Adsorption and desorption behaviour of Pb(II) on a natural kaolin: equilibrium, kinetic and...

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Research ArticleReceived: 2 January 2009 Revised: 9 February 2009 Accepted: 12 March 2009 Published online in Wiley Interscience: 28 April 2009

(www.interscience.wiley.com) DOI 10.1002/jctb.2192

Adsorption and desorption behaviour of Pb(II)on a natural kaolin: equilibrium, kineticand thermodynamic studiesQiang Tang, Xiaowu Tang, Zhenze Li,∗ Yunmin Chen, Naiyu Kouand Zufeng Sun

Abstract

BACKGROUND: Pb(II) is common in both waste-waters and gas emissions. In developing countries, public health problems havebeen reported concerning Pb(II) pollution, so that stringent measures are required to deal with it.

MAJOR RESULTS: The adsorption and desorption behaviour of Pb(II) has been investigated on a natural Chinese kaolin.Several factors, including initial concentration, pH, equilibration time, dosage and temperature correlated positively with Pb(II)adsorption. The Pb(II) adsorption capacity of natural kaolin was 165.117 mg g−1. A kinetic study shows that Pb(II) adsorption onpurified kaolin equilibrates within 35 min. The enthalpy changes of Pb(II) adsorption on purified kaolin were 63.683, 20.488 and21.371 kJ mol−1 with entropy changes 262.250, 112.210 and 105.120 J mol−1 K−1 for solutions containing 50, 100 and 200 mgL−1 Pb(II) respectively, indicating an endothermic and spontaneous adsorption process. The desorption of Pb(II) from kaolinwas difficult with more than 85% Pb(II) removal. Based on X-ray diffraction (XRD) analysis, the Pb(II) adsorption on natural andpurified kaolin was attributed mainly to the magnesite component and complexation with the mineral surface.

CONCLUSIONS: Natural kaolin exhibits a satisfactory performance for adsorption of Pb(II) from aqueous solution. The optimumconditions for adsorption were: ionic strength = 0.01 mol L−1; pH ≥ 7.2; dosage = 10 g L−1; temperature = 25 ◦C; duration ≥ 16h (Ci = 80 mg L−1); and the optimum conditions for desorption were ionic strength = 0.1 mol L−1 and pH ≤ 5.0.c© 2009 Society of Chemical Industry

Keywords: natural kaolin; adsorption; desorption; Pb; kinetics; thermodynamics; mechanism

INTRODUCTIONAlong with the rapid development of industrial economies,the environment is faced with severe anthropogenic pollution.Arbitrary discharge of mining drainage, industrial effluent anddomestic sewage lead directly to massive soil contamination thatresults in a potential threat to living organisms. Metal pollutionreduces diversity by more than 99.9%, indicating the highly toxiceffect of metal contamination, especially for rare taxa.1 Pb(II) is acommon contaminant of surface soil from atmospheric depositsfrom the exhaust of automobile engines and waste incineratorsand from the discharge of industrial waste-water. The lead fromsuch sources has the potential to accumulate in the human bodyand cause chronic diseases and brain damage. What is worse, theeffect of such pollution may last for two millennia.2 Thus world-wide the concentration of lead in drinking water has been strictlylimited.3 Owing to the various routes for lead pollution in theenvironment, it is important to develop methods of reducing theexposure of living organisms to such pollutants.4,5 In developingcountries, public health problems have been reported relatedto lead pollution in drinking water. The adsorption of lead fromaqueous solution has been investigated using various adsorbents,such as activated carbon prepared from coconut shell,6 tree fern,7

expanded perlite,8 biopolymeric sorbents,9 and sawdust,10 buttheir adsorption capacities range only from 8.62 to 39.80 mg g−1,

too low to satisfy our needs. Therefore, it is necessary to developnovel methods for lead removal from aqueous solution.

According to statistical data, kaolin resources are widelydistributed in more than 60 countries, including Great Britain,USA, Brazil, Australia and China. However, most of these depositsinclude kaolin with a certain amount of impurities and high puritykaolinite is rare and precious.11 The kaolin deposits operatedby ECC International in Cornwall, UK has been reported to beexhausted. Therefore this study is not merely a local issue butworld-wide and it is believed that this paper and the results willinterest readers in many other regions of Europe, USA and Asia.Kaolin has been used as an important additive in many industries,including ceramics, paper manufacture, rubber processing, plasticand oil paint manufacture. High purity kaolin is always required andthose sources with impurities are comparatively less valuable. Inview of the enormous mineral reserves and the increasing market

∗ Correspondence to: Zhenze Li, MOE Key Laboratory of Soft Soils andGeoenvironmental Engineering, Department of Civil Engineering, ZhejiangUniversity, Yuhangtang Road 388, Hangzhou, 310058, P. R. China.E-mail: [email protected]

MOE Key Laboratory of Soft Soils and Geoenvironmental Engineering,Department of Civil Engineering, Zhejiang University, Yuhangtang Road 388,Hangzhou, 310058, P. R. China

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www.soci.org Q Tang et al.

competition, it is necessary to broaden the scope of applicationsof kaolin by developing new technologies.

