Bioavailable and Bioaccessible Fractions of Potentially Toxic Elements in Copper Mining Wastes in the Southeastern Amazon

Abstract

The Brazilian Amazon presents several artisanal and industrial Cu mines that generate significant amounts of waste. The objective of this study was to evaluate the risks to the environment and human health based on the bioavailable and bioaccessible concentrations of potentially toxic elements (PTEs; Ba, Co, Cr, Cu, Mo, Ni, Pb, and Zn) in artisanal and industrial Cu mining areas in the Carajás Mineral Province (CMP), eastern Amazon. For this purpose, samples (0–20 cm depth) were collected from natural forest soils (considered as a reference), as well as areas where artisanal mining wastes (artisanal overburden–AO and artisanal rock waste–AR) and industrial mining tailings (IT) were deposited. Total PTE concentrations were obtained via acid digestion, bioavailable concentrations were obtained via sequential extraction, and oral bioaccessible concentrations were obtained via the simple bioaccessibility extraction test. Environmental indices were obtained from PTE concentrations. The results indicated contamination by elements such as Mo, Cr, Ni, and Cu, mainly in AR, which had the highest contamination levels. Sequential extraction revealed that most PTEs are in residual form, suggesting low environmental risk from the bioavailable fraction. The bioaccessible concentrations of Cr and Ni were associated with health risks for children in AR. The results of this study will be important for protecting the environment and public health in artisanal mining areas in the region.

1. Introduction

Anthropogenic activities have drastically modified the landscape and increased pressure on soil quality in the Carajás Mineral Province (CMP) [1,2], including mining, which is among the main causes of enrichment of potentially toxic elements (PTEs) in the region’s soils [3,4,5]. In the CMP, there are industrial Cu mining complexes with annual production capacities ranging from 140,000 to 200,000 tons of concentrated metal, as well as several artisanal mining areas, whose production information is not well known [6].

While industrial mining employs more appropriate methods of processing and waste disposal, artisanal mining adopts rudimentary procedures with greater potential for contamination by PTEs [7,8,9]. In the CMP, for instance, artisanal Cu mining has already been associated with high total levels of Cu, Cr, and Ni contamination, representing a much more alarming scenario than that observed in industrial Cu mines [6]. Such impacts can be even more serious considering the frequent human occupation in artisanal mining areas, which differ from industrial mines where access is usually restricted.

Total PTE contents can be used to calculate pollution indices such as the geoaccumulation index (Igeo) and the pollution load index (PLI), which may have some limitations. For example, Igeo does not allow for the assessment of the multielement effect and includes the use of a constant (1.5) that may underestimate the level of contamination [10]. In addition, Igeo and PLI do not consider the differences in toxicity between PTEs, nor their bioavailability [11]. Despite these limitations, such indices have been widely used to assess PTE pollution levels in mining areas in several countries, such as Brazil [12], India [13], and China [14], mainly due to their ease of calculation and high applicability [10].

Knowledge of the bioavailable levels of PTEs is crucial for adequately characterizing environmental risks since they represent immediate damage to the ecosystem and directly affect the food chain [15,16,17]. The bioavailable fraction of PTEs can be obtained by sequential extractions, such as the Community Bureau of Reference (BCR) method, which allows the contents of these contaminants to be determined in different fractions, including acid-soluble or exchangeable and bound to carbonates (F1), reducible or bound to oxides (F2), oxidizable or bound to sulfides and organic matter (F3), and residual (F4), which constitutes the fraction strongly bound to the crystalline structures of the minerals [18,19]. The two most reactive forms (F1 + F2) are usually considered the bioavailable fraction of PTEs in the ecosystem [20,21].

Furthermore, considering that only part of the total PTE content is released in the human body, it is essential to determine the bioaccessible fraction to understand the actual risk to human health and avoid overestimations [22,23,24]. The oral bioaccessible fraction of PTEs can be obtained in vitro via the Simple Bioaccessibility Extraction Test (SBET), which simulates the gastrointestinal environment of human digestion [25]. This method provides a practical approach to estimating the potential exposure of humans to PTEs through ingestion, offering valuable insights for risk assessment and environmental health studies [14,26].

Despite the importance of understanding the bioavailable and bioaccessible fractions of PTEs, such studies are scarce in the CMP. Given the direct contact of the population with areas impacted by mining in the region, it is essential to know the damage associated with these elements, especially with studies that consider their bioavailable and bioaccessible fractions. Therefore, the objectives of this study were to evaluate the risks to the environment and human health based on the bioavailable and bioaccessible concentrations of Ba, Co, Cr, Cu, Mo, Ni, Pb, and Zn in artisanal and industrial Cu mining areas in the CMP, eastern Amazon. The results of this study could contribute to protecting the environment and the health of the region’s inhabitants.

