Geochemical assessments and classification of coal mine spoils for better understanding of potential salinity issues at closure

Environmental ScienceProcesses & Impacts

PAPER

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View Article OnlineView Journal

aCentre for Mined Land Rehabilitation, The

4072, Australia. E-mail: [email protected]

3346 4060bEnvironmental Systems and Reporting, C

Mitsubishi Alliance, Mackay, QLD 4740, Au

Cite this: DOI: 10.1039/c3em30672k

Received 14th August 2012Accepted 12th April 2013

DOI: 10.1039/c3em30672k

rsc.li/process-impacts

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Geochemical assessments and classification of coal minespoils for better understanding of potential salinityissues at closure

Jin Hee Park,a Xiaofang Li,a Mansour Edraki,*a Thomas Baumgartla and Bernie Kirschb

Coal mining wastes in the form of spoils, rejects and tailings deposited on a mine lease can cause various

environmental issues including contamination by toxic metals, acid mine drainage and salinity. Dissolution

of salt from salinemine spoil, in particular, during rainfall events may result in local or regional dispersion of

salts through leaching or in the accumulation of dissolved salts in soil pore water and inhibition of plant

growth. The salinity in coal mine environments is from the geogenic salt accumulations and weathering

of spoils upon surface exposure. The salts are mainly sulfates and chlorides of calcium, magnesium and

sodium. The objective of the research is to investigate and assess the source and mobility of salts and

trace elements in various spoil types, thereby predicting the leaching behavior of the salts and trace

elements from spoils which have similar geochemical properties. X-ray diffraction analysis, total

digestion, sequential extraction and column experiments were conducted to achieve the objectives.

Sodium and chloride concentrations best represented salinity of the spoils, which might originate from

halite. Electrical conductivity, sodium and chloride concentrations in the leachate decreased sharply with

increasing leaching cycles. Leaching of trace elements was not significant in the studied area.

Geochemical classification of spoil/waste defined for rehabilitation purposes was useful to predict

potential salinity, which corresponded with the classification from cluster analysis based on leaching

data of major elements. Certain spoil groups showed high potential salinity by releasing high sodium

and chloride concentrations. Therefore, the leaching characteristics of sites having saline susceptible

spoils require monitoring, and suitable remediation technologies have to be applied.

Environmental impact

This work showed that geological classication well represented spoil groups that have potential salinity issues. In this way, salinity can be easily predictedwithout measuring EC or element concentrations. Cluster analysis of spoils based on leaching characteristics is the rst attempt to classify spoils having highsalinity potential. Monitoring of salinity from spoil will help to understand water dynamics on the spoil deposition and plan for spoil management.

Introduction

Coal mining activities generate considerable amounts of over-burden materials and waste rock (or spoil), which are dumpedon the land surface within a mining lease. Waste dumps ofunweathered material can potentially create environmentalchallenges to rehabilitation because they are physically, nutri-tionally and biologically poor in nature.1 The wastes are usuallypiled without specic treatment, and as such can be activesources of drainage water that contaminate environments

University of Queensland, St Lucia, QLD

.au; Fax: +61 7 3346 4056; Tel: +61 7

entral Queensland Office, BHP Billiton

stralia

Chemistry 2013

surrounding the mining area. Hazardous materials such astoxic metals, acids and salts can be leached from spoils undermoist, oxidizing conditions.2

Spoils typically consist of argillaceous and arenaceous rockssuch as sandstone, siltstone and mudstone, as well as coal andcoal shale.3 The sandstones and mudstones contain variousaluminosilicate minerals such as kaolinite, illite andmuscovite,which can adsorb trace elements. Oxidative weathering ofsuldes contained in the form of pyrite, chalcopyrite ormarcasites generates acidic drainage from spoil piles, simulta-neously increasing the bio-availability of metals such as Cu, Fe,Mn, Pb and Zn.4,5 Quartz, the most common mineral in thesedimentary rocks associated with coal, is an inert material anddoes not signicantly affect the acid–alkali balance. Siderite, aniron-rich carbonate mineral, is common in the shales andsandstones associated with spoils but it does not contribute to

Environ. Sci.: Processes Impacts

http://dx.doi.org/10.1039/c3em30672k

http://pubs.rsc.org/en/journals/journal/EM

Fig. 1 Photos of spoil piles.

