Coprecipitation reduction step

Sequestration by Fe° occurs mainly by adsorption, reduction, and coprecipitation, although other processes may be involved such as pore diffusion and polymerization. In most cases, adsorption is the initial step and... 

Procedure. The procedure is outlined in Figure 1. The first step is the separation of the desired nuclides by coprecipitation with calcium fluoride. The optimum nitric acid concentration for effective carrying on calcium fluoride is between O.IN and 0.2N. Reduction of the plutonium to Pu(III) was necessary to obtain quantitative carrying on calcium fluoride. Plutonium (IV) is known to form colloidal or non-ionic species in neutral solution, and in this form may be incompletely carried by calcium fluoride. Bisulfite was effective and gave complete reduction in 3.5 hr at 50° C or overnight at room temperature. The concentration of calcium must be at least 0.1 mg/ml for quantitative carrying. 

The support is often coprecipitated simultaneously with the active metals in a single process step. This can be done by mixing in basic precursors of the support, such as sodium sihcate, sodium aluminate, or mixtures thereof, with the base precipitant. These can alternatively be added separately. Also, more acidic compounds like aluminum nitrate can be used as support precursors and, optionally, mixed in with the metal precursor. Compared to preformed supports, the amount of support precursor can be quite low and this is why coprecipitates may contain very high metal levels up to 80-90%, a level not attainable by impregnation. 20-30 mol% of silicate or aluminate relative to the metal is usually sufficient to obtain a high metal surface area after reduction. It may be debated whether the resulting metal silicate, aluminate, or alumina, are real supports or that names like stabilizer or spacer are more appropriate. 

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