Surface complexation model reactions

For the interpretation of the results using the surface-complexation model, reactions 2.47-2.53 have to be taken into account. In addition, the surface acid-base properties and the neutralization reactions of the layer charge have to be included as in Section 2.4.2 the parameters determined there are treated as fixed, input data. In the case of copper- and zinc-montmorillonite, the copper and zinc concentration of the solution and solid also have to be determined, and these data have to be taken into consideration. That is, the quantity of the total sorbed valine and the copper or zinc ion concentrations versus pH function can be fitted, and KH2Valx, KAioH2Vai> and KSi0CuVal stability constants can be computed. The results of the parameter fit for copper- and zinc-montmorillonites as well as the obtained stability constants are shown in Figures 2.17 and 2.18, and in Table 2.12, respectively. 

Surface complexation modeling reactions can be significantly constrained using XAFS-derived information on surface reaction products. 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

Molecular simulation methods can be a complement to surface complexation modeling on metal-bacteria adsorption reactions, which provides a more detailed and atomistic information of how metal cations interact with specific functional groups within bacterial cell wall. Johnson et al., (2006) applied molecular dynamics (MD) simulations to analyze equilibrium structures, coordination bond distances of metal-ligand complexes. 

To be useful in modeling electrolyte sorption, a theory needs to describe hydrolysis and the mineral surface, account for electrical charge there, and provide for mass balance on the sorbing sites. In addition, an internally consistent and sufficiently broad database of sorption reactions should accompany the theory. Of the approaches available, a class known as surface complexation models (e.g., Adamson, 1976 Stumm, 1992) reflect such an ideal most closely. This class includes the double layer model (also known as the diffuse layer model) and the triple layer model (e.g., Westall and Hohl, 1980 Sverjensky, 1993). 

In a second example, we calculate how pH affects sorption onto hydrous ferric oxide, expanding on our discussion (Section 10.4) of Dzombak and Morel s (1990) surface complexation model. We start as before, setting the dataset of surface reactions, suppressing the ferric minerals hematite (Fe203) and goethite (FeOOH), and specifying the amount of ferric oxide [represented in the calculation by Fe(OH)3 precipitate] in the system... 

The rate law is based on a surface complexation model Liger et al. (1999) developed for the hematite nanoparticles (see Chapter 10, Surface Complexation ). The >FeOH surface sites react by protonation and deprotonation to form >FeOII2h and >FeO-, by complexation with ferrous iron to form >FeOFe+ and >FeOFeOH, and to make a complex >Fe0U020H with uranyl. Table 28.1 shows the reactions and corresponding log K values. The nanoparticles are taken to have a specific surface area of 109 m2 g-1, and a site density of 0.06 per Fe2C>3. 

To see how we can use the surface complexation model to trace the kinetics of this reaction, we simulate an experiment conducted at pH 7.5 (Liger et al, 1999, their Fig. 6). They started with a solution containing 100 mmolar NaNC>3, 0.16 mmolar FeS04, and 0.53 g l-1 of hematite nanoparticles. At t = 0, they added enough uranyl to give an initial concentration of 5 x 10-7 molar, almost all of which sorbed to the nanoparticles. They then observed how the mass of sorbed uranyl, which they recovered by NaHCC>3 extraction, varied with time. 

To run the simulation, we save the surface complexation model to a dataset FeOH U02.dat , decouple the relevant redox reactions, set the system s initial composition, and define the rate law. The procedure in REACT is... 

In REACT, we prepare the calculation by disenabling the redox couple between trivalent and pentavalent arsenic (arsenite and arsenate, respectively). As well, we disenable the couples for ferric iron and cupric copper, since we will not consider either ferrous or cupric species. We load dataset FeOH+.dat , which contains the reactions from the Dzombak and Morel (1990) surface complexation model, including those for which binding constants have only been estimated. The procedure is... 

We employ the LLNL thermodynamic data for aqueous species, as before, omitting the PbC03 ion pair, which in the dataset is erroneously stable by several orders of magnitude. The reactions comprising the surface complexation model, including those for which equilibrium constants have only been estimated, are stored in dataset FeOH+.dat . 

Fig. 32.3. Comparison of the simulation results from Figure 32.1 (solid lines), which were calculated using a surface complexation model, with a parallel simulation in which sorption is figured by the reaction Kd approach (dashed lines). In each case, the retardation factor Rf for Pb++ transport is two.

The relative importance of the EDL for reactions other than adsorption is not well understood. Surface complexation models have recently been applied to processes in which adsorption represents the first step in a sequence of reactions. For example, Stumm et al. (22) have applied a model with an EDL component in their studies of the role of adsorption in dissolution and precipitation reactions. The effect of surface charge and potential on precipitation and the... 

Surface complexation models for the oxide-electrolyte interface are reviewed two models for surface hydrolysis reactions are considered (diprotic surface groups and monoprotic surface groups) and four models for the electric double layer (Helmholtz,... 

Empirical Models vs. Mechanistic Models. Experimental data on interactions at the oxide-electrolyte interface can be represented mathematically through two different approaches (i) empirical models and (ii) mechanistic models. An empirical model is defined simply as a mathematical description of the experimental data, without any particular theoretical basis. For example, the general Freundlich isotherm is considered an empirical model by this definition. Mechanistic models refer to models based on thermodynamic concepts such as reactions described by mass action laws and material balance equations. The various surface complexation models discussed in this paper are considered mechanistic models. 

