Environmental Geochemistry of Potentially Toxic Metals in Phosphate Rocks, Products, and Their Wastes in the Algerian Phosphate Mining Area (Tébessa, NE Algeria)

Abstract

This study is focused on the environmental geochemistry of potentially toxic metals (PTMs)-bearing products and wastes in a mining area in Djebel Onk (NE Algeria) and their potential ecological and health risk assessment. Representative samples from (i) commercial products and (ii) grain size classes of wastes were mineralogically characterized using X-ray diffraction. The major and PTMs elements in the samples were chemically analyzed via ICP-AES and ICP-MS, respectively. The results reveal that the samples were mostly composed of carbonate fluorapatite (P₂O₅ > 24 wt %) and yielded PTM concentrations within the average range of phosphorites in neighboring countries and worldwide deposits as well. The concentrations of V, Cd, As, Ni, and Pb in the products were found to be within the acceptable values according to various standards, such as the Canadian and German Fertilizer Ordinance policies. Geochemically, PTMs distribution is linked to the main sub-composition of phosphate rock (apatite, clay, and dolomite). The Enrichment Factors (EF) display very-high-to-extremely-high enrichment of PTMs (Cr, Zn, As, Sr, Mo, Cd, Sb, Tl, Th, and U), while V, Co, Cu, Ni, Zr, Ga, Nb, and Pb show minor and moderate-to-high enrichments. Among all the PTMs, Cd, Tl, and U display a very high ecological risk (PERI) and contribute most to the total risk index (RI). The human health risk assessment of PTMs represented by the Hazard Index (HI) indicates that the non-carcinogenic risks are below the threshold values (HI < 1), while the HI values are higher for children than they are for adults. However, the cancer index (life time cancer risk) for Cr, Ni, As, and Cd for children and Cd for adults is greater than the acceptable threshold. These results are useful for phosphate beneficiation via removing these PTMs from the commercial product for efficient waste management.

1. Introduction

Potentially toxic metals (PTMs) pollution remains a major global concern due to the risks it poses to both the environment and human health. It is noteworthy to point out that even low concentrations of the majority of these elements are highly toxic. They are characterized by extensive sources, non-degradable properties, and cumulative behaviors, which are mainly dependent on their speciation and persistence in the environment [1,2,3]. During recent decades, research on PTMs has focused on understanding their spatial distribution and extent and the identification of different sources of contamination [2,4,5,6].

Phosphorites have gained a great deal of interest, although they are relatively enriched by numerous trace elements (e.g., [7,8,9,10,11]). They are economically important due to their multiple functions in the manufacturing of fertilizers and in industries (e.g., [3,12]). Large resources of phosphorites have been found in North Africa, with resources that total 50 billion metric tons (~75% of the world reserves [13]). In Algeria, the amount of phosphorite resources is estimated at 2 billion metric tons [13], and these are mostly situated in the Tébessa region in the north-eastern part of the country.

In this country, phosphate resources have been mined from the Djebel Onk basin since 1965; these deposits are geologically dated to the Paleocene–Eocene transition (~56 Ma., [11,12,14,15]). They were extensively studied from the point of view of geology and geochemistry (e.g., [11,12,16,17]). The authors have noticed that phosphorites yield several trace metal elements that were incorporated in the phosphate minerals and matrix during their deposition and may have been enhanced due to post-depositional conditions.

It is worth mentioning that the extraction and beneficiation of phosphate may potentially result in negative environmental impacts [9], especially when it is extracted using open-cast mining methods. Airborne, mine-derived resources are constantly threatening human health and the environment via the atmospheric deposition of polluted dust from solid phosphate waste [18]. Furthermore, anthropogenic air pollution is enhanced by Saharan wind currents [19], which allow the erosion and transport of Saharan dust mixed with mine dust. Raw phosphates are moved out of the mine using open-air trucks after their extraction and procession, leading the production of huge quantities of powder [2,3]. Additionally, the use of on-land disposal mine tailings is widespread, and these piles are generally found in urban areas without protection. For instance, Ferreira da Silva et al. [16] have found that the levels of Cd exceed the permissible limits set by the European Commission for growing crops. In the case of Algerian P-wastes, Boumaza et al. [3] reported the presence of certain hazardous trace metals that exceed the soil standards, including U, Cd, Cr, Mo, V, and Tl.

Except from a few studies that have put an emphasis on the investigation of trace metal elements during phosphate treatment (e.g., [16]) and in wastes [3]), there has been no comprehensive ecological risk and health impact assessment of phosphate mining activities and operations. The novel contribution of this research is that it provides new results concerning PTMs in the phosphate industry from upstream to downstream, i.e., from the phosphate rock parent, products, and their wastes. Therefore, the present study was undertaken to enhance our knowledge on PTMs, with new analyses of actual wastes and commercial products to assess their fractionation throughout the process, with particular attention being paid to the evaluation of potential ecological and health risks. In detail, the research objectives aim to: (i) mineralogically and chemically characterize the phosphate wastes and the commercial product, (ii) identify the PTMs as well as their distribution and fractionation throughout the treatment process, and (iii) assess potential ecological and health risks.

2. Study Area and Features of Phosphate ore Deposits

2.1. Study Area Description

The Djebel Onk mining complex is located in the city of Bir El Ater, south of Tébessa region in south-east of Algiers (34.7078° N, 8.0024° E), 4 km from the urban area, and 21 km from the Algerian–Tunisian border. The population is estimated to total ~100,000 inhabitants, according to administrative estimates for the year, 2019 (Figure 1). The studied region has total area of 1986.34 km².

Based on climatological data provided by the National Water Resources Agency (ANRH) and a meteorological station in Tébessa over the periods 1998–2014 and 1972–2015, respectively, the study area is characterized by a semi-arid climate, with relatively cold winters and dry summers [20,21]. The effect of a drought is exacerbated by frequent occurrences of sirocco wind, which is characterized by hot, dry winds (south–north wind direction) that occur between June and July. The mean minimum temperature in January (coldest month) is 6.5 °C, while the maximum one is recorded in July (43.5 °C), which is the hottest month, with an average annual temperature of 15.85 °C. Additionally, the number of rainy days varies from 66 to 107 per year, and the annual rainfall ranges from 200 to 400 mm. While frost and snow are uncommon, the temperature frequently goes below 0 °C. In contrast, thunderstorms are strong and frequent, especially in the months of September and August.

Plant communities that characterize the ecosystem in the study area are typical of the steppe rangelands [22] and are dominated by Globularia alypum (L.), Stipa tenacissima (L.) (Halfa grass) (Figure 2B), Artemisia herba-alba Asso, Artemisia campestris (L.) (Figure 2A) and Rosmarinus officinalis L. The study area also includes planted stands established, such as olive (Figure 2A,C), Fig trees, and prickly pears [20,22,23], and natural trees, such as Aleppo pine, cypress (Figure 2B), and eucalyptus. In addition, wheat and barley fields and the cultivation of vegetables are widespread. On the other hand, the study area is considered to be a pastoral area where sheep and goats are raised. In the area of Djebel Onk, we noted the presence of gerbils (gerboa), Iberian hares, foxes, jackals, weasels, porcupines, and hyenas [24]. In addition to mammals, avian fauna, which are common in the area, consist of wild pigeons, falcons, partridges, and owls. There are also sparrows, crows, and wild geese.

As one part of Algeria’s phosphate accumulates area, the Djebel Onk phosphate mining area (Bir El Ater) is a very important phosphate-producing industry, with a mining history of over 50 years. Due to this long period of continuous phosphate mining, land resources have been destroyed in some parts, with negative effects on the regional ecosystems, such as air pollution and the contamination of water, soil, fauna, and flora (e.g., [25,26]).

2.2. Geology of Phosphate Deposit

The Tébessa region (NE Algeria) hosts one of the largest phosphate ore deposits, containing ~2.2 billion metric tonnes [13]. It is located a few kilometres away from the Algerian–Tunisian border and ~600 km south-east of Algiers (Figure 1). Currently, the phosphate raw material is extracted from the Kef Essenoun deposit, with an estimated annual production total of about 1.3 million tons in 2018 [27]. The local geology has been studied extensively by many geologists (e.g., [14,15,28,29]). Recent geochemical investigations were conducted to identify the depositional conditions that promoted phosphate deposition (e.g., [11,12,30,31,32]). Here, we briefly provide a description of the geological features of the studied phosphate deposit. The Kef Essenoun deposit is included in a stratigraphic succession dated to the Maastrichtian to the Lutetian and is overlain by Miocene and Quaternary materials (Figure 3 [14,33]). The phosphorite layer is dated to the Upper Thanetian, displaying a ~35 m thick stratum and from a 10 to 15° dip towards the south [15,34]. Generally, two main phosphorite facies are identified, including black and dark colors based on the matrix and organic matter content [11,31]. This layer itself is divided into three sub-layers based on P₂O₅ concentrations (basal, main, and upper). The main sub-layer is recognized due to its thickness of ~25 m on the one hand and its high P₂O₅ contents on the other one hand (>28 wt% [11]); therefore, it is selected for extracting phosphate raw material.

