Soil contaminated with hazardous waste materials at Rio Tinto mine (Spain) is a persistent secondary source of acid and heavy metals to the environment
Abstract
Mineralogical analysis and laboratory-based leaching tests coupled with speciation modeling were undertaken to quantify the potential for short-term acid generation and the release of trace elements from soils heavily contaminated with mine waste at Rio Tinto. Three different waste materials were considered as case studies: roasted pyrite, copper slags, and leached sulfide ores. The results showed elevated values of net acid generation (up to 663 mmol H+/kg), the major pools being potential sulfidic acidity and acidity retained in jarosite. Remarkable contents of As and toxic heavy metals were found especially in the slag-contaminated soil. Copper, Zn, and Pb were the most abundant metals in the acid leach solutions resulting from mine soil-water interaction, with peak values of 55.6 mg L−1 , 2.77 mg L−1 , and 2.62 mg L−1 , respectively. Despite the high total contents of trace elements occurring in soil, the mobile fraction was limited to maximum release values of 12.60% for Cd and 10.27% for Cu, according to the test leaching. Speciation calculations indicated that free metal ions (M2+) and sulfate species (MSO4 0 ) accounted for most of the dissolved load. Acid soil drainage is a secondary source of acid and heavy metals in the mine site and, therefore, an effective land reclamation program should ensure that acidity and metal mobility are reduced to environmentally sustainable levels.
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Citation: Fernández-Landero, S.; Fernández-Caliani, J.C.; Giráldez, M.I.; Morales, E.; Barba-Brioso, C.; González, I. Soil Contaminated with Hazardous Waste Materials at Rio Tinto Mine (Spain) Is a Persistent Secondary Source of Acid and Heavy Metals to the Environment. Minerals 2023,13, 456. https://doi.org/ 10.3390/min13040456 Academic Editors: Saglara S. Mandzhieva and Maria Economou-Eliopoulos Received: 7 February 2023 Revised: 20 March 2023 Accepted: 21 March 2023 Published: 23 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article Soil Contaminated with Hazardous Waste Materials at Rio Tinto Mine (Spain) Is a Persistent Secondary Source of Acid and Heavy Metals to the Environment Sandra Fernández-Landero 1, Juan Carlos Fernández-Caliani 1,* , María Inmaculada Giráldez 2, Emilio Morales 2, Cinta Barba-Brioso 3and Isabel González 3 1Department Earth Sciences, University of Huelva, 21071 Huelva, Spain; [email protected] 2Department Chemistry, University of Huelva, 21071 Huelva, Spain; [email protected] (M.I.G.); [email protected] (E.M.) 3Department Crystallography, Mineralogy and Agricultural Chemistry, University of Seville, 41071 Seville, Spain; [email protected] (C.B.-B.); [email protected] (I.G.) *Correspondence: [email protected] Abstract: Mineralogical analysis and laboratory-based leaching tests coupled with speciation modeling were undertaken to quantify the potential for short-term acid generation and the release of trace elements from soils heavily contaminated with mine waste at Rio Tinto. Three different waste materials were considered as case studies: roasted pyrite, copper slags, and leached sulfide ores. The results showed elevated values of net acid generation (up to 663 mmol H + /kg), the major pools being potential sulfidic acidity and acidity retained in jarosite. Remarkable contents of As and toxic heavy metals were found especially in the slag-contaminated soil. Copper, Zn, and Pb were the most abundant metals in the acid leach solutions resulting from mine soil-water interaction, with peak values of 55.6 mg L −1 , 2.77 mg L −1 , and 2.62 mg L −1 , respectively. Despite the high total contents of trace elements occurring in soil, the mobile fraction was limited to maximum release values of 12.60% for Cd and 10.27% for Cu, according to the test leaching. Speciation calculations indicated that free metal ions (M 2+ ) and sulfate species (MSO 40 ) accounted for most of the dissolved load. Acid soil drainage is a secondary source of acid and heavy metals in the mine site and, therefore, an effective land reclamation program should ensure that acidity and metal mobility are reduced to environmentally sustainable levels. Keywords: Technosol; mine wastes; soil contamination; acidity; leaching test; metal release; speciation; ICP-OES/MS; acid soil drainage; Iberian Pyrite Belt 1. Introduction Soil is a vital and non-renewable resource that is increasingly under pressure from a range of human activities, with mining being one of the leading anthropogenic forces driving environmental degradation. Acid generation and metal release from the oxidative dissolution of sulfide minerals in mine wastes is a well-documented issue in many historic mining districts [ 1 – 3 ]. Acid mine drainage (AMD) and heavy metal-bearing mineral particles arising from abandoned mine lands may pose a serious threat to human health and ecosystems [ 4 – 8 ]. Hence, a great deal of work has been undertaken to characterize the mineralogy, geochemistry, and environmental impacts of mine wastes [ 9 – 13 ]. However, there are still gaps in the research on mining-affected soil systems that need to be addressed. The pollutants enter surrounding soils by seepage and run-off from mine wastes, AMD discharges, and atmospheric fallout, resulting in a legacy of mine-related anthropogenic soils that can be classified as Technosols according to IUSS Working Group WRB [ 14 ]. Mine Technosols contaminated with waste materials may have lost their natural resilience and adaptive capacity to retain harmful contaminants and, therefore, can generate acidic Minerals 2023,13, 456. https://doi.org/10.3390/min13040456 https://www.mdpi.com/journal/minerals
