Assessment of NORM in Mining Residues and Environmental Materials from Sefwi Awaso Bauxite Mine, Ghana

1. INTRODUCTION

Mining has been identified as one of the potential sources of exposure to Naturally Occurring Radioactive Materials (NORM), and they exist in various kinds of natural sources (UNSCEAR 2000). The preeminent concentration of these terrestrial radionuclides regularly originates in geological materials such as igneous rocks and ores. Human activities around such geological settings might lead to a higher concentration of radionuclides in the environment, thereby increasing human exposure to ionizing radiation. If these activities are not well managed or controlled, consequently, under adverse conditions, they may present a radiation risk to individuals. Mining and various industrial activities involving minerals and raw materials, including phosphate, monazitic sand, bauxite, coal, and metals, can elevate the effective dose received by workers and the public from natural sources such as soil, water, and air (El Zrelli R. et al. 2019, Hamed M.M. et al. 2016, Goronovski A. et al. 2021, Galhardi J.A. et al. 2017, Huang Y.J. et al. 2015 and El Hajj T.M. et al. 2019). Radionuclides may be unevenly distributed among the numerous components generated during the mining of minerals from the Earth's crust and their subsequent physical or chemical processing (Liu H. et al. 2012 and >van der Sloot H.A. et al. 2017).

Mining entails the extraction of ores along with the accompanying waste material, which includes non-mineralized or low-grade rocks (Michalik B. et al. 2018)>. Mined ores are further treated to separate precious minerals from gangue, resulting in concentrates and tailings. Mining waste and mineral processing tailings necessitate proper disposal and management due to their potential significant environmental impact (Goronovski A. et al. 2018). These materials may also comprise trace elements, including radionuclides.

The radioactive materials that occur naturally in the environment are thorium and uranium decay chains and potassium (Alashrah, S. and Taher, A. E. 2017). The emanation of radiation from soil and rock is due to both the decay of the parent radionuclides to their daughter radionuclides and also depends on the mineral composition of the Earth's crust. Many industries, particularly the mining industries, have operated for a long time without knowing that their operations could give rise to NORM in environmental materials as well as in mining residues (Brian, H. et. al. 2012 and >Darko, E. O. et. al. 2005).

Bauxite is considered radioactive primarily due to the presence of naturally occurring radionuclides, particularly ²³⁸U, ²³²Th, and ⁴⁰K. These elements are concentrated in bauxite because the geological processes that form bauxite deposits also facilitate the accumulation of these radioactive materials. Significant activity concentrations of these radionuclides have been found in bauxite in a number of investigations that show increased levels in comparison to surrounding soils (Adams Udoji >Itodo et. al. 2023). The mineral makeup and unique geological circumstances of mining regions affect the concentration of these elements in bauxite, resulting in increased radiation levels that call for monitoring for any exposure-related health hazards (Aydan Altıkulaç 2022).

Radioactive substances are unique among all contaminants that adversely influence the quality of surface and groundwater and provide a dual hazard from toxicity and ionizing radiation. Many of these radionuclides have a large energy potential, which makes them both advantageous and dangerous. Radioactive materials are extremely valuable for energy and medicinal applications because of their special characteristics. Compounds and their byproducts can, however, be released into the environment through the mining, manufacture, use, and disposal of these materials, endangering both people and ecosystems (Kate Campbell 2021).

The ways that alpha, beta, and gamma radiation are exposed and how they affect tissue vary greatly. Exposure to ionizing radiation raises the risk of developing cancer and is mutagenic in all forms (WHO 2005 and EPA 2009). Although alpha particles have the lowest penetration depth, they are the most ionizing. The health implications, however, can be severe if an alpha-emitting source is consumed because alpha particles are the most harmful type of radiation. Although less harmful than alpha particles, beta particles can nonetheless damage cells and cause DNA mutations due to their greater penetration. Even when the source is not ingested orally, gamma radiation is very penetrating and can result in serious cell damage and mutagenesis.

The study undertaken was mainly to assess the NORM in water and soil from the mine and the environs of Sefwi Awaso Bauxite Mine and to estimate the health risk to both mine workers and community members residing in the vicinity of the mining area. Specific objectives were to identify and estimate the levels of ²³⁸U, ²²⁶Ra, ²³²Th, and ⁴⁰K in bauxite mining and also to estimate the exposure of workers and the public due to the consumption of water from the area as a result of the mining activities.