From a mineralogical perspective, many authors, such as Kechiched et al. [11,12], Bezzi et al. [16], and Boumaza et al. [3], have indicated that the main phosphate mineral in Kef Essenoun phosphate rocks is carbonate fluorapatite (CFA). The matrix consists of quartz, calcite, dolomite, and clays, together with organic matter and minor sulfide minerals.

3. Materials and Methods

3.1. Samples and Analytical Techniques

In the current study, six samples were collected before and after the treatment of phosphate of the Kef Essenoun main sub-layer. These samples included commercial products from the dry (CP1) and wet (CP2) samples, in addition to wastes. To avoid the weathering effect, the samples, CP1 and CP2, were taken directly from phosphate silos, while the wastes were sampled directly via rejection (dry and wet) before they were expelled to exterior piles. Additionally, data from Boumaza et al. [3] on raw phosphate rock (RP) were used for comparison. This sampling methodology enables the tracking of the distribution of PTMs and associated ecological and health risks throughout the phosphorite treatment, including the rock, products, and wastes. The sampling methodology, for both final products and wastes, consisted of collecting quantities of ~0.5 kg for 30 min over a period of 5 h. These samples were stored in plastic bags before being transferred to the laboratory. Note that the samples were dried in an oven at a temperature of 105 °C for 24 h, and then each single sample type was individually packed in an airtight plastic bag.

Grain size classification was performed to investigate the relationship between particle size fractions and concentrations of PTMs. The samples were dry sieved using an electronically controlled electromagnetic sieve shaker, specifically the FRITSCH Analysette 3. The retained particle size fractions were: 0.08 mm, 0.08–0.5 mm, 0.5–2 mm, and >2 mm. During this process, the sieves were thoroughly cleaned with distilled water, as suggested by Thorne and Nickless [35]. After homogenizing and dividing each sample with a Rotary Sample Divider, ~20 g of each sample was ground to obtain a fine powder using an electric mill, the FRITSCH Pulverisette type.

The mineralogical analysis of samples was carried out using a Siemens D5000. Data were collected within the range 2–100° (2θ), with step size of 0.01° and step time of 10 s and Cu Kα1 radiation (λ = 1.54056 Å), running at a voltage of 45 kV and current of 35 Ma. Mineralogical analysis was performed at the Chemical and Materials Analysis Laboratory (FiLAB) in Dijon Cedex (France).

Major oxides were analyzed via inductively coupled plasma-atomic emission spectrometry (ICP-AES) using a Perkin Elmer OPTIMA 3300 RL. Note that acid digestion was performed using a mixture of HNO₃-Br₂-HF-HCl. The loss on ignition value (LOI) was determined when the samples were heated at 1100 °C for 1 h. For potentially toxic metals (PTMs) determination, ~ 0.25 g of powdered and dried samples were studied via inductively coupled plasma-atomic mass spectrometry (ICP-MS) using a PerkinElmer Elan 6100 DRC after digestion in a mixed solution (HClO₄-HNO₃-HCl-HF) at the Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements (IMGRE) in Moscow (Russia). The detection limits for PTMs were as follows: 1 mg/kg (Mo, Zn, Rb, Ba, and Cr; 0.5 mg/kg for As and Sr); 0.2 mg/kg (Sn, Pb, and Co; 0.1 mg/kg for W, Nb, Zr, Cd, V, Ni, Cu, Ga, U, and Sb); 0.05 mg/kg (Cs, Hf, Th, and Tl). To ensure precise and accurate results were obtained, calibration solutions based on High Purity Standard ICP-MS-68B (Solution A + Solution B) (100 mg/l) in addition to accuracy control using a High Purity Standard CRM-TMDW solution were used. The analytical values obtained were within 95% confidence limits of the suggested values for this certified material.

Considering the compositional nature of geochemical data, centered log ratio (clr) transformation was conducted on the raw data following Aitchison’s [36] rules and several studies on compositional data (CoDa) (e.g., [37,38,39]). Principal Components Analysis (PCA), representing one of the most popular statistical analyses used to unravel underlying structures, was applied to the newly clr-transformed data instead of raw data to avoid the closure effect, which is usually associated with geochemical data. Additionally, hierarchical cluster analysis (HCA) was also conducted on the clr-transformed data using the Euclidean distance for a distance calculation and Ward’s method for agglomerative clustering, as suggested by many authors (e.g., [40]). The transformation of data into clrs and PCA analysis were performed using CoDaPack software [41], while XLSTAT software [42] was used for HCA analysis.

3.2. Indices of PTMs Risk Assessment

3.2.1. Enrichment Factor (EF)

To assess the risk of potentially toxic metals (PTMs), the Enrichment Factor (EF) is used by many authors to evaluate the occurrence and intensity of possible anthropogenic contaminants (e.g., [43]). To calculate EF values, Fe and Al can be used as normalizing elements since they are conservative tracers in most studies [5,44]. For this study, Al was considered as the reference element for the EF calculation.

The EF was calculated using the following formula [45]:

$$EF=\frac{\left[\left(\frac{C}{Al}\right)\right]sample}{\left[\left(\frac{C}{Al}\right)\right]background}$$

(1)

where the $(\frac{C}{Al})$ sample represents the ratio of metal content relative to the reference element (Al) measured in the sample. The $(\frac{C}{Al})$ background is the ratio calculated using values of the Upper Continental Crust [46].

For the classification of EFs, the following ranges were suggested after Sutherland [47]:

• Depletion-to-minimal enrichment for the values: EF < 2;
• Moderate enrichment for the values: 2 < EF < 5;
• Significant enrichment for the values: 5 < EF < 20;
• Very high enrichment for the values: 20 < EF < 40;
• Extremely high enrichment for the values: EF > 40.

3.2.2. Potential Ecological Risk Index (PERI)

Hakanson [48] proposed a methodology for assessing the ecological risk by evaluating the potential ecological risk index (PERI). This index reflects a quantitative expression of the risk degree based on the toxicity of some common PTMs and the environment’s response [49,50]. This is how it is defined:

$$RI={\displaystyle \sum E_r^i}={\displaystyle \sum T_r^i}(C_s^i/C_n^1)$$

(2)

where RI represents the sum of all the potential risk factors from all PTMs, and $E_r^i$ consists of the potential ecological risk index for a single PTM, as calculated using:

$$E_r^i=C_r^i\times T_r^i$$

(3)

$T_r^i$ is the toxic response factor of the contamination of a single toxic metal. $C_r^i$ represents the pollution index for a given PTMs, which can be expressed as:

$$C_r^i=C_s^i/C_n^i$$

(4)

where $C_s^i$ is the present concentration of PTM in the sample, and $C_n^1$ is the reference value of PTMs. The toxic response factors were taken according to Hakanson [48] as follows: 2 for Cr and V, 3 for Zn, 7 for Sb, 5 for Co, Cu, Pb, Ni, and U, 10 for As and Tl, and 30 for Cd. The potential ecological risks for each single element ($E_r^i$) were classified as the following:

• Low risk: $E_r^i$ < 40;
• Moderate risk: 40 < $E_r^i$ < 80;
• Considerable risk: 80 < $E_r^i$ < 160;
• High risk: 160 < $E_r^i$ < 320.

On the other hand, the potential ecological risk index (PERI) was classified as follows:

• Low risk: RI < 150;
• Moderate risk: 150 < RI < 300;
• Considerable risk: 300 < RI < 600;
• High risk: RI ≥ 600.

3.2.3. Potential Human Health Risk

In this study, health risk assessment models were used, employing the method developed by the United States Environmental Protection Agency [51]. The chronic daily intake (CDI) was calculated for three main pathways (ingestion, dermal contact, and inhalation) for both adults and children according to the equations listed below.

$$CD{I}_{ing}=C\times \frac{IR\times EF\times ED}{BW\times AT}\times {10}^{-6}$$

(5)

$$CD{I}_{der}=C\times \frac{AF\times SA\times ABS\times EF\times ED}{BW\times AT}\times {10}^{-6}$$

(6)

$$CD{I}_{inh}=C\times \frac{IR\times EF\times ED}{PEF\times BW\times AT}$$

(7)

The Hazard Index (HI) that represents the total non-carcinogenic risk effects posed by all exposure pathways was estimated by summing up all the hazard quotients (HQ) [52,53,54], as expressed in Equations (8) and (9):

$$HQ=\frac{CDI}{RfD}$$

(8)

$$HI={\displaystyle \sum HQ=H{Q}_{ing}+H{Q}_{der}+H{Q}_{inh}}$$

(9)

The Hazard Index (HI) categorizes health risks into two types: HI < 1 refers to no significant risk of non-carcinogenic effects. The value of HI ≥ 1 indicates a possibility of a non-carcinogenic risk, the likelihood of which tends to increase as the HI value increases [51,55]. Similarly, the aggregate carcinogenic risk, as represented by the lifetime cancer risk (CR), was computed by summing the individual cancer risk from all exposure pathways using the following equations:

$$CR=CDI\times CSF$$

(10)

$${\displaystyle \sum CancerRisk=LCR=CI=C{R}_{ing}+C{R}_{der}+C{R}_{inh}}$$

(11)

Note that the value of 1.0 × 10⁻⁴ is an acceptable threshold value of the cancer risk, whereas the tolerable LCR for regulatory purposes ranges between 1.0 × 10⁻⁶ and 1.0 × 10⁻⁴ [55]. All the reference parameters that were used to calculate the different chronic daily intakes (CDI), hazard quotients (HQ), and cancer risks (CR) for adults and children are listed in Tables S1 and S2.

