Naofumi Kozai

Environment. Superselective clay for radium uptake.

A legacy of the nuclear industry that threatens many areas of the world is contamination by radioactive radium from the solid and liquid wastes left after extracting uranium from its ores. Here we describe a highly charged, synthetic clay that specifically removes radium and immobilizes it at room temperature for safe disposal. This material may prove to be useful in decontaminating drinking water wells and soils of this toxic isotope.

Novel function for anionic clays: selective transition metal cation uptake by diadochy

Here we report for the first time, the extremely high and selective uptake of transition metal cations such as Cu2+, Ni2+, Co2+ and Zn2+ by two anionic clays of nominal composition, [Mg6Al2(OH)16 ]2+[CO3·4H2O]2– and [Mg2Al(OH)6 ]+[NO3·2H2O]–. We postulate that the principal mechanism of this selective cation uptake is by substitution in the anionic clay structure for mainly Mg through a process known as diadochy. This newly discovered function for anionic clays is useful for the decontamination and immobilization of the above transition metals at room temperature and may find applications in the remediation of metal contaminated soils, filtration of drinking water as well as decontamination of waste waters.

Chemical states of fallout radioactive Cs in the soils deposited at Fukushima Daiichi Nuclear Power Plant accident

The chemical states of radioactive Cs (caused by Fukushima Daiichi Nuclear Power Plant accident) in the contaminated soils have been characterized by the desorption experiments using appropriate reagent solutions and size fractionation of the contaminated soils. More than 65% of radioactive Cs remained in the residual fraction of the soil samples after treatment of 1 mole L−1 NH4Cl solution and 1 mole L−1 CH3COOH solution. Approximately 70% of radioactive Cs in the residual fraction were associated with the size fractions larger than the elutriated one, even though mica-like minerals were present in the elutriated one. These results strongly suggest that radioactive Cs was irreversibly associated with soil components other than mica-like minerals in the contaminated soil.

Adsorption behavior of radioactive cesium by non-mica minerals

We studied the adsorption behavior of radioactive cesium (Cs) by the non-mica minerals kaolinite, halloysite, chlorite, montmorillonite, mordenite, MnO2, TiO2, Al2O3, and FeOOH to elucidate the environmental behavior of radioactive Cs fallout from the Fukushima Daiichi Nuclear Power Plant in the Tohoku region of Japan. The adsorption and desorption experiments of Cs on the minerals were carried out at the Cs concentrations 1 × 10−4, 1 × 10−5, and 2 × 10−9 mole L−1 at pH 5.5. The desorption of Cs from the minerals was examined using 0.1 mole L−1 LiCl, NaCl, KCl, RbCl, and CsCl solutions. The sequential desorption was examined using a 0.1 mole L−1 LiCl solution, a 1 mole L−1 KCl solution, and a 1 mole L−1 HCl solution. The distribution coefficient (K d) for the minerals at the Cs concentration 10−9 mole L−1 was in the order of mordenite > illite > montmorillonite, sericite, MnO2, kaolinite, and halloysite > chlorite, TiO2, Al2O3, and FeOOH, differing from the order observed at higher Cs concentrations. After the sequential desorption by the three reagent solutions, the residual fraction of Cs was higher at the Cs concentration 10−9 mole L−1 than at higher concentrations. Approximately 40%, 40%, 50%, and 25% of the adsorbed Cs were residual in montmorillonite, mordenite, MnO2, and kaolinite, respectively, after the sequential desorption. These results strongly suggest that (1) radioactive Cs at 10−9 mole L−1 is more strongly associated with the non-mica minerals than at higher concentrations of 1 × 10−4 and 1 × 10−5 mole L−1, and (2) the non-mica minerals montmorillonite, mordenite, kaolinite, and MnO2 contributed to the fixation of the radioactive Cs fallout on Fukushima soil.

Characterization of saline groundwater at Horonobe, Hokkaido, Japan by SEC-UV-ICP-MS: Speciation of uranium and iodine

The saline groundwater collected at a depth of about 500 m in Horonobe, Japan, where an underground research laboratory (URL) has been built, is rich in saline (Na 4900 ppm, Cl 7600 ppm), iodine (42 ppm), and methane gas. We analyzed the colloids and ions of this groundwater mainly by employing a size exclusion chromatography (SEC) coupled on-line to ultraviolet-visible (UV-Vis) detection and inductively coupled plasma mass spectrometry (ICP-MS) technique and focused on the speciation of uranium and iodine, both of which are of particular importance for radioactive waste disposal. For this purpose, the groundwater sample was introduced to SEC columns after being passed through a 0.45 μm filter but without further pretreatment, such as isolation of colloids. The chromatographic profiles obtained with two different SEC columns were compared. This study revealed that uranium present in the groundwater at several tens of ppt was associated with low molecular weight silica species with neutral charge. The silica species were virtually free of metal elements such as Na, K, Mg, Ca, and Al. This study also found that almost all of the iodine in the groundwater was iodide (I(-)). The groundwater contained an unidentified organic colloid that was not a carrier for the radioactive waste-relevant elements Se, Sr, I, Cs, Th, and U.

