Indian Journal of Chemical Technology Vol. 9, Jan uary 2002, pp. 32-40

A preliminary examination of the adsorption characteristics of Pb(II) ions using sulphurised activated carbon prepared from bagasse pith

K Anoop Krishnan & T S Anirudhan'"

Department of Chemistry, University of Kerala. Kariavattom, Trivandrum 695 58 1, India

Received 30 December 2000: revised 12 November 2001 ; accepted 20 November 2001

Bagasse pith, a sugar indus try waste, has been converted into sulphurised act ivated carbon by carbon isation at 200°C under N2 for 2h, followed by steam acti vation in presence of S02 and H2S at 400°C for 2h. The adsorp tion characteristics of Pb(ll) ions on activated carbon have been examined from aq ueous solutions using batch technique. The effect of ag itat ion period, initial concentration of sorbate, p H, ionic strength, temperature and particle size of the adsorben t has been stud ied to optimise the cond iti ons for maximum removal of Pb( ll ) ions. The maximum removal takes place in the p H range of 4.0-8.0. With an initi al concentrat ion of Pb(ll) at 50 mg/L at 30°C and pH 6.0, its removal has been found to be 99.8 %. T he process of uptake is governed by a pseudo-second-order kinetics. Kinet ic parameters as a function of initia l concentrati on and temperature have been ca lcul ated. Decrease in ionic strength and increase in temperature of the solution has been found to improve the uptake of Pb(ll ). Studies show that the adsorpti on decreases with an increase in particle size of the adsorbent. Sorption data of Pb(ll) in the concentration range 50-1000 mg/L have been correlated with Langmuir isotherm model. Sulphurised ac ti vated carbon had adsorption capacities for Pb(ll) from 200.08 mg/g at 30°C to 243.93 mg/g at 60°C, which is much higher than the values for the adsorbent materials reported in the literature. The adsorbent has been satisfactoril y used for the removal of Pb(ll) from synthetic wastewaters. The adsorbed Pb(ll ) ions arc completely recovered with 0.2 M HCI.

The recovery of lead from water and wastewater has become important environmental issue since this metal is highly toxic to mammals and aquatic lives if discharged into the natural water resources. The source of lead pollution is from the discharge of various industries such as petroleum refinery , e lectroplating, paper and pulp industries , pestic ides and chemical manufacturing industries. 1 Several methods have been suggested for the removal of metals from wastewaters incl uding precipitation, solvent extraction, reverse osmosis, complex ing, coagul ation , ion exchange and adsorpt ion. The selection of suitable method for the elimination of metal pollution is determined by economical factors. With suitable adsorbents and under optimum operation conditions, the adsorption process is sti ll an economically appealing method for the removal of metals from wastewater. In recent years adsorption systems using activated carbon have been widely employed in the purification o f metal bearing wastewater. A w ide range of lignocellu losic agricultural by-products has successfully been converted into activated carbon . These include rice husk, coconut coir, j ute stick, sawdust, tea leaves ctc2

·6

•

*For correspondence (E-11Iail: tsani@rcdiffmail.com)

Sugar industry generates large amount of pollution load particularly in terms of suspended solids, organic matter, press mud and bagasse pith. Bagasse pith does not find any use as such and causes disposal problem. If this can be utili zed for wastewater treatment, it may be a new milestone in our economic development. Removal of dyes, phenols, organic matter and metals from wastewater by adsorpt ion o n bagasse fly ash have been studied by a number of workers7

.10

.

