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 Table of Contents 
ORIGINAL ARTICLE
Year : 2019  |  Volume : 42  |  Issue : 3  |  Page : 107-111  

Accumulation of 210Po in Medicinal Plants in the Environment of Mangalore, Southwest Coast of India


1 Department of Studies and Research in Physics, Payyanur College, Kannur University, Kannur, Kerala, India
2 Department of Physics, SJC Institute of Technology, Chickballapur, Karnataka, India
3 Department of PG Studies in Physics, Mangalore University, Mangalore, Karnataka, India

Date of Submission03-Apr-2019
Date of Decision14-Apr-2019
Date of Acceptance04-Jun-2019
Date of Web Publication06-Nov-2019

Correspondence Address:
V Prakash
Department of Studies and Research in Physics, Payyanur College, Kannur - 670 327, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_13_19

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  Abstract 


Systematic studies on the accumulation of210Po in 12 medicinal plants and activity concentration in associated soils have been carried out. The activity of210Po was measured using a ZnS (Ag) alpha counting system. The mean210Po activity concentration was found to be 27.8 and 8.3 Bq/kg for plant and soil, respectively. The plant-to-soil mean activity ratio of210Po was found to be 3.8. A good correlation was observed between the activity concentration of210Po in plant and soil. The absorbed gamma dose rates in the study area were also measured using a portable scintillometer and found to vary in the range of 34.8–52.2 nGy/h, with a mean value of 43.5 nGy/h. The results of these systematic investigations are presented and discussed in detail.

Keywords: 210Po activity, alpha counting system, dose rate, medicinal plants, soil


How to cite this article:
Prakash V, Rajashekara K M, Narayana Y. Accumulation of 210Po in Medicinal Plants in the Environment of Mangalore, Southwest Coast of India. Radiat Prot Environ 2019;42:107-11

How to cite this URL:
Prakash V, Rajashekara K M, Narayana Y. Accumulation of 210Po in Medicinal Plants in the Environment of Mangalore, Southwest Coast of India. Radiat Prot Environ [serial online] 2019 [cited 2019 Nov 12];42:107-11. Available from: http://www.rpe.org.in/text.asp?2019/42/3/107/270438




  Introduction Top


Mangalore, an important region of the southwest coast of India, is heading to become a major industrial activity center with oil refineries, petrochemical complexes, chemical and fertilizer factories, thermal power stations, and a host of other industries. Although the information on the radiation level and radionuclide distribution in different environmental matrices of the region is available in the literature, systematic studies on uptake of radionuclides by plants from soil are sparse. The uptake of radionuclides within the soil to plant is a part of the biochemical cycling. The mobility and availability of radionuclides depend on several factors such as geochemical, biological, and climatic conditions. The prediction of radionuclides uptake by plants from a given growth medium should be based on several biotic and abiotic parameters that control their behavior in soil. The risk to both the environment and human health of a given radionuclide is a function of its mobility and phytoavailability. Therefore, the studies on behavioral properties of radionuclides in soils have gained importance in environmental studies.

In view of this, about 12 Ayurvedic medicinal plants and associated soils collected from Moodabidri, near Mangalore, were analyzed for the concentration of210 Po. The radionuclide210 Po is one of the most toxic alpha-emitting natural radionuclides of238 U series and an important contributor to the internal exposure of human population. The main sources of the210 Po entering the environment are natural decay of238 U series and the exhalation of radon gas from the surface layers of the earth's crust into the atmosphere as a result of the turbulent diffusion and convection processes. Plants may get radioactive nuclides in two ways: (a) by the deposition of radioactive fallout on the plant directly and (b) by absorption from the soil.[1]

The main routes of radionuclides intake by human body are inhalation, ingestion, and injection. The ingestion, i.e., the intake of radionuclides by means of consumption of food and Ayurvedic drugs, is usually the most important route by which these radionuclides enter the human system. The main ingredients of Ayurvedic drugs are natural herbs, and the study of concentration of radionuclides in such herbs has got great significance. The210 Po decays with a half-life of 138 days by alpha emission (5.3 MeV) and causes considerable biological damage. It is estimated that the equivalent dose resulting from a single disintegration of210 Po is thousand times greater than210 Pb, a toxic radionuclide of238 U series decay.[1] Morgan et al. have included210 Po in the group of most toxic radioisotopes.[2]

The activity of210 Po was measured using a ZnS(Ag) alpha counting system. The effect of physicochemical parameters such as soil pH, organic matter content, moisture content, and electrical conductivity on the activity concentration of210 Po was studied. The absorbed gamma dose rates were also measured in the study area using a portable plastic scintillometer. Keeping in view the biological hazards of210 Po, the activity of this radionuclide in plants and associated soils has been measured. The results of these investigations are presented and discussed in this paper.


