Long-Term Sustainability of Marble Waste Sludge in Reducing Soil Acidity and Heavy Metal Release in a Contaminated Mine Technosol

Citation: Fernández-Caliani, J.C.; Giráldez, I.; Fernández-Landero, S.; Barba-Brioso, C.; Morales, E. Long-Term Sustainability of Marble Waste Sludge in Reducing Soil Acidity and Heavy Metal Release in a Contaminated Mine Technosol. Appl. Sci. 2022,12, 6998. https://doi.org/ 10.3390/app12146998 Academic Editors: Svetlana Bortnikova and Nataliya V. Yurkevich Received: 24 May 2022 Accepted: 8 July 2022 Published: 11 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 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/). applied sciences Article Long-Term Sustainability of Marble Waste Sludge in Reducing Soil Acidity and Heavy Metal Release in a Contaminated Mine Technosol Juan Carlos Fernández-Caliani 1,* , Inmaculada Giráldez 2, Sandra Fernández-Landero 1, Cinta Barba-Brioso 3 and Emilio Morales 2 1Department of Earth Sciences, University of Huelva, Campus de El Carmen, 21071 Huelva, Spain; [email protected] 2Department of Chemistry, University of Huelva, Campus de El Carmen, 21071 Huelva, Spain; [email protected] (I.G.); [email protected] (E.M.) 3Department of Crystallography, Mineralogy and Agricultural Chemistry, University of Seville, Campus de Reina Mercedes, 41071 Seville, Spain; [email protected] *Correspondence: [email protected] Abstract: A field-based experiment was set up to evaluate the effectiveness of a single application of marble waste sludge (MWS) on chemical immobilization of potentially hazardous trace elements (PHE) within the soil profile of a mine Technosol under natural assisted remediation for 12 years. Results showed that MWS amendment significantly reduced soil acidity and PHE mobility compared to unamended soil, thus improving soil health and plant growth. The amendment application had a sustained acid-neutralizing action, as soil pH remains relatively constant at between 5.8 and 6.4 throughout the entire profile (70 cm depth). In addition to diluting pollutants, the treatment triggered a redistribution of trace elements among the various operationally defined geochemical pools, shifting the PHE speciation from water-soluble forms to fractions associated with carbonates (29% Cd), metal oxides (40–48% Zn, Cd, Cu, and Ni), organic matter (22% Cu and Ni), and insoluble secondary oxidation minerals and residual phases (80–99% As, Cr, Sb, Tl, and Pb), thereby effectively limiting its potential environmental significance. MWS treatment to immobilize PHE in the contaminated mine Technosol was effective and persistent while in the untreated soil metal release is continuing over time. Keywords: waste recycling; soil amendment; acid neutralization; chemical partitioning; metal immobilization; mine soil 1. Introduction Soils contaminated with potentially hazardous trace elements (PHE) arising from mining wastes and industrial discharges pose serious threat to human health and ecosystems [ 1 , 2 ]. Management of these environmentally significant soils [ 3 ] may require the use of sustainable remediation strategies to mitigate the dispersion of contaminants and their associated risks. Conventional soil clean-up technologies [ 4 , 5 ], especially those involving surface capping and landfilling, are often financially inefficient in extensive mine lands where treatments should be affordable and self-sustaining. In situ stabilization or chemical fixation of PHE induced by the addition of immobilizing agents, such as soil amendments made of waste materials and by-products [ 6 – 8 ], is a cost-effective and eco-friendly option to reclaim abandoned mine lands following circular economy principles and modern waste management trends. These amendments can be referred as Technosols according to the World Reference Base for Soil Resources [ 9 ], because they form a new soil layer [10]. The immobilization technique involves PHE removal from soil solution either through precipitation, adsorption, ion exchange, or complexation mechanisms [ 11 ], thus reducing Appl. Sci. 2022,12, 6998. https://doi.org/10.3390/app12146998 https://www.mdpi.com/journal/applsci Appl. Sci. 2022,12, 6998 2 of 17 to some extent contaminant uptake by plants and leaching into groundwater or nearby watercourses [ 12 ]. Since contaminant solubility is linked to its mobility and bioavailability, chemical stabilization has the potential to reduce