Evaluation of Radioactivity and Heavy Metals Content in a Basalt Aggregate for Concrete from Sicily, Southern Italy: A Case Study

Abstract

In the present paper, an investigation on the natural and anthropic radioactivity and heavy metals content in a basalt aggregate for concrete from Sicily, Southern Italy, was performed as a case study. In particular, the evaluation of the specific activity of radium-226, thorium-232, potassium-40 and caesium-137 radionuclides was performed by using High-Purity Germanium (HPGe) γ-ray spectrometry, together with the estimation of several indexes developed to evaluate the radiological risk for the population related to radiation exposure, i.e., the alpha index (I_α), the radium equivalent activity (Ra_eq), the absorbed γ-dose rate (D) and the annual effective dose equivalent outdoor (AEDE_out) and indoor (AEDE_in). Moreover, measurements of the average heavy metals (arsenic, cadmium, copper, mercury, nickel, lead, antimony, thallium and zinc) concentrations in the analyzed sample were performed by using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Furthermore, with the aim to investigate any possible chemical pollution, the Enrichment Factor (EF), Geo-accumulation Index (I_geo), Contamination Factor (CF) and Pollution Load Index (PLI) were assessed. Finally, the identification of the source of the aforementioned radioisotopes of natural origin was carried out by X-ray diffraction (XRD), thus identifying the major mineralogical phases present in the investigated basalt aggregate for concrete.

1. Introduction

Basalt is the most widespread magmatic or igneous effusive rock. With andesite, another type of volcanic rock, it makes up almost all, about 98%, of the rocks made up of the lava that erupted on the Earth’s surface and was then subjected to a process of crystallization. Generally, when not greatly weathered, basalt has colorations that from dark gray can tend to black [1]. For several years, it has been employed in casting procedures to produce ceramic plates and panels for architectural purposes [2]. In addition, fused basalt coatings for iron pipes show an extremely high abrasion strength in manufacturing operations [3]. Basalt is also used in many countries in the construction of highway and airport pavements [4], and it also finds application in organic farming, in the form of micronized rock flour, to revitalize and nourish soils and plants that have lost fertility, such as intensive and extensive farming that deplete soils of natural elements [5].

Furthermore, fragmented basalt aggregates, which are compact, finely grained, very dark green or black rocks produced when melted lava from the depths of the Earth’s crust ascends and crystallizes, are also recognized as a natural resource for the manufacturing of cheap and eco-friendly construction materials with acceptable strength and durability features, well suited to the framework of sustainable development [6,7]. In detail, the partial replacement of Portland cement with basalt aggregates in concrete, when available, can lead to a more cost-efficient solution [8].

Basaltic rock in Italy can be found close to the Mt. Etna volcano (eastern Sicily, Southern Italy) [9], the edifice of which grew on a sedimentary substrate more than 1.5 km thick [10]. The origin of Mt. Etna’s magmatism is probably related to extensive melting of the mantle, according to what is reported in [11].

In the present paper, a multi-technique approach including the use of High-Purity Germanium (HPGe) γ-ray spectrometry, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and X-ray diffraction (XRD) was employed with the aim to evaluate the radioactivity (radium-226, thorium-232, potassium-40 and caesium-137) and the heavy metals (arsenic, cadmium, copper, mercury, nickel, lead, antimony, thallium and zinc) content of the investigated basalt aggregate for concrete, picked up in a surrounding area of the Mt. Etna volcano [12], and to relate the natural radionuclides’ specific activity to its mineralogical composition.

Furthermore, in order to assess any possible radiological hazard for the population, the calculation of the alpha index (I_α), the radium equivalent activity (Ra_eq), the absorbed γ-dose rate (D) and the annual effective dose equivalent outdoor (AEDE_out) and indoor (AEDE_in) was performed [13,14,15]. Of note, in Italy, the current legislation states that building materials or additives of natural igneous origin are subject to radiometric checking before being placed on the commercial market [16].

