Metal surface complexation constant

In the diffuse layer model, all intrinsic metal surface complexation constants were optimized with the FITEQL program for both the strong and weak sites using the best estimates of the protonation constant, log X +(int) = 7.29, and the dissociation constant log K-(int) = —8.93 obtained with Eq. (6.61) (Dzombak and Morel, 1990). Thus, individual values of log A . (int) and log A . (int) and best estimates of log (int) and log A j (int) are unique in that they represent a self-consistent thermodynamic database for metal adsorption on hydrous ferric oxide. 

According to the Langmuir model (Eq.2) the adsorption capacity qm for Cd is 2.5 times grater than for Zn and adsorption capacity qm for Pb is 2 times grater than Zn when granular activated carbon is used. When natural zeolite is used as adsorbent, the adsorption capacity qm for Zn is 5 times lower than Cd and Pb. So, qm varied in the order Cd (II)> Pb(II) >Zn(II) for GAC, and Pb(II) = Cd(II)>Zn(II) for the natural zeolite as adsorbent. Ricordel et al (2001) and Tsoi and Zhao (2004) reported a similar relationship when different adsorbents were used. This can be explained on the basis of their ionic radii, hydration energy, ionic mobility and diffusion coefficient. The explanations of different authors were given on the basis of the surface covered by the adsorbed metal ions or on the basis of metal surface complexation constants and thermodynamic parameters values. 

The second and third method allow the measurement of surface complexation constants at various transition metal loadings and consequently yield apparent composition dependent constants. In the first method on the contrary a truly thermodynamic constant is obtained under standard state conditions. 

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are... 

The results of a newly proposed model for adsorption at the oxide/water interface are discussed. The modeling approach is similar to other surface complexation schemes, but mass-law equations are corrected for the effect of the electrostatic field. In this respect, this model bridges the gap between those models that emphasize physical interactions. The general applicability of the model is demonstrated with comparisons of calculations and experimental data for adsorption of metal ions, anions, and metal-ligand complexes. Intrinsic ionization and surface complexation constants can be determined with an improved double extrapolation technique. 

Surface complexation is a typical multi-component reaction, similar to cation exchange. The database for surface complexation includes complexation constants for major elements in groundwater such as and S04 , but not for and HCOs". In the first instance, constants for these ions can be estimated with linear free energy relations (LFER s) in which the properties of similar chemical systems are compared and interpolated (Dzombak and Morel, 1990). Thus, the surface complexation constant for is expected to lie in between the ones for and for Zn, in line with the known differences of the association constants of these heavy metals with OH in water. For the weak sites, the LFER gives ... 

Figure 10,26 Correlation plot for some metal cations, of their first hydrolysis constants ( /fii) versus intrinsic surface complex constants i Ku) for their adsorption by Si02(am) assuming the constant capacitance model. The equation of the solid line is log = 0.09 -( 0.62 log A. Hydrolysis and adsorption reactions are written A,i -t- H2O = +...

Smith, R. W., and E. A. Jenne. 1991. Recalculation, evaluation, and prediction of surface complexation constants for metal adsorption on iron and manganese oxides. Envir. Sci. Technol. 25 525-31. 

The intrinsic equilibrium constants for the diffuse layer model are similar to those for the constant capacitance model where P is replaced by Equations (6.10) and (6.11) describe surface protonation and dissociation, respectively. Metal surface complexation is described by two constants similar to tliat defined in Eq. (6.12) for strong and weak sites ... 

Another standardized database for the diffuse layer model was developed for montmorillonite by Bradbury and Baeyens (2005). Surface complexation constants for strong and weak sites and cation exchange were fit to adsorption data for various metals using constant site densities and protonation-dissociation constants in a nonelectrostatic modeling approach. Linear free energy relationships were developed to predict surface complexation constants for additional metals from their aqueous hydrolysis constants. 

Table 24.4 Surface complexation constants of metal ion adsorption onto two activated carbon cloths, calculated using the DLM model [33]. na not available...

Reaction 10.54 is the ligand exchange of metal-coordinated water by the conjugate base, R , and its tendency to proceed is determined by the basicity of the organic ligand relative to that of water. This reaction can be described by a surface complexation constant, K. ... 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

The data of Loukidou et al. (2004) for the equilibrium biosorption of chromium (VI) by Aeromonas caviae particles were well described by the Langmuir and Freundlich isotherms. Sorption rates estimated from pseudo second-order kinetics were in satisfactory agreement with experimental data. The results of XAFS study on the sorption of Cd by B. subtilis were generally in accord with existing surface complexation models (Boyanov et al. 2003). Intrinsic metal sorption constants were obtained by correcting the apparent sorption constants by the Boltzmann factor. A 1 2 metal-ligand stoichiometry provides the best fit to the experimental data with log K values of 6.0 0.2 for Sr(II) and 6.2 0.2 for Ba(II). 

Borrok et al. (2004a) used potentiometric titration to measure Cd sorption by different bacterial consortia, and a surface complexation approach to determine thermodynamic stability constants. When the data were modeled by adopting a single set of stability constants, a similar sorption behavior was shown by a wide range of bacterial species. Further, current models that rely on pure strains of laboratory-cultivated bacterial species appear to overestimate the extent of metal biosorption in natural systems. 

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

Stability constants (ethylendiamine, glycinate, oxalate), surface complex formation constants and solubility products (sulfides) of transition ions. The surface complex formation constant is for the binding of metal ions to hydrous ferric oxide =Fe-OH + Me2+ =FeOMe++ H+ K. ... 

We resume the problem discussed in Example 2.2 and solve the same problem, but now we correct for electrostatic effects. Sumarizing the problem Calculate the pH dependence of the binding of a) a metal ion Me2+, and b) of a ligand A to a hydrous oxide, SOH, and compare the effect of a charged surface at an ionic strength I = 0.1. A specific surface area of 10 g m 2 10 4 mol surface sites per gram ( 6 sites nnrr2) concentration used 1 g e-1 (10 4 mol surface sites per liter solution). As before (Example 2.2) the surface complex formation constants are log Kj = -1 and log K = 5, respectively. 

We have argued that (inner-sphere) surface complex formation of a metal ion to the oxygen donor atoms of the functional groups of a hydrous oxide is in principle similar to complex formation in homogeneous solution, and we have used the same type of equilibrium constants. How far can we apply similar concepts in kinetics ... 

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