Colloidal precipitates coprecipitation

Adsorbed, occluded, and included impurities are said to be coprecipitated. That is, the impurity is precipitated along with the desired product, even though the solubility of the impurity has not been exceeded. Coprecipitation tends to be worst in colloidal precipitates... 

After pH adjustment, the water enters a precipitation/coprecipitation tank, where chemical reagents (such as iron or aluminum salts) are carefully added to form the precipitates. The resulting precipitates are often colloidal or are otherwise too fine grained to readily settle out of solution. They may also have repulsive surface charges that prevent them from agglomerating and settling. As shown in Figure 7.1,... 

Adsorbed, occluded, and included impurities are said to be coprecipitated. That is, the impurity is precipitated along with the desired product, even though the solubility of the impurity has not been exceeded. Coprecipitation tends to be worst in colloidal precipitates (which have a large surface area), such as BaS04, Al(OH)3, and Fe(OH)3. Figure 7-3 shows that phosphate coprecipitated with calcium carbonate in coral is proportional to the concentration of phosphate in seawater. By measuring P/Ca in ancient coral, we can infer the concentration of phosphate in the sea at the time the coral lived. 

FeS is a black colloidal precipitate that can not readily be settled out. It is difficult to flocculate and generally requires coprecipitation of an analogous mass of metallic hydroxide by addition of an iron or aluminum salt... 

Ferric hydroxide coprecipitation techniques are lengthy, two days being needed for a complete precipitation. To speed up this analysis, Tzeng and Zeitlin [595] studied the applicability of an intrinsically rapid technique, namely adsorption colloid flotation. This separation procedure uses a surfactant-collector-inert gas system, in which a charged surface-inactive species is adsorbed on a hydrophobic colloid collector of opposite charge. The colloid with the adsorbed species is floated to the surface with a suitable surfactant and inert gas, and the foam layer is removed manually for analysis by a methylene blue spectrometric procedure. The advantages of the method include a rapid separation, simple equipment, and excellent recoveries. Tzeng and Zeitlin [595] used the floation unit that was devised by Kim and Zeitlin [517]. 

Uranium coprecipitated with aluminium phosphate, precipitate dissolved in nitric acid Adsorption onto colloidal ferric hydroxide... 

Some metals are irreversibly adsorbed, probably via incorporation into the mineral phases, such as amorphous iron oxyhydroxides, as shown in Figure 11.6d. Some of these amorphous phases form by direct precipitation from seawater. As noted earlier, hydrothermal fluids are an important source of iron and manganese, both of which subsequently precipitate from seawater to form colloidal and particulate oxyhydroxides. Other metals tend to coprecipitate with the iron and manganese, creating a polymetallic oxyhydroxide. It is not clear the degree to which biological processes mediate the formation of such precipitates. Since the metals are incorporated into a mineral phase, this type of scavenging is better referred to as an absorption process. 

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. 

Reactions of dissolved species with particulate and colloidal suspended matter include adsorption/desorption, complexation, ion-exchange, precipitation/dissolution, coprecipitation during coagulation and flocculation (Morgan, 1966 Stumm and Morgan, 1981 Parks, 1975). These processes are particularly important at the land-sea boundary in estuaries (Duinker, 1980 Martin et al., this volume). The interaction with particles > 0.45 ym is not discussed here. 

The life of an Avicel suspension can be extended by coprecipitating the rodlike structures with a protective colloid after trituration. Avicel-RC19 is limit cellulose that has been physically modified by coprecipitation with CMC to facilite dispersibility. Avicel-RC water suspensions simulate the properties of a hydrosol. At low aqueous concentrations, the apparendy hydrated crystallites assemble into a thixotropic, heat- and acid-stable structure whose viscosity depends direcdy on pH to about pH 10, whereupon it declines precipitously. The suspension coalesces at low pH. The addition of salt after mixing increases viscosity above what it would be if the salt were added at the time of mixing or shearing. 

In the geological and soil science literature, ion exchange and precipitation are frequently considered as adsorption and thermodynamically described by adsorption equations, or isotherms. This is not correct because, as shown previously, the processes are principally different adsorption is directed by the decrease of surface energy, and it takes place on the free surface sites ion exchange is just a competitive process on an already covered surface, determined by the ionic composition of the liquid phase. Precipitation, including colloid formation, is governed by the composition of the liquid phase, the crystal structure (coprecipitation), or primary chemical forces. 

The methods for the recovery of protactinium include coprecipitation, solvent extraction, ion exchange, and volatility procedures. All of these, however, are rendered difficult by the extreme tendency of protactinium (V) to form polymeric colloidal particles composed of ionic species. These cannot be removed from aqueous media by solvent extraction losses may occur by adsorption to containers and protactinium may be adsorbed by any precipitate present. 

Aquatic sediments are formed in all surface waters by the settling of coarse and fine inorganic and organic particles. They are present in rivers, in lakes and in the oceans, and radionuclides deposited on the surface of the earth will sooner or later come into contact with these sediments. They may enter the sediments by sorption of molecularly-dispersed species (ions, molecules), by precipitation or coprecipitation, by coagulation of colloids (in particular carrier colloids) followed by sedimentation of the particles formed, or by sedimentation of coarse particles (suspended matter). By desorption, the radionuclides may be remobilized and released again into the water. 

Chang PL, Yen FS, Cheng KC, Wen HL (2001) Examirratiorrs on the critical and primary crystallite sizes during 0-to a-phase transformation of rrltrafine alumina powders. Nano Letters 1 253-261 Charlet L, Manceau A (1992) X-ray absorption spectroscopic study of the sorption of Cr(IIl) at the oxide/water interface. 11 Adsorptiorr, coprecipitation and surface precipitation on ferric hydrous oxides. J Colloid Interface Sci 148 425-442... 

Adsorption is a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas—that is, coagulated colloids (see Feature 12-1 for definition of specific area). Although adsorption does occur in crystalline solids, its effects on purity are usually undetectable because of the relatively small specific surface area of these solids. 

The extent of mixed-ci-ystal contamination is governed by the law of mass action and increases as the ratio of contaminant to analyte concentration increases. Mixed-crystal fomiation is a particularly troublesome type of coprecipitation because little can be done about it when certain combinations of ions are present in a sample matrix. This problem is encountered with both colloidal suspensions and crystalline precipitates. When mixed-crystal formation occurs, the interfering ion may have to be separated before the final precipitation step. Alternatively, a different precipitating reagent that does not give mixed crystals with the ions in question may be used. 

In groundwaters distant from the engineered portions of a high-level waste repository the mobilities of U and Th may be limited by adsorption at low U and Th concentrations and by the solubilities of U and Th solids at higher concentration. In the same groundwaters, the mobilities of Ra, I, Tc, Np, and Pu are more likely to be limited (if they are limited) by adsorption and/or by coprecipitation in the structures of major mineral precipitates, or by the instability or filtration of colloids. Explain these statements and discuss the possible detailed behavior of each element. 

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