Adsorption of Cd(II) and Pb(II) from aqueous solutions on activated alumina

Journal of Colloid and Interface Science 333 (2009) 14–26

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Adsorption of Cd(II) and Pb(II) from aqueous solutions on activated alumina Tarun Kumar Naiya, Ashim Kumar Bhattacharya, Sudip Kumar Das ∗ Department of Chemical Engineering, University of Calcutta, 92, A P C Road, Kolkata-700 009, India

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 11 August 2008 Accepted 3 January 2009 Available online 10 January 2009

The ability of activated alumina as synthetic adsorbent was investigated for adsorptive removal of Cd(II) and Pb(II) ions from aqueous solutions. Various physico-chemical parameters such as pH, initial metal ion concentration, and adsorbent dosage level and equilibrium contact time were studied. The optimum solution pH for adsorption of Cd(II) and Pb(II) from aqueous solutions was found to be 5. Kinetics data were best described by pseudo-second order model. The effective particle diffusion coefficient of Cd(II) and Pb(II) are of the order of 10−10 m2 /s. Values of mass transfer coefficient were estimated as 4.868 × 10−6 cm/s and 6.85 × 10−6 cm/s for Cd(II) and Pb(II) adsorption respectively. The equilibrium adsorption data for Cd(II) and Pb(II) were better fitted to Langmuir adsorption isotherm model. The thermodynamic studies indicated that the adsorption was spontaneous and exothermic for Cd(II) adsorption and endothermic for Pb(II). The sorption energy calculated from Dubinin–Radushkevich isotherm were 11.85 kJ/mol and 11.8 kJ/mol for the adsorption of Cd(II) and Pb(II) respectively which indicated that both the adsorption processes were chemical in nature. Desorption studies were carried out using dilute mineral acids. Application studies carried out using industrial waste water samples containing Cd(II) and Pb(II) showed the suitability of activated alumina in waste water treatment plant operation. © 2009 Elsevier Inc. All rights reserved.

Keywords: Activated alumina Pseudo-second order Effective diffusivity Langmuir Desorption Application studies

1. Introduction Heavy metals are considered to be non-biodegradable and have great environmental, public health and economic impacts [1]. Increasing concentration of these metals in the water constitutes a severe health hazard due to their toxicity, persistent in nature particularly when it exceeds the permissible limits. These heavy metals introduced into natural water resources by waste water discharged from industries such as smelting, metal plating, cadmium– nickel and lead storage batteries, phosphate fertilizer, mining, galvanizing, paints, pigments, insecticides, cosmetics, stabilizer and alloy manufacturing. The health effects of Cd(II) on human include nausea, vomiting, diarrhea, muscle cramp, salivation, loss of calcium from bones, yellow coloration of teeth (cadmium ring formation), reduction of red blood cells, damage of bone marrow, hypertension, kidney failure following oral ingestion, lung irritation, chest pain, and loss of sense of smell after inhalation. Chronic cadmium poisoning produces proteinuria and affects the proximal tubules of kidney, causing formation of kidney stones [2–4]. Pb(II) is a highly toxic substance, exposure to which can produce a wide range of adverse health effects for both adults and children. Very low levels of exposure to young children under the age of six can

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Corresponding author. Fax: +91 33 2351 9755. E-mail address: [email protected] (S.K. Das).

0021-9797/$ – see front matter doi:10.1016/j.jcis.2009.01.003

© 2009

Elsevier Inc. All rights reserved.

Table 1 Tolerance limits for heavy metal concentration in drinking water and discharge into inland surface waters. Heavy metal

Cd(II) Pb(II)

EPA [7]

WHO [8]

Drinking water (mg/L)

IS 10500 1992 [6] Discharge in inland surface water (mg/L)

Discharge in public sewers (mg/L)

Drinking water (mg/L)

Drinking water (mg/L)

0.01 0.05

2.00 0.10

1.00 0.10

0.005 0.015

0.003 0.01

result in reduced IQ, learning disabilities, attention deficit disorders, behavioral problems, stunted growth, impaired hearing, and kidney damage. At high levels of exposure, a child may become mentally retarded, fall into a coma, and even die from lead poisoning. In adults, lead can increase blood pressure and cause fertility problems, nerve disorders, muscle and joint pain, irritability, and memory or concentration problems [5]. The tolerance limits for heavy metal concentration in potable water and discharge into inland surface waters is shown in Table 1 [6–8]. Precipitation, ion exchange, electrochemical precipitation, solvent extraction, membrane separation, concentration, evaporation, reverse osmosis and bio-sorption and emulsion per traction technology are the conventional methods for the removal of heavy metals from the aqueous solution [9–19]. These technologies suffer from cost effectiveness and ineffectiveness when the metals are

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

15

Fig. 2. Scanning electron micrographs (SEM) of activated alumina (500×).

Fig. 1. XRD study of activated alumina. Table 2 Chemical composition and characteristics of activated alumina used for adsorption studies. Constituent

Percent by weight (%)

Loss on ignition Fe2 O3 Al2 O3 SiO2 Na2 O

6.7 0.03 93.1 0.03 0 .1

dissolved in large volume of solution of relatively low concentration. Adsorption process seems to be most versatile and effective method for removal of heavy metal if combined with appropriate regeneration steps. It solves the problem of sludge disposal and renders the system more viable. In the last few years, several approaches have been studied in this area. The literature review suggested the use of different natural and synthetic adsorbents for the removal of heavy metals from waste water [20–28]. The advantage of activated alumina as adsorbent lies in the fact that it has high surface area, mechanical strength and amphoteric properties. The present study deals with a series of batch adsorption experiments to investigate and explore the feasibility of activated alumina (Al2 O3 ) as an adsorbent for removal of Cd(II) and Pb(II) from aqueous solutions. 2. Materials and methods All the chemicals used in our experiments were of analytical grade. Activated aluminum oxide (Al2 O3 ) was obtained from Titan Biotech Limited, India. The physicochemical characterization of activated alumina was performed using standard procedures. The sample was subjected to characterization by a variety of methods such as X-ray diffraction (XRD), chemical analysis, particle size, BET surface area, and scanning electron microscope (SEM). The structure of activated alumina was studied using X-ray diffractograms (XRDs) obtained from an X-ray diffractometer (Model No. XRD 3000P, Seifert, Germany). The X-ray diffraction analysis was done by using Cu Kα as a source and Ni as a filter media, K radiation maintained at 1.52 Å. The diffraction pattern is shown in Fig. 1. The chemical analysis of the activated alumina was carried out using Vogel’s method [29] and shown in Table 2. Fig. 2 shows the

