Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Chemodynamics of soft Charged nanoparticles in aquatic media: Fundamental concepts
    Town, R.M. ; Buffle, J. ; Duval, J.F.L. ; Leeuwen, H.P. van - \ 2013
    The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 117 (2013)33. - ISSN 1089-5639 - p. 7643 - 7654.
    metal speciation dynamics - aqueous-solutions - solvent exchange - complexes - relaxation - nmr - ion - approximation - temperature - membranes
    Understanding the extraordinary ionic reactivity of aqueous nanoparticles
    Leeuwen, H.P. van; Buffle, J. ; Duval, J.F.L. ; Town, R.M. - \ 2013
    Langmuir 29 (2013)33. - ISSN 0743-7463 - p. 10297 - 10302.
    soft - chemodynamics - size - coagulation - relaxation - complexes - ligands - binding - cu(ii)
    Nanoparticles (NPs) are generally believed to derive their high reactivity from the inherently large specific surface area. Here we show that this is just the trivial part of a more involved picture. Nanoparticles that carry electric charge are able to generate chemical reaction rates that are even substantially larger than those for similar molecular reactants. This is achieved by Boltzmann accumulation of ionic reactants and the Debye acceleration of their transport. The ensuing unique reactivity features are general for all types of nanoparticles but most prominent for soft ones that exploit the accelerating mechanisms on a 3D level. These features have great potential for exploitation in the catalysis of ionic reactions: the reactivity of sites can be enhanced by increasing the indifferent charge density in the NP body
    Chemodynamics of metal complexation by natural soft colloids: Cu(II) binding by humic acid
    Town, R.M. ; Duval, J.F.L. ; Buffle, J. ; Leeuwen, H.P. van - \ 2012
    The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 116 (2012)25. - ISSN 1089-5639 - p. 6489 - 6496.
    physicochemical heterogeneity - solvent exchange - copper-binding - simple ligands - fulvic-acids - ion-binding - relaxation - substances - kinetics - model
    The chemodynamics of Cu(II) complexation by humic acid is interpreted in terms of recently developed theory for permeable charged nanoparticles. Two opposing electric effects are operational with respect to the overall rate of association, namely, (i) the conductive enhancement of the diffusion of Cu2+, expressed by a coefficient f(e nu), which accounts for the accelerating effect of the negative electrostatic field of the humic particle on the diffusive transport of metal ions toward it, and (ii) the ionic Boltzmann equilibration with the bulk solution, expressed by a factor f(B), which quantifies the extent to which Cu2+ ions accumulate in the negatively charged particle body. These effects are combined in the probability of outer-sphere metal site complex formation and the covalent binding of the metal ion by the complexing site (inner-sphere complex formation) as in the classical Eigen mechanism. Overall "experimental" rate constants for CuHA complex formation, k(a), are derived from measurements of the thermodynamic stability constant, K*, and the dissociation rate constant, k(d)(*), as a function of the degree of metal ion complexation, theta. The resulting k(a) values are found to be practically independent of theta. They are also compared to theoretical values; at an ionic strength of 0.1 mol dm(-3), the rate of diffusive supply of metal ions toward the particles is comparable to the rate of inner-sphere complex formation, indicating that both processes are significant for the observed overall rate. As the ionic strength decreases, the rate of diffusive supply becomes the predominant rate-limiting process, in contrast with the general assumption made for complexes with small ligands that inner-sphere dehydration is the rate-limiting step. The results presented herein also resolve the discrepancy between experimentally observed and predicted dissociation rate constants based on the above assumption.
    Chemodynamics of soft nanoparticulate complexes: Cu(II) and Ni(II) complexes with fulvic acids and aquatic humic acids
    Town, R.M. ; Leeuwen, H.P. van; Buffle, J. - \ 2012
    Environmental Science and Technology 46 (2012)19. - ISSN 0013-936X - p. 10487 - 10498.
