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Fly ash-brine interactions: Removal of major and traceelements from brineOjo O. Fatoba a , Leslie F. Petrik a , Wilson M. Gitari c & Emmanuel I. Iwuoha ba Environmental and Nano Sciences Research Group, Chemistry Department, University ofthe Western Cape, Bellville, South Africab SensorLab, Chemistry Department, University of the Western Cape, Bellville, South Africac Department of Ecology and Resources Management, School of Environmental Sciences,University of Venda, Thohoyandou, Limpopo, South AfricaVersion of record first published: 29 Nov 2011.

To cite this article: Ojo O. Fatoba , Leslie F. Petrik , Wilson M. Gitari & Emmanuel I. Iwuoha (2011): Fly ash-brineinteractions: Removal of major and trace elements from brine, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 46:14, 1648-1666

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Journal of Environmental Science and Health, Part A (2011) 46, 1648–1666Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2011.623647

Fly ash-brine interactions: Removal of major and traceelements from brine

OJO O. FATOBA1, LESLIE F. PETRIK1, WILSON M. GITARI3 and EMMANUEL I. IWUOHA2

1Environmental and Nano Sciences Research Group, Chemistry Department, University of the Western Cape, Bellville, South Africa2SensorLab, Chemistry Department, University of the Western Cape, Bellville, South Africa3Department of Ecology and Resources Management, School of Environmental Sciences, University of Venda, Thohoyandou,Limpopo, South Africa

Fly ash and brine contain major and trace elements such as Na, Cl, Ca, SO4, K, Mg, P, Si, Al, Fe, Mn, Cr, V and Ti in significantquantities. This study focuses on the leachability of species from fly ash and the removal of major and trace species from brine as thetwo waste streams interact. Another objective is to evaluate the effect of the interactions on the brine quality. Batch reaction testswere carried out on two different fly ashes and brine at different L/S ratios and different reaction times, and the supernatant analysedfor major and trace species. Chemical analysis revealed that the unreacted brine solution contained high concentration of speciessuch as Na, K, Ca, Mg, Cl and SO4, while species such as As, Ba, Cd, Co, Cr, Mn, Pb and Ti were present in trace quantities.Analysis of the supernatants after the batch reaction tests (fly ash-brine interaction experiments) revealed that major species such asNa, Mg, Cl and SO4, and trace elements such as As, Co, Pb, Zn, Ni and Cu were significantly removed from the brine solution whileCa, Ba, Sr, Cr and Mo were leached into the brine solution from the fly ashes. The removal of species from the brine solution wasmost prominent at L/S ratio 1:1. This indicates that the L/S ratio of the ash-brine system has a significant effect on the release ofspecies from fly ash or the removal of species from brine solution. The final pH of the fly ash-brine solutions and the contact timeswere also observed to have a significant effect on the leaching from fly ash as well as the removal of major and trace species from thebrine. The study also showed that some contaminant species can be removed from brine solution using fly ash.

Keywords: Brine, fly ash, batch reaction test, chemical interactions, leaching, major and trace species, brine quality, mineralogy,chemical composition, coal combustion.

Introduction

Fly ash is a by-product generated from combustion of coalto generate electricity. Fly ash is generated in large quan-tities in most of the countries in the world as a result ofan increase in the demand for electricity. Several countriesincluding South Africa generate huge amounts of fly ashannually during power generation. For instance, accordingto the American Coal Ash Association,[1] USA generated102 million tons (Mt) of coal combustion products in 1996,out of which fly ash was 59 million tons, but only 25 %of this fly ash was utilized.[2] In South Africa, accordingto the ESKOM report,[3] ESKOM generates approximately34 Mt of fly ash annually of which only 5 % is currentlybeneficially utilized, the rest being disposed of in ash dams,

Address correspondence to Ojo O. Fatoba, Environmental andNano Sciences Research Group, Chemistry Department, Univer-sity of the Western Cape, Private Bag X17, Bellville 7535, SouthAfrica; E-mail: ofatoba@uwc.ac.zaReceived February 3, 2011.

landfills or ponds.[4] SASOL Synfuels at Secunda produces3 Mt fly ash and 7 Mt gasification ash annually.

Apart from the generation of fly ash, power stations gen-erate significant quantities of brine effluents. The treatmentof wastewaters in power stations for water reuse throughdifferent methods such as reverse osmosis (RO), electrodialysis reversal (EDR) and other membrane methods re-sults in the production of brine, a hyper-saline solution.[5]

As a result of wastewater treatment, ESKOM (Tutukapower station) generates about 2.52 million litres (Ml) perday.[6] Brine contains elements such as Na, K, Mg, Ca, Fe,Si and Se in large quantities.[6]

Fly ash and brine pose a major environmental threatdue to their chemical compositions and the possible re-lease of the contaminants in these waste materials to thesurrounding soils, surface water and groundwater. Studieshave shown that major and minor species are released fromfly ash when in contact with aqueous solution,[7–17] andthe possibility of the leached contaminants migrating tothe surface water and groundwater exists. Due to the chal-lenges faced by power stations in disposing fly ash, studieshave been carried out on the beneficial ways of utilizing

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Fly ash-brine interactions 1649

fly ash in order to reduce the disposal problem and theenvironmental implications. These include synthesis of ze-olites,[4,18] treatment of acid mine drainage (AMD),[19] landremediation or rehabilitation for agricultural purposes[20,21]

and in construction works as additive to cement.[22,23]

In the search for ways to reduce the disposal problems ofbrine and fly ash, some power generating utilities use brinestreams to hydraulically transport fly ash to ash dumpswhile others use brine for dust suppression in ash dumps.For instance, in SASOL-Secunda, the fly ash is mixed withabout 60 % brine solution to form slurry before it is trans-ported hydraulically to the ash dumps. At ESKOM-Tutuka,the fly ash is conditioned with about 16 % brine for dustsuppression before conveying the ash to the dump for dis-posal, where it is further conditioned with brine. Despitethe co-disposal of fly ash and brine solution, the inter-action chemistry of the species in the system is not fullyunderstood. The interaction of some species in the fly ashand brine could result in precipitation of salts due to super-saturation or adsorption. It is necessary to study the chem-ical interaction chemistry in fly ash-brine systems. This willgive an insight into the effect of the interaction on the brinequality as well as the environmental implications overtimewhen fly ash and brine are co-disposed. The aims of thisstudy are: (1) to evaluate the chemical interactions of flyash and brine components, and (2) to evaluate the effect ofthe interactions on brine quality.

Materials and methods

Fly ash and brine

Fresh fly ash samples from coal-fired power utilities oper-ated by ESKOM (Tutuka) and SASOL (Secunda) in SouthAfrica were used in this study. The fly ash from SASOL(Secunda) was collected directly from the hoppers, whilethe fly ash from ESKOM (Tutuka) was collected from theconveyor belt conveying the conditioned ash to the ashdump. At Tutuka, the fly ash is conditioned with about 16% brine solution to suppress dust before conveying it tothe ash dump. The samples were kept in plastic contain-ers which were tightly closed to prevent ingress of air, andstored at room temperature (20 ± 5◦C). The brine sampleused in this study was collected at ESKOM (Tutuka) powerstation. The sample was stored in plastic containers, tightlyclosed and refrigerated at 4◦C.

Chemical and mineralogical analysis of fly ash

Chemical and mineralogical analysis of the fly ash sampleswere done using X-ray fluorescence (XRF) and XRD witha Philips 1404 Wavelength Dispersive Spectrometer fittedwith a Rh tube, and a Philips PANalytical instrument witha PW3830 X-ray generator operated at 40 kV and 25 mA

respectively. The samples were oven-dried at 105◦C for 12hours to remove the water.

Batch reaction tests

The batch reaction tests were carried out on the fly ashsamples to evaluate the interaction chemistry of the fly ash-brine system at different liquid/solid (L/S) ratio and at dif-ferent reaction times. The batch reaction tests were carriedout according to the German leach test (DIN 38414 S4)[24]

but with slight modification. 50 g of fly ash was reactedwith varying volumes of brine to give specific L/S ratios(L/S 1:1, 1.5:1 and 5:1). The fly ash-brine mixtures wereagitated using a mechanical shaker at 250 rpm for periodsvarying from 5 minutes to 24 hours at room temperature.The pH and EC of the supernatants were measured beforefiltration using Hanna HI 991301 pH meter with portablepH/EC/TDS/Temperature probe. The supernatants werefiltered using a 0.45 µm pore nucleopore membrane filterafter which the filtrates for cation analysis were acidified.The filtrates for cation and anion analysis were stored at4◦C until the analysis. All experiments were done in tripli-cate.

The major, minor and trace elements in unreactedbrine (UB) samples and in the supernatants from thefly ash-brine batch reaction tests were determined usinginductively couple plasma atomic emission spectrometry(ICP-AES) (Varian Liberty) and inductively coupledplasma mass spectrometry (ICP-MS) (Agilent 7500ce).The anions were determined by the use of a DionexDX-120 Ion Chromatograph (IC) with an Ion Pac AS14Acolumn and AG14-4 mm guard column.

Total dissolved solid (TDS)

The total dissolved solid (TDS) of the brine sample wasdetermined gravimetrically by weighing known amountsof brine into an evaporating dish and oven-dried at 105◦Cuntil a constant weight was achieved. The weight of the drysolid residue was used to calculate the total dissolved solidof the brine solution.

Results and discussion

Chemical composition of fly ash

The chemical compositions of fly ashes from Secunda andTutuka power stations are shown in Table 1. The fly asheswere observed to contain high concentrations of Al2O3,SiO2, CaO and Fe2O3 as their major elements. Accord-ing to the American Society for Testing and Materials(ASTM),[25] which classifies fly ashes based on the percent-age composition of Al2O3, SiO2 and Fe2O3 in the ash, flyashes from Secunda and Tutuka can be classified as Class F(SiO2 +Al2O3+Fe2O3≥ 70 %). The concentrations of the

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1650 Fatoba et al.

Table 1. XRF analysis showing the chemical compositions (major elements as oxides in % w/w and trace elements in mg/L) ofSecunda and Tutuka fly ash samples (n = 3).