As one of the most important clay minerals in the naturalenvironment, kaolin plays a key role in immobilizing and retardingthe transfer of contamination.12 – 16 Recently, kaolin has beenused for treating waste-water polluted by heavy metals. Ikhsanet al.13 reported the adsorption behaviour of transition elementson kaolin; Katsumata et al.12 used kaolin to remove heavy metalsfrom plating waste-water; Adebowale et al.17 studied the effect ofvarious factors on the adsorption of Cd(II) and Pb(II); Sari et al.18

reported the adsorption, kinetic and thermodynamic propertiesof Pb(II) on Turkish kaolin, and the adsorption of Pb(II), Cd(II)and Ni(II) on bentonite and kaolinite was studied by Gupta andBhattacharyya.19 In addition to study of the adsorption mechanismof Pb(II) on kaolin,16 the improved adsorption performance20

gained by the addition of sodium tetraborate has also beenreported. In general, the reported adsorption capacities ofthese adsorbents are rather low thus their application does notseem to be economically practical. Thus it appears importantto study the use of natural kaolin which is very common inmineral deposits. Because of the abundant world-wide reservesof kaolin, the possibility of using it to remove lead from aqueoussolution is important. Therefore in this study, the adsorptionand desorption behaviour of lead on kaolin were investigatedand in addition to discussion of the governing mechanismsseveral influencing factors have been studied, including slurryconcentration, temperature, equilibration time, solution pH andmineral composition, etc. have been studied.

EXPERIMENTALMaterialsThe kaolin used in the experiment was purchased from SinopharmLtd (China). All other inorganic reagents were of analytical grade,and a stock solution (0.1 mol L−1) of Pb(II) was prepared bydissolving lead nitrate in deionized water (DW).

Characterization of the adsorbentThe elemental constituents (Zn, Pb, Fe, Mg, Ca, K, Na, Al) ofthe kaolin were determined by atomic absorption spectroscopy(AAS, Hitachi 180-80, Japan) following complete digestion of a 1 gsample in a 1 : 1 mixture of HF/HNO3 (10 mL).

The specific surface area of the natural kaolin was determinedby N2 adsorption (Autosorb 1-MP apparatus, QuantachromeCorporation, USA). The results were analysed by BET adsorptiontheory to predict the specific surface area, volume of micropores,average pore size, etc.

X-ray diffraction (XRD) patterns of the adsorbents (natural kaolin,purified kaolin and Pb(II)-loaded kaolin) were obtained by D/MAX-RA apparatus (Rigaku Corporation, Japan). The Pb(II) loaded kaolinwas prepared by equilibrating natural kaolin (1.0 g) with a 200 mgL−1 Pb(II) solution (40 mL) for 24 h at 25 ◦C. The mixture wasseparated by centrifuging at 3000 rpm for 5 min.

Pretreatment of the adsorbentSince the sample is a natural kaolin without treatment, some impu-rities exist which could affect the test results. Thus pretreatment ofthe adsorbent included: mixing natural kaolin (50 g) with 0.1 molL−1 nitric acid (500 mL) with a magnetic stirrer at 500 rpm for36 h, letting the slurry settle to separate and then flushing theprecipitated adsorbent three times with DW (100 mL) until the pH

of the solution was close to or higher than 5.0. The sample wasthen oven dried at 105 ◦C for 24 h, cooled to room temperatureand stored in sealed in plastic bags for further tests.

Adsorption testEffect of slurry concentrationThe slurry concentration of the adsorbent (natural kaolin) in theaqueous solution was increased from 0.5 to 1, 5, 10, 20 and 40 g L−1

to investigate the effect of slurry concentration. Pb(II) solutionswith initial concentrations 60, 120, 240 mg L−1 were prepared andtransferred into stoppered conical flasks. After mixing with theadsorbent, the flasks were put into a thermostated agitator. Batchadsorption tests were conducted under isothermal conditions(25 ◦C) for 24 h at 180 rpm. The solution pH was not adjustedin this test. At the end of the test, the slurry was separated bycentrifuging at 3000 rpm for 5 min. The supernatant was collectedand the equilibrium Pb(II) concentration was determined by AAS.Duplicates and controls were also analysed for Pb(II).

Adsorption kineticsPb(II) solutions with varied initial concentration (80, 160, 320 mgL−1 for natural kaolin and 50, 100, 200 mg L−1 for purified kaolin)were prepared. The slurry concentration of the adsorbent wasfixed at 10 g L−1 in this test. Natural kaolin (0.5 g) was mixed withPb(II) solutions (50 mL) in a conical flask and placed in the agitatorto equilibrate at 25 ◦C. The test was stopped after specific timesthat increased from 0.5 to 1, 2, 4, 8, 16, 24, 48, 96 and 168 h. Themixture was transferred to polypropylene centrifuge tubes andcentrifuged at 3000 rpm for 5 min. The Pb(II) concentration in thesupernatant was determined and duplicate and blank tests wereconducted to obtain an average value for each test.

For the purified kaolin, the duration of tests varied from 0.5 to 1,2, 4, 6 and 24 h with the other processes following the above test.