4. Results and Discussion

4.1. Mineralogical Composition of the Studied Samples

XRD analyses revealed that the raw phosphate (RP) from Kef Essenoun mainly consists of carbonate fluorapatite (CFA), which represents the main phosphate mineral phase. The samples also contain dolomite, calcite, quartz, gypsum, and clay minerals, which represent the matrix phase (Figure 4). This mineralogical composition is consistent with that reported in Algerian–Tunisian phosphorites by many authors (e.g., [11,16,56] and the references therein). Similar mineral phases have also been found in commercial products (CP) and some wastes, except for the fine fraction (<0.08 mm), which contains clay minerals (Figure 4). The similarity in the mineralogical composition of both the input raw phosphate (RP) and output products (CP and wastes) suggests that the treatment processes of phosphate ores do not significantly alter the primary mineralogy.

4.2. Major Elements

Chemical composition of the phosphate mine wastes, as well as the raw phosphate (RP) and commercial products (CP), are presented in terms of oxides, including Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃, and are shown in Table 1.

Additionally, Boumaza et al. [3] have analyzed raw phosphate and found high concentrations of P₂O₅ and CaO (30 wt% and 49.5 wt%, respectively), which reflect high proportions of fluorapatite (CFA).

To achieve high P-amounts, the treatment processes for increasing the P₂O₅ contents depend on the grain size characteristics and on decreasing the gangue content (including calcareous and clay matrix) in the commercial product. The results showed that the commercial product samples contained relatively high concentration of P₂O₅ and CaO, averaging 30.04 wt% and 48.25 wt % values, respectively. These results are considered to be satisfactory compared to the levels of marketable grades (28 wt%–30 wt% [57]). In terms of P₂O₅ contents, the wet process slightly enhances P₂O₅ in the marketable ore more compared to that of the dry process (30.15 w% and 29.94 wt%, respectively). Moreover, Al₂O₃, SiO₂, K₂O, TiO₂, and Fe₂O₃ show lower concentrations after the wet process (Al₂O₃ = 0.38 wt%, SiO₂ = 2.19 wt%, K₂O = 0.12 wt%, TiO₂ = 0.029 wt%, and Fe₂O₃ = 0.37 wt%). The LOI values are quite similar in commercial products subjected to both methods (wet and dry), ranging from 11.98 wt% to 12.42 wt%.

In this work, the wastes showed higher P₂O₅ and CaO contents (26.19 ± 1.79 wt% and 46.21 ± 2.25 wt%, respectively). The second most abundant major oxides are represented by SiO₂, MgO, Na₂O, and Al₂O₃, showing contents of 5.20 ± 2.42 wt%, 3.09 ± 0.53 wt%, 1.12 ± 0.06 wt%, and 1.08 ± 0.53 wt%, respectively. The total amount of other major oxides (K₂O, TiO₂, MnO, and Fe₂O₃) was generally less than <1%. These results globally reflect a similarity with Tunisia’s mine wastes (Table 1). On the other hand, the highest P₂O₅ and CaO contents characterize the average and coarse wastes, reflecting CFA minerals in the same wastes. The oxides Al₂O₃, SiO₂ and Fe₂O₃ mainly mirror the fine wastes, which are indicative of the abundance of aluminosilicates in fine wastes (1.87 wt %, 8.78 wt %, and 0.66 wt %, respectively). Similarly, MgO (3.27%) is generally known to be a component derived from dolomite and magnesian clay [58]. The LOI values range between 14.30 and 17.27 wt%.

The CaO/P₂O₅ ratios show that the values for raw and commercial phosphates vary from 1.60 to 1.65, which are relatively similar to those reported in pure CFA (1.57–1.62 [58,59]). In comparison to Morocco and Tunisia phosphorites, the CaO/P₂O₅ ratio in raw phosphate is higher than those of Moroccan phosphorites [60] and slightly lower than those of Tunisian phosphorites [10]. The higher values of the CaO/P₂O₅ ratio obtained for the wastes (1.69–1.84) reflect large contents of carbonates as gangue minerals. Table 1 shows that the CaO/P₂O₅ ratios of the waste samples are lower than those of the wastes from the P mining location in Morocco [61] and slightly higher than the corresponding ratios of wastes from Tunisia [9].

Table 1. Major element composition (wt %) of analyzed samples (RP: raw phosphate; CP: commercial products and wastes).

	Present Study							Morocco		Tunisia
Elements	RP (*)	Commercial Product		Wastes (mm)
		CP1	CP2	<0.08	0.08–0.5	0.5–2	>2	P-Rock 1	P-Waste 2	P-Rock 3	P-Waste 4
Na2O	1.26	1.29	1.29	1.07	1.19	1.07	1.13	0.48	0.46	1.39	1.03
MgO	1.63	1.33	1.49	3.27	2.32	3.55	3.21	0.57	3.28	0.72	1.86
Al2O3	0.76	0.44	0.38	1.87	0.71	0.88	0.84	1.73	0.89	1.16	1.27
SiO2	3.57	2.31	2.19	8.78	3.54	4.39	4.07	8.25	11.60	9.03	10.65
P2O5	30.00	29.94	30.15	24.41	28.51	25.23	26.62	34.80	16.90	26.53	28.85
K2O	0.21	0.13	0.12	0.5	0.17	0.22	0.21	0.20	0.12	0.18	0.22
CaO	49.50	48.17	48.33	43.04	48.25	46.4	47.16	42.65	43.00	44.45	44.67
TiO2	0.032	0.04	0.029	0.055	0.038	0.047	0.037	0.04	0.07	0.047	0.08
MnO	0.003	0.003	0.003	0.004	0.003	0.003	0.003	0.005	n.a.	0.004	n.a.
Fe2O3	0.52	0.38	0.37	0.66	0.48	0.5	0.5	0.27	0.38	0.50	0.49
LOI	11.80	12.42	11.98	15.92	14.3	17.27	15.74	8.90	n.a.	12.74	10.43
CaO/ P2O5	1.65	1.61	1.60	1.76	1.69	1.84	1.77	1.23	2.54	1.67	1.55

(*) Data used are from Boumaza et al. [3], ¹ data from [60], ² data from [61], ³ data from [10], and ⁴ data from [9].

Table 1. Major element composition (wt %) of analyzed samples (RP: raw phosphate; CP: commercial products and wastes).

	Present Study							Morocco		Tunisia
Elements	RP (*)	Commercial Product		Wastes (mm)
		CP1	CP2	<0.08	0.08–0.5	0.5–2	>2	P-Rock 1	P-Waste 2	P-Rock 3	P-Waste 4
Na2O	1.26	1.29	1.29	1.07	1.19	1.07	1.13	0.48	0.46	1.39	1.03
MgO	1.63	1.33	1.49	3.27	2.32	3.55	3.21	0.57	3.28	0.72	1.86
Al2O3	0.76	0.44	0.38	1.87	0.71	0.88	0.84	1.73	0.89	1.16	1.27
SiO2	3.57	2.31	2.19	8.78	3.54	4.39	4.07	8.25	11.60	9.03	10.65
P2O5	30.00	29.94	30.15	24.41	28.51	25.23	26.62	34.80	16.90	26.53	28.85
K2O	0.21	0.13	0.12	0.5	0.17	0.22	0.21	0.20	0.12	0.18	0.22
CaO	49.50	48.17	48.33	43.04	48.25	46.4	47.16	42.65	43.00	44.45	44.67
TiO2	0.032	0.04	0.029	0.055	0.038	0.047	0.037	0.04	0.07	0.047	0.08
MnO	0.003	0.003	0.003	0.004	0.003	0.003	0.003	0.005	n.a.	0.004	n.a.
Fe2O3	0.52	0.38	0.37	0.66	0.48	0.5	0.5	0.27	0.38	0.50	0.49
LOI	11.80	12.42	11.98	15.92	14.3	17.27	15.74	8.90	n.a.	12.74	10.43
CaO/ P2O5	1.65	1.61	1.60	1.76	1.69	1.84	1.77	1.23	2.54	1.67	1.55

(*) Data used are from Boumaza et al. [3], ¹ data from [60], ² data from [61], ³ data from [10], and ⁴ data from [9].

