Research - (2020) Volume 7, Issue 5
Received: 05-Sep-2020
Published:
25-Nov-2020
, DOI: 10.37421/2380-2391.2020.7.278
Citation: Boisa Ndokiari, Cookey Grace, Okpulor Happiness. “Potentially Toxic Elements (PTEs) in Surface Soils of Gemstones and Lead Mining Communities: Differential Mineral Phases and Exposure Risks.” J Environ Anal Chem 7 (2020): 278.
Copyright: © 2020 Ndokiari B, et al. This is an open-access article distributed under the terms of the creative commons attribution license which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.
Proper human health risk assessments are rarely conducted for mining communities, consequently when there is need for remediation to be conducted and compensation to be paid to impacted population they are typically done, assuming equal exposure risks from varying mining activities. This study was conducted to compare the distribution of potentially toxic elements in surfaces soils of gemstone and lead mining towns in Nigeria, and their anticipated exposure risks. To achieve this thirty-two surface soil samples were collected from Eggon, a gemstone mining town in Nasarawa State and Enyigba, a lead mining town in Ebonyi State. The samples were air dried, disaggregated and sieved through a 2000 μm mesh with the <2000 fraction retained for aqua regia digestion. Potentially toxic elements bound in the samples were quantified with Agilent Technologies 4210 (MP-AES). Mineral phases were identified and quantified using using a inXitu’s portable transmission XRD/XRF instrument (Terra) with a miniature X-ray tube and a CCD detector for collection both XRD signatures. The PTEs, As, Cd, Mn, Pb and Zn were higher in surface soils of lead mining town. At the gemstone mining town most the PTEs indicated similar concentrations at both mine site and residential areas, while at the lead mining town of the PTEs indicated higher concentrations at mine sites and mine road. At the lead mining town the estimated daily exposure doses for Pb were above threshold values irrespective of the exposure scenario. Mineralogy data indicated most of the PTEs in mineralized phases.
Surface soils • Gemstones • Lead • Mining • Mineralogy • Exposure risk
Several emerging and industrial cities in developing countries are suffering from the impact of old technologies associated unacceptable emissions and wastes. Due to emerging environmental advocacy groups, governments are under intense pressure to remediate impacted land areas and compensate local human population. The desire by responsive governments to alleviate pollution impact is limited by the lack data relating to pollution legacies different industrial outfits. This study aims to generate data related to impacts anticipated on communities exposed to gemstones and lead mining activities. Both industries are classed as mining but data related their ranking is not available.
Previous studies [1-4] employed several geostatistic, multivariate statistical analysis tools like ANOVA, cluster analysis, inter-metal correlations and others for the identification of both natural and anthropogenic activities that introduce potentially toxic metals in surface soils. In addition to the use of statistical tools, mineralization has also been employed to tracing sources of potentially toxic metals in soil and underground media [5]. In this study surface soil from residential setting, mining site and mine wastes were sampled from two mining towns, Enyigba in Ebonyi state and Eggon in Nasarawa state both in Nigeria were analyzed for acid digestible fraction of arsenic, cadmium, lead, manganese and zinc and their likely mineralization.
Study area
Enyigba is located in south eastern Nigeria bordered by Abakiliki and Ikwo in the north and south, respectively (Figure 1). The economy of the town has been dominated by artisan mining for Pb and Zn, and farming. The population of Enyigba was estimated to be around 8,000 and that of Eggon 110, 613 based on the 2006 census figure [6,7]. Eggon is located in northern Nigeria bordered by Akwanga and Lafia in the north and south, respectively (Figure 1). The economy of Eggon has been dominated by gemstone mining and farming. The major ore deposits at Enyigba are suggested to be Pb-Zn minerals, while deposits at Eggon are quartz, mica and granite [8]. There is a potential of both locations in the future becoming major mining town with associated metalliferous wastes, since Nigeria is desperately working to diversify its economy due to dwindling foreign exchange from crude oil export.
Figure 1. Map of the two study locations, Eggon and Enyigba in Nigeria.
Sample collection
A total of 32 surface soil and mine waste samples were collected from Eggon in Nasarawa State and Enyigba in Ebonyi State with a stainless steel trowel. Sampling focused upon mine sites, mine roads, streets and residential areas in both towns. Each sample consisted of 3 closely spaced subsamples, which were bulked to produce composite. In the laboratory the samples were air dried, disaggregated and sieved through a 2000 μm mesh with the <2000 fraction retained for digestion.