Effect of temperatureSamples of purified kaolin slurry (10 g L−1) were separately mixedwith three sets of lead nitrate solution with increasing initial Pb(II)concentration from 60 to 120 and 240 mg L−1. The slurries wereseparately equilibrated at increased temperatures from 15 to 45 ◦Cin increments of 10 ◦C. The mixtures were agitated at 180 rpm for24 h. The equilibrium Pb(II) concentration was determined usingthe same method as above.

Effect of pHThe initial concentration of Pb(II) solutions were 5.0 and 50 mgL−1 while the ionic strength of the solution (I) was adjusted to 0.1and 0.01 mol L−1 with potassium nitrate. The dosage of naturalkaolin was fixed at 0.2 g per 100 mL, the mixture was agitatedat 180 rpm with a magnetic stirrer for 24 h at a temperature of25 ◦C to reach equilibrium. The solution pH, measured by a glasselectrode/potentiometer, was adjusted by adding 0.01 mol L−1

HNO3 or KOH solutions automatically using a pH controller.The adsorbent was mixed with Pb(II) solution in a beaker and

the solution pH was adjusted to pH = 2.0±0.2, and then increasedto 10.0 ± 0.2 in several steps. At each pH step, the solution pH waskept constant. At the end of the 24 h, the stirrer was stopped forabout 10–15 min to allow the adsorbent to settle before 100 µLsupernatant was sampled and diluted to 1 mL for determinationof the Pb(II) concentration. The amount of liquid sampled was notallowed to reach 1% of the total volume.

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Adsorption/desorption behaviour of Pb(II) on a natural kaolin www.soci.org

Desorption testDesorption of the loaded adsorbent could be achieved by eitherdecreasing the solution pH or the solute concentration. In thiswork both effects were investigated.

Effect of pHThe effect of pH on Pb(II) desorption from natural kaolin wasstudied following the method described earlier except that thesolution pH was decreased from 10.0 to 2.0. The change of Pb(II)concentration in solution with pH was recorded to show thedesorption behaviour.

Effect of initial concentrationSince the sorption of Pb(II) on natural kaolin did not reach equilib-rium within 24 h, based on the adsorption kinetic study, purifiedkaolin was chosen as the adsorbent for this desorption test. Theadsorbent was first loaded with lead by taking weighed samples(0.4 g) and mixing in 50 mL polypropylene centrifuge tubes withPb(II) solution (40 mL) with increasing initial concentration from25 to 50 and 100 mg L−1 and ionic strength adjusted to 0.01 molL−1 with KNO3. The tubes were agitated at 180 rpm for 3 h at 25 ◦C.Afterwards, the samples were centrifuged at 3000 rpm for 5 minand the supernatant was analysed for Pb(II) by AAS. A sample ofthe supernatant (10.0 mL) was removed and replaced with freshwater (10 mL) of the same ionic strength (I = 0.01 mol L−1). Themixture was further equilibrated in an agitator at 180 rpm for 3 h at25 ◦C and following centrifugation the supernatant was analysedfor Pb(II). A portion (10 mL) of the supernatant was again removedand replaced by water and the process repeated nine times.

RESULTS AND DISCUSSIONCharacterization of the adsorbentTable 1 shows the background of the elemental constituents andthe correlated basic parameters. The results show that the majorelements in the kaolin are Al (21.58%), Fe (2.500%), Ca (3.052%),Na (1.352%), Mg (0.476%) and K (0.112%), Pb was not found. Theadsorbent also contains silica but as this mineral is not involvedin the adsorption process its concentration was not determined.From the N2 adsorption test, the specific surface area of the naturalkaolin is 7.977 m2 g−1, while the total pore volume and the averagepore diameter are 0.05189 mL g−1 and 260.2 µm, respectively. Ac-cording to the mass content of iron, kaolin can be divided into twokinds: hard kaolin and soft kaolin.21 The kaolin used here could beviewed as hard kaolin because the iron content reaches about 2.5%.

Figure 1 shows the XRD pattern of kaolin before and after purifi-cation and also the pattern of Pb(II)-loaded kaolin. From the charac-teristic bands, it can be seen that kaolin composes just a part of theadsorbent and other minerals such as mica, white mica, K nephelineand magnesium carbonate are also present. After adsorption withPb(II), the bands related to white mica and magnesium carbonatedisappear while new bands corresponding to lead oxide and leadcarbonate compounds (Cerrusite) were observed. Pattern c showsthe spectra of kaolin purified by nitrate acid and it is obvious thatthe carbonate originally present has been effectively removed.

