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.
Proton binding properties of humic substances originating from natural and contaminated materials
Zomeren, A. van; Costa, A. ; Pinheiro, J.P. ; Comans, R.N.J. - \ 2009
Environmental Science and Technology 43 (2009)5. - ISSN 0013-936X - p. 1393 - 1399.
nica-donnan model - acid-base properties - metal-ion binding - organic-matter - fulvic-acids - bottom ash - soil - parameters - copper
Humic substances (HS) are ubiquitous organic constituents in soil and water and can strongly adsorb metal contaminants in natural and waste environments. Therefore, understanding and modeling contaminant-HS interactions is a key issue in environmental risk assessment. Current binding models for HS, such as the nonideal competitive adsorption (NICA)-Donnan model, are developed and calibrated against natural organic matter from soils and surface waters. The aim of this study is to analyze the proton binding properties of humic and fulvic acid samples originating from secondary materials, waste materials and natural samples in order to assess whether the charge development of these HS can be described with generic NICA-Donnan parameters. New proton binding parameters are presented for HS isolated from several natural and contaminated (waste) materials. These parameters are shown to be similar to those of HS originating from natural environments, suggesting that the NICA-Donnan model and generic binding parameters are adequate to describe proton binding to HS in both natural and contaminated materials. These findings widen the range of environments to which the NICA-Donnan model can be applied and justify its use in geochemical speciation modeling of metal mobility in contaminated (waste) materials
Partitioning of organic matter and heavy metals in a sandy soil: Effects of extracting solution, solid to liquid ratio and pH
Fest, P.M.J. ; Temminghoff, E.J.M. ; Comans, R.N.J. ; Riemsdijk, W.H. van - \ 2008
Geoderma 146 (2008)1-2. - ISSN 0016-7061 - p. 66 - 74.
donnan membrane technique - low-molecular-weight - humic substances - contaminated soils - model parameters - fulvic-acids - trace-metals - bottom ash - copper - speciation
In sandy soils the behavior of heavy metals is largely controlled by soil organic matter (solid and dissolved organic matter; SOC and DOC). Therefore, knowledge of the partitioning of organic matter between the solid phase and soil solution is essential for adequate predictions of the total dissolved metal concentration in the soil solution. At present, only a few studies have incorporated solid/liquid partitioning of organic matter in metal mobility predictions. In order to gain more insight in the behavior of DOC, we have studied the effect of the extraction solution, solid to liquid ratio (SLR), pH and storage time on the extractability of DOC and related metal concentration in the soil solution of a sandy soil. Furthermore, the composition of the DOC was measured and free metal concentrations were analyzed in the soil solution with the Donnan Membrane Technique. Extraction solution, SLR and pH affected the extracted amount of both DOC and metals. The DOC concentrations were highest in pore water and decreased further from water extracts to CaCl2 extracts. In general, with increasing SLR the metal/DOC ratio decreased, which indicated that the increased DOC at higher SLR had a lower average metal binding capacity than DOC released at the lowest SLR. Storage time of the samples and changes in the field greatly affected the extracted amount of DOC; the DOC concentration of samples taken 20 years ago and stored for a long period of time are 10 times higher than for samples from the same area and stored for less than one year. The extracted DOC is comprised mainly of fulvic acids (FA) and hydrophilic acids. Humic acids (HA) played a minor role in the DOC, which is in contrast to the often used assumptions on the DOC compositions in modeling studies. The composition of DOC is also affected by the soil pH; at low pH the contribution of FA to the total DOC concentration is lower because more FA is adsorbed to Fe-oxides in the soil. Speciation calculations assuming that DOC comprised of HA, FA and citric acid (as a model substance for hydrophilic acids) showed that for Cu the citrate-bound Cu could be up to 20% of the total Cu in solution. This study shows that more research is needed to gain more insight in the composition, concentration and behavior of DOC under field conditions in order to improve the prediction of metal leaching in the field.
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.
Ion binding to natural organic matter : General considerations and the NICA-Donnan model
Koopal, L.K. ; Saito, T. ; Pinheiro, J.P. ; Riemsdijk, W.H. van - \ 2005
Colloids and Surfaces. A: Physicochemical and Engineering Aspects 265 (2005)1-3. - ISSN 0927-7757 - p. 40 - 54.