SEM micrographs (Model S415A, Hitachi, Japan) of the activated alumina. It indicated that the surface was porous in nature. The particle size of 250–350 μm is analyzed in particle size distribution analyzer (Model 117.08, MALVERN Instruments, USA). The results are shown in Table 3. The effective diameter (d p ) is calculated to be 291.45 μm. The surface area was determined by BET (Brunauer–Emmett–Teller nitrogen adsorption technique) method using a surface area analyzer (Model 1750 SORPTY, Carlo Erba, Italy) and was found to be 126 m2 /g. The bulk density of activated alumina was determined as 0.81 g/cm3 . The point of zero charge of the activated alumina was determined by the solid addition method [30]. To a series of 100 ml conical flasks 45 ml of KNO3 solution (0.1 M and 0.01 M) was transferred. The pH values of the solutions were adjusted by adding 0.1 N HCl or 0.1 N NaOH solutions. The total volume of the solution in each flask was made exactly to 50 ml by adding additional KNO3 solution (0.1 M or 0.01 M as the case). The pH of the solutions was noted (pH0 ). 1 g of activated alumina was added to each flask. The suspensions were manually shaken and allowed to equilibrate for 48 h with occasionally shaken manually. The pH values of the supernatant liquid were noted. The difference between the initial (pH0 ) and final pH (pHf ) values (pH = pH0 − pHf ) was plotted against pH0 . The point of intersection of the resulting curve will give the point of zero charge and it is 6.51. All the other necessary chemicals used in the study were of analytical grade. Cadmium nitrate tetrahydrate [Cd(NO3 )2 , 4H2 O] and lead nitrate [Pb(NO3 )2 ] were obtained from E. Merck India Limited, Mumbai, India. Stock solution of the above heavy metals was made by dissolving exact amount of respective metal salt. The range of concentration of the metal components prepared from stock solution was varied between 3 mg/L and 300 mg/L. The test solutions were prepared by diluting 1 g/L of stock metal solution with double distilled water. The necessary amount of activated alumina was taken in a 250 ml stopper conical flask containing 100 ml of desired concentration of the test solution for the batch adsorption studies at the desired pH value. Different initial concentration of metal solutions was prepared by proper dilution from stock 1000-ppm metal standard. pH of the solution was monitored in a 5500 EUTECH pH Meter using FET solid electrode calibrated with standard buffer solutions by adding 0.1 M HCl and 0.1 M NaOH solution as per required pH value. Necessary amount of activated alumina was then added and contents in the flask were shaken for the desired contact time in an electrically thermo stated reciprocating shaker at 120 strokes/min at 30 ◦ C. The time required for reaching the equilibrium condition estimated by drawing samples at regular intervals of time till equilibrium was reached. The contents of the flask were filtered through filter paper and the filtrate was analyzed

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Table 3 Particle size distribution of the activated alumina (250–350 μm). Adsorbent

250–275 μm

275–285 μm

285–295 μm

295–305 μm

305–315 μm

315–325 μm

325–335 μm

335–350 μm

Activated alumina

2.0%

40.4%

30.5%

9.2%

7.8%

6.5%

2.2%

1.4%

Fig. 3. Effect of pH on the adsorption of Cd(II) ions. Initial Cd(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 7.5 gm/L, contact time 2 h.

for remaining metal concentration in the sample using atomic absorption spectrophotometer (VARIAN SPECTRA AA 55, USA) as per procedure laid down in APHA, AWWA, WEF, Standard Methods for Examination of Water and Wastewater, 1998 edition [31]. All the investigations were carried out in triplicate to avoid any discrepancy in experimental results with the reproducibility and the relative deviation of the order of ±0.5% and ±2.5% respectively. The effect of pH on adsorption was conducted by batch adsorption process described above maintaining the solution pH adjusted to 2.0 ± 0.1 to 8.0 ± 0.1 under thermostated conditions of 30 ◦ C. The effect of temperature on adsorption isotherm was conducted under isothermal condition at 30 ◦ C, 40 ◦ C and 50 ◦ C where temperature varied within ±0.5 ◦ C. 3. Results and discussion 3.1. Effect of pH The pH of the solutions has an important variable governing metal adsorption. In general, adsorption of cations is favored at pH > pHpzc . The effect of pH on the adsorption of Cd(II) by activated alumina was studied by varying pH of the solution over the range of 2–8 using different concentration. The calculation from the solubility product equilibrium constant (K sp ) demonstrated that the best pH range of 2–8 for Cd(II) for adsorption [30]. Fig. 3 illustrated that removal efficiency increased with increase pH for each of the initial concentration. The uptake of Cd(II) by activated alumina increased as the pH increased from 2 to 8. Although a maximum uptake was noted at a pH of 8, as the pH of the solution increased to >7, Cd(II) started to precipitate out from the solution. Therefore experiments were not conducted over pH 7. The increased capacity of adsorption at pH > 7 may be a combination of both adsorption and precipitation on the surface of

Fig. 4. Effect of pH on the adsorption of Pb(II) ions. Initial Pb(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 5 gm/L, contact time 2 h.

