The use of construction waste to remediate a thermally active spoil heap
Abstract
This article presents the results of experimental research on the possible use of construction and demolition waste (CDW) to improve the properties of unburnt tailings originating from the thermally active spoil heap in Heˇrmanice (Ostrava, Czech Republic). Mining activity anywhere in the world generally entails a lot of negative impacts on the environment, which are of a long-term nature. One of the most pressing challenges in the remediation of the consequences of mining activity is the thermal activity of spoil heaps associated with the high acidity of the tailings. Active acidity (pH/H2O), exchangeable acidity (pH/CaCl2 ), hydrolytic acidity (Ha), and elemental composition of tailings and CDW have been monitored. Based on an acidity study, it has been proven that compared to burnt tailings (pH/H2O = 8.4, pH/CaCl2 = 8.9 and Ha = 1.4 mmol kg−1 ), unburnt tailings show acidic properties (pH/H2O = 3.7, pH/CaCl2 = 3.6 and Ha = 205 mmol kg−1 ). The bioavailability of two selected potentially toxic elements (PTEs), namely Al and Fe, was examined based on the elemental composition. BCR sequential extraction analysis was used to determine their bioavailability. It has been proven that mixing CDW with tailings has a positive effect on the pH value, which has a positive effect on the further development of the entire site. The increase in the pH value is provably dependent on the amount of construction waste added, so it can be said that the increasing amount of construction waste will result in improved parameters of the burnt tailings. The results of the BCR analysis show that aluminum from the tailings will be released both from the reducible and oxidisable fractions, where it will be mainly bound to sulphides. The relatively high concentration of Fe in the oxidisable fraction (2002 mg Fe kg−1 ) suggests that Fe is bound to sulphides in the tailings, and it is due to the high residual pyrite and sulphide content in the dumped material, as expected. This work has found no limits where CDW no longer positively affects the acidity of unburnt tailings. For practical application, however, it is important that the mixture of CDW and tailings is properly mixed and then used for remediation.
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Citation: Pertile, E.; Dvorský, T.; Václavík, V.; Syrová, L.; Charvát, J.; Máˇcalová, K.; Balcaˇrík, L. The Use of Construction Waste to Remediate a Thermally Active Spoil Heap. Appl. Sci. 2023,13, 7123. https://doi.org/ 10.3390/app13127123 Academic Editor: Rafael López Núñez Received: 27 February 2023 Revised: 22 May 2023 Accepted: 12 June 2023 Published: 14 June 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/). applied sciences Article The Use of Construction Waste to Remediate a Thermally Active Spoil Heap Eva Pertile , Tomáš Dvorský* , Vojtˇech Václavík , Lucie Syrová, Jakub Charvát, Kateˇrina Máˇcalová and Lukáš Balcaˇrík Department of Environmental Engineering, Faculty of Mining and Geology, VSB—Technical University of Ostrava, 17. Listopadu 15/2172, 708 00 Ostrava, Czech Republic; [email protected] (E.P.); [email protected] (V.V.); [email protected] (L.S.) *Correspondence: [email protected]; Tel.: +420-597-323-593 Abstract: This article presents the results of experimental research on the possible use of construction and demolition waste (CDW) to improve the properties of unburnt tailings originating from the thermally active spoil heap in Heˇrmanice (Ostrava, Czech Republic). Mining activity anywhere in the world generally entails a lot of negative impacts on the environment, which are of a long-term nature. One of the most pressing challenges in the remediation of the consequences of mining activity is the thermal activity of spoil heaps associated with the high acidity of the tailings. Active acidity (pH/H 2 O), exchangeable acidity (pH/CaCl 2 ), hydrolytic acidity (H a ), and elemental composition of tailings and CDW have been monitored. Based on an acidity study, it has been proven that compared to burnt tailings (pH/H 2 O = 8.4, pH/CaCl 2 = 8.9 and H a = 1.4 mmol kg −1 ), unburnt tailings show acidic properties (pH/H 2 O = 3.7, pH/CaCl 2 = 3.6 and H a = 205 mmol kg −1 ). The bioavailability of two selected potentially toxic elements (PTEs), namely Al and Fe, was examined based on the elemental composition. BCR sequential extraction analysis was used to determine their bioavailability. It has been proven that mixing CDW with tailings has a positive effect on the pH value, which has a positive effect on the further development of the entire site. The increase in the pH value is provably dependent on the amount of construction waste added, so it can be said that the increasing amount of construction waste will result in improved parameters of the burnt tailings. The results of the BCR analysis show that aluminum from the tailings will be released both from the reducible and oxidisable fractions, where it will be mainly bound to sulphides. The relatively high concentration of Fe in the oxidisable fraction (2002 mg Fe kg −1 ) suggests