate of the metal; and (iv) lead uptake by intestinal Caco-2 cells, illustrating the simplification of the bulk depletion model for a system with different geometry where internalization is the rate-controlling step.
- Scanned stripping chronopotentiometry of metal complexes: lability diagnosis and stability computationPublication . Pinheiro, J. P.; Van Leeuwen, H. P.A method is presented for analyzing the dynamic speciation features of metal complexes based on stripping chronopotentiometry at a scanned deposition potential (SSCP). The shift in the SSCP half-wave deposition potential, DEd;1=2, is straightforwardly related to the complex stability, K, irrespective of the degree of lability; the limiting wave height, s , quantifies the metal species accumulated in the electrode and thus depends on both the lability and mass transport properties of the metal complex species in solution. For complexes with a lower diffusion coefficient than the free metal ion, K calculated from DEd;1=2 will be the same as that derived from the relative s values so long as the system is fully labile. Discrepancies between DEd;1=2-derived and s -derived K values indicate loss of lability. We show that this approach is a sensitive indicator of lability, as illustrated by analysis of cadmium and lead binding by carboxylated nanospheres.