Specifically sorbed species

For the case at hand, it is easy to see that the amount of = MOH2 and sMO" species on the solid surface control the surface s charge. The concentration of this charge, difference between positive and negative site concentrations ... 

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 ... 

The distribution coefficient approach - commonly referred to as the K approach - is the most widely applied method in environmental geochemistry for predicting the sorption of contaminant species onto sediments. The distribution coefficient Kd itself is simply the ratio under specific conditions of the sorbed to the dissolved mass of a contaminant. Sorbed and dissolved mass are expressed in units such as... 

The description of the partial pressure exerted by a sorbate, or a mixture of sorbates, when they reside on the sorbent surface, at some given temperature is what we speak of as adsorption equihbrium. For a single adsorbate (adsorbing molecular species) we require three state variables to completely describe the equilibrium the temperature, the sorbed phase concentration or loading and the partial pressure exerted by the sorbed phase are very convenient variables to use. As more adsorbable compounds are added to the problem we require additional information to adequately describe the problem. That information is the specification of the mole fractions of the adsorbable compounds in both the gas and sorbed states. 

Ion exchange shares many characteristics with adsorption, such as mass transfer from the fluid to the solid phase there are, however, some significant differences. Specifically, although both processes can be characterized as sorption processes, the sorbed species are ions in ion exchange, whereas electrically neutral substances are sorbed hi adsorption. Moreover, in ion exchange, the ions removed from the liquid phase are replaced by ions from the solid phase. So, there actually occurs an exchange of ions and not only a removal... 

The sorption of a penetrant by a class (a) membrane can be treated most simply, if the processes of sorption in the polymer matrix and into each kind of specific sorption site within it can be treated as mutually independent and hence additive. Under these conditions, each sorbed species may similarly be expected to contribute additively to the total diffusion flux. Hence, if there are N — 1 kinds of specific sorption sites, the overall sorption and permeability or diffusion coefficients can be written as... 

Here, S and P, or DXj characterize the ith sorbed penetrant species (i = 1 refers to the polymer matrix and i = 2, 3,. .., N identify each kind of specific sorption site). 

It must not, however, be forgotten that conventional techniques (e.g., 13C Fourier transform NMR) can be applied to certain solids of catalytic significance, such as sheet silicates since in many of these systems rapid motion of intercalated or otherwise sorbed organic species secures sharp absorption lines which provide much information about the individual atomic environments. Organic species attached to high surface area solids (such as zeolites, silica, alumina, magnesia, as well as other oxides and their mixtures) are specific examples (6). 

While surface charge is by no means the only factor responsible for electrolyte adsorption (94), particularly organic electrolytes (9, 27), the extent of adsorption of the less specifically adsorbed species, such as the simple aquo ions and, for example, primary amines and alkyl sulfonates, decreases rapidly when the sign of the oxides surface charge is changed to that of the sorbing species (6, 10). 

The multireaction approach, often referred to as the multisite model, acknowledges that the soil solid phase is made up of different constituents (clay minerals, organic matter, iron, and aluminum oxides). Moreover, a heavy metal species is likely to react with various constituents (sites) via different mechanisms (Amacher et al 1988). As reported by Hinz et al. (1994), heavy metals are assumed to react at different rates with different sites on matrix surfaces. Therefore, a multireaction kinetic approach is used to describe heavy metal retention kinetics in soils. The multireaction model used here considers several interactions of one reactive solute species with soil matrix surfaces. Specifically, the model assumes that a fraction of the total sites reacts rapidly or instantaneously with solute in the soil solution, whereas the remaining fraction reacts more slowly with the solute. As shown in Figure 12.1, the model includes reversible as well as irreversible retention reactions that occur concurrently and consecutively. We assumed that a heavy metal species is present in the soil solution phase, C (mg/L), and in several phases representing metal species retained by the soil matrix designated as Se, S, S2, Ss, and Sirr (mg/kg of soil). We further considered that the sorbed phases Se, S, and S2 are in direct contact with the solution phase (C) and are governed by concurrent reactions. Specifically, C is assumed to react rapidly and reversibly with the equilibrium phase (Se) such that... 

The term molecular sieve describes a material having pores that closely match the dimensions of a specific molecule. The best-known molecular sieves are composites of microcrystalline zeolites embedded in an inert clay binder. Zeolites are composed of regular clusters of tetrahedral aluminosilicates, with varying percentages of bound cations and water molecules, whose crystal structures incorporate small molecule-sized cavities. Because zeolite pore size is different for each of the numerous different crystal structures in this family, the size-selective nature can be tailored for specific applicatimis. Studies of the transport of liquid and gaseous organic species in molecular sieves indicate that the diffusion rate and equilibrium concentration of sorbed analyte are sensitive functions of their molecular dimensions, as well as zeolite pore size and shsqre [110]. 

From the previous sections it is evident that radionuclides of high specific activity often represent very small amounts (microamounts, non-weighable amounts < 1 pg) of matter, especially if the half-lives are short. Handling of such microamounts requires special precautions, because in the absence of measurable amounts of carriers the radionuclides are microcomponents and their chemical behaviom may be quite different from that observed for macrocomponents. This aspect is of special importance if the system contains liquid/solid, gas/solid or liquid/liquid interfaces. The percentage of radionuclides sorbed on the walls of a container depends on the chemical form (species) of the radionuclide, its concentration and specific activity, and on the properties of the container material. At high specific activity of a radionuclide in solution, the surface of a glass beaker generally offers an excess of surface sorption sites. 

Metal ion oxides are often important in minimizing the solubility of arsenic in the environment in general and more specifically for localizing the impact of arsenic contamination near contaminated sites, especially old mines (La Force et al., 2000 Roussel et al., 2000b Webster eta/., 1994). Organic arsenic species tend to be less strongly sorbed by minerals than inorganic species. 

Aqueous radionuclide species and other solutes can sorb to mineral surfaces by forming chemical bonds directly with the amphoteric sites or may be separated from the surface by a layer of water molecules and be bound through longer-range electrostatic interactions. In the TLM, complexes of the former type are often called inner-sphere complexes those of the latter type are called outer-sphere complexes (Davis and Kent, 1990). The TLM includes an inner plane (o-plane), an outer plane (/8-plane), and a diffuse layer that extends from the /8-plane to the bulk solution. Sorption via formation of inner-sphere complexes is often referred to chemisorption or specific... 

Concentrations of the major solution and surface species corresponding the calculated isotherms in Figure 7 are shown on Figures 8a (no surface. omplexation) and 8b (surface complexation of Na+ and Ca2+). Without sur- iC-e complexation (Figure 8a), Na+ dominates sorption in the diffuse layer at. gh TOTNa concentrations, although Ca2+ quickly becomes dominant as TOTNa decreases. With specific sorption, the surface complex of Na+ and its Tuse-layer species constitute roughly similar fractions of the total sorbed... 

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