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Article Assessment of Characteristics of Mine Drainage Treated with

Saba Shirin 1,* , Aarif Jamal 1, Christina Emmanouil 2,* and Akhilesh Kumar Yadav 1

1 Department of Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221 005, India; [email protected] (A.J.); [email protected] (A.K.Y.) 2 School of Spatial Planning and Development, Faculty of Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece * Correspondence: [email protected] (S.S.); [email protected] (C.E.); Tel.: +91-94159-75284 (S.S.)

Abstract: (AMD) occurs naturally in abandoned mines, and it contains hazardous toxic elements in varying concentrations. In the present research, AMD samples collected from an abandoned mine were treated with fly ash samples from four thermal power plants in Sin- grauli Coalfield in the proximate area, at optimized concentrations. The AMD samples were analyzed for physicochemical parameters and content before and after fly ash treatment. Morphological, geochemical and mineralogical characterization of the fly ash was performed using SEM, XRF and XRD. This laboratory-scale investigation indicated that fly ash had appreciable neutralization poten- tial, increasing AMD pH and decreasing elemental and sulfate concentrations. Therefore, fly ash may be effectively used for AMD neutralization, and its suitability for the management of coalfield AMD pits should be assessed further.

Keywords: abandoned mine; fly ash; acid mine drainage; SEM  

Citation: Shirin, S.; Jamal, A.; Emmanouil, C.; Yadav, A.K. 1. Introduction Assessment of Characteristics of Acid Mining activities exert an inherently high impact on the environment, and as such a Mine Drainage Treated with Fly Ash. common framework for responsible mining is urgently needed [1]. Among the various Appl. Sci. 2021, 11, 3910. https:// pollution sources related to mining activities, acid mine drainage (AMD) is created in doi.org/10.3390/app11093910 areas that have previously been mined and that contain pyritic materials in spoil piles or mine shafts; in the chemically reacts with and , facilitated Received: 31 March 2021 by the Thiobacillus sp. , resulting in the creation of AMD. Therefore, AMD is a Accepted: 22 April 2021 dangerous industrial byproduct, highly acidic and rich in sulfates [2]. The of Published: 26 April 2021 this secondary waste may reduce environmental risks, as well as increase the economic profitability of the mining industry [3]. In any case, while mine waste piles accumulate, Publisher’s Note: MDPI stays neutral AMD is continuously formed, and this affects industrial environments well after closure with regard to jurisdictional claims in of mining activities. In and around coal mines, the pollution of surface water by AMD published maps and institutional affil- iations. and other mine effluents may have serious environmental implications [4–6]. In many industrial settings, the consequences of AMD are considered moderate to severe [7,8]. There are various methods for treating AMD, such as using carbonate, sodium hydroxide, sodium or anhydrous ammonia, as well as zeolite- processes. The fresh zeolite, for example, presents a high ammonia removal efficiency from Copyright: © 2021 by the authors. mine effluents [9–11]. These chemicals raise the pH to acceptable levels and decrease the Licensee MDPI, Basel, Switzerland. of dissolved , but the cost of the treatment of AMD is usually very high This article is an open access article since the latter is generated up to 300 m3/day for 2000 tons/day or less of mine production distributed under the terms and conditions of the Creative Commons capacity [12]. On another note, fly ash is another deleterious byproduct generated by Attribution (CC BY) license (https:// coal burning. Its potential (metal ) may be higher in low pH batches [13]. creativecommons.org/licenses/by/ In the literature, neutralization of AMD has been attempted through various treatment 4.0/). systems, a simple one being the addition of . This method efficiently increases AMD

