Carbonates, surface complex formation

Stipp and Hochella (1991), on the basis X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED), have shown that CaC03 exposed to water, contains at the surface =C03H and =CaOH functional groups and van Capellen (1991) has proposed a surface complex formation model for carbonates. Similarly, Ronngren et al. (1991) have proposed =SH and =ZnOH functional groups for the surface of hydrous ZnS s). 

The phenomena of surface precipitation and isomorphic substitutions described above and in Chapters 3.5, 6.5 and 6.6 are hampered because equilibrium is seldom established. The initial surface reaction, e.g., the surface complex formation on the surface of an oxide or carbonate fulfills many criteria of a reversible equilibrium. If we form on the outer layer of the solid phase a coprecipitate (isomorphic substitutions) we may still ideally have a metastable equilibrium. The extent of incipient adsorption, e.g., of HPOjj on FeOOH(s) or of Cd2+ on caicite is certainly dependent on the surface charge of the sorbing solid, and thus on pH of the solution etc. even the kinetics of the reaction will be influenced by the surface charge but the final solid solution, if it were in equilibrium, would not depend on the surface charge and the solution variables which influence the adsorption process i.e., the extent of isomorphic substitution for the ideal solid solution is given by the equilibrium that describes the formation of the solid solution (and not by the rates by which these compositions are formed). Many surface phenomena that are encountered in laboratory studies and in field observations are characterized by partial, or metastable equilibrium or by non-equilibrium relations. Reversibility of the apparent equilibrium or congruence in dissolution or precipitation can often not be assumed. 

Speciation of Pb(II) in Glatt river. The concentrations given for CO2, Pb(II), Cu(II) and [Ca2+] as well as for the pollutants EDTA and NTA are representative of concentrations encountered in this river, The speciation is calculated from the surface complex formation constants determined with the particles of the river and the stability constants of the hydroxo-, carbonate-, NTA- and EDTA-complexes.The presence of [Ca2+] and [Cu2+] is considered. 

Surface complex formation models for carbonates and sulfides, respectively, have been proposed by van Capellen et al. (1993) and by Ronngren et al. (1991). 

J. P. Chen and M. S. Lin, Surface charge and metal ions adsorption on an H-type activated carbon experimental observation and modeling simulation by the surface complex formation approach, Carbon 39, 1491-1504 (2001). 

Surface Complex Formation on Carbonates. There are various possibilities for functional groups on the surface of carbonates, sulfides, phosphates, and similar compounds. By using a very simple approach similar to the one used for hydrous oxides (chemisorption of H20), one could postulate surface groups for carbonates (e.g., FeC03) as shown in List III. 

It is generally accepted that the perovskite-catalyzed combustion of soot with O2 occurs through a four-step mechanism involving (i) adsorption of O2 on the catalyst, (ii) delivery of activated oxygen species to the carbon surface, (iii) formation of surface oxygen complexes (SOCs) on the carbon surface, and (iv) desorption of SOCs [27], This general mechanism is valid not only for perovskites but also for some other soot combustion catalysts. 

The presence of high-molecular weight p-sulfur with chain structure seemed improbable since the sulfur was not extractable with boiling toluene. The p-sulfur is known to convert to the soluble ring structure (Sg) rather rapidly at 115°. Wibaut (119) thought the formation of a carbon-sulfur complex similar to the surface oxide formed with oxygen very likely. He was not able, however, to analyze definite surface groups. Hofmann and Nobbe (123) established that the sulfur content was dependent on the specific surface area. Enoksson and Wetterholm (124) confirmed by X-ray diffraction that no crystalline sulfur was present in exhaustively extracted charcoal with 13% sulfur content. 

A definite theoretical explanation of this behavior is not available. It is important to realize that the preference of a metal for 3C as opposed to 2C complexes or for 5C as opposed to 3C complexes may be either intrinsic or induced by adsorption of less reactive carbonaceous fragments and carbon (for simplicity, we shall refer to both of these as carbon ) on the metal (alloy) surface. Also, the choice of the reaction conditions (apparent contact time, poisoning or self-poisoning of the catalyst, etc.) influences the temperature range in which the catalysts can be tested, and since the selectivity in various complex formations is also temperature dependent, one must always analyze which aspects of the product distributions are intrinsic properties of a metal and which are induced by often unavoidable side reactions. 

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