the adsorbent. It is considered that activated alumina had a maximum adsorption capacity at a pH = 5, if the precipitated amount is not considered in the calculation. Therefore, the optimum pH for Cd(II) adsorption is 5. The same trend has also been reported in the removal of Cd(II) ions by other adsorbent materials such as cassava waste biomass [32], anaerobic granular biomass [33], chitosan-coated perlite beads [34], saw dust [35]. In order to evaluate the influence of this parameter on the adsorption of Pb(II), the experiments were carried out with range of initial pH 3–7 in the different initial concentration. The pH range was chosen as 3–7 in order to avoid precipitate in the form of lead chloride and lead hydroxides, which has been estimated to occur at pH < 2.0 for PbCl2 and pH > 6.5 for Pb(OH)2 [36]. The effect of pH on adsorption efficiencies are shown in Fig. 4. Removal of Pb(II) increases with increasing solution pH and a maximum value was reached at an equilibrium pH of around 5 in each of the cases. The same trend has also been reported in the removal of Pb(II) ions by other vegetable materials such as spent grain [37], Pinus sylvestris [38], and crop milling waste [39]. The metal ions in the aqueous solution may undergo solvation and hydrolysis. The process involved for metal adsorption is as follows [40]: M2+ + nH2 O = M(H2 O)n2+ , M(H2 O)n2+



+

(1) +

= M(H2 O)n−1 (OH) + H , + Ka  M2+ + nH2 O  M(H2 O)n−1 (OH) + H+ .

(2) (3)

The pK a value for Cd(II) and Pb(II) are 10.1 and 7.7 respectively. Perusal of the literature on metal speciation shows that the dominant species is M(OH)2 at pH > 6.0 and M2+ and M(OH)+ at pH < 6.0. Maximum removal of metal was observed at pH 5 for adsorption. On further increase of pH adsorption decreases probably

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

Fig. 5. Plot for initial pH vs final pH. Initial metal ion conc. 10 mg/L, adsorbent dosage 10 g/L, contact time 3 h.

17

Fig. 6. Effect of adsorbent concentration on adsorption of Cd(II) ions. Initial Cd(II) ion conc. 10 mg/L, 25 mg/L, 50 mg/L, initial pH 5, contact time 2 h.

due to the formation of hydroxide of cadmium and lead because of chemical precipitation [28,41,42]. The optimum pH value for adsorption was found to be 5. Effect of activated alumina on the solution pH was studied for both the metal ion using initial solution pH of 4, 5 and 6. It was observed that after 3 h pH was constant. So after attainment of equilibrium final pH was measured and results are shown in Fig. 5. It is evident that there was an increase in solution pH in each case. 3.2. Effect of adsorbent concentration The effect of adsorbent dosage in terms of percent removal of Cd(II) and Pb(II) ions at C 0 = 10, 25, 50 mg/L were studied and results are represented in Figs. 6 and 7 respectively. The rate of Cd(II) ion removal was found very fast with increase in the adsorbent dosage from 1 to 5 g/L, from 5 to 10 g/L the rate of removal is slow and beyond 10 g/L it remains constant. Similarly for Pb(II) ion removal increases sharply as adsorbent dosage increases from 1 to 2.5 g/L and after that the rate is slow up to adsorbent dosage 5 g/L and beyond this value it remains constant. Increase in adsorption with increase in adsorbent dosage attributed to the availability of larger surface area and more adsorption sites. At low adsorbent concentration, i.e., less than 1 g/L, the absorbent surface become saturated with the metal ions and the residual metal ion concentration in the solution is large. With an increase in adsorbent dosage, the metal ion removal increased to higher metal ion uptake by the increased amount of adsorbent. For higher adsorbent dosage, the incremental metal ion removal becomes very low as the surface metal ion concentration and the solution metal ion concentration comes to equilibrium with each other. 3.3. Effect of initial metal ion concentration Effect of initial metal ion concentration in terms of distribution coefficient K d , as defined in Eq. (4) are shown in Fig. 8. With increase in initial metal ion concentrations, more metal ions are left un-adsorbed in the solution due to the saturation of the binding sites. This indicates that energetically less favorable sites become

Fig. 7. Effect of adsorbent concentration on adsorption of Pb(II) ions. Initial Pb(II) ion conc. 10 mg/L, 25 mg/L, 50 mg/L, initial pH 5, contact time 2 h.

involved with increasing ion concentration in aqueous solution. Metal ion adsorption is attributed to different mechanisms of ion exchange as well as to the adsorption process. During the ion exchange process, the metal ion had to be moved through the pores of the adsorbent mass, but also through the channels of the lattice, they have to be replaced exchangeable cations. Diffusion was faster through pores and was retarded when the ion moves through the smaller diameter channels. Here, the metal ion adsorption mainly is attributed to ion-exchange reactions in the micro pores of the adsorbents.

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T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

Fig. 8. Effect of initial concentration on the adsorption of metal ions. Cd(II) pH 5, adsorbent conc. 7.5 gm/L, contact time 2 h. Pb(II) pH 5, adsorbent conc. 7.5 gm/L, contact time 2 h.

Kd =

(C 0 − C e ) Ce



V M

Fig. 9. Effect of contact time on the adsorption of metal ions. Initial Cd(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 7.5 gm/L, pH 5.

 .

