Indian Journal of Chemical Technology Vol. 7, May 2000, pp. 127-131
/.2 1 .,
- '
Adsorption kinetics of
_ _ •a
_ _-cresol
....on aE,tivate<;i carbon from palm seed
a_ coat.
S Rengaraj, R Sivabalan, Banumath' Arabindoo &.YJMurugesan*
-..l - - -
Department of Chemistry, Anna University, Madras 600 025, ndia '- ~ Received 27 July 1999; accepted 13 March 2000
he. activated carbon prepared from palm seed coat by dolomite process showed more than 95% removal of o-cresol from aqueous solution under optimum conditions. The adsorption capacity calculated by Freundlich adsorption isothenn gave 19.58 mglg of o-cresol removal. The kinetic studics revealed the overall rate constant decrease with increasing con- centration of o-cresol. The activated carbon showed reversible uptake of o-cresol and thus have a good application potential for the removallrecovery of o-cresol from aqueous solution. Film diffusion was found to be the rate limiting step. Recovery of used carbon was studied by in silll chemical regeneration technique and showed good res~ ) \ \
vt;.r
Environmental problems arising out of polluted water compounds. The high cost and difficulty IJ1 influence the relevance of separation of organic procurement of activated carbon paved the way to matter. Most of the industries in world dump their produce indigenous low cost activated carbons from wastewater in river, pond or sea which pollutes the agricultural waste like coconut sheilS, rice husk and water and increase the pollutional load. Hence it is groundnut husk6, tamarind nue etc. [n the search for increasingly important to treat polluted water by new and low cost agricultural wastes as source physico-chemical processes. Phenol and its material for activated carbon, attempts have been derivatives in wastewater are the most prevalent made to produce activated carbon from palm seed forms of pollutants in the chemical industriesl. They coal by dolomite process. Extensive characterisation are harmful and are present in effluents of several studies were performed as per the standard methods to major industries such as petroleum refineries, assess the quality of activated carbon. Adsorptive petrcchemicals, synthetic resins, steel, paints, property of the activated carbon was tested by pharmaceuticals, plywood and textile, processing adsorption of o-cresol from aqueous solution. A industries as well as many organo-chemical thorough study of adsorption kinetics revealed the industries2. Discharge of phenolic wastes may cause potentiality of this carbon as an adsorbent.
serious problems as they impart carbolic odour to the water course and can be toxic to fish and human beings3. Some phenolic compounds have been found to accelerate tumour formation and to be ciliostatic4.
Removal of phenolic compounds from wastewater is, therefore, of utmost importance to prevent pollution of water in the receiving water course.
The various processes for the removal of phenolics from the wastewater can be grouped into two broad categories. One category includes processes like microbiological degradation, chemical treatment, incineration etc. which aims at destroying the phenolics while the other category such as solvent extraction and adsorption deals with the recovery of phenolics. Among the various methods, the use of activated carbon in the tertiary level treatment is a promlslJ1g one for the removal of phenolic
* For correspondence
Experimental Procedure
Preparation of activated carbon-Activated carbon from palm seed coat (Borassus flabellyfer L.) was prepared by the procedure reported by Rengaraj et aI.8,9. Initially the materials were washed thoroughly with water to remove earthy matter and dried at I 10°e. The dried material was carbonised with dolomite at 500°C for 3 h. The carbonised material was washed well with dilute hydrochloric acid and water to remove dirt and then dried at I 10°e.
The dried material was subjected to thermal activation in carbon dioxide atmosphere at 850°-900°C for 30 min. The material was ground and sieved for the average diameter of -0.5 mm.
Characterisation of activated carbon-The activated carbon was characterised by adopting the standard procedures1o-12. The moisture content of the carbon was carried out by heating a known weight of
128 TN 01 AN 1. CHEM. TECHNOL.; MAY 2000
120
~IOO
~ '0 ~ > 80
0
E
.,
60...
'0
...
.,
40....
U 0 I 20
0 2
PSCC
Volume • 100 ml
Concentration of a-cresol. 25 mall Carbon do ••• 0.50
Equilibration lime • 24 h
4 6 8
pH
10 12 14
Fig. l-Effect of pH on the o-cresol removal by palm seed coat carbon
120,---,
~ 80
~ ~ 60
...
