W.H. van Riemsdijk

Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach: I. Model description and evaluation of intrinsic reaction constants

At the solid/solution interface of (hydr)oxides various types of surface groups exist, each reacting according to its own affinity constant (K) for protons. A model is presented that estimates the value of the log K of various types of surface groups (singly, doubly, and triply metal-coordinated O(H) and OH(H) surface groups) of (hydr)oxides. The intrinsic affinity constants (K) depend on many factors, e.g., the valence of the central cation (Me) of the (hydr)oxides, its electron configuration, and the MeH distance of the reacting surface group. Besides these also the number of surrounding ligands, the number of central cations coordinating with a ligand, and the type of reacting ligand (an oxo or hydroxo species) determine the proton affinity constant. Proton adsorption reactions can in principle be considered as a two-step proton adsorption reaction, forming OH and OH2 species at the surface. Analysis of the calculated affinity constants shows, however, that generally a surface group will react in a limited pH range (for instance pH 3–10) only according to a one-step protonation reaction (1-pK model). A general MUltiSIte Complexation model (MUSIC) is presented, which is based on crystallographic considerations. The new site binding model (MUSIC) can unify the classical 2-pK model and the recently presented 1-pK model, both being special cases of the model described here. The surface charge of a surface with more than one type of surface group can be described with one proton adsorption reaction and one discrete K for each type of surface group.

Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach: II. Application to various important (hydr)oxides

Abstract At the solid/solution interface of metal (hydr)oxides various types of O(H) and OH(H) groups are present, which differ in the number of coordinating metal ions. The σ0-pH curves of metal (hydr)oxides are strongly determined by the composition and the relative extent of the various crystal planes of (hydr)oxides. The charging behavior is discussed for gibbsite (Al(OH)3), goethite (FeOOH), hematite (Fe2O3), rutile (TiO2), and silica (SiO2). New experimental σ0-pH data for goethite and gibbsite are presented. Several important (hydr)oxides exhibit crystal faces which do not develop surface charge over a relatively wide pH range. An uncharged crystal face may be due to the presence of surface groups which are not reactive (inert) in the pH range under consideration, like the 001 face of gibbsite and the 0001 face of hematite, or caused by the presence of two types of interacting charged surface groups of which the charge of one type is fully compensated by the other like at the 100 face of goethite. The charging behavior of silica and the 001 face of gibbsite is determined by one type of reactive surface group with a large ΔpK for the consecutive protonation steps. The crystal structure imposes the presence of uncharged surface groups and this results in a quite different shape of σ0-pH curves for gibbsite and silica in comparison with the commonly observed σ0-pH curves of metal (hydr)oxides. The MUltiSIte Complexation (MUSIC) model as developed by T. Hiemstra, W. H. Van Riemsdijk, and G. H. Bolt, (J. Colloid Interface Sci.132 (1989)) leads to a rather good prediction of σ0-pH curves for various metal (hydr)oxides using predicted affinity constants for the various types of surface groups and Stern layer capacitance values and pair formation constants estimated from the literature.

Metal ion binding to humic substances: application of the non-ideal competitive adsorption model.

The application of a new model to describe metal ion binding by humic acids is discussed. Metal ion binding is always of a competitive nature since the proton is always present. Although of great practical importance, the combination of a chemically heterogeneous system with competitive binding poses difficult problems from both experimental and theoretical points of view. The new Non-Ideal Competitive Adsorption model (NICA model) used here is able to account for the non-ideal binding to heterogeneous ligands. A good description of the binding of H, Ca, Cd, and Cu to a purified peat humic acid is achieved over a wide range of free metal ion concentrations (-2 > log Me 2+ > -14) and pH (2 < pH < 10). The results show that binding of metal ions to humic acid is strongly influenced by the intrinsic chemical heterogeneity of the humic material itself as well as by ion-specific non-ideality. The results indicate that copper competes much more efficiently with protons bound to the phenolic type groups than calcium and cadmium.

Phosphate and sulfate adsorption on goethite: single anion and competitive adsorption.

Abstract The adsorption of phosphate and sulfate on goethite is studied individually and in combination at solution concentrations of phosphate ranging from 10 −8 to 10 −4 M and of sulfate ranging from 10 −5 to 10 −3 M. For single anion adsorption the influence of pH, ionic strength, and anion:goethite ratio was determined. The anion adsorption data were described well with a model in which surface complexation and electrostatic interaction is taken into account. In systems with both anions, the influence of phosphate on sulfate adsorption was much stronger than vice versa, which reflects the higher affinity of phosphate for the goethite surface. In spite of the rather small competitive effect of sulfate on phosphate adsorption expressed per unit surface area of goethite, a considerable increase in the solution concentration of phosphate was observed at relatively low pH in the presence of sulfate. The relative increase in the phosphate solution concentration was larger at a higher ratio of total concentrations of sulfate and phosphate in the system and when lowering the pH. The data indicate that the competitive interaction of phosphate and sulfate for adsorption may have an important effect on the bioavailability of these anions. The competitive adsorption data were predicted well using model parameters derived for single anion adsorption

Metal ion binding by natural organic matter: from the model to the field.

With the newly developed NICCA-Donnan model, we estimate the activity of toxic metal ions from simple measurements like total metal concentration and organic matter content. The model evaluates Cu and Cd binding from three field systems, a mountain lake and two sandy soils, using model parameters calibrated for natural organic matter analogues with laboratory measurements. This is possible because the model includes site binding heterogeneity, electrostatic effects, competitive binding, and ion specific nonideality. The predictions derived closely matched the field observations when site binding densities are adjusted. The partition coefficients between soil and soil solution were also predicted for Cd and Cu under conditions where the organic matter controls the metal binding in both soil and soil solution. The model calculations show that in soil solutions 50% of the Cd and 99.99% of the Cu is bound to the dissolved organic matter. The model can be used to evaluate the effects of variations in the chemical conditions (e.g., acidification or total metal loading) on the free metal ion concentration in solution.

