Fuel 88 (2009) 1714–1719

Contents lists available at ScienceDirect

Fuel

journal homepage: www.elsevier .com/locate / fuel

Adsorption of heavy metal cations using coal fly ash modifiedby hydrothermal method

Marisa Nascimento a,*, Paulo Sérgio Moreira Soares b, Vicente Paulo de Souza b

a Fluminense Federal University, Department of Metallurgical Engineering - Av. dos Trabalhadores 420, 27255-250 Volta Redonda, Rio de Janeiro, Brazilb Centre for Mineral Technology, Metallurgical Process Coordination - Av Pedro Calmon 900 21941-908, Rio de Janeiro, Brazil

a r t i c l e i n f o

Article history:Received 30 June 2008Received in revised form 21 December 2008Accepted 10 January 2009Available online 4 February 2009

Keywords:Coal fly ashAdsorption isothermZeolite

0016-2361/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.fuel.2009.01.007

* Corresponding author. Tel.: +55 2133937209.E-mail addresses: marisa.nascimento@gmail.co

com.br (M. Nascimento).

a b s t r a c t

In this study, the adsorption properties of synthetic zeolite produced from Brazilian coal fly ashwere investigated for some heavy metal cations (Zn, Cu, Mn and Pb). The batch method has beenemployed, using metal concentrations in solution ranging from 100 to 3000 mg/l. Preliminary statisti-cal analysis has indicated that temperature and time of synthesis of zeolites were the most importantvariables that affect the their adsorption capacity. Results lead to the conclusion that a hydrothermaltreatment can increase from 2 to 25 times the adsorption capacity (CA) of the coal ash comparing toits original capacity. The ion-exchange characteristic of the zeolites was determined using Langmuir,Freundlich and Dubinin–Kaganer–Radushkevich isotherms. The adsorption increases as cation concen-trations in aqueous solution increases. The preference order observed for adsorption isPb > Cu > Zn > Mn.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Toxic soluble heavy metal species cause serious problems toecosystems and this has been a great motivation for the growingnumber of research works on effluent treatment processes. Someof these processes are based on adsorptive properties of materialsthat may successfully immobilize heavy metals.

Several natural or synthetic materials, such as bagasse, fly ash[1] carbonates, phosphate rocks, alkaline agents, zeolites, clay min-erals and organics, have been recently tested as to their ability toimmobilize metals [2,3]. Zeolites have been widely used in heavymetals adsorption due to their unique physical and chemical prop-erties (crystallinity, thermal stability, well-defined molecular sizecage structure, ion-exchange capacity, etc) [4,5].

Zeolites are microporous crystalline hydrated aluminosilicatesthat can be considered as inorganic polymers built from aninfinitely extending three-dimensional network (similar to ahoneycomb) of tetrahedral TO4 units, where T is Si or Al, whichform interconnected tunnels and cages.

The interstitial cations are not held firmly and can migrate quitefreely through the voids of the lattice and are therefore readilyexchanged [6].

Literature review shows that zeolites can be produced byhydrothermal methods from coal fly ashes [5,7]. But, the detailed

ll rights reserved.

m, marisa.nascimento@bol.

reaction mechanism of hydrothermal synthesis has not beenclarified adequately. However one knows that the efficiency of zeo-litic materials synthesized from fly ash as adsorbates for heavymetals in solution depends on the synthesis conditions [4,8–16]as well as on the cations concentration in solution and temperaturein the adsorption process.

On the other hand, many tons of fly ashes are produced as solidwastes every year in coal fired power stations. In southern Brazil,the coal consumption is about 3.7 � 106 t/year, whereas theamount of ashes produced are around 1.7 � 106 t/year [17]. Thesefigures make production of zeolites from coal fly ashes attractivefor effluent treatment.

This paper presents a study on adsorption of selected heavy me-tal cations in a synthetic zeolite produced from Brazilian coal flyash. Temperature and time of zeolite synthesis were identified pre-liminarily as the most important variables that affect the adsorp-tion of cations in solution.

