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 Table of Contents 
ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 4  |  Page : 181-188  

Assessment of artisan's exposure to heavy metals from tantalite ore mining sites in Oke-Ogun, Oyo State, Nigeria


1 Department of Physics, Federal University of Agriculture, Abeokuta, Ogun, Nigeria
2 Department of Applied Sciences, College of Technological Studies, Public Authority for Applied Education and Training, Shuwaikh, Kuwait

Date of Submission07-Aug-2018
Date of Decision16-Nov-2018
Date of Acceptance24-Nov-2018
Date of Web Publication6-Feb-2019

Correspondence Address:
Dr. Ajetunmobi Abayomi Ekundayo
Department of Physics, Federal University of Agriculture, Ogun
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_57_18

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  Abstract 


Mining activities cause disequilibrium in the bound states of heavy metals in rocks, soils and mineral ore. These heavy and toxic metals find their ways into the food chain thereby posing health risks along the chain. Furthermore, the work scenarios in mining activities have exposure pathways such as inadvertent ingestion and inhalation of dust and materials from the sites . The dose levels due to heavy metals in tantalite samples from Oke-Ogun, Oyo State have been estimated. Twelve samples of tantalite ore [(Fe, Mn) Ta2O6], were obtained from the mining sites. 500mg of the pulverized samples were pelletized for their elemental analysis using proton Induced X-ray Emission technique. Hazard indices due to exposure to carcinogenic elements in Tantalite were estimated based on United State Environmental Protection Agency (USEPA) dose model. Chromium, Lead and Arsenic are the only carcinogenic elements found in the samples and average concentrations 311.05, 2533.26 and 127.35 ppm respectively and are higher than permissible limit recommended by Abundance elements in average crustal rock (AEACR) (100, 10.1 and 2-0 ppm respectively). The summation of hazard indices values estimated for carcinogenic elements are greater than unity. The estimated Life time average daily dose for the carcinogenic elements is 6.86E-06 mg/kg. The cancer risk estimated for the carcinogenic elements in Komu and Eluku is of the order Cr (1.32E-05) > Pb (5.83E-08). The total risk for these elements is 1.32E-05 and this value is within the permissible limit range of 10-4 - 10-6 recommended by USEPA, 1998.

Keywords: Artisan, exposure, heavy metals


How to cite this article:
Ekundayo AA, Mustapha AO, Okeyode IC, Michael GA, Darwish AA. Assessment of artisan's exposure to heavy metals from tantalite ore mining sites in Oke-Ogun, Oyo State, Nigeria. Radiat Prot Environ 2018;41:181-8

How to cite this URL:
Ekundayo AA, Mustapha AO, Okeyode IC, Michael GA, Darwish AA. Assessment of artisan's exposure to heavy metals from tantalite ore mining sites in Oke-Ogun, Oyo State, Nigeria. Radiat Prot Environ [serial online] 2018 [cited 2019 Nov 22];41:181-8. Available from: http://www.rpe.org.in/text.asp?2018/41/4/181/251679




  Introduction Top


Heavy metals are natural constituents of the earth crust and continuous human activities such as mining have changed their geochemical and biochemical equilibrium.[1] The in-balance equilibrium due to mining activities in the selected locations for the research will definitely affect the terrestrial ecosystem in the environment as a result of large volumes of sand excavated from the mining sites and the resulting accumulating tailings due to mining activities. These tailings usually contain high concentration of heavy metals from ores and the surrounding rocks which eventually finds its way into the soil, thereby increasing metallic concentration of the soil and continuous accumulation of these tailings at the sites will eventually find its way to farmland through flooding, thereby accessing the food chain. Heavy metal has been reported to be found in food crops and known to be of potential health hazard to man.[2] High concentration of these metals in the soil may cause long-term risk to ecosystem and humans.[3]

Exposure pathways at the sites include inadvertent ingestion of particles of these ore and tailings, inhalation of dust particles of the ore by the dealers during the crushing of tantalite to determine the grade of the tantalite, and through dermal contact with the ores and tailings. Excess of heavy metals in the body has a negative health effect. It accumulates on the wall of coronary arteries and can cause obstruction in easy flow of blood in these arteries. This can result to cardiovascular blockages.[4] Heavy metal inhalation may affect the normal functioning of the body immune system.[4] Other adverse effects of mining activities have been listed by various research works [5],[6],[7],[8] to include ecological disturbance; destruction of natural flora and fauna; pollution of air, land, and water; instability of soil and rock masses; landscape degradation; and radiation hazards. Action plan to address these challenges will be enhanced if relevant information about the elemental components of tantalite that is mined at the selected sites is known. This will assist in planning some remedial measures and policies aimed at resolving the occupational and public health effect of mining of tantalite in the selected area and by extension, Nigeria at large. The research work is aimed at estimating the dose received by workers and the public from the heavy metals in tantalite as a result of the work activities in the site and the surrounding villages using USEPA model.[9] The research works will also broaden the existing database on this subject of interest.


