Preparation of activated carbon from dried pods of Prosopis cineraria with zinc chloride activation...

RESEARCH ARTICLE

Preparation of activated carbon from dried pods of Prosopiscineraria with zinc chloride activation for the removalof phenol

Kaushik Nath & Suresh Panchani & M. S. Bhakhar & Sandip Chatrola

Received: 9 July 2012 /Accepted: 12 November 2012 /Published online: 5 December 2012# Springer-Verlag Berlin Heidelberg 2012

Abstract Utilization of agrowaste materials for the produc-tion of activated carbon, as an excellent adsorbent with largesurface area, is well established industrially, for dephenola-tion of wastewater. In the present work, dried pods ofProsopis cineraria—a novel and low-cost agrowaste mate-rial—were used to prepare activated carbons by zinc chlo-ride activation. Batch adsorption experiments were carriedout to study the effects of various physicochemical param-eters such as initial phenol concentration, adsorbent dose,initial solution pH, and temperature. Pseudo-first-ordersecond-order and diffusion kinetic models were used toidentify the possible mechanisms of such adsorption pro-cess. The Langmuir and Freundlich equations were used toanalyze the adsorption equilibrium. Maximum removal ef-ficiency of 86 % was obtained with 25 mgL−1 of initialphenol concentration. The favorable pH for maximum phe-nol adsorption was 4.0. Freundlich equation represented theadsorption equilibrium data more ideally than the Langmuir.The maximum adsorption capacity obtained was 78.32 mgg−1 at a temperature of 30 °C and 25 mgL−1 initial phenolconcentration. The adsorption was spontaneous and endo-thermic. The pseudo-second-order model, an indication ofchemisorption mechanism, fitted the experimental data bet-ter than the pseudo-first-order Lagergren model. Regenera-tion of spent activated carbon was carried out usingPseudomonas putida MTCC 2252 as the phenol-degradingmicroorganism. Maximum regeneration up to 57.5 % wasrecorded, when loaded phenol concentration was 25 mgL−1.The data obtained in this study would be useful in designing

and fabricating an efficient treatment plant for phenol-richeffluents.

Keywords Phenol . Adsorption . Activated carbon .

Bioregeneration . Isotherm

Introduction

Phenol is one of the Environmental Protection Agency(EPA)’s priority pollutants and is a known human carcino-gen of considerable health concern, even at low concentra-tion. It is persistent in the environment when released inlarge quantities, or if it is continuously released from asource (Sathishkumar et al. 2009). Wastewater from coalgasification sites, petrochemical units, pesticide, paper andpulp, resin, tanning, textile, plastic, rubber, pharmaceutical,and dye manufacturing industries are the major sources ofphenol. Ingestion of a small amount of phenol (thresholdlimit value 5 ppmv) by human beings following dermalexposure may cause gastrointestinal irritation, nausea, oreffects ranging from erythema and deep necrosis (Busca etal. 2008). It is toxic for aquatic organisms, and an environ-mental concern level of 0.02 μgL−1 is suggested for waterby the World Health Organization; however, USEPA (1987)recommends a maximum allowable limit of 0.001 mgL−1.

Among the various available technologies from literaturefor the treatment of phenol from water and gaseous streams(Busca et al. 2008), adsorption onto the surface of activatedcarbon and other adsorbents is still by far the most widelyused method for treating domestic and industrial effluents.Large Brunauer–Emmett–Teller (BET) surface area, con-trollable pore structure, thermal stability, and low acid–basereactivity of activated carbon have made it an indispensableadsorbent for wastewater treatment process (Foo andHameed 2010). Activated carbon adsorption has been

Responsible editor: Hailong Wang

K. Nath (*) : S. Panchani :M. S. Bhakhar : S. ChatrolaDepartment of Chemical Engineering,G H Patel College of Engineering & Technology,Vallabh Vidyanagar 388 120 Gujarat, Indiae-mail: [email protected]

Environ Sci Pollut Res (2013) 20:4030–4045DOI 10.1007/s11356-012-1325-y

recommended by the USEPA as one of the best availabletechnologies in removal of organic compounds. Notwith-standing its prolific use, adsorptive removal of phenols fromwastewaters, using commercial activated carbons, has sev-eral problems associated with it. For example, apart from thephenols, there are other constituents like resinous and as-phaltic materials of the wastewater deposited upon the ac-tive carbon and cannot be removed readily there from. Inaddition, a treatment of the activated carbon for reuse ispractically mandated by the high cost of the adsorbent. Inview of the tedious procedures for the preparation andvariable performance of carbon regeneration, low-cost po-tential and single-use adsorbents are desirable.

In comparison with physical activation, chemical activa-tion offers several advantages which include single-stepactivation, low activation temperatures, low activation time,higher yields, and better porous structure (Lim et al. 2010).Chemical activation by ZnCl2, in particular, improves thepore development in the carbon structure, and because of theeffect of chemicals, the yields of carbon are usually high. Inpractice, agrowaste materials have emerged as a better can-didate for the production of commercial activated carbonsover the conventional raw materials such as bituminouscoal, peat, and lignite (Ahmaruzzaman 2008; Lin and Juang2009). Utilization of agricultural wastes is of great signifi-cance in India where more than 200 million tons of agricul-tural residues is generated annually (Reddy 2006). Theutilization of these wastes provides additional employmentsand income to marginal farmers and landless agriculturallaborers, especially in developing countries. Literature sur-vey indicates that there have been many attempts to obtainlow-cost activated carbons from a large number of agricul-tural wastes for adsorptive dephenolation. Some of the re-cent works are cited later in “Results and discussions”section.

Prosopis cineraria, locally known as “Khejri,” is one ofthe indigenous trees in the arid and semi-arid areas of westernRajasthan and Gujarat of India. Its pods grow soon after theflowering season of March to May and develop rapidly toattain full size. The brown-colored dried pods are semicircularto oval in shape (Fig. 1) and generally constitute a major feedrequirement of desert livestock. This work reports the resultsobtained on the preparation of activated carbons from thedried pods of P. cineraria, a novel and low-cost agrowastematerial, with zinc chloride activation and their ability toremove phenol from dilute aqueous solutions. The influencesof several operating parameters, such as initial phenol con-centration, adsorbent dose, initial solution pH, and tempera-ture on batch adsorption, were investigated. Pseudo-first-order, second-order, and diffusion kinetic models were usedto identify the possible mechanisms of such adsorption pro-cess. The Langmuir and Freundlich equations were used toanalyze the adsorption equilibrium. Bioregeneration of the

spent carbon was carried out using Pseudomonas putidaMTCC 2252 (DSM 549).