2. Materials and methods

The fly ash used in this study originated from a coal thermopower station located in southern Brazil that operates with pulver-ized Brazilian coal. Ashes and synthetic zeolites produced byhydrothermal treatment were previously characterized chemicallyand mineralogically and subsequently used in adsorption experi-ments. These experiments were performed with synthetic solu-tions of sulphate salts of Pb(II), Cu(II), Zn(II), and Mn(II) preparedwith analytical grade reagents in deionized water.

M. Nascimento et al. / Fuel 88 (2009) 1714–1719 1715

2.1. Zeolite preparation

The coal fly ash was submitted to hydrothermal treatment atdifferent temperatures, reaction times, NaOH concentrations, so-lid/liquid ratios (S/L) and aluminum/silicon (Al/Si) ratios as shownin Table 1. A Plackett–Burmann experimental design (25�2 frac-tional factorial design) was used in order to identify the mostimportant variables in a preliminary analysis [18,19]. The testswere performed in duplicate.

The molar ratio Al/Si was modified by adding analytical gradeAl2O3 in the reaction medium.

The hydrothermal treatment was carried out in a 450 ml reactorPARR-4562, made of Nickel 200 equipped with a turbine impellerand stirred constantly at 300 rpm. The reaction products were fil-tered and washed with water to remove the excess of sodiumhydroxide.

The mineralogical characterization of the zeolites as well as ofthe ashes was carried out by X-ray diffraction (XRD) in a Bruker– AXS D5005 powder diffractometer with Goebel mirror and CoKa(35 kV/40 mA) radiation.

2.2. Preliminary evaluation of the response variable

The zeolites as well as the coal fly ash were washed up to pH 9and subsequently dried at 60 �C for 24 h before the adsorptionexperiments. The adsorption capacity was determined by contact-ing, 50 ml of Mn and Cu solutions (100 mg/l) with 0.5 g of zeolitesin plastic bottles. The solution pH of each test was chosen conve-niently, between 4 and 5 to avoid cation precipitation.

The bottles were shaken for 2 h at 180 rpm in a KS501 IKA sha-ker and the solids were filtered with Whatman filter paper. Theconcentrations of metal ions of all tests were determined in a Var-ian Atomic Absorption Spectrometer - model Spectra 50B.

The adsorption capacity was defined as a percentage and calcu-lated by the equation

adsorption capacity ð%Þ ¼ ½ðCi � CeÞ=Ci� � 100 ð1Þ

Where Ci and Ce are, respectively initial and final concentrations ofthe metal ion in solution.

2.3. Adsorption experiments

The adsorption experiments for the heavy metals, Cu2+, Pb2+,Zn2+, and Mn2+

, with synthesized zeolite were carried out usingthe shaking device and methodology described previously in Sec-tion 2.2. A concentration range from 100 to 3000 mg/l was usedfor each cation. Only the zeolitic material from test 8 (Table 1)was used as adsorbent.

The obtained data were plotted and adjusted with isothermadsorption models to analyze the cations adsorption onto theadsorbents.

Table 1Experimental conditions of the hydrothermal experiments.

Tests Concentration(mol/l)

Temperature (�C) Time (h) Al/Si ratio S/L ratio

1 2.0 100 0.5 1.00 1/62 5.0 100 0.5 0.51 1/83 2.0 150 0.5 0.51 1/64 5.0 150 0.5 1.00 1/85 2.0 100 6.0 1.00 1/86 5.0 100 6.0 0.51 1/67 2.0 150 6.0 0.51 1/88 5.0 150 6.0 1.00 1/6

3. Results and discussion

3.1. Characterization and preparation

The XRD analysis of the ashes used for zeolite preparation isshown in Fig. 1. Mullite (Al6Si2O13), quartz (SiO2) and maghemite(Fe2O3) were identified as the main constituents of the ashes. Thequantitative chemical analysis show 56.8% SiO2, 24.5% Al2O3, anda SiO2/Al2O3 ratio of 2.32.

