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Separation and Purification Technology 51 (2006) 240–246

Kinetics and thermodynamics of Cd(II) adsorption ontopyrite and synthetic iron sulphide

Mehmet Erdem ∗, Arzu OzverdiDepartment of Environmental Engineering, Fırat University, 23279 Elazıg, Turkey

Received 26 December 2005; received in revised form 1 February 2006; accepted 2 February 2006

bstract

In this paper, the Cd(II) adsorption abilities of pyrite and synthetic iron sulphide (SIS) were studied. Experiments were carried out as a functionf pH, Cd(II) concentration, contact time and temperature. Maximum adsorption yields for pyrite and SIS were determined to be 34.2% and 65.8%nder the conditions of initial Cd(II) concentration of 100 mg/l, pH 5.4, contact time of 120 min and adsorbent dosage of 20 g/l, respectively.he adsorption data fitted both the Langmuir and Freundlich adsorption models. Adsorption capacities of SIS and pyrite at 25 ◦C were found

o be 3.05 and 2.08 mg Cd(II)/g, respectively. The first-order and pseudo second-order rate expressions were applied to experimental data and it

as determined that the adsorption process followed the first-order kinetic model. In addition, activation energy values and some thermodynamicarameters such as �G◦, �H◦ and �S◦ for the cadmium adsorption processes were calculated from the isotherm and kinetic data. The adsorptionf Cd(II) on to SIS and pyrite was found to be endothermic and spontaneous.

2006 Elsevier B.V. All rights reserved.

isfbemiOa[[mtaius

eywords: Cadmium; Adsorption; Kinetics; Isotherms; Pyrite; Iron sulphide

. Introduction

Heavy metal pollution in wastewaters is an extremely impor-ant environmental problem. The main sources of heavy metalsor wastewaters are mining, metal industries and some otherndustrial areas using metals and metal salts. Lead, chromium,admium, copper, zinc and mercury are among the most fre-uently encountered metal contaminants [1].

Heavy metals are extremely toxic and threaten to the livingy joining the food chain. When they release into the waters,ost of them are strongly retained and their adverse effects can

ast for a long time. Thus, it is important to apply an effectivereatment method to wastewaters polluted with heavy metals.

In order to remove the toxic metal ions from wastewater,urrent methods are chemical precipitation, ion exchange, sol-ent extraction, adsorption and reverse osmosis techniques [2].hemical precipitation, especially as metal hydroxide or sul-

hide, is widely practiced, having the advantages of simplicitynd inexpensive chemicals. However, it is not effective to reduceeavy metal concentration to low level required by water qual-

∗ Corresponding author. Tel.: +90 424 2370000; fax: +90 424 2415526.E-mail address: merdem@firat.edu.tr (M. Erdem).

aFnatap

383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2006.02.004

ty standards [3] and generation of a voluminous toxic wasteludge is a major problem encountered. Therefore, in the lastew decades, adsorption process has received much concern andecome an alternative to conventional precipitation technique,specially for wastewaters that contain low concentrations ofetals and complex forming substances [2]. Activated carbon

s the most widely used adsorbent in the wastewater treatment.wing to high-cost of activated carbon, usage of the low-cost

dsorbents such as agricultural wastes [4], metallurgical slags5,6], fly ashes [7,8] and various minerals have been investigated9–17]. In order to remove cadmium from aqueous solutions byinerals such as perlite [18], low-grade phosphate [19], clinop-

ilolite [20], goethite and aluminum oxide [21], montmorrilonitend kaolinite [22], akaganeite-type nanocrystals [23], a lot ofnvestigations have been done. Currently, a few studies on thesage of pyrite as an adsorbent have been reported. But, thesetudies have been generally focused on the removal of molybdatend tetrathiomolybdate and influence of cadmium sorption oneS2 oxidation [24,25]. Pyrite and synthetic iron sulphide haveot been systematically investigated and evaluated for the metal

dsorption purposes. That is, any data have been reported forhe adsorption of Cd(II) onto pyrite, one of the abundant miner-ls in the nature, and synthetic iron sulphide. Starting from thisoint, in this study, it has been investigated Cd(II) adsorption

