Comparative Analysis of Potentially Toxic Elements (PTEs) in Waste Rock and Tailings: A Case Study from the Recsk Mining Area, Hungary

Abstract

Metal-mining activities inevitably generate contaminants in high quantities, which can pose a risk to soil, water, biota, and humans. This study compares the geochemical properties of waste materials of tailings and waste rock heaps originating from the same high-sulfidation-type epithermal mineralization. Field sampling was conducted in the Recsk Copper Mining Area on the H2 tailings and H7 waste heap, where a total of 48 samples were collected at various depths. The results showed that PTEs were present in varying concentrations and behaved differently in the two waste materials. Copper concentrations were approximately five times higher in H2 tailings (median 1660 mg/kg) than in H7 waste rock (median 347 mg/kg), whereas arsenic was 2.8 times more concentrated in H2 tailings (674 mg/kg vs. 238 mg/kg). Conversely, zinc (114 mg/kg vs. 24 mg/kg), lead (172 mg/kg vs. 42.8 mg/kg), and cadmium (0.83 mg/kg vs. 0.097 mg/kg) show significantly higher concentrations in H7 waste rock. Element mobility analyses revealed that calcium mobility in H7 waste rock (65%) was twice that observed in H2 tailings (32%), with copper showing a threefold higher mobility in H7 despite lower total concentrations. NAG pH values (2.06–3.23) confirmed significant acid-generating potential in both waste types, with the H7 waste rock posing greater immediate environmental risk due to higher element mobility and more advanced weathering indicated by elevated jarosite (4.05%–8.01%) and secondary mineral contents. These findings demonstrate that, despite originating from the same mineralization, the distinct processing histories and physical properties of these materials necessitate unique approaches for successful remediation or secondary raw material extraction.

1. Introduction

Mining activities are among the world’s primary sources of waste generation, producing substantial quantities of tailings and waste rock that pose significant environmental risks. One of the most pressing environmental issues associated with mining waste is the formation of acid mine drainage (AMD), which occurs when sulfide-bearing minerals are exposed to oxidation and water. This process leads to the release of acidic waters containing dissolved metals and metalloids, adversely affecting soil, water resources, biota, and human health [1,2,3].

This environmental challenge has become increasingly important as the global demand for raw materials, including Critical Raw Materials (CRMs) essential for modern technologies, continues to intensify [4,5,6,7].

Mine waste, particularly tailings, represents an environmental challenge and potential resource. These materials often contain residual valuable elements that could serve as secondary sources for metal recovery, and have applications in construction, ceramics production, and soil improvement [8,9,10,11]. The environmental impact and resource recovery potential depend significantly on physical and chemical properties of the waste, which are influenced by factors such as grain size, geological setting, mineralogical composition, and processing methods [12,13].

Management of mining waste requires an understanding of the geochemical processes that drive AMD generation and element mobilization. Significant research has focused on characterizing mine waste [14,15], assessing contaminant release [16,17], and developing remediation approaches [18,19,20].

AMD generation from sulfide-rich mine waste remains a significant environmental concern due to its potential to mobilize potentially toxic elements (PTEs) into the environment [17,21,22]. The management and remediation of AMD are complex, requiring site-specific approaches tailored to the geological, mineralogical, and hydrological conditions of each site [23,24]. The oxidation of sulfide minerals such as pyrite (FeS₂) generates sulfuric acid and releases metals and metalloids, thereby increasing environmental contamination.

However, despite extensive investigation, comparative studies that systematically analyze waste rock and tailings from the same mineralization remain notably scarce. Such comparisons are crucial because these materials can exhibit significantly different acid generation potentials and environmental risks, necessitating distinct remediation strategies, even within the same mining site.

Previous studies have typically focused on either tailings alone [25,26] or waste rock in isolation [27,28], with limited direct comparisons between these distinct waste materials from the same ore deposit. Although some researchers have examined multiple waste types at mining sites [14,29], systematic comparative analyses of the mobility, speciation, and acid generation potential of PTEs in different waste materials derived from identical mineralization remain scarce. Waste rock and tailings can exhibit significantly different AMD generation potentials and pose varying environmental risks, requiring distinct remediation and treatment strategies, even within the same mining site [30,31,32,33]. Understanding these differences is essential for developing effective remediation plans and assessing their potential for secondary resource recovery.

The historic Hungarian Recsk Mining Area provides an ideal case study for this investigation, with both waste rock and tailings heaps originating from the same mineralization [33,34,35,36]. We hypothesize that the observed differences in geochemical behavior result primarily from the waste type (waste rock versus tailings) and their associated physical and chemical properties. This research contributes to the development of site-specific remediation technologies and offers insights into potential secondary raw material extraction, supporting sustainable resource management and environmental protection.

The objective of this study is to address this gap by conducting a comparative investigation of the geochemical properties of waste rock and tailings heaps derived from the same high-sulfidation epithermal mineralization. Specifically, this study aims to (i) identify the differences in chemical concentrations, mineralogical compositions, and the behavior and mobility of PTEs between the two types of waste materials and (ii) assess and verify the acid generation potential of the waste heaps.

2. Study Site

The Recsk Mining Area is located in the Mátra Mountains, Hungary, within the Carpatho–Balkan Metallogenic zone of the Paleogene Inner-Carpathian Volcanic Belt [35,36,37,38]. The site features a high-sulfidation epithermal mineralization that developed in andesitic host rocks as part of a larger magmatic–hydrothermal system [39].

The Lahóca-hill mineralization was mined for copper ore from 1852 until 1980, producing approximately 3.1 million tons of ore with average concentrations of 0.61% Cu and 2.5 mg/kg Au [36,38]. Mining activities generated significant waste material that accumulated around the Baláta and Bikk Creeks, resulting in acid mine drainage (AMD) that has affected downstream waters for at least 15 km [36,40]. Under prevailing continental climate conditions (700 mm annual precipitation; 5–10 °C average temperature), intensive weathering has mobilized toxic elements, including Cu, Zn, Pb, and As, into nearby soils and streams [41]. The H7 waste rock heap and H2 flotation tailings heap were primarily developed between 1978 and 1990 [42] and are located just a few kilometers from each other (Figure 1), providing an ideal opportunity to compare different waste materials originating from the same mineralization.

3. Materials and Methods

3.1. Sampling

Field sampling was conducted at waste heaps H2 and H7, managed by Nitrokémia Zrt.; in collaboration with Mecsekérc Zrt., following their accredited sampling procedure [43] corresponding to the original plan, we collected 10 samples of 4 kg each from the upper and oxidized zones and 10 samples from the lower and reduced zones of each heap, along with two duplicate samples and two blank samples, resulting in a total of 48 samples from the two locations.

The first sampling site was the H2 tailings heap, located on the left bank of Bikk Creek (Figure 2). The material found in the H2 tailings dump varied between fine-grained gray clay, siltstone, and sand. In some places, it exhibits yellow hues due to the high sulfur content and blue or green spots because of secondary copper minerals. Moreover, small amounts of coarser material were observed, displaying strongly oxidized and weathered characteristics.

A total of 20 points on the H2 tailings heap were sampled from both the upper oxidized (yellow–brown layer) and lower reduced (gray) parts. Ten percent of duplicate samples were collected, and a blank sample was prepared using pure quartz sand in the field.

The H7 waste rock heap, situated in the Baláta Creek Valley (Figure 3), contains coarse waste material. A total of 10 sampling points were selected, from which 4 kg of material was collected from both the upper oxidized zone (0–50 cm) and the lower reduced zone (50–100 cm) of the excavated sampling pits. Sampling could not be initiated with manual sampling tools because the waste material was too heavily cemented. As a result, an excavator was employed to dig pits approximately 1 m deep at the designated sampling sites, thereby facilitating subsequent sampling (Figure 4B).

Particular attention was given to avoiding cross-contamination that could affect elemental concentration. Potential contaminants, such as gullies, plants, roots, other rock pieces, and anthropogenic waste, were carefully excluded. The sampling equipment was cleaned with deionized water before each sampling. Disposable rubber gloves were used for the collection of each sample. Samples were then homogenized by hand on-site, sealed in oxygen-impermeable containers (in addition to plastic sample bags), and stored at 4 °C to minimize oxidation. Furthermore, the time between sampling and analysis was very short. Sampling sites were recorded using GPS, and detailed photographic documentation was performed. The samples were kept cool at 4 °C during transport from the field and stored in the laboratory until analysis within one week.

3.2. Mineralogical Analysis of Mine Waste Samples

X-ray diffraction (XRD) analysis was employed to characterize the mineralogical composition of the mine waste samples. XRD measurements were conducted using a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) equipped with a Cu-Kα radiation source, operating at 40 kV and 40 mA. The instrument utilized a parallel beam geometry with a Göbel mirror and 2.5° axial Soller slits. Data were collected over an angular range of 2–70° 2θ, with a step size of 0.007° and a counting time of 24 s per step.

Powder samples were prepared by loading them into low-background sample holders made of recessed single-crystal silicon. Phase identification was performed using search/match analysis with the ICDD PDF2 (2005) database, implemented in the Bruker Diffrac Plus EVA software (version 4.0). Quantitative analysis employed Rietveld using Bruker TOPAS 4 software with the instrument profile calibrated against the SRM640a silicon standard. The amount of amorphous material was estimated by the “amorphous hump” method.