Secunda fly ash Tutuka fly ash

Major elements(%w/w)

Trace elements(ppm)

Major elements(%w/w)

Trace elements(ppm)

Al2O3 26.03 V 219.7 Al2O3 24.5 V 204.0CaO 9.30 Cr 129.8 CaO 5.9 Cr 145.3Cr2O3 0.02 Co 10.5 Cr2O3 0.0 Co 21.0Fe2O3 2.36 Ni 58.5 Fe2O3 5.4 Ni 62.2K2O 0.81 Cu 37.0 K2O 0.8 Cu 32.8MgO 2.25 Zn 32.1 MgO 1.4 Zn 48.5MnO 0.05 Ga 40.8 MnO 0.1 Ga 33.6Na2O 0.61 Rb 33.8 Na2O 0.3 Rb 42.2P2O5 0.89 Sr 3268.1 P2O5 0.5 Sr 1164.4SiO2 49.58 Y 81.9 SiO2 51.8 Y 74.4TiO2 1.58 Zr 419.2 TiO2 1.5 Zr 382.8LOI 4.35 Nb 35.8 LOI 8.0 Nb 33.7Sum 97.82 Ba 2292.0 Sum 100.1 Ba 983.7

La 119.8 La 108.3Ce 259.4 Ce 199.7Nd 86.9 Nd 67.9Pb 41.0 Pb 50.3Th 47.6 Th 39.5U 2.0 U 8.9Cl 73.9 Cl 730.26

major elements (as oxides) in the two fly ashes vary. Forexample, the CaO (9.3 %w/w), Al2O3 (26.0 %w/w) andMgO (2.3 %w/w) concentrations in Secunda fly ash washigher than in Tutuka fly ash. On the other hand, the con-centrations of SiO2 (51.8 %w/w) and Fe2O3 (5.4 %w/w)in Tutuka fly ash were higher than in Secunda fly ash. Thevariation in the concentrations of these oxides in the flyashes could be due to the different types of coal burned.

The dissolution of the major oxides (such as Ca and Mgoxides) in the fly ash has a significant effect on the pHof the fly ash solution. For example, the concentrations ofCa and Mg (reported as oxides) are higher in Secunda flyash. This probably accounts for the difference observed inthe natural pH (Fig. 1) of the two fly ash solutions. It wasobserved that the concentration of the trace elements inTutuka fly ash was slightly higher than in Secunda fly ash.Except for elements such as Sr, Ba, Th, V, Ga, Cu, Y, Zr,Nb, La, Ce, Nd, the concentration of other trace elementswere found to be higher in Tutuka fly ash. Generally, theconcentrations of some trace elements such as Sr, Ba, V,Zr and Ce present in the two fly ashes were higher whencompared to the concentration of other trace elements suchas Cr, Co, Ni, Cu and Pb among others (Table 1).

The chloride content of Tutuka fly ash was observedto be higher than that of Secunda fly ash. The signifi-cant difference in the concentration of Cl in Tutuka flyash when compared to Secunda fly ash is likely to be theresult of preconditioning the Tutuka fly ash with brine so-lution, which is high in Cl (Table 2), before the sample wascollected.

Loss on ignition (LOI) of Tutuka fly ash was higher thanwhat was recorded in Secunda fly ash. This is an indicationthat the percentage of unburned carbon in Tutuka fly ashis greater than in Secunda. Variation in the LOI could beattributed to the different combustion temperatures of thepower stations, or the different milling conditions of coal,and the oxidation of combustible constituents of coal.

Mineralogical compositions of the fly ashes

The XRD patterns of fresh Secunda and Tutuka fly ashesused in this study are shown in Figure 1. It was observedthat the major crystalline mineral phases of the two flyashes are quartz (SiO2) and mullite (3Al2O3.2SiO2). Themineralogical compositions of Secunda and Tutuka flyashes are similar except for the CaO (lime) content in Se-cunda and Calcium Magnesium Aluminium Silicate Chlo-ride ((Ca19.44Mg2.56)(Si.75Al.25)8O36Cl2) in Tutuka fly ash.The peak intensity of CaO (lime) observed in Secunda flyash was low when compared to the intensity of other peakswhich could indicate that the amount of CaO in the fly ashis lower than the phases such as quartz.

The non-appearance of the CaO peak in Tutuka fly ashcould also indicate that its concentration in the fly ash sam-ple is very low and due to the limitation of XRD techniqueto detect phases that are very low. The XRF analysis (Table1) revealed that the Ca (expressed as CaO) concentrationof the two fly ashes varied slightly with Secunda fly ashhaving 9.3 %w/w while Tutuka fly ash contains 5.9 %w/w.CaO has been observed to be one of the major contributors

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Fly ash-brine interactions 1651

Fig. 1. XRD spectra of Secunda (A) and Tutuka (B) fresh fly ash; M = Mullite, Q = quartz, C = Calcium Magnesium AluminumSilicate Chloride, L = CaO.

to the high pH values of fly ash when dissolved in aqueoussolution. The difference observed in the pH values of Se-cunda and Tutuka fly ash solutions (Fig. 2, A1 and Fig. 2,B1) could be attributed to the difference in the CaO con-centration. The Calcium Magnesium Aluminium SilicateChloride ((Ca19.44Mg2.56)(Si.75Al.25)8O36Cl2) identified inTutuka fly ash could be attributed to the preconditioning

Table 2. Chemical compositions (in mg/L except EC (mS/cm)and pH) of unreacted brine (UB) solution from Tutuka powerstation (n = 3).

Major elements (mg/L) Minor elements (mg/L)

B 2.24 ± 0.024 Al 0.045 ± 0.0063Ca 106.99 ± 1.69 As 0.0068 ± 0.000053K 106.2 ± 0.85 Ba 0.057 ± 0.0019Mg 163.36 ± 0.85 Cd 0.00017 ± 0.000012Na 4804.88 ± 2.72 Co 0.014 ± 0.00037Si 13.11 ± 0.075 Cr 0.014 ± 0.0014Cl 2424 ± 16.97 Cu 0.26 ± 0.087SO4 8858 ± 86.27 Fe 0.24 ± 0.2pH 7.75 ± 0.031 Mn 0.0017 ± 0.000089EC (mS/cm) 16.69 ± 0.50 Mo 0.039 ± 0.0011TDS 15400 ± 282.84 Ni 0.12 ± 0.00071

P 0.82 ± 0.025Pb 0.0039 ± 0.0036Se 0.0049 ± 0Sr 3.055 ± 0.0098Ti 0.00069 ± 0.00055V 0.019 ± 0.000035Zn 0.13 ± 0.014

of Tutuka fly ash with brine before disposal. The fresh Tu-tuka fly ash was moistened with brine in order to suppressdust before disposal and irrigated with brine after disposalat the dump (see later). This treatment could have lead tothe chemical interactions between the species of brine andfly ash before the sample was taken.

Chemical compositions of brine

The chemical composition (major and minor species) ofbrine used in this study is presented in Table 2. It wasobserved that the concentrations of species such as Ca, Na,Mg, K, Cl and SO4 were high in the brine solution. Theconcentrations of Na (4804.9 mg/L), Cl (2424 mg/L) andSO4 (8858 mg/L) in the brine were significantly higher thanother major species. The concentrations of trace elementssuch as As, Se, Cd, Cu, Cr, Fe, Mn, Co, etc were very lowin the brine solution. The pH of the brine solution is nearneutral (7.75) while the electrical conductivity (EC) andtotal dissolved solids (TDS) values were found to be 16.69mS/cm and 15400 mg/L respectively. The pH indicatesthat the brine solution is neutral in nature while the ECand TDS values not surprisingly indicate the presence ofionic species and high amounts of dissolved salts in thebrine solution.

Batch reaction tests

pH, Electrical Conductivity (EC) and Total Dissolved Solids(TDS). The batch reaction experiment was carried out atdifferent liquid/solid ratios (L/S 1:1, 1.5:1 and 5:1) and

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1652 Fatoba et al.

7.75

7

8

9

10

11

12

13

14

UB 5 10 20 30 60 240 480 840 1080 1440

pH

Reac�on �me (min)

UB pH (1:1) pH (1.5:1) pH (5:1)

A1

7.75

7

8

9

10

11

12

UB 5 10 20 30 60 240 480 840 1080 1440

pH

Reac�on �me (min)

UB pH (1:1) pH (1.5:1) pH (5:1)

B1

15400

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

UB 5 10 20 30 60 240 480 840 1080 1440

TDS

(mg/

L)

Reac�on �me (min)

UB TDS (1:1) TDS (1.5:1) TDS (5:1)

A2

15400

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

UB 5 10 20 30 60 240 480 840 1080 1440

TDS

(mg/

L)

Reac�on �me (min)

UB TDS (1:1) TDS (1.5:1) TDS (5:1)

B2

Fig. 2. pH and TDS of Secunda (A1–A2) and Tutuka (B1–B2) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and 5:1 as a function ofreaction time. (UB = unreacted brine) (color figure available online).

at different contact times ranging from 5 minutes to 1440minutes. This was done in an attempt to determine theeffect of the L/S ratio and the contact time on the releaseof species from fly ash into the brine, and the removal ofspecies from the brine solution. Figure 2 shows the pH andTDS profiles of Secunda (Fig. 2, A1 and A2) and Tutuka(Fig. 2, B1 and B2) fly ash-brine batch reaction tests. ThepH of the brine solutions after contact with the fly ashesincreased abruptly from 7.75 to 11.26 for Secunda ash-brine systems and to ≈9 for Tutuka ash-brine systems after5 minutes’ reaction time.

These pH values were stable irrespective of the L/S ratioof the solutions until 30 minutes’ reaction time when afurther significant increase was observed in the pH values ofthe solutions of the fly ashes. Thereafter, the pH was stablein the case of Secunda fly ash-brine solutions but a gradualincrease in the pH of Tutuka fly ash-brine solutions wasobserved until the end of the batch reaction experiments.The observed increase in the pH of the brine solutions aftercontacting with the fly ashes could be due to the dissolutionand hydrolysis of some basic oxides such as CaO and MgOthat are present in significant quantities in the fly ashes(Table 1). A slight variation was observed in the pH values

of Secunda and Tutuka fly ash-brine solutions at the samereaction time.

