Co-precipitation behavior

TABLE IV-14. Co-precipitation Behavior of Trace Amounts of Plutonium and Neptunium in Principal Valence States. 

Schwartz, R. W. 1989. Chemical processing of PbTi03 by co-precipitation and sol-gel methods the role of powder and gel characteristics on crystallization behavior. PhD dissertation. University of Illinois at Urbana-Champaign. 

Co-precipitation of Re S with platinum sulfide from cone, hydrochloric acid solutions of microamounts of technetium and rhenium is suitable for the separation of technetium from rhenium , since technetium is only slightly co-precipitat-ed under these conditions (Fig. 7). At concentrations of 9 M HCl and above, virtually no technetium is co-precipitated with platinum sulfide at 90 °C, whereas rhenium is removed quantitatively even up to 10 M HCl. The reduction of pertechnetate at high chloride concentration may be the reason for this different behavior, because complete co-precipitation of technetiiun from sulfuric acid solutions up to 12 M has been observed. However, the separation of weighable amounts of technetium from rhenium by precipitation with hydrogen sulfide in a medium of 9-10 M HCl is not quantitative, since several percent of technetiiun coprecipitate with rhenium and measurable amounts of rhenium remain in solu-tion . Multiple reprecipitation of Re S is therefore necessary. 

The oldest, most well-established chemical separation technique is precipitation. Because the amount of the radionuclide present may be very small, carriers are frequently used. The carrier is added in macroscopic quantities and ensures the radioactive species will be part of a kinetic and thermodynamic equilibrium system. Recovery of the carrier also serves as a measure of the yield of the separation. It is important that there is an isotopic exchange between the carrier and the radionuclide. There is the related phenomenon of co-precipitation wherein the radionuclide is incorporated into or adsorbed on the surface of a precipitate that does not involve an isotope of the radionuclide or isomorphously replaces one of the elements in the precipitate. Examples of this behavior are the sorption of radionuclides by Fe(OH)3 or the co-precipitation of the actinides with LaF3. Separation by precipitation is largely restricted to laboratory procedures and apart from the bismuth phosphate process used in World War II to purify Pu, has little commercial application. 

Figure 7. Precipitation behavior of U(VI) peroxo complex ion with cobalt(III) complex salt U taken, 5.0 mg total volume, 5 mL HtOt concn., 15% pH 7.5-9.0 aging time, 5 min 1, precipitation with [Co(NHs)6]Cls 2, precipitation with [Co(en)s]Cls 3, precipitation with [Co-(tn)s]Cls...

The reasons of such behavior are connected with the differences in phase formation of solid product. Primary co-precipitation products appear to be soluble hydroxo-complexes of metals. Depending on rates of hydrolysis stages, the clusters of separate hydro-complexes or the clusters... 

Some studies have reported conservative behavior during estuarine mixing. In the unpolluted Krka Esmary of Yugoslavia, Seyler and Martin (1991) observed a linear increase in total arsenic with increasing salinity, ranging from 0.13 xgL in freshwaters to 1.8 JLgL offshore. Other studies however, have observed nonconservative behavior in estuaries due to processes such as diffusion from sediment pore waters, co-precipitation with iron oxides, or anthropogenic inputs (M. O. Andreae and T. W. Andreae, 1989 Andreae et al., 1983). The flocculation of iron oxides at the freshwater-saline interface as a result of increase in pH and salinity can lead to major decrease in the arsenic flux to the oceans (Cullen and Reimer, 1989). 

Dithiocarbamates (RCS2") have been applied widely in the analytical chemistry of trace metals in natural water 1, 2, 3). The ammonium salt of the dithiocarbamate of pyrrolidine (APDC) is a water-soluble compound which forms water-insoluble uncharged chelates with a variety of metals. At the very low metal concentration levels found in natural waters, dithiocarbamate chelates form colloids (4). At higher concentration levels the chelates precipitate (5). Since the metal ion in the chelate is eflFectively shielded by hydrophobic groups, precipitation in a multi-metal system is relatively nonselective the chelates of several metals precipitate together rather than as separate phases. If one transition metal is present in suflBcient concentration to form particles, other metals will be incorporated into the particles regardless of their concentration. The analytical implication of this behavior is clear— APDC chelates of trace transition elements can be co-precipitated by addition of one transition element up to suflBcient concentration to form chelate particles. 

Many trace contaminants co-precipitate with Fe(III), behavior that can be problematic in many hydrometallurgical processes. However, such behavior is useful in removing arsenate from solutions, and has been tested for a broader range of contaminants. 2 There are unresolved questions on the long-term stability of such residues under ambient condilions.- - The theoretical basis behind cationic precipitation with Fe(III) or Al(ni) is essentially the same as the previous case—the formation of insoluble compounds, which have specific solubility products (A sp). ... 