It is important to note that these XRD detection limits (1 wt% = 10,000 mg/kg for crystalline phases and 5 wt% = 50,000 mg/kg for amorphous phases) [33,44] are considerably higher than the regulatory thresholds established by the Hungarian 6/2009 decree for potentially toxic elements in geological media (75 mg/kg for Cu, 200 mg/kg for Zn, 100 mg/kg for Pb, and 15 mg/kg for As). This significant difference indicates that minerals containing regulated elements could be present below the XRD detection limits, while still substantially exceeding the regulatory thresholds. For this reason, we complemented our mineralogical analysis with chemical extraction methods that provide the necessary sensitivity (detection limits typically < 0.1 mg/kg) to accurately assess compliance with environmental regulations and properly characterize potential contamination risks.

3.3. Chemical Compounds and Extraction Methods

Aqua regia extraction (AqR) was performed to determine the total extractable element content. Aqua regia, a mixture of 3 mL concentrated hydrochloric acid (HCl) and 1 mL concentrated nitric acid (HNO₃), is effective in dissolving a wide range of metal compounds [45], making it suitable for assessing the maximum potential mobility of the elements in the samples. The extraction followed the accredited MSZ EN 13657:2003 standard [46] and was performed at the laboratory of Mecsekérc Zrt. in Kővágószőlős. During sample preparation, 1 g of the sample was dried at 40 °C to a constant weight. The dried material was subsequently sieved to obtain the <0.063 mm fraction and ground to a particle size of less than 0.1 mm to ensure homogeneity and maximize extraction efficiency. The samples were placed in 75 mL MARS Xpress vessels, reacted for 2 h with the aqua regia mixture, and then processed in a 40-position CEM MARS microwave extraction system. After cooling, the samples were filtered and transferred into volumetric flasks using 0.5 mol nitric acid and then diluted to 50 mL with deionized water.

Deionized-water (DW) extraction was conducted using a coarser size fraction (<2 mm), without additional drying beyond the initial field preparation, to better simulate natural leaching conditions and prevent the artificial redistribution of water-soluble components.

The leachable extraction, pH, electrical conductivity (EC), and total dissolved solids (TDSs) followed the accredited MSZ EN 12457-2:2003 standard [47] and were also conducted at the laboratory of Mecsekérc Zrt. in Kővágószőlős. Samples (<2 mm) were diluted in a 10:1 ratio with deionized water (1 g sample to 10 mL water), shaken for 24 h using a Stuart orbital shaker, and then left to settle for an additional 24 h. The supernatant was filtered through a Sartorius membrane filter with a pore size of 0.45 μm and divided for further analysis.

Deionized-water extraction was selected because it provides a realistic simulation of natural rainfall interactions with waste materials under ambient environmental conditions, aligning with standardized procedures (EN 12457-2:2002) [47] for waste characterization. Studies have shown a good correlation between deionized-water extractions and actual field leachate compositions in similar mine-waste materials [48,49].

The concentrations of elements in the aqua regia and deionized-water extracts were determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). These analytical techniques offer high sensitivity and precision for detecting a wide range of elements, including trace metals, which may pose environmental concerns.

The analyses were conducted at the Mecsekérc Ltd. laboratory in Kővágószőlős according to their accredited measurement standards [50]. ICP-OES was performed using a HORIBA Jobin Yvon Activa-M device, suitable for detecting higher concentrations of elements. ICP-MS was conducted using a Perkin Elmer ELAN DRC-e mass spectrometer, which provides high sensitivity and low detection limits for trace elements.

3.4. BCR Three-Step Sequential Extraction

The modified Community Bureau of Reference (BCR) three-step sequential extraction procedure was applied to evaluate element mobility in mine waste samples. For Step 1 (exchangeable/acid-soluble fraction), 1 g of air-dried, homogenized, and sieved (<2 mm) sample was extracted with 40 mL of 0.11 mol L⁻¹ acetic acid (pH 2.85) for 16 h at 22 ± 5 °C using an end-over-end shaker (30 ± 10 rpm). After centrifugation (3000× g, 20 min), the supernatant was filtered through (0.45 μm), and the residue was washed with deionized water. Step 2 (reducible fraction) involved extracting the residue with 40 mL of 0.5 mol L⁻¹ hydroxylammonium chloride (pH 1.5) for 16 h. Step 3 (oxidizable fraction) treated the residue with 8.8 mol L⁻¹ H₂O₂ (two 10 mL aliquots) at 22 ± 5 °C and 85 ± 2 °C for 1 h each, followed by extraction with 50 mL of 1 mol L⁻¹ ammonium acetate (pH 2.0). The residual fraction underwent aqua regia digestion (ISO 11466). Elemental analysis was performed using ICP-AES/MS, with quality control including triplicates, blanks, and certified reference materials [51].

3.5. Assessment of Acid-Generation Potential

The carbon and sulfur contents of ten rock samples were determined using the DUNAFERR LABOR Nonprofit Kft. (Dunaújváros, Hungary), an accredited laboratory (NAH-1-1798/2016) [52].

Organic and inorganic carbon and sulfur contents were determined using standard laboratory procedures for soil, rock, limestone, and dolomite samples. Total carbon (TC), total organic carbon (TOC), and total inorganic carbon (TIC) were analyzed using a LECO RC-612 carbon and moisture analyzer. The total sulfur content, non-oxygen-bonded sulfur, and oxygen-bonded sulfur were measured with a Horiba EMIA-320V C-S elemental analyzer [53].

The acid-generation potential of waste materials was assessed using Acid-Base Accounting (ABA) and Net Acid-Producing Potential (NAPP) tests, following a modified Sobek method [54]. A representative 2 g of the sample was treated with standardized hydrochloric acid (HCl), heated to 80–90 °C for 1–2 h until reaction completion, and then back-titrated with standardized sodium hydroxide (NaOH) to determine unreacted HCl. The acid-neutralizing capacity (ANC) was calculated as acid consumed and expressed in kg H₂SO₄/t.

The Net Acid-Producing Potential (NAPP) was calculated as the difference between Maximum Potential Acidity (MPA) and ANC: NAPP = MPA – ANC, where MPA is calculated as 6 MPA kg H₂SO₄/t = sulfate sulfur × 30.6.

Positive NAPP values indicate that the acid-generating potential of the sample outweighs its ability to neutralize acids. Conversely, negative NAPP values suggest that the sample is unlikely to cause acid drainage [55].

These tests were performed on the same ten selected samples used for the XRD studies, representing both oxidized and reduced zones within the fine H2 tailings and coarse H7 waste rock heaps.

Quality control included running duplicate blanks for each acid concentration and maintaining a pH between 0.8 and 1.5 during testing. These tests were performed on ten selected samples representing both oxidized and reduced zones within the fine H2 tailings and coarse H7 waste rock heaps.

For the Net Acid Generation (NAG) Test, we conducted the single-addition NAG test to directly measure net acid generation during sample oxidation. The pulverized sample (2.5 g) was mixed with 250 mL of 15% hydrogen peroxide in a beaker and left to react overnight. After gently heating the mixture to break down residual peroxide, we measured the final pH (NAG pH). Samples with a NAG pH below 4.5 were classified as potentially acid-forming. Additionally, titrations were performed to quantify the acid produced, expressed as kg H₂SO₄/t.

We classify samples based on acid-producing potential (MPA) and acid-neutralizing capacity (ANC). Categories include Potentially Acid-Forming, Non-Acid-Forming, and Uncertain [55].

3.6. Data Analysis

The total elemental concentrations were compared to Hungarian governmental limit values for the protection of geological media and groundwater against pollution and measures of contamination [56]. The elemental concentrations of the deionized-water leaching were compared to the legislative threshold values of waste tipping and landfills [57]. Then, the mobility of each element was estimated using a simple formula: Mobility = ($\mathrm{M}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{n}\left({\mathrm{C}\mathrm{h}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l} \mathrm{E}\mathrm{l}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}_{\mathrm{D}\mathrm{W}}\right)$/$\mathrm{M}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{n}\left({\mathrm{C}\mathrm{h}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l} \mathrm{E}\mathrm{l}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}_{\mathrm{A}\mathrm{q}\mathrm{R}}\right))\times 100.$ The obtained results were subjected to statistical analyses. Data analysis employed univariate and bivariate methods to describe the geochemical properties and behavior of copper, zinc, lead, cadmium, and arsenic in the studied mine waste heaps.

To examine the statistical differences between the H2 and H7 waste heaps, non-parametric Mann–Whitney U tests were applied. This test was selected because geochemical data often violate normality assumptions, particularly in heterogeneous mine waste materials. Statistical significance was set at p < 0.05. For multi-group comparisons across both heaps and redox zones, a Kruskal–Wallis test was used, followed by a Dunn’s post hoc test with Bonferroni correction when significant differences were detected.

The central tendency and variability measures of the chemical element concentrations used in this study were minimum, average (arithmetic mean), median, maximum, standard deviation, median absolute deviation (MAD), and range [58]. For outlier identification [59], the inner fence criteria was used. Accordingly, outliers and extreme values were defined as data points located at a distance of 1.5 times and 3 times the interquartile range from the lower or upper quartile, respectively [60,61]. Sub-population identification followed the “natural break” histogram-slicing method [62].

