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Chemical Engineering Journal 178 (2011) 317– 323

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

omparative study of lead sorption onto natural perlite, dolomite and diatomite

ohammad Irania,∗, Mehdi Amjadib, Mohammad Ali Mousaviana

Department of Chemical Engineering, Faculty of Engineering, University of Tehran, Tehran, IranSchool of Chemical Engineering, Iran University of Science & Technology, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 5 July 2011eceived in revised form 3 October 2011ccepted 4 October 2011

eywords:erliteolomite

a b s t r a c t

In this work, natural clays of Iran including perlite, dolomite and diatomite have been used as an adsor-bent for removing lead from aqueous solution in a batch system. The BET analysis of clays showedthat the pore size, surface area and pore volume of diatomite are greater than those of perlite anddolomite. The effect of several variables like pH, contact time, initial concentration and temperatureon lead sorption by perlite, dolomite and diatomite was investigated. Kinetic data were analyzed usingpseudo-first-order, pseudo-second-order and double-exponential models. The equilibrium experimen-tal data were tested with Freundlich, Langmuir and Dubbin–Radushkevich (D–R) isotherm models. It

iatomiteeaddsorption

was observed that the maximum lead adsorption capacity of lead of natural clays followed the order ofdiatomite (25.01 mg g−1) > dolomite (19.69 mg g−1) > perlite (8.906 mg g−1). Thermodynamic parametersshowed that the lead sorption onto the dolomite and diatomite is endothermic and lead sorption by per-lite is exothermic. The results indicated that the lead sorption by studied adsorbents is spontaneous andthermodynamically feasible. Because of low-cost and local availability of natural clays, these adsorbentshave suitable potential for removal of lead ions in practical process.

. Introduction

The amount of heavy metals in water supplies has steadilyncreased over the last years as a result of over population andndustrialization. Industries such as plating, ceramics, glass, min-ng and battery manufacturing are considered the main sourcef heavy metals in local water streams. For the removal ofoxic heavy metals a number of various technologies, such aslectrochemical separation [1], precipitation [2,3], membrane fil-ration [4], ionic exchange [5] and solvent extraction [6] haveeen used. However, these techniques are associated with prob-

ems such as excessive time requirements, high costs and highnergy consumption. In this way, adsorption method can beonsidered as an effective and widely used process due to itsimplicity, moderate operational conditions and economical fea-ibility.

In principle, any solid material with a microporous structurean be used as an adsorbent. The most important property of anydsorbent is the surface area and structure. Furthermore, the chem-cal nature and polarity of the adsorbent surface can influence

he attractive forces between the adsorbent and adsorbate. Theighly developed structure of activated carbon allows wide usages an adsorption media for a large number of organic and inorganic

∗ Corresponding author. Tel.: +98 021 88765937.E-mail address: irani mo@ut.ac.ir (M. Irani).

385-8947/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.cej.2011.10.011

Published by Elsevier B.V.

materials, including trace concentrations of heavy metals. Acti-vated carbon, however, is not suitable in practice due to the highcosts associated with production and regeneration [7]. Thus, theuse of alternative low cost materials for heavy metal removal isrequired. Up to date, several inorganic and organic adsorbentshave been proposed for the adsorption method, including zeo-lites, clay minerals, trivalent and tetravalent metal phosphatesand biosorbents. A comprehensive review of the various adsor-bents has been presented by Bailey et al. [8]. In recent years, thesearch for locally available low-cost adsorbents has been intensi-fied. Materials, such as natural materials, rock minerals, agriculturalor industrial waste by-products can be utilized as low-cost adsor-bents.

The objective of the present work is to evaluate and compare theadsorption ability of three different mineral adsorbents which areabundant in Iran, namely perlite, dolomite and diatomite for leadion removal and to determine the influence of operating parame-ters on the adsorption performance.

Lead is a hazardous waste and is highly toxic to humans, plantsand animals. It causes plant and animal death as well as anemia,brain damage, mental deficiency, anorexia, vomiting and malaisein humans [9]. Lead is a substitute for calcium in bony tissues andaccumulates there. The presence of lead in drinking water is known

to cause various types of serious health problems leading to death inextreme cases [10]. The permissible limit of lead is 0.1–0.05 mg L−1

in water [11]. Thus the removal of lead from wastewater and indus-trial effluents is a vital necessity.

