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Analysis of technetium species and fractions in natural seaweed using biochemical separation and ICP-MS measurement Keliang Shi, Xiaolin Hou, Jixin Qiao, Xuejie Sun, Per Roos, and Wangsuo Wu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03837 • Publication Date (Web): 04 Nov 2016 Downloaded from http://pubs.acs.org on November 16, 2016
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Analysis of technetium species and fractions in natural seaweed using biochemical separation and ICP-MS measurement Keliang Shi1,2,3 *, Xiaolin Hou 2, 4 *, Jixin Qiao 2, Xuejie Sun 1, Per Roos 2, Wangsuo Wu 1,3
1
Radiochemistry Lab, School of Nuclear Science and Technology, Lanzhou University, 730000, Lanzhou, P.R.China.
2
Center for Nuclear Technologies, Technical University of Denmark, DTU Risø Campus, DK-4000 Roskilde, Denmark.
3
Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, 730000, Lanzhou, P.R.China.
4
Xi’an AMS Center, State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, 710061, Xi’an, P. R. China
ABSTRACT
An extremely high accumulation and retention of technetium in marine plants especially brown seaweed makes it a unique bio-indicator of technetium. In the present work, a novel approach was developed for the speciation analysis of technetium in seaweed, wherein a series of biochemical
*
Corresponding author. Tel.: +86 9318913278 or +45 21325129. E-mail address:
[email protected] and
[email protected] 1
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separations was exploited to isolate different species of technetium. Inductively coupled plasma mass spectrometry (ICP-MS) was applied for the measurement of
99
Tc after thorough radiochemical
pre-concentration and purification. The results show that the distribution of technetium species in seaweed is relatively dispersive. Besides the inorganic species of TcO4-, most of technetium (>75%) combined with organic components of seaweed such as algin, cellulose and pigment. This investigation could provide important fundamental knowledge for studying the processes and mechanisms of 99Tc accumulation in the natural seaweed. INTRODUCTION Technetium-99 (99Tc) is a potential long-term environmental hazard due to its long half-life (2.1×105 y) and high abundance in nuclear wastes1-3. It is mainly produced through thermal neutron fission of (fission yield of 6.1%) and
239
235
U
Pu (fission yield of 5.9%). Significant amount of 99Tc has been released
into the environment from spent nuclear fuel reprocessing plants and nuclear weapons testing, of which the majority of 99Tc were directly discharged into the oceans4. Accordingly, increasing attentions and efforts have been paid to investigate the behavior of technetium in marine environment5-7. Seaweed, especially brown seaweed like Fucus, has been proved to be very effective to uptake technetium from seawater (concentration factor is in order of 105) and thus widely used as bio-indicator for monitoring
99
Tc in the marine environment8-11. Although there have been several studies on the
enrichment of technetium in seaweed12,13, the mechanisms regarding the uptake and accumulation of technetium in seaweed is still not clear. To better understand the uptake and accumulation processes of technetium in natural seaweed, knowledge on the distribution of technetium in different species in seaweed are crucial. So far, the bioavailable form of technetium to plants is known as TcO4-14. Once being taken up by seaweed, TcO4- might be transported to the leaves via the xylem sap and reduced to low oxidation state (mainly in Tc(IV) form) in the chloroplast by electrons through the photosynthesis electron transport15. Subsequently, reduced technetium is complexed with ligands present in the plant cell and finally 2
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immobilized. It was reported that most of organically associated technetium in seaweed existed in reduced state, while TcO4- only existed in inorganic form13. Earlier study16 has observed that the reduced technetium mainly existed as protein associated species in the leaves of tomato plants. Minor reduced technetium has also been detected in pigments, hemicellulose and cellulose of maple leaves17. Presumably, these technetium species should also be formed in seaweed. Therefore, two questions need to be answered regarding: i) how technetium combines with different substances in seaweed and ii) which species of technetium is dominant. To achieve this, establishment of reliable methods for speciation analysis of technetium in seaweed is essential. This work aims to develop a method for speciation analysis of 99Tc in natural seaweed by biochemical approaches for separation of different species of technetium and ICP-MS for measurement of 99Tc after sample pretreatment and extraction chromatographic purification, in order to interpret and discuss the possible uptake mechanism of technetium in seaweed. EXPERIMENTAL SECTION Reagents and materials.
99m
Tc tracer was obtained from 2-4 GBq commercial
99
Mo-99mTc
generators (Amersham, UK) and purified using alumina cartridges according to the method reported by Hou et al.18. A
99
Tc standard solution (in the form of NH4TcO4 with an activity of 4.17 Bq/g) was
supplied by Center for Nuclear Technologies, Technical University of Denmark. Tris-HCl (purchased from AMRESCO®, USA, purity >99.5%) buffer solution (pH=7.5) was used for protein extraction. TEVA extraction chromatographic resin (100~150 μm particle size), purchased from TRISKEM International (Bruz, France), was applied for the separation and purification of technetium prior to the 99
Tc measurement by ICP-MS. AG1-×4 anion exchange chromatographic resin (100~200 mesh particle
size, Bio-Rad Laboratory, USA) was used to separate TcO4- from other species of technetium. All other chemicals used in the experiment were of analytical grade and prepared using deionized water (18.2 MΩ·cm).