Environmental Science: Processes & Impacts Paper

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neutralizing capacity. Although calcite and dolomite can bufferacidity, these carbonates are not usually dominant in coal-bearing strata. Complex processes of lithology, climate,hydrology and the local inventory of acid-generating suldesand acid-neutralizing carbonates control the generation of aciddrainage.6

Mobile constituents such as Na, Ca, Mg, Cl and SO4 cancause salinization of surface and ground waters near mine sitesand local soils.6 Among those constituents Na and Cl are themost abundant ions followed by Ca and Mg, which are essentialfor plant nutrition.7 As an extreme example, in Poland largevolumes of highly saline water containing 4000 t per day of Cland SO4 were withdrawn and discharged to surface waters,which caused an ecological hazard to the local environment.8 Itwas reported that 2.4 million tons of salt as NaCl were dis-charged every year.9

Salinity results from both natural and human inducedprocesses that accumulate dissolved salts in the soil water.Weathering processes of parent materials containing solublesalts can release mainly chlorides of Na, Ca and Mg, and to alesser extent, sulfates and carbonates.10 However, it is difficultto identify sources of salinity at coal mine sites where aciddrainage is minimal and naturally saline soils and water arepresent. The analysis of spoils and aqueous leachates of spoilscan be used to characterize the composition of salinity sources.6

As part of the closure planning for a coal mine, geochemicalexperiments and assessments were conducted to predictpotential salinity issues for surface water and groundwater inthe nal landscape of the site, taking into account the abun-dance and geogenic salt contents of different spoil materials.

The objective of the research is to test the hypothesis thatdifferent spoil types classied according to geology, mineralogyand physical properties produce different levels and types ofsalinity. Static and kinetic leaching tests were used to identifythe potential release of salts, the source and mobility of salts,and trace elements in various spoil types and to predict theleaching characteristics of the salts and trace elements fromthose spoils. Especially, leaching characteristics indicate thequantity of solution/rainfall needed to wash off salts and alsothe effect of residence time on the salinity of the leachate.

Materials and methodsSample collection and classication

Samples from spoil areas, coal rejects dumps and tailings damsat a coal mine in Queensland, Australia (produced coal types areprimarily coking and thermal coals) were collected to representthe major types of spoil or soil on site. The sampling strategywas based on the volumes and areas covered by each spoil types.The choice of number of samples taken from spoil piles wasbased on the proportional area of the spoil piles. Fig. 1 showssome spoil piles of the sampling sites. For spoil types whichcovered larger areas we collected more number of samples. Theexpected reactivity of spoils was also considered in samplingprogram and derived from a spoil classication chart (Table 1shows the major spoil types). For example, for the basalt, whichis relatively inert, we took only one sample (C1), whereas more


replicate samples were taken from other spoil piles (G1, G2 andG3). The focus of the investigation was on the generation ofsalts and release of trace elements. Properties of spoil pilesamples differed in color, particle size, signs of weathering andvegetation cover. These properties were visually inspectedbefore sampling to ensure representative samples werecollected. Seventeen spoils, one coal reject and one coal tailingsamples were collected. Spoil samples were classied toreworked Cainozoic sediments, in situ weathered Tertiary sedi-ments, Tertiary basalt and Permian sedimentary rocks based ongeological properties. Coal tailings and rejects were grouped tocoal enriched spoil/waste products. The samples sometimesconsisted of mixtures of spoil types, but were classied as thedominant type.

X-ray diffraction analysis

Mineralogical identication of coal mine spoils was undertakenat the University of Queensland's Centre for Microscopy andMicroanalysis using a Bruker D8 Advance X-ray diffractometerequipped with a copper target, diffracted-beam mono-chromator, and scintillation counter detector. Conditions forrunning the samples were 40 kV, 30mA, 3–80� 2q, 0.05� step sizeor increment, with 10 seconds per step. Soware used for themineral identication was DIFFRACplus Evaluation Search/Match Version 8.0 and Siroquant for semi-quantitative analysis.

Total digestion

Total digestion of samples followed the United States Environ-mental Protection Agency (USEPA) method 200.2. A homoge-nized sample (1 g) was digested in 2 mL diluted nitric acid(50%) and 10 mL of diluted hydrochloric acid (20%) for 30 min.Aer cooling, the sample was digested a further 30 min with 2mL of 30% hydrogen peroxide to breakdown high organiccompounds in the sample. The sample was allowed to cool,diluted to 50 mL with reagent grade water and total metalconcentrations were analyzed by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) and total major cationswere analyzed by ICP-AES following American Public HealthAssociation (APHA) 3120 and USEPA SW 846-6010 methods. A10 g sample of soil was mixed with 50 mL Milli-Q water in anend over end shaker for 1 h to extract water soluble salts. Thesolution was ltered and analyzed for Cl� and NO3

� concen-trations by APHA 4500-Cl method and APHA 4500-NO3 method,respectively. A 1 g of soil was digested in 30 mL of 30% HCl, Sconcentration was analyzed by ICP-AES and reported as SO4

2�.