Finally, with the aim to estimate the level of environmental chemical pollution by the heavy metals, the Enrichment Factor (EF), Geo-accumulation Index (I_geo), Contamination Factor (CF) and Pollution Load Index (PLI) were assessed [17,18].

2. Materials and Methods

2.1. Sample Collection

The GPS coordinates of the specimen location are 37.53247 (latitude) and 15.037817 (longitude) (Figure 1).

Five aliquots of the basalt aggregate for concrete were collected in February 2022, from basalt outcrops, at depths of a few centimeters, and reduced to a coarse grain size by mechanical procedures. The sampling was performed from a relatively flat, clean, dry, hard surface, avoiding locations where surface dust or salts are likely to have accumulated.

After the collection, they were stored in labeled plastic containers, with proper precautions taken to avoid contamination [19], and subsequently transported to the laboratory.

2.2. HPGE γ-ray Spectrometry Measurements

Each aliquot of the basalt aggregate for concrete was first dried, in order to completely remove the moisture and to obtain constant mass. After, it was inserted into a Marinelli hermetically sealed container of 250 mL capacity. After 40 days, the secular radioactive equilibrium between ²²⁶Ra and its daughter products was reached, and then the sample was ready for gamma spectrometry measurement with a live time of 70,000 s. Spectra were analyzed with the aim to assess the activity concentration of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs. In detail, the ²²⁶Ra activity concentration was calculated by using the 295.21 keV and 351.92 keV ²¹⁴Pb and 1120.29 keV ²¹⁴Bi gamma-ray lines, and the ²³²Th-specific activity was determined by using the 911.21 keV and 968.97 keV ²²⁸Ac γ-ray lines. In particular, for the ²¹⁴Bi radionuclide, the TCS coincidence summation correction was applied [20] by using the MEFFTRAN code [21]. Continuing, for ⁴⁰K, the evaluation was performed from its γ-line at 1460.8 keV and, finally, in order to investigate the anthropic radioactivity content, the ¹³⁷Cs-specific activity was evaluated through its γ-line at 661.66 keV.

The experimental set-up was composed of a positive biased Ortec HPGe detector (GEM), whose operating parameters are reported in

Table 1. The HPGe GEM operating parameters.

HPGe GEM
Parameter	Value
Full Width at Half Maximum	1.85 keV
Peak-to-Compton ratio	64:1
Relative Efficiency	40% (at the 1.33 MeV 60Co γ-line)
Bias Voltage	4500 V
Energy Range	50 keV–2 MeV

[22].

The detector was located inside lead wells to screen the environmental background radioactivity and, for efficiency and energy settings, a multi-peak Marinelli γ-source (BC-4464) of 250 mL capacity, energy range 60–1836 keV, custom made to replicate the exact designs of the specimens in a water-equivalent epoxy–resin matrix, was employed.

The Gamma Vision (Ortec) software was used for data acquisition and analysis [22].

The specific activity (Bq kg⁻¹ dry weight, d.w.) of the investigated radioisotopes was calculated as follows [23]:

$$\mathrm{C}=\frac{{\mathrm{N}}_{\mathrm{E}}}{{\mathsf{\epsilon }}_{\mathrm{E}}\mathrm{t}{\mathsf{\gamma }}_{\mathrm{d}}\mathrm{M}}$$

(1)

where N_E is the net area of a peak at energy E; ε_E and γ_d are the efficiency and yield of the photopeak at energy E, respectively; M is the mass of the sample (kg); and t is the live time (s) [24]. Moreover, with the density of the basalt aggregate for concrete being higher than 1.1, the self-absorption correction on the activity concentration value was performed according to [25,26].

The Italian Accreditation Body (ACCREDIA) certified the quality of the γ-ray spectrometry experimental results [27], thus ensuring continuous verification that the performance properties of the method are preserved [28].