318 M. Irani et al. / Chemical Engineering

Table 1Chemical composition of natural perlite, dolomite and diatomite of Iran.

Component Perlite (wt%) Dolomite (wt%) Diatomite (wt%)

SiO2 73.32 0.41 89.2Al2O3 12.62 0.12 4.10K2O 5.02 2.10 0.63Na2O 2.96 0.71 1.21CaO 0.66 31.69 0.50Fe2O3 0.67 0.26 1.50MnO 0.66 – –MgO 0.21 23.64 0.30P2O5 0.13 0.11 0.11SO3 – 0.03 –L.O.I.a 3.75 40.93 2.45

tadiuaka

vPPicm

igoottcapirtTtaitt

2

2

rOAipTbT

sorption of lead ions decreases at pH values greater than 6.

a L.O.I., loss of ignition.

Perlite is a glassy and volcanic rock varying in color from grayo black. Perlites of different types (expanded and unexpanded)nd different origin would have different properties because of theifferences in their compositions [12]. Perlite acts as an excellent

nsulator, both thermal and acoustical, resists fire and is classified asltra-light weight materials. Perlite is very cheap and easily avail-ble in Iran. The cost of expanded perlite is less than US $0.2 perg in Iran. This could make it a viable candidate as an economicaldsorbent for removing heavy metals.

The efficiency of raw dolomite [CaMg(CO3)2], a material that isery cheap and abundant around the world, in removing Cu2+ andb2+ ions from aqueous solution matrices has been reported byehlivan et al. [13]. Miyaneh area has the main source of dolomiten Iran. Dolomite is a very cheap material and the total productionost is $8 per ton. For this reason, it is an economic adsorbent foretal ion removing processes.In recent researches, silica-based adsorbents were widely used

n the removal of heavy metal ions [14–16]. Diatomite is fine-rained, low-density biogenic sediment, which consists essentiallyf amorphous silica (SiO2·nH2O) derived from opalescent frustulesf diatoms. Due to the presence of silanol groups that spread overhe matrix of silica, diatomite can react with many polar func-ional groups [17]. It has been commonly used in water purification,larifications of liquors and juices, filtration of commercial fluidsnd separation of various oils and chemicals [18]. The adsorptionroperties of diatomite toward heavy metal ions were reported

n the literature [19–22]. Kamelabad area (Iran) is particularlyich in the diatomite resources. This work presents a compara-ive study on lead adsorption potential of three different clays.he influence of various experimental parameters on lead adsorp-ion onto the perlite, dolomite and diatomite has been investigatednd the optimum conditions for the maximum adsorption of leadons from aqueous solutions have been determined. The nature ofhe adsorption process with respect to its kinetics, isotherms andhermodynamic aspects has been also evaluated.

. Experiments

.1. Adsorbents

The adsorbents, used for the adsorption of lead, were localow minerals. Perlite, dolomite and diatomite were obtained fromshlogh Chay, Miyaneh and Kamelabad mines in Iran, respectively.t first, natural clays were washed by distilled water to remove

mpurities, and dried at 110 ◦C for 5 h. Then dried samples wereowdered in a ball mill and sieved to obtain a grain size of 100 �m.

he chemical composition of all the three clays was determinedy X-ray fluorescence (XRF) analysis and the results are given inable 1.

Journal 178 (2011) 317– 323

2.2. Adsorbate

The average pore diameter, specific surface area and porevolume of the prepared membranes were measured by theBrunauer–Emmet–Teller (BET) method. The solutions of lead ionswere prepared by dissolving weighed amounts of lead nitrates(Aldrich) in distilled water. The concentration of lead ions in theadsorption medium was determined by using an inductivity cou-pled plasma atomic emission spectrophotometer (ICP-AES, ThermoJarrel Ash, Model Trace Scan). Analytical wavelength was set to220.553 nm for lead ions.