Effect of slurry concentrationThe effect of adsorbent dosage is shown in Figs 2 and 3. Theefficiency of removal of Pb(II) by purified kaolin at initial concen-trations (Ci) = 60, 120 and 240 mg L−1 was examined. The test

Table 1. Basic parameters of natural kaolin

Element

Measured concentrationin digestion

solution mg L−1 (40 mL) Weight percent %

K 2.8 0.112

Na 33.8 1.352

Ca 11.9 0.476

Mg 76.3 3.052

Al 53 750 21.58

Fe 62.5 2.500

Pb NA NA

Parameter Test data R2

Specific surface area 7.977 m2 g−1

Total pore volume 0.05189 mL g−1 0.9999

Average pore diameter 260.2 µm

Figure 1. XRD pattern of (a) natural kaolin, (b) kaolin loaded with lead, and(c) purified kaolin.

was conducted at 25 ◦C without adjusting the solution pH. Withincreasing solid–solution ratio, the equilibrium Pb(II) concentra-tion decreased but not linearly as the initial rapid decrease in Pb(II)gradually flattened out at higher slurry concentrations. Althoughthe amount of Pb(II) removed per gram of adsorbent decreased(Fig. 2), the total amount removed from solution still increasedwith increasing slurry concentration, as shown by the decrease ofequilibration concentration (Fig. 3). The initial Pb(II) concentrationalso affected the removal efficiency with the final percentageadsorption increasing with increasing initial concentration.

From these data an optimum dose of adsorbent (10 g L−1) wasobtained and in the following experiments this dosage was chosenfor further tests.

Adsorption kineticsThe variation of equilibrium concentration of Pb(II) with time atdifferent initial Pb(II) concentrations is shown in Fig. 4. The timerequired to reach equilibrium increased with increasing initial Pb(II)concentration, i.e. 16, 48 and 96 h for C0 = 80, 160 and 320 mgL−1. The solution pH of the slurry with 160 mg L−1 Pb(II) was alsomeasured and is shown in Fig. 4. The pH increased from 5.735

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Figure 2. Effect of dose on the adsorption amount per gram (naturalkaolin).

Figure 3. Variation of equilibration concentration with the dosage (naturalkaolin).

(1 h) to 5.815 (5 h) and 7.255 (78 h). This increase of pH promotesabsorption of Pb(II).

Figure 5 shows the amount of Cs of Pb(II) on kaolin withincreasing equilibration time. From the plot, it is obviousthat initially adsorption increased almost linearly but thenbecame constant at equilibrium. The time required to reach theadsorption equilibrium increased from 18–100 h as the initialsolute concentrations increased from 50–200 mg L−1.

The test data were further analysed with three kinetic models(i.e. pseudo-first-order kinetics, pseudo-second-order kinetics, andintraparticle diffusion model).

The pseudo-first-order kinetic equation is:22

log(Ce − Cs) = log(Ce) − k1

2.303t (1)

where Ce and Cs are the amounts of solute adsorbed per unitadsorbent at equilibrium and at any time t, respectively (mg g−1),and k1 the pseudo-first-order rate constant (h−1).

The pseudo-second order kinetic equation is:22

t

Cs= 1

k2Ce2 + 1

Cet (2)

Figure 4. Variation of equilibrium concentration of lead with equilibrationtime (natural kaolin).

Figure 5. Effect of time on the amount of lead adsorption per gram (naturalkaolin).

where k2 is the pseudo-second-order rate constant (g mg−1 h−1).The equation for the intraparticle diffusion model is:23

Cs = kintt1/2 + C (3)

where kint is the relevant rate constant (mg g−1 h−1/2) and C is theintercept.

Table 2 shows the predicted kinetic parameters for Pb(II)adsorption on kaolin. The correlation coefficient values indicatethat both the pseudo-first-order kinetic equation (R2 = 0.982,0.991 and 0.762) and the pseudo-second-order kinetic equation(R2 = 0.96, 0.974 and 0.813) fit the test data well with lowinitial concentrations. The equation of the intraparticle diffusionmodel can be considered as a good replacement when the initialconcentration was extremely high (i.e. Ci = 320 mg L−1).

In addition, from the listed values of k1 and k2 it ob-vious that both decrease as the initial metal ion concen-trations increase (k1 decreased from 0.143 to 0.045 h−1; k2

decreased from 0.023 to 0.002 g mg−1 h−1), which meansthat the system needs more time to reach equilibrium. How-ever, analysis of the intraparticle diffusion model shows thatthe rate constant kint increased with increasing initial Pb(II)


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Table 2. Predicted kinetic constants of Pb(II) adsorption on naturalKaolin

Ci (mg L−1) 80 160 320

The pseudo-first-order kinetics equation

Ce (mg g−1) 0.095 16.273 30.323

k1 (h−1) 0.143 0.061 0.045

R2 0.982 0.991 0.762

The pseudo-second-order kinetic equation

1/Ce 0.114 0.054 0.030

1/(k2 × C2e ) 0.573 0.751 0.466

R2 0.960 0.974 0.813

Ce (mg g−1) 8.804 18.643 33.704

k2 (g mg−1 h−1) 0.023 0.004 0.002

The intraparticle diffusion model

kint (g mg−1 h−1/2) 0.587 1.449 2.251

C 2.533 1.546 5.911

R2 0.620 0.793 0.959

Figure 6. Adsorption isotherms at different equilibration times for naturalkaolin.

concentration. Thus the higher the Pb(II) concentration, thehigher the diffusion rate, consistent with Fick’s diffusionlaw.