(4)

The K d values increased with the decreasing concentration of metal ion. In other words K d values increased as the dilution of metal ion proceeds. This effect can be explained as at low metal ion/adsorbent ratios, metal ion adsorption involves higher energy sites. As the metal ion/adsorbent ratio increases, the higher energy sites are saturated and adsorption begins on lower energy sites, resulting in decreases in the percentage removal of metal ion [43]. 3.4. Effect of contact time and adsorption rate kinetics mechanism The effect of shaking time on the adsorption of Cd(II) and Pb(II) ion at initial metal ion concentration 10, 25 and 50 mg/L are shown in Figs. 9 and 10 respectively. During the experiment contact time was varied from 0 min to 5 h. The initial rapid adsorption gives away a very slow approach to equilibrium. In present studies, for both Cd(II) and Pb(II) equilibrium was achieved by 1 h of contact time. 3.5. Adsorption kinetics study The study of adsorption kinetics describes the solute removal rate and evidently this rate controls the residence time of adsorbate removal at the solid–solution interface including the diffusion process. The mechanism of adsorption depends on the physical and chemical characteristics of the adsorbent as well as on the mass transfer process [44]. With the maximum shaking speed of 120 rpm, it was assumed to offer no mass transfer (both external and internal external) resistance to the overall adsorption process. Therefore kinetic can be studied through the residual metal ion concentration in the solution. The results obtained from the experiments were used to study the kinetics of metal ion adsorption. The rate kinetics of metal ion adsorption on activated alumina at initial metal ion concentration 10, 25 and 50 mg/L respectively were analyzed using pseudo-first order [45,46], pseudo-second order [46,47], and intraparticle diffusion models [48]. The conformity

Fig. 10. Effect of contact time on the adsorption of metal ions. Initial Pb(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 5.0 gm/L, pH 5.

between experimental data and the model predicted values was expressed by correlation coefficients, r 2 and chi-square test, χ 2 . Lagergren model The pseudo-first order equation, Lagergren [45], is generally expressed as dq dt

= K 1 (qe − q).

Apply the following boundary conditions

(5)

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

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Table 4 Kinetic parameter for the removal of Cd(II) by activated alumina. Pseudo-first order model C0 (mg/L)

qexp (mg/g)

10 25 50

k1 × 102 (min−1 )

qcal (mg/g)

1.3127 3.178 6.164

0.8417 1.9384 3.9765

5.5664 5.2002 5.3637

Correlation coefficient r2

Chi square

0.9886 0.9954 0.9908

3.1536 8.5652 14.7638

χ2

Pseudo-second order model C0 (mg/L)

k2 × 102 (g/mg min)

qexp (mg/g)

qtheo (mg/g)

h (mg/g min)

Correlation coefficient r2

Chi square

10 25 50

19.642 7.733 3.884

1.3127 3.178 6.164

1.3195 3.2842 6.1968

0.351 0.810 1.532

0.9995 0.9994 0.9992

0.0371 0.1385 0.1718

C0 (mg/L)

C (mm)

K id (mg/g min0.5 )

χ2

Intraparticle diffusion model

10 25 50

Fig. 11. Lagergren plot for the adsorption of Cd(II) ions. Initial Cd(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 7.5 gm/L, pH 5.

0.6440 1.5644 2.9275

qexp (mg/g)

0.1060 0.2527 0.5094

1.3127 3.178 6.164

qtheo (mg/g) 2.4799 5.9409 11.7437

Correlation coefficient r2

Chi square

0.9334 0.9291 0.9333

2.5302 5.8388 12.295

χ2

Table 5 Kinetic parameter for the removal of Pb(II) by activated alumina. Pseudo-first order model C0 (mg/L)

qexp (mg/g)

10 25 50

k1 × 102 (min−1 )

qcal (mg/g)

1.9724 4.763 9.146

1.5722 4.0998 8.3354

7.1646 7.5515 7.5677

Correlation coefficient r2

Chi square

0.9894 0.9908 0.9888

6.4006 5.0680 1.6373

χ2

Pseudo-second order model k2 × 102 (g/mg min)

C0 (mg/L)

qexp (mg/g)

10 25 50

12.273 4.887 2.341

1.9724 4.763 9.146

C0 (mg/L)

C (mm)

K id (mg/g min0.5 )

qtheo (mg/g) 2.011 4.857 9.345

h (mg/g min) 0.496 1.153 2.044

Correlation coefficient r2

Chi square

0.9996 0.9997 0.9994

0.1245 0.2685 0.5735

Correlation coefficient r2

Chi square

χ2

Intraparticle diffusion model

10 25 50

1.2331 3.0802 6.2626

0.1489 0.3645 0.6440

qexp (mg/g) 1.9724 4.763 9.146

qtheo (mg/g) 3.8122 9.3954 17.4188

0.8706 0.8724 0.8026

χ2 3.7243 14.8862 27.2303

Pseudo-second order model The pseudo-second order adsorption kinetic rate equation is expressed as [47] dq dt Fig. 12. Lagergren plot for the adsorption of Pb(II) ions. Initial Pb(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 5 gm/L, pH 5.

t = o,

q = o,

t = t,

q = q.

(7)

with the boundary conditions t = o,

q = o,

t = t,

q = q.

The integrated form of Eq. (7) becomes

The integrated form of Eq. (10) becomes log(qe − q) = log qe −

= K 2 (qe − q)2 ,

K 1t 2.303

.



1 (6)

The values of the model parameter as well as correlation coefficient and chi square can be obtained from the plot log(qe − q) vs t (Figs. 11 and 12) and results are depicted in Tables 4 and 5.

(qe − q)

=

1



+ K 2t .

qe

(8)

This is the integrated rate law for a pseudo-second order reaction, Eq. (8) can be rearranged to obtain t q

=

1 K 2 qe2

+

t qe

.

(9)

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T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

Fig. 13. Pseudo-second-order plot for the adsorption of Cd(II) ions. Initial Cd(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 7.5 gm/L, pH 5.

Fig. 14. Pseudo-second-order plot for the adsorption of Pb(II) ions. Initial Pb(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 5 gm/L, pH 5.

Fig. 15. Weber and Moris (intraparticle diffusion) plot for the adsorption of Cd(II) ions. Initial Cd(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 7.5 gm/L, pH 5.

