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Equilibrium, kinetic, and thermodynamic studies for biosorption of cadmiumand nickel on grapefruit peel

Meisam Torab-Mostaedi a,*, Mehdi Asadollahzadeh a,b, Alireza Hemmati b, Amir Khosravi a

a Nuclear Fuel Cycle Research School, Nuclear Science and Technology Research Institute, P.O. Box 11365-8486, Tehran, Iranb Department of Chemical Engineering, Iran University of Science and Technology, P.O. Box 16765-163, Tehran, Iran

1. Introduction

Heavy metals are released continuously from different sources,industrial, agricultural and domestic wastes into the environment,which create problem for human as well as aquatic organisms. Thepollution of water resources due to the indiscriminate disposal ofheavy metals has been causing worldwide concern for the last fewdecades. Toxic heavy metals of particular concern in treatment ofindustrial wastewaters include cadmium, lead, mercury, nickel,zinc, copper and chromium.

Cadmium and nickel are usually encountered together inindustrial wastewaters from mine drainage, electroplating, smelt-ing, alloy manufacturing, pigments, nickel–cadmium and solarbattery production. Combination of cadmium with hydroxylradicals, superoxide anions, nitric oxide and peroxides in variousorgans can cause cancers such as the lung, kidney, pancreas,prostate, and liver. Besides, cadmium is known to get accumulated inthe kidneys causing its malfunction at overdoses spilling proteins inthe urine and disrupting protein metabolism. An essential amount ofnickel may be benefit organisms as a component in a number ofenzymes, participating in important metabolic reactions (methanebiogenesis, hydrogen metabolism, ureolysis, and acidogenesis).However, an over permissible value of nickel exposure causes

various diseases, such as renal edema, lung cancer, pulmonaryfibrosis, skin dermatitis, and gastrointestinal discomfort [1].

In view of the toxicity and in order to meet regulatory safedischarge standards, it is essential to remove heavy metals fromwastewaters/effluents before it is released into the environment[2]. A number of technologies have been developed over the yearsto remove toxic heavy metals from wastewater. Conventionalmethods for the removal of heavy metal ions from wastewaterinclude ion-exchange, solvent extraction, chemical precipitation,membrane filtration, electrochemical treatment and adsorption[3]. All these conventional methods have some advantages anddisadvantages such as high consumption of reagent and energy,low selectivity, high operational cost and generation of secondarypollutants [4]. Therefore, environmental and public healthengineers have been searching for an inexpensive and efficientfor treatment of metal-containing wastewaters. It is known foryears that biomaterials can bind different heavy metal ions. Themain advantages of this technique are the reusability ofbiomaterial, low operating cost, improved selectivity for specificmetal of interests, short operation time and no production ofsecondary compounds which might be toxic [5]. Biosorbents forremoval of heavy metal ions mainly come under the followingcategories: bacteria, fungi, algae, industrial waste, agriculturalwastes, and other polysaccharide materials [6,7]. Fruit peels aretypically generated in large quantities by the fruit juice industry.These materials have received little scientific attention, in spite oftheir high quantity of pectin, which contains carboxylate groups.

Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 295–302

A R T I C L E I N F O

Article history:

Received 5 June 2012

Received in revised form 20 October 2012

Accepted 4 November 2012

Available online 5 January 2013

Keywords:

Biosorption

Cadmium

Nickel

Grapefruit peel

Isotherm

Kinetic

A B S T R A C T

The biosorption of cadmium and nickel onto grapefruit peel from aqueous solution has been investigated

using batch technique. Experiments are carried out as a function of solution pH, biosorbent dosage, contact

time and temperature. The equilibrium adsorption data are fitted to Langmuir and Freundlich isotherm

models and the model parameters are evaluated. The Freundlich model fits the equilibrium data better than

the Langmuir model. The maximum uptakes of Cd(II) and Ni(II) by grapefruit peel are found to be 42.09 and

46.13 mg/g, respectively. The kinetics of the biosorption process is found to follow the pseudo-second-

order kinetic model. Thermodynamic parameters depict the endothermic nature of biosorption and the

process is spontaneous and favorable. Release of cations and protons from the biosorbent during sorption of

Cd(II) and Ni(II) reveals that the main sorption mechanism is ion exchange. FTIR analysis demonstrates that

carboxyl and hydroxyl groups were involved in the biosorption of the metal ions. The recovery of the Cd(II)

and Ni(II) from grapefruit peel is found to be more than 97% using 0.1 M HCl. The results suggest that

grapefruit peel can be used effectively for the removal of Cd(II) and Ni(II) ions from wastewaters.

