Applied Geochemistry 25 (2010) 1621–1631

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Applied Geochemistry

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Mobility of trace and potentially harmful elements in the environmentfrom high sulfur Indian coal mines

B.P. Baruah ⇑, Puja KhareCoal Chemistry Division, North-East Institute of Science and Technology (CSIR), Jorhat 785 006, Assam, India

a r t i c l e i n f o

Article history:Received 12 May 2009Accepted 13 August 2010Available online 21 August 2010Editorial handling by A. Kolker

0883-2927/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.apgeochem.2010.08.010

⇑ Corresponding author.E-mail address: bpbaruah@yahoo.com (B.P. Baruah

a b s t r a c t

Coal mine rejects and sulfide bearing coals are prone to acid mine drainage (AMD) formation due to aque-ous weathering. These acidic effluents contain dissolved trace and potentially harmful elements (PHEs)that have considerable impact on the environment. The behavior of these elements in AMD is mainly con-trolled by pH. The focus of the present study is to investigate aqueous leaching of mine rejects for pre-diction of acid producing potential, rates of weathering, and release of PHEs in mine drainage. Minereject (MR) and coal samples from the active mine sites of Meghalaya, India typically have high S contents(1.8–5.7% in MR and 1.7–4.7% in coals) with 75–90% of the S in organic form and enrichment of most ofthe PHEs in rejects. Aqueous kinetic leaching experiments on mine rejects showed high acid producingpotential and release of trace and potentially harmful elements. The elements (Sb, As, Cd, Cr, Co, Cu,Pb, Mn, Ni, V and Zn) in mine sample leachates are compared with those in mine waters. The concentra-tions of Al, Si, P, K, Ti, Mn, Fe, Co, Ni, Cu, Zn and Pb are found to increase with leaching time and are neg-atively correlated with pH of the solution. The processes controlling the release of these elements are acidleaching, precipitation and adsorption. The critical loads of PHEs in water affected by AMD are calculatedby comparing their concentrations with those of regulatory levels. The Enrichment Factors (EFs) and soilpollution indices (SPIs) for the elements have shown that PHEs from coal and mine reject samples aremobilized into the nearby environment and are enriched in the associated soil and sediment.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Coal mining activities have a significant negative impact on theenvironment, particularly on aquatic resources. Sulfide minerals,mainly pyrite and marcasite, present in coal and mine rejects arelargely responsible for acid mine drainage (AMD) formation dueto aqueous weathering. A number of reactions between the acidicdischarge and other mineral phases present give rise to enhanceddissolution and mobilization of S and hazardous toxic elementsduring coal mining. In order to study the genesis of the problemand suitable remediation approaches, it is necessary to investigatethe physico-chemical characteristics of the coal of mine rejects,and of associated geological materials.

Potential acidity of drainage from mine wastes is predicted bygeochemical tests such as static or kinetic leaching techniques(Ferguson and Erickson, 1988). Kinetic aqueous leaching tech-niques are the widely accepted methods for the assessment of acidproducing potentials and potentially harmful element (PHE) mobi-lization from coal and mine rejects (Kaasalainen and Yli-Halla,2003; Ren et al., 2004). These tests can predict weathering of sam-ples in order to assess the potential to generate acidity, the rate of

ll rights reserved.

).

acid formation, sulfide oxidation, in situ neutralization, and disso-lution of trace and PHEs and control (Campbell et al., 2001).

Sub-bituminous Indian coals of Tertiary age are prone to acidmine drainage formation (Baruah et al., 2005, 2006; Baruah,2009; Baruah and Khare, 2007). These coals have different charac-teristics in comparison to other Indian coals. The characteristics ofthese coals are low ash content and high S content, with high or-ganic S, due to the influence of marine sources during diagenesis(Chandra et al., 1983; Singh and Singh, 2000). These coals have ahigh content of volatile matter and vitrinite contents, yielding dou-ble the amount of tar in comparison to other Indian coals. Thesecoals are perhydrous in nature with a high H/C ratio. The miningof these coals generates a large volume of reject materials. The re-jects and coals dumped near the pit entrance are exposed to theenvironment. Pyrite present in these materials is oxidized andhydrolyzed resulting in the formation of acid mine drainage, acidic,SO2�

4 -rich drainage water. The continuous leaching of mine wastesand aqueous discharge from the mining sites in the state of Megha-laya is a major environmental concern.