that Fe is bound to sulphides in the tailings, and it is due to the high residual pyrite and sulphide content in the dumped material, as expected. This work has found no limits where CDW no longer positively affects the acidity of unburnt tailings. For practical application, however, it is important that the mixture of CDW and tailings is properly mixed and then used for remediation. Keywords: thermally active spoil heap; mine waste; construction demolition waste; remediation; mining activity; sequential extraction analysis 1. Introduction Coal mining and the recent mass closure of mines are associated with a negative impact on natural objects and technical structures. In addition to the extraction of mineral raw materials, the mining activity also produces a large amount of waste material, which is generated during all the phases of development and mining activity in the mines, as well as all the technological operations related to the processes of enrichment and purification of the extracted raw material. In the past, most of these waste materials were deposited on the surface. That is why numerous spoil heaps were created near coal mines or in their immediate vicinity. A huge amount of tailings and other types of waste materials were deposited in these spoil heaps. In coal basins around the world, there are still hundreds of Appl. Sci. 2023,13, 7123. https://doi.org/10.3390/app13127123 https://www.mdpi.com/journal/applsci
Appl. Sci. 2023,13, 7123 2 of 19 objects representing the locations with waste from coal mining [ 1 – 3 ]. For example, there are approximately 281 spoil tips and 46 spoil heaps in the Ostrava-Karvina Coalfield. These are anthropogenic tailing spoil heaps formed at different times during the nearly 200-year long tradition of coal mining in the Czech parts of the Upper Silesian Coal Basin [ 4 ]. However, there are countries in which this number is even higher. For example, in neighbouring Poland, there are approximately 250 such objects, and in China, the number of sites where tailings are stored exceeds 1700 [2]. Spoil heaps of mine tailings significantly affect the character of the landscape and are perceived as a negative aesthetic element that stands out from the surrounding relief of the landscape. Whether they are overgrown with invasive vegetation over time or they are after more or less successful remediation, these are still areas with limited options for further use. In addition to the adverse effect on the landscape, the presence of surface coal waste dumps is closely related to their impact on the environment [ 2 , 5 , 6 ]. From the point of view of their environmental burden, spoil heaps can be particularly problematic due to their thermal activity, and also the chemical composition of the deposited material which, as a result of the age of the spoil heap and its thermal activity, cannot be clearly determined or more precisely estimated. In the past, a significant impact of carbon tailings stored in spoil heaps on the individual components of the environment was not assumed. Carboniferous tailings as such do not contain any contaminants, but under certain conditions these can be uncontrollably released from the tailings. The main risk factor that is closely associated with carboniferous tailings is the possibility of endogenous combustion [ 7 ]. The chemical reactivity of rocks deposited in spoil heaps is mainly determined by the amount of unstable minerals present (e.g., pyrite, carbonates, and feldspars). Pyrite weathers very easily. It occurs due to the infiltration of precipitation or the penetration of air volume into the spoil heap, resulting in the formation of sulphuric acid, or sulphates. This acidification process is more visible especially in places of endogenous combustion. The degree of thermal transformation of the waste affects the leachability of metals. Significantly more elements (Al, Co, Cr, Cu, Ni) are leached from the deeply thermally transformed samples (sinters) than from samples of thermally intact (unburnt) waste and slightly thermally transformed (burnt) waste. This is due to the presence of a large amount of glaze in sinter, and this glaze is subject to devitrification and the release of metals contained in it [ 8 ]. Changes in temperature also affect the chemical status of organic substances found in the coal waste dump. This leads to the formation of phenols and their derivatives, which can pass into water and subsequently cause its pollution [ 9 – 11 ]. The material deposited on spoil heaps is also characterized by different particle sizes [ 12 ]. If the material is porous, the heat spreads more easily and much faster. With fine-grained materials, air movement is slower and the temperature therefore increases gradually. The geotechnical condition of the spoil heap or other types of waste or sludge materials that might have been deposited on the spoil heaps in the past must also be taken into consideration. They can represent a secondary source of contamination of the rock environment [8,11,13]. Spontaneous combustion usually triggers the process of long-lasting subsurface fires. Due to the