2. Material and Methods

2.1. Study Site and Sampling

The studied areas are located in the municipality of Canaã dos Carajás, which has an estimated population of more than 39,000 inhabitants and is located in the CMP, the state of Pará, northern Brazil. The municipality of Canaã dos Carajás occurs in the transition between the Rio Maria granite-greenstone terrain and the Carajás Basin [27]. This region has a relief characterized by mountainous terrain and a hot and humid climate, type Aw according to the Koppen classification [28]. The average annual temperature is 27.2 °C, and the average annual precipitation is approximately 2000 mm.

In this study, soils and wastes from artisanal and industrial mining areas were evaluated. The Archean units at these sites include Serra Dourada Granite (natural forest soils and artisanal mining areas) and Bom Jesus Granite (industrial mining area) [29]. The Serra Dourada Granite is mainly composed of quartz, alkali feldspar, plagioclase, biotite, and chlorite, as well as accessory minerals including zircon, epidote, pyrite, allanite, magnetite, and hematite [30]. The Bom Jesus Granite, in turn, is composed of monzogranite to syenogranite with biotite as the main mafic mineral, in addition to accessory minerals such as zircon, titanite, allanite, magnetite, ilmenite, and apatite [31].

The studied soils and mine wastes were identified as follows: (i) natural forest soil (NF), classified as Dystrophic Red–Yellow Argisol, collected as a reference for the natural environment (latitude 6°24′36.61″ S, longitude 49°52′18.34″ O); (ii) artisanal overburden (AO), composed of the material resulting from the stripping of the areas (latitude 6°24′36.75″ S, longitude 49°52′22.37″ O); (iii) artisanal rock waste (AR), collected in piles close to the exploration areas, formed after excavation (latitude 6°24′36.46″ S, longitude 49°52′25.30″ O); and (iv) industrial tailings (IT), collected in an industrial exploration tailings dam resulting from the flotation of Cu ore (latitude 6°27′11.66″ S, longitude 50°4′46.71″ O) (Figure 1).

Five composite samples (approximately 2.5 kg each) were collected per evaluated area (NF, AO, AR, and IT). Each of these samples was formed from the collection and mixing of five subsamples (approximately 0.5 kg each). The samples were collected from the 0–20 cm layer using a stainless-steel Dutch auger to avoid contamination. After collection, the samples were air-dried, processed through a sieve (Ø = 2 mm), homogenized, and stored for laboratory analyses.

2.2. Quantification of Total PTE Contents

Total PTE concentrations were obtained via the EPA method 3051A [32]. For this purpose, 9 mL of nitric acid (HNO₃) and 3 mL of hydrochloric acid (HCl) were added to 0.5 g of sieved soil (Ø = 100 mesh), followed by digestion in a microwave oven (model MARS 5, CEM Corporation, Matthews, NC, USA). The digestion process lasted 5 minutes and 30 seconds up to 175 °C (heating ramp), after which the mixture remained at this temperature for 4 minutes and 30 seconds (holding). After cooling, the extracts were slowly filtered and transferred to certified volumetric flasks. The volume was then adjusted to 50 mL.

For analytical quality control, samples were analyzed in triplicate, and in each batch, a blank sample and a certified sample of reference material were included (144 ERM-CC141). The concentrations were quantified via microwave plasma atomic emission spectrometry (MP-AES) (model 4210, Agilent, Santa Clara, CA, USA). The recovery rates ranged from 88 to 92%.

2.3. Assessment of Contamination Levels

From the total concentrations of PTEs, Igeo and PLI were calculated to study contamination in mining waste disposal areas. In these calculations, the concentrations of PTEs in the forest area were taken as a reference for the natural environment (background values) (Table S1). Igeo is commonly used to study the degree of contamination caused by a given element in relation to the natural environment [33] and is calculated according to Equations 1:

$$Igeo={Log}_2\left({\displaystyle \frac{C_{PTE}}{1.5B_{PTE}}}\right)$$

(1)

where C_PTE is the PTE concentration in the sample (mg kg⁻¹), B_PTE is the PTE concentration in the reference area (mg kg⁻¹), and 1.5 is a constant used to minimize geochemical variations. The Igeo values were interpreted according to [33]: Igeo ≤ 0 indicates uncontaminated material, 0 < Igeo ≤ 1 indicates slightly contaminated material, 1 < Igeo ≤ 2 indicates moderately contaminated material, 2 < Igeo ≤ 3 indicates moderately to highly contaminated material, 3 < Igeo ≤ 4 indicates highly contaminated material, 4 < Igeo ≤ 5 indicates highly to extremely contaminated material, and Igeo > 5 indicates extremely contaminated material.