2.3. Batch adsorption experiments

The adsorption of lead ions was studied as a function of pH,contact time, dosage of adsorbent, initial concentration of lead andtemperature in a batch system. The effect of pH of the solutionon the lead sorption was studied in the range of 2–7. The initialpH of the solution was adjusted with 0.1 M HNO3 or 0.1 M NaOH.The experiments were carried out in 250 mL Erlenmeyer flasks con-taining 0.2 g of the adsorbent in 100 mL of lead solutions at 25 ◦Con a rotary shaker at 150 rpm for 4 h. The effect of contact time onlead sorption, were done by placing 0.2 g of adsorbent in 100 mLof lead solutions (100 mg L−1) at 25 ◦C. For examining the effectof initial lead concentration and temperature, 0.2 g of the sampleswas rinsed in 100 mL of lead solutions with concentrations vary-ing in the range of 10–500 mg L−1 at three different temperatures(25 ◦C, 35 ◦C and 45 ◦C) for 4 h. Each experiment was repeated threetimes and the results were given as averages. The amount of leadadsorbed was calculated from the following equation:

qe = (C0 − Ce)V1000M

(1)

where qe is the adsorption capacity in mg g−1, C0 and Ce are theinitial and equilibrium concentrations of metal solution in mg L−1,V is the volume of the solution in mL and M is the weight of the dryabsorbents in g.

2.4. Determination of the point of the zero charge (pHpzc)

The pHpzc for the adsorbents was determined by the followingmethod. 50 mL of 0.1 M NaCl was transferred in series of flasks;the pH of solutions was adjusted in the range of 2–7 by adding0.1 M HNO3 or/and 0.1 M NaOH solutions. Then 0.2 g adsorbent wasadded on the solution. After that, the solutions were shaken for 5days at 25 ◦C. Finally, the pH of the solutions was measured. pHpzc

was reported at the pH in which the initial pH equals the final pH.Similar method is applied by Hameed et al. [23] for determiningpHpzc.

3. Results and discussion

3.1. Effect of pH

The effect of pH on the lead sorption by different adsorbentsis investigated and results are shown in Fig. 1. Since at pH rangeabove 7, precipitation occurs in the copper solutions, the exper-iments are not conducted beyond a pH of 7. Similar trends werereported by other researchers [24,25]. It can be seen from Fig. 1 thatthe lead sorption by different adsorbents is increased when the pHof the solution increases from 2 to 6. At the pH of 6, the adsorptioncapacity of lead by adsorbents reaches its maximum value and the

In low pH, the protonation of the anion functional groups ofadsorbents reduces the ability of the functional groups in chelatingwith the lead ions. Also the hydrogen ion competition with lead ions

M. Irani et al. / Chemical Engineering Journal 178 (2011) 317– 323 319

2 3 4 5 6 71

1.5

2

2.5

3

3.5

4

4.5

5q

(mg

/ g)

PerliteDolomiteDiatomite

F

fsccrTtaATbWiflcopolgfalto

3

doeaafa

ok

q

we

0 50 10 0 15 0 20 0 25 00

1

2

3

4

5

time (min)

q (m

g/g)

Perlitedolomitediatom ite

t e xadsD1 xads

D2

where D1 and D2 (mg L−1) are rate constants of the rapid andslow steps; kD1 and kD2 (min−1) are constants controlling the

pH

ig. 1. Effect of pH on the lead sorption by natural perlite, dolomite and diatomite.

or active sites on the surface of adsorbents leads to fewer activeites available to bind lead ions and consequently the removal effi-iency of lead ions is decreased. With increasing the pH, the positiveharge density on the surface sites of adsorbents is decreased whichesults in an increase of the lead sorption by all three adsorbents.he effect of pH on the lead sorption could be also explained withhe calculation of pHpzc values of the natural diatomite, dolomitend perlite which are obtained as 3.6, 3.9 and 4.3, respectively.t pH values lower than pHpzc, the adsorbent surface is positive.here is an electrostatic repulsion between positive charge adsor-ent surface and the lead ions, which leads to the lower sorption.hen pH value is equal to pHpzc, the surface charge of adsorbents

s neutral. At pH values greater than pHpzc, the adsorbent sur-ace becomes negatively charged; it causes more attraction of theead ions onto the surface adsorbents and increases the adsorptionapacity of lead by the adsorbents. Also, as can be seen, the pHpzc

f diatomite is lower than that of dolomite and perlite. When theHpzc value is low, the more lead ions are attracted to the active sitesn the surface of adsorbents. So, the further adsorption capacity ofead onto the diatomite can be attributed to the more functionalroups of diatomite which leads to the more available active sitesor lead sorption by the diatomite in comparison with dolomitend perlite. At pH values greater than 6, the formation of hydroxy-ated complexes of the lead ions in the form of Pb(OH)2 decreaseshe adsorption capacity of different adsorbents [26]. Therefore, theptimum pH for further adsorption studies is selected as 6.