� 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +98 21 88221117; fax: +98 21 88221116.

E-mail address: mmostaedi@aeoi.org.ir (M. Torab-Mostaedi).

Contents lists available at SciVerse ScienceDirect

Journal of the Taiwan Institute of Chemical Engineers

jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1876-1070/$ – see front matter � 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jtice.2012.11.001

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The grapefruit is cultivated in both north and south of Iran, withapproximately 50,000 ton annual production. Grapefruit has anagronomic importance because the fruits are widely used in natureor in a processed form as concentrated juice. Grapefruit peelcontains several water soluble and insoluble monomers andpolymers [8,9]. The water soluble fraction contains glucose, fructose,sucrose, and some xylose, while pectin, cellulose, hemicellulose, andlignin constitute between 50 and 70% of the insoluble fraction. Thesepolymers are rich in carboxyl and hydroxyl functional groups, whichmay cationic metal ions in aqueous solution [10].

In this work, grapefruit peel has been used as an abundant andinexpensive biosorbent for removing Cd(II) and Ni(II) fromaqueous solution by batch method. The effects of various operatingparameters such as initial pH of solution, biosorbent dosage,contact time and temperature on the biosorption process areinvestigated. The Langmuir and Freundlich models are used to fitthe equilibrium isotherms. The biosorption mechanisms of Cd(II)and Ni(II) onto grapefruit peel are also evaluated in terms ofthermodynamics and kinetics. The mechanism of the biosorptionprocess is examined by different experimental approaches, such asFT-IR spectroscopy, chemical blocking of functional groups andconcomitant release of cations from grapefruit peel during thesorption of Cd(II) and Ni(II).

2. Experimental

2.1. Preparation of biosorbent

Grapefruit fruits were purchased from a local market. The whitepeel waste was removed from the yellow coating. The peals wasthen cut into small pieces, was extensively washed with doubledistilled water to remove extraneous materials. The fruit peelswere then dried at 70–75 8C in a convection oven until theyreached a constant weight, which was accomplished after 24 h. Thedried material was then ground and sieved to obtain a particle sizelower than 355 mm.

2.2. Chemicals

All the chemicals used in the study were of analytical reagent(AR) grade. Stock solutions of cadmium and nickel (1000 mg/L)were prepared by dissolving Cd(NO3)2 and Ni(NO3)2 in doubledistilled water. The stock solution was then diluted with distilledwater to obtain the desired Cd(II) and Ni(II) concentrations.

2.3. Major cation content of the biosorbent

1.0 g of the biosorbent was mixed with 100 mL of 0.1 M HCl todetermine the amount of alkali and alkaline earth metals andthereby the ion exchange capacity. The suspension was stirred by amagnetic stirrer for 60 min at 25 8C, and then separated bycentrifugation. The biosorbent residue was resuspended in fresh0.1 M HCl solution and it was repeated twice. The resulting filtratewas analyzed for release of Mg2+, Ca2+, Na+ and K+ using an atomicabsorption spectrophotometer. Replacement of 0.1 M HCl withnanopure water (approximately 18 V resistance) in a similarprocedure was served as a control.

2.4. Blocking of carboxyl and hydroxyl functional groups

Blocking of carboxyl groups of grapefruit peel was carried outaccording to the Gardea-Torresdey et al. [11], which involved thefollowing reaction with methanol:

RCOOH þ CH3�!Hþ

RCOOCH3 þ H2O (1)

For this purpose, the biosorbent (9.0 g) was suspended in amixture of anhydrous methanol (633 mL) and concentrated HCl(5.4 mL), with continuous agitation for 6 h on a magnetic stirrer at200 rpm. Hydroxyl groups of the biosorbent were blocked inreaction with formaldehyde according to procedure of Chen andYang [12]:

2R � OH þ HCHO ! ðR � OÞ2CH2 þ H2O (2)

For this purpose, the biosorbent (5.0 g) was suspended in HCHO(100 mL) and agitated at 200 rpm for 6.0 h. In both cases, thebiosorbent was then separated by centrifugation, washed repeat-edly with nanopure water, freeze–dried and used in biosorptionexperiments.