Fig. 2 shows the XRD of all products of the hydrothermal treat-ments. One can be observed that treatments resulted in formationof zeolitic phases as zeolite A, cubic analcime, philipsite, hydroxy-cancrinite and Na8(AlSiO4)6(OH)2 � 2H2O. Table 2 shows theadsorption capacity obtained for the zeolitic products. The resultsshow that a hydrothermal treatment can increase 10– 25 timesthe% adsorption when comparing with the value of the originalashes. The adsorption of Mn+2 is higher than 85% and the absorp-tion of Cu+2 reaches up to 99%. In the case of a solution with100 ppm of Mn the adsorption increased about 25� when com-pared with the results of the test for the same Mn concentrationperformed with ashes with no hydrothermal treatment. Theincreasing was about 16� for Cu+2. The tests 1 and 2 showed smalladsorption capacity for cations tested. In Fig. 2, these tests did notshow zeolitic phases.

Figs. 3 and 4 show the results of preliminary statistical analysisand the effects of the variables. The F-test was used to identify themost significant variables in the hydrothermal process. The signif-icance level (p-value) adopted was 0.05. Temperature and timewere the most significant variables in the synthesis of the zeolites.An increase of the level of these variables tends to increase theadsorption capacity for Mn and Cu.

Several authors suggest that changes in synthesis temperatureslead to different zeolitic phases. The differences among adsorptioncapacities of the various zeolites synthesized may be credited tothe different zeolitic phases present in the products of the hydro-thermal synthesis [3,4,9,11]. An increase in the reaction time tendsto promote a better crystallization of the phases formed, which itmight also explain the increase in the of adsorption capacity [9,10].

3.2. Adsorption Studies

The adsorption of different metal ion concentrations onto syn-thetic zeolite at 25 �C was studied for Cu2+, Pb2+, Zn2+, and Mn2+

in the range 100–3000 mg/l keeping all other variables constant.The results are shown in Fig. 5. The adsorption for Cu2+, Pb2+,Zn2+, and Mn2+ increases with increasing metal concentration in

Fig. 1. XRD pattern of coal fly ash. Mineral abbreviations: M, mullite; Q, quartz.

1 0 2 0 3 0 4 0 5 0

0

1 0 0

2 0 0

3 0 0

4 0 0

L C 0 1

M

M

MMM

M

Q

M

MQM

cps

2 - th e ta1 0 2 0 3 0 4 0 50

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

MQM

QMM

M

Q

M

QM

L C 0 2

cps

2 - th e ta

1 0 2 0 3 0 4 0 5 00

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

MM

MPM

M P

M

P

Q

M

P

QPM

P

L C 0 3

cps

2-theta1 0 2 0 3 0 4 0 5 0

0

1 0 0

2 0 0

3 0 0

4 0 0

MM

Z

MM

Z

M

Z

M

Q

C Z

QZC

M

C Z

L C 0 4

cps

2-theta

1 0 2 0 3 0 4 0 5 00

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

ZM

M

MZMZ

AA

Q

MZ

A

A

QM

AA

Z

AA

L C 0 5

cps

2-theta1 0 2 0 3 0 4 0 5 0

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

Z

MM

MZQM

Z

M

Z

MZ

Q

M

Z

QM

Z

L C 0 6

cps

2-theta

1 0 2 0 3 0 4 0 5 00

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

A nA n

Q

MM

A nM

M

A n

A nQ

P

A n

PP

M

A n

P

L C 0 7

cps

2-theta1 0 2 0 3 0 4 0 5 0

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

MZMZM M

Z

Z

Z

Z

Z

Q

C

C

L C 0 8

cps

2-theta

Fig. 2. XRD pattern of modified coal fly ash. Phases abbreviations: M, mullite; Q, quartz; Z, zeolite [Na8(AlSiO4)6(OH)2 � 2H2O]; C; hydroxycancrinite; A, zeolite A; An,analcime; P, zeolite P. The legends LC01 to LC08 are related with tests 1–8 from Table 1.