Q
~ 40 ....
U
o I 20
PSCC
Volume' 100ml
Concentration of a-cresol. 25 mg/l Equmbrotion time • 24 h
p~ • 6.2
O~_,--,__,--,_~,--.--r__r--._~
o 100 200 300
Carbon dosage mg 1100 mL
Fig. 2-Effect of carbon dosage on the removal of o-cresol the sample in an air oven maintained at 105 ± 5°C for about 4 h. The residue was ignited in a muffle furnace at 1000°C for abou't 3 h to determine the ash content.
Iron in the ash content was estimated by atomic absorption spectrophotometer (Perkin-Elmer-2380), Freshly boiled water was digested \yith dried carbon and pH was determined for the clear solution using pH meter (Systronics-335) . Decolourising power of the carbon was determined using methylene blue solution. The amount of carbon required for 90%
removal of phenol was taken as phenol number.
Surface area measurement was' carried out using Micromeritics Pulse Chemisorb 2700 equipment. The characteristics study results are presented in Table I.
Batch experiments-IOO mL each of o-cresol solution (25 mg/L) adjusted to different pH valUles were taken in 250 mL leak-proof reaction bottles and known amount of palm seed coat carbon was added. The solution was equilibrated for 24 h at 27± 1°C in a mechanical shaker. After the equilibration period, the
carbons were filtered and analysed for o-cresol following the standard colorimetric procedure!l, Desorption studies were conducted using dilute sodium hydroxide solution by providing an equilibration time of 24 h. Kinetic experiments were conducted using a known weight of carbon dosage and employing o-cresol concentration in the range of 10-60 mg/L. After regular intervals of time, suitable aliquots were analysed for o-cresol concentration and recorded. The rate constants were calculated by using
h . I . 13
t e conventtona rate expressIOn .
Results and Discussion
The adsorbent prepared from palm seed coat was studied in the removal efficiency of o-cresol from aqueous solution under different experimental conditions like pH, carbon dosage and contact time.
The experimental results and the relevant observations are discussed in the fo'llowing sections.
Effect of pH--In order to optirnise the pH for maximum removal efficiency, experirnents were conducted with 100 mL of 25 mg/L of o-cresol solution containing 0.5 g palm seed coat carbon in the pH range 1-13 and thf' results are depicted in Fig. 1. It is evident that the removal of o-cresol by palm seed coat carbon is maximum in the pH range 3-10 and above pH 10 the removal efficiency decreases. The pH of the aqueous solution of o-cresol affects its uptake on activated carbon and in general the uptake decreases at lower as well as at higher pH valuesl4. At lower pH range the uptake of o-cresol is less due to the presence of
J-t
ions suppressing the ionisation of o-cresol and hence its uptake on polar adsorbents is reduced. In the higher pH range all o-cresol fonn salts which readily ionise leaving negative charge on the 0-cresol amity. At the same time the presence of OH- ions on the adsorbent prevents the uptake of phenolate ionsl5. Hence in the subsequent studies experiments were performed in the solution pH (6.2) .
Effect of carbon dosage-Fig. 2 shows the amount of o-cresol removed as a function of carbon dosage at the solution pH (6.2) . Carbon dosage was varied from 0.5 to 109 and equilibrated for 24 h. From the result it is evident that optimum carbon dosage of 2.0 giL is required for 96% removal of o-cresol (25 mglL) . The results also reveal that o-cresol removal efficiency increases upto the optimum dosage beyond which the removal efficiency is negligiblel6.
Effect of contact time-Known volume of o-cresol in the solution pH with optimum carbon dosage was kept in the mechanical shaker and equilibrated for the
RENGARAJ et at. . ADSORPTION KINETICS OF a-CRESOL 129
120~---.
u
.,
80 i;~ 60
~
'0 ; 40
~
U o I 20
I ·
PSCC
Conc.nlralion rA o-cr ... ol' 2~ m9/L Valum • • IOOmL
Carbon do ••• 29 I L pH • 6.2
2 3 4 5 6 24
Contact time (h)
Fig. 3 -Etfect of contact time for the removal of o-cresol by palm seed coat carbon
120
~tOO
v Cl .&J ~ 80
0
'"
~ 60
PSCC
Volume of NaOH • IOOmL
~ Cl 40
~
U
I
0 20
0 1
0 0.04 0.08 0.12 0.16 0.2
Normality of sodium hydroxide
Fig. 4--Etfect of concentration of sodium hydroxide on the de- sorption of o-cresol from palm seed coat carbon
period ranging 0.25 - 24 h. The experimental adsorption rate curve for o-cresol is shown in Fig. 3.