The AMD problem in these Indian coal mines has not been ade-quately studied so far, however some studies have reported theoccurrence of the problem in NE Indian coal mines (Singh, 1987,1988; Baruah et al., 2005, 2006; Baruah, 2009; Khare and Baruah,2008; Khare et al., 2007, 2008; Bhattacharjee et al., 2007). For

1622 B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631

effective site specific management of the AMD and associated re-lease of toxic elements, laboratory simulation studies are essential.Mine rejects subjected to kinetic leaching in low pH environmentreveal information on weathering rates, and rates of acid produc-tion and aqueous leaching of trace and PHEs. Enrichment Factorsand soil pollution indices show the impact of these elements onwater, soil and sediment. The present investigation reports detailedcharacterization of coal, soil, sediments, simulated and actual AMDwaters, and the processes taking place during AMD formation andits impact on the environment. In addition, the study will help informulation of a management action plan for the study area.

2. Experimental

2.1. Sample collection

Freshly mined coal and reject samples were collected from coal-fields of Jaintia (Lat. 25�100 and 25�280N, Long. 92�080 and 92�340E)and West Khasi Hills (Lat. 25�100 and 25�420N, Long. 91�140 and91�550E), Meghalaya, India (Fig. 1). The coal samples from coalbearing sites will be henceforth termed Bapung (MB), Khileriat(MK), Mondiati (MM) and Sutunga (MS) of Jaintia Hills, Ukium(MU), Lateher (ML) and Challang (MC) of West Khasi Hills, andthe mine rejects from Jaintia Hills as MR1 and MR2 respectively.The samples were ground to a mesh size of �72 BS (0.211 mm),preserved in desiccators, and used in all subsequent analyses.

Soil samples from the mining sites were collected from depthsof 5–7.5 and 15–20 cm below the surface of the soil, dependingon the depth of the soil horizon (Herr and Gray, 1997).

Fig. 1. Location map o

Sediment samples were collected by submerging sample con-tainers to the stream bed and scooping sediment directly into con-tainers. Three sediment samples were collected from each locationand considered as one set of replicate samples (Herr and Gray,1997). A minimum of three sets of replicate samples were takenfrom each location. The air-dried samples of �72 BS (0.211 mm)mesh size were used for all subsequent analyses.

2.2. Leaching experiments

Kinetic leaching experiments were conducted for a period of75 days to obtain information on the rate of sulfide oxidation,and the rate of release of trace elements and PHEs and acid produc-tion. Coal-bearing mine reject samples having similar S contentswere selected because of their continuous exposure to naturalweathering in the mine site.

The leaching studies were carried out in a glass column havingan 11 cm internal diameter and a height of 105 cm. The base plateof the column was covered with glass beads (2–3 mm diameter) toa depth of �30 mm to prevent water build-up during the move-ment of air and water through the sample. The material to be lea-ched was poured into the column; a layer of glass beads was placedon top of the sample to a depth of 20–40 mm for uniform distribu-tion of the water that was periodically introduced from the top ofthe column. The leaching experiments were conducted at a con-stant temperature of 25 �C.

Simulation of AMD development was done by taking 500 gm ofrepresentative mine reject sample and adding 1 L of deionizedwater (in each addition). The experiments were initially carried

f sampling sites.

Table 1Proximate and S analysis of coal and mine rejects (%, as received basis).

Proximate analysis TS Forms of sulfur

M Ash VM FC PS SS OS

MM 2.7 11.5 34.6 51.2 3.51 0.33 0.63 2.55MS 2.9 20.0 35.6 41.5 2.90 0.15 0.29 2.46MK 3.1 11.5 41.4 44.0 4.66 0.24 0.37 4.05MB 1.5 11.5 40.5 46.5 4.23 0.26 0.43 3.54MU 2.4 2.7 48.7 46.2 4.4 – – –ML 2.3 4.3 56.0 37.4 2.8 – – –MC 6.8 4.4 44.2 44.6 1.7 – – –MR 1 1.3 46.8 25.7 26.2 5.7 0.39 0.45 4.86MR 2 0.30 70.4 14.4 14.9 1.8 0.07 0.05 1.68

PS: pyritic sulfur, SS: sulfate sulfur, OS: organic sulfur, MR: mine rejects.

B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631 1623

out by addition of water on an hourly basis initially up to 7 daysfollowed by addition up to 11 days on a daily basis. Monitoringof pH was done continuously.

2.3. Coal and water sample analysis

The proximate and S analyses of the coal and reject sampleswere done by proximate (TGA 701, Leco, USA) and sulfur(S-144DR, Leco, USA) analyzers, respectively. Using microproces-sor-based water and soil analysis kit model 1160E (Environmental& Scientific Instrument Co., India), pH, Eh, conductivity, acidity andTDS of the leachate solutions at different time intervals were re-corded. Sulfate ion concentrations were determined by standardgravimetric analysis. The forms of S in coal and reject samples wereestimated by standard wet chemical methods (IS 1352, 1959).

