Surface complexation constants predicting

Smith, R. W., and E. A. Jenne. 1991. Recalculation, evaluation, and prediction of surface complexation constants for metal adsorption on iron and manganese oxides. Envir. Sci. Technol. 25 525-31. 

Another standardized database for the diffuse layer model was developed for montmorillonite by Bradbury and Baeyens (2005). Surface complexation constants for strong and weak sites and cation exchange were fit to adsorption data for various metals using constant site densities and protonation-dissociation constants in a nonelectrostatic modeling approach. Linear free energy relationships were developed to predict surface complexation constants for additional metals from their aqueous hydrolysis constants. 

Analysis of the predictive capabilities of the monomer surface complexation constants over a range of moderate and high surface coverage data. Model failure at high coverage was indicative of the formation of surface precipitates. 

Furthermore, most experimental work applies to simple laboratory systems, and a great deal more work will be needed to understand adsorption quantitatively in complex natural systems. Particularly lacking is a mechanistic understanding of the role of the solid phase, and any ability to predict surface complexation constants. A promising start in this direction is provided by Sverjensky (1993,1994), and Sverjensky and Sahai (1996). 

However, the predicted uranium concentration at the edge of the plume is far higher than the observation. Observational data indicate that the maximum uranium concentration outside of the plume is less than 92 pCi/L, while the model predicts it to be in excess of 1000 pCi/L. This discrepancy between observed and predicted uranium concentrations is an indication that the model parameters used to predict U partitioning may not accurately represent conditions at the site. Examination of Dzombak and Morel s (1990) work reveals that they have used a predicted surface complexation constant rather than retrieve a value from experiments. Newer experimental data show that uranium(VI) adsorption onto ferrihydrite can be fitted by a two-site model with a bidentate complex (Waite et al 1994). However, this type of modeling ability is not included with minteqa2. Another explanation could be that co-precipitation is ignored, but apparently is a major attenuation mechanism at a similar site (Opitz et al. 1983). 

Simulations of three representative Cs-smectites revealed interlayer Cs+ to be strongly bound as inner sphere surface complexes, in agreement with published bulk diffusion coefficients [78]. Spectroscopic and surface chemistry methods have provided data suggesting that in stable 12.4 A Cs-smectite hydrates the interlayer water content is less than one-half monolayer. However, Smith [81] showed using molecular simulations of dry and hydrated Cs-montmorillonite that a 12.4 A simulation layer spacing was predicted at about one full water monolayer. The results of MD computer simulations of Na-, Cs-and Sr-substituted montmorillonites also provide evidence for a constant water content swelling transition between one-layer and two-layer spacings [82]. 

The complexation constants of the individual major seawater ions with otFeOOH determined in single salt solutions can be used to predict the titratable charge and surface species distribution of goethite in seawater. This prediction can then be compared with the experimentally determined charge of goethite in a mixed seawater type electrolyte. 

Accurate predictions of the transport of As in groundwater requires site specific data to model adsorption/desorption reactions. In complex mixtures of minerals, it may not be possible to quantify the adsorption properties of individual minerals. Therefore, it has been suggested that adsorption properties of composite materials should be characterized as a whole (Davis and Kent, 1990). Previously published data for adsorption by pure mineral phases such as the surface complexation database for adsorption by ferrihydrite (Dzombak and Morel, 1990) can be a useful starting point for modeling adsorption of solutes in groundwater however, these equilibrium constants may not reflect the adsorption properties of composite oxide coatings on aquifer solids. For example, incorporation of Si, and to a lesser extent, A1 into Fe oxyhydroxides has been shown to decrease adsorption reactivity towards anions (Ainsworth et al., 1989 Anderson and Benjamin, 1990 Anderson et al, 1985). Therefore, equilibrium constants will likely need to be modified for site-specific studies. 

When only little information is available on both the surface structure and the type of complexes formed it is reasonable to investigate the adsorption with the simple one-pKn model of a pseudo homogeneous surface and to describe the complexation with Eqs. (55) to (59). Review [29] and Refs. [52-54] can be consulted for details of this type of description. In review [29] also the bond valence concept is used to describe complexation. No attempts have been made to predict complexation constants so that these have to be treated as adjustable parameters. 

Figure 4.4. Predicted adsorption of as a function of concentration on an Al hydroxide having acid dissociation constants of = 10 and K2 = 10 and a complexation constant of = 10 . Surface bonding by the bidentate mechanism is neglected for this calculation.

The specific adsorption of H, OH, cations, and anions on hydrous oxides and the concomitant establishment of surface charge can be interpreted in terms of the formation of surface complexes at the oxide-water interface. The fixed charge of the solid surface and the pH of its isoelectric point can be measured experimentally by determining the proton balance at the surface (from alkalimetric titration curve) and by the analytical determination of the extent of adsorbate adsorption. Equilibrium constants established for the surface coordination reactions can be used to predict pHiEp, to calculate adsorption isotherms, and to estimate concentration-pH regions for which the hydrous oxide dispersions are stable from a colloid-chemical point of view. 

---