Assessments of natural radioactivity and determination of heavy metals in soil around industrial dumpsites in Sango-Ota, Ogun state, Nigeria.

The activity concentration of natural radionuclides in soil samples from industrial dumpsites in Sango-Ota were determined using gamma-ray spectrometry with NaI(Tl) detector. The mean activity concentration of (226)Ra, (232)Th and (40)K was 3.0 ± 1.2, 33.3 ± 9.8 and 122.1 ± 20.6 Bqkg(-1), respectively. Radium equivalent activities were calculated to assess the hazards arising from the use of the soil sample in agriculture. All the calculated values were lower than the world average. The mean concentration of heavy metals in the soil samples were 33.6, 2.9, 3.8, 2.7, 48.9, 1,5, 34.5 and 0.8 mg l(-1) for Cu, Mg, Ca, P, Fe, Pb, Zn and Cd, respectively. The concentrations of Cd, Cu and Pb were higher than the natural permissible range in soil. Therefore, the government should discourage the use of the soil around dumpsites for planting because of the presence of heavy metals in the sites.

Introduction

Human beings are continuously exposed to ionizing radiation that stem from both natural and man-made sources. The principal origins of ionizing radiation exposure are natural sources. The main constituents of natural radiation are cosmic rays and gamma ray emitters in soils, building materials, water, food, and air. Natural background radiation, which is equivalent to 2.4 mSv per year, makes up approximately 80% of human exposure in a year.[1] The spatial distribution of natural radiation is highly variable and dependent on the local geological formations.[2] The natural radioactivity of soil samples is usually determined from the 226Ra, 232Th, and 40K contents. The study of the levels of radionuclide distribution in the environment is essential especially in providing radiological information.[3]

Human activities generate wastes. Industrialization and population increment in most cities in Nigeria results in changing the composition and quality of waste generated as this can be seen in most cities of developing countries.[4] Soil from waste dumpsites may contain heavy metals and naturally occurring radionuclides in significant amounts.[5] Hazardous waste can cause and has caused pollution, damage to health and even death. The external radiation exposure pathways to the population have been extensively studied.[67] Several studies in Nigeria have measured the activity concentration of natural radionuclides in soil to ascertain the levels of contamination.[891011121314] It was revealed that staple food stuffs consumed in Nigeria contain traces of radionuclide and as a result of this, refuse dumpsites were identified as a liable recipient in containment of radioactive materials as inhabitants cultivate and plant legumes, vegetables, and so on in the field around the dumpsites. The transportation of heavy metal as well as radionuclides in soil from this sites are possible via root-uptake and then to human through breathing and ingestion. Therefore, accurate measurement of elemental composition and radionuclide levels in soil sample from these dumpsites will provide information from which average radiation to the public from these dumpsites can be estimated. The aims of this study were to: (i) determine the activity concentrations of 226Ra, 232Th, and 40K in the representative soil samples from industrial dumpsites; (ii) to estimate the absorbed dose rates and the effective dose from the exposure to the natural radionuclides; and (iii) to determine the heavy elements contained in the representative soil samples. This study was carried out in Ota, an industrial and commercial area located between latitude 6˚ 41’ North and longitude 3˚ 14’ East [Figure 1]. Its elevation is 74.259 m above the sea level and has a large concentration of land space with land mass of about 1460 sq km and has grown into an industrial city due to the economic development planning and lobbying by the Manufacturers Association of Nigeria which led to the official designation of Ota as an industrial town. Wastes in these dumpsites were mostly generated from industries such as pharmaceuticals, medical, breweries, beverages, aluminum roofing sheet, iron, and steel companies. The infectious medical wastes as well as the industrial toxic wastes are disposed together in the dumpsites. The study area is underlain by the Precambrian basement complex of South-Western Nigeria and is at the fringes of the border between Nigeria and the Republic of Benin.

Figure 1

Map of Ota town showing sampling points S1 – S10

Experimental Procedures

Sample collection and preparation

Soil samples were collected from 10 industrial dumpsites in Ota. A total of 10 samples were collected from each dumpsite at a depth of about 20-30 cm at the various locations. The samples were processed following the standard procedures.[15] Soil samples were well mixed after removing extraneous materials such as roots, pieces of stones, and gravel. Samples were weighed and then dried in an oven at 105°C overnight and reweighed to find the water content. The samples were crushed and were made to pass through a 0.2-mm sieve. Sieved samples were weighed and a mass of 200 g of each sample was placed in a plastic container. The plastic containers were hermetically sealed with adhesive tape[16] for 30 days for secular equilibrium to take place.[17]