Generally, adsorption of metal ions or other contaminants onto lignocellulosic based activated carbon has been limited and surface modification of carbon is desirable to enhance adsorption . In thi s regard, various modification strateg ies have been employed. Yousef and Mostafa 11 reported the steam activation in presence of ZnCh and HN03 oxidant to improve the metal binding capacity. Our laboratory has an ongoing program to develop surface modi fied activated carbons from lignocellulosic materials. Copper and lan thanum impregnated activated carbons from saw du st and coconut husk were found to exhibit very high adsorption capacity for the removal of heavy metals from wastewater 12.u. Since heavy metals, show a high affinity towards sulphur groups, a method of improving adsorption efficiency of activated carbons cou ld be based on immobi lizing sulphur on the carbon. The abi lity of polysulphide

ANOOP KRISHNAN & ANIRUDHAN: ADSORPTION CHARACTERISTICS OF Pb(II) IONS 33

Table 1- Characteri stics of the acti vated carbons

No. Parameter Value SA-C SA-SOr H2S-C

I Apparent density (g/ml) 1.06 1.25

2 Cation exchange capacity 2.06 3.02 (meq/g)

3 Surface area (m2/g) 536.5 500.5

4 pH,rc 5.6 4.3

5 Porosity (ml/g) 0.52 0.43

6 Sulphur content (W %) 8.9

7 Total ac id groups (meq/g) 3.2 2.0

8 Particle size (mesh size) -80+230 -80+230

treated sawdust and coconut husk for the removal of heavy metals from wastewater has been reported in the literature 14

•15

• The purpose of this paper is to explore and test the ability of sulphurised activated carbon prepared from sugar cane bagasse for the removal of Pb(II) ions from wastewater.

Experimental Procedure

Adsorbent preparation The bagasse pith collected from local sugar mill

was dried naturally and was used as precursor for preparing activated carbons. A matri made (India) muffle furnace was used for preparing activated carbon. Precursor was carbonised at 200°C for 2 h (C-200). Steam activation of this carbon was done using the method described in the earlier publication 16

•

Steam produced by a steam generator, entered in the carbon at a rate of 3 mL/min. The sample was then heated at a fixed rate of approximately 10°C/min to 400°C for 2 h. Before utilizing this steam pyrolysed activated carbon (SA-C) was treated with 1.0 M HCI to remove ash content and then washed with distilled water. The material was dried at 100°C and particles of -80+230 mesh size was used. Sulphurised activated carbon was prepared by steam activation of C-200 at 400°C in presence of S02 and H2S for 2 h (SA- S02- H2S-C). Steam was passed continuously at 400°C, however, the gases S02 and H2S were passed intermittently at every 5 min from gas generators. The product was washed with distilled water and dried at 100°C. The particles of -80+230 mesh size was used for adsorption experiments.

The surface area (BET) measurements of the activated carbons were obtained from N2 adsorption at 77K using a Quantasorb surface area analyser (Model QS-7). A mercury porosimeter was used to determine the porosity of the activated carbons . The surface

charge and density of these carbons were determined using potentiometric titration method 17 and specific gravity bottle respectively. Conductometric titration 18

was carried out to determine the total number of acidic groups. The amount of sulphur contained in the adsorbent material was estimated using the method described by Rump and Krist 19

• The results are presented in Table I along with other characteristics.

Adsorption experiments Aqueous Pb(II) solution of 1000 mg/L was

prepared by dissolving accurately weighed quantity of pure lead chloride in distilled water. A weighed amount (100 mg) of adsorbent was shaken together with 50 mL of Pb(II) solution of desired concentration in stoppered glass flask using a water bath shaker maintained at 30°C. The pH and ionic strength of the solution were adjusted to constant values. Samples were then drawn from the bottle at predetermined time intervals and residual metal concentration was estimated using a Perkin Elmer atomic absorption spectrophotometer. The amount of adsorption was calculated based on the difference of the metal concentration in aqueous solution before and after adsorption, the volume of aqueous solution (50 mL) and the weight of the adsorbent (100 mg) . The effect of pH on metal adsorption on SA-C and SA-S02-H2S­C was studied in the pH range of 2.0-8.0. Kinetic experiments were performed using initial metal concentration of 50, 100, 150 and 250 mg/L at different time intervals. The effect of temperature on the extent of adsorption kinetics was also studied using an initial concentration of 250 mg/L at 30,40,50 and 60°C. Adsorption isotherms were also determined using varying concentration of metal ions (50-1000 mg/L). Each test was carried out in duplicate and the average results are presented in the work.