4.3. Potentially Toxic Metals Composition

Deposits of mining wastes from phosphate treatment processes represent one of the main sources of potentially toxic metals (PTMs) contamination in mining environments [3,62,63].

The concentrations of potentially toxic metals (PTMs) in raw phosphate, commercial products, and mining wastes were determined in order to estimate and evaluate their potential risks to both human health and the environment.

Table 2. Potentially toxic metals composition (mg/kg) of the analyzed samples (RP: raw phosphate; CP: commercial products and wastes).

	RP (*)	Commercial Product		Wastes
		CP1	CP2	<0.08	0.08–0.5	0.5–2	>2
Al	4022.31	2323.70	2011.15	9896.99	3757.68	4657.41	4445.71
V	63.50	50	39.10	90.55	59.77	59.16	56.74
Cr	107	92.20	53.70	276.78	116.93	117.10	103.07
Co	1.68	1.67	1.99	2.48	1.71	1.56	1.98
Ni	19.30	17	15.70	26.88	16.93	16.36	16.79
Cu	11.10	10	9.14	14.46	9.77	8.57	9.17
Zn	175	165	146	197.05	168.9	145.55	161.09
Ga	6.83	7.66	7.89	8.78	7.59	7.39	7.72
As	6.08	5.43	5.54	5.16	5.31	5.30	4.99
Rb	4.52	3.89	3.43	7.23	4	4.21	4.43
Sr	1785	1901	1853	1741.04	1686.53	1596.09	1608.02
Zr	26	26.60	23.80	29.06	25.89	25.24	25.08
Nb	1.26	1.29	0.987	1.84	1.39	1.54	1.36
Mo	6.87	5.37	5.74	5.76	5.08	5.80	5.77
Cd	20.80	20.80	19.30	26.51	22.18	21.61	22.78
Sn	0.11	0.101	0.095	0.17	0.11	0.12	0.12
Sb	0.84	0.769	0.763	0.79	0.76	0.76	0.76
Cs	0.32	0.291	0.267	0.40	0.31	0.35	0.34
Ba	29.40	28.80	25.60	37.3	28.61	24.96	27.52
Hf	0.28	0.213	0.168	0.33	0.22	0.22	0.22
W	0.06	0.130	0.069	0.09	0.07	0.08	0.10
Tl	3.79	3.77	4.07	4.41	3.61	3.80	3.56
Pb	2.50	2.63	2.46	6.35	2.73	2.86	2.69
Th	17.30	18.20	16.70	16.47	18.05	14.62	16.88
U	47.30	46.30	46.30	37.75	43.06	39.51	40.55

(*) Data used are from Boumaza et al. [3].

presents the concentrations of PTMs in commercial products (CP) and wastes samples.

Before discussing the new data, it is worth mentioning that Boumaza et al. [3] reported that raw phosphate has a high concentrations of V, U, Cr, Zn, and Sr (63.50 mg/kg, 47.30 mg/kg, 107 mg/kg, 175 mg/kg, and 1785 mg/kg, respectively), which are the most abundant PTMs. The second most abundant PTMs are Ni, Ga, Cu, As, Zr, Mo, Cd, Th, and Ba (contents ranging from 6.08 mg/kg to 29.40 mg/kg), while concentrations of Co, Rb, Nb, Tl, and Pb vary from 1.26 mg/kg to 4.52 mg/kg. The other PTMs, such as Sn, Sb, Hf, and W, are less abundant (<1 mg/kg).

Generally, the occurrence and distribution of PTMs are strongly influenced by several rock components (i.e., CFA, matrix, and accessory minerals) (e.g., [10,64]). The incorporation of PTMs can occur due to ionic isomorphic substitutions that take place in the apatite lattice (i.e., replacement of Ca²⁺ cation by several PTMs), and also, via adsorption onto the surfaces of minerals (e.g., [8,65,66]).

In this study, the mean abundances of PTMs in decreasing order in both commercial product samples (CP) are: Sr (1877 mg/kg), Zn (155.50 mg/kg), Cr (72.95 mg/kg), U (46.30 mg/kg), V (44.55 mg/kg), Ba (27.20 mg/kg), Zr (25.20 mg/kg), and Cd (20.05), while contents of Co, Ni, Cu, Ga, As, Rb, Nb, Mo, Tl, Pb, and Th vary between 1.14 mg/kg and 17.45 mg/kg. The contents of other PTMs (Sn, Sb, Hf, and W) total less than 1 mg/kg. These results highlight the high level of Sr (>1500 mg/kg), which does not necessarily imply the replacement of Ca²⁺ with Sr²⁺ (0.99°A), but may reflect the adsorption and fixation of apatite crystals (e.g., [16]). The presence of other PTMs in commercial product samples (CP) may be explained by the crystal-chemical control of apatite and the occurrence of clays and several silicates, iron oxides–hydroxides, or dolomite [60,67].

The examined waste samples show high contents of Sr, Zn, Cr, V, U, Ba, Zr, Cd, and Ni, with averages of 1657.92 ± 68.41 mg/kg, 168.15 ± 21.57 mg/kg, 153.47 ± 82.47 mg/kg, 66.56 ± 16.05 mg/kg, 40.22 ± 2.22 mg/kg, 29.60 ± 5.36 mg/kg, 26.32 ± 1.86 mg/kg, 23.27 ± 2.21 mg/kg, and 19.24 ± 5.10 mg/kg, respectively. Moreover, the concentrations of V, Cr, Zr, Cd, and Ba are higher compared to those in the RP and CP samples. Additionally, Co, Cu, Ga, As, Rb, Nb, Mo, Tl, Pb, and Th are moderately abundant (ranging from 1.36 mg/kg to 18.05 mg/kg). On the other hand, other elements, such as Sn, Sb, Cs, Hf, and W, showed contents of less than 1 mg/kg.

Based on these results, it is clear that the contents of the PTMs vary considerably depending on the particles size, whereby grain size may control the variation. For instance, except for Th and U, the concentrations of the other PTMs are higher in the fine fractions (<0.08 mm), which are rich with fine particles of dolomite, silica, and clay, which are associated with a significant number of PTMs. Furthermore, transition metals, such as V, Cr, Co, Ni, Cu, Zn, Zr, Mo, Cd, Ba, and Pb, can be associated with various gangue mineral species (e.g., [3,16,68]). On the other hand, the 0.08–05 mm fraction, which is generally rich in phosphates (P₂O₅ = 28.51 wt% and CaO = 48.25 wt%), had comparatively high contents of the following elements: Sr, Zn, U, and Th, which are associated with apatite minerals. Remarkably, the samples with the highest U content also yield the highest P₂O₅ amounts, which indicates a strong relationship between both elements. Note that U is known to be easily incorporated into the apatite lattice [7,11,64,69].

Compared to phosphorites worldwide, most PTMs concentrations in raw phosphate (e.g., V, Cr, Ni, Cu, Zn, As, Mo, Cd, Sb, Pb, and U) are generally lower or similar to the PTMs contents in phosphorites from North Africa (Tunisia [9,10] and Morocco [60,61]) and the Average World Phosphorite value [7] (

Table 3. Comparison of the average PTMs composition (in mg/kg) of raw phosphate (RP), commercial products (CP), and their wastes from Djebel Onk mine (the present study) compared to those of other worldwide phosphate rocks wastes.

