Natural radioactivity and mineralogical composition of different particle size fractions of a river sediment from Calabria, southern Italy: a case study

The natural radionuclides (238U, 232Th and 40K) content and the mineralogical characteristics of three different particle size fractions of a sediment sample from the Amendolea river, Calabria region, southern Italy, were investigated. High Purity Germanium (HPGe) gamma spectrometry was employed for the radioactivity measurements and specific activity data were reported. X-Ray Diffraction (XRD) measurements were carried out to identify the mineral contents of each particle size fraction. This case study was developed with the aim of evaluating the specific activities of natural radionuclides and their relation to specific minerals. Data obtained can provide useful information in view of a deep understanding of the adsorption of radioactive elements and the influence of the sediment granulometry on the radionuclides specific activities.


Introduction
Natural radioactivity provides the greatest contribution to the dose received by the population. From this point of view, important radionuclides are those belonging to the radioactive families 238 U, 232 Th and 235 U, together with the radioisotope of potassium, 40 K [1]. Together with their product of decay, they are commonly found in different concentrations in environmental matrices, especially in water, rocks and soils [2][3][4][5].
Among the environmental matrices, river sediments are of course particularly interesting, due to their mineralogical composition, particle size distribution and because they are reliable long-term indicators of river pollution by radionuclides [6]. Furthermore, knowing the distribution of the latter in terms of activity concentration in river sediments is extremely important, as it allows us to evaluate any temporal variations of specific activities within the river [7].
On the other side, geochemical studies of river sediments are also particularly useful, as the geomorphology of the investigated site greatly influences the chemical composition of this type of sediment, as well as the mineralogical composition reflects the geological history of the transport and the sorting process [8][9][10][11].
In this article, a case study was taken into account; in particular, natural radioactivity and minerals composition of three different particle size fractions of a sediment sample of the Amendolea river, Calabria region, southern Italy, were investigated with the aim to evaluate the influence of sediment particle sizes on natural radionuclides concentrations and the influence of the mineralogical composition on the 226 Ra, 232 Th and 40 K activity concentration. River sediments can be employed as building materials and thus the knowledge of their granulometric distribution, as well as the radioactivity concentration in each particle size fraction and the mineralogy of the investigated sediment, strictly correlated with its natural radioactive content, is extremely useful from a radiological point of view [12].
The schematic block diagram of figure 1 summarizes what was performed in the present study.

Sample collection and treatment
The investigated river sediment sample, around 1 kg, was collected in the sampling point indicated in figure 2   The sampling procedure is described in [10]. In laboratory, the sample was dried at 105° C in an oven and sieved to obtain the various particle size fractions: pelite (< 65 µm), fine sand (65 µm -250 µm) and coarse sand (250 µm -1 mm). Each size fraction was packed in a hermetically sealed 20 mL polyethylene plastic vial. After 40 days, the secular radioactive equilibrium between 226 Ra and their daughter products was attained and samples were ready for gamma spectrometry counting [13].

Radioactivity analysis
For the gamma spectrometry analysis, the pelite, fine sand and coarse sand fractions were counted for 70000 s. The 226 Ra and 232 Th activity concentrations were obtained by the 295.21 keV and 351.92 keV 214 Pb, the 1120.29 keV 214 Bi and the 911.21 keV and 968.97 keV 228 Ac γ-lines, respectively. For the 40 K specific activity evaluation, its γ-line at 1460.8 keV was employed.
The experimental setup was composed by an Ortec HPGe positive biased detector (GEM), with FWHM of 1.85 keV, peak to Compton ratio of 64:1 and relative efficiency of 40 % at 1.33 MeV ( 60 Co), and integrated digital electronics.
The ANGLE 4 code was employed for the efficiency transfer factors calculations [15]. The Gamma Vision (Ortec) software was used for data acquisition and analysis [16]. The activity concentration of each identified radionuclide was calculated using the following formula where NE, εE and γd account for the net area, efficiency and yield of a photopeak at energy E, respectively, M is the mass sample (kg) and t is the live time (s).
The quality of the gamma spectrometry experimental results was certified by the Italian Accreditation Body (ACCREDIA) [18].