For desorption study 100 mg of metal loaded adsorbent was agitated with 50 mL of the extractant solutions listed on Fig. 9. After mixing 4 h the suspension was filtered and extractant filtrate analysed for Pb(II) . Comparison of this value with the Joss observed in the initial sorption step was used to compute the percentage recovery values.

Results and Discussion

Sorbent characterization Infra-red spectroscopy was used to determine the

surface functional groups of the carbons. Carbon samples SA-C and SA-SOrH2S-C were dried over night to remove any water retained which could

34 INDIAN J. CHEM. TECHNOL., JANUARY 2002

] t\0

• S,\..C

105 • SA-S01-H 1S..C

0.0 1 r-.; 1\aCl 70 0.001 N NaCl

.... 35 E u --u u :t

0 -35 b

-70

-105

-14 0

3 5 7 9 11

pH

Fig. !- Effect of pH on the surface charge density of different activated carbons.

Table 2- FTIR data of sulphuri sed and sulphur free activated carbons

Band position. em·'

3764 3762

2925 2925

2854 2854

1724 1730

1600 1608

1357 1360

11 67

!I ll

460

Possible assignments

0-H stretching of hydroxyl group

C=C-H stretch ing

C=C-H stretching

C=O stretching of -COOH group

C=O stretching of carbonyl group

C-H deforming

C=S stretching

S=O stretching

s-s stretching

interfere with absorption of hydroxy l groups on the surface. This was followed by encapsulation into dry KBr disc. The spectra of the samples were recorded on a Perkin- Elmer Ff-IR spectrophotometer in the wave length range 400-4000 cm·t. Some fundamental absorption frequencies of the carbon samples are shown in Table 2. The asymmetric absorption band observed at 3764 cm· t for SA-C and 3762 cm· 1 for SA-S02-H2S-C indicate the presence of exchangeable -OH groups in both carbons. Sulphurised and su lphur free carbons contain carboxyl (bands at 1724 cm·t for

SA-C and 1730 cm· 1 for SA-SOrH2S-C) functional groups. The IR spectra show bands at 1600 and 1608 cm· 1 for SA-C and SA-S02-H2S-C respectively indicating the presence of conjugated hydrogen bonded carbonyl groups as suggested by Hallum and Drushell 20

. Additional peaks at 1167, 1111 and 460 cm·t in the spectra of SA-S02-H2S-C represent the C=S, S=O and S-S stretching vibrations which are due to the sulphur groups bonded to activated carbon21

•

The variation of 0"0 as a function of pH at an ionic strength of 0.01 and 0.001 N NaCl was determined using potentiometric titration method 17 employi ng the following equation

... (I)

where F is the Faraday constant, CA and C8 are the concentration of acid and alkali after each addition during ti tration . [H+] and [OH-] are the equilibrium concentration of H+and OH- ions bound to the suspension surface (equ/cm2

) and A is the surface area of suspension in cm2/g. The point of intersection of 0"0

aga inst pH curves for di fferent concentrations of electrolyte gives the pHzpc (Fig. 1 ). The pHzpc of the sulphur free SA-C sample .and SA-S02-H2S-C was

ANOOP KRISHNAN & ANIRUDHAN : ADSORPTION CHARACTERISTICS OF Pb( ll ) IONS 35

120 Sorb<nt dose Equilibrium time

100 Tt:mpc:raturt:

~ . 80

= ·~ 60 0. ... 0

"' "0 40 < • SA-C

• SA-SO,-H,S-C

20 . . 50 mg!L - 100 mg!L

0

0 2 4 6 8 10 12

pH

Fig. 2-Effect of pH on the removal of Pb(ll) by different activated carbons.

found to occur at 5.6 and 4.3 respectively . Based on these results, it is assumed that carbon surface is modified with sulphur groups. Although sulphurised carbon contains sulphur groups or linkage; the surface area and porosity values remain considerably higher with respect to SA-C. Only a small decrease in surface area and porosity was observed for sulphurised carbon. This suggests that most of the functional groups or pores are not reacted or clogged with sulphur atoms.

