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Technical Note

Effect of ashing temperature on accurate determination of plutonium in soil samples Zhongtang Wang, Guosheng Yang, Jian Zheng, Liguo Cao, Haijun Yu, Yanbei Zhu, Keiko Tagami, and Shigeo Uchida Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b01472 • Publication Date (Web): 04 May 2015 Downloaded from http://pubs.acs.org on May 11, 2015

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Analytical Chemistry

1

Effect of ashing temperature on accurate determination of

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plutonium in soil samples

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Zhongtang Wang1, Guosheng Yang1, Jian Zheng1*, Liguo Cao1, 2, Haijun Yu3,

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Yanbei Zhu4, Keiko Tagami1, Shigeo Uchida1

6 7 1

8

Research Center for Radiation Protection

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National Institute of Radiological Sciences,

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4-9-1 Anagawa, Inage, Chiba 263-8555, Japan

11 12

2

School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China

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3

College of Materials Science & Engineering,

Beijing University of Technology, Beijing, 100124, China

17 18 19

4

National Metrology Institute of Japan, AIST,

1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan

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______________________________________________________________________

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*Corresponding author. Tel.: +81 43 2064605; Fax: +81 43 2064601.

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E-mail address: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT:

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Acidic leaching method using HNO3 is widely employed to release the global

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fallout Pu from soil samples for further chemical separations in radioecology and

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toxicology studies, and in many applications using Pu as a useful tracer. In the method’s

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sample ash treatment step to decompose organic matter in soil, various ashing

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temperatures (400 – 900˚C) are used. However, the effect of ashing temperature on the

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accurate Pu analysis has not been well investigated. In this study, two standard reference

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soils (IAEA-soil-6 and IAEA-375) were used to determine the ashing temperature effect

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(from 375 to 600˚C) on HNO3 leaching method. The Pu analytical results of both

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standard reference materials showed that lower 239+240Pu activity was observed when the

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ashing temperature exceeded 450˚C, and the

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the ashing temperature was raised. Approximately 40% of the Pu content could not be

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leached out by concentrate HNO3 after ashing for 4 h at 600˚C. The Pu loss was

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attributed to the formation of refractory materials, which are insoluble in HNO3 solution.

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This hypothesis was confirmed by the XRD analysis of soil samples which revealed that

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plagioclase-like silicate materials were formed after high temperature ashing. To ensure

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the Pu release efficiency in HNO3 leaching, we recommend 450˚C as the ideal ashing

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temperature. This recommendation is also useful for analysis of other important

239+240

Pu activity continue to decrease as

2


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artificial radionuclides (e.g. 137Cs, 90Sr, 241Am) for which an ashing process is needed to

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decompose the organic content in soil samples.

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The global fallout plutonium in soil, introduced by nuclear detonations in the last

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century, has been extensively studied not only for the purpose of radiological

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assessments,1-4 but also in various studies where Pu is used as a geochemical tracer,

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such as soil erosion,5-8 sediment dating,9-10 desertification studies,11 and estimation of

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aerosol residence time in the stratosphere.12 Usually, in these applications, large amount

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of soil samples are needed for Pu analysis. To shorten the analytical time and improve

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efficiency, a quick digestion method, HNO3 leaching method, was commonly

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employed.13-17

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Generally, the HNO3 leaching method uses concentrate HNO3 (or 8 M HNO3) to

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dissolve the Pu component in the soil samples after high temperature ashing, and that is

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followed by chemical separation and Pu measurement. However, during the ashing step,

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which is intended to decompose the organic matter and avoid interferences with the

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subsequent chemical separation, various ashing temperatures (400 – 900˚C) have been

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used by different researchers.13, 18-21 Different ashing temperatures may cause additional

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uncertainties for Pu analysis, e.g. a low temperature cannot decompose the organic

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matters thoroughly; while high temperature may produce some refractory particles,

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which cannot be dissolved by simple HNO3 leaching.22 It was observed that, at an

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ashing temperature of 900˚C, the

239+240

Pu activities in a series of soil and sediment

4


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standard samples were lower than the certified values.18 Thus, an appropriate ashing

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temperature should be identified and accepted by researchers to improve the reliability

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and accuracy of the HNO3 leaching method.

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In present study, the IAEA reference soil samples, IAEA-soil-6 and IAEA-375,

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were used to evaluate the effect of different ashing temperatures (375 - 600˚C) on Pu

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analysis. Lower

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increased. When ashed at high temperature, the formation of refractory fractions was

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proposed to be responsible for the incomplete release of Pu from soil samples. This

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proposed cause was discussed by examining the chemical change in two ways via:

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X-ray diffraction analysis and comparison of the Pu results with those of the total

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digestion method. Finally, an optimal ashing temperature for HNO3 leaching method

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was recommended.