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Table 1 Definition and description of spoil/waste mapping categories defined for rehabilitation purposes (Emmerton, 2010, personal communication)

Geology Stratigraphic unit Features Samples

Reworked Cainozoic sediments A Highly weathered, ne sandy, eitherloamy or clayey materials that aretypically neutral to alkaline,dispersive, saline

A1, A2

In situ weathered Tertiary sediments B Pallid or mottled (white, red,purple), deeply weathered kaoliniticclays, acidic and saline

B1, B2

Tertiary basalt C Fresh or weathered basalt, non-reactive

C1

Permian sedimentary rocks,predominantly fresh

D Predominantly fresh, labile, negrained Permian sedimentary rocks,mudstones, siltstones, shales

D1

E Predominantly fresh, relativelystable, grey Permian siltstones

E1, E2, E3, E4, E5

F Similar to E, weathered, negrained, lithic sandstones/siltstones

F1, F2, F3

G Fresh Permian sedimentary rocks,rich in pyrite, acid generation

G1, G2, G3

Coal enriched materials, tailings,rejects

H Coal enriched spoil/waste products,dark, carbonaceous, siltstones andshales

H1, H2

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To measure total carbon concentration, the dried and pulver-ized sample was combusted in a LECO furnace in the presenceof strong oxidants/catalysts. The evolved carbon as CO2 wasmeasured by an infra-red detector. Quality control was con-ducted using duplicate, blank and spiked matrix were testedand showed that relative percent deviation of laboratory dupli-cates was less 10%, and recovery of spiked matrix rangedbetween 90% and 110%.

Sequential extraction

The sequential extraction experiment was conducted based onprocedures developed by Dold and Fontbote,11 and CardosoFonseca and Martin.12 Five grams of samples (<2 mm) wereplaced into 250 mL tubes, and 250 mL of Milli-Q water wasadded. Samples were shaken in an end-over-end shaker for 1 h at150 rpm. The supernatant liquid was separated from the solidphase by centrifugation at 4000 rpm for 10 min. It was thenltered with 45 mm syringe lter into polyethylene vessels andstored at 4 �C before analysis. Exchangeable fraction wasextracted using 1 M ammonium acetate (pH 4.5). Ammoniumacetate solution (50mL) was added to the residue, shaken for 2 hat room temperature, and centrifuged at 4000 rpm for 10 min tocollect the leachate. The remaining residue was washed with 10mL of Milli-Q water, and the washings were discarded aercentrifugation. To extract iron oxyhydroxide fraction, 200 mL of0.2 M ammonium oxalate solution (pH 3.0) was added to theresidue and shaken for 1 h in darkness at room temperature. Theleachate was collected and the residue was washed same asabove. Residue samples were digested with strong acids asdescribed in above in the section ‘total digestion’. Major cationconcentrations in the extracted solution were analyzed by ICP-AES and metal concentrations were analyzed by inductivelycoupled plasma mass spectroscopy (ICP-MS).


Column leaching test

A column leaching experiment was conducted to simulate theweathering of primarily saline substrate by rainfall with wet anddry cycles. Medium sized Buchner funnels (diameter: 120 mm;height: 60 mm) were lled with spoil materials to a thickness of30mm (approx. 350 cubic centimeters) with a lter paper placedat the bottom of the funnels to avoid losing ne particles in theleachate. Deionised (DI) water (350 mL) was carefully added tothe funnels without disturbing the soil surface. Before drainagehad lowered the water level to the spoil surface, 250 mL of DIwater was added to avoid an unsaturated situation in the spoilduring each of the leaching tests. Total leached volume of waterranged 4–6 pore volumes, which was equivalent to the half ofannual rainfall (576 mm per year) of the sampling site. Elevensuccessive 50 mL aliquots of leachate water were collected (total550 mL of leachate), and the time was recorded to calculate theow rate. The rst collected aliquot was passed through a0.45 mm membrane lter and acidied for metals analysis. pHand electrical conductivity (EC) were measured for eachleachate aliquot except the very rst 50 mL to avoid contami-nation. The second to the h aliquots (total 200 mL) werecombined for major cations and anions analysis. The rest of theleachates was discarded. Aer the leaching event, the spoil wasdried and leached again according to the procedure describedabove. The wetting and drying leaching cycles were repeated upto ve times. Major cations in the leachate were analyzed byICP-AES and metal concentrations were analyzed using ICP-MS.Sulfate concentration was analyzed by ICP-AES as S and repor-ted as SO4

2�. Chloride and NO3� concentrations were analyzed

according to APHA 4500-Cl method and APHA 4500-NO3

method, respectively.Themineral saturation index (SI) was calculated using Visual

MINTEQ 3.0 beta based on the spoil leachate chemistry. Mineral



Table 2 Summary of mineralogy of selected coal mine spoils

Spoil

Minerals

Quartz Rectorite Albite Microcline Anatase Kaolinite Muscovite Dolomite Goethite Calcite Jarosite

A1 O O O OB1 O O O O OD1 O O O O OE1 O O O O OF1 O O O O O O OF3 O O O O OG3 O O O O O O OH2 O O O O O O O

Table 3 Total digestion and sequential extraction of mine spoils (unit: mg kg�1)