2.4. Elemental analysis

The elements present in coal and leachate samples were deter-mined by Energy Dispersive X-ray Fluorescence (polarized) [ED-XRF (P)] spectrometer, Spectro, Xepos with Pd window X-ray tube.ED-XRF has been used previously for the determination of traceand PHEs in coal samples (Ward et al., 1999).

For the analysis of solid samples, pellets were prepared by mix-ing 4 g of sample powder with 0.9 g of Hoechst wax in a homoge-nizer (Swing Mill MM 301) and then pressing the mixture in amanual hydraulic press under 10 tonnes of pressure to form a32 mm pellet. For the analysis of water samples, 4 gm of samplewas poured into a sample cup with an outer diameter of 32 mm.The bottom of the sample cup was covered with a 4 lm prolenefilm.

2.5. Quality assessment/quality control

The ED-XRF instrument was calibrated by the TURBOQUANTcalibration method, which can determine 49 elements (Na to U)in solid samples with accuracies between 10% and 20% and 29(Al to Bi) in water. The matrix effect of the samples is correctedusing the Compton method. As a reference, validation and calibra-tion of the system was performed using samples calibrated by theparent office of SPECTRO Analytical Instruments, GmbH, Germany.Normalization values and error corrections are achieved by a spe-cialized software program (SPECTRO X-LABPRO).

Blanks and duplicate sample analyses were performed for about10% of all samples according to standard operating procedures.Detection limits of individual elements ranged between 0.0003%and 0.15% in pellets and between 1 and 100 ng cm�3 in water sam-ples. The estimated precision of XRF analysis ranged between 0.1%and 30% for individual elements, with most being <5%.

Table 2Trace and potentially harmful elements (ppm) in coal and mine rejects of Meghalaya.

Elements Coal Mine rejects

MB MK MM MS MU ML MC MR1 MR2

Sb 6.0 13.4 6 22.7 6.0 19.7 21.8 12.6 11.1As 1.0 1.2 3.0 1.2 1.0 1.0 1.0 1.3 25.9Cd 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.1 5.0Cr 36.2 41.2 55.5 51.2 17.9 24.4 27.9 259 361Co 10.5 12.3 15.0 9.9 11.8 3.0 11.5 22 20.9Cu 2.8 13.4 6.4 3.6 6.1 8.4 40.0 23.6 32.9Pb 6.1 12.0 6.5 13.7 2.4 4.9 7.4 149 98Mn 46.9 56.2 31.6 38.6 86.6 88.5 65.3 104 87Ni 2.8 4.6 4.1 4.5 2.0 2.4 9.8 36.4 58V 60.8 70.3 74 71.4 18.8 34.1 48.5 50 55Zn 19.2 13.6 8.5 11.4 12.2 21.9 36.6 35.8 55

3. Results and discussion

3.1. Characteristics of the coal and mine reject samples

The coals (Table 1) have low ash, high volatile matter and S con-tents, while the rejects have high ash and low volatile matter con-tents. Sulfur contents in coal and mine rejects were found to becomparable, and most of the S is in the organic form (75–90%).The characteristics of these coals are attributed to their formation/deposition over platform areas in estuarine and lagoonal environ-ments, which were subjected to recurrent marine transgressionsand regressions during the Eocene Period (Singh and Singh, 2000).

The concentrations of trace and potentially harmful elements(Mn, V, Cr, Zn, Sb, Cu, Co, Pb, Cd, Ni and As) in the Meghalaya coalsand mine rejects are shown in Table 2. Most of these are found to

be enriched in rejects in comparison to the coal samples. The dis-tribution of trace elements did not show any significant variationamong the coals with the exception of Sb and Cu. The relative con-centration of these elements in the coal samples is as follows:Mn > V > Cr > Zn > Sb �Cu > Co > Pb > Cd > Ni > As.

Correlation coefficients can be used to demonstrate the associ-ation of trace and potentially harmful elements in the coal and re-ject samples. The Pearson correlations derived (Table 3) for fixed C,total S, ash content, trace elements and PHEs of the samples showsignificant correlation between ash yield and As (r = 0.81), Cr(r = 0.98), Co (r = 0.82), Cu (r = 0.50), Pb (r = 0.86), Ni (r = 0.97)and Zn (r = 0.77), respectively, indicating their association withmineral matter. Significant correlation of S is also observed withAs (r = 0.51), Cd (r = 0.61), Cu (r = 0.50) and Zn (r = 0.77), whichmay be due to their presence either as sulfides in coal/rejects oras trace inclusions or impurities within pyrite. A negative correla-tion of As, Cd, Cr, Co, Cu, Pb, Mn and Ni with fixed C further indi-cates that these elements are associated with ash. XRD analysisof mine rejects indicated the presence of quartz, kaolinite, calcite,illite and pyrite.