2.5. Batch biosorption experiments

The biosorption of cadmium and nickel on grapefruit peel wasstudied using the batch procedure. All experiments were carriedout in triplicate and the mean value was used in all cases. For eachexperimental run, 100 mL aqueous solution of known concentra-tion of Cd(II) and Ni(II) was taken in polyethylene round-bottomflasks containing determined amount of the grapefruit peel. Theseflasks were agitated at a constant rate of 180 rpm in temperaturecontrolled shaker. The effect of pH on biosorption process wasinvestigated in a pH range of 2.0–8.0. The effect of biosorbentdosage was studied by varying the dosage in the range of 1–6.0 g/L.The samples were taken at 5, 10, 20, 30, 45, 60, 75, 90 and 120 minto determine optimal contact time. For the adsorption isothermstudies, the initial metal concentration was varied over a range of10–200 mg/L. The biosorption experiments were also carried out at20, 30, 40 and 50 8C to determine the adsorption thermodynamicparameters. The supernatants were filtered through a filter paperand the filtrate was analyzed for Ni(III) and Cd(II) concentration byan inductively coupled plasma atomic emission spectrophotome-ter (ICP-AES). For each experiment a blank, containing 100 mL ofonly Cd(II) or Ni(II) solution without any biosorbent was shakensimultaneously to determine any adsorption of the metal onto thewall of the flasks.

The calculation of the biosorption uptake (qe) is given by themass balance equation, Eq. (3).

qe ¼ðCo � CeÞ � V

M(3)

where V (L) is the volume of metal solution in contact with thebiosorbent; Co and Ce (both in mg/L) are the initial and equilibriumconcentrations of the metal in the solution, respectively; and M (g)is the mass of the biosorbent. The percent biosorption of the metalions was calculated as follows:

Biosorption ð%Þ ¼Co � C f

Co

� �� 100 (4)

where Cf is the final metal ion concentration. The average absolutevalue of relative error, AARE, is used to compare the predictedresults with the experimental data. This is defined as follows:

AARE ¼ 1

NDP

XNDP

i¼1

jPredicted value � Experimental valuejExperimental value

in which NDP is the number of data points.

2.6. Desorption experiments

The desorption studies of Cd(II) and Ni(II) from the biosorbentwere done by using 0.1 M HCl as the desorbing agent. Afterdetermination of metal contents of the metal solutions, thebiosorbent was washed with excess of acid solution and nanopure

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water in order to reuse for next experiment. Consecutive sorption–desorption cycles were repeated to determine reusability potentialof the biosorbent.

3. Results and discussion

3.1. Chemical composition and cation exchange capacity of grapefruit

peel

The composition of grapefruit peel based on the dry weightpercentage is 18.76% pectin, 10.52% cellulose, 6.02% hemicellulose,4.63% lignin and 3.4% protein [13]. This composition indicates thepresence of many carboxylic (–COOH) and hydroxyl (–OH) acidicgroups capable of ion exchange with metal ions. The total amountof cations released by grapefruit peel during acid washing was1.307 mequiv./g, which comprised 0.778, 0.336, 0.110, and0.053 mequiv./g of Mg2+, Ca2+, K+ and Na+, respectively. The totalcation content can be considered as a measure of the approximatecation exchange capacity (CEC) of a sorbent material.

3.2. SEM analysis

The SEM analysis of grapefruit peel reveals importantinformation on the surface morphology (Fig. 1). As seen inFig. 1, the biosorbent has some cavities in its structure capable ofuptaking metal ions. After metal binding, the surface of grapefruitpeel flattened in comparison of raw sample. Apart from that, nofurther significant morphological changes were apparent in theSEM images.