.2. Effect of contact time and adsorption kinetics

Fig. 2 shows the effect of contact time on the lead adsorption byifferent adsorbents. It can be seen that after 1 h, the concentrationf lead ions in aqueous solutions reaches more than 80% of theirquilibrium concentration for all of the three adsorbents. After 2 h,lmost all of the available sites of adsorbents are saturated and thedsorption capacity does not change anymore with time. There-ore, 2 h is selected as equilibrium time of lead sorption by studieddsorbents for further experiments.

Kinetic models, namely pseudo-first-order, pseudo-second-rder and double-exponential are used to describe the adsorptioninetics of lead ions onto the different adsorbents.

The pseudo-first-order kinetic model is given as [27]:

t = qe(1 − exp(−k1t)) (2)

here qt and qe (mg g−1) are the adsorption capacity at time t andquilibrium time, respectively and k1 (min−1) is the pseudo-first

Fig. 2. Effect of contact time on the lead sorption by natural perlite, dolomite anddiatomite.

order model rate constant. The constants of the pseudo-first orderkinetic model are obtained by plot of qt versus t (Fig. 3), and theresults are presented in Table 2.

The pseudo-second-order kinetic model by Ho and McKay [28]is given as:

qt = k2q2e t

1 + k2qet(3)

where k2 (g mg−1 min−1) is the adsorption rate constant. Fig. 4shows the plot of qt versus t for the pseudo-second-order modelfor the adsorption of lead by studied adsorbents. The parameters ofpseudo-second-order kinetic model are also given in Table 2. The R2

values of pseudo-first-order and pseudo-second-order equationsindicate that each two models of pseudo-first-order and pseudo-second-order describe the experimental data well.

The double-exponential kinetic model indicates that diffusionis controlling step in adsorption process onto the adsorbents. Thismodel included two separate regions; a rapid sorption stage whereexternal surface diffusion of lead occurs and the latter region,slower sorption stage where internal surface diffusion of lead ionsoccurs onto the adsorbents [29,30]. The double-exponential kineticmodel is given as [29]:

q = q − D1 exp(−k t) − D2 exp(−k t) (4)

Fig. 3. Pseudo-first-order kinetic plots for lead sorption onto the natural perlite,dolomite and diatomite.

320 M. Irani et al. / Chemical Engineering Journal 178 (2011) 317– 323

Table 2Kinetic parameters of lead sorption onto the adsorbents.

Adsorbent Pseudo-first-order model Pseudo-second-oder model Double-exponential kinetic model

qexp (mg g−1) qe (mg g−1) k1 (min−1) R2 qe k2 (g mg−1 min−1) R2 qe D1 (mg L−1) kD1 (min−1) D2 (mg L−1) kD2 (min−1) R2

Perlite 4.2 4.298 0.0230 0.993 5.257 0.00455 0.981 4.276 3.506 0.0239 5.258 0.0239 0.997Dolomite 4.35 4.433 0.0236 0.995 5.402 0.00460 0.985 4.421 3.654 0.0243 5.346 0.0243 0.998Diatomite 4.85 4.932 0.0259 0.993 6.105 0.00365 0.988 4.955 4.004 0.0218 5.966 0.0218 0.997

Table 3Isotherm parameters for lead adsorption onto the adsorbents at different temperatures.

Adsorbent T (◦C) Freundlich isotherm Langmuir isotherm D–R isotherm

kF (mg g−1) n R2 qmax (mg g−1) KL (L mg−1) R2 qDR (mmol/g) BDR (mol2 J−2) R2

Perlite 25 4.593 6.188 0.981 8.906 0.5166 0.994 0.040 5.446 × 10−9 0.98335 4.094 6.043 0.981 8.184 0.4226 0.995 0.037 5.492 × 10−9 0.98045 3.863 6.333 0.980 7.513 0.4238 0.994 0.034 5.932 × 10−9 0.968

Dolomite 25 4.484 3.044 0.992 18.55 0.1280 0.994 0.088 9.477 × 10−9 0.98035 5.610 3.472 0.981 18.63 0.1962 0.985 0.093 7.901 × 10−9 0.98545 6.527 3.691 0.964 19.69 0.2502 0.980 0. 102 7.095 × 10−9 0.969

