Models CD-MUSIC

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. 

In another study, maximum adsorption of fulvic acid from a podzol B horizon by goethite was found to lie at ca. 0.6 0.3 and 0.05 mg m at pH 4 7 and 10, respectively and this was successfully described with the CD-MUSIC model (Films et al.,... 

Charge Distribution Multisite Complexation (CD-MVSIC) Model The charge distribution multisite complexation model (CD-MUSIC) model of the oxide-solution interface (Hiemstra et al., 1989 Hiemstra and van Riemsdijk, 1996) contains the following assumptions ... 

In the CD-MUSIC model the reactive surface functional group is defined as where n is the number of metal ions in tire solid phase coordi-... 

Figure 6.8. Fit of the CD-MUSIC model to cadmium adsorption by goethite. Open symbols represent total dissolved Cd at le-6 mol L. Closed symbols represent total dissolved Cd at le-4 mol L. (From Venema et al., 1996a.)...

In the CD-MUSIC model, C l is obtained from titration data and C2 is chosen to provide a good fit to the salt dependency of specifically adsorbing ions (Hiemstra and van Riemsdijk, 1996). It should be noted that the diffuse layer model does not contain any capacitance parameters. 

X-RAY REFLECTIVITY X-ray reflectivity measurements can provide important information about mineral-water interfaces in situ by accurately determining die position of an adsorbed monolayer relative to the substrate surface. By measuring x-ray reflectivity of calcite, with and without lead, Sturchio et al. (1997) established that the lead ions were located in the surface atomic layer. X-ray reflectivity measurements found rubidium to be specifically adsorbed to the rutile surface at the tetradentate site (Zhang et al., 2004). These authors were able to include this information in the CD-MUSIC model to obtain an accurate description of rubidium adsorption. 

An alternative type of component additivity approach is to use different adsorption models for different adsorbents (Weng et al., 2001, 2002). These authors described Cd2+, Zn2+, and NP+ binding in soils by combining the diffuse layer model for hydrous ferric oxide, the CD-MUSIC model for goethite, the Donnan model for illite, and the nonideal competitive adsorption (NICA) Donnan model (Kmniburgh et al., 1999) for humic acid. 

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... 

Weerasooriya. R., Aluthpatabendi, I), and Tobschall, H.J., Charge distribution multi-site complexation (CD-MUSIC) modeling of Pb(II) adsorption on gibbsite. Colloids Surf. A. 189, 131, 2001. 

Conclusion We conclude that the CD-MUSIC model can be considered as a frame work, which is based as much as possible on structural information and well established concepts taken from mineralogy and colloid chemistry, and can be used... 

This interpretation has been supported by a study of oxalate sorption on corundum modelled by the CD-MUSIC model involving ATR-IR spedroscopy (Johnson et al., 2004). A mononuclear bidentate complex was found up to 14 pmol/m, whereupon oxalate additionally adsorbed as an outer-sphere complex. Sorption of oxalate has also been studied on boehmite and corundum by Yoon et al. (2004) The peaks assigned to the inner-sphere complex in previous works (near 1286,1418,1700 and 1720 cmi) were claimed to arise from the presence of several species. Evidence for this phenomenon comes from the observation that peaks at 1286 and 1418 cm-i are shifted to 1297 and 1408 cm-i as the oxalate surface coverage increases. The authors finally postulated the existence of two species species "A" at 1286 and 1418 cmi, and species "B" at 1297 and 1408 cm-i, respectively, which were... 

DETAILED SURFACE DESCRIPTION CD-MUSIC MODEL 12.3.1 Model Formulation... 

In the CD-MUSIC model, the activity of the surface species are expressed as coverage Oi (fraction of surface sites occupied by species i, see Chapter 4), and the electrostatic contribution (Equation 12.1) is given by... 

As in the other surface complexation models, the chemical part consists in several adsorption reactions. In the application of the CD-MUSIC model, the choice of these reactions has been carefully based on spectroscopic evidence of course, that can be done also in other models such as TLM, BSM, and so on, but here it is rather essential for proper elucidation of the charge distribution. The original paper (Hiemstra and Van Riemsdijk 1996) applied the model to phosphate adsorption on goethite. Based on spectroscopic studies, they proposed the existence of monodentate-bound species (Reaction 12.44), an unprotonated bidentate-bound species (Reaction 12.45), and a protonated bidentate species, following ... 

The model parameters are the capacitances Q and Cj (as in the TLM), the intrinsic protonation and complexation constants, and f. The model has shown good predictive capabilities, as shown in Figure 12.15, where data from different authors (Hiemstra and Van Riemsdijk 1996 Bowden et al. 1980) are seen to fit well by the CD-MUSIC model with the same parameter values. 

In spite of the successful results, because of the requirement of detailed knowledge of surface species, the application of the CD-MUSIC model has been until present... 

Finally, S0 et al. (2012) studied the competitive adsorption of arsenate and phosphate onto calcite in batch experiments and modeled the results using the CCM and the CD-MUSIC model. It was found that both models predicted well the separate adsorption of both anions, but the CCM underpredicted the competition when combining the models for single sorbate systems instead the CD-MUSIC model predicted satisfactorily the competitive adsorption of phosphate and arsenate onto calcite in the binary systan by combining the parameters of the separate systems. 

Antelo, J., S. Fiol, C. Perez, S. Marino, F. Arce, D. Gondar, and R. L6pez. 2010. Analysis of phosphate adsorption onto ferrihydiite using the CD-MUSIC model. Journal of Colloid and Interface Science 3iAl, no. 1 112-119. doi 10.1016/j.jcis.2010.03.020. 

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