Sample Fraction Ca Mg Na K Cl NO3� SO4

2� Al As Cr Cu Co Ni Pb Zn Mn Fe C

A1 1 175 200 2300 0 2600 2.8 1925 1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0 1252 5765 2450 175 70 10 0.0 60 40 0.0 0.5 0.1 1.8 1.8 1.3 3.3 209 225 03 0 740 0 0 0 0.0 0 113 1.0 0.8 4.5 1.6 2.9 2.2 7.4 106 2264 04 2160 815 65 830 0 0.0 725 3615 2.5 6.5 16 6.5 16 12 44 122 14 200 0Sum 8100 4205 2540 900 2610 2.8 2710 3769 3.5 7.9 20 9.9 20 16 55 437 16 689 125Total 15 500 7060 2470 840 2760 0.0 3030 2700 6 7.0 20 10 21 14 58 469 24 600 1.75

B1 1 50 100 675 0 925 9.3 150 1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0 502 2735 1560 50 20 0 0.0 20 34 0.0 0.1 0.8 0.2 0.7 0.0 4.3 2 0 03 0 320 0 0 0 0.0 0 82 0.2 0.2 1.5 2.9 1.9 0.2 4.1 19 132 04 840 255 0 230 0 0.0 610 3555 3.5 23 11 6.0 33 0.0 39 24 18 350 0Sum 3625 2235 725 250 925 9.3 780 3672 3.7 23 13 9.1 35 0.2 47 46 18 482 50Total 4520 3000 660 260 1010 0.0 270 2740 <5 24 12 9.0 29 <5 53 62 29 600 0.3

C1 Total 13 700 22 500 370 230 <10 0.0 140 4300 <5 34 18 29 112 <5 68 796 54 800 2.46D1 1 0 0 800 100 350 0.0 475 16 0.0 0.0 0.4 0.0 0.1 0.0 0.6 0 13 250

2 2160 1805 325 125 0 0.0 20 48 0.5 0.3 1.9 1.3 2.1 0.6 5.5 94 1299 03 0 700 0 0 0 3.8 80 254 3.8 1.4 14 4.6 9.5 4.4 26 201 9480 04 1815 1020 100 1105 0 0.0 2840 2930 3.0 4.0 9.0 8.0 20 12 43 261 18 550 0Sum 3975 3525 1225 1330 350 3.8 3415 3248 7.3 5.7 26 14 31 17 75 555 29 342 250Total 3750 3790 1100 1050 390 0.0 3210 2800 8.0 4.0 22 11 26 14 61 547 27 900 1.72

E1 1 0 0 100 0 0 0.0 150 4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0 752 450 450 20 55 0 0.0 15 9 0.0 0.1 0.1 0.0 0.3 0.4 1.7 5 8 03 0 80 0 0 0 0.0 0 42 0.9 0.6 3.0 2.2 2.2 6.0 11 108 2208 04 80 255 0 570 0 0.0 720 1170 0.0 0.0 0.0 3.0 6.0 8.5 36 84 6440 0Sum 530 785 120 625 0 0.0 885 1226 0.9 0.7 3.2 5.3 8.5 15 49 197 8656 75Total 620 910 120 620 10 0.0 360 1280 <5 2.0 8.0 6.0 10 17 54 246 12 000 1.24

F1 1 100 125 1925 25 1700 3.8 1325 1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0 1502 6325 2135 155 60 10 0.0 50 25 0.0 0.4 0.3 1.7 2.0 1.5 3.3 172 461 03 0 1000 0 0 0 0.0 40 45 2.1 2.0 9.0 3.5 7.3 2.1 25 79 2760 04 6260 1315 75 820 0 0.0 795 2720 0.0 5.0 12 7.5 15 15 42 199 14 065 0Sum 12 685 4575 2155 905 1710 3.8 2210 2791 2.1 7.4 21 13 24 18 70 449 17 286 150Total 12 600 5340 2140 760 2030 0.0 1660 2110 <5 6.0 19 11 21 13 63 455 21 400 1.1

F3 1 0 75 2775 0 2875 13 1150 5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 3 1502 5985 3575 325 50 30 0.0 50 10 0.0 0.2 0.1 0.5 0.5 0.5 1.4 71 64 03 0 1260 0 0 0 0.0 0 62 0.2 0.1 1.4 1.6 1.7 0.9 2.1 214 334 04 4770 1235 60 480 0 0.0 270 3355 3.0 6.5 13 9.0 19 10 47 207 20 650 0Sum 10 755 6145 3160 530 2905 13 1470 3432 3.3 6.8 14 11 21 11 51 492 21 050 150Total 10 400 6810 2710 550 3240 0.0 1380 2720 <5 6.0 14 10 19 12 48 651 25 300 0.8

G3 1 525 325 0 0 0 3.5 3125 3 0.0 0.0 0.2 0.3 1.1 0.0 2.7 19 0 02 505 240 0 20 0 0.0 1140 110 0.0 0.2 0.5 0.2 1.0 0.0 2.0 15 14 03 0 0 0 0 10 0.0 560 124 1.4 0.3 1.4 0.0 0.1 0.3 0.6 1 1072 04 0 65 610 1060 0 0.0 9455 915 8.0 1.0 7.5 3.5 5.5 20 42 188 24 750 0Sum 1030 630 610 1080 10 3.5 14 280 1152 9.4 1.5 9.6 4.0 7.7 20 47 223 25 836 0Total 1400 720 910 1100 20 0.0 17 100 1230 10 3.0 11 4.0 9.0 20 51 229 34 200 1