    nuclear-magnetic-resonance - dissolved organic-matter - metal-ion binding - dynamic speciation analysis - deposition potential sscp - aqueous-solutions - copper-binding - water exchange - stripping chronopotentiometry - dissociation kinetics
    The dynamics of metal complexation by small humic substances (fulvic acid and aquatic humic acid, collectively denoted as "fulvic-like substance", FS) are explored within the framework of concepts recently developed for soft nanoparticulate complexants. From a comprehensive collection of published equilibrium and dissociation rate constants for CuFS and NiFS complexes, the association rate constant, k(a), is determined as a function of the degree of complexing site occupation, theta. From this large data set, it is shown for the first time that k(a) is independent of theta. This result has important consequences for finding the nature of the rate limiting step in the association process. The influence of electric effects on the rate of the association process is described, namely (i) the accelerating effect of the negatively charged electrostatic field of FS on the diffusion of metal ions toward it, and (ii) the extent to which metal ions electrostatically accumulate in the counterionic atmosphere of FS. These processes are discussed qualitatively in relation to the derived values of k(a). For slowly dehydrating metal ions such as Ni(H2O)(6)(2+) (dehydration rate constant, k(w)), k(a) is expected to derive straight from k(w). In contrast, for rapidly dehydrating metal ions such as Cu(H2O)(6)(2+), transport limitations and electric effects involved in the formation of the precursor outer-sphere associate appear to be important overall rate-limiting factors. This is of great significance for understanding the chemodynamics of humic complexes in the sense that inner-sphere complex formation would not always be the (sole) rate-limiting step.
    Electric relaxation processes in chemodynamics of aqueous metal complexes: From simple ligands to soft nanoparticulate complexants
    Leeuwen, H.P. van; Buffle, J. ; Town, R.M. - \ 2012
    Langmuir 28 (2012)1. - ISSN 0743-7463 - p. 227 - 234.
    dynamic speciation - physicochemical parameters - humic substances - particles - dispersions - coagulation - compilation - binding - flux
    The chemodynamics of metal complexes with nanoparticulate complexants can differ significantly from that for simple ligands. The spatial confinement of charged sites and binding sites to the nanoparticulate body impacts on the time scales of various steps in the overall complex formation process. The greater the charge carried by the nanoparticle, the longer it takes to set up the counterion distribution equilibrium with the medium. A z+ metal ion (z > 1) in a 1:1 background electrolyte will accumulate in the counterionic atmosphere around negatively charged simple ions, as well as within/around the body of a soft nanoparticle with negative structural charge. The rate of accumulation is often governed by diffusion and proceeds until Boltzmann partition equilibrium between the charged entity and the ions in the medium is attained. The electrostatic accumulation proceeds simultaneously with outer-sphere and inner-sphere complex formation. The rate of the eventual inner-sphere complex formation is generally controlled by the rate constant of dehydration of the metal ion, kw. For common transition metal ions with moderate to fast dehydration rates, e.g., Cu2+, Pb2+, and Cd2+, it is shown that the ionic equilibration with the medium may be the slower step and thus rate-limiting in their overall complexation with nanoparticles
    Chemodynamics of soft nanoparticulate metal complexes in aqueous media: Basic theory for spherical particles with homogeneous spatial distributions of sites and charges
    Leeuwen, H.P. van; Town, R.M. ; Buffle, J. - \ 2011
    Langmuir 27 (2011)8. - ISSN 0743-7463 - p. 4514 - 4519.
    speciation dynamics - physicochemical parameters - flux - compilation - ions
    A theoretical discussion is presented to describe the formation and dissociation rate constants for metal ion binding by soft nanoparticulate complexants. The well-known framework of the Eigen mechanism for metal ion complexation by simple ligands in aqueous systems is the starting point. Expressions are derived for the rate constants for the intraparticulate individual outer-sphere and inner-sphere association and dissociation steps for the limiting cases of low and high charge densities. The charge density, binding site density, and size of the nanoparticle play crucial roles. The effects of the electrostatic potential and particle radius on the overall complexation reaction are compared with those for simple ligands. The limitations of the proposed approach for nanoparticulate ligands are discussed, and key issues for future developments are identified
    Chemodynamics of aquatic metal complexes: From small ligands to colloids
    Buffle, J. ; Leeuwen, H.P. van - \ 2009
    Environmental Science and Technology 43 (2009)19. - ISSN 0013-936X - p. 7175 - 7183.