Diatomite 25 7.254 3.198 0.997 24.21

−9

35 8.533 3.530 0.999 24.81

45 9.906 3.801 0.991 25.01

0 50 100 15 0 200 2500

1

2

3

4

5

6

time (min)

q (m

g/g)

Exprimenta l data- perliteExprimenta l data-dol omiteExprimental da ta-diat omite Pseudo-s econd -ord er-p erlit e Pseudo-s econd -ord er-d olo mite Pseudo- second -order-di atomite

Fd

mps(

Fd

ig. 4. Pseudo-second-order kinetic plots for lead sorption onto the natural perlite,olomite and diatomite.

echanism; xads (g L−1) is the adsorbent concentration. The

arameters of the double-exponential kinetic model are pre-ented in Table 2 which were calculated by plotting qt versus tFig. 5).

0 50 10 0 15 0 20 0 25 00

1

2

3

4

5

6

time (min)

q (m

g /g

)

Expri mental data-p erliteExpri mental data-dolo miteExprimental data-diato mite Double-ex pon ential-p erlite Double-ex pon ential-dolo mite Double-ex pon ential-diato mite

ig. 5. Double-exponential kinetic plots for lead sorption onto the natural perlite,olomite and diatomite.

0.2031 0.961 0.106 6.200 × 10 0.8590.3320 0.948 0.108 5.972 × 10−9 0.8110.6001 0.934 0.111 5.738 × 10−9 0.715

The high values of correlation coefficient of double-exponentialkinetic model (R2 > 0.997) indicate that this model is suitable forsorption kinetic of adsorbents. By examining the values of constantparameters of double-exponential kinetic model, it can be foundthat the both external diffusion and internal diffusion are effec-tive in the lead adsorption by three adsorbents. By comparison ofvalues of correlation coefficient for pseudo-first-order (R2 > 0.993),pseudo-second-order (R2 > 0.981) and double-exponential models,it was found, the adsorption kinetics was best described by double-exponential model.

3.3. Effect of initial concentration and isotherm models

The effect of initial concentration on lead removal by studiedadsorbents is shown in Fig. 6. It can be seen that the adsorp-tion capacity of lead ions by studied adsorbents increases with anincrease in the initial lead concentration, and then approaches afixed value. The increase of adsorption capacity with raising theinitial lead concentration is due to an increase of the numbers oflead ions available to binding sites of adsorbents. At higher leadconcentration, the active sites become saturated and adsorptioncapacity approaches a constant value.

Furthermore, it can be seen from Fig. 6 that increasing adsorp-tion capacity of lead ions onto the dolomite and diatomite withraising the temperatures shows that the removal of lead ions bythese two adsorbents is favorable at higher temperatures. More-over, decreasing the adsorbent capacity of lead onto the perlite withraising the temperatures indicates the favorability of lead sorptionat lower temperatures.

Three isotherm models namely Freundlich, Langmuir andDubbin–Radushkevich are used to describe the equilibrium dataof lead sorption by studied adsorbents. The parameters of thesemodels were calculated by plotting of qe versus Ce at different tem-peratures (25, 35 and 45 ◦C) and the results are presented in Table 3.The Freundlich isotherm model is given as [31]:

qe = kF C1/ne (5)

where kF (mg g−1) and n are the Freundlich parameters. The n valuesgreater than one indicate that the adsorption of lead is favorableat studied conditions. From Table 3, it can be concluded that theadsorption is favorable in the studied conditions.

M. Irani et al. / Chemical Engineering Journal 178 (2011) 317– 323 321

0 20 40 60 80 1000

2

4

6

8

10

Initial concentration (mg / L)

q (m

g / g

)

Perlite,T=25 CPerlite,T=35 CPerlite,T=45 C

0 20 40 60 80 10 00

5

10

15

20

Initial concentration (mg / L)

q (m

g / g

)

Dolomite,T=25 CDolomite,T=35 CDolomite,T=45 C

0 20 40 60 80 10 00

5

10

15

20

25

30

Initial concentration (mg / L)

q (m

g / g

)

Diat omite,T=25 CDiat omite,T=35 CDiat omite,T=45 C

a

b

c

Fp

q

wb(t1

Table 4Comparison of adsorption capacity (mg g−1) of natural clays of Iran such as perlite,dolomite and diatomite for lead sorption with other adsorbents reported in theliterature.