content of sulphide sulphur and other combustible substances (debris of conveyor belts, wooden parts of mine supports, etc.), carboniferous tailings represent a constant risk of combustion (e.g., by inappropriate intervention in old spoil heaps). That is why the remediation work also plays an important role, because it should lead to thermal phenomena inhibition on the spoil heap, and should be adapted to the local morphology, technology, composition and volume of the deposited waste material [ 7 , 11 ]. An increase in the temperature of the subsoil and the surface is a very significant negative factor related to thermal processes, and it leads to changes in the topography, soil cover, and plant cover, as well as the atmosphere [ 2 , 14 ]. If fires occur in the tailing dumps, the emission of pollutants into the air will increase significantly. As a result of the increased temperature and more intense evaporation, the material on the spoil heap sheds its moisture, and dry particles get into the air more easily. If the coal contained in the waste turns into ash during fire, its
Appl. Sci. 2023,13, 7123 3 of 19 tendency to be carried away by the wind increases, especially during the operations related to fire extinguishing, removal, and transport of burnt material, or during remediation works. Since the waste material contains sulphur, SO 2 and H 2 S are generated. This is evidenced by the yellow efflorescence on the surface of the spoil heap body (Figure 1). As a result of wind erosion, spoil heaps are therefore a secondary source of air-borne dust (including respirable PM10 particles) [15,16]. Appl.Sci.2023,13,xFORPEERREVIEW3of20 particlesgetintotheairmoreeasily.Ifthecoalcontainedinthewasteturnsintoashdur‐ ingfire,itstendencytobecarriedawaybythewindincreases,especiallyduringtheop‐ erationsrelatedtofireextinguishing,removal,andtransportofburntmaterial,orduring remediationworks.Sincethewastematerialcontainssulphur,SO2andH2Saregenerated. Thisisevidencedbytheyellowefflorescenceonthesurfaceofthespoilheapbody(Figure1). Asaresultofwinderosion,spoilheapsarethereforeasecondarysourceofair‐bornedust (includingrespirablePM10particles)[15,16]. Figure1.Heřmanicespoilheap. Theformationofvegetationgroupsisalwaysapositivephenomenon,becausethe decompositionofphytomasssupportstheformationofhumus.Inplaceswithvegetation, endogenouscombustioncanturnintoanopenfireandthusconfoundtheinvestments spentforrecultivation.Thiswasthecase,forexample,inthermallyactiveHeřmaniceor Hedvikaspoilheaps(Ostrava,Moravian‐SilesianRegion,CzechRepublic).Itistherefore notanisolatedcasewhenalreadyreclaimedandadaptedareasofformerspoilheapsstart tospontaneouslyheatup[2].Itisverydifficulttodealwithhotspotsandpreventendog‐ enouscombustionofoldspoilheapsthatcontainlargeamountsofcoaldebrisandwere oftenplacedinaloose,unconsolidatedconfigurationthatallowsoxygentoeasilyinteract withthewaste.Regularmonitoringmakestheiridentificationpossible,butapparently, theonlywaytodealwiththemistointensifythecombustionbyimprovingtheaccessof airandthusallowingthetailingstoburnthrough[17].Thisisthereasonwhymanyenti‐ tiesdecidedtorecultivatecoalwastedumpsonlyaftertheyhadbeenburned,extin‐ guished,orpartiallyexcavated.Researchhasshownthatinthecaseofburningcoalwaste dumps,theintroductionofvegetationhasnoeffectunlesstheobjectissufficientlypro‐ tectedagainstfire.Insuchsituations,spontaneoussuccessionistheonlysolution[2]. TheremovalofdamagescausedbyminingactivitiesisnotrelatedtotheOstrava‐ KarvinaCoalfieldonly.Inanumberoflegislativeandlegalmeasuresofothercountries, thereisanobligationtorecultivatedamagedareasonthesurface.Thisisaglobalproblem andalong‐termprocess[18].Theseanthropogenicgeomorphologicalbodiesare,onthe onehand,aburdenfortheenvironment;butontheotherhand,theycanbeanimportant asourceofcheapandavailableaggregates.Thecurrentpressuretolimittheuseofpri‐ maryrawmaterials,theextractionofwhichcausesadditionalunpleasantimpactsonthe landscapeandtheenvironment,necessarilyleadstotheuseofwasteassecondaryraw materials.IntheCzechRepublic,thisproblemwassolvedlegislativelyin2001,because miningwasteisnotclassifiedaswaste,butasaproductusedinlandreclamationorroad construction. Figure 1. Heˇrmanice spoil heap. The formation of vegetation groups is always a positive phenomenon, because the decomposition of phytomass supports the formation of humus. In places with vegetation, endogenous combustion can turn into an open fire and thus confound the investments spent for recultivation. This was the case, for example, in thermally active Heˇrmanice or Hedvika spoil heaps (Ostrava, Moravian-Silesian Region, Czech Republic). It is therefore not an isolated case when already reclaimed and adapted areas of former spoil heaps start to spontaneously heat up [ 2 ]. It is very difficult to deal with hot spots and prevent endogenous combustion of old spoil heaps that contain large amounts of coal debris and were often placed in a loose, unconsolidated configuration that allows oxygen to easily interact with the waste. Regular monitoring makes their identification possible, but apparently, the only way to deal with them is to intensify the combustion by improving