environmental risk [ 13 ], although the immobilizing effects may not last long enough and, consequently, repeated amendment additions may be needed to sustain the stabilization process. Liming is a widely used practice for reducing the mobility of PHE in contaminated soils with different degrees of success [ 14 , 15 ]. Most research on the use of lime-based waste materials and their efficiencies for PHE immobilization in soil has been conducted under laboratory conditions in short-term studies [ 11 , 12 ], as the liming effects were observed only after the first few years of treatment. These results from lab-scale experiments need to be validated by further field work to verify long-term stability of immobilized PHE in amended soils [ 16 – 18 ], where natural processes controlling PHE mobility can be monitored over long time scales under realistic conditions. Long-term field monitoring provides direct evidence of the ageing process of amendments [19]. Specifically, the waste sludge (i.e., micronized calcium carbonate) generated by marble processing plants has been proven as an effective nanomaterial for acid soil neutralization and for enhanced PHE retention capacity of soils affected by mining activities [ 20 – 23 ]. In fact, a field trial in the historic mining district of Tharsis (southwestern Spain) showed the potential of marble waste sludge (MWS) as a promising soil additive for assisting re-vegetation of legacy mine lands [ 24 ]. The most remarkable features of the MWS are alkalinity (pH = 8.8–9.2), fineness (mean particle size = 8.24 µ m), and purity (~95 wt% of calcite). Using this MWS, a single amendment of 22 cmol c of lime per kilogram of soil was applied by Fernández-Caliani and Barba-Brioso [ 24 ] to raise the pH from 3.2 to a pH level suitable for plant regrowth. The acid neutralizing effect of the soil amendment effectively reduced the concentrations of Al, Fe, sulfate and PHE, notably Cd, Cu, Pb and Zn, in the most labile and phytoavailable fraction enough to allow the settlement of spontaneous plant species (Lolium perenne, Plantago coronopus and Spergularia rubra). Even though the reported early findings showed promise, substantial uncertainty exists regarding the long-term effectiveness of PHE stabilization in soil. To address this uncertainty, a mini plot field experiment was conducted to evaluate the sustainability of a single application of MWS on chemical immobilization of PHE in the mine soil of Tharsis, after a period of 12 years post-application. The specific objectives were to ascertain: (1) the effect of acid buffering capacity of the liming waste on the soil leachate chemistry; and (2) the influence of the MWS addition on fractionation of PHE and their environmental availability over a long time span. 2. Location and Description of the Experimental Site The experimental plot was established at the historic mine site of Tharsis, in southwestern Spain (Figure 1), a world-class massive sulfide deposit hosted primarily by black shale sequences from the latest Devonian [ 25 ]. The biggest deposit is exposed in the Filón Norte open pit (Figure 1a), with total original reserves of about 90 million tons containing average grades of 46.5% S, 0.7% Cu, 2.7% Zn plus Pb, 35 g/t Ag, and 0.9 g/t Au [ 26 ]. Besides, a wide variety of PHE (As, Cd, Hg, Sb, and Tl, among others) are usually present in the ore as trace elements. Tharsis has a long history of metal extraction dating back from prehistoric (third millennium B.C.) and Roman to modern times [ 27 ]. Past mining and smelting activities have left an extensive legacy of heavily contaminated soils. The surface soil has been subjected to chemical loading for decades in the absence of legal and regulatory frameworks for mine closure and environmental remediation. In the vicinity of the mining area, the soil is adversely affected by acid mine drainage (AMD) emanating from the waste dumps, thus being unable to support plant life [ 22 , 28 , 29 ]. However, small agricultural holdings have persisted by adopting traditional farming practices that enable cultivation of food crops [ 30 ], all at risk of contamination [ 31 ]. Elevated levels of PHE in soil and vegetation can be detected up to 2–3 km downwind of the Tharsis mines [32,33]. Appl. Sci. 2022,12, 6998 3 of 17 The experimental plot (1 × 1 m) is regarded as representative of an area impacted by AMD discharges arising from the mine wastes of Filón Norte pit (Figure 1b), with an extension of about 1.5 ha. The site (UTM coordinates: X = 666,533 and Y = 4,163,175) is located 250 m above sea level and subject to a Mediterranean climate (Csa in the Köppen climate classification system) characterized by hot, dry summers and mild winters. During the experimental period (2007 to 2019), the average minimum and maximum temperatures were 5 ◦ C and 34 ◦ C, respectively, and the average annual rainfall was 389 mm, according to data recorded in the nearest weather station. 