Nitric oxide isotopic exchange

Firestone, M. K., Firestone, R. B., and Tiedje, ]. M. (1979). Nitric oxide as an intermediate in denitrification Evidence from nitrogen-13 isotope exchange. Biochem. Biophys. Res. Commun. 91, 10-16. 

Ye, R. W., Toro-Suarez, 1., Tiedje, J. M., and Averill, B. A. (1991). H2 0 isotope exchange studies on the mechanism of reduction of nitric oxide and nitrite to nitrous oxide by denitrifying bacteria. J. Biol. Chem. 266, 12848-12851. 

Challenger and Masters (7) presented excellent evidence to support their viewpoint that the increase in G(Cem) induced by nitrate ion was because of the oxidation of nitrate ion by OH radical nitrate ion simultaneously reduced the G-value for isotopic exchange between radioactive cerium (III) and inactive cerium (IV) induced by oxidation of cerium-(III) by OH radical. Although we previously presented (15, 23) excellent evidence that any reaction of OH with nitrate ion could not be the cause of the increase in G(Cem), no alternative mechanism was previously proposed for the isotopic exchange results of Challenger and Masters (7). We now propose that the lower G-value for cerium (III)-cerium(IV) exchange observed in 1.0M nitric acid is caused by the reaction of a significant fraction of the OH and N03 radicals with nitrous acid. 

A viable rate model derived from a mechanism should be able to predict these measured kinetic parameter values. Unlike NO oxidation, the standard SCR on Fe-exchanged zeolites is not appreciably inhibited by water. Metkar et al. [42] proposed that the inhibition of the NO oxidation is mitigated by the reactive removal of the responsible inhibiting species. This mechanistic picture is supported by earlier work on Fe-zeolites. A fundamental study was carried out by Sun et al. [22] for SCR on Fe/MFI catalyst using isotopically labeled nitric oxide, NO. Their data showed that the preferred route to molecular nitrogen involve N atoms from NH3 and NO, giving the mixed product ( NN). On the other hand, undesired oxidation of NH3 led to the unlabeled product (N2). From these data. 

It has been shown by isotopic labelling that the exchange reaction of nitrosoalkanes with nitric oxide occurs via oxygen atom transfer, and not by a displacement mechanism (Scheme 132). 

Plutonium Purification. The same purification approach is used for plutonium separated from sediments or seawater. In case reduction may have occurred, the plutonium is oxidized to the quadrivalent state with either hydrogen peroxide or sodium nitrite and adsorbed on an anion exchange resin from 8M nitric acid as the nitrate complex. Americium, curium, transcurium elements, and lanthanides pass through this column unadsorbed and are collected for subsequent radiochemical purification. Thorium is also adsorbed on this column and is eluted with 12M hydrochloric acid. Plutonium is then eluted from the column with 12M hydrochloric acid containing ammonium iodide to reduce plutonium to the non-adsorbed tervalent state. For seawater samples, adequate cleanup from natural-series isotopes is obtained with this single column step so the plutonium fraction is electroplated on a stainless steel plate and stored for a-spectrometry measurement. Further purification, especially from thorium, is usually needed for sediment samples. Two additional column cycles of this type using fresh resin are usually required to reduce the thorium content of the separated plutonium fraction to insignificant levels. 

In another study carried out at a different nuclear establishment, the isotopes and were determined (along with calculated concentrations) in a series of human urine samples, using a concentrated aqua regia wet oxidation method to dissolve the uranium and destroy the organic matter. The uranium was selectively separated from the matrix using anion exchange, eluted with dilute nitric acid, and then aspirated into the ICP mass spectrometer. Using this method, a detection limit of 6 ng/L was achieved, with excellent spike recoveries at the 200 ng/L level, which met both plant and industry standard (American National Standards Institute 13.30) internal dose assessments for total uranium. ... 

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