Surface speciation

Greater adsorption of trace metals is found at higher pH and C02(g) concentrations. Sites available for Zn2+ sorption are less than 10% of the Ca2+ sites on the calcite surface, and Zn adsorption is independent of surface charge. This indicates a surface complex with a covalent character (Zachara et al., 1991). Furthermore, the surface complex remains hydrated and labile because Zn2+ is rapidly exchangeable with Ca2+, Zn2+ and ZnOH. At the dolomite-solution interface, the carbonate(C03)-metal (Ca/Mg) complex dominates surface speciation at pH > 8, but at pH 4-8, hydroxide (OH) -metal (Ca/Mg) dominates surface speciation (Pokrovsky et al., 1999). Calcite has an observed selectivity sequence Cd > Zn > Mn > Co > Ni > Ba = Sr, but their sorption reversibility is correlated with the hydration energies of the metal sorbates. Cadmium and Mn dehydrate soon after adsorption to calcite and form a precipitate, while Zn, Co and Ni form surface complexes, remaining hydrated until the ions are incorporated into the structure by recystallization (Zachara et al., 1991). 

Pokrovsky O., Schott J., Thomas F. Dolomite surface speciation and reactivity in aqutic systems. Geochim Cosmochim Acta 1999 63 3133-3143. 

Arai Y, Sparks DL (2002) Residence time effects on arsenate surface speciation at the aluminum oxide-water interface. Soil Sci 167 303-314... 

Sjoberg, E.L. Rickard, D.T. 1984. Calcite dissolution kinetics surface speciation and the origin of the variable pH dependence, Chemical Geology, 42, 119-136. 

In analyzing the kinetics of surface reactions, it will be illustrated that many of these processes are rate-controlled at the surface (and not by transport). Thus, the surface structure (the surface speciation and its microtopography) determine the kinetics. Heterogeneous kinetics is often not more difficult than the kinetics in homogeneous systems as will be shown, rate laws should be written in terms of concentrations of surface species. 

Stumm, W., and E. Wieland (1990), "Dissolution of Oxide and Silicate Minerals Rates Depend on Surface Speciation", in W. Stumm, Ed., Aquatic Chemical Kinetics, John Wiley and Sons, New York, 367-400. 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Surface speciation can be expected to have a tremendous impact on rates of precursor complex formation. Rj, the rate of precursor complex formation, may depend upon the extent of surface protonation, since ligand exchange rates of >MeOH2, >MeOH, and >MeO may vary substantially ... 

Simple ligands can adsorb on iron oxides to form a variety of surface species including mononuclear monodentate, mononuclear bidentate and binuclear mono or bi-dentate complexes (Fig. 11.2) these complexes may also be protonated. How adsorbed ligands (and cations) are coordinated to the oxide surface can be deduced from adsorption data, particularly from the area/adsorbed species and from coadsorption of protons. Spectroscopic techniques such as FTIR and EXAFS can provide further (often direct) information about the nature of the surfaces species and their mode of coordination. In another approach, the surface species which permit satisfactory modelling of the adsorption data are often assumed to predominate. As, however, the species chosen can depend upon the model being used, this method cannot provide an unequivocal indication of surface speciation confirmation by an experimental (preferably spectroscopic) technique is necessary. 

Gabriel, U., Charlet, L., Schlapfer, C. W., Vial, J. C., Brachmann, A. Geipel, G. 2001. Uranyl surface speciation on silica particles studied by time-resolved laser-induced fluorescence spectroscopy. Journal of Colloid and Interface Science, 239, 358-368. 

Huertas, F. J., Chou, L. Wollast, R. 1998. Mechanism of kaolinite dissolution at room temperature and pressure Part I. Surface speciation. Geochimica et Cosmochimica Acta, 62, 417-731. 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

A deeper perception of the mechanistic implications of equation (9.2) can be had if the rational activity coefficients are described on the molecular level using the methods of statistical mechanics. This approach is the analogue of the statistical mechanical theory of activity coefficients for species in aqueous solution (Sposito, 1983). Fundamental to it is the prescription of surface speciation and the dependence of the rational activity coefficient on surface characteristics. Three representative molecular models of adsorption following this paradigm are summarised in Table 9.8. Each has been applied with success to describe the surface reactions of soil colloids (Goldberg, 1992). 

This is not to say that no progress has been made in surface speciation kinetics related to soils (Sparks and Suarez, 1991). Adaptation of elementary reaction mechanisms established for aqueous complexes, for example, has proved to be an effective means of classifying kinetic surface species and establishing rate laws for... 

W. Stumm and E. Wieland, Dissolution of oxide and silicate minerals Rates depend on surface speciation, Chap. 13 in Aquatic Chemical Kinetics, ed. by W. Stumm, Wiley, New York, 1990. 

W. Stumm and R. Wollast. Coordination chemistry of weathering Kinetics of the surface-controlled dissolution of oxide minerals, Rev. Geophys. 28 53 (1990). See also A. E. Blum and A. C. Lasaga, The role of surface speciation in the dissolution of albite, Geochim. Cosmochim. Acta 55 2193 (1990). 

It is apparent that in order to use our rate model, a thermodynamic evaluation of the bulk fluid and surface speciation is... 

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