Materials and methods

Precursor materials

Pods of P. cineraria, collected from western Gujarat, werefirst washed with distilled water to remove the water-solubleadherent impurities. This was followed by drying in an airoven at 80 °C for 6 h to get rid of the moisture and othervolatile impurities. The dried seeds were grounded with ballmill and sieved to a particle size range of 150 to 200 μmprior to activation. The finer fractions were used for char-acterization and production of activated carbons. The prop-erties of the precursor materials are given in Table 1.

Preparation of activated carbon

Chemical activation of the powdered seeds was done withzinc chloride (ZnCl2). An important factor in the ZnCl2 acti-vation is the degree of impregnation (the ratio of the weight ofactive agent added to the weight of carbonizing material)(Mohanty et al. 2005a). Carbons were prepared from 1.0 to0.25 at 700 °C for 1 h. Ten grams of dried seeds was wellmixed with 100 mL of concentrated solution that contained10 g of ZnCl2. The mixing was performed at 50 °C for 1 h.After mixing, the slurry was subjected to vacuum drying at100 °C for 24 h. The resulting chemical-loaded samples wereplaced in a crucible and heated to the final carbonizationtemperature under a nitrogen flow rate of 150 mLmin−1

STP. The products were washed sequentially with 0.5 NHCl, hot water, and finally cold distilled water to removeadhered impurities and then dried at 110 °C.

Characterization of activated carbon

The moisture and ash content of the activated carbon wascharacterized following standard procedures (APHA 1989).To determine its bulk density, activated carbon powder (200mesh sieve) was taken in a 10-mL calibrated measuringcylinder of about 1.0 cm diameter. Sufficient quantity ofcarbon powder was added to occupy a volume of 10 mLunder the condition, and it was subsequently weighed. Thebulk density has been expressed as grams per liter, i.e., theweight of the activated carbon filling a graduated cylinderon gentle tapping, divided by the volume of the cylinder(10 mL). Freshly boiled water was digested with driedcarbon, and the pH was estimated for the clear solutionusing a pH meter (Systronics-335). Decolorizing power ofthe carbon was determined using a methylene blue solution.The amount of carbon required for 90 % removal of phenol

Environ Sci Pollut Res (2013) 20:4030–4045 4031

was taken as the phenol number. Iodine number is definedas milligrams of iodine adsorbed by a gram of carbon, usingthe procedure as stated in APHA (1989). Iodine number iscommonly used in industries as a measure of adsorptioncapacity, reflecting the amount of micropores in the carbon.

Iodine number approximates the internal surface area ofactivated carbon and measure availability of pores greaterthan 1.0 nm in diameter. The BET surface area was calcu-lated from N2 adsorption isotherms by using the BET equa-tion. The surface characteristics of the activated carbonswere analyzed using scanning electron microscopy (SEM)(ESEM EDAX XL-30). The surface organic functionalgroups were studied by Fourier transform infrared spectros-copy (Perkin Elmer Spectrum GX). The spectra wererecorded from a wave number of 400–4,000 cm−1. The totalpore volume (Vtotal) and the average pore diameter (dav)were derived from the Barrett, Joyner, and Halenda (BJH)method. All the chemicals used were of analytical reagentgrade procured from Sigma, USA and Merck, India. All thesolutions were prepared in double distilled water.

Fig. 1 Photograph ofP. cineraria plant and itsdry pods

Table 1 Propertiesof the precursorP. cineraria

Parameters Values

C (%) 44.43

H (%) 5.52

N (%) 3.1

Moisture (%) 3.4

Ash content (%) <1

Volatile matter (%) 23

4032 Environ Sci Pollut Res (2013) 20:4030–4045

Batch adsorption study

Batch adsorption kinetics and equilibrium studies were carriedout using the bottle point isotherm technique by placing aknown quantity of the adsorbent in glass bottles containing100 mL of an aqueous solution of phenol with a predeterminedconcentration. All chemicals and reagents used were of analyt-ical grade. All experiments were conducted using double dis-tilled water. Stock phenol solution of strength 1,000 mgL−1

was prepared by diluting 0.93 mL standard phenol solution to1 L with double distilled water. Fresh stock solution, as re-quired, was prepared every day andwas stored in a brown colorglass container to prevent photo-oxidation. The bottles wereplaced over a magnetic stirrer, and temperature was maintaineduntil equilibrium was attained. At the end of adsorption pro-cess, the adsorbent particles were filtered out throughWhatman42 filter paper, and the equilibrium concentration of phenol inthe supernatant was then analyzed. For pH study, the pH of thesolution was adjusted using dilute NaOH and sulfuric acid. Theadsorption amount was calculated as follows:

q ¼ v c1�c2ð Þw

ð1Þ

where q is the adsorption amount (in milligrams per gram),w is the weight (in grams) of the activated carbon preparedfrom P. cineraria pods, v is the volume of solution, and c1 andc2 are the concentrations (in milligrams per liter) of phenolbefore and after adsorption, respectively. After regular inter-vals of time, suitable aliquots were analyzed for phenol con-centration and recorded.

Estimation of phenol concentration

The concentration of residual phenol in the sorption mediumwas determined spectrophotometrically (model Systronics106) based on the rapid condensation of phenol with 4-aminoantipyrene, followed by oxidation with potassium ferri-cyanide (12) The absorbance of the colored complex of phe-nol with 4-aminoantipyrene was read at wavelength λ0500 nm. The pH of the feed solution was measured by a pHelectrode (Systronics).

Langmuir and Freundlich isotherms

The Langmuir and Freundlich models are most widely usedto describe the experimental data of adsorption isotherms. Inthe present work, both the models were used to describe therelationship between the amount of phenol adsorbed and itsequilibrium concentration for activated carbon. The Lang-muir model is expressed as:

qe ¼ QbCe

1þ bCeð2Þ

A linear form of this equation is

Ce

qe¼ 1

Qbþ Ce

Qð3Þ

where qe is the amount of adsorbate adsorbed per unitweight of adsorbent (in milligrams per gram) and Ce is theequilibrium concentration of the adsorbate (in milligramsper liter). The constants Q and b are Langmuir constants.The values of Q and b are calculated from the intercept andslope of the plot of Ce/qe versus Ce.