Enrichment Factors (EFs) are used to assess the enrichment ordepletion of trace elements and PHEs in coal and mine rejects incomparison to those elements in the Earth’s crust. Enrichment Fac-tors were calculated by comparing the concentration of specificelement in the coal and mine rejects with its average concentrationin the crust/Clarke value. The EF is given by:

EF ¼ Concentration of elements in coal=mine rejectsConcentration of elements in Earth’s crust

ð1Þ

The EF values have been grouped into four classes, <0.10 (deple-tion of elements), 0.10–1.0 (no trend), 1.0–10.0 (slightly enriched)and 10.0 (highly enriched). Enrichment Factors calculated for theMeghalaya coals and rejects are reported in Table 4. EnrichmentFactor values for Mn, Ni, Cu were below 0.10 indicating depletion

Table 3Pearson correlation coefficients between fixed C, total S, ash content, trace and potentially harmful elements of coal and mine rejects of Meghalaya.

FC TS Ash Sb As Cd Cr Co Cu Pb Mn Ni V Zn

FC 1.00TS 0.16 1.00Ash �0.92 �0.08 1.00Sb �0.01 �0.35 �0.15 1.00As �0.79 0.51 0.81 �0.15 1.00Cd �0.42 0.61 0.43 �0.04 �0.13 1.00Cr �0.94 �0.03 0.98 �0.13 0.80 0.49 1.00Co �0.66 0.25 0.82 �0.37 0.53 0.58 0.82 1.00Cu �0.58 0.53 0.50 0.32 0.47 0.23 0.54 0.49 1.00Pb �0.83 0.26 0.86 �0.07 0.46 0.82 0.90 0.82 0.48 1.00Mn �0.65 �0.01 0.44 0.03 0.25 0.54 0.51 0.28 0.43 0.62 1.00Ni �0.94 �0.16 0.97 �0.07 0.83 0.43 0.99 0.80 0.65 0.86 0.54 1.00V 0.08 0.04 0.17 0.05 0.07 �0.07 0.07 0.21 �0.09 0.00 �0.73 0.01 1.00Zn �0.82 0.48 0.77 0.15 0.74 0.28 0.81 0.55 0.84 0.68 0.59 0.88 �0.15 1.00

Table 4Average Enrichment Factors (EF) of trace and potentially harmful elements.

EF Enrichment Coals MR Soil Sediment

0.10 Depleted Mn, Ni, Cu Mn, Cu, Mn, V, Cu Mn, V, Cu0.10–1.0 No trend As, Cr, Co, Zn, Pb, V Cr, Zn, V, Cr, Ni, Zn, Co Cr, Ni, Zn Co1.0–10.0 Slightly Enriched – Sb, As, Pb, Ni, Co Pb Pb>10.00 Highly Enriched Sb, Cd Cd Sb, As, Cd Sb, As, Cd

1624 B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631

of these elements in Meghalaya coals and rejects. The values for As,Cr, Co, V, Zn and Pb lie in between 0.10 and 1.00 showing no trend.However, highly enriched values were found for Sb and Cd. Thehigh EFs for Cd and Sb in coal and mine reject samples from thecoalfields of Meghalaya have significance in relation to contami-nant build-up. The ratios of EFs of trace and PHEs present in rejectsand coals (Fig. 2) show that they are enriched in rejects and themaximum enrichment found for Pb (16 times). The enrichmentin rejects may be attributed to the difference in geochemical nat-ure of host materials. The occurrence of Pb has also been reportedin Tertiary Indian coals in the form of galena (PbS) and may alsohave a terrigeneous aluminosilicates origin (Saikia et al., 2009).These minerals occur as cell filling of coal-forming plants and areof chemical sedimentary origin.

3.2. Kinetic leaching of mine rejects

The mine rejects from Mondiati (MM), Jaintia Hills, Meghalayarepresenting a class of similar geological material from the coalmines have been subjected to leaching for different periods. Thecoals and the mine rejects have similar sulfur contents, but therejects were found to have higher concentration of trace and

0

2

4

6

8

10

12

14

16

18

Sb As Cd Cr Co

Fig. 2. Ratio of Enrichment Factors (EFs) of trace and potenti

potentially harmful elements. The mine rejects are more exposedto natural weathering conditions and therefore were selected forsimulation experiments. Compositional parameters and concentra-tion of trace and potentially harmful elements in aqueous leach-ates were determined over different time periods.

