Assessment of the levels of natural radionuclides and heavy metals and their transfer factor from soil to crops/vegetables in some agricultural soil in barkin ladi area, plateau state, Nigeria

Received: 07-Aug-2022, Manuscript No. PULJECT-22-5492; Editor assigned: 13-Aug-2022, Pre QC No. PULJECT-22-5492(PQ); Accepted Date: Aug 30, 2022; Reviewed: 14-Aug-2022 QC No. PULJECT-22-5492(Q); Revised: 16-Aug-2022, Manuscript No. PULJECT-22-5492(R); Published: 24-Sep-2022, DOI: 10.37532/ pulject.2022.6(5);01-05.

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Abstract

Geothermal Heat Pumps (GSHPs), or Direct Expansion (DX) ground source heat pumps, are a highly efficient renewable energy technology, which uses the earth, groundwater or surface water as a heat source when operating in heating mode or as a heat sink when operating in a cooling mode. It is receiving increasing interest because of its potential to reduce primary energy consumption and thus reduce emissions of the greenhouse gases (GHGs). The main concept of this technology is that it utilises the lower temperature of the ground (approximately<32°C), which remains relatively stable throughout the year, to provide space heating, cooling and domestic hot water inside the building area. The main goal of this study is to stimulate the uptake of the GSHPs. Recent attempts to stimulate alternative energy sources for heating and cooling of buildings has emphasised the utilisation of the ambient energy from ground source and other renewable energy sources. The purpose of this study, however, is to examine the means of reduction of energy consumption in buildings, identify GSHPs as an environmental friendly technology able to provide efficient utilisation of energy in the buildings sector, promote using GSHPs applications as an optimum means of heating and cooling, and to present typical applications and recent advances of the DX GSHPs. The study highlighted the potential energy saving that could be achieved through the use of ground energy sources. It also focuses on the optimisation and improvement of the operation conditions of the heat cycle and performance of the DX GSHP. It is concluded that the direct expansion of the GSHP, combined with the ground heat exchanger in foundation piles and the seasonal thermal energy storage from solar thermal collectors, is extendable to more comprehensive applications.

Keywords

Soil; Heavy metals; Radionuclides; Transfer-factor

Introduction

Agriculture has been the backbone of the economy of many Adeveloping countries. Many countries in Africa have laid down policies on the provision of sustainable food security. When people have sufficient food to eat, many of the nutrition-related problems are avoided and healthy citizens are available to work for the growth of their economies.

of the occurrence of threats to the health of the soil . Soils contaminated with radionuclides lose their ability to produce good quality crops and thus can be classified as degraded. The issues related to the degradation of radioactively contaminated soils are being considered as an exceptional type of chemical contamination, with additional, specific features related to ionizing radiation. Heavy metals when present in high concentrations are hazardous contaminants in food and the environment as they are nonbiodegradable having long biological half-lives. The implications associated with metal (embracing metalloids) contamination are of great concern, particularly in agricultural production systems due to their increasing trends in human foods and the environment. Environmental contamination by heavy metals has become a worldwide problem in recent years due to the fact that heavy metals unlike some other pollutants are not biodegradable. Consequently, they are not detoxified but are bioaccumulated in the environment. Soil pollution by heavy metals has serious health implications especially with regards to crops/vegetables grown on such soils. Heavy metals occupy a special position in soil chemistry because they play very important physiological roles in nature. Generally, the topsoil layer contains the largest amount of pollutants. The contaminant concentration in soil mainly depends on the adsorption properties of soil matter. The solubility of heavy metal ions in the soil is mainly influenced by many factors such as pH, conductivity, and moisture content, higher levels of radionuclide/heavy metals concentration in crops, vegetables, and water have an adverse effect on the health of people exposed to these radionuclides/heavy metals (WHO, 2007) [1-5].

The Transfer Factor (TF) is a value used in the evaluation studies on the impact of routine releases of radionuclides/heavy metals into the environment for most important agricultural products. The soil to plant transfer factor is regarded as one of the most important parameters in the environmental safety assessment needed for nuclear facilities. This parameter is necessary for environmental transfer models which are useful in the prediction of the radionuclide concentrations in agricultural crops for assessment of dose to man IAEA (1994). This work is therefore aimed at assessing the concentration of radionuclides/ heavy metals and their transfer factor from soil to vegetables/crops cultivated in some parts of Barkim-Ladi area in Plateau state, Nigeria [6-9].