H2 1 200 0 0 0 0 0.0 2950 234 0.0 0.0 1.6 0.1 0.2 0.0 1.1 5 43 02 40 0 0 0 0 0.0 790 397 0.2 0.1 2.0 0.0 0.1 0.0 0.1 1 313 03 0 0 20 80 0 0.0 1340 264 12 0.0 3.1 0.0 0.0 0.5 0.3 0 1316 11004 30 0 575 1155 0 0.0 17 050 2445 9.0 0.0 11 0.0 0.0 26 0.0 0 12 550 0Sum 270 0 595 1235 0 0.0 22 130 3339 21 0.1 18 0.1 0.3 26 1.5 6 14 222 1100Total 270 50 880 1610 <10 0.0 34 300 2840 20 <2 19 <2 <2 31 6.0 6 26 200 8.08

Environ. Sci.: Processes Impacts This journal is ª The Royal Society of Chemistry 2013


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Table

4Flow

rate,initialE

Can

dpH

andelem

entco

ncentrationsin

thefirstleachates

(unit:m

gL�

1)

Flow

rate

(mm

min

�1)

EC

(mScm

�1)

pHCa

Mg

Na

KCl

NO3�

SO42�

Al

As

Cr

Cu

Co

Ni

PbZn

Mn

FeC

A1

0.02

511

.57.14

240

409

2810

1636

804.21

3630

0.02

<0.001

0.00

10.02

90.00

20.00

6<0

.001

0.00

80.10

60.11

21A2

0.00

713

.57.24

171

347

3770

1266

3025

1200

0.05

<0.001

0.00

10.00

40.00

10.00

4<0

.001

0.00

70.02

90.14

21B1

0.03

38.07

6.48

5714

372

42

1260

13.2

280

0.12

<0.001

<0.001

0.17

90.01

00.00

8<0

.001

0.02

20.09

80.22

22B2

0.18

024

.03.25

246

715

3140

1576

502.58

224.4

<0.001

0.00

60.50

60.28

90.57

6<0

.001

3.75

3.56

0.88

4C1

2.71

0.03

778.42

42

2<1

10.07

20.03

<0.001

0.00

10.00

3<0

.001

<0.001

<0.001

0.00

50.00

30.06

6D1

0.03

13.88

7.61

3410

169

815

609

0.41

888

0.02

0.00

20.00

10.02

40.00

10.01

3<0

.001

0.00

70.00

8<0

.05

88E1

0.66

60.78

37.43

812

676

90.33

193

0.02

0.00

1<0

.001

0.00

2<0

.001

0.00

3<0

.001

0.01

60.05

6<0

.05

7E2

0.86

11.41

7.60

3932

9616

1612

.636

0<0

.01

0.00

60.00

20.00

10.03

40.03

0<0

.001

0.00

90.07

4<0

.05

14E3

1.25

2.92

6.38

267

271

166

3214

2.34

2010

<0.01

<0.001

0.00

1<0

.001

0.11

90.15

6<0

.001

0.01

50.56

80.69

4E4

0.11

71.13

7.30

2421

6413

60.06

213

<0.01

0.01

10.00

40.00

20.01

80.02

7<0

.001

0.01

00.08

0<0

.05

18E5

1.25

11.4

6.59

274

791

1030

2753

31.9

5240

0.18

0.00

50.00

1<0

.001

0.00

20.00

8<0

.001

0.02

40.07

50.12

3F1

0.03

620

.47.46

121

370

2490

2430

7020

.930

000.17

0.00

90.00

10.00

30.00

10.00

4<0

.001

0.01

00.04

90.13

20F2

0.01

13.16

8.20

4459

658

754

22.06

677

0.03

0.00

50.00

10.01

10.00

10.00

7<0

.001

0.01

00.03

0<0

.05

75F3

0.00

124

.08.83

8365

833

6010

5940

4.18

2620

0.01

<0.001

0.00

20.00

60.00

20.01

1<0

.001

0.00

70.01

70.14

41G1

0.73

712

.22.86

6032

2216

40.22

327

3.0

0.00

10.00

30.06

60.05

90.31

30.01

20.64

84.21

0.12

2G2

0.90

23.82

2.47

457

9713

184

0.08

1640

290.00

20.00

60.34

20.10

40.39

70.00

81.59

8.01

3.5

3G3

0.79

85.39

2.39

298

408

1518

90.05

2510

590.00

30.03

01.08

0.68

22.64

0.00

36.86

45.9

174

H1

0.48

37.53

3.04

312

443

566

112

60.05

4970

294

0.00

30.03

70.03

31.01

1.75

<0.001

4.34

27.2

4.1

11H2

3.48

1.70

2.56

7750

336

15<1

<0.01

405

590.04

30.00

50.43

50.01

30.03

2<0

.001

0.18

00.79

927

2



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saturation index is dened by SI¼ log(IAP/Ksp), where IAP is theion activity product of the dissolved mineral constituents in asolubility reaction and Ksp is the corresponding solubilityproduct for the mineral. Therefore, SI > 0 indicates supersatu-ration while SI < 0 implies undersaturation.13 The relationshipsbetween various attributes of the samples were determined byanalyzing the similarity of major element concentrations of theleachates using Statistica 11. Hierarchical cluster analysis wasconducted by implementing Pearson correlation as a distancemeasure and nearest neighbour method for clustering.

Results and discussionGeochemical characterization

X-ray diffraction shows that quartz, rectorite and kaolinite arepresent in all samples (Table 2). This is consistent with previousstudies on the spoils from the Bowen Basin14 and some othersites.15 Other minerals are common in natural soils (e.g.microline, muscovite, dolomite and calcite), except for jarosite,which may originate from the oxidation of pyrite and could bethe source of acidity in the leachates.16