The Eh and pH values over different leaching time periods(Fig. 3) indicate that the leachates were highly acidic with variablepH. Values of pH varied with time (Fig. 3) showing an increasefrom 2 to 4 up to 6 days, to 5 from 9 to 38 days, and then decreasedto 3.8 from 38 to 75 days. The first 6 h represented the initial flushof pore water. The period from 7 h to 5 days represented acid gen-eration and leaching, characterized by low pH and increased Eh.During the period from 5 to 38 days, consumption of carbonatestakes place; Fig. 4 shows some increase in Ca during this time per-iod along with increase in pH. Carbonates are depleted after38 days, and the leachate is again acidic.

The carbonates and aluminosilicates present in the rejects takepart in neutralization, but are insufficient to completely buffer theacidity. The dissolution rates of carbonates (10�5–10�6 molm�2 s�1) (Sanchez Espana et al., 2005) are higher than those of alu-minosilicates (10�8–10�10 mol m�2 s�1). The buffering capacity ofcarbonates is greater, whereas aluminosilicates dissociate and

Cu Pb Mn Ni V Zn

ally harmful elements present in mine rejects and coals.

Fig. 3. Plot of Eh and pH with time of leaching of mine rejects.

Fig. 4. Concentration of major elements in leachate water of mine rejects.

0.01

0.1

1

10

100

1000

10000

1 ho

ur

2 ho

ur

3 ho

ur

4 ho

ur

5 ho

ur

6 ho

ur

7 ho

ur

2 da

ys

3 d

ays

4 d

ays

5 d

ays

6 d

ays

9 d

ays

10 d

ays

11 d

ays

16 d

ays

25

days

38 d

ays

50

days

60

days

75 d

ays

Time of leaching

Log

Con

cent

ratio

n

Conducttivity TDS Acidity

Fig. 5. Plot of conductivity (mS), TDS (ppt), total acidity (mg L�1) and SO4 (mg L�1) in leachate water from mine rejects.

B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631 1625

1626 B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631

participate in increasing acidity, for which variable pH is observed.This is further evidenced by the increase in concentration of Ca andSi (Fig. 4). During the period from 5 to 10 days, the increase in pHfrom 4 to 5 days shows reduction in acidity due to pronounced dis-solution of carbonates and aluminosilicates.

Total dissolved solids in the leachate water range between0.014 and 1.471 ppt. A plot of TDS versus time (Fig. 5) shows thatTDS decreases gradually with time and again shows an increaseafter 25 days of leaching. Electrical conductance (EC) shows a sim-ilar variation. The highest EC and TDS values were observed whenpH was lowest indicating dissolution of minerals present in minerejects. The dissolved elements in the leachate water are likely tobe present in the form of either individual free ions or as SO4 ionicspecies (Sanchez Espana et al., 2005). The increase in EC and TDS inthe test water is due to the presence of these ionic species.

The variation of Eh with time was large and did not show anytrend (Fig. 6). The Eh values probably reflect the variation in thedominant Fe2+/Fe3+ redox couple. Eh values are likely to be mean-ingful in high Fe and low pH waters. As shown in Fig. 6, the leach-ate water has a high concentration of Fe. Eh values were more than400 mV up to 9 days of leaching, and subsequently decreased to

2 2.5 3 3.5

Eh (m

V)

Fe +++

Pyrite

Fe++

0

100

200

300

400

500

600

700

Fig. 6. Eh–pH diagram

1.35

1.37

1.39

1.41

1.43

1.45

1.47

1.49

1.51

1.53

1.55

1 ho

ur

3 ho

ur

5 ho

ur

7 ho

ur

Leach

Mol

ar S

/Fe

ratio

Fig. 7. Plot of molar S/Fe ratios in leachate w

200 mV. An Eh–pH diagram (Fig. 6) shows that Fe2+ ions dominatein leachate water of mine rejects.

In the plot of molar SO2�4 /total Fe ratio with increasing reaction

time (Fig. 7), the slope of the plot indicates the ratio of SO2�4 to Fe

(molar), which was found to be 0.58. These values are significantlyless than 2 mol of SO2�

4 per mole of Fe which normally is expectedfrom stoichiometric dissolution of pyrite. In acid mine drainage, alarge number of secondary Fe phases also precipitate, resultingfrom the oxidation of Fe sulfides. Jarosite and amorphous Fe oxy-hydroxides are insoluble in water and remain in the solid phaseafter flushing, leading to a lower SO2�

4 /Fe ratio. It has also reportedthat in AMD systems with pH: 3–4, Eh: 50–450 mV, Fe: 31–5610 mg L�1, Al: 339–2370 mg L�1 and SO2�

4 : 124–4530 mg L�1,the poorly crystalline (jarosite), as well as more crystalline (goe-thite, hematite) Fe phases are always super saturated in the acidsolutions and tend to precipitate (Sanchez Espana et al., 2005).These parameters, obtained in this study are similar to those inAMD waters, where formation of jarosite, goethite, and hematitemay take place. The presence of these precipitates in the study areahas also been reported by Baruah (2009, 2010) for similar Tertiarycoals.

