Surface complexation models interactions

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. 

Burnett PGG, Daughney CJ, Peak D (2006) Cd adsorption onto Anoxybacillus flavithermus Surface complexation modeling and spectroscopic investigations. Geochim Cosmochim Acta 70 5253-5269 Chenu C, Stotzky G (2002) Interactions between microorganisms and soil particles an overview. In Huang PM, Bollag J-M, Senesi N (eds) Interactions... 

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. 

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. 

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. 

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... 

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. 

The adsorption data is often fitted to an adsorption isotherm equation. Two of the most widely used are the Langmuir and the Freundlich equations. These are useful for summarizing adsorption data and for comparison purposes. They may enable limited predictions of adsorption behaviour under conditions other than those of the actual experiment to be made, but they provide no information about the mechanism of adsorption nor the speciation of the surface complexes. More information is available from the various surface complexation models that have been developed in recent years. These models represent adsorption in terms of interaction of the adsorbate with the surface OH groups of the adsorbent oxide (see Chap. 10) and can describe the location of the adsorbed species in the electrical double layer. 

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. 

Here, the adsorption of valine on different cation-exchanged montmorillonites is described (Nagy and Konya 2004). A discussion of the kinds of interactions that are possible in the ternary system of montmorillonite/valine/metal ions will be presented, and a description how the metal ions can affect these interactions. The interlayer cations (calcium, zinc, copper ions) were chosen on the basis of the stability constants of their complexes with valine. The adsorption of valine on montmorillonite is interpreted using a surface-complexation model. 

Metal mobility in soils is governed by interfacial processes, such as dissolution. The role of DOM in such processes will be determined by the nature t organic matter-surface associations. The surface complexation model pro-odes a conceptual framework for estimating the contributions of specific DOM components, particularly LMW organic ligands, to the mobilization f metals in soils. With this framework, the effects of humic substances on mineral dissolution can be interpreted to provide some insight into hu--mate-surface interactions. 

Weerasooriya, R,. Dharmasena, B., and Aluthpatabendi, I ),. Copper-gibbsite interactions An application of l-pK surface complexation model. Colloids Surf. A, 170, 65. 2000. 

However, as we noted early in this chapter, numerous assumptions are employed in the field applications of surface complexation models. Davis et al. (1998) noted that surface complexation models are mainly developed from well-controlled laboratory experiments. It is unclear how the models can be applied to soil and sediments where the double layers of the heterogenous particles may interact and the competitive adsorption of many different ions can cause significant changes in the electrical properties of mineral-water interfaces. 

The model used to evaluate surface chemistry in these systems is the constant capacitance surface complexation model. This model has been used to describe the adsorption of cations (41) and anions (4,8) onto oxides similar to those used in our experiments. A significant difference between those studies and the present study is that we have adapted the model to simulate some of the interactions that might occur between particles in a binary suspension. 

Details on the adsorbent preparation and experimental and analytical techniques are presented elsewhere (9). This paper briefly reviews the experimental results for the Fe(OH)3 and Si02 suspensions and describes a conceptual and mechanistic model for particle interactions which is qualitatively consistent with the experimental observations. Similar results were obtained for binary Al(OH)3 and Si02 suspensions (9). The constant capacitance surface complexation model is then used to test the mechanistic model and estimate the quantitative influence of the particle-particle interactions on adsorbate distribution. 

A major advantage of the reviewed surface complexing models is that they take into consideration the effect of protonation, deprotonation and surface ion interaction reactions. However, they are built on an assumption... 

The Non-Electrostatic (NE) surface complexation model (Davis et al., 1998 Stumm et al., 1976) This is the simplest approach, neglecting any contributions from electrostatic interactions. The sorption is considered to be purely chemical, thus only reaction constants serve as fitting parameters. 

To date, potentiometric titration is still a main approach to study the surface acid base chemistry of clay minerals. Only some papers deal with the dissolution of a solid matrix resulting in various hydrolyzed aluminum species, silicic acid and their product hydrous aluminosilicates, though their interaction with a clay surface should be considered in the modeling description. The surface complexation model (SCM) was successfully applied in a recent paper [6] to interpret surface acid-base reactions involving the dissolution of illite clays during prolonged titration. Voluminous literature on ion adsorption and surface complexation... 

Scientists fi-om different disciplines (e.g., soil science, geology) are interested in interactions of solutes with minerals (e.g., heavy metals as an example of hazardous substances but also nutrients, etc.). Surface complexation models are tools which allow to describe such interactions. 

Environmental scientists such as those involved in performance assessment (e.g., for nuclear repositories) would be pleased to be able to predict the interactions of solutes (e.g., radionuclides) with backfill and geologic materials. However, in this area, which in many countries receives much public attention, even the aqueous solution equilibria for the pertaining conditions of a favored repository concept cannot be accurately described (e.g., metal ions in brine solutions, which require Pitzer formalism, or in highly alkaline backfill pore waters, which have traditionally not received much attention in aqueous solution studies because of the limitations of glass electrodes and solid phase formation). Databases for surface complexation applications are also required for many other purposes, but the major drawback of such potential databases is that no agreement exists on the actual surface complexation model to be used. This may ultimately lead to particular difficult situations whenever one of the following occur ... 

Payne, T.E. 1999. Uranium (VI) interactions with mineral surfaces Controlling factors and surface complexation modeling. Ph.D. Diss. Univ. of New South Wales, Australia. 

A theoretical model for the adsorption of metals on to clay particles (<0.5 pm) of sodium montmorillonite, has been proposed, and experimental data on the adsorption of nickel and zinc have been discussed in terms of fitting the model and comparison with the Gouy-Chapman theory [10]. In clays, two processes occur. The first is a pH-independent process involving cation exchange in the interlayers and electrostatic interactions. The second is a pH-dependent process involving the formation of surface complexes. The data generally fitted the clay model and were seen as an extension to the Gouy-Chapman model from the surface reactivity to the interior of the hydrated clay particle. 

Figure 6.16 Proposed chemical interaction of Pt complexes with an alumina surface, which involves surface-ligand exchange of either surface OH for Cl ligands (model B1) or Cl ligands from the CPA complex for surface hydroxyls (model B2). (From Shelimov, B., Lambert, J.-F., Che, M., and Didillon, B., J. Mol. Catal. 158, 2000, 91.)...

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

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