Surface Complexation Theory Research Articles

Cu and Zn ternary surface complex formation with SO 4 on ferrihydrite and schwertmannite

The use of adsorption data from single sorbate systems to model metal adsorption in SO 4-rich waters, such as acid mine drainage, can lead to inaccurate predictions of metal speciation. The adsorption of Cu and Zn on ferrihydrite, for example, is enhanced at low pH values in the presence of SO 4. This effect can only be accurately modeled using the diffuse layer model and surface complexation theory if ternary surface complexes, ≡FeOHCuSO 4 or ≡FeOHZnSO 4, are taken into consideration. Intrinsic adsorption constants for the formation of these ternary complexes on ferrihydrite have been derived from experimental data. When included in the model, Cu and Zn adsorption in the presence of SO 4 is accurately predicted for a wide range of metal, ferrihydrite and SO 4 concentrations. Adsorption of Cu and Zn onto the SO 4-rich Fe oxyhydroxide, schwertmannite, could also be accurately predicted and is indistinguishable from adsorption onto ferrihydrite in the presence of high solution SO 4 concentrations (e.g. 0.01 mol kg −1 SO 4).

Kinetics of montmorillonite dissolution in granitic solutions

Experiments measuring smectite dissolution rates in granitic solutions were carried out in a semi-batch reactor at 20, 40, and 60°C. The pH conditions of the solutions range from 7.6 to 8.5. Solid samples were confined within a dialysis membrane and introduced in the solution. The solution was renewed every 7 days and the dissolution reaction was investigated by the variation of Si concentration in the solutions. The average rates at pH∼8 were 10−14.13, 10−13.70, and 10−13.46 mol m−2 s−1, at 20, 40, and 60°C, respectively, and the activation energy for the dissolution reaction at pH ∼8 was 30.5±1.3 kJ mol−1. Comparison of the present results with other studies reveals that the montmorillonite dissolution rate depends strongly on the pH of the solution, with a minimum value at pH 8–8.5. At room temperature, the dissolution rate was found to be linearly dependent on proton (acidic conditions) or hydroxyl (basic conditions) activity in solution:Rate=10−11.39aH+0.34pH<8Rate=10−12.31aOH−0.34pH>8.5The comprehension of the dissolution mechanism can be improved by using surface complexation theory. Correlation between speciation of surface sites and kinetic results indicated that at room temperature the dissolution rate was directly proportional to the surface concentration of >AlOH2+ and >AlO− surface complexes, under acidic or alkaline conditions, respectively.Rate=10−8.0{>AlOH2+}pH<8Rate=10−8.2{>AlO−}pH>8.5A multiple variable model is proposed to take into account simultaneously the effect of pH on dissolution rates and on activation energy. The rates estimated using the model are in good agreement with experimental dissolution rates.

Benzenecarboxylate surface complexation at the goethite (α-FeOOH)/water interface: II. Linking IR spectroscopic observations to mechanistic surface complexation models for phthalate, trimellitate, and pyromellitate

A study combining information from infrared spectroscopy and adsorption experiments was carried out to investigate phthalate, trimellitate, and pyromellitate complexes at the goethite (α-FeOOH)/water interface. Infrared spectra showed evidence for inner-sphere complexes below pH 6 and outer-sphere complexes in the pH range 3 to 9. Normalized infrared peak areas were used as a semi-quantitative tool to devise diagrams showing the molecular level surface speciation as a function of pH. Surface complexation models that simultaneously predict these diagrams, the proton balance data and the ligand adsorption data were developed with surface complexation theory. Surface complexation modeling was carried out with a Charge Distribution Multisite Complexation Model (CD-MUSIC), assuming goethite particles with surfaces represented by the {110} plane (90% of total particle surface area) and by the {001} plane (10% of total particle surface area). Inner-sphere complexes were described as mononuclear chelates at the {001} plane, whereas outer-sphere complexes were described as binuclear complexes with singly coordinated sites on the {110} plane. The Three-Plane Model (TPM) was used to described surface electrostatics and to distribute the charges of the inner- and the outer-sphere complexes on different planes of adsorption.

Benzenecarboxylate Surface Complexation at the Goethite (α-FeOOH)/Water Interface: I. A Mechanistic Description of Pyromellitate Surface Complexes from the Combined Evidence of Infrared Spectroscopy, Potentiometry, Adsorption Data, and Surface Complexation Modeling

An investigation combining IR spectroscopy, potentiometric titrations, and adsorption experiments was carried out to study pyromellitate (1,2,4,5-benzenetetracarboxylate) sorption at the goethite (α-FeOOH)/water interface. The IR spectra show evidence of outer-sphere complexation throughout the pH range from 3 to 9. Below pH 6 additional IR spectroscopic features appear, which are tentatively assigned to inner-sphere complexes. A normalized IR peak area plot for each peak indicative of inner- and of outer-sphere complexes as a function of pH provided a semiquantitative surface speciation scheme. This scheme was successfully reproduced using surface complexation theory with a multisite complexation model calibrated on potentiometric titration and on adsorption data. The surface speciation was described with a binuclear outer-sphere complex on the {110} plane of goethite and a mononuclear inner-sphere complex on the {001} plane. Furthermore, as the IR spectra also indicated partial protonation of pyromellitate complexes at low pH, a partially protonated outer-sphere species on the {110} plane was included in the model.