1 Figure 1. Figure 1. Location map of the study area showing a satellite view of the mining area of Tharsis ( a ), and a field picture of the field experimental site (b). 3. Materials and Methods 3.1. Soil Sampling Two undisturbed soil cores were extracted by hand augering to a depth of 70 cm from the experimental mini plot amended with MWS in 2007 [ 24 ], and from an adjacent site of contaminated soil in which no amendment was applied (control soil) for comparative purposes. The cores were obtained using an Eijkelkamp auger (100 cm in length and 30 mm in diameter) after 12 years of MWS amendment application. The two soil cores were sectioned transversely into seven 10-cm thick slices that are assumed to be representative Appl. Sci. 2022,12, 6998 4 of 17 samples of incremental depths within the soil profile (0–10, 10–20, 20–30, 30–40, 40–50, 50–60, and 60–70 cm) in both treated and control areas. 3.2. Analytical Procedures The soil samples were air-dried, gently crushed to pass through a 2–mm sieve, and mixed thoroughly. Soil texture was determined by particle-size distribution analysis using a laser diffraction analyzer (Mastersizer 2000, Malvern Panalytical, Malvern, Worcestershire, UK). Soil reaction (pH), redox potential (Eh) and electrical conductivity (EC) were determined by stirring soil in deionized water at the ratio of 1:2.5 (g mL −1 ). All measurements were performed in triplicate and averaged. Aliquots of the sieved material (<2 mm) were ground in an agate mortar until getting a fine powder (<63 µm) for analyses. Major crystalline phases were identified by X-ray powder diffraction (XRPD) using a BRUKER AXS D8-Advance diffractometer (Bruker Corp., Karlsruhe, Germany), with monochromatic CuK α radiation operated at 40 kV and 30 mA. Scans of randomly oriented bulk powders were run from 3 to 70 ◦ 2 θ with a step size of 0.02 ◦ and a counting time of 0.6 s per step. The XRPD raw data were processed with the software MacDiff 4.2.6 (from R. Petschick, Goethe-Universität Frankfurt, Frankfurt, Germany) to precisely determine the position, intensity, and area of the diffraction peaks. Semi-quantitative estimates of mineral abundance were made by empirical intensity factors weighting the integrated peak area values of diagnostic reflections [ 34 ], with absolute errors averaging less than 3% [ 35 ]. Samples were examined by environmental scanning electron microscopy (ESEM) on a JEOL JSM-IT500 instrument (JEOL, Tokyo, Japan) coupled with energy dispersive X-ray spectroscopy (EDS). Back-scattered electron (BSE) images and EDS spectra were acquired at 20 kV accelerating voltage to assist in the identification of accessory minerals of environmental concern. The soil samples were subjected to multi-acid digestion (HF-HClO 4 -HNO 3 -HCl) followed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis to determine total concentrations of As, Cd, Cr, Cu, Ni, Pb, Sb, Tl, and Zn, all which are of environmental significance. The analyses were carried out at Activation Laboratories Ltd. (Ancaster, ON, Canada), which is compliant to the ISO/IEC 17,025 standard. Reliability of the analytical procedure was evaluated by the use of reagent blanks, certified reference materials (GRX-4, GRX-6, SDC-1, DNC-1a, OREAS-45d), and replicates to check accuracy and precision of the data. The average relative standard deviation (RSD) of the analyses was between 1.6 and 11.7%. Chemical partitioning of PHE in the topsoil of both the amended and the control soil was determined using an improved Community Bureau of Reference (BCR) sequential extraction method [ 36 ]. This protocol fractionates PHE into three operationally defined pools: (F1) exchangeable and weak acid soluble fraction (extraction with 20 mL 0.11 mol L −1 acetic acid, CH 3 COOH); (F2) reducible fraction (extraction with 20 mL 0.5 mol L −1 hydroxylammonium chloride, NH 2 OH · HCl); (F3) oxidizable