Electrodriven Selective Transport of Cs+ Using Chlorinated Cobalt

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Electrodriven Selective Transport of Cs+ Using Chlorinated Cobalt Dicarbollide in Polymer Inclusion Membrane: A Novel Approach for Cesium Removal from Simulated Nuclear Waste Solution Sanhita Chaudhury, Arunasis Bhattacharyya, and Asok Goswami* Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India S Supporting Information *

ABSTRACT: The work describes a novel and cleaner approach of electrodriven selective transport of Cs from simulated nuclear waste solutions through cellulose tri acetate (CTA)/poly vinyl chloride (PVC) based polymer inclusion membrane. The electrodriven cation transport together with the use of highly Cs+ selective hexachlorinated derivative of cobalt bis dicarbollide, allows to achieve selective separation of Cs+ from high concentration of Na+ and other fission products in nuclear waste solutions. The transport selectivity has been studied using radiotracer technique as well as atomic emission spectroscopic technique. Transport studies using CTA based membrane have been carried out from neutral solution as well as 0.4 M HNO3, while that with PVC based membrane has been carried out from 3 M HNO3. High decontamination factor for Cs+ over Na+ has been obtained in all the cases. Experiment with simulated high level waste solution shows selective transport of Cs+ from most of other fission products also. Significantly fast Cs+ transport rate along with high selectivity is an interesting feature observed in this membrane. The current efficiency for Cs+ transport has been found to be ∼100%. The promising results show the possibility of using this kind of electrodriven membrane transport methods for nuclear waste treatment.

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INTRODUCTION Cs is one of the most abundant fission products present in nuclear waste. It is of major environmental concern because of the long half-life (30 y) and high gamma energy (662 keV) of its radioisotope 137Cs. Recovery of Cs is required for its industrial use as gamma irradiators and removal of this radionuclide simplifies subsequent waste handling and storage in geological repositories. Once released, the radioactive cesium isotopes persist in the environment, with the potential to cause adverse health effects. Transport of Cs from the simulated Hanford tank waste leachate through sands/sediments has been studied.1−3 Liu et al.1 have reported the effect of temperature on Cs+ sorption and desorption in subsurface sediments at the Hanford Site, U.S.A. Parajuli et al.4 have studied the effective removal of Cs from ash samples (in Fukushima) by methods like washing with water or acid treatment at high temperature. Presence of bulk concentration of Na+ in the nuclear waste solution is the major challenge in recovery/removal of Cs+. Literature reports on separation of Cs from nuclear waste solutions include the use of number of techniques viz., precipitation with sodium phosphotungstic acid,5 ion exchange with silicotitanates,6 phosphomolybdates7 and solvent extraction using different macrocyclic ionophores,8,9 protonated form of the hexachlorinated derivative of cobalt bis (dicarbollide) (HCCD).10−14 The selective adsorption/extraction of Cs+ over Na+ using polyphenol enriched biomass based adsorbents or some macrocyclic carrier based solvent extraction methods has also been studied.15−18 In order to achieve quantitative © 2014 American Chemical Society

separation of Cs from nuclear waste solution, though, several solvents (nitrobenzene, FS-13)10−14 and macrocyclic ionophores (bis(octyloxy) calix[4]arene-monocrown-6, calix[4]-bis2,3-naptho-crown-6)8,9 have been synthesized, but high cost of their synthesis and purification necessitates the need for exploring the methods which requires low ligand inventory. Other major disadvantage associated with the conventional separation methods (solvent extraction, ion exchange) is the generation of large amount of secondary waste. In general, membrane based separations are advantageous over other separation methods in terms of low ligand requirement and low secondary waste generation. Among different types of membrane, the liquid membranes (bulk liquid membrane, supported liquid membrane-SLM and polymer inclusion membranes-PIM) have drawn attention of the researchers because of the scope of carrier mediated selective transport of species of interest. The problem of solvent leaching from the porous support in SLM can be overcome by incorporating the ligand in the polymer matrix, that is, PIM, resulting in higher lifetime of the membrane.19 Use of different macrocyclic and macromolecular carrier based PIMs20−23 or SLMs24 for facilitated transport of Cs+ are well reported in literature. The major drawback associated with these ligands is Received: Revised: Accepted: Published: 12994

