research article comparative biosorption studies of hexavalent...

Hindawi Publishing CorporationAdvances in Physical ChemistryVolume 2013, Article ID 159712, 9 pageshttp://dx.doi.org/10.1155/2013/159712

Research ArticleComparative Biosorption Studies of Hexavalent Chromium Iononto Raw and Modified Palm Branches

Mona A. Shouman, Nady A. Fathy, Soheir A. Khedr, and Amina A. Attia

Surface Chemistry and Catalysis Laboratory, National Research Centre, El-Dokki, Cairo 12622, Egypt

Correspondence should be addressed to Amina A. Attia; [email protected]

Received 29 March 2013; Revised 20 May 2013; Accepted 9 June 2013

Academic Editor: Marc Koper

Copyright © 2013 Mona A. Shouman et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The waste of palm branches (PB) was tested for its ability to remove chromium (VI) from aqueous solution by batch and columnexperiments. Palm branches chemically modified with an oxidizing agent (sulphuric acid) then coated with chitosan and surfactant(hexadecyl trimethyl ammoniumbromide surfactant,HDTMA), respectively, were carried out to improve the removal performanceof PB. The results of their Cr (VI) removal performances are pH dependent. The adsorption data could be well interpreted by theLangmuir, Freundlich, and Flory-Huggins isothermmodels.Themaximumadsorption capacity obtained from the Langmuirmodelfor the chitosan coated oxidized palm branches is 55mg/mg. The adsorption process could be described by pseudo-second-orderkinetic model. The intraparticle diffusion study revealed that film diffusion might be involved. The biosorbents were successfullyregenerated using 1M HCL solution.

1. Introduction

Increased industrialization and human activities have createdimpact on the environment through the disposal of wastecontaining heavy metals. Heavy metals are toxic even inextremely minute quantities on human health and to thefauna and flora of receiving water [1]. It is known that legalstandards on environment control are becoming strict and,as a result, the discharge of heavy metals into aquatic bodiesand sources of potable water is being