Fig. 16. Weber and Moris (intraparticle diffusion) plot for the adsorption of Pb(II) ions. Initial Pb(II) conc. 10 mg/L, 25 mg/L, 50 mg/L, adsorbent conc. 5 gm/L, pH 5.

Intraparticle diffusion model The intraparticle diffusion model (Fig. 6) is based on the theory proposed by Weber and Moris [48]. According to this theory

Kinetic parameters and fitting parameters can be obtained from the plots (Figs. 15 and 16). The values of rate constants and correlation coefficients for each model are shown in Tables 4 and 5. However, the correlation coefficients, r 2 , showed that the pseudosecond order model, an indication of chemisorptions mechanism, fits better with the experimental data than the pseudo-first order model. In addition, the chi-square test was also carried out to support the best fit adsorption model. The equation for evaluating the best fit model is to be written as

q = K id t 0.5 .

χt2 =

The kinetics parameters as well as fitting parameters can be experimentally determined by plotting t /qt vs t (Figs. 13 and 14) and the results are shown in Tables 4 and 5.

(10)

 (qt − qtm )2 qtm

.

(11)

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

Thus based on the high correlation coefficient and low χt2 value, it can be said that adsorption of Cd(II) and Pb(II) onto activated alumina follow pseudo-second order model. The values of pseudo-second order rate constant, k2 , were found to decrease with increase in the initial concentration for both the metal adsorption. The value of the rate constants, k2 , were found to decrease (from 12.273 to 2.341 g mg−1 min−1 for Cd(II) adsorption and from 19.642 to 3.884 g mg−1 min−1 for Pb(II) adsorption) with increase in the initial Cd(II) and Pb(II) ion concentration from 10 to 50 mg L−1 .

21

properties of the adsorbents – pore size along the length of pore, orientation, electric field and the interaction of the adsorbate–van der Walls attractive forces, surface diffusion characteristics and adsorption mechanism, all affect the diffusion. Meso-pores are found to occupy most of the pore length of activated alumina. The diffusion within the pores of wider path and weaker retarding forces of electrostatic interaction accounts for the greater D e and one within the pore of narrower mesh widths and stronger retarding forces accounts for lower D e . For the present systems, the value of D e falls well within the values reported in literature, especially for chemisorptions system (10−9 to 10−17 m2 /s) [52].

3.6. Mass transfer analysis 3.8. Richenberg model Mass transfer analysis for the removal of Cd(II) and Pb(II) from aqueous solutions by activated alumina were carried out using the following equation as proposed by McKay et al. [49]:



ln

Ct C0

−

 Ct

− 

C0

 1+M K bq  M K bq



= ln

1 + M K bq

The plot of ln slope



1

M K bq 1 + M K bq

1 1+ M K bq







−

1 + M K bq M K bq



β S s t . (12)

β S s and the values of mass transfer coefficients

3.7. Determination of diffusivity Adsorption data obtained from kinetic study could be described well by the models given by Boyd et al. [50]. Diffusion found to be rate controlling in the adsorption of Cd(II) and Pb(II) onto the particles of spherical shape. As the volume of the solution is much higher compare to the volume of the activated alumina particle and if the concentration of metal ions is assumed constant from the surface to the center of the adsorbent particle the previous equation can be expressed as



α 6  1

π

2

n=1

n2

exp −

D e t π 2 n2 r2

 .

(13)

Applicability of Vermeulen’s approximation is limited and highly depends on the ratio of the initial metal ion concentration in activated alumina (C 0 )a and in the solution C 0 and of the volume of adsorbent (V a ) and the volume of the solution (V s ). For the criterion of ‘infinite solution volume’ that is given by the ratio (C 0 )a V a  C 0 ( M ) V s , where the concentration of metal ion in the solution remains negligible throughout the process, and for the range 0  F (t )  1 in the solution of divalent exchangeable ions, Eq. (13) can be simplified [51] as



ln

1 1 − F 2 (t )



=

π

2

R a2

Det.



versus t results a straight line of

(β ) calculated from the slops of the plots. Value of mass transfer coefficient were estimated as 4.868 × 10−6 cm s−1 (r 2 = 0.975) and 6.85 × 10−6 cm s−1 (r 2 = 0.967) for Cd(II) and Pb(II) adsorption respectively for the initial metal ion concentration of 10 mg/L. The values of mass transfer coefficients (β ) and correlation coefficient obtained from the study indicate that the velocity of the adsorbate transport from bulk to the solid phase was quite fast.

F (t ) = 1 −

The rate of sorption is determined by applying well-known equation for the diffusion and mass transfer phenomena. For the fast reaction, the sorption may be due to film diffusion [53] and occur within the microspores of the adsorbent. In that case Reichenberg equation is applied, i.e.,

(14)

The plot of ln[1/(1 − F 2 (t ))] versus t provides a line from whose slope π 2 D e /r 2 the diffusion coefficient, D e can be calculated. The value of diffusion coefficient as calculated from the equation was found to be 1.096 × 10−10 m2 /s and 1.39 × 10−10 m2 /s for the for the adsorption of Cd(II) and Pb(II) onto activated alumina respectively. The adsorbent–adsorbate and adsorbate–adsorbate interactions have their impact on the diffusion process and affect the value of D e . The pores of activated alumina have different sizes along its length and adsorbent has a wide pore size distribution. The

F (t ) = 1 −



6

e − Bt .

π2

(15)

Above equation may be written as:





Bt = −0.4977 ln 1 − F (t ) .

(16)

The plot of Bt vs time is linear with a correlation coefficient

(r 2 ) of 0.988 for Cd(II) adsorption and 0.989 for Pb(II) adsorption for the initial metal ion concentration of 10 mg/L for both metal ions, thereby indicating that sorption was controlled by film diffusion. 3.9. Bangham’s equation Bangham’s equation [54] used to check whether pore diffusion is the only rate controlling step or not in the adsorption system using the kinetic data as per the following equation:





log log



C0

 = log

C 0 − qt m

k0 m 2.303V

 + α log(t ).