How the Kd Approach Undermines Ground Water Cleanup

AbstractEnvironmental scientists have long appreciated that the distribution coefficient (the “Kd” or “constant Kd”) approach predicts the partitioning of heavy metals between sediment and ground water inaccurately; nonetheless, transport models applied to problems of environmental protection and ground water remediation almost invariably employ this technique. To examine the consequences of this practice, we consider transport in one dimension of Pb and other heavy metals through an aquifer containing hydrous ferric oxide, into which many heavy metals sorb strongly. We compare the predictions of models calculated using the Kd approach to those given by surface complexation theory, which is more realistic physically and chemically. The two modeling techniques give qualitatively differing results that lead to divergent cleanup Strategies. The results for surface complexation theory show that water flushing is ineffective at displacing Pb from the sorbing surface. The effluent from such treatment contains a persistent “tall” of small but significant levels of contamination. Subsurface zones of Pb contamination, furthermore, do not migrate rapidly or far in flowing ground water. These results stand in sharp contrast to the predictions of models constructed using the Kd approach, yet are consistent with experience in the laboratory and field.

Sorption of cesium, barium and europium on magnetite

Laboratory experiments have been performed on magnetite according to the batch method. The sorption of three cations of three different oxidation states has been investigated under various pH: Cs (I) and Ba (II) which have long lived isotopes, and Eu (III) which can be considered as a chemical analogue of Am (III). Results showed that magnetite has a high capacity of retention for Ba 2+ and Eu 3+, especially under pH conditions expected in a deep geological disposal. Cs + is not sorbed on a pure magnetite but it can be sorbed if magnetite contents some impurities such as silica. In this case the sorption was strongly dependent on the presence of other alkaline metal ions such as sodium. A good fitting of these experimental results has been obtained by using the surface complexation theory ( fiteql 3.2 code) with the diffuse layer model (DLM) (leading to the species ≡XOHBa 2+, ≡XOBaOH, ≡XOHEu 3+, ≡XOEu(OH) 2 with log K respectively equal to 4.05–11.50 6.43 and −14.87).

Comparison of 1-pK and 2-pK Versions of Surface Complexation Theory by the Goodness of Fit in Describing Surface Charge Data of (Hydr)oxides

1-pK and 2-pK formulations exist in surface complexation theory with respect to the description of basic charging behavior of (hydr)oxide electrolyte interfaces. The 2-pK approach has been commonly used with at least four different electrostatic models, whereas the 1-pK model has been primarily used with only one of those electrostatic models. In this paper the 1-pK approach is combined with three more electrostatic models. The eight resulting possible combinations have been tested on several sets of data. Applying the 2-pK basic Stern model (BSM) and the triple-layer model (TLM) was not satisfactory: due to the high number of adjustable parameters involved in these model variations the optimization procedure of the applied computer codes did not converge. This was taken as an a priori justification to exclude even more complicated (four-layer) models. For the remaining six models which could be successfully applied in the present paper, the goodness of fit parameter given by a computer code was used to compare the quality of the description of the chosen experimental data by the respective models. A purely diffuse layer model (DLM) generally gave the poorest fit to experimental data when combined with the 1-pK approach and was only sligthly better when combined with the 2-pK formalism. Either the 1-pK BSM or the 2-pK constant capacitance model (CCM) gave the best fit to the data in all the examples. However, it was found in two cases that some arbitrary constraint was necessary to define a unique (and thus meaningful) parameter set for the 2-pK CCM. The 1-pK TLM version allowed in more than half of the examples to determine a unique parameter set, which is impossible with the 2-pK TLM. It is concluded that the 1-pK BSM should be considered as the first choice model with respect to the goodness of fit and the uniqueness of the estimated parameters. The 2-pK CCM is still a good choice for a constant ionic strength case when the experimental data allow a determination of unique parameters and if only goodness of fit is used as a criterion.

