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  • Author or Editor: K. Hirose x
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Abstract  

Thorium isotopes in seawater are determined by means of adsorption of the Xylenol Orange /XO: H6A/ complex onto XAD-2 resin at pH=3 and the XO concentration of 10–5M, and subsequent purification using an anion-exchange resin, and finally with alpha-spectrometry. The dissolved232Th concentration in the western North Pacific surface water is found to range from 0.8 to 1.2 Bq –1. The adsorbed species of the Th-XO complex under the experimental conditions has a composition of Th/H2A/2 according to the mass balance analysis.

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Abstract  

Artificial radionuclides in deposition and airborne dust samples in 1986 were measured at Tsukuba and 11 stations in Japan. In May 1986, the Chernobyl radioactivity was observed in rain and air samples in Japan. The Chernobyl-derived Pu isotopes, which are characterized by higher238Pu/239,240Pu (85) and241Pu/238Pu (0.5) activity ratios than those of the nuclear test-derived Pu and90Sr, were detected in deposition and airborne dust samples in Japan, as well as volatile radionuclides such as131I and137Cs. However, the activities of Pu isotopes and90Sr observed in Japan were about two and three orders of magnitude lower than those expected from the activity ratios in the total release at Chernobyl, which means that the residence time of Pu in the air was shorter than that of137Cs. In order to understand the fractionation between the Chernobyl radionuclides we studied about individual wet and dry deposition. The results suggest that this cause is due to the difference of the particle size of radionuclide-bearing particles, which may be related to the release process of Chernobyl radionuclides.

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Abstract  

A sequential leaching experiment for chemical speciation of particulate uranium in seawater was carried out using solution chemistry techniques. Mass action analysis on the leaching processes reveals that the major species of particulate uranium in seawater is an organic complex of uranium(VI), which is dissociated completely in the leaching solution of pH 1.0. Inert uranium, which is not dissolved in the leaching solution of pH 1.0, exists is suspended matter although it is a minor constituent. The result suggests that this species in an organic complex of uranium(IV). The analysis of the experimenmt deduces that an organic binding site in suspended matter reacts as a polydentate ligand, which has more than two stepwise protonation constants within the pH range of 2.0 to 8.2. The conditiona stability constant of the organic uranium complex in suspended matter under the conditions of seawater is within the following range: 12<log Kc,U<16. These findings suggest stronglt that the organic binding site in suspended matter can form stable chelates with metallic elements.

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Abstract  

A new method has been developed to measure a strong ligand in oceanic dissolved organic matter (DOM) by Th complexation in acidic media and the adsorption of the Th complex onto XAD-2 resin. Th reacts quantitatively with organic binding site of DOM in strong acid media (around 0.1M H+ solution), which is equilibrated within 24 hours. According to mass action analysis, Th forms a 11 complex with the binding site in DOM, whose conditional stability constant is 106.7M–1. The conditional stability constant of the Th complex in DOM is in good agreement with that determined for oceanic Particulate Matter (PM) under similar experimental conditions. This finding suggests that the chemical properties of the strong ligand in DOM are similar to these in biogenic PM. The Th-complexing capacity in DOM, which corresponds to the total concentration of the strong organic ligand, can be determined (2–3 nM in surface waters) in a small volume (about 200 ml). The method has a detection limit of approximately 0.05 nM for the thorium-complexing capacity of DOM by using230Th.

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Abstract  

Uranium shows relatively conservative behaviour in seawater because of the formation of stable carbonato complexes, whereas particulate uranium, involved in suspended particles with the particle size of more than 0.45 m, is a minor constituent. It was found that particulate uranium, with a range from 0.24 to 39 Bq·1–1, varies spatially and temporally. Its highest concentration occurs in the tropical region of the western North Pacific during the winter of 1983, corresponding to the 1983 El Nifio event. A leaching experiment revealed that major species of particulate uranium are labile organic complexes. Mass balance considerations suggest that particulate uranium in open ocean waters correlates with the presence of particulate organic matter (POM). A high peak of particulate uranium in 1983 may indicate that POM, i.e., primary productivity, increased in the western tropical Pacific during the El Niño event.