Materials and Methods

Materials/Equipment

The materials that were used for the research work are: Canberra Model 727/727R Lead Shield Gamma-ray Spectrometer with NaI(Ti) detector, EDX 3600B-Energy Dispersive X-ray Fluorescence spectrometer, Jiangsu Skyray Information Technology Co. Ltd. China Oven Gallenham England, Beakers, Mortar, and pestle.

Area of study

This study was carried out in some parts of Barkin Ladi, a Local Government Area of Plateau State. A total of (7) soil and 7 vegetables/crops samples were collected for radionuclides and (4) soil samples with (4) vegetables/crops samples for heavy metals analysis. The samples were taken from Mazat, Bistchi, and Foron districts.

Sample collection

At each sampling point, about 0.50 kg of the soil sample was collected from a depth of about 0 cm- 15 cm from the surface of the soil, using a clean stainless-steel spoon at a distance of 1m away from each other, and within an area of one square meter from each sampling site. The vegetables were also collected directly from the farm land where the soil was collected.

Sample analysis

Analysis for the radionuclides was achieved by weighing the sample and transferring to radon-impermeable cylindrical plastic containers of uniform size (70 mm height by 60 mm diameter) and was sealed for about 30 days. This was done in order to allow Radon and its short-lived progenies to reach secular radioactive equilibrium prior to gamma spectroscopy. The samples were placed symmetrically on top of the detector and measured for a period of 29000 seconds. The net area under the corresponding peaks in the energy spectrum was computed by subtracting counts due to the Compton scattering of higher peaks and other background sources from the total area of the peaks. The use of ED-XRF was employed in the analysis of the heavy metals. ED-XRF spectrometry is an elemental analysis technique with broad applications in science and industry. XRF is based on the principle that individual atoms, when excited by an external energy source, emit x-ray photons of characteristic energy or wavelength. By counting the number of photons of each energy emitted from a sample, the elements present may be identified and quantitated.

TransferFactor(TF)

Transfer Factor (TF) is a useful parameter for radiological assessment. It is defined as the steady-state concentration ratio between one physical situation and another. As a case in point, it is the ratio of the concentration of an element in dry vegetation to that in dry soil. Equation (1) will be used to determine the transfer factor between vegetables and soil as illustrated below:

Where TF is Transfer Factor of soil to vegetables, Cv is the concentration of radionuclides in Bq/kg dry vegetables weight. Cs is the concentration of radionuclides in Bq/kg dry soil weight (Tables 1- 3).

TABLE 1 Results of the activity concentration of ⁴⁰K, ²²⁶Ra and ²³²Th n soils and vegetables/crops bq/kg and their transfer factor

TABLE 2 Results of concentration of heavy metals in Soil and Vegetables in Mazat, Bisitchi and Foron (mg/kg)