  Materials and Method Top


Determination of elemental components of tantalite

Elemental components of pulverized samples of tantalite from selected mining sites were determined using particle-induced X-ray emission (PIXE) technique at the Centre for Energy Research and Development (CERD), Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. In the PIXE technique, particle beam hits a sample and characteristic X-rays are emitted from the sample elements. These X-rays were measured with detectors and from the recorded spectra of the elemental composition of the sample were analyzed. PIXE offers its maximum sensitivity or minimum detection limits in two atomic number regions, 20 < Z < 35 and 75 < Z < 85.

Sample preparation

A total of nine samples were purchased from the miners and dealers of tantalite at the sites. Appropriate codes (labels) were used to identify these samples and the samples were taken to CERD, Obafemi Awolowo University, Ile-Ife, for analysis. At CERD, the samples were broken into pieces and crushed into powder form using a small porcelain mortar and piston. They were made into pellets of 500 mg and 13-mm diameter using pellet making set which consists of a speck, cop, dice, and hydraulic press.[1],[2]

Particle induced X-ray emission analysis for elemental components of tantalite

The PIXE analysis was carried out using proton beams produced by ion beam analysis (IBA) facility of the 1.7 MeV Tandem accelerator located at the CERD, University of Ife, Nigeria.

Calibration of particle-induced X-ray emission

The PIXE system was calibrated using an in-house calibration system and the National Institute of Standards and Technology geological standard for coltan was used for the quality assurance control of the result.[10] The IBA facility used in this analysis consists of 5SDH modeled, NEC Tandem Pelletron accelerator complete with an end station made up of aluminum chamber of about 150 cm in diameter and 180-cm high.

The samples were irradiated by a 4-mm diameter beam of protons with energy of 2.5 MeV and beam current of 0.2 nA for 900 s. The chamber of the accelerator has four ports and a window. Port 2 that is inclined at 135° to the horizontal was used for PIXE detector. This detector is an ESL X30-150 model of a Canberra Si (Li) Detector (Resolution 175 eV at 5.9 keV), coupled with a Canberra Inspector -2000 Digital Signal Processor. Mirion Technologies (GDS) Inc. Irvine, California, United States Canberra Genie 3.1 software was used for acquisition of the PIXE data,[11],[12] while the Cups Computer Code was used for fitting the experimentally generated PIXE spectrum before quantitative analysis.[13] The software (Genie 3.1) automatically estimates the limit of detection for all the elements detected in the tantalite samples from all the mining sites [Plate 1].



Health risk assessment method

The model used for health risk assessment for estimating human exposure to metals in this study is based on criteria of the United States Environmental Protection Agency [9] and the Dutch National Institute of Public Health and Environmental Protection.[14] The model has been used in different works to estimate the dose from heavy metals for different materials.[15],[16],[17],[18] Hence, it is not out of place to use the model to investigate the health risk associated with the exposure of the artisan miners and the public to heavy metals in tantalite. The health risk assessment focused on two separate sets of people: children and adults. Exposure to metals can occur through three main paths: (a) inadvertent ingestion of tantalite during the packing and carrying the tantalite up and out of the underground mines, (b) direct inhalation of tantalite dust through the mouth and nose during the process of crushing of tantalite and magnetic separation of tantalite from other metallic ores (removal of impurities from tantalite), and (c) dermal contact with tantalite during harvesting or removal the tantalite from the concealing rocks, crushing of tantalite, and other work activities in the sites. Equations 1–4 proposed by the model were used to calculate the daily exposure due to the exposure pathways of consideration.







The lifetime average daily dose (LADD) (mg/kg/day) for the carcinogenic heavy metals in the study was used to estimate the cancer risk as a result of exposure to the carcinogenic elements in tantalite from two of the mining sites. The equation is given by Equation 4.