(17)

Adsorption kinetics would be limited by pore diffusion if the experimental data are well represented by Eq. (17). Values of Bangham parameters, k0 and α are 2.714 × 10−3 L g−1 and 0.759 for Cd(II) adsorption and 3.032 × 10−3 L g−1 and 0.799 for Pb(II) adsorption with a correlation coefficients of 0.848 and 0.876 respectively for Cd(II) and Pb(II) adsorption. From the correlation coefficients it can be concluded that above Equation does not give a good fit of the experimental data, indicating thereby that the diffusion of adsorbate into the pores of the adsorbent is not solely rate-limiting step [55]. 3.10. Adsorption isotherms Adsorption isotherms are used for describing adsorption equilibrium for waste water treatments. 3.10.1. Langmuir adsorption isotherm The Langmuir equation is based on the assumption of a structurally homogeneous adsorbent where all sorption sites are identical and energetically equivalent. The Langmuir adsorption isotherm [56] applied to equilibrium adsorption assuming monolayer adsorption onto a surface with a finite number of identical sites and is represented as follows: Ce qe

=

1 qmax b

+

Ce qmax

.

(18)

22

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

Fig. 17. Langmuir plot for the adsorption of metal ions. Cd(II) pH 5, adsorbent dosage 7.5 g/L, contact time 2 h. Pb(II) pH 5, adsorbent dosage 7.5 g/L, contact time 2 h.

Linear plots of C e /qe vs C e (Fig. 17) were employed to determine the value of qmax (mg/g) and b (L/mg). The data obtained with the correlation coefficients (r 2 ) were listed in Table 6 for both the metal ions. Weber and Chakraborti [57] expressed the essential characteristics and the feasibility of the Langmuir isotherm in terms of a dimensionless constant separation factor or equilibrium parameter R L , which is defined [58] as: RL =

1 1 + bC 0

(19)

.

The R L value indicates the shape of the isotherm as follows: R L value

Type of isotherm

RL > 1

Unfavorable

RL = 1

Linear

0 < RL < 1

Favorable

RL = 0

Irreversible

According to McKay et al. [59], R L values between 0 and 1 indicate favorable adsorption. The R L values for the adsorption on activated alumina at initial concentration of 10 mg/L (lowest concentration studied) and 100 mg/L (highest concentration studied) are 0.419 and 0.067 respectively for Cd(II) adsorption and they were 0.661 and 0.162 respectively for Pb(II) adsorption. The data obtained represent a favorable adsorption.

Fig. 18. Freundlich plot for the adsorption of metal ions Cd(II) pH 5, adsorbent dosage 7.5 g/L, contact time 2 h. Pb(II) pH 5, adsorbent dosage 7.5 g/L, contact time 2 h.

3.10.2. Freundlich adsorption isotherm The Freundlich adsorption isotherm [60] is an empirical equation employed to describe heterogeneous systems, in which it is characterized by the heterogeneity factor, n. The linear form of Freundlich adsorption isotherm takes the following form: log qe = log K f +

1 n

(20)

log C e .

The Freundlich isotherm constants K f and n are constants incorporating factors affecting the adsorption process like adsorption capacity and intensity of adsorption. The constants K f and n were calculated from Eq. (20) and Freundlich plots (Fig. 18). The values for Freundlich constants and correlation coefficients (r 2 ) for the adsorption process are presented in Table 6. The values of n between 1 and 10 (i.e. 1/n less than 1) represent a favorable adsorption. The values of n, which reflect the intensity of adsorption, also reflected the same trend. The n values obtained for the adsorption process represented a beneficial adsorption.

χe2 =

 (qe − qem )2 qem

(21)

.

From Table 6, it is seen that experimental data including chisquare test, carried out using Eq. (21), for both Cd(II) and Pb(II) adsorption are better fitted to Langmuir than Freundlich adsorption isotherm. Therefore uptake of both Cd(II) and Pb(II) ions preferably follows the monolayer adsorption process.

Table 6 Langmuir and Freundlich adsorption isotherm constants. Heavy metal

Cd(II) Pb(II)

Langmuir constants

Freundlich constants

qmax (mg g−1 )

b (L mg−1 )

r2

χ2

SD

Kf (mg/g)/(mg/L)1/n

n

r2

χ2

SD

35.06 83.33

0.1389 0.0515

0.994 0.974

0.965 0.075

0.525 0.012

4.34 3.82

1.81 1.44

0.978 0.994

1.69 0.339

0.085 0.043

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

Fig. 19. Dubinin–Radushkevich isotherm of metal ions. Cd(II) pH 5, adsorbent dosage 7.5 g/L, contact time 2 h. Pb(II) pH 5, adsorbent dosage 7.5 g/L, contact time 2 h.

3.10.3. Dubinin–Radushkevich (D–R) isotherm The D–R isotherm [61] was employed in the following linear form: ln C abs = ln X m − λε 2 .

(22)

The Polanyi potential [62], ε , can be expressed as



ε = R T ln 1 +

1 Ce



.

(23)

A plot of ln C abs vs ε 2 in Fig. 19 gave a straight line from which values of λ for Cd(II) and Pb(II) was evaluated. Using the calculated value of λ, it was possible to evaluate the mean sorption energy, E, from E=√

1

−2λ

.

