Accumulation of Natural Radionuclides by Some Edible Wild Mushrooms in Ekiti State , Southwestern , Nigeria

Activity concentrations of primordial radionuclides such as 40K, 226Ra and 232Th were determined in edible mushrooms samples in Ekiti State Nigeria. The edible mushrooms collected are Termitomyces striatus, Psathyrella atroumbonata, Termitomyces robustus, Pleurotus tuber-regium and Pleurotus squarrosulus. The activity measurements were carried out by gamma spectrometry. The average concentrations of 40K varied from 254.17 ± 46.78 to 416.07 ± 68.43 Bq∙kg−1, 226Ra concentrations varied from 2.68 ± 0.82 to 21.64 ± 7.23 Bq∙kg−1 and 232Th concentrations varied from 8.57 ± 3.25 to 10.98 ± 4.31 Bq∙kg−1. The concentrations were converted to effective dose. Effective doses calculated were found to be below maximum permissible levels. Therefore, no health risk is envisaged for those that normally consumed these mushrooms.


Introduction
Anthropogenic ( 90 Sr, 137 Cs and 134 Cs) and primordial ( 40 K, 226 Ra and 232 Th) radionuclides are available in diverse environments such oceans, rivers, streams, soils, rocks, vegetable, animals and human body [1].As a result, human being are constantly bombarding by radiation [2] due to human activities such as use of phosphate fertilizer in cultivation, burning of fossil fuels to generate heat and electricity, mining, milling operations and building materials [2] [3].Food is one of the sources by which man is exposed to radiation.Therefore, radioactivity measurements in food stuffs (especially mushrooms) have widely reported in literature [3]- [11].
Mushrooms are known to accumulate radionuclides efficiently [12].However, there are no studies that determine primordial and anthropogenic radionuclides composition in edible mushrooms in Nigeria, therefore dearth of record abound.Mushrooms are highly rich in fiber, proteins, vitamins and minerals such as potassium, phosphorus, iron and sodium and are widely consumed in Ekiti State.As a result 95% of families in Ekiti and its environs consume up to 4 kg•y −1 .Mushrooms are commonly found in cocoa plantation and thick forest.Samples are as seen in Figure 1 and Figure 2. Farmers in Ekiti usually use chemicals and herbicides to control weeds, pests and fungi.It is imperative to ascertain the level of radionuclides in these mushrooms that could absorb them in the atmosphere though their surface cells of from soil through their roots.

Sample Collection, Preparation and Counting
Edible mushrooms were obtained from different towns and villages in the state.Ten samples of each species were collected.The species are Termitomyces striatus, Psathyrella atroumbonata, Termitomcyes robustus, Pleurotus tuber-regium and Pleurotus squarrosulus.Fifty samples were collected in all.Samples were washed, cleaned and dried in air.They were then oven-dried at 80˚C for 16 hrs to remove the moisture content [13].Dried samples of the same species were grounded together using a domestic blender.Thirty (30 g) of each dried mushrooms were placed in polyethylene beakers previously cleaned with 10% nitric acid.The beakers were then sealed and allowed to stand for at least 4 weeks so that the 226 Ra series was able to reach radioactive secular equilibrium.The activity concentrations of natural radionuclides 40 K, 226 Ra and 232 Th in the samples were determined using a NaI (Tl) γ-ray spectrometer system.The 226 Ra activity determination was based on 1.76 MeV gamma rays from 214 Bi.The activity of 232 Th was determined through its 2.62 MeV gamma rays from 208 Tl.The activity of 40 K (non-series) radionuclides was determined through its 1.46 MeV gamma rays.In situ measurement was carried out at the point of obtaining mushroom samples and after laboratory exposure using gamma scout detector that is calibrated across a wide scale (0.01 to 50 μSv•hr −1 ).This was done to ensure safety at point of collection and laboratory.The values obtained are presented as shown in Table 1.

Internal Dose from Ingested Mushrooms.
The maximum permitted concentration level recommended by International Atomic Energy Agency (IAEA) is 10 kBq•kg −1 DM (dried matter) for mushrooms [14].This limit must not be exceeded in all the samples.A possible risk of radioactivity for human being that consume these mushrooms is expressed by the effective dose (H) given in mSv•y −1 .The acceptable limit recommended by International Commission for Radiological Protection (ICRP) has been 1 mSv for adult yearly.The contribution to the annual effective dose to an adult that consume these species of mushrooms is calculated as follows [15].
where Y is annual consumption of mushrooms in kg DM per person, Z is activity concentration in Bq•kg −1 DM, IDCF is the internal dose conversion factors of 6.2 × 10 −3 , 0.28 × 10 −3 and 0.23 × 10 −3 μSv•Bq −1 of 40 K, 226 Ra and 232 Th respectively.Table 2 shows the results of dose that will accrue to adult consuming these species of