The pH value of Secunda fly ash-brine solution wasfound to be consistently higher than that of Tutuka flyash-brine solution (Table 3; Fig. 2, A1 and B1). This couldbe attributed to the variation observed in the concentra-tions of CaO and MgO in the fly ashes. The concentrationsof basic oxides (CaO and MgO) were higher in Secunda flyash (9.3 % CaO and 2.3 % MgO) than in Tutuka fly ash (5.9% CaO and 1.4 % MgO) (Table 1).The pH of fly ash solu-tions has been observed to depend on the relative quantitiesof the soluble basic or acidic oxides in the fly ash.[7,26]

In their studies, Choi et al.[27] and Reardon et al.[28] at-tribute the high pH value of fly ash solutions to the disso-lution and hydrolysis of basic oxides such as calcium andmagnesium oxides, while the low pH value observed in theirstudies was said to be accounted for by the dissolution ofsoluble acids such as B2O3, and the salts containing hy-drolysable constituents, such as Fe2(SO4)3 and Al2(SO4)3in the fly ash.

There was no significant difference observed in the pHvalues of the fly ash-brine solutions with respect to thedifferent L/S ratios in the case of Secunda, but a significant

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Tab

le3.

The

%sp

ecie

sle

ache

dor

rem

oved

from

Secu

nda

flyas

h-br

ine

batc

hre

acti

onte

stat

L/S

1:1

asa

func

tion

ofco

ntac

tti

me.

Para

met

er/c

onta

ctti

me

(min

)U

B5

1020

3060

240

480

840

1080

1440

Al

0.04

510

.210

4.08

248

.98

−476

.13

−427

.71

−858

.06

−584

.7−1

739.

14−4

48.5

4−8

97.2

2A

s∗0.

0068

73.5

382

.35

70.5

996

.093

100

100

95.7

782

.62

90.9

580

B∗

2.24

100

100

100

−20.

2236

.16

53.7

380

.29

85.8

281

.91

85.7

4B

a0.

057

−777

.19

−742

.11

−759

.65

−396

.35

−482

.95

−682

.62

−563

.35

−477

.41

−325

.76

−304

.52

Ca

106.

99−1

593.

7−1

584.

27−1

557.

44−1

990.

21−1

230.

29−8

96.5

1−8

50.4

1−7

79.1

9−8

05.1

7−7

77.6

2C

o∗0.

014

98.9

399

.78

99.9

993

.29

96.4

995

.82

95.3

594

.59

95.7

594

.52

Cr

0.01

4−1

757.

14−1

685.

71−1

971.

43−1

836.

27−4

186.

13−6

376.

88−6

973.

69−1

3923

.12

−151

97.6

9−1

7982

.37

Cu∗

0.26

98.1

599

.86

99.9

993

.73

95.0

3493

.91

92.1

194

.58

94.9

394

.71

Fe∗

0.24

54.1

750

41.6

7−3

6.49

17.4

410

.88

−19.

44−2

6.81

32.6

26.

76K

∗10

6.2

−8.5

9−6

.6−5

.53

6.42

7.12

3.45

6.63

1.37

−0.8

9−2

.26

Mg∗

163.

3699

.93

99.9

299

.95

100

100

100

99.8

899

.88

100

100

Mn∗

0.00

1773

.53

63.5

329

.41

−231

.34

−142

.55

−115

.58

−242

.12

−314

.99

−132

.84

−169

.67

Mo

0.03

9−6

17.9

5−5

92.3

1−6

43.5

9−3

27.3

5−3

74.5

6−8

19.9

9−8

36.4

9−9

71.0

47−1

387.

65−1

560.

3N

a∗48

04.8

814

.029

18.6

718

.56

14.8

112

.082

11.1

514

.01

12.4

76.

4510

.3N

i∗0.

1255

55.8

355

45.4

945

.98

47.9

543

.24

44.5

955

.55

53.8

2P

b∗0.

0039

58.4

661

.54

61.5

4−1

0.36

−37.

98−6

5.51

−85.

06−1

21.8

97.

14−1

0.33

Sr3.

06−1

207.

69−1

113.

39−1

198.

53−9

15.7

2−1

052.

42−1

210.

21−1

390.

7−1

762.

69−2

148.

32−2

328.

65Z

n∗0.

1382

.31

84.6

284

.62

26.8

852

.85

47.6

522

.09

11.1

842

.219

.13

Cl∗

2424

5.8

7.96

14.1

220

.96

23.9

324

.01

25.4

127

.48

24.7

529

.13

SO∗ 4

8858

28.6

631

.17

32.8

430

.89

44.0

9653

.96

56.0

454

.55

53.8

956

.56

pH7.

75−4

5.29

−44.

39−4

4−6

4.13

−63.

23−6

6.06

5−6

3.48

−63.

0967

7−6

4−6

5.29

EC

∗16

.69

11.6

87.

1311

.32

2.16

15.0

416

.72

22.0

515

.99

21.6

38.

87T

DS∗

1540

048

.44

45.7

148

.18

42.7

950

.32

51.2

954

.42

50.7

854

.29

46.7

5

UB

=un

reac

ted

brin

e,N

egat

ive

(−)=

%le

ache

d,Po

siti

ve(+

)=

%re

mov

al,A

ster

isk

(∗ )=

para

met

erre

mov

ed.(

All

valu

esar

ein

%ex

cept

UB

,whi

chis

inm

g/L

.)

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1654 Fatoba et al.

difference was observed in Tutuka especially at L/S 5:1.The difference observed in the pH at L/S 5:1 of Tutuka flyash-brine system could be attributed to the wide range offly ash: brine ratio, and the pre-treatment of Tutuka fly ashbefore sampled. This indicates that the L/S ratio of the flyash-brine systems had impact on the pH of the solutions.

The TDS values of the brine solutions after contact withthe fly ashes at different L/S ratios (Fig. 2, A2 and B2)were observed to decrease significantly when compared tothe value of the unreacted brine (UB) except at L/S 5:1for Secunda fly ash-brine systems. The decrease in the TDSvalues of the fly ash-brine solutions was maintained fromthe beginning of the batch reaction tests (5 minutes) to theend (1440 minutes) for most of the L/S ratios (Fig. 2, A2and B2). At L/S 5:1 for Secunda fly ash-brine solution,the TDS was observed to increase gradually for the entireperiod of the batch reaction test. The TDS values of bothSecunda and Tutuka fly ash-brine solutions were found tobe lower at L/S 1:1 than the values at other L/S ratios (L/S1.5:1 and 5:1).

The decrease observed in the TDS after the batch reac-tions at different L/S ratios indicates that some of the dis-solved salts such as Na, SO4 and Cl in the unreacted brinesolution may have been removed by the fly ashes either byadsorption or precipitation as a result of super-saturationoccurring due to dissolution of fly ash components. Thereduction in the TDS of the fly ash-brine solutions wasmore prominent in Tutuka fly ash-brine solution where theminimum value of 3310 mg/L (78.5 % removal) (Table 4)was observed at 1080 minutes for the solution at L/S 1:1.The gradual increase observed at L/S 5:1 for Secunda flyash-brine solution could be an indication that there wasno precipitation occurring due to undersaturation of thespecies in the fly ash-brine systems as a result of the highL/S ratio employed. Instead of precipitation, more sol-uble salts were released into the brine solution from flyash.

The electrical conductivity (EC) of Tutuka fly ash-brinesolutions decreased rapidly (10.6 mS/cm) at 5 minutes’contact time when compared with the EC of the unreactedbrine (UB) (16.7 mS/cm). The EC of Tutuka fly ash-brinesolutions thereafter decrease gradually for the remainingperiod of the reaction test (graphs not shown). The ECof Secunda fly ash-brine solutions was found to decreasegradually with an increase in the contact time until at 1440minutes where a slight increase in the EC was observed.The rate at which the EC of Tutuka fly ash-brine solutiondecreased with an increase in the contact time was morethan that of Secunda fly ash-brine solution. This indicatesthat more ions were removed from the brine solution byTutuka fly ash. It was reported by Wong et al. [29] thatprecipitation of soluble salts could cause a decrease in theEC of fly ash solution. The decrease observed in the EC ofthe fly ash-brine solutions could indicate the precipitationof soluble salts from the fly ash-brine solution in the formof transient mineral phases.

Sodium (Na) and Chloride (Cl). The concentrations of Naand Cl in the leachates of the fly ash-brine batch reactiontests at different reaction times and L/S ratios are shownin Figure 3. It was observed that the concentrations ofNa in the brine solutions decreased significantly with anincrease in the contact time (Fig. 3, A3 and B3). At 10and 20 minutes’ contact time, the lowest concentration ofNa in the solutions was observed for Tutuka and Secundafly ash-brine systems, respectively. Concentrations of Na inthe fly ash-brine solutions at L/S 1:1 was lower than whatwas observed at other L/S ratios (1.5:1 and 5:1) for both flyashes. Comparison of the amount of Na in unreacted brine(UB) with Na concentrations in the fly ash-brine solutionsat all contact time indicates the removal of Na ions frombrine during the fly ash-brine interactions. The removalof Na in the systems could be due to the precipitation oftransient phases as a result of super-saturation of Na in thefly ash-brine systems.

The percentage removal of Na from the brine solutionranged between 6–19 % and 19–32 % for Secunda and Tu-tuka fly ash-brine systems respectively (Tables 3 and 4). Thepercentage Na removal obtained shows that Tutuka fly ashhas the capacity to remove more Na ions from the brinesolution than Secunda fly ash. Reduction in the concen-tration of Na in brine after contacting brine with fly ashwas observed in the study carried out by Soong et al.[30]

The reduction in Na concentration was attributed to theprecipitation of NaCl.

However, it was observed by Kirby and Rimstidt,[31] thatNa could be locked up in some solid phases such as car-bonate and sulphate if formed in the system. Therefore,the removal of Na could be attributed to either the super-saturation of Na ions in the solutions of the fly ash-brinesystems leading to precipitation of NaCl or as a result ofbeing locked up in the solid phases.

The Cl concentration in the Secunda fly ash-brine solu-tions at L/S 1:1, 1.5:1 and 5:1 was found to decrease gradu-ally at the beginning of the test (Fig. 3, A4) and a minimumobserved at 20 minutes’ contact time in the case of L/S1.5:1 and 5:1 solutions. After 30 minutes’ contact time, theCl concentration was almost stable for the remaining pe-riod of the test irrespective of the L/S ratio (Fig. 3, A4). Inthe case of Tutuka fly ash-brine systems, the concentrationof Cl (Fig. 3, B4) decreased abruptly at 5 minutes’ contacttime when compared to the UB, after which an increase(but below levels of UB) in its concentration was observedat 30 minutes. The concentration of Cl in the Tutuka flyash-brine solutions became stable thereafter.