Appl. Sci. 2021, 11, 3910. https://doi.org/10.3390/app11093910 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 3910 2 of 10

pH value, but it necessitates large lime quantities and produces unstable secondary wastes. Other chemical [14,15], biological and microbiological [16–18] methods have also been applied, but clogging due to metal hydroxide precipitation may interfere with the biological and microbiological activity [19]. As such, a satisfactory and economic for the long-term treatment of AMD is still needed. Ideally, the material applied should be an undesirable byproduct not used for any other application [20]. Especially in India, which mostly depends on thermal power plants to meet its requirements, fly ash production is significant. India’s total installed power-generation capacity was 350,162 MW in March 2019, out of which the thermal power plants contributed to 222,927 MW; i.e., 63.7% of the total power-generation capacity of India [21]. Fly ash is indeed an undesirable byproduct of coal combustion, disposed of in specially designed landfills in an environmentally safe manner. This waste has been proven to be appropriate for AMD treatment due to its alkaline properties [22–25]. As an extravagant example, water quality from a heavily affected by AMD, with low macroinvertebrate productivity and diversity, significantly improved after impoundment works that directed from a fly ash pond to the stream. Species diversity and benthic production increased, and a few opportunistic fish feeding on insects appeared [26]. Focusing on these two kinds of toxic industrial waste, fly ash produced from a thermal power plant was characterized by its geochemical and mineralogical profile. Its suitability to successfully neutralize AMD pits was then assessed. These preliminary results may be utilized to manipulate fly ash to aid the neutralization of AMD, reducing its potential for metal mobilization.

2. Materials and Methods 2.1. Field Investigation and Sample Collection The Singrauli Coalfield is located between latitudes 24◦120 N and 23◦470 N in India, and four combustion plants are present (Figure1). The fly ash used in this study was collected from these plants in a dry state from the plants’ electrostatic precipitators. The chute hoppers were carefully opened, and gunny sacks made of strong poly-coated cotton with a 30 kg capacity were used to collect the dry fly ash. Each bag’s mouth was sealed Appl. Sci. 2021, 11, x FOR PEER REVIEWimmediately after collection, inserted into another poly-coated bag and brought3 of 11 to the laboratory (Banaras Hindu University).

Figure 1. The s studytudy area area.. The Gorbi mine (Figure 2) is an abandoned mine at the north of the Singrauli field that has been shown to produce AMD (pH 2.54, in-house data). Pit I, Pit II and Pit III of the mine were used for the collection of AMD for the present experiment. Further infor- mation on the AMD pits is given in Table 1. The sample collection and the physical and species chemical analysis of AMD followed Indian standard guidelines [27–30]. All sam- ples were brought to the laboratory (BHU) in sealed gunny sacks and stored until further analysis.

Table 1. Details of examined pits. Sl. No. Pit No. Pit Area in Ha Depth Water Volume 1 I 40.00 20.00 million m3 2 II 8.00 4.00 million m3 50 m * 3 III 26.00 13.00 million m3 74.00 37.00 million m3 * Approximate depth was considered. Appl. Sci. 2021, 11, 3910 3 of 10

The Gorbi mine (Figure2) is an abandoned mine at the north of the Singrauli field that has been shown to produce AMD (pH 2.54, in-house data). Pit I, Pit II and Pit III of the mine were used for the collection of AMD for the present experiment. Further information on the AMD pits is given in Table1. The sample collection and the physical and species Appl. Sci. 2021, 11, x FOR PEER REVIEWchemical analysis of AMD followed Indian standard guidelines [27–30]. All samples4 were of 11

brought to the laboratory (BHU) in sealed gunny sacks and stored until further analysis.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 11

Figure 2. The Gorbi abandoned mine mine,, showing P Pitsits I, II and III ( (viavia Google Earth).