July 28, 2014 October 7, 2014 October 9, 2014 October 9, 2014 dx.doi.org/10.1021/es503667j | Environ. Sci. Technol. 2014, 48, 12994−13000

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potentials. The current efficiency for the transport process has been found out from the simultaneous measurement of the current profile and Cs+ transport rate. The stability of this membrane in terms of leaching of carrier molecule has been studied. The novelty of the method in terms of cation transport rate vis-à-vis selectivity is discussed.

the requirement of anion for charge compensation of the metal ion. Literature reports25−29 are available where the charge compensation of the metal ions has been achieved by incorporating anionic functional groups either in the extractant or in the polymer backbone of the membrane. In our earlier work, cation driven loading30−32 of crown ethers within Nafion117 cation exchange membranes has been done to make Cs+ ion selective membranes. In this case, however, the Cs+ selectivity (from crown ether) and anion for charge compensation (−SO3− groups in Nafion) are provided by two different moieties. CCD− is an anion, which is highly selective for Cs+. The two requirements, viz. charge compensation and selectivity can therefore be fulfilled by this single moiety (CCD−). It is also reported to display good stability toward radiation.10 The solvent extraction of Cs+ using protonated form of CCD− (HCCD) as extractant and trifluoromethylphenyl sulfone (FS13)12,14 or nitrobenzene13 as the solvent is well reported in literature. Selective transport of cesium using HCCD in SLM have been reported with a decontamination factor (DF, defined as the ratio of product to impurity in the receiver divided by that in the feed) of ∼300 over different transition metal ions.33 One of the major disadvantage of this study is the requirement of 8−9 M HNO3 for stripping. Moreover; neither of these membrane based studies20−24,33 have addressed the selective separation of Cs+ from Na+ ions from nuclear waste solution. In general, the cation transports through carrier facilitated ion exchange membrane requires the counter transport of another cation from receiver to feed compartment to maintain electrical neutrality. Alternatively, use of 8−9 M HNO3, as strippant in the receiver phase, is reported in the literature.33 If an electric field is applied across the membrane, the anion in the feed compartment is oxidized at the anode and the cation is forced to move to the receiver compartment without requiring any counter transport. The use of electric field, thus, eliminates the need of adding salt or stripping agent in the receiver side and also enhances the cation transport rate. The membrane based analyte preconcentration/separation method under the influence of electric field (electromembrane extraction) is gaining increasing attention.32,34−38 However, mutual separation of ions of similar charge state by electrodriven transport is difficult. In our earlier work,32,39 an attempt was made to impart selectivity for Cs+ transport by using ion selective reagents (crown ether, inorganic ion exchanger) in the membrane and the corresponding DF for Cs+ over Na+ in the receiver phase has been found to be low. It is also to be mentioned that electro-driven transport is expected to be more effective with ionic carriers like HCCD as compared to any other chelating or neutral macrocyclic ligands. In the present work, an attempt has been made to use HCCD as a carrier in 2-nitro-phenyl-octyl ether (NPOE) plasticized cellulose tri acetate (CTA)/poly vinyl chloride (PVC) based PIM to achieve electrodriven selective separation of Cs+ from Na+ and other metal ions present in the nuclear waste solution. In view of the different possible compositions of the nuclear waste solutions, the transport selectivity have been studied with different feed compositions of the simulated nuclear waste solutions. The selectivity of the membrane for removal of Cs from other metal ion has been determined using radiotracer and inductively coupled plasma- atomic emission spectroscopic (ICP-AES) measurement. In order to obtain the minimum applied potential required for efficient transport, the electrodriven transport has been studied under varied applied