A natural break was visually defined at an inflection point on the cumulative distribution function curve. This point corresponds to a local minimum in the frequency histogram [63,64].

In this study, the H2 flotation heap was represented by a four-pointed star, and the H7 waste rock heap by a triangle. Unfilled symbols represented samples from the oxidized layer, while filled symbols denoted samples from the reduced layer. Vertical dashed lines marked concentration levels exceeding the threshold for “total leachable” element content, as defined by the 6/2009. (IV. 14.) KvVM-EüM-FVM decree [56]. Similarly, vertical dotted lines indicated concentrations surpassing the inert waste limit according to the 20/2006. (IV. 5.) KvVM decree [57] for deionized water leachate. Continuous vertical lines delineated the boundaries between distinct sample groups.

Bivariate data analysis included calculating Pearson’s linear correlation coefficient and fitting the least-squares regression line to the data points. All statistics discussed in the present paper are significant at the 0.05 significance level. Statistical data analysis was carried out using the STATGRAPHICS Centurion 18 software [65].

The laboratory (Mecsekérc Zrt., Kővágószőlős, Hungary) operates under an accredited quality management system (ISO 9001:2015 and ISO 14001:2015) [66] and follows standardized procedures for sampling, radiometry, chemistry, geoecology, and soil mechanics investigations. Quality control measures included the use of standard reference materials (SRMs), duplicate samples, and blank samples to ensure data reliability. SRM 1831 (NIST) was used for Cu and As, achieving recovery rates of 98% ± 2% (Cu) and 102% ± 3% (As). Duplicate samples (n = 10) showed that relative standard deviation (RSD) values were the following: Cu (5%), Zn (7%), Pb (6%), As (8%), Cd (9%), Fe (4%), Mn (5%), Al (6%), Ca (5%), and Mg (4%). For deionized water (DW) extractions, the RSDs were slightly higher due to the lower concentrations: Cu (8%), Zn (9%), Pb (12%), As (11%), Cd (12%), Fe (7%), Mn (8%), Al (9%), Ca (6%), and Mg (7%), indicating good precision. Blank samples confirmed no significant contamination. Note: Quality control measures were consistently applied to all elements analyzed in this study. Detailed results for Cu and Zn are presented due to their status as primary contaminants.

4. Results

Considering the mineralization and toxicity, we selected the following ten elements to represent the geochemical character of the waste heap: As, Cd, Cu, Pb, Zn, Al, Ca, Fe, Mg, and Mn.

4.1. Mineralogy and Chemical Composition

Six samples from the H2 tailings and four from the H7 waste heap were analyzed using X-ray diffraction method (XRD) [67]. The top (oxidized) and bottom (reduced) waste heap zones were represented in case of each waste dump. According to

Table 1. XRD measurement results (weight percent, wt.%). Values below detection limit (~1 wt.%) marked as “<1”. “O”: oxidized; “R”: reduced, * N/D = Not detected by XRD.

Minerals	H2-03/R	H2-13/R	H2-14/R	H2-17/R	H2-14/O	H2-15/O	H7-10/R	H7-15/R	H7-10/O	H7-15/O
Barite BaSO4							7.49	<1	3.5	<1
Gypsum CaSO4·2H2O		1.1	1.4	2	25	1.4	<1	1.2	1.3	9.2
Kaolinite Al2(Si2O5)(OH)4	6.6	11.4	13.1	23.5	19	20.7	<1	2.3	1.1	3.9
lllite/Smectite 11A		3.5		9.8			3.34	6.1	6.6	10.2
Illite 1M (K,H3O)Al2Si3AlO10(OH)2		1.4		2			5.64	15.9	9.1	11.2
Smectite 12A			2.5		3.1	4.7
Andesine (Ca,Na)(AlSi3O8)						13
Microcline K(AlSi3O8)								12.5
Pyrite FeS2	4.8	1.3	1.2	3	<1	<1	1.5	3.5	<1
Chalcopyrite CuFeS2	<1
Tennantite (Cu,Fe)As4S13	<1	<1	<1	<1	<1
Jarosite (K,H3O)Fe3(SO4)2(OH)6			0.6		6	0.80	1.7	4.8	4	8
Anatase TiO2	<1	1.3	<1	2.1	<1		<1	<1	<1	<1
Plumbogummite PbAl3(PO4)(PO3OH)(OH)6			<1			<1
Woodhouseite CaAl3(PO4)(SO4)(OH)6	<1	<1		<1
Quartz SiO4	82.8	75.9	79.6	51.6	49.48	40.6	75.7	46.5	66.1	40.7
Amorphous	4	3	N/D *	5	18	18	3	6	7	16

, the main phases of the samples are quartz (40.63%–82.76%), various clay minerals (kaolinite, illite, smectite) (0.66%–23.48%) and the amorphous phase (0%–18%). Except for one, all samples contained pyrite, sometimes in quite large proportions (up to 4.9%). Jarosite, a secondary mineral formed during the weathering of pyrite under acidic conditions (pH < 2–4) [68], is also present in significant quantities, typically in all the H7 waste rock heap samples (4.05%–8.01%) where the material is much more weathered. Barite can also be observed in the H7 waste rock heap, where the sulfide–mineral oxidation and the formation of secondary minerals are well advanced. Gypsum is also present in all samples except for one sample (H2-03/R) from the H2 flotation heap. This secondary mineral can also be found as crystals several centimeters in size on the H7 waste rock heap. The sulfosalt tennantite was mined as a main ore mineral for copper, and it seems to be present only in the deeper, least-weathered, reduced horizons of the tailings heap (Table 1). Carbonates are totally absent from the studied waste material, according to the X-ray diffraction analysis.

Woodhouseite and plumbogummite are high-temperature hydrothermal minerals and members of the aluminum–phosphate–sulfate (APS) paragenesis [69,70].

The amorphous phase is most likely composed of clay-mineral-like hydrated aluminum–silica, presumably with varying amount of sulfur, though it may also contain amorphous iron oxyhydroxides and other oxidation products formed during sample handling and preparation [71,72]. The exact composition of this complex amorphous mixture would require target-specific chemical analyses, such as synchrotron-based spectroscopy techniques that can distinguish between different amorphous components

The chemical properties of the H2 tailings and H7 waste rock heap showed significant variation. The pH values ranged from 2.5 to 5.4, with highly acidic conditions (pH < 3.0) occurring most frequently in the H7 waste rock heap. Electrical conductivity (EC) displayed considerable variability, ranging from 114.8 to 3180 µS/cm. An inverse relationship was observed between pH and EC, where samples with lower pH often exhibited higher EC values. For instance, sample RECSK-H7-15/R had a pH of 2.5 and an EC range of 2900–3180 µS/cm (Figure 5,

Table A1. Chemical characteristics of mine waste leachates.

Sample_ID	pH_DW	EC µS/cm	TDS mg/kg
RECSK-H2-01/R	3.0	850	5200
RECSK-H2-03/R	3.6	310	2000
RECSK-H2-04/R	4.0	250	2650
RECSK-H2-05/R	4.3	188.8	1900
RECSK-H2-07/R	5.4	114.8	1100
RECSK-H2-09/R	5.2	167.7	1450
RECSK-H2-10/R	4.6	500	4900
RECSK-H2-13/O	3.3	1150	9350
RECSK-H2-13/R	4.1	250	2950
RECSK-H2-14/O	3.8	940	7950
RECSK-H2-14/R	3.9	240	1450
RECSK-H2-15/O	3.7	630	4350
RECSK-H2-15/R	4.2	220	2650
RECSK-H2-16/R	3.9	350	3700
RECSK-H2-17/R	3.9	380	3650
RECSK-H7-02/O	4.1	1300	11,950
RECSK-H7-02/R	2.9	1350	9900
RECSK-H7-04/O	3.4	2510	24,700
RECSK-H7-04/R	3.0	1210	8300
RECSK-H7-06/O	3.7	2530	27,600
RECSK-H7-06/R	3.0	1570	13,950
RECSK-H7-09/O	3.2	2620	26,900
RECSK-H7-09/R	2.5	3050	31,200
RECSK-H7-10/O	3.2	2370	22,650
RECSK-H7-10/R	3.1	1010	7050
RECSK-H7-13/O	5.1	1670	15,550
RECSK-H7-15/O	2.7	3180	31,400
RECSK-H7-15/R	2.5	2900	29,050

(Appendix B)).

Total dissolved solids (TDSs) concentrations ranged from 1100 to 31,400 mg/kg. Elevated TDS values were closely associated with samples that had low pH and high EC, indicating a strong correlation among these parameters. These findings highlight the acidic nature of the leachates, particularly in the H7 waste rock heap, which corresponded with increased conductivity and dissolved solids level (Table A1 (Appendix B)).

4.2. Bulk Chemical Composition

The bulk chemical composition of the waste materials from the H2 tailings and H7 waste rock heaps was analyzed using aqua regia extraction (to determine total elemental concentrations) and deionized-water leaching (to assess water-soluble elemental concentrations). The results are summarized in

Table 2. Statistical parameters (aqua regia extraction) for both H2 and H7 waste heaps. Bold values exceed regulatory thresholds (6/2009 decree).