2.2.Table Fly 1. AshDetails Characterization of examined pits. The fly ash samples were air-dried for 2 to 3 days in a drying oven (Thermo Scientific Sl. No. Pit No. Pit Area in Ha Depth Water Volume Heraeus VT6060P) and mixed by the coning and quartering method, followed by obtain- ing a representative1 sample I for analysis [31] 40.00. Mineral identification in fly 20.00ash was million charac- m3 2 II 8.00 4.00 million m3 terized using X-ray fluorescence Figure(XRF), 2.X -Theray Gorbi diffraction abandoned (XRD)50 mine m and * , showing scanning Pit eslectron I, II and m IIIi- (via Google Earth). 3 III 26.00 13.00 million m3 croscopy (SEM). 74.00 37.00 million m3 X-ray fluorescence (XRF) was used2.2. for Fly the Ash nondestructive Characterization chemical analyses of the * Approximate depth was considered. fly ash. Mineral phase recognition by X-ray diffractionThe fly ash method samples depends were air on-dried the diffraction for 2 to 3 days in a drying oven (Thermo Scientific peaks2.2. Fly at Ash different Characterization 2θ values relating to theHeraeus mineral VT6060P)’s d-spacing and mixedvalue. Theby the X- rayconing plot and of a quartering method, followed by obtain- diffractionThe fly angle ash samples(2θ) versus were intensity air-dried ofing for radiation 2a torepresentative 3 days reveals in a the drying sample interplanar oven for (Thermoanalysis spacing [31] Scientific and,. Mineral in identification in fly ash was charac- turn,Heraeus the VT6060P)type of mineral and mixed present. by the coningterized and using quartering X-ray fluorescence method, obtaining (XRF), aX represen--ray diffraction (XRD) and scanning mi- tativeX- sampleray diffraction for analysis (XRD) [31 of]. oven Mineral-driedcroscopy identification fly-ash (SEM). samples in fly of ash less was than characterized 75 μ was carried using outX-ray to fluorescencedetect the mineral (XRF), composition X-ray diffraction of its (XRD) variousX-ray and fluorescence mineral scanning phases. electron (XRF) Mineral was microscopy used phase for recog- (SEM). the nondestructive chemical analyses of the nitionX-ray by XRD fluorescence depends on (XRF) the diffraction was usedfly forpeaks ash. the Mineral at nondestructive different phase 2θ recognition values chemical relating by analyses X to-ray the diffraction of min- the method depends on the diffraction eralfly ash.’s d- Mineralspacing phasevalue. recognitionAccording byto X-rayBraggpeaks’ diffractions law,at different as each method 2θ mineral values depends hasrelati a on nghighly the to diffractionthe ordered mineral ’s d-spacing value. The X-ray plot of a crystallinepeaks at different structure, 2θ valuesits d-spaci relatingng values to thediffraction are mineral’s characteristics angle d-spacing (2θ) of versus value.a set intensityof The diffraction X-ray of radiation plot peaks. of a reveals the interplanar spacing and, in ANdiffraction Ultima angleIV (Austin, (2θ) versus TX 78717 intensity, USA) of withturn, radiation athe Cu type K reveals-alpha of mineral c theentral interplanar present. instrumentation spacing and,facility in wasturn, used the typein the of current mineral work. present. The scatteringX angle-ray diffraction2θ ranged from(XRD) 5 °of to oven 85° at-dried a scanning fly-ash samples of less than 75 μ was carried rate ofX-ray 5°/min diffraction with a (XRD)step size of oven-driedof 0.05°,out and fly-ash to the detect target samples the used mineral of lesswas thancomposition a Cu 75-Kaµ was (⅄ =of carried 1.5418A its various out°) mineral phases. Mineral phase recog- target.to detect the mineral composition of its variousnition by mineral XRD depends phases. Mineralon the diffraction phase recognition peaks at different 2θ values relating to the min- by XRDFurthermore, depends on the the fly diffraction ash samples peaks eral were at’s differentd examined-spacing 2 θ value. undervalues According high relating-resolution to to the Bragg mineral’s scanning’s law, as each mineral has a highly ordered electrond-spacing microscopy value. According (JOEL model to Bragg’s JSM law,-6480crystalline as LV, each Tokyo mineralstructure, Japan) has its to a highlyd understand-spacing ordered values their crystalline aresurface characteristics of a set of diffraction peaks. microstructure,-morphology. its d-spacing Since values fly ash are is characteristics nonconductive,AN Ultima of IV ait set (wasAustin, of coated diffraction TX 78717with peaks. ,a USA platinum) AN with Ultima layera Cu K-alpha central instrumentation facility forIV (Austin,180 s as TXneeded 78717, for USA) a current with aof Cu 50 K-alphawasmA usedat vacuum central in the instrumentationcurrentbefore acquiring work. The facility the scatt SEMer wasing micro- usedangle 2θ ranged from 5° to 85° at a scanning graph.in the current work. The scattering anglerate 2 θofranged 5°/min fromwith 5a◦ stepto 85 size◦ at aof scanning 0.05°, and rate the of target used was a Cu-Ka (⅄ = 1.5418A°) 5◦/min with a step size of 0.05◦, and thetarget. target ⅄ used ⅄ was⅄ ⅄ a Cu-Ka⅄ ⅄ (⅄ = 1.5418A◦) target. 2.3. NeutralizationFurthermore, Experimental the fly ash Setup samples wereFurthermore, examined under the fly high-resolution ash samples were scanning examined under high-resolution scanning electronFor microscopythe experiment, (JOEL 1000 model mL of JSM-6480 AMDelectron (pH LV, 2.54)Tokyo,microscopy were Japan) collected (JOEL to understand model into containers. JSM their-6480 surface LVFour, Tokyo Japan) to understand their surface sets of AMD were prepared in separatemicro beakers.-morphology. The AMD Since sample fly ash’s physicochemical is nonconductive, it was coated with a platinum layer characteristics were measured accordingfor to Indian180 s as standard needed guidelinesfor a current, as ofmentioned 50 mA at [27 vacuum– before acquiring the SEM micro- 30]. In brief, fly ash samples (100 g; one graph.from each λ plant) were added to the beakers. Each sample was subjected to mine water leaching for 35 days. Hence, four different types of samples were produced. Finally,2.3. the Neutralization leachate samples Experimental were collected Setup and filtered (Whatman Grade 42 filter paper) into samplingFor the bottles experiment, and assessed 1000 mL for of the AMD aforemen- (pH 2.54) were collected into containers. Four tioned physicochemical parameters. sets of AMD were prepared in separate beakers. The AMD sample’s physicochemical characteristics were measured according to Indian standard guidelines, as mentioned [27– 30]. In brief, fly ash samples (100 g; one from each plant) were added to the beakers. Each sample was subjected to mine water leaching for 35 days. Hence, four different types of leachate samples were produced. Finally, the leachate samples were collected and filtered (Whatman Grade 42 filter paper) into sampling bottles and assessed for the aforemen- tioned physicochemical parameters. Appl. Sci. 2021, 11, 3910 4 of 10