2.3. Evaluation of the Radiological Health Risk

Several indexes developed over the years to evaluate the radiological risk for the human beings related to radiation exposure, i.e., the alpha index (I_α), the radium equivalent activity (Ra_eq), the absorbed γ-dose rate (D) and the annual effective dose equivalent outdoor (AEDE_out) and indoor (AEDE_in), were calculated to estimate the potential radiation risk to humans.

2.3.1. Alpha Index

The alpha index was calculated with the following formula [29]:

I_α = C_Ra/200

(2)

where C_Ra is the mean activity concentrations of radium-226 in the basalt aggregate for concrete.

The alpha index allows to assess the alpha radiation exposure to the indoor radon exhaled from construction materials. The activity concentration of radium-226 must be lower than 200 Bq kg⁻¹, to prevent exposure to indoor radon-specific activity higher than the threshold value of 200 Bq m⁻³ [16], and then I_α must be less than unity for the risk of exposure to radiation to be minimal.

2.3.2. Radium Equivalent Activity

The radium equivalent activity is an index that describes the specific activities of radium-226, thorium-232 and potassium-40 in a single term [30,31]:

Ra_eq (Bq kg⁻¹) = C_Ra + 1.43C_Th + 0.077C_K

(3)

where C_Ra, C_Th and C_K are the mean activity concentrations of radium-226, thorium-232 and potassium-40 in the basalt aggregate for concrete, respectively.

This index must be lower than 370 Bq kg⁻¹ for the safe utilization of the basalt aggregate for concrete as building material [32].

2.3.3. Absorbed γ-Dose Rate

This parameter was calculated with the following formula [33]:

D (nGy h⁻¹) = 0.462C_Ra + 0.604C_Th + 0.0417C_K

(4)

2.3.4. Annual Effective Dose Equivalent Outdoor and Indoor

The annual effective dose equivalent for an individual was calculated using the equations below, with occupation factors of 20% and 80% for outdoor and indoor environments, respectively [34]:

AEDE_out (mSv y⁻¹) = D (nGy h⁻¹) × 8760 h × 0.7 Sv Gy⁻¹ × 0.2 × 10⁻⁶

(5)

AEDE_in (mSv y⁻¹) = D (nGy h⁻¹) × 8760 h × 0.7 Sv Gy⁻¹ × 0.8 × 10⁻⁶

(6)

Both must be lower than 1 mSv y⁻¹ for the radiological health risk to be negligible [16].

2.4. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Measurements

For the ICP-MS analysis, approximately 0.5–1.0 g of sample, together with 3 mL of ultrapure (for trace analysis) HNO₃ (67–69%) and 9 mL of ultrapure (for trace analysis) HCl (32–35%) (aqua regia), was directly introduced into a 100 mL TFM vessel. A Milestone microwave system, Ethos 1, was used for the acid digestion, as follows: (i) 15 min at 1500 W and 180 °C; (ii) 10 min at 1500 W and 180 °C; (iii) 10 min at 1000 W and 120 °C, with 20 min cooling [35]. The mixture was filtered and filled to 50 mL with distilled H₂O and diluted 10 times. Opportune dilutions of two certified materials were employed in order to prepare calibration solutions for the analytes in 0.5 % (v/v) HNO₃ and 0.5 % (v/v) HCl [18].

For the measurements, a Thermo Scientific iCAP Qc ICP-MS was used [36]. The instrument was operated in a single collision cell mode, with kinetic energy discrimination (KED), using pure He as the collision gas. All samples were presented for analysis using a Cetac ASX-520.

2.5. Evaluation of the Level of Heavy Metals Contamination

In order to assess the level of heavy metals contamination in the basalt aggregate for concrete, the pollution indices reported in the following were calculated.

2.5.1. The Enrichment Factor

This index was evaluated as follows [37]:

$$\mathrm{E}\mathrm{F}=\frac{\left\{{\mathrm{C}}_{\mathrm{x}}/{\mathrm{C}}_{\mathrm{F}\mathrm{e}}\right\}\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}{\left\{{\mathrm{C}}_{\mathrm{x}}/{\mathrm{C}}_{\mathrm{F}\mathrm{e}}\right\}\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}}$$

(7)

where C_x is the concentration of the potential enrichment element and C_Fe is the concentration of the normalizing element, usually iron [37].