The relationship between the phenol uptake capacity qe(in milligrams per gram) of adsorbent and the residualphenol concentration Ce (milligrams per liter) at equilibriumis given by the Freundlich isotherm as:

1nqe ¼ 1nk þ 1

n1nCe ð4Þ

where the intercept ln k is a measure of adsorbent capacityand the slope 1/n is the sorption intensity. The values of kand n were calculated from the intercept and slope of theplot of ln qe versus ln Ce, respectively.

Adsorption kinetics

In the present study, adsorption of phenol on activatedcarbon prepared from P. cineraria pods has been describedby first- and pseudo-second-order models and also by adiffusion model.

Pseudo-first-order model

Lagergren proposed a pseudo-first-order kinetic model (Ho2004). The integral form of the model is

log qe � qð Þ ¼ log qe � K1

2:303t ð5Þ

where q is the amount of phenol sorbed (in milligramsper gram) at time t (in minutes), qe is the amount ofphenol adsorbed at equilibrium (in milligrams per gram),and K1 is the equilibrium rate constant of pseudo-first-order adsorption (in minutes). The plot of log (qe−qt)versus t gives a straight line for the first-order adsorptionkinetics.

In the non-linear differential form of pseudo-first-order

equation d qtd t ¼ k1 qe � qtð Þ� �

, the parameter k1 (qe−qt) doesnot represent the number of available sites, and the param-eter log (qe) is not always equal to the intercept of the plot oflog (qe−qt) versus t, whereas in a true first-order model, thevalue of log (qe) should be equal to the intercept. Thus,pseudo-first-order kinetic Eq. (5) is used for estimating k1alone, which is considered as mass transfer coefficient in thedesign calculations (Gupta and Babu 2009).


Pseudo-second-order model

The expression for the pseudo-second-order rate equation isgiven by

d qtd t

¼ k2 qe � qtð Þ2 ð6Þ

where k2 (in grams per milligram per minute) is the second-order rate constant for the pseudo-second-order adsorptionkinetics (20). The integral forms of the above equation forthe boundary conditions are t00 to t and qt00 to qt

1

qe � qtð Þ ¼1

qeþ k2t ð7Þ

Equation 7 can be written in a linear form, as given by Eq. (8)

t

qt¼ 1

k2qe2þ 1

qe

� �t ð8Þ

The slope of the plot (t/qt) versus t gives the value of qe andfrom the intercept k2 can be calculated.

Diffusion model

The intraparticle diffusion model is based on the theoryproposed by Weber and Morris (1963). According to thistheory

q ¼ kdffiffit

p ð9Þwhere kd is the rate constant of intraparticle diffusion (mgg−1min−1/2). The applicability of this model can be exam-ined by the linear plot of q versus t1/2. The pore diffusioncoefficient for the intraparticle transport of phenol was cal-culated assuming spherical geometry of the sorbent usingfollowing equation:

t1=2 ¼ 0:03r02

Dð10Þ

where ro is the radius of the sorbent, D is the pore diffusioncoefficient (in square centimeters per second) and t1/2 is thetime for half sorption.

Bioregeneration study

Bioregeneration of spent activated carbon loaded with phe-nolic impurities was carried out using P. putidaMTCC 2252(DSM 549) as the phenol-degrading microorganism inshake flask batch rector. The culture was maintained on agarslants using the growth media described below. These stockcultures were stored in refrigerators at 4 °C. The growthmedia contained phenol as the only carbon source and atconcentration of 100 mgL−1. The media were buffered atpH 7.0±0.2, using KH2PO4 (1 gL

−1) and KH2PO4 (1 gL−1).

Trace nutrients were supplied using MgSO4 (0.2 gL−1),CaCl2 (0.02 gL−1), NH4NO3 (1.00 gL−1), and FeCl3(0.05 gL−1) and yeast extract (5 mgL−1). The incubationperiod was 48 h under aerobic condition at about 25 °Ctemperature. Bioregeneration efficiency of the spent carbonwas expressed as a ratio between adsorption values on theregenerated and fresh sorbents. The original capacity (A0) ofthe carbon for a particular adsorbate is considered to be thatquantity of solute adsorbed from solution per unit weight ofcarbon (experimental conditions chosen to ensure equilibra-tion and exhaustion). The capacity of the regenerated carbon(Ar) is the quantity of the same solute adsorbed from solu-tion per unit weight of regenerated carbon (again ensuringboth equilibration and exhaustion) (Martin and Ng 1984).Regeneration efficiency can be calculated using followingequation:

REð%Þ ¼ Ar

A0� 100 ð11Þ

Results and discussions

Physicochemical characteristics of carbon

The physicochemical characteristics of the activated carbonprepared from P. cineraria pods are presented in Table 2. Themeasured BET surface area of the prepared activated carbonof the present study was 595.05m2g−1and is much higher thanvarious low-cost agricultural by-product-based adsorbents,namely maize cob carbon (468 m2g−1) (Sathishkumaret al. 2009), palm pith carbon (188 m2g−1) (Sathishkumaret al. 2007), peanut hull carbon (208 m2g−1) (Periasamyand Namasivayam 1996), and cassava (270 m2g−1)(Rajeshwarisivaraj et al. 2001). The moisture content of the

Table 2 Properties of activated carbon prepared from P. cineraria

S No. Parameters Values

1 pH 7.5

2 Moisture (%) 12.57

3 Bulk density (gmL−1) 0.65

4 Porosity (%) 65.32

5 Ash content (%) 2.3

6 Fixed carbon content (%) 45

7 Specific gravity 1.07

8 Iodine number (mgg−1) 576

9 Decolorizing power (mgg−1) 92.5

10 BET surface area (m2g−1) 595.05

11 BJH adsorption average pore width (Ǻ) 51.15

12 BJH adsorption average pore volume (cm3/g) 0.36


carbon was found to be 12.57 %. If the moisture content of theadsorbent is high, it tends to dilute the action of carbon and itnecessitates utilizing some extra load of carbon (Sathishkumaret al. 2009). The total ash content was 2.3 %, which representsthe good quality of carbon. Carbons with high ash content areconsidered to have less sorption capacity. The decolorizingcapacity was 92.5 mgg−1, which indicates that the carbonprepared had good adsorption capacity and it could be usedfor adsorption of organics. The shape of the isotherm indicatesthe mesoporous nature of the activated carbon prepared fromP. cineraria. The BJH pore size distribution curve (Fig. 2) forthe activated carbon provides a pore size of 50Ǻ at a maxi-mum pore volume of 0.35 cm3g−1. The adsorption hysteresisrelates to the type IVisotherm (Fig. 3). Themesoporous natureof the activated carbon, nanopore size, and the good surfacearea is an indication of effective adsorption of phenol on theadsorbent.