micro-morphology. Since fly ash is nonconductive, it was coated with a platinum layer for 180 s as needed for a current of 50 mA at vacuum before acquiring the SEM micrograph.

2.3. Neutralization Experimental Setup For the experiment, 1000 mL of AMD (pH 2.54) were collected into containers. Four sets of AMD were prepared in separate beakers. The AMD sample’s physicochemical char- acteristics were measured according to Indian standard guidelines, as mentioned [27–30]. In brief, fly ash samples (100 g; one from each plant) were added to the beakers. Each sample was subjected to mine water leaching for 35 days. Hence, four different types of leachate samples were produced. Finally, the leachate samples were collected and filtered (Whatman Grade 42 filter paper) into sampling bottles and assessed for the aforementioned physicochemical parameters.

3. Results and Discussion 3.1. XRF Analysis of Fly Ash

The major minerals phases in the fly ash samples were found to be SiO2, Al2O3, CaO, MgO, MnO, Na2O, K2O and FeO, as shown in Table2. Different percentages of the minerals were found in each of the four fly ash samples. The Anpara TPP sample showed the highest concentration of Al2O3 and CaO (and K2O), and Shaktinagar TPP showed the highest concentration of SiO2 (and MnO). Na2O, K2O, CaO and MgO can be considered as acid-neutralizing groups, and Al2O3 and F2O3 as amphoteric groups.