2.5.2. The Geo-Accumulation Index

This pollution index is [38]:

$${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}=\mathrm{L}\mathrm{o}{\mathrm{g}}_2\left[{\mathrm{C}}_{\mathrm{n}}/\left(\mathrm{k}{\mathrm{B}}_{\mathrm{n}}\right)\right]$$

(8)

where C_n is the concentration of the potential harmful element in the sample, B_n is the geochemical background value in the average shale of element n and k is the correction factor of the background matrix [38].

2.5.3. The Contamination Factor

This index is given by [39]

$$\mathrm{C}\mathrm{F}={\mathrm{C}}_{\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}/{\mathrm{C}}_{\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}$$

(9)

where C_metal and C_background are the heavy metals concentration and background values, respectively [38].

2.5.4. The Pollution Load Index

The n-th root of the product of the Contamination Factor of heavy metals is the Pollution Load Index [40]:

$$\mathrm{P}\mathrm{L}\mathrm{I}={\left(\mathrm{C}{\mathrm{F}}_1\times \mathrm{C}{\mathrm{F}}_2\times \mathrm{C}{\mathrm{F}}_3\times \dots .\times \mathrm{C}{\mathrm{F}}_{\mathrm{n}}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}$$

(10)

where n is the number of metals [40].

2.6. XRD Analysis

X-ray diffraction analyses were performed by using a Panalytical Empyrean Diffractometer with Cu K_α radiation on a Bragg–Brentano theta-theta goniometer, equipped with a solid-state detector, PIXcel [41].

The generator settings were 40 kV and 40 mA. The measurements were performed in glass slide holders ensuring a uniform dispersion of properly compressed specimens. The continuous scan mode was employed in order to span the 2Θ incidence angle from 5° to 60° with a scan velocity of 1.2° per minute. The total runtime for each analysis was about 45 min.

The observed peak positions were then compared with reference spectra from RRUFF database, with the aim to identify the crystalline mineralogical constituents of the analyzed basalt aggregate for concrete [42].

3. Results and Discussion

3.1. The Specific Activity of the Radioisotopes

The average specific activity (the mean value of the 5 analyzed aliquots) of radium-226, thorium-232, potassium-40 and caesium-137, in the investigated basalt aggregate for concrete, was found to be (58.6 ± 6.6) Bq kg⁻¹ dry weight (d.w.), (40.7 ± 5.3) Bq kg⁻¹ d.w., (498 ± 57) Bq kg⁻¹ d.w. and lower than the minimum detectable activity (0.24 Bq kg⁻¹ d.w.), respectively.

Table 2. The specific activity C_Ra, C_Th, C_K and C_Cs of, respectively, ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs, in the five analyzed aliquots, together with the average values.

Aliquot ID	CRa (Bq kg−1 d.w.)	CTh (Bq kg−1 d.w.)	CK (Bq kg−1 d.w.)	CCs (Bq kg−1 d.w.)
1	53.6 ± 6.1	36.4 ± 4.9	498 ± 57	<0.18
2	63.6 ± 7.1	44.9 ± 5.7	510 ± 66	<0.24
3	58.6 ± 6.6	35.7 ± 5.1	491 ± 50	<0.21
4	61.9 ± 6.8	45.7 ± 5.5	505 ± 64	<0.27
5	55.3 ± 6.4	40.7 ± 5.3	486 ± 48	<0.30
Average	58.6 ± 6.6	40.7 ± 5.3	498 ± 57	<0.24

reports the radium-226-, thorium-232-, potassium-40- and caesium-137-specific activity in the five analyzed aliquots, together with the average values.