fraction (digestion with 5 mL 8.8 mol L −1 hydrogen peroxide, H 2 O 2 , and extraction with 25 mL 1 mol L −1 ammonium acetate, CH 3 COONH 4 ); and (F4) residual fraction (open vessel, hot plate acid digestion with 10 mL HF, 3.8 mL HNO 3 , 5 mL HClO 4 , and 1.2 mL HCl). Following each extraction step, the extract was centrifuged at 5000 rpm for 10 min, then the supernatant was filtered through a 0.45- µ m membrane filter, and stored at 4 ◦ C until analysis. The PHE concentrations in the filtered soil extract were chemically analyzed by ICP-MS using an Agilent 7900 ICP-MS instrument (Agilent Technologies, Santa Clara, CA, USA), with detection and quantification limits of 0.01 µg L−1and 0.03 µg L−1, respectively, for all the determinations. The sequential extraction procedure was repeated at least twice to ensure repeatability of results. The validity of the method was verified by carrying out analysis of a reference material (BCR-701) certified for extractable metal contents in the three steps of the modified BCR scheme [ 37 ]. The results showed a close agreement between observed and expected values for most PHE. The mean recovery was 102.6% with a standard deviation of 10.2% for PHE extracted in step 1, 102.2 ± 5.0% for those extracted in step 2, and 92.8 ± 16.0% Appl. Sci. 2022,12, 6998 5 of 17 for those extracted in step 3. Moreover, the cumulative amount of PHE released from the three steps (F1 + F2 + F3) plus residual fraction (F4) compared well with the total PHE concentrations determined in soil samples, indicating acceptable levels of accuracy. Easily leachable and potentially leachable fractions of PHE were assessed by singlereagent extraction tests with deionized water and with ethylene diamine tetraacetic acid (EDTA), respectively. Aliquots of each soil sample (<2 mm) were placed in 50-mL polypropylene centrifuge tubes and shaken for 2 h, with deionized water and with 0.05 mol L −1 EDTA (adjusted to pH 7.0) at a soil:solution ratio of 1:10 (w/v) [ 38 , 39 ]. Afterwards, the suspensions were centrifuged for 10 min at 5000 rpm, and then filtered through a 0.45- µ m membrane filter. The PHE concentrations were measured in the filtrates by ICP-MS (Agilent 7900 ICP-MS instrument). Blank analyses were carried out to correct for contamination. The extraction procedure was repeated twice to assess reproducibility. Precision values were generally better than 10% RSD. 3.3. Analytical Analysis The data obtained were statistically processed using the Statistica 10.0 (Stat Soft Inc., Tulsa, OK, USA) and MS Excel 2016 (Microsoft Co., USA) software packages. For statistical and graphical purposes, all values below detection limits (non-detects) were replaced with half of the detection limit [ 40 ]. Statistical differences between measured variables in treated and control samples were evaluated using Student’s t-test and analysis of variance (oneway ANOVA). Prior to analysis, the homogeneity of variance was assessed by Levene’s test. In this study, an α -value of 0.05 was used as critical level of significance for all statistical testing. 4. Results and Discussion 4.1. Mineral Composition and Soil Reaction The soil affected by AMD discharges from the sulfide mine wastes is silt loam in texture (~68% silt, 25% sand and 7% clay), and mineralogically composed of quartz, clay minerals (illite and kaolinite) and jarosite, with minor feldspars, gypsum and hematite, as determined by XRPD analysis of control samples (Figure 2). Appl. Sci. 2022, 12, 6998 5 of 18 leased from the three steps (F1 + F2 + F3) plus residual fraction (F4) compared well with the total PHE concentrations determined in soil samples, indicating acceptable levels of accuracy. Easily leachable and potentially leachable fractions of PHE were assessed by single-reagent extraction tests with deionized water and with ethylene diamine tetraacetic acid (EDTA), respectively. Aliquots of each soil sample (<2 mm) were placed in 50-mL polypropylene centrifuge tubes and shaken for 2 h, with deionized water and with 0.05 mol L−1 EDTA (adjusted to pH 7.0) at a soil:solution ratio of 1:10 (w/v) [38,39]. Afterwards, the suspensions were centrifuged for 10 min at 5000 rpm, and then filtered through a 0.45-µm membrane filter. The PHE concentrations were measured in the filtrates by ICP-MS (Agilent 7900 ICP-MS instrument). Blank analyses were carried out to correct for contamination. The extraction procedure