Adsorbent Lead adsorption capacity (mg g−1) Ref.

Expanded perlite 13.39 [33]Raw dolomite 21.74 [13]Natural diatomite 24.89 [34]Raw diatomite 24 [35]Celtek clay 18.08 [36]Olive stone 5.9 [37]Montmorillonite 10.40 [38]Kaolinitic clay 6.46 [39]Silica ceramic 2.7 [40]Natural calcite 19.92 [41]Kaolinite 11.52 [42]Bentonite 16.66 [43]Sawdust 21.05 [44]Perlite 8.906 This work

ig. 6. Effect of initial concentration of lead on adsorption capacity of natural (a)erlite, (b) dolomite and (c) diatomite.

Langmuir isotherm model is given as [32]:

e = qmbCe

1 + bCe(6)

here qm (mg g−1) is the maximum adsorption capacity of adsor-

ent that is related to the monolayer adsorption capacity and bL mg−1) is the Langmuir model constant. It can be seen from Table 3hat the maximum adsorption capacity of lead ions increases from8.55 to 19.69 mg g−1 for dolomite, and from 24.21 to 25.01 mg g−1

Dolomite 19.69 This workDiatomite 25.01 This work

for diatomite with raising the temperature from 25 to 45 ◦C. Also,the maximum adsorption capacity of lead onto the perlite decreasesfrom 8.91 to 7.51 mg g−1 with increasing the temperature from 25to 45 ◦C.

In Table 4 the maximum adsorption capacity of perlite, dolomiteand diatomite is compared with other adsorbents reported inthe literature [13,33–44]. It can be concluded that the maximumadsorption capacity of lead ions by diatomite is higher than allof the adsorbents in these literatures, therefore, diatomite has asignificant potential for adsorption of lead from aqueous solutions.

Another important feature of Langmuir isotherm is the determi-nation of favorability of sorption process using the dimensionlessconstant separation factor (RL) which is defined as [45]:

RL = 11 + bC0

(7)

where C0 (mg L−1) is the initial concentration of lead ions. Thevalue of RL indicates the type of isotherm that is either unfavorable(RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0).The values of RL for lead sorption by perlite, dolomite and diatomitehave been calculated as 0.019–0.191, 0.038–0.439 and 0.016–0.329respectively. These results indicate the favorability of the lead ionsorption by studied adsorbents.

The D–R isotherm is expressed as [46]:

qe = qDR exp(−BDRε2DR) (8)

where qDR (mg g−1) and BDR (mol2 J−2) are the D–R isothermconstants and εDR is the Polanyi potential that is equal toRT ln(1 + (1/Ce)), where R is the gas constant (8.314 J mol−1 K−1) andT is the absolute temperature (K). The value of BDR is related to theadsorption free energy that can be calculated from the followingequation:

E = 1√2BDR

(9)

The value of free energy determines the type of adsorption mech-anism. Physisorption processes have adsorption energy in therange 1–8 kJ mol−1, if E value lies between 8 and 16 kJ mol−1,the adsorption process is ion exchange and chemisorption mech-anism occurs while E value is in the range of 20–40 kJ mol−1

[47]. The adsorption free energy was calculated from 9.58 to

9.18 kJ mol−1, 7.18 to 8.395 kJ mol−1 and 8.98 to 9.34 kJ mol−1 withthe increase in temperature from 25 ◦C to 45 ◦C for the adsorp-tion of lead ions by perlite, dolomite and diatomite, respectively.These results are indicating an ion-exchange process for perlite

322 M. Irani et al. / Chemical Engineering Journal 178 (2011) 317– 323

Table 5Thermodynamic parameters of lead adsorption onto the adsorbents.

Adsorbent kC �H◦ (kJ mol−1) �S◦ (kJ mol−1 K−1) �G◦ (kJ mol−1)

298 K 308 K 318 K 298 K 308 K 318 K

0.045 −4.500 −3.919 −3.6050.103 −5.025 −5.927 −7.0880.154 −9.068 −10.672 −12.143

aai

Ltid

3

attff

k

�

wtac

b

l

lms

lipfpa�d

tcepr

3

FBu

Fig. 7. BJH desorption pore size distribution plot for the natural perlite, dolomiteand diatomite.

Table 6Physical properties of natural clays.