239+240

Pu activities were observed when the ashing temperature was

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EXPERIMENTAL SECTION

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Standard Soil Samples. Two soil standard reference materials (IAEA-soil-6 and

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IAEA-375) were used to investigate the ashing temperature effect on the release

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efficiency of Pu. IAEA-soil-6 standard samples were collected in Upper Austria, and

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their Pu content was assumed to be from global fallout.23 The approximate composition

5



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of IAEA-soil-6 reference material is summarized in Table S-1. It is noted that this

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material was characterized by a high content of SiO2 (38.5%). IAEA-375 soil samples,

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were collected in Russia in July 1990, and their Pu content was assumed to be from

83

global fallout and radionuclides released in the Chernobyl accident.14,24 The elemental

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composition of the IAEA –375 reference material is summarized in Table S-2. It is

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noteworthy that the latter may be contaminated with hot particles resulting from the

86

Chernobyl accident; significantly elevated activities may be observed for anthropogenic

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radionuclides in some sub-samples.25

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Instrumentation. The ashing process for soil samples was conducted in a muffle

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furnace (FUW 253PA, Advantec, Tokyo, Japan). An example of temperature increment

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conditions was, heating from room temperature to 450˚C during 10 min, followed by

91

maintaining the temperature at 450˚C for 4 h.

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To evaluate the ashing temperature effect on Pu release in soil samples by HNO3

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leaching, 15 samples of IAEA-soil-6 (ca. 1.5 g amount for each sample) and 18 samples

94

of IAEA-375 (ca. 2 g amount for each sample) were transferred to individual 30 mL

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ceramic crucible (CW-B1, ASONE Corp., Tokyo, Japan), and then ashed at 375 - 600˚C.

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After heating at the target temperature for 4 h, the samples were cooled to room

97

temperature in the muffle furnace and ready for subsequent acid leaching/digestion.

6


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Weight loss ranges of 0.9-4.4 % and 9.2-11.7% were found after ashing for IAEA-soil-6

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and IAEA-375 samples respectively, and the actual values depended on the ashing

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temperature. The crystal structures of the ashed soils were obtained using powders

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X-ray diffraction (XRD) by a Bruker D8 Advance diffractometer (Karlsruhe, Germany)

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using Cu Kα radiation.

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In order to measure Pu isotopes, a high efficiency sample introduction system

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(APEX-Q) equipped with a conical concentric nebulizer was combined with

105

SF-ICP-MS (Element 2, Thermo Scientific, Bremen, Germany). Details of this

106

analytical system have been described in previous study.26 Low resolution mode was

107

used to take advantage of the maximal instrument sensitivity. All the measurements

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were made in the self-aspiration mode with an uptake rate of ~ 0.2 mL/min to reduce

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the risk of contamination from the peristaltic pump tubing. The SF-ICP-MS was

110

optimized on a daily basis using 0.1 ng/mL U standard solution to provide optimum

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intensities and peak shapes. The instrument detection limit of Pu was as low as 0.14

112

fg/mL.

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Reagents. All solutions were prepared using analytical grade reagents, including

114

HCl, HNO3, HF, H3BO3, HBr, HClO4, H2O2, NaNO2, NH4I and NH2OH∙HCl. Ultrapure

115

grade HNO3 obtained from Tama Chemicals (Tokyo, Japan) was used for preparation of

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the final sample solution for ICP-MS measurement. Milli-Q water (18.2 MΩ cm) was

117

used for sample preparation. The two anion-exchange resins, AG 1X8 (100-200 mesh,

118

Cl-form) and AG MP-1M (100-200 mesh, Cl-form) were obtained from Bio-Rad

119

(Hercules, CA, USA).

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Brunswick Laboratory, NJ, USA) was used to spike the soil samples as a yield tracer.

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The mixed Pu isotope standard solution (NBS-947) with certified 240Pu/239Pu atom ratio

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of 0.242 was employed for mass bias correction.

242

Pu (CRM 130, Pu spike assay and isotopic standard, New

123

Pu Separation Method. Three acidic leaching or digestion methods were used in

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this study: HNO3 leaching, HNO3-HF leaching and HNO3-HF-HClO4 digestion.