Table 2. Characteristics of the fly ash samples.

Parameters

Sampling Location SiO2 Al2O3 CaO FeO MgO MnO Na2OK2O Concentration in % Anpara TPP 45.9 27.7 17.4 3.2 2.9 1.6 0.6 0.7 Renusagar TPP 40.5 23.0 16.1 8.1 8.0 2.2 1.4 0.7 Shaktinagar TPP 44.7 26.3 15.6 7.9 1.7 2.1 1.3 0.4 Vindhyachal TPP 44.0 20.7 14.9 8.4 5.8 4.8 1.2 0.2 Note: TPP = thermal power plant.

The following chemical reactions were expected after mixing with AMD: ◦ (i) Silica (SiO2) is stable below 870 C. It will not react with any base or acid except hydrofluoric acid. Hence, SiO2 will not have an impact on AMD. (ii) Alumina (Al2O3) will also not have any impact on AMD, at least at room . (iii) Hematite (Fe2O3) will have negligible impact on AMD, up to the most extreme temperature and pressure found in the field. (iv) Lime (CaO) present in fly ash will react with H2O to give Ca(OH)2. (v) Magnesia (MgO) present in fly ash will react with H2O to give Mg (OH)2, which will also neutralize acidic (vi) Na2O will react with H2O to give NaOH. (vii) K2O will react with H2O to give KOH.

Thus, considering the ability of CaO, MgO, Na2O and K2O to provide hydroxyl radicals in the solution, the acid-neutralization potential of fly ash may be calculated. The amount of SO3 in the fly ash was only 0.18%, and for the sake of simplicity, its effect may be disregarded [32].

3.2. XRD Analysis of Fly Ash The mineral phase resulting peaks (Figure3) were compared to the Joint Committee on Powder Diffraction Standards (JCPDS) data. The crystalline phases present in the fly ash consisted of quartz (SiO2), mullite (Al6SiO2), hematite (Fe2O3) and (CaO). Similar results (material rich in quartz, mullite, hematite and quicklime) were noted in Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 11 Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 11

3.2. XRD Analysis of Fly Ash 3.2. XRD Analysis of Fly Ash Appl. Sci. 2021, 11, 3910 The mineral phase resulting peaks (Figure 3) were compared to the Joint Committee5 of 10 The mineral phase resulting peaks (Figure 3) were compared to the Joint Committee on Powder Diffraction Standards (JCPDS) data. The crystalline phases present in the fly on Powder Diffraction Standards (JCPDS) data. The crystalline phases present in the fly ash consisted of quartz (SiO2), mullite (Al6SiO2), hematite (Fe2O3) and calcium oxide (CaO). ash consisted of quartz (SiO2), mullite (Al6SiO2), hematite (Fe2O3) and calcium oxide (CaO). Similar results (material rich in quartz, mullite, hematite and quicklime) were noted in the theSimilar XRD results analysis (material of a fly rich ash samplein quartz, from mullite, a coal-fired hematite boiler’s and quicklime) electrostatic were precipitators, noted in the XRD analysis of a fly ash sample from a coal-fired boiler’s electrostatic precipitators, gen- generallyXRD analysis in bituminous of a fly ash fly sample ashes [from22]. a coal-fired boiler’s electrostatic precipitators, gen- erally in bituminous fly ashes [22]. erally in bituminous fly ashes [22].