M. Erdem, A. Ozverdi / Separation and Purifi

Nomenclature

A frequency factorb Langmuir constant (l/mg)C0 initial Cd(II) concentration (mg/l)Ce equilibrium Cd(II) concentration (mg/l)EA activation energy (kJ/gmol)�G◦ Gibbs free energy change (kJ/gmol)�H◦ enthalpy change (kJ/gmol)kad adsorption rate constant (min−1)kF Freundlich constant (mg/g)m amount of adsorbent (g)n Freundlich constantQ◦ maximum adsorption capacity (mg/g)qe amount of Cd(II) adsorbed per g of adsorbent at

equilibrium (mg/g)q amount of Cd(II) adsorbed per g of adsorbent at

any time (mg/g)r dimensionless equilibrium parameterR universal gas constant (8.314 J/gmol K)R2 correlation coefficient�S◦ entropy change (kJ/gmol)t time (min)

pFtp

2

(ws

spfajecm

odtcaters [2,28], 100 mg Cd(II)/l has been selected as the suitableinitial metal concentration value. pH adjustments were madeby using HNO3 and NaOH solutions in various concentrations.

TC

P

S

T temperature (K)x amount of adsorbed Cd(II) (mg)

roperties of pyrite and synthetic iron sulphide. Langmuir andreundlich adsorption isotherms have also been tested at various

emperatures and adsorption kinetics and some thermodynamicarameters have been determined.

. Experimental

The pyrite sample, collected from pyrite-bed, Keban-ElazıgTurkey), and synthetic iron sulphide (SIS), catalogue number ofhich is Merck-3908, were used as adsorbents in this study. The

amples in gross particles (pyrite) and pellets (SIS) were crushed

Fc1

able 1hemical and mineralogical compositions of the pyrite and SIS

Chemical compositions

Constituents w/w

yrite Fe 41.45%S 36.43%Al 2.20%Ca 3.58%Mg 1.18%Si 5.47%Cu 3700 mg/kgCo 300 mg/kgNi 200 mg/kgMn 700 mg/kgZn 350 mg/kg

IS Fe 61.03%S 32.82%

cation Technology 51 (2006) 240–246 241

eparately. The visible impurities were removed by hand fromyrite sample and then the samples were ground and sieved. Theractionated materials in the particle size of <53 �m were driedt 60 ◦C for 6 h, and then they were stored in a tightly closedar throughout the study. Characterizations of the samples werexplained in our previous studies [26,27], however, results of thehemical and mineralogical compositions of the samples deter-ined in previous study mentioned above are given in Table 1.In order to prepare the experimental solutions, stock solution

f Cd(II) (10 g/l) was prepared by dissolving its nitrate salt inistilled water. The working solutions were prepared by dilutinghe stock solution with distilled water. Taking into considerationommon concentration values of cadmium in actual wastew-

ig. 1. Effect of pH on the adsorption of Cd(II) by SIS and pyrite [initial Cd(II)oncentration: 100 mg/l; SIS dosage: 20 g/l; pyrite dosage: 20 g/l; contact time:20 min; temperature: 25 ◦C].

Mineralogical compositions

Minerals Formula

Pyrite FeS2

Quartz SiO2

Calcite CaCO3

Dolomite CaMg(CO3)2

Natrosilite Na2Si2O5

Brussite CaHPO4·2H2OChamosite (Fe,Al,Mg)6(Si,Al)4O10(OH)8

Troilite FeSIron FeWustite FeO (minor amount)

2 Purifi

Ag

piacFwir

bflpuw

3

3

tpio

ttuistis(iH

iioc

F

C

ibo5te

3

titaota

3

muSaa

F2

42 M. Erdem, A. Ozverdi / Separation and

ll the chemicals used in the study were of analytical reagentrade.