Sample SBET (m2/g) Pore volume (cm3/g) Average porediameter (nm)

Perlite 1.92 0.035 1.11

Perlite 6.15 4.62 3.91 −17.892

Dolomite 7.60 10.12 14.60 25.682

Diatomite 38.87 64.56 98.77 36.756

nd diatomite, respectively. Furthermore, the lead sorption mech-nism by dolomite is physisorption at lower temperatures andon-exchange at higher temperatures.

By comparing the correlation coefficients, it was found that theangmuir isotherm model fitted the equilibrium data of lead sorp-ion onto the perlite and dolomite better than Freundlich and D–Rsotherm models. Also the equilibrium data of diatomite were wellescribed by the Freundlich isotherm model for lead ions.

.4. Adsorption thermodynamics

Temperature affects on removal efficiency of metal ions by thedsorbent. Furthermore, temperature plays an important role inhe adsorption process which determines the nature of adsorp-ion with calculation of the thermodynamic parameters. The Gibbsree energy change of the adsorption process (�G◦) is calculated byollowing equations:

C = limCel→0

Ces

Cel(10)

G◦ = −RT ln kC (11)

here R is the gas constant (8.314 J mol−1 K−1), T is an absoluteemperature (K) and kC is the adsorption equilibrium constant. Ces

nd Cel are the values of solid phase concentration and liquid phaseoncentration at equilibrium in mg L−1.

The enthalpy (�H◦) and entropy (�S◦) changes were calculatedy the following equation:

n kC = �S◦

R− �H◦

RT(12)

�H◦ and �S◦ were obtained from the slope and intercept ofn kC versus 1/T plot (figure is not presented). The calculated ther-

odynamic parameters of lead sorption by studied adsorbents arehown in Table 5.

It can be seen from Table 5 that �G◦ of lead sorption by per-ite, dolomite and diatomite is negative at studied conditions. Thisndicates the feasibility and spontaneous nature of lead sorption byerlite, dolomite and diatomite. Also the Gibbs free energy changeor lead sorption by perlite is more negative with decreasing tem-erature which shows that the lower temperature is preferred fordsorption of lead by perlite. Furthermore, more negative value ofG◦ with raising temperature indicates that the lead sorption by

iatomite and dolomite is more favorable at higher temperature.The negative value of �H◦ indicates the exothermic nature of

he lead sorption by perlite. Also the positive values of enthalpyhange show the adsorption of lead by dolomite and diatomite isndothermic. The positive values of �S◦ for adsorption of lead byerlite, dolomite and diatomite indicate the increased disorder andandomness at the solid–solution interface of lead with adsorbents.

.5. BET analysis

The pore size distributions of adsorbents are shown inig. 7. Based on BJH theory, the average pore diameter, therunauer–Emmet–Teller (BET) surface area (SBET) and the pore vol-me of adsorbents are shown in Table 6. It can be seen that the peak

Dolomite 2.71 0.047 1.24Diatomite 4.11 0.053 1.38

position of diatomite shifted slightly toward the greater pore diam-eter in comparison with dolomite and diatomite that indicates thepore size of diatomite is greater than that of perlite and dolomite.Also, as can be seen from Table 6, the average of pore diameter,SBET and pore volume of diatomite are greater than those of per-lite and dolomite. More lead sorption onto the diatomite can beattributed to the further pore size, surface area and pore volumewhich causes the lead ions to easily diffuse into the pores onto theinternal surface of diatomite.

4. Conclusion

In the present study, the adsorption of lead ions from aque-ous solutions using natural clays of Iran such as perlite, dolomiteand diatomite was investigated. The optimal conditions for leadsorption by dolomite, perlite and diatomite were found to be pHof 6, temperature of 45 ◦C, pH of 6, temperature of 25 ◦C and pHof 6, temperature of 45 ◦C, respectively. Results indicated that thelead sorption onto all of the studied clays follows the double-

exponential kinetic model and equilibrium is reached after 2 h.From the obtained results, the experimental data of perlite anddolomite followed well Freundlich model, and Langmuir modeldescribed well the equilibrium data of diatomite for lead sorption.

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he adsorption capacity of natural clays of Iran for lead sorption wasound to be in order of diatomite > dolomite > perlite. The calculatedhermodynamic parameters indicated that the adsorption of leadnto the perlite was spontaneous and exothermic and the naturef lead sorption onto the dolomite and diatomite was endothermicnd spontaneous.

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