Activity determination

The analysis of radionuclide concentrations was performed by gamma-ray spectrometry with sodium iodide detectors. The counting assembly was a scintillation detector and a Canberra multichannel analyzer. The detector was a 7.6 × 7.6 cm2 NaI (Tl) manufactured by Bicron. A cylindrical lead shield of approximately 5 cm thickness with a fixed bottom and a movable cover shielded the detector from background radiation. The spectrometer was tested for its linearity and then calibrated for energy using gamma sources supplied by the International Atomic Energy Agency, Vienna. This was achieved by collection of spectra data from standard sources with energies in the range 0.511-2.62 MeV. The channel numbers of the photopeaks corresponding to the different gamma energies were recorded after 900s and the energy-channel linear relationship obtained is shown in Figure 2. The detection efficiency calibration of the system was carried out using a reference standard gamma source prepared by Rocketdyne Laboratories, Canoga Park, CA, USA, which is traceable to a mixed standard gamma source (ENV94084-200 g) by Analytic Inc., Atlanta, GA, USA. The values of efficiency obtained were 0.440, 0.321, and 0.141 for 40K, 226Ra, and 232Th, respectively. The efficiency calibration curve for the NaI (Tl) detector is shown in Figure 3. The detector assembly has a resolution of ~8% at 0.662 MeV of 137Cs. The reference sources have activity concentration of 479.15, 566.47, and 11.60 Bqkg-1 for 40K, 226Ra, and 232Th, respectively. The background count was determined by counting an empty container of the same dimensions as the one containing the samples and subtracting from the gross count. The counting time was set at 36 000 s (10 h) to obtain the gamma spectrum with good statistics.

Figure 2

Graph of the energy-channel dependence

Figure 3

The efficiency calibration curve for the Nai(Tl) detector

From the net area, the activity concentrations in the samples were obtained using equation (1):

C is the activity concentration of the radionuclide in the sample given in Bq kg-1, Cn is the count rate under the corresponding peak, ε is the detector efficiency at the specific g-ray energy, Pg is the absolute transition probability of the specific g-ray, and Ms is the mass of the sample (kg).

The detection limit (DL) of a measuring system describes its operating capability without the influence of the sample. The DL given in Bq kg-1, which is required to estimate the minimum detectable activity in a sample, was obtained using equation (2):

Where LLD is the Lowest limit of detect, Cb is the net background count in the corresponding peak, tb is the background counting time (s), and f is the factor that converts cps (counts per second) to activity concentration (Bq kg-1).

The DLs obtained were 17.3 Bq kg-1, 4.2 Bq kg-1, and 5.1 Bq kg-1 for 40K, 226Ra, and 232Th, respectively. The concentration of 226Ra was determined by a 1.764 MeV gamma ray from 214Bi. The gamma-ray energy of 2.614 MeV from 208Tl was used to determine the activity concentration of 232Th and a gamma ray of 1.460 MeV from 40K was used to determine the concentration of 40K in the samples.

Elemental analysis procedure

The main material used in the determination of the elemental composition of the samples is the Flame Atomic Absorption Spectrophotometer model S4 series, Model (GBC 906) (USA). Soil samples are digested. A total of 1 g of fine particles of dried soil samples from dumpsites was weighed into a 100 mL digestion tube. Each of the tubes was labeled to avoid mix-up. A total of 5 mL each of nitric acid and hydrogen peroxide were added, while SO4 was added in small amount and the mixture was thoroughly stirred until white fumes evolved. The process continued until the solution is clear. The solution was decanted and diluted with deionized water up to 100 mL before being filtered. The procedure was repeated for all the samples. Before metal concentrations were determined, standard solutions were prepared in each case and were used to eliminate sample standard matrix indifferences. The instrument was calibrated for all the measurement carried out using deionized and standard solutions; these standard solutions are different for different elements. The concentration of the samples were obtained from the readings of Atomic Absorption Spectrophotometer (AAS)

Results and Discussions

Figure 4 shows the portion of the x-ray spectrum of sample 2 from site S4. The 226Ra, 232Th, and 40K activity concentrations measured in soil samples from 10 industrial dumpsites in Ota are shown in Table 1. The activity concentrations of 232Th and 40K in soil samples ranged from 16.1 ± 2.4-49.2 ± 5.2 Bqkg-1 and 88.8 ± 13.4-161.7 ± 16.1 Bqkg-1 with mean values 33.3 ± 9.8 and 122.1 ± 20.6 Bqkg-1, respectively. All the values of 226Ra measured in the samples were below the DL. From the data obtained, it is clear that the activity concentration of 40K in all the samples were higher than the activity concentration of 226Ra and 232Th. The mean value of the activities concentrations of 226Ra, 232Th, and 40K were found to be lower than the world average of 35, 30, and 400 Bqkg-1, respectively.[2]

Figure 4

Portion of the gamma ray spectrum of sample 2 from site S4

Table 1

Activity concentration of 226Ra, 222Th, and 40K (Bqkg-1) in the dumpsites

The absorbed dose rate, radium equivalent, internal hazard index, and annual effective dose calculated from the activity concentrations of the natural radionuclides in soil samples were presented in Table 2.

Table 2

The absorbed dose rate, internal hazard index, radium equivalent, and annual effective dose estimated in the dumpsites

Absorbed dose rate

The total absorbed rate in air (nGy h-1) at 1 m above the ground due to the activity concentration of 226Ra, 232Th, and 40K was calculated using the formula.[2] We assumed here that the contribution from other naturally occurring radionuclides and cosmic radiation at the locations were insignificant.