    debye-huckel theory - dynamic speciation - physicochemical parameters - humic substances - bio interfaces - outer-sphere - kinetics - flux - adsorption - binding
    Recent progress in understanding the formation/dissociation kinetics of aquatic metal complexes with complexants in different size ranges is evaluated and put in perspective, with suggestions for further studies. The elementary steps in the Eigen mechanism, i.e., diffusion and dehydration of the metal ion, are reviewed and further developed. The (de)protonation of both the ligand and the coordinating metal ion is reconsidered in terms of the consequences for dehydration rates and stabilities of the various outer-sphere complexes. In the nanoparticulate size range, special attention is given to the case of fulvic ligands, for which the impact of electrostatic interactions is especially large. In complexation with colloidal ligands (hard, soft, and combination thereof) the diffusive transport of metal ions is generally a slower step than in the case of complexation with small ligands in a homogeneous solution. The ensuing consequences for the chemodynamics of colloidal complexes are discussed in detail and placed in a generic framework, encompassing the complete range of ligand sizes
    Chemodynamics and bioavailability in natural waters
    Buffle, J. ; Wilkinson, K.J. ; Leeuwen, H.P. van - \ 2009
    Environmental Science and Technology 43 (2009)19. - ISSN 0013-936X - p. 7170 - 7174.
    dynamic speciation - nutrient uptake - metal flux - diffusion - transport - limitation - complexes - bacteria - biofilm - systems
    Metal Flux in ligand mixtures. 2. Flux enhancement due to kinetic interplay: Comparison of the reaction layer approximation with a rigorous approach
    Zhang, Z. ; Buffle, J. ; Town, R.M. ; Puy, J. ; Leeuwen, H.P. van - \ 2009
    The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 113 (2009)24. - ISSN 1089-5639 - p. 6572 - 6580.
    reaction-diffusion processes - complex-systems - dynamic speciation - alga chlamydomonas - phytoplankton - interfaces - lability - code
    The revisited reaction layer approximation (RLA) of metal flux at consuming interfaces in ligand mixtures, discussed in the previous paper (part 1 of this series15) is systematically validated by comparison with the results of rigorous numerical simulations. The current paper focuses on conditions under which the total metal flux is enhanced in the ligand (and complex) mixture compared to the case where the individual fluxes of metal complexes are independent of each other. Such an effect is exhibited only in ligand mixtures and results from the kinetic interplay between the various complexes with different labilities. It is exemplified by the Cu/NTA/N-(2-carboxyphenyl)glycine system (see part 1 paper), in which we show that the flux due to the less labile complex (CuNTA) is increased in the presence of a ligand (2-carboxyphenyl)glycine) that forms labile Cu complexes, even when the latter is in negligible proportion in the bulk solution. This paper first explains how the so-called composite and equivalent reaction layer thicknesses computed by RLA can be determined graphically from the concentration profiles of free metal and its complexes, as obtained by rigorous calculations. This approach allows comparison between the latter and RLA predictions. Comparison between these reaction layer thicknesses is then done using the chemical system mentioned above. The mechanism of flux enhancement with this system is studied in detail by following the change of the concentration profiles and reaction layer thicknesses with the increase of concentration of the ligand forming labile complexes. The mechanism of flux enhancement is well explained by the RLA and is validated by the concentration profiles obtained by rigorous numerical simulations. Based on this validation, the RLA is used to predict the conditions of the individual complex labilities and degree of complexation required to get flux enhancement in a two-ligand system. Due to compensation effects between kinetic and thermodynamic factors, a maximum flux enhancement is observed in a specific range of ratios of the lability indices of the two complexes. Flux enhancement might play a significant role in metal uptake in environmental or biological systems and should be considered in data interpretation
    Biophysical Chemistry of Fractal Structures and Processes in Environmental Systems
    Buffle, J. ; Leeuwen, H.P. van - \ 2008
    Chichester : John Wiley and Sons (Series on analytical and physical chemistry of environmental systems ) - ISBN 9780470014745 - 323
    fractals - uitvlokking - coagulatie - fractal meetkunde - fractals - flocculation - coagulation - fractal geometry
    This book aims to provide the scientific community with a novel and valuable approach based on fractal geometry concepts on the important properties and processes of diverse environmental systems. The interpretation of complex environmental systems using modern fractal approaches is compared and contrasted with the more classical approaches. The book will provide the fundamental knowledge necessary for solving practical environmental problems. Furthermore, it examinea how the fractal approach has been applied in order to understand the structure and reactivity of natural, environmental systems including flocs, sediments, soils, microorganisms and humic substances.