For the estimation of Hazard Index (HI) due to the exposure to noncarcinogenic elements in the tantalite samples from the sites, the doses calculated for each metal for the three exposure pathways were divided by the corresponding reference dose for the pathways, (Acute reference dose; mg/kg/day) and this gives the hazard quotient (HQ).

For exposure to carcinogenic elements in the tantalite, the dose estimated for the pathways were multiplied by the corresponding Slope Factor (SF) (mg/kg/day)–1 to give the estimation for the level of cancer risk. The hazard index (HI) is then the sum of HQ.[19],[20],[21],[22],[23]

Carcinogenic risk is the probability of an individual developing any type of cancer from lifetime exposure to carcinogenic hazards. The acceptable or tolerable risk for regulatory purposes is in the range 10−6–10−4.[9] These values indicate that one additional case in a population of 1,000,000 to one in 10,000 people is acceptable.[24] The values of parameters used in the study for estimating doses due to the exposures to heavy metals in tantalite are presented in [Table 1] and [Table 2] and the values of these parameters are substituted appropriately in equations.
Table 1: Parameters and their values used for the estimation of daily dose in the study

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Table 2: Toxicological characteristics of investigated heavy metals in the study (USEPA, 2001)

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The exposure point concentration, mg/kg/day (C) in Equations 1–4, is an estimate of reasonable maximum exposure [17],[19],[20],[25] and was calculated as the upper limit of the 95% confidence limit for mean of the log-transformed values of the concentration of the heavy metals (mg/kg/day) using SPSS-17, IBM, Armink, New York, Unted States (released 2008, version 17). Equation 5 was used to obtain the values of exposure point concentration C.



Where y is the arithmetic mean of the log-transformed data of the values of the concentration of the heavy metals in mg/kg/day, S is the standard deviation of the log-transformed data, S 2 is the variance of the log-transformed values of the concentration of heavy metals, H is the value from the H-statistic table,[21] and n is the number of samples.

This scope of the study is estimations of dose from heavy and toxic metals for two out of four of the mining sites. This is due to the limited number of samples that were analyzed for SP and GB mining site.


  Results and Discussion Top


The section presents the result of the elemental composition and concentrations of tantalite ores from the sites using proton-induced X-ray emission. [Table 3], [Table 4], [Table 5], [Table 6], [Table 7] and [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8] show the results and pictorial representation of the results, respectively. Chromium, lead, and arsenic are the only carcinogenic elements found in the samples and average concentrations 311.05, 2533.26, and 127.35 ppm, respectively, and are higher than permissible limit recommended by abundance elements in average crustal rock [22] (100, 10.1, and 2-0 ppm, respectively).
Table 3: Elemental composition and concentration samples of tantalite from Mining sites in Oke-Ogun

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Table 4: Exposure dose, hazard quotient risk for each noncarcinogenic element, and exposure pathways for adult and children in Ko mining site

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Table 5: Exposure dose, hazard quotient risk for each noncarcinogenic elements, and exposure pathways for adult and children in EL mining site

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Table 6: Estimated lifetime average daily dose (mg/kg/day) and cancer risk of carcinogenic elements Cr and Pb for KO miming site

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Table 7: Estimated lifetime average daily dose (mg/kg/day) and cancer risk of carcinogenic elements Cr, Ni, As, and Pb for EL miming site

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Figure 1: Estimated dose of adult for different exposure pathways due to heavy metals in tantalite for KO mining site

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Figure 2: Hazard quotient and HI values for adult for different exposure pathways due to heavy metals for KO mining site

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Figure 3: Estimated dose for children for different exposure pathways due to heavy metals in tantalite for KO mining site

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Figure 4: Hazard quotient and HI values for children for different exposure pathways due to heavy metals in tantalite for KO mining site

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Figure 5: Estimated dose of adult for different exposure pathways due to heavy metals in tantalite for EL mining site

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Figure 6: Hazard quotient and HI values for adult for different exposure pathways due to heavy metals in tantalite for EL mining site

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Figure 7: Estimated dose of children for different exposure pathways due to heavy metals in tantalite for EL mining site

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Figure 8: Hazard quotient and HI values of children for different exposure pathways due to heavy metals in tantalite for EL mining site

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The order of health index (HI) values is of the order Mn > Zn, in addition, the estimated exposure dose is of the order Ding > Dderm > Dinh. Similarly, the order of the HQs for the exposure pathways for the site is HQing > HQderm > HQinh. The HQ value of ingestion for children is 1.43 times greater than that of adult and accounted for about 58.26% of the HQ for ingestion, while the adult accounted for about 40.86% of the total value of the HQ for ingestion. This result may be attributed to the special behavioral patterns of children, particularly frequent hand-to-mouth contact or “syndrome.”