It is seen that the contact time required to reach equilibration time is 4 h after which there is no change in the percentage removal of o-cresol.
Desorption of o-cresol-Sodium hydroxide has been found to be a good reagent for the desorption of o-cresol. This may be attributed to the formation of sodium salt of o-cresol which facilitates the desorption from the carbon surface'? From the Fig. 4 it is seen that approximately 0.1 N NaOH is required for quantitative desorption of o-cresol from palm seed coat carbon.
Adsorption isotherm-The results reveal that the adsorption of o-cresol on palm seed coat carbon under optimum conditions (Table 2) at room temperature (27 ± 1°C) obeys Freundlich adsorption isotherm.
Freundlich adsorption isotherm represents the relationship between the amollnt of o-cresol adsorbed
Table l-Characteristics of palm seed coat carbon
Control tests Values
Bulk density (glcc) 0.34
Moisture content (%) 18.01
Ash content (%) 1.07
Matter soluble in water (%) 0.65
Matter soluble in acid (%) 1.21
pH 3.68
Decolourising power (mglg) 90.00
Phenol number (mg) 5.53
Ion exchange capacity (mglg) 0.046
Surface area (m2/g) 577.00
Iron content (%) Nil
Table 2--Optimum conditions for maximum removal of o-cresol by activated palm seed coat carbon at 27 ± 10C Dose of adsorbent (giL)
pH
Contact time (h) Percentage removal (%)
Initial concentration of o-cresol = 25 mglL
2.0 6.2 4 96
per unit mass of the adsorbent (x/m) and the concentration of o-cresol at equilibrium (Ce) . k and n are constants representing the adsorption capacity and intensity of adsorption respectively. The data obtained in this study fit Freundlich adsorption isotherm (Fig.
5) . The plot of log (x/m) versus log Ce for various initial" concentration is found to be linear indicating the applicability of Freundlich adsorption isotherm.
Higher values of k (\9.58) indicates greater affinity for o-cresol and the n value (4.0) shows good adsorption character of the carbon's.
Kinetics of adsorption-Kinetics of sorption describe the solute uptake rate which in tum governs the residence time of sorption reaction. It is one of the important characteristics in defining the efficiency of sorption. Hence in the present study, the kinetics of 0-
cresol removal has been carried out to understand the behaviour of this low cost adsorbent. The adsorption of o-cresol from an aqueous solution follows a reversible first order kinetics, when a single species is considered on a heterogeneous surface. The heterogeneous equilibrium between the o-cresol solution and the activated carbon may be expressed as
k, A<::>B
k2
130 INDIAN 1. CHEM. TECHNOL., MAY 2000
1.6
1.2 E
"'-
;;. 0.8 .2
0.4
0
PSCC
-0.8 -0.4 o
log Ce
0.4 0.8
Fig. 5--Freundlich adsorption isotherm for o-cresol and palm seed coat carbon system
o
~-I :l I
;;
-2
__ 10 mv/l ___ 20"'ll/l
~ :3Omv/L -iT- 4O"'II/l
... ~mv/L
..."... 60mv / l
PSCC
-3;----r---,---.----.---~--~--~--~
o 2
Time(h)
3 4
Fig. 6--Kinetic tits for the adsorption of o-cresol and palm seed coat carbon system
where k) is the forward rate constant and kJ is the backward rate constant. A represents
~ -cresol
remaining in the aqueous solution and B represents 0-
cresol adsorbed on the surface of activated carbon.
Since the reaction in both directions is of first order the resultant rate expression for the reaction may
b~
expressed as In (I -ul ) = -kt
where u( = (Co - C() / (Co - Ce)
Co, C( and Co are the concentration in mg/L of o-cresol initially, at any time t and at equilibrium respectively.