Selenium oxyanions: Highly selective uptake by a novel anion exchanger

We report the extremely high and selective uptake of selenium oxyanions by a novel anion exchanger, Ni 1 - x Zn 2 x (OH) 2 (OCOCH 3 ) 2 x . nH 2 O (0.15 < x < 0.25). The tested Ni-Zn basic salt (x = 0.24) exhibited very high selectivity for Se(IV) [K d = 9.0 x 10 4 cm 3 /g with an initial Se(IV) concentration of 1 x 10 - 4 M] in the presence of 0.1 M Cl - solution. The uptake of Se(IV) on the Ni-Zn basic salt was irreversible when treated with solutions containing 1 N Cl - , 1 N NO 3 -, or 1 N PO 4 3-. This novel exchanger also showed high K d (2.6 x 10 3 cm 3 /g) for Se(VI), and therefore it is expected to be useful for decontamination and removal of selenium oxyanions from contaminated water.

Characterization of Fe-montmorillonite: A Simulant of Buffer Materials Accommodating Overpack Corrosion Product

Candidate materials as engineered barriers for high-level radioactive waste disposal include carbon steel for the overpack and bentonite for the buffer material. If these materials are selected, corrosion products from the overpack will migrate into the compacted bentonite. 1–3) Potential diffusing species of the corrosion products include Fe 2+ ions. The Fe2+ ions will be sorbed on montmorillonite, which is the major mineral component of bentonite. Some Fe 2+ ions may form more stable hydroxides and oxides. Radioactive nuclides leached from waste forms will migrate in such altered montmorillonite. An anticipated shortcoming of the alteration of montmorillonite is degradation of swelling property, which is liable to cause acceleration of nuclide migration. On the other hand, an expected advantage may be the immobilization of nuclides with high sorption affinities for iron compounds. However, the characteristics of such altered montmorillonite including retentivity for radioactive nuclides have yet to be clarified. Here we briefly report characterization of homoionic Fe 2+montmorillonite (Fe-montmorillonite) and its oxidized product that do retain Se(VI) for which natural montmorillonite such as Na +-type and Ca 2+-type has little retentivity. We used Fe-montmorillonite as a simulant of the buffer material in which Fe2+ ions diffused.

Effects of Citrate, NTA, and EDTA on the Reduction of U(VI) by Shewanella putrefaciens

Reduction of U(VI) by Shewanella putrefaciens in the presence of citrate, NTA, and EDTA was examined to elucidate the effects of strong complexation agents on biological reduction. Uranium(VI) and lactic acid were supplied as the sole electron acceptor and donor, respectively, under anaerobic conditions at 30°C at initial pH 7.0. In the control medium containing no citrate, NTA, or EDTA, the concentration of dissolved U decreased with time and precipitates appeared. X-ray absorption near-edge structure analysis showed that the precipitates contained U(IV). In the presence of citrate, NTA, or EDTA, the concentrations of dissolved U scarcely changed. The UV-VIS spectra of the media showed that concentrations of U(VI) species decreased and those of U(IV) species increased with time. These results indicate that the presence of strong complexation agents inhibits biological reductive UO2 precipitation by forming soluble U(IV)-organic complexes. The initial reduction rate of U(VI) in the citrate medium in which polynuclear U(VI)-citrate complexes were formed was much slower than those in the NTA and EDTA media. Formation of the polynuclear complexes may be one of the reasons for retardation of the U(VI) reduction by S. putrefaciens. Our results indicate that the presence of strong complexation agents affects the chemical species of U(IV) and the reduction rate of U(VI).

Observation of Microstructures of Compacted Bentonite by Microfocus X-Ray Computerized Tomography (Micro-CT)

In this study, nondestructive, three-dimensional images of the internal microstructures of compacted bentonite samples in dry and water-saturated states were examined with the micro-CT.

Association of Actinides with Microorganisms and Clay: Implications for Radionuclide Migration from Waste-Repository Sites

We conducted a series of basic studies on the microbial accumulation of actinides to elucidate their migration behavior around backfill materials used in the geological disposal of radioactive wastes. We explored the interactions of U(VI) and Pu(VI) with Bacillus subtilis, kaolinite clay, and within a mixture of the two, directly analyzing their association with the bacterium in the mixture by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The accumulation of U by the mixture rose as the numbers of B. subtilis cells increased. Treating the kaolinite with potassium acetate (CH 3 COOK) removed approximately 80% of the associated uranium while only 65% was removed in the presence of B. subtilis. TEM-EDS analysis confirmed that most of the U taken from solution was associated with B. subtilis. XANES analyses revealed that the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both was U(VI). The amount of Pu sorbed by B. subtilis increased with time, but did not reach equilibrium in 48 h; in kaolinite alone, equilibrium was attained within 8 h. After 48 h, the oxidation state of Pu in the solutions exposed to B. subtilis and to the mixture had changed to Pu(V), whereas the oxidation state of the Pu associated with both was Pu(IV). In contrast, there was no change in the oxidation state of Pu in the solution nor on kaolinite after exposure to Pu(VI). SEM-EDS analysis indicated that most of the Pu in the mixture was associated with the bacteria. These results suggest that U(VI) and Pu(VI) preferentially are sorbed to bacterial cells in the presence of kaolinite clay, and that the mechanism of accumulation of U and Pu differs. U(VI) is sorbed directly to the bacterial cells, whereas Pu(VI) first is reduced to Pu(V) and then to Pu(IV), and the latter is associated with the cells. These results have important implications on the migrations of radionuclides around the repository sites of geological disposal. Microbial cells compete with clay colloids for radionuclides accumulation, and because of their higher affinity and larger size, the microbes accumulate radionuclides and migrate much slower than do the clay colloids. Additionally, biofilm coatings formed on the fractured rock surfaces also accumulate radionuclides, thereby retarding radionuclide migration.