The Effect of Ionic Strength on the Adsorption of H+, Cd2+, Pb2+, and Cu2+byBacillus subtilisandBacillus licheniformis:A Surface Complexation Model

To quantify metal adsorption onto bacterial surfaces, recent studies have applied surface complexation theory to model the specific chemical and electrostatic interactions occurring at the solution-cell wall interface. However, to date, the effect of ionic strength on these interactions has not been investigated. In this study, we perform acid–base titrations of suspensions containingBacillus subtilisorBacillus licheniformisin 0.01 or 0.1 M NaNO3, and we evaluate the constant capacitance and basic Stern double-layer models for their ability to describe ionic-strength-dependent behavior. The constant capacitance model provides the best description of the experimental data. The constant capacitance model parameters vary between independently grown bacterial cultures, possibly due to cell wall variation arising from genetic exchange during reproduction. We perform metal–B. subtilisand metal–B. licheniformisadsorption experiments using Cd, Pb, and Cu, and we solve for stability constants describing metal adsorption onto distinct functional groups on the bacterial cell walls. We find that these stability constants vary substantially but systematically between the two bacterial species at the two different ionic strengths.

Ionic Strength Effects on Cation Sorption to Oxides: Macroscopic Observations and Their Significance in Microscopic Interpretation

Sorption of metal ions at oxide mineral–water interfaces is a complex process involving many possible variables, one of them being ionic strength. Depending on the system, adsorption of metal ions at these interfaces has been reported to increase or decrease with increasing ionic strength. However, in most cases it appears to be insensitive to ionic strength. Insensitivity to ionic strength has been taken as an indication for inner sphere surface complexation. Decrease of sorption with increasing ionic strength has been interpreted as outer sphere surface complexation. Promotive effects of ionic strength have not been discussed at all in terms of conventional surface complexation theory. The present paper shows that the classical example of Ba sorption to goethite, where increasing ionic strength decreases Ba adsorption, can be alternatively explained by an inner sphere mechanism if ion pair formation in solution between Ba and nitrate of the swamping electrolyte is taken into account. This suggests that macroscopic observations in cation uptake experiments should not be used to obtain mechanistic interpretations, even for ionic strength effects. Promotive effects of ionic strength on Cd sorption to goethite are also discussed. The present modeling exercises with the help of a conventional one-site surface complexation model suggest that promotive ionic strength effects may be explained in terms of more effective shielding of highly charged surface complexes at high ionic strength.

Description of sorption equilibria for ions onto activated carbon using the surface complexation theory

The surface of activated carbon contains a large variety of ionizable groups which lead to the sorption of either anions and/or cations from the aqueous phase. The amphoteric sorption of simple inorganic species has been described by means of the theory of surface complexation. Each kind of species is assumed to be arranged in an ordered Stern layer parallel to the surface. For the sorption of each kind of ion a set of two characteristic equilibrium parameters can be evaluated from equilibrium data. Evaluation is possible both from simple or from multicomponent systems. The parameters remain unchanged also in systems with additional ions. Evaluation of data for two commercially available carbons reveals a very satisfactory prediction of the carbon loading.

Description of sorption equilibria for ions onto activated carbon using the surface complexation theory

The surface of activated carbon contains a large variety of ionizable groups which lead to the sorption of either anions and/or cations from the aqueous phase. The amphoteric sorption of simple inorganic species has been described by means of the theory of surface complexation. Each kind of species is assumed to be arranged in an ordered Stern layer parallel to the surface. For the sorption of each kind of ion a set of two characteristic equilibrium parameters can be evaluated from equilibrium data. Evaluation is possible both from simple or from multicomponent systems. The parameters remain unchanged also in systems with additional ions. Evaluation of data for two commercially available carbons reveals a very satisfactory prediction of the carbon loading.

Modeling potentiometric titration behavior of glauconite

Potentiometric titration behavior and the effects of dissolution on the titration experiment of a complex natural clay mineral, glauconite, were investigated and interpreted according to surface complexation theory. Considerable dissolution was detected in the time frame of the titration experiments, with the amount of individual cations released from glauconite a function of solution variables and dissolution kinetics. Dissolution effects can be accounted for in model simulations of titration behavior by considering hydrolysis of these cations. Proton surface charge can be calculated by subtracting supernatant titration curves from those of glauconite suspension. Surface complexation models were used to describe titration data. The nonelectrostatic model (NEM) and constant capacitance model (CCM) were implemented using single-site, single-site with dissolution correction, and multi-site with dissolution correction approaches. CCM typically produced better fits of experimental data than NEM based on a statistical fitting parameter. In general, model descriptions are good for background electrolyte concentration ≥ 0.01 M. In most instances, the correction for dissolution effects yielded a modest improvement in model simulation of experimental data over that obtained by a simple single-site model, but complexity of the model calculations increases greatly when the numerous additional chemical reactions are considered. Incorporation of ion exchange interactions produced no substantive improvement over the other approaches. Consistent with these results, speciation calculations indicate that the > XOH group dominates over other surface species in the pH range of interest, and surface protonation-deprotonation are the most influential reactions with respect to the surface chemistry of the mineral.