When compared with the concentration in the UB, theconcentration of Cl in the fly ash-brine solutions decreasedsignificantly in both cases. The decrease in the concentra-tion of Cl in the brine solution after contacting the fly ashesindicates that the fly ashes were able to significantly removethe Cl ion from the brine solution. The percentage removalof Cl from the brine solutions ranged between 5 % and 29% for Secunda fly ash-brine solutions and between 21 and

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Tab

le4.

The

%sp

ecie

sle

ache

dor

rem

oved

from

Tut

uka

flyas

h-br

ine

batc

hre

acti

onte

stat

L/S

1:1

asa

func

tion

ofco

ntac

tti

me.

Para

met

er/c

onta

ctti

me

(min

)U

B5

1020

3060

240

480

840

1080

1440

Al

0.04

5−6

55.1

−126

7.35

−838

.78

−210

1.36

−592

1.58

−383

6.81

−861

7.77

−924

6.09

−506

0.97

−405

5.3

As

0.00

68−6

1.76

−27.

94−3

5.29

−81.

89−6

5.3

−99.

075

−125

.24

−112

.88

−139

.69

−128

.38

B∗

2.24

71.4

391

.52

93.7

538

.28

40.6

2−4

.055

76.9

80.3

154

.14

46.5

7B

a0.

057

−285

.96

−303

.51

−338

.59

−529

.98

−383

.97

−371

.88

−354

.25

−288

.014

−241

.79

−210

.07

Ca

106.

99−8

3.91

−95.

07−9

5.97

−105

.64

−128

.81

−179

.11

−103

.79

−72 .

62−1

14.6

7−1

27.9

7C

o∗0.

014

60.7

167

.14

61.4

329

.42

45.9

155

.14

58.4

571

.58

70.3

273

.52

Cr

0.01

4−1

9471

.43

−136

14.2

9−1

3900

−163

32.0

8−1

8475

.14

−236

05.2

−190

37.2

8−1

8804

.62

−232

03.4

7−2

4090

.75

Cu∗

0.26

91.9

293

.08

91.1

579

.54

83.0

6486

.86

83.7

787

.29

86.7

586

.17

Fe∗

0.24

37.5

5059

.58

9.18

−0.2

5−1

1.29

−33.

2713

.69

−13.

64−2

6.38

K10

6.2

−31.

98−2

6.51

−26.

74−3

1.73

−36.

06−3

7.33

−37.

66−3

2.86

−36.

65−3

8.51

Mg∗

163.

3690

.19

97.0

694

.47

96.6

99.6

698

.99

100

100

100

99.7

6M

n∗0.

0017

51.1

854

.71

62.3

5−1

75.4

5−2

05.9

2−3

91.6

2−1

90.6

9−4

83.5

9−2

01.9

9−2

47.8

6M

o0.

039

−243

8.46

−187

4.36

−192

5.64

−203

2.42

−232

1.19

−269

4.49

−237

1.61

−223

0.53

−262

4.22

−274

2.37

Na∗

4804

.88

30.1

231

.73

28.7

221

.65

21.5

719

.53

23.8

924

.74

21.2

119

.83

Ni∗

0.12

21.6

731

.67

26.6

718

.09

15.3

715

.65

16.7

122

.25

11.9

311

.67

Pb∗

0.00

3981

.54

88.3

183

.08

37.0

743

.43

34.9

4−3

.11

49.6

946

.43

51.0

7Sr

3.06

−187

.07

−208

.35

−204

.75

−222

.84

−231

.48

−256

.44

−214

.39

−184

.37

−212

.13

−230

.018

Zn∗

0.13

85.3

892

.31

92.3

139

.91

56.6

756

.88

58.5

954

.24

46.9

826

.71

Cl∗

2424

45.4

843

.73

42.1

326

.24

24.6

722

.36

25.1

724

.83

23.4

321

.2SO

∗ 488

5833

.01

35.4

132

.96

35.6

732

.74

29.4

433

.89

34.4

934

.32

31.2

pH7.

75−2

2.45

−23.

09−2

3.48

−40.

77−4

4.65

−44

−49.

68−4

8.39

−47.

48−4

8.65

EC

∗16

.69

36.7

938

.35

49.0

755

.06

55.0

654

.76

64.4

762

.85

63.3

959

.26

TD

S∗15

400

64.9

463

.83

70.5

274

.03

73.6

473

.31

79.2

278

.25

78.5

176

.1

UB

=un

reac

ted

brin

e,N

egat

ive

(−)=

%le

ache

d,Po

siti

ve(+

)=

%re

mov

al,A

ster

isk

(∗ )=

para

met

erre

mov

ed.(

All

valu

esar

ein

%ex

cept

UB

whi

chis

inm

g/L

.)

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1656 Fatoba et al.

4804.88

3000

3200

3400

3600

3800

4000

4200

4400

4600

4800

5000

UB 5 10 20 30 60 240 480 840 1080 1440

Na

(mg/

L)

Reac�on �me (min)

UB Na (1:1) Na (1.5:1) Na (5:1)

A3

4804.88

3000

3200

3400

3600

3800

4000

4200

4400

4600

4800

5000

UB 5 10 20 30 60 240 480 840 1080 1440

Na

(mg/

L)

Reac�on �me (min)

UB Na (1:1) Na (1.5:1) Na (5:1)

B3

2424

1200

1400

1600

1800

2000

2200

2400

2600

UB 5 10 20 30 60 240 480 840 1080 1440

Cl (m

g/L)

Reac�on �me (min)

UB Cl (1:1) Cl (1.5:1) Cl (5:1)

A4

2424

1200

1400

1600

1800

2000

2200

2400

2600

UB 5 10 20 30 60 240 480 840 1080 1440

Cl (m

g/L)

Reac�on �me (min)

UB Cl (1:1) Cl (1.5:1) Cl (5:1)

B4

Fig. 3. Na and Cl concentration of Secunda (A3–A4) and Tutuka (B3–B4) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and 5:1 as afunction of reaction time. (UB = unreacted brine) (color figure available online).

45 % for Tutuka fly ash-brine (Tables 3 and 4). This in-dicates that Tutuka fly ash can more significantly removeCl ions from the brine solution more than Secunda flyash. An observation of the Tutuka trends show similarityin the trends of Na and Cl, which could indicate removalas NaCl[30] due to super-saturation of Na and Cl in thesystems, but for Secunda (Fig. 3, A4), apart from the pre-cipitation of NaCl, different salts bearing Na and Cl couldbe responsible for the removal of Cl.

Calcium (Ca), Magnesium (Mg) and Sulphate (SO4). Theconcentrations of Ca, Mg and SO4in the solutions of Se-cunda and Tutuka fly ash-brine reactions are shown inFigure 4 (A5–A7 and B5–B7), respectively. The Ca con-centration in Secunda fly ash-brine solutions increased sig-nificantly upon interaction with fly ash at the beginningof the experiment, irrespective of the L/S ratio. The Caconcentration increased to a maximum (2236.5 mg/L) at30 minutes (Table 3) for Secunda fly ash-brine solution atL/S 1:1 (Fig. 4, A5). The concentration of Ca thereafterreduced gradually until at 840 minutes when the concen-tration was almost stable in the solutions at L/S 1:1 and1.5:1.

Despite the gradual decrease observed in the concen-tration of Ca, the value in the fly ash-brine solution wasstill higher than its initial concentration in the UB. Thisshows that the Ca containing phases in Secunda fly ashdissolved readily. The Ca concentrations in Tutuka fly ash-brine solutions gradually increased from the beginning ofthe experiment with increase in contact time except at 480,840 and 1080 minutes where the concentration was slightlyreduced (Fig. 4, B5). The increase observed in Ca concen-tration in the solutions of the fly ash-brine systems whencompared with the Ca concentration in the unreacted brine(UB) shows that Ca was significantly leached from the flyashes.

The percentage of Ca leached into the brine solutionfrom the fly ashes ranged from 700 to about 2000 % forSecunda fly ash-brine systems and from 72 to about 180 %for Tutuka fly ash-brine systems (Tables 3 and 4).The grad-ual decrease in Ca concentration after the initial increasein the solution of Secunda fly ash-brine indicates that someCa ions were being removed from the system. This decreasecould be attributed to the formation of Ca containing sec-ondary mineral phases such as CaSO4 and Ca(OH)2 in thefly ash-brine systems.

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Fly ash-brine interactions 1657

106.990

200400600800

10001200140016001800200022002400

UB 5 10 20 30 60 240 480 840 1080 1440

Ca (m

g/L)

Reac�on �me (min)

UB Ca (1:1) Ca (1.5:1) Ca (5:1)

A5

106.99

0

50

100

150

200

250

300

350

UB 5 10 20 30 60 240 480 840 1080 1440

Ca (m

g/L)

Reac�on �me (min)

UB Ca (1:1) Ca (1.5:1) Ca (5:1)

B5

163.36

0

20

40

60

80

100

120

140

160

180

UB 5 10 20 30 60 240 480 840 1080 1440

Mg

(mg/

L)

Reac�on �me (min)

UB Mg (1:1) Mg (1.5:1) Mg (5:1)

A6

163.36

-20

0

20

40

60

80

100

120

140

160

180

UB 5 10 20 30 60 240 480 840 1080 1440

Mg

(mg/

L)

Reac�on �me (min)

UB Mg (1:1) Mg (1.5:1) Mg (5:1)

B6

8858

3500

4500

5500

6500

7500

8500

9500

UB 5 10 20 30 60 240 480 840 1080 1440

SO4

(mg/

L)

Reac�on �me (min)

UB SO4 (1:1) SO4 (1.5:1) SO4 (5:1)

A7

8858

3500

4500

5500

6500

7500

8500

9500

UB 5 10 20 30 60 240 480 840 1080 1440

SO4

(mg/

L)

Reac�on �me (min)

UB SO4 (1:1) SO4 (1.5:1) SO4 (5:1)

B7

Fig. 4. Ca, Mg and SO4 concentration of Secunda (A5–A7) and Tutuka (B5–B7) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and5:1 as a function of reaction time. (UB = unreacted brine) (color figure available online).

