Electrostatic sorption and surface complexation

Figure 6. Schematic representation of electrostatic sorption and surface complexation involved in Na -Ca2+ exchange at the mineral-water interface. =SOHn represents a surface hydroxyl (variable-charge) site =Sp represents a site of fixed (permanent) negative charge.

The surface complexation approach is distinct from the Stern model in the primacy given the specific chemical interaction at the surface over electrostatic effects, and the assignment of the surface reaction to the sorption reactions themselves (Dzombak and Morel, 1990). 

Surface complexation models can be extended to account explicitly for electrostatic sorption by calculating excess counterion concentrations in the diffuse layer in addition to specific sorption. Counterions in the diffuse layer (e.g., Ca ) can then be treated as distinct from those in bulk solution (e.g., Ca2+) and those that are specifically sorbed (e.g., =Sp-Caf). The total sorption is given by the sum of the concentrations of specifically sorbed and electrostatically sorbed species ... 

Marmier, N. and Fromage, E. Comparing electrostatic and nonelectrostatic surface complexation modeling of the sorption of lanthanum on hematite, J. Colloid Interf. Sci., 212, 252, 1999. 

A number of different surface complexation models have been applied to describe and predict divalent metal ion sorption data over the past 20 to 30 yr. All of Ihe models incorporate surface acidity and the formation of metal ion complexes with surface hydroxyl groups via equilibrium mass law expressions such at those presented in Tabic 7-2. In addition, each model employs a description of the elec-Irical double layer lo correcl for electrostatic effects at the mineral/water interface (as shown in Fig. 7 4 lor (he triple layer model and described in Table 7-3). These... 

There appears to be little point in a detailed evaluation of the more ambiguous hypotheses of heavy metal fixation. However, it may be noted that a number of investigators have considered the possibility of either the sorption of complex ions on clay mineral surfaces or reaction of heavy metal cations with clay surfaces in some manner other than simple electrostatic sorption (31, 66, 67, 156, 157, 217), However, the solubility products of Cu(OH)2 and Zn(OH)2 in aqueous suspensions of montmorillonite (19) have been found to be quite similar to those previously found for solutions in contact with only the pure hydroxides. This would indicate that metal ion-clay mineral surface complex formation is not important otherwise the apparent solubility would have been greater in the presence of montmorillonite. 

Several models have been developed to describe reactions between aqueous ions and solid surfaces. These models tend to fall into two categories (1) empirical partitioning models, such as distribution coefficients and isotherms (e.g., Langmuir and Freundlich isotherms), and (2) surface-complexation models (e.g., constant-capacitance, diffuse-layer, or triple-layer model) that are analogous to solution complexation with corrections for the electrostatic effects at the solid-solution interface (Davis and Kent, 1990). These models have been described in numerous articles (Westall and Hohl, 1980 Morel, Yeasted, and Westall, 1981 James and Parks, 1982 Barrow, 1983 Westall, 1986 Davis and Kent, 1990 Dzombak and Morel, 1990). Travis and Etnier (1981) provided a comprehensive review of the partitioning and kinetic models typically used to define sorption of ions by soils. The reader is referred to the cited articles for details of the models. 

Pagnanelli, R, L. Bomoroni, E. Moscardini, and L. Toro. 2006. Non-electrostatic surface complexation models for protons and leadfll) sorption onto single minerals and their mixture. Chemosphere 63, no. 7 1063-1073. doi 10.1016/j.chemosphere.2005.09.017. 

After precipitation, adsorption is the most important sink for U in natural systems. Uranium sorption in soils is primarily controlled by pH and carbonate levels (see Davis et al., 2002, this publication). At high pHs, where anionic uranyl-car-bonate complexes predominate, U is only weakly sorbed due to electrostatic repulsion by negatively charged mineral surfaces. When carbonate concentrations are low or absent uranyl-hydroxy surface complexes are observed to form. Minor irreversible sorption of U typically occurs when Fe and Mn oxides are present. Colloidal transport appears to be a less important transport mechanism for U relative to other actinides (see review of Jove Colon et al., 2001). 

The pronounced effects of aqueous chemistry on actinide sorption behavior suggest that sorption modeling should account for changing physicochemical conditions. A number of different modeling approaches of varying complexity can be used to incorporate the effects of chemistry on radionuclide sorption. A class of models that has been used with success in modeling pH-dependent sorption for actinides and other metals is the electrostatic surface complexation model (SCM). These models are equilibrium representations of sorption at the mineral-water interface and are discussed in detail elsewhere (Davis Kent, 1990 Dzombak Morel, 1990 Hayes etal., 1991 Seme etal., 1990 Turner, 1995), with only a brief overview presented here. 

In the absence of dyes, APA- and AdPA-grafted silica bind La(III) with, respectively, 0.20 and 0.27 mmol/g sorption capacity, resulting in formation of 1 2 (La L) complexes. 50% of introduced cation is bonded at pH=5 (APA), pH=6.1 (AdPA) and complete adsorption occurs at pH=6 (APA), pH=6.5 (AdPA). The grafted support in absence of La adsorbs the chosen dyes at pH<4 due to the electrostatic interaction with the -NH, groups on the surface, present as a result of grafting procedure. The adsorption of dyes at pH>4 is insignificant. 

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