125

Detailed descriptions of their procedures can be found elsewhere: HNO3 leaching

126

method from Bu et al.;

127

HNO3-HF-HClO4 digestion method from Zhang et al.29 For the HNO3 leaching method,

128

in brief, 1-3 g ashed soil samples and 20 mL conc. HNO3 were transferred to a capped

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120 mL Teflon vessel and digested at 160˚C for 4 h, after 0.57 pg 242Pu was added as a

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yield tracer. After filtration and adjusting the acidity to 8 M HNO3, 0.41g NaNO2 was

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added to take Pu to the tetravalent state.27 In HNO3-HF leaching method, an ashed soil

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sample (1-3 g) was mixed with 0.57 pg

133

and digested by heating on a hot plate after adding 20 mL 10 M HNO3-1 M HF. After

27

HNO3-HF leaching method from Zheng et al.;28 and

242

Pu tracer in a capped 120 mL Teflon vessel

8


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leaching, the solution was heated to dryness, and the residues were dissolved in 30 mL 8

135

M HNO3. NaNO2 was also added to adjust the oxidation state, and then 0.3 g boric acid

136

was added to convert unreacted HF to BF4-.28 The HNO3-HF-HClO4 digestion method

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used 40 mL HNO3 and 10 mL HF for digestion after adding 0.57 pg

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samples were then evaporated to dryness, followed by adding 3 mL HClO4. Next, 3 mL

139

conc. HNO3 was added and the solution was evaporated to dryness, this step was

140

repeated twice. The residue was dissolved in 50 mL 1 M HNO3, and 4 mg Fe3+ was

141

added as carrier. Then, 2.5 mL NH2OH·HCl (80 g/L) was added to change Pu to Pu(Ⅲ)

142

and Pu was further co-precipitated with Fe(OH)2. The precipitate was dissolved in 1.5

143

mL conc. HNO3 and then 50 mL 8 M HNO3 was added to the solution along with 0.4 g

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NaNO2 to adjust oxidation state of Pu to Pu (IV).29 After acid leaching or digestion,

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samples were subject to the subsequent two-stage anion chromatographic chemical

146

separation as described by Bu et al.27 In brief, Pu was separated from sample matrix

147

using an AG 1X8 anion-exchange column. The obtained Pu fraction was further purified

148

using an AG MP-1 M anion-exchange column with HBr for Pu elution. After removing

149

any trace of HBr, the sample was finally dissolved in 4% HNO3, in preparation for the

150

SF-ICP-MS analysis. Details on SF-ICP-MS measurements of Pu isotopes were

151

reported elsewhere.26

9


242

Pu tracer. The


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153

RESULTS AND DISCUSSION

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Effect of Ashing Temperature on the Release of Pu Using HNO3

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Leaching Method. The Pu analytical results are shown in Table S-3 for IAEA-soil-6

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samples (temperature from 400 to 600˚C), treated by HNO3 leaching method. A

157

239+240

158

the IAEA recommended

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atom ratios derived from the current analysis ranged from 0.183 to 0.203, consistent

160

with the reported range of 0.17 - 0.22.13-15,17,30-35

Pu activity range from 0.625 to 1.087 mBq/g was obtained, equal to or lower than 239+240

Pu activity value: 0.96 - 1.11 mBq/g. The

240

Pu/239Pu

161

For a better understanding of the ashing temperature effect on the release of Pu in

162

HNO3 leaching method, Pu analytical results (temperature-averaged) of IAEA-soil-6

163

samples are plotted in Figure 1. For samples ashed at temperatures not exceeding 450˚C,

164

the

165

mBq/g. As the ashing temperature was increased, however, an obvious decreasing trend

166

was observed. The lowest activity (average: 0.638±0.011 mBq/g) was found for the

167

600˚C ashed samples, in which only approximately 62% of the Pu was released, by

168

comparison, the IAEA recommended average activity was 1.035 mBq/g. In contrast to

169

239+240

239+240

Pu activities were generally consistent with the reported range: 0.96 – 1.11

Pu activity, the

240

Pu/239Pu atom ratios did not show any significant difference at

10


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various ashing temperatures, and all the ratios were within the reported range. This may

171

indicate no isotopic discrimination occurred during the ashing process, and all Pu

172

isotopes were lost at the same rate.