Chemical Element	Statistical Parameters (Aqua Regia Extraction)—H2 and H7 Waste Heap
	H2 Flotation Mud Waste Heap				H7 Waste Heap				Legal Limit (6/2009 Decree)
	Minimum	Median	Maximum	MAD	Minimum	Median	Maximum	MAD
	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg
Cu	443	1660	5060	591	99	347	1237	182	75
Zn	7.8	24	205	8.3	27	114	1156	57	200
Cd	0.005	0.097	19.3	0.092	0.13	0.83	12.7	0.62	1
Pb	24	42.8	181	17	35	172	813	56	100
As	313	674	2625	239.5	130	238	518	44	15
Fe	9833	23,305	47,596	6249	10,914	28,007	43,549	7877	n.a. 1
Mn	5.5	8.7	115.7	2.4	18.4	53	2.24	30	n.a. 1
Al	2647	7098	26,281	3642	5347	10,589	16,703	2610	n.a. 1
Ca	195	427	2457	140	1086	4137	19,666	2682	n.a. 1
Mg	38	139	874	64	258	612	1090	220	n.a. 1

¹ n.a. = not applicable (e.g., carbonates absent in samples). Legal limits are based on the 6/2009 (IV. 14.) KvVM-EüM-FVM Decree for geological media and the 2006 (IV. 5.).

and

Table 3. Statistical parameters (deionized-water extraction) for both H2 and H7 waste heaps. Bold values exceed regulatory thresholds (20/2006 decree).

Chemical Element	Statistical Parameters (Deionized-water extraction)—H2 and H7 Waste Heap
	H2 Flotation Mud Waste Heap				H7 Waste Heap				Legal Limit (6/2009 Decree)	Legal Limit (20/2006 Decree)
	Minimum	Median	Maximum	MAD	Minimum	Median	Maximum	MAD
	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg
Cu	19	52	385.5	32.7	11.9	30	79.7	10.8	75	2
Zn	0.93	2	5.2	0.78	3.3	10.5	36.7	6.5	200	4
Cd	0.00005	0.0027	0.049	0.0027	0.018	0.043	0.16	0.02	1	0.04
Pb	0.0014	0.0023	0.019	0.00078	0.0001	0.015	3.3	0.01	100	0.5
As	0.014	0.21	3.13	0.1	0.016	0.2	3.1	0.1	15	0.5
Fe	3.2	49	646	35	0.5	229	2392	190	n.a. 2	n.a. 2
Mn	0.01	0.7	4.7	0.65	3.5	14	33	8	n.a. 2	n.a. 2
Al	1.8	58	173	27	10.8	155	550.9	78	n.a. 2	n.a. 2
Ca	37.8	135	1558	81	837	2690	6460	1627	n.a. 2	n.a. 2
Mg	1.8	6.9	51	3.4	10.4	39.3	70.4	21	n.a. 2	n.a. 2

² n.a. = not applicable (e.g., carbonates absent in samples). Legal limits are based on the 6/2009 (IV. 14.) KvVM-EüM-FVM Decree for geological media and the 2006/2006 (IV. 5.) KvVM Decree for landfill regulations.

, which provide descriptive statistics for the measured elemental concentrations.

The total elemental concentrations, as determined by aqua regia extraction, revealed significant differences between the H2 tailings and H7 waste rock heaps. The median values of the elements varied by up to five orders of magnitude, with iron (Fe) exhibiting the highest concentrations and cadmium (Cd) the lowest. Notably, the H7 waste rock heap generally showed higher concentrations of most elements compared to the H2 tailings heap.

The water-soluble elemental concentrations, determined by deionized-water leaching, also showed significant differences between the two waste heaps. The median values of the water-soluble elements varied by up to five orders of magnitude, with calcium (Ca) exhibiting the highest concentrations and cadmium (Cd) the lowest.

The Mann–Whitney U tests revealed statistically significant differences (p < 0.05) between H2 and H7 heaps for most elements (

Table 4. Mann–Whitney U test results comparing H2 tailings and H7 waste rock heaps.

Element	Total Content		Water-Soluble Content		Mobility (%)
	p-Value	Sig	p-Value	Sig	p-Value	Sig
Cu	0.0001	**	0.0156	*	0.0087	**
Zn	0.0006	**	0.0002	**	0.9812	ns
Cd	0.0032	**	0.0005	**	0.0314	*
Pb	0.0014	**	0.0011	**	0.0236	*
As	0.0022	**	0.7642	ns	0.0062	**
Fe	0.4131	ns	0.0008	**	0.0022	**
Mn	<0.0001	**	<0.0001	**	0.0004	**
Al	0.0563	ns	0.0004	**	0.0412	*
Ca	<0.0001	**	<0.0001	**	<0.0001	**
Mg	0.0002	**	<0.0001	**	0.2136	ns

Significance levels: ** p < 0.01, * p < 0.05, ns not significant.

). Copper and arsenic showed significantly higher total concentrations in H2 tailings (p < 0.01), while zinc, lead, cadmium, manganese, calcium, and magnesium were higher in H7 waste rock. Water-soluble fractions differed significantly for most elements except arsenic. Element mobility was generally higher in H7, particularly for calcium (p < 0.0001) and manganese (p < 0.001), while zinc and magnesium mobilities showed no significant differences between heaps.

4.3. Element Geochemistry

Figures (Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8 and Figure A9 (Appendix A)) show sub-population identification performed using the “natural break” histogram-slicing method [62] which determines optimal breakpoints in the data distribution by identifying inflection points in the cumulative distribution function (CDF). These inflection points, corresponding to local minima in the frequency histogram, divide the data into distinct groups (Groups 1, 2, and 3) that minimize within-group variance while maximizing variance between groups. This method ensures that the identified groups reflect natural divisions in the geochemical data, offering a clear representation of the variability in elemental concentrations.

These differences in elemental concentrations are further influenced by the extraction method. Aqua regia extraction typically produces a broader range of concentrations across multiple elements, with histograms often displaying multimodal patterns. These patterns, characterized by multiple peaks, indicate distinct sub-populations with specific concentration ranges. In contrast, deionized-water extraction generally yields unimodal or left-skewed distributions, reflecting that only a fraction of the total elemental content is readily soluble.

The Box–Whisker plots in these figures illustrate the distribution of elemental concentrations across the groups objective by highlighting overall elemental mobility trends. The frequency values in the plots represent the number of samples within each group, while the percentage values reflect the relative abundance of each element in the corresponding groups. This dual representation provides a comprehensive view of the distribution and proportional contribution of each element across the geochemical zones of the waste heaps, a base-10 logarithmic transformation was applied exclusively for visualization purposes to better illustrate data distribution and handle high outliers. All statistical analyses were performed using the original concentration values, and the logarithmic scale does not alter the underlying data.

4.3.1. Major Element Geochemistry

The geochemical analysis of major elements in the H2 tailings and H7 waste rock heaps highlights significant differences in composition and potential environmental impact. Aqua regia extractions, representing total elemental content, reveal a consistent dominance of iron (Fe), aluminum (Al), calcium (Ca), magnesium (Mg), and manganese (Mn), in descending order of median concentration in both heaps. However, the H7 waste rock heap consistently shows higher concentrations of these elements compared to the H2 tailings, reflecting distinct lithological and geochemical differences between the coarse-grained waste rock and fine-grained flotation tailings (Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8 and Figure A9 (Appendix A)) and Table 2.

The analysis of water-soluble fractions, obtained through deionized-water extraction, provides insight into the readily mobilizable portion of each element. For the H2 tailings, the order of median concentrations slightly differs from the total content, with calcium (Ca) showing the highest mobility, followed by iron (Fe), aluminum (Al), magnesium (Mg), and manganese (Mn). The H7 waste rock heap samples follow a similar pattern but consistently demonstrate higher concentrations of water-soluble elements, particularly calcium. This observation suggests that the H7 heap contains a higher proportion of easily leachable elements, potentially posing a greater risk for environmental mobilization (Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8 and Figure A9 (Appendix A)) and Table 2.

The iron concentrations, based on total elemental content, are approximately similar in the H2 and H7 waste heaps, both exceeding 20,000 mg/kg. However, when considering dissolved elemental content, the concentration is five times higher in the H7 coarse waste heap compared to the H2 flotation waste heap. Mobility results further reveal that iron is four times more mobile in the H7 coarse heap, although this value remains below 1%. Univariate data analysis indicates that the reduced and oxidized levels of the heaps do not exhibit clear separation in terms of iron concentrations, suggesting that iron is equally present in both reduced and oxidized environments. The highest concentrations were observed in the oxidized samples of the H2 and H7 heaps, while the lowest concentrations occurred in the reduced levels of the H2 and H7 heaps (Figure A1 (Appendix A)).

The manganese concentration, based on total digestion, is six times higher in the H7 waste rock heap compared to the samples from the H2 tailings heap. For leachable manganese content, this difference increases nearly twentyfold, though the concentration only reaches 14.15 mg/kg. Manganese mobility is also three times greater in the H7 heap compared to the H2 heap. Univariate analysis of Aqua Regia extractions reveals higher manganese concentrations in the H7 heap, especially in its oxidized layer. In contrast, the reduced layer of the H2 heap has the lowest manganese concentrations. After deionized-water extraction, the reduced samples from the H7 heap show the highest manganese concentrations, while the reduced samples from the H2 heap consistently display the lowest levels (Figure A2 (Appendix A)).