the access of air and thus allowing the tailings to burn through [ 17 ]. This is the reason why many entities decided to recultivate coal waste dumps only after they had been burned, extinguished, or partially excavated. Research has shown that in the case of burning coal waste dumps, the introduction of vegetation has no effect unless the object is sufficiently protected against fire. In such situations, spontaneous succession is the only solution [2]. The removal of damages caused by mining activities is not related to the OstravaKarvina Coalfield only. In a number of legislative and legal measures of other countries, there is an obligation to recultivate damaged areas on the surface. This is a global problem and a long-term process [ 18 ]. These anthropogenic geomorphological bodies are, on the one hand, a burden for the environment; but on the other hand, they can be an important a source of cheap and available aggregates. The current pressure to limit the use of primary raw materials, the extraction of which causes additional unpleasant impacts on the landscape and the environment, necessarily leads to the use of waste as secondary raw materials. In the Czech Republic, this problem was solved legislatively in 2001, because mining waste is not classified as waste, but as a product used in land reclamation or road construction. In Europe, the two economic sectors producing the largest volume of waste are mining and quarrying (28.1%), and building industry and demolition (34.7%) [ 19 ]. In the Czech Republic , construction and demolition waste constitutes a significant part of the
Appl. Sci. 2023,13, 7123 4 of 19 total amount of waste produced in terms of weight. In the years 2014–2020, the production of construction waste accounted for more than half of the total production of waste (see Table 1). On the contrary, with regard to the decline in mining activity, waste from mining and quarrying accounted for only 0.05% of the total amount of waste produced in 2020 [ 20 ]. Table 1. Production of waste in the years 2014–2020 [20]. Year Total Production of Waste t Production of Construction and Demolition Waste t Share of Construction and Demolition Waste on the Total Production % 2014 32,028,422 19,124,592 59.7 2015 37,338,298 24,916,868 65.1 2016 34,242,076 20,669,215 60.4 2017 34,553,461 20,153,879 58.3 2018 37,940,560 21,498,561 56.7 2019 37,310,939 23,551,255 63.1 2020 38,486,186 24,955,252 64.8 In compliance with the EU communication COM/2014/398 “Towards a circular economy: A zero waste program for Europe”, which introduces the main objectives and measures in the field of circular economy, the landfilling of recyclable waste will not be allowed from the year 2025, and it will be completely prohibited from the year 2050 [ 21 ]. As a corrective measure, it is recommended to increase the neutralization potential by liming, especially in locations with an increased concentration of pyrite, or with the occurrence of jarosite, which can be an important indicator in this regard. An increase in the neutralization potential can also slow down the kinetics of pyrite oxidation. However, a decrease in tailings acidity by surface application of lime can be very difficult and time-consuming given the limited movement of lime through the spoil heap body. The aim of this article is to assess the effect of the admixture of construction and demolition waste (CDW) on the properties of tailings with an acidic character. According to the current practices of the application of CDW to thermal waste, it is used to fill up the depressions created, for example, by burning through hot spots, but this does not achieve the ultimate goal that would lead to a pH adjustment. One of the objectives was therefore to check whether a greater effect could be achieved by mixing CDW and unburnt tailings, and to check whether the percentage of unburnt tailings and CDW would have an effect on the final situation. In addition, unburnt mine waste could be used in line with the 3Rs (Reduce, Reuse, and Recycle) concept of the circular economy, thus removing old environmental burdens. 2. Materials and Methods 2.1. Description of the Mining Site Tailings from the largest and most thermally active complex (Figure 2) in the entire Ostrava-Karvina Coalfield, the Heˇrmanice spoil heap (area of 881,993 m 2 , volume of 20,106 m3 , repose height 20–30 m ), have been chosen for the experiments. This is the youngest thermally active spoil heap of a terraced character, which was created from the mid-19th century, when tailings from the Ida Mine were deposited here. Intense thermal activity was recorded in the body of the spoil heap in 2004, when there were open fires at the roots of trees planted as part of remediation. In 2005, a grouting wall was built here to prevent these thermal processes, but the combustion process is still taking place here at this time. In 2009, the process of removal was started in order to build an air barrier between the rock and the burning material. Construction waste was also deposited here in an uncontrolled manner, but it was not incorporated into the tailings in any way.