4 4.5 5 5.5pH

Hematite

for leachate water.

3 da

ys

5 d

ays

9 da

ys

11 d

ays

25 d

ays

ing time

ater with leaching time of mine rejects.

B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631 1627

With low pH and Eh values up to 500 mV as observed, the fol-lowing reactions may take place and control the solubility of Feand Al in leachate water (Eqs. (2)–(7)) (Baruah, 2010)

Fe3þ þ SO2�4 þ OH� $ FeðOHÞSO4ðSÞ ð2Þ

Fe2þ $ Fe3þ þ e� ð3ÞAl3þ þ SO2�

4 þH2O$ AlðOHÞSO4 þHþ ð4Þ

4AlðOHÞ3 þ 2Hþ þ SO4 þ 3H2O

! Al4ðOHÞ10ðSO4Þ � 5H2O ðbasaluminiteÞ ð5Þ

AlðOHÞ3 þ SO2�4 þ 2Hþ þ 3H2O! AlOHðSO4Þ

� 5H2O ðJurbaniteÞ ð6Þ

3AlðOHÞ3 þ 3Hþ þ Kþ þ 2SO2�4 ! KAl3ðOHÞ6ðSO4Þ2 ðaluniteÞ

þ 3H2O ð7Þ

Physico-chemical characteristics of both simulated and AMDwaters (Fig. S-I) show similarities. They are highly acidic with highconductivity, TDS and SO2�

4 concentrations. Sulfate concentrationin mine water was found to be higher than that in the simulatedwater. This may be due to the fact that in actual field sites, otherprocesses such as bacterial action may accelerate acid formation.

3.3. Acid production rate

Iron sulfides are normally stable and insoluble under reducingconditions, but on exposure to atmospheric conditions, oxidationtakes place generating acidity, forming sulfates and liberating met-als and metalloids.

The rate of SO2�4 release can be used to approximate the rates of

Fe sulfide dissolution and acid production (Eqs. (8)–(11), Nord-strom and Alpers, 1999)

FeS2ðsÞ þ 14O2 þ 4H2OðlÞ ! 4Fe2þðaqÞ þ 8SO2�

4 ðaqÞ þ 8HþðaqÞ ð8Þ

4Fe2þ þ O2 þ 4Hþ ! 4Fe3þðaqÞ þ 2H2OðlÞ ð9Þ

4Fe3þðaqÞ þ 12H2OðlÞ ! 4FeðOHÞ3ðsÞ þ 12HþðaqÞ ð10Þ

The sum of Eqs. (8)–(10) gives

4FeS2ðsÞ þ 15O2ðsÞ þ 4H2OðlÞ ! 4FeðOHÞ3ðsÞ þ 8SO2�4 ðaqÞ þ 16HþðaqÞ

ð11Þ

If it is assumed that the total sulfide released is from Fe sulfideoxidation and that all the SO2�

4 produced remained in solution(Lapakko, 1988, 1990), the concentration of H+ will depend uponthe amount of FeS2 oxidized as well as the amount of Fe that is pre-cipitated. The rate of acid production can be obtained from Eq. (12)

�2dðFeÞ=dt ¼ �2dðSÞ=dt ¼ 2dðSO2�4 Þ=dt ¼ dðHþÞ=dt ð12Þ

Dissolution of Ca and Mg carbonate minerals present in the re-jects neutralizes acid and releases Ca and Mg into the solution asshown in reaction (13)

CaCO3ðsÞ þ 2HþðaqÞ ! H2CO3ðaqÞ þ Ca2þðaqÞ ð13Þ

If it is assumed that this reaction represents dissolution of thecarbonate minerals and consequently the acid consumption, thenthe rate of acid consumption can be given by Eq. (14)

�ndðCaCO3Þ=dt ¼ ndðCa2þÞ=dt ¼ �dðHþÞ=dt ð14Þ

where n represents the number of moles of Ca or Mg carbonates dis-solved. This is a function of the amount of pyrite oxidized. Using

Eqs. (12) and (14), the total acid production and consumption werecalculated. For that the slope of a regression line fitted to data from3 h to 76 days was used to estimate the rate of acid production andconsumption (Fig. S-II). The initial two flushes were not taken intoaccount in the calculation, because they were not representative oflong term behavior of up to 76 days. The calculated release rate ofacid production was 4.41 kg/MT/h and the consumption rate was1.52 kg/MT/h, which reveals that the rates of SO2�

4 production ismuch higher compared to the rates of carbonate mineral dissolu-tion. The Meghalaya mine reject samples showed zero alkalinityat the start of the experiment.