2. MATERIALS AND METHODS

2.2. Materials

2.2.1. Collection of Soil, Bauxite Ore, and Red Mud Samples

Soil, mining residue, and bauxite ore samples were collected within the perimeter of the Awaso bauxite mine and 5 km away from the surrounding communities. A simple random sampling technique was adopted where the samples were collected using a manual sampling tool, a shovel, into a polythene bag. The samples were aggregated from three different but close (within a meter radius) locations into one representative sample with a single coordinate. These coordinates were plotted as shown in Fig. (1). A total number of eight aggregated samples of all kinds, comprising soil, ore, and red mud samples, were collected at 5 cm depth using the hand shovel. Labeled polythene bags were used in the collection of the soil, ore, and red mud samples, respectively. The Awaso bauxite mine contains a plant that is mostly used for crushing. Thus, the boulders are crushed and washed on a machine, producing red mud, which is made from the collection of residues.

2.2.2. Preparation of Samples for Laboratory Analysis

The soil samples were first placed on metallic trays and air-dried for 7 days and then ground using a steel ball mill into fine particles and sieved with a 500 μm mesh size to have a uniform matrix size. The sieved soils were then placed in the oven to dry for 3 hours at 105 °C to completely remove excess moisture from the sample. The samples were then transferred to 500 ml labeled Marinelli beakers and tightly sealed and stored for 30 days, allowing short-lived daughters of radionuclides for both the ²³⁸U and ²³²Th decay series to attain secular equilibrium with their long-lived parent radionuclides.

3. EXPERIMENTAL

3.2. Calculation of Activity Concentration

The following expression was used to calculate the activity concentrations for natural radionuclides in soil, water, ore, and red mud samples (eq. 1):

(1)

where, A is the activity concentration, N_D is the net count area of the radionuclides in the sample, p is the gamma-ray emission probability, η(E) is the absolute counting efficiency of the high-purity Germanium detector system,T_c is the time taken for the sample to be counted, and m is the mass of the sample.

3.3. Calculation of Absorbed Dose Rate

The equation (2) below was used to determine the absorbed dose rate from soil samples (Inayatullah et. al. 2016).

(2)

where, D_γ is absorbed dose rate, DCF is dose conversion factors; 0.462, 0.0417 and 0.604 (nGyh^-1/Bqkg^-1) respectively for ²²⁶Ra, ⁴⁰K and ²³²Th, and A is Activity concentration of the radionuclide.

3.4. Calculation of Annual Effective Dose

To estimate the annual external effective dose, three factors were taken into consideration, i.e., the conversion coefficient from absorbed dose in the air to an effective dose; the outdoor occupancy factor of 0.2, which assumes an average semi-urban-rural lifestyle of the inhabitants where most adults spend their time in the farms away from the mine and the community, with children in schools most often; the number of hours per year (8760 h) and the annual estimated average effective dose received by a member of public is calculated using a conversion factor of 0.7 SvGy^-1, given as follows (UNSCEAR 2000) (eq. 3):

(3)

Where, E_γ is the annual effective dose and D_γ is the absorbed dose rate in soil.

3.5. Committed Effective Dose

The committed effective dose of adult persons by water ingestion is determined by the activity concentration of ⁴⁰K, ²²⁶Ra, and ²³²Th in the sample, and also the amount of water a person takes in liters per year. According to WHO guidelines for drinking water (WHO 2004), an individual is recommended to take 730 Ly^-1, and the ingestion dose coefficient for ²²⁶Ra, ²³²Th, and ⁴⁰K is 280, 230, and 6.2 nSvBq^-1, respectively (UNSCEAR 2000).

The equation (4) below is used to calculate the committed effective dose.

(4)

where, S_ing(w) is the committed effective dose, A_sp is the activity concentration of the radionuclides in the sample in Bq.l^-1, I_w is the intake of water in Ly^-1 and DFC_ing is the ingestion dose coefficient in SvBq^-1.

4. RESULTS AND DISCUSSION

Table 1 indicates the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K radionuclides present in the soil samples, bauxite ore, and red mud collected at the Awaso bauxite mining zone where the actual mining and processing takes place.

As shown in Table 1, the activity concentrations due to terrestrial gamma rays from ²²⁶Ra, ²³²Th, and ⁴⁰K in the red mud were recorded as (45 ± 5) Bq kg^-1, (64 ± 7) Bq kg^-1, and (125 ±19) Bq kg^-1 respectively. The values for both ²²⁶Ra and ²³²Th are above the world median values of 35 and 30 Bqkg^-1, while the ⁴⁰K figure is reported to be low as compared to 400 Bq kg^-1 reported in UNSCEAR publication (UNSCEAR 2000).