A A Anpara; Renusagar Shaktinagar; Anpara; Renusagar Vindhyachal C Shaktinagar; Vindhyachal D C B D B D A C B D A C B A A=Quartz A C B=CalciumA=Quartz Oxide D C B=Calcium Oxide D C=Mullite B D=HematiteC=Mullite B D=Hematite

Intensity (a.u) A

Intensity (a.u) A C D C D B B

10 20 30 40 50 60 70 80 10 20 Bragg's30 diffraction40 angle50 '2'(degree60 ) 70 80 Bragg's diffraction angle '2'(degree)

Figure 3. X-ray diffractograms of selective fly ash samples from various thermal power plants using the abbreviations for FigureFigure 3. 3.X-ray X-ray diffractograms diffractograms of of selective selective fly fly ash ash samples samples from from various various thermal thermal power power plants plants using using the the abbreviations abbreviations for for minerals followed by Kretz [33]. mineralsminerals followed followed by by Kretz Kretz [ 33[33].]. 3.3. SEM Analysis of Fly Ash 3.3.3.3. SEMSEM AnalysisAnalysisof ofFly Fly AshAsh SEM images of 2 mm size fly-ash particles are shown in Figure 4. Large particle ag- SEMSEM imagesimages ofof 2 mmmm size fly fly-ash-ash particles particles are are shown shown in in Figure Figure 4.4 .Large Large particle particle ag- glomerations were frequently seen, and several spherical particles, together with irregu- agglomerationsglomerations were were frequently frequently seen, seen, and and several several spherical spherical particles, particles, together together with with irregu- irreg- larly shaped particles, were also observed. The rounded particles were mostly glassy, ularlylarly shaped shaped particles, particles, were also observed. The rounded particles werewere mostlymostly glassy, glassy, while the angular particles were typically made of crystalline solids such as quartz, mul- whilewhile the the angular angular particles particles were were typically typically made made of of crystalline crystalline solids solids such such as quartz,as quartz, mullite, mul- lite, magnetite, hematite, etc. A similar composition was noted for Canadian [25] and Brit- magnetite,lite, magnetite, hematite, hematite etc., Aetc. similar A similar composition composition was was noted noted for Canadianfor Canadian [25 ][25] and and British Brit- ish coal fly ash samples [34], with their crystalline portion comprising mainly quartz and coalish coal fly ashfly ash samples samples [34 ],[34 with], with their their crystalline crystalline portion portion comprising comprising mainly mainly quartz quartz and and mullite. Both ashes mainly consisted of an amorphous phase; here, the amorphous phase mullite.mullite. BothBoth ashesashes mainlymainly consistedconsisted ofof an an amorphous amorphous phase; phase; here, here, the the amorphous amorphous phase phase was mostly dominant in the Shaktinagar and Renusagar fly ashes (Figure 4). waswas mostly mostly dominant dominant in in the the Shaktinagar Shaktinagar and and Renusagar Renusagar fly fly ashes ashes (Figure (Figure4). 4).

Figure 4. Cont. Appl. Sci. 2021, 11, 3910 6 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 11

