Proton adsorption

The kinetics of proton adsorption by alumina was studied in [167,168]. The potential was studied as a function of exposure time (1-14 days) in [169]. 

The kinetics of OH and alkali metal ion uptake by A,-MnO2 was studied in [177]. 

The kinetics of proton adsorption by silica was studied in [168]. After addition of quartz to a solution [178], the fast stage (the first 4 minutes) was followed by a slower, linear decrease of pH with time. 

The kinetics of proton adsorption/desorption on anatase was studied using a pressure jump technique in [179], and rate constants were calculated. Reference [161] presents the kinetics of proton adsorption for TiO2 

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Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

Hiemstra et al. (1989) have elaborated on a multisite proton adsorption model taking into account the various types of surface groups intrinsic log K values for the protonation of various types of surface groups can be estimated with this model. 

Table 3.2 Thermodynamic data for proton adsorption T = 20° C All quantities in kJ mol1 (from Lyklema, 1987)...

Hiemstra, T W. H. van Riemsdijk, and H. G. Bolt (1989), "Muitisite Proton Adsorption Modeling at the Solid/Solution Interface of (Hydr)Oxides, I. Model Description and Intrinsic Reaction Constants", J. Colloid Interf. Sci. 133, 91-104. 

The surface proton adsorption which occurs after Step 2, however, complicates the determination of the heat content change resulting from anion adsorption. In order to make this correction, the heat associated with proton adsorption must be determined from the previous potentiometric-calorimetric titrations. Proton adsorption on goethite is exothermic, and Figure 1 provides an average value of -29.6 kj/mol near pH 4. This value, when multiplied by the moles of protons required to return to pH 4 after anion adsorption, allows correction for the heat associated with proton adsorption. This correction, however, is based on the assumption that the proposed two-step anion adsorption mechanism described above represents the only surface reactions which occur during anion adsorption. As such, the results obtained by this procedure are model dependent and are best used for comparative purposes. 

Figure 1. Proton adsorption-desorption heats ( 1 Standard Deviation or SD) as a function of pH for goethite at 10 g/L solid concentration in 0.05 M NaNO. All titrations were NaOH titrations starting from pH 4 followed by acid titrations back to pH 4.

This may be due to the higher acidity of the surface sites (see Kint) values in Table I) and hence higher activation energy of proton adsorption for the silica-alumina and Y-zirconium phosphate. 

T. Hiemstra, W. H. van Riemsdijk, and G. H. Bolt, Multisite proton adsorption modeling at the solidZsolution interface of (hydr)oxides A new approach 1. Model description and evaluation of intrinsic reaction constants, J. Colloid Interface Sci. 133(1), 99-104 (1989). 

The process in which a hydrated ion is dehydrated to form an adsorbed ion on an electrode interface, and its reverse process, are ion transfer processes across the compact layer on the electrode interface. As an example, we consider proton adsorption (cathodic proton transfer) and proton desorption (anodic proton transfer) at metallic electrodes represented by Eqn. 9-59 ... 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

Tab. 10.3 Enthalpies of adsorption and kinetics of proton adsorption and desorption on iron oxides.

In addition to proton adsorption, interactions between the ions of the inert electrolyte (counter ions, section 10.3) and the oxide surface lead to ion pair formation which influences the electrochemical properties of the oxides and the determination of pKa values. Ion pair formation involves outer sphere surface complexes (see Chap. 11), e.g. 

The equilibrium constants for ion pair formation are formulated in the same way as those for proton adsorption some values for the iron oxides are listed in Table 10.4. 

Fig. 12.3 The three subsequent reaction steps of the dissolution of an Fe " oxide by an organic ligand ligand adsorption, iron detachment and proton adsorption (site restoration) (Stumm, Furrer, 1987, with permission).

In view of its importance, reductive dissolution of Fe oxides has been widely studied. Reductants investigated include dithionite, thioglycolic acid, thiocyanate, hydrazine, ascorbic acid, hydroquinone, H2S, H2, Fe ", tris (picolinato) V", fulvic acid, fructose, sucrose and biomass/bacteria (Tab. 12.3). Under the appropriate conditions, reductive dissolution may also be effected photochemically. As with protonation, the extent of reduction may be strongly influenced by ligand and proton adsorption on the oxide surface. 

Astumian, R.D. Sasaki, M. Yasunga,T. Schelly, Z.A. (1981) Proton adsorption-desorption kinetics on iron oxides in aqueous suspensions, using the pressure jump method. J. Phys. Chem. 85 3832—3835 Atkins, P.W. (1990) Physical chemistry. 4 Ed. 

Onari, S. Arai,T. Kudo, K. (1977) Infrared lattice vibrations and dielectronic dispersion in a- Fe203. Phys. Rev. B16 1717 Onoda, G.Y. de Bruyn, P.L. (1966) Proton adsorption ot the ferric oxide/aqueous solution interface. I. A kinetic study of adsorption. Surface Sd. 4 48—63... 

T. Akrmoto, S. (1984) Mossbauer study on the high pressure phase ofiron(III) oxide. Solid State Commun. 50 97—100 Szczypa, J. Matysiak, J. Kosmilski, M. (1994) Standard enthalpies of proton adsorption on hematite in various solvent systems. Abstracts of 8 Int. Conf. Colloid Surf. Chem. Adelaide... 

Ashida, M., Sasaki, M., Kan, H., Yasunaga, T., Hachlya, K, and Inoue, T. (1978). Kinetics of proton adsorption-desorption at Ti02-H20 interface by means of pressure jump technique. J. Colloid Interface Sci. 678, 219-225. 

Proton-promoted dissolution reactions are exemplified for carbonates, silicates, and metal oxyhydroxides by Eqs. 3.15, 3.18-3.20, 3.25, 3.39, 3.46, 3.53, 3.56, and 3.59c. The typical response of the rate of dissolution to varying pH is illustrated in Fig. 3.2, and this response is often hypothesized to be a result of the proton adsorption-bond-weakening structural detachment sequence described in connection with Eq. 3.60.36 This sequence can be represented by the following generic reaction scheme ... 

Experimental data on the pH dependence of proton adsorption by metal oxyhydroxides can often be represented mathematically by the logarithmic relationship ... 

This subsequence is useful to consider if the time scale for proton adsorption-desorption reactions is comparable to or longer than that for outer-sphere surface complexation. It is a special case of the abstract scenario listed third in Table 4.3. Under the conditions given there, the protonation-proton dissociation reaction (A = SOH, B = H C = SOH2) is assumed to be much faster than outer-sphere surface complexation-dissociation, such that (kf ka, kb kd, k f kf, k b kb here)... 

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