3.4. Environmental impacts

With intense coal mining, a high degree of contamination, withlow pH values, high SO2�

4 and trace element and PHE loadings areobserved. To study the environmental impact on the nearby min-ing sites, simulated AMD is compared with site effluents. The pos-sible trace element and PHE leaching, critical load (CL) and soilpollution index (SPI) for the elements evaluated shows a high de-gree of contamination in the nearby environment from coal miningactivities.

3.4.1. Element leachingSome of the major (Fe, Al, Si and Ca), trace and PHEs of envi-

ronmental concern (Sb, As, Cd, Cr, Co, Cu, Pb, Mn, Ni and Se) havealso been found in leachate water and are gradually released withtime (Fig. 8). The mobilization of these elements depends uponthe association in the solid phase to which they are bound andto the chemical reactions taking place during the leaching pro-cess. To know the factors affecting the leaching of these elementsfrom rejects, a correlation matrix was drawn (Table 5). Alumin-ium, P, S, K, Ti, Cr, Co, Zn show negative correlations with pHindicating that the concentrations of these elements increase inan acidic environment. This inverse correlation with pH is consis-tent with their pH dependent solubility. As the acidity of theleachate water is increased, the dissolution of mineral matter oc-curs. The correlation between Al, Mg, Na, K and Ca is indicative ofthe dissolution of aluminosilicate minerals in the acidic water.The correlation between Ca and Mg suggests dissolution of car-bonates present in the rejects. A good correlation of Fe with S(r = 0.99) indicates dissolution of pyrite. The Al, Si, P, Cl, K, Ti,Mn, Co and Ni concentrations in the leachates increase as pyriteoxidation progresses. These elements have a strong correlationwith SO2�

4 in the leachate. The release of these elements dependson pyrite oxidation and dissolution (Yue and Zhao, 2008),whereas Cd, Sn, Sb and Te contents in the leachates are mainlycontrolled by adsorption on Fe hydroxides, which is indirectlyinfluenced by pH. This indicates that trace element contents inleachate water are controlled by three factors: the oxidation rateof pyrite, the acidity of the leachate water and the mineralogy ofthe rejects.

In order to assess the actual field conditions, a comparison be-tween the elements released during leaching experiments withthose of AMD water collected from the mine site has been made(Fig. 9). The cumulative concentration of trace elements and PHEsin leachate water was found to be higher than the AMD water, per-haps due to the dilution effect at the mine site due to heavy precip-itation occurring throughout the year.

3.4.2. Impact assessmentThe seriousness of the pollution caused by PHEs is mainly

based on two fundamental reasons: (1) they can be retainedby organic tissues through bioaccumulation and then transmit-ted to other species at a higher level of the trophic chain, thus

1 ho

ur

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ays

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ys

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ys

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ays

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ays

16 d

ays

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ays

38 d

ays

Leaching time

Con

cent

ratio

n m

g L-1

(Log

sca

le) Cr Mn Co Ni Cu

0.1

1

10

100

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ur

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Leaching time

Conc

entra

tion

mg

L-1 (L

og s

cale

)

Zn Cd Sn Sb Pb

0.1

1

10

100

Fig. 8. Trace and potentially harmful element concentrations in leachate water from mine rejects.

Table 5Correlation matrix between the trace and potentially harmful elements leached from the mine rejects.

pH Al Si P Cr Mn Fe Co Ni Cu Zn Cd Sb

pH 1.00Al �0.47 1.00Si �0.34 0.96 1.00P �0.63 0.95 0.84 1.00Cr �0.82 0.34 0.15 0.53 1.00Mn �0.40 0.96 0.87 0.94 0.33 1.00Fe �0.44 0.97 0.88 0.95 0.35 1.00 1.00Co �0.47 0.97 0.86 0.96 0.39 0.99 0.99 1.00Ni �0.39 0.97 0.90 0.94 0.29 1.00 1.00 0.99 1.00Cu �0.35 0.97 0.90 0.91 0.28 0.99 0.98 0.97 0.99 1.00Zn �0.50 0.97 0.88 0.97 0.38 0.97 0.99 0.99 0.97 0.95 1.00Cd 0.69 �0.25 �0.07 �0.41 �0.72 �0.26 �0.26 �0.32 �0.22 �0.21 �0.27 1.00Sb 0.66 �0.36 �0.29 �0.44 �0.64 �0.26 �0.28 �0.32 �0.26 �0.25 �0.32 0.71 1.00

1628 B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631

causing biomagnification and (2) small amounts of PHEs can betoxic to aquatic and terrestrial biota.

As a part of this study, an impact assessment was done bydetermination of PHE contents in the water bodies, soil and sedi-ments from the affected areas.