1716 M. Nascimento et al. / Fuel 88 (2009) 1714–1719

aqueous solutions. These results indicate that energetically lessfavorable sites become involved when the concentration of metalin solution increases. The metal uptake can be credited to different

mechanisms of both ion-exchange and adsorption. During the ion-exchange process, metal ions have to move through the pores ofthe zeolite, but also through channels of the lattice, and they have

Table 2Results of Cu2+ and Mn2+ adsorption.

Test Cu2+ Mn2+

Ce (mg/l) Adsorption capacity (%) Ce (mg/l) Adsorption capacity (%)

1 37.43 62.57 42.80 57.202 34.36 65.64 66.19 33.813 1.66 98.34 15.98 84.024 0.37 99.63 26.48 73.525 1.11 98.89 24.19 75.816 0.02 99.98 21.77 78.237 0.11 99.89 15.25 84.758 0.22 99.78 14.40 85.60Coal fly ash 93.95 6.05 96.60 3.40

% adsorption - Mn

0.68

-1.85

2.25

4.59

5.01

p=.05

Standardized Effect Estimate (Absolute Value)

(4)Al/Si Ratio

(1)concentration (mol/L)

(5)S/L ratio

(3)Time (h)

(2)Temperature (C)

Fig. 3. Diagram of standardized effects to fatorial planning - % Mn Adsorption (p-level = 0.05).

M. Nascimento et al. / Fuel 88 (2009) 1714–1719 1717

to replace exchangeable cations (mainly sodium and calcium). Dif-fusion is faster through the pores and is retarded when ions movethrough channels of small diameter. In this case the metal ion up-take can mainly be credited to ion-exchange reactions in the por-ous of the zeolite samples [6].

The preferred order observed for adsorption was Pb > Cu >Zn P Mn. In the literature, similar results were obtained whenthe adsorption capacity of a large variety of zeolite minerals forcadmium, copper and zinc and revealed that zinc had the lowestadsorption for all zeolites synthesized [3,5,7–16]. Zeolites obtainedunder same conditions as phillipsite and chabazite had limitedadsorption capacity (CA) for zinc as compared to copper [20]. Theadsorption characteristics of Zn(II) onto pure fly ash showed thatthe solution pH was the key factor affecting the adsorption charac-teristics [11–23].

We used the Langmuir [24], Freundlich [25], and Dubinin–Kaganer–Radushkevich (DKR) isotherms to analyze the adsorp-tions of Cu2+, Zn2+, Pb2+, Mn2+ onto the adsorbent.The Langmuirisotherm can be represented

1=We ¼ 1=bþ 1=ðK 0CeÞ ð2Þ

where We is the amount adsorbed (mg/g) and Ce is the equilibriumconcentration of the adsorbate (mg/l). The b (constant related with

monolayer adsorption capacity) and K0 (constant related with max-imum energy of adsorption) are Langmuir constants. Table 2 pre-sents the Langmuir constants b and K0 calculated for all ionstested. The Langmuir fits quite well the adsorption behaviour withcorrelation coefficients R2 between 0.94 and 0.99. According to the b(mg/g) parameter, sorption on zeolitic material is produced follow-ing the sequence Pb > Cu > Mn P Zn.

The Freundlich isotherm can be represented as

logðWeÞ ¼ log bþm logCe ð3Þ

where We is the amount adsorbed (mg/g) and Ce is the equilibriumconcentration of the adsorbate (mg/l). The b (constant related toadsorption capacity) and m (constant related with the adsorptionintensity) are Freundlich constants. The Table 2 lists the calculatedFreundlich constants b and m of all ions tested. Because the valuesof R2 were 0.91–0.99, it appears that these adsorption events fit Fre-undlich isotherm quite well.

An essential characteristic of Langmuir Isotherm can be ex-pressed in a dimensional constant called equilibrium parameter(RL):

RL ¼ 1=ð1þ K 0C0Þ ð4Þ

% adsorption - Cu

-0.17

-0.15

0.24

3.23

3.32

p=.05Standardized Effect Estimate (Absolute Value)

(4)Al/Si Ratio

(5)S/L ratio

(1)concentration (mol/L)

(2)Temperature (C)

(3)Time (h)

Fig. 4. Pareto chart of standardized effects to fatorial planning - % Cu Adsorption (p-level = 0.05).