Total element concentration can be used to predict potentialsalinity although sequential extraction results better explainsalinity than total concentration. For the major total cation andanion concentrations, weathered unconsolidated Cainozoic(unit A) and Permian sedimentary rocks (units D and F) have thehighest Cl (up to 3240 mg kg�1) and Na (up to 2710 mg kg�1)concentrations which may result in high salinity (Table 3).

Units A, C (basalt) and F have the highest Ca and Mgconcentrations, which can be attributed to the presence ofcarbonate minerals, plagioclase and ferromagnesian minerals(basalt only).17 The origin of Ca and Mg in carbonates can besupported by the close correlation of their total concentrations(R2 ¼ 0.88) and also by the column leaching results showinglittle release of these cations, which otherwise would be inter-preted as surface exchange reactions. Nevertheless, thesequential extraction results show substantial amounts of Caand Mg in the exchangeable fraction (see Potential mobility ofelements section). The coal reject sample (H2) has the highest Kconcentration, which may be related to the presence ofmuscovite and jarosite [KFe3(OH)6(SO4)2] possibly formedthrough oxidation and acid mine drainage processes.18,19

Sulfate concentration is relatively high in unit G and Hsamples. Samples with high sulfate concentration show low pHof the leachate in the column experiment (Table 4), conformingwith other evidence that oxidation of pyrite mineral canincrease sulfate concentration and reduce pH.20

Total Cd and Hg concentrations are below detection limits(1 mg kg�1 and 0.1 mg kg�1, respectively) in all samples (dataare not shown). The concentrations of Cr, Co and Ni are rela-tively higher in the basalt sample (C1) as would be expectedfrom the geochemistry of basalt compared to sedimentaryrocks.21 However, the average concentrations of potentially toxicmetals such as Cu, Ni, Pb and Zn in this study are generallyaround or below the background values of those in soil(Table 3). Chalcophile elements (i.e. elements with suldeaffinity) such as Zn and Cu do not show a clear correlation with




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sulde concentration, which means that some of the heavymetals may originate from themineral matter or coal fraction ofthe soil.22

Potential mobility of elements

The sequential extraction results show that Na is mainly asso-ciated with the water soluble fraction except in acidic spoils (G3and H2) (Table 3). Hydrogen ions in acidic soil may substitutewater soluble or exchangeable fractions of Na, and only theresidual fraction of Na remains. Chloride is also associated withthe water soluble or exchangeable fraction. Sodium and Cl play

Fig. 2 EC and pH of the leachates from selected samples with time. (Wetting andindicate each leaching cycles.)


a signicant role in soil salinity, and high water soluble andexchangeable Na and Cl concentrations indicate saline soil.Potassium is mainly associated with the residual andexchangeable fraction with the exception of H2 (coal reject)where K is related to feldspar, oxyhydroxides and hydratedsulfates (jarosite) of ferric iron formed as the result of theoxidation of coal rejects.19 Calcium and Mg associated with theresidual and exchangeable fraction can be attributed tocarbonate minerals and feldspar.23,24

As would be expected, the sulfate concentration in the acidproducing samples (units G and H) are higher than others, butthe high concentrations of sulfate in the water soluble fraction

drying cycles were repeated up to 5 times. First, second, third, fourth and fifth




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of Cainozoic and Permian boxcut materials (samples A1, F1 andF3) show that these spoils have the potential to contribute tosalinity by dissolution of secondary sulfate salts. The watersoluble fraction of metals (Al, Cu, Co, Ni, Zn and Mn) is higherin acidic soils (G3 and H2) than other soils. It is well known thatacidic soil increases metal solubility through modication ofsurface charge, altering the speciation of metals, and inu-encing the reduction and oxidation reactions of the metals.25,26

However, metal concentrations in the acidic samples are notsignicant in terms of soil contamination.