was repeated twice to assess reproducibility. Precision values were generally better than 10% RSD. 3.3. Analytical Analysis The data obtained were statistically processed using the Statistica 10.0 (Stat Soft Inc., Tulsa, OK, USA) and MS Excel 2016 (Microsoft Co., USA) software packages. For statistical and graphical purposes, all values below detection limits (non-detects) were replaced with half of the detection limit [40]. Statistical differences between measured variables in treated and control samples were evaluated using Student’s t-test and analysis of variance (one-way ANOVA). Prior to analysis, the homogeneity of variance was assessed by Levene’s test. In this study, an α-value of 0.05 was used as critical level of significance for all statistical testing. 4. Results and Discussion 4.1. Mineral Composition and Soil Reaction The soil affected by AMD discharges from the sulfide mine wastes is silt loam in texture (~68% silt, 25% sand and 7% clay), and mineralogically composed of quartz, clay minerals (illite and kaolinite) and jarosite, with minor feldspars, gypsum and hematite, as determined by XRPD analysis of control samples (Figure 2). Figure 2. XRPD diffractograms of topsoil samples of amended soil (T10) and control soil (ST10). Figure 2. XRPD diffractograms of topsoil samples of amended soil (T10) and control soil (ST10). Appl. Sci. 2022,12, 6998 6 of 17 In addition, a suite of accessory minerals was identified by ESEM-EDS, including barite, monazite, zircon, rutile, plumbogummite, cuprospinel and poorly crystallized iron oxides. Consistently, the contaminated mine soil has a poor buffering capacity to neutralize acid that is being produced, or has been produced, by AMD because it is completely free of carbonates and depleted in other acid-neutralizing minerals. Jarosite typically occurs as clusters of euhedral crystals of less than 1 µ m in size, coexisting with iron oxides nanoparticles and soil clays, as observed by ESEM at high magnification (Figure 3a,b). The EDS spectra of some isolated crystals of a jarosite-group mineral that appear brighter in the ESEM-BSE images revealed a remarkable amount of Pb, by virtue of which they can be ascribed to plumbojarosite (Figure 3c). Twelve years after amendment with MWS, the soil showed a silt loam texture with high contents of fine-grained calcite in the topsoil layer (0–20 cm) and lower proportions of the other constituents relative to control soil, with the exception of gypsum. It is interesting to note that most calcite crystals remain unaltered, maintaining their original rhombohedral morphology despite the long time since marble slurry application. Nevertheless, some of them showed signs of partial dissolution on their surface (Figure 3d), and some others appear to be coated by iron oxyhydroxides (Figure 3e). Direct precipitation of gypsum (Figure 3f) was induced by partial dissolution of the carbonates added to the soil, thus providing the Ca 2+ ions necessary to combine with the sulfate ions. The neutralization reaction can explain not only the increase in gypsum concentration observed in the amended soil layer but also the occurrence of metal oxide coatings, as the Fe 3+ solubility is controlled by the precipitation of ferric iron oxyhydroxides [2]. Appl. Sci. 2022, 12, 6998 6 of 18 In addition, a suite of accessory minerals was identified by ESEM-EDS, including barite, monazite, zircon, rutile, plumbogummite, cuprospinel and poorly crystallized iron oxides. Consistently, the contaminated mine soil has a poor buffering capacity to neutralize acid that is being produced, or has been produced, by AMD because it is completely free of carbonates and depleted in other acid-neutralizing minerals. Jarosite typically occurs as clusters of euhedral crystals of less than 1 µm in size, coexisting with iron oxides nanoparticles and soil clays, as observed by ESEM at high magnification (Figure 3a,b). The EDS spectra of some isolated crystals of a jarosite-group mineral that appear brighter in the ESEM-BSE images revealed a remarkable amount of Pb, by virtue of which they can be ascribed to plumbojarosite (Figure 3c). Twelve years after amendment with MWS, the soil showed a silt loam texture with high contents of fine-grained calcite in the topsoil layer (0–20 cm) and lower proportions of the other constituents relative to control soil, with the exception of gypsum. It is interesting to note that