Minerals 2023,13, 456 2 of 21 effluents and release sulfate and potentially toxic elements (PTEs) into surface and pore waters [ 15 ]. This can be referred to as acid soil drainage (ASD) in analogy to AMD. Soil erosion and leaching are intensive during rain events leading to the dispersion of PTEs, either bound to suspended particles or as dissolved species. Knowledge of ASD is important for understanding the mobility and environmental availability of PTEs at historically contaminated sites, and also for developing sustainable remediation options to reduce the dispersion of contaminants and their associated risks [ 16 ]. Accurate prediction of ASD is challenging but essential to minimize the environmental impact of these soils. The goal of this work was to assess the potential mobilization of acid and PTEs from mine Technosols contaminated with various types of mining and metallurgical wastes in the mining district of Rio Tinto (Spain). These drastically disturbed mine soils have been subjected to chemical loading for over two centuries in the absence of legal and regulatory frameworks. The sources and relative contributions of acid and PTEs from the mine Technosols are currently not well understood. Thus, the specific objectives of the study were: (1) to quantify the existing acidity and to predict the acid-forming potential from sulfide oxidation, and (2) to determine the element concentrations leached from the mine soils and then to evaluate the mobility of PTEs and their contribution for water pollution. 2. Site Description and Historical Background Rio Tinto mine, located in Huelva province, Spain (Figure 1), is an internationally renowned mine not only for the giant size of its ore body but also because of its long history of mining and extractive metallurgy, dating back to pre-Roman times. It is the largest of the volcanogenic massive sulfide (VMS) deposits of the Iberian Pyrite Belt (IPB), with over 500 million tons of VMS ores averaging 45 wt.% S, 40 wt.% Fe, 0.8 wt.% Cu, 2.1 wt.% Zn, 0.8 wt.% Pb, 0.5 g/t Au, and 26 g/t Ag [ 17 ], and about 2 billion tons of low-grade stockwork mineralization including zones of economic copper ore (186 million tons at 0.38 wt.% Cu according to Noble [ 18 ]) that are currently being mined at Cerro Colorado open pit. Previous research on the geology and metallogeny of the Rio Tinto VMS deposit has been described in detail recently ([19] and references therein). Minerals2023,13,x.https://doi.org/10.3390/xxxxxwww.mdpi.com/journal/minerals Figure1.LocationmapoftheRioTintominingarea(viewfinderSIGPAC),andrepresentativephotographsoftheextensiveminewastes:(a)roastedpyritepiles;(b)slagdumps;and(c)heapleaching grounds(photoA.Delgado). Figure2.Satelliteview(GoogleEarth)oftheRioTintocoppermineshowing:themining-affected area(a);thelocationofthesamplingsitesatPeñadeHierro(b)andZarandas-LaNaya(c);andthe methodfortakingsoilsampleswithhandauger(d). Figure 1. Location map of the Rio Tinto mining area (viewfinder SIGPAC), and representative photographs of the extensive mine wastes: ( a ) roasted pyrite piles; ( b ) slag dumps; and ( c ) heap leaching grounds (photo A. Delgado).
Minerals 2023,13, 456 3 of 21 The Rio Tinto mining area is also an example of large-scale environmental degradation caused by long-lasting mining, ore processing, and smelting activities. It is generally accepted that Rio Tinto was extensively mined since at least the Late Bronze Age [ 20 ], being a major source of copper, gold, and silver in the Roman era, as evidenced by a large number of ancient slags widespread in the area (about 6 million tons according to Rothenberg and García-Palomero [21]). Modern mining started in the middle of the 19th century with the extraction of cupreous pyrites, and major technological changes took place involving the smelting of high-grade ores in blast furnaces and converters. Slags produced during smelting were accumulated in significant amounts over time. Further details of the smelting works are given in Salkield [ 22 ]. The low-grade copper ores that were deemed unsuitable for smelting were converted into water-soluble sulfates by open-air roasting in heaps locally known as teleras. The roasted mineral (i.e., pyrite ash) was washed with acidic water to leach out the soluble metal-sulfate salts, leaving a solid residue enriched in ferric iron (red waste), while the leach liquors were conducted into cementation tanks where dissolved copper was precipitated by iron scrap chips. The red wastes resulting from that ancient process can be found today at the Planes and Peña de Hierro sites (Figure 1a). In 1873 the British Rio Tinto Company (RTC) was formed to operate the mines and the scale of the operations was dramatically increased. The ore body was extensively worked from several open-pit and underground mines, focusing on copper-rich ores and massive pyrite for the manufacture of sulfuric acid. During the period of RTC operations (1873–1954), the total output of pyrite ores was almost 110 million tons [ 22 ]. The pyrite concentrates were transported by railway to Huelva port causing adverse impacts on the local soil environment due to hazardous cargo spills [ 23 ]. Pyrite smelting operated until 1970 to melt massive sulfide concentrates to produce blister copper, and at the same time to recover the excess sulfur in the elemental form. The smelting operations produced a vast volume of slag (at least 10 million tons with an average grade of 0.44 wt.% Cu according to Lottermoser [ 24 ]) placed along the banks of the Tinto River, in the industrial complex of Zarandas (Figure 1b). Heap leaching was another widely used extraction method for low-grade ores since the early 20th century. An average of about 2000 tons of ore per day was delivered from the mines to the heaps by railway wagons, with La Naya becoming the main center of operations (Figure 1c). In more recent times, high-grade copper stockwork mineralization and gold-bearing gossan, which overlies the VMS deposit, were exploited by opencast mining. The Rio Tinto mine was last operated in 2001 and restarted operations in 2015 owing to the rise in the copper price. The current open pit mining operation is focused on the Cerro Colorado deposit, including Filón Sur and Filón Norte orebodies. Extensive opencast workings and smelting activities as well as the lack of environmental protection have resulted in land contamination arising from past waste disposal practices. The natural soil was irreversibly damaged or transformed into degraded land unable to support plant life over a bare area of about 1000 ha (Figure 2). Huge quantities of mine and metallurgical wastes, including sulfide-rich waste dumps, ore stockpiles, slag deposits, roasted pyrite residues, heap leaching grounds, and tailings impoundments were left without any environmental control. These hazardous materials have the potential for releasing a variety of dissolved metals and metalloids that are usually present in the ores as minor or trace elements, such as Cu, Pb, Zn, As, Cd, Sb, and Tl, thus causing serious environmental pollution to the surrounding soils and vegetation [25–30].
