Models chemical surface complexation

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

Surface-complexation models require a high degree of detail about the heterogeneous systems. Unfortunately, the chemical detail required to use surface-complexation models will often exceed our knowledge of interactions taking place in natural systems. Consequently, geochemists have often resorted to semi-empirical, macroscopic descriptions, which are more easily utilized. 

In surface-complexation models, the relationship between the proton and metal/surface-site complexes is explicitly defined in the formulation of the proposed (but hypothetical) microscopic subreactions. In contrast, in macroscopic models, the relationship between solute adsorption and the overall proton activity is chemically less direct there is no information given about the source of the proton other than a generic relationship between adsorption and changes in proton activity. The macroscopic solute adsorption/pH relationships correspond to the net proton release or consumption from all chemical interactions involved in proton tranfer. Since it is not possible to account for all of these contributions directly for many heterogeneous systems of interest, the objective of the macroscopic models is to establish and calibrate overall partitioning coefficients with respect to observed system variables. 

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 main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

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). 

Goldberg, S. 1992. Use of surface complexation models in soil chemical systems. ln Sparks, D. L. (ed) Advances in Agronomy, 47. Academic Press, Inc., San Diego, 233-329. 

Surface complexation model A computer code or geochemical model that provides an explanation and attempts to predict the partitioning of a chemical species between the surface of an adsorbent and the associated solvent. The models consider a number of factors, including pH and ionic strength (see (Langmuir, 1997), 369-395 for details compare with charge distribution multisite complexation model). 

Goldberg, S. (1992) Use of surface complexation models in soil chemical systems. Adv. 

Deposition-precipitation is often practised with silica as the support. Especially suitable is aerosil silica, which consists of very small non-porous spheres, so that the precipitation process is not affected in any way by diffusion processes. It is well known that most hydrolysed metal species have a high affinity for the silica surface, thus fulfilling the condition for obtaining surface precipitation only. In the colloid-chemical literature, the initial adsorption of the (partially) hydrolysed metal ions with a silica surface is often described in terms of a surface-complexation model, involving negatively charged surface sites, which exist on silica at pH above 2 (= pzc of silica), and positively charged metal species ... 

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 thermodynamic equilibrium models, including surface complexation models, require the solution of a complex mathematical equation system. For this reason, many computer programs (e.g., CHEAQC, CHEMEQL, CHESS, EQ3/6, F1TEQL, Geochemist s Workbench, H ARPHRQ, JESS, MINTEQ and its versions, NETPATH, PHREEQC, PHRQPITZ, WHAM, etc.) have been developed to calculate the concentration and activity of chemical species, estimate the type and amount of minerals formed or dissolved, and the type and amount of sorbed complexes. 

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. 

A)jS, whether sampled from probability distribution functions or calculated by regression equations or surface-complexation models, can be used in many contaminant transport models. Alternate forms of the retardation factor equation that use a (Equation (3)) and are appropriate for porous media, fractured porous media, or discrete fractures have been used to calculate contaminant velocity and discharge (e.g., Erickson, 1983 Neretnieks and Rasmuson, 1984). An alternative approach couples chemical speciation calculations... 

Papelis C., Hayes K. F., and Leckie J. O. (1988) HYDRAQL A Program for the Computation of Chemical Equilibrium Composition of Aqueous Batch Systems Including Surface-complexation Modeling of Ion Adsorption at the Solution Oxide/Solution Interface. 306. Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, CA, 131pp. 

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 ... 

Spurred by the work of Stumm (e.g., 11-13), surface complexation modeling has emerged as a powerful tool for describing chemical sorption of ions onto reactive mineral surfaces. In surface complexation models, ions and individual functional groups on the surface are considered to react to form... 

Anderson, P, R., and M. M. Benjamin. 1990a. Constant-capacitance surface complexation model. In Chemical modeling of aqueous systems //, ed D. C. Melchior and R. L. Bassett. Am. Chem. Soc. Symp. Ser. 416, pp. 272-81. Washington DC Am. Chem. Soc. 

Surface complexation models of the solid-solution interface share at least six common assumptions (1) surfaces can be described as planes of constant electrical potential with a specific surface site density (2) equations can be written to describe reactions between solution species and the surface sites (3) the reactants and products in these equations are at local equilibrium and their relative concentrations can be described using mass law equations (4) variable charge at the mineral surface is a direct result of chemical reactions at the surface (5) the effect of surface charge on measured equilibrium constants can be calculated and (6) the intrinsic (i.e., charge and potential independent) equilibrium constants can then be extracted from experimental measurements (Dzombak and Morel, 1990 Koretsky, 2000). 

Various empirical and chemical models of metal adsorption were presented and discussed. Empirical model parameters are only valid for the experimental conditions under which they were determined. Surface complexation models are chemical models that provide a molecular description of metal and metalloid adsorption reactions using an equilibrium approach. Four such models, the constant capacitance model, the diffuse layer model, the triple layer model, and the CD-MUSIC model, were described. Characteristics common to all the models are equilibrium constant expressions, mass and charge balances, and surface activity coefficient electrostatic potential terms. Various conventions for defining the standard state activity coefficients for the surface species have been... 

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