SEM analysis

SEM technique was used to observe the surface physicalmorphology of the activated carbon derived from P. ciner-aria pods after chemical activation. Figure 4 depicts theSEM photographs of the raw materials, raw materials with

ZnCl2, and activated carbon with different magnifications. Itcan be seen from the micrographs that the external surfaceof the chemically activated carbon is full of cavities. Thereason for the formation of the cavities on the ZnCl2-acti-vated carbon is not clear. According to the micrographs, itseems that the cavities resulted from the evaporation ofZnCl2 during carbonization, leaving the space previouslyoccupied by the ZnCl2 (Mohanty et al. 2005a, b). Thecarbonization temperature for chemical activation was toolow to cause the agglomeration of the char structure. Sincethe carbonization temperature for physical activation washigh (500 °C), caking and agglomeration occurred on thechar structure and thus resulted in the formation of charswith an intact external surface.

FTIR analysis

In order to determine the functional groups responsible forphenol uptake by activated carbon prepared from P. ciner-aria pods, Fourier transform infrared spectroscopy (FTIR)was used. The major adsorption bands identified in thespectra associated to varied vibrations that take place inthe oxygenated functional groups present at the surface ofthe activated materials are listed in Table 3. The FTIR

Fig. 2 Nitrogen adsorption anddesorption isotherm of theactivated carbon prepared fromP. cineraria


spectrums presented in Fig. 5 revealed that there were var-ious functional groups detected on the prepared activatedcarbon. The broad band at about 3,402 cm−1 found in thespectrum of the activated carbon can be assigned to O–Hstretching vibration of hydroxyl functional groups includinghydrogen bonding. The position of hydrogen-bonded OHgroups is usually in the range of 3,200–3,650 cm−1 foralcohols and phenols. This functional group has also beendetected on the surface activated carbons prepared frompalm shell (Guo and Lua 2003) as well as commercialgranular activated carbons (Hameed et al. 2008). The bandlocated at 2,924 cm−1 is attributed to the C–H stretching inaliphatic structure. The band at 2,377 cm−1 is ascribed toC≡C vibrations in alkyne groups. Other distinct peaksdetected on the activated carbon that were located at1,701, 1,571, 1,461 (also 1,410), and 1,164 cm−1, respec-tively, are assigned to C0O vibration in carbonyl group,C0C vibration in aromatic group, C–O vibration in carbox-ylate group, and C–O vibration in alcohol group. From theband assignment, the oxygen groups present in the P. ciner-aria dry pods include ethers, esters, alcohols, and phenolgroups. A close comparison of the spectra reveals that aftercarbonization by chemical activation method, some of thefunctional groups, originally detected in the precursor, P.cineraria, disappeared and a few new peaks were detected atwhich might be due to the overlapping of C–O–C stretching,C–O stretching, and O–H bending modes of alcoholic,

phenolic, and carboxylic groups. The band located at1,571 cm−1, which is attributed to C0C vibration in aromaticgroups, is stronger than that of P. cineraria (located at1,531 cm−1). This probably indicates increased aromatiza-tion upon heat treatment.

Effect of initial adsorbate concentration

Phenol adsorption is significantly influenced by the initialconcentration of phenol in aqueous feed solution. In thepresent work, the effect of initial concentration of phenolon adsorption by activated carbon derived from the P. cin-eraria pods was studied at four different initial concentra-tion (10, 25, 50, and 100 mgL−1) at a fixed dose ofadsorbent (10 gL−1). Maximum contact time of adsorptionwas 140 min. During the experiment, pH (4.0) and temper-ature (30 °C) were also kept constant. The results of theseexperiments are depicted in Fig. 6. The general trend of theplot indicates that except for 10 mgL−1 concentration, thepercentage removal of phenol increased with the decrease ininitial concentration of phenol. Maximum removal efficien-cy of 86 % was obtained with 25 mgL−1 of initial phenolconcentration. However, with higher initial concentration ofphenol, the percentage removal dropped significantly. Foran initial phenol concentration of 50 and 100 mgL−1, re-moval efficiency was 83 and 68 %, respectively. For aninitial concentration of 10 mgL−1, the extent of phenol

Fig. 3 BJH Pore distributioncurve of the activated carbonprepared from P. cineraria


removal was only 81 %. This was probably due to slow porediffusion of the solute into the bulk of the adsorbent at avery low concentration. But with very high feed concentra-tion, the active adsorption sites become saturated very fastresulting in reduction of the extent of adsorption. Figure 6also reveals that up to 100 mgL−1 of initial phenol concen-tration in the feed, the maximum percentage removal, wasaccomplished after a contact time of 120 min and thereafterno appreciable change took place. In the initial stages ofadsorption of phenol, the concentration gradient between thefilm and the available pore sites is large, rendering the faster

rate of adsorption. Similar trends of the equilibrium contacttime of phenol adsorption are reported by other researchers(Idris and Saed 2003; Al-Asheh et al 2003).

Effect of adsorbent dose

The effect of the adsorbent dose on removal of phenol wasstudied by varying the dose of adsorbent from 5 to 40 gL−1

at fixed pH (4.0±0.5), temperature (30 °C), and contact time(120 min) (Fig. 7) for various initial feed concentrations.Percentage removal of phenol initially increased with theincrease in the dose of adsorbent, and maximum adsorptionwas recorded at adsorbent dose of 15 gL−1 beyond which noappreciable increase was noticed. This could be due to theincreased availability of active adsorption sites and surfacearea resulting from the conglomeration of the adsorbentsespecially at higher adsorbent dose (Ucer et al. 2006). Theplots of percentage removal of phenol versus dose of adsor-bent were found to be almost exponential for different initialconcentrations of phenol indicating that the amount of phe-nol adsorbed varied in accordance with a fractional powerterm of the dose of adsorbent [for example (dose)n, wheren0fraction]. This suggests that the adsorbed phenol eitherblocked that access to the initial pores or caused particles toaggregate, thereby reducing the active site availability. Theresults of this section also indicated that, in order to obtainthe optimal adsorbent dosage, higher initial phenol concen-trations should be tested in conjunction with appropriateadsorbent dosage (Lin et al. 2009).

