Precipitation, in the context of our review, can be considered as the formation of a crystalline solid phase from a liquid phase containing electrolytes. Taking precipitation of BaSO4 as an example, when the ionic product, Kss, is greater than the solubility product, Ksp, precipitation may occur; i.e., Kss = [Ba2+][SO4−2] > Ksp. There is an incubation period for precipitation, defined as the time between the formation of the supersaturated solution and the occurrence of visible turbidity. The incubation time is related to the concentration of the supersaturated solution by the following equation:
t = Const. × C−n.
The value of n was found to be 3.3 for CaC2O4, 4.7 for Ag2CrO4, and 9 for CaF2 [Nie55]. The kinetics of precipitation is discussed in depth by Nielsen in a monograph [Nie64] and in an article by O'Rourke and Johnson [Oro55].
The size of the particles formed is influenced by the degree of supersaturation when precipitation occurs. The less supersaturation, the bigger the particle size will be. Bigger particle size is preferable for fast and easy filtration. However, under low supersaturation conditions, the incubation time will be long.
The factors that affect the purity of the precipitate need to be considered along with the kinetics and particle size mentioned above. Contamination of the precipitate may occur even when the ionic product of the contaminant does not exceed its solubility product. This phenomenon, known as coprecipitation, may be classified into four major categories:
Formation of mixed crystals: Chemically similar electrolytes known to form isomorphous crystals will form mixed crystals. For example, BaSO4 and PbSO4 form mixed crystals.
Occlusions: During the rapid growth of crystal, the adsorbed ions may not be desorbed before the next larger crystal grows. Thus, the adsorbed impurity is trapped inside the crystal. This phenomenon is known as occlusion.
Mechanical entrapment: Several small crystals physically lying close together during growth may form a crystalline mass; during this process, part of the solution may be entrapped.
Adsorption: On the surface of the crystal, depending on the conditions of the precipitation, various ions could be adsorbed. Most of the adsorbed contaminant can be removed by proper washing of the precipitate. Unfortunately, this is not the case with coprecipitation due to the other three phenomena mentioned above.
The faster the growth, the larger the chance of contamination by occlusion and mechanical entrapment. Coprecipitation can be minimized by slow precipitation from homogeneous solution. For example, the distribution coefficient for coprecipitation of americium with lanthanum oxalate was 6.3 when the precipitation rate was slow, but decreased to 1 at fast rates [Wal67a]. Precipitation from a homogeneous solution has been reviewed by Gordon [Gor55] and by Cartwright and coworkers [Car67]. The theory of coprecipitation is discussed by Kolthoff [Kol32] and Nielsen [Nie83]. Information on adsorption is presented by Kolthoff [Kol36] and Nielsen [Nie83]. The various factors affecting the formation of precipitates have been discussed in detail by Walton [Wal67b].
From the point of view of ultrafast separation, it is imperative that the required radioactive material be precipitated rapidly from the solution and filtered quickly. These conditions are contradictory to the requirement for obtaining a pure solid phase [Gor59]; also, these conditions will lead to crystals of small size that will not have the required filtration characteristics. Because of these limitations, precipitation has not been used extensively in fast radiochemical separations. Figure 1 shows the elements for which a fast precipitation procedure has been reported in the