Appl. Sci. 2023,13, 7123 5 of 19 Appl.Sci.2023,13,xFORPEERREVIEW5of20 thistime.In2009,theprocessofremovalwasstartedinordertobuildanairbarrierbe‐ tweentherockandtheburningmaterial.Constructionwastewasalsodepositedherein anuncontrolledmanner,butitwasnotincorporatedintothetailingsinanyway. Figure2.ThermalactivityinHeřmanicespoilheap. 2.2.SamplingandPre‐AnalyticalSteps Samplingoftailingsfromthesurfaceofthespoilheapbodywascarriedoutoverits entirearea(30samplingpoints)wherethermalactivityistakingplace.First,approx.10– 20cmoftheupperlayerofthespoilheap,whichwasnotpartofthesampletaken,was removed.Tailingsamplesweighingapprox.5kgweretakenfromthedepthof20–50cm. Bycarefulmixing,thetailingsamplesfromallthesamplingpoints(seeFigure3)were homogenizedintoonecomplexsample,whichwasstoredinaclosableplasticcontainer aftercooling.Thetailingsampleswerefirstdriedinalaboratoryatlaboratorytempera‐ ture(±23°C)andthenhomogenizedusingaRetschjawcrushertypeBB200WC(Haan, Germany).Afterhomogenization,thetailingsweresievedthroughaRetschstainlesssteel sievewiththemeshsizeof2mmandsubsequentlydriedtoaconstantweightinavacuum dryerVO29MEMMERT(Schwabach,Germany).Thedriedtailingsampleswerekeptin adesiccator. Thesamplesofburnttailingsweretakenfromtheplacewherethetailingsarere‐ moved(pointsmarkedwithatriangle)forthepurposeofcomparisonofthechemical compositionofburntandunburnttailings;seeFigure3.Theprocedurefortreatingburnt tailingsamplesisidenticaltotheprocedurefortreatingunburnttailingsamples,whichis describedabove. Figure 2. Thermal activity in Heˇrmanice spoil heap. 2.2. Sampling and Pre-Analytical Steps Sampling of tailings from the surface of the spoil heap body was carried out over its entire area (30 sampling points) where thermal activity is taking place. First, approx. 10–20 cm of the upper layer of the spoil heap, which was not part of the sample taken, was removed. Tailing samples weighing approx. 5 kg were taken from the depth of 20–50 cm. By careful mixing, the tailing samples from all the sampling points (see Figure 3) were homogenized into one complex sample, which was stored in a closable plastic container after cooling. The tailing samples were first dried in a laboratory at laboratory temperature ( ± 23 ◦ C) and then homogenized using a Retsch jaw crusher type BB200 WC (Haan, Germany). After homogenization, the tailings were sieved through a Retsch stainless steel sieve with the mesh size of 2 mm and subsequently dried to a constant weight in a vacuum dryer VO29 MEMMERT (Schwabach, Germany). The dried tailing samples were kept in a desiccator. The samples of burnt tailings were taken from the place where the tailings are removed (points marked with a triangle) for the purpose of comparison of the chemical composition of burnt and unburnt tailings; see Figure 3. The procedure for treating burnt tailing samples is identical to the procedure for treating unburnt tailing samples, which is described above. A sample of a mixture of construction and demolition waste was taken during the demolition of a civic amenity building, and it contained the remains of bricks, plaster, and tiles without admixture of heterogenous materials (remains of cables, paper, plastics). The samples were treated in the same way as the tailing samples. A mixture of tailings and construction waste in different ratios was used to prepare samples for the determination of the physical-chemical parameters and sequential extraction analysis (SEA). The mixed samples contained tailings and a mixture of construction waste in the ratios of 9:1 (90% tailings + 10% construction mix), 7:3 (70% tailings + 30% construction mix) and 8:2 (80% tailings + 20% construction mix). A higher ratio of construction waste was not examined. The effort was to maintain a higher ratio of tailings, which is the main treated material.
Appl. Sci. 2023,13, 7123 6 of 19 Appl.Sci.2023,13,xFORPEERREVIEW6of20 Figure3.SamplingpointsofHeřmanicespoilheap. Asampleofamixtureofconstructionanddemolitionwastewastakenduringthe demolitionofacivicamenitybuilding,anditcontainedtheremainsofbricks,plaster,and tileswithoutadmixtureofheterogenousmaterials(remainsofcables,paper,plastics).The samplesweretreatedinthesamewayasthetailingsamples. Amixtureoftailingsandconstructionwasteindifferentratioswasusedtoprepare samplesforthedeterminationofthephysical‐chemicalparametersandsequentialextrac‐ tionanalysis(SEA).Themixedsamplescontainedtailingsandamixtureofconstruction wasteintheratiosof9:1(90%tailings+10%constructionmix),7:3(70%tailings+30% constructionmix)and8:2(80%tailings+20%constructionmix).Ahigherratioofcon‐ structionwastewasnotexamined.Theeffortwastomaintainahigherratiooftailings, whichisthemaintreatedmaterial. 