Speaker
Description
[Introduction]
Radiostrontium is a critical target for environmental monitoring in nuclear emergencies due to its significant long-term health risk by accumulation in bones. The main isotopes, 90 Sr (half-life 28.9 years) and 89Sr (half-life 50 days), are pure beta emitters whose conventional analysis are complex, time-consuming, and require hazardous reagents. In this study, we investigated sorption characteristics of a Sr sorbent, for rapid and safe analysis of radiostrontium in seawater. The Sr sorbent, Pureceram MAq (Nippon Chemical Industrial Co., Ltd.; hereafter P-MAq), is a water-insoluble white powder. It has a particle size range of 5–125 $\mu$m (average 25 $\mu$m) and a complex structure of mixed crystalline and amorphous phases.
[Experimental]
A fixed amount of simulated seawater was prepared by acidifying with 0.1 M HCl and adding sulfate ions, and then 85Sr or 90Sr-90Y was spiked. A certain amount of P-MAq was added to this solution, and was stirred for 4 hours. The fractional activity in the P-MAq and supernatant were measured with a NaI scintillation counter or a liquid scintillation counter (LSC). Additionally, recoveries of major elements in seawater (Na, Mg, K, and Ca) were measured with an ICP-AES. The sorption rates for Cs, Pb, Ra, and Ba were investigated using 134Cs, 210Pb, 228Ra, and 133Ba, those were measured with a HP Ge detector. The effects of Ca (0–20 mM) and sulfate (0–25 mM) concentration on Sr sorption also investigate in different concentrations.
[Results and Discussions]
A Sr recovery of over 90% was obtained by stirring 150 mg of P-MAq in 100 mL of simulated seawater for 4 hours. Major elements in seawater, Na, Mg, K, and Ca showed adsorption rate below 2%. The adsorption rates of Cs and Y were 0.001% and 7.4%, respectively. On the other hand, Pb, Ra, and Ba showed high adsorption rate of over 98% due to formation of insoluble sulfates. Therefore, radionuclides such as 140Ba require attention during post-accident monitoring. Though natural radioactivity, such as Pb and Ra, will be negligible for small volumes (50-100mL).
The results revealed that high Ca concentrations reduced Sr recovery. At typical seawater levels (9-12mM Ca), Sr recovery was 91-89%, compared to 98% without Ca.
On the other hand, the Sr sorption rate was almost 0% without sulfate ions. So that, Sr is presumed to precipitate as sulfate before substituting barium in sorbent to form strontium silicate.
To apply for environmental samples, 90Sr and 89Sr are measured using LSC or a low-background gas flow counter (LBC). In LSC, centrifuged P-MAq was mixed with scintillators. For LBC, P-MAq was collected on a filter by suction filtration. In this case, the sorbent mass is less than 75 mg to minimize the self-absorption, which restricts the sample volume to 50 mL. This method achieves a detection limit of 0.6 Bq L-1 for 90Sr with a 60-minute measurement for a 50 mL seawater sample. This sensitivity is 50 times lower than the permissible limit of effluent standard for wastewater (30 Bq L-1)
[Conclusion]
We have proposed a safe and rapid method for detecting radiostrontium in seawater using P-MAq within six hours without using hazardous reagents. This method provides sufficient sensitivity not only for emergency monitoring but also for routine environmental surveillance to confirm safety around nuclear facilities.