	Present Study			Morocco		Tunisia		AWP 5	ASC 6	CFIA 7	GFO 8
	RP (*)	CP	Waste	P-Rock 1	P-Waste 2	P-Rock 3	P-Waste 4
Al	4022.31	2169.93	5689.45	n.a.	n.a.	n.a.	n.a.	n.a.	80,000	n.a.	n.a.
V	63.5	44.55	66.56	158	56	67	n.a.	130	130	44	n.a.
Cr	107	72.95	153.47	209	137	241.37	427.5	125	90	1060	300
Co	1.68	1.83	1.93	n.a.	n.a.	n.a.	0.3	7	19	151	n.a.
Ni	19.3	16.35	19.24	43.47	34	22.12	55	53	68	181	80
Cu	11.1	9.57	10.49	3.66	31.8	12.37	23.75	21.7	45	757	n.a.
Zn	175	155.5	168.15	156	195	289.5	226	195	95	1868	n.a.
Ga	6.83	7.78	7.87	3.85	n.a.	n.a.	n.a.	n.a.	19	n.a.	n.a.
As	6.08	5.49	5.19	n.a.	8	2.55	11	23	13	75	40
Rb	4.52	3.66	4.97	4.27	n.a.	4.63	n.a.	n.a	140	n.a.	n.a.
Sr	1785	1877	1657.92	627	769	1788.75	1606	1900	300	n.a.	n.a.
Zr	26	25.2	26.32	24	n.a.	35.5	n.a.	70	160	n.a.	n.a.
Nb	1.26	1.14	1.53	<1	n.a.	1.62	n.a.	n.a.	11	n.a.	n.a.
Mo	6.87	5.56	5.6	n.a.	n.a.	4.87	14.1	n.a.	2.6	20	n.a.
Cd	20.8	20.05	23.27	<2	33	54.27	50.3	18	0.3	20	50
Sn	0.11	0.1	0.13	n.a.	n.a.	n.a.	n.a.	n.a.	6	n.a.	n.a.
Sb	0.84	0.77	0.77	n.a.	n.a.	0.4	n.a.	n.a	1.5	n.a.	n.a.
Cs	0.32	0.28	0.35	n.a.	n.a.	n.a.	n.a.	n.a.	5	n.a.	n.a.
Ba	29.4	27.2	29.6	25	79.2	34.63	178	350	580	n.a.	n.a.
Hf	0.28	0.19	0.25	n.a.	n.a.	0.46	n.a.	n.a.	2.8	n.a.	n.a.
W	0.06	0.1	0.09	n.a.	n.a.	n.a.	n.a.	n.a.	1.8	n.a.	n.a.
Tl	3.79	3.92	3.85	n.a.	n.a.	n.a.	n.a.	n.a.	1.4	11	1
Pb	2.5	2.55	3.66	<1	13	n.a.	2.5	1–100	20	505	150
Th	17.3	17.45	16.51	<1	n.a.	7.6	n.a.	n.a.	12	n.a.	n.a.
U	47.3	46.3	40.22	137	0	31.9	103.2	120	3.7	n.a.	50

n.a.: data not available; (*) data used are from Boumaza et al. [3]; ¹ data from [60]; ² data from [61]; ³ data from [10]; ⁴ data from [9]; ⁵ AWP: Average World Phosphorite [7]; ⁶ ASC: Average Shale Composition [70]; ⁷ CFIA: Canadian Food Inspection Agency [71]; ⁸ GFO: German Fertilizer Ordinance [72,73].

). However, Sr is more significantly enriched in phosphorites as compared to those of other phosphates and to the ASC—Average Shale Composition [70]. The Nickel, Cu, As, Zn, Mo, Pb, Cr, Cd, Sr, and Ba contents in the waste samples were found to be much lower than those reported in wastes from Morocco [61] and Tunisia [9].

Note that Cd, Cr, and Pb are considered to be non-essential for plant life and dangerous to humans’ health. Therefore, based on their concentrations, they may be classified as toxic to plants. As fertilizers are the final commercial products of phosphate manufacturing, the concentrations of toxic metals present in the studied samples were compared to the limit values suggested by the Canadian Food Inspection Agency [71] and German Fertilizer Ordinance [72].

In the commercial product, the average concentrations of V and Cd (44.55 mg/kg and 20.05 mg/kg, respectively) are close to meeting Canadian fertilizer requirements (44 mg/kg and 20 mg/kg, respectively [71]). In addition, the concentration of Tl in the commercial product (3.92 mg/kg) is higher than the permissible limit set by the German Fertilizer Ordinance (1 mg/kg [72]), but it ranges within the permissible value of Canadian fertilizers limit (11 mg/kg [71]). The concentrations of As, Ni, Pb, and Tl remained below the legal limits according to the Canadian and German Fertilizer Ordinance. Currently, no legal limits have been defined in Canada and Germany yet for Rb, Sr, Zr, Nb, Sn, Sb, Cs, Hf, W, Ba, and Th.

4.4. Geochemical Controls on Potentially Toxic Metals Distribution

Principal component analysis of clr-transformed raw data has proven to be robust in dealing with compositional data, which are skewed in most cases. The closure effect can influence the obtained correlations, leading to an erroneous interpretation (e.g., [74,75]). The authors of several recent studies have applied PCA and HCA to clr-data in order to unravel and interpret the sub-compositions (e.g., [3,40,76]). A biplot [76] is a graphical representation that gathers both variables and factors, allowing a accurate interpretation. Note that a biplot is also adopted to deal with compositional data [77].

Using clr-transformed data, the extraction of principal components shows that PC1, PC2, and PC3 dominate the total variance, with 85.24% (PC1 = 73.29%; PC2 = 11.95%; PC3 = 7.94%). The remaining components have a minor effect on the total variance (e.g., PC4 = 4.15%; PC5 = 1.55%; PC6 = 1.12%) (Table S6). The biplot (Figure 5A) shows that major PTMs are mainly clustered into three main sub-compositions. Using major element clusters and mineralogical analysis, PTMs controls can be defined. Notably, PC1 appears to control two major components of apatite and clays, which are abundant in phosphates raw materials from the main sub-layer [11,32].

The Ca-P sub-composition, which reflects carbonate fluorapatite (CFA), shows highly negative loading towards PC1 and is enhanced in the commercial product (CP1 and CP2). This component controls several PTMs that can be associated with CFA, such as Sr, As, Th, and U, and are interpreted as being caused by substitution in the apatite lattice (e.g., [8,65]). On the other hand, the clay sub-composition, with high Al and Si contents, displays highly positive loading towards PC1 and mainly controls the finest fractions of wastes (e.g., <0.08 mm). This sub-composition relatively enhances some PTMs like Cr and Pb. The third sub-composition is represented by a relatively highly dolomitic component, especially in the wastes, showing a position between the two previous sub-compositions. It displays highly positive loading towards PC3, with less control on the PTMs. This sub-composition displays similar behavior to that of the raw phosphate samples, but is enriched with Mg, characterizing the coarse fraction of the samples (RP, 0.5–2 mm and >2 mm). Furthermore, some PTMs, such as Zn, Cd, Sn, Cs, and Co, display low levels of loading, which can be interpreted by studying other controls on their distribution, such as the occurrence of sulfide minerals that are largely reported in phosphorites from Algerian–Tunisian basins (e.g., [10,11,12]).

Moreover, the HCA of clr-transformed data (Figure 5B) can be used to interpret the clustering of studied samples. The HCA results are in agreement with the PCA and mineralogical analysis findings. In detail, HCA revealed three clusters of samples with similar behavior, such as the clayey sample (a: <0.08 mm), a high Ca–P composition represented by the final product (b: CP1 and CP2), and raw phosphates and coarse wastes (c and d: 0.08–0.5 mm, 0.5–2 mm, and >2 mm, RP).

4.5. PTMs Enrichment Assessment

The Enrichment Factor (EF) is an index that is commonly used to assess the enrichment degree of PTMs in environmental media at Djebel Onk. The EF results were compared to those reported in the literature to help determine appropriate technical remediation actions [44,78].

The phosphate rock samples (i.e., raw phosphate: RP) display extremely high EFs (>40) for Zn, Sr, Mo, Cd, Sb, Tl, Th, and U, with values ranging from 41.60 to 5834.61. Cr and As yield very high enrichment levels (20 < EF < 40), while V, Co, Cu, Ni, Zr, Ga, Nb, and Pb display moderate-to-high enrichment levels (2 < EF < 20). On the other hand, Rb, Sn, Cs, Ba, W, and Hf are generally depleted (EF < 2). As stated above and considering the presence of Cd in CFA according to the PCA results (see Figure 5A), the extremely high enrichment levels of Cd (EF = 5834.61) is explained by the extensive substitution of Ca²⁺ for Cd²⁺ in the apatite structure, reflecting crystal-chemical control [65,79].

Table 4 clearly shows that the EF values vary depending on the two processes of enrichment used (wet and dry processes) for all identified PTMs. Generally, the EF values for Zn, As, Sr, Mo, Cd, Sb, Tl, Th, and U in the commercial product (CP1 and CP2) were significantly higher than 40 (i.e., EF > 40), suggesting an “extremely high enrichment” level. However, high enrichment values (i.e., 20 < EF <40) were recorded for Cr in the commercial product (CP1 and CP2). The EF values for V, Ni, Cu, Ga, Zr, and Pb were between 5 and 20, suggesting a significant level of enrichment. The EF values for Co, Nb, Cs, and W were between two and five, indicating a moderate level of enrichment. Those for the remaining elements in CP1 and CP2 were less than two, suggesting a “minimal enrichment” level.

The enrichment of some PTMs in the dry process may be attributed to their association or melting point of toxic metals into the endogangue, which makes them difficult to be extracted using the same method. While in the wet method, these PTMs cannot float with carbonates due to their weight or because of the type of reagent of floatation used [16].

Taking into account the obtained results (Table 4); they clearly show that the extent of the PTMs’ enrichment differed among the waste types studied, most likely due to differences in the treatment mode, leading to their generation. Mine wastes are extremely enriched with Zn, Sr, Mo, Cd, Tl, and U, As, Sb, and Th elements. V, Cr, Ni, Cu, and Ga show significant levels of enrichment, while Zr, Nb, and Pb are moderately enriched. On the other hand, the remaining elements (e.g., Rb, Zr, Sn, Cs, Ba, Hf, and W) were not enriched in the mine wastes (EF < 2). The enrichment of these PTMs in mine wastes can be explained by the abundance of fine particles, which are relatively rich in iron oxides–hydroxides, silica, dolomites, and clay minerals. These PTMs accumulate through mechanisms of adsorption and fixation, as well as accumulation onto their surfaces [60,67].