2.3.CalculationsandInstrumentalConditions Torecalculatetheresults,itwasnecessarytodeterminethedrymatter.Thedetermi‐ nationwasmadeusingthegravimetricmethod.Thecalculationofdrymatterandwater contentwascarriedoutaccordingtoEquations(1)and(2). Calculationofmassmoisturecontent: 𝑤 ∙ ,(1) where: w—massmoisturecontentin%, m1—originalsampleweighting, m2—sampleweightafterdryinging. Calculationofdrymatter: s=100−w,(2) where: s—drymatterin%, w—massmoisturecontentin%. Figure 3. Sampling points of Heˇrmanice spoil heap. 2.3. Calculations and Instrumental Conditions To recalculate the results, it was necessary to determine the dry matter. The determination was made using the gravimetric method. The calculation of dry matter and water content was carried out according to Equations (1) and (2). Calculation of mass moisture content: w=(m1−m2)·100 m2, (1) where: w—mass moisture content in %, m 1—original sample weight in g, m 2—sample weight after drying in g. Calculation of dry matter: s= 100 −w, (2) where: s—dry matter in %, w—mass moisture content in %. Determination of the pH value in the aqueous leachate was carried out according to ˇ CSN ISO 10390 (836221) Soil quality—Determination of pH. Exchangeable acidity was determined according to ˇ CSN EN ISO 14254 (836223) Soil quality—Determination of exchangeable acidity in calcium chloride leaching. The potential exchange reaction also includes adsorbed H + protons and Al 3+ and Fe 3+ ions. An inoLab ® pH 7110 laboratory pH meter from Xylem Analytics Germany Sales GmbH (Weilheim, Germany) was used to measure both parameters. When leaching protons from tailing samples using a neutral salt solution (CaCl 2 ), it is not possible to displace all H + protons from the sorption complex. Hydrolytically alkaline sodium acetate salts were therefore used to displace all bound protons. Hydrolytic acidity H a (mmol kg −1 ) was determined by titration using sodium acetate solution. Sodium ions displace hydrogen ions from the sorption complex, which form acetic acid in the tailing solution, the amount of which is determined by titration with a
Appl. Sci. 2023,13, 7123 7 of 19 measured sodium hydroxide solution. The hydrolytic acidity was then calculated according to the equation (see Equation (3)): Ha=a·f·cNaOH ·1000 ·K g, (3) where: H a—hydrolytic acidity in mmol kg−1, a—consumption of NaOH during titration in mL, f—factor 0.1 M NaOH, c NaOH —concentration of NaOH, 1000 —conversion to 1 kg of soil, K—correction to sodium acetate, g—weight of soil in g. 2.3.1. Sequential Extraction Analysis (SEA) The reactivity or mobility of potentially toxic elements (PTEs) in soils, sediments, or other materials and their potential toxicity depends on the phase in which the risk element is contained, as well as the physical and chemical processes these phases undergo. A variety of sequential extraction types have been developed to determine PTE mobility. The number of individual steps in these types of sequential extractions is quite heterogeneous [ 22 – 27 ]. In our experiment, the sequential extraction analysis used was created by the European Commission in the program called Standards, Measurement and Testing Programme, formally called the BCR ( Bureau Community of Reference ). It is used in different modifications and consists of three steps [ 28 – 35 ]. After each step, i.e., after 16 h , the samples were centrifuged using an EBA 21 centrifuge from Schoeller Instruments ( Prague, Czech Republic ) to separate the solid and liquid phases. The centrifugation was carried out for 20 min at 3000×grpm . The individual steps of the sequential BCR extraction analysis used are summarized in Table 2. Table 2. Overview of extraction agents and conditions for BCR analysis. Step Isolated Fractions Agent Volume mL Temperature ◦CTime 1Exchangeable fraction and fraction bound to carbonates 0.11M CH3COOH 40 22 ±2 shaking 16 h 2Fraction bound to Fe/Mn oxides and hydroxides (reducible fraction) 0.1M NH2OH HCl acidified 2.0M HNO340 22 ±2 shaking 16 h 3 Fraction bound to organic matter and sulphides (oxidizable fraction) 8.8M H2O2,pH=2 1.0M NH 4 OAc, pH = 2 10 50 22 ±2 85 ±2 22 ±2 leaching 1 h leaching 1 h shaking 16 h 2.3.2. Analytical Methods The F-AAS (Flame atomic absorption spectrometry) method was applied to determine the concentration of selected hazardous metals in individual fractions using an AAS contrAA ® 700 atomic absorption spectrometer from Analytik Jena GmbH company (Jena, Germany). The chemical composition of the samples was determined semi-quantitatively by Xray fluorescence on the XEPOS (Spectro, Kleve, Germany) energy dispersion spectrometer. After trituration, the samples were placed in a plastic cuvette with a Mylar protective foil and then analyzed in a protective atmosphere (He). The phase composition and microstructural properties were determined using X-ray powder diffraction (XRD) technique. XRD patterns were obtained using a Rigaku SmartLab diffractometer (Rigaku, Tokyo, Japan) with a D/teX Ultra 250 detector. The X-ray source
Appl. Sci. 2023,13, 7123 8 of 19 was a Co tube (CoK α , λ1 = 0.178892 nm, λ2 = 0.179278 nm) operating at 40 kV and 40 mA. The powder samples were finely ground with agate mortar and pressed with a microscope glass in a rotating sample holder and measured in the reflection mode (Bragg-Brentano geometry) prior to the analysis. The samples were rotated (30 rpm) during the measurement to eliminate the preferred orientation effect. The XRD patterns were collected within the range of 2 θ 5 ◦ –90 ◦ with a step size of 0.01 ◦ and a speed of 0.5 ◦ deg.min −1 . The measured XRD patterns were evaluated using PDXL 2 software (version 2.4.2.0) and compared with the PDF-2 database, 2015 release (ICDD, Newton Square, Worcester, MA, USA). 