Effect of pH on metal adsorption The influence of pH on the adsorption of Pb(II) by

sulphur free and sulphurised carbons is shown in Fig. 2. It is clear that sulphurised carbon is effective for the quantitative removal of Pb(II) over the pH range 4.0-8.0. However, sulphur free carbon is effective within a narrow pH range of 6.0-8.0. Below and above the pH range adsorption is minimum. The maximum adsorption of 99.76 % (24.94 mg/g) and 98.02 % (49.01 mg/g) is observed with sulphurised carbon at pH 6.0 for an initial concentration of 50 and 100 mg/L respectively. On comparing the adsorption of Pb(II) on sulphur free and sulphurised carbons under identical conditions it is fo und that in the latter case adsorption is about 18.92 and 27.30% more than the former. Perusal of the literature on Pb(II) speciation diagram22 shows that in the presence of cr the dominant Pb(II) species at pH >6.0 is Pb(OHh and at pH <6.0 is Pb2+ and Pb(OHt. The increase in Pb(II) adsorption above pH 6.0 for both sulphur free and sulphurised carbons may be due to the retention of Pb(OHh species in the micropores of the carbon particles23

• The maximum sorption efficiency in the pH range 4.0-6.0 for su lphurised carbon may be due

to the interaction of Pb2+ or Pb(OHt with surface sulphur groups present in sulphurised carbon. According to Pearson theorl 4

, during acid-base reaction, hard acids prefer to coordinate with hard base and soft ac ids to soft bases. Positively charged Pb(II) species are soft acids and as a rule the interaction of Pb2+ or Pb(OHt with surface sulphur groups (soft bases) is likely to favour at pH range 4.0-6.0. The sites responsible for sorption process are not exclusively due to the sulphur groups, other sites on the carbon surface can also contribute to the adsorption process. At low pH, particularly below the pH of zero point charge, the positively charged Pb2+ or Pb(OHt species present in the solution may exchange with H+ from -COOH groups of the carbon. A surface complexation model has also been proposed by Corapcioglu and Huang23 for metal adsorption on activated carbon. Based on the model the following reactions are proposed for the adsorption of Pb(ll) on activated carbon.

.. . (2)

. .. (3)

or

... (5)

Here, surface hydroxyl groups, i.e., _co- and .COH are Lewis bases; whereas the metal ions are Lewis acids. The data clearly indicate that steam activated carbon in presence of S02 and H2S is an effective method to improve the adsorption efficiency of the carbon. As such subsequent investigations with Pb(II) ions were made only with sulphurised carbon.

Effect of contact time and initial concentration The uptake of metal ions from aqueous media by

sulphurised carbon increase with time and at some point it reaches a constant value beyond which no more of the metal is removed from solution (Fig. 3). A quasi-stationary state was obtained within 4 h of shaking and is independent of initial concentration. The amount of metal adsorbed at thi s equil ibrium time reflects the maximum adsorption capacity of the adsorbent under particular operating conditions. For practical consideration 4 h time was presumed to represent the equilibrium time for the adsorption of

36 INDI AN J. CHEM . TECHNOL., JANUARY 2002

150 Sorbent dose Initial concn.

120 pH

90 F ~

60 t)J)

e.o e 30

1 r.. 0 0 "' ~ Sorbent dose 04

Temperature Q ~ 100 pH 0 E -( 80 -

60 -40 I-F*'

20 ~

0

0 50

: 2 giL : 250 mgiL :6.0

-

: 2 giL : 30'C :6.0

+ 50 mg!L ... IOO mg!L

100 150

Time, min

(B)

+ 30°C .A. 40°C Ill SO"C • 60°C

(A)

II I 50 mg!L

• 250 mg!L

200 250 300

Fig. }-Effect of agitation time on the adsorption of Pb(II ) onto SA-S02-H2S-C at (A) different concentrations and (B) different temperatures.

metal ions. When the initial concentration increased from 50 to 250 mg/L, the amount of Pb(II) uptake per unit weight of the adsorbent (mg/g) increases from 24.93 to 96.78 mg/g whereas the percentage adsorption decreases from 99.72 to 77.42 %. It is also evident from the figure that for all concentrations the adsorption is rapid in the initial stages and then gradually decreases until the equilibrium IS

established in 4 h.

