Surface site density

Ns Surface site density (sites per unit area)... 

The expressions of Equations 9 and 10 which give the normalized surface site densities as a function of x and E are still valid when substituted into Equation 29 an equation is obtained which can be solved as before to give ... 

Surface site densities used in the computation of the oxide site concentrations presented in this paper were determined by either rapid tritium exchange or acquired from published values (18). Reported total site densities for hydrous metal oxides show relatively little variation generally they range by less than a factor of 3. Since [M], [SOM], [H] and x are known or can be determined from experimental data, uncertainties in estimates of the total site concentration are directly translated into uncertainties in the calculated partitioning coefficient. 

Christl, I. R. Kretzschmar (1999) Competitive sorption of copper and lead at the oxide-water interface Implications for surface site density. Geochim. Cosmochim. Acta 63 2929-2938... 

Using Eq. 11 -34 (Box 11.1) and the goethite surface site density, its pA i and pK values, and the ionic strength of the solution, find the intensity of surface charging of the goethite at the three pH values of interest (note that [Pg.446]

What is the physical significance of d> We have used V to denote the total surface site density, which is the number of sites N divided by the area a. With this definition it is usual to define... 

Equation 11.131 assumes that the total surface site density T is constant. Section 17.7.1 uses the equation above in the formulation of a transient stagnation flow problem, for example. 

The following input conditions and parameters will be assumed in this analysis total pressure is 1 atm, surface temperature is 700 K, total surface site density is T = 1.3 x 10-9 mol/cm2, density of the deposited material CdTe(bulk) is 5.85 g/cm3. 

The protein s intrinsic properties (size, molecular weight, 3-D structure, surface site density, conformational stability) are all very important and must be fully characterized and understood in order to interpret adsorption data. 

Additional information on adsorption mechanisms and models is in Stollenwerk (2003), 93-99 and Prasad (1994). Foster (2003) also discusses in considerable detail how As(III) and As(V) may adsorb and coordinate on the surfaces of various iron, aluminum, and manganese (oxy)(hydr)oxides. In adsorption studies, relevant laboratory parameters include arsenic and adsorbent concentrations, adsorbent chemistry and surface area, surface site densities, and the equilibrium constants of the relevant reactions (Stollenwerk, 2003), 95. Once laboratory data are available, MINTEQA2 (Allison, Brown and Novo-Gradac, 1991), PHREEQC (Parkhurst and Appelo, 1999), and other geochemical computer programs may be used to derive the adsorption models. 

The protein is now represented as, B, for bivalent. The units of Kai are in M-1, while Ka2 has units of dm2/moles (1/number density).85 The total surface site density [S]s can be expressed as ... 

Here, k(is the rate constant vk the stoichiometric coefficients of the kth component in the ith reaction of reagents and products, correspondingly A, the equilibrium constants the number of sites occupied by surface substance j of condensed phase n F," the surface site density in the th phase standard state and hk and sk the enthalpy and the entropy of the Arth substance. 

As discussed previously (Section 9.06.3.1.1), plots of pH sorption edges (see Figure 3) are useful in summarizing the sorption of radionuclide by substrates that have amphoteric sites (i.e., SOH, SO , SOHJ). The pH sorption edges of actinides are similar for different aluminosilicates (quartz, a-alumina, clinoptUolite, montmorillo-nite, and kaolinite). For example, Np(V) and U(VI) exhibit similar pH-dependent sorption edges that are independent of specific aluminosilicate identity (Bertetti et al, 1998 Pabalan et al., 1998). Under similar solution conditions, the amount of radionuclide adsorbed is primarily a function of the surface area. This observation has led several workers to propose that the amount of actinide sorption onto natural materials can be predicted from the surface site density and surface area rather the specific molecular structure of the surface (Davis and Kent, 1990 Turner and Pabalan, 1999). 

Methods for measurement of parameters used in SCM s have been described in the literature. Only a brief summary is presented here. Surface complexation model parameters that can be measured directly include, (1) the solid concentration, (2) surface site density, (3) surface area, and (4) equilibrium constants for the mass action equations describing all relevant adsorption reactions. The relation between surface charge and potential is calculated in geochemical equilibrium models. 

Reasonable estimates of surface site density are important for accurate modeling of solute adsorption (Davis and Kent, 1990). The most common methods for estimating surface site density include measurement of adsorption maxima of protons, cations, or anions at pH values where adsorption of the specific solute is greatest. Most measurements of surface site density for Fe, Al, and Mn oxides range from 1 to 7 pmoles/m (Davis and Kent, 1990). Dzombak and Morel (1990) reviewed the adsorption literature for ferrihydrite and selected a surface site density of 3.84 j,moles/m in constructing a coherent thermodynamic database for adsorption of cations and anions by ferrihydrite. 

Deriving equilibrium constants for surface complexation reactions based on experimental data can be facilitated with paramater optimization programs such as FITEQL (Westall, 1982). Ideally, surface site density is known however, it can also be adjusted simultaneously with LogK s to... 

TABLE 10.2 Some measured or estimated surface areas (S ) and maximum surface-site densities (Ns) (chiefly negative) for geological materials... 

What are the absolute and relative abundances of important sorbent solids and what fraction of their surface areas are exposed to flowing water Any adsorption model we select that assumes a finite number of sorption sites, requires, as input, the area of a sorbing phase exposed to a given volume of water I.e.g., Cs(g/L) x 5 (m /g)] and a surface site density [ (sites/m-)] for that phase. Can we measure or estimate these values Such measurements and estimates are extremely difficult for metal adsorption by modern stream sediments, which may be mix-... 

The sorptive capability of solids for charged species is proportional to their surface area per weight and the density of charged sites on their surfaces (their surface-site density or exchange capacity). Explain this statement as it applies to the sorptive capacity of natural hematite versus that of ferrihydrate. 

For calculations of sorption, MINTEQA2 requires that the concentration of sorption sites be available in units of mol/L. Explain how this information can be obtained from the surface-site density (W sites/m ), speeiftc surface area of the sorbing solid (S j, m /g), concentration of solid in contact with a liter of soil water (C g/] ), and Avogadro s number N ). 

A vermiculite clay has the structural formula K.Mgo.sKMgaoFe sFeJ sXAl, sSi65)02o(OH)4. Calculate its cation-exchange capacity in meq/IOO g and its surface-site density in mol/g. Assuming the surface area of the clay is 30 mVg, what is its surface-site density in nm and /umol/m ... 

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