The PLI allows quantitative evaluation of the degree of integrated pollution of the PTEs [34], which can be calculated according to Equations 2 and 3:

$$CF={\displaystyle \frac{C_{PTE}}{B_{PTE}}}$$

(2)

$$PLI={({CF}_1\times {CF}_2\times {CF}_3\times \dots {CF}_n)}^{1/n}$$

(3)

where CF is the contamination factor, C_PTE is the total concentration of PTE in the sample (mg kg⁻¹), B_PTE is the concentration of PTE in the reference area (mg kg⁻¹), and n is the number of PTEs under study. PLI values are evaluated at two levels: a PLI ranging from 0 to 1 indicates uncontaminated material and a PLI greater than 1 indicates contaminated material [34].

2.4. Chemical Fractionation and Bioaccessibility of PTEs

Following the methodology proposed by [35], sequential BCR (Community Bureau of Reference) extraction was used to determine the concentrations of PTEs in the following fractions: acid-soluble or exchangeable and bound to carbonates (F1), obtained with 0.11 M acetic acid (CH₃COOH); reducible or bound to oxides (F2), extracted with 0.5 M hydroxylamine (NH₂OH · HCl) at pH 2; oxidizable or associated with organic matter and sulfides (F3), obtained with 1 M ammonium acetate (NH₄CH₃CO₂) at pH 2, after the oxidation of organic matter with 8.8 M hydrogen peroxide (H₂O₂); and residual (F4), which represents the fraction associated with the crystalline structures of the minerals, found from the difference between the total concentration and the sum of the concentrations found in the exchangeable, reducible, and oxidizable fractions [5,36]. All PTE fractions were quantified via MP-AES (model 4210, Agilent, Santa Clara, CA, USA).

Based on the results obtained from the chemical fractionation, the risk assessment code (RAC), the individual contamination factor (ICF), the global contamination factor (GCF), and the mobility factor (MF) were calculated according to the methodology described by [5]. In the calculations, the bioavailable fraction was represented by the sum of the concentrations obtained in the F1 and F2 fractions, which represents an immediate risk to the biota.

The oral bioaccessible concentrations of the PTEs were determined according to the Simple Bioaccessibility Extraction Test (SBET) [5,36]. All analyses were performed in triplicate, with the addition of 100 mL of 0.4 M glycine (C₂H₅NO₂) solution at pH 1.5 to 1 g of soil or mining waste. This mixture was placed in a water bath (37 °C) with stirring (30 rpm) for one hour. The samples were subsequently centrifuged (400 rpm) and filtered. The concentrations were quantified via MP-AES (model 4210, Agilent, Santa Clara, CA, USA).

2.5. Risk Assessment

The potential ecological risk factor (RF) and the potential ecological risk index (RI) were used to understand the risks of PTEs to the environment. These indices were created to assess the impact of PTEs on the basis of their total concentrations [37]. However, in this study, the bioavailable fraction (F1 + F2) was used to improve the accuracy of the environmental risk assessment, as shown in Equations 4 and 5:

$$RF=T\times \left({\displaystyle \frac{{BAC}_{PTE}}{{BC}_{PTE}}}\right)$$

(4)

$$RI={RF}_1+{RF}_2+{RF}_3+\dots {+RF}_n$$

(5)

where T is the toxicity response factor (5 for Co, Cu, Ni, and Pb; 2 for Ba and Cr; and 1 for Mo and Zn) [6], BAC_PTE is the bioavailable fraction of PTE in the sample (mg kg⁻¹), BC_PTE is the total concentration of PTE in the reference area (mg kg⁻¹), and n is the number of elements under study. The results were interpreted according to [37]: RF ≤ 40 indicates low risk, 40 < RF ≤ 80 indicates moderate risk, 80 < RF ≤ 160 indicates considerable risk, 160 < RF ≤ 320 indicates high risk, and 320 < RF indicates very high risk; and RI ≤ 150 indicates low risk, 150 < RI ≤ 300 moderate risk, 300 < RI ≤ 600 considerable risk, and 600 < RI high risk.