Effect of temperature Fig. 4 shows the experimental results obtained from

a series of contact time studies for Pb(Il) adsorption with an initial concentration of 250 mg/L at pH 6.0 in which temperature was varied from 30 to 60°C. The adsorption of metal ions has been found to increase from 96.78 mg/g (77.42 %) to 116.21 mg/g (92.96%) with an increase in temperature from 30 to 60°C. The equilibrium time is independent of solution temperature. The increase in adsorption capacity of sulphurised activated carbon with temperature indicates an endothermic process. The increase in

Sorbent du~~ · 2 giL (R) lnltlal CUIICI\ . . 250 mg/L pll 1>0

2

+ 30°C

!)..[ ... 40"C

E • SO'T 01 • 60''C c

E 0 Sorhcm do!)r: ' 2 g/1 (A )

0" Temper at uri! JO"C ...... 10 pi I 6 0

• 50 mg/1 X ... IOOmli/L

• 150 mg/1.

6 • 250 m~/L

4

2

()

() 50 100 I 50 200 250 300

Time, min

Fig. 4--Pseudo-second-order kinetic plots for the sorption of Pb(II) onto SA-S02-H2S-C at (A) different concentrat ions and (B) different temperatures.

adsorption with temperature may be attributed to either increase in the number of active surface sites available for adsorption on the adsorbent or the desolvation of the adsorbing species. At higher temperature the possibility of diffus ion of solute within the pores of the adsorbent may not ruled out as reported by earlier workers for the adsorption of

. . d b 2s 26 D'ff . . catiOns on activate car ons · . I . usion process IS

an endothermic process and hence obviously greater adsorption will be observed at higher temperatures.

Adsorption kinetics The kinetics is another important aspect in any

evaluation of sorption as a unit operation . The kinetic constants of metal adsorption, which could be used to optimise the residence time of an industrial wastewater in sulphurised activated carbon column, were computed using the above experimental data. Earlier workers23

'27 proposed a surface complexation

mechanism for heavy metal removal onto activated carbons. By virtue of its surface composition, sulphurised activated carbon is assumed to have C=S,

ANOOP KRI SHNAN & AN IRUDHAN : ADSORPTION CHAR ACTERISTICS OF Pb( II ) IONS 37

Tab le 3- Kinetic parameters for the sorpt ion of Pb( II) on SA-S02-H2S-C

Co k, qe h mg/L g/mg mi n mg/g mg/g mi n

50 7.0 Ix l0.3 24 .93 4.37

100 3.81 x 1 o·3 49. 11 9. 19

150 3.15x l 0"3 68.54 14.80

250 2.9 1x l0.3 96.78 27.26

S=O, S-S, C=O, C-OH and C- o · groups. The si mplest way to describe the kinetics of metal removal can be expressed as

. . . (6)

where P is the number of active si tes occupied on the adsorbent, M is the concentration of free metal in solution. k1 and k2 are the adsorption and desorption rate constants respectively. The rate equation for the sorption reaction is expressed as

.. . (7)

where nr1 and n pe are the number of active sites occupied on the sorbent at time t and the number of the equilibrium sites available on the adsorbent. To calculate the rate constant k 1 for the adsorption process, following equation developed by Ho and McKa/8 is used,

t 1 t -=--+-ql klqe 2 qe

.. . (8)

where qe is the amount of metal ion adsorbed at equilibrium (mg/g) and q1 is the amount of metal ion on the adsorbent at any time t (mglg). The product k1q/ is actually the initial sorption rate represented as h = k1qe2

• The straight line plots of t!q, versus t at different concentrations and temperatures (Fig. 4) suggest the applicability of Ho and McKay equation to the present system and also explain that the process of adsorption follows pseudo-second-order kinetics. The values of k1, h and qc were calculated from the slope and intercepts of the plots and are presented in Table 3. The initial sorption rate, h increases with an increase in the initial metal concentration while h raises from 4.37 to 27.26 mg/g min, the initi al concentration Co varies from 50 to 250 mg/L.

