Coprecipitation phenomena

Quinones, J., Grambow, B., Loida, A. Geckeis, H. 1996. Coprecipitation phenomena during spent fuel dissolution. Part 1 Experimental procedure and initial results on trivalent ion behavior. Journal of Nuclear Materials, 238, 38-43. 

In soils, the solid phases typically are mixtures of chemical compounds. The generic term used to describe the processes by which these mixtures form is coprecipitation, the simultaneous precipitation of a compound in conjunction with other compounds by any mechanism at any rate. Three broad categories of coprecipitation phenomena have been identified in soils mixed solid formation, adsorption, and inclusion. 

It is possible to calculate from (22-10) the theoretical degree of separation of two metals at a given pH value. The theoretical separation may not actually be achieved, however, owing to coprecipitation phenomena (Chapter 9). 

Disproportionation of the praseodymium and terbium oxides may also be induced by prolonged exposure to air, at room temperature [26,189]. As a result, Ln(OH)3 and Ln02 phases are formed. It would be therefore important to verify the likely occurrence of this phase segregation effect, because of its influence on the actual structural constitution and chemical behavior of the resulting material. Likewise, the formation of Ln(OH)3 (Ln Pr, Tb) may strongly favor the occurrence of dissolution/coprecipitation phenomena during the preparation of praseodymia (terbia) supported metal catalysts. These phenomena may induce nanostructural effects similar to those commented on above. 

Adsorption-coprecipitation phenomenon using fluorescein, dichlorofluorescein and tetrabromofluo-rescein (eosin) essentially impart the fluoresceinate ion that is absorbed on the AgCl particles. At the equivalence point, the AgCl particles change from white to pink due to the coprecipitation of silver fluoresceinate. In short, the adsorption indicator method is quite rapid and capable of providing very accurate results for the estimation of Cl with AgN03. 

In support of that explanation, X-ray analysis of the catalyst after use indicated the presence of MgO. Hence, the catalytically active phase was finely divided copper in intimate contact with magnesia, quasi as carrier. The same phenomenon was observed with the Zintl-phase alloys of silver and magnesium. Such catalysts were then deliberately prepared by coprecipitation of copper and silver oxides with magnesium hydroxide, followed by dehydration and reduction. Table I shows that these supported catalysts had the same activation energies as those formed by in situ decomposition of copper and silver alloys with magnesium. 

Natural carbonate minerals do not form from pure solutions where the only components are water, calcium, and the carbonic acid system species. Because of the general phenomenon known as coprecipitation, at least trace amounts of all components present in the solution from which a carbonate mineral forms can be incorporated into the solid. Natural carbonates contain such coprecipitates in concentrations ranging from trace (e.g., heavy metals), to minor (e.g., Sr), to major (e.g., Mg). When the concentration of the coprecipitate reaches major (>1%) concentrations, it can significantly alter the chemical properties of the carbonate mineral, such as its solubility. The most important example of this mineral property in marine sediments is the magnesian calcites, which commonly contain in excess of 12 mole % Mg. The fact that natural carbonate minerals contain coprecipitates whose concentrations reflect the composition of the solution and conditions, such as temperature, under which their formation took place, means that there is potentially a large amount of information which can be obtained from the study of carbonate mineral composition. This type of information allied with stable isotope ratio data, which are influenced by many of the same environmental factors, has become a major area of study in carbonate geochemistry. 

An additional complication has been demonstrated by Reeder and Grams (1987). They found that coprecipitation of Mg and Mn in calcites is different for different crystal faces. This results in the phenomenon of sector zoning. 

Although several peroxidase enzymes obtained from plant, animal, and microbial sources have been investigated for their ability to catalyze the removal of aromatic compounds from wastewaters, the majority of studies have focused on using HRP. In particular, it has been shown HRP can transform phenol, chlorophenols, methoxyphenols, methylphenols, amino-phenols, resorcinols, and various binuclear phenols [7], HRP was also used for the treatment of contaminants including anilines, hydroxyquinoline, and arylamine carcinogens such as benzidines and naphthylamines [7,8]. In addition, it has been shown that HRP has the ability to induce the formation of mixed polymers resulting in the removal of some compounds that are either poorly acted upon or not directly acted upon by peroxidase [7], This phenomenon, termed coprecipitation or copolymerization, has important practical implications for wastewaters that usually contain many different pollutants. This principle was demonstrated when it was observed that polychlorinated biphenyls (PCBs) could be removed from solution through coprecipitation with phenols [20]. However, this particular application of HRP does not appear to have been pursued in any subsequent research. 

