Surface complexation model structure

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. 

The surface complexation models used are only qualitatively correct at the molecular level, even though good quantitative description of titration data and adsorption isotherms and surface charge can be obtained by curve fitting techniques. Titration and adsorption experiments are not sensitive to the detailed structure of the interfacial region (Sposito, 1984) but the equilibrium constants given reflect - in a mean field statistical sense - quantitatively the extent of interaction. 

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. 

EXAFS, extended X-ray absorption fine structure eXANES, X-ray absorption near-edge structure SCM, surface complexation model... 

The models describing hydrolysis and adsorption on oxide surfaces are called surface complexation models in literature. They differ in the assumptions concerning the structure of the double electrical layer, i.e. in the definition of planes situation, where adsorbed ions are located and equations asociating the surface potential with surface charge (t/> = f(5)). The most important models are presented in the papers by Westall and Hohl [102]. Tbe most commonly used is the triple layer model proposed by Davis et al. [103-105] from conceptualization of the electrical double layer discussed by Yates et al. [106] and by Chan et al. [107]. Reviews and representative applications of this model have been given by Davis and Leckie [108] and by Morel et al. [109]. We will base our consideration on this model. 

In the area of interfacial charging at the solid/liquid interface of metal oxide aqueous suspensions, the "surface complexation or site binding concept is commonly used [3-20]. This concept is characterised by consideration of specific ionic reactions with surface groups, rather than assuming simple binding of ions to the surface or their accumulation at the interface (adsorption). In the past decade several different models were introduced on the basis of the surface complexation model (SCM) they differ in the assumed structure of the electrical interfacial layer (EIL) and in the proposed mechanisms and stoichiometries of surface reactions leading to surface charge. 

Spadini, L. et al., Hydrous ferric oxide Evaluation of Cd-HFO surface complexation models combining Cd EXAFS data, potentiometric titration results and surface site structures identified from mineralogical knowledge, J. Colloid Interf. Sci., 266, 1, 2003. 

Pokiovsky, O.S., Pokrovsky, G.S., and Schott, J., Gallium (III) adsorption on carbonates and oxides X-ray absorption fine structure study and surface complexation modeling, J. Colloid Interf. Sci.. 279, 314, 2004. 

Van Cappellen P, Charlet L, Stumm W, Wersin P (1993) A surface complexation model of the carbonate mineral-aqueous solution interface. Geochim Cosmochim Acta 57 3503-3518 van der Vegt HA, Pinxteren HM, Lohmeier M, Vlieg E, Thornton JMC (1992) Surfactant induced layer-by-layer growth of Ag on Ag(l 11). Phys Rev Lett 68 3335-3338 van Hove MA (1999) Atomic scale surface structure determination comparison of techniques. Surf Interface Anal 29 36-43... 

In this chapter we present a general method for solving surface/ solution equilibrium problems described by a surface complexation model, applicable for arbitrary surface layer charge/potential relationships and arbitrary surface/solution interface structures. 

The paper summarizes eiforts started to deliver a profound chemical base for risk assessment, namely to properly take into account the physico-chemical phenomena governing the contamination source term development in time and space. One major aspect there is the substitution of conventional distribution coefficients (IQ values) for the empirical description of sorption processes by surface complexation models, in combination with other thermodynamic concepts. Thus, the framework of a Smart Kd is developed for complex scenarios with a detailed explanation of the underl3dng assumptions and theories. It helps to identify essential processes and the associated most critical parameters, easing further refinement studies. The presented case studies cover a broad spectrum of contamination cases and successfully demonstrate the applicability of the methodology. The necessity to create a mineral-specific sorption database to support the Smart IQ approach is derived and a first prototype for such a digital database introduced, combining numeric data with a knowledge base about the relevant theories, experimental methods, and structural information. 

MOLECULAR HYPOTHESES. If experimental methods exist for measuring the composition of the solid and aqueous solution phases in a suspension of adsorbent particles, then Eqs. 5.82 to 5.85 constitute a description of the surface chemistry that requires no additional molecular hypotheses. Surface complexation models represent molecular theories that try to calculate the composition of the solid phase in a suspension, given the composition of the aqueous solution phase and a set of hypotheses concerning the detailed structure of the interfacial region. The specific focus of these models is the equilibrium constants in Eq. 5.87. Consider, for example, Eq. 5.87a. The equilibrium constant soc be expressed... 

Another major disadvantage of the commonly used surface complexation models, and of most equilibrium-based sorption models, is that three-dimensional surface products are not included as possible complexes. However, there are several exceptions. Farley et al. (5) and James and Healy (6) considered surface precipitation in successfully modeling sorption of hydrolyzable metal ions. Dzombak and Morel (7) modified the diffuse layer surface complexation model to include surface precipitation. However, these applications relied solely on macroscopic data without molecular-level identification of the sorption complex structure. Recently, Katz and Hayes (8,9) employed triple layer models, that included a surface solution model, a surface polymer model, and a surface continuum model to describe molecular level data for Co sorption on y-AljOj over a wide range of surface coverages (0.1 to 100%). 

Let us now concentrate our attention on the surface of the support grains, more precisely on the surface of the support nanoparticles that constitute the grains. In spite of the progress with respect to the surface structure of the oxidic supports thanks to crystallography or IR spectroscopy, rather hypothetical homogeneous surface ionization (or surface binding or surface complexation) models have been used to describe their protonation/deprotonation behavior in aqueous solutions. 

Inorganic (coordination) chemists are interested in the structure of surface complexes. Advanced surface complexation models can take into account the details of surfaces and bonding mechanisms to describe the molecular structure within a thermodynamic fi-amework. 

In this survey, it is not attempted to show how successful surface complexation models are. Rather, it is attempted to show what can be done with them, what will be done with them in the future and what should not be done with them. The experimental aspects (e.g., the input data to the models) are discussed whenever judged important (certainly without full coverage but rather with focus on those aspects, which have not yet been addressed or details which have been of interest to the present author). In particular, the importance of combining methods and data is stressed. In recent years, there has been an increased interest in linking surface complexation models, which are traditionally based on macroscopic (adsorption, titration, and other) data, with structural information obtained with modem spectroscopic methods such as x-ray absorption spectroscopy (XAS). It is expected that the closer the agreement of the thermodynamic formulation of a surface chemical reaction with the actual structure of a surface complex is, the more reliable a prediction of the system behavior under more or less strongly varied conditions will be. 

Similar discrepancies as documented in the previous section for macroscopic data can be found for the spectroscopic approaches, which are now available for studying the structure of surface complexes in situ (i.e., wet samples). With respect to inverse modeling these studies would attempt to resolve the stmcture of the surface complexes in a certain system and to impose such structures in the surface complexation model. This would avoid extensive discussion about the mode of bonding (e.g., inner versus outer sphere monodentate versus polydentate). 

A second distinction among the surface complexation models arises from the postulated structure of the electrical double layer. 

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