TABLE 3 Transfer factor of the heavy metals from soil to vegetables

Result and Discussion

The activity concentration of ⁴⁰K, ²²⁶Ra and ²³²Th expressed in Bq/Kg for the samples obtained from some agricultural farmland in Mazat and Bisitchi District in Barkin Ladi. The soil and vegetables samples were collected directly from the farmland in Ramabohan, Kaper, and Bisitchi. The result of the analysis is as presented in table 1 above. The result of ⁴⁰K, ²²⁶Ra, and ²³²Th in the soil samples ranges from 203.56 ± 0.80 to 217.39 ± 0.54, 69.39 ± 0.16 to 98.07 ± 0.88, and 69.52 ±0.79 to 97.88 ± 0.67 respectively. The result obtained from this study were compared with other studies in Nigeria and elsewhere in the world on agricultural soil samples and was found to be lower than those of Jibiri et al. (2011) and Masok et al. (2015) in some ex-tin mining locations of Jos Plateau, Nigeria. A similar study in Toro show a similar result for ²³²Th while ²²⁶Ra was lower compared to those of this study, ⁴⁰K was found to be higher than the result obtained in this study. This study also has results higher than those of Babatunde et al. (2019), Araromi et al. (2016), Rahamat and Lihan, (2022), in Malaysia. the high concentrations of the radionuclides in the present studies may be as a result of artisanal mining activities taking place close to the farm lands. ²³²Th and ²²⁶Ra concentration were higher than the recommended world average of 45 Bq/Kg and 35 Bq/Kg respectively while the concentration of ⁴⁰K in all the samples were lower than the world average of 420 Bq/Kg (UNSCEAR, 2000). The concentration of the radionuclides in the vegetables for 40K, has values that ranges from 151.75 Bq/Kg ± 0.48 Bq/Kg in Ramabohan Potato Farmland 1 (RP1) to 271.39 Bq/Kg ± 0.53 Bq/Kg in Spinach collected from Bisitchi farm land 2 (VB2), 226Ra ranges from 60.76 ± 0.35 in spinach from bisitchi farm land 1 (VB1) to 80.57 ± 0.64 in potato collected from Ramabohan farm land 2 ( RP2), 232Th varies from 34.05 Bq/Kg ± 0.90 Bq/Kg in spinach collected from bisitchi farm 2 ( VB2) to 90.16 Bq/Kg ± 0.59 Bq/Kg in potato collected from Kaper farm land 1 (KP1)[6-9] . The result obtained are higher than those reported by Aswood et al. (2013) and Ademola, (2019). The transfer of the radionuclides from soil to vegetables is also as presented in Table1.

The results recorded a very high transfer factor in all the radionuclides analysed. ⁴⁰K recorded the lowest TF of 0.60 in spinach cultivated in Foron whereas the highest TF of 0.99 was recorded in potato cultivated in Ramabohan. The high TF value of potassium is not a risk because it has an insignificant contribution to internal dose as ⁴⁰K content is homeostatically controlled, UNSCEAR, (2000). ²²⁶Ra also range from 0.73 to 0.93 whereas the transfer factor of ²³²Th range from 0.44 to 0.93. The TF is higher in potato than those of the spinach analyzed in the two districts. Uptake of the isotopes from the soil by the vegetables depends on various interrelated soil properties, including texture, clay content, cation exchange capacity, exchangeable cations, pH, and organic matter content. It also varies depending on the chemical and physical forms of the radionuclides, plant species and stage of growth.The concentrations (Mg/kg) of Cr, Ni, Cu, Zn, Mn Zr and V in both soil and vegetable samples are presented in Table 2 above. The data revealed that all the analyzed heavy metals accumulated by the vegetable and soil at different concentrations. Zr has a higher concentration in both the soil and vegetables compared to the other metals. The concentration of Zr in the study area has the highest concentration of 7,770.00mg/kg in Foron Siol (FS1) whereas the least was recorded in Ramabohan Soil (RS1), likewise, the spinach cultivated in Foron (FV1) recorded 666.00 mg/kg with spinach in Bisitchi recoding 1,258.00 mg/kg respectively. Zircon often contains uranium and thorium and other radioactive elements in it. Earlier studies on the natural radioactivity on beach sand has proven that Zr contains 0.1 to 0.5% uranium and thorium Van Schumus, (1995) and Bergamini, (1985).

Chromium regulates carbohydrate, nucleic acid and lipoprotein metabolism. This metal also potentiates insulin action. In addition, Cr activates several enzymes. However, chronic exposure of Cr may damage the liver, kidney, and lungs, Malaysian Food Regulation (1985). The range of Cr in the soil is 2,288.00 mg/kg to 1,156.00 mg/kg. This is higher than that of Daniel et al. (2014). The concentration of Cr in the vegetables also range from 6,936.00 mg/ kg in potato sample KP1 to 1,360.00 mg/kg in spinach sample BV1. This value is higher than the 2 mg/kg permissible limit while the concentration of Cr in the soil is higher than the allowable limits of 60 mg/kg .