Human health risks were estimated from the ingestion of PTEs, both for adults and children. These analyses are commonly performed using only total concentrations, which may cause an overestimation of the results. Thus, total and bioaccessible (oral) concentrations were used to obtain the risk indices (HI) and modified risk indices (HI_m), respectively [38], following Equations (6)–(9):

$$ADD=(C\times {\displaystyle \frac{IR\times EF\times ED}{BW\times AT}}\times CF)$$

(6)

$$Modified ADD=(OBC\times {\displaystyle \frac{IR\times EF\times ED}{BW\times AT}}\times CF)$$

(7)

$$HI={\displaystyle \frac{ADD}{Rfd}}$$

(8)

$$Modified HI={\displaystyle \frac{Modified ADD}{Rfd}}$$

(9)

where ADD is the average daily dose (mg kg⁻¹ day⁻¹); C is the total concentration (mg kg⁻¹); OBC is the oral bioaccessible concentration (mg kg⁻¹); IR is the intake rate, 100 mg day⁻¹ for adults and 200 mg day⁻¹ for children [39]; EF is the exposure frequency, 350 days year⁻¹ for adults [40] and children [41]; ED is the exposure duration, 24 years for adults and 6 years for children [42]; BW is the body weight, 70 kg for adults [43] and 15 kg for children [44]; AT is the average time, 8760 days for adults and 2190 days for children [45]; and CF is a conversion factor (10⁻⁶) [46]. The reference daily dose (Rfd) values were 3 × 10⁻³ mg kg⁻¹ day⁻¹ for Cr [39], 2 × 10⁻¹ mg kg⁻¹ day⁻¹ for Ba [46], 2 × 10⁻² mg kg⁻¹ day⁻¹ for Co [47], 4 × 10⁻² mg kg⁻¹ day⁻¹ for Cu [48], 5 × 10⁻³ mg kg⁻¹ day⁻¹ for Mo [47], 2 × 10⁻² mg kg⁻¹ day⁻¹ for Ni [49], 3.5 × 10⁻³ mg kg⁻¹ day⁻¹ for Pb [49], and 3 × 10⁻¹ mg kg⁻¹ day⁻¹ for Zn [50].

To obtain the carcinogenic risk indices of Cr, Ni, and Pb, the ADD and modified ADD were multiplied by the following slope factors: 0.5 kg day mg⁻¹ for Cr [49], 0.91 kg day mg⁻¹ for Ni [44], and 8.5 × 10⁻³ kg day mg⁻¹ for Pb [39].

3. Results and Discussion

3.1. Contamination Assessment

Igeo and PLI are among the most widely used indices for assessing contamination by PTEs. These indices allow the assessment of the impacts of PTEs based on a reference site, that is, an area without significant anthropogenic impact (NF in this study) that reflects local conditions prior to alteration by mining [12,51]. According to the Igeo values found in this study, in AO, contamination is absent for Ba, Cr, Co, Ni, Pb, and Zn; low for Mo; and moderate for Cu (Figure 2). In IT, contamination is absent for Ba, Cr, Co, Pb, and Zn; low for Cu; moderate for Ni; and strong to extreme for Mo. In AR, contamination is absent for Ba, Co, Pb, and Zn; moderate for Mo; strong for Cr and Ni; and strong to extreme for Cu [33]. The PLI values, which consider the joint effect of the PTEs, indicate that only AO has no contamination (<1) [34].

The Igeo values of Ba, Co, Pb, and Zn were low, suggesting that there is no significant enrichment of these PTEs in the mining wastes based on the reference area (Figure 2). On the other hand, it is possible to state that mining activities contribute to contamination by Cr, Cu, Ni, and Mo. The high concentrations of Cu, Cr, and Ni are directly related to the occurrence of mafic and ultramafic complexes resulting from supergene alterations in the CMP [1,52]. The mobilization of rocks in these complexes, followed by the deposition of wastes on the surface, contributes to the accumulation of PTEs in the soil. Molybdenum was the only PTE with heavy to extreme contamination in the IT, which can be explained by the occurrence of molybdenite in the source material found in the region [53].

According to Igeo, AR presents a greater number of PTEs at higher levels of contamination. Therefore, special attention should be given to this material, considering that it is deposited in piles without adequate management. The action of rainfall and wind may directly contribute to the dispersion of particles with the potential to cause damage to the environment and human health [5]. This should be monitored not only in terms of total concentrations but also considering the reactivity of PTEs through chemical fractionation studies, given that environmental changes can increase the content of PTEs in bioavailable fractions [54].

3.2. Chemical Fractionation

Sequential extraction revealed that all PTEs were predominant in the residual fraction, with the exceptions of Ba in AO, Cu in AR, and Zn in AR and IT (Figure 3). These results indicate that most PTEs have low reactivity in mining wastes and may be associated with low environmental risk due to the strong association of the elements with the crystalline structures of the minerals [55,56]. The predominance of Cr, Cu, Pb, and Zn in the residual fraction was also observed in Cu mine tailings in Tongling, China, suggesting greater stability for these elements in the environment [57].

