Labilities of aqueous nanoparticulate metal complexes in environmental speciation analysis
Town, R.M. ; Leeuwen, H.P. van - \ 2014
Environmental Chemistry 11 (2014)2. - ISSN 1448-2517 - p. 196 - 205.
ombrotrophic peat bog - humic-acid - fulvic-acid - stripping chronopotentiometry - ionic-strength - dissociation kinetics - diffusive gradients - dynamic speciation - solvent exchange - organic-matter
An inherent property of a dispersion of charged nanoparticles is that their charges and reactive sites are spatially confined to the particle body which is at a different potential from that in the bulk medium. This feature has important consequences for the reactivity of nanoparticulate complexants: the diffusive rate of reactant supply is lower as compared to molecular complexants, whereas the local concentration of reactant ions may be enhanced if the particle’s electric field has the opposite charge sign. These effects are most dramatic for soft nanoparticles for which the electrostatic accumulation mechanisms operate on a 3-D level. We show how the interplay of these effects governs the reactivity of charged nanoparticulate metal complexes (M-NPs) at the surface of an analytical speciation sensor. A theoretical framework is presented that describes the lability of M-NP species over a range of effective timescales for different electrochemical and other dynamic speciation analysis techniques. The concepts are illustrated by electrochemical stripping data on metal complexes with natural soft nanoparticles of humic acid.
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
Strategies in the application of the Donnan membrane technique.
Weng, L.P. ; Vega, F.A. ; Riemsdijk, W.H. van - \ 2011
Environmental Chemistry 8 (2011)5. - ISSN 1448-2517 - p. 466 - 474.
metal-ion concentrations - soil solution - chemical speciation - organic-matter - heavy-metals - sandy soil - dynamic speciation - humic substances - model parameters - trace-metals
e Donnan membrane technique (DMT) can be applied to measure free ion concentrations both in laboratory and in situ in the field. In designing DMT experiments, different strategies can be taken, depending on whether accumulation is needed. (1) When the free ion concentration is above the detection limit of the analytical technique (e.g. ICP-MS), no accumulation is needed and no ligand is added to the acceptor. Measurement can be based on the Donnan membrane equilibrium. (2) When an accumulation of less than 500 times is needed, an appropriate amount of ligand can be added to the acceptor and measurement can be based on the Donnan membrane equilibrium. (3) When an accumulation factor of larger than 500 times is needed, a relatively large amount of ligand is added to the acceptor and measurement can be based on the transport kinetics. In this paper, several issues in designing the DMT experiments are discussed: choice of DMT cell, measurement strategies and ligands and possible implication of slow dissociation of metal complexes in the sample solution (lability issue). The objective of this paper is to give better guidance in the application of DMT for measuring free ion concentrations in both synthetic and natural samples.
Diffusion of neutral and ionic species in charged membranes: Boric acid, arsenite, and water
Goli, E. ; Hiemstra, T. ; Riemsdijk, W.H. van; Rahnemaie, R. ; Malakouti, M.J. - \ 2010
Analytical Chemistry 82 (2010)20. - ISSN 0003-2700 - p. 8438 - 8445.
self-diffusion - humic-acid - atomistic simulation - dynamic speciation - nafion membranes - organic-matter - coefficients - boron - equilibrium - transport
Dynamic ion speciation using DMT (Donnan membrane technique) requires insight into the physicochemical characteristics of diffusion in charged membranes (tortuosity, local diffusion coefficients) as well as ion accumulation. The latter can be precluded by studying the diffusion of neutral species, such as boric acid, B(OH)30(aq), arsenite, As(OH)30(aq), or water. In this study, the diffusion rate of B(OH)30 has been evaluated as a function of the concentration, pH, and ionic strength. The rate is linearly dependent on the concentration of solely the neutral species, without a significant contribution of negatively charged species such as B(OH)4-, present at high pH. A striking finding is the very strong effect (factor of 10) of the type of cation (K+, Na+, Ca2+, Mg2+, Al3+, and H+) on the diffusion coefficient of B(OH)30 and also As(OH)30. The decrease of the diffusion coefficient can be rationalized as an enhancement of the mean viscosity of the confined solution in the membrane. The diffusion coefficients can be described by a semiempirical relationship, linking the mean viscosity of the confined solute of the membrane to the viscosity of the free solution. In proton-saturated membranes, as used in fuel cells, viscosity is relatively more enhanced; i.e., a stronger water network is formed. Extraordinarily, our B(OH)3-calibrated model (in HNO3) correctly predicts the reported diffusion coefficient of water (DH2O), measured with 1H NMR and quasi-elastic neutron scattering in H+-Nafion membranes. Upon drying these membranes, the local hydronium, H(H2O)n+, concentration and corresponding viscosity increase, resulting in a severe reduction of the diffusion coefficient (DH2O ˜ 5-50 times), in agreement with the model. The present study has a second goal, i.e., development of the methodology for measuring the free concentration of neutral species in solution. Our data suggest that the free concentration can be measured with DMT in natural systems if one accounts for the variation in the cation composition of the membrane and corresponding viscosity/diffusion coefficient.
Effects of Lability of Metal Complex on Free Ion Measurement Using DMT
Weng, L.P. ; Riemsdijk, W.H. van; Temminghoff, E.J.M. - \ 2010
Environmental Science and Technology 44 (2010)7. - ISSN 0013-936X - p. 2529 - 2534.
donnan membrane technique - soil solution - in-situ - dynamic speciation - trace-metals - copper - waters - dissociation - equilibrium - dialysis
Very low concentrations of free metal ion in natural samples can be measured using the Donnan membrane technique (DMT) based on ion transport kinetics. In this paper, the possible effects of slow dissociation of metal complexes on the interpretation of kinetic DMT are investigated both theoretically and experimentally. The expressions of the lability parameter, , were derived for DMT. Analysis of new experimental studies using synthetic solution containing NTA as the ligand and Cu2+ ions shows that when the ionic strength is low (=0.2 mM Ca(NO3)2) the dissociation rate of NTACu becomes the limiting step in Cu transport of the DMT measurement. In natural waters, dissolved organic matter (DOM) is the most important source of ligands that complex metals. By comparing the fraction of labile species measured using other dynamic sensors (DGT, GIME) in several freshwaters, it is concluded that in most waters ion transport in DMT is controlled by diffusion in the membrane. Only in very soft waters (
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