In comparison with phosphorites from around the world, the EF values of these PTMs do not appear to be different from the values obtained from other Moroccan and Tunisian phosphate rocks, except for Cd in Tunisian phosphate rocks, which display the maximum value (11,631 [9]). On the other hand, the EF values of the Djebel Onk phosphate rocks samples (RP) show that the studied samples display relatively higher ratios than the Average World Phosphorite value (AWP) (six times higher for Sr, five times higher for Cr and Mo, three times higher for Cd, and two times higher for Zn). The other elements, such as V, Co, Ni, Cu, As, Pb, and U, show similar EF trends to those of the AWP.

The enrichment of these toxic metals in the environment is of the greatest concern because of their high level of toxicity and significant threat to humans’ health [3,4]. Note that EF values greater than 1.5 are a cause for alarm, as they indicate potentially toxic metals concentration above natural background values [80]. Consequently, the analyzed samples in the present study can be considered to be highly enriched with V, Cr, Co, Ni, Cu, Zn, Ga, As, Sr, Zr, Nb, Mo, Cd, Sb, Tl, Pb, Th, and U.

Table 4. Enrichment factors (EF) for the Djebel Onk samples compared to those of other worldwide phosphate deposits.

	Present Study							MOP 1	TUP 2	USP 1	AWP 2
	RP (*)	Commercial Product		Wastes
		CP1	CP2	<0.08	0.08–0.5	0.5–2	>2
V	16.35	19.85	15.52	7.69	13.37	10.67	10.72	10.6	n.a.	n.a.	11.5
Cr	29.36	38.58	22.47	24.78	27.57	22.27	20.54	28	91	15	11.6
Co	2.45	3.72	4.43	1.18	2.14	1.58	2.10	0.5	0.4	3.6	3.4
Ni	10.37	13.93	12.86	4.71	7.81	6.09	6.55	10	24	8	9.7
Cu	10.01	13.75	12.57	4.25	7.57	5.36	6.00	12	18	7	22.9
Zn	65.94	94.81	83.90	24.22	54.68	38.02	44.08	49	70	34	24.9
Ga	9.85	16.85	17.36	4.13	9.41	7.39	8.09	n.a.	n.a.	n.a.	n.a.
As	31.98	43.55	44.44	8.85	23.99	19.32	19.06	26	48	35	41
Rb	1.36	1.78	1.57	0.71	1.03	0.88	0.97	n.a.	n.a.	n.a.	n.a.
Sr	140.82	228.71	222.94	44.81	114.32	87.29	92.13	49	104	76	26
Zr	3.40	5.31	4.75	1.24	2.91	2.29	2.38	n.a.	n.a.	n.a.	n.a.
Nb	2.65	4.14	3.17	1.26	2.51	2.25	2.08	n.a.	n.a.	n.a.	n.a.
Mo	157.67	187.95	200.90	43.12	100.17	92.27	96.17	85	267	134	23
Cd	5834.61	8897.78	8256.11	2425.75	5345.41	4201.94	4640.37	2843	11,631	3056	1711.2
Sn	1.32	1.85	1.74	0.67	1.14	1.00	1.05	n.a.	n.a.	n.a.	n.a.
Sb	53.02	74.02	73.44	16.26	41.21	33.25	34.83	n.a.	n.a.	n.a.	n.a.
Cs	1.65	2.29	2.10	0.67	1.37	1.25	1.27	n.a.	n.a.	n.a.	n.a.
Ba	1.19	1.78	1.58	0.49	0.99	0.70	0.81	n.a.	n.a.	n.a.	n.a.
Hf	1.33	1.55	1.22	0.51	0.90	0.73	0.76	n.a.	n.a.	n.a.	n.a.
W	0.80	2.63	1.40	0.39	0.80	0.74	0.96	n.a.	n.a.	n.a.	n.a.
Tl	106.31	161.27	174.11	40.35	87.00	73.89	72.52	n.a.	n.a.	n.a.	n.a.
Pb	3.71	5.96	5.57	3.08	3.48	2.94	2.90	4	4	15	25.2
Th	41.60	66.73	61.23	12.92	37.29	24.37	29.47	n.a.	n.a.	n.a.	n.a.
U	442.27	660.20	660.20	115.14	345.92	256.08	275.34	661	794	880	380

n.a.: data not available. (*) data used from Boumaza et al. [3]. ¹ MOP: Morocco Phosphates; USP: USA Phosphates [81]; ² TUP: Tunisia Phosphates; AWP: Average World Phosphorite [9].

Table 4. Enrichment factors (EF) for the Djebel Onk samples compared to those of other worldwide phosphate deposits.

	Present Study							MOP 1	TUP 2	USP 1	AWP 2
	RP (*)	Commercial Product		Wastes
		CP1	CP2	<0.08	0.08–0.5	0.5–2	>2
V	16.35	19.85	15.52	7.69	13.37	10.67	10.72	10.6	n.a.	n.a.	11.5
Cr	29.36	38.58	22.47	24.78	27.57	22.27	20.54	28	91	15	11.6
Co	2.45	3.72	4.43	1.18	2.14	1.58	2.10	0.5	0.4	3.6	3.4
Ni	10.37	13.93	12.86	4.71	7.81	6.09	6.55	10	24	8	9.7
Cu	10.01	13.75	12.57	4.25	7.57	5.36	6.00	12	18	7	22.9
Zn	65.94	94.81	83.90	24.22	54.68	38.02	44.08	49	70	34	24.9
Ga	9.85	16.85	17.36	4.13	9.41	7.39	8.09	n.a.	n.a.	n.a.	n.a.
As	31.98	43.55	44.44	8.85	23.99	19.32	19.06	26	48	35	41
Rb	1.36	1.78	1.57	0.71	1.03	0.88	0.97	n.a.	n.a.	n.a.	n.a.
Sr	140.82	228.71	222.94	44.81	114.32	87.29	92.13	49	104	76	26
Zr	3.40	5.31	4.75	1.24	2.91	2.29	2.38	n.a.	n.a.	n.a.	n.a.
Nb	2.65	4.14	3.17	1.26	2.51	2.25	2.08	n.a.	n.a.	n.a.	n.a.
Mo	157.67	187.95	200.90	43.12	100.17	92.27	96.17	85	267	134	23
Cd	5834.61	8897.78	8256.11	2425.75	5345.41	4201.94	4640.37	2843	11,631	3056	1711.2
Sn	1.32	1.85	1.74	0.67	1.14	1.00	1.05	n.a.	n.a.	n.a.	n.a.
Sb	53.02	74.02	73.44	16.26	41.21	33.25	34.83	n.a.	n.a.	n.a.	n.a.
Cs	1.65	2.29	2.10	0.67	1.37	1.25	1.27	n.a.	n.a.	n.a.	n.a.
Ba	1.19	1.78	1.58	0.49	0.99	0.70	0.81	n.a.	n.a.	n.a.	n.a.
Hf	1.33	1.55	1.22	0.51	0.90	0.73	0.76	n.a.	n.a.	n.a.	n.a.
W	0.80	2.63	1.40	0.39	0.80	0.74	0.96	n.a.	n.a.	n.a.	n.a.
Tl	106.31	161.27	174.11	40.35	87.00	73.89	72.52	n.a.	n.a.	n.a.	n.a.
Pb	3.71	5.96	5.57	3.08	3.48	2.94	2.90	4	4	15	25.2
Th	41.60	66.73	61.23	12.92	37.29	24.37	29.47	n.a.	n.a.	n.a.	n.a.
U	442.27	660.20	660.20	115.14	345.92	256.08	275.34	661	794	880	380

n.a.: data not available. (*) data used from Boumaza et al. [3]. ¹ MOP: Morocco Phosphates; USP: USA Phosphates [81]; ² TUP: Tunisia Phosphates; AWP: Average World Phosphorite [9].

4.6. Ecological Risk Assessment

Given the occurrence of potentially toxic metals (PTMs) in raw phosphate, commercial products, and wastes, the potential ecological risk ($E_r^i$) for each potentially toxic metal and the total potential ecological risk index (RI) were calculated using contents of V, Cr, Co, Ni, Cu, Zn, As, Cd, Sb, Tl, Pb, and U (

Table 5. Potential ecological risk indexes for PTMs of the studied samples.