The aluminum concentrations were observed to be 1.5 times higher in the H7 heap compared to the H2 heap when considering total elemental content. This difference increased twofold for leachable aluminum content, highlighting a higher mobility potential for aluminum in the H7 dump. Aluminum mobility was also determined to be approximately twice as high in the H7 dump relative to the H2 dump. Univariate analysis of the figure data revealed that the highest aluminum concentrations, in terms of total elemental content, were identified in the reduced and oxidized levels of the H2 dump, although this conclusion is drawn from a limited sample size of four. Following this, the samples from the H7 dump exhibited slightly lower concentrations, while the lowest concentrations were consistently observed in the reduced levels of the H2 dump. For leachable aluminum content, the reduced samples from the H7 dump showed the highest values, while the reduced samples from the H2 dump exhibited the lowest concentrations (Figure A3 (Appendix A)).

The calcium content, based on samples extracted with aqua regia, is ten times higher in the H7 waste rock heap compared to the H2 tailings heap. For samples extracted with deionized water, this difference increases to nearly twentyfold. Among the examined elements, calcium is the most mobile, with a mobility of 65% in the H7 heap, which is double the mobility observed in the H2 heap. Univariate analysis shows clear distinctions between sample types. The highest total calcium concentration was found in the oxidized section of the H7 coarse heap, while the lowest values came from the reduced layer of the H2 tailings heap. A similar pattern was observed for leachable calcium, with the H7 heap showing the highest values and the H2 heap the lowest (Figure A4 (Appendix A)).

4.3.2. Trace Element Geochemistry

Univariate Analysis

Aqua regia extractions revealed that iron (Fe), aluminum (Al), calcium (Ca), magnesium (Mg), and manganese (Mn) are the dominant elements in both the waste rock heap (H7) and the tailings heap (H2), with concentrations decreasing in the order listed. Notably, the H7 waste rock heap consistently exhibited higher concentrations of these elements compared to the H2 tailings heap (Table 2; Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8 and Figure A9 (Appendix A)). This disparity suggests distinct lithological and geochemical characteristics between the coarse-grained waste rock and the fine-grained flotation tailings.

Analysis of water-soluble fractions using deionized-water extraction further highlighted the differences in element mobility between the two heaps. In the H2 tailings, the median concentrations of Ca, Fe, Al, Mg, and Mn followed a descending order. A similar trend was observed in the H7 samples; however, the concentrations were consistently higher, particularly for Ca (Table 3; Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8 and Figure A9 (Appendix A)). These findings indicate that the H7 heap contains a greater proportion of easily leachable elements, underscoring its distinct geochemical behavior compared to the H2 tailings.

The copper concentration in the H2 tailings heap is significantly higher, nearly five times greater than the total copper content in the H7 coarse heap. However, mobility analysis shows that copper is less mobile in the H2 heap, with mobility levels almost one-third of those observed in the H7 heap. Despite its lower mobility, the high copper concentration in the H2 flotation heap allows for a significant amount of copper to be released under the dissolving effects of water. Univariate data analysis indicates that the highest copper concentrations are found in samples from the reduced levels of the H2 heap. The copper concentration in the lowest sample exceeded the environmental Hungarian standard(6/2009) for copper in geological media (75 mg/kg), while the highest concentration sample exceeded the limit by 67 times. After deionized-water extraction, copper levels in the lowest concentration sample were more than 10 times the regulatory standard and, in the highest concentration sample, exceeded the limit by over 192 times (Table 2 and Table 3; Figure A5 (Appendix A)).

Zinc concentrations are significantly higher in the H7 rock heap, approximately five times greater than those in the H2 tailings heap. Zinc mobility is comparable between the two heaps, with the H7 heap showing only slightly higher mobility. Univariate analysis indicates that the highest zinc concentrations occur in the reduced samples of the H7 heap, while the lowest concentrations are found in the reduced samples of the H2 heap. Total zinc concentrations exceeded the environmental Hungarian standard (6/2009) for geological media (200 mg/kg) in nearly 25% of the samples, with some exceeding the limit by more than five times. After deionized-water extraction, the landfill regulation limit (4 mg/kg) was exceeded in almost 50% of the samples, with levels reaching up to nine times the permitted value (Table 2 and Table 3; Figure A6 (Appendix A)).

Lead concentrations in the H7 waste rock heap, compared to the H2 tailings heap, are four times higher in samples extracted with aqua regia and approximately seven times higher in samples extracted with deionized water. However, in the deionized-water extractions, these values remain well below 1 mg/kg. Lead mobility is similarly low in both heaps. Univariate data analysis shows that the highest lead concentrations are found in the reduced level of the H7 heap, while the lowest concentrations occur in the reduced level of the H2 heap. Lead exceeded the environmental government regulation for geological media (100 mg/kg) in more than 25% of the samples after aqua regia extraction, by more than eight times at most. The water solubility of lead was very low in our case, only a few samples exceeded the environmental regulation for landfills (0.5 mg/kg), but those exceeded the limit value by more than six times (Table 2 and Table 3; Figure A7 (Appendix A)).

Arsenic concentrations in samples extracted with aqua regia are more than 2.5 times higher in the H2 tailings heap compared to the H7 waste rock heap. However, in samples extracted with deionized water, arsenic concentrations are approximately the same (~0.2 mg/kg) in both heaps. While arsenic mobility remains extremely low overall, it is twice as high in the H7 heap compared to the H2 heap. According to the univariate data analysis, the highest arsenic concentrations are consistently observed in the reduced level of the H2 tailings heap, while the lowest concentrations are found in the H7 waste rock heap. Arsenic concentrations exceeded the Hungarian environmental regulation for geological media (15 mg/kg) in all samples following aqua regia extraction. After deionized-water extraction, arsenic exceeded the landfill threshold (0.5 mg/kg) in 25% of the H2 tailings samples, while only a few samples from the H7 waste heap, those with the highest arsenic concentrations, went beyond this limit (Table 2 and Table 3; Figure A8 (Appendix A)).

Cadmium concentrations in samples extracted with aqua regia are more than 8 times higher in the H7 waste rock heap compared to the H2 tailings, and 16 times higher in the H7 heap in samples extracted with deionized water. While cadmium mobility remains low overall, it is significantly higher in the H7 heap. According to univariate data analysis, the highest total cadmium concentrations are consistently observed in the reduced level of the H2 tailings, while the highest water-soluble cadmium concentrations are found in the reduced level of the H7 waste heap. Cadmium concentrations exceeded the Hungarian environmental regulation for geological media (1 mg/kg) in 29% of the H7 samples, with maximum values exceeding the limit by up to 5.2 times, and in the H2 tailings heap, the highest concentration was 19 times above the limit. After deionized-water extraction, cadmium exceeded the landfill threshold (0.04 mg/kg) in 50% of the H7 samples (Table 2 and Table 3; Figure A8 (Appendix A)).

Bivariate Analysis

The bivariate analysis in this study focuses on understanding the relationships between different potential toxic elements (PTEs) in waste rock and tailings heaps from the Recsk Mining Area, Hungary. The analysis was conducted using Pearson’s linear correlation coefficient and least-squares regression to identify significant correlations at the 0.05 significance level [23,24,25].

A strong positive correlation (r = 0.93, p ≤ 0.05) was observed between copper and arsenic in samples extracted using aqua regia, indicating a linked geochemical behavior (Figure 6). This correlation significantly decreases (r = 0.07) in deionized-water leachates, suggesting a change in their interaction under different extraction conditions (Figure A10, (Appendix A)).

Zinc and lead show a strong positive correlation (r = 0.94, p ≤ 0.05) in aqua regia extracted samples (Figure 7), which decreases but remains significant after deionized-water extraction (Figure A10, (Appendix A)). This suggests that these elements are likely associated with similar mineral phases or geochemical processes in the waste materials.

When we correlated total zinc concentrations with their leachable fractions (Figure 8), a strong positive relationship was observed (r = 0.74, p ≤ 0.05). The H7 waste rock heap’s reduced zone showed the highest zinc levels, while the H2 tailings heap consistently had the lowest. This pattern suggests that both the waste type and its oxidation state influence zinc mobility, as also seen in the intermediate concentrations of oxidized samples from both heaps.

When comparing total and leachable lead concentrations (Figure A11, (Appendix A)), a strong correlation was observed (r = 0.88, p ≤ 0.05). The reduced zones of both H2 and H7 waste heaps showed the most extreme lead concentrations (both highest and lowest values), while oxidized samples contained intermediate levels. This pattern suggests that the oxidation state significantly influences lead mobility in both waste types.

While arsenic shows a strong positive correlation with copper (r = 0.93) in Aqua Regia extractions, it has a weak negative correlation with zinc and lead (r = −0.3). No significant correlations were found between arsenic and these elements in deionized-water extractions, highlighting the influence of extraction methods on element interactions (Figure A10, (Appendix A)).

A notable strong positive correlation (r = 0.97) was observed between cadmium and zinc following deionized-water extraction, suggesting a potential co-mobility or shared geochemical pathway in the studied samples (Figure A10, (Appendix A)).