Many models, which could be classified as "surface complexation models (6-8)," have been used to describe reactions at the oxide-solution interface. Although there are differences in the way these models are formulated, they all have two features in common ... 

At equilibrium the rate of all elementary reaction steps in the forward and reverse directions are equal therefore, this condition provides a check point for studying reaction dynamics. Any postulated mechanism must both satisfy rate data and the overall equilibrium condition. Additionally, for the case of reactions occurring at charged interfaces, the appropriate model of the interface must be selected. A variety of surface complexation models have been used to successfully predict adsorption characteristics when certain assumptions are made and model input parameters selected to give the best model fit (12). One impetus for this work was to establish a self-consistent set of equilibrium and kinetic data in support of a given modeling approach. 

The semi-empirical descriptions of adsorbate/solid interactions are based on net changes in system composition and, unlike surface complexation models, do not explicitly identify the details of such interactions. Included in this group are distribution coefficients (Kp) and apparent adsorbate/proton exchange stoichiometries. Distribution coefficients are derived from the simple association reaction... 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

The surface complexation models differ from the above equations in that they explicitly define the chemical reaction involved in the adsorption process. A crucial feature of these models is the treatment of adsorption as an interaction of adsorbing species with well defined coordination sites (the surface OH groups) in a manner analogous to complexation reactions in solution. A further feature of these models is that the chemical free energy of adsorption predominates with electrostatic effects having but a secondary role. 

The surface complexation models quantify adsorption with experimentally determined equilibrium constants. Another, less widely used approach considers the relationship between the equilibrium constant for the adsorption reaction and the associated free energy change (James and Healy, 1972). Attempts have been made to determine the chemical contribution to the overall adsorption free energy by fitting adsorption isotherms to the experimental data values of -50, -33 and —45 kj mol were found for the change in chemical free energy associated with adsorption of Cr, Ni and Zn, respectively, on ferrihydrite (Crawford et al., 1993). Values ranging from -21 to 241 kJ mol were found for Ni on hematite the actual value depended upon the hydrolysis species that were assumed to exist (Fuerstenau and Osseo-Assare, 1987). 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

The site binding model based on reactions (1), (2), (14) and (15), often called surface complexation model (SCM), was, beside the simple site binding models (for example two layer model or constant capacitance model) readily applied to a description of the edl on the metal oxide-electrolyte solution interface. Reactions (14) and (15) describe the adsorption of so-called back-... 

As seen from Equations 1.54-1.56, the intrinsic stability constants of surface reactions are dependent on two factors a chemical and an electric contribution. The chemical contribution is taken into consideration by the mass balance the electric contribution is treated by the charge balance. There are several surface complexation models that mainly differ in the description of the electric double layer that is used to calculate the surface potential, which is done by different double-layer models. These models have been mentioned previously in this chapter. Since, however, the terminology usually used in electrochemistry, colloid chemistry and, especially, in the discussions of surface complexation models is different, they are repeated again ... 

The edge charges can also bond ions with opposite charges. This process, however, is not directed clearly by electrostatic forces chemical properties play an important role. The ions are sorbed with no hydrate shell, that is, inner-sphere complexation occurs. These reactions and the surface complexation models for their quantitative treatment are shown in general in Chapter 1, Table 1.7. 

More complex model reactions (selective butadiene hydrogenation) Apart from being accessible to surface spectroscopy, model catalysts also have the advantage that the nanoparticle morphology and surface structure can be accurately measured. This advantage allows the determination of the relative abundance of specific surface sites and calculation of surface site statistics, as shown, for example, in Table II. Knowledge of the exact number and type of available surface sites then allows calculation of more accurate (and perhaps more meaningful) turnover frequencies of catalytic reactions. 

In the surface complexation model, Stumm and co-workers (Furrer and Stumm, 1983, 1986 Stumm and Furrer, 1987 Stumm and Wieland, 1990) suggested that adsorption or desorption of protons on an oxide surface polarizes the metal-oxygen bonds, weakening the bonding between the cation and the underlying lattice and explaining the pH-dependence of rates. Surface complexation reactions for an oxide mineral can be written as follows (Schindler, 1981) ... 

Surface complexation models (SCM s) provide a rational interpretation of the physical and chemical processes of adsorption and are able to simulate adsorption in complex geochemical systems. Chemical reactions at the solid-solution interface are treated as surface complexation reactions analogous to the formation of complexes in solution. Each reaction is defined in terms of a mass action equation and an equilibrium constant. The activities of adsorbing ions are modified by a coulombic term to account for the energy required to penetrate the electrostatic-potential field extending away from the surface. Detailed information on surface complexation theory and the models that have been developed, can be found in (Stumm et al., 1976 ... 

Methods for measurement of parameters used in SCM s have been described in the literature. Only a brief summary is presented here. Surface complexation model parameters that can be measured directly include, (1) the solid concentration, (2) surface site density, (3) surface area, and (4) equilibrium constants for the mass action equations describing all relevant adsorption reactions. The relation between surface charge and potential is calculated in geochemical equilibrium models. 

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