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In the case of IAEA -375 samples (Table S-4), unlike the IAEA-soil-6 case, no 239+240

174

obvious ashing temperature effect was observed on the temperature-averaged

175

activities, which appeared to be irregularly distributed with large uncertainties. In

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addition, some sub samples showed significantly higher

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(0.335-0.456), compared to the literature value: 0.22-0.31.13,15,18,20,33,36 These

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discrepancies might be attributed to the influence of hot particles released from the

179

Chernobyl accident. As pointed out by the IAEA data sheet of IAEA-375, some

180

evidence has been presented to suggest that this material may be contaminated with hot

181

particles resulting from the Chernobyl accident.25 But the frequency of the occurrence

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of these hot particles is unknown, the IAEA suggested that Pu analysis of IAEA-375

183

would be especially welcomed, as to receive additional data about hot particles.37

184

Fortunately, due to the smaller sample amount (about 2g) and larger number of samples

185

adopted in the current study, compared to other studies,13,20,36 hot particle signals were

186

successfully observed. Among the 18 analyzed IAEA-375 samples, 6 samples

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(400_sub_2, 450_sub_2, 500_sub_2, 550_sub_1, 550_sub_2 and 600_sub_1 in Table

11


240

Pu

Pu/239Pu atom ratios


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S-4) were considered to be seriously affected by hot particles, taking into account their

189

much higher

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values, 0.22-0.31 for

191

239+240

192

for IAEA-375 samples is plotted in Figure 2, in which the samples heavily affected by

193

hot particles were clearly separated from the less (or non-) hot particle-affected samples.

194

Taking the 240Pu/239Pu atom ratio as a Pu source indicator, a higher ratio means more hot

195

particles were mixed in the soil sample, thus resulting in a linearly increased

196

activity. The highest 240Pu/239Pu atom ratio was found to be 0.456 ± 0.041, which was in

197

good agreement with the reported values of 0.45-0.52 for Chernobyl hot particles.38

198

240

Pu/239Pu atom ratios and 240

239+240

Pu activities compared to the reported

Pu/239Pu atom ratio;13,15,18,20,33,36 and 0.26-0.34 mBq/g for

Pu activity.25 The correlation between 240Pu/239Pu atom ratio and 239+240Pu activity

239+240

Pu

To eliminate the hot particle’s influence on IAEA-375 samples, only those samples

199

whose Pu analytical results (240Pu/239Pu atom ratios and

200

consistent with the reported values, were considered for the evaluation of ashing

201

temperature (Figure 3). Similar to the IAEA-soil-6 case, the

202

decreased as the ashing temperature increased, with the exception of the 600oC ashed

203

samples, which might be slightly influenced by hot particles. Unlike the IAEA-soil-6

204

case, IAEA-375 samples ashed at 400 and 450oC showed lower 239+240Pu activities than

205

the IAEA recommended range. This was probably due to the fact that the IAEA

12


239+240

Pu activities) were

239+240

Pu activities

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recommended 239+240Pu activity was based on measurements of large-amount samples in

207

which the hot particle’s influence might be included and hence the recommended values

208

were overestimated. The highest Pu loss was found for the sample ashed at 550˚C, in

209

which only 55% of the Pu was measured, compared to the IAEA recommended activity

210

(0.30 mBq/g).

211

As discussed above, both standard reference materials had a decreasing trend for

212

239+240

213

Pu content was measured in these samples. Since all the experimental conditions were

214

the same except ashing temperature, we hypothesized that the high ashing temperature

215

is responsible for the Pu loss by forming some refractory fractions in which some

216

portion of the Pu was trapped and could not be leached out by HNO3.

Pu activity when the ashing temperature exceeded 450˚C, indicating that a smaller

217

XRD Analytical Results. To verify our hypothesis, X-ray diffraction (XRD)

218

analysis was performed to identify the phase change in the soil samples after ashing

219

treatment. Soil samples ashed at 400 and 600˚C, together with non-ashed sample, were

220

subjected to XRD measurements, in which the diffraction data were recorded in the 2θ

221

range of 10–80˚ with a step of 0.03˚ and a count time of 1 s. As shown in Figure 4a, the

222

spectrums obtained from soil samples, non-ashed and ashed at 400˚C, had exactly the

223

same distribution pattern, indicating that no phase change was observed after 400˚C

13



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heating. However, in the 600˚C case, three new peaks were observed between the angle

225

25˚ and 30˚. New phases, for which the three new peaks stand, were formed in soil

226

samples during the 600˚C ashing process.

227

For a better understanding of these newly formed components, a comparison

228

between the present XRD spectrums and a soil sample spectrum reported by Mukai et

229

al.39 was carried out and the results are shown in Figure 4. Mukai et al. collected a litter

230

soil sample from a forest in Fukushima Prefecture and analyzed this sample by XRD.

