Surfaces complexation reactions

Fowle and Fein (1999) measured the sorption of Cd, Cu, and Pb by B. subtilis and B. licheniformis using the batch technique with single or mixed metals and one or both bacterial species. The sorption parameters estimated from the model were in excellent agreement with those measured experimentally, indicating that chemical equilibrium modeling of aqueous metal sorption by bacterial surfaces could accurately predict the distribution of metals in complex multicomponent systems. Fein and Delea (1999) also tested the applicability of a chemical equilibrium approach to describing aqueous and surface complexation reactions in a Cd-EDTA-Z . subtilis system. The experimental values were consistent with those derived from chemical modeling. 

Following our convention, we place surface complexes to the left of the reactions and the uncomplexed sites to the right. For use with the gwb programs, file FeOH.dat contains the database of surface complexation reactions prepared by... 

Table 28.1. Surface complexation reactions considered by Liger et al. (1999) in describing the kinetics of the catalytic oxidation ofuranyl by ferrous iron...

TLM Activity Coefficients. In the version of the TLM as discussed by Davis et al. (11), mass action equations representing surface complexation reactions were written to include "chemical" and "coulombic" contributions to the overall free energy of reaction, e.g., the equilibrium constant for the deprotonation reaction represented by Equation 2 has been given as... 

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Table 9.11 Rate parameters for the surface complexation reactions in equation (9.11) (goethite adsorbent) at pH 5...

In analogy with Eqs. 1.50 and 2.5, the overall surface ligand-exchange reactions in Eqs. 3.46, 3.53, 3.56, 3.65, 3.66, and 4.15b can be dissected into steps by applying the concept of the Eigen-Wilkins-Wemer mechanism, discussed in Section 2.1. Following this perspective, one would decompose the overall surface complexation reaction in Eq. 4.15b into a set of coupled reactions (cf. Eq. 1.50) ... 

Model Adjustable Parameters Protolysis Reaction Surface Complexation Reaction Surface Mass Balance... 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

Koretsky, C. (2000). The significance of surface complexation reactions in hydrologic systems a geochemist s perspective. Journal of Hydrology, 230, 127-71. 

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) ... 

In a more recent study. Nelson and Yang [494] pre.sented a surface complex-ation model to describe the effect of pH on adsorption equilibria of chlorophe-nols, i.e., the electrostatic effect they also discussed the potential importance of 7t-7t interactions and donor-acceptor complex formation but could not distinguish between the two and concluded, somewhat vaguely, that [t]hese proposed mechanisms provide plausible explanations for the surface complexation reactions between chlorophenols (neutral or anionic forms) and the surface of activated carbon (acidic or basic sites). ... 

Surface complexation reactions (surface hydrolysis, the formation of co-ordinative bonds at the surface with metals and with ligands). 

The specific adsorption of an anion at the oxide/electrolyte surface, which changes the surface charge, may be viewed as a surface complexation reaction. Thus, fluoride ions that are adsorbed at the silicon oxide surface centers form Si-F complex ... 

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 ... 

Deriving equilibrium constants for surface complexation reactions based on experimental data can be facilitated with paramater optimization programs such as FITEQL (Westall, 1982). Ideally, surface site density is known however, it can also be adjusted simultaneously with LogK s to... 

The one-dimensional reaction-transport model PHREEQC version 2 (Parkhurst and Appelo, 1999) was used to simulate chemical changes, resulting from elution of ground water through the cores. PHREEQC has the capability to model advective transport of water in combination with a variety of chemical reactions including homogeneous aqueous reactions, mineral equilibria, and surface-complexation reactions. Version 2 of PHREEQC has added capabilities to simulate kinetic reactions. 

The interfacial aqueous coordination chemistry of natural particles, in particular their surface complexation reactions, owes much of its development to the research of Werner Stumm. Beginning with the tentative interpretation of specific adsorption processes in terms of chemical reactions to form inner-sphere surface complexes, his seminal questions spawned a generation of research on the detection and quantitation of these surface species. The application of noninvasive spectroscopy in this research is exemplified by electron spin resonance and extended X-ray absorption fine structure studies. These studies, in turn, indicate the existence of a rich variety of surface species that transcend the isolated surface complex in both structure and reactivity, thereby stimulating future research in molecular conceptualizations of the particle-water interface. 

Carroll-Webb, S. A., and J. V. Walther. 1988. A surface complex reaction model for the pH-dependence of corundum and kaolinite dissolution rates. Geochim. Cosmochim. Acta 52 2609-23. 

The Rossendorf expert system for surface and sorption thermodynamics, RES T (Brendler et al., 2003, 2004), is a mineral-specific digitized thermodynamic database that contains mineral properties, specific surface areas, ion sorption data, surface complexation reactions, and bibliographic information based... 

In the generalized composite (GC) approach it is assumed that the adsorption behavior of a complex mineral assemblage can be described by surface complexation reactions written with generic surface functional groups that represent average properties of the assemblage as a whole rather than as specific mineral phases (Davis et al., 1998). The GC approach uses the overall surface area value and fits surface complexation constants to tlie experimental adsorption data. The number of site types and surface complexes are chosen to provide good data... 

---