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EXPERIMENTAL SECTION Reagents and Chemicals. HCCD has been procured from Katchem, Czech Republic. Radiotracers 22Na, and 137Cs used in the present study have been obtained from Board of Radiation and Isotope Technology, Mumbai, India. All other reagents which have been used are of analytical reagent grade. The 0.1 M salt solutions have been prepared by dissolving a known amount of the respective salt in 250 mL deionized water. Preparation of Membrane. A mixture of 0.08 g CTA, 5 mg HCCD and 0.2 mL of NPOE has been dissolved in 5 mL dichloromethane and homogenized by sonication. The solution has been poured into a flat Petri dish and allowed to evaporate at room temperature. The resulting PIM has been peeled out by spreading a few mL of water on it and subsequently used for the transport studies. The carrier concentration in the membrane matrix is limited by the solubility of HCCD in the plasticizer NPOE. Maximum 5 mg (0.01 mili moles) HCCD can be dissolved in 0.2 mL NPOE, that is, the maximum weight percentage of the carrier is 1.71. Increase in the amount of the NPOE resulted in overplasticization of the membrane. The PVC based membrane has been prepared using 0.28 g PVC, 5 mg HCCD and 0.2 mL NPOE. Tetra hydro furan (10 mL) has been used as the solvent for this preparation. The thickness of the synthesized CTA and PVC based membranes, measured using digimatic micrometer (Mitutoyo Corporation, Japan), are 41 and 48 μm, respectively. Transport Studies with CTA based PIM. Two compartment electrodriven permeation experiments using CTA based membranes for different feed compositions have been carried out. Electric field has been applied across the membrane using two Pt electrodes and the current has been monitored using a digital multimeter. The active surface area of the membrane is 1.77 cm2. The volumes of the feed and the receiver compartment are 32 and 3.2 mL respectively. In order to achieve volume reduction in the receiver compartment, the volume ratio of feed to receiver compartment has been kept at 10. Deionized water spiked with NaOH (0.004 mmol) has been used in the receiver compartment. The solutions in both the compartments have been stirred continuously to avoid any film controlled diffusion at the membrane interface. The cations (Na+/ Cs+) in the feed compartment have been tagged with 22 Na and 137Cs tracer. The amount of Na+ and Cs+ transferred from the feed to the receiver side has been monitored by taking out 200 μL of aliquots from both the compartments at regular time intervals and counting the 22Na and 137Cs activity in high purity germanium (HPGe) detector. The minimum detection limit40 for the gamma measuring system, used in the present work, is 0.07 counts/s in the 500−700 keV energy range and the count rates for all the samples are well above this limit. In order to obtain the minimum applied potential required for significant Cs+ transport rate, the permeation experiments have been carried out at different applied potentials in the range of 1−7 V. A mixture of 0.0025 M NaCl and 0.0025 M CsCl has been used in the feed compartment. The cation transport rate has been monitored by counting the radiotracers. It has been observed that at applied potential in the range of 1−2 V, the 12995

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Cs+ transport rate is very slow, while the rate increases significantly at 3 V. The data indicates that further increase in potential has resulted in loss of Cs+ selectivity while there is no significant gain in time for ∼100% cation transport. Thus, the rest of the experiments have been carried out at a working applied potential of 3 V. Experiments have also been carried out with pure 0.0025 M NaCl and 0.0025 M CsCl in the feed compartment and tagging the cation with respective radiotracers to obtain the current efficiency. In order to study the transport selectivity at different neutral feed compositions, two compartment permeation experiments have also been carried out with varying proportions of Cs+ and Na+ (1:1 and 1:25) in the feed. For these experiments the CsCl concentration (0.0025 M) has been kept constant and the NaCl concentration (0.0025 M/0.0625 M) has been varied. In order to obtain the transport profiles, the cations (Na+/ Cs+), they have been tagged with their corresponding radiotracers in the feed compartment. Transport experiment with the CTA based PIM has also been carried out for a solution having the same Cs+ to Na+ ratio as that in the effluent solution (SRELW) of resorcinol formaldehyde polycondensate resin (intermediate level nuclear waste treated with the resin). The feed composition has been kept at 0.1 M NaNO3 and 3.5 × 10−4 M CsNO3 in 0.4 M HNO3,41 while in the receiver compartment 3.2 mL deionized water spiked with NaOH (0.004 mmol) has been used. The transport rates of the cations (Cs+/ Na+) have been monitored by the same method as described in the previous section and the acid transport has been monitored by acid−base titration. In order to obtain the transport profile of salt and HNO3, the same experiment has been repeated at zero applied potential. The salt transport has been monitored by measuring the gamma activity of 137Cs and 22Na in the receiver compartment. The stability of the CTA based membrane has been studied using UV−visible spectrophotometry. Transport Studies with PVC-Based PIM. The experiment to study the transport selectivity of Cs+ from a SHLW42 has been carried out at potential using HCCD as carrier in a PVC based PIM. The composition of the SHLW has been determined from ICP-AES measurement and is given in Table 1. The SHLW solution is in 3 M HNO3. It can be seen from the table that apart from cesium, the initial feed solution contains high amount of sodium, manganese, potassium, and iron. For this study, a two compartment (each of 25 mL) glass cell with the membrane active surface area of 5.3 cm2 has been used. The receiver compartment contains 25 mL deionized water spiked with NaOH (0.004 mmol). The transport of the Cs+ ion has been monitored using radiotracer technique, while the transport of other cations have been monitored by atomic emission spectroscopy using Jobin−Yvon Ultima high resolution ICP-AES. The acid transport has been monitored by acid−base titration.