Sampling	Potential Ecological Risk Indices for Single PTM $\left({\mathit{E}}_{\mathit{r}}^{\mathit{i}}\right)$												Total Ecological Risk Index (RI)
	V	Cr	Co	Ni	Cu	Zn	As	Cd	Sb	Tl	Pb	U
RP (*)	1.32	2.32	0.50	2.05	2.00	2.61	12.70	6933.3	14.70	42.10	0.75	87.60	7101.95
CP1	1.03	2.00	0.48	1.81	1.79	2.46	11.31	6933.3	13.46	41.89	0.77	85.74	7096.08
CP2	0.81	1.17	0.58	1.67	1.63	2.18	11.54	6433.3	13.35	45.22	0.72	85.74	6597.94
<0.08	1.87	6.02	0.72	2.86	2.58	2.94	10.75	8836.7	13.83	49.00	1.87	69.91	8999.00
0.08–0.5	1.23	2.54	0.49	1.80	1.74	2.52	11.06	7393.3	13.30	40.11	0.80	79.74	7548.69
0.5–2	1.22	2.55	0.45	1.74	1.53	2.17	11.04	7203.3	13.30	42.22	0.84	73.17	7353.56
>2	1.17	2.24	0.57	1.79	1.64	2.40	10.40	7593.3	13.30	39.56	0.79	75.09	7742.28

(*) Data used are from Boumaza et al. [3].

).

The range of the total ecological risk index (RI) of the studied samples ranged from 6597.94 to 8999.00, indicating a very high ecological risk (RI ≥ 600) according to the risk classification [48]. Irrespective of the type of sample, the size of the contribution from Cd to the total potential ecological risk index (RI) was extremely large compared to those of other PTMs in the samples. The results pointed out that all samples were dominated by Cd, followed by U and Tl. Considering the classification of risk index suggested by Hakanson [48], the $E_r^i$ values for Cd indicate a “very high potential ecological risk” ($E_r^i$ ≥ 320) and a “considerable potential ecological risk” for U (80 < $E_r^i$ < 160), while the $E_r^i$ values for Tl display a “moderate potential ecological risk” (40 < $E_r^i$ < 80).

Cadmium and U are considered to be non-essential for living organisms and can cause serious environmental problems due to their high toxicity ([2,3,9] and the references therein). Previous studies have shown that phosphate rock wastes contain many potentially toxic metals (e.g., [2,3,60,82]). Some of these potentially toxic metals can infect soils, sediments, and water, especially if the manufacturing process does not allow their removal. A study on some toxic metal elements and an ecological risk assessment in the Djebel Onk phosphate mining area revealed that agricultural soils and sediments display had a high Cd level, which may pose risks at the local scale [25]. This is due to the fact that Cd is one of the main emissions from phosphate mining activities [83]. In addition, our earlier study in the vicinity of the phosphate mine area, including wells and surface and spring water, showed that Cd and U were among the main contaminants, exceeding the international standards for PTMs [6]. On the other hand, the individual $E_r^i$ values of the other toxic metals were less than 40, indicating a low potential risk. Although they are low risk, Sb, As, Cr, and Ni still pose a greater risk compared to that of the remaining PTMs, i.e., elements that contribute the most to the total ecological risk index. The average $E_r^i$ values of 12.91, 11.88, 2.97, and 1.98 were recorded for Sb, As, Cr, and Ni, respectively (Table 5).

4.7. Potential Human Health Risk

A risk assessment of the measured potentially toxic metals (PTMs) concentrations in raw phosphate, commercial products, and mine wastes is classically conducted to evaluate whether any potential adverse health effects are posed to mineworkers and inhabitants due to phosphate processing.

The average chronic daily intake (CDI), hazard quotient (HQ), and Hazard Index (HI) for the non-carcinogenic risk of PTMs from the three exposure pathways to both adults and children are shown in

Table 6. Average chronic daily intake (CDI; mgkg⁻¹day⁻¹), hazard quotient (HQ), and cumulative Hazard Index (HI) for non-carcinogenic risk in the studied samples.

Adults
PTM	CDIing	CDIder	CDIinh	HQing	HQder	HQinh	HI
V	8.2 × 10−5	3.3 × 10−7	7.7 × 10−9	1.2 × 10−2	4.7 × 10−3	1.1 × 10−6	1.6 × 10−2
Cr	1.9 × 10−4	7.5 × 10−7	1.8 × 10−8	6.2 × 10−2	1.2 × 10−2	6.1 × 10−4	7.5 × 10−2
Co	2.4 × 10−6	9.7 × 10−9	2.3 × 10−1	1.2 × 10−4	6.1 × 10−7	4.0 × 10−5	1.6 × 10−4
Ni	2.5 × 10−5	1.0 × 10−7	2.4 × 10−9	1.3 × 10−3	1.9 × 10−5	1.2 × 10−7	1.3 × 10−3
Cu	1.4 × 10−5	5.5 × 10−8	1.3 × 10−9	3.4 × 10−4	4.6 × 10−6	3.2 × 10−8	3.5 × 10−4
Zn	2.2 × 10−4	9.0 × 10−7	2.1 × 10−8	7.5 × 10−4	1.5 × 10−5	7.0 × 10−8	7.6 × 10−4
As	7.8 × 10−6	3.1 × 10−8	7.4 × 10−10	2.6 × 10−2	2.5 × 10−4	2.5 × 10−6	2.6 × 10−2
Sr	2.4 × 10−3	9.7× 10−6	2.3 × 10−7	4.1 × 10−3	8.1 × 10−5	3.8 × 10−7	4.1 × 10−3
Mo	7.4 × 10−6	3.0 × 10−8	7.0 × 10−10	1.5 × 10−3	1.6 × 10−5	1.4 × 10−7	1.5× 10−3
Cd	3.0 × 10−5	1.2 × 10−7	2.8 × 10−9	3.0 × 10−2	1.2 × 10−2	2.8 × 10−6	4.2 × 10−2
Sb	1.0 × 10−6	4.0 × 10−9	9.5 × 10−11	2.5 × 10−3	5.0 × 10−4	2.4 × 10−7	3.0 × 10−3
Ba	3.9 × 10−5	1.6 × 10−7	3.7 × 10−9	5.6 × 10−4	3.2 × 10−5	2.6× 10−5	6.1 × 10−4
Tl	5.2 × 10−6	2.1 × 10−8	4.9 × 10−10	6.6 × 10−2	2.1 × 10−3	6.2 × 10−6	6.8 × 10−2
Pb	4.5 × 10−6	1.8 × 10−8	4.3 × 10−10	1.3 × 10−3	3.5 × 10−5	1.2 × 10−7	1.3 × 10−3
U	5.9 × 10−5	2.3 × 10−7	5.5 × 10−9	9.8 × 10−2	4.6 × 10−4	9.2 × 10−6	9.8 × 10−2
Children
PTM	CDIing	CDIder	CDIinh	HQing	HQder	HQinh	HI
V	7.7 × 10−4	2.1 × 10−6	2.1 × 10−8	1.1 × 10−1	3.1 × 10−2	3.1 × 10−6	1.4 × 10−1
Cr	1.7 × 10−3	4.9 × 10−6	4.9 × 10−8	5.8 × 10−1	8.1 × 10−2	1.7 × 10−3	6.6 × 10−1
Co	2.3 × 10−5	6.4 × 10−8	6.4 × 10−10	1.1 × 10−3	4.0 × 10−6	1.1 × 10−4	1.3 × 10−3
Ni	2.4 × 10−4	6.7 × 10−7	6.7 × 10−9	1.2 × 10−2	1.2 × 10−4	3.2 × 10−7	1.2 × 10−2
Cu	1.3 × 10−4	3.6 × 10−7	3.6 × 10−9	3.2 × 10−3	3.0 × 10−5	9.0 × 10−8	3.2 × 10−3
Zn	2.1 × 10−3	5.9 × 10−6	5.9 × 10−8	7.0 × 10−3	9.8 × 10−5	2.0 × 10−7	7.1 × 10−3
As	7.3 × 10−5	2.0 × 10−7	2.0 × 10−9	2.4 × 10−1	1.7 × 10−3	6.8 × 10−6	2.4 × 10−1
Sr	2.3 × 10−2	6.4 × 10−5	6.4 × 10−7	3.8 × 10−2	5.3 × 10−4	1.1 × 10−6	3.8 × 10−2
Mo	6.9 × 10−5	1.9 × 10−7	1.9 × 10−9	1.4 × 10−2	1.0 × 10−4	3.9 × 10−7	1.4 × 10−2
Cd	2.8 × 10−4	7.9 × 10−7	7.9 × 10−9	2.8 × 10−1	7.9 × 10−2	7.9 × 10−6	3.6 × 10−1
Sb	9.4 × 10−6	2.6 × 10−8	2.6 × 10−10	2.4 × 10−2	3.3 × 10−3	6.6 × 10−7	2.7 × 10−2
Ba	3.6 × 10−4	1.0 × 10−6	1.0 × 10−8	5.2 × 10−3	2.1 × 10−4	7.1 × 10−5	5.5 × 10−3
Tl	4.9 × 10−5	1.4 × 10−7	1.4 × 10−9	6.1 × 10−1	1.4 × 10−2	1.7 × 10−5	6.3 × 10−1
Pb	4.2 × 10−5	1.2 × 10−7	1.2 × 10−9	1.2 × 10−2	2.3 × 10−4	3.4 × 10−7	1.2 × 10−2
U	5.5 × 10−4	1.5 × 10−6	1.5 × 10−8	9.1 × 10−1	3.0 × 10−3	2.6 × 10−5	9.2 × 10−1

. Meanwhile, the average lifetime cancer risks (LCR/CI) of various PTMs from the different exposure pathways (i.e., ingestion, inward breath, and dermal contact) are shown in

Table 7. Average carcinogenic risks for different exposure pathways for adults and children in the studied samples.