Effect of initial pH

Sorption of phenol from the aqueous medium depends con-siderably on the initial solution pH, which affects the surface

(a)

(b)

(c)

Fig. 4 Scanning electron micrographs of raw material (a); raw mate-rial with ZnCl2 and activated carbon (c) (magnification, ×500)

Table 3 FTIR assignments of principal bands observed on carbonsurfaces

Wave number (cm−1) Assignment

3,402, 3,388 O–nbH vibration in hydroxyl groups

2,926, 2,924 C–H stretching in aliphatic structure(–CH2, –CH2–CH3

2,377, 2,375 C≡C vibration in alkyne groups

2,145 C0O stretching

1,701, 1,629 C0O vibration in carbonyl group

1,571, 1,531 C0C vibration in aromatic group

1,461, 1,436, 1,410 C−O vibration in carboxylate group

1,269, 1,240 C−O vibration in esters, ethers,or phenol groups

1,164, 1,034 C−O vibration in alcohol (R–OH) group

669 O−H stretching

522, 519 C−O−H twist


charge of the adsorbent and the degree of ionization of theadsorbate. The effects of pH on phenol removal by activatedcarbon derived from the P. cineraria pods were studied atvarious pH values ranging from pH 2 to 8 with keeping thedose of adsorbent 10 gL−1 and temperature 30 °C. Theamount of phenol adsorbed was found to show a decliningtrend with higher as well as with lower pH, with maximumremoval of phenol (about 85 %) observed at pH 4.0, for allthe adsorbents studied (Fig. 8). Percentage adsorption ofphenol increased with the increase in pH from 2.0 to 4.0and decreased thereafter. Maximum adsorption of phenol atall the concentrations took place at pH 4.0. As evident fromTable 3, the surface of the activated carbon contains a largenumber of functional groups. The pH dependence of phenoladsorption can largely be related to the type and ionic stateof these functional groups and also on the phenol chemistry

in solution. Adsorption of phenol up to pH 4.0 suggests thatthe positively charged C6H5OH2

+ ions (formed by C6H5OHin acidic media) bind through electrostatic attraction tonegatively charged functional groups on the surface of acti-vated carbon because at this pH, more functional groupscarrying negative charge would be exposed. But on raisingthe pH above 4.0, the concentrations of C6H5OH2

+ andC6H5O

− both are to be considered. Experimental resultsrevealed that C6H5O

− is relatively important at pH>4.0and these negatively charged ions are no longer attractedby the activated carbon on its surface resulting in the reduc-tion of phenol removal. Hence, it can be concluded thatabove pH 4.0, other mechanism like physical adsorptionon the surface of adsorbent could have taken an importantrole in the adsorption of phenol and exchange mechanismmight have influenced markedly (Mohanty et al. 2005b).

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.035.30

35.5

36.0

36.5

37.0

37.5

38.0

38.5

39.0

39.5

40.0

40.5

41.0

41.51

Wave number (cm -1)

% T

Activated carbon

37383681

34022924

2377

1701

1571

14611410

1269

1164

519

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.034.6

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49.3

Wave number ( cm-1)

% T

Dry pods of Prosopis cineraria3907

3752

3388

2926

2375

2145

1629

1531

1436

1240

1034

669

522

Fig. 5 Fourier transforminfrared spectra of dry pods ofP. cineraria and activatedcarbon


At lower pH values, the uptake of phenol is less due tothe presence of H+ ions suppressing the ionization of phenoland hence its uptake on polar adsorbent is reduced. More-over, additional protons in the solution at low pH resulted incompetition for the carbonyl sites, thereby reducing theextent of adsorption at low pH (Srihari and Das 2008). Inthe higher pH range, phenol forms salts which readily ionizeleaving negative charge on the phenolic group. At the sametime, the presence of OH− ions on the adsorbent prevents theuptake of phenolate ions. High pH environments led to highconcentration of OH−, which can increase the hindrance tothe diffusions of phenolate ions and thus reduce the chancesof their adsorption (Rengaraj et al. 2002a). Owing to the

amphoteric character of the carbon surface, i.e., to the acidicand/or basic functional groups, the surface properties maybe influenced by the pH value of the coexisting liquid bulkphase. In fact, the pH value, at which the surface charge iszero, is called the point of zero charge (PZC). For typicalamphoteric carbons, the surface is positively charged at pH<pHPZC and negatively charged at pH>pHPZC (Dabrowskiet al. 2005). Since the pKa value for phenol is 9.89, in thepresent study, phenol could be expected to become nega-tively charged phenolate ion above this pH, and repulsionbetween the surface layer and subsequently the anionicphenolate might have been resulted in reduced adsorption.Although in the present study experiments were not

Time (min)

Ext

ent o

f ph

enol

rem

oval

(%

)

0 20 40 60 80 100 120 140 1600

10

20

30

40

50

60

70

80

90

10 mg L 1

25 mg L 1

50 mg L 1

100 mg L 1

Fig. 6 Effect of initialadsorbate concentration on thepercent removal of phenol byadsorption onto activatedcarbon prepared from P.cineraria (dose of adsorbent10 gL−1, initial pH 4.0±0.5,temperature 30 °C)

Adsorbent dose (g L 1)

Ext

ent o

f ad

sorp

tion

(%)

5 10 15 20 25 30 35 4045

50

55

60

65

70

75

80

85

90

95

10 mg L 1

25 mg L 1

50 mg L 1

100 mg L 1

Fig. 7 Effect of adsorbent doseon the percent removal ofphenol by adsorption ontoactivated carbon prepared fromP. cineraria (initial pH 4.0±0.5,temperature 30 °C, time120 min)


conducted beyond pH 8, the results agree well with thoseobtained for the adsorption of phenols on to bentonite clay(Banat et al. 2000) and activated charcoal (Halouli andDrawish 1995). It is worthwhile to mention that the pH ofthe solution was monitored before and after the adsorptionand no noticeable change in pH was observed. However, thedetails for the effect of pH on phenol adsorption mechanismneed to be further investigated.

