Adsorption of Cesium Ion on Various Clay Minerals and Remediation of Cesium Contaminated Soil in Japan

Transcription

1 Resources Processing 60 : (2013) Original Paper RESOURCES PROCESSING Adsorption of Cesium Ion on Various Clay Minerals and Remediation of Cesium Contaminated Soil in Japan Toyohisa FUJITA*, Li Pang WANG, Koji YABUI, Gjergj DODBIBA, Katsunori OKAYA, Seiji MATSUO and Kiyoshi NOMURA The University of Tokyo, Graduate School of Engineering, Hongo, Bunkyo-ku, Tokyo , Japan Abstract It has been estimated that as much as 29 million m 3 of contaminated soil and fallen leaves should be removed and stored in interim storage facilities as a result of the Fukushima nuclear crisis that followed the earthquake in In this research, a soil remediation method was investigated in order to reduce the intermediate storage. In other words, screening and washing of cesium (Cs) contaminated soil, as well as Cs adsorption onto various clay minerals have been tested. It was found that screening and washing were effective in reducing radioactivity; clay minerals such as zeolite, biotite, montmorillonite and muscovite adsorbed Cs well. In addition, flotation can be applied to these minerals, so they can be removed from soil, which would lead in further reduction of radioactivity. If the radioactivity of contaminated soil can be reduced sufficiently, the soil can return to the nature. Key words: Cesium, Adsorption, Clay mineral, Cesium contaminated soil, Remediation 1. Introduction On March 12 th 2011, Fukushima nuclear crisis occurred following the off shore Pacific Ocean Earthquake. As a result, radioactive substances, such as Cs, I, Sr and Pu leaked out and contaminated the soil. The distribution of cecium-134 and 137 of east Japan on October 13 th, is shown in Fig. 1. Cesium (Cs) was spread in the vast area of east Japan by the wind. The government judged that the Cs contamination was the most problematic issue and set the guidelines for cleaning the contaminated soil. It has been estimated that as much as 29 million m 3 of soil and fallen leaves should be removed and stored in interim storage facilities. The concept for safekeeping facility of Cs contaminated soil is shown in Fig. 2 that is submitted by the Ministry of the Environment in Japan 2. The containers with Cs-contaminated soil are piled up from more than 30 cm deep to the surface and the area is guarded by fences. However, the amount is Paper presented at the 10 th Japan/Korea International Symposium on Materials Science and Resources Recycling, May 2012, Doejeon Korea Accepted 29 January, 2013 * tfujita@sys.t.u-tokyo.ac.jp huge and therefore a vast storage space is required. The exposed radiation to the human being should be less than 1 msv/year (0.11 µsv/hr). Also the food standard containing Cs started from April in 2012, that is, the general food and drinking water should be less than 100 Bq/kg and 1 Bq/kg, respectively 2. The Cs-137 is accumulated on the surface of soil for a long time. The soil on grounds containing Cs has to be removed from 5 cm depth to the surface. In this research, the remediation method of soil on the field and ground to reduce the intermediate storage was put forward and its effectiveness was investigated experimentally, in other words screening and washing of Cs contaminated soil, as well as Cs adsorption onto various minerals. The flotation of the ground soil is performed to investigate the possibility of minerals separation. 2. Experiment and Results 2.1 Sieving and washing of soil The flow sheet for Cs contaminated soil of washing process is shown in Fig. 3. The soil collected from the ground is first sieved and the fine particle fraction enters the water washing process 13

2 FUJITA, WANG, YABUI, DODBIBA, OKAYA, MATSUO and NOMURA Fig. 3 The flow sheet for washing of Cs contaminated soil Fig. 1 Distribution of cecium-134 and 137 of east Japan on October 13 th, 2012 (By Asahi Shinbun) Fig. 2 Concept for safekeeping facility of Cs contaminated soil (From the Ministry of the Environment in Japan) because the Cs concentration of soil increases proportionally to the specific surface area of soil particles. Water washing can remove Cs ion from the surface of soil because Cs easily dissolves in water. Next, the some minerals adsorbing large amount of Cs can be separated from low Cs adsorbed minerals by flotation. Sometimes, magnetic separation is also effective. Low Cs concentration soil can be returned to the area where it was collected. The effect of sieving on the radioactivity of the as-received Cs contaminated soil is shown in Fig. 4. The soil was collected in the field of Towa town of Fukushima prefecture and the radiological dosage was 0.25 µsv/h on 25 th, January The 14 Fig. 4 Sieving result of radioactivity of as received Cs contaminated soil large particle fraction of more than 105 µm is about 80% and the radioactivity is lower than 200 Bq/kg. While the fine fraction of less than 105 µm is more than 400 Bq/kg. The sieving of 105 µm is very effective. The radiological dosage RESOURCES PROCESSING