most calcite crystals remain unaltered, maintaining their original rhombohedral morphology despite the long time since marble slurry application. Nevertheless, some of them showed signs of partial dissolution on their surface (Figure 3d), and some others appear to be coated by iron oxyhydroxides (Figure 3e). Direct precipitation of gypsum (Figure 3f) was induced by partial dissolution of the carbonates added to the soil, thus providing the Ca2+ ions necessary to combine with the sulfate ions. The neutralization reaction can explain not only the increase in gypsum concentration observed in the amended soil layer but also the occurrence of metal oxide coatings, as the Fe3+ solubility is controlled by the precipitation of ferric iron oxyhydroxides [2]. Figure 3. ESEM-BSE images showing salient mineralogical and textural features of the amended soil after 12 years of amendment application: (a) aggregate of jarosite with pseudocubic habit of euhedral crystals; (b) iron oxide nanoparticle aggregate; (c) isolated pseudocubic crystal of plumbojarosite; (d) calcite grain with dissolution grooves on its surface; (e) iron oxide coating calcite grain; and (f) newly-formed crystal of gypsum. Soil reaction measured on control samples was ultra-acid, with pH values ranging from 3.1 to 3.5 (Table 1), whereas in the amended plot the soil samples had pH values in the range of 5.8–6.4, with an average of 6.2. Therefore, the soil solution appears to be neutralized to slightly acidic reaction throughout the entire sampled profile (0–70 cm) in the response to MWS amendment. Moreover, the long-term measurement of soil redox potential changed noticeably from strongly oxidizing conditions (Eh = 535–647 mV) to moderately oxidizing conditions (Eh = 343–525 mV) after treatment. The mean values of soil electrical conductivity were 2.62 mS cm−1 in the amended soil and 3.04 mS cm−1 in the unamended soil, indicating a low soluble salt content in both soil solutions. Figure 3. ESEM-BSE images showing salient mineralogical and textural features of the amended soil after 12 years of amendment application: ( a ) aggregate of jarosite with pseudocubic habit of euhedral crystals; ( b ) iron oxide nanoparticle aggregate; ( c ) isolated pseudocubic crystal of plumbojarosite; ( d ) calcite grain with dissolution grooves on its surface; ( e ) iron oxide coating calcite grain; and ( f ) newly-formed crystal of gypsum. Soil reaction measured on control samples was ultra-acid, with pH values ranging from 3.1 to 3.5 (Table 1), whereas in the amended plot the soil samples had pH values in the range of 5.8–6.4, with an average of 6.2. Therefore, the soil solution appears to be neutralized to slightly acidic reaction throughout the entire sampled profile (0–70 cm) in the response to MWS amendment. Moreover, the long-term measurement of soil redox potential changed noticeably from strongly oxidizing conditions (Eh = 535–647 mV) to moderately oxidizing conditions (Eh = 343–525 mV) after treatment. The mean values of soil electrical conductivity were 2.62 mS cm −1 in the amended soil and 3.04 mS cm −1 in the unamended soil, indicating a low soluble salt content in both soil solutions. Appl. Sci. 2022,12, 6998 7 of 17 Table 1. Electrochemical properties and semiquantitative mineral composition of the samples. Sample Depth (cm) Electrochemical Parameters Mineral Composition (wt %) pH Eh (mV) EC (mS/cm) Quartz Clay Minerals * Feldspars Jarosite Gypsum Hematite Calcite Amended soil T10 0−10 6.3 343 3.32 10−15 5−10 <5 10−15 10−15 <5 55−60 T20 10 − 20 6.4 458 3.01 30−35 25−30 <5 10−15 5−10 <5 25−30 T30 20 − 30 6.3 421 2.85 40−45 30−35 <5 15−20 <5 <5 n.d. T40 30 − 40 5.8 472 1.72 40−45 30−35 <5 15−20 <5 <5 n.d. T50 40 − 50 6.1 502 1.95 35−40 30−35 <5 15−20 <5 <5 n.d. T60 50 − 60 6.3 407 2.90 35−40 35−40 <5 15−20 <5 <5 n.d. T70 60 − 70 6.2 525 2.61 40−45 30−35 <5 15−20 <5 <5 n.d. Control soil ST10 0−10 3.2 619 4.02 40−45 35−40 5−10 10−15 <5 <5 n.d. ST20 10 − 20 3.3 647 3.10 35−40 25−30 <5 15−20 10−15 <5 n.d. ST30 20 − 30 3.1 644 3.07 40−45 25−30 5−10 15−20 <5 <5 n.d. ST40 30 − 40 3.3 632 2.90 40−45 30−35 5−10 15−20 <5 <5 n.d. ST50 40 − 50 3.3 627 1.92 40−45 30−35 <5 15−20 <5 <5 n.d. ST60 50 − 60 3.5 600 2.98 35−40 40−45 <5 10−15 5−10 <5 n.d. ST70 60 − 70 3.4 535 3.26 30−35 45−50 <5 10−15 <5 <5 n.d. * illite and kaolinite; EC (electrical conductivity); n.d. (not detected). As compared to soil electrochemical properties after one year of treatment, the soil pH declined but not to detrimental levels, from 6.8 to 6.3, and the electrical conductivity remained practically constant at 3.3–3.5 mS cm −1 twelve years after amendment application. Given the long lapse of time since the treatment, the soil system seems to be well buffered by carbonate equilibria, and thereby further potential acid production could be consumed with little or no decrease in pH through acid-neutralization reactions [41]. 