PHRQKIN, a program simulating dissolution and precipitation kinetics in groundwater solutions

The PHRQKIN program has been developed to handle the chemical kinetics of aluminium silicate minerals in weathering reactions. The dissolution model used in the program is based on transition state theory and surface complexation theory. The precipitation reaction may be handled either kinetically or with equilibrium. The equilibrium and speciation calculations in the program are handled by the equilibrium code PHREEQE. The PHRQKIN program was used in simulations, in order to test the ability of the program to simulate the dissolution of albite under well-controlled conditions. Also the kinetic data-base used in the simulations was verified by simulation of experiments performed on albite dissolution. The simulation results agreed well with the experimental data from the literature.

Physical surface-complexation models for sorption at the mineral–water interface

NONE of the traditional models of surface complexation of ions at oxide–water interfaces, such as the constant-capacitance, double-diffuse-layer and triple-layer models1–5, provides an explicit, quantitative treatment of ion solvation. Here I show that this process can be included quantitatively in surface-complexation theory by describing it using the Born theory of ion solvation6,7. In this way, the standard Gibbs free energy of sorption can be decomposed into three terms: the standard coulombic term, a Born solvation contribution and a term intrinsic to the ion alone. Consideration of the Born solvation term shows that the equilibrium constant for sorption depends linearly on the inverse of the dielectric constant of the solid. By this means, all three contributions to the free energy can be estimated empirically or calculated theoretically. Inclusion of this physical description of ion solvation should facilitate the application of the theory of ion sorption to complex natural oxide and silicate minerals.

Thorium sorption in seawater suspensions of aluminium oxide particles

The partitioning of thorium between solid and solution phases in seawater suspensions of aluminium oxide particles was studied in controlled laboratory experiments to determine whether partitioning is consistent with current models of trace-metal adsorption (surface complexation models). Experimental conditions (i.e., thorium and particle concentrations, pH, temperature, salinity) were chosen to be as realistic as possible for coastal seawater while minimizing nonadsorptive processes; 234Th was used as a tracer of thorium, and filtration/ultrafiltration techniques were used to prepare suspensions with minimal colloidal material, as well as to define and to separate solid and solution phases. A comparison of the experimental results with relationships predicted by surface complexation models shows that thorium sorption in the alumina suspensions was consistent with surface complexation theory: Sorption kinetics were consistent with a (pseudo-) first-order reversible reaction at constant particle concentration, the pseudo-first-order forward rate constants had a first-order dependence on particle concentration, and K d values were independent of particle concentration. Thorium sorption consisted of two distinct reversible reactions, both of which were consistent with surface complexation theory. Second order rate constants were within the range of rate constants reported for the adsorption of divalent metal ions ontoγ-Al 2O 3 surfaces.

Application of the surface complex formation model to exchange equilibria on ion exchange resins Part III: Anion exchangers

The surface complexation theory considers the sorption of ions as a local equilibrium reaction, which is caused by the amphoteric behaviour of the surface. Application of this theory to exchange equilibria with ordinary and chelating weak-acid ion exchange resins which can release protons has opened the way for a comprehensive description. Binary equilibria are described by a logarithmic equilibrium parameter, which is a linear function of the composition of the resin phase. Multicomponent equilibria are considered as a superposition of several binary equilibria. The approach considers the pH-dependent dissociation of functional groups. Furthermore, chemical reactions in the liquid phase in which exchangeable species are involved, can readily be taken into account. In contrast to cation exchange resins, the behaviour of anion exchangers is characterized by the uptake of protons enabling them to also adsorb anions. As a consequence, anion exchange equilibria can be described in similar way. The evaluation of data demonstrates that similar relationships exist between a logarithmic equilibrium parameter and the resin phase composition. As for cation exchangers, multicomponent equilibria can be predicted with excellent accuracy from binary parameters.

Measurements of Metal Adsorption in Oxide-Clay Mixtures: “Competitive-Additivity” Among Mixture Components

ABSTRACTAn important question concerning the transport of radionuclides from nuclear waste repositories is whether the adsorption of metals by rocks and soils can be predicted from the properties of the constituent minerals. Attempts by previous researchers to use sorption models based on linear adsorption or weighted "sorptive additivity" have met with limited success. In this study, a “competitive-additivity” model based on surface complexation theory was used to model the pH-dependent adsorption of lead by goethite/Ca-montmorillonite mixtures using complexation constants obtained from single sorbent systems. Measurements of lead adsorption by goethite, Ca-montmorillonite, and goethite-Ca-montmorillonite mixtures (and similar studies of copper and zinc adsorption) demonstrate that the two adsorbents compete for adsorption of metals over wide ranges of pH and concentrations of adsorbents and metals. The adsorption behaviors of the mixtures are determined by the relative concentrations of the adsorbents and their respective affinities for the adsorbate metal. Particle-particle interactions such as heterocoagulation of the oxide and clay do not appear to be significant for the majority of the adsorption sites in this system.