Nuclear tests and operation of nuclear facilities are usually accompanied with the release of artificial radionuclides by means of adhering to the fallout in the environment. With the fallout being deposited on the ground, part of artificial radionuclides would be absorbed by plants, finally reaching human bodies via the food chain^[1]. ⁹⁰Sr is a product of nuclear fission. It is present in significant amount in spent nuclear fuel and radioactive waste from nuclear reactors and nuclear fallout from nuclear tests. ⁹⁰Sr is one of the most dangerous man-made radionuclides with a long half-life of 28 years. It has a high affinity to bone^[2]. Previous literatures have revealed that ⁹⁰Sr was apt to accumulate in teeth and skeleton and became a potential contributor to human exposure dose^[3].

Tea is an aromatic beverage, which is the most widely consumed drink across the world. Tea consumption in China was reportedly 1.44 million tons in 2015. However, it is easy for tea plants to absorb ⁹⁰Sr from soils in the growth process, and ⁹⁰Sr in fallout is prone to contaminate the tea leaves as well^[4]. Therefore, activity concentration of ⁹⁰Sr in tea is closely related to people′s health. Not only can we reveal the extent to contamination caused by nuclear activities, we can also assess the potential risk of ⁹⁰Sr in tea to consumers′ health by investigating ⁹⁰Sr radioactivity. In this study, 26 kinds of tea produced in 16 typical regions were collected, and their ⁹⁰Sr activity concentration was analyzed using the method of di (2-ethylhexyl) phosphate (HDEHP) extraction chromatography. The results further enriched the baseline data for ⁹⁰Sr level in Chinese tea.

Materials and methods 1. Chemicals and instruments

Nitric acid used in this work was of electronic grade. Yttrium nitrate and strontium chloride were both of high purity grade. Other reagents, such as hydrogen peroxide (30%), ammonium hydroxide, hydrochloric acid, and oxalic acid dehydrate were all of analytical grade. Chromatographic columns with an inner diameter of 8-10 mm were made by Beijing Glassware Factory. Extraction resin (HDEHP≥50%) was prepared by Beijing Research Institute of Chemical Engineering and Metallurgy. Ultrapure water (18.2 M cm) from a Milli-Q system (Millipore, USA) was used.

In addition, the activity concentration in all samples was measured using low-level α/β counter (BH1217B, CNNC Beijing Nuclear Instrument Factory), and the pH values of solutions were measured by a pH meter (Mettler Toledo, Switzerland).

2. Tea samples and pretreatment

Twenty six kinds of tea were collected in 2016 from 26 cities of 16 provinces. The detailed information was provided in Table 1. Pretreatment of tea samples was described as follows: 10 g of tea ash was placed in a 100 ml crucible. Subsequently, strontium carrier (1 ml, 50 mg Sr/ml), yttrium carrier (1 ml, 20 mg Y/ml), 10 ml nitric acid (15 mol/L) and 6 ml hydrogen peroxide (30%) were added to the crucible. Afterwards the crucible was heated on electric heating plate until the mixture was evaporated to dry, and then transferred into muffle furnace, kept at 600 ℃ for 2 h. The sample was leached by 2 mol/L hydrochloric acid. After centrifugation, the leach liquor was retained. 10 g of oxalic acid was added to the leach liquor and then adjusted to pH 3.0 by ammonium hydroxide to form the precipitate. The mixture was filtered. The obtained filter cake was heated at 800 ℃ in muffle furnace for 2 h. Afterwards, the filter cake was dissolved with 5 ml 6 mol/L nitric acid and 35 ml 1 mol/L nitric acid. After 0.5 ml bismuth carrier (50 mg Bi/ml) and 0.4 ml sodium sulfide (0.3 mol/L) were added, the red precipitate appeared right now. And then the suspension was filtered to separate the liquor.

3. Separation using HDEHP extraction chromatography

The solution passed through the column filled with HDEHP extraction resin at a rate of 2 ml/min. Then the column was rinsed with 30 ml 1 mol/L hydrochloric acid and 30 ml 1.5 mol/L nitric acid to get rid of all interfering ions. Finally, yttrium ions were eluted from the resin using 30 ml 6 mol/L nitric acid.

4. Measurement and calculation

5 ml saturated solution of oxalic acid was added into the solution and the solution was adjusted to pH 3.0 until white precipitate was formed. After vacuum drawing and filtering, the filter cakes were paved on measuring plate and the counts of the samples were measured using low-level α/β counter. The activity concentration in samples was calculated using the following formula^[5]:

$A = \frac{{\rm{N}}}{{60{{\rm{E}}_f}{\rm{m}}{{\rm{Y}}_Y}{e^{ - {\rm{\lambda }}\left( {{{\rm{t}}_3} - {{\rm{t}}_2}} \right)}}}}$

(1)

Where A is activity concentration in the sample, Bq/kg; N is the net count rate, count/min; E_f is detection efficiency for ⁹⁰Y, %; m is the weight of the sample, kg; Y_Y is recovery of yttrium carrier, %; e^{－λ(t₃－t₂)}is decay correction for ⁹⁰Y.

5. Uncertainty evaluation

In order to completely express the results of ⁹⁰Sr analysis, the uncertainty of ⁹⁰Sr activity concentrations in tea samples were presented. The uncertainty of ⁹⁰Sr activity concentrations mainly originate from ⁹⁰Y β-counting, detection efficiency of equipment and chemical recovery of the method. The detailed evaluation process can refer to our previous work^[6]. The uncertainty of each activity concentration was also listed in Table 1.