Furthermore, summation of hazard index (ΣHI) value for the site for KO mining site was used to access the overall potential for noncarcinogenic effects from different pathways for children resident in the villages around the mining site. The dealers are reported to crush the tantalite outside their houses, thereby allowing the release of the dust form of these metals into the surrounding air where children play around.

In addition, magnetic separation of the tantalite from other metallic species is carried out using bare hands and a sieve. This procedure may also contribute to inhalation exposure pathway. For KO site, the summation of the hazard index (ΣHI) is <1. This implies that there is no significant risk of noncarcinogenic effect as a result of the miners' children exposure to the heavy metals in the tantalite from the environment.

For EL site, the estimated hazard index for elements vanadium (V), manganese (Mn), and zinc (Zn) is in increasing order of V (2.23E-02) > Mn (2.78-03) > Zn (3.52E-05) for children. In addition, the estimated exposure dose is of the order Ding (1.56E-04) > Dderm (2.29E-08) > Dinh(1.78E-08). The explanation for this follows the same trend as that of KO mining site. The order of the HQs for the exposure pathways for the site is HQing (1.20E-02) > HQderm(7.50E-04 > HQinh (1.24-E-02). For EL site, the summation of the hazard index is ΣHI (2.51E-02) <1. This implies that there is no significant risk of noncarcinogenic effect as a result of the miners' exposure to the heavy metals in the tantalite from this site during their work activities.

The order of hazard index (HI) values is of the order V (2.68E-02) > Mn (4.12E-03) > Zn (7.80E-05) and the summation of hazard index for the site is <1. The estimated exposure dose is of the order Ding (3.64E-02) > Dderm(3.56E-07) > Dinh (1.02E-08). Similarly, the order of the HQs for the exposure pathways for the site is HQing > HQderm > HQinh.

The HQ value for children is 2.3 times greater than that of adult and accounted for 70% of the HQ for ingestion, while the adult accounted for about 30% of the total value of the HQ for ingestion. This shows that the children stand the chance of being contaminated by these heavy metals in tantalite as of results of the activities of the dealers and miners.

Finally, for EL mining site, the summation of the hazard index for children is ΣHI <1. This implies that there is no significant risk of noncarcinogenic effect for children in the villages surrounding the mining site. In the study of Li H,[27] hazard index values for all studied metals were lower than the safe level of 1 and Pb exhibited the highest risk value (0.125) in the case of children. The carcinogenic risk for Cd, Co, Cr, and Ni was all below the acceptable level and the result is similar to the present study.

In KO mining sites, two elements are known to be carcinogenic of all the elements in tantalite from the site, that is, Pb and Cr [Table 5]. The estimated LADD for all the elements is of the same value (6.86E-06). The cancer risk estimated for the carcinogenic elements are of the order (1.33E-05) > Pb (5.83E-08). The total risk for these elements is 1.32E-05 and this value is within the permissible limit range of 10−4–10−6 recommended by USEPA.[9] This implies that both the adult and the children are not at risk of cancer as a result of being exposed to theses carcinogenic heavy metals in the tantalite from KO mining site.

For EL mining sites, four elements are known to be carcinogenic of all the elements in tantalite samples from the site (Cr, Ni, As, and Pb) [Table 6]. The estimated LADD for all the elements is of the same value (6.86E-06). The cancer risk estimated for the carcinogenic all the elements are of the order of As (1.39E-04) > Ni (1.74E-05) > Cr (1.32E-05) > Pb (5.83E-08).

The estimated cancer risk for element As is highest with a value of 1.39E-04 and contributed to about 89.80% of the total cancer risk with a value of 1.47E-03. Followed by asernic As is nickel Ni (1.74E-05) which contributed to about 81.76% of the total estimated cancer risk. The sum of all the estimated cancer risk for all the elements is higher than the permissible limit with a range of 10−4–10−6. This implies that both the adult and the children have slightly more risk of cancer as a result of being exposed to theses carcinogenic heavy metals in the tantalite of EL mining compared to standard range.


  Conclusions Top


The work activities at the sites do not expose the artisan miners to any risks from both the carcinogenic and noncarcinogenic elements in the tantalite from Oke-Ogun, Oyo State, tantalite-mining sites.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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