Ca is known to be associated with the surface of fly ashparticles as soluble salt, as well as forming part of the maincomponents of the aluminosilicate glass fractions, alongwith other major element like K and Al.[7] Ca-containingphases such as CaO, which is one of the major componentsof fly ash, are observed to be highly soluble when in contactwith aqueous solutions.[31] Therefore the increase in theconcentration of Ca in the brine solution could be ascribedto the dissolution of the easily soluble salts on the surfaceof fly ash particles, and as a result of the dissolution of CaOthat is present in the fly ashes (Table 1).

The concentration of Ca in Secunda fly ash-brine solu-tions was observed to be significantly higher than the con-centration observed in Tutuka fly ash-brine solutions. Thisobservation agrees with what was observed in the XRF dataof the fly ashes, where the concentration of Ca (expressed asCaO) was found to be higher in Secunda fly ash (Table 1).The variation in the Ca concentration of the fly ashes maybe due to the type of feed coal, combustion sequence andmethod of collection, storage and climate.[32,33] The Tutukafly ash has been irrigated with brine before the ash samplewas taken, and this may have affected the Ca containing

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1658 Fatoba et al.

phases in the fly ash, which in turn may affect the amountof Ca available for dissolution.

The concentration of Mg in the batch reaction tests in-volving unreacted brine (UB) and fly ashes from Secundaand Tutuka power utilities is shown in Figure 4 (A6 andB6). The Mg concentration was reduced to the minimumat the beginning of the batch reaction tests (at 5 minutes),for Secunda fly ash-brine systems (Fig. 4, A6). This sig-nificant reduction in Mg concentration continued until theend of the reaction. Slight differences were observed in theremoval of Mg from Tutuka fly ash-brine systems. The Mgconcentration in Tutuka fly ash-brine solutions was onlyreduced to the minimum at 60 minutes’ contact time forL/S 1:1 and 240 minutes for L/S 1.5:1.

The low concentration of Mg in Tutuka fly ash-brinesolutions at L/S 1:1 and 1.5:1 continued thereafter till theend of the experiment. At L/S ratio 5:1, the concentrationof Mg was found to be reducing gradually with an increasein contact time (Fig. 4, B6). The reduction observed inthe concentration of Mg in the solutions of Secunda andTutuka fly ash-brine batch reactions when compared withthe concentration of Mg in the unreacted brine (UB) showsthat Mg was significantly removed from the brine aftercontact with fly ash. The percentage removal of Mg fromthe solutions of the fly ash-brine systems ranged between99 and 100 % for Secunda fly ash and from 90–100 % forTutuka fly ash (Tables 3 and 4). Mg in alkaline solution (pH>10) has been observed to exist as insoluble Mg(OH)2,[7,26]

and the precipitation of this insoluble Mg containing phasefrom solution could account for its low concentration in thesolutions of the fly ash-brine systems.

The rate at which Mg was removed from Secunda fly ash-brine solutions was higher than that of Tutuka fly ash-brinesolutions at the beginning of the experiment (Fig. 4, A6 andB6) despite the higher concentration of MgO in Secunda flyash (Table 1). This observed difference could be attributedto the variation in the pH of the two fly ash-brine systemsat the beginning of the experiment (Fig. 1, A1 and B1). ThepH of Secunda fly ash-brine systems was found to be higher(>11) than that of Tutuka fly ash-brine systems (9–9.5),which could indicate that Secunda fly ash-brine systems hasmore hydroxyl ions (OH−)which favours the precipitationof more Mg(OH)2. The solution pH plays a major role inelements’ solubility, complexation and leaching from wastematerials.[34,35]

The concentration of SO4 in Secunda fly ash-brine so-lution (Fig. 4, A7) decreased gradually at the beginningof the experiment (5–30 minutes) after which a significantdecrease was observed for the entire period of the batch re-action tests at L/S 1:1 and 1.5:1. The concentration of SO4in the Tutuka fly ash-brine solution was almost stable afterthe initial decrease at 5 minutes contact time (Fig. 4, B7). Asignificant amount of SO4 was removed from the solutionsof both fly ash-brine systems when compared with the SO4concentration in the UB (Table 3). The percentage SO4 re-moval was found to be between 28 and 57 % for Secunda fly

ash-brine systems, and the percentage removal in Tutukafly ash brine systems was between 29 and 36 % (Tables 3 and4). The variation observed in the removal of SO4 from thefly ash-brine solutions could be attributed to the differencein the pH of the solutions (Fig. 1, A1 and B1) or as a resultof the lower concentration of species such as Ca observed inTutuka fly ash solution (Fig. 4, B5), which could affect theprecipitation of SO4 and Ca-containing solid phases suchas CaSO4 in the fly ash-brine systems (details elsewhere).

Boron (B) and Barium (Ba). The concentrations of B andBa in the solutions of Secunda and Tutuka fly ash-brinebatch reaction tests are shown in Figure 5. The B con-centration in the fly ash-brine solutions of Secunda andTutuka, when compared to its concentration in the UB,was found to decrease abruptly at the beginning of the ex-periment up to 20 minutes’ contact time irrespective of theL/S ratio. The B concentration thereafter increased at 30minutes (Fig. 5, A8 and B8). B concentration in Secundafly ash-brine solution gradually decreased after 30 minutesuntil the end of the batch reaction tests (Fig. 5, A8). Theconcentration of B, after the initial B removal in the caseof Tutuka fly ash-brine systems, was found to be stablebetween 20 and 240 minutes at L/S 1:1 and 1.5:1. Theconcentration thereafter reduced gradually.

The B concentration decreased compared to the unre-acted brine (UB) after contacting with Tutuka fly ash andthe trends were inconsistent as observed in Figure 5 (B8).The removal trends observed in the Tutuka fly ash-brinesolutions were not the same with respect to the L/S ratioof the solutions after 240 minutes. The reason for the trendscould be attributed to the formation of B-containing tran-sient phases at L/S 1:1 and 1.5:1 at 480 and 840 minutes. AtL/S 5:1, the concentration of B increased throughout theperiod of the experiment after the initial decrease. Boronwas removed from Secunda fly ash-brine systems in therange of 36 to 100 % while in Tutuka fly ash-brine systemsthe percentage B removal was between 38 and 92 % (Tables3 and 4). Boron has been observed to exist as an oxy-anionin solution at high pH,[10,26] thereby having the tendency tointeract with other positively charged ions in solution.

The formation of ettringite [Ca6Al2(SO4)3.26H2O] in thesolution of fly ash at high pH has been identified to removeboron from the solution. According to Iwashita et al.[36]

and Hassett[37] the ettringite phase, when formed, has thetendency of trapping oxy-anionic species such as borateby replacing its sulphate component with the borate in thesolution. Fly ash with high Ca contents has been observedto reduce the concentration of B in solution due to thepossible co-precipitation of B with CaCO3.[38] The removalof boron from the fly ash-brine solutions could be as aresult of boron entrapment by an ettringite phase or dueto its interaction with Ca in the solution. The fluctuationobserved in the removal of boron from Tutuka fly ash-brinesolution could be as a result of formation and dissolutionof transient phases in the systems overtime.

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2.24

0

0.5

1

1.5

2

2.5

3

UB 5 10 20 30 60 240 480 840 1080 1440

B (m

g/L)

Reac�on �me (min)

UB B (1:1) B (1.5:1) B (5:1)

B8

0.0570

0.1

0.2

0.3

0.4

0.5

0.6

0.7

UB 5 10 20 30 60 240 480 840 1080 1440

Ba (m

g/L)

Reac�on �me (min)

UB Ba (1:1) Ba (1.5:1) Ba (5:1)

A9

0.0570

0.1

0.2

0.3

0.4

0.5

0.6

UB 5 10 20 30 60 240 480 840 1080 1440

Ba (m

g/L)

Reac�on �me (min)

UB Ba (1:1) Ba (1.5:1) Ba (5:1)

B9

2.24

0

0.5

1

1.5

2

2.5

3

UB 5 10 20 30 60 240 480 840 1080 1440

B (m

g/L)

Reac�on �me (min)

UB B (1:1) B (1.5:1) B (5:1)

A8

Fig. 5. B and Ba concentration of Secunda (A8–A9) and Tutuka (B8–B9) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and 5:1 as afunction of reaction time. (UB = unreacted brine) (color figure available online).

The trends observed for Ba concentration (Fig. 5, A9and B9) shows that Ba is immediately leached from thefly ashes into the brine solution (UB). It was observedthat the concentration of Ba in Secunda and Tutuka flyash-brine batch reaction increased at the beginning ofthe tests (5 minutes). This increase continued until at20 and 30 minutes when a decrease in the concentrationwas observed for Secunda and Tutuka fly ash-brinesystems respectively. The concentration of Ba in Secundafly ash-brine solution at L/S 5:1 was almost stableafter the increase observed at 20 minutes reaction time(Fig. 5, A9).

However, in the case of Tutuka fly ash, a gradual decreasewas noted. Ba was significantly leached into the solutionsof the fly ash-brine systems with the percentage rangingfrom 304 to 777 % in the case of Secunda fly ash-brinesystems while the percentage leached into Tutuka fly ash-brine was found to be between 210 and 530 % (Tables 3 and4). The leaching of Ba into the solution of brine could beas a result of the dissolution of soluble Ba-containing saltson the surface of the fly ash. Reardon et al.[28] observedthat the oxides of Ba when in contact with water dissolvereadily thereby increasing Ba concentration in solution.The decrease observed in the leaching trend over time could

indicate the formation of secondary mineral phases such asbarite.

Iron (Fe) and Manganese (Mn). Figure 6 shows the re-lease and removal of Fe and Mn as a result of the in-teractions between brine and fly ashes from Secunda andTutuka power utilities. The trends of release and removalof Mn and Fe in the fly ash-brine solutions of the two flyashes were almost the same. The concentrations of Fe andMn (Fig. 6, A10–A11 and B10–B11) in the solutions ofSecunda and Tutuka fly ash-brine systems were found todecrease slightly from levels in UB at the beginning of theexperiment (5–20 minutes). When compared to their con-centrations in the UB, Fe and Mn concentrations at 5–20minutes were observed to be lower in the systems except forTutuka fly ash-brine solution at L/S 5:1 where an increasein Mn concentration was observed at 10 minutes.