Figure 6 shows the amount of lead adsorption for un-purified kaolin for different contact times. This shows thatthe amount of adsorption per unit of adsorbent reached198 mg g−1 after 168 h, which is much more than after 24 h(17 mg g−1).

Isothermal equations were applied to evaluate the test results,and the data are listed in Table 4. The predicted amount ofadsorption per unit of adsorbent reached 165.117 mg g−1.

Figure 7 shows the adsorption kinetics of Pb(II) on purifiedkaolin. In the first 35 min the equilibrium concentration of Pb(II)reduced sharply from 50, 100 and 200 mg L−1 to 19.9, 53.6and 118.0 mg L−1, and then remained stable for the rest ofthe time. Thus adsorption reached equilibration within 35 min. Itis also obvious that although adsorption is faster the capacityof purified kaolin was significantly less than natural kaolin(Fig. 2).

Figure 7. Variation of equilibrium concentration of lead with equilibrationtime (purified kaolin).

Figure 8. Isothermal adsorption lines at different temperatures (purifiedkaolin).

Effect of temperature on Pb(II) sorption and the thermody-namic behaviourFigure 8 shows the effect of temperature on the removal efficiencyof Pb(II). It can be seen that higher temperatures lead to increasedadsorption as well as increased removal efficiency, denoting theendothermic nature of Pb(II) adsorption on kaolin.

Thermodynamic parameters such as enthalpy change (�H◦),Gibb’s free energy change (�G◦) and entropy change (�S◦) canbe estimated with the following Gibb’s free energy equations:

�G◦ = −RT ln KD (4)

�G◦ = �H◦ − T�S◦ (5)

ln KD = �S◦

R− �H◦

RT(6)

where R is the universal gas constant (8.314 J mol−1K−1),T the absolute temperature (K), and KD the equilibrium partitioncoefficient (mL g−1).

The resulting thermodynamic parameters (Table 3) can bedetermined from the slope and intercept of the linearized testdata (Fig. 9). The Gibb’s free energy changes of Pb(II) adsorption


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Table 3. Thermodynamic parameters for adsorption of Pb(II) on kaolin

Ci mg L−1 50 100 200

Temperature 1/T ln Kd G (kJ mol−1) ln Kd G (kJ mol−1) ln Kd G (kJ mol−1)

278 0.003597 4.291 −9.917 4.653 −10.755 3.334 −7.705

288 0.003472 4.565 −10.551 4.697 −10.857 3.562 −8.232

298 0.003356 5.535 −12.793 5.419 −12.526 4.368 −10.096

308 0.003247 7.198 −16.638 5.783 −13.367 4.360 −10.077

318 0.003145 7.318 −16.914 5.489 −12.686 4.369 −10.098

�S◦ (J mol−1 K−1) 262.250 112.210 105.120

�H◦ (kJ mol−1) 63.683 20.488 21.371

R 0.961 0.872 0.903

Table 4. Predicted constants of isothermal models for Pb(II) adsorption on kaolin

Adsorbent+loading metal Purified kaolin+Pb Unpurified kaolin+Pb

Temperature (K) 278 288 298 308 318 298(24 h) 298(168 h)

Langmuir model

Q0 (mg g−1) 5.475 6.524 10.850 8.907 8.476 15.151 165.117

b (L mg−1) 0.045 0.038 0.044 0.306 0.369 1.734 0.548

R2 0.815 0.908 0.981 0.989 0.816 0.942 0.917

Freundlich model

KF (mg g−1) 1.535 1.363 1.807 4.111 3.875 8.097 10

n 4.486 3.571 2.921 5.972 5.703 6.873 2.394

R2 0.273 0.528 0.853 0.923 0.999 0.976 0.938

D-R model

qm (mg g−1) 12.9 18.7 35.7 15.7 14.5 22.9 247

k (mol2 kJ−2) 4.00E-03 4.00E-03 4.00E-03 2.00E-03 2.00E-03 1.00E-03 2.00E-03

E (kJ mol−1) −11.180 −11.180 −11.180 −15.811 −15.811 −22.361 −15.811

R2 0.278 0.521 0.796 0.808 0.961 0.962 0.864

Figure 9. Fitting of experimental data with Gibb’s free energy equations.

with initial concentrations of 50, 100 and 200 mg L−1 ranged from−9.91 to −16.91; −10.75 to −12.69, and 7.70 to −10.10 kJ mol−1,respectively, over the temperature range 278 to 318 K, indicatinga spontaneous adsorption process that would be enhanced by

increasing the temperature. Because the correlation coefficientsbetween the curve fitted to the thermodynamic parameterand the test data are 0.961, 0.872 and 0.903, the Gibb’s freeenergy equations fit the test data well. The enthalpy changesof Pb(II) adsorption on purified Kaolin were 63.683, 20.488 and21.371 kJ mol−1 while the entropy changes were 262.250, 112.210and 105.120 J mol−1 K−1 These values are close to those reportedby Gupta.14 The adsorption of Pb(II) can thus be viewed asan endothermic and preferred process while the adsorptionaffinity appeared to decrease with increasing solute concentrationbecause of the decreasing entropy change.