Adsorption experiments were carried out by shaking 2 g/l ofyrite and 2 g/l of SIS with 100 ml aqueous solutions of Cd(II)n series of batch reactors at a constant speed of 200 rpm inn orbital shaker. Samples were withdrawn after predeterminedontact time interval and filtered through 0.45 �m filter paper.inal pH of the filtrates was measured by a pH meter. The filtratesere acidified with 1 ml of HNO3 solution to prevent the precip-

tation of metal ions and then they were analyzed to determineesidual concentration of Cd(II).

The concentration of metals in the solutions was determinedy ATI-Unicam 929 atomic absorption spectrophotometer usingame atomization technique. pH measurements were done by aH meter (Orion SA720). Standard solutions were prepared bysing analytical chemicals. All dilutions were made by distilledater.

. Results and discussion

.1. Effect of pH

Since pH is an important parameter affecting metal adsorp-ion, the effect of the initial pH on Cd(II) removal by eitheryrite or SIS was investigated at the pH range of 3.5–6 by takingnto account precipitation pH value of Cd(II) [29]. The resultsbtained depending on equilibrium pH are given in Fig. 1.

In the studies carried out with SIS, it has been determinedhat the adsorption yield of Cd(II) at equilibrium pH of 3.7 (ini-ial pH of 2) is 100% and they decrease with the increasing pHp to 4.1 and then they increase at low rate again. When takingnto account that cadmium ions have positive charge, it can betated that the cadmium ions can be more effective adsorbed athe high pH values. On the contrary, the Cd(II) adsorption yieldn the presence of SIS was rather high at pH below 4.1. This

ituation is associated with H2S generation in acidic media (Eq.1)). H2S is known as an efficient reagent for metal precipitationn the form of metal sulphides. Therefore, due to generation of

2S in solutions having pH below 4.1, Cd(II) precipitates as

fs

l

ig. 2. Effects of temperature and contact time on the Cd(II) adsorption by SIS and p0 g/l; pH 6].

cation Technology 51 (2006) 240–246

ts sulphide (Eq. (2)) and consequently cadmium removal yieldncreases. But, the mechanism of Cd(II) removal from aque-us solution in this situation is not adsorption, it is completelyhemical precipitation:

eS(s) + 2H+(aq) → H2S(g) + Fe2+

(aq) (1)

d2+(aq) + H2S(g) →↓ CdS(s) + 2H+

(aq) (2)

The Cd(II) adsorption yields for pyrite increased with thencreasing pH up to 5.9. At equilibrium pHs of 5.4 and 5.9 foroth adsorbents, the Cd(II) adsorption yields were close to eachther. For that reason, it can be stated that the optimum pH is.4. This pH value corresponding initial pH of 6 is in agree withhe results of some earlier studies [18–20], thus, and subsequentxperiments were carried out at this pH.

.2. Effect of temperature

The data of adsorption at different temperature indicated thathe Cd(II) adsorption yield onto both adsorbents increased by thencreasing temperature (Fig. 2). This increase by the increasingemperature indicates that the adsorption process is endothermicnd higher temperature favours Cd(II) removal by adsorptionnto SIS and pyrite. The removal efficiency of Cd(II) by adsorp-ion on SIS and pyrite also increased with contact time and itttained a maximum value at 120 min except for 45 ◦C for SIS.

.3. Adsorption kinetics

In order to determine kinetic parameters and explain to theechanism of the adsorption processes, lots of researchers have

sed first and pseudo second-order rate expressions [18,30,31].imilarly, in order to determine the rate constants (kad) of Cd(II)dsorption onto SIS and pyrite, some kinetic analyses were madet various temperatures depending on contact time by using

ollowing first-order rate expression of Lagergren and pseudoecond-order rate expression:

n(qe − q) = ln qe − kadt (3)

yrite [initial Cd(II) concentration: 100 mg/l; SIS dosage: 20 g/l; pyrite dosage:

M. Erdem, A. Ozverdi / Separation and Purification Technology 51 (2006) 240–246 243

te [ini

(mrv04affisp

stttirtt

vr[tpe(wls

od

r

wtfu

Fig. 3. Lagergren plot for the adsorption of Cd(II) onto SIS and pyri

1

Ce= 1

C0+ kadt (4)

Straight lines will be obtained of the left-hand sites of Eqs.3) and (4) versus t suggest the applicability of these kineticodels. Data obtained in this study were fitted to the first-order

ate expression of Lagergren (Fig. 3). The correlation coefficientalues (R2) for first-order rate expression were found greater than.97 for all temperatures studied. The values of kad at 25, 35 and5 ◦C for both adsorbents were calculated to be 0.0298, 0.0396nd 0.1295 min−1 for SIS and 0.0308, 0.0347 and 0.0419 min−1

or pyrite, respectively, from the slopes of the straight lines ingure. The increase in rate constants depending on temperaturehows that the rate-limiting step is surface adsorption and therocess is endothermic.

The Arrhenius Equation expresses the first-order rate con-tant of the adsorption reaction as a function of temperature andhe activation energy of the process is calculated from this equa-ion. The magnitude of activation energy may give an idea abouthe type of adsorption. There are two types of adsorption, phys-

cal and chemical. In the physical adsorption, the equilibrium isapidly attained and reversible. Since the effective forces for thisype of adsorption are weak, its activation energy value is low. Onhe contrary, chemical adsorption has higher activation energy

wauf

Fig. 4. Arrhenius plots for the Cd(II)

tial Cd(II) concentration: 100 mg/l; adsorbent dosage: 20 g/l; pH 6].

alue (between 8.4 and 83.7 kJ/mol) and it is specific. Also, theate in chemical adsorption varies with increasing temperature32]. In order to determine adsorption type of Cd(II) adsorp-ion onto SIS and pyrite, Arrhenius Equation was used. For thisurpose, ln kad values were plotted versus 1/T and activationnergy values were calculated from the slope of the line obtainedFig. 4). Activation energy values for the adsorption processhich SIS and pyrite were used as an adsorbent were calcu-

ated to be 57.47 and 12.09 kJ/gmol, respectively. These valuesuggest that chemical forces govern the adsorption process.

In order to predict whether the adsorption is favourabler unfavourable, the dimensionless equilibrium parameter wasetermined by the following equation:

= 1

1 + bC0(5)

here C0 is the initial Cd(II) concentration (mg/l) and b ishe Langmuir isotherm constant. Value of r < 1 represents theavourable adsorption and value greater than one representsnfavourable adsorption. The values of r for Cd(II) adsorption

ere found to be 0.027, 0.003 and 0.002 for SIS and 0.287, 0.284

nd 0.269 for pyrite at 25, 35 and 45 ◦C, respectively. These val-es indicate that the both Cd(II) adsorption processes are highlyavourable.

adsorption onto SIS and pyrite.

244 M. Erdem, A. Ozverdi / Separation and Purification Technology 51 (2006) 240–246

s for C

3

aFtmwdwa

l

wasct

SaatmytpTtwprs

(

Fig. 5. Langmuir adsorption isotherm

.4. Adsorption isotherms

Adsorption isotherms are commonly used for describingdsorption equilibrium for wastewater treatment. Langmuir andreundlich adsorption isotherms are classical models to describe

he equilibrium between metal ions adsorbed onto adsorbent andetal ions in solution at a constant temperature. These modelsere also applied in this study. For this purpose, the experimentalata obtained under the various concentrations and temperaturesere plotted in linearized forms of Langmuir and Freundlich

dsorption isotherms (Eqs. (6) and (7), respectively):

Ce

x/m= 1

bQ◦ + Ce

Q◦ (6)

nx

m= ln kf + n ln Ce (7)

here Ce is equilibrium concentration (mg/l), x/m the amount

dsorbed at equilibrium (mg/g), Q◦, b, kf and n are isotherm con-tants. Q◦ and kf are defined as adsorption maxima or adsorptionapacity (mg/g) for Langmuir and Freundlich isotherms, respec-ively.

mCaa

Fig. 6. Freundlich adsorption isotherms for

d(II) adsorption onto SIS and pyrite.