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Seaweed samples, collected at Hvide Sanda (west coast of Jutland, Denmark) in 2011, were used for the method development. The samples were washed with seawater at the sampling spot to remove the mud, sand and attached particles. Then they were packed into clean plastic bags, transferred to the laboratory and preserved at -20 oC. Prior to analysis, the samples were defrosted at room temperature, smashed by a grinder. A part of the samples was used for the speciation analysis while the others were dried at 60 oC for determination of total technetium. Separation procedures for different species of technetium in seaweed. Different species of technetium in fresh seaweed were obtained using biochemical separation methods. The schematic diagram of operational procedure is shown in Figure 1. The prepared sample (10-20 g fresh weight) was firstly leached with 50 mL acetone for 3 times at room temperature (3 h for the first time and 1 h for the second and third time, respectively). The same leaching scheme was applied in the following stages. After centrifugation, the supernatants were combined and distillated to remove acetone. The residue after acetone leaching and centrifugation was then sequentially leached with 50 mL 0.05 mol/L tris-HCl buffer solution (containing 1% CaCl2) to extract water-soluble technetium. The water-soluble protein in the tris-HCl leachate was salted out via addition of (NH4)2SO4 till 95% saturation. The supernatant obtained after centrifugation was then directly loaded to an anion exchange column (AG1-×4 resin, 7 mL) to separate TcO4-. The residue obtained after tris-HCl treatment was then leached with 50 mL of 0.2 mol/L HCl at 60 oC and the leachate was separated via centrifugation to obtain the acid-soluble technetium fraction. Thereafter, 50 mL of 0.3 mol/L Na2CO3 was used to leach the residue at 60 oC to obtain algin through the formation of soluble sodium alginate. At last, the residue was leached with 50 mL of 0.2 mol/L NaOH at 60 oC. After centrifugation, the supernatant was combined with another supernatant obtained after separating alginic acid precipitate to get alkali-soluble technetium fraction. The final residue is regarded as Tc-cellulose. Radiochemical procedure for
99
Tc determination. For the determination of
99
Tc in different
fractions of seaweed obtained, organic substances in each fraction need to be decomposed to release 4
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technetium as inorganic ions for further radiochemical separation and the ICP-MS measurement. Solid fractions, containing Tc-protein, Tc-pigment, Tc-algin and Tc-cellulose, were dried and analyzed using the same method as for total 99Tc determination in seaweed. The detailed analytical procedure has been reported in our previous work19 and the main steps have been summarized as follows: the solid sample was incinerated at 700 oC for 3 h, and then digested with 8 mol/L HNO3. After the enrichment of 99Tc with Fe(OH)3/Fe(OH)2 co-precipitation, 99Tc in solution was separated and purified using two TEVA columns (2 mL). The eluate (8 mol/L HNO3 media) was evaporated and prepared for the measurement by ICP-MS. To avoid the loss of technetium in these samples during dry ashing, NaCl powder (about 0.5 g) was added and the sample was pretreated with 0.01 mol/L NaOH and dried before ashing. The TcO4- fraction was evaporated to about 10 mL, 6 mol/L NaOH was added to pH 10~12, and 30% H2O2 was then added. The same column separation procedure as for total technetium analysis was followed. The fraction of other water-soluble technetium was treated with 30% H2O2 to decompose the organic compounds and to oxidize the released technetium to TcO4-, then the solution was adjusted to pH 8~9 with 6 mol/L NaOH and loaded to an anion exchange column (7 mL, AG1-×4 resin) to concentrate technetium. TcO4- was eluted from the column using 70 mL of 10 mol/L HNO3 and further purified with the same procedure as for the TcO4- fraction mentioned above. For acid-soluble and alkali-soluble fractions, 30% H2O2 was added to decompose organic matters in acidic media to release technetium to solution as TcO4-. After removal of excess H2O2 by heating, 0.1 g Fe3+ (as FeCl3 solution) was added. TcO4- was reduced by KHSO3, and co-precipitated with Fe(OH)2, then followed the same procedures as for total 99Tc determination. 99m
Tc was used to monitor the chemical yield of technetium and detected by a well type NaI γ detector.
The concentration of 99Tc was measured using an ICP-MS system (X Series II, Thermo Fisher Scientific, Waltham, MA) equipped with an Xs- skimmer cone and standard concentric nebulizer. Prior to sample measurement, the ICP-MS instrument was tuned to optimal 99Tc signal using a 99Tc standard solution
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(0.1 μg/L), typical response for 99Tc ranges from 1×105 to 3×105 cps per μg/L. The typical operational conditions of the instrument for 99Tc measurement are listed in Table 1. RESULTS AND DISCUSSION Stability of technetium during solid sample dry ashing. Dry ashing is a simple and effective method to decompose organic matter in solids to be able to extract technetium from the sample using acid solution. However, technetium might be lost through the formation of volatile technetium species such as Tc2O7 during heating at high temperature19. On the contrary, organic matter cannot be completely incinerated at low temperature (e.g.,