The worldwide average specific activity of radium-226, thorium-232 and potassium-40 is 35, 30 and 400 Bq kg⁻¹, respectively [32]. In light of this, the experimental results here reported show that, in our case, the average specific activity of all the detected radioisotopes is higher than the average worldwide value. These results need a more critical interpretation, which will be provided further below in terms of the mineralogical composition of the basalt aggregate for concrete itself.

Regarding caesium-137, the mean specific activity turned out to be lower than the minimum detectable activity, ruling out an anthropic contamination.

3.2. Radiological Hazard Effects Assessment

With reference to the values of the radiological hazard indices, the alpha index, obtained by using Equation (2), was found to be 0.29, less than unity and thus avoiding exposure to the indoor radon concentration of more than 200 Bq m⁻³. The radium equivalent activity was calculated through Equation (3) with the aim to ascertain the suitability of the investigated basalt aggregate for concrete for use as a structural material component. The obtained value was 155 Bq kg⁻¹, lower than 370 Bq kg⁻¹, set as the threshold limit for building materials, thus ensuring again that the analyzed sample may not be harmful if employed for civil construction.

The absorbed γ-dose rate, as obtained through Equation (4), was found to be equal to 65.3 nGy h⁻¹, a value attributable to the lithologic component of the sampling site [43], and it was used to evaluate, through Equations (5) and (6), the annual effective dose equivalent outdoor and indoor due to the activities of the radium-226, thorium-232 and potassium-40 in the investigated sample. The obtained values were 88.8 µSv y⁻¹ and 355 µSv y⁻¹, respectively, lower than the threshold value of 1 mSv y⁻¹ [16].

3.3. Heavy Metals Content

Table 3. Heavy metals content (mg kg⁻¹ d.w.) for the analyzed basalt aggregate for concrete.

ICP-MS Analysis
		Threshold limit
CAs	0.87	20
CCd	0.03	2
CCu	70.8	120
CHg	0.04	1
CNi	9.09	120
CPb	8.90	100
CSb	0.06	10
CTl	0.02	1
CZn	50.1	150

reports the heavy metals content (mg kg⁻¹ d.w.) for the analyzed basalt aggregate for concrete.

Of note, the obtained results are lower than the threshold limits [44]; hence, they can be regarded as no pollutants and do not compromise the well-being of the environment.

3.4. Evaluation of the Heavy Metals Contamination Level

3.4.1. EF

In agreement with [45], an EF < 2 indicates minimal enrichment. In particular, 0.5 < EF < 1.5 shows a natural-origin metal, while EF > 1.5 suggests a more likely anthropic one [45]. Moreover, the values between 2 and 5 indicate moderate enrichment; between 5 and 20, significant enrichment; between 20 and 40, high enrichment; and an EF higher than 40, extremely high enrichment.

The obtained EF values, reported in

Table 4. Calculated values of EF, I_geo, CF and PLI for the investigated sample.

Metal	Index of Contamination
	EF	Igeo	CF	PLI
As	0.07	−4.49	0.07	0.14
Cd	0.10	−3.91	0.10
Cu	1.58	0.07	1.57
Hg	0.10	−3.91	0.10
Ni	0.13	−3.49	0.13
Pb	0.45	−1.75	0.45
Sb	0.04	−5.23	0.04
Tl	0.01	−6.71	0.01
Zn	0.53	−1.51	0.53

, were found to be <2 in all cases, indicating no or minimal enrichment.

3.4.2. I_geo

The I_geo values must be interpreted as follows [46]:

• I_geo ≤ 0 denotes no contamination;
• For 0 < I_geo ≤ 1, no/a medium degree of contamination;
• For 1 < I_geo ≤ 2, a medium degree of contamination;
• For 2 < I_geo ≤ 3, a medium/high degree of contamination;
• For 3 < I_geo ≤ 4, a high degree of contamination;
• For 4 < I_geo ≤ 5, a high/very high degree of contamination;
• I_geo > 5, a very high degree of contamination.

The obtained I_geo values, reported in Table 4, were found to be <0 with the only exception being copper, probably because it is often used as a soil defense product, as well as the soil texture and its high pH [46,47].