4.2. Total Trace Element Concentrations Results of the total concentrations of PHE analyzed in the amended and control (no amendment) soil samples are listed together in Table 2, and displayed in Figure 4to graphically show the variation of concentration with depth. Table 2. Total trace element concentrations (in mg kg−1) analyzed by ICP-OES. Element (mg kg−1) As Cd Cr Cu Ni Pb Sb Tl Zn Detection limit 3 0.3 1 1 1 3 5 5 1 Amended soil T10 268 1.4 34 1210 38 655 64 BDL 851 T20 689 0.7 46 808 20 1800 35 BDL 296 T30 1290 BDL 60 607 18 2310 22 7 176 T40 1450 BDL 64 816 17 2420 26 11 153 T50 1860 BDL 61 636 18 2660 20 9 134 T60 1390 BDL 72 734 19 2380 20 11 217 T70 1830 BDL 60 743 20 2620 37 9 162 Control soil ST10 711 0.3 69 756 20 2010 129 BDL 157 ST20 964 0.5 68 474 14 1990 20 BDL 129 ST30 1130 BDL 61 504 20 2080 34 BDL 121 ST40 980 BDL 70 1630 22 2080 34 6 156 ST50 1090 BDL 59 1350 20 2080 42 12 141 ST60 1110 BDL 81 825 13 3410 36 BDL 171 ST70 1060 BDL 73 777 15 3320 42 6 132 BDL (below detection limit). Appl. Sci. 2022,12, 6998 8 of 17 Appl. Sci. 2022, 12, 6998 8 of 18 Figure 4. Vertical distribution of trace elements along the two sampling sections (amended and control) of the soil profile. Total concentrations of As, Cr, and Pb increased significantly (p < 0.01) with depth in the amended soil up to a maximum of 1860 mg kg−1 As at 40–50 cm depth, 72 mg kg−1 Cr (50–60 cm), and 2660 mg kg−1 Pb (40–50 cm). Compared to the control, the concentrations of As, Pb, and Cr in the amended soil were noticeably low at the upper sampling layers (up to 20 cm depth), and then increased with depth to levels comparable to even higher than baseline concentration. Such vertical distribution pattern may be related to a dilution effect caused by the incorporation of MWS to the plough layer. It is nevertheless noteworthy that As showed a contrasted soil profile distribution, with increasing downward concentrations suggesting leaching from the upper horizons and subsequent re-deposition. The levels of Cu, Cd, Ni, and Zn in the surface horizon were significantly (p < 0.01) higher than in the lower sampling depths. Notwithstanding the diluting effect due to MWS addition, the amounts of Cu (1210 mg kg−1), Cd (1.4 mg kg−1), Ni (38 mg kg−1), and Zn (851 mg kg−1) found in the topsoil (0–10 cm) of the amended mini plot were higher by a factor of 5.4 for Zn, 4.7 for Cd, 1.9 for Ni, and 1.6 for Cu relative to control soil (Figure 5). This increased upward trend is clearly indicative of metal enrichment in the amended surface layer. It was further found that the vertical distribution of such PHE in the two sampling sections (i.e., MWS-amended and control) was rather similar, with the concentration factor values ranging from 0.5 to 1.5, except for Cu that seemed to be more depleted in the 30–50 cm depth layer. Figure 4. Vertical distribution of trace elements along the two sampling sections (amended and control) of the soil profile. Total concentrations of As, Cr, and Pb increased significantly (p< 0.01) with depth in the amended soil up to a maximum of 1860 mg kg −1 As at 40–50 cm depth, 72 mg kg −1 Cr (50–60 cm), and 2660 mg kg −1 Pb (40–50 cm). Compared to the control, the concentrations of As, Pb, and Cr in the amended soil were noticeably low at the upper sampling layers (up to 20 cm depth), and then increased with depth to levels comparable to even higher than baseline concentration. Such vertical distribution pattern may be related to a dilution effect caused by the incorporation of MWS to the plough layer. It is nevertheless noteworthy that As showed a contrasted soil profile distribution, with increasing downward concentrations suggesting leaching from the upper horizons and subsequent re-deposition. The levels of Cu, Cd, Ni, and Zn in the surface horizon were significantly (p< 0.01) higher than in the lower sampling depths. Notwithstanding the diluting effect due to MWS addition, the amounts