Temperature k, qc h oc g/mg min mg/g mg/g min

30 2.9 1x l0.3 96.78 27.26

40 3.95x io·3 99 .98 39.48

50 4.87x l0"3 109.11 57.98

60 6.l lx l0·3 11 6.21 82.5 1

Simi larl y the values of rate constant kt. were found to increase from 2.9 l x l0.3 to 7.0 l x l0.3 g/mg min for a decrease in the initial Pb(Il) concentration from 250 to 50 mg/L, as would be anticipated. It also appears from the Table that the values of h increase with increase in temperature. The values of h increase from 27.26 to 82.51 mg/g min with the increase in temperature from 30 to 60°C. The same variation in temperature, the value of k1 increases from 2.9 lx l0.3

to 6.1lx 10·3 g/mg min. The perusal of data in Table 3 reveals that for the equilibrium time, the metal ion adsorbed qc is higher at higher temperature and for greater values of initial metal ion concentration.

Since the rate constant is an important design and kinetic parameter, it is useful if k1 can be correlated against any particular system variable, in this case, initial metal concentration. The linearised CJk 1 and Co values exhibited a correlation coefficient 0.98. The relationship between k1 and Co can be expressed as

c k = - 0 1 3.96xl02 ~0 -l.27xl04

... (9)

This fundamental equation can be used to predict the k1 values for any concentration with in the test limit. An examination of temperature effects on the rate at which Pb(II) is sorbed from solution also allows for the evaluation of activation energy (Ea) for the sorption reaction . The value of Ea was determined using the Arrhenius equation. A plot of In k 1 versus liT was found to be linear. From the slope Ea value for the sorption process was calculated to be -20.12 kJ/mol. Relatively low Ea value further confirmed that Pb(II) adsorption is a diffusion controlled process.

Effect of ionic strength Fig. 5 shows the influence of ionic strength on the

removal of Pb(Il) ions by sulphurised activated carbon. It was found that adsorption of Pb(II) from solution decreased from 98 .22 to 62.03 % with an increase in ionic strength from 0.00 I to 0.5 M NaCl. Adsorption is sensitive to the change in concentration

38 INDIAN J. CHEM. TECHNOL., JANUARY 2002

120

100

~ " 80

.s c. 60 ... 0 <n ~ 40 Sorbent dose :2 giL < Temperature : 30' C

Equi librium time : 4 h 20 Ini tial concn. : ! OOmg/L

pll :6.0

0 0.001 0.005 0.0 1 0.05 0. 1 0.5

Ionic strength, M

Fig. 5---Effect of ionic strength on the adsorption of Pb(Il ) onto SA-S0yH2S-C.

120

100

~ .. 80 c .s::

60 c. ... 0 <n

Sorbent dose :2 giL ~ 40 < Equilibrium time : 4 h Initial concn. : 100 mg/L

20 Temperature : 30°C pH : 6.0

0 50 100 150 200 250 300 350

Geometric mean size, micron

Fig. 6--Effect of particle size on the adsorpt ion of Pb(ll) onto SA-SOy H2S-C.

of the supporting electrolyte if the electrostatic attraction is significant for mEtal removal29

. Based on the results of the experiments , electrostatic attraction plays an important role in the adsorption of metal ions onto activated carbon. At high ionic strength, the increased amount of NaCl can help to swamp the surface of the carbon, wh ich decrease Pb(II) ions access to the carbon surface fo r adsorption . The cation competi tion and modification of Pb(II) speciation are less important factors since NaCl was used in the adjustment of the ionic strength.