3. Results and Discussion Tailings as a secondary product of coal mining are no longer considered waste in the Czech Republic, according to Act No. 185/2001 Coll., on waste and on the amendment of certain other laws. The handling of tailings, their storage, and use are mainly covered by mining legislation (Act No. 89/2016 Coll. Act of the Federal Assembly on the Protection and Use of Mineral Resources, the so-called “Mining Act” as amended). An optimal proposal for landscape regeneration in the area affected by mining activity must be based primarily on information describing the impact of tailings from mining and coal processing on the environment. 3.1. Mineralogical-Petrographic Characteristics of Tailings The mineralogical-petrographic characteristics of tailings are important for assessing their potential impact on the environment. The representation of different types of rocks in the tailings is determined by the layer unit in which coal was mined. The content of combustible substances in the spoil heap material affects both the petrographic composition and the granulometry. During the study of spoil heap material in the Ostrava-Karvina Coalfield (Czech Republic), it was proven that the petrographic composition of the tailings is practically identical within the individual spoil heaps. The main rock types on the spoil heaps are various types of aleuropelites (black and black-gray to gray siltstones to very fine-grained sandstones with root soil content), which come from the immediate vicinity of the coal seams. The decay of siltstone is relatively fast. Fine-grained, medium-grained to coarse-grained sandstones are also represented in the spoil heap material in large and variable quantities. These rocks are somewhat more resistant to weathering. In sandstones especially, the cement of their sand grains is corroded to form sand eluvia. The rate of disintegration of sandstones, therefore, depends on the nature of their cement. Sandstones with carbonate and clay cement are especially less resistant, while silicified sandstones are very stable. The most common type of carbonates is ankerite or a representative of the dolomite-ankerite isomorphic series. As a rock-forming mineral, ankerite occurs relatively rarely in the cement of some sandstones, while its occurrence is more common in seam parting. A significant part of the so-called pelosiderites is made up of ankerite rather than siderite. Calcite is very rare in Ostrava-Karvina Coal Area (OKCA) rocks. Small amounts of clay minerals (of the illite-smectite mixed structure type) may show swelling when in contact with water [36]. The mining material is characterized primarily by rocks and rock material (clay stones, mudstones, sandstones, etc.). There are also large amounts of silica (SiO 2 ) in it, alumina (Al 2 O 3 ), iron oxides (Fe x O y ), potassium oxide (K 2 O), carbon (C) and calcium oxide (CaO), sodium oxide (Na 2 O), or titanium oxide (TiO 2 ) [ 37 , 38 ]. The mineralogical composition of the tailings sample from the thermally active Heˇrmanice spoil heap is presented in Table 3. SiO2is also the most abundant (43%), and the accompanying aluminosilicates are present in smaller amounts. 3.2. Chemical Composition of Tailings At the Heˇrmanice spoil heap, which is thermally active, both the tailings found in the upper part of the spoil heap body and the burnt tailings that are being removed have been analyzed. In unburnt tailings, which were in the centre of attention, the prevailing
Appl. Sci. 2023,13, 7123 9 of 19 elements included Si (27%), Fe (25%), and Al (11%). It constituted the most decisive share of inorganic components. Considering the percentage presence of silicon, it can be assumed that acidification does not occur due to the loss of Si, but due to the lack of basic cations during the decomposition of aluminosilicates. The content of Si (48%) and Al (19%) in burnt tailings increased, while the content of Fe (15%) decreased. Table 3. Mineralogical composition of tailings using XRD method in %. Mineral Formula Wt Quartz SiO243 Muscovite KAl2(AlSi3O10)(F,OH)219 Clinochlore Mg 3.75 Fe 2+1.25 Si 3 Al 2 O 10 (OH) 813 Albite NaAlSi3O88 Potassium K 2 Feldspar KAlSi3O8−CaAl2Si2O810.5 Minority 4.5 In unburnt tailings from the surface of the Heˇrmanice spoil heap body, the total sulphur content was 3.5%. Raclavskáet al. also states that the total sulphur content of the anhydrous sample in OKC coal is generally low, and that the average in the individual layers and areas are usually lower than 1%, and reach an average of 0.78% in working seams [ 36 ]. Pešek presents a much larger range of total sulphur values of 0.4–4.8% with a mean value of 2.1% [ 39 ]. In burnt tailings, the total sulphur content is reduced to 0.5% due to endogenous combustion. Pešek also states that similar to sulphur content, fluoride and chloride content is highly variable but generally low, and phosphorus, fluorides, and chlorides are bound to the occurrence of hydroxylapatite. However, the chloride content in the black coal of the Upper Silesian Basin is usually lower than 0.025% [ 39 ]. Unburnt tailings from the Heˇrmanice spoil heap contained 0.3% of P and 0.5% of chlorides. Their higher representation in tailings in the Heˇrmanice spoil heap may be related to secondary halite contamination. The content of elements such as Na (<0.01%), Ca (0.7%), Mg (0.2%) in the unburnt tailings is very low. Daniels, Stewart, and Zipper (2018) report that heavy metals such as copper, nickel, and