Surface complex models

Pabalan RT, Turner DR (1997) Urarrinm(6+) sorption on montmorillotrite experimental and surface complexation modeling stndy. Aqnat Geochem 2 203-226... 

Kent, D.B., Tripathi, V.S., Ball, N.B., Leckie, J.O., and Siegel, M.D., Surface-Complexation Modeling of Radionuclide Adsorption in Subsurface Environments, U.S. Nuclear Regulatory Commission Report NUREG/CR-4807, 1988, p. 113. 

Burnett PGG, Daughney CJ, Peak D (2006) Cd adsorption onto Anoxybacillus flavithermus surface complexation modeling and spectroscopic... 

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. 

Batch adsorption experiments by Yee and Fein (2002) using aqueous Cd, B. subtilis, and quartz as a function of pH showed that the thermodynamic stability constants, determined from binary systems, could successfully describe the distribution of Cd between the aqueous phase and the bacterial and mineral surfaces. The constants could also be used to estimate the distribution of mass in systems, and construct a surface complexation model. 

The data of Loukidou et al. (2004) for the equilibrium biosorption of chromium (VI) by Aeromonas caviae particles were well described by the Langmuir and Freundlich isotherms. Sorption rates estimated from pseudo second-order kinetics were in satisfactory agreement with experimental data. The results of XAFS study on the sorption of Cd by B. subtilis were generally in accord with existing surface complexation models (Boyanov et al. 2003). Intrinsic metal sorption constants were obtained by correcting the apparent sorption constants by the Boltzmann factor. A 1 2 metal-ligand stoichiometry provides the best fit to the experimental data with log K values of 6.0 0.2 for Sr(II) and 6.2 0.2 for Ba(II). 

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... 

Daughney CJ, Fein JB (1998) The effect of ionic strength on the adsorption of H+, Cd2+, Pb2+, and Cu2+ by Bacillus subtilis and Bacillus licheniformis a surface complexation model. J Colloid Interface Sci 198 53-77... 

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... 

Saving the revised surface complexation model in dataset Fe0H U02s.-dat , the procedure 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... 

It is useful to compare the capacity for each metal to be sorbed (the amount of each that could sorb if it occupied every surface site) with the metal concentrations in solution. To calculate the capacities, we take into account the amount of ferric precipitate formed in the calculation (0.89 mmol), the number of moles of strongly and weakly binding surface sites per mole of precipitate (0.005 and 0.2, respectively, according to the surface complexation model), and the site types that accept each metal [As(OH)4 and ASO4 sorb on weak sites only, whereas Pb++, Cu++, and Zn++ sorb on both strong and weak]. 

We construct in this section a model of how inorganic lead reacts as it infiltrates and contaminates an aquifer, and then as the aquifer is flushed with fresh water during pump-and-treat remediation (Bethke, 1997 Bethke and Brady, 2000). We assume groundwater in the aquifer contacts hydrous ferric oxide [Fe(OH)3, for simplicity] which sorbs Pb++ ions according to the surface complexation model of Dzombak and Morel (1990), as discussed in Chapter 10. 

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.

K( results predict that flushing only a few pore volumes of clean water through the aquifer can displace the contamination, suggesting pump-and-treat remediation will be quick and effective. Models constructed with the surface complexation model, in contrast, depict pump-and-treat as a considerably slower and less effective remedy. 

Fig. 32.4. Chromatographic separation of metal contaminants in a groundwater flow at 25 °C, due to differential sorption. According to the surface complexation model used, Hg++ in the simulation sorbs more strongly to the ferric surface in the aquifer than Pb++, which sorbs more strongly than Zn++. Plot at top shows concentrations of the metal ions in groundwater, and bottom plot shows sorbed metal concentrations.

Davis, J. A. and D. B. Kent, 1990, Surface complexation modeling in aqueous geochemistry. In M. F. Hochella and A. F. White (eds.), Mineral-Water Interface Geochemistry. Reviews in Mineralogy 23, 177-260. 

Dzombak, D. A. and F. M. M. Morel, 1990, Surface Complexation Modeling. Wiley, New York. 

Sverjensky, D. A., 1993, Physical surface-complexation models for sorption at the mineral-water interface. Nature 364, 776-780. 

Dzombak, A. Morel, M. 1990. Surface complexation modeling hydrous ferric oxide. Wiley-lnterscience, New York. 

Dzombak, D. A., and F. M. M. Morel (1990), Surface Complexation Modeling Hydrous Feme Oxide, Wiley-lnterscience, New York. (This book addresses general issues related to surface complexation and its modeling, using the results obtained for hydrous ferric oxide as a basis for discussion. 

The central ion of a mineral surface (in this case we take for example the surface of a Fe(lll) oxide and S-OH corresponds to =Fe-OH) acts as Lewis acid and exchanges its stuctural OH against other ligands (ligand exchange). Table 2.1 lists the most important adsorption (= surface complex formation) equilibria. The following criteria are characteristic for all surface complexation models (Dzombak and Morel, 1990.)... 

Surface Complexation Models and Mean Field Statistics... 

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. 

Amrhein, C., and D. L. Suarez (1988), "The Use of a Surface Complexation Model to Describe the Kinetics of Ligand-Promoted Dissolution of Anorthite", Geochim. Cosmochim. Acta 52, 2795-2807. 

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 ... 

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