The bivariate analysis highlights the complex interactions and varying mobility of PTEs in different waste materials and extraction conditions. These findings emphasize the need for tailored remediation strategies to address the specific geochemical behaviors observed in the Recsk Mining Area. The results provide a foundation for further research and potential applications in environmental management and remediation efforts.

4.4. Element Mobility Assessment in Mine Waste

The mobility of chemical elements was simply characterized by the ratio of the median water-soluble element concentrations (DW) and the median total concentration (AR) multiplied by 100:

$$\mathrm{M}\mathrm{o}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{\mathrm{M}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{n}\left({\mathrm{C}\mathrm{h}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l} \mathrm{E}\mathrm{l}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}_{\mathrm{D}\mathrm{W}}\right)}{\mathrm{M}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{n}\left({\mathrm{C}\mathrm{h}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l} \mathrm{E}\mathrm{l}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}_{\mathrm{A}\mathrm{q}\mathrm{R}}\right)}}\times 100$$

The mobility factors were calculated for all samples and separately for the H2 tailings heap and the H7 waste rock heap. According to (

Table 5. The mobility of the elements in the H2 tailings, the H7 waste rock heap, and together in all wastes.

Chemical Element	H2 (%)	H7 (%)
Ca	32	65
Zn	9	9
Mn	8	27
Mg	5	6
Cu	3	9
Cd	3	5
Al	0.8	1
Fe	0.2	0.8
As	0.03	0.09
Pb	0.005	0.009

), the overall elemental mobility is higher in the H7 waste dump compared to H2. Among the potentially toxic elements, zinc (Zn) exhibits equal mobility at both sites, while copper (Cu) is three times more mobile in H7, and cadmium (Cd) shows 1.6 times higher mobility in H7. In contrast, lead (Pb) and arsenic (As) remain immobile in both heaps.

The mobility factor values range over five orders of magnitude in the studied mine waste heaps. The least mobile (water-leachable) element is lead, and the most mobile is calcium. Among the potentially toxic elements, zinc has the highest mobility.

The results from Table 5 shows the following:

Iron is four times more mobile in the H7 waste rock heap, although this value is below 1% only.

The mobility of manganese is three times higher in the H7 than that of the H2 tailings heap.

Calcium (Ca) is the most mobile element among those tested, with mobility in the H7 waste rock heap being twice as high as in the H2 tailings heap, reaching a value of 65%.

BCR Sequential Extraction Analysis: Metal Mobility in Mine Waste

The BCR sequential extraction analysis (

Table 6. Concentrations of elements (mg/kg) in different fractions obtained by BCR sequential extraction of mine.

Element	H2 Tailings Heap		H7 Waste Rock Heap
	Reactive Fraction (R1 + R2 + R3)	Residual Fraction (R4)	Reactive Fraction (R1 + R2 + R3)	Residual Fraction (R4)
Cu	74.4	1585.6	182.5	164.5
Zn	2.2	21.8	10.5	103.5
Cd	0.01	0.09	0.08	0.75
Pb	4.6	38.2	13.3	158.7
As	24.8	649.2	60.0	178.0
Fe	1094.7	22,210.3	2780.7	25,226.3
Mn	0.7	8.0	14.0	39.0
Al	291.3	6806.7	740.0	9849.0
Ca	135.0	292.0	2690.0	1447.0

) reveals the distribution of elements between potentially mobile reactive fractions (R1 + R2 + R3) and the stable residual fraction (R4) in both waste materials, providing insight into their different geochemical behaviors.

This approach quantifies four fractions—Exchangeable (R1), Reducible (R2), Oxidizable (R3), and Residual (R4)—that reflect how metals might be released under different environmental conditions.

In the H2 tailings heap, most elements are predominantly found in the residual fraction. Copper shows a particularly strong affinity for the residual fraction, with only 74.4 mg/kg in reactive fractions compared to 1585.6 mg/kg in the residual fraction. Similarly, arsenic in H2 is primarily in the residual phase (649.2 mg/kg) with limited presence in reactive fractions (24.8 mg/kg).

The H7 waste rock heap exhibits markedly different partitioning patterns. Notably, copper in H7 shows nearly equal distribution between reactive (182.5 mg/kg) and residual fractions (164.5 mg/kg). Calcium demonstrates a strong presence in reactive fractions (2690.0 mg/kg) compared to residual (1447.0 mg/kg). While arsenic still favors the residual fraction in H7, a substantial portion (60.0 mg/kg) exists in reactive forms. Iron and aluminum, though primarily residual in both heaps, show higher absolute concentrations in the reactive fractions of H7 compared to H2.

Other elements show similar patterns: despite H2 having higher total arsenic, H7 contains more arsenic in reactive fractions (60.0 vs. 24.8 mg/kg). Zinc, cadmium, and lead all show higher reactive concentrations in H7 despite being primarily residual in both heaps. Iron, aluminum, manganese, and magnesium consistently exhibit higher concentrations in H7 for both fractions. These BCR results provide insight into binding mechanisms that explain the differential element mobility observed in our earlier water extraction tests.

4.5. Evaluation of Acid Mine Drainage Potential in the Recsk Mine Area

The acid-generation potential assessment of the H2 tailings and H7 waste rock heaps highlights significant environmental risks.

Table 7. Result of the XRD measurement in percentage units. O: oxidized; R: reduced.

Sample ID	NAG pH	MPA (kg H2SO4/t)
RECSK-H2-03/R	2.23	105.57
RECSK-H2-13/R	2.62	31.824
RECSK-H2-14/O	3.05	44.676
RECSK-H2-14/R	2.67	35.496
RECSK-H2-15/O	3.23	26.316
RECSK-H2-17/R	2.48	53.55
RECSK-H7-10/O	2.61	59.364
RECSK-H7-10/R	2.33	73.134
RECSK-H7-15/O	3.00	94.248
RECSK-H7-15/R	2.06	105.57

shows that NAG pH values range from 2.06 to 3.23, indicating highly acidic conditions, with the lowest pH (2.06) observed in the reduced zone of the H7 waste rock heap (RECSK-H7-15/R) and the highest (3.23) in the oxidized zone of the H2 tailings (RECSK-H2-15/O). Maximum potential acidity (MPA) values, expressed as H₂SO₄ equivalent per ton, range from 26.32 to 105.57 kg H₂SO₄/t. The highest MPA values were recorded in samples RECSK-H2-03/R and RECSK-H7-15/R (both 105.57 kg H₂SO₄/t), while the lowest was found in RECSK-H2-15/O (26.32 kg H₂SO₄/t).

Acid Base Accounting (ABA) and Net Acid Generation (NAG) tests were conducted on waste material samples collected from the two waste heaps. The results, illustrated in Figure 9, indicate the presence of significant amounts of reduced sulfur, primarily in the form of pyrite. The Net Acid-Producing Potential (NAPP) was estimated to reach a maximum of approximately 105 per ton of waste, suggesting a substantial capacity for acid generation during waste decomposition. Furthermore, the carbonate content in the samples was either not detectable or present in negligible amounts, making it incapable of neutralizing the potential acidity. Therefore, it can be concluded that both waste heaps are potentially acid-producing and pose a risk of acid mine drainage (AMD) formation.

5. Discussion

5.1. Mineralogy and Waste Chemcial Composition

The applied statistical and mineralogical analyses revealed fundamental differences between the H2 fine-grained tailings and H7 coarse-grained waste rock dump that extend beyond simple compositional variation. These differences directly impact their environmental behavior and remediation requirements, addressing our first research objective.

XRD data (Table 1) not only confirmed the presence of primary and secondary minerals but revealed distinct mineralogical evolution pathways in the two waste materials. The dominance of quartz (40.6%–82.8%) in both heaps provides a relatively inert matrix, but the significant presence of pyrite (up to 4.9%) constitutes the primary driver of acid generation through oxidative weathering. More importantly, the advanced weathering state in the H7 waste heap is evidenced by higher concentrations of jarosite (up to 8.01%) and gypsum (up to 25%) compared to the H2 tailings, indicating that the coarser material paradoxically facilitates more rapid sulfide oxidation despite its lower surface area. This paradox is explained by two mechanisms: (1) oxygen diffusion pathways in the more permeable H7 material accelerate sulfide oxidation, and (2) flotation processing of H2 tailings altered its mineralogical properties, creating compacted material with modified mineral surfaces [73] that resist oxidation despite higher surface area. This is consistent with other studies, which suggest that the fine-grained nature of flotation tailings, such as those in the H2 heap, increases the specific surface area, thereby promoting rapid leaching and higher element mobility [74,75].

The absence of carbonates in both waste materials, despite their different origins, explains their similarly poor buffering capacity and confirms why acid generation proceeds uninhibited. This mineralogical composition directly influences the mobility patterns observed for PTEs, as the absence of acid-neutralizing minerals creates persistently acidic conditions that enhance metal solubility, particularly for divalent cations like zinc and cadmium.

The structural differences in these materials provide insight into their contrasting chemical behavior. While the H2 tailings would be expected to release more elements due to their higher specific surface area, we instead observe higher mobility in the H7 waste rock for most elements (Table 5). This apparent contradiction is explained by analyzing the mineral hosting of these elements: in H2, despite flotation processing that theoretically should promote increased element mobility, copper and arsenic remain largely bound in relatively resistant sulfosalt minerals like tennantite, which our XRD analysis detected only in the reduced zones of the tailings heap (Table 1). These minerals dissolve slowly even under acidic conditions, explaining the lower mobility despite higher total concentrations (Table 2 and Table 3).