zinc are often linked with pyrite and other sulphide minerals. Increased levels of heavy metals in the soil solution can be toxic to plant roots and microbes, and they can also pose a risk to water quality [40]. However, the content of heavy metals in the OKCA carboniferous rocks is very low and their representation does not exceed the volume of other industrial emissions from an ecological point of view. That is why it does not represent a serious environmental burden (see Table 4). Table 4. Elemental analysis of samples using XRF method in %. % Burnt Tailings Tailings % Burnt Tailings Tailings Na <0.01 <0.01 Mo 0.0007 0.0011 Ag <0.0002 <0.0002 Nb 0.0066 0.0054 Al 19.04 11.16 Nd 0.0313 0.0292 As 0.001 0.020 Ni 0.03 0.03 Ba 0.2815 0.678 P0.19 0.30 Bi <0.00010 <0.00010 Pb 0.01142 0.0272 Br 0.0006 0.0082 Pr 0.0066 <0.00020 Ca 1.99 0.68 Rb 0.0608 0.0543 Cd 0.0017 0.00047 S0.54 3.50 Ce 0.0342 0.011 Sb 0.0021 0.00043 Cl 0.01 0.49 Se <0.00005 0.0006 Co 0.01 0.005 Si 48.09 26.70 Cr 0.06 0.04 Sn 0.0036 0.00081 Cs 0.0110 0.014 Sr 0.0388 0.0532 Cu 0.02 0.03 Ta 0.0118 0.0115
Appl. Sci. 2023,13, 7123 16 of 19 and 30% of CDW, Fe loses its affinity to sulphur and will probably be preferentially bound in the form of oxides and hydroxides in the mixture. 4. Conclusions The use of a mine waste dump after deep coal mining causes a number of practical problems. The most important problem to be solved is thermal activity. The pace and dynamics of succession in burning coal waste dumps depends on the stage of the fire, topography, and character of the substrate. Coal waste varies in colour, from light gray to black. A large part of the incoming solar radiation is thus retained as heat, and in sunny weather, the temperature on the surface of the spoil heap often exceeds the air temperature several times, which is fatal for plants. Another problem related to the thermal activity of the spoil heaps is their burning temperature, which reaches up to 1300 ◦ C in some hot spots, specifically at the Heˇrmanice spoil heap. The evaporated PTEs are mostly sorbed on the smallest particles of the generated ash during subsequent cooling (after entering the air). This results in larger particles being depleted of toxic metals. The remaining metals mainly enrich PM x due to sorption and condensation of vapours on fine PM x particles. Since the dust particle has the largest specific surface, the surface of the particles also has the highest concentration of toxic metals. In the event that the body of the landfill contaminates its surroundings (PM x , AMD) and intensive thermal activity occurs, such as in the case of the thermally active Heˇrmanice spoil heap, it is most often recommended to completely remove it. If the spoil heap is completely removed, then the area can be considered as a perfectly normal foundation soil with regard to the location of the body and on the bedrock. Dynamically developing, more flexible, and advanced technologies make it possible to apply the 3R principle (reduction, reuse, and recycling) of the circular economy in practice to the mining industry as well. The excavated burnt tailings, together with a larger fraction of unburnt tailings, can be used as construction material for embankments, roads, railways, and other constructions. They can also be used as raw material for civil engineering, industry, and reclamation. Forest reclamation and the so-called controlled succession are mainly used in the Ostrava-Karvina Coalfield. The land can be used, for example, as recreational areas, golf courses, hippodromes, city parks, construction sites, etc. In a circular economy, it is important to identify all possible ways of reuse of resources. Mixing alkaline construction waste with unburnt tailings from the surface of the spoil heap significantly affected its acidity, which has a positive effect on the increase of active and exchangeable acidity and the decrease of hydrolytic acidity. Construction and demolition waste (CDW) is also a rich source of a whole range of basic cations, which can be used to enrich the tailings. This can speed up and positively affect the reclamation of mountain landscapes. The excavated area can then be used, for example, for development in the form of residential units, shopping centres, manufacturing plants and many others. The area can also be used to build recreational centres, such as a tennis hall, football field, cultural facilities, and others. Author Contributions: Conceptualization, E.P., T.D. and V.V.; methodology, E.P. and T.D.; validation, E.P., T.D. and V.V.; formal analysis, K.M., V.V. and T.D.; investigation, L.S., J.C., K.M. and L.B.; resources, J.C.; writing—original draft preparation, E.P., V.V., T.D. and L.S.; writing—review and editing, E.P. and T.D.; visualization, T.D.; supervision, E.P.; project administration, V.V. and J.C.; funding acquisition, J.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by: VSB-TUO, Faculty of Mining and Geology—grants number SP2022/57; VSB-TUO, Faculty of Mining and Geology—grants number SP2023/017. Project CZ.11.4.120/0.0/0.0/15_006/0000074 TERDUMP Cooperation VŠB-TUO/GIG Katowice on the survey of burning dumps on both sides of the common border. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable.
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