Hibiscus sabdariffa Linn is to the family of the plant called malvaceae and it is usually called “bissap” in Burkina Faso, Senegal, Togo, Mali and Côte d’Ivoire [1]. The fruit of the plant is used in nutrition for making juice extract [2] in Africa. It is also well known traditionally that fruits including juice from H. sabdariffa are rich in vitamin C. These leaves are also used to treat diseases like kidneystone, liver damage, hypertension and leukemia [3]. The works of Mensah and Golomeke have allowed discovering that extracted juice of H. sabdariffa has antioxidant properties [4]. In Côte d’Ivoire, the most populations drink “bissap” juice, it is said to be rich in vitamin B6, calcium, potassium and other several elements that are yet to be isolated and characterized. In order to assess the radiological risks linked to the consumption of this plant and its derivatives by the population, we focused this study to determine the natural radioactivity levels from this plant and also evaluated the Average Annual Committed Effective Dose (AACED) due to the ingestion of radionuclides from H. sabdariffa.

Sample preparation

Samples of leaves of H. sabdariffa imported to Côte d’Ivoire were collected from the local markets of Côte d’Ivoire (Figure 1), these different varieties of leaves shown in Table 1. Samples were dried in the range of 25˚C at ambient air and grinded into a fine powder with a particle size < 1 mm, 500 mg of each samples were put into standard Marinelli beaker and the activity concentrations for the natural radionuclide’s in the measured samples were computed. Samples were placed in Marinelli beaker sealed off and kept for one month, the time to achieve radioactive secular equilibrium between ²²⁶Ra and ²²²Rn and daughter nuclei. In order to determine the background distribution due to naturally occurring radionuclide’s in the environment around the detector, an empty Marinelli beaker container was counted in the same manner as the samples.

Instrumentation and Calibration: Direct instrumental analysis without pre-treatment (non-destructive) was used for the measurement of gamma rays for the water samples using a High Purity Germanium detector (HPGE). The gamma spectrometry system consists of an n-type HPGE detector coupled to a computer based multi-channel analyzer (MCA). The relative efficiency of the detector is 25% with energy resolution of 1.8 keV at gamma-ray energy of 1332 keV of ⁶⁰Co. The identification of individual radionuclides was performed using their gamma-ray energies and the quantitative analysis of radionuclides was performed using gamma-ray spectrum analysis software, GENIE 2000. The detector is mounted in a cylindrical lead shield (100 mm) lined with copper, cadmium and plexiglass (3 mm each) to reduce the background radiation. The detector is cooled in liquid nitrogen at a temperature of −196˚C (77˚K). In order to determine the background distribution in the environment around the detector, ten empty Marinelli beakers were thoroughly cleaned and filled with distilled water and counted for 36,000 s in the same geometry as the samples. The background spectra were used to correct the net peak area of gamma rays of measured isotopes. The energy calibration of the spectrometry chain consists of establishing the relationship between channels and energy in the spectra. This was done using a set of gamma-emitting radionuclides (⁶⁰Co and ¹³⁷Cs). The main parameter necessary to quantify radionuclide activity is the full energy peak efficiency. The efficiency calibrations were carried out by counting standard radionuclides of known activities with well-defined energies in the energy range of 60 keV to −2000 keV. After carrying out these operations, we determined the specific activity of the radionuclides like uranium (²²⁶Ra), thorium (²³²Th) and potassium (⁴⁰K). The ²²⁶Ra activity was measured by taking the mean activity of the two separate photo peaks of the daughter nuclides: ²¹⁴Pb at 351.9 keV and ²¹⁴Bi at 609.3 keV, the activity of ²³²Th was determined using photo peaks of ²²⁸Ac at 911.1 keV and the photopeak of ²¹²Pb at 238.6 keV and the activity of ⁴⁰K was directly determined using its gamma rays emitted at 1460.8 keV.

After measurement and subtraction of the background, the activity concentrations were calculated. The analytical expression used in the calculation of the activity concentrations in Bq/Kg for water samples is as shown in Equation (1)

A = N e λ t d P ⋅ T c ⋅ γ ⋅ M (1)

where N is the net counts of the radionuclide in the samples,t_d is the delay time between sampling and counting, P is the gamma-ray emission probability (gamma-ray yield), γ is the absolute counting efficiency of the detector system, T_c is the sample counting time, m is the mass of the sample (kg), exp(λt_d) is the decay correction factor for delay between time of sampling and counting and λ is the decay constant of the parent radionuclide.

At energy E, M is the dry-weight of samples (kg).

The average annual committed effective dose, E_ave, for ingestion of NORMS in the medicinal plants was calculated using the expression in Equation (2):

E a v e = I p ⋅ D C F i n g ⋅ A S p (2)

where DCF_ing is the dose conversion factor for ingestion, for each radionuclide (i.e., 4.5 × 10⁻⁵ mSv/Bq, 2.3 × 10⁻⁴ mSv/Bq and 6.2 × 10⁻⁶ mSv/Bq for ²³⁸U, ²³²Th and ⁴⁰K respectively for an adult) [5], I_p is the consumption rate from intake of NORMS in medicinal plants and A_Sp is the activity concentration in the plant sample.