(24)

Although the Freundlich isotherm provides the information about the surface heterogeneity and the exponential distribution of the active sites and their energies, it does not predict any saturation of the surface of the adsorbent by the adsorbate. Hence, infinite surface coverage could be predicted mathematically. In contrast, D–R isotherm relates the heterogeneity of energies close to the adsorbent surface. If a very small sub-region of the sorption surface is chosen and assumed √ to be approximately by the Langmuir isotherm, the quantity λ can be related to the mean sorption energy, E, which is the free energy for the transfer of 1 mol of metal ions from the infinity to the surface of the adsorbent. The estimated value of E was 11.85 and 11.8 kJ/mol for Cd(II) and Pb(II) adsorption respectively, which is the range expected for chemisorptions (8–16 kJ/mol) [63]. 3.11. Adsorption thermodynamics 3.11.1. Effect of temperature on thermodynamic properties for adsorption of Cd(II) and Pb(II) Adsorption experiments to study the effect of temperature were carried out at 30–50 ◦ C at optimum pH value of 5 and adsorbent dosage level of 7.5 g/L. The equilibrium contact time for adsorption was maintained at 2 h. The increase or decrease in adsorption with

23

Fig. 20. Determination of thermodynamic parameter for the adsorption of Cd(II) and Pb(II).

rise in temperature may be due to the strengthening or weakening of adsorptive forces between the active sites of the adsorbents and adsorbate species and between the adjacent molecules of the adsorbed phase respectively. The variation in the extent of adsorption with respect to temperature has been explained based on thermodynamic parameters viz. changes in standard free energy, enthalpy and entropy. The thermodynamic equilibrium constant (K C0 ) for each metal ion was calculated by determining the apparent equilibrium constant K C at different initial concentration and extrapolating to zero. Ca . (25) K C = Ce The Gibbs free energy (G 0 ) for the adsorption process was obtained using the formula

G 0 = − R T ln K C0 .

(26)

For determination of enthalpy and entropy change of the adsorption process by studying the temperature dependence of adsorption of each metal ions on the activated alumina was evaluated using the equation ln K C0 = −

H 0

+

S0

. (27) RT R From the slope and intercept of the plot (Fig. 20), the values of  H 0 and  S 0 had been computed. The values of the thermodynamic parameters thus calculated are recorded in Table 7. From Table 7 it is clear that Cd(II) and Pb(II) adsorptions are exothermic and endothermic in nature respectively. The heat of adsorption value between 20 and 400 kJ/mol indicates the chemisorption process. Hence both the metal ion adsorption is chemical in nature [64]. The negative value  S 0 (Table 7) for the adsorption of Cd(II) suggests decreased randomness at the solid/solution interface and a decrease in the degree of freedom of the adsorbed species. The positive value of  S 0 is an indicative of increased randomness at the adsorbent–adsorbate interface during the adsorption of Pb(II). The negative value of G 0 confirms the feasibility of both the metal ion adsorption process.

24

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

Table 7 Changes in Gibbs free energy (G 0 ), heat of adsorption ( H 0 ) and entropy ( S 0 ). T (K)

−G 0

−0.051

303 313 323

5.989 5.852 4.951

0.912

0.087

303 313 323

5.022 6.228 6.745

0.921

H 0 (kJ/mol)

S0

Cd(II)

−21.59

Pb(II)

21.22

Metal ion

(kJ/mol K)

r2

(kJ/mol)

Table 8 Desorption of Cd(II) and Pb(II) from metal loaded activated alumina. Conc. of HCl (M)

Percentage desorption of Cd(II) (%)

Conc. of HNO3 (M)

Percentage desorption of Pb(II) (%)

0.01 0.025 0.05 0.075 0.1 0.125 0.15 0.20

20.20 25.2 70.85 92.60 99.0 99.15 99.2 99.2

0.01 0.025 0.05 0.075 0.10 0.125 0.150 0.20

62.35 80.5 94.55 96.80 97.9 98.7 98.8 98.9

3.12. Desorption studies for Cd(II) and Pb(II)–activated alumina system Desorption studies were carried out to understand the regenerative capability of the activated alumina. Various concentration of mineral acids, 0.01–0.15 M HCl and 0.01–0.15 M HNO3 , were used as desorbing media for the batch desorption studies from the Cd(II) and Pb(II) loaded activated alumina respectively. Desorption experiments were performed maintaining the process condition similar to the batch experiments. Results of batch desorption experiment studies were depicted in Table 8. It was evident from Table 8 that maximum desorption efficiency was 99.1% for Cd(II) with 0.5 M HCl and 98.7% for Pb(II) using 0.125 M HNO3 . Hydrogen ions may replace the Cd(II) and Pb(II) ions on the metal loaded adsorbent thus functioning as a cation exchanger. The metal ion loaded activated alumina creates disposal problem as it is hazardous in nature. This problem may be overcome to some extent by using elution methods. The elution of the heavy metals allows recovery of the metal ions in the concentrated solution and the regenerated adsorbents. The concentrated metal solution may be suitable for recovery of the metal. The regenerated

adsorbent may be recycled for reuse and ultimately the adsorbents must be incinerated. Table 9 shows the performance of the regenerated adsorbent. Adsorption/desorption cycle of the activated alumina decreases as the number of cycle increases. More than 90% metal ion removal is possible using three cycles for Cd(II) and two cycles for Pb(II). 3.13. Application studies using industrial effluents Industrial effluent containing Cd(II) was collected from electroplating unit located near Kolkata, India. Application studies for Pb(II) containing effluents were carried out using an effluent sample from a battery manufacturing unit at Shyamnagar, near Kolkata, India. The characteristics of Cd(II) and Pb(II) containing effluent samples were shown in Tables 10 and 11 respectively. Batch adsorption studies using Cd(II) and Pb(II) effluents was carried under optimum conditions similar to the batch adsorption studies using synthetic sample. After batch adsorption Cd(II) and Pb(II) percentage removal was found to be more than 98% and 98.5%. The results of the application studies were shown in Tables 10 and 11 respectively. Cd(II) and Pb(II) concentrations in the treated effluents were 0.05 and 0.04 mg/L respectively thereby meeting the IS 10500 1992 [6]. 4. Summary In this study, batch adsorption experiments for the removal of Cd(II) and Pb(II) from aqueous solutions have been carried out using activated alumina as synthetic adsorbent. The adsorption characteristics have been examined at different pH values, initial metal ion concentrations, contact time and different adsorbent dosage levels. The obtained results can be summarized as follows: 1. The pH experiments showed that the governing factors affecting the adsorption characteristics of all adsorbents are competition of the H+ ions with metal ions at low pH values, maximum adsorption at pH 5–6 and at higher pH precipitation of hydroxyl species onto the adsorbents is more predominant (pH 6–11). 2. Increase in mass of adsorbent leads to increase in metal ion adsorption due to increase in number of adsorption sites. Maximum uptake was obtained at adsorbent dose of 7.5 g/L for both Cd(II) and Pb(II), which may be considered as optimum adsorbent dosage level at the specified conditions.

