Instrumental Neutron Activation Analysis (INAA) Assessment of REE of Soil from Mining Site Umuahia, Abia State, South East, Nigeria

: In this work, the main goal was to examine the REE concentration and distribution pattern in soil from Umuahia, Abia State, Southeast, Nigeria, in a clay mining site compared with a non-contaminated site approximately 3 km away. The REE were determined by Instrumental Neutron Activation Analysis (INAA) at IPEN, Brazil. The main mineralogy of the samples was determined by XRD. The following REE were determined: Ce, Eu, La, Lu, Nd, Sm, Tb and Yb. Soil samples from the mining area present quartz and kaolinite as their main constituents, with REE concentrations comparable with that of the Upper Continental Crust and the North American Shale Composite. The calculated Geoaccumulation index (IGeo) indicate that REE have natural origin and ∑LREE N /∑HREE N ratio show an enrichment of the light over heavy REE, in samples of the clay mining area.


INTRODUCTION
Soil is the basic and needful part of the earth's crust, the natural medium for the growing of earth's crops and plants, having lots of purposes and uses in everyday activities [1].Soil is also the host of mineral ores, among them, those containing rare earth elements (REE) [2].
Rare earth elements are the members of fifteen metallic elements that compose the lanthanide series.Some authors include scandium and yttrium in this group [3].REE have resembling electronic structures providing them similar chemical and physical properties [4], they are also called technology elements or metals because of their wider application in the world of technology as well as in pure science and medicine [5].There are more than 160 minerals containing these elements, most of them are in the form of silicates, carbonates, oxides and phosphates [6,7].The estimated average concentration of the REE in the Earth's crust ranges from around 130 µg g -1 to 240 µg g -1 which is significantly higher than other commonly exploited elements [8].In trace amounts these elements may be found, besides soil, in plants, water, and the atmosphere.They can also be spread in environments due to anthropogenic activities and be incorporated into the human body through several exposure pathways specifically via food ingestion [9].The process of discarding old fashioned devices containing REE, applications of phosphate fertilizers containing REE, mining, and scattering/spreads from indigenous rocks may increase the probability of environmental REE contamination [5,10].
The possible impacts of REE on ecosystems are relatively unknown, therefore, the propensity for exposure and transfer into living organisms needs to be assessed to understand ecosystems and human health threats since the escalation of soil contamination may be continuously increasing because of the input from various human activities such as p. 4 mining, industry, agriculture, highways, and road development [11].Besides that, soil contamination can also originate from geogenic processes such as landslides, terrestrial runoffs, and land erosion [10].
Rare earth elements are classified as: light rare earth elements (LREE) comprising the group formed by La, Ce, Pr, Nd, Sm, Eu and heavy rare earth elements (HREE), including Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu [12].Also, REE may be classified as middle rare earth elements (MREE) founded on their atomic mass and effective ion radius and comprises the elements Sm, Eu, Gd and Tb [13].REEs have significant applications in modern healthcare, telecommunication, agriculture, petroleum refining, automotive, electronic components, semiconductor and defense, etc. [5].
In Nigeria, the mining sector boasts more than 40 different types of commercially  [14].During exploitation of these minerals, soil, air, and water pollution may occur especially when these operations are done in an artisanal manner as seen in most mining areas in Nigeria [15].
The main aim of this work is to investigate the REE distribution in Nigerian soil samples from a clay mining area with the goal of assessing their concentrations, distribution pattern, and possible contamination.For comparison, soil samples from a farmland, located at approximately 3 km away, out of the influence of the mining site and considered non-contaminated, was chosen as the control site.Elemental concentrations and mineralogy were determined by instrumental neutron activation analysis (INAA) and X-ray diffraction (XRD) respectively.

Study Area
The study area is located in Umuahia South, Abia State, Southeast, Nigeria, where the mining of clay is being carried out.The coordinates of the sampling points are presented in supplementary material S1.Umuahia South is one of the local government areas of Abia State and the sampling location is shown in Figure 1.It has a mean area of 140 km² and a population of 138,570 according to the 2006 census.For comparison, four points were chosen as a control area in farming land and a built-up area, about 3 km from the clay mining site.
The mining site is about 5 km from Umuahia town and about 30 km from Aba, Enyimba City of Abia state.The most common occupation of Umuahia is trading and some are civil servants.The average annual temperature is 26.4°C.About 2333 mm of precipitation falls annually.The vegetation is ordinarily considered part of tropical rain forest which is the dominant natural vegetation in most parts of southern Nigeria.