  Materials and Methods Top


In the present study, 12 medicinal plants and associated soils collected from Moodabidri were subjected to210 Po analysis. The collection of samples was done following standard procedures and techniques.[3]

Plant samples

The vegetation samples of medicinal plants, which are commonly found in the region, were carefully collected to avoid contamination. Approximately 5 kg total weighed samples were collected in big polythene bag, brought to the laboratory, washed with running water to free it from pollutants, and cut into pieces. Then, the samples were dried in open air to reduce the volume.

Soil samples

The soil samples were mixed well after removing the extraneous materials, such as plant materials, root, mat portions, pieces of stones, and gravel. The physicochemical parameter such as soil organic matter was measured employing standard method.[4] The samples were transferred to a porcelain tray and dried in an oven at 110°C till a constant dry weight is obtained. Moisture percentage of the sample was estimated by weight loss method. The dried sample was then sieved through a 250-μ sieve and taken for analysis.

The air-dried vegetation samples were transferred to the porcelain tray and dried in the oven at 110°C till a constant dry weight is obtained. Moisture percentage of the sample was estimated by the weight loss method. Since the activity in vegetation sample is low, wet ashing method was employed as210 Po is very volatile and the sample ashed at 400°C–450°C cannot be used for210 Po analysis.[5]

Ambient gamma radiation

The ambient gamma radiation survey was carried out using the portable plastic scintillometer, consisting of a plastic scintillator of diameter 5.5 cm and height of 15 cm. The scintillation is covered with a thin (5 mg/cm2) coating of ZnS(Ag) to improve the energy response below 60 keV. The scintillometer has a sensitivity of the order of 10−10 A/μR/h and reading an accuracy of 0.2 μR/h. It has the ability to integrate the dose rate for some short duration (1–10 min) to improve accuracy and also has an excellent flat energy response from 20 to 1.2 MeV. The calibration of the scintillometer was checked periodically using133 Ba source (strength 1.1 × 105 Bq). The dose rates in air were recorded at 1 m above the ground level. A number of readings were recorded around each measurement point, and the average radiation level was taken as a representative level of the location. The recorded dose rates include both terrestrial and cosmic ray components.

Electrochemical deposition method

One of the characteristics of210 Po has been its easy deposition either by chemical or by electrodeposition on noble metals. Over 99% of210 Po present in the sample can be very easily deposited from hydrochloric acid solution. The problem of ferric ions present in the solution that hinders the deposition of210 Po can overcome by adding ascorbic acid.

210 Po activity was determined by the electrochemical deposition method.[5] The samples were dried at 110°C about 24 h to obtain a constant dry weight. The dried samples of about 20 g were leached with 4M HNO3, and then, organic matter present in the samples was destroyed by adding 3:1 HNO3+ HClO4 mixture in small increments till white residue appears. The samples were then converted into 1M HCl medium. The solution was kept on a hot plate cum magnetic stirrer for electroplating. The210 Po in the solution was deposited on a brightly polished and background counted (both sides) silver disc using magnetic stirrer at 97°C for continuous 6 h. Then, the disc was removed from the solution, washed with distilled water, rinsed with alcohol, and dried under infrared lamp. The counts were noted for 2000 s on both sides using the ZnS(Ag) alpha counter of 0.3 cpm background and 30% efficiency for the210 Po activity. From the total counts obtained on both sides, the activity of210 Po was calculated following the expression.[5]



where S is the net counts per se cond, SD is the standard deviation, E is the efficiency (%) of alpha counter, Ep is the plating efficiency (%), determined as 90% using210 Po standard, and W is the weight of the dry sample taken for analysis in grams.

Transfer of 210Po from soil to plant

Transfer factor (TF), a measure of uptake of radionuclides from soil to plant, can be defined as the ratio of activity concentration of radionuclides in plant (Cplant) to the activity concentration in soil (Csoil). It is reported that TF varies from place to place for a given type of plant and for a given radionuclide.[6]

TF = Cplant/Csoil (2)

Physicochemical parameter

The adsorption of radionuclides on the soil grain will be enhanced by the presence of organic matter content in the soil. However, the amount of radionuclides absorbed by the plants constitutes a small fraction of the total radiation content of the soil.[7] The weight loss on ignition method[4],[8] was followed to obtain organic matter content in the soil. The samples were treated at an ignition temperature of 550°C for 24 h.

Oven drying method was followed to obtain moisture content of the soil. It is based on removing moisture content of the soil by oven drying until a constant dry weight is obtained. The moisture content (%) is calculated following weight loss method. The samples were treated at a temperature of 110°C for 24 h.

The soil samples were dissolved in distilled water (1:4 w/v) and allowed to settle down the particles. The pH of the suspension was determined using a pH meter. Electrical conductivity of the soil was determined in the filtrate of the water extract using a conductivity meter.