6. Quality control

In order to verify the accuracy of the method, two reference materials were used to perform the quality control of the experimental procedure. One reference material was IAEA-156 (Radionuclides in clover), and the other was IAEA-330 (Radionuclides in spinach). The results in Table 2 were in accordance with the certified value, which showed that the used method presented a good accuracy.

Results 1. Activity concentration

At present, extraction chromatography method is a most popular and relatively fast method for ⁹⁰Sr determination. The basic process includes Di (2-ethylhexyl) phosphate (HDEHP) extraction chromatography separation, ⁹⁰Y precipitation and counting by instrument. Compared with other methods, such as fuming nitric acid method and ion-exchange method, the used method is also convenient to operate, and does little harm to operators. Analytical results of the ⁹⁰Sr activity concentration in IAEA certified reference materials have shown the method to have a good accuracy.

All tea samples were collected from typical regions. These typical regions included some eastern regions and some western regions, some common regions and some special regions of China. Special regions are those where nuclear facilities are sited or there used to be nuclear tests.

Activity concentration of ⁹⁰Sr in all tea samples were measured using HDEHP extraction chromatography method. The results and places of origin information were listed in Table 1. The results showed that activity concentration of ⁹⁰Sr were, respectively, from 0.28 to 3.78 Bq/kg in all tea samples across the country, 0.69-3.78 Bq/kg in eastern provinces, such as Jiangsu, Shandong, Zhejiang, Anhui and Fujian, and 0.28-2.23 Bq/kg in middle and southern provinces, such as Henan, Hubei, Hunan, Hainan, Guangdong and Guangxi. Apart from, the values for western and southwestern provinces were 0.82-1.96 Bq/kg, such as in Sichuan, Yunnan, and Xinjiang. These data revealed that there was no significantly statistical difference in ⁹⁰Sr activity concentration in tea among various regions of China. In addition, activity concentrations in the samples collected in special regions were not significantly higher than in general areas.

Table 3 presents some historical results of ⁹⁰Sr in tea from various regions in different years. It can be seen from Table 3 that three surveys on ⁹⁰Sr activity concentration in tea on China mainland were conducted in 1974, 1984, and 1996. The results of the three surveys were 13-30 Bq/kg, (12.71±0.33) Bq/kg and (17±9) Bq/kg, respectively, which are all higher than the value of 0.28-3.78 Bq/kg in this work (2016). Similar surveys had been conducted as well in other countries or regions. Two surveys were carried out in Japan in 1981 and 2015, and the ⁹⁰Sr activity concentrations were 3.1-4.5 Bq/kg and 0.60-3.7 Bq/kg, respectively. A similar investigation also was made in China′s Taiwan Provincee in 1968, with ⁹⁰Sr activity concentration being 4.74-31.34 Bq/kg. India also surveyed ⁹⁰Sr activity concentration in tea samples from 1961 to 1979, with a obtained result of 5.4-37.5 Bq/kg. All these data is able to objectively reflect the level of radioactive contamination in the locality at that time.

2. Hygienic evaluation

According to the ICRP Publication 72, the exposure dose due to intake of radionuclides can be calculated by the following formula^[13]:

$E\left( \tau \right) = {A_0}e\left( \tau \right)$

(2)

Where E (τ) is committed effective dose, Sv; e (τ) is committed effective dose coefficient, Sv/Bq; A₀ is intake of certain nuclides, Bq.

As for ⁹⁰Sr, the value of committed effective dose coefficient, e(τ) = 2.8×10^-8 Sv/Bq, can be acquired by looking up GB18871-2002^[14] or international basic safety standards^[15]. According to the statistics from Euromonitor (survey institution) and the International Tea Association (ITC) in 2011, China's per capita consumption of tea was 566 g/year^[16]. Taking Biluochun tea for example, the activity concentration is 1.45 Bq/kg. Provided that ⁹⁰Sr in tea could be assimilated into people′s body totally, the average annual intake of ⁹⁰Sr can be figured out like this: A₀ = 1.45×566×10^-3 Bq = 0.82 Bq. Therefore, the annual committed effective dose caused by ⁹⁰Sr in Bilunchun tea should be evaluated as: E(τ) = A₀e(τ)=2.3×10^-2 μSv. Similarly, the annual committed effective dose caused by ⁹⁰Sr in other types of tea can also be computed using this formula, and the results were listed in Table 1, from which the annual doses from drinking tea was evaluated as (0.44×10^-2-6.00×10^-2) μSv. All these doses are far less than the public annual dose limit of 1 mSv^[14].

Discussion

The activity concentration of ⁹⁰Sr in all the tea samples are much lower than the limit of 96 Bq/kg for foodstuff and 77 Bq/kg for vegetables specified in GB 14882-1994^[17].

26 kinds of tea produced in 16 typical regions were selected, and their ⁹⁰Sr activity concentration was analyzed using HDEHP extraction chromatography method. The results revealed that the activity concentration of Chinese tea from typical regions ranged from 0.28 to 3.78 Bq/kg, contributing possible exposure doses of 0.44×10^-2-6.00×10^-2 μSv to human body. These were far less than the annual dose limit of 1 mSv, suggesting less impact on people's health. These results further enriched the baseline data for ⁹⁰Sr level in Chinese tea.

Conflict of interest statement None
Contribution statement of authors Chen Fei designed and performed the experiment, and wrote the paper; Yin Liangliang, Kong Xiangyin, Xu Ying, Zhang Yao and Shao Xianzhang participated in the processing of data; Ji Yanqin modified this paper