X-Ray Diffraction (XRD) analysis
A Malvern Panalytical Empyrean Diffractometer, equipped with a solid-state detector, PIXcel, was employed for the XRD tests on about 1 g of finely powdered sample for each investigated fraction [19]. Acquisition conditions were 40 kV and 40 mA. XRD spectra were recorded from 2 to 54 degrees, with step size of 0.007 degrees and counting time of 20 seconds.
The background correction of the Cu Ka2 component with a digital filter and the identification of the crystalline mineral components through the ICDD JCPDS database were also performed [20].

Radioactivity analysis
The specific activity of detected radionuclides, 226 Ra, 232 Th and 40 K, in the investigated sediment sample, is shown in figure 3 for pelite, fine sand and coarse sand.
Obtained values are strictly correlated with the sampling point. It is, in fact, well-established that the radionuclides activity concentration in river sediments varies from location to location, considering that the river bottom can exhibit large variations in chemical and mineralogical properties and rare-earth elements [21].
As we can see from an inspection of the figure,  The adsorption of radioactive elements is then highest in the pelite fraction and it decreases from fine to coarse-grained materials [22]. Moreover, the decrease in the specific activity values for 226 Ra, 232 Th and 40 K with increasing particle size, seems to indicate that the three particle size fractions are different from a mineralogical point of view and, consequently, have a different radionuclides content. The assessment of the influence of the mineralogical composition on the activity concentration of detected radioisotopes was then performed through the XRD analysis of the three particle size fractions of the investigated sample.  From this figure we can notice that for each investigated size fraction, quartz (SiO2) is the most abundant mineral. Biotite, K(Mg;Fe)(AlSi3O10)(OH)2, and microcline feldspar, KAlSi3O8, are also present in all samples. Kaolinite mineral, Al2Si2O5(OH)4, was identified only in the pelite fraction, whereas albite, (Na,Ca)Al(Si,Al)3O8, and sylvite (KCl) mineral components were detected only in the pelite and coarse sand samples. Finally, the muscovite mineral, KAl2(Si3Al)O10(OH,F)2, was found in the pelite.
Worth of note, kaolinite is detected only for the pelite fraction, not for the fine and coarse sand ones. Now, according to the literature [23], the kaolinite mineral is shown to be responsible for 232 Th and 40 K concentrations and its highest dispersion is produced by particles of 25 -35 μm diameter [24]. In the light of this we can hypothesize, then, that kaolinite plays the major role, more than microcline feldspar and quartz, in the observed trend of the specific activity of these radionuclides as detected by radioactivity measurements in the investigated particle size fractions. No specific correlation between some particular mineralogical phase and specific activity of 226 Ra was found in literature. Nevertheless, being 226 Ra in secular equilibrium with 238 U, its decreasing radioactivity concentration with increasing particle could be related to the uranium concentration. An investigation of the elemental composition of each particle size fraction, by means of spectroscopic techniques (like as X-Ray Fluorescence and ICP-MS), with the aim to quantify the uranium content in each of them and thus to verify a possible decrease in concentration by increasing the particle size, can be helpful in clarifying this aspect. It will be planned in the next development of the research.

Conclusions
In the present paper, the 226 Ra, 232 Th and 40 K specific activity for three different particle size fractions of a sediment sample of Amendolea River (Calabria region, southern Italy), taken as case study, was 10  quantified with HPGe gamma spectrometry, in order to investigate the influence of the granulometry on the radionuclides specific activity. The highest concentrations of detected radionuclides were found in the pelite fraction, because the adsorption of radioactive elements has a strong dependence on the sediment nature and decreases from fine to coarse-grained materials.
At the same time, XRD tests were performed to identify the mineralogical composition of each particle size fraction, with the aim of correlating the radioactivity results to specific minerals. From the results, the main role of kaolinite in increasing the radioactivity concentration for 232 Th and 40 K in pelite fraction with respect to, in the order, fine and coarse sand was hypothesized. As far as 226 Ra is concerned, being this radioisotope in secular equilibrium with 238 U, the observed decreasing level of its activity concentration with increasing particle size was supposed to be related to the uranium concentration in each of them.
The obtained data can be used as a baseline for future investigations about radioactivity background levels in river sediments samples of the investigated region and they can provide useful information to better understand the adsorption of radioactive elements and the variation of the radionuclides specific activity with the sediment granulometry.