Effect of particle size The experimental results for the adsorption of

Pb(II) at different size of carbon (20-80, 80- I 20, I 20-170 and 170-230 mesh size) at a fixed adsorbent dose

5

Sorbent dose Eq uili bri um time

4 pH

~ 3 'iJJ .j -., u 2

0

0 200

; 2 giL :4 h : 6. 0

400

c., mg/L

•

600

+ 30°C .A. 40'C • 50'' C • 60°C

800

Fig. 7-Langmui r plots for the adsorption of Pb( JI ) onto SA-S02-H2S-C at different temperatures.

(2 g/L) and initial concentration of I 00 mg/L are shown in Fig. 6. As shown in Fig. 6, decrease in particle size has a favourable effect on Pb(II) removal by activated carbon. Thi s is because adsorption being a surface phenomenon, the smaller adsorbent size offered comparatively large surface area. Therefore, more functional groups on acti vated carbon can be used to bind metal ions.

Adsorption isotherm The isotherm data were used in the application of

the Langmuir model of adsorption process . The Langmuir isotherm is based upon the assumption of monolayer adsorption onto a surface containing fi nite number of adsorption sites of uni form energies of adsorption with no transmigration of adsorbate in the plane of the surface. The linear fo rm of the Langmuir isotherm i~ represented as

... (10)

where qe is the amount of metal adsorbed in mg/g, C is the equilibrium solution concentration in mg/L and Q0 and b are Langmuir constants related to adsorption capacity and energy of adsorption respectively. The sorption equilibrium values expressed in the Langmuir model are shown in Fig. 7. The values of Q0 and b for all temperatures were calculated using the least-square method (through a regression analysis) and arc presented in Table 4 along with their coefficient of correlation (r2

) . The increase in Q" and b values with ri se of temperature indicates the

ANOOP KRISHNAN & ANIRUDHAN : ADSORPTION CHARACTERI STICS OF Pb(ll) IONS 39

Table 4-- Langmuir constants for the adsorption of Pb(ll ) on SA-SOrH2S-C

Temperature, °C b, Llmg Qo, mg/g r2

30 0.03 11 200.08 0.9871

40 0.0332 217.41 0.9870

50 0.0481 232.56 0.99 18

60 0.0676 243 .93 0.9978

Table 5-Composition of Pb(ll) containing synthetic wastewater

Composition, mg/L 20

Pb: 50.0; Na: 25 .0; K: 25 .0; Mg: 10.0; Ca: 10.0; NH 4: 10.0; Cl: 35.25; NOf 50.0; S04: 40.; N03: 3 1.0; CHr COO: 33.0

120 Temperature : JO'C • 25 mg!L Equ ilibrium time : 4 h • 50 mg/L

100 pH : 6.0

~ w r-

~ 80 .,

...: <'l ... 60 c E "' " 40

20

0

,. ~; :..

i ,'

I''! ,' f. h ;~ ~ 1'.

~

~ "·

I" !~ r: 1:; I ~ ~: li' i .,. ['• .

25 50 75 100 ISO 200 250

Sorbent dose, mg/ SO mL

Fi g. 8--Effect of sorbent dose on the removal of Pb(ll ) from synthetic wastewater by SA-SOrH2S-C.

endothermic nature of adsorption. i.e., adsorption is favoured at higher temperature. This is because the thickness of the boundary layer surrounding the adsorbent decreases with temperature, so the mass transfer resistance of adsorbate in the boundary layer decreases. Thus, the diffusion rate of ions in the external mass transport process increases with temperature.