Effect of temperature and thermodynamic properties

The adsorption studies were carried out at three differenttemperatures, 30, 40, and 50 °C, and keeping the feedconcentration (25 mg L−1) and adsorbent dose (15 gL−1)constant. It was observed that with an increase in tem-perature, adsorption capacity increased, thereby indicatingthe adsorption as an endothermic process. Maximumadsorption was recorded at 50 °C temperature. The ob-served experimental findings may be explained by thefact that with the increase in temperature, the number ofactive sites available for adsorption must have increasedresulting in enhancement of percentage adsorption ofphenol on to activated carbon derived from the P. ciner-aria pods. Another possibility of decreasing the thicknessof the boundary layer surrounding the adsorbent with risein temperature cannot also be ruled out since this tendsto reduce the mass transfer resistance of adsorbate in theboundary layer (Coskun et al. 2006). Moreover, as dif-fusion is an endothermic process, the increase in uptakeof phenol may also be due to an enhanced rate of intra-particle diffusion of sorbate. An increase in pore diffu-sivity of adsorbate with temperature leads to increasedremoval of phenol by its adsorption. The thermodynamic

parameters of the adsorption were determined using theequations below.

The change of enthalpy (ΔHo) and entropy (ΔSo) can beobtained from the slope and intercept of a van’t Hoff equa-tion of Go versus T:

$G� ¼ $H� � T$S� ð12Þ

where ΔGo is standard Gibbs free energy change, joules; Ris universal gas constant, 8.314 Jmol−1K−1; and T is abso-lute temperature, kelvin.

logqeCe

� �¼ S�

2:303Rþ �H�

2:303RTð13Þ

where qe is the maximum amount of phenol adsorbed perunit mass of the activated carbon (in milligrams per gram).Ce is the equilibrium concentration (in milligrams per liter),R is the gas constant (8.314 Jmol/K), and T is temperature inkelvin. Equation (13) was used with the experimental data toevaluate entropy of adsorption ΔSo and enthalpy of adsorp-tion ΔHo from a plot of log (qe/Ce) versus 1/T. The value ofGibbs free energy is then calculated from Eq. (12). Thevalues of these thermodynamic parameters, for four initialfeed concentrations (viz. 10, 25, 50, and 100 mgL−1 ofphenol), are given in Table 4. It is observed that ΔGo atall temperatures are negative. ΔGo values are ranging from−6.69 to 11.56 kJmol−1 depending on temperature rangingfrom 303 to 323 K. It indicates that the adsorption of phenolon the activated carbon is spontaneous in nature. The pos-itive values of enthalpy infer that the reaction is an endo-thermic process and so the reaction consumes energy. Thenegative value of ΔSo suggests that there is little reductionof randomness as a result of probable change in internal

pH

Ext

ent o

f ph

enol

rem

oval

(%

)

2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 820

25

30

35

40

45

50

55

60

65

70

75

80

85

90

10 mg L 1

25 mg L 1

50 mg L 1

100 mgL 1

Fig. 8 Effect of initial pH onthe percent removal of phenolby adsorption onto activatedcarbon prepared fromP. cineraria (dose of adsorbent10 gL−1, temperature 30 °C)


structure of activated carbon during the adsorption of phenol(Manju et al. 1998). It also indicates the probability of afavorable sorption.

Adsorption isotherm study

Adsorption isotherms are important to describe the adsorp-tion mechanism for the interaction of phenol on the surfaceof the activated carbon. Adsorption data of phenol on acti-vated carbon at three different temperatures viz., 30, 40, and50 °C were fitted with both the Langmuir and Freundlichisotherms. The coefficients of these two isotherm modelshave been shown in Table 5.

The Langmuir isotherm model was chosen for the esti-mation of maximum adsorption capacity corresponding tocomplete monolayer coverage on the agrowastes. The sorp-tion capacity, Q, which is a measure of the maximumsorption capacity corresponding to complete monolayercoverage, showed that all the agro-based adsorbents seemedto have high monolayer adsorption capacities. The adsorp-tion coefficient, b, is related to the apparent energy (surfaceenergy) of sorption. A relatively lower b value (0.003 to0.004) (Table 4) implies low surface energy in all the sys-tems, thereby indicating a probable stronger bonding be-tween phenol and sorbents (Aksu and Yener 2001). In fact,fairly low to moderate b values have been reported in manyof the sorbent–phenol systems, involving bentonite, palm-seed-coat-activated carbon, and rice husk activated carbon(Banat et al. 2000; Rengaraj et al. 2002a, b; Mahvi et al.2004). To account for possible multilayer adsorption andnonlinear energy distribution for the adsorption sites, theFreundlich isotherm was also studied for the same adsorb-ents. The Freundlich parameters (k and n) indicate whetherthe nature of sorption is either favorable or unfavorable. Thehigher value of k (the Freundlich constant) indicates higher

the phenol uptake from aqueous solution (Lin et al. 2009),assuming the adsorption as multilayered with exponentialenergy distribution. The values of n were 4.65, 3.24, and3.32 at three different temperatures viz. 50, 40, and 30 °C,respectively (Table 5). It is known that when the n value isgreater than 1.0, conditions are favorable to adsorptionsatisfy the condition(s) of heterogeneity (Khalid et al.2000). Hence, it indicates that adsorption of phenol on toactivated carbon agrees well with the Freundlich isothermmodel. Correlation coefficients are also calculated by fittingthe experimental adsorption equilibrium data for the phenol-activated carbon system, using both the isotherms. The highvalue of coefficient of determination (R200.98 to 0.99)obtained in Freundlich isotherm indicates a good agreementbetween the experimental values and isotherm parameters.

Adsorption kinetics

The kinetic parameters are useful in predicting the adsorp-tion rate which can be used in design and modeling ofadsorption operation. Kinetics of adsorption describes thesolute uptake rate which in turn governs the residence timeof adsorption reaction. It is one of the important character-istics in defining the efficiency of adsorption (Rengaraj et al.2002a, b). Various researchers have used several kineticmodels, where the adsorption has been treated as a first-order pseudo-first-order and pseudo-second-order process.In the present study, adsorption of phenol on activatedcarbon prepared from P. cineraria pods has been describedby first- and pseudo-second-order models and also by adiffusion model.