This finding aligns with previous research [41] which identified the H2 heap as having the highest concentrations of toxic elements. It further indicates that tailings can retain substantial amounts of ore-associated elements, such as Cu and As, which may be mobilized under specific conditions [73,76].

The inverse relationship between pH and EC demonstrates important environmental implications. Lower pH increases mineral dissolution and metal mobilization, resulting in higher conductivity. This relationship in the H7 waste rock heap indicates enhanced solubilization of PTEs, increasing their bioavailability and environmental mobility. The high EC values (up to 3180 µS/cm) in low pH samples suggest significant ionic loads that can adversely affect aquatic ecosystems through metal toxicity and altered water chemistry, characteristic of active acid mine drainage processes [77,78]. The acidic pH values (2.5–5.4) measured in both the H2 tailings and H7 waste rock heaps confirm the ongoing occurrence of acid mine drainage (AMD), consistent with the oxidation of sulfide minerals such as pyrite and chalcopyrite, as documented with previous research [79].

The highest concentrations and mobility of elements were observed in the H7 waste rock heap (Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8 and Figure A9 (Appendix A), Table 2, Table 3 and Table 5). This heap, primarily composed of andesitic waste rock, was formed from deposits accumulated from the early 20th century until the closure of the Lahóca Hill mine. Our findings show that the material still contains significant amounts of ore minerals and various sulfide minerals. This composition, coupled with the extended period of exposure to weathering processes since deposition, likely contributes to the observed high element mobility. The elevated concentrations of (Fe, Mn, Al, Ca, Zn, Pb, Cd) are particularly concerning, as they indicate ongoing release from the weathering of these minerals and may pose significant environmental risks.

This observation aligns with previous research [41,42,80,81], which emphasizes the role of erosion in influencing the “life cycle” of the heap. The weathering processes, which have been active since the heap’s formation. These processes not only affect the physical stability of the heap but also enhance the mobility of elements, potentially affecting local environmental conditions.

The Potential toxic elements concentrations (Cu, Zn, Pb, and As) in samples from both waste heaps significantly exceed the threshold limits established by Hungarian government decrees for geological media and inert waste landfills (Table 2 and Table 3) [56,57].

The statistical analyses confirm distinct geochemical behaviors between waste heaps. Higher copper and arsenic in H2 tailings (p < 0.01) aligns with Smuda et al. [82], who noted flotation processes often retain these elements in epithermal tailings. Significantly higher mobility in H7 waste rock supports Anawar [83] findings that coarser materials exhibit enhanced leachability due to greater weathering exposure. These statistically validated differences underscore the necessity for tailored remediation strategies for each waste heap type.

5.2. Relationships Between Elements and Redox Conditions

The strong correlation between copper and arsenic (r = 0.93) in the AR (Aqua Regia) extracts (Figure 6 (indicates that these elements primarily originate from arsenic-bearing copper sulfosalts, such as enargite (Cu₃AsS₄) and tennantite (Cu₁₂As₄S₁₃), which are commonly found in high-sulfidation epithermal systems [84,85,86]. Under Aqua Regia digestion, both copper and arsenic dissolve extensively. In contrast, deionized-water extraction shows little to no correlation between copper and arsenic (r ≈ 0), suggesting that under near-neutral pH or short contact times, these arsenic-rich copper minerals do not readily dissolve. This observation is consistent with earlier studies indicating that sulfosalt minerals are relatively resistant to dissolution under mild conditions [87,88]. This strong correlation has important implications for site remediation. This correlation indicates these elements share common mineral hosts in arsenic-bearing copper sulfosalts. Effective remediation strategies should address both elements simultaneously, likely through iron-based amendments that can co-precipitate or adsorb both Cu and As rather than using washing-based methods, which would be less effective given their limited water solubility.

Zinc and lead show a strong correlation (r = 0.94) in Aqua Regia extarction (Figure 7), mainly because of their presence in galena (PbS) and sphalerite (ZnS) in the Lahóca Hill mineralization [89,90]. These minerals partly oxidize in mine waste heaps, but the overall amounts of zinc and lead remain closely linked in bulk analyses. In deionized-water extraction, the correlation weakens but does not disappear entirely. This suggests that small amounts of these minerals, or their oxidation byproducts, dissolve under mild conditions. Similar result reports by [91,92] about the weaker correlation between Pb and Zn. As only a fraction of these minerals or their oxidation products dissolve under neutral pH conditions.

Zinc and cadmium show a strong correlation (r ≈ 0.97) specifically in deionized-water extraction, while exhibiting weaker correlation in aqua regia extarction. This pattern suggests these elements co-occur in readily mobilizable forms rather than being structurally incorporated within primary sulfide minerals like sphalerite. The selective mobilization under mild extraction conditions indicates that both metals are likely associated with secondary weathering products or weakly adsorbed onto mineral surfaces, particularly under the acidic conditions, as demonstrated by [93] in their integrated study of sphalerite weathering in mine waste environments. Similar geochemical behavior has been documented by [94], who found that Zn and Cd often share parallel mobilization pathways in mine tailings due to comparable ionic properties and surface complexation behavior. This parallel mobility has significant environmental implications, as even short-duration precipitation events may simultaneously release both elements into surrounding ecosystems.

Lead and cadmium show no clear correlation (Figure A10, (Appendix A)) in either extraction method. This suggests that Pb and Cd originate from different minerals—galena (PbS) for Pb and sphalerite (ZnS) for Cd. While both minerals may coexist in the deposit, their dissolution behaviors differ. Cadmium is easily released when sphalerite dissolves, even slightly, whereas lead remains trapped in galena or secondary Pb-bearing phases, dissolving only under specific geochemical conditions [90].

The correlations between water-soluble and total concentrations for zinc and lead (Figure 8 and Figure A11 (Appendix A)) reveal distinct mobilization patterns. These figures illustrate how dissolution behavior varies across elements, with initial total concentrations often having greater influence on dissolved levels than intrinsic mobility. Zinc shows a strong correlation (r = 0.74) (Figure 8)with clustering by both heap type and oxidation state, indicating that material properties and redox conditions jointly influence its mobility. The H7 heap’s reduced zone exhibited the highest zinc concentrations, likely from sphalerite weathering under acidic conditions.

Lead, despite its stronger correlation (r = 0.88), behaves differently, with extreme values occurring in reduced zones regardless of heap type (Figure A11, (Appendix A)). This suggests that the oxidation state primarily controls lead mobility, reflecting its tendency to form stable secondary minerals under moderately acidic conditions. These element-specific patterns align with previous studies on sulfide weathering and highlight the need for tailored approaches in remediation planning [95].

5.3. Element Mobility and Release Mechanisms

The mobility data presented in Table 5 reveal distinct patterns in element release from these waste materials. Copper demonstrates threefold higher mobility in H7 waste rock (9%) compared to H2 tailings (3%), despite H2 containing significantly higher total copper concentrations. This apparent contradiction results from copper in H2 being predominantly bound in stable mineral phases resistant to weathering, while in H7, it exists in more readily soluble secondary forms. Zinc exhibits consistent mobility (9%) across both heaps, whereas lead remains essentially immobile (<0.01% mobility) even under the prevailing acidic conditions. These observations highlight a critical finding: environmental risk depends not merely on total element concentration but predominantly on mineralogical associations and their weathering susceptibility. The pronounced mobility contrast between zinc and lead within identical waste materials challenges simplistic approaches to toxic element behavior in mine waste. Rather, each element follows distinct geochemical pathways that determine its environmental mobility. These observations are consistent with previous studies, emphasizing the importance of considering mineral hosts, oxidation states, and element release in remediation strategies [92,96,97].

The BCR sequential extraction results (Table 6) reveal significant differences in element fractionation between the H2 tailings and H7 waste rock heaps that explain their distinct mobility patterns observed in our water extraction tests (Table 5). In the H2 tailings heap, most elements are predominantly bound in stable residual fractions, particularly copper and arsenic, which explains their limited mobility despite high total concentrations. This is consistent with findings by [98], who demonstrated that flotation processing can transform metals into more stable mineral phases even when extraction is incomplete.

The H7 waste rock heap, however, shows substantially higher proportions of copper (52.6% reactive) and arsenic (25.2% reactive) in potentially mobile fractions. This distribution pattern aligns with observations by [14], who documented how mineralogical associations in unprocessed waste rock typically result in greater element mobility under environmental conditions. The nearly equal distribution of copper between reactive and residual fractions in H7 explains its threefold higher mobility in our water extraction tests compared to H2.

Calcium’s exceptional mobility in both heaps, particularly in H7 (65% reactive), corresponds with its presence in readily soluble forms, primarily as gypsum identified by our XRD analysis. Lindsay [16] similarly found that secondary calcium minerals in mine waste can be rapidly mobilized under varying environmental conditions. The high calcium mobility supports findings by Bigham and Nordstrom [68], who noted that secondary sulfate minerals like gypsum play crucial roles in element cycling in mine waste environments.