Minerals 2023,13, 456 4 of 21 Minerals2023,13,x.https://doi.org/10.3390/xxxxxwww.mdpi.com/journal/minerals Figure1.LocationmapoftheRioTintominingarea(viewfinderSIGPAC),andrepresentativephotographsoftheextensiveminewastes:(a)roastedpyritepiles;(b)slagdumps;and(c)heapleaching grounds(photoA.Delgado). Figure2.Satelliteview(GoogleEarth)oftheRioTintocoppermineshowing:themining-affected area(a);thelocationofthesamplingsitesatPeñadeHierro(b)andZarandas-LaNaya(c);andthe methodfortakingsoilsampleswithhandauger(d). Figure 2. Satellite view (Google Earth) of the Rio Tinto copper mine showing: the mining-affected area ( a ); the location of the sampling sites at Peña de Hierro ( b ) and Zarandas-La Naya ( c ); and the method for taking soil samples with hand auger (d). 3. Materials and Methods 3.1. Sample Selection and Preparation Three different Technosols containing abundant artifacts in the form of mine waste particles were selected for this study: (1) Technosol contaminated with roasted pyrite wastes from the Peña de Hierro mine site (hereafter referred to as PH; Figure 2b); (2) Technosol contaminated with copper slags from the old smelter Fundición Piritas at Zarandas (hereafter referred to as ZA; Figure 2c); and (3) Technosol contaminated with leached sulfide ores from the leach heap grounds of La Naya (hereafter referred to as LN; Figure 2c). These areas have experienced considerable environmental damage and were selected on the basis of being representative of mine-related anthropogenic soils in the mining district of Rio Tinto . Within each selected area, soil samples were collected with an Edelman hand auger from the surface layer (0–20 cm) at three randomly selected sites and stored in airtight polyethylene bags. At each sampling site, a composite sample of about 10 kg was obtained by bulking five topsoil samples taken in crossing directions and spaced approximately 1.5 m around the central point (Figure 2d). The composite samples were thoroughly mixed to ensure optimal homogenization, dried at room temperature (20 ± 1 ◦ C), gently disaggregated, and sieved to 2 mm. A portion of the sieved sample was ground in an agate mortar and passed through a 63 µm sieve for analysis. 3.2. Soil Characterization Methods Soil reaction (pH), redox potential (Eh), and electrical conductivity (EC) were determined by stirring 10 g of soil in 25 mL of deionized water, after shaking for 5 min, and left to stand for 30 min. Soil pH was also determined in 1 M KCl suspension at the soil-to-solution ratio of 1:2.5 (m/v), and in a suspension of soil after digestion by 30% hydrogen peroxide (H 2 O 2 ) in a 1:20 (m/v) soil-to-solution ratio [ 31 ]. The reaction was allowed to continue until the bubbling stopped (about 6 h after the addition of H 2 O 2 ). The pH values in water
Minerals 2023,13, 456 5 of 21 (pH H2O ), KCl (pH KCl ), and H 2 O 2 (pH ox ) suspensions were measured with a digital pH meter properly calibrated with two buffer solutions at pH 4 and 7. All measurements were performed in triplicate and averaged. Mineralogical analysis of the bulk sample was performed by X-ray diffraction (XRD) with a BRUKER AXS D8-Advance powder diffractometer (CITIUS, University of Seville) using monochromatic Cu-K α radiation. The instrument was operated under standard conditions: 40 kV voltage and 30 mA current in the angular range of 3–70 ◦ 2 θ with a step size of 0.015 ◦ and a scan speed of 0.1 s per step. Intensity factors weighting integrated peak area values of diagnostic reflections in combination with the 100% approach [ 32 ] were applied to obtain semiquantitative mineral abundances. To assist in the identification of metal-bearing accessory minerals and poorly crystallized phases, the samples were coated with a thin layer of sputtered carbon and examined by field-emission scanning electron microscopy (FESEM) with a JEOL JSM-IT500HR instrument (University of Huelva) fitted with an energy dispersive X-ray spectroscopy (EDS) detector (Oxford Instruments). The samples were observed in both secondary electron (SE) and backscattered electron (BSE) modes. The working distance was set at 10 mm, accelerating voltage at 20 kV, and probe current at 2.5 nA. Quantitative analysis of the elemental composition of selected phases was conducted on polished sections by electron probe microanalysis (EPMA) using a JEOL JXA-8200 SuperProbe (University of Huelva) equipped with four wavelength dispersive X-ray spectrometers. The instrumental operating conditions were accelerating voltage at 20 kV, beam current at 20 nA, and beam diameter up to 5 µ m allowing point-by-point element determination. A set of synthetic materials and well-characterized minerals was used as standards for calibration. The composite bulk soil samples were analyzed for the determination of the total contents of PTEs of environmental significance (As, Cd, Co, Cr, Cu, Ni, Pb, Sb, Tl, and Zn). First, a 0.25 g sample was digested with four acids beginning with HF, followed by a mixture of HNO 3 and HClO 4 , and then heated in several ramping and holding cycles. After incipient dryness was attained, samples were brought back into the solution using aqua regia. The analysis was conducted by inductively coupled plasma optical emission spectrometry (ICP-OES) with an Agilent 735 instrument at Activation Laboratories (Ancaster, ON, Canada), which is accredited to ISO 9001 and ISO/IEC 17,025 standards. ICP-OES has a proven history as an appropriate