The lower concentration of these species at the beginningof the tests is an indication that they were removed from theunreacted brine solution immediately after contacting flyash. The percentage removal of Fe at the beginning of thetests ranged between 41 and 54 % (Secunda fly ash-brinesolutions) and 37 and 60 % (Tutuka fly ash-brine solu-tions). The initial removal of Mn was much higher with the

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0.24

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

UB 5 10 20 30 60 240 480 840 1080 1440

Fe (m

g/L)

Reac�on �me (min)

UB Fe (1:1) Fe (1.5:1) Fe (5:1)

A10

0.24

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

UB 5 10 20 30 60 240 480 840 1080 1440

Fe (m

g/L)

Reac�on �me (min)

UB Fe (1:1) Fe (1.5:1) Fe (5:1)

B10

0.0017

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

UB 5 10 20 30 60 240 480 840 1080 1440

Mn

(mg/

L)

Reac�on �me (min)

UB Mn (1:1) Mn (1.5:1) Mn (5:1)

A11

0.0017

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

UB 5 10 20 30 60 240 480 840 1080 1440

Mn

(mg/

L)

Reac�on �me (min)

UB Mn (1:1) Mn (1.5:1) Mn (5:1)

B11

Fig. 6. Mn and Fe concentration of Secunda (A10–A11) and Tutuka (B10-B11) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and 5:1as a function of reaction time. (UB = unreacted brine) (color figure available online).

percentage ranging between 29 and 74 % for Secunda flyash-brine solutions and between 51 and 62 % in the solu-tion of Tutuka fly ash-brine systems (Tables 3 and 4). Agradual increase in the concentrations of Fe and Mn in thesolutions (above that of UB) was observed at 30 minutes’contact time for almost all the leachate solutions irrespec-tive of their L/S ratios.

It has been observed that Mn and Fe, when in alkalinesolution, form oxy-hydroxides, which coat around the sil-icate grains of fly ash.[39] The formation of the transientoxy-hydroxides could account for the reduction observedin the concentrations of Fe and Mn in the fly ash-brinesolutions at the beginning of the batch reaction tests, whilethe gradual increase in the concentrations of Fe and Mnin the systems thereafter could be attributed to the disso-lution of the transient oxy-hydroxides and the Fe and Mnminerals in the fly ashes.

Copper (Cu), Zinc (Zn) and Nickel (Ni). Figure 7(A12–A14 and B12–B14) presents the concentrations ofCu, Zn and Ni in the solutions of Secunda and Tutukafly ash-brine batch reaction tests at different reaction timesand at different L/S ratios. The concentration of Cu im-mediately decreased on contact with the fly ashes and was

very low in Secunda fly ash-brine solutions at the three L/Sratios. A slight increase (not up to its concentration in theUB) at 30 minutes in Tutuka fly ash-brine systems was ob-served. For Zn, initial removal was followed by an increasein the concentration (not up to its concentration in the UB)at 30 minutes in both fly ash-brine systems. Considering theeffect of the L/S ratios, Ni concentration in the solutionsof the fly ash-brine systems had the lowest values at L/S1:1.

The concentration of Ni was almost stable from the be-ginning of the test to the end. In comparison, the concen-trations of Cu, Zn and Ni in the fly ash-brine solutionswere found to be lower than their concentrations in the un-reacted brine (UB), which indicates that these species wereremoved from UB by contacting with the fly ashes. Thepercentage removal of Cu, Zn and Ni from the solutionsof Secunda fly ash-brine systems ranged between 92 and100 %, 11 and 85 %, 43 and 56 %, respectively (Table 3),yet the removal of these species in Tutuka fly ash-brine sys-tems ranged between 79 and 92, 26 and 92, 11 and 32 %respectively (Table 4).

The removal was observed to be more prominent in Se-cunda fly ash-brine solutions, where the concentration ofCu in the fly ash-brine solution was almost below detection

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Fly ash-brine interactions 1661

0.26

0

0.05

0.1

0.15

0.2

0.25

0.3

UB 5 10 20 30 60 240 480 840 1080 1440

Cu (m

g/L)

Reac�on �me (min)

UB Cu (1:1) Cu (1.5:1) Cu (5:1)

B12

0.12

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

UB 5 10 20 30 60 240 480 840 1080 1440

Ni (

mg/

L)

Reac�on �me (min)

UB Ni (1:1) Ni (1.5:1) Ni (5:1)

A14

0.12

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

UB 5 10 20 30 60 240 480 840 1080 1440

Ni (

mg/

L)

Reac�on �me (min)

UB Ni (1:1) Ni (1.5:1) Ni (5:1)

B14

0.13

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

UB 5 10 20 30 60 240 480 840 1080 1440

Zn (m

g/L)

Reac�on �me (min)

UB Zn (1:1) Zn (1.5:1) Zn (5:1)

A13

0.13

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

UB 5 10 20 30 60 240 480 840 1080 1440

Zn (m

g/L)

Reac�on �me (min)

UB Zn (1:1) Zn (1.5:1) Zn (5:1)

B13

0.26

0

0.05

0.1

0.15

0.2

0.25

0.3

UB 5 10 20 30 60 240 480 840 1080 1440

Cu (m

g/L)

Reac�on �mme (min)

UB Cu (1:1) Cu (1.5:1) Cu (5:1)

A12

Fig. 7. Ni, Cu and Zn concentration of Secunda (A12–A14) and Tutuka (B12–B14) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and5:1 as a function of reaction time. (UB = unreacted brine) (color figure available online).

limit at some stage. This slight difference observed in theremoval capacity of the Secunda and Tutuka fly ash-brinesystems could be attributed to the difference in the final pHvalues of the systems (Fig. 2, A1 and B1). Astrup et al.[40]

observed that the release of heavy metals is controlled bythe pH of the solution. Several studies[33,41–43] have shownthat, at high pH, the surfaces of fly ash are negativelycharged.

According to Alinnor,[44] most of the positively chargedheavy metal such as Ni, Cu, Pb and Zn in solution havethe tendency to adsorb to the surfaces of the fly ash due

to electrostatic forces of attractions between the negativelycharged fly ash surface and the positively charged metalions. Studies[39,45] have shown that heavy metals such Cu,Ni and Zn are strongly adsorbed to the surface of Fe orMn oxy-hydroxides formed in alkaline system. The re-moval of Cu, Ni and Zn in the fly ash-brine systems couldalso be as a result of adsorption on the fly ash or onFe and Mn oxy-hydroxides in the systems. The slow in-crease observed at some stage in the trends for Cu, Ni andZn could be attributed to their desorption from the flyash.

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1662 Fatoba et al.

0.0068

0

0.005

0.01

0.015

0.02

0.025

0.03

UB 5 10 20 30 60 240 480 840 1080 1440

As (m

g/L)

Reac�on �me (min)

UB As (1:1) As (1.5:1) As (5:1)

A15

0.0068

0

0.005

0.01

0.015

0.02

0.025

0.03

UB 5 10 20 30 60 240 480 840 1080 1440

As (m

g/L)

Reac�on �me (min)

UB As (1:1) As (1.5:1) As (5:1)

B15

0.014

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

UB 5 10 20 30 60 240 480 840 1080 1440

Co (m

g/L)

Reac�on �me (min)

UB Co (1:1) Co (1.5:1) Co (5:1)

A16

0.014

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

UB 5 10 20 30 60 240 480 840 1080 1440

Co (m

g/L)

Reac�on �me (min)

UB Co (1:1) Co (1.5:1) Co (5:1)

B16

0.0065

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

UB 5 10 20 30 60 240 480 840 1080 1440

Pb (m

g/L)

Reac�on �me (min)

UB Pb (1:1) Pb (1.5:1) Pb (5:1)

A17

0.0065

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

UB 5 10 20 30 60 240 480 840 1080 1440

Pb (m

g/L)

Reac�on �me (min)

UB Pb (1:1) Pb (1.5:1) Pb (5:1)

B17

Fig. 8. As, Co and Pb concentration of Secunda (A15–A17) and Tutuka (B15–B17) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and5:1 as a function of reaction time. (UB = unreacted brine) (color figure available online).

Arsenic (As), Cobalt (Co) and Lead (Pb). The concen-trations of As, Co and Pb in the solutions of Secundaand Tutuka fly ash-brine batch reaction test are shownin Figure 8 (A15–A17 and B15–B17). It was observedthat the concentrations of As, Co and Pb in the flyash-brine solutions were significantly reduced except forTutuka fly ash-brine solution (Fig. 8, B15) where theconcentration of As was immediately increased above theconcentration in the unreacted brine (UB). The decreasein the concentrations of these species in the other caseswhen compared with their concentrations in UB indicates

their removal from the brine solution after contacting flyash.

The percentage removal of As, Co and Pb in the solutionsof Secunda fly ash-brine systems ranged from 70 to 100 %,93 to 100 % and 7 to 62 %, respectively (Table 3), yet thepercentage removal of Co and Pb from Tutuka fly ash-brinesystems was found to range from 29 to 74 % and 34 to 88%, respectively (Table 4). Comparing the removal of speciesby the fly ashes, the concentrations of As and Co in Tutukafly ash-brine solutions were higher than what was observedin the Secunda fly ash-brine solutions (Table 3), and the

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Fly ash-brine interactions 1663

0.0390

0.1

0.2

0.3

0.4

0.5

0.6

0.7

UB 5 10 20 30 60 240 480 840 1080 1440

Mo

(mg/

L)

Reac�on �me (min)

UB Mo (1:1) Mo (1.5:1) Mo (5:1)

A18

0.0390

0.2

0.4

0.6

0.8

1

1.2

UB 5 10 20 30 60 240 480 840 1080 1440

Mo

(mg/

L)

Reac�on �me (min)

UB Mo (1:1) Mo (1.5:1) Mo (5:1)

B18

0.0140

0.5

1

1.5

2

2.5

3

UB 5 10 20 30 60 240 480 840 1080 1440

Cr (m

g/L)

Reac�on �me (min)

UB Cr (1:1) Cr (1.5:1) Cr (5:1)

A19

0.0140

0.5

1

1.5

2

2.5

3

3.5

4

UB 5 10 20 30 60 240 480 840 1080 1440

Cr (m

g/L)

Reac�on �me (min)

UB Cr (1:1) Cr (1.5:1) Cr (5:1)

B19

Fig. 9. Mo and Cr concentration of Secunda (A18–A19) and Tutuka (B18–B19) fly ash-brine batch reaction at L/S 1:1, 1.5:1 and 5:1as a function of reaction time. (UB = unreacted brine) (color figure available online).

concentration of Pb in Secunda fly ash-brine solution wasslightly higher.