of Cu (1210 mg kg −1 ), Cd (1.4 mg kg −1 ), Ni (38 mg kg −1 ), and Zn (851 mg kg −1 ) found in the topsoil (0–10 cm) of the amended mini plot were higher by a factor of 5.4 for Zn, 4.7 for Cd, 1.9 for Ni, and 1.6 for Cu relative to control soil (Figure 5). This increased upward trend is clearly indicative of metal enrichment in the amended surface layer. It was further found that the vertical distribution of such PHE in the two sampling sections (i.e., MWS-amended and control) was rather similar, with the concentration factor values ranging from 0.5 to 1.5, except for Cu that seemed to be more depleted in the 30–50 cm depth layer. Appl. Sci. 2022,12, 6998 9 of 17 Appl. Sci. 2022, 12, 6998 9 of 18 Figure 5. Soil profile distribution of the concentration factor (calculated by dividing the total content of each trace element in the amended soil by that in the unamended soil) for: (a) Cd, Cu, Ni, and Zn; and (b) As, Cr, Sb, and Pb. A similar distribution pattern was observed when comparing the ratio between the concentrations of PHE in the amended topsoil 12 years after treatment and those previously reported for the surface soil before treatment [24]. Zinc, Cd, Ni, and to a lesser extent, Cu were found enriched in the amended surface layer (Figure 6), clearly indicating retention by the soil, while As, Cr, and Pb were relatively depleted by the dilution effect of the soil amendment. Figure 6. Quotient between total concentration (C) of trace elements in the amended surface soil 12 years after amendment application (this study) and before treatment [24]. 4.3. Chemical Partitioning Trace elements were unevenly distributed among the four operationally defined soil fractions, as determined by sequential chemical extraction (Table 3), which has been proven to be a useful procedure for assessing the long-term mobility of PHE [37]. Figure 5. Soil profile distribution of the concentration factor (calculated by dividing the total content of each trace element in the amended soil by that in the unamended soil) for: ( a ) Cd, Cu, Ni, and Zn; and (b) As, Cr, Sb, and Pb. A similar distribution pattern was observed when comparing the ratio between the concentrations of PHE in the amended topsoil 12 years after treatment and those previously reported for the surface soil before treatment [ 24 ]. Zinc, Cd, Ni, and to a lesser extent, Cu were found enriched in the amended surface layer (Figure 6), clearly indicating retention by the soil, while As, Cr, and Pb were relatively depleted by the dilution effect of the soil amendment. Appl. Sci. 2022, 12, 6998 9 of 18 Figure 5. Soil profile distribution of the concentration factor (calculated by dividing the total content of each trace element in the amended soil by that in the unamended soil) for: (a) Cd, Cu, Ni, and Zn; and (b) As, Cr, Sb, and Pb. A similar distribution pattern was observed when comparing the ratio between the concentrations of PHE in the amended topsoil 12 years after treatment and those previously reported for the surface soil before treatment [24]. Zinc, Cd, Ni, and to a lesser extent, Cu were found enriched in the amended surface layer (Figure 6), clearly indicating retention by the soil, while As, Cr, and Pb were relatively depleted by the dilution effect of the soil amendment. Figure 6. Quotient between total concentration (C) of trace elements in the amended surface soil 12 years after amendment application (this study) and before treatment [24]. 4.3. Chemical Partitioning Trace elements were unevenly distributed among the four operationally defined soil fractions, as determined by sequential chemical extraction (Table 3), which has been proven to be a useful procedure for assessing the long-term mobility of PHE [37]. Figure 6. Quotient between total concentration (C) of trace elements in the amended surface soil 12 years after amendment application (this study) and before treatment [24]. 4.3. Chemical Partitioning Trace elements were unevenly distributed among the four operationally defined soil fractions, as determined by sequential chemical extraction (Table 3), which has been proven to be a useful procedure for assessing the long-term mobility of PHE [37]. Appl. Sci. 2022,12, 6998 16 of 17 References 1. Plumlee, G.S.; Morman, S.A. Mine wastes and human health. Elements 2011,7, 399–404. [CrossRef] 2. Grantcharova, M.M.; Fernández-Caliani, J.C. 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