tool to assess metal contamination [33]. Quality control of the analytical method included the use of reagent blanks, replicate analysis of two samples, and a range of certified reference materials (OREAS 13b, OREAS 98, OREAS 101b). Analysis of the reference materials yields values that deviated from the certified contents by less than 10% (relative standard deviation, RSD). Reproducibility of the analytical data was better than 2% RSD for most of the elements reported and always better than 5% RSD. 3.3. Static Test Methods Acid-forming potential (net acidity) was quantified for acid soil drainage prediction using an acid-base accounting (ABA) approach that involves determining the total acidgenerating capacity and the acid-neutralizing capacity of the soils. In the context of acid sulfate soils, the total acid-generating capacity comprises the potential sulfidic acidity in addition to the existing acidity, which in turn includes actual (soluble plus exchangeable) and retained acidity [ 34 ]. Thus, the global ABA equation can be expressed in the following form: Net acidity = Potential sulfidic acidity + actual acidity + retained acidity − acid neutralizing capacity. Potential sulfidic acidity (PSA) was determined by measuring the pyrite sulfur (S pyr ) extracted with 6 M HNO 3 solution heated at 120 ◦ C for 30 min, performed on the residue remaining after sulfate extraction with hot dilute HCl acid, according to the standard test method ASTM D2492-02 optimized for Fe-rich soils. The S pyr concentration of the extract was analyzed by ICP-OES using an Agilent 5110 instrument (University of Huelva,
Minerals 2023,13, 456 6 of 21 Supplementary Table S1), and then used to calculate the potential pool of sulfidic acidity. The weight percent of S pyr was multiplied by the stoichiometric factor of 625 to give the PSA value in mmol H + /kg, based on the theoretical assumption that each mole of pyrite releases two moles of sulfuric acid [35,36]. Acidity existing in the soil as a consequence of previous sulfide oxidation, resulting either from the dissolution of readily soluble sulfates (actual acidity) or from acid adsorbed onto negative exchange sites of the soil particles (exchangeable acidity), was measured directly by titrating a 1 M KCl soil suspension with 0.1 M NaOH to a pH endpoint of 7.0 [ 37 ]. Additional existing acidity stored in sparingly soluble hydroxy-sulfate minerals, particularly jarosite (a conspicuous product of pyrite oxidation), was calculated from the concentration of sulfate sulfur (S jar ) extracted with a 0.2 M NH 4 -oxalate solution adjusted to pH 3.0 and heated at 80 ◦ C for 2 h [ 38 ]. The extract solutions were analyzed for S jar using the Agilent 5110 ICP-OES. The weight percent of NH 4 -oxalate extracted sulfur was multiplied by the stoichiometric factor of 468 to convert the S jar values to equivalent acidity units (mmol H + /kg) on the basis that one mole of sulfur in jarosite releases 1.5 moles of acidity. Alternatively, a direct approach to give an indication of acid-forming potential was made for measuring net acid generation (NAG) using an H 2 O 2 -based static test. For the NAG test, 2 g of dry soil was treated with 40 mL of 30% H 2 O 2 to accelerate the oxidation of pyrite, followed by measurement of the pH of the extract solution and titration with 0.1 M NaOH until pH 7 was reached, thus allowing the product to react with the acid-neutralizing minerals, if any. 3.4. Leaching Test The leaching test was performed according to European Standard EN-12457-4 [ 39 ] in order to determine the element concentrations leached from the mine Technosols, and then to assess the mobility of PTEs and their potential impact on water resources. Following this test procedure (Figure 3), a dry mass of soil was placed in contact with distilled water under a liquid-to-solid (L/S) ratio of 10 L kg −1 , and kept under agitation for 24 h. The solid residue was separated by filtration using a 0.45 µ m membrane filter, and the PTE concentrations (As, Cd, Cr, Cu, Ni, Pb, and Zn) in the leachates were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent 7700 instrument (University of Huelva, Supplementary Table S1). Multi-element standard-2A Agilent solutions were used for external calibration, and 10 µ g L −1 of Ge, Rh, Sc, and Tb were used as internal standards. The accuracy and reproducibility of the analytical data were better than 10% RSD for most analyzed elements. Minerals2023,13,x 2of5 Figure3.Simplifiedflowdiagramofthestaticleachingtest. Figure4.BSEimagesofpolishedsectionsshowingindetailsometexturalandcompositionalfeaturesoftheTechnosols.(a)Aggregatesofhematite(Hem)withinclusionsofquartz(Qz)(sample PH-2);(b)Pb-richbarite(Pb-Brt)occurringascrackfillingsinbarite(samplePH-2);(c)slagartifact (sampleZA-1)composedoflath-shapedfayalite(Fa)anddisseminatedmagnetite(Mag);(d)Copper nanoparticle(CuNP)embeddedinthefine-grainedmatrixoftheslag;(e)quartz(Qz)grainscoated byPb-As-bearingjarosite(Jrs)rims(sampleLN-3);(f)Barite(Brt)crystalsenclosedbyanglesite (Ang)overgrowths(sampleLN-3). Figure 3. Simplified flow diagram of the static leaching test. The eluate solution recovered from the leaching test was also analyzed for sulfate, nitrate, and chloride anions by ion chromatography with a Metrohm 883 Basic IC plus instrument (University of Huelva), and the leaching conditions were recorded in terms of pH, electrical conductivity and redox potential (Eh) values.