Copper presented the highest bioavailable concentrations (F1 + F2) in mining waste disposal areas, with values of 612.8 mg kg⁻¹ in AO, 9201.6 mg kg⁻¹ in AR, and 121.3 mg kg⁻¹ in IT (Table S2). These results suggest a greater potential for Cu release in AO and AR than in IT. The higher total concentrations of this metal in AO and AR may directly contribute to higher bioavailable levels [58]. A similar trend was observed in soils naturally rich in PTEs in the Amazon, with a positive relationship between total and bioavailable Cu concentrations [59].

The bioavailable concentrations of Ba also deserve attention in AO and AR, with values of 58.5 and 20.5 mg kg⁻¹, respectively. These results can be explained by the higher biological activity associated with the higher OM content (Table S3). This may have contributed to the release of Ba from more stable forms to more mobile and bioavailable forms [5]. High bioavailable concentrations of Ba can alter physiological processes and the metabolic activity of plants, in addition to causing negative impacts on the soil microbiota [60,61]. Elevated levels of Ba in bioavailable forms were also detected in artisanal mining areas at the Serra Pelada mine in northern Brazil [5].

To understand the risks associated with PTEs in mining wastes, several indices (RAC, ICF, and MF) were calculated from the results obtained via sequential extraction. The RAC values indicated a low risk for all the elements, except for Cu in AO, which had a medium risk (10.5%), and Mo in IT, which had a high risk (34.6%) (

Table 1. Risk assessment code (RAC), individual contamination factor (ICF), mobility factor (MF), and global contamination factor (GCF) in copper mining waste disposal areas in the southeastern Brazilian Amazon.

Element	Index	Areas
		AO (n = 5)	AR (n = 5)	IT (n = 5)
Ba	RAC (%)	7.0	2.8	NC
	ICF	1.1	0.2	NC
	MF (%)	48.7	14.2	0.7
Co	RAC (%)	0.4	NC	NC
	ICF	0.1	NC	NC
	MF (%)	7.9	3.8	NC
Cr	RAC (%)	NC	NC	NC
	ICF	NC	NC	0.1
	MF (%)	NC	0.8	7.3
Cu	RAC (%)	10.5	9.0	4.9
	ICF	0.5	1.1	0.1
	MF (%)	29.0	48.3	8.2
Mo	RAC (%)	NC	NC	34.6
	ICF	NC	NC	0.5
	MF (%)	NC	NC	34.6
Ni	RAC (%)	NC	0.1	1.8
	ICF	NC	NC	0.1
	MF (%)	3.6	3.5	4.7
Pb	RAC (%)	NC	NC	NC
	ICF	0.1	NC	0.1
	MF (%)	7.1	NC	6.9
Zn	RAC (%)	4.1	9.2	6.0
	ICF	0.2	1.0	0.4
	MF (%)	16.4	25.8	11.1
All	GCF	1.9	2.3	1.2

NC—not calculated because the concentrations were below detection limits.

). The RAC considers the risk of the most bioavailable fraction (F1) to the environment [5]. Therefore, these results reinforce that most PTEs have a low potential to cause damage from the exchangeable fraction (F1).

In AO, the ICF indicated low contamination for Co, Cu, Pb, and Zn and moderate contamination for Ba. In AR, contamination was low for Ba, Cu, and Zn. In IT, contamination was low for Cr, Cu, Mo, Pb, and Zn. The GCF indicated low contamination in all areas studied. These results suggest that the combined bioavailability of PTEs presents low risk levels in areas altered by mining, especially in IT.

The MF values followed the order Ba > Cu > Zn > Co > Pb > Ni in AO, Cu > Zn > Ba > Co > Ni > Cr in AR, and Mo > Zn > Cu > Cr > Pb > Ni > Ba in IT. The highest MF values were found for Ba in AO and Cu in AR, corresponding to 48.7 and 48.3%, respectively. The other PTEs had relatively lower MF values, ranging from 0.7% (Ba in IT) to 34.6% (Mo in IT). The higher the MF, the greater the mobility of the PTE and its availability to biological systems [21,62]. Therefore, Ba and Cu should be monitored in artisanal mining areas, where wastes are randomly disposed of in the environment.

Considering the potential for the mobilization of PTEs by mining, attention should be given to these contaminants, especially Ba, Cu, and Zn in artisanal mining areas. Environmental agents (such as water and temperature) may promote the mobilization of PTEs to fractions of greater reactivity and mobility [63]. For example, high temperatures (common in the Amazon) can promote the release of PTEs from the fraction bound to organic matter [54,64]. Other environmental variations that can contribute to the release of bioavailable forms of PTEs include changes in redox potential, acidity, salinity, and biological activity [65].