Electrolyte adsorption on heterogeneous surfaces: adsorption models

Abstract Several heterogeneous electrolyte adsorption models are derived. The underlying assumptions for these models and their applicability are discussed. It is shown that the pristine point of zero charge (PPZC) may well be a function of the degree of surface heterogeneity. On the other hand model calculations show that the shape of the surface charge (σ0)-pH curves is very insensitive towards the degree of heterogeneity. It thus follows that σ0-pH curves for metal oxides, exhibiting some degree of heterogeneity, may be described with a homogeneous electrolyte adsorption model. Interestingly it appears that the titration curves of TiO2 in KNO3 can be described equally well using a “one-pK” model as with a “two-pK” model.

Determination of the chemical speciation of trace metals in aqueous systems by the Wageningen Donnan Membrane Technique

Abstract In order to determine the ‘free’ metal ion concentration in aqueous solutions the so-called Wageningen Donnan Membrane Technique (WDMT) has been developed. This involves a continuous flow system in which the donor side and the acceptor side of the WDMT cell are continuously flushed with solution across the membrane. The new cell design allows pseudo equilibrium to be reached for the free metal ions via a Donnan equilibrium across a negatively charged ion-exchange membrane within a reasonable time span. The donor solution contains both ‘free’ and complexed metal ions. The concentration of the cations in the acceptor solution is either equal to the concentration of the ‘free’ cation concentration in the donor solution, or it can be calculated using simple correction factors. The optimization experiments have been performed with cadmium and copper, in the presence of various (in)organic complexing agents, at various pH values, and different salt concentrations. In multi-component systems, like the systems used for the experiments with calcium, cadmium, copper, protons and EDTA, the results are in good agreement with the speciation calculations. The transport of the different cations and anions across the membrane can be very well explained by the simplified theory presented. Effects of difference in salt concentration on metal concentrations can be corrected. For the more complicated systems with natural dissolved organic matter (e.g. humic acid) the concentrations measured in the acceptor solution are also in good agreement with the speciation calculations performed for the donor solution.

Long-term kinetics of phosphate release from soil.

Long-term phosphate (P) desorption from soil is described using two discrete P pools in the soil : one available and one strongly fixed pool. The P release kinetics for each pool are described with a first-order rate equation. A new desorption method is used with hydrous iron oxide inside dialysis tubing acting as a P sink. The widely used iron-impregnated filter paper desorption method overestimates initial P desorption by a factor of up to 4 and underestimates the quantitative progression of desorption as a function of time. P desorption continued with substantial rates for periods longer than 1600 h. A wide range in P desorbability was observed : 15-70% of oxalate-extractable P (P ox ) desorbed after 1600 h. P desorbability decreased with increasing Fe ox +Al ox content of the sample. The relative size of the quickly desorbing pool increased with increasing initial degree of P saturation α 0 =P 0X [Fe 0X + Al 0X ] of the soils. This fact is of direct importance for the estimation of P losses from phosphate-rich soils. This study furthermore provides evidence that all oxalate-extractable P potentially is desorbable : no irreversibly fixed P OX exists.

Metal ion adsorption on heterogeneous surfaces; Adsorption models.

The sensitivity of metal ion adsorption on metal oxides for the degree of random surface heterogeneity (smeared out potential) using three electrolyte adsorption models is investigated. It is shown that the effect of random surface heterogeneity is hard to detect when the metal ion adsorption constants are adjusted together with the adjustment of Kq, i.e., a shift in the position of the energy distribution curve. This shift is necessary in order to keep the experimentally determined pristine point of zero charge at a fixed position. Some reasons for the low sensitivity are indicated. Cadmium adsorption isotherms for hematite and amorphous iron oxide in combination with CdH exchange data obtained at several pH values can be described satisfactorily with a simple homogeneous surface one- or two-pK basic Stern model. For both iron oxides the predominant surface species of cadmium is SO−CdOH+.

Analysis of proton binding by a peat humic acid using a simple electrostatic model

Detailed potentiometric titration data were collected for a purified peat humic acid (PPHA) over a range of pH (pH 3.5–10.5) and KNO3 background electrolyte concentrations (0.001–0.3 M). The data were analyzed following the master curve approach which includes both an electrostatic double layer model and a model for the intrinsic heterogeneity of the PPHA. Spherical and cylindrical double layer models gave equally good fits to the data. A salt dependence observed around pH 5 could not be completely removed by taking into account the electrostatic interactions. Hysteresis was observed to a much greater extent in the first titration cycle compared with the second cycle. This suggested that some slow and only partly reversible aggregation was occurring possibly as a result of the aggregation created during the purification of the humic acid. Titration curves for fully redispersed samples fitted the master curve approach (surface charge vs. surface pH) reasonably well but still displayed an ionic strength dependence at a pH of less than 5 which could not be accounted for using the simple electrostatic model. Heterogeneity analysis of the master curve showed that the affinity distribution had two peaks centred at log KHint ∼ 4 and log KHint ∼ 8 to 9. The total number of weak acid sites titrated between pH 3.5 and 10.5 was approximately 3.5 eq kg−1 but the total number of sites estimated from the isotherm analysis was 5.3–5.8 eq kgt1¯. Double Toth and double Langmuir-Freundlich isotherms fitted the data almost equally well but the implied distribution of sites between the more acidic “car☐ylic” sites and the weakly acidic “phenolic” sites varied with the isotherm chosen. An important source of uncertainty in the analysis was in estimating the charge on the humic acid at its initial pH of about pH 3.