Adsorption isothermsFigure 8 shows the adsorption isotherms of Pb(II) on purifiedkaolin at different temperatures acquired from the thermodynamicstudies. Three isothermal equations were applied to evaluate thetest results.

The Langmuir isotherm:22

1

qe= 1

Q0 + 1

bQ0Ce(7)

where Ce is the equilibrium concentration of solute solution (mgL−1), Q0 is the maximum sorption capacity of the adsorbent (mgg−1), and b (L mg−1) the Langmuir constant.


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The Freundlich isotherm can be expressed as:

qe = KFCe1/n (8)

where KF is the Freundlich constant (mg g−1) indicating theadsorption capacity and strength of the adsorptive bond, andn the heterogeneity factor.

The D-R model assumes a uniform pore-filling sorption and canpredict the free sorption energy change by which the sorptiontype can be judged.24 – 26 The D-R model is written as:

ln qe = ln qm − kε2 (9)

where qm is the maximum sorption capacity (mol g−1), k a modelconstant related to the free sorption energy and ε the Polanyipotential:

ε = RT ln(1 + (1/Ce)) (10)

The mean free energy of sorption (E) is:

E = − 1√2k

(11)

The adsorption is basically a surface adsorption associated withion exchange when |E| is between 8 and 16 kJ mol−1; a rangeof values from 1.0 to 8.0 kJ mol−1 suggest a physical adsorptionmechanism.27,28

Table 4 lists the isothermal constants for Pb(II) adsorption onkaolin based on the linearization of test data as shown in Figs 6and 8. Based on the correlating coefficients, the Langmuir modelwas found to fit the test data better than the Freundlich29 andD-R models under most conditions with R2 > 0.816 except forthat at 318 K, which could be best fitted by the Freundlich modelwith R2 = 0.999. The predicted Pb(II) adsorption capacity of thepurified kaolin from the Langmuir model increased from 5.475to 6.524 and 10.850 mg g−1 with increasing temperature 278,288, 298 K, respectively. Further increase in the equilibratiumtemperature from 298 to 308 and 318 K in turn decreased theadsorption capacity from 10.850 to 8.907 and 8.476 mg g−1. Thisphenomenon has not been observed in the literature. Interestingly,the authors found that whatever model was used, the predictedadsorption capacity of purified kaolin reached a maximum valueat 298 K. In addition, the free adsorption energy at 308 and 318 Kwere both determined as −15.811 kJ mol−1 by the D-R model withR2 > 0.808. This value is close to the predicted Gibb’s free energiesshown in Table 3, confirming the spontaneous adsorption processin ion exchange forms.

Effect of pHAdsorption testFigures 10 and 11 show the relation between the amount of Pb(II)adsorption on kaolin and the equilibrium pH (pHe). In Fig. 10,initial Pb(II) concentration 50 mg L−1, and ionic strength 0.01and 0.1 mol L−1, the solution pH has an obvious effect on Pb(II)adsorption on natural kaolin. A sharp rise occurred at pH 5.63and 6.72 for solutions with ionic strength at 0.01 and 0.1 mol L−1

respectively. Before this significant increase the adsorbed Pb(II)increased gradually from 3.87 to 23.08% on increasing pH from1.88 to 2.50 for I = 0.01 mol L−1 and from 0 to 27.57% with pH from5.63 to 6.72 for I = 0.1 mol L−1 solutions. The ionic strength alsoaffected the adsorption and a 10 times increase in ionic strengthresulted in a pH increase of 1.0–1.5 units of the adsorption edge.

Figure 10. Effect of pH and ionic strength on adsorption of Pb(II) on naturalkaolin (ionic strength I = 0.01, 0.1 mol L−1; initial Pb(II) concentrationC0 = 50.0 mg L−1; T = 15–25 ◦C; time = 24 h).

Figure 11. Effect of pH and initial Pb(II) concentration on adsorptionof Pb(II) on natural kaolin (ionic strength I = 0.1 mol L−1; initial Pb(II)concentration C0 = 50.0 mg L−1; T = 15–25 ◦C; time = 24 h).

Figure 10 also shows the precipitation behaviour of Pb(II) insolution without kaolin. When the solution pH was increasedto 5.5, Pb(II) began to precipitate and at pH = 7.4, the Pb(II)adsorption amount reached a peak (96.4%). Further increase in pHdid not increase the removal of Pb, with 3.6% still remaining free insolution. However, when mixed with kaolin, the percentage Pb(II)adsorption reached a maximum >99% at pH > 8.0. At the lowerionic strength as 0.01 mol L−1, the adsorption edge was close tothat of precipitation. However, when the ionic strength rose to0.1 mol L−1, adsorption occurred at higher pH than precipitation.

Figure 11 shows the effect of pH and initial Pb(II) concentrationon the adsorption on natural kaolin. With C0 = 5.0 mg L−1,Pb(II) adsorption reached its first plateau (31.9%) at pH 3.0, thenproceeded to the second plateau (88.0%) at pH 7.68, finallydecreasing a little to 80% at pH 8.9. For C0 = 50.0 mg L−1,the first plateau (19.2%) occurred at pH 3.85, and then reacheda maximum (98.6%) at pH 8.0 without any decrease with furtherincrease in pH.