  Results and Discussions Top


The results of the activity of210 Po in the plant species and associated soils are given in [Table 1]. It can be seen from [Table 1] that the activity of210 Po in plant species is found to vary from 6.3 to 56.9 Bq/kg with a mean value of 27.8 Bq/kg and that in soil is found to vary from 2.2 to 19.6 Bq/kg with a mean value of 8.3 Bq/kg. The variation of210 Po activity in the plant and associated soil is shown in [Figure 1]. Studies on the activity of210 Po in different plants show no systematic variation. The210 Po concentration was found to vary widely among the plant species and with soil type.
Table 1: Activity of 210Po in plants and soils, activity ratio, and dose rate

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Figure 1: Variation of210Po activity in plants and associated soil

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The maximum activity of 56.9 and 19.6 Bq/kg was measured for the plant Barringtonia acutangula (Samudraphalam) and associated soil, respectively. The minimum activity of 6.3 and 2.2 Bq/kg was measured for the plant Ficus benghalensis (Vatavruksha) and associated soil, respectively. This clearly indicates that soil is one of the main sources of radionuclides in the plants. The selective enrichment of polonium in some medicinal plants indicates the use of such plants in the monitoring of radioactive contamination.

[Table 1] also gives the210 Po activity ratio of plant to soil (TF). The TF of210 Po is found to vary from 1.8 to 7.6 with a mean value of 3.8. It is observed from the study that TF factor is different for different sets of plants and associated soils. The physical and chemical properties of the soil and environmental conditions, viz., temperature, pressure, and precipitation, have influenced the TF value. The various activities of human beings in the environment also influenced the TF values. It is evident from the ratio that210 Po activity in the plant is almost three times higher than that in the associated soil. It clearly indicates that, in addition to the natural decay of238 U in soil,210 Po also comes from atmospheric deposition due to the decay of222 Rn. The210 Po can deposit on the vegetation by the direct method.[9] The plants which are growing in the fresh outdoor contain natural radionuclides and in which most of them are210 Po. This is due to the direct deposition of222 Rn daughters from atmospheric precipitation. Eighty percent of the radioactive materials in the plants are210 Po as a result of the direct deposition of222 Rn daughters from atmospheric precipitation.[10],[11] A good correlation was observed between the activity concentration of210 Po in the soil and plant with a correlation coefficient, r = 0.91 [Figure 2]. Usually, a constant ratio of plant-to-soil concentration implies a linear relation.[12] However, variations in the soil properties such as organic matter content, mineralogical composition, pH, conductivity, and fertility components affect the uptake of radionuclides resulting in a nonlinear relation.[13],[14]
Figure 2: Correlation between210Po activity in plant and soil

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A portable plastic scintillometer was used to measure the absorbed gamma dose rate in air at 1 m above the ground level, and the dose rate values are also given in [Table 1]. The dose rate varies in the range 34.8–52.2 nGy/h with a mean value of 43.5 nGy/h. The gamma dose rate obtained from the present study shows a uniform pattern, except in some sampling stations.

[Table 2] gives the physicochemical parameters of the soil. It is clear from [Table 2] that the activity level of the radionuclides in plants is conditional, the content being affected by the number of parameters such as original mineral content, organic matrix of corresponding plant, geochemical characteristics of the soil, pH, and the ability of plants to accumulate some of the radionuclides selectively.[15] The rainfall, atmospheric dusts, plant protection agents, and fertilizers were the additional sources of radionuclides like210 Po and it could be absorbed through the leaf blades.
Table 2: Physicochemical parameters of the soil

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It is clear from [Table 2] that pH of the soil varies from 4.82 to 5.63 with a mean value of 5.29. It means soil from the region shows acidic nature. Moisture content of the soil ranges from 2.5 to 4.5 with a mean value of 3.8. Organic matter content of the soil ranges from 12 to 15.5 with a mean value of 13.4. Same time, the conductivity of the soil ranges from 16 to 57 μS/cm with a mean value of 32 μS/cm.

Comparison of activity and correlation study

The activities of210 Po have been compared with the values reported for other regions and are given in [Table 3]. It is evident from [Table 3] that the mean210 Po activity in the present study area is low compared to the values reported for Goa region and Black Forest and high compared to the high background radiation area, Kerala. The210 Po concentration in the soil varied from place to place and with soil type. This could be due to nature of soil and its physicochemical characteristics such as soil pH, humidity, and organic matter content.
Table 3: Comparison of 210Po activity with other environs

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  Conclusions Top


The210 Po concentration was found to vary widely among the plant species and with soil type. The plant to soil ratio of210 Po indicates that, in addition to natural decay of238 U in soil,210 Po also comes from atmospheric deposition due to the decay of222 Rn. The210 Po activity concentration varied with soil structure and soil type. The activity concentration210 Po in the plants almost certainly depends on the physicochemical parameters of the corresponding soil. The study clearly indicates that the activities of polonium in the selected medicinal plants were below the reported value elsewhere. Hence, the selected medicinal plants can be used for the preparation of Ayurvedic drugs without substantial effect of polonium concentration. The study also confirms the selective enrichment of polonium in selected medicinal plants and can be used as bioindicators for the future monitoring of radioactive contamination.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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