In the second column of Table 2 of radionuclides, the highest ²²⁶Ra activity is (1.74) × 10⁻³ Bq/Kg of H. sabdariffa leaves from samples sold at the “marché de Bingerville”, the lowest concentration (1.14) × 10⁻⁴ Bq/Kg measured corresponds to leaves of this species at the Marcory site. An average concentration activity level of (5.99 ± 0.31) × 10⁻⁵ Bq/Kg in ²²⁶Ra is noted. Column three (3) of this table reports the activity concentrations in ²³²Th, the maximum value is (6.72) × 10⁻⁴ Bq/Kg of H. sabdariffa . leaves at the Cocody site and the minimum is (6.08) × 10⁻⁵ Bq/Kg at the “marché de Marcory”. The mean value is (1.59) × 10⁻⁴ Bq/Kg in ²³²Th. The radionuclide ⁴⁰K has the most significant activity concentrations. The concentrations vary from (4.65) × 10⁻³ to (5.44) × 10⁻⁴ Bq/Kg respectively in Treichville site and “Grand-marché de Koumassi” with an average of (1.95) × 10⁻³ Bq/Kg

Figure 2 shows the histograms of the radionuclides. We note a significant concentration of ⁴⁰K, when detected at sites S01, S02, S03, S05, S07 and S010 in the ten (10) collection markets. We also note a lower concentration than ²²⁶Ra in market S06. The regular absorption and accumulation of this radionuclide over a given period of time may be one of the reasons for its higher concentration.in plants. Chethan Rao’s studies in 2012 showed that ⁴⁰K is an important radionuclide in plant metabolic activities [6]. The high concentration of potassium activity recorded may explain the use of the leaves of this plant for the treatment of hypertension since patients suffering from hypertension have a low concentration of potassium in their blood [7]. Among ²²⁶Ra and ²³²Th, ²²⁶Ra is relatively higher, and it may be due to the fact that radium is chemically similar to calcium and calcium being one of the nutrients plants may absorb with radium, which will result in relatively more concentration of radium. The low ²³²Th content of the plant can be explained by their insolubility and low specific activity. Ions of ²³²Th bind weakly to soil particles that are not absorbed by the plant [8] (Table 3).

At the AACED level, the highest and lowest values are respectively at the Marcory site (251.492) nSv/y and the large at “marché Gouro d’Adjamé” (2.806) nSv/y. It is noted that these values are insignificant compared to the value of 0.3 mSv/y which can cause diseases to adults who regularly use H. sabdariffa leaves during the year. There is thus no radiological risk to consume this plant (Table 4).

The average annual committed effective dose due to ²³⁸U, ²³²Th and ⁴⁰K for this study was relatively low compared to that which was reported for medicinal plants from [9] and [10], almost similar than [11] for herbal tea.

In this work, the AACED results obtained show that there is no radiological fear to use this plant because they are far below the world value for an individual which is 0.30 mSv/y. The element potassium which has an important role in the treatment of hypertension and radium which is similar to calcium which strengthens the strength of skeletal bones, leads us to conclude that this plant has therapeutic and nutritional virtues.

The datasets supporting the conclusions of this article are included within the article.

This research was supported by the Laboratory of Nuclear physics and Radiation protection of the University Felix Houphouet-Boigny of Cocody, Abidjan, Côte d’Ivoire.

ADA, DL, and BO designed the experiment. ADA, BO, and DL carried out the experimental studies and collection, analysis, and interpretation of data. ADA wrote the manuscript. BO helped to draft and revised the manuscript. The authors read and approved the final manuscript.

The authors thank the Institute for Radiation Protection (RPI) of the Ghana Atomic Energy Commission (GAEC) for the use of their facilities.

The authors declare no conflicts of interest regarding the publication of this paper.

Alfred, A.D.D., Koudou, D. and Jean-Claude, B.O. (2021) Determination of Natural Radioactivity of the Hibiscus sabdariffa Linn (Roselle) Used in Côte d’Ivoire (Ivory Coast). Advances in Materials Physics and Chemistry, 11, 59-66. https://doi.org/10.4236/ampc.2021.113006

⁶⁰Co: Cobalt 60

¹³⁷Cs: Cesium 137

Bi: Bismuth

Ac: Actinium

Pb: Lead

²³²Th: Thorium

²³⁸ U: Uranium

²²⁶ Ra: Radium

⁴⁰K: Potassium

NORMS: Naturally Occurring Radioactive Materials

H. sabdariffa: Hisbiscus sabdariffa

HPGE: High Purity Germanium detector

MCA: multi-channel analyzer

AACED: The average annual committed effective dose