3 Adsorption of Cesium Ion on Various Clay Minerals and Remediation of Cesium Contaminated Soil in Japan Fig. 5 Washing by water of as-received Cs contaminated soil (Water temperature 288 K, Dried soil after one hour stirring) Fig. 6 Langmuir and Freundlich isotherms: adsorption of Cs by zeolite (Experimental conditions: Solution: water; Adsorbent s dosage in solution: 0.1 g/ L; Contact time; 120 min; Initial ph: 7.5; Particle size: µm; Temperature: 288 K) was measured by Becquerel monitor (Berthold LB200). The result of washing by water of the asreceived Cs contaminated soil is shown in Fig. 5. The soil and water suspension is stirred for one hour at the temperature of 288 K. The radioactivity of washed soil reduced 40% comparing no washed soil as the Cs ion is dissolved in water. After washing, the concentrated Cs ion in water can be removed by the adsorbents like zeolite, ferro cyanide compounds, etc. 2.2 Adsorption of Cs onto various minerals In the soil there are many kinds of minerals, organic materials, carbon and water. If the large adsorbent minerals of Cs are removed from the soil, the radioactivity of soil will reduce. In this experiment, the various materials are ground and measured the Cs adsorption isotherms. The chemical components of materials are listed in Table 1. The particle size of each sample ranged from 74 µm to 149 µm. The ph condition of adsorption is regulated at 7.5. After 2 hours of continuous agitation the remaining concentration of Cs was measured. Fig. 6 shows Cs adsorption Langmuir and Freundlich isotherms on zeolite. The maximum Cs adsorption capacity as indicated by the Langmuir isotherm was 196 mg/g. Fig. 7, on the other hand, shows Cs adsorption Langmuir and Freundlich isotherms on biotite. The maximum Cs adsorption rate by Langmuir isotherm was 51 mg/g. As well as these minerals, the largest Cs adsorption by Langmuir isotherm was investigated on the other materials. Isotherm parameters obtained from adsorption of Cs using various materials are listed in Table 2. Zeolite has Table 1 Chemical components of various minerals used for Cs adsorption Zeolite (wt.%) SiO 2 Al 2O 3 CaO Fe 2O 3 K 2O Biolite K(Mg,Fe) 3(Al,Fe)Si 3O 10(OH,F) 2 Montmorillonite (Na,Ca) 0.33(Al,Mg) 2Si 4O 10(OH) 2 nh 2O Muscovite KAl 2(AlSi 3)O 10(OH) 2 Kaolinite Al 4Si 4O 10(OH) 8 Magnetite Fe 3O 4 Sodium feldspar NaAlSi 3O 8 Calcite CaCO 3 Hematite Fe 2O 3 Carbon from zelkova C Quartz SiO 2 15

4 FUJITA, WANG, YABUI, DODBIBA, OKAYA, MATSUO and NOMURA Fig. 7 Langmuir and Freundlich isotherms: adsorption of Cs by biotite (Experimental conditions: Solution: water; Adsorbent s dosage in solution: 0.1 g/ L; Contact time; 120 min; Initial ph: 7.5; Particle size: µm; Temperature: 288 K) 2.3 Purification of soil by flotation The soil collected from school ground in Fukushima prefecture is dried and put into the Denver type flotation cell. The soil size was less than 100 µm and the pulp density was 1 wt%. Flotation conditions were: ph 2.5, the surfactant dosage was 1 kg/ton of DAA. Silica does not float whereas mica (muscovite and biotite) floats in those conditions 3 to reduce the radioactivity that adsorbs Cs. The chemical component of each fraction measured by XRF is listed in Table 3. The Si and Ca concentration in tailings are larger than ones in froth. On the other hand, Fe percentage in the tailing is smaller than one of froth. The result seems that the silica does not float well while the mica and iron oxide float. Furthermore, according to the concentration criterion for gravity concentration 4 : the largest Cs adsorption ability of about 200 mg/ g. Next, biotite, montmorillonite and muscovite also have high Cs adsorption ability of about 50 mg/g. The feldspar, hematite, calcite and carbon shows small adsorption amount of approximately 30 mg/g. The Cs adsorption on quarts is the smallest of 13 mg/g. Therefore, if zeolite, biotite, montmorillonite and muscovite were removed from other materials, the Cs content in soil could be reduced. (D h -D f ) / (D l -D f ) where D h is the specific gravity of the heavy minerals, D l is the specific gravity of the light minerals, and D f is the specific gravity of the fluid medium. Gravity concentration is relatively easy when the quotient is greater than 2.5, and is not generally commercially feasible when the quotient is below Since the specific gravity of quartz is 2.6 whereas zeolite is 1.8 (measured by pycnometer), the concentration criterion of quartz Table 2 Isotherm parameters obtained from adsorption of Cs using various minerals (Particle size: µm) Langmuir Freundlich Table 3 R 2 q max (mg/g) K a (L/mg) R 2 1/n K F Zeolite Quartz Kaolinite Muscovite Biotite Hematite Magnetite Calcite Sodium feldspar Montmorillonite Carbon from zelkova Flotation result of Cs contaminated soil (Experimental conditions: ph 2.5, DAA 1 kg/ton, Soil 100 µm, Pulp density 1 wt.%) wt.% Al Si P K Ca Ti Mn Fe Froth Tailing As received soil RESOURCES PROCESSING

5 Adsorption of Cesium Ion on Various Clay Minerals and Remediation of Cesium Contaminated Soil in Japan and zeolite when water is used as the fluid medium is (2.6 1) / (1.8 1) = 2, which suggests that gravity concentration might also be applied to separate them. 3. Conclusions The remediation of cesium-contaminated soil in Japan was investigated, aiming to reduce the volume of the Cs contaminated soil. At first, to eliminate the Cs concentration in the soil, the sieving with a 105 µm screen and the washing with water are effective. Next, the adsorptions of materials are measured. The zeolite, biotite, montmorillonite, muscovite showed higher Cs adsorption capacity. On the other hand, the adsorption capacity on quartz was relatively small. The flotation by DAA as surfactant at ph 2.5 was effective in removing larger Cs adsorbed minerals. Acknowledgments The present study was supported in part by the 21st Global COE program, Mechanical Systems Innovation, Ministry of Education, Culture, Sports, Sciences and Technology of Japan, as well as the JST CREST of Innovative Technology and System for Sustainable Water Use Research Area (Shibusawa group). References 1. Asahi Shinbun, October 13 th, Home page of the Ministry of Environment: K. Tomita: Mineral processing of nonmetal minerals, Yogyo Kyokai, pp (1974) 4. B.A. Wills: Mineral processing technology, 3 rd edition, pp. 302 (1985) 17