contaminated sandy soil - adsorption energy-distribution - chemical-equilibrium models - simple electrostatic model - heavy-metal biosorption - isolated cell-walls - humic substances - proton binding - affinity distributions - fulvic-acids
The general principles of cation binding to humic matter and the various aspects of modeling used in general-purpose speciation programs are discussed. The discussion will focus on (1) the discrimination between chemical and electrostatic interactions, (2) the binding site heterogeneity, (3) the models for the electrostatic interactions, such as the Donnan model and (4) the complications inherent to ion binding in multi-component systems, including a different stoichiometry for different ions. The basic NICA (non ideal competitive adsorption) model addresses issues (1), (2) and (4) and ensures thermodynamic consistency. The electrostatic interactions (3) can be included using the ion activity adjacent to the binding sites. To compute this activity it is necessary to use an electrostatic model that relates the particle charge and the characteristic particle potential. Several simplifications to arrive at such a relation are analyzed. The Donnan model is proposed as the most convenient model. A short derivation of the NICA-Donnan model is presented to illustrate that the derivation is quite straightforward once it is clear which assumptions should be made. The main parameters of the NICA model are the site density, the median affinity constants for the different ions, the ion stoichiometry and the width of the generic affinity distribution. All these parameters are intrinsic properties that are independent of pH, salt concentration or metal concentration. The key parameter of the Donnan model is the Donnan volume, for which an empirical relation with one adjustable parameter is advised. The NICA-Donnan model has been used in several studies. To illustrate the capabilities of the model some results obtained for purified humic acid are presented together with model predictions. Key features of the model are its ability to adjust to different heterogeneities and binding stoichiometries and its predictive abilities with respect to metal ion-proton and metal-metal ion competition. With the application the model should be regarded as semi-empirical because assumptions have been made for its derivation that are not met in practice. Both for this reason and since it is difficult, if not impossible, to find a unique set of parameters for a given system the interpretation of the parameter values may be difficult. A brief discussion is given on the generic NICA-Donnan description of fulvic and humic acids presented by Milne et al. [C.J. Milne, D.G. Kinniburgh, E. Tipping, Environ. Sci. Technol. 35 (2001) 2049; C.J. Milne, D.G. Kinniburgh, W.H. van Riemsdijk, E. Tipping, Environ. Sci. Technol. 37 (2003) 958]. Finally a brief literature review is presented on the use of the NICA-Donnan model for other heterogeneous natural sorbent systems
An FTIR-DRIFT study on river sediment particle structure: Implications for biofilm dynamics and pollutant binding
Galle, T. ; Lagen, B. van; Kurtenbach, A. ; Bierl, R. - \ 2004
Environmental Science and Technology 38 (2004)17. - ISSN 0013-936X - p. 4496 - 4502.
chromatography-mass spectrometry - particulate organic-matter - chemical-equilibrium model - humic substances - suspended sediment - blackwater river - macoma-balthica - humber rivers - fulvic-acids - waste-water
Diffuse reflectance infrared Fourier transform (DRIFT) spectrometry was applied to a set of sediment samples collected by traps over one and a half years in a midmountainous river. Dynamic changes in hydrological and life-cycle conditions generated sediment particles of different Corg content and organic composition. Periods in the midst of or shortly after flood events left particles poor in Corg content with spectral features that were enriched in carboxylic and aromatic signals. These are characteristic of terrestrial oxidized vascular plant debris. Low-flow conditions saw the consequent build-up of amide, aliphatic, and polysaccharide moieties as expected for autochthonous biofilm derived material. A peak ratio of two bands representing the alternation of these two types of organic matter showed that flood particle Corg had a higher affinity for metals than the high Corg of mature biofilms, probably owing to higher COO- contents in the first. The relative dietary bioavailability of the metals from sediment Corg, which is related to the nutritional value of the substrate, is therefore probably lower in the aftermath of a flood than in prolonged low-flow situations. This needs to be accounted for in future metal speciation and bioavailability modeling approaches.
Application of the NICADonnan model for proton, copper and uranyl binding to humic acid
Saito, T. ; Nagasaki, S. ; Tanaka, S. ; Koopal, L.K. - \ 2004
Radiochimica Acta 92 (2004)9-11. - ISSN 0033-8230 - p. 567 - 574.
metal-ion binding - fulvic-acids - humate interactions - substances - complexation - adsorption - parameters - heterogeneity - uranium(vi) - goethite
Humic acids are natural organic materials that play an important role in the migration of heavy metal and actinide ions in aquatic and soil systems. In the present study, the binding of protons, copper ions and uranyl ions to the purified Aldrich humic acid (PAHA) is investigated and the results are modeled with the Non-Ideal Competitive Adsorption (NICA) model extended with electrostatic interactions according to the Donnan model (NICA-Donnan model). The NICA part of the model enables one to describe the competitive ion binding to a heterogeneous substrate taking into account a different stoichiometry per ion. The NICA-Donnan model can describe the binding of the ions to PAHA in large concentration ranges (3
Measuring and modeling zinc and cadmium binding by humic acid
Osté, L.A. ; Temminghoff, E.J.M. ; Riemsdijk, W.H. van - \ 2002
Analytical Chemistry 74 (2002)4. - ISSN 0003-2700 - p. 856 - 862.
metal-ion binding - diffusive gradients - organic-matter - fulvic-acids - trace-metals - thin-films - soil - speciation - complexation - substances
Free metal ions in aqueous and terrestrial systems strongly influence bioavailability and toxicity. Most analytical techniques determine the total metal concentration, including the metal ions bound by dissolved organic matter. Ion activity can be measured with ion-specific electrodes (ISEs) for some metals, but an electrode for Zn is not commercially available. As a result, very few data are available on Zn binding by natural dissolved organic matter. The aim of this study is to determine free Zn concentrations in purified humic acid solutions using the recently developed Donnan membrane technique. However, several analytical aspects of the Donnan membrane technique had to be clarified before reliable data could be composed. Cd was chosen for validation. This study shows that free Cd concentrations as measured by the Donnan membrane technique agreed well with Cd ISE measurements. It is also shown that the Donnan membrane technique could be used at high pH. The Donnan membrane technique provided consistent results in a range of p[Cd2+] = 3-9 and p[Zn2+] = 3-8 at pH 4, 6, and 8. Metal speciation in humic acid solutions was also calculated with the consistent NICA-Donnan model using generic parameters. The model could excellently describe the experimental data without adjusting any of the parameters (R2Cd = 0.971, R2Zn = 0.988)