Minerals 2023,13, 456 7 of 21 4. Results and Discussion 4.1. Soil Components with Emphasis on Heavy Metal-Bearing Particles Combined results from XRD and FESEM-EDS analysis of the bulk fraction (<2 mm) revealed that the mine soils impacted by waste materials have a contrasting mineral composition (Table 1). The chemical composition of randomly selected heavy metal-bearing particles obtained from EPMA is also listed in Table 1. The soil contaminated with roasted pyrite wastes (PH) is mineralogically composed of well-crystalized iron oxides (hematite), jarosite, and quartz, with subordinate amounts of phyllosilicates (mica and kaolinite), feldspars, and barite. Hematite is responsible for the distinctive weak red color (Munsell 10R 4/2) of this Technosol. The essential mineralogy is in line with that described in the literature for historic pyrite ash wastes of the Rio Tinto mining area [ 40 , 41 ]. The XRD patterns displayed a high background signal indicating the occurrence of amorphous or poorly ordered material in the PH samples. Moreover, the FESEM-EDS examination (Figure 4) allowed the identification of anglesite, often forming overgrowths on barite crystals, and confirmed the presence of abundant iron oxide aggregates with a porous texture (Figure 4a). EPMA analysis of representative hematite particles (Table 1) showed that they are almost chemically pure with only trace amounts (less than 0.5 wt.%) of Cu, As, Sb and S. The EPMA study also revealed Pb-rich barite veins filling pre-existing cracks within barite (Figure 4b). Table 1. Mineral composition of the Technosols and EPMA analysis of selected mineral particles. Technosol Peña de Hierro (PH) Zarandas (ZA) La Naya (LN) Major minerals Hem + Jrs + Qz Mca + Chl/Kln + Qz ±Jrs ±Py Qz + Py + Mca ±Jrs Accessories Mca + Fsp + Brt + Ang Fsp + Brt ±Hem ±Jrs ±Gp Fsp + Brt + Ang ±Jrs ±Hem ±Gp Oxides (wt.%) Hem1Hem2 Hem 3Ang1 Ang 2Jarosite-like minerals Ang Jarosite-like minerals Pb-rich phosphate Fe2O399.74 99.17 97.96 45.78 27.94 42.38 53.73 38.02 37.45 7.21 4.34 SO30.05 0.03 0.03 25.60 27.05 17.93 14.01 23.44 21.97 26.17 23.66 22.88 5.29 4.48 As2O50.04 4.00 9.88 2.66 4.97 0.70 3.14 1.67 2.34 Sb2O50.04 0.05 0.02 0.05 0.05 0.15 3.55 0.20 0.11 0.34 0.44 0.07 0.13 P2O512.13 11.67 PbO 73.97 72.19 1.40 12.55 8.30 1.98 73.11 14.88 12.31 32.60 31.69 CuO 0.05 0.05 0.03 0.24 0.24 0.68 0.21 0.45 0.20 0.13 0.13 ZnO 0.06 0.06 K2O 1.24 0.53 1.19 1.40 1.54 3.68 0.39 0.41 Mineral abbreviations: Ang (anglesite); Brt (barite); Chl (chlorite); Fsp (feldspar); Gp (gypsum); Hem (hematite); Jrs (jarosite-like minerals); Kln (kaolinite); Mca (mica); Py (pyrite); Qz (quartz). The slag-contaminated soil samples from Zarandas (ZA) are made up of quartz, phyllosilicates (mica, chlorite, and/or kaolinite), and feldspars along with variable quantities of jarosite (up to 25 wt.%) and accessory hematite, barite, and gypsum. This Technosol contains numerous copper slag fragments consisting of feathery crystals of fayalite with the interstices filled with glassy material (Figure 4c), in which tiny crystals of magnetite and more rarely matte particles and spherical copper nanoparticles are disseminated, as detected by FESEM-BSE-EDS (Figure 4d). The fayalitic slag often had a vesicular texture with jarosite-lined voids. In addition, the EDS spectra indicated the occurrence of Asbearing iron mineral particles attributable to amorphous ferric arsenate or scorodite, and complex phase mixtures containing S-Fe-Pb-As-(Cu)-(Sb). The results of EPMA analyses (Table 1) of selected micro-areas suggest that the chemical composition of such phases may be compatible with jarosite-like minerals (Pb-jarosite and Pb-As-jarosite) that appear mixed with iron oxides. Interestingly, the sample ZA-3 showed a strikingly high content of pyrite (20–25 wt.%), most likely sourced from sulfide ores used for manufacturing sulfuric acid in a plant adjacent to the smelter site. The presence of readily water-soluble sulfate salts in the slag-contaminated soil (Figure 5a) and the widespread occurrence of efflorescent sulfate minerals at seepage points of the slag dumps [ 24 ], support the idea that these metallurgical wastes are chemically active long after the mining period.
Minerals 2023,13, 456 8 of 21 Minerals2023,13,x 2of5 Figure3.Simplifiedflowdiagramofthestaticleachingtest. Figure4.BSEimagesofpolishedsectionsshowingindetailsometexturalandcompositionalfeaturesoftheTechnosols.(a)Aggregatesofhematite(Hem)withinclusionsofquartz(Qz)(sample PH-2);(b)Pb-richbarite(Pb-Brt)occurringascrackfillingsinbarite(samplePH-2);(c)slagartifact (sampleZA-1)composedoflath-shapedfayalite(Fa)anddisseminatedmagnetite(Mag);(d)Copper nanoparticle(CuNP)embeddedinthefine-grainedmatrixoftheslag;(e)quartz(Qz)grainscoated byPb-As-bearingjarosite(Jrs)rims(sampleLN-3);(f)Barite(Brt)crystalsenclosedbyanglesite (Ang)overgrowths(sampleLN-3). Figure 4. BSE images of polished sections showing in detail some textural and compositional features of the Technosols. ( a ) Aggregates of hematite (Hem) with inclusions of quartz (Qz) (sample PH-2 ); ( b ) Pb-rich barite (Pb-Brt) occurring as crack fillings in barite (sample PH-2); ( c ) slag artifact (sample ZA-1) composed of lath-shaped fayalite (Fa) and disseminated magnetite (Mag); ( d ) Copper nanoparticle (CuNP) embedded in the fine-grained matrix of the slag; ( e ) quartz (Qz) grains coated by Pb-As-bearing jarosite (Jrs) rims (sample LN-3); ( f ) Barite (Brt) crystals enclosed by anglesite (Ang) overgrowths (sample LN-3). Minerals2023,13,x 2of5 Figure3.Simplifiedflowdiagramofthestaticleachingtest. Figure4.BSEimagesofpolishedsectionsshowingindetailsometexturalandcompositionalfeaturesoftheTechnosols.