Samples Preparation
Soil samples were collected randomly covering the entire mining sites.All the soil samples were collected using an auger sampler at a depth of 5-15 cm for every sampling point, and the soil samples were lowered into the labeled plastic bags [16].At each point, about 1 to 1.5 kg was collected.Every bag with a soil sample was properly sealed and then transported to the laboratory for preparation processes.
At the laboratory, the soil samples were air dried in trays for 7 days and then oven dried at a temperature of 105°C for between 3-4 hours until they presented a constant weight [17], at the central laboratory of Federal University Wukari.After drying the soil samples were pounded using a mortar and pestle.A sieve of mesh size 2 mm recommended in [18] was used to obtain uniform particle sizes.The sieved prepared soil samples were flown to Nuclear and Energy Research Institute (IPEN from the Portuguese Instituto de Pesquisas Energéticas e Nucleares) São Paulo, Brazil, for analysis.At IPEN, approximately 80 mg and 120 mg of reference materials (Syenite, Table Mountain, STM-2 from USGS and Standard Reference Material SRM-1646a from NIST) and samples were prepared and weighed for irradiation.Each of the samples and reference materials were tightly sealed in clean polyethylene bags, to avoid contamination.Chemical standard solutions were also prepared, pipetted in filter paper strips.

Analysis of Samples
The samples, reference materials and pipetted samples were placed in separated polyethylene and wrapped in aluminum foils, packaged in aluminum containers, named rabbits, and hermetically sealed for irradiation.In the irradiation scheme, samples were irradiated for 8 hours and allowed to decay for seven days before the first counting set, and for 15 days for the second counting set.Samples, reference materials and pipetted standards were counted for 7200 seconds.

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For the gamma counting a p-types high pure germanium detector (HPGe), from Canberra (USA), with relative efficiency ≥ 40%, resolution ≤ 1.800 keV FWHM at 1.33 MeV was used.Daily verification performance was done using the isotopes 57 Co, with an energy of 122 keV and 60 Co with an energy of 1332 keV.The data analysis was done using the stationary in house gamma ray software programme known as CAX, which identifies the required gamma ray peaks of the interested nuclide elements [19].
For X-Ray diffraction analysis, a SmartLab SE diffractometer from Rigaku was used with a Cu Tube X-Ray generator (λ=1.54Å).The measurements were performed in powder samples in a range from 10º to 70º at a speed of 1º/min and step of 0.02º.

Geo-accumulation Index (IGeo)
Geo-accumulation index is applied in the determination of the extent of anthropogenic influence and compares various metals existing in the earth's crust [20].It was first applied by [21] and has been used by many authors.Mathematically, it can be computed using Equation 1: Where IGeo is the geoaccumulation index, CREE is the concentration of the REE in the material soil sample (µg g -1 ), Bn is the concentration of the REE in control or reference value (µg g -1 ), and the factor 1.5 is employed to reduce the possible variations in the background data because of lithological variations in material sample concentration.
Lithological variation is the natural factor that affects the spatial elemental distribution in the soil [22].The following are the range of categories of the geo-accumulation index [23].The upper continental crust (UCC) REE concentrations were applied for values of Bn.

RESULTS AND DISCUSSIONS
The mineralogical characterization of the soil samples from the clay mining area and from the control site are depicted in the diffractograms shown in Figures 2 and 3  It can be seen in Figure 4 that no significant difference exists between samples from the clay site mining, control site, UCC and NASC indicating that the REE present in the samples are of natural origin.This conclusion is corroborated by the IGeo values shown in Table 2, where it can be seen that, with just a few exceptions, the values are < 0. Only the samples SUA3 and SUA7 presented Igeo values > 1 for Lu and SUCA2 for Ce.As no evidence of a pollution source could be identified, these values may probably be attributed to a variation in soil matrix, such as organic matter, changes in pH and redox potential [24].
The REE concentrations found are in good agreement with those found by [25] for soils from Nigerian semi-savanna and rain forest regions.values indicating Ce enrichment [26].Europium anomaly, denoted by Eu*, may also occur.

