Coprecipitation systems

General design of precipitation/coprecipitation systems for arsenic treatment in water... 

Figure 7.1 A typical precipitation/coprecipitation system. (Modified after US EPA (2002b, 18) with the permission of the United States Environmental Protection Agency.)...

An important advantage of the Fe(II)-HMDTC coprecipitation system originally proposed as a batch procedure by Eidecker and Jackwerth [27,28] is its strong tolerance to common matrix constituents, including K, Na, Ca, Mg, Fe, and Al, for a whole series of trace metals and nonmetals. The merits of the batch procedure are fully preserved in... 

Fang et al.[2] determined lead in whole blood digests by flame AAS using an on-line coprecipitation system based on coprecipitation with Fe(Il)-HMDTC The precipitate is collected with a knoned reactor in a fllterless FI system (for details cf. Sec 9. 3). A 44-fold signal enhancement was achieved with a sampling frequency of 90 h. ... 

K. Osseo-Asare, F. J. Arriagada, and J. H. Adair, "Solubility Relationships in the Coprecipitation Synthesis of Barium Titanate Heterogeneous Equihbria in the Ba—Ti—C2O4—H2O System," in G. L. Messing, E. R. Fuller, Jr., and Hans Hausin, eds.. Ceramic Powder Science Vol. 2,1987, pp. 47-53. 

The GdAlgB O QiCe ", Tb " is synthesized by a soHd-state firing of the rare-earth coprecipitated oxide plus boric acid and MgCO at 900° C in a slightly reducing atmosphere. As in the case of the lanthanum phosphate phosphor, a flux is usually used. The synthesis of this phosphor is further comphcated, however, by the fact that it is a ternary system and secondary phases such as gadolinium borate form and must then react to give the final phosphor. 

Chromium Removal System. Chlorate manufacturers must remove chromium from the chlorate solution as a result of environmental regulations. During crystallization of sodium chlorate, essentially all of the sodium dichromate is recycled back to the electrolyzer. Alternatively, hexavalent chromium, Cr, can be reduced and coprecipitated in an agitated reactor using a choice of reducing agents, eg, sodium sulfide, sulfite, thiosulfate, hydrosulfite, hydrazine, etc. The product is chromium(III) oxide [1333-82-0] (98—106). Ion exchange and solvent extraction techniques have also... 

One problem associated with the peroxotungstate-catalyzed epoxidation system described above is the separation of the catalyst after the completed reaction. To overcome this obstacle, efforts to prepare heterogeneous tungstate catalysts have been conducted. De Vos and coworkers employed W catalysts derived from sodium tungstate and layered double hydroxides (LDH - coprecipitated MgCU, AICI3, and NaOH) for the epoxidation of simple olefins and allyl alcohols with... 

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

Morse JW, Bender ML (1990) Partition coefficients in calcite Examination of factors irrflnencing the validity of experimental resnlts and their application to natural systems. Chem Geol 82 265-277 Mucci A, Morse JW (1990) The chemistry of low temperature abiotic calcites Experimental studies on coprecipitation, stability and fractionation. Rev Aquatic Sci 3 217-254 Musgrove ML, Barmer JL, Mack LE, Combs DM, James EW, Cheng H, Edwards RL (2001) Geochronology of late Pleistocene to Holocene speleothems from central Texas Implications for regional paleoclimate. Geol Soc Am Bull 113 1532-1543... 

Attractive interactions are also the reason for the self-assembly of PS-fo-PB-fo-PMMA at the interface of poly(styrene-co-acrylonitrile), SAN, and poly(2,6-dimethylphenylene ether), PPE. In this blend, PS and PPE are miscible on one side and PMMA and SAN are miscible on the other one, with negative / parameters. This blend, in which the rubbery domain is located at the interface between SAN/PMMA and PPE/PS, was originally prepared by coprecipitation of all components from a common solution [195]. From a processing point of view, in this system the difficulty was to get the dispersion of PPE in SAN via melt mixing of SAN, PPE and the triblock terpolymer. 

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]. 

The latter concept implies providing local life support systems for unfriendly environments. By now, Ukrainian scientists and engineers have developed a variety of processes for potable water treatment by adsorption, electrochemical oxidation, electrocoagulation, electro-coprecipitation, electrodialysis, electrofloatation, floatation, membrane techniques etc. Each family must get small units for water purification, air cleaning and removal of hazardous substances from the food as soon as possible, for it may take decades to introduce cleaner production on a national scale. Here, we should follow the example of Western business people who bring with them to Ukraine devices enabling a safe existence in this unfriendly environment. 

Carbonate minerals in natural systems precipitate in the presence of various other solutes This trace amounts of all components present in the solution may get incorporated into the solid carbonate minerals ("coprecipitation"). 

This straightforward application is obvious for type I systems only, for which coprecipitation diagrams like Figure 3 can be calculated and experimentally verified. As can be seen from Figures 4 and 5, the apparent distribution coefficient, X, for systems of... 

The System Calciumhydroxyapatite - Leadhydroxyapatite. In this system there is at least one and presumably two miscibility gaps around 1200°C (66). At room temperature there is one large miscibility gap. The solubility product for leadhydroxyapatite (67) is about 10 so that for this system D is about 30000 in favour of lead incorporation into the apatite. This means that upon precipitation, practically all the lead will precipitate before any calcium coprecipitates. 

Equilibrium between simple salts and aqueous solutions is often relatively easily demonstrated in the laboratory when the composition of the solid is invariant, such as occurs in the KCI-H2O system. However, when an additional component which coprecipitates is added to the system, the solid composition is no longer invariant. Very long times may be required to reach equilibrium when the reaction path requires shifts in the composition of both the solution and solid. Equilibrium is not established until the solid composition is homogeneous and the chemical potentials of all components between solid and aqueous phases are equivalent. As a result, equilibrium is rarely demonstrated with a solid solution series. 

The present study examines the approach to equilibrium in the very soluble salt system KCl-KBr-H20. Soluble salt reactions are known to be relatively rapid and there is greater likelihood for equilibrium to be established. Solubility in the KCl-KBr-H20 system has been well studied at 25°C (6-8) and has been assumed previously to attain equilibrium (3,8). By examining the compositional dependence of the experimental distribution coefficient, Stoessell and Carpenter (9) concluded equilibrium was not established during coprecipitation of trace Br in KC1. 

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