3.4.3. CF

According to [47], a CF ≤ 1 indicates no contamination; 1 < CF ≤ 3, a low or medium degree of contamination; 3 < CF ≤ 6, a high degree of contamination; and CF > 6, a very high degree of contamination.

The obtained CF values, reported in Table 4, are <1 in all cases except copper, showing again a very moderate degree of contamination for this metal.

3.4.4. PLI

According to [48], a PLI value higher than 1 indicates chemical pollution.

In our case, the PLI was found to be <1, thus revealing no pollution by the investigated heavy metals.

3.5. XRD Analysis

The X-ray diffraction analysis result is shown in Figure 2.

Minerals recognition was performed by comparing the measured diffraction peak positions to the RRUFF database. An XRD analysis put in evidence the presence of Altaite (PbTe, RRUFF ID: R060939), Anorthite (Ca(Al₂Si₂O₈), RRUFF ID: R040059), Augite ((Ca,Mg,Fe)₂Si₂O₆, RRUFF ID: R061108), Pyrochlore ((Na,Ca)2Nb2O6(OH,F), RRUFF ID: R060151) and Hematite (Fe₂O₃, RRUFF ID: R040024), superimposed to a glassy groundmass, in the investigated basalt aggregate for concrete.

Of note, we can reliably hold the detected mineralogical phases to account for the radionuclides content previously discussed. In particular, the high specific activity of radium-226, thorium-232 and potassium-40 radioisotopes, if compared with the average worldwide value, can be explained taking into account that, from the diffractogram, it is possible to evince in particular the presence of pyrochlore, in the composition of which Niobium appears. This REE represents an element that, from a geochemical point of view, is a marker of occurrence of crustal contamination, i.e., a migration of isotopes of various elements from the Earth’s crust to the magma, which then solidified and became basalt [49,50]. Therefore, in light of this, it is possible to justify the specific activity values of the detected natural radioisotopes, which are higher than the average worldwide value in all cases [51].

4. Conclusions

The natural and anthropic radioactivity content of a basalt aggregate for concrete from Sicily, Southern Italy, was analyzed through High-Purity Germanium (HPGe) γ-ray spectrometry. Moreover, calculations of the alpha index (I_α), the radium equivalent activity (Ra_eq), the absorbed γ-dose rate (D) and the annual effective dose equivalent outdoor (AEDE_out) and indoor (AEDE_in) were performed in order to estimate the radiological hazard for human beings. Of note, the obtained values turned out to be lower than the maximum recommended ones for humans, thereby rationally excluding any significant health impact related to exposure to ionizing radiation. Additionally, the mean specific activity of caesium-137 turned out to be lower than the minimum detectable activity.

Next, the concentration levels of the heavy metals in the analyzed basalt aggregate for concrete were investigated through Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The resulting values were found to be below the threshold levels established by Italian legislation and thus do not reasonably represent a health risk to humans. In addition, the calculation of various pollution indices was carried out in order to assess the ecological risk from heavy metals imposed on the ecology of the ecosystem. The obtained results show a very minimal enrichment only for copper, probably due to the use of this metal as a soil defense product, as well as the soil texture and its high pH, and in general no pollution by the assessed heavy metals.

Finally, X-ray diffraction (XRD) was applied to recognize the mineralogical/geochemical composition of the investigated sample and to relate it to the natural radioactivity content. From the results, we can conclude that the analyzed basalt aggregate for concrete was characterized by the presence of Altaite, Anorthite, Augite, Pyrochlore and Hematite, superimposed to a glassy groundmass. Moreover, the natural radionuclides’ specific activity reported in the present study underlined a high value of the activity concentration of radium-226, thorium-232 and potassium-40 radionuclides with respect to the average worldwide value. This can be explained by the occurrence of crustal contamination, put in evidence by the presence, in the diffractogram, of pyrochlore, in the composition of which Niobium appears.