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If Eu has been present exclusively as Eu(III) it will behave as its closest lanthanides neighbors.Conversely, if Eu is present in its reduced form, as Eu(II), this result is the so-called Eu anomaly [27].According to the used equation, positive Eu* will present values higher than 1 and negative Eu* will present values lower than 1.These changes in redox conditions may the responsible for the higher observed Ce values normalized by the chondrite in the above-mentioned samples, since oxidizing conditions favors the tetravalent and more insoluble Ce form.No significant fractionation was observed for Eu in the samples.

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The values for Ce* and Eu* anomaly is presented in Table 3. Sample SUCA2 is the one with the higher values for Ce*, and SUA12 is the one that present the highest Eu* value.for the samples SUCA2, the ∑LREEN/∑HREEN are very close to the unity in the control site, while the samples of the clay mining area generally present values slightly higher.The high value of this ratio may arise out of the higher affinity of LREEs for the clay fraction in the soil system [28].In the attempt to characterize the clay samples according to their REE pattern, principal component and classification analysis was applied.The result is shown in Figure 7.

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The samples with positive loading for the first and second factors are characterized by the higher values of the ∑REE, ∑LREE, and ∑LREEN/∑HREEN ratio.This factor also includes sample SUCA2 that present a high Ce* value.Samples with positive loading for the first factor and negative loadings for the second factor (fourth quadrant) are characterized by the higher values of LaN/YbN, LaN/LuN, LaN/LuN ratios.ND -not determined.The subscript N indicates that the concentration is normalized by the UCC.Ce* was calculated by the formula Ce*/CeN = ((La/LaN) 2/3 ×(Nd/NdN) 1/3 ) -1 [29].Eu* was calculated by the formula Eu* = EuN/(SmN 2 × TbN) 1/3 [26].

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All the other samples compose a more homogeneous group.The samples presenting enrichment in LREE are probably related to the amount of kaolinite that is a detrital mineral, generally derived from highly weathered soils [30], since REE may be adsorbed as impurities or incorporated as substituting elements into the crystalline structure of this host minerals [31].

CONCLUSIONS
The samples of soil collected in a clay mining area, from Umuahia, Abia Stae, Southeast, Nigeria, as well as, soil samples from an area chosen as control, approximately 3 km away, from a farmland area, was analyzed to determine the REE concentration, their distribution pattern, mineralogy and possible anthropogenic contamination.
It was determined that the soil samples are composed basically of silicate and kaolinite minerals.The obtained concentrations are in agreement with the ∑REE of 130 µg g -1 to 240 µg g -1 and comparable with the global standards UCC and NASC.IGeo values, calculated using the UCC values as normalizer indicate that no anthropogenic influence can be observed in these samples, being the incorporation of the REE in the soil samples a result of geological

Figure 2 : 9 Figure 3 :
Figure 2: Diffractogram of soil samples from the clay mining area

Figure 5 :Figure 6 :
Figure 5: Normalized REE from clay mining and control areas by upper continental crust (UCC)

Figure 7 :
Figure 7: Principal component and classification analysis obtained for the soil samples from the clay mining and control area based in the data presented in Table3 viable minerals.Only clay-rich sediments are mined in more than 80 clay deposits in all parts of the country.Examples are, clay deposits in Abak, Akwa Ibom State, Uruove near Ughelli in Delta State, Ifon in Ondo State, Mokola in Oyo State, Sokoto in Sokoto State, Gombe in Gombe State, Dangara in Niger State, Umuahia in Abia State, Onitsha in Anambra State and Kutigi in Niger State, etc

Table 1 :
Concentrations, in µg g -1 , of the REE in the samples from the clay mining area (SUA) and from the control site (SUCA), upper continental crust (UCC), chondrite and North American Shale Composite (NASC)

Table 2 :
IGeo values calculated for the samples from the clay mining area (SUA) and from control area (SUCA)

Table 3 :
Normalization values of the REE in the soil samples from the clay mining and control area