The concentration of Zn in the soil of the study area range from Not Detected (ND) to 560.10 mg/kg whereas the concentration in the vegetables varies from 1,043.90 mg/kg to 9,636.00 mg/kg. Concentrations of Zn found in contaminated soils frequently exceed to those required as nutrients and may cause phytotoxicity. Zn concentrations in the range of 150 mg/kg-300 mg/kg have been measured in polluted soils. High levels of Zn in soil inhibit many plant metabolic functions and will result in retarded growth and cause senescence. Zinc toxicity in plants limited the growth of both root and shoot. The Zn concentrations in this study exceed the permissible limits of 60 mg/kg highly detrimental to human health than too much Zn in the diet. The recommended dietary allowance for Zn is 15 mg/day for men and 12 mg/day for women Agency for Toxic Substances and Disease Registry, but high concentration of Zn in vegetables may cause vomiting, renal damage, cramps, etc.

Manganese is a very essential trace heavy metal for plants and animals’ growth. Its deficiency produces severe skeletal and reproductive abnormalities in mammals. High concentration of Manganese (Mn) causes hazardous effects on lungs and brains of humans. The concentration of Mn in the soil of the study area has the highest value of 2,709.00 mg/kg recorded in Kaper Soil (KS1) while the least is 1,006.79 recorded in Foron soil FS1. Mn content in the vegetables varies from 417.96 mg/kg in spinach collected from Bisitchi VB1 to 10,062.00 mg/kg in cabbage collected in Foron VF1. The concentration of Mn is more in the vegetables than in the soil except for vegetables in Bisitchi where the value is higher in the soil than the vegetable. Higher proportions of Mn in the vegetable samples are another confirmation of the high absorption of Mn by the tissues from the soils on which they grow and other nonanthropogenic sources.

Copper is an essential micronutrient involved in a number of biological processes needed to sustain life. However, it can be toxic when present in excess. The concentration of Cu in the soil varies from 576.80 mg/kg to 3, 625.60 mg/kg which is higher than those reported by Mokgolele and Likuku, (2016). This value exceeds the maximum permissible limit of 100mg/kg for Cu in horticultural soils. The results of Cu in vegetables in this study has a very high value of 22,248.00 mg/kg in spinach VF1 while the lowest was recorded in VB1 as 2,307.20 mg/kg. All results recorded in the vegetable samples are greater than the soil. The maximum permissible concentration of Cu in plants recommended by the World Health Organization (WHO) is 73 mg/kg.

Generally, the transfer factor expresses the bioavailability of metal at a particular position on a species of plant. All the samples have significant differences in the transfer factors of metals relative to the availability of the same metals in the soil. The transfer factor as seen in Table 3 are very high. The TF in Cr ranges from 1.18 to 5.84, Mn from 0.85 to 10.00, Ni from 1.12 to 4.00, Cu from 0.64 to 38.57, Zn from Null to 5.00 respectively. Cu has the highest TF of 38.57 in spinach cultivated in Foron, and the least TF which is Mn is recorded in cabbage cultivated in Bisitchi. When the transfer factor is less than one, it may be a probability that soil is the main source of metal bioaccumulation in plants. However, it is more revealing that, when the value is higher than one, the total concentrations of metals in soil do not necessarily correspond to the metal bioavailability in plants. The bioavailability of heavy metals depends on a number of physicochemical properties such as pH, organic matter contents, cation exchange capacity, redox potential, soil texture, and clay contents.

Conclusion

The concentration of the radionuclides ⁴⁰K, ²²⁶Ra, and ²³²Th as well as the heavy metals Cr, Mn, Ni, Cu, Zn, and Zr were analysed from soil in agricultural farmland and vegetables/crops in Barkin Ladi Local Government Area of Plateau State. The transfer factor was also determined. The result of the analysis shows that the concentrations of ²²⁶Ra and ²³²Th in the soil and vegetables were all above the recommended limits whereas the concentration of ⁴⁰K was below the maximum permissible limits. All the heavy metals analysed also show higher concentrations above the recommended standard. The TF was all above 0.5 except for Zr where the TF is 0.086. This is an indication that there is a high rate of absorption of both the radionuclides and heavy metals by the crops and vegetables. The soil is thus said to be polluted with both heavy metals and radionuclides

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