The results show that the rate of removal of As and Cofrom the unreacted brine solution (UB) after contacting Se-cunda fly ash was higher than Tutuka fly ash. The ability ofSecunda fly ash to remove more species could be attributedto its higher pH values, which could in turn favour the pre-cipitation or co-precipitation of species from the solutionor their adsorption to the fly ash surfaces. The difference inthe final pH values of the fly ashes could be associated withthe slight variations observed in the chemical compositionsof the fly ashes either due to the pre-treatment of the flyashes before sample collection (as described before) or dueto the type of coal burnt.

As and Pb concentrations in alkaline environment issaid to be controlled by adsorption and precipitation reac-tions.[10,46] As exists as arsenate (AsO3−

4 ) in strongly basicconditions and has the tendency of co-precipitating withaluminium and iron. The removal of As from wastewaterwas suggested to be as a result of co-precipitation with alu-minum, iron, calcium, and manganese in solution.[47] Asdescribed, the low concentrations of these species in solu-tions could be as a result of adsorption to the surface offly ashes particles or incorporation during the formation ofsecondary mineral phases.

Molybdenum (Mo) and Chromium (Cr). The concentra-tions of Mo and Cr are shown in Figure 9 (A18 and A19,B18 and B19). It was observed that the concentrations ofMo and Cr in Secunda fly ash-brine solutions increasedgradually above UB levels with increase in the reactiontime (Fig. 9, A18 and A19). The release pattern of Mo andCr in the solutions of Tutuka fly ash-brine systems wasslightly different. Their concentrations were almost stableirrespective of the L/S ratio after the initial increase in theconcentrations (Fig. 9, B18 and B19). The trends of in-creased concentrations compared to UB indicate that Moand Cr were leached out from the fly ashes into the solu-tions from the beginning of the reaction tests to the end.

Interestingly, it was observed that the pattern of releaseof Mo into the solutions at L/S 1:1, 1.5:1 and 5:1 werethe same as the patterns observed for the release of Crinto the solutions except that the concentrations of Cr inthe solutions were higher than the concentrations of Moin the solutions. Although the total concentrations of Mowas not determined by the XRF analysis, the differencein the concentrations of Mo and Cr in the fly ash-brinesolutions could be due to the difference in their concentra-tions in the fly ashes (Table 1). It has been observed that athigh pH and oxidizing conditions, some species such as Moand Cr are likely to form oxyanions (MoO2−

4 and CrO2−4 ,

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1664 Fatoba et al.

respectively).[40,48] The oxyanions of Mo and Cr are ob-served to be mobile in solution because of their inabilityto adsorb to hydrous iron (hydr) oxides or hydrous alu-minum (hydr)oxides.[49] The mobility of the oxyanions ofMo and Cr in the solution could account for the gradualincrease observed in the solutions of Secunda and Tutukafly ash-brine systems.

Concentrations and percentages of species removed andleached

Tables 3 and 4 show the percentage of species leached fromthe fly ashes or removed from the brine solution (UB) atL/S 1:1 during the batch reaction experiments. The choiceof L/S 1:1 was based on the fact that the removal andleaching of species was prominent in the leachates of thefly ash-brine systems at this L/S ratio. The % leached orremoval of the species was calculated based on the followingequation;

% Removal or Leached

= Initial concentration in UB − Final concentration in the leachateInitial concentration in UB

× 100

i.e.,Change in species concentration after batch reaction test

Initial concentration of species in UB× 100

In Secunda fly ash-brine solutions (Table 3), As, B, Co,Cu, Mg, Na, Ni, Zn, Cl and SO4 were significantly re-moved from the brine solution with percentage rangingfrom 70–100 %, 36–100 %, 93–100 %, 92–100 %, 99.9–100%, 6–19 %, 43–56 %, 11–85 %, 5–29 %, 28–57 %, respec-tively while Ba, Ca, Cr, Mo and Sr were leached from the flyash in significant quantities into the brine solution. Speciessuch as Al, Fe, K, Mn and Pb were inconsistent in theiravailability in the fly ash-brine solution. The trend observedin Tutuka fly ash-brine solutions was similar to that of Se-cunda fly ash-brine solutions.

Species such as B, Co, Cu, Mg, Na, Ni, Zn, Cl and SO4were also removed from the brine solution during inter-actions with Tutuka fly ash (Table 4), and the percentageremoval were 38–94 %, 29–74 %, 79–93 %, 90–100 %, 19–32%, 11–32 %, 26–92 %, 21–45 % and 29–36 %, respectively.Al, As, Ba, Ca, K, Cr, Mo and Sr were leached from thefly ash into the brine solution. As observed in the case ofSecunda fly ash-brine solution, the leaching and removalof some species such as Fe, Mn and Pb were observed to beinconsistent.

The results as shown in Tables 3 and 4 also revealedthe optimum contact time for the removal of species fromthe UB by the fly ashes. It was observed that the contacttimes where the highest % removal of species such as B,Co, Cu, Fe, Mn, Na, Ni, Pb and Zn in Secunda fly ash-brine systems ranged from 5–20 minutes after which the% removal of these species decreased in the systems. At60 and 240 minutes, the % removal of As and K were at

the maximum in Secunda fly ash-brine batch systems whilethe optimum contact times for Mg, Cl and SO4 were 1440minutes (Table 3).

The trends observed in the case of Tutuka fly ash-brinebatch reaction systems indicated that the optimum contacttimes for the removal of B, Cu, Fe, Mn, Na, Ni, Pb, Zn andCl were between 5 and 20 minutes. The % removal of SO4,Co and Mg was at the peak at 30, 840 and 1440 minutesrespectively (Table 4). The decrease in the % removal afterthe optimum contact times for some species could indicatethe dissolution of the transient mineral phases formed dur-ing the batch reaction test. The stable or gradual increaseobserved in the % removal of some species such as Mg andSO4 could imply that the mineral phases formed are slightlysoluble or insoluble.

Conclusion

The different sources of the fly ashes coupled with the dif-ferent pre-treatment before the samples were taken accountfor the variation observed in the pH values and the chem-ical compositions of the fly ashes when compared. Thechemical and mineralogical characteristics of the fly ashesused in this study revealed their reactivity potentials. Bothfly ashes used for this study contain high concentrations ofCa and Mg (reported as oxides) which are readily solublewhen in contact with aqueous solution. The hydrolysis anddissolution of basic oxides such as CaO and MgO in flyash have likely contributed to the highly alkaline pH of flyash solutions. The high pH (>10) of the fly ash-brine batchsystems was observed to have a significant effect on thereactivity of the species in both the fly ashes and the brinesolution.

The significant decrease observed in the TDS value(15400 mg/L) of the brine solution (UB) after contactingthe fly ashes revealed the removal of some soluble speciesduring the interactions (batch reaction tests) by the flyashes. The TDS value was reduced to the range of 7020mg/L (54.4 % reduction) after contacting Secunda fly ashwhile its contact with Tutuka fly ash reduced the TDS toabout 3200 mg/L (72.2 % reduction). Even though the re-sults of the batch leaching tests showed that some speciesleached from the fly ash into the fly ash-brine systems (Ta-bles 3 and 4), the overall TDS values of the systems afterthe interactions revealed greater removal of species thanleaching.

The fly ashes were found to have the ability of removingsome species from the brine solution while some speciesoriginating from the fly ash samples were also leached intothe brine solution. The L/S ratio of the fly ash-brine sys-tems and the contact time also play a major role in theinteractions of the species in the fly ash-brine systems. Theremoval of major and minor species was found to be signif-icant at L/S 1:1. The optimum contact time for the removalof some species from the fly ash-brine systems was between

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Fly ash-brine interactions 1665

5 and 20 minutes, after which more species were graduallyreleased into the systems. This indicates the instability ofmost of the likely mineral phases formed during the inter-actions.

Species such as B, Na, Mg, Cl and SO4 which consti-tute the major species of the brine solution were signifi-cantly removed; 36.2–100 % B, 6.5–18.7 % Na, 99.9–100% Mg, 5.8–29.1 % Cl and 28.7–56.6 % SO4 were removedby Secunda fly ash, while the removal of these species byTutuka fly ash ranged from 38.3–93.8 % B, 19.5–31.7 %Na, 90.2–100 % Mg, 21.2–45.5 % Cl and 29.4–35.7 % SO4.Minor elements such as As, Co, Cu, Pb, Ni and Zn wereremoved from the brine after which some of these specieswere gradually released into the fly ash-brine systems. Thisalso shows the instability of the species removed from thebrine solution by the fly ashes. Despite the ability of the flyashes to transiently remove major and minor species frombrine, some species such as Ca, Ba, Sr and K were signif-icantly leached into the brine solution from the fly ashesthereby increasing their concentration in the solutions. Crand Mo were also leached into the brine solution from thefly ashes.

The fly ash-brine batch reaction experiments give an in-sight into the possible leachability or removal of speciesby the formation of new mineral phases when fly ash andbrine are co-disposed. The fly ash-brine co-disposal tech-nique practised by some coal-fired industries could not atthis point be considered the best disposal option until thestability of the species captured in the fly ash is ascertained.One of the shortcomings of the practice, as observed inthis study, includes the leaching of some species such asCr and Mo, which could pose an environmental risk. Thepossible mineral formation responsible for the removal ofspecies during the interactions, and the stability of theselikely formed mineral phases will be considered in futurework.

Acknowledgments

We express our gratitude to SASOL-ESKOM ash-brine co-disposal study initiative for providing financial support toperform this study, SASOL-Secunda R&D and ESKOM-Tutuka R&D who provided the fly ash samples used for thisstudy, and the Chemistry Department of the University ofthe Western Cape, South Africa for the assistance providedduring this study.

References

[1] American Coal Ash Association, website: http://www.acaa-usa.org, 1998 (accessed 2009).

[2] Duchesne, J.; Reardon, E.J. Lime treatment of fly ash; character-ization of leachate composition and solid/water reactions. WasteMgmt. 1999, 19, 221–231.

[3] ESKOM Annual Report (2007), http://www.eskom.co.za/annreport07/annreport07/ (accessed 2009).