Langmuir and Freundlich adsorption isotherms of Cd(II) onIS and pyrite are shown in Figs. 5 and 6. The isotherm constantsnd correlation coefficients calculated at different temperaturesre also tabulated in Tables 2 and 3. Experimental data fittedo the both isotherms. But, the correlation coefficients of Lang-

uir adsorption isotherm showed that the Langmuir isothermielded the best fitted to experimental data. As seen from theable, Langmuir adsorption capacity values increased by the tem-erature. The other Langmuir parameter b shows a similar trend.his situation confirms the finding that chemical forces govern

he adsorption process. Similar result was found by Kandah,ho has investigated zinc and cadmium adsorption on low-gradehosphate [19]. In addition, Mathialagan and Viraraghavan haveeported that the adsorption data of the cadmium from aqueousolutions onto perlite fits to Freundlich isotherm [18].

It has been determined that the Cd(II) adsorption capacityQ◦) of SIS is higher than that of the pyrite. For example, while

aximum adsorption capacity for SIS was 9.47 mg adsorbedd(II)/g SIS, it for pyrite was 3.43 mg adsorbed Cd(II)/g pyritet 45 ◦C. Therefore, it can be concluded that the SIS for Cd(II)dsorption is more appropriate than the pyrite. In the earlier

Cd(II) adsorption onto SIS and pyrite.

M. Erdem, A. Ozverdi / Separation and Purification Technology 51 (2006) 240–246 245

Table 2Langmuir constants and correlation coefficients of Cd(II) adsorption onto SIS and pyrite

Temperature (◦C) SIS Pyrite

b (l/mg) Q◦ (mg/g) R2 b (l/mg) Q◦ (mg/g) R2

25 0.3596 3.05 0.998 0.0248 2.08 0.97135 2.8011 8.40 1 0.0252 2.78 0.92645 4.1247 9.47 0.998 0.0271 3.43 0.975

Table 3Freundlich constants and correlation coefficients of Cd(II) adsorption onto SIS and pyrite

Temperature (◦C) SIS Pyrite

kf 1/n R2 kf 1/n R2

25 1.2433 0.1765 0.974 0.5351 0.2351 0.93235 7.1836 0.0375 0.983 0.5188 0.2946 0.93545 7.4529 0.0559 0.940 0.5575 0.318 0.956

Table 4Thermodynamic parameters for the adsorption of Cd(II) onto SIS and pyrite

Temperature (◦C) SIS Pyrite

−�G◦ (kJ/gmol) �S◦ (kJ/gmol K) −�G◦ (kJ/gmol) �S◦ (kJ/gmol K)

25 26.279 0.413 19.654 0.077534

st[m0CaSaipk

bcF[ararwd

3

ap

sc

l

l

�

wot

t3tTatcef

4

5 32.418 0.4195 34.493 0.413

tudies related to Cd(II) adsorption, it has been determined thathe adsorption capacities of perlite [18], low-grade phosphate19], clinoptilolite [20], goethite and aluminium oxide [21],ontmorrilonite and kaolinite [22] are 0.64, 7.54, 7.41, 72, 31,

.72, 0.32 mg/g, respectively. When taking into considerationd(II) adsorption capacities of the adsorbents mentioned abovend the values obtained from the present study to be 3.05 mg/gIS and 2.08 mg/g pyrite (at 25 ◦C), it can be seen that the Cd(II)dsorption efficiencies of the minerals mentioned decreasen the order of goethite > aluminium oxide > low-grade phos-hate > clinoptilolite > SIS > pyrite > montmorrilonite > perlite >aolinite.

Although a significant number of low-cost mineral adsor-ents have been studied, the higher adsorption capacities foradmium have been reported in the biosorption researches.or instance; the Cd(II) adsorption capacities of S. platensis33], marine macroalgae [34], aerobic granules [35], marinelga Laminaria japonica [36] and C. vulgaris [37] in theseesearch have been reported to be 98.04, 64–95, 566, 146.12nd 85.3 mg Cd(II)/g biomass, respectively. According to theseesults, it can be stated that the applicability to the realastewater of mineral adsorbents mentioned above is probablyifficult.