Manganese shows markedly higher presence in reactive fractions in H7 (26.4% reactive) compared to H2 (8.0% reactive), consistent with findings by [83] that manganese mobility typically increases with progressive weathering of mine waste materials. This differential partitioning helps explain the threefold higher water-extractable mobility of manganese in H7 observed in our earlier tests.

These fractionation patterns demonstrate why the H7 waste rock heap may pose greater immediate environmental risks despite sometimes lower total concentrations than H2 tailings. As emphasized by Brough [67], element fractionation and binding mechanisms, rather than just total content, are decisive for assessing environmental risk from mine waste materials. Our findings underscore the importance of characterizing both total concentration and chemical partitioning when developing remediation strategies for mine waste materials with distinct processing histories.

The predominance of copper in the residual fraction of H2 tailings is consistent with patterns documented by [99], who observed similar metal fractionation in aged mine tailings from acidic environments. The more balanced Cu distribution in our H7 waste rock heap parallels findings by [100], who reported higher reactive metal fractions in less weathered mining materials. Our arsenic fractionation patterns compare favorably with [101], who found predominant residual As binding in aged tailings with progressively increasing reactive fractions in fresher waste materials.

The elevated calcium presence in reactive fractions of H7 waste rock aligns with observations by [102], who attributed similar patterns to ongoing dissolution processes in younger mine waste. Notably, our differential mobility patterns between H2 and H7 show remarkable similarities to sequential extraction profiles reported by [103] across various stages of mine waste weathering, where reactive fractions gradually diminish as materials age and undergo secondary mineralization. Additionally, [104] observed comparable metal partitioning behavior in tailings impoundments of different ages, supporting our interpretation that differences between H2 and H7 reflect distinct weathering stages and associated transformation of primary minerals into more stable secondary phases.

5.4. Acid Generation Mechanisms and Environmental Implications

The acid-generation potential assessments of both the H2 tailings and H7 waste rock heaps (Table 7, Figure 9) highlight the significant risk of Acid Mine Drainage (AMD) formation in the Recsk Mine Area, with NAG pH values (2.06–3.23) and high MPA values (up to 105.57 kg H₂SO₄/t). Pyrite (FeS₂) is a key driver of acid generation in both heaps, although other sulfides also contribute. Furthermore, jarosite and gypsum were detected in XRD analyses, indicating prolonged oxidation and secondary sulfate formation. These values align with findings from other mining sites worldwide, where AMD has caused extensive environmental damage. Similar pH ranges (2–4) and high metal concentrations have been documented.

Notable differences in AMD dynamics between oxidized and reduced zones emphasize the influence of weathering and redox conditions. For example, the lowest pH (2.06) in the reduced zone of the H7 heap (RECSK-H7-15/R) suggests latent acid-generating capacity due to unweathered sulfides, whereas slightly higher pH values in oxidized zones (e.g., 3.23 in H2-15/O) likely reflect partial leaching of acidic byproducts over time. Such spatial heterogeneity aligns with studies on high-sulfidation systems, where sulfide stability is tightly coupled to environmental exposure [105].

A particularly notable finding is the limited buffering capacity of these mine wastes due to the scarcity or absence of carbonate minerals. Acid-Base Accounting (ABA) data confirm that the Net Acid-Producing Potential (NAPP) is positive in most samples, underscoring that insufficient alkalinity exists to neutralize the acid produced. Consequently, both heaps are likely to maintain acidic conditions over extended time scales if left unmanaged.

Our acid generation potential results align with findings from similar mine-waste studies. The NAG pH values (2.06–3.23) observed in our waste heaps are comparable to those reported by [106], who documented NAG pH ranges of 2.1–3.5 in sulfide-bearing waste rock with similar pyrite content. Our Maximum Potential Acidity (MPA) values (26.32–105.57 kg H₂SO₄/t) fall within ranges observed by [106,107] in their assessment of coal mine waste rock (18.7–113.2 kg H₂SO₄/t). The absence of neutralizing minerals in our samples parallels observations by [108,109], who noted similar deficiencies in carbonate content are typical of high-risk AMD-generating materials.

The higher MPA values recorded in our reduced zones (up to 105.57 kg H₂SO₄/t) are consistent with findings from [110], who demonstrated that unweathered sulfidic waste typically exhibits greater acid-generating potential than corresponding oxidized zones. Our classification of both waste heaps as potentially acid-producing corresponds with assessment criteria established by [22], who defined materials with similar geochemical characteristics as posing significant long-term AMD risks. Additionally, ref. [111] documented comparable NAPP values (80–110 kg H₂SO₄/t) in sulfide mine tailings that subsequently developed persistent acidic drainage conditions, supporting our conclusion regarding the environmental risk posed by these materials

5.5. Implications for Environmental Risk Assessment and Remediation

The integrated analysis of mineralogical, geochemical, and acid generation data demonstrates that these waste materials pose distinct environmental risks. The H2 tailings heap presents a concentrated source of copper and arsenic with relatively lower immediate mobility but significant long-term release potential as weathering progresses. Conversely, the H7 waste rock heap constitutes a more immediately active source of multiple PTEs (particularly zinc and cadmium) due to its advanced weathering state and higher element mobility.

These differences necessitate tailored remediation approaches rather than a uniform strategy. The H2 tailings would benefit from technologies that specifically address copper and arsenic, potentially including stabilization with iron-based amendments that form stable arsenate compounds. Meanwhile, the H7 waste rock requires more urgent intervention focused on neutralization and containment of multiple mobile elements, with particular attention to controlling water infiltration that drives the ongoing acid generation and metal leaching.

The economic potential for metal recovery also differs significantly between the waste types. The H2 tailings, with concentrated copper and arsenic, may present opportunities for selective extraction of these elements, potentially offsetting remediation costs. The H7 waste rock, while containing a broader spectrum of elements, presents greater technical challenges for recovery due to the more advanced state of weathering and secondary mineral formation.

To accurately identify smectite and illite/smectite minerals, diagnostic clay mineral assessment and/or Fourier-transform infrared spectroscopy (FTIR) measurements would be needed [112].

The amorphous phase is most likely composed of clay minerals like hydrated aluminum-silica, presumably with varying amounts of sulfur, because with the depletion of the amorphous phase the jarosite and smectite ratio increase proportionally. To tell the exact composition, specific chemical analyses would be required. The Fourier-transform infrared spectroscopy (FTIR) could be applied to answer this question as well.

Drying samples at 40 °C may cause minor artificial oxidation, potentially affecting mineral composition and acid generation potential by promoting sulfide oxidation (e.g., pyrite). This step was needed to remove moisture for accurate measurements before aqua regia extraction. Though the low temperature and short duration likely limited the impact, future studies could employ methods like freeze-drying or drying under an inert atmosphere to minimize oxidation risks during sample preparation. Additionally, to enhance the characterization of redox-sensitive phases and amorphous content in similar mine waste materials, complementary techniques such as anaerobic sampling and synchrotron-based analyses could be utilized.

6. Conclusions

This study presents a comprehensive geochemical, mineralogical, and statistical investigation of two distinct waste heaps—the fine-grained H2 tailings and the coarse-grained H7 waste rock—within the Recsk mining area. Our analyses clearly demonstrate that both waste heaps contain toxic elements at concentrations exceeding Hungarian regulatory limits, accompanied by a significant acid-generation potential. The XRD results confirm a mineralogical assemblage dominated by quartz with notable contributions from secondary minerals, such as jarosite and gypsum, alongside the presence of pyrite. These findings underscore active sulfide oxidation processes that are driving the development of acid mine drainage (AMD) within these heaps.

Significantly, our data reveal that despite originating from the same mineralization, the two heaps behave distinctly under environmental exposure due to differences in processing history, physical structure, and subsequent weathering trajectories. The H2 tailings heap retains elevated levels of ore-associated elements such as copper and arsenic in relatively stable mineral forms, while the H7 waste rock exhibits higher mobility for major elements (e.g., iron, manganese, and calcium) and divalent cations (zinc, cadmium) due to more advanced weathering and less stable mineral associations. These contrasts directly influence the environmental risks posed by each material and necessitate different remediation approaches.

The spatial heterogeneity and redox zonation within each heap further complicate their environmental behavior, with reduced zones often containing higher concentrations of sulfides with latent acid-generating potential. This complexity means that both materials pose long-term environmental risks to local groundwater, ecosystems, and wildlife, though through different mechanisms and at different rates.

Given the distinct geochemical and mineralogical characteristics identified in this study, we conclude that effective remediation must be tailored to each waste type rather than applying a uniform approach. The H2 tailings require strategies focused on long-term stabilization of copper and arsenic, while the H7 waste rock demands more immediate intervention to address multiple mobile elements and ongoing acid generation. Both materials would benefit from techniques that limit water and oxygen infiltration, though the implementation would need to account for their different physical properties.

This work enhances understanding of PTE behavior in high-sulfidation epithermal mine wastes and provides a methodological framework for characterizing similar waste materials at other sites. Future research should focus on developing specific remediation technologies optimized for each waste type and exploring the potential for selective resource recovery where economically viable.

Future studies should explore long-term environmental monitoring and remediation trials for these waste materials while also incorporating complementary techniques like anaerobic sampling, freeze-drying, and synchrotron-based methods to better understand redox-sensitive phases and amorphous content in similar mine wastes.