Cytochromes/( J from Latin frons for leaf) and were discovered almost half a century ago hy Hill and Scarisbrick and by Hill, respectively. In fact, the redox behavior of these chloroplast c)do-chromes led Robin Hill and Fay Bendall to formulate the so-called Z-scheme for oxygenic photosynthesis. In 1972, Nelson and Neumann" isolated a partially purified complex from a digitonin-fraction-ated PS-I particle obtained from lettuce chloroplasts. The complex was found to contain Cyt/, Cyt and non-heme iron, which led the authors to note its similarity to the Cyt-bci complex (i.e., complex III) of mitochondria. In 1975 Sugahara, Shaw and this author isolated a complex from spinach TSF-I particles and by investigation of its spectroscopic and EPR properties, showed that it also contained Cyt/, Cyt b and nonheme iron, consistent with its being a bjcomplex. The fraction also displayed a distinct EPR signal characteristic of a copper protein, apparently due to plastocyanin co-precipitated during fractionation. 

It can be seen that magnesium addition to calcium oleate solution or vice versa does shift the transmission curve to lower oleate concentrations, indicating co-precipitation of calcium and magnesium oleates due to synergistic effects. Overlapping of both the curves in Fig. 4.36 is also in line with the behavior of the dolomite and francolite systems in microflotation and dissolution experiments. Simultaneous measurements of dissolved species in the supernatant of the precipitated solutions (Fig. 4.37) showed Ca and Mg levels to remain almost constant until solubility products of their oleate are reached (2 x 10 " mol/1... 

Carbonates - which are common constituents in some sods - have a major influence on the pH of soils, and therefore also on trace element behavior. Trace elements may co-precipitate with carbonates, being incorporated in their structure or may be sorbed onto carbonate particles. Trace metals can occur as complex carbonate cations or anions, depending on soil pH (Table 5.4). 

Several blends prepared by co-precipitation followed by crystallization from the melt exhibit a double melting behavior, due to the occurrence of the secondary crystallization process. The amorphous component causes a retarded crystallization of some of the crystallizable chains, which form lamellae smaller than and located between the primary ones constituting the spherulites (see Figure 3.22). 

Once the ability of the IC-ICP-MS system to repeatedly separate the three selenium species was confirmed, efforts focused on investigating process variables which would influence the concentration of various selenium species, specifically selenocyanate (SeCN) in the SSW. One such process variable studied was pH. The study of the pH effects on the conversion of the SeCN to other selenium species, such as selenate and selenite, was undertaken to define the pH boundaries fiir any pre-treatment or treatment conditions for the SSW. Knowledge of tbrs behavior would influence the design of any systems, e.g., iron co-precipitation, built to remedate the Se in the SSW streams. 

Yam] Yamamoto, K., Kimura, Y, Mishima, Y, Precipitation Behavior and Phase Stability of Inter-metallic Phases in Fe-Cr-W-Co Ferritic Alloys , Mat. Sci. Forum, 475-479, 845-848 (2005) (Morphology, Phase Relations, Experimental, Electronic Structure, 14)... 

The separation of Pu by co-precipita ion usually takes advantage of the aforementioned oxidation and reduction cycles to effect purification. The procedure may be illustrated with the carrying of Pu(III) and Pu(IV) and the non-carrying of Pu(VI) on LaFg. Only those elements with co-precipitation and oxidation-reduction behavior very similar to that of Pu interfere. The oxidation-reduction cycle may be repeated as many times as needed to get any desired degree of purity. The use of LaFg precipitation is also a valuable group separation and volume reduction step, since not very many elements have acid-insoluble fluorides. 

The preparation and solution properties of poly(vinylpyrrolidone-co-acrylamide), synthetic water-soluble copolymer which has non-precipitating behavior in hard brines at high temperatures, is discussed. One sample of the copolymer has remained in synthetic seawater for over six years at 121 C (250 F) without... 

Previous work in our laboratory [16] has shown the presence of co-ions like aluminum and nickel in the Fe(III)/As(V) co-precipitation system to have a stabilizing effect on As(V) [17]. Thus for Fe(III)/As(V)=4 containing Ni(II)/As(V)=0.5 the As(V) concentration in solution after 210 days of pH=8 adjusted aging was down to 0.4 mg/L from 0.63 mg/L without nickel. The origin of the stabilizing role of Ni(II) is currently under investigation [18]. The partial substitution of aluminum for ferric (Fe(III)/Al(III)/As(V)=3/l/l) also exhibited better stability behavior than Fe(III)/As(V) (=4) alone. Thus after 225 days only 0.44 mg/L As(V) was found in solution in contrast to 0.65 mg/L As(V) without aluminum. Further work is planned to evaluate the stabilizing roles of Ni and A1 under anoxic conditions. 

The interpretation of the results of experiments performed in recent years has yielded contradictory conclusions as to the sources and the mechanisms of contamination buildup. A th one exception, the measures taken on the basis of these results have not resulted in a clear success on the contrary, in some cases a deterioration of the situation has resulted. The question as to the reasons for such consequences emerges and it seems that the failure of many attempts has mainly been due to the fact that only macrochemical aspects (e. g. effect of pH and temperature on the solubility of the corrosion product oxides) have been taken into consideration. In reality, because of the very low mass concentrations of the essential radionuclides and their mother elements in the coolant, severe deviations in behavior from that of macroamounts are to be expected, an effect which is well known in radiochemistry. In particular, in the behavior of Co and Co trace-chemical mechanisms such as surface adsorption onto oxide particles, co-precipitation together with other elements, as well as ion exchange and isotope exchange with other constituents of the corrosion product oxides can be assumed to play an important role, but in most of the investigations performed up to now these have not been considered. 

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