Table 2 is also a representation of values for radionuclide activity concentration (²²⁶Ra, ²³²Th, and ⁴⁰K) in the entire soil samples as collected at the six different locations within the Awaso township away from the center of mining activities in the study area. The activity concentrations at SS1 and SS5 are higher compared to the rest of the sampled location points. As indicated in Fig. (1), samples SS1 and SS5 locations are closer to the mine, whilst the rest of the locations are spaced further away from the mine site. The results obtained for the red mud and bauxite ore indicate that the values for ²²⁶Ra and ²³²Th are higher in them, whilst ⁴⁰K is lower in both samples. All the activity concentrations found in the soils from the study area are lower than the average activity concentration found in soil samples around the world, which is 35, 30, and 400 Bq kg^-1 for the radionuclides mentioned and higher for ore and red mud (UNSCEAR 2000).

As depicted in Table 3, the absorbed dose rate for the soils in the mining enclave is higher than that for the red mud and the bauxite ore, considering the higher radionuclide levels present in the sample, especially ⁴⁰K and their dose conversion factors. The arithmetic mean for the absorbed dose for the samples sampled within the Awaso community (soils) is (21 ± 3) nGyh^-1, which is the potential energy deposited into tissue with respect to exposure to ionizing radiation. The arithmetic mean for the absorbed dose for the samples sampled within the mining zone (soil, bauxite, and red mud) is (61 ± 4) nGyh^-1. Thus, the potential exposure is higher within the mining zone than the exposure in the community.

It is also illustrated in Table 3 that the annual effective dose was calculated for the samples, and it is observed that the dose for bauxite ore is higher than the one for the red mud and the soils. The calculated annual effective dose for the samples within the mine is 75 ± 5 µSv. The calculated annual effective dose for the samples within the community is 26 ± 3 µSv. These numbers are below the recommended dose limit of 20 mSv for workers and 1 mSv for members of the public, as stated by (ICRP 2007) in radiation protection principles for public radiation exposure control.

Using a radar plot, as shown in Fig. (2), allows for an intuitive visual comparison of absorbed doses and annual effective doses across different soil types or materials from the mining zone and the average soil within the township. This shows that soil from the mining zone has potential radiation exposure risks compared to soils within the township especially if considered as a building material in any form or shape.

Table 4 illustrates the comparison of activity concentrations of the bauxite ore in this work to those published by other investigations in different locations across the world. The activity concentrations recorded in this study are lower than what is reported from the countries representing some of the regions of the world. The overall effect is that bauxite ore contains higher radionuclides, ²²⁶Ra, ²³²Th, than the world average (UNSCEAR 2000). On the other hand, the activity concentration of ⁴⁰K for other studies, including Ghana, is within the world average figure of 400 Bq kg^-1, though it was not detectable in some countries (UNSCEAR 2000). The results obtained for bauxite ore indicate that the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K were (39.42 ± 4) Bq kg^-1, (97 ± 11) Bq kg^-1, and (15 ± 2) Bq kg^-1, respectively, in this study. It is also evident from this study that the activity concentration for both ²²⁶Ra and ²³²Th was above the world average.

CONCLUSION

The different radionuclides that were found to be connected with NORMs in bauxite mining were discovered to be ²²⁶Ra, ²³²Th, and ⁴⁰K. The level of exposure was determined by analyzing soil samples, bauxite ore samples, red mud samples, and water samples collected at the location where the study was carried out. According to the findings of the study, the levels of radioactivity in the soil for ²²⁶Ra, ²³²Th, and ⁴⁰K at all of the locations within the mine and the surrounding communities where soil samples were collected were well within the threshold for the global average activity concentration of soil. The activity concentration that was found to be present in red mud and the bauxite ore was found to be higher than the concentration averagely expected to be present in the soil. The levels of radioactivity in the water were found to be within the average range for the world for ²²⁶Ra, ²³²Th, and ⁴⁰K, and the committed effective dosage was calculated and compared to the world averages, respectively. In general, the results that were achieved for this study are comparable with the results that were obtained for other studies, such as those carried out in bauxite mines (UNSCEAR 2000 and WHO 2004). It also shows that the level of natural radioactivity in the soil, water, bauxite ore, and red mud in the study area is not considerable. As a result, it does not represent any significant risk of exposure to the general public or the personnel who work in the study region. The current data can be used to demonstrate that the bauxite mining operation does not significantly increase the activity concentration of ²²⁶Ra, ²³²Th, and ⁴⁰K beyond normal values. This can be interpreted as an indirect demonstration that the bauxite mining operation does not significantly increase the annual dose received by the Awaso community.

REFERENCES