Analysis of 210Po, 210Bi, and 210Pb in atmospheric and oceanic samples by simultaneously auto-plating 210Po and 210Bi onto a nickel disc
Introduction
Products of the naturally occurring 238U decay series include 210Bi and 210Po which are produced by the decay of 210Pb. These three radionuclides are useful tracers to determine the mean residence times of atmospheric aerosols and to trace the particle dynamics and removal processes of heavy metals and particulate organic carbon (POC) from sea waters (Chen et al., 2016; Roca-Martí et al., 2016; Su et al., 2017). For instance, Fry and Menon (1962), Poet et al. (1972), Tsunogai and Fukuda (1974) and Moore et al. (1976), had found a large difference between the atmospheric residence times of aerosols estimated from 210Bi/210Pb and 210Po/210Pb activity ratios. However, simultaneous determination of these three nuclides in seawater or marine particle samples has not yet been widely reported because of the short half-life of 210Bi (~5 days), which makes it difficult to separate 210Bi from 210Pb within that short period of time.
The classical method to determine 210Po in environmental samples is based on its spontaneous deposition on a silver or nickel disk and the later measurement by alpha spectrometry (Blanchard, 1966; Fleer and Bacon, 1984; Lee et al., 2014; Murray et al., 2005; Su et al., 2017; Wei et al., 2011). 210Pb concentrations are typically obtained by measuring either the ingrowth of 210Po in a sample stored for six to twelve months after all Po in the plating solution had been cleared (Bacon et al., 1988; Baskaran et al., 2013), or by measuring the 210Bi ingrowth after removing the Pb by electro-deposition (Narita et al., 1989), ion-exchange (Peck and Smith, 2000), or solvent extractions (Jia et al., 2000, 2001).
For land-based laboratories, 210Bi is usually separated from 210Pb by co-precipitation (Fry and Menon, 1962; Poet et al., 1972; Tsunogai and Fukuda, 1974; Nevissi, 1991), solvent extraction (Blais and Marshall, 1988), ion-exchange (Harada et al., 1989; Vreček et al., 2004), electro-deposition (Narita et al., 1989; Tokieda et al., 1994), or auto-deposition onto noble metals and subsequently measured using beta counter (Blanchard, 1966; Ehinger et al., 1986). These methods, however, present a series of limitations: Separation of 210Bi from 210Pb by co-precipitation using BiOCl may contain contamination of small amounts of 210Pb (Narita et al., 1989); Using solvent extraction techniques to extract 210Pb and/or 210Bi in the presence of various amounts of 210Bi and/or 210Po and to measure 210Pb and 210Bi by a liquid scintillation counter (LSC) is effective and time-saving (Blais and Marshall, 1988; Długosz-Lisiecka, 2019), but LSC requires high concentration (~100 Bq) and is expensive and not available in many marine laboratories (Lee et al., 2014); Ion-exchange is useful for quantitative separation of 210Bi and 210Pb, but the procedure is long and tedious, requiring up to 2 days (Narita et al., 1989; Tokieda et al., 1994), and may not be suitable to be used on boats or ships especially when sea conditions are very bad. The separation of 210Bi by applying a controlled potential electrolysis requires to add nitric and perchloric acids and evaporate it to nearly dryness to decompose the reducing agent and to change the medium to a fluoroborate solution for electrodepositing Pb onto an anode as PbO2 (Tokieda et al., 1994). In addition, many electrodeposition apparatuses are needed in order to increase the sample preparation and output in the ocean.
Therefore, a rapid sensitive spontaneous deposition method is usually chosen because of its relative simplicity and quickness, in addition, both 210Bi and/or 210Po can be spontaneously deposited onto a nickel disc and/or Ag disc quantitatively at temperatures above 80 °C with negligible interfering deposition of 210Pb (Blanchard, 1966; Helmkamp et al., 1979; Church et al., 1994; Vesterbacka and Ikäheimonen, 2005; Lee et al., 2014). However, under most modestly equipped marine laboratories, especially on waterborne vessels, maintaining a near-constant temperature in an aqueous solution for hours or in a heated water bath (80–90 °C) may potentially be a safety hazard and limit the number of samples that could be processed without temperature-controlled devices. Another concern is the volatility of 210Po at high plating temperatures (Lee et al., 2014; Matthews et al., 2007). Therefore, the objective of this study is to determine the optimum conditions for the deposition of both 210Po and 210Bi at room temperature (~25 °C) and to apply this technique to environmental samples. The outcome of this research will make possible onboard, rapid and simultaneous plating of 210Po-210Bi by providing the most appropriate and reliable plating conditions when working with high temperatures may not be available.
Section snippets
Reagents and standards
A210Pb–210Bi-210Po standard in radioactive equilibrium was obtained from the Beijing Geological Research Institute of Nuclear Industry (GBW04127). 209Po and 207Bi tracer were purchased from Eckert & Ziegler Isotopes Products (refs. 1526-81-1 and 1835-28-1, respectively). All chemicals/reagents used in this study were of analytical grade (purchased from Sinopharm Chemical Reagent Co., Ltd). Silver sheet (99.9% pure), nickel sheet (99.99%), copper sheet (99.5%) and stainless-steel discs, were
Results and discussion
The specific activities of 210Pb, 210Bi and 210Po in natural environment samples are typically low, 20–240 mBq/g (Persson and Holm, 2011), and even less than 20 mBq/g (Gao et al., 2017; Qiao et al., 2017). Due to those low specific activities, the volumes required to measure those radionuclides in marine samples (i.e. seawater and suspended materials in deep ocean) tend to be large, which can be a limiting factor. Therefore, improvements in recoveries of 210Po and 210Bi from limited sample
Conclusion
This study investigates the optimum conditions for the auto-plating of both 210Po and 210Bi at room temperature (25 ± 1 °C) to facilitate the sample processing for the determination of these radionuclides when high temperature plating is a limiting factor. The results indicate that the optimum conditions are achieved when the plating is conducted using 25 mm nickel disc, for 16 h with a 60 mL HNO3 or HCl media solution with the presence of 150–300 mg ascorbic acid per 100 mg Fe added when Fe(OH)
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This research was supported by the Key Projects of Intergovernmental Science and Technology Innovation Cooperation of the Ministry of Science and Technology in China (2018YFE0109900). The authors wish to thank members of RIC group for their assistance with sampling and data analysis. We are grateful to the crew of the ship “Runjiang II” for sampling and good cooking. The first author wants to say thanks to all the doctors, nurses and medical researchers for fighting with COVID-2019.
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