Fig. 5. Adsorption isotherms of Cu2+, Zn2+, Pb2+, Mn2+.

1718 M. Nascimento et al. / Fuel 88 (2009) 1714–1719

where C0 is the major initial concentration of metal (mg/l) and K’ isthe Langmuir constant. RL between 0 and 1 indicate adsorption isfavorable. The value 1/m > 1 represents the same role in the Freund-lich isotherm (Table 3).

The DKR equation can be represented as

ln We ¼ ln Wm � b�2 ð5Þ

where We is the amount adsorbed (mg/g), Wm is the DKR monolayercapacity, b(mol2/J2) is a constant related to the sorption energy, and

� is the Polanyi potential, which is related to equilibrium concentra-tion through the expression

� ¼ RT lnð1=CeÞ ð6Þ

where T is the temperature (K), R is the gas constant (kJ/molK) andCe is a equilibrium concentration of Cu+2 in solution. When ln We

was plotted against e2, a straight line was obtained. We calculatedthe value of b and it is related to the sorption energy, E, throughthe following relationship:

Table 3Langmuir, Freundlich and DKR constants for Cu2+, Zn2+, Pb2+, Mn2+.

Adsorvates Cu(II) Zn(II) Pb(II) Mn(II)

Langmuir b (mg/g) 76.9 59.2 194.7 60.4K0 (L/mg) 2.13E-3 0.88E-3 1.41E-3 0.65E-3R2 0.96 0.99 0.96 0.94RL 0.14 0.19 3.63E-2 0.34

Freundlich b 1.60 0.24 2.04 0.16m 0.45 0.66 0.57 0.71R2 0.91 0.99 0.93 0.961/m 2.22 1.52 1.75 1.41

DKR Wm 7.20 1.87 11.68 1.39b (mol2/J2) �4.51E-9 �1.15E-8 �2.54E-8 �2.46E-8R2 0.93 0.98 0.90 0.95E (kJ/mol) 10.53 6.59 4.44 4.51

M. Nascimento et al. / Fuel 88 (2009) 1714–1719 1719

E ¼ 1=ð�2bÞ1=2 ð7Þ

Table 3 provides the DKR parameters for the adsorptions of theions tested. Because the values of R2 were 0.90–0.98, it appearsthat the adsorptions of all the ions followed the DKR isothermquite well.

The magnitude of E can be related to the reaction mechanism,governed by ion-exchange or physical forces may affect the sorp-tion mechanism. The values of E (4.44–10.33 kJ/mol) were thoseexpected for an ion-exchange mechanism [26].

4. Conclusions

The study demonstrates the application of Brazilian fly ash forzeolite synthesis by hydrothermal treatment. Statistical analysisshown that an increase of time and temperature in zeolite synthe-sis tends to increase the capacity of adsorption of ions tested ontoproducts.

The products obtained are effective adsorbent for removal ofPb2+, Zn2+, Mn2+ and Cu2+ ions from aqueous solutions. The perfor-mance of adsorption increased up to 25�when compared with theoriginal ashes. The results obtained by isotherm models were con-sistent with an ion-exchange adsorption mechanism.

References

[1] Gupta VK, Jain CK, Ali I, Sharma M, Saini VK. Removal of cadmium and nickelfrom wastewater using bagasse fly ash-a sugar industry waste. Water Res2003;37:4038–44.

[2] Chen ZS, Lee GJ, Liu JC. The effects of chemical remediation treatments on theextractability and speciation of cadmium and lead in contaminated soils.Chemosphere 2000;41:235–42.

[3] Querol X, Alastuey A, Moreno N, Ayuso EA, Sáchez AG, Cama J, et al.Immobilization of heavy metals in polluted soils by the addition os zeoliticmaterial synthesized from coal fly ash. Chemosohere 2006;62:171–80.