3.3. FTIR analysis of biosorbent

The FTIR spectrum is carried out as a qualitative analysis todetermine the main functional groups that are involved in theadsorption process. The FTIR spectrum of the grapefruit peel isshown in Fig. 2. From this figure, it is possible to verify the complexnature of this material. The broad and intense absorption peak at3440 cm�1 indicates the presence of free or hydrogen bonded O–Hgroups (alcohols, phenols, and carboxylic acids) as in pectin,cellulose and lignin on the surface of the biosorbent [14]. The O–Hstretching vibrations occur within a broad range of frequenciesindicating the presence of ‘‘free’’ hydroxyl groups and bonded O–Hbands of carboxylic acids. The peak observed at 2930 cm�1 can beattributed to the C–H stretching vibrations of aliphatic acids [15].The peak observed at 1739 cm�1 corresponds to the stretchingvibration of C55O bond due to the non-ionic carboxyl groups (–COOH and –COOCH3), and may be assigned to carboxylic or theiresters [16]. Asymmetric stretching vibration of ionic carboxylicgroups appears at 1629 cm�1. The peak at 1382 cm�1 may beassigned to symmetric stretching of –COO� of pectin [16]. Thestrong C–O band at 1069 cm�1 due to –OCH3 group, also confirmsthe presence of lignin structure in grapefruit peel [16]. Theadditional peak at 620 cm�1 can be assigned to bending modes ofaromatic compounds. It is clear from the FTIR spectra of grapefruitpeel that carboxyl and hydroxyl groups are present in abundance.The adsorption of Ni(II) and Cd(II) onto the grapefruit peel maylikely be due to the electrostatic attraction between these groupsand the cationic metal molecules. At pH above 4, the carboxylicgroups are deprotonated and negatively charged carboxylateligands (–COO�) bind the charged Ni(II) and Cd(II) molecules.

3.4. Blocking of functional groups

To verify and quantify the participation of functional groups inbiosorption of Cd(II) and Ni(II) ions, batch experiments usinggrapefruit peel with blocked carboxyl and hydroxyl groups werecarried out. The results are given in Table 1. The amount of bothCd(II) and Ni(II) ions adsorbed decreases significantly whencarboxyl groups are esterified. Decrease of approx. 76% and 74%in the sorption of Cd(II) and Ni(II) reflects important role ofcarboxyl groups and ion exchange in the sorption process.Nevertheless, 24.19% Cd(II) and 26.08 Ni(II) ions are still adsorbedby the biosorbent after the blocking of carboxyl groups, indicatingthe presence of other metal binding sites. As seen in Table 1, adecrease of 26.60% and 27.22% in the biosorption of Cd(II) andNi(II), respectively, is noted when hydroxyl groups of grapefruitpeel are blocked. This decrease may be attributed to the blockage ofgrapefruit peel hydroxyl groups and confirms their role in sorption

Fig. 1. Typical SEM micrograph of (A) natural grapefruit peel and (B) grapefruit peel

after biosorption of metal ions.

Fig. 2. FTIR spectra of natural grapefruit peel.

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of heavy metals. Interactions of metals with vicinal hydroxylgroups of pectin and cellulose have been suggested. Theseobservations, along with the possible assigned functional groupsin the FTIR spectra, confirm that the carboxyl and hydroxyl groupsare principally responsible for the removal of Cd(II) and Ni(II) bygrapefruit pee, with carboxyl groups being responsible for most ofthe metal uptake.

3.5. Sorption mechanism

To investigate the mechanism of Cd(II) and Ni(II) sorption bygrapefruit peel, the release of alkali (K+ and Na+) and alkaline earthmetals (Ca2+ and Mg2+) and protons H+ during sorption aredetermined. The net release of the alkali and alkaline earth metalsdue to sorption of Cd(II) and Ni(II) are calculated by subtracting theamount of their release in nanopure water used as the control. Theamount of protons H+ released during the sorption process iscalculated from the amount of NaOH added to keep the pH ofsolution at 5.0 at equilibrium. The results show Ca2+, Mg2+, K+ andNa+, from grapefruit peel due to the uptake of Cd(II) and Ni(II)(Table 2). It is observed that the biosorption of Cd(II) and Ni(II) ongrapefruit peel (85.56% for Cd(II) and 84.20% for Ni(II)) is mainlydue to exchange with alkali and alkaline cations. The remainingremoval of Cd(II) and Ni(II) ions is attributed to ion exchange withproton, released during metal sorption which resulted in thedecrease of pH of metal solution at equilibrium. It can be concludedthat the sorption process for both metal ions on grapefruit peeloccurs mainly through ion exchange involving alkali and alkalineearth metals and, to a lesser extent, with protons bound to nativecarboxylate groups and the concomitant complexation with –OHgroups of polysaccharide component [13].