Where D is the dose rate (nGy h-1) at 1 m above the ground due to 226Ra, 232Th, and 40K in the soil sample. ARa, ATh and AK are the activity concentrations of 226Ra, 232Th, and 40K in Bqkg-1, respectively. The value of LLD (4.2 Bkg-1) was used in the calculation of ARa since all the values obtained for 226Ra in this study were below the DL.

The absorbed dose rate calculated for this study ranged from 16.53-42.21 nGyh-1 with a mean value of <29.79 nGyh-1. The highest value in this study is lower than the world average of 51 nGy h-1.[2]

Radium equivalent activity and internal hazard index

It is important to assess the gamma radiation hazards to human associated with the use of the soil from the site for filling or for use in construction. Radium equivalent activity (Raeq) was calculated in this study because it gives a single index to describe the gamma output from different mixture of radium, thorium, and potassium in the samples. The Raeq was calculated using equation described by.[1819]

where ARa, ATh, and AK are the activity concentrations in Bqkg-1 of 226Ra, 232Th, and 40K respectively.

The values obtained in the samples were presented in Table 2 column 3. Raeq in the dumpsites varies between 34.06 and 87.01 Bqkg-1 with a mean of <61.02 Bqkg-1. The values obtained in all the dumpsites were smaller than the suggested maximal permissible value of 370 Bqkg-1 which is equivalent to annual effective dose of 1.5 mSv. The radiation hazard due to internal exposure from radon and its short-lived decay products to the respiratory organs can be estimated using the following formula:

Where Hin is the interhazard index and ARa, ATh and AK are the activity concentrations in Bqkg-1 of 226Ra, 232Th, and 40K respectively.

The internal hazard index ranged from 0.1-0.25 with a mean of <0.18 which is less than 1 as desirable.

The results obtained in this study compared well with the similar investigation carried out in Botswana.[20] The mean absorbed dose rate in this study (29.79 nGyh-1) is lower than the results obtained in Lagos[21] (56.40 nGyh-1) and in Portharcourt[22] (38.17 nGyh-1), Nigeria. The activity concentrations and the absorbed dose rate obtained in this study when compared with other studies from normal living areas in Abeokuta[9] (43, 84 and 329 Bqkg-1; 88 nGyh-1), Ibadan[23] (26, 54 and 619 Bqkg-1; 69 nGyh-1) for 238U, 226Th, 40K and absorbed dose rate, respectively and Lagos[10] (absorbed dose rate = 41 nGyh-1), south-western Nigeria were found to be lower. All the results obtained in this study are lower than the world average given by United Nations Scientific Committee on the Effect of Atomic Radiation, 2000.

Elemental composition of the soil samples

The concentrations of iron (Fe), copper (Cu), magnesium (Mg), calcium (Ca), phosphorus (Ph), lead (Pb), zinc (Zn), and cadmium (Cd) in soil samples from the 10 dumpsites survey were presented in Table 3. The mean concentrations of Fe, Zn, Mg are 33.6, 48.9 and 34.5 mg l-1, respectively. These values are within the natural range of these metals in soil which is 15-250, 0.05-1.5, and 5.0-175, respectively.[24] The mean concentrations of Cu and Cd are 33.6 and 0.8 mgl-1, respectively in the soil sample. These values are higher than their natural range in the soil which is 0.035-0.400 and 0.01-0.7 mg/L, respectively.[25] Pb was detected in only three of the 10 dumpsites and was higher in 2 of the dumpsites. Calcium and phosphorous were also detected in the soil samples and the concentrations of the two were below the natural range in the soil. Calcium had the highest value of concentration in parts per million out of the eight elements detected in the soil samples. This may be due to the primary and secondary mineral components in the soil of the area, which is essential for many plants functions. Calcium has not been considered as a major pollutant in the environment compared with other elements.

Table 3

Concentrations (mgl-1) of heavy elements in the soil samples from the dumpsites

Conclusion

The radionuclide content and heavy metal concentration of soil samples from 10 dumpsites in Ota Industrial city has been measured using gamma-ray spectrometry with NaI (Tl) detector and flame atomic absorption spectrophotometer. The average outdoor gamma radiation dose values obtained due to the activity concentrations of the radionuclides in the samples from these dumpsites are lower when compared with the results obtained for normal living areas in the same region.[91023] However, no matter how small radiation exposure could be, it has effect on human being and exposure to this radiation must be reduced. The results of heavy metal concentrations in the soil showed the presence of heavy metals with cadmium and copper having concentrations above their natural range in the soil. At lower concentrations, these metals may be beneficial to the ecosystem but they are regarded to be toxic at higher concentrations. However, heavy metal are potentially toxic with prolonged exposure.[5] Therefore, it is imperative to monitor their accumulation in soil samples before inhabitants can cultivate the land around the dumpsites for agricultural purposes to prevent the transportation of these metals into human system. The government should discourage the use of the soil/fields around these dumpsites for agricultural purposes because of the presence of heavy metals in the sites at present.