The applicability of the pseudo-first-order, pseudo-second-order and diffusion models were examined by eachlinear plot (not shown) of log (qe−qt) versus t, (t/qt) versus t,and q versus t1/2, respectively, for three different initial

Table 4 Thermodynamicparameters for the adsorptionof phenol on activated carbon

Concentration of phenol (mg/L) ΔHo (kJmol−1) −ΔSo (kJmol−1K−1) ΔGo (kJmol−1)

303 K 313 K 323 K

25 20.81 0.076 −11.04 −8.56 −7.95

50 18.56 0.055 −12.12 −8.78 −7.56

100 19.11 0.042 −11.56 −6.69 −7.41

Table 5 Langmuir and Freund-lich adsorption isotherm con-stants for adsorption of phenolon activated carbon preparedfrom P. cineraria pods

Temperature (°C) Langmuir constant Freundlich constants

Q (mgg−1) b (lmg−1) R2 k n R2

50 4.24 2.79×10−3 0.89 62.23 4.65 0.98

40 4.63 3.34×10−3 0.88 59.52 3.24 0.98

30 5.37 4.20×10−3 0.91 58.32 3.36 0.99


adsorbate concentrations (viz. 25, 50, and 100 mgL−1). Rateconstants for the first-order, pseudo-second-order, anddiffusion-controlled adsorption of phenols on to the activat-ed carbon are presented in Table 6. To quantify the applica-bility of each model, the correlation coefficient R2 wascalculated from these plots. The linearity of these plotsindicates the applicability of the three models. Usually amodel with R2 closer to unity represents the data moreaccurately than a model with R2 lower than 1. Comparingthe statistical indicators (R2) for the two models, it is clearthat the pseudo-second-order model, an indication of chem-isorption mechanism, fits better the experimental data (R2>0.997) than the pseudo-first-order Lagergren model (R2 is inthe range of 0.956–0.973). The intraparticle diffusion hadalso some role in the adsorption of phenol by activated

carbon. The value of D, according to Eq. (10), was foundout to be 3.39×1010cm2s−1 for initial phenol concentrationof 25 mgL−1.

Bioregeneration studies

The increased utilization of activated carbons in recent yearshas been accompanied by the development and broaderapplication of a number of new regeneration processeswhich increase the service period of the activated carbon.A variety of thermal, chemical, and electrochemical regen-eration techniques are suggested in literature for exhaustedactivated carbon, especially carbon saturated with organicpollutants like phenol. However, in most of the regenerationprocesses, degradation of the sorbed contaminants (here, itis phenol) does not take place; rather, the impurities arereleased to the environment. Thermal regeneration is alsocharacterized by 5–10 % carbon loss due to oxidation andattrition. In the present work, a microbial regenerationprocess was attempted, where microorganisms regeneratethe surface of spent activated carbon using organic sub-strate, already adsorbed, as a source of food and energy.The process of substrate removal includes the adsorptiononto activated carbon surface, adsorption onto the biofilm,and microbiological oxidation (Xiaojian et al. 1991). Becauseof the combination of microorganisms and the adsorptive

capacity of activated carbon, a synergic effect can beobserved. This indicates that the total effect of carbonadsorption and biological oxidation acting together isgreater than the sum of their effects each taken separate-ly. Probably such synergy is attributed to stimulation ofbiological activity through removal of inhibitory substan-ces or concentration of nutrients and to bioregenerationof the adsorptive capacity of activated carbon (Xiaojianet al. 1991). Biological reaction rates become higher onactivated carbon due to an enrichment of the organics bycarbon adsorption. According to exoenzymatic hypothe-sis, some enzymes excreted by the bacteria could easilydiffuse into the micropores and react with the adsorbedsubstrate (Aktas and Cecen 2007) leading to its degrada-tion. The surfaces of activated carbon are excellent forcolonization by microorganisms. Its adsorptive propertiesserve to enrich substrate, nutrients, and oxygen concen-trations; the craggy surface provides recesses that aresheltered from fluid shear forces; and the variety of func-tional groups on the surface can enhance attachment ofmicroorganisms.

Bioregenerability of activated carbon loaded with phenolwas studied in shake flask batch reactors using P. putidaMTCC 2252 (DSM 549) as the organism. Virgin activatedcarbons were initially loaded with phenol by contactingactivated carbon with phenol and agitating until equilibriumwas reached. The supernatant was removed by centrifuga-tion. Then, by measuring the amount of phenol remaining inthe supernatant, the loading on the activated carbon wascalculated. Loaded activated carbon was mixed with thesuspension culture of P. putida. The average bioregenerationefficiency of spent carbon as calculated by using Eq. (11)was 57.5 %. This was low as compared to the percentregeneration by conventional method using 1(N) NaOHsolution (Table 7). A perusal of Table 7 indicates that whenthe loaded phenol concentration of the spent activated car-bon is low, it gives slightly better regeneration than that ofhigher phenol concentration. On the other hand, bioregen-eration could not yield appreciable result. According toHutchinson and Robinson (1990), the growth thresholdconcentration in the fermentation broth during regenerationprevents complete desorption of the adsorbate (Hutchinson

Table 6 Rate constants for first- and second-order and diffusioncontrolled adsorption of phenols on to the activated carbon

Concentration (mgL−1) k1a k2

b Dc×1010 (cm2s−1)

25 0.04 0.137 3.39

50 0.042 0.080 4.15

100 0.039 0.058 3.81

aR2 00.956–0.973bR2 ≥0.99cR2 00.90–0.99

Table 7 Comparison of percent bioregeneration of spent carbon andchemical regeneration

S No Loaded phenolconcentration(mgmL−1)

Bioregeneration(%)

Regeneration by1(N) NaOH (%)

1 25 57.5 78.5

2 50 52.5 75.5

3 100 43.4 64.6


and Robinson 1990). Considerable amounts of phenol re-main adsorbed even when the liquid-phase concentration ofphenol contacting the adsorber is reduced to the growththreshold level. Therefore, it is obvious that effluent levelslower than the growth threshold concentration may not beachievable using bioregenerated activated carbon due to theresidual adsorbate remaining following bioregeneration.The use of mixed microflora could possibly be an answerto this problem. However, the chances of microbial foulingof the pore openings and/or adsorption of microbially pro-duced compounds cannot be ruled out (Hutchinson andRobinson 1990).

Comparison with other precursors

Activated carbons, prepared from various agrowaste precur-sors, their activation process, and maximum adsorption ca-pacity in the temperature range of 30−40 °C are presented inTable 8. However, it is somewhat difficult to compare ad-sorption performance of various adsorbents due to a lack ofconsistency in the literature data. Adsorption capacities wereevaluated at different pHs (and not necessarily at the opti-mum pH), temperatures, and concentration ranges. Thecomparison of adsorption performance depends not only

on the parameters related to the experimental conditionsand the effluent but also on the analytical method used fordecontamination tests (batch method, column, reactors, etc.)(Lim et al. 2010). The influences of impregnation variables,concentration of activating agents, heat treatment tempera-ture, and surface functional groups are some of the otherimportant factors, which drive the differences in maximumadsorbent capacity among various precursors. Results givenin Table 8 indicate that except for datestone (adsorptioncapacity 111.1 mgg−1) and lignin from Eucalyptus grandiskraft black liquors (adsorption capacity 227.3 mgg−1), themaximum adsorption capacity obtained in this study(78.32 mgg−1) is higher compared with those obtained frommany other agrowaste precursors of activated carbon.