3.3. Oral Bioaccessibility

The bioaccessible concentrations of the PTEs followed the order Cu > Ba > Zn > Cr in AO, Cu > Ba > Cr > Ni > Zn in AR, and Cu > Ba > Ni > Cr > Zn in IT (

Table 2. Oral bioaccessible concentrations of potentially toxic elements in copper mining waste deposition areas in the southeastern Brazilian Amazon.

Element (mg kg−1)	AO (n = 5)	AR (n = 5)	IT (n = 5)
Ba	114 ± 0.8	31.5 ± 1.8	15.1 ± 0.4
Co	<DL	<DL	<DL
Cr	3.9 ± 0.3	27.9 ± 5.9	4.5 ± 0.3
Cu	860 ± 15.9	15,000 ± 1957.6	230 ± 9.8
Mo	<DL	<DL	<DL
Ni	<DL	14.1 ± 0.8	7.2 ± 1.0
Pb	<DL	<DL	<DL
Zn	12.5 ± 2.2	3.4 ± 0.8	3.6 ± 0.1

DL—detection limit. The values are presented as the means ± standard deviations.

). The highest bioaccessible concentrations were found for Cu, corresponding to approximately 860, 15,000, and 230 mg kg⁻¹ in AO, AR, and IT, respectively. These results indicate that artisanal mining generates wastes with a greater potential for Cu absorption in the human body when compared to industrial mining [66,67]. Despite being an essential element, high concentrations of Cu in the human body can cause several health problems, such as gastrointestinal disorders, central nervous system problems, and liver and kidney damage [68].

Barium had high bioaccessible concentrations in AO (Table S2). This result deserves attention, considering that Ba is not essential for the human body and can cause several negative effects on the organism, including high blood pressure, gastrointestinal dysfunctions, cardiac arrhythmias, and respiratory failure [69]. In the human body, the bioaccessibility of this element tends to be greater in the acidic environment of the stomach, which favors the release of Ba ions [70]. The oral bioaccessible concentrations of Ba in this study are lower than those reported in a barite mine in Nigeria, which ranged from 4.2 to 33.2 mg kg⁻¹ [71].

3.4. Environmental Risk Assessment

The RF values indicated that most PTEs presented a low risk from individual exposure, except for Cu in AR, which was associated with considerable risk (

Table 3. Ecological risk factors and indices of potentially toxic elements in copper mining waste deposition areas in the southeastern Brazilian Amazon.

Element	AO (n = 5)		AR (n = 5)		IT (n = 5)
	RF	Degree	RF	Degree	RF	Degree
Ba	0.8 ± 0.0	Low	0.3 ± 0.0	Low	NC	Low
Co	0.2 ± 0.0	Low	0.3 ± 0.3	Low	NC	Low
Cr	NC	Low	0.2 ± 0.0	Low	0.1 ± 0.0	Low
Cu	7.0 ± 0.1	Low	104.9 ± 0.9	Considerable	1.4 ± 0.1	Low
Mo	NC	Low	NC	Low	9.5 ± 1.8	Low
Ni	0.2 ± 0.0	Low	2.7 ± 0.0	Low	1.2 ± 0.0	Low
Pb	0.3 ± 0.0	Low	NC	Low	0.3 ± 0.0	Low
Zn	0.1 ± 0.0	Low	NC	Low	0.1 ± 0.0	Low
RI	8.6	Low	108.4	Low	11.1	Low

The values are presented as the means ± standard deviations. NC—not calculated because the concentrations were below the detection limits. RF—risk factor. RI—risk index.

). The IR values, in turn, indicated a low risk for all materials. These results suggest that PTEs present a low current risk to the environment from simultaneous exposure, due to their low bioavailable concentrations. However, the total concentrations deserve attention because of the potential for release into the environment [72].

Considering the bioavailability of Cu, it is important to implement environmental remediation measures to control the impacts of these elements on the ecosystem. Among these measures, phytoremediation may be interesting because it mitigates the contamination and dispersion of PTEs in the ecosystem [73,74], with results that can be enhanced by the association with microorganisms [75,76]. The application of biochar can also contribute to mitigating the impacts caused by PTEs [77,78], especially in the Amazon, which has several production chains that generate materials with potential for this purpose.

3.5. Human Health Risk Assessment

The estimation of risks to human health is particularly important in the areas studied, given the direct contact of the population with artisanal mining wastes, as well as the occupation of workers in industrial mining areas. The threshold values for non-carcinogenic and carcinogenic risk indices correspond to 1 [79] and 10⁻⁴ [80], respectively. In this study, potential non-carcinogenic health risks for children were evidenced by the total Cr and Cu concentrations in AR (

Table 4. Human health risk indices by total concentrations of potentially toxic elements in copper mining waste deposition areas in the southeastern Brazilian Amazon.