3.6. Effect of pH

Hydrogen ion concentration in the adsorption process isconsidered as one of the most important parameters that influencethe adsorption behavior of metal ions in aqueous solutions. This ispartly because hydrogen ions themselves are strongly competingwith metal ions for adsorption sites. The effect of pH on biosorptionof cadmium and nickel ions onto grapefruit peel is shown in Fig. 3.Studies beyond pH 8 are not attempted because precipitation ofthe ions as hydroxides would be likely [17]. As can be seen in Fig. 3,biosorption efficiency increases with the increase in the pH valueuntil it reaches maximum adsorption capacities at pH 5.0. Theminimum biosorption at low pH 2.0 may be due to the fact thathigh concentration and high mobility of H+ ions, the hydrogen ions

are preferentially adsorbed rather than the metal ions. At higherpH values, the lower number of H+ and greater number of ligandswith negatives charges results in greater metal ions biosorption.The weakly acidic carboxyl groups are regarded as the main ligandsinvolved in the metal uptake by grapefruit. Since the pKa ofcarboxylic acids ranges from 3.5 to 5.5, more carboxyl groups willbe deprotonated at pH over this range, and thus resulting in morenegative binding sites. Consequently, the attraction of positivelycharged metal ions would be enhanced. Decrease in biosorption athigher pH values is due to the formation of soluble hydroxylatedcomplexes of the metal ions and their competition with the activesites, as a consequence, the retention would decrease again.Therefore, an optimum pH 5.0 is selected for further experiments.

3.7. Effect of biosorbent dosage

The effect of biosorbent dosage on the removal of Cd(II) andNi(II) is shown in Fig. 4. As seen in this figure, percentage removalof Cd(II) and Ni(II) increase with increase in biosorbent dosagefrom 1 to 4 g/L. There is no significant increase in removal of Cd(II)and Ni(II) when biosorbent dosage increases beyond 4 g/L. Thissuggests that after a certain biosorbent dosage, the maximumbiosorption is attained and hence the amount of ions remainsconstant even with further increase in the surface area of thebiosorbent, which in turn increases the number of binding sites.However, at high sorbent dosages the available metal ions areinsufficient to cover all the exchangeable sites on the biosorbent,resulting in low metal uptake [18,19]. Therefore, the optimumgrapefruit peel dosage is selected as 4 g/L and this is used for allfurther experiments.

3.8. Effect of contact time

The effect of contact time on the biosorption of Cd(II) and Ni(II)from aqueous solution is shown in Fig. 5. It is observed that the

Table 1Biosorption of Cd(II) and Ni(II) by grapefruit peel and grapefruit peel with

chemically blocked carboxyl and hydroxyl groups (Co: 50 mg/L; pH 5; adsorbent

dosage: 4 g/L; time: 60 min).

Metal Metal ion adsorbed

Native grapefruit

peel (mg/g)

Grapefruit peel

with blocked

carboxyl groups

(mg/g)

Grapefruit peel

with blocked

hydroxyl groups

(mg/g)

Cd(II) 10.43 2.524 (24.19%) 7.697 (73.40%)

Ni(II) 11.82 3.083 (26.08%) 8.603 (72.78%)

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8 9

Bios

orp�

on (%

)

pH

Ni (II)Cd (II)

Fig. 3. Effect of pH on the biosorption of Cd(II) and Ni(II) ions on grapefruit peel (Co:

50 mg/L; contact time: 150 min; adsorbent dosage: 1 g/L).

Table 2Release of Ca2+, Mg2+, K+, Na+ and H+ after sorption of Cd(II) and Ni(II) by grapefruit peel (Co: 50 mg/L; pH 5; adsorbent dosage: 4 g/L; time: 60 min).

Metal ions Total metal sorbed (mg/g) Net amount of cations and protons released (mg/g)

Ca2+ Mg2+ K+ Na+ H+ Sum

Cd(II) 10.43 5.440 2.455 0.696 0.333 1.440 10.374

Ni(II) 11.82 6.107 2.654 0.829 0.364 1.812 11.764

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biosorption efficiency of Cd(II) and Ni(II) ions increase withincrease in contact time. The biosorption of Cd(II) and Ni(II) ions israpid in the first 30 min, as a result of free binding sites on thebiomass. The biosorption approaches equilibrium within 60 minfor both metal ions as the binding sites on the biomass are beingused up. The optimum contact time is therefore selected as 60 minfor further experiments. Biosorption of both metal ions seems tofollow two-phase sorption mechanism [20,21]. The initial fastphase occurs due to a larger surface area of the biosorbent beingavailable for the adsorption of the metal ions. The subsequent slowphase occurs due to quick exhaustion of the adsorption sites.