    Impact of ligand protonation on eigen-type metal complexation kinetics in aqueous systems
    Leeuwen, H.P. van; Town, R.M. ; Buffle, J. - \ 2007
    The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 111 (2007)11. - ISSN 1089-5639 - p. 2115 - 2121.
    deposition potential sscp - stripping chronopotentiometry - dissociation kinetics - speciation analysis - planar diffusion - lability - association - cadmium - ions - thermodynamics
    The impact of ligand protonation on metal speciation dynamics is quantitatively described. Starting from the usual situation for metal complex formation reactions in aqueous systems, i.e., exchange of water for the ligand in the inner coordination sphere as the rate-determining step (Eigen mechanism), expressions are derived for the lability of metal complexes with protonated and unprotonated ligand species being involved in formation of the precursor outer-sphere complex. A differentiated approach is developed whereby the contributions from all outer-sphere complexes are included in the rate of complex formation, to an extent weighted by their respective stabilities. The stability of the ion pair type outer-sphere complex is given particular attention, especially for the case of multidentate ligands containing several charged sites. It turns out that in such cases, the effective ligand charge can be considerably different from the formal charge. The lability of Cd(II) complexes with 1,2-diaminoethane-N,N'-diethanoic acid at a microelectrode is reasonably well predicted by the new approach.
    Environmental Colloids and Particles
    Buffle, J. ; Leeuwen, H.P. van; Wilkinson, K.J. ; Lead, J.R. - \ 2007
    Chichester : John Wiley and Sons (IUPAC Series on Analytical and Physical Chemistry of Environmental Systems 10) - ISBN 9780470024324 - 685 p.
    Roles of dynamic metal speciation and membrane permeability in metal flux through lipophilic membranes: General theory and experimental validation with nonlabile complexes
    Zeshi, Zhang ; Buffle, J. ; Leeuwen, H.P. van - \ 2007
    Langmuir 23 (2007)9. - ISSN 0743-7463 - p. 5216 - 5226.
    supported liquid-membrane - selective separation - copper speciation - kinetics - extraction - lability - waters - ion - voltammetry - interfaces
    The study of the role of dynamic metal speciation in lipophilic membrane permeability in aqueous solution requires accurate interpretation of experimental data. To meet this goal, a general theory is derived for describing 1:1 metal complex flux, under steady-state and ligand excess conditions, through a permeation liquid membrane (PLM). The theory is applicable to fluxes through any lipophilic membrane. From this theory, fluxes in the three rate-limiting conditions for metal transport are readily derived, corresponding, namely, to (i) diffusion in the source solution, (ii) diffusion in the membrane, and (iii) the chemical kinetics of formation/dissociation of the metal complex in the interfacial reaction layer. The theory enables discussion of the reaction layer concept in a more general frame and also provides unambiguous criteria for the definition of an inert metal complex. The theoretical flux equations for fully labile complexes were validated in a previous paper. The general theory for semi- or nonlabile complexes is validated here by studying the flux of Pb(II) through PLMs in contact with solutions of Pb(II)-NTA and Pb(II)-TMDTA at different pHs and flow rates.
    Quasi-reversible Faradaic depolarization processes in the electrokinetics of the metal/solution interface
    Duval, J.F.L. ; Buffle, J. ; Leeuwen, H.P. van - \ 2006
    The Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical 110 (2006)12. - ISSN 1520-6106 - p. 6081 - 6094.
    metal/electrolyte solution interface - lateral electric-field
    Bipolar faradaic depolarization of the metal/solution interface is quantitatively analyzed for the case where the solution is subject to lateral flow and contains a quasi-reversible redox couple. Transversal convective diffusion of the electroactive species and a position-dependent degree of reversibility of the interfacial electrontransfer (e.t.) reaction are among the primary features that govern depolarization. The spatial distributions of species concentrations and electric potential are numerically simulated. The system is characterized by nonlinear coupling between the transport (diffusion and flow) and the electric potential distribution under conditions of finite local currents. The resulting picture is that the reversibility of the e.t. reaction varies with position on the surface, with the highest reversibility downstream. This, in itself, generally leads to strongly asymmetric profiles of the faradaic current density along the surface. The impact on the electrokinetic properties of the interface is huge. For example, the steady-state streaming potential is depressed by the contribution from the bipolar faradaic process to the back current to an extent that varies from insignificant to complete, depending on the e.t. rate constant and concentrations of the electroactive species
    Roles of metal ion complexation and membrane permeability in the metal flux through lipophilic membranes. Labile complexes at permeation liquid membranes
    Zhang, Z. ; Buffle, J. ; Leeuwen, H.P. van; Wojciechowski, K. - \ 2006
    Analytical Chemistry 78 (2006)16. - ISSN 0003-2700 - p. 5693 - 5703.
