[en] The chemoautotrophic conversion of nitrite to nitrate by Nitrobacter spp. leads to the release of -64.4 to -87.4 kJ/mol of free enthalpy. The reaction optimum is in the pH range 7.1 to 7.4. From the theory of kinetic isotope effects a value of 0.989703 (at 25 ⁰C, inverse) is derived for fractionating of nitrogen isotopes. On the other hand nitrate is used as an electron acceptor in denitrifying under anaerobic conditions. In intensively used agricultural areas depletions occur in the soil zone and high enrichments of ¹⁵N are observed in the nitrate of the ground water. These processes are evaluated and the term 'mixing isotope effect' is introduced for interpretation

No abstract available

[en] In the last years an increasing nitrate content in surface waters and groundwaters is world-wide observed, esp. in intensively areas fit for agricultural use. Demonstrating by some examples it can be derived a correlation between the nitrate content and the corresponding δ¹⁵N-NO₃^--value which is understood as a biological isotope effect. These isotope effects are in general very small compared with the measured isotope effects under laboratory conditions. The interpretation of this fact is tried by introducing a diffusion process. The results contribute an answer to the problem what degree of conversion of the nitrate will be reached under natural conditions, esp. by denitrification processes in groundwaters. (57 refs.) (author)

[en] Isotopic variations of nitrogen in soils may have their roots in three different forms: fluctuations of the isotopic composition in nitrogen compounds of the precipitations (inputs), the result of isotope effects in ion exchange processes and lastly of isotope effects in metabolic processes. Arable fields may be changed by anthropogenic processes, especially by applying nitrogenous fertilizers. The different isotopic compositions of the nitrogen compounds in soils may be used as natural tracers, i.e. tracing substances in soils and ground waters. (author)

[en] Starting from terms and definitions, some selected applications of stable isotopes, especially in the field of tracer techniquies, in chemistry, biochemistry, biology, medicine, and agriculture are dealt with. Analytical methods for stable isotopes are only outlined. An extensive bibliography with about 700 references has been included

No abstract available

[en] Contaminated media at Superfund sites typically consist of complex mixtures of organic and inorganic chemicals. These mixtures are difficult to characterize, both analytically and toxicologically, especially the complex mixtures of polycyclic aromatic hydrocarbons. The current approach to risk assessment assumes that all contaminants in the soil are available for human exposure. EPA protocol uses solvent extraction to remove chemicals from the soil as a basis for estimating risk to the human population. However, contaminants that can be recovered with a solvent extract may not represent chemicals that are available for exposure. A system using aqueous extraction provides a more realistic picture of what chemicals are bioavailable through leaching and ingestion. A study was conducted with coal tar contaminated soil spiked with benzo(a)pyrene, and trinitrotoluene. Samples were extracted with hexane:acetone and water titrated to pH 2 and pH 7. HPLC analysis demonstrated up to 35% and 29% recovery of contaminants from aqueous extracts with an estimated cancer risk one order of magnitude less than that for solvent extracts. Analysis using the Salmonella/microsome assay showed that solvent extracts were genotoxic with metabolic activation while aqueous extracts showed no genotoxicity. These results suggest that aqueous extraction may be useful in determining what contaminants are available for human exposure, as well as what compounds may pose a risk to human health

[en] A method is described which is suitable for separating nickel from large amounts of uranium and making the spectrophotometric end-determination accessible. The separation of the nickel from the uranium matrix is conducted by means of adsorption on a column of the strongly acidic cationexchanger Dowex 50 X8 from a mixture of 95 vol.% tetrahydrofuran and 5 vol.% of 12 M nitric acid, which also is 0.1 M with respect to trioctylphosphineoxide. Following the elution with 6 M hydrochloric acid, the nickel is separated by chloroform extraction of its dimethylglyoxime complex from the coeluted foreign ions. After the back-extraction with 1 M hydrochloric acid, the nickel is determined spectrophotometrically with application of the dimethylglyoxime method. The procedure was employed for the determination of the nickel in U₃O₈ standard specimens (0.64 to 23.3 p.p.m. Ni) and Yellow Cake (25.0 to 58.0 p.p.m. Ni). An excellent agreement was attained in the findings in all instances. (author)

[en] The separation of Cd(II), Co(II), Mn(II), Ni(II), Pb(II), Rb(I), Cr(III), Zn(II), Mg(II), Ca(II), Sr(II), Ba(II) and Fe(III) from an uranium matrix is achieved by means of adsorption on the strongly acidic cation exchanger Dowex 50 X8 from a mixture of 80 volume % hexon, 10 volume % acetone and 10 volume % 12M nitric acid. Following elution of the adsorbed elements by means of 2M nitric acid, these elements are determined by atomic absorption-spectrophotometric methods. The procedure was used for the determination of the above-named elements in sodium-diuranate samples (Yellow Cake specimens) whereby concentrations in the range of 0.5ppm (Cd and Rb) up to over 1 mg/g (Ca and Fe) were determined

[en] A method is described for the determination of uranium in minerals and rocks by spectrophotometry and fluorimetry. After treatment of the sample with hydrochloric acid, uranium is separated from matrix elements by adsorption on a column of the strongly basic anion-exchange resin Dowex 1 X8 from an organic solvent system consisting of IBMK, tetrahydrofuran and 12M hydrochloric acid (1:8:1 v/v). Following removal of iron, molybdenum and co-adsorbed elements by washing first with the organic solvent system and then with 6M hydrochloric acid, the uranium is eluted with 1M hydrochloric acid. In the elute, uranium is determined by means of the spectrophotometric arsenazo III method or fluorimetrically. The suitability of the method for the determination of both trace and larger amounts of uranium was tested by analysing numerous geochemical reference samples with uranium contents in the range 10^-1 to 10⁴ ppm. In practically all cases very good agreement of results was obtained. (author)