Surface site concentration

Calculate the specific surface site concentration (moles of surface sites per gram) for quartz (Si02) particles 1 jum in diameter. Assume a site density, ris, of 5 sites/nm (1 nm = 10 m). What will be the concentration of silica surface sites if the suspended quartz is present at 2 ppm (mg/L) ... 

This method for computing the F-function is due to Harame (25) it has the virtue of being easy to generalize to more complex cases. Note that the substitution of Equation 12 into Equations 9 and 10 gives the exact solution of the initial system of equations for the surface site concentrations. Together with Equation 4 which can now be written... 

Adsorption to the surfaces of ettringite and monosulphate is an alternative immobilization mechanism that has been examined for SeO , ScO and of AsO (Myneni et al. 1997 Baur 2002 Baur Johnson 2002a). The studies estimated a maximum surface site concentration of approximately 0.03-0.1 mol/kg... 

Fig. 20. Schematic adsorption isotherms with a constant surface site concentration ([A]s in Fig. 12 is here constant), but with adsorption time as a variable. At very short times, adsorption is diffusion controlled. At short times, the protein has insufficient time to conformationally adjust to the interface, thus adsorption can be reversible and of the Langmuir type. At longer times, conformational adjustments begin leading to the commonly observed semi- orir-reversible behavior of protein adsorption. Other nomenclature same as Fig. 12...

Titrations. Values for surface site concentration listed in Table III are averages of titrations performed in triplicate. 

Congruent dissolution of Fe(III) (hydr)oxides would be expected to release arsenic until the surface site concentration is depleted below the level of adsorbed arsenic (in combination with other high-affinity anions, such as phosphate). Secondary mineralization of iron, however, convolutes this view, leading to the potential for arsenic to be incorporated on or in the newly forming solids. 

The value of i oo would, however, be lower as compared to i max due to the reduction in surface site concentration [S] caused by deactivation. 

Figure 1. Na on Quartz Comparison of experimental (29) and simulated adsorption and zeta-potential data. Modeling parameters for surface ionization and sodium adsorption are given in Table I. Surface site concentration Z SOH = 7.06 x 10 M.

However, it is interesting that in two references the variation of TOF with d has been used to estimate a further characteristic of the active ensemble of atoms. Topsoe et al. (345) have reasoned that since N2 adsorption seems to be rate-determining for the synthesis of NH3 on Fe/MgO, it would be logical to measure the surface site concentration by N2 chemisorption. When a new TOF is defined in this way, it is found to be insensitive to particle size. Recall that we have classified this system as showing antipathetic structure sensitivity, with FE measured as usual on H2 chemisorption. 

The values of surface site concentrations used were reported as ... 

The concentration of sites can be incorporated into the rate constant by rewriting the product of the surface site concentration and the surface rate constant simply as a rate constant ... 

We can do this because the surface site concentration is also a constant. Thus the overall rate for this catalytic reaction is ... 

The extent of arsenic sorption in natural waters will be influenced by many factors, relating to both the sorbent and the water composition. As(V) and As(III) have different affinities for various sorbent phases that may be present in sediment, soils, and aquifers. Thus the redox speciation of arsenic and the characteristics of available sorbents will strongly affect the extent of arsenic sorption as will the pH and concenPations of co-occurring inorganic and organic solutes in the aqueous phase. Since sorption is a surface phenomenon and is limited by the availability of surface sites on the sorbing phase(s), the extent of competition between arsenic and other sorbates will depend not only on the affinity of each sorbate for the surface but also on their concentrations relative to each other and to the surface site concentration. Elevated concenPations of phosphate have been used to desorb arsenic from clays (51) and from soils contaminated with arsenical pesticides (113). 

Particle-speciflc surface area It is necessary to know the area of the solid in contact with the liquid in order to estimate surface site concentrations (e.g., fi om specific surface site densities) or if speciation calculations are carried out on the basis of surface specific units, the experimental data pertaining to the surface must be transformed from molar concentrations using the specific surface area. For the constant capacitance model, the whole treahnent can be done on a mass-specific basis. In principle for this model, the specific surface area only has to be involved if surface-specific site densities can be evaluated. The specific surface area is usually measured by gas adsorption. For in situ methods, other probe molecules are used (e.g., EGME method). Furthermore, microscopic methods can be used to determine the shape and size of particles from particle size distributions, which can, for example, be obtained with setups for microelectrophoresis or with acoustophoretic methods, specific surface area can be calculated for either known or assumed particle geometries. Problems in... 

InFig. 9, aplotofEq. (114) results in predicted adsorption as a function of pH with total cation and surface-site mole fraction held constant. For the Gibbs plot in Fig. 10 of equation (115), adsorption is plotted as the areal surface concentration, Tg, as a function of increasing total cation mole fraction [or equilibrium solution mole fraction with Eq. (102)] at constant pH and total surface-site concentration. 

Equation (159) for divalent cation adsorption can be seen to be identical in form to the adsorption equation (100) for monovalent cation adsorption. Therefore, since formation of the surface complexes SOMOH and (SO)2 M seem to have a minimal effect on the calculated adsorption of divalent cations, the simple adsorption equation (159) should accurately predict both monovalent and divalent cation adsorption at the aqueous S—MO interface. Following die monovalent cation adsorption derivation, the adsorption equation for divalent cations in terms of total cation and surface site concentration, ch and cj, respectively, would be... 

No variable representing surface site concentrations) appears explicitly. 

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