Evaluation of adsorption properties of U(VI) for various inorganic adsorbents

Large amounts of highly contaminated water over 800,000 m³ accumulated in the reactor, turbine building and the trench in the facility were generated from the nuclear accident of Fukushima NPS (BWR) caused by the Great East Japan Earthquake. At present, the cold shutdown is completed stably by the circulating injection cooling system (SARRY, KURION) for the decontamination of radioactive nuclides such as¹³⁴Cs and¹³⁷Cs using zeolites and crystalline silicotitanate (CST). Further, the Advanced Liquid Processing System (ALPS) is under operation for the decontamination of 62 nuclides such as⁹⁰Sr,¹²⁹I and⁶⁰Co, etc. However, the adsorption behaviors of actinoids through the decontamination systems are complicated, and especially their adsorption properties for zeolites and CST, major inorganic adsorbents, are not yet clarified. In near future, the decontamination of actinoids leached from the crushed fuel debris will be an important subject. In this study, the practical adsorption properties of U(VI) for various inorganic adsorbents were evaluated under different solution conditions. The adsorption properties (distribution behaviors and adsorption kinetics) were evaluated by batch adsorption method; 19 kinds of inorganic adsorbents including zeolites and CST (crystalline silicotitanate) were contacted with U(VI)) solutions. The conditions of 5 kinds of U(VI) solutions were as follows; Solution 1: [U(VI)]= 50 ppm, initial pH= 0.5∼ 5.5 Solution 2: [U(VI)]= 50 ppm, [NaCl]= 0.1 M, initial pH= 4.0 Solution 3: [U(VI)]= 50 ppm, [CaCl2]= 0.1 M, initial pH= 4.0 Solution 4: [U(VI)]= 4.84 mM, [NaCl]= 0.1 M, initial pH= 3.18 Solution 5: [U(VI)]= 4.86 mM, 2,994 ppm boric acid/30% seawater, initial pH= 4.25 The uptake (%) and distribution coefficient (Kd. cm³/g) were estimated by counting the radioactivity using NaI(Tl) scintillation counter and liquid scintillation counter. In the simple Solution 1, the Kd values for zeolites increased linearly with equilibrium pH up to pH 7. The Kd value for tin hydroxide had a maximum profile around pH 7 and a relatively large Kd value above 10⁴ cm³/g was obtained. In the presence of NaCl and CaCl2 (Solution 2 and 3), relatively large Kd values above 10² cm³/g were obtained, other than mordenite and clinoptilolite, and the effect of [Ca²⁺] on U(VI) uptake was larger than that of [Na⁺]. In Solution 4 containing high concentration of U(VI), the uptake(%) was considerably lowered, while that for zeolite A, X and Y was estimated over 20%. Similar tendency was observed in Solution 5, and, in the case of granulated potassium titanate, yellow precipitate was observed on the surface due to the increase of equilibrium pH up to 5.25. The adsorption behavior of U(VI) on inorganic adsorbents is mainly governed by three steps; ion exchange, surface precipitation of hydrolysis species and sedimentation depending on equilibrium pH, and hence it should be noted the change of U(VI) chemical species. These basic adsorption data are useful for the selection of inorganic adsorbents in the Fukushima NPS decontamination process.