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Figure 12. Desorption isotherm of Pb(II) on purified kaolin.

Figure 13. Effect of initial equilibrium concentration on amount ofdesorbed Pb(II) on purified kaolin.

Desorption testEffect of pH. Figure 10 shows that the desorption curves alwayslie above the adsorption curves, which indicates that it was hardto desorb to the adsorbed Pb(II) from kaolin. At pH > 7.0 the twocurves were close together but with decrease in pH the two curvestended to move slightly apart but the difference was not verylarge.

In Fig. 11, the first inflection point of the desorption curves ofPb(II) for solutions with C0 = 5.0 and 50.0 mg L−1 occurred atpH 5.60 and 6.68 with adsorption percentages of 32.4 and 50.1%,respectively. With further decrease in pH, a second inflection pointwas observed at pH 2.80 and 3.55 with adsorption percentage at40.0 and 27.4% for solutions with initial Pb(II) concentration at 5.0and 50.0, respectively.

Whatever the solution conditions were, the end of the des-orption curves coincided with each other (adsorption percentage12.0–14.0%, pH = 2.3–2.4), which means that the desorption ofPb(II) depends on the solution pH, initial Pb(II) concentration andionic strength at pH > 2.5. It was also shown that the completeremoval of Pb(II) absorbed on kaolin would not be possible.

Table 5. Equilibrium adsorption amount of kaolin for Pb(II)

Equilibrium constants(Langmuir)

Remark

Author Q0 (mg g−1) b (L mg−1) (Reference)

Srivastava et al., 2005 13.14 0.06198 35

Shahwan et al., 2005 15.15 0.13 36

Bhattacharyya andGupta, 2008

5.20 – 37

Omar and Al-Itawi, 2007 113.63 0.0132 31

Adebowale et al., 2006 16.6 0.009 17

Alkana et al., 2008 6.09 7.39 34

Gupta andBhattacharyya, 2008

11.5 20.7 19

Sari et al., 2007 31.75 0.049 18

This paper 165.117 0.917 Natural kaolin

This paper 10.85 0.044 Purified kaolin

Effect of concentration. Figure 12 shows the effect of initial Pb(II)concentration on the desorption of Pb(II) from kaolin. It canbe seen that the adsorbed Pb(II) began to be desorbed as theequilibrium solution (Pb(II)) concentration decreased, and thenflattened out when the equilibrium concentration approached40% of the initial value. The slopes of the desorption isothermswere less than the adsorption isotherms, once again indicatingthat the adsorbed solute was hard to remove. Hysteresis in the twoisotherms suggests two different modes of interaction betweenPb(II) and kaolin: a weakly bound mode that is easy to desorb and astrongly bounded mode that is hard to desorb even under extremesolution conditions of very low pH or high ionic strength.16

Figure 13 shows a linear variation between the initial equilibriumPb(II) concentration and the amount of desorbed Pb(II) frompurified kaolin. This was best fitted by Equation (12), indicatingthat these parameters were closely linked. Merikepeld reported asimilar linear relation between desorbed Cd(II) and the adsorbedamount of Cd(II) on Apatite.30

�Cs = 0.0207C0 − 0.3174 (12)

DISCUSSION OF ADSORPTION/DESORPTIONMECHANISMThis work investigated the adsorption/desorption behaviour ofPb(II) on kaolin. The results indicate that various factors, i.e.mineral component, ionic strength, pH, temperature, initial Pb(II)concentration and time, all have a great effect on adsorption,among which the mineral component and pH appear to be mostimportant.

Concerning the mineral component, the carbonate containedin the natural kaolin used in this study contributed significantly toPb(II) adsorption. After acidification, the carbonate was removed asverified by the XRD patterns (Fig. 1), and as a result the adsorptionof Pb(II) on kaolin significantly decreased and became consistentwith the reported data in Table 5 (113.63 mg g−1).31 This datawas consistent with that of the natural kaolin used in this study:the samples used in these two studies contain impurities thatcontribute significantly to Pb(II) adsorption.

Pattern b (Fig. 1) is the XRD pattern of unpurified kaolin withadsorbed Pb(II). Comparing patterns a and b, it is seen that thecarbonate (dolomite and magnesite) in the kaolin disappeared


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following adsorption of Pb(II) while new substances appeared(lead carbonate (Cerussite), lead silicate and lead hydroxide). Moreexactly, considering the initial pH of solution (5.735), Pb(II) may bein the form of Pb(OH)+ (Fig. 4) and the existence of lead hydroxideindicates the following precipitation reaction:32

Pb2+ + H2O → Pb(OH)+ + H+ (13)

Pb(OH)+ + H2O → Pb(OH)2(s) + H+ (14)

After equilibration, the pH of the solution referred to in Fig. 4reached 7.255 where 96.4% of Pb(II) was precipitated, followingthe Pb(II) precipitation curve shown in Fig. 10. Before massprecipitation occurred (pH range 5.7 to 7.2), a rapid adsorptionstage existed as shown by the adsorption kinetics (Fig. 11). Thisphenomenon could be caused by the carbonate content:

Pb2+ + MgCO3(s) → PbCO3(s) + Mg2+ (15)

Pb(OH)+ + MgCO3(s) → PbCO3(s) + Mg2+

+ OH− (16)

Equations (15) and (16) were confirmed by the existence oflead carbonate as shown in Fig. 1. In the forward reaction(Equation (16)), hydroxyl ions are produced, which would increasethe pH value (Fig. 4). This reaction was a slow process sincethe increase of pH occurred over nearly 20 h. The production oflead hydroxide and lead carbonate covered the surface of thereagent (magnesium carbonate) and prevented further reaction,corresponding to the much longer equilibration time required fornatural kaolin than the purified mineral.