3.9. Adsorption isotherm

Biosorption isotherms describe the relationship between themass of the adsorbed component per biosorbent mass and theconcentration of this component in the solution. Determination ofequilibrium parameters provides important information that allows

for future design of adsorption. In the present study, the Langmuirand Freundlich adsorption isotherm models are used to evaluate theequilibrium data. Langmuir isotherm assumes monolayer adsorp-tion onto a surface containing a finite number of uniform adsorptionsites without migration of adsorbed molecules on the surface. Thismodel can be written as follows [22]:

qe ¼qmKLCe

1 þ KLCe(5)

where qe (mg/g) is the amount of metal ions adsorbed per unitmass of adsorbent, Ce (mg/L) is the equilibrium metal ionconcentration in the solution, qm (mg/g) is the monolayer sorptioncapacity of the sorbent (mg/g), and KL is the Langmuir sorptionconstant (L/mg) relating the free energy of sorption and a highvalue indicates a higher affinity.

The essential characteristics of Langmuir isotherm can beexpressed in terms of dimensionless constant separation factor forequilibrium parameter, RL, which is defined as:

RL ¼1

1 þ KLCo(6)

The value of RL indicates the type of Langmuir isotherm to beirreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) orunfavorable (RL > 1).

The Freundlich isotherm model proposes a monolayer sorptionwith a heterogeneous energetic distribution of active sites,accompanied by interactions between adsorbed molecules. TheFreundlich model can be expressed as [23]:

qe ¼ KFC1=ne (7)

where Kf is a constant relating the sorption capacity and 1/n is anempirical parameter relating the sorption intensity, which varieswith the heterogeneity of the material.

The Langmuir and Freundlich isotherm plots for Ni(II) and Cd(II)are given in Figs. 6 and 7, respectively. The calculated results of theLangmuir and Freundlich isotherm constants are given in Table 3. Itis observed that the Freundlich isotherm model shows a better fitto adsorption data than the Langmuir isotherm model for thebiosorption Ni(II) and Cd(II) onto grapefruit peel. The value of n forthe biosorbent used found to be greater than 1, indicating favorableadsorption of Ni(II) and Cd(II) ions on the biosorbent.

The dimensionless constant RL ranges from 0.125 to 0.74 forCd(II) and from 0.074 to 0.615 for Ni(II), so the biosorption for both

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7

Bios

orp�

on (%

)

Biosorbent dosag e (g/L )

Ni (II)Cd (I I)

Fig. 4. Effect of biosorbent dosage on the biosorption of Cd(II) and Ni(II) ions on

grapefruit peel (Co: 50 mg/L; contact time: 150 min).

20

30

40

50

60

70

80

90

100

0 20 40 60 80 10 0 120 140

Bios

orp�

on (%

)

Time (min)

Ni (II)

Cd (I I)

Fig. 5. Effect of contact time on uptake of Cd(II) and Ni(II) ions by tangerine peel (Co:

50 mg/L; pH 5; adsorbent dosage: 4 g/L).

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60

q e(m

g/g)

Ce (mg/L)

Exper imental dataLan gmuir modelFreundl ich model

Fig. 6. Isotherm models of Ni(II) biosorption by grapefruit peel.

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metal ions by grapefruit peel can be considered to be favorable.The monolayer maximum adsorption capacities are calculatedas 46.13 and 42.09 mg/g for Ni(II) and Cd(II), respectively.Compared with several alternative adsorbents in the literaturepresented in Table 4, it is found that grapefruit peel has higheruptake capacities for both metal ions than most other materials,indicating that grapefruit peel is promising biosorbent for theeffective removal of Ni(II) and Cd(II) ions from aqueoussolutions.

3.10. Adsorption kinetics

The pseudo-first order equation is generally expressed asfollows [54]:

dqt

dt¼ k1ðqe � qtÞ (8)

where qe and qt are the sorption capacity at equilibrium and at anytime, t, respectively, in mg/g, and is the pseudo-first order rateconstant, in min�1.