Table 1. Initial Feed Concentrations and Final Receiver Concentrations of Different Elements Present in the SHLW42 Feed Solution, along with the Corresponding DF of Cs over these Elements element

initial feed conc. (mg/L)

final receiver Conc. (mg/L)

DF of Cs

Cs Na K Cr Mn Fe Sr Zr Mo Ba Y La Ce Pr Nd Sm

220 4620 220 112 480 690 28 84 62 23 60 168 55 76 92 45

220 59 BDLa BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL

80 >2200 >1120 >4800 >6900 >284 >835 >618 >232 >600 >1680 >550 >757 >920 >453

a

BDL: Below Detection Limit of ICP-AES (0.1 mg/L).

Figure 1. Basic principle of electrodriven cation transport through PIM.

about the limiting current density as well as the membrane resistance. This resistance includes membrane resistance (Rmem), diffusion boundary layer resistance (RDBL), as well as electrical double layer resistance (RDL). A detailed knowledge of the individual components can be obtained from impedance measurement and is important in designing and optimizing electromembrane process. This has been reported in greater detail by several researchers.44−47 These experiments have been carried out with conventional ion exchange membranes (Neosepta) for which the Rmem is of the order of few Ω and is comparable to RDL and RDBL. However, the Rmem for the PIMs are reported to be much higher (of the order of kΩ).48,49 In the present work, the Rmem for the CTA based PIM, as obtained from electrochemical impedance measurement, has been found to be 1 kΩ-cm2. The impedance measurement within the frequency region 1 MHz- 1 Hz has been carried out following the method as described in ref 50 and the impedance spectra for the CTA based PIM has been given in the Supporting Information (SI) Figure S3). With PIM, under stirring condition, the RDLand RDBLare expected to be negligible with respect to the high membrane resistance. Thus, in the

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RESULTS AND DISCUSSION CTA based PIM for Cs+ Transport from Low Acidic/ Neutral Feed Solution. The basic principle of electrodriven cation transport is shown in Figure 1. In presence of Cl− ion in the feed solution, the anodic reaction involves oxidation of Cl− (2Cl−→ Cl2 + 2e−),43 whereas for the experiments with NO3− in the feed solution, water is oxidized (2H2O → 4H+ + O2 + 4e−) at the anode in preference to NO3−. Ideally the measurement of potential drop across the membrane with varying current density provides information 12996

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This indicates that no unwanted process like water electrolysis contributes to the integrated current. The current efficiency for Cs+ transport from the feed containing 1:1 mixture of CsCl and NaCl (0.0025 M NaCl + 0.0025 M CsCl) has been obtained as 90%. Of the remaining, 1.8% has been accounted for the Na+ transport. The results of electrodriven selective transport of Cs+ from SRELW using the CTA based PIM of HCCD are summarized in Figure 3. The gamma spectra of the initial and final feed

present case, the electrodriven cation transport is expected to be solely governed by membrane resistance only. The cation (Cs+/ Na+) transport profiles for CTA based PIM for different Cs+ to Na+ ratio (1:1 or 1:25) in the neutral feed solution are shown in Figure 2. The error bars associated with

Figure 2. Cation (Cs+/ Na+) transport profiles obtained for CTA based PIM at 3 V for different neutral feed compositions. The ratios indicate the Cs+: Na+ ratio in the initial feed solution.

the experimental data points are of the size of the symbols. The high selectivity of the membrane for Cs+ over Na+ is evident from the figure, where for 1:1 mixture of CsCl and NaCl, >90% Cs+ is transported within ∼14 h, with negligible Na+ transport (1.8%). The DF for the separation of Cs+ over Na+ is calculated using the following relation r DF = 2 r1 where, r1 = ((CCS)feed)/((CNa)feed) and r2 = ((CCS)receiver)/ ((CNa)receiver), (CM)feed is the metal concentration in the initial feed solution and (CM)receiver is the metal concentration final receiver solution. Figure 2 also shows that the cation transport is unaffected inspite of wide variation in the feed composition. As shown in Table 2, the DF for different Cs+: Na+ feed compositions are nearly same (∼50). The pH of the both the compartments have been found to remain constant during the experiment.

