Coprecipitation Reactions and Solid Solutions

Coprecipitation Reactions and Solid Solutions of Carbonate Minerals... 

Kinetics of Carbonate Coprecipitation Reactions and Solid Solution Formation The uptake of a cation into a carbonate has been studied by Davis et al. (1987), by Wersin et al. (1989), and by Stipp and Hochella (1991), Morse and Mackenzie (1990) have reviewed extensively the geochemistiy of dolomites and magnesian calcites. 

X-ray diffraction and microscopic studies revealed that calcined stoichiometric mixtures of coprecipitated hydroxides do indeed form spinels and solid solutions. With some mixtures, complete reaction was not always easily attained. For example, in the CuO Fe203 system, excess copper oxide and Fe203 peaks were found in x-ray diffraction patterns in addition to the major spinel phase. Calcined mixtures usually produced pure spinel compounds. 

Another method of segregating and removing a portion of the dissolved organic matter includes incorporation into a solid phase, which can then be removed by filtration or centrifugation. This incorporation can result either from coprecipitation with a solid phase formed in a reaction in the solution or, as discussed in the next section, from adsorption of the organic material onto a pre-existing solid phase. 

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. 

Equilibrium between simple salts and aqueous solutions is often relatively easily demonstrated in the laboratory when the composition of the solid is invariant, such as occurs in the KCI-H2O system. However, when an additional component which coprecipitates is added to the system, the solid composition is no longer invariant. Very long times may be required to reach equilibrium when the reaction path requires shifts in the composition of both the solution and solid. Equilibrium is not established until the solid composition is homogeneous and the chemical potentials of all components between solid and aqueous phases are equivalent. As a result, equilibrium is rarely demonstrated with a solid solution series. 

Another, and on the face of it, rather different example, is the coprecipitation of solid solution compounds, such as CulnSi and CulnSei—semiconductors of particular interest due mainly to their applicability for photovoltaic cells. It was shown, by X-ray diffraction, that the precipitate resulting from reaction between H2S and an aqueous solution containing both Cu" and In " ions was, at least in part (depending on the concentrations of the cations), single-phase CulnSi [3]. Two factors were found to be necessary for this compound formation (1) the presence of sulphide on the surface of the initially precipitated colloidal solid metal sulphide and (2) one of the cations being acidic and the other basic. The monovalent Cu cation is relatively basic, while the trivalent In cation is relatively acidic. It is not clear what the physical reason is for this latter requirement. A difference in practice between acidic and basic cations is that, in an aqueous solution of both cations, the acidic cation is more likely to be in the form of some hydroxy species (not to be confused with hydrated cations), while the basic cation is more likely to exist as the free cation. 

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