A key factor which dominates the adsorption of phenolon activated carbon is the available surface area. In general,naturally occurring adsorbents have less surface area whichresults in a lower adsorption capacity. In contrast, the BETsurface area of the activated carbon prepared from the drypods of P. cineraria in this study was 595.05 m2g−1, whichis much higher than similar types of activated carbons asreported in literature. Moreover, dry pods used in the presentstudy have a lesser bulk density (0.65 gcm−1) because ofwhich almost all the surface was available for adsorption.

Table 8 Performance of activated carbons from various precursors for adsorption of phenol from aqueous solutions

S No Precursor Adsorbate Activation Maximumadsorbentcapacity (mgg−1)

Reference

1 Terminalia arjuna nuts Phenol Chemical activationby ZnCl2

93 % for 10 g/Linitial concentration

Mohanty et al. (2005a, b)

2 Tectona grandis sawdust Phenol Chemical activationby ZnCl2

2.82 Mohanty et al. (2005a, b)

3 Palm seed coat phenol Thermal activationby CO2

18.3 Rengaraj et al. (2002a)

4 Rubber seed coat Phenol Thermal activationby CO2

15.5 Rengaraj et al. (2002b)

5 Particle board waste Phenol Thermal activationwith steam

50.0 Girods et al. (2009)

6 Raw date pits Phenol Physical activationby CO2

46.076 Banat et al. (2004)

7 Tamarind nut shell Phenol Chemical activationby H2SO4

4.237 Goud et al. (2005)

8 Rapeseed Phenol Physical activationby CO2

80 Valente Nabais et al. (2009)

9 Lignin from Eucalyptusgrandis kraft black liquors

Phenol Chemical activationby H3PO4

227.3 Gonzalez-Serrano et al. (2004)

10 Date stones Phenol ZnCl2 activation 111.5 Alhamed (2009)

11 Black gram husk Phenol Chemical activationby H2SO4

11.17 Srihari and Das (2008)

Green gram husk 10.23

12 Oil palm empty fruit bunch Phenol Thermal activationby CO2

4.81 Alam et al. (2009)

13 Rice husk Phenol ZnCl2 activation 27.58 Kalderis et al. (2008)Bagasse 24.68

14 Prosopis cineraria pods Phenol ZnCl2 activation 78.32 Present study


This is one of the reasons for the higher adsorption capacityof phenol on the prepared activated carbon. Method ofactivation also plays an important role on the performanceof the activated carbon toward its adsorption capacity. It hasbeen reported that activated carbons prepared from chemi-cally activated naturally occurring precursors shows largercapacity as compared to other methods (Gupta and Babu2009). During pyrolysis, dehydration of the cellulosic ma-terial takes place which results into charring and aromatiza-tion of the carbon skeleton and the creation of themesoporous structure. The presence of oxygen and hydro-gen in surface groups as evident from the FTIR analysis(Table 3) affects strongly the adsorptive properties of theactivated carbon. The presence of such functional groups,mainly oxygen groups as well as with various surface im-purities, imparts chemical heterogeneity in activated carbon.This contributes to the unique adsorption property of acti-vated carbon.

The cost of adsorbent for phenol and its derivativesdeveloped from agrowaste materials is scarcely reported inliterature because it strongly depends on local availability,processing required, treatment conditions, and both recycleand lifetime issues (Mohanty et al. 2005a, b). The cost mustinclude the cost of production and regeneration. Whenabundantly available agrowaste materials are used as pre-cursor, regeneration of sorbent might not be economical asthe cost of regeneration chemical might be significant. Butfrom the environmental point of view, spent activated car-bon sorbed with phenolic impurities should be regeneratedbiologically so that the phenol can be degraded. In India,where a large amount of P. cineraria pods are wasted, theuse of it as a precursor of activated carbon appears to be aneconomic and efficient option for wastewater treatmentprocesses.

Conclusions

The production of value-added products like activated car-bon from agrowaste materials is very important from bothenvironmental and economical scope. P. cineraria pod is anovel agrowaste precursor for the production of activatedcarbon by ZnCl2 activation. Activated carbon thus preparedhas a BET surface area of 595.05 m2g−1 and has been foundto be an effective adsorbent for the dephenolation of waste-water. The maximum removal of phenol was found to be78.32 mgg−1 at a temperature of 30 °C and 25 mgL−1 initialphenol concentration. Maximum removal efficiency of 86 %was obtained with 25 mgL−1of initial phenol concentration.Also, percentage removal of phenol increased with the in-crease in the dose of adsorbent, and maximum adsorptionwas recorded at adsorbent dose of 15 gl−1 beyond which nofurther increase was noticed. The favorable pH for

maximum phenol adsorption was 4.0. Thermodynamic stud-ies indicate that the process was spontaneous and endother-mic. The fit of the adsorption data more closely in toFreundlich isotherm confirmed the multilayer adsorptionwith exponential energy distribution. The best correlationcoefficient using the pseudo-second-order kinetic modelindicated that phenol adsorption process with the presentactivated carbon followed the pseudo-second-order rate ex-pression. Preliminary investigation showed that spent acti-vated carbon was amenable to microbial regeneration usingP. putida as the phenol-degrading microorganism; however,the maximum regeneration of only 57.5 % could beachieved in this study, when loaded phenol concentrationwas 25 mgL−1.

In essence, bioregeneration can compete with other con-ventional regeneration processes of granular and powderedactivated carbon, due to its distinct advantages like theability of simultaneous biodegradation and regeneration,safety, and ecofriendly nature. Further work is necessary tostudy the mechanism of bioregeneration and to improve itsefficiency. The data obtained in this study would be usefulin designing and fabricating an efficient treatment plant forphenol rich effluents. However, future study should focus onthe larger-scale demonstration in packed column reactor,detailed economical, and environmental assessments of theproposed phenol removal technology.

Acknowledgments Financial grant from Council of Scientific andIndustrial Research, Govt. of India, New Delhi under the scheme no.22 (0542)/10/EMR-II is gratefully acknowledged. The authors alsoacknowledge the help received from Sophisticated InstrumentationCentre for Applied Research & Testing (SICART), Vallabh Vidyana-gar, Gujarat to conduct SEM and FTIR analysis.

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