Figure 3. Gamma spectra of the (a) initial (SRELW) and (b) final feed solution as well as (c) final receiver (93% Cs transport) solution, showing the selective transport of 137Cs over 22Na through CTA based PIM. The spectra have been normalized for the same counting time.

Table 2. Decontamination Factors (DF) of Cs+ over Na+ in Electrodriven (3 V) Transport Experiments Using PIMs no

base polymer

feed composition

DF (at >90% Cs transport)

I II III IV

CTA CTA CTA PVC

Cs: Na= 1:1(Neutral) Cs: Na= 1:25 (Neutral) SRELW (0.4 M HNO3) SHLW (3 M HNO3)

51 52 410 80

solution as well as final receiver solution, obtained for a fixed counting time are shown in Figure 3a, b, and c, respectively. Figure 3a shows that the intensities of the 511 keV of 22Na and 662 keV of 137Cs peaks in the initial feed solution are comparable. As seen in Figure 3c, the quantitative transport of 137 Cs (93%) as well as the negligible transport of 22Na (0.2%) in the receiver phase indicates excellent selectivity for Cs+ over Na+. This is again confirmed from the spectra of the final feed solution (Figure 3b). From the peak areas in the gamma spectra, the % cation transport and hence the DF, as shown in Table 2, has been calculated to be 410. During the experiment, the pH of the feed solution remained constant, while the pH of the receiver solution decreased from 7 to 1.0. It is also to be mentioned that, due to presence of large amount of Na+ as well as H+ (0.4 M HNO3), long time (59 h) is required for significant (93%) Cs+ transport and this has also been associated with 2.3% H+ transport. The H+ transport can be

The current efficiencies for the transport of Cs+ and Na+ have been determined by measuring the current along with the cation transport at different time intervals. The current profiles, obtained for CTA based PIM for different neutral feed compositions have been given in the SI (Figure S1). These have been used to obtain the current efficiency for the transport of Cs+ and Na+ from the feed compartment. The current efficiency has been found to be 95% and 100% for the feed solutions of 0.0025 M CsCl and 0.0025 M NaCl, respectively. 12997

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therefore, is a challenge for use of ion-exchange membrane in separating ions of similar charge. In our earlier work30−32 with Nafion-Crown ether composite membrane, it has been observed that high selectivity is achieved only at the cost of cationic mobility in the membrane.32 The results described in the present work indicate that HCCD-PIM, inspite of its high selectivity for Cs+, allows the cation to be transported within reasonable time under electric field. The work addresses the decontamination of Cs+ over Na+, which is the major challenge in nuclear waste treatment and has not been addressed in any membrane based separation methods. The use of stripping agent (8 M HNO3), as described in previous literatures, has been avoided by using electric field in the present study. The results of the present work indicate high DF for Cs+ over different metal ions as well as high current efficiency. From these aspects, this is indeed a unique, energy efficient and environment friendly approach for the separation of Cs+ from nuclear waste solution.