.5. Adsorption thermodynamics

The increasing adsorption efficiency with increasing temper-ture can be explained on the basis of some thermodynamicarameters such as the changes in Gibbs free energy (�G◦),

stt

20.354 0.077321.208 0.0776

tandard enthalpy (�H◦) and entropy (�S◦), which can be cal-ulated from the following relationships:

n1

b= �G◦

RT(8)

n b = ln b0 − �H◦

RT(9)

G◦ = �H◦ − T �S◦ (10)

here b is Langmuir constant which is related with the energyf adsorption, b0 a constant, R the ideal gas constant and T isemperature (K) [38].

The standard enthalpy changes (�H◦) of the Cd(II) adsorp-ion onto SIS and pyrite were determined to be 96.783 and.468 kJ/gmol from the ln 1/b versus 1/T, respectively. The posi-ive values of �H◦ suggest the endothermic nature of adsorption.he Gibbs’ free energy (�G◦) and entropy values (�S◦) for thedsorption process were calculated from Eqs. (8) and (10) andabulated in Table 4. The negative Gibbs’ free energy valuesonfirm that the adsorption is spontaneous. The increase in freenergy change with the rise in temperature shows an increase ineasibility of adsorption at higher temperatures.

. Conclusions

In this study, the Cd(II) adsorption abilities of synthetic ironulphide and pyrite were tested by using equilibrium, kinetic andhermodynamic aspects. The results indicated that the adsorp-ion capacities of the adsorbents were changed depending on pH,

2 Purifi

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46 M. Erdem, A. Ozverdi / Separation and

ontact time and temperature. The optimum equilibrium pH andquilibrium time were determined to be 5.4 and 120 min, respec-ively.

The kinetic studies indicated that the adsorption of Cd(II) onyrite and SIS followed first-order kinetic model. The first-orderate constants for Cd(II) adsorption on pyrite and SIS were foundo be 2.98 × 10−2 and 3.08 × 10−2 min−1 at 25 ◦C, respectively,nd it increases with the temperature. The activation energy val-es of the process suggest that the chemical forces govern thedsorption process.

Experimental data fitted to the Langmuir and Freundlichdsorption isotherms. That the adsorption capacities increaseith the temperature and positive enthalpy values of the pro-

ess show that the adsorption process is endothermic. Standardibbs free energy values showed that the adsorption process was

pontaneous.Taking into consideration present findings, it can be stated

hat the SIS and pyrite are mineral based adsorbent having lowd(II) adsorption capacity. However, these minerals, generate2S in acidic solution, can be evaluated as a precipitation reagent

or Cd(II) removal from the wastewaters having low pH (pH <3)uch as acidic metal plating wastewater.

eferences

[1] J.M. Moore, S. Ramamoorthy, Heavy Metals in Natural Waters, Springrer-Verlag Corporation, New York, 1984.

[2] J.W. Patterson, Wastewater Treatment Technology, Ann Arbor Inc., NewYork, 1975.

[3] T. Maruyama, S.A. Hannah, J.M. Cohen, J. Water Pollut. Contam. F 47(1975) 440.

[4] S.W. Peternele, A.A. Winkler-Hechenleitner, P.A.E. Gomez, Bioresour.Technol. 68 (1999) 95.

[5] N. Ortiz, M.A.F. Pires, J.C. Bressiani, Waste Manage. 21 (2001) 631.

[6] D. Feng, J.S.J. Deventer, C. Aldrich, Sep. Purif. Technol. 40 (2004) 61.[7] K.K. Panday, G. Prasad, V.N. Singh, J. Chem. Technol. Biotechnol. 34A

(1984) 367.[8] B. Bayat, J. Hazard. Mater. B95 (2002) 275.[9] G. Suraj, C.S.P. Iyer, M. Lalithambika, Appl. Clay Sci. 13 (1998) 293.