[4] Petrik, L.; White, R.; Klink, M.; Somerset, V.; Key, D.L.; Iwuoha,E.; Burgers C.; Fey, M.V. Utilization of Fly Ash for Acid MineDrainage Remediation, 2005, WRC Report No. 1242/1/05; SouthAfrica.

[5] Turek, M. Electrodialytic desalination and concentration of coal-mine brine. Desalination 2004, 162, 355–359.

[6] Mooketsi, I.O.; Ginster, M.; Matjie, H.R.; Riedel, J.K. Leachatecharacteristics of ash residues from a laboratory-scale brine en-capsulated simulation process. World of Coal Ash (WOCA), 2007,Covington, Kentucky, USA.

[7] Gitari, W.M.; Fatoba, O.O.; Petrik, L.F.; Vadapalli, V.R.K. Leach-ing characteristics of selected South African fly ashes: Effect of pHon the release of major and trace species. J. Environ. Sci. Health Pt.A 2009, 44(2), 206–220.

[8] Praharaj, T.; Powell, M.A.; Hart, B.R.; Tripathy, S. Leachabilityof elements from sub-bituminous coal fly ash from India. Environ.Inter. 2002, 27, 609–615.

[9] Kim, A.G. The effect of alkalinity of Class F PC fly ash on metalrelease. Fuel 2006, 85, 1403–1410.

[10] Jankowski, J.; Ward, C.R.; French, D.; Groves, S. Mobility of traceelements from selected Australian fly ashes and its potential impacton aquatic ecosystems. Fuel 2006, 85, 243–256.

[11] Hassett, D.J.; Pflughoeft-Hassett, D.F.; Heebink, L.V. Leaching ofCCBs: observations from over 25 years of research. Fuel 2005, 84,1378–1383.

[12] Paul, M.; Seferinoglu, M.; Aycık, G.A.; Sandstrom, A.V.; Smith,M.L.; Paul, J. Acid leaching of ash and coal: time dependenceand trace element occurrences. Inter. J. Miner. Proc. 2006, 79, 27–41.

[13] Querol, X.; Juan, R.; Lopez-Soler, A.; Fernandez-Turiel, J.L.; Ruiz,C.R. Mobility of trace elements from coal and combustion wastes.Fuel 1996, 75, 821–838.

[14] Querol, X.; Umana, J.C.; Alastuey, A.; Ayora, C.; Lopez-Soler, A.;Plana, F. Extraction of soluble major and trace elements from flyash in open and closed leaching systems. Fuel 2001, 80, 801–813.

[15] Seferinoglu, M.; Paul, M.; Sandstrom, A.; Koker, A.; Toprak,S.; Paul, J. Acid leaching of coal and coal-ashes. Fuel 2003, 82,1721–1734.

[16] Ohki, A.; Nakajima, T.; Yamashita, H.; Iwashita, A.; Takanashi,H. Leaching of various metals from coal into aqueous solutionscontaining an acid or a chelating agent. Fuel Proc. Technol. 2004,85, 1089–1102.

[17] Ugurlu, A. Leaching characteristics of fly ash. Environ. Geol. 2004,46(6–7), 890–895.

[18] Querol, X.; Moreno, N.; Alastuey, A.; Juan, R.; Andres, J.M.;Lopez-soler, A.; Ayora, C.; Medinaceli, A.; Valero, A. Synthesisof high ion exchange zeolites from fly ash. Geol. Acta 2007, 5(1),49–57.

[19] Gitari, M.W.; Petrik, L.F.; Etchebers, O.; Key, D.L.; Iwuoha, E.;Okujeni, C. Treatment of acid mine drainage with fly ash: removalof major contaminants and trace elements. J. Environ. Sci. HealthPt. A 2006, 41(8), 1729–1747.

[20] Kumpiene, J.; Lagerkvist, A.; Maurice, C. Stabilization of Pb- andCu-contaminated soil using coal fly ash and peat. Environ. Poll.2007, 145, 365–373.

[21] Yunusa, I.A.M.; Eamus, D.; DeSilva, D.L.; Murray, B.R.; Burchett,M.D.; Skilbeck, G.C.; Heidrich, C. Fly-ash: An exploitable resourcefor management of Australian agricultural soils. Fuel 2006, 85,2337–2344.

[22] Yılmaz, B.; Olgun, A. Studies on cement and mortar containing low-calcium fly ash, limestone, and dolomitic limestone. Cem. Concr.Res. 2008, 30, 194–201.

[23] Lee, C.Y.; Lee, H.K.; Lee, K.M. Strength and microstructural char-acteristics of chemically activated fly ash-cement systems. Cem.Concr. Res. 2003, 33, 425–431.

[24] Institut fur Normung, “DIN 38414 S4: German Standard Proce-dure for Water, Wastewater, and Sediment Testing-Group S (Sludge

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn C

ape]

at 0

2:11

25

Janu

ary

2013

1666 Fatoba et al.

and Sediment); Determination of Leachability (S4),” Berlin, Ger-many, 1984.

[25] American Society for Testing and Materials. Standard specifica-tion for fly ash and raw or calcined natural pozzolan for use as amineral admixture in Portland cement concrete. ASTM C618-88.,Philadelphia, PA, 1988.

[26] Fatoba, O.O. Chemical compositions and leaching behaviour ofsome South African fly ashes. Unpublished MSc Thesis (2008), Uni-versity of the Western Cape, South Africa.

[27] Choi, S.-K.; Lee, S.; Song, Y.-K.; Moon, H.-S. Leaching charac-teristics of selected Korean fly ashes and its implications for thegroundwater composition near the ash disposal mound. Fuel 2002,81, 1083–1090.

[28] Reardon, E.J.; Czank, C.A.; Warren, C.J.; Dayal, R.; Johnston,H.M. Determining controls on element concentrations in fly ashleachate. Waste Mgmt. Res. 1995, 13, 435–450.

[29] Wong, J.W.C.; Fang, M.; Li, G.X.; Wong, M.H. Feasibility of co-composting coal ash residues with sewage sludge. Environ. Technol.1997, 18, 563–568.

[30] Soong, Y.; Fauth, D.L.; Howard, B.H.; Jones, J.R.; Harrison, D.K.;Goodman, A.L.; Gray, M.L.; Frommell, E.A. CO2 sequestrationwith brine solution and fly ashes. Energy Conv. Mgmt. 2006, 47,1676–1685.

[31] Kirby, C.S.; Rimstidt, J.D. Interaction of municipal solid waste ashwith water. Environ. Sci. Technol. 1994, 28(3), 443–451.

[32] Adriano, D.C.; Page, A.L.; Elseewi, A.A.; Chang, A.C.; Straughan,I. Utilization and disposal of fly ash and other coal residues interrestrial ecosystems: A review. J. Environ. Qual. 1980, 9, 333–344.

[33] Steenari, B.-M.; Schelauder, S.; Lindqvist, O. Chemical and leach-ing characteristics of ash from combustion of coal, peat and woodin a 12MW CFB – A comparative study. Fuel 1999, 78, 249–258.

[34] van der Sloot, H.A.; Heasman, L.; Quevauviller, P. Harmonizationof leaching/extraction tests. Elsevier, Amsterdam, The Netherlands,1997, Volume 70, pp. 75–99.

[35] Chandler, A.J.; Eighmy, T.T.; Hartlen, J.; Hjelmar, O.; Kosson, D.S.;Sawell, S.E.; Vehlow, J. Municipal solid waste incinerator residues.Stud. Environ. Sci. 1997, 67, 974–981.

[36] Iwashita, A.; Sakaguchi, Y.; Nakajima, T.; Takanashi, H.; Ohki,A.; Kambara, S. Leaching characteristics of boron and seleniumfor various coal fly ashes. Fuel 2005, 84, 479–485.

[37] Hassett, R.P. Digestive enzyme activity is present in maturecopepods despite the absence of the corresponding substrates inthe diet during development. J. Plank. Res. 1994, 16(4), 413–420.

[38] Hollis, J.F.; Keren, R.; Fal, M. Boron release and sorption by flyash as affected by pH and particle size. J. Environ. Qual. 1988, 17,181–184.

[39] Drever, I.J. The Geochemistry of Natural Waters: Surface andGroundwater Environments. 3rd Edition, Prentice Hall, Inc., En-glewood Cliffs, NJ, 1997.

[40] Astrup, T.; Mosbaek, H.; Christensen, T.H. Assessment oflong-term leaching from waste incineration air-pollution-controlresidues. Waste Mgmt. 2006, 26(8), 803–814.

[41] Elliot, H.A.; Denneny, C.M. Soil adsorption of calcium from so-lution containing organic ligands. J. Environ Qual. 1982, 11, 658–663.

[42] Cho, H.; Oh, D.; Kim, K. A study on removal characteristics ofheavy metals from aqueous solution by fly ash. J. Hazard. Mater.2005, B127, 187–195.

[43] Lee, M.-K.; Saunders, J.A. Effects of pH on metals precipitationand sorption: Field bioremediation and geochemical modeling ap-proaches. Vadose Zone J. 2003, 2, 177–185.

[44] Alinnor, I.J. Adsorption of heavy metal ions from aqueous solutionby fly ash. Fuel 2007, 86, 853–857.

[45] Shim, Y.S.; Kim, Y.K.; Kong, S.H.; Rhee, S.W.; Lee, W.K.The adsorption characteristics of heavy metals by various par-ticle sizes of MSWI bottom ash. Waste Mgmt. 2003, 23, 851–857.

[46] Goh, K.-H.; Lim, T.-T. Geochemistry of inorganic arsenic and sele-nium in a tropical soil: effect of reaction time, pH, and competitiveanions on arsenic and selenium adsorption. Chemosphere 2004, 55,849–859.

[47] Smedley, P.L.; Kinniburgh, D.G. A review of the source, behaviourand distribution of arsenic in natural waters. Appl. Geochem. 2002,17, 517–568.

[48] Appelo, C.A.J.; Postma, D. Geochemistry, Groundwater and Pollu-tion, A.A. Balkema, Rotterdam; 1994.

[49] Hyks, J.; Astrup, T.; Christensen, H. Long-term leaching fromMSWI air-pollution-control residues: Leaching characterizationand modelling. J. Hazard. Mater. 2009, 162, 80–91.

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