explained on the basis of leakage of HNO3 through the membrane. Otherwise, in the case of electrodriven H+ transport, the pH of the receiver phase would not have changed as the OH− generated at the cathode would have neutralized the transported H+. The leakage of HNO3 has been confirmed from the experiment at zero applied potential. The transport profiles of HNO3 along with that of CsNO3/ NaNO3 at zero applied potential have been shown in the SI (Figure S2), which shows that after 59 h, 2.4% of initial HNO3 in the feed has been transported to the receiver compartment. On the other hand, no 137Cs/22Na radioactivity has been observed in the receiver compartment (below the detection limit of gamma spectrometry), showing absence of the salt transport during the experiment. The lower DF in neutral feed solution as compared to that in the SRELW can be explained on the basis of presence of large amount of H+ in SRELW. In general, it has been observed from solvent extraction studies that the distribution coefficient (Kd) of Cs+ decreases with increasing HNO3 concentration12 with HCCD as extractant. However, no such study exists for Na+. HCCD being an acidic extractant, it is expected to show lower Kd (with increase in acidity) in case of Na+ also. Apparently, the increase in DF from neutral to 0.4 M HNO3 feed solution shows that the decrease in Kd is more prominent for Na+ than that for Cs+. The slower transport rate of Cs+ in SRELW as compared to the neutral solution can also be explained based on the decrease in Kd with increase in acidity. The pH dependence of the Cs+ transport rate, as shown in SI Table S2 also supports this. In order to study the carrier leaching from the membrane, the UV−visible spectra of CTA based membranes containing different weight% of HCCD have been acquired. From UV− visible calibration of the membranes, it has been observed that there is a 12% decrease in the carrier concentration of the membrane after 48 h of continuous use. This indicates moderate stability of the CTA based PIM of HCCD. PVC-Based PIM for Cs+ Transport from High Level Waste Solution. Separation of Cs+ from SHLW is important from waste management aspect. The SHLW contains a large number of metal ions (Table 1) at 3 M HNO3 concentration, where CTA based PIM cannot be used due to the acid hydrolysis of the CTA matrix. PVC based membrane has thus been prepared for Cs+ transport from SHLW. The results obtained for the transport study with SHLW using HCCD as carrier in PVC based PIM are summarized in Table 1. The Cs+ concentrations in feed and receiver compartments have been measured using radiotracer technique, whereas those of the other metal ions have been determined using ICP-AES technique. The data given in Table 1, clearly indicates that, the membrane exclusively transports Cs+ from a SHLW solution. The results show that, inspite of presence of large quantity of Na+, only 1.2% of the initial Na+ has been transported to the receiver side at the end of the transport process. The compositions of other elements in the final feed solution remain unaltered as those in the final receiver solution are below 0.1 mg/L. As shown in Table 2, the DF for Cs+ has been calculated to be 80 over Na+ and in the range of ∼500− 5000 over other metal ions. Quantitative (>99%) Cs+ transport takes place in ∼42 h. This is accompanied by only 3.3% (of initial feed proton conc.) HNO3 transport to the receiver phase. In general, in ion-exchange membrane, enhanced selectivity of an ion is usually accompanied by reduction in mobility of the ion. Enhancement in the permeability with high selectivity,

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ASSOCIATED CONTENT

S Supporting Information *

The current profiles obtained using CTA based PIM at 3 V for different neutral feed compositions and the transport profile of HNO3 and CsNO3/ NaNO3 obtained using CTA based PIM at zero applied potential with SRELW in the feed compartment have been given in the Supporting Information. The pH dependence of the Cs+ transport rate as well as the electrochemical impedance spectra of the CTA based PIM have also been mentioned in the Supporting Information. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-22- 25593688; fax: +91-22-25505150/25505151; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Dr. S. K. Thulasidas, Radiochemistry Division, BARC, for carrying out ICP-AES analysis.

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REFERENCES

(1) Liu, C.; Zachara, J. M.; Qafoku, O.; Smith, S. C. Effect of temperature on Cs+ sorption and desorption in subsurface sediments at the Hanford site, U.S.A. Environ. Sci. Technol. 2003, 37 (12), 2640− 2645, DOI: 10.1021/es026221h. (2) Zhuang, J.; Flury, M.; Jin, Y. Colloid-facilitated Cs transport through water-saturated Hanford sediment and Ottawa sand. Environ. Sci. Technol. 2003, 37 (21), 4905−4911, DOI: 10.1021/es0264504. (3) Rod, K. A.; Um, W.; Flury, M. Transport of strontium and cesium in simulated hanford tank waste leachate through quartz sand under saturated and unsaturated flow. Environ. Sci. Technol. 2010, 44 (21), 8089−8094, DOI: 10.1021/es903223x. (4) Parajuli, D.; Tanaka, H.; Hakuta, Y.; Minami, K.; Fukuda, S.; Umeoka, K.; Kamimura, R.; Hayashi, Y.; Ouchi, M.; Kawamoto, T. Dealing with the aftermath of Fukushima Daiichi nuclear Accident: Decontamination of radioactive cesium enriched ash. Environ. Sci. Technol. 2013, 47 (8), 3800−3806, DOI: 10.1021/es303467n. (5) Singh, S. P. N. U.S. DOE Report SD-RE-PCP-011; Rockwell Hanford Operations: Richland, WA, 1983. 12998

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