3.4.2.1. Water. A more appropriate assessment of element mobil-ity can be obtained from the leaching data rather than usingthe total contents in the reject. Therefore, an attempt has beenmade to estimate the critical loads of PHEs (As, Cd, Cr, Cu, Pb,

Mn, Hg, Mo and Ni) released into the environment. Critical loads(CLs) were calculated according to Baruah (2007) and Baruah andKhare (2009)

CL ¼ Cumulative concentration of element in leached water ðmgÞVolume of water ðLÞ � limits ðmg=LÞ

If the critical load is less than 1, it indicates that the elementis within permissible limits. If the CL > 1, the element needs tobe removed from the AMD effluents before surface discharge.The critical loads show that the concentration of all these

0

50

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150

200

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300

350

400

Con

cent

ratio

n m

g L-1

Cr Mn Co Ni Cu Zn Cd Sn Sb PbElements

LW MW1 MW2

Fig. 9. Comparison of some selected elements present in leachate water and mine waters collected from mine sites. LW: leachate water, MW1: mine water site 1, MW2: minewater site 2.

Con

cent

ratio

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g L-1

(log

scal

e)

As Cd Cr Cu Pb Mn Hg Mo NiElements

1

0.0001

0.001

0.01

0.1

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100

1000AMD DW IW AL

Fig. 10. Comparison of trace and potentially harmful element concentrations in AMD water with standards for drinking water (DW), irrigation water (IW) and water qualityfor aquatic life (AL).

1

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1000000

S V Cr Mn Fe Co Ni Cu Zn As Pb Cd Sn SbElements

Con

cent

ratio

n m

g L

-1 (l

og s

cale

)

Sediment Soil

Fig. 11. Trace and potentially harmful element concentrations in soil and sediments of AMD affected areas.

B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631 1629

1630 B.P. Baruah, P. Khare / Applied Geochemistry 25 (2010) 1621–1631

elements (Fig. 10) exceeded permissible target concentrations fordrinking water, irrigation water and water quality in aquatic life(USEPA, 1994).

3.4.2.2. Soils and sediments. Soil and sediments from AMD affectedareas have low pH values (3.7–5.4). All the sediments containedreddish brown Fe-oxide/hydroxide coating. The S, trace and poten-tially harmful elements (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Pb, Cd, Snand Sb) concentrations of soil and sediment are plotted in Fig. 11.Enrichment Factors (EF) were used to quantify the extent of con-tamination (Sinex and Helz, 1981; Covelli and Fontolan, 1997). EFis calculated by comparing the concentration of a specific elementin the sediment or soil versus its average concentration in theEarth’s crust. EFs are a better assessment of onsite pollution (Kaba-ta-Pendias and Pendias, 2001) and geological variation of elemen-tal abundances (rock types and mineralization). EF values of PHEspresent in soil and sediments of the AMD affected area (Table 4)are high for the elements Sb, Cd and As.

3.4.2.3. Soil Pollution Index (SPI). The Soil Pollution Index (SPI) wascalculated by dividing the existing concentration of PHEs by theircritical concentration in the soil; the critical levels were assumedto be the reference base line for each pollutant (element) (Singhand Srivastava, 1997; Kabata-Pendias and Pendias, 1984). Soilpollution index was used to evaluate the degree of PHE contam-ination. High SPI values for Cr, Cu, Cd and Pb were observed inAMD affected areas of Meghalaya (Fig. S-III). These high valuesmay be due to accumulation from the aqueous discharge in thecoal mines.

4. Conclusions

The present investigation shows that the coal mine rejects un-der study have high acid generating potential and high concentra-tions of PHEs. Aqueous leaching of mine rejects revealed that acidicleaching, precipitation and adsorption are the major processes con-trolling leaching of these elements. Mine water, sediment and soilsamples collected in this study contain concentrations of trace andpotentially harmful elements that exceed limits set by interna-tional regulatory agencies.

There is, therefore, an urgent need not only to monitor the lev-els of contamination in the area adjacent to the mining site understudy, but also to investigate the concentrations of these elementsin living organisms in order to prevent possible toxicity. Moreover,preliminary results indicate the need for a study that defines nat-ural background levels in the areas so that anthropogenic and nat-ural contributions can be quantified.

Acknowledgements

The work was supported by the Ministry of Environment & For-ests, New Delhi (REF No. 19-50/2002-RE) and Council of Scientific& Industrial Research, New Delhi under the CSIR Task Force Pro-gram. The authors are thankful to the Director, NEIST, Jorhat forencouragement to carry out the work. The authors are also gratefulto the anonymous reviewers and Dr. A. Kolker for suggestionswhich improved the quality of the article.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.apgeochem.2010.08.010.

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