Coprecipitation reaction solid carbonates

FIGURE 1.37 The Arrhenius plot of Ce08Sm0i2Oli9 from different methods (A) solid-state reaction [95] (B) sol-gel process [116] (C) oxalate coprecipitation [91] (D) carbonate coprecipitation, our work and (E) glycine-nitrate process [157]. 

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. 

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. 

The p-block metal oxides were synthesized by solid-state reactions at high temperatures. Usually a mixture of carbonates and component single metal oxides that were mechanically mixed was used as a starting material and calcined in air for 16 h at temperatures between 1273 and 1723 K. In some cases, coprecipitates were prepared as a mixture. For example, for the synthesis of calcium indate, Caln204, Ca(N03 )2-4H20 and In(N03 >3 3H2O were dissolved in a water-ethanol mixture, and an oxalic acid ethanol solution was added [18]. The coprecipitate was aged at 353 K, dried at 393 K, and calcined at different temperatures from 1273 to 1573 K. The formation of p-block metal oxides was confirmed by the x-ray diffraction patterns reported previously in the literatures. 

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