In order to justify the validity of a treatment process, the adsorptive capacity of this activated carbon must be compared with other sorbents examined for the treatment of Pb(ll) under simi lar conditions. The maximum adsorption capacity, Q0 of sulphurised activated carbon used in the present study at 30°C was found to be 200.08 mg/g which is very much higher than the values reported in the literature. The Q0 values for the adsorption of Pb(II) on china cla/0

, wollastonite30, bagasse fly ash31

, pyrophyllite32,

polymer grafted tin(IV) oxide gel33, carboxymethy­

lated lignin34 (from sugar cane bagasse), polymerised

0 NaNO, NaC\ Na, SO, HCI HN03 H,SO, NaCI+HCI

Reagent

Fig. 9-Effcct of various reagents on the desorption of Pb(ll ) from spent SA-S02-H2S-C.

sawdust35 and tea leaves carbon6 were found to be 0.40, 0.27, 2.73, 33.33, 84.25, 103.84, 124.5 1 and 150.8 1 mg/g respectively.

Test with synthetic wastewater The composition of synthetic wastewater is given

in Table 5. Fig. 8 shows the effect of the mass of activated carbon on the adsorption of Pb(II) from synthetic wastewaters. The removal of Pb(Il) was found to increase with increase in mass of carbon equilibrated with Pb(II) wastewaters. This may be due to the increased adsorbent surface area with increase of mass of adsorbent. It is evident that quantitati ve removal of Pb(ll) ions from 50 mL synthetic samples containing 25 and 50 mg/L metal ions and several other ions a minimum adsorbent dosage of 2 giL is sufficient for the removal of 95.72 and 93.12 % of the Pb(II) respectively. The amounts of adsorbent dose for the complete removal of Pb(II) from the same 50 mL samples were found to be 200 and 250 mg respectively.

Desorption and regeneration studies A suitable method of desorption is needed for the

recovery of metals and regeneration of carbons. For this purpose different reagents as extractant were tested. The results of the experiments are shown in Fig.9. The relatively inexpensive HCI eluted almost all the bound Pb(II) from the carbon. In contrast NaN03, NaCI, Na2S0 4, HN03 , H2S04 and NaCI+HCI were not efficient in the removal of Pb(II) ions. Similar results were obtained in the case of desorption of Hg(II) from polysulphide treated coconut husk 15

.

The hydrogen ions from HCI easily displace Pb(II)

40 INDIAN J. CHEM. TECHNOL., JANUARY 2002

ions bounded to sulphurised activated carbon during the extraction stage. An efficiency of 96.74% was obtained by us ing 0.2 M HCl solution and is therefore suitable fo r the extraction of Pb(ll) into the aqueous phase.

After three cyc les the adsorption capacity of the su lphurised activated carbon was reduced by 9.3 1% and on the other hand recovery of Pb(Il) ions in 0.2 M 1-ICl decreased from 96.74% in the first cycle to 85 .32% in the third cycle. The small fraction of the adsorbed Pb(II ) not recoverable by regeneration presumably represents the metal ions which are bound through stronger interactions and as a result, the adsorption efficiency is reduced in subsequent cycles.

Conclusion The sulphuri sed act ivated carbon appears to be a

suitable adsorbent medium for the removal of Pb(Il) from aqueous solutions, with both kinetics and capacity being highly favourable. Sorption of Pb(II) increases with increase in pH and reaches a plateau value around 4.0. The kinetics of adsorption follows a pseudo-second-order reaction rate with lower concentration and high temperature being favoured for an efficient removal from the aqueous solution . The overall sorption process is complex and includes both surface complexation and ion exchange. Decreasi ng ionic strength, the Pb(II) adsorption is enhanced. The experimental adsorption data fits Langmuir isotherm equation. The process is endothermic in nature. 100% Removal of Pb(II) from synthetic wastewater could be achieved. Desorption of Pb(II) from spent adsorbent was 96.74% at 0.2 M 1-ICI. Additional research is needed to evaluate the sorption efficiency of this adsorbent using live industri al wastewaters . Further investigation will be the extension of this study to other lignocellu los ic materials such as sawdust, coconut husk and jute fiber after converting them to sulphurised activated carbon to check metal ions such as Hg, Cd, Co and Ni to find the optimum adsorption condition and system for each metal ion.

Acknowledgement The authors are thankful to the Professor and Head,

Department of Chemistry, University of Kerala, Trivandrum for providing laboratory facilities.

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