Tjisse Hiemstra

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.

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

Adsorption of fulvic acid on goethite

The adsorption of fulvic acid by goethite was determined experimentally as a function of concentration, pH, and ionic strength. The data were described with the CD-MUSIC model of Hiemstra and Van Riemsdijk (1996), which allows the distribution of charge of the bound fulvate molecule over a surface region. Simultaneously, the concentration, pH, and salt dependency of the binding of fulvic acid can be described. Using the same parameters, the basic charging behavior of the goethite in the absence of fulvic acid could be described well. The surface species used in the model indicate that inner sphere coordination of carboxylic groups of the fulvate molecule is important at low pH, whereas at high pH the outer sphere coordination with reactive groups of the fulvate molecule with high proton affinity is important.

Physical chemical interpretation of primary charging behaviour of metal (hydr) oxides

The primary charging behaviour of metal (hydr)oxides is of great practical and theoretical importance. It has been shown in the literature that it is possible to describe this charging behaviour with widely different models. This ability to describe the experimental observations has not contributed to a consensus with respect to the physical interpretation. The most popular models for interpretation are, in essence, all a combination of a description for adsorption of protons (“site-binding model”) with a double layer model. The classical one- and two-pK site-binding models assume that the surface can be treated as chemically homogeneous. Recently, a multisite complexation model has been formulated (MUSIC model) that allows for a priori estimation of the proton affinity constants for various types of reactive groups present on metal (hydr)oxides. Large differences between experimental capacitance (dσo/dpH) as well as model constants for the Stern layer capacitance (0.2–4 F m−2 can be found for different metal (hydr)oxides, and are discussed. A physical model for the compact part of the double layer on metal (hydr)oxides is derived and compared with that for AgI. From the model it follows that for well crystallized non-porous metal (hydr)oxides the Stern layer capacitance is expected to be smaller than or equal to around 1.7 F m−2. Analysis of primary charging curves of non-porous colloids like gibbsite (Al(OH)3), rutile (TiO2) and goethite (FeOOH) in combination with the MUSIC model approach leads to a Stern layer capacitance of 1.2±0.4 F m−2, in agreement with the proposed double layer structure. Salt dependency of the charging curves as well as the value of the capacitance support the assumption of ion-pair formation. For silica a higher capacitance is derived which can easily be interpreted in the light of the surface structure since the reactive groups are not confined to a layer of densely packed reactive surface groups. Goethites with a specific surface area of less than 50 m2 g−1, prepared by rapid neutralization of an iron salt, charge significantly better. This is interpreted as being due to the presence of other crystal faces and/or porosity.

The relationship between molecular structure and ion adsorption on variable charge minerals

Ion adsorption modeling is influenced by the presumed binding structure of surface complexes. Ideally, surface complexes determined by modeling should correspond with those derived from spectroscopy, thereby assuring that the mechanistic description of ion binding scales from the nanoscopic molecular structure to the macroscopic adsorption behavior. Here we show that the structure of adsorbed species is a major factor controlling the pH dependency of adsorption. An important aspect of the pH dependency is the macroscopic proton-ion adsorption stoichiometry. A simple and accurate experimental method was developed to determine this stoichiometry. With this method, proton-ion stoichiometry ratios for vanadate, phosphate, arsenate, chromate, molybdate, tungstate, selenate and sulfate have been characterized at 1 or 2 pH values. Modeling of these data shows that the macroscopic proton-ion adsorption stoichiometry is almost solely determined by the interfacial charge distribution of adsorbed complexes. The bond valence concept of Pauling can be used to estimate this charge distribution from spectroscopic data. Conversely, the experimentally determined proton-ion adsorption stoichiometry allows us to successfully predict the spectroscopically identified structures of, for example, selenite and arsenate on goethite. Consequently, we have demonstrated a direct relationship between molecular surface structure and macroscopic adsorption phenomena.