(a)Aggregatesofhematite(Hem)withinclusionsofquartz(Qz)(sample PH-2);(b)Pb-richbarite(Pb-Brt)occurringascrackfillingsinbarite(samplePH-2);(c)slagartifact (sampleZA-1)composedoflath-shapedfayalite(Fa)anddisseminatedmagnetite(Mag);(d)Copper nanoparticle(CuNP)embeddedinthefine-grainedmatrixoftheslag;(e)quartz(Qz)grainscoated byPb-As-bearingjarosite(Jrs)rims(sampleLN-3);(f)Barite(Brt)crystalsenclosedbyanglesite (Ang)overgrowths(sampleLN-3). Figure 5. Field pictures of the Technosol ZA showing: ( a ) sulfate precipitate occurring as greenish blue efflorescence (white arrow) on the slag-contaminated soil; and ( b ) grass plants growing in the vicinity of the old smelter site on patches of soil with a moderately acid reaction. The Technosol contaminated with leached ores from the heap leaching grounds of La Naya (LN) is comprised of mixtures of quartz, pyrite, mica, and jarosite-group minerals, with minor hematite, feldspars, barite, and anglesite. The FESEM-EDS study showed actively oxidizing crystals of pyrite with pitted surfaces. Jarosite was commonly found forming aggregates of euhedral pseudo-cubic crystals, but also appeared as coatings and rims covering grains of quartz or feldspars (Figure 4e). BSE images on polished sections revealed the occurrence of barite crystals enclosed by anglesite (Figure 4f). The jarosite-like
Minerals 2023,13, 456 9 of 21 minerals contain significant amounts of Pb and As, and minor levels of Sb and Cu, as determined by EPMA analysis (Table 1), which also allowed us to infer the presence of Pb-rich phosphate particles with additional amounts of Fe, S, and As. Therefore, the mining-affected soils are variable in mineral composition. Besides quartz, feldspars, and phyllosilicates, which are remnants of the original soil, a variety of heavy metal-bearing particles have been inferred as both primary contaminants of mine wastes (pyrite, hematite, copper slags) and secondary products of oxidation, dissolution, and precipitation reactions (jarosite-group minerals, amorphous iron oxyhydroxides, scorodite, anglesite, and gypsum). Overall, this mineral assemblage is consistent with the mineralogy of the alluvium contaminated by metal mining in the Rio Tinto area [16]. 4.2. Total Acid Generating Capacity and Net Acidity The soils of the areas under study were mostly extremely acidic in reaction when suspended in water, with pH H2O values below 3.5, although the slag-contaminated soil samples (Technosol ZA) spanned a range of values between 3.0 and 5.4 (Table 2). A few patches of grass were seen at the site growing on the least acid soil (Figure 5b). They are all well aerated with Eh values consistently over +500 mV reflecting strongly oxidizing conditions. The soil electrical conductivity was always lower than 2 mS cm −1 , which is indicative of a relatively low soluble salt concentration in the soil solution. Similar results were obtained when the soil pH was measured in a 1M KCl solution (pH KCl = 2.4–5.3), with the exception of the sample ZA-2 whose average pH KCl value was 1.2 units lower than that measured in water. Nonetheless, the pH measured after complete oxidation of the sample with H 2 O 2 varied noticeably among the different mine soils, depending on the relative abundance of pyrite. The lowest pH ox levels (1.7–1.9) were found in the Technosol LN, where pyrite is a ubiquitous mineral, revealing a latent acid-generating capacity after sulfide oxidation. The Technosol PH showed pH ox values close to those measured in water and KCl solution due to the lack or scarcity of sulfide minerals, while the pH ox of the Technosol ZA varied between around 2.0 (samples ZA-1 and ZA-2) and 5.1 (sample ZA-3). However, it must be pointed out that the sample ZA-3 had a pH ox of 3.0 after 2 min of contact with H 2 O 2 , indicating a likelihood of potential sulfidic acidity. Thus, all samples are potentially acid generating as the pH values after H 2 O 2 -reaction were less than 4.0 in the short term. Table 2. Soil pH values, sulfur speciation, and major pools of acidity. Sample pH Sulfur Speciation (wt.%) Acidity (mmol H+/kg) H2O KCl H2O2Stotal Spyr Sjar PSA TAA RA TAG NAG PH-1 2.96 3.00 2.83 2.60 0.18 1.71 111 90 800 1001 144 PH-2 2.51 2.57 2.45 2.75 0.15 0.78 93 27 367 487 104 PH-3 2.14 2.58 2.41 2.63 0.16 1.07 97 36 503 636 93 ZA-1 2.96 2.75 1.86 2.53 0.16 2.44 102 19 1146 1267 442 ZA-2 4.56 3.36 2.20 0.55 0.11 0.24 66 10 110 186 632 ZA-3 5.35 5.34 5.08 6.80 3.54 1.78 2214 7 835 3056 240 LN-1 2.68 2.49 1.85 2.97 2.00 0.32 1248 26 151 1425 663 LN-2 2.51 2.44 1.65 3.15 1.42 1.66 890 30 780 1700 609 LN-3 3.02 3.01 1.78 1.67 1.26 0.33 786 31 153 971 463 PSA (Potential Sulfidic Acidity); TAA (Titratable Actual Acidity); RA (Retained Acidity); TAG (Total Acid Generation); NAG (Net Acid Generation). Even though pH is a primary index for soil acidity, it gives no indication of the total acid-generating capacity of the Technosols. The measurement of total sulfur (S total ) is an option widely used for estimating the maximum potential acidity from sulfide sources [ 34 ]. According to the S total content measured in soil (Table 2), the potential acid release should be in the range of 340–4250 mmol H + /kg. But this conservative approach overestimates the potential acid risk of the studied soils, in which a fraction of sulfur is in the form of jarosite and non-acid-producing sulfate minerals, such as gypsum and barite, and additionally,