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Abstract  

A new method of plutonium speciation in large volume of sea water was developed by using adsorption of Pu(IV)-Xylenol Orange chelate and Pu-Arsenazo chelate on XAD-2 resin, respectively. The tetravalent plutonium ion reacts selectively with Xylenol Orange in acid solution and that adsorbed on XAD-2 resin. Total plutonium can be collected onto the resin in the form of its Arsenazo-III complex. The determination of plutonium then was carried out by alpha-ray spectrometric method after decomposition of organic complexes and ion exchange separation. The present method is confirmed for convenient and rapid preconcentration procedure for plutonium shipboard chemistry.

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Abstract  

Plutonium isotope concentrations in the surface air at Tsukuba, Japan are reported during the period from 1981 to the end of 1986. The239,240Pu concentration in the surface air, which showed a marked seasonal variation with a spring maximum and fall minimum, decreased until the end of 1985 according to the stratospheric residence time of 1.15 years. In May 1986, elevated239,240Pu concentrations with high238Pu/239,240Pu activity ratios were observed. The serial trend of plutonium concentration in the surface air is similar to the concentrations of the Chernobyl-released radionuclides. These findings suggest that a significant part of the plutonium in the surface air in May 1986 was due to the Chernobyl fallout. The size distribution of plutonium bearing particles indicates that plutonium isotopes were mechanically released in the Chernobyl accident. The surface air concentration of plutonium from the Chernobyl accident was much lower than the concentrations of the volatile fission products, and increased the monthly mean239,240Pu concentration by only 0.03 Bqm–3.

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Abstract  

The wet and dry deposition of gamma-emitting nuclides are presented for Tsukuba and eleven stations in Japan following the nuclear reactor accident at Chernobyl'. In Japan fallout from the reactor at Chernobyl' was first detected on May 3, 1986, a week after the accident. Abruptly high radioactive deposition, which mainly consists of131I,132I,103Ru,137Cs and134Cs, was observed in early May. The cumulative amount of131I,103Ru and137Cs in May at Tsukuba were 5854±838 Bq·m–2, 364±54 Bq·m–2 and 130±26 Bq·m–2 (decay was corrected to April 26), respectively. The monthly137Cs deposition in May corresponds to 2.5% of the cumulative137Cs deposition during the period from 1960 through 1982. Most of the Chernobyl' radioactivities, especially131I, are scavenged from the atmosphere by the wet removal process.

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Abstract  

Plutonium was determined in monthly atmospheric deposition samples from 2000 to the end of 2003 collected at Tsukuba, Japan, together with monthly thorium deposition. Monthly 239,240Pu and thorium depositions showed a typical seasonal variation with a maximum in spring. The 230Th/232Th activity ratios in the deposition samples differed from that of surface soils near Tsukuba. These results reveal that a significant part of the 239,240Pu deposition observed in Japan is attributable to resuspension of deposited plutonium originating from the East Asian continent deserts and arid areas.

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Abstract  

To clarify environmental effects of the Chemobyl radionuclides, long-lived Chernobyl radioactivity (239,240Pu,238Pu,241Pu and90Sr) in deposition samples in May 1986 was measured at 11 stations in Japan. Temporal variation of weekly deposition of90Sr differed from that of volatile radionuclides such as131I and137Cs, which may reflect the released process at Chernobyl. On the other hand, the geographical distributions of the monthly deposition of long-lived radionuclides were similar to those of volatile radionuclides, in which maximum deposits of90Sr and plutonium were observed in Akita, a northwestern Japan Sea coast site of Honshu Island. Higher241Pu deposition in most of the stations, as well as high238Pu/239,240Pu activity ratios were observed. The241Pu/238Pu activity ratios in deposition samples were nearly equal to that in the total release, which is clear evidence that Chernobyl-derived plutonium was transported to Japan in May 1986 together with volatile radionuclides although the contribution of Chernobyl Pu was about three orders of magnitude lower than137Cs.

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