    copper speciation - natural-waters - trace-metals - fatty-acid - separation - voltammetry - system - enrichment - transport - as(iii)
    The various physicochemical factors that influence the flux of carrier-transported metal ions through permeation liquid membranes (PLM) are studied systematically. Understanding PLM behavior is important (i) to optimize the application of PLM as metal speciation sensors in environmental media and (ii) because PLM may serve as bioanalogical devices that help to elucidate the environmental physicochemical processes occurring at the surface of biological membranes. Diffusion of free and complexed metal ions in solution, as well as diffusion of the metal carrier complex in the membrane, is considered. The respective roles of diffusion layer thickness, ligand concentration, complex stability, carrier concentration, and membrane thickness are studied experimentally in detail and compared with theory, using various labile complexes, namely, Pb(II)-diglycolate, Cu(II)-diglycolate, and Cu(II)-N-(2-carboxyphenyl)glycine. Conditions where either membrane diffusion or solution diffusion is rate limiting are clearly discriminated. It is shown in particular, that, by tuning the carrier concentration or membrane thickness, either the free metal ion concentration or the total labile metal species are measured. PLM can thus be used to determine whether models based on the free ion activity in solution (such as BLM or FIAM models) are applicable to metal uptake by microorganisms in a real natural medium
    Model predictions of metal speciation in freshwaters compared to measurements by in situ techniques
    Unsworth, E.R. ; Warnken, K.W. ; Zhang, H. ; Davison, W. ; Black, F. ; Buffle, J. ; Cao, J. ; Cleven, R.F.M.J. ; Galceran, J. ; Gunkel, P. ; Kalis, E.J.J. ; Kistler, D. ; Leeuwen, H.P. van; Martin, M. ; Noel, S. ; Nur, Y. ; Odzak, N. ; Pauy, J. ; Riemsdijk, W.H. van; Sigg, L. ; Temminghoff, E.J.M. ; Tercier-Waeber, M.L. ; Topperwien, S. ; Town, R.M. ; Weng, L.P. ; Xue, Hanbin - \ 2006
    Environmental Science and Technology 40 (2006)6. - ISSN 0013-936X - p. 1942 - 1949.
    donnan membrane technique - permeation liquid-membrane - dissolved organic-matter - biotic ligand model - humic substances - natural-waters - ion-binding - thin-films - diffusive gradients - acute toxicity
    Measurements of trace metal species in situ in a softwater river, a hardwater lake, and a hardwater stream were compared to the equilibrium distribution of species calculated using two models, WHAM 6, incorporating humic ion binding model VI and visual MINTED incorporating NICA-Donnan. Diffusive gradients in thin films (DGT) and voltammetry at a gel integrated microelectrode (GIME) were used to estimate dynamic species that are both labile and mobile. The Donnan membrane technique (DMT) and hollow fiber permeation liquid membrane (HFPLM) were used to measure free ion activities. Predictions of dominant metal species using the two models agreed reasonably well, even when colloidal oxide components were considered. Concentrations derived using GIME were generally lower than those from DGT, consistent with calculations of the lability criteria that take into account the smaller time window available for the flux to GIME. Model predictions of free ion activities generally did not agree with measurements, highlighting the need for further work and difficulties in obtaining appropriate input data
    Comparison of analytical techniques for dynamic trace metal speciation in natural freshwaters
    Sigg, L. ; Black, F. ; Buffle, J. ; Cao, J. ; Cleven, R. ; Davison, W. ; Galceran, J. ; Gunkel, P. ; Kalis, E.J.J. ; Kistler, D. ; Martin, M. ; Noel, S. ; Nur, Y. ; Odzak, N. ; Puy, J. ; Riemsdijk, W.H. van; Temminghoff, E.J.M. ; Tercier-Waeber, M.L. ; Toepperwien, S. ; Town, R.M. ; Unsworth, E.R. ; Warnken, K.W. ; Weng, L.P. ; Xue, H.B. ; Zhang, H. - \ 2006
    Environmental Science and Technology 40 (2006)6. - ISSN 0013-936X - p. 1934 - 1941.