Multiple activated complex dissolution of metal (hydr) oxides: A thermodynamic approach applied to quartz

The rate of proton-promoted dissolution of metal (hydr)oxides can be related to the charge and speciation of surface groups at the solid/solution interface. In the literature this dissolution is thought to be rate limited by the decomposition of only one charged activated complex. A new model for the description of the dissolution kinetics of metal (hydr)oxides has been developed in which all potential activated complexes are in principle able to decompose. The new model with multiple activated complexes (MAC) uses the classical activated complex theory. Variable charge theory and lattice statistics are combined in our MAC model with a thorough analysis of the thermodynamics of surface reactions presented in this paper. The MAC model predicts the pH and salt dependency of the dissolution rates on the basis of the thermodynamics-derived quantities (Gibbs free energies, enthalpies, and entropies) for proton (de/ad)sorption reactions without any recourse to the dissolution data if the surface geometry and surface composition of the metal (hydr)oxides are known. The MAC model is applied to quartz. The relevant surface characteristics of quartz are relatively simple. The kinetics of dissolution of silica and quartz are evaluated quantitatively. On the basis of surface geometry five activated complexes are distinguished, differing in the charge of the surrounding ligands. For well-cleaned quartz the MAC theory can describe the pH and salt dependency of the kinetics of proton-promoted dissolution adjusting only one parameter which determines the absolute level of the rate of dissolution. It follows from the model that all activated complexes may be of importance in determining rate. The relative importance of the various activated complexes is strongly pH dependent. The model predicts that the experimentally determined activation energy is a function of the solution composition. For each decomposition reaction the activation enthalpy has been determined.

Modeling the binding of fulvic acid by goethite: the speciation of adsorbed FA molecules

Abstract Under natural conditions, the adsorption of ions at the solid–water interface may be strongly influenced by the adsorption of organic matter. In this paper, we describe the adsorption of fulvic acid (FA) by metal(hydr)oxide surfaces with a heterogeneous surface complexation model, the ligand and charge distribution (LCD) model. The model is a self-consistent combination of the nonideal competitive adsorption (NICA) equation and the CD-MUSIC model. The LCD model can describe simultaneously the concentration, pH, and salt dependency of the adsorption with a minimum of only three adjustable parameters. Furthermore, the model predicts the coadsorption of protons accurately for an extended range of conditions. Surface speciation calculations show that almost all hydroxyl groups of the adsorbed FA molecules are involved in outer sphere complexation reactions. The carboxylic groups of the adsorbed FA molecule form inner and outer sphere complexes. Furthermore, part of the carboxylate groups remain noncoordinated and deprotonated.

Factors controlling phosphate interaction with iron oxides.

Factors such as pH, solution ion composition, and the presence of natural organic matter (NOM) play a crucial role in the effectiveness of phosphorous adsorption by iron oxides. The interplay between these factors shows a complicated pattern and can sometimes lead to controversial results. With the help of mechanistic modeling and adsorption experiments, the net macroscopic effect of single and combined factors can be better understood and predicted. In the present work, the relative importance of the above-mentioned factors in the adsorption of phosphate was analyzed using modeling and comparison between the model prediction and experimental data. The results show that, under normal soil conditions, pH, concentration of Ca, and the presence of NOM are the most important factors that control adsorption of phosphate to iron oxides. The presence of Ca not only enhances the amount of phosphate adsorbed but also changes the pH dependency of the adsorption. An increase of dissolved organic carbon from 0.5 to 50 mg L can lead to a >50% decrease in the amount of phosphate adsorbed. Silicic acid may decrease phosphate adsorption, but this effect is only important at a very low phosphate concentration, in particular at high pH.

Natural and pyrogenic humic acids at goethite and natural oxide surfaces interacting with phosphate

Fulvic and humic acids have a large variability in binding to metal (hydr) oxide surfaces and interact differently with oxyanions, as examined here experimentally. Pyrogenic humic acid has been included in our study since it will be released to the environment in the case of large-scale application of biochar, potentially creating Darks Earths or Terra Preta soils. A surface complexation approach has been developed that aims to describe the competitive behavior of natural organic matter (NOM) in soil as well as model systems. Modeling points unexpectedly to a strong change of the molecular conformation of humic acid (HA) with a predominant adsorption in the Stern layer domain at low NOM loading. In soil, mineral oxide surfaces remain efficiently loaded by mineral-protected organic carbon (OC), equivalent with a layer thickness of ≥ ~0.5 nm that represents at least 0.1-1.0% OC, while surface-associated OC may be even three times higher. In natural systems, surface complexation modeling should account for this pervasive NOM coverage. With our charge distribution model for NOM (NOM-CD), the pH-dependent oxyanion competition of the organo-mineral oxide fraction can be described. For pyrogenic HA, a more than 10-fold increase in dissolved phosphate is predicted at long-term applications of biochar or black carbon.