Element	Value	Adults			Children
		AO (n = 5)	AR (n = 5)	IT (n = 5)	AO (n = 5)	AR (n = 5)	IT (n = 5)
Non-carcinogenic risks
Cr	Mean	5.9 × 10−3	6.4 × 10−1	2.1 × 10−2	5.5 × 10−2	6.0	2.0 × 10−1
	Standard deviation	5.6 × 10−4	3.0 × 10−2	1.5 × 10−3	5.2 × 10−3	2.8 × 10−1	1.4 × 10−2
Ba	Mean	8.2 × 10−4	9.9 × 10−4	3.3 × 10−4	7.7 × 10−3	9.3 × 10−3	3.0 × 10−3
	Standard deviation	8.4 × 10−5	5.9 × 10−5	3.0 × 10−5	7.8 × 10−4	5.5 × 10−4	2.8 × 10−4
Co	Mean	1.7 × 10−3	4.3 × 10−3	1.8 × 10−3	1.6 × 10−2	4.0 × 10−2	1.7 × 10−2
	Standard deviation	1.4 × 10−4	8.7 × 10−5	4.9 × 10−4	1.3 × 10−3	8.1 × 10−4	4.6 × 10−3
Cu	Mean	7.2 × 10−2	6.5 × 10−1	5.1 × 10−2	6.8 × 10−1	6.1	4.8 × 10−1
	Standard deviation	2.4 × 10−3	1.0 × 10−1	4.3 × 10−2	2.3 × 10−2	9.7 × 10−1	4.0 × 10−1
Mo	Mean	2.2 × 10−4	7.9 × 10−4	3.9 × 10−3	2.0 × 10−3	7.3 × 10−3	3.7 × 10−2
	Standard deviation	2.4 × 10−5	6.4 × 10−6	7.3 × 10−4	2.2 × 10−4	6.0 × 10−5	6.8 × 10−3
Ni	Mean	2.0 × 10−3	3.4 × 10−2	1.1 × 10−2	1.9 × 10−2	3.1 × 10−1	1.0 × 10−1
	Standard deviation	7.6 × 10−5	1.2 × 10−3	4.0 × 10−3	7.1 × 10−4	1.1 × 10−2	3.7 × 10−2
Pb	Mean	6.6 × 10−3	1.7 × 10−3	5.6 × 10−3	6.1 × 10−2	1.6 × 10−2	5.2 × 10−2
	Standard deviation	6.7 × 10−4	4.3 × 10−5	2.3 × 10−3	6.3 × 10−3	4.0 × 10−4	2.2 × 10−2
Zn	Mean	1.1 × 10−4	2.5 × 10−5	7.4 × 10−5	1.1 × 10−3	2.3 × 10−4	6.9 × 10−4
	Standard deviation	1.1 × 10−5	5.1 × 10−6	5.9 × 10−6	1.0 × 10−4	4.8 × 10−5	5.5 × 10−5
Carcinogenic risks
Cr	Mean	8.9 × 10−6	9.6 × 10−4	3.2 × 10−5	8.3 × 10−5	9.0 × 10−3	3.0 × 10−4
	Standard deviation	8.4 × 10−7	4.5 × 10−5	2.2 × 10−6	7.8 × 10−6	4.2 × 10−4	2.1 × 10−5
Ni	Mean	3.6 × 10−5	6.1 × 10−4	2.0 × 10−4	3.4 × 10−4	5.7 × 10−3	1.8 × 10−3
	Standard deviation	1.4 × 10−6	2.2 × 10−5	7.3 × 10−5	1.3 × 10−5	2.1 × 10−4	6.8 × 10−4
Pb	Mean	2.0 × 10−7	5.0 × 10−8	1.7 × 10−7	1.8 × 10−6	4.6 × 10−7	1.5 × 10−6
	Standard deviation	2.0 × 10−8	1.3 × 10−9	7.0 × 10−8	1.9 × 10−7	1.2 × 10−8	6.5 × 10−7

). In addition, potential carcinogenic risks were found for adults by Cr and Ni in AR and only Ni in IT, as well as for children by Ni in AO and Cr and Ni in AR and IT.

To avoid overestimating the risks to human health, the indices were also calculated from the oral bioaccessible concentrations (

Table 5. Human health risk indices due to oral bioaccessible concentrations of potentially toxic elements in copper mining waste deposition areas in southeastern Brazilian Amazon.