Correlation of geochemistry with water quality

Potential salinity issues of sampling site can be estimated bymeasuring EC of the leachate. Saline soils are generally denedby having values of EC greater than 4 mS cm�1 in the saturationextract even though the terminology committee of the SoilScience Society of America has lowered the standard to 2 mScm�1.7 Electrical conductivity in this experiment was measuredin leachatewith a soil : water ratio of approx. 1 : 2, whichwas less

Fig. 3 Major element concentrations in the leachates of selected samples with timeanalyzed up to 4 cycles. First, second, third and fourth indicate each leaching cycles


than the saturation extract. Stevens et al.27 showed the goodcorrelationbetweenEC in1 : 5¼ soil : water extract andEC in thesaturation extract for the Northern Adelaide Plains soils. EC inthe saturation extract couldbe estimatedbymultiplyingEC in the1 : 5 ¼ soil : water extract by 8.2, which allows estimation ofsaline soil using the ECmeasurement in the soil extract. Ten outof the 19 samples showed EC greater than 4 mS cm�1 without amultiplying dilution factor. Therefore, the samples can beregarded as highly saline soils. The highest EC was found in B2andF3 (24.0mScm�1), and the lowest ECwas found inC1 (0.0377mScm�1) (Table 4). Electrical conductivity is representative of theoverall change of the dissolution/precipitation and adsorption/desorption processes within the sample. The EC generallydecreased with increasing leaching volume while pH showed adifferent pattern depending on the spoil type. Changes in EC andpH of selected samples with time are presented in Fig. 2.

Electrical conductivity dropped sharply aer the rst fewleachate aliquots and in general gradually decreased insuccessive leaching cycles. In the case of H2, EC increased in thethird leachate aliquot, which might be attributed to insufficient

. (Wetting and drying cycles were repeated up to 5 times and major elements were.)



Table 5 Saturation index of typical salts in the leachates obtained by VisualMINTEQ 3.0 beta

Spoil

Salt

Gypsum Halite KCl

A1 �0.424 �3.749 �5.579A2 �0.988 �3.370 �5.450B1 �1.650 �4.689 �6.829B2 �2.589 �3.392 �5.293C1 �4.217 �10.18 �21.47D1 �1.435 �5.017 �6.268E1 �2.215 �7.760 �8.389E2 �1.413 �7.382 �7.742E3 �0.335 �7.297 �7.597E4 �1.755 �7.963 �8.237E5 �0.234 �5.005 �6.175F1 �0.744 �3.866 �5.468F2 �1.368 �5.081 �6.636F3 �1.042 �3.476 �5.586G1 �1.275 �8.618 �8.339G2 �0.188 �8.927 �8.369G3 �0.339 �8.544 �8.049H1 �0.142 �5.873 �8.215H2 �1.222 —a —a

a Saturation index is not available as Cl is <1 mg L�1 in the H2 leachate.


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interaction between solid and leaching solution in the rst andsecond leaching aliquots due to the high ow rate (3.48 mmmin�1). Samples A1, A2, B1, D1, F1, F2 and F3 showed low owrates and it was further reduced aer the rst leaching cycle(Table 4). High Na concentration dispersed clay particles whichblocked soil pores. Because of the very slow ow rates, leachingdata were obtained only up to the second cycle for thosesamples.

The pH values generally increased with increasing leachingvolume (Fig. 2). Leaching may result in a slight increase in soilpH due to a decrease of the salt concentration.7 However, pHwas not changed signicantly during an individual leachingcycle (Fig. 2). In most samples pH changes were limited to lessthan 0.5 unit. Units B, G, and H samples showed acidic pH(Table 4). The acidic pH of samples could be due to oxidation ofpyrite, and to some extent to the dissolution of Al and Fe saltswhich contain residual (or secondary) acidity.

The sodic saline groups (units A, B, D and F) have releasedhigh Cl and Na concentrations as it was expected from total andsequential extraction results. These groups showed a reducedow rate in the leaching column experiment, which is attrib-uted to the dispersive behaviour of some of these spoils causedby high Na concentration (Table 4). Various factors includingexchangeable cations, Al and Fe oxides, organic matter, claycontent, ionic strength and pH inuence the dispersibility ofcolloids.28 Especially, it is known that high exchangeable Naconcentration is associated with plugging of pores by dispersedclay and reduced soil hydraulic conductivity.29,30 When thesaline soil is leached with low concentration salt water, the soilshows low permeability to water and air due to dispersion.7

The water soluble fraction of major cations and anions wasremoved during the leaching cycles. Even though the totalsulfate concentration was not high in A1 and F1, the sulfateconcentration in the rst leachate was the highest because of ahigh water soluble sulfate fraction as shown in the sequentialleaching results (Table 3 and Fig. 3). When the total sulfateconcentration was high (G3 and H2), sulfate was releasedcontinuously in successive leaching cycles. More than 99% ofthe total Cl released during four leaching cycles was extracted inthe rst cycle. Samples with high EC showed high concentra-tions (more than 500 mg L�1) of Cl in the rst leaching cycle.Leaching of Na showed similar trends to Cl (Fig. 3). Thepercentages of Ca and Mg released in the rst cycle were rela-tively lower than Na and Cl, which might be related to theirchemical forms. Calcium and Mg were mainly found in theexchangeable fraction while Na and Cl were present in the watersoluble fraction.