FigureFigure 4. 4.SEM SEM images image (2s (2µ m)μm) of of fly fly ashes ash fromes from Anpara, Anpara, Shaktinagar, Shaktinagar, Renusagar Renusagar and Vindhyachaland Vindhyachal TPPs. TPPs. 3.4. AMD Quality Analysis and Neutralization Potential 3.4. InAMD the Quality present Analysis research, and AMD Neutralization analysis revealedPotential low pH, high TDS concentrations, electricalIn the conductivity present research, and various AMD elemental analysis revealed (Table low3). Considerable pH, high TDS neutralization concentrations, of AMDelectrical was achievedconductivity with and the various addition elemental of fly ash, ions as shown (Table in 3). Table Considerable4. Fly ash showedneutralization a pH ofof 12, AMD and was as such achieved the pH with of AMDthe addition was elevated of fly ash, from as 2.5shown to 6.0 in within Table 4 35. Fly days ash (Figure showed5 ). a TherepH of was 12, and a decreasing as such the trend pH of in AMD all chemical was elevated species from of the 2.5 leachate. to 6.0 within The 35 pH days remained (Figure relatively5). There constantwas a decreasing after 35 daystrend ofin theall chemical experiment. species These of the results leachate. may The be attributedpH remained to therelatively fly ash Ca-bearingconstant after minerals 35 days [23 of]. the It hasexperiment also been. These postulated results that may CaO be attributed reacts with to the the otherfly ash siliceous Ca-bearing and aluminousminerals [23] materials. It has also of the been fly postulated ash and forms that cementitiousCaO reacts with complexes the other thatsiliceous stabilize and AMD’s aluminous leaching materials elements of the [25 ].fly Comparison ash and forms of the cementitious four samples complexes showed that that thestabilize highest AMD CaO’s contentleaching was elements found [25] in the. Comparison Anpara TPP of the fly four ash. samples However, showed silicates that and the aluminumhighest CaO and content ferrous was oxides found also in contribute the Anpara to TPP acid fly buffering ash. However, at low pH silicates [23]. and alumi- numElectrical and ferrous conductivity oxides also and contribute TDS of the to treated acid buffering AMD were at low reduced. pH [23] All. of the showed an initial decrease in the sulfate concentrations. This was due to the formation −1 of CaSO4 upon reaction with the fly ash. Al concentration decreased from 33.7 mg L to 21.0 mg L−1. It was believed that the aluminum reacted with the silica in the ash and formed more mullite. In general, the heavy-metal concentrations decreased. B, Ba, Be, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sr and Zn concentrations were all low and decreased to undetectable Appl. Sci. 2021, 11, 3910 7 of 10

levels. In brief, after the described laboratory experiment, the physicochemical parameters of the AMD from the abandoned mine were under the permissible limits for release in water catchments.

Table 3. Physicochemical characteristics of the AMD water.

Parameter Unit Concentration pH (AMD water) – 2.5 pH (Fly ash) – 12.0 EC µS/m 2990 TDS ppm 2270 Acidity (CaCO3) 3097 Na 200 Mg 45 Al 33.7 K 43.5 Ca 158 Fe 166 Fe2+ 145 Fe3+ 156 2- SO4 190 NO3- 45 Almg/L 37.5 B 0.07 Ba 0.02 Be <0.005 Cd 0.05 Co 0.58 Cr <0.005 Cu 0.06 Mn 45 Ni 0.70 Pb 0.05 Sr 0.74 Zn 3.0

Table 4. Physicochemical characteristics of the AMD after addition of fly ash.

Thermal Power Plant Parameter * Unit Anpara Renusagar Shaktinagar Vindhyachal Concentration pH – 6.0 6.0 6.0 6.0 EC µS/m 765.0 780.0 735.0 752.0 TDS ppm 682.0 675.0 695.0 672.0 Na 20.0 15.0 21.0 19.0 Mg 53.0 54.0 51.0 49.0 Al 29.0 21.0 27.0 25.0 K 12.0 10.0 9.0 13.0 Ca 55.0 49.0 53.0 52.0 mg/L Fe 3.1 4.5 2.1 4.7 Fe2+ 32.0 32.0 30.0 31.0 Fe3+ 21.0 20.0 19.0 20.0 2− SO4 32.0 29.0 24.0 50.0 − NO3 15.0 12.0 24.0 33.0 * All other species below limit of detection. Appl. Sci. 2021, 11, 3910 8 of 10 Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 11

Figure 5. VariationVariation of pH in relation to duration treatment of the AMD withwith flyfly ash.ash.

