the forest trees show large nonsystematic variations in the 13C/12C ratio over the 500 years, the freestanding trees show smaller fluctuations, which can be correlated to climatic changes. Since industrialization of these areas in 1850 A.D., the 13C/12C record for the free-standing trees has been dominated by a systematic decrease of about 2%0, which is comparable with other mean 13C/12C tree ring data. The calculated Northern Hemisphere 13C/12C record derived from a total of about 50 trees reflects the recent 13C/12C change in atmospheric CO2 observed from 1956 to 1980 A.D. INTRODUCTION Whether the stable carbon isotope record in tree rings can be used for reliable determination of past •3C/•2C levels of atmospheric CO2 is a matter of some contention. Earlier measurements have shown that the atmospheric •3C/•2C information in tree rings is often masked by scatter, which is supposed to depend on local physical as well as climatological factors [Freyer, 1980; Francey and Farquhar, 1982]. To eliminate some of the scatter, a larger number of freestanding trees from various parts of the world should be analyzed. More than 50 individual trees from the Northern and Southern hemispheres have been measured so far by various authors. Despite the scatter, some common trends can be derived from most of the measurements: •3C/•2C data in tree rings decrease over the second half of the past century and the first quarter of this century and, in some cases, increase during the 1940-1960 period [for a summary of data see, e.g., Stuiver, 1978; Bell, 1979; Freyer, 1979a]. From about 1960 to 1975 a further decrease of the •3C/•2C ratio is observed in our extensive tree ring record obtained from measurements on 22 free-standing trees of different origin in the Northern Hemisphere [Freyer, 1978, 1979a]. This decrease in mean 13C/12C tree ring data compares favorably [Freyer, 1981] with the 13C/12C decrease in atmospheric CO2 between 1956 and 1978 that was determined directly by Keeling et al. [1979, 1980]. The overall 13C/12C decrease obtained from some of the earlier tree ring records has been used for model evaluation of the past atmospheric CO2 increase and total anthropogenic CO2 production since industrialization of the study areas [Freyer, 1978; Stuiver, 1978]. However, a comparison of these 13C/12C decreasing data with natural preindustrial •3C/ •2C variations in tree tings has not as yet been shown. This The oak trees were taken as dated cores from the collection of B. Becker (University of Hohenheim, Germany). These trees had grown in the forest area of the German Spessart (50.0øN, 9.3øE) in the Forstamt Rothenbuch (Qu 16-Qu 18), the Forstamt Lohr (Qu 19) and the Forstamt Altenbuch (Qu 20). The free-standing Scots pine trees (Pi 7-Pi 11) were felled and sampled by O. Zackrisson (University of Umefi, Sweden) on mire complexes in Muddus National Park, Sweden (67.0øN, 20.2øE) at distances of 5 to 10 km N or NNW of Lake Muddusjaur. Sites were selected where no dry snags could be found that would indicate that no other tree had gr...

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The sorption kinetics and equilibrium isotherms of zirconium, uranium, and molybdenum ions onto synthetic polyaniline/SiO2 composite (PAn/SiO2) have been studied using batch-sorption techniques. This study was carried out to examine the sorption behavior of the PAn/SiO2 for the removal of Zr(IV), U(VI), and Mo(VI) ions from an aqueous solution. The influence of some parameters on the sorption process was also studied. The maximum sorption for Zr(IV), U(VI), and Mo(VI) ions was achieved at 60 min shaking time. Langmuir isotherm model is the most representative for discussing the sorption process with a maximum sorption capacity of 24.26, 21.82, and 13.01 mg/g for Zr(IV), U(VI), and Mo(VI) ions, respectively. Kinetic modeling revealed that the sorption of all ions follows the pseudo-second-order kinetic model. The results demonstrated that both the external and intra-particular diffusion are taken into account in determining the sorption rate. Thermodynamic parameters like ΔG°, ΔH°, and ΔS° for the sorption process were evaluated. The synthetic composite has been successfully applied for the removal and recovery of U(VI) ions from real solution (monazite leachate) using a chromatographic column packed with PAn/SiO2 composite with a breakthrough capacity equal to 239.70 mg/g.