Coprecipitation is a phenomenon in which otherwise soluble compounds are removed from solution during precipitate formation. It is important to understand that contamination of a precipitate by a second substance whose solubility product has been exceeded does not constitute coprecipitation. 

However, during impregnation of the support, dissolution of alumina followed by surface precipitation has been evidenced [1]. This phenomenon is increased when solubilized aluminum reacts with species such as cobalt, to form hydrotalcite-like coprecipitates, or molybdenum to form Andemon-lype heteropolyanion AMo6024H3 [2,3]. After calcination, these species do not lead to easily sulfiding species and thus decrease the overall yield in CoMoS active phase [4,5], which is of course detrimental for catalytic activity. 

The corresponding x-ray patterns from the coprecipitated set of materials are shown in Figure 3. The very strong similarity between the sets suggests that the formation of the cubic phase Is a thermodynamically controlled phenomenon rather than a diffusion-controlled process. In the coprecipitated set, small peaks associated with traces of unreacted barium carbonate at 26 24° can be seen. Also, Y-123 peaks, which persist to x 0.158 In the composite set, appear to collapse to single peaks, even at the lowest level of Bl-2212 incorporation... 

At x 0.429( the Region B particles corresponding to the cubic phase dominate the structures as seen by S.E.M. As before, for both composite and coprecipitated samples, this phase appears (by EDX) to consist primarily of Y, B1 and Ba, with traces of Ca and Cu. The remainder of the copper Is mostly present as small, smooth chunks, which. In the composite material, contain only traces of the other elements, and In the coprecipitated sample, contain large concentrations of both strontium and calcium. It seems likely that both phases In these complex systems exist as solid solutions, and that the exact partition of the elements between the phases Is a klnetlcally controlled phenomenon, determined by the starting materials from which they were synthesized. 

The optimum conditions for the precipitation may also vary when a batch procedure is adapted to work in a continuous mode. Thus the optimum pH range for the coprecipitation of lead with Fe(II )-HMDTC (pH 2-3) was found to be much narrower in the FI procedure than in the original batch procedure (pH 1-7) [21]. This was also considered to be a kinetic effect due to insufficient time for equilibrium in the continuous mode, so that coprecipitation can only be complete under the most favorable conditions. This phenomenon could, however, be made use of to overcome certain interferences from coexisting elements by kinetic discrimination, and should not be simply regarded as a drawback of the continuous approach. In manual batch procedures one always waits for a reaction to complete, unfortunately too often allowing interfering side reactions to fully develop. The equilibrium is therefore often achieved with a significant loss in selectivity. 

On washing the coprecipitates, the amounts of excess sodium ion are retained, and may be readily exchanged for ammonium or other ions by treating with solutions of their salts. This phenomenon is well known, and both natural and synthetic zeolites have been thoroughly studied in this respect (Refs. 24-27). However, these investigators have been more concerned with the behavior of synthetic zeolites than with the problem of determining the causes of zeolitic phenomena. 

It is possible, however, that a mixed Cr(III), Zn(II), and Ni(II) precipitate forms and that this mixed precipitate is responsible for the enhanced adsorption. It was earlier argued that the enhanced removal of Ni(II) in the presence of Zn(II) and Cr(III) by HFO could be due to coprecipitation of a mixed hydroxy species, although it was not strictly necessary to invoke such a phenomenon. In this case [Ni(II) adsorption onto HCO in the presence of Zn(II)], the concept of a mixed hydroxide precipitate is more likely. 

The coprecipitation of hyaluronic acid and acidified protein in the form of a stringy mucin clot can be prevented by the depolymerizing action of hyaluronidase. This phenomenon forms the basis of the mucin clot prevention test (M. C. P.) developed by Robertson et al. (163) and modified by McClean (104,107). The method is well suited for serial determinations but gives only relative values since the assay results vary inversely with the concentration of the substrate (104). 

X 10 mol/L. However, this is an assertion based only on theoretical principles. In practice, in several examples, a phenomenon of coprecipitation also exists, which consists of an entrapment of the solution within the precipitate. Moreover, an adsorption of ions that are not those engaged in the precipitate may occur at the precipitate s surface. Eventually, the formation of a solid solution is also possible. 

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