Table 9 Performance of fresh and recycled activated alumina. Adsorbent

Fresh First recycle Second recycle Third recycle

Conc. of Cd(II) solution (mg/L) Initial

Final

Percent removal (%)

Initial

Conc. of Cd(II) solution (mg/L) Final

Percent removal (%)

10 10 10 10

0.264 0.575 0.979 1.948

97.36 94.25 90.21 80.52

10 10 10 10

0.126 0.790 1.397 2.743

98.74 92.10 86.03 72.57

Table 10 Application studies of Cd(II) containing effluent using activated alumina. Test parameter

Untreated effluent

Treated effluent

Remarks

pH Conductivity (μmhos/cm) Cd(II) (mg/L)

4.7 2760 3.8

5.8 2605 0.05

Fe (mg/L) Ca (mg/L) Mg (mg/L) Chloride (mg/L) TSS (mg/L)

1.06 180 48 28 32

0.65 150 26 26 29

– – Successfully meet the IS 10500 1992 [6] norms of Cd(II) for discharge water – – – – –

T.K. Naiya et al. / Journal of Colloid and Interface Science 333 (2009) 14–26

25

Table 11 Application studies of Pb(II) containing effluent using activated alumina. Test parameter

Untreated effluent

Treated effluent

Remarks

pH Conductivity (μmhos/cm) Pb(II) (mg/L)

2.7 1737 2.84

6.15 1590 0.04

Fe (mg/L) Ca (mg/L) Mg (mg/L) Chloride (mg/L) TSS (mg/L)

1.2 214 64 18 26

0.72 175 54 15.6 22

– – Successfully meet the IS 10500 1992 norms [6] of Pb(II) for discharge water – – –

3. Adsorption of Cd(II) and Pb(II) were found to follow the pseudo-second order model. 4. The effective diffusivity calculated using Vermeulen’s approximation was indicate that the interaction between both Cd(II) and Pb(II) on activated alumina are chemical in nature. 5. Langmuir adsorption model was better fitted for the adsorption of both the adsorbate. The highest monolayer adsorption capacity was obtained 35.06 mg/g for adsorption of Cd(II) and 83.33 mg/g at optimum pH 5.0. 6. Sorption energy calculated from Dubinin–Radushkevich (D–R) isotherm indicated that the adsorption processes are chemical in nature for both the metal ion. 7. Thermodynamic parameters studies showed that the both Cd(II) and Pb(II) ions adsorption were spontaneous in nature. The adsorption process is exothermic for Cd(II) and endothermic for Pb(II). 8. Dilute HCl and HNO3 solutions have good potential to dissolve Cd(II) and Pb(II) respectively by batch desorption technique maintaining the conditions similar to batch adsorption studies. 9. Greater than 90% metal ion removal is possible using three adsorption/desorption cycles for Cd(II) and two adsorption/desorption cycles for Pb(II). 10. Studies on batch adsorption using industrial effluent indicate that activated alumina has a good potential to remove Cd(II) and Pb(II) from waste water sample. Appendix A. Nomenclature b B C abs Ca Ce C final C initial C0 Ct E G 0 K2 K0 K1 KC Kf K id K bq M m n

Langmuir constant (L mg−1 ) Time dependent factor Concentration of metal ion on adsorbent (mmol/g) Concentration of metal ion on the adsorbent at equilibrium (mg L−1 ) Concentration of metal ion in solution at equilibrium (mg L−1 ) Final concentration of metal ion in solution (mg L−1 ) Initial concentration of metal ion in solution (mg L−1 ) Initial concentration of metal ion in solution (mg L−1 ) Concentration of metal ion after time t (mg L−1 ) Mean sorption energy (kJ mol−1 ) Gibbs free energy (kJ mol−1 ) Pseudo-second order rate constant of adsorption [(mg/g) min] Constant in Eq. (17) Lagergren rate constant (min−1 ) Thermodynamic equilibrium constant Measure of adsorption capacity (mg/g) Intraparticle rate constant [(mg/g) min1/2 ] Constant obtained by multiplying qmax and b Mass of the adsorbent per unit volume (g L−1 ) Amount of adsorbent added in g Freundlich constants, intensity of adsorption

q qe qmax qt qtm qem r2 RL Ss t V F (t ) Xm

Amount adsorb per g of the adsorbent (mg/g) Amount adsorb per g of the adsorbent at equilibrium Maximum adsorption capacity (mg/g) Amount adsorb per g of adsorbent at time t (min) Amount of metal ion adsorbed per g of adsorbent according to kinetic model (mg/g) Amount of metal ion adsorbed per g of adsorbent according to isotherm model (mg/g) Correlation coefficient Separation factor External surface area of the adsorbent per unit volume (m−1 ) Time (min) Volume of the solution (ml) Amount adsorbed per g of adsorbent at time/amount adsorbed per g of adsorbent at equilibrium Maximum adsorption capacity (mmol/g)

Greek letters

α β λ

ε

Constant in Eq. (17) Mass transfer coefficient (m/s) Constant related to energy (mol2 /kJ2 ) Polanyi potential (kJ2 /mol2 )

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