Similar leaching trends of Na and Cl might be related to thedissolution of halite.31 Strong correlations were found betweentotal Na and total Cl (R2 ¼ 0.81), and total Na and EC (R2 ¼ 0.73)of the leachates indicating that soluble Na–Cl salts were themajor salinity source in these sites. Speciation analysis furtherindicated that NaCl dissolution was not saturated in all thesamples (Table 5). These results indicate that soluble Na–Clsalts are an important source for base cations and control theEC in the rst leachates. Calcium and Mg showed similarleaching patterns which were related to the dissolution of


carbonates and similarity of surface exchange (or sorption/desorption) characteristics of these two cations.32

The saturation index indicates the degree of saturation ofimportant salts in spoil leachates and predicts the order ofprecipitation of salts.33 Values greater than zero indicatesupersaturation, whereas values less than zero representundersaturation. Results show that all major salinity salts(gypsum, halite and KCl) are undersaturated in all samples(Table 5). Gypsum is relatively close to saturation in mostsamples and may precipitate during surface water movement. Aconsiderable amount of gypsum is usually present in salinesoils while soluble carbonates are not present.7 Halite is foundto dominate in the sampled spoil site, although it is far fromsaturation in most samples. However, it should be noted thatthese index values were based on the leachate solution chem-istry which is arbitrarily determined by the water/spoil ratio inthis study, and may not correspond to the real extent of saltsaturation in a specic precipitation event.

The low pH groups (unit G and H) showed the highestconcentrations of Al, Cr, Cu, Co, Ni, Pb, Zn, Mn and Fe inleachates (Table 4). Of these metals, Cu, Ni and Zn levels were ofenvironmental concern, especially Zn. High Zn concentrationswere found in G3 (6.86 mg L�1), B2 (3.75 mg L�1) and H1 (4.34mg L�1). Among the spoils, G3 was of particular environmentalrisks since all the highest Zn, Cu (1.08 mg L�1), Ni (2.64 mg L�1)and Mn (45.9 mg L�1) were found in this leachate. However, inmost cases, the leaching of trace elements was not signicant(Table 4). Dang et al.34 also reported that only a small fraction ofheavy metals was leached to the environment during chemicalweathering of mine spoils while most of them remained in theresidual fraction. This could be explained by coal mine spoils–water interaction processes which rstly break down metal



Fig. 4 Cluster analysis of samples based on major element concentrations of theleachate.


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suldes, then adsorb metals by the iron oxyhydroxide colloidsand organic matter.

Cluster analysis based on concentrations of major elementsleached from columns showed that samples could be broadlydivided into three groups (Fig. 4). The rst group includes unitsE, G and H, and units A, B, D and F belong to the second group.Spoils which belong to the rst group are predominantly freshand coal enriched materials. Therefore, they did not releasemuch salts. The second group showed relatively higher salinitythan the rst group. Spoils in the second group are mainlyweathered and highly weathered. The basalt sample is fresh andits minerals are not reactive in the prevailing thermodynamicconditions and time frame of the leaching experiment. There-fore, it falls into separate category (the third group) based oncluster analysis. Cluster analysis well reected the geochemicalclassication of spoil/waste dened for rehabilitation purposes.

Conclusions

Cluster analysis of the samples based on major elementconcentrations of the leachate showed that classication ofwastes by geology and apparent physical features well predictedthe potential and extent of salinity issues. Therefore, the spoilsand rejects can be managed depending on their geochemicalclassication. The leaching of elements was correlated to thegeochemical properties of mine wastes. Electrical conductivitydecreased markedly with the progress of leaching at higher EClevels, indicating rapid removal of salts in the eld. Sampleswith high total, water soluble and exchangeable Na concentra-tions represented high salinity in the column leaching experi-ment and the dissolution of halite and the release of Na and Clwas regarded as a major source of salinity at the coal mine site.Contamination by trace elements on the site was not an issuebecause the leaching of trace elements was not signicant. Thestudy conrmed that the salinity problem could be predicted bymapping the spoil/waste groups without the need for analysingEC or element concentrations of the leachate. Mining waste


classication by spoil/waste mapping can provide criticalguidance for the development of an effective and cost-efficientplan for sustainable rehabilitation and successful mine closure.

Acknowledgements

We would like to thank Bevan Emmerton of B.R. EmmertonPTY. Ltd. and Jon Burgess for the spoil classication based ongeology and for their guidance during eld sampling.

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Documents

Geochemical assessments and classification of coal mine spoils for better understanding of potential salinity issues at closure