However, the adsorption mechanism of purified kaolin differssignificantly from that of natural kaolin. Heavy metals like Pb(II)could be adsorbed on kaolin by two major mechanisms: edgeadsorption, and cleavage plane surface adsorption.33 Because ofcrystal lattice defects in most cases, the overall charge on theedge of the particles becomes negative but it also depends on thesolution pH. The charge on the basal faces of the kaolin particles,which are strong adsorption sites, is permanently negative andindependent of change in solution pH.34 The kaolin mineral crystaledge consists of an aluminium hydroxide bond and the chargeswill change with the pH of the solution, these are thus variablecharge sites and can be combined in the form of outer-spherecomplexes.35 As shown in Figs 10 and 11, Pb(II) adsorption showstwo stages of adsorption with increasing pH, the first the reactionoccurring between pH = 3 and Ph = 4, and the second betweenpH = 7 and pH = 8. This is consistent with the proposed adsorptionmechanism, with reaction between Pb(II) and permanent negativecharge sites leading to the first stage adsorption; whereas thesecond stage is the result of complexation of Pb(II) and variablecharge sites.

The result of adsorption/desorption at different ionic strengths(Fig. 10) showed that adding electrolyte reduced adsorption andcould add to the mobility of Pb (II) in soil, which might increasethe pollution of groundwater. In solutions with ionic strength0.01 mol L−1, Pb(II) desorption required a pH < 4.69, while at ionicstrength 0.1 mol L−1 a pH of 6.68 gave a similar result. Becauseof the sensitivity of goethite and carbonate to solution pH, alower pH would make these minerals dissolve and thus changethe soil mineral component and physicochemical properties. Asa result, soil flushing with strong acid solution is not advisable toremediate contaminated soils. However, adding an electrolyte tothe flushing liquid can avoid the problem of a strong acid solution(pH < 3.0–4.0) and could be feasible in practical applications.

The results of desorption of Pb(II) show that once Pb(II) wasadsorbed, it was retained by strong bonds and was difficult todesorb. Thus completely removing Pb(II) from kaolin could not beachieved by flushing with acidified water.

As it is so hard to completely desorb Pb(II) even under veryharsh conditions (pH = 2–3), it is necessary to remove residualPb(II) from kaolin? This will inevitably depend on the remediationstandard for polluted soils. For environmental considerations,how will the strongly adsorbed Pb(II) on kaolin interact with theaqueous solution and could it be continually desorbed into thesurroundings? These questions cannot be answered in this paperand further studies are needed to determine these unknownfactors and their effects.

CONCLUSIONS(1) The existence of carbonate increases the adsorption capacity

from 12.8 to 243.6 mg g−1, while the equilibrium time isincreased from<35 min to>16 h. Natural kaolin was useful forPb(II) removal from aqueous solution and could be refreshedby acid washing. Purified kaolin has the potential for recycling.

(2) When the initial Pb(II) concentration was raised from 80to 320 mg L−1, the adsorption equilibrium time increasedfrom 16 to 96 h. During the process in which initial Pb(II)concentration was increased from 5.0 to 50 mg L−1, themaximum adsorption increased from 88.0% to 98.6%.

(3) The Pb(II) adsorption front on kaolin increases by 1.0–1.5 pHunits with a 10 times increase in ionic strength from 0.01 to0.1 mol L−1.

(4) Adsorbed Pb(II) on purified kaolin could be desorbed bydecreasing the solution pH to 2–3 (85.5–88.0% desorption)and by decreasing the equilibrium Pb(II) concentration(40% desorption).

(5) Pb(II) adsorption on natural kaolin includes precipitationand adsorption, while that on purified kaolin includes anion exchange strongly bounded mode (20%) and surfacecomplexation type weakly bounded mode (20%).

(6) The optimum conditions for Pb(II) adsorption were ionicstrength 0.01 mol L−1; pH≥7.2; dosage 10 g L−1; temperature25 ◦C; duration 20 h (Ci = 80 mg L−1). The optimumconditions for Pb(II) desorption were ionic strength 0.1 molL−1 and pH ≤ 5.0.

ACKNOWLEDGEMENTSThe authors would like to express their sincere gratitude to the KeyProject of National Natural Science Foundation of China (NSFC)(Grant 50538080) and National Science Fund for DistinguishedYoung Scholars (Grant 50425825) for the financial support to thisstudy.

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