    permeation liquid-membrane - in-situ measurements - voltammetry cle-adsv - biotic ligand model - stripping chronopotentiometry - thin-films - microelectrode - bioavailability - complexes - systems
    Several techniques for speciation analysis of Cu, Zn, Cd, Pb, and Ni are used in freshwater systems and compared with respect to their performance and to the metal species detected. The analytical techniques comprise the following: (i) diffusion gradients in thin-film gels (DGT); (ii) gel integrated microelectrodes combined to voltammetric in situ profiling system (GIME-VIP); (iii) stripping chronopotentiometry (SCP); (iv) flow-through and hollow fiber permeation liquid membranes (FTPLM and HFPLM); (v) Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE-SV). All methods could be used both under hardwater and under softwater conditions, although in some cases problems with detection limits were encountered at the low total concentrations. The detected Cu, Cd, and Pb concentrations decreased in the order DGT ¿ GIME-VIP ¿ FTPLM ¿ HFPLM ¿ DMT (> CLE-SV for Cd), detected Zn decreased as DGT ¿ GIME-VIP and Ni as DGT > DMT, in agreement with the known dynamic features of these techniques. Techniques involving in situ measurements (GIME-VIP) or in situ exposure (DGT, DMT, and HFPLM) appear to be appropriate in avoiding artifacts which may occur during sampling and sample handling
    Humic substances are soft and permeable : evidence from their electrophoretic mobilities
    Duval, J.F.L. ; Wilkinson, K.J. ; Leeuwen, H.P. van; Buffle, J. - \ 2005
    Environmental Science and Technology 39 (2005)17. - ISSN 0013-936X - p. 6435 - 6445.
    fluorescence correlation spectroscopy - ion complexation equilibria - fulvic-acids - proton-binding - physicochemical description - polyelectrolyte properties - diffusion-coefficients - colloidal particles - organic-matter - ph
    Due to the complexity of the humic substances (HS), mathematical models have often been employed to understand their roles in the environment. Since no consensus exists with respect to the structure and conformation of the HS, models have alternatively given them properties corresponding to impermeable hard spheres or fully permeable polyelectrolytes. In this study, the hydrodynamic permeability of standard HS (Suwannee River fulvic, humic, and peat humic acids) are evaluated as a function of pH and ionic strength. A detailed theoretical model is used to determine the softness parameter (lambda0), which characterizes the degree of flow penetration into the HS on the basis of measured values of electrophoretic mobilities, diffusion coefficients, and electric charge densities. Their motion in an electric field is evaluated by a rigorous numerical evaluation of the governing electrokinetic equations for soft particles. The hydrodynamic impact of the polyelectrolyte chains is accounted for by a distribution of Stokes resistance centers and partial dissociation of the hydrodynamically immobile ionogenic groups distributed throughout the polyelectrolyte. The results demonstrate thatthe studied HS are small (radius ca. 1 nm), highly charged (500-650 C g(-1) when all sites are dissociated), and very permeable (typical flow penetration length of 25-50% of the radius, depending on pH). The HS also coagulate slightly when lowering the pH of the solution. Modeling of the HS as hard spheres with a charge and slip plane located at the surface is thus physically inappropriate, as are a number of analytical theories for soft particles that hold for low to moderate electrostatic potentials and large colloids. The shortcomings of these simpler approaches, when interpreting the electrophoretic mobilities of HS, are highlighted by comparison with rigorous theoretical predictions.
    Dynamic speciation analysis and bioavailability of metals in aquatic systems
    Leeuwen, H.P. van; Town, R.M. ; Buffle, J. ; Cleven, R.F.M.J. ; Davison, W. ; Puy, J. ; Riemsdijk, W.H. van; Sigg, L. - \ 2005
    Environmental Science and Technology 39 (2005)22. - ISSN 0013-936X - p. 8545 - 8556.
    chemische speciatie - metalen - risicoschatting - biologische beschikbaarheid - waterbodems - chemical speciation - metals - risk assessment - bioavailability - water bottoms - in-situ measurements - deposition potential sscp - anodic-stripping voltammetry - permeation liquid-membrane - association dissociation kinetics - integrated microelectrode arrays - ion-selective electrodes - lower detection limit - biotic ligand model - natural-w
    Dynamic metal speciation analysis in aquatic ecosystems is emerging as a powerful basis for development of predictions of bioavailability and reliable risk assessment strategies. A given speciation sensor is characterized by an effective time scale or kinetic window that defines the measurable metal species via their labilities. Here we review the current state of the art for the theory and application of dynamic speciation sensors. We show that a common dynamic interpretation framework, based on rigorous flux expressions incorporating the relevant diffusion and reaction steps, is applicable for a suite of sensors that span a range of time scales. Interpolation from a kinetic spectrum of speciation data is proposed as a practical strategy for addressing questions of bioavailability. Case studies illustrate the practical significance of knowledge on the dynamic features of metal complex species in relation to biouptake, and highlight the limitations of equilibrium-based models.
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