Anionic surface species, sorption

Suarez et al. (36) use a combination of FTIR spectroscopy, electrophoretic mobility and pH titration data to deduce the specific nature of anionic surface species sorbed to aluminum and silicon oxide minerals. Phosphate, carbonate, borate, selenate, selenite and molybdate data are reviewed and new data on arsenate and arsenite sorption are presented. In all cases the surface species formed are inner-sphere complexes, both monodentate and bidentate. Two step kinetics is typical with monodentate species forming during the initial, rapid sorption step. Subsequent slow sorption is presumed due to the formation of a bidentate surface complex, or in some cases to diffusion controlled sorption to internal sites on poorly crystalline solids. 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

The TLM (Davis and Leckie, 1978) is the most complex model described in Figure 4. It is an example of an SCM. These models describe sorption within a framework similar to that used to describe reactions between metals and ligands in solutions (Kentef fll., 1988 Davis and Kent, 1990 Stumm, 1992). Reactions involving surface sites and solution species are postulated based on experimental data and theoretical principles. Mass balance, charge balance, and mass action laws are used to predict sorption as a function of solution chemistry. Different SCMs incorporate different assumptions about the nature of the solid - solution interface. These include the number of distinct surface planes where cations and anions can attach (double layer versus triple layer) and the relations between surface charge, electrical capacitance, and activity coefficients of surface species. 

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

Incorporation in cement minerals will lead to a similar relationship, which may be described by a distribution ratio with the exception that uptake may be much greater than that of surface sorption ( ). Such mechanisms may apply to C-S-H, AFt, and AFm phases. The mechanism of incorporation may be by isomorphic substitution of a particular species within a crystal lattice. A good example here is the exchange of SO in ettringite for another anion. Another possibility is the adsorption to sites within a crystal structure, as may occur at silicate sites within C-S-H. 

Interferences with arsenic adsorption and ion exchange Dissolved organics and anions may interfere with arsenic adsorption and ion exchange in both natural environments and water treatment systems. In some cases, chemical species directly compete with arsenic for adsorption sites. They may also desorb and replace arsenic. Vanadium is one element that could interfere with the adsorption of arsenic onto mineral surfaces. In most cases, vanadium is not abundant in water. However, alkaline (pH 7.0-8.8) groundwaters in the loess aquifers of La Pampa, Argentina contain up to 12mgL 1 of vanadium (Smedley et al., 2005). The vanadium readily hinders the sorption of As(V) onto iron (III) (oxy)(hydr)oxides (Chapter 3). 

For simplicity, in this equation, we have assumed that activities are equal to concentrations and brackets refer to activities. C is a units conversion constant = Vy m relating void volume Vy (mL) in the porous media and the mass m (g) of the aquifer material in contact with the volume Vy, is the formation constant for an aqueous uranyl complex, and the superscripts i, j, k describe the stoichiometry of the complex. The form that the sorption binding constant takes is different for the different sorption models shown in Figure 4 (e.g., see Equation (5)). Leckie (1994) derives similar expressions for more complex systems in which anionic and cationic metal species form poly dentate surface complexes. Equation (7) can be derived from the following relationships for this system ... 

Sorption to mineral surfaces (as opposed to NOM) is generally viewed as more of a displacement than a dissolution phenomenon. Because mineral surfaces tend to be more polar than NOM, sorption to the former is more substantial for polar and ionic compounds than for those that are more hydrophobic (Curtis et al., 1986 Chiou, 1998). Furthermore, since most NOM and mineral surfaces exhibit either a neutral or negative charge, sorption to soils and sediments is considerably stronger for pesticide compounds that are positively charged in solution—such as paraquat or diquat—than for neutral species, and weaker still for anions. As a consequence, measured values in soils exhibit little dependence upon pH for pesticide compounds that are not Brpnsted acids or bases (Macalady and Wolfe, 1985 Haderlein and Schwarzenbach, 1993). 

There is clear evidence that the dissolution of oxide minerals is promoted by the specific sorption of solutes at the mineral-solution interface. Moreover, it has been found that comparatively simple rate laws are obtained if the observed rates are plotted against the concentrations of adsorbed species and surface complexes (Pulfer et al., 1984 Furrer and Stumm, 1986). For example, in the presence of ligands (anions and weak acids) surface chelates are formed that are strong enough to weaken metal-oxygen bonds and thus to promote rates of dissolution proportional to their surface concentrations. Simple rate laws have been also observed with H+—or OH —promoted dissolution of oxides in a manner that can be predicted from knowledge of the oxide composition and the surface concentrations of protons and hydroxyl radicals. 

One mechanism that is consistent with the observed properties of the particles in these suspensions involves the dissolution of amorphous Si02 and adsorption of soluble silicate on the Fe(OH)3 surface. This process could occur in parallel with the heterocoagulation mentioned earlier. Soluble silicate species might then compete with Se03 or PO4 for surface sites as suggested by Goldberg (8) for the P04/silicate/goethite system. Sorption of silicate species onto Fe(OH)3 need not affect cationic adsorbates. Benjamin and Bloom (10) demonstrated that adsorption of cations is often minimally affected by anion adsorption even under conditions where anion-anion competition is severe (11). 

Such Fe(in) and Al speciation has important implications for the geochemical evolution of AMD. At typical conditions of pH (1.5 ) and sulfate concentrations (0.01-0.1 M SO "), the mineralogy of the precipitates will be dominated by sulfates and hydroxysulfates, instead of oxides or hydroxides, as described in the next section. Further, this metal speciation can imply important differences in their sorption behavior. For example, at sulfate activities of 10 " to 10 , aluminum is present as free aqueous cation, being essentially conservative. However, at higher activities of the SO " anion (from 10 to >10 ), Al forms bisulfate anionic species (A1(S04)2) which can be sorbed onto positively charged mineral surfaces at low pH [14]. 

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