PTM	Adults				Children
	CRing	CRder	CRinh	CI	CRing	CRder	CRinh	CI
Cr	9.3 × 10−5	3.7 × 10−7	8.8 × 10−9	9.4 × 10−5	8.7 × 10−4	2.4 × 10−6	2.4 × 10−8	8.7 × 10−4
Ni	4.3 × 10−5	1.7 × 10−7	4.1 × 10−9	4.3 × 10−5	4.0 × 10−4	1.1 × 10−6	1.1 × 10−8	4.1 × 10−4
As	1.2 × 10−5	4.7 × 10−8	1.1 × 10−9	1.2 × 10−5	1.1 × 10−4	3.1 × 10−7	3.1 × 10−9	1.1 × 10−4
Cd	1.9 × 10−4	7.6 × 10−7	1.8 × 10−8	1.9 × 10−4	1.8 × 10−3	5.0 × 10−6	5.0 × 10−8	1.8 × 10−3
Pb	3.9 × 10−8	1.5 × 10−10	3.6 × 10−12	3.9 × 10−8	3.6 × 10−7	1.0 × 10−9	1.0 × 10−11	3.6 × 10−7

.

Among these exposure pathways, maximum PTMs intake occurred via ingestion. The results reveal that the highest chronic daily intake (CDI) values are for Sr, and the lowest ones are for Sb. Increased PTMs intake via the ingestion pathway was found to be in agreement with a previous study that reported data from many worldwide locations, such as Malaysia [50], Mongolia [84], Tunisia [2], Turkey [85], Jordan [4], China [86], and Iran [87]. Furthermore, children showed increased PTMs intake and exposure compared to those of adults (see Table 6).

To study the non-carcinogenic risk, this study reported the average hazard quotients (HQ) of all PTMs through three exposure pathways (ingestion, dermal contact, and inhalation). The results show higher average values for children compared to those for adults. The average total HI values (i.e., the sum of HQ_ingestion, HQ_dermal, and HQ_inhalation) for all PTMs have a range of 1.6 × 10⁻⁴–9.8 × 10⁻² for adults, whereas the average total HI values for children were found to have a range of 1.3 × 10⁻³–9.2 × 10⁻¹. Notably, all HI average values were less than one (HI < 1) in both adults and children, indicating no significant non-carcinogenic risk of PTMs in the studied area, according to the United States Environmental Protection Agency [51,52,55,88].

Regardless of whether the subjects were adults or children, the average Hazard Index (HI) values for PTMs were found to be in the order of U > Cr > Tl > Cd > As > V > Sr > Sb > Mo > Pb > Ni > Zn > Ba > Cu > Co. Although there are unobserved non-carcinogenic risks, Cr, Cd, and U have a higher potential health risks than other toxic metals do. This finding is also consistent with previous works by Khelifi et al. [2] and Al-Hwaiti et al. [4], who studied the health risks of potential toxic metals of phosphate dust and phosphate mine tailings to adults and children. In addition, they found that the children’s average HI values were higher than those for adults. Our previous study on groundwater and surface and spring water in an area affected by a phosphate mine [6] showed higher average HIs among children, almost twice as high as those among adults, indicating that children are more susceptible to non-carcinogenic health effects. However, these researchers found that the non-carcinogenic risks due to PTMs resulted in HI values of less than 1 (HI < 1). Hence, it can be expected that health risks to the mineworkers and local residents are negligible and may not have any immediate non-cancerous health effects.

The total cancer risk (i.e., lifetime cancer risk), which is based on the concentrations of As, Cd, Cr, Ni, and Pb, is classified as carcinogenic by the International Agency for Research on Cancer [89]. As shown in Table 7, the average total cancer risk (CRs) values for all the study samples were an order of magnitude higher for children than they were for adults and across all exposure pathways (oral, dermal, and inhalation). Additionally, the average CI values ranged from 3.6 × 10⁻⁷ to 1.8 × 10⁻³ (children) and from 3.9 × 10⁻⁸ to 1.9 × 10⁻⁴ (adults). Notably, the CI for Cr, Ni, As, and Cd (children) and Cd (adults) exceeded the range recommended by United States Environmental Protection Agency of 1.0E-06 to 1.0 × 10⁻⁴ [51,52,88], i.e., CI >1.0 × 10⁻⁴ for both children and adults. It indicates that the long-term exposure to particles in the phosphate mining area can increase the potential risk of developing cancer and emphasize the need for the ongoing monitoring of these toxic metals in the study area.

Regardless of the type of risks (carcinogenic or non-carcinogenic), the results showed that the oral ingestion presented the highest risk, followed by dermal contact, and then the inhalation of particles. Additionally, the results indicated that children are more susceptible to potential health risks due to the presence of PTMs in the mining area than adults are, which is consistent with previous studies that have found children to be at-high-risk groups in the vicinity mining area and phosphorous chemical factories [2,50,63,86,90]. This result is mostly due to children’s behavioral patterns, as well as their physiological characteristics and poor hygiene habits, such as chewing their fingernails, finger or hand sucking, food consumption habits, frequent pica behavior, and high respiration rate [91,92,93].

4.8. Limitations of Health Risk Assessment

Many authors have evaluated the health risk based on calculating the risk quotient (HQ) and risk index (HI) (e.g., [94]). Although we only evaluated a theoretical case scenario, based on certain assumptions regarding exposure pathways (ingestion, dermal contact, and inhalation), these assumptions may not accurately reflect real-world exposure scenarios. Therefore, this may imply a relative overestimation or underestimation of the risk, which is related to the total concentrations applied. Furthermore, some limitations should be considered, such as the duration of exposure, the element itself and its form, co-occurring contaminants, nutritional status, lifestyle factors, genetic variations, gender, life stage (e.g., pregnancy and lactation), and age of the exposed population [94,95,96], as well as, variations in individual susceptibility [94]. However, the current risk assessment contributes, in part, to understanding the potential implications of PTMs dispersion within the studied material, thus evaluating the impact of human activity on the well-being of nearby communities. The present study provides insights into the environmental impact of phosphate mining activities. The obtained results can be useful for local communities living near mines, mineworkers, and policymakers. In addition, the existing regulations and waste management techniques could be improved in light of these results.

5. Conclusions

The authors of the present study have investigated potentially toxic metals (PTMs) in input and output products and their wastes resulting from the processing of phosphate ores in the Djebel Onk mining area, with the aim to assess their potential ecological and health risks. The main conclusions are as follows:

(1)

The mineralogical characterization showed that the samples share similar mineral assemblages, including carbonate fluorapatite, dolomite, calcite, quartz, gypsum, and clays.

(2)

The geochemical analyses revealed that the distribution and fractionation of potentially toxic metals (such as V, Cr, Co, Ni, Cu, Zn, Sr, Zr, Cd, Ba, Tl, and Pb) are related to the main components of phosphatic rock (clay, carbonate fluorapatite, dolomite, and sulfides).

(3)

The analyzed samples yielded PTM values similar to those of neighboring deposits in the North African region. The concentrations of V, Cd, As, Ni, and Pb in the commercial product were found to be within the acceptable values of the Canadian and German Fertilizer Ordinance.

(4)

The potential ecological risk (PERI) in the samples studied showed a very high ecological risk for Cd, Tl, and U, which generally have a higher risk compared to those of other toxic metals, contributing much more to the total risk index (RI).

(5)

In terms of health risks, the non-carcinogenic risks were found to be below the threshold values (HI < 1), revealing a non-significant non-carcinogenic risk due to PTMs. In addition to this, the cancer index (lifetime cancer risk) of Cr, Ni, As, and Cd for children and Cd for adults were found to be greater than the acceptable threshold value of 1.0E−04. Regardless of the type of risk (carcinogenic or non-carcinogenic), children were found to be more susceptible to potential health risks, owing to the presence of PTMs, than adults are, as the risk pathways were found to be in the order of oral ingestion > dermal contact > the inhalation of particles.

(6)

Both ecological and human risk assessments are critical to ensure that mining operations continue without harming the environment and people. The results of this study revealed the environmental impact of phosphate mining activities, which could be useful to local communities living near mines, mineworkers, policymakers amending existing regulations, and relevant agencies planning future improvement efforts.