Aqua regia digestion and analysis
Aqua regia digestions (HCl:HNO3 in the ratio 3:1 v/v) were performed on 0.5 g sub-sample in digestion tubes mounted on hot plate in fume cupboard. The filtrates obtained from the digestion were stored in the refrigerated prior to analysis with Agilent Technologies 4210 (MP-AES).
Soil ingestion exposure assessment at Eggon and Enyigba
It is possible to estimate human exposure risk associated with the ingestion of surface soils at the different study locations. PTEs daily intake (PTE DI) from ingestion of surface based on total concentrations of the different PTEs of interest in this study were obtained using the equation.
PTE DI=(EC × SIR × ED)/1000BW
Where , EC is the concentration of PTEs (μg/g), SIR is the soil ingestion rate (mgd-1) which for this study was set at 200 mgd-1 for 2-6 year child against that (100 mgd-1) suggested by USEPA [9] because of dusty nature of tropical Africa. ED (unitless) is the exposure duration. ED was set at different values, 1 for residential [10]. In Nigeria artisan working mother that are nursing children below 6 year go along with such kids. Also most casual workers in developing countries work for at least 12 hrs per day. The absence in most cases of functional roads and lack motorized means of transport at rural setting in developing countries, suggest that workers with their children spend between 4-hours on roads per day. Consequently the ED for children at mine site and street/road were set at 0.5 and 0.17, respectively. BW is the body weight of the children between 2-6 years. The default BW for children of the age grade was set at 17.8 kg [9].
X-ray diffraction analysis
To identify possible origins of the potentially toxic elements in streets and residential areas a limited number of samples (n=15) were selected for mineralogy. Subsamples were prepared and scanned using a inXitu’s portable transmission XRD/XRF instrument (Terra 575, USA) with CoKa (1.7903000 A) α radiation. The instrument is fitted with a miniature X-ray tube and a CCD detector for collection both XRD signatures. The scan was run from 5 to 55° (2-theta scale), with increments of 0.02° and a counting time of 1.0 second per step. The operating conditions were 40 kV and 40 mA. Peak identification was carried out with the Match, 2003-2019 CRYSTAL IMPACT, software package (Bonn, Germany).
Quality control
Analytical accuracy of aqua regia extractions was assessed against a reference material, BCR 143R (aqua regia certified sewage sludge amended soil) and the inclusion of blank digestion for every run. Acceptable results were obtained for the aqua regia extractable concentrations of PTEs compared to certified values.
This study was conducted to compare the distribution of potentially toxic elements in surfaces soils of gemstone and lead mining towns in Nigeria, and their associated exposure risks. Thirty-two surface soil samples were collected from Eggon, a gemstone mining town in Nasarawa State and Enyigba, a lead mining town in Ebonyi State. At Eggon Nordstromite Coronadite, Altaite, Cuprite and Antimony lead manganese oxide mineral phases where identified. Gemstone mineral found at Eggon were Muscovite; a hydrated phyllosilicate commonly found in granite, Argentopyrite with monoclinic crystal system, and Brianroulstonite; with a trigonal crystal system. Published literature has not previously indicated existence of Brianroulstonite mineral in Nigeria. The mineral phases observed in Enyigba surface soils were, Arsenopyrite, Berlinite, Cadmium lead (IV) oxide, Manganese lead antimony selenide, Magnesite, Pyrite, Quartz, Ramsdellite, Wulfenite and Zincochromite.
Both mining towns indicated varying concentrations of As, Cd, Mn, Pb and Zn for the different exposure scenarios (residential, mine road, streets and mine site), with the lead mining town, Enyigba indicating higher concentrations than the gemstone mining town, Eggon. Consequently the different mining towns indicated varying potential exposure doses for the PTEs studied. As indicated estimated daily exposure doses above existing threshold value, (0.3 μg Cd Kg-1 BW d-1) at both gemstone and lead mining communities at residential areas, suggesting the likelihood As poisoning to human population. Pb indicated estimated daily exposure doses above existing threshold value, (3.6 μg Cd Kg-1 BW d-1) at both and lead mining community alone irrespective of the exposure scenario, suggesting the likelihood Pb poisoning to human population.
The results of this study have highlighted the need to conduct risk assessment for individual mining towns, since they may indicate varying concentrations for PTEs and also different mineral phases.
We thank the Head of Department, Dr. J.L. Konne for providing conducive atmosphere for conducting the research.
Journal of Environmental Analytical Chemistry received 1781 citations as per Google Scholar report