Minerals 2023,13, 456 16 of 21 not have a significant correlative effect on the amount of extractable PTEs. This is in good agreement with the results obtained from mine soil samples elsewhere in the IPB [25,59]. Minerals2023,13,x 5of5 Figure9.Ficklindiagramplottingthesumofmetalconcentrationsleachedfromthesoilsamples againstthepHoftheleachates. Tofurthercomparetheleachtestresults,thelinechartofFigure8showstheproportionofthetotalmetalpoolthatwasreleasedfromthesoilandmaybeavailableforplant uptakeandleachedbypercolation.NotwithstandingthehightotalcontentsofPTEsoccurringinsoil,theaveragevaluesofthewater-extractableproportion(Figure10)were relativelylow(lessthan5.5%),suggestingthatmetalmobilitywaslimitedforallelements, withmaximumreleasevaluesof12.60%forCdand10.27%forCu.Itisimportanttonote thatsomeofthesepercentagescorrespond,nevertheless,toelevatedcontentsofPTEs, whichisanissueofconcern.TheextremelyhighdegreeofcontaminationoftheTechnosolsdidnothaveasignificantcorrelativeeffectontheamountofextractablePTEs.This isingoodagreementwiththeresultsobtainedfromminesoilsampleselsewhereinthe IPB[25,59]. Figure10.AveragevaluesofthetraceelementfractionextractedfromtheTechnosolsbyapplying thestandardEN-12457-4leachingtest. Figure 10. Average values of the trace element fraction extracted from the Technosols by applying the standard EN-12457-4 leaching test. In most samples PTEs showed the following order of decreasing mobility: Cu > Zn > Cd > Ni > Pb > Cr > As, although Pb was more efficiently leached from the Technosol LN. Accordingly, Cd, Cu, and Zn are the most labile PTEs in mine soils, as they are likely stored in soluble sulfate salts. The fraction of As extracted in the course of the leach test was found to be practically negligible (less than 0.1%), despite the high level of contamination occurring in the Technosols. The low leachability of As with water is consistent with the fact that most of this contaminant could be accommodated by arsenate-for-sulfate substitution in the crystal structure of jarosite [ 60 , 61 ], which is sparingly soluble in water under the acidic and oxidizing conditions prevailing in the mine sites. Besides, the solubility of As could have been limited by adsorption on, or coprecipitation with, iron oxyhydroxide minerals [ 62 ] and precipitation of arsenic minerals like scorodite [ 63 ], as inferred from the mineralogical analysis. Similarly, the low concentration of Pb in the leach solutions indicated that this heavy metal resides to a large extent in the fixed pool, tightly bonded to soil constituents such as jarosite-like minerals, anglesite, and Pb-bearing phosphate phases. From these findings, it can be arguably claimed that the environmental availability of As and Pb in the short term is very low because, in general, they were not leached abundantly from the mine soils. According to the aqueous speciation calculations performed using the PHREEQC code [ 64 ] with the wateq4f thermodynamic database, the most labile PTEs in soil occurred in the leach solutions in the form of a variety of ionic species (Table 5). Free metal ion (M 2+ ) was the largely dominant form over the pH range of 2.5 to 5.7, with median values accounting for 100% of dissolved Cd, around 85% of dissolved Cu, Ni, and Zn, and about 70% of dissolved Pb. Sulfate species (MSO 40 ) accounted for most of the remaining species, comprising between 8% and 29% of the median dissolved load of Ni, Cu, Zn, and Pb. Other inorganic complexes such as chloride, nitrate, and hydroxyl ions were not predicted to be significant complexing ligands, because their concentrations in the acid solutions leached from the Technosols were not high enough to be competitive with sulfate ions for PTE speciation. Arsenic release from the soil was present as oxyanion species in the form of arsenate, mainly H 2 AsO 4- and, to a lesser extent, H 3 AsO 4 . Similar speciation patterns have been reported in previous studies on the modeling of PTE behavior in acidic drainage waters [56,65].
Minerals 2023,13, 456 17 of 21 Table 5. Distribution of aqueous species of trace elements of environmental significance in the acid leach solutions indicated as a percent of the total dissolved species. Element Species (%) PH-1 PH-2 PH-3 ZA-1 ZA-2 ZA-3 LN-1 LN-2 LN-3 Median As H2AsO4−93.52 65.01 77.15 91.04 100 95.23 86.82 90.53 95.09 91.04 HAsO42−4.77 H3AsO46.48 34.99 22.85 8.96 13.18 9.47 4.91 8.96 Cd Cd2+ 100 100 100 100 100 100 100 100 100 100 Cu Cu2+ 90.29 75.31 82.04 73.28 96.26 93.59 82.76 86.57 89.49 86.57 CuSO49.68 24.69 17.95 26.71 3.61 5.58 17.21 13.43 10.51 13.43 CuOH+0.13 0.28 Cu(OH)20.22 CuCl+0.03 0.01 0.01 0.02 0.03 0.01 Cu2(OH)22+ 0.32 Ni Ni2+ 91.32 83.42 74.71 96.58 94.86 83.68 87.4 83.68 NiSO48.68 16.58 25.29 3.42 5.14 16.32 12.6 8.68 Pb Pb2+ 77.33 51.49 62.29 48.79 90.44 86.15 62.89 69.81 74.66 69.81 PbSO421.87 47.53 36.87 50.18 9.56 13.85 36.4 29.84 25.06 29.84 PbCl+0.44 0.06 0.09 0.34 0.06 0.17 0.06 Pb(SO4)22−0.36 0.91 0.75 1.03 0.37 0.28 0.12 0.36 Zn Zn2+ 88.78 72.13 79.45 69.91 95.86 93.52 80.5 84.72 88.09 84.72 ZnSO410.9 27.02 19.9 29.12 4.11 6.38 19.17 15.05 11.82 15.05 ZnOH+0.01 0.02 Zn(SO4)22−0.29 0.84 0.64 0.97 0.01 0.05 0.31 0.22 0.09 0.29 ZnCl+0.03 0.01 0.01 0.01 0.02 0.03 0.01 The leach solutions from the Technosol ZA displayed a more complex speciation pattern. The activity of sulfate species decreased noticeably with increasing pH from 3.2 (sample ZA-1) to 5.7 (sample ZA-3), while the contribution of hydroxyl complexes of Cu and Zn in the aqueous solution was shown to increase, although to a still relatively very low level. Therefore, the distribution of major species seems to be related to the solution pH. The saturation index (SI) values for solid phases were computed with PHREEQC by comparing measured solution activity, expressed as an ion activity product (IAP) with the theoretical solubility product constant (Ksp), as follows: SI =logIAP Ksp The geochemical modeling approach of the mine soil-water interaction indicated a slight supersaturation with respect to anglesite (SI = 0.16–0.21) in the Technosol LN, which is consistent with the observed mineralogy, and also with copper-rich secondary minerals, like brochantite (SI = 2.15) and antlerite (SI = 1.19), that may form from evaporation at the site ZA-3. The extracts were undersaturated (SI < 0) with all the Cd- and Zn-bearing minerals in the database, so it is unlikely that such phases are present as precipitates under the prevailing conditions. 