Surface complexation models proton uptake

A significant problem in surface complexation models is the definition of adsorption sites, The total number of proton-exchangeable sites can be determined by rapid tritium exchange with the oxide surface (25). Although surface equilibria are usually written in terms of one surface site, e.g. Equations 5, 6, 8, 9, adsorption isotherms for many ions show that the number of molecules adsorbed at maximum surface coverage (fmax) is less than the total number of surface sites. For example, uptake of Se(VI) and Cr(VI) ions on Fe(0H)3(am) at T ax 1/3 and 1/4 the total... 

More recently, Brennsteiner et al. [ 175] noted that the electrochemical removal efficiency for nickel is dependent on the pH of the contaminant solution. Maximum efficiency was achieved at pH = 7.0, but only when the carbon electrode was preplated with a layer of copper the role of surface chemistry was not investigated. Seco et al. [172] did characterize the surface chemistry of a commercial activated carbon (pHp r = 6.1) and studied its uptake of heavy metals (Ni, Cu, Cd, Zn), as well as of some binary systems. They interpreted the monotonic uptake increase with pH to be consistent with the surface complexation model a decrease in competition between proton and metal species for the surface sites and a decrease in positive surface charge, which results in a lower cou-lombic repulsion of the sorbing metal. In the binary uptake studies, they concluded that Ni (as well as Cd and Zn) is not as strongly attracted to the. sorbent as Cu. 

The above observations regarding the ionic strength effects (Figs 5.105-5.116) do not represent any general trends, namely, they are only valid for certain set of TLM parameters. Figures 5.112 and 5.117-5.119 show the ionic strength effect on the uptake curves calculated for the same electrostatic position of Pb (CD model,/ 0.5), and the same number (two in the present example) of protons released per one adsorbed Pb, but using different sets of TLM parameters from Table 5.18. The stability constants of the alumina-Pb surface complex are presented in Table 5.22. 

Figure 5.132 presents the ionic strength effect on the model uptake curves calculated for one proton released per one adsorbed Pb, using the diffuse layer model Kosmulski, for model parameters cf. Table 5.13). The model curves are significantly steeper, and the ionic strength effect is less significant than in the analogous Pb adsorption model (inner sphere, one proton released) combined with TLM (Fig. 5.126). The calculated stability constant of the surface complex is higher by three orders of magnitude for the diffuse layer model (Table 5.28) than for TLM (Table 5.27).

Also for other diffuse layer models (cf. Table 5.13) the calculated stability constant of silica-Pb surface complex is considerably higher than for their TLM counterparts obtained from the same experimental data. The difference between the highest and the lowest K in Table 5.28 by almost two orders of magnitude is more significant than the discrepancies between the stability constants calculated for different diffuse layer models obtained for alumina (cf. Table 5.22). In spite of different K, the course of the calculated uptake curves obtained for different diffuse layer models and one proton released per one adsorbed Pb with... 

The discussed above examples are limited to adsorption of divalent metal cation. They indicate that the numerical value of the stability constant of the surface complex depends on the assumed model of primary surface charging. In this respect the significance of comparison of the stability constants of analogous surface complexes from different sources is questionable, when these stability constants were calculated using different models of primary surface charging. On the other hand the choice of the model of primary surface charging has rather limited effect on the shape of the calculated uptake curves. The shape of calculated uptake curves (slope, ionic strength effect) and the numerical value of the stability constant of the surface complex are both affected by the model of specific adsorption (electrostatic position of the specifically adsorbed cation and the number of protons released per one adsorbed cation). 

Model M9 Mono- and multinuclear complexes and precipitation. The best model yet achieved comprises reactions 203, 204, 221, 902, and precipitation. Simulations of uptake data and Nco are shown in Figures 5A,B, 5C, and 6. Model M9 fits both uptake and Nco data well. The fitting exercise reinforces our inferences and the conclusion offered by Hayes and Katz (10), namely that uptake data alone are inadequate to derive a robust sorption model. Spectroscopic information is needed to support selection of reactions. Future models must account for the mixed-cation hydroxide precipitates observed by Scheidegger et al. (36), Towle et al. (22), and Thompson et al (35), but we found it unnecessary to invoke a co-precipitate or variable activity precipitate. Obviously, M9 is not a unique solution, A continuum of multimeric species seems more likely than a single species. More data are needed to test for protonation/deprotonation of all proposed surface complexes and precipitates, and sorption by the precipitates themselves. 

The above conclusions are also valid for specific adsorption of cations whose valence is different from two, and for specific adsorption of anions. Figure 5.133 shows the calculated uptake curves of trivalent Gd on alumina. Identical model as in Fig. 5.120 was used 1-pK-diftuse layer model (Sprycha) for primary surface charging, and one proton released per one adsorbed Gd, except the logarithm of the stability constant of Gd-alumina complex equals 3.76 (from the condition pHso S at 10 g alumina/dm and lO " mol dm inert electrolyte). The uptake curves in Fig. 5.133 are steeper than corresponding curves in Fig. 5.120. This result is not surprising in view of higher valence of Gd (cf. Eq. (5.1)), and indeed, the experimental uptake curves are usually steeper for trivalent than for divalent cations. 

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