[4] Moreno N, Querol X, Ayora C, Alastuey A, Fernández-Pereira C. Potentialenvironmental applications of pure zeolitic material synthesized from fly ash. JEnviron Eng 2001;127:994–1002.

[5] Hsu T, Yu C, Yeh C. Adsorption of Cu+2 from water using raw and modified coalfly ashes. Fuel 2008;87(7):1355–9.

[6] Grimshaw RW, Harland CE. Ion-exchange: introduction to theory andpractice. London: The Chemical Society; 1975. p. 82.

[7] Fungaro DA, Izidoro JC, Almeida RS. Remoção de compostos tóxicos de soluçãoaquosa por adsorção com zeólita sintetizada a partir de cinzas de carvão.Ecética Química 2005;30:31–5.

[8] Erdem E, Karapinar N, Donat R. The removal of heavy metal cations by naturalzeolites. J Colloid Interf Sci 2004;280:309–14.

[9] Murayama N, Yamamoto H, Shibata J. Mechanism of zeolite synthesis fromcoal fly ash by alkali hydrothermal reaction. Int J Miner Process 2002;64:1–17.

[10] Juan R, Hernández S, Andrés JM, Ruiz C. Synthesis of granular zeolitic materialswith high cation exchange capacity from agglomerated coal fly ash. Fuel2007;86:1811–21.

[11] Molina A, Poole C. A comparative study using two methods to produce zeolitesfrom fly ash. Miner Eng 2004;17:167–73.

[12] Hui KS, Chao CYH. Synthesis of MCM-41 from coal fly ash by a green approach:influence od synthesis pH. J Hazard Mater 2006;B137:1135–48.

[13] Penilla RP, Bustos AG, Elizalde SG. Immobilization of Cs, Cd, Pb, Cr by syntheticzeolites from spanish low-calcium coal fly ash. Fuel 2006;85:823–32.

[14] Hui KS, Chao CYH, Kot SC. Removal of mixed heavy metal ions in wastewaterby zeolite 4A and residual products from recycled coal fly ash. J Hazard Mater2005;B127:89–101.

[15] Wang S, Li L, Zhu ZH. Solid-state conversion of fly ash to effective adsorventsfor Cu removal from wastewater. J Hazard Mater 2007;B139:254–9.

[16] Wang S, Soudi M, Li L, Zhu ZH. Coal ash conversion into effective adsorbents forremoval of heavy metals and dyes from wastewater. J Hazard Mater2006;B133:243–51.

[17] Zamzow MJ, Murphy JE. Removal of metal cations from water using zeolites.Sep Sci Technol 1992;27(14):1969–84.

[18] Montgomery D, Calado V. Planejamento de experimentos usando o Statistica.Rio de Janeiro: e-papers; 2003.

[19] Hogg RV, Ledolter J. Engineering statistics2. New York: Macmillan PublishingCompany; 1987.

[20] Colella C, Gennaro M, Langella A, Pansini M. Evaluation of natural philipsiteand chabazite as cation exchangers for copper and zinc. Sep Sci Technol1998;33(4):467–81.

[21] Ören A, Kaya A. Factors affecting adsorption characteristics of Zn2+ on twonatural zeolites. J Hazard Mater 2006;131(1–3):59–65.

[22] Weng C, Huang CP. Adsorption characteristics of Zn(II) from dilute aqueoussolution by fly ash. Colloid Surface A 2004;247:137–43.

[23] Bayat B. Comparativestudy of adsorption properties of Turkish fly ashes I. Thecase of nickel(II), copper(II) and zinc(II). J Hazard Mater 2002;B95:251–73.

[24] Langmuir I. Adsorption of gases on plane surfaces of glass, mica and platinum. JAm Chem Soc 1918;40:1361–403.

[25] Freundlich H. Adsorption in solution. Phys Chem 1906;57:384–410.[26] Helfferich F. Ion exchange. New York: McGraw Hill; 1962.