5. Conclusions This paper has highlighted the importance of determining the various forms of acidity and the potential for the release and mobilization of PTEs at historically contaminated mine sites in order to assess the risk of water contamination by leaching through the soil. Specifically, it has been shown that soils contaminated with hazardous mine waste at Rio Tinto have become secondary source areas of acidic effluents and harmful contaminants to the surrounding environment. These soils have a remarkable acid-production capacity due to sulfide oxidation and an existing acidity, albeit often neglected, that may be slowly released by hydrolysis of jarosite-like minerals. It was also found that they contain anomalous
Minerals 2023,13, 456 18 of 21 contents of As, Pb, Cu, and Zn, mostly linked to heavy metal-bearing particles inherited from the wasteland, but also structurally bound in soil-forming minerals thus limiting their environmental significance. Notwithstanding, there still is a persistent reservoir of readily leachable Cu, Cd, Zn, and Pb, in the form of free metal ions and sulfate species that are being flushed into the Tinto River system. The extreme acidity and metal release will likely be maintained over time due to the occurrence of actively oxidizing pyrite. Ultimately, in light of these findings, an effective mine land reclamation program should not be limited to abandoned mine wastes but should also ensure that acidity and metal mobility in the mining-affected soils are reduced to environmentally sustainable levels. Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/min13040456/s1, Table S1: ICP-OES and ICP-MS operating conditions. Author Contributions: S.F.-L.: investigation; formal analysis; data curation; software; visualization; writing-original draft preparation. J.C.F.-C.: investigation; conceptualization; methodology; visualization; validation; original draft preparation; supervision; project administration; M.I.G.: investigation; formal analysis; methodology; validation; writing-review and editing; resources. E.M.: investigation; formal analysis; validation; writing-review and editing. C.B.-B.: investigation; formal analysis; validation; writing-review and editing. I.G.: investigation; formal analysis; validation; writing-review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Regional Government of Andalusia (Spain) and the European Regional Development Fund Andalusia 2014–2020 through Project P-18-TP-3503, in collaboration with DSM Soluciones Medioambientales. Data Availability Statement: The authors confirm that the data supporting the findings of this study are available within the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations ABA: Acid-Base Accounting. AMD: Acid Mine Drainage. ASD: Acid Soil Drainage. ASTM: American Standard Test Method. BSE: Back Scattered Electron. EC: Electrical Conductivity. EDS: Energy Dispersive X-ray Spectroscopy. EPMA: Electron Probe Micro Analysis. FESEM: Field-Emission Scanning Electron Microscopy. ICP-MS: Inductively Coupled Plasma Mass Spectrometry. ICP-OES: Inductively Coupled Plasma Optical Emission Spectrometry. IPB: Iberian Pyrite Belt. ISO: International Organization for Standardization. IUSS: International Union of Soil Sciences. NAG: Net Acid Generation. PLI: Pollution Load Index. PSA: Potential Sulfidic Acidity. PTE: Potentially Toxic Trace Elements. RA: Retained Acidity. RSD: Relative Standard Deviation. RTC: Rio Tinto Company. SE: Secondary Electron. SI: Saturation Index. TAA: Titratable Actual Acidity. TAG: Total Acid Generation. VMS: Volcanogenic Massive Sulfide. WRB: World Reference Base. XRD: X-ray Diffraction. References 1. Salomons, W. Environmental impact of metals derived from mining activities. J. Geochem. Explor. 1995,53, 53–56. 2. Nordstrom, D.K. Hydrogeochemical processes governing the origin, transport and fate of major and trace elements from mine wastes and mineralized rock to surface waters. Appl. Geochem. 2011,26, 1777–1791. [CrossRef] 3. Hudson-Edwards, K. Tackling mine wastes. Science 2016,352, 288–290. [CrossRef] 4. Plumlee, G.S.; Morman, S.A. Mine wastes and human health. Elements 2011,7, 399–404. [CrossRef] 5. Sánchez de la Campa, A.; De la Rosa, J.D.; Fernández-Caliani, J.C.; González-Castanedo, Y. Impact of abandoned mine waste on atmospheric respirable particulate matter in the historic mining district of Rio Tinto (Iberian Pyrite Belt). Environ. Res. 2011 ,111, 1018–1023. [CrossRef] [PubMed] 6. Castillo, S.; De la Rosa, J.; Sánchez de la Campa, A.; González-Castanedo, Y.; Fernández-Caliani, J.C.; González, I.; Romero, A. Contribution of mine wastes to atmospheric metal deposition in the surrounding area of an abandoned heavily polluted mining district (Rio Tinto mines, Spain). Sci. Total Environ. 2013,449, 363–372. [CrossRef] [PubMed] 7. Romero, A.; González, I.; Martín, J.M.; Vázquez, M.A.; Ortiz, P. Risk assessment of particle dispersion and trace element contamination from mine-waste dumps. Environ. Geochem. Health 2015,37, 273–286. [CrossRef]
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