ElectricalThe amount conductivity of fly ash requiredand TDS toof reachthe treated acceptable AMD limits were isreduced. approximately All of the 4–5 leacha- times teshigher show thaned an the initial lime decrease requirement; in the therefore, sulfate concentrations. high amounts This of fly was ash due can to be the disposed formation of. ofIt shouldCaSO4 up beon noted reaction that flywith ash the may fly containash. Al concentration metals of toxicological decreased concern, from 33.7 such mg asL−1 Cd, to 21.0Cr, Co, mg Ni L−1 and. It was Pb [ 35believed]; these that metals the may aluminum leach into reacted the environment with the silica during in the the ash AMD and formedneutralization more mullite. process, In creating general, additional the heavy- problems.metal concentrations If high amounts decreased. of fly B, ash Ba, are Be, used, Cd, Co,more Cr, metals Cu, Mn, are Ni, released Pb, Sr intoand theZn environmentconcentrations [23 were]. So, all only low 30% and ofdecreased the produced to undetect- fly ash ablewas usedlevels. in In this brief, experiment. after the described Nevertheless, laboratory these elements’experiment, leaching the physicochemical can be limited pa- by rametersmaintaining of the near-neutral AMD from pH the conditions abandoned and mine adding were materials under that the canpermissible absorb and limits adsorb for releasethem. It in is water known catchments. that the mine waste material itself can achieve the latter [22]; furthermore, low AMDThe amount pH is counterbalancedof fly ash required by to high reach fly acceptable ash pH, and limits vice is versa approximately [34], so near-neutral 4–5 times higherconditions than canthe belime achieved. requirement; The remaining therefore, materialshigh amounts (treated of fly AMD) ash can may be be disposed processed of. Itfurther should (for be example,noted that with fly zeolite),ash may andcontain the solidmetals residue of toxicological may be used concern as a mine, such backfill as Cd, Cr,material Co, Ni for and a true Pb zero-waste[35]; these environmentalmetals may leach management into the environment scheme [24]. during the AMD neutralization process, creating additional problems. If high amounts of fly ash are used, 4. Conclusions more metals are released into the environment [23]. So, only 30% of the produced fly ash was usedAMD in neutralization this experiment. by optimized Nevertheless, amounts these of elements fly ash’ mayleaching become can very be limited useful by in maintainingterms of the safenear and-neutral economic pH conditions disposal ofand waste, adding especially materials when that thermalcan absorb power and stations adsorb them.and coal It is mines known are that near the each mine other. waste As shown material in theitself present can achieve research, the AMD latter pH [22] was; further- raised moreconsiderably, low AMD after pH the is counterbalanced addition of fly ash. by high Metal fly concentrations ash pH, and vice and versa sulfate [34 ions], so near were- neutralreduced, conditions and AMD can physicochemical be achieved. The characteristics remaining materials were improved. (treated This AMD) study may concludes be pro- cessedthat fly further ash may (for be carefullyexample, placed with zeolite), in abandoned and the mine solid pits residue containing may AMD be used and neutralizeas a mine backfillthis acidic material liquid for waste. a true Nevertheless, zero-waste environmental additional research management should focus scheme on the[24] new. waste created, its stability, its leaching potential and its general environmental impact. 4. Conclusions Author Contributions: S.S.: primary investigation, data curation, methodology, conceptualiza- tion, writing—originalAMD neutralization draft preparation;by optimized A.J.: amounts investigation, of fly data ash analysis; may become C.E.: conceptualization, very useful in termswriting—review of the safe and and editing; economic A.K.Y.: disposal conceptualization, of waste, especially writing—original when thermal draft preparation.power stations All andauthors coal have mines read are and near agreed each to other. the published As shown version in the of present the manuscript. research, AMD pH was raised considerably after the addition of fly ash. Metal concentrations and sulfate ions were re- Funding: This research received no external funding. duced, and AMD physicochemical characteristics were improved. This study concludes Institutional Review Board Statement: Not applicable. Appl. Sci. 2021, 11, 3910 9 of 10

Informed Consent Statement: Not applicable. Acknowledgments: The authors would like to express their highest gratitude to the Indian Institute of Technology (BHU) and other organizations that assisted in this research work: NCL, Singrauli; the thermal power plants of Singrauli Coalfield; and the Ministry of Human Resource Development (Govt. of India). Conflicts of Interest: The authors declare no conflict of interest.

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