The straight line plot of In [I - urJ versus t indicates the adsorption process follow first order kinetics (Fig. 6) . The straight line portions of the curves were used for calculating the slope values which give the overall rate constant k of the process. The forward (k) and backward (k2) rate constants were calculated using the following equation
Table 3 -Rate constants for the removal of o-cresol by palm seed coat carbon
o-cresol Overall rate Forward rate Backward rate
(mg/L) constant constant constant
k = k( + k2 (h'1) kl (h'1) k2 (h·l)
10 0.9793 0.9695 0.0098
20 0.6430 0.6390 0.0040
30 0.4503 0.4470 0.0033
40 0.3200 0.3000 0.0200
50 0.3240 0.3080 0.0160
60 0.3350 0.3120 0.0230
Table 4--Diffusion co-efficients for the removal of o-cresol with palm seed coat carbon system
o-cresol concentration Film diffusion (mg/L) coefficient (cm2/s)
10 0.2569 x 10.8 20 01169'6 x 10.8 30 0.1185x I0·8 40 0.0795 x 10.8
50 0.0815 x 10.8
60 0.0828 x 10.8
Pore diffusion coefficient (cm2/s)
0.9733 x 10.8 0.6391 x 10.8 0.4475 x 10.8 0.3180 x 10.8 0.3220 x 10.8 0.3329 x 10.8
kc = k)/k2 where kc is the equilibrium constant
The data are furnished in Table 3. It is evident that the forward rate constant is much higher than the backward rate constant suggesting that the rate of adsorption is clearly dominant.
In order to. assess the nature of the diffusion process responsible for adsorption of o-cresol on palm seed coat carbon, attempts were made to calculate the coefficients of the process, If film diffusion is to be the rate determining step in the adsorption of o-cresol on palm seed coat carbon surface, the value of film diffusion coefficient (Dr) should be in the range 10.6 -
10.8 cm2/s. If pore diffusion is to be the rate limiting, the pore diffusion coefficient (Op) should be in the
. flO')) 10.13 2/ )9 A ' ,
range 0 - cm s . ssumlllg sphencal geometry for the sorbent, the overall rate constant of the process can be correlated to the pore diffusion coefficient and film diffusion coefficient independently in accordance with the expressions13,
')
Pore diffusion coefficient Dp = 0.03 x ~
I) 12
Film diffusion coefficient Dr = 0.23 x roa x
~
t) /2 c
RENGARAJ et al. .. ADSORPTION KINETICS OF o-CRESOL 131
where ro is the radius of the adsorbent, EJ is the film thickness, c is the amount adsorbed and c is the initial concentration. Employing the appropriate data and the respective overall rate constants, pore and film diffusion coefficients for various concentrations of 0-
cresol were calculated for palm seed coat carbon. The results are presented in Table 4. It is evident that the removal of o-cresol follows film diffusion process since the coefficient values are in the range 10.6 - 10-8 cm2/s
Conclusion
Palm seed coat, an agricultural waste of India, is a potential source for the preparation of activated carbon. The characterisation studies reveals its adsorption capacity. Palm seed coat carbon is capable of removing o-cresol from an aqueous solution to the extent of 96% at the solution pH. The adsorption process obeys Freundlich adsorption isotherm. The values of adsorption capability (k) and intensity of adsorption (n) indicate the, greater affinity for o-cresol by this carbon (Table 2) . The kinetics of adsorption of o-cresol by palm seed coat carbon follow first order reversible kinetics. The low value of k2 (desorption process)' indicates that the adsorbed o-cresol remains almost stable on the adsorbent can be recovered from the adsorbent by desorption with sodium hydroxide.
The regenerated adsorbent can be reactivated and reused. The results also demonstrate that an intraparticle diffusion mechanism plays a significant role in the adsorption and it is apparent that the adsorption rate is controlled by the film diffusion process. It is concluded that the activated carbon prepared from palm seed coat could be exploited for commercial applications in the tertiary level treatment of potable water as well as industrial effluents.
Acknowledgement
This work was supported by a researoh grant from Tamil Nadu State Council for Science and
Technology, Chennai. One of the authors (SR) gratefully acknowledge the Council of Scientific and Industrial Research, New Delhi for the award of Senior Research Fellow.
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