Precipitation isotopes

If macroamounts of other elements are to be separated from microamounts of radioactive substances by precipitation, isotopic or non-isotopic hold-back carriers may be added to suppress coprecipitation of the radioactive substances. 

Buhay, W. M., 1997. Inferring precipitation isotopic compositions from lake sediment organics and pore water Workshop on water and climate studies in Canada using isotope tracers Past, present, future. University of Waterloo, Waterloo. 

The extraction process for radioisotopes is a combined chemical/distillation process in which the noble gases and iodine are first condensed from the target fill gas. Next, the UO2 fuel and fission products are dissolved from the inside of the target in an acidic solution and a wet chemistry process is used to selectively precipitate isotopes of interest. The precipitates are further washed, filtered, redissolved and purified. Finally, the isotope solution is prepared for shipment. 

The plutonium usually contains isotopes of higher mass number (Fig. 1). A variety of industrial-scale processes have been devised for the recovery and purification of plutonium. These can be divided, in general, into the categories of precipitation, solvent extraction, and ion exchange. 

Silicon [7440-21-3] Si, from the Latin silex, silicis for flint, is the fourteenth element of the Periodic Table, has atomic wt 28.083, and a room temperature density of 2.3 gm /cm. SiUcon is britde, has a gray, metallic luster, and melts at 1412°C. In 1787 Lavoisier suggested that siUca (qv), of which flint is one form, was the oxide of an unknown element. Gay-Lussac and Thenard apparently produced elemental siUcon in 1811 by reducing siUcon tetrafluoride with potassium but did not recognize it as an element. In 1817 BerzeHus reported evidence of siUcon occurring as a precipitate in cast iron. Elemental siUcon does not occur in nature. As a constituent of various minerals, eg, siUca and siUcates such as the feldspars and kaolins, however, siUcon comprises about 28% of the earth s cmst. There are three stable isotopes that occur naturally and several that can be prepared artificially and are radioactive (Table 1) (1). 

Cesium isotopes can be recovered from fission products by digestion in nitric acid, and after filtration of waste the radioactive cesium phosphotungstate is precipitated using phosphotungstic acid. This technique can be used to prepare radioactive cesium metal or compounds. Various processes for removal of Cs isotopes from radioactive waste have been developed including solvent extraction using macrocycHc polyethers (62) or crown ethers (63) and coprecipitation with sodium tetraphenylboron (64). 

Fig. 18-6 Characteristic air-mass trajectory and corresponding per mil isotopic composition of precipitation, along a transect from the subtropics to a polar ice sheet. This is a highly schematic view of the true atmospheric system.

The global atmospheric circulation acts as an enormous filtration system, which depletes high-latitude precipitation of heavy isotope-bearing water molecules. Because of this system, measurements of the stable isotopic composition of the ice sheets and of ocean-floor sediments reveal very important paleo-environmental information (see Sections 18.2.2,18.3.2, and 18.3.3). Here we examine this filtration system at a physical level. This system was first understood by a great Danish geochemist named Willi Dansgaard (Dansgaard, 1964). 

H2 0 vapor, and Oq moles of HDO vapor. Write nj to signify either of the heavy-isotope-bearing vapors (n or nig). Define the isotopic ratio of the vapor as Rv/ = npn (,. If the condensation of the vapor to form liquid water or solid ice is an equilibrium process, and this condensation is quickly removed from the air mass as precipitation, the corresponding isotopic ratio in the precipitation (RpPis proportional to that of the vapor... 

Now consider the simultaneous evolution of and c)D. Suppose an air mass having initial compositions 3, = 0 cools and precipitates, always as an equilibrium process. According to the preceding analysis, measurements of both isotopic ratios in precipitation will plot on a curve with a slope approximately given by... 

Equilibrium isotopic fractionations explain the gross behavior of and i5D in precipitation of mid to high latitudes. 

Fig. 18-23 Observed correlation of isotopic composition of precipitation with ground temperature (gray diamonds Jouzel et ah, 1987), and predictions of simple isotopic models. A, prediction with constant a B, prediction with temperature-dependent a.

Jouzel, J. and Merlivat, L. (1984). Deuterium and oxygen 18 in precipitation modelling of the isotopic effects during snow formation. /. Geophys. Res. 89(D7), 11749-11757. 

Emrich, K., Emhalt, D.H. and Vogel, J.C. 1970 Carbon isotope fractionation during the precipitation of calcium carbonate. Earth and Planetary Science Letters 8 363-371. 

The second analytical method uses a combustion system (O Neil et al. 1994) in place of reaction with BrF,. This method was used for the crocodiles because they were represented by very thin caps of enamel. The enamel was powdered and sieved (20 mg), pretreated in NaOCl to oxidize organic material and then precipitated as silver phosphate. Approximately 10-20 mg of silver phosphate were mixed with powdered graphite in quartz tubes, evacuated and sealed. Combustion at 1,200°C was followed by rapid cooling in water which prevents isotopic fractionation between the CO2 produced and the residual solid in the tube. Analyses of separate aliquots from the same sample typically showed precisions of 0.1%o to 0.4%o with 2 to 4 repetitive analyses even though yields are on the order of 25%. 

Gat, J.R. 1980 The isotopes of hydrogen and oxygen in precipitation. In Fritz, P. and Fontes, J., eds.. Handbook of Environmental Isotope Geochemistry. Amsterdam, Elsevier 21-47. 

Hydrogen isotopic relationship between modern bone collagen and precipitation... 

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