Element	Value	Adults			Children
		AO (n = 5)	AR (n = 5)	IT (n = 5)	AO (n = 5)	AR (n = 5)	IT (n = 5)
Non-carcinogenic risks
Cr	Mean	1.8 × 10−3	1.3 × 10−2	2.1 × 10−3	1.6 × 10−2	1.2 × 10−1	1.9 × 10−2
	Standard deviation	1.3 × 10−4	2.7 × 10−3	1.4 × 10−4	1.2 × 10−3	2.5 × 10−2	1.3 × 10−3
Ba	Mean	7.8 × 10−4	2.2 × 10−4	1.0 × 10−4	7.3 × 10−3	2.0 × 10−3	9.6 × 10−4
	Standard deviation	7.7 × 10−6	1.2 × 10−5	2.8 × 10−6	7.1 × 10−5	1.1 × 10−4	2.6 × 10−5
Co	Mean	NC	NC	NC	NC	NC	NC
	Standard deviation	NC	NC	NC	NC	NC	NC
Cu	Mean	2.9 × 10−2	5.1 × 10−1	7.9 × 10−3	2.7 × 10−1	4.8	7.3 × 10−2
	Standard deviation	7.7 × 10−4	6.7 × 10−2	3.4 × 10−4	7.2 × 10−3	6.3 × 10−1	3.1 × 10−3
Mo	Mean	NC	NC	NC	NC	NC	NC
	Standard deviation	NC	NC	NC	NC	NC	NC
Ni	Mean	NC	9.7 × 10−4	4.9 × 10−4	NC	9.0 × 10−3	4.6 × 10−3
	Standard deviation	NC	5.3 × 10−5	7.2 × 10−5	NC	5.0 × 10−4	6.7 × 10−4
Pb	Mean	NC	NC	NC	NC	NC	NC
	Standard deviation	NC	NC	NC	NC	NC	NC
Zn	Mean	5.7 × 10−5	1.5 × 10−5	1.7 × 10−5	5.3 × 10−4	1.4 × 10−4	1.5 × 10−4
	Standard deviation	1.4 × 10−5	3.7 × 10−6	3.2 × 10−7	1.3 × 10−4	3.5 × 10−5	3.0 × 10−6
Carcinogenic risks
Cr	Mean	2.6 × 10−6	1.9 × 10−5	3.1 × 10−6	2.5 × 10−5	1.8 × 10−4	2.9 × 10−5
	Standard deviation	1.9 × 10−7	4.0 × 10−6	2.0 × 10−7	1.8 × 10−6	3.8 × 10−5	1.9 × 10−6
Ni	Mean	NC	1.8 × 10−5	9.0 × 10−6	NC	1.6 × 10−4	8.4 × 10−5
	Standard deviation	NC	9.7 × 10−7	1.3 × 10−6	NC	9.0 × 10−6	1.2 × 10−5
Pb	Mean	NC	NC	NC	NC	NC	NC
	Standard deviation	NC	NC	NC	NC	NC	NC

NC—not calculated because the concentrations were below the detection limits.

). The results indicated a low non-carcinogenic risk (HI < 1) for all the PTEs, with the exception of Cu for children in AR, which had an index of 4.8, well above the value indicated as safe by the [81]. The assessment also revealed that there are carcinogenic risks associated with Cr and Ni for children in AR. No PTE was associated with risks in IT.

The incorporation of oral bioaccessibility allowed us to identify the actual risks related to PTEs in the studied areas. Owing to its high bioaccessible concentrations, Cu poses a risk to the health of children in AR, indicating the need for special monitoring. Among the carcinogenic elements, Cr and Ni present current risks for children in AR, which reinforces the potential for damage associated with this material. Based on the results, the IT area is not associated with health risks related to bioaccessible concentrations.

In this study, only ingestion was considered in the risk assessment, considering that this is the main route of entry of PTEs into the human body [82]. New studies should be carried out to assess the risks associated with inhalation and dermal contact, aiming to understand the impacts of PTEs through these two alternative routes. Given the expansion of artisanal mining in the Amazon [83], it will also be important to monitor other areas and types of waste to protect the biota and the health of the population from the negative impacts of PTEs.

4. Conclusions

Environmental indices suggest that AR is the material with the highest contamination level, especially for Mo, Cr, Ni, and Cu. Chemical fractionation revealed that most PTEs are predominant in the residual fraction, which indicates that the elements are strongly associated with the crystalline structures of the minerals, except for Ba in AO, Cu in AR, and Zn in AR and IT. Barium and Cu presented relatively high oral bioaccessible concentrations in all the materials studied. The environmental risk assessment indicates that AR is the most hazardous material among the wastes studied. Health risks to adults and children have been evidenced in AR, including cancer risks from oral bioaccessible concentrations of Cr and Ni. The results of this study can support environmental monitoring and the development of remediation techniques in the studied areas, especially in artisanal mining sites.