[[[

cation Technology 51 (2006) 240–246

10] P.V. Brady, H.W. Papenguth, J.W. Kelly, Appl. Chem. 14 (1999) 569.11] M. Lehmann, A.I. Zouboulis, K.A. Matis, Environ. Pollut. 113 (2001) 121.12] Y. Seida, Y. Nakano, Y. Nakamura, Water Res. 35 (2001) 2341.13] S.H. Abdel-Halim, A.M.A. Shehata, M.F. El-Shahat, Water Res. 37 (2003)

1678.14] O. Yavuz, Y. Altunkaynak, F. Guzel, Water Res. 37 (2003) 948.15] M.J.S. Yabe, E. Oliveira, Adv. Environ. Res. 7 (2003) 263.16] Z. Wu, Z. Gua, X. Wanga, L. Evansc, H. Guo, Environ. Pollut. 121 (2003)

469.17] S.T. Singh, K.K. Pant, Sep. Purif. Technol. 36 (2004) 139.18] T. Mathialagan, T. Viraraghavan, J. Hazard. Mater. B94 (2002) 291.19] I.M. Kandah, Sep. Purif. Technol. 35 (2004) 61.20] V.O. Vasylechko, G.V. Gryshchouk, Yu.B. Kuz’ma, V.P. Zakordonskiy,

L.O. Vasylechko, L.O. Lebedynets, M.B. Kalytovs’ka, Microporous Meso-porous Mater. 60 (2003) 183.

21] S.K. Srivastava, G. Bhattacharjee, R. Tyagi, N. Pant, N. Pal, Environ. Tech-nol. Lett. 9 (1988) 1173.

22] S.K. Srivastava, R. Tyagi, N. Pal, Environ. Technol. Lett. 10 (1989) 275.23] E.A. Deliyanni, K.A. Matis, Sep. Purif. Technol. 45 (2005) 96.24] B.C. Bostick, S. Fendorf, G.R. Helz, Environ. Sci. Technol. 37 (2003)

285–291.25] B.C. Bostick, S. Fendorf, B.T. Bowie, P.R. Griffiths, Environ. Sci. Technol.

34 (2000) 1494.26] M. Erdem, F. Tumen, Tr. J. Eng. Environ. Sci. 20 (1996) 363 (in Turkish).27] M. Erdem, H.S. Altundogan, A. Ozer, F. Tumen, Environ. Technol. 22

(2001) 1213.28] M. Sittig, Pollutant Removal Handbook, Noyes Data Corporation, England,

1973.29] E. Jackson, Hydrometallurgical Extraction and Reclamation, John Wiley

and Sons, New York, 1986.30] Y.S. Ho, D.A.J. Wase, C.F. Forster, Environ. Technol. 17 (1996) 71.31] M.R. Unnithan, T.S. Anirudhan, Ind. Eng. Chem. 40 (2001) 2693.32] J.M. Smith, Chemical Engineering Kinetics, third ed., McGraw-Hill, Sin-

gapore, 1981.33] N. Rangsayatorn, E.S. Upatham, M. Kruatrachue, P. Pokethitiyook, G.R.

Lanza, Environ. Pollut. 119 (2002) 45.34] P. Lodeiro, B. Cordero, J.L. Barriada, R. Herrero, M.E. Sastre de Vicente,

Bioresour. Technol. 96 (2005) 1796.35] Y. Liu, S.F. Yang, H. Xu, K.H. Woon, Y.M. Lin, J.H. Tay, Process. Biochem.

38 (2003) 997.36] P. Yin, Q. Yu, Z. Lin, P. Kaewsarn, Sep. Purif. Technol. 45 (2005) 25.37] Z. Aksu, Sep. Purif. Technol. 21 (2001) 285.38] J.M. Smith, H.C. Van Ness, Introduction to Chemical Engineering Ther-

modynamics, fourth ed., McGraw-Hill, Singapore, 1987.