231

Their results showed the presence of quartz, plagioclase and hornblende in the soil

232

sample. Two obvious quartz peaks appeared at the same locations in both spectrums

233

(from 20˚ to 30˚) (Fig. 4b and Fig. 4c), providing a valid basis for comparing these two

234

spectrums in detail. Similarly, the three new peaks observed in this study also appeared

235

in Mukai et al.'s spectrum, in which the middle peak was identified as plagioclase.

236

Plagioclase, commonly presented in the Earth's crust, refers to a series of silicate

237

minerals, which are known to be insoluble by nitric acid leaching. Thus, it was

238

concluded, in the 600˚C ashing temperature case, that the lost Pu content during ashing

239

was probably wrapped up by these newly formed plagioclase-like refractory fractions.

240

Validation of the Hypothesis Using Different Leaching/Digestion

241

Methods. To validate the hypothesis, various leaching/ digestion, including conc.

14


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242

HNO3, HNO3-HF, and HNO3-HF-HClO4, were utilized to treat IAEA-soil-6 samples

243

after ashing at 550˚C. The

244

method was 0.67 ± 0.02 mBq/g, significantly lower than the reported range: 0.96 – 1.11

245

mBq/g. For the HNO3-HF leaching and HNO3-HF-HClO4 digestion methods, silicate

246

fractions formed during ashing were dissolved by HF, resulting in the

247

(0.96 ± 0.03 mBq/g for HNO3-HF leaching; 0.99 ± 0.02 mBq/g for HNO3-HF-HClO4

248

digestion) being within the reported range. Consequently, based on the above discussion,

249

it was confirmed that high ashing temperature (500-600˚C), can lead to formation of

250

some refractory silicates which remain insoluble in conc. HNO3, and Pu loss in the

251

HNO3 leaching method.

239+240

Pu activity obtained from the conc. HNO3 leaching

239+240

Pu activity

252

253

Conclusions

254

In summary, standard reference soil samples, IAEA-soil-6 and IAEA-375, were used to

255

evaluate the effect of ashing temperature on Pu analysis, for the widely employed HNO3

256

leaching method. Both standard reference materials showed Pu loss (without isotopic

257

composition change) when the ashing temperature exceeded 450˚C, Pu release

258

decreased as the ashing temperature was increased, and approximately 40% of the Pu

259

was lost after 600˚C ashing. The results of XDR analysis for IAEA-soil-6 samples

15



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260

revealed that refractory materials (silicate, e.g. plagioclase-like) were generated after the

261

600˚C ashing, however, these refractory fractions remain insoluble in concentrate HNO3.

262

Thus, the Pu content wrapped in these refractory fractions could not be released by

263

HNO3 leaching, resulting in the Pu loss observed for this method. To ensure the Pu

264

release efficiency, the findings of this study suggest that the temperature for sample

265

ashing in the HNO3 leaching method should be controlled below 500˚C, and 450˚C is

266

recommended. This suggestion is also useful for the determination of other artificial

267

radionuclides (e.g.

268

particle information in IAEA-375 standard reference soil was presented, and it should

269

be useful for studies using this soil as a quality control material for the validation of

270

analytical methods and for the assessment of a laboratory’s analytical work.

137

Cs,

90

Sr,

241

Am), if an ashing process is needed. In addition, hot

271

272

Acknowledgement

273

The authors thank Mr. W. T. Wu for useful discussion. This work was supported by the

274

Agency for Natural Resources and Energy (METI), Japan.

275

276

Supporting Information Available

277

Supporting information contains four Tables (S-1, S-2, S-3 and S-4) as noted in text.

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This information is available free of charge via the Internet at http://pubs.acs.org/.

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Figure Captions

Figure 1. Pu analytical results of IAEA-soil-6 samples treated by HNO3 leaching method (ashed at 400 – 600˚C). Figure 2. The correlation between 240Pu/239Pu atom ratio and IAEA-375 soil samples affected by hot particles.

239+240

Pu activity in

Figure 3. Pu analytical results of IAEA-375 samples treated by conc. HNO3 leaching (ashed at 375 – 600˚C). Figure 4. The spectrum of XRD analysis after ashing at different temperature (a); XRD results derived from Mukai et al.39 (b); and XRD spectrums from this study (20o-30o) (c).

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Figure 1.

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Figure 2.

Figure 3.

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Figure 4.

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Effect of Ashing Temperature on Accurate ... - ACS Publications

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