After integrating and applying the boundary conditions, forqt = 0 at t = 0 and qt = qt at t = t, the equation becomes:

qt ¼ qeð1 � e�k1tÞ (9)

The pseudo-first order kinetics model is expressed by thefollowing equation [55]:

dqt

dt¼ k2ðqe � qtÞ

2 (10)

where k2 is the pseudo-second order rate constant, in g/mg min.Integrating this equation for the boundary conditions, gives:

qt ¼q2

e k2t

1 þ qek2t(11)

The constants of kinetic models are determined experimentallyby non-linear regression. The values of rate constant are given inTable 5 along with correlation coefficient and AARE values.Calculated correlations are closer to unity for pseudo-second-orderkinetic model; therefore, the biosorption kinetics could well beestimated more favorably by pseudo-second-order kinetics modelrather than pseudo-first order kinetics for Ni(II) and Cd(II) ions.

0

5

10

15

20

25

30

35

40

0 15 30 45 60 75 90

q e(m

g/g)

Ce (mg/L)

Exper imental dataLan gmuir modelFreundl ich model

Fig. 7. Isotherm models of Cd(II) biosorption by grapefruit peel.

Table 3Langmuir and Freundlich constants for the adsorption of nickel and cadmium on grapefruit peel.

Metal ions Langmuir model Freundlich model

qm (mg/g) KL (L/mg) R2 AARE (%) 1/n KF R2 AARE (%)

Ni(II) 46.13 0.0626 0.924 31.91 0.468 5.76 0.981 12.53

Cd(II) 42.09 0.0349 0.984 17.67 0.512 3.43 0.994 9.83

Table 4Comparison of adsorption potential of various adsorbents for cadmium and nickel removal from aqueous solution.

Cadmium Nickel

Adsorbent qm (mg/g) Reference Adsorbent qm (mg/g) Reference

Eleocharis acicularis 33.71 [24] Padina australis 27.03 [41]

Escherichia coli 10.30 [25] Escherichia coli 6.90 [25]

Hypnea valentiae 28.65 [26] Pine bark 6.28 [42]

Aspergillus niger 4.0 [27] Yohimbe bark 8.80 [43]

Rice hush ash 3.02 [28] Rice hush ash 4.87 [47]

Pleurotus platypus 34.96 [29] Orange peel 9.82 [44]

Pomelo peel 21.83 [30] Cassava peel 57.07 [45]

Papaya wood 17.22 [31] Wallnut sawdust 6.43 [46]

Activated carbon 3.4 [32] Maple sawdust 0.294 [47]

Olive stone 7.33 [33] Tea factory waste 15.26 [48]

Expanded perlite 1.79 [34] Expanded perlite 2.24 [34]

Red mud 106.04 [35] Deactivated protonated yeast 9.01 [49]

Corncob 5.04 [36] Activated sludge 23.9 [50]

S. platensis 73.64 [1] S. platensis 69.04 [1]

Akaganeite-type nanocrystals 17.02 [37] Baker’s yeast 11.38 [51]

Sugarcane bagasse 6.97 [38] Sugarcane bagasse 2.0 [38]

Oscillatoria sp. H1 30.10 [39] Grape stalks wastes 10.66 [52]

Fungus (Trametes versicolor) biomass 166.60 [40] Marine macroalgae 181.0 [53]

Grapefruit peel 42.09 Present study Grapefruit peel 46.13 Present study

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3.11. Adsorption thermodynamics

The thermodynamic parameters obtained for the sorptionprocess are calculated by using the following equation:

ln KD ¼DS�

R�DH�

RT(12)

where KD is the distribution coefficient (mL/g), DH8 is the enthalpychange (kJ/mol), DS8 is the entropy change (J/mol K), T is thetemperature (K), and R is the universal gas constant (8.314 J/mol K). The Gibbs free energy change (DG8) values (kJ/mol) arecalculated from the following equation:

DG� ¼ DH� � TDS� (13)

The enthalpy change of biosorption, DH8, and the entropy changeof biosorption, DS8, can be obtained the slopes and intercepts oflinear regression of ln KD vs. 1/T (Fig. 8). These thermodynamicparameters are given in Table 6. The negative DG8 values confirmthe feasibility of the biosorption process and the spontaneousnature of biosorption. The positive values of DH8 confirm theendothermic nature of the process and the positive values of

entropy, DS8, indicate the affinity of the biosorbent for cadmiumand nickel.

3.12. Desorption efficiency and reusability

The regeneration of the biosorbent is one of the key factors inassessing of its potential for commercial applications. Njikam andSchiewer [56] suggested that mineral acids such as HCl, which ischeap and relatively harmless are promising desorbing agents forefficient citrus peels regeneration. Therefore, 0.1 M HCl is used asthe desorbing agent for the elution of Cd(II) and Ni(II) from metal-loaded grapefruit peel. More than 97% of the desorbed Cd(II) andNi(II) is desorbed from the metal-loaded biosorbent. The reusabil-ity of the biosorbent is also investigated in five consecutivesorption–desorption cycles (Fig. 9). These results indicate that thegrapefruit peel offers potential to be used repeatedly for removal ofCd(II) and Ni(II) from aqueous solutions.

4. Conclusions

This work demonstrates the feasibility of using grapefruit peelfor the biosorption of Ni(II) and Cd(II) ions. The batch studyparameters such as pH of solution, biosorbent dosage, contacttime, and temperature are found to be effective on the biosorptionprocess. The desired pH value for biosorption is found to be 5.0. Themaximum uptakes of Cd(II) and Ni(II) by grapefruit peel areestimated to be 42.09 and 46.13 mg/g, respectively. Freundlichisotherm model shows better fit to the sorption data of both Ni(II)and Cd(II) than the Langmuir isotherm model. The biosorption rateof both metal ions is very fast and the equilibrium is achieved after60 min. The kinetics of the process is well described using thepseudo-second order model. The calculated thermodynamicparameters show the feasibility, endothermic and spontaneousnature of the biosorption of Ni(II) and Cd(II) ions onto grapefruitpeel.

FTIR spectra reveals that the carboxyl and hydroxyl groups inthe grapefruit peel are responsible for Cd(II) and Ni(II) ionsbiosorption. The recovery tests indicate that 0.1 M HCl solution isable to elute more than 97% of Cd(II) and Ni(II) from the biosorbent.The biosorbent could be regenerated and reused for at least fiveadsorption–desorption cycles of Cd(II) and Ni(II) without anysignificant decrease in its metal uptake capacity. Based on the

Table 5Kinetic parameters for the biosorption of Ni(II) and Cd(II) onto grapefruit peel.

Pseudo-first-order kinetic model

k1 (min�1) qe (mg/g) R2 AARE (%)

Ni(II) 0.0897 11.36 0.709 11.23

Cd(II) 0.0691 10.27 0.885 9.64

Pseudo-second-order kinetic model

k2 (g/mg min) qe (mg/g) R2 AARE (%)

Ni(II) 0.0104 12.61 0.868 7.11

Cd(II) 7.96 � 10�3 11.68 0.953 5.57

5

6

7

8

9

10

11

3 3.1 3.2 3.3 3.4 3.5

lnK D

1/T × 103 (K-1)

Ni (II )

Cd (I I)

Fig. 8. Plots of ln KD vs. 1/T for the nickel and cadmium biosorption on grapefruit

peel.

Table 6Thermodynamic parameters for uptake of nickel and cadmium on grapefruit peel.

Metal ion DG8 (kJ/mol) DH8(kJ/mol)

DS8(J/mol K)

20 8C 30 8C 40 8C 50 8C

Ni(II) �18.35 �21.10 �23.86 �26.63 62.52 276.19

Cd(II) �16.33 �18.12 �19.91 �21.70 36.11 179.66

80

84

88

92

96

100

1 2 3 4 5

Deso

rp�o

n (%

)

Cycle numbe r

Cd (I I)

Ni (II)

Fig. 9. Reusability of grapefruit peel with repeated sorption–desorption cycle (Co:

50 mg/L; pH 5; adsorbent dosage: 4 g/L; time: 60 min).

M. Torab-Mostaedi et al. / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 295–302 301

Author's personal copy

results of the present work, the grapefruit peel can be used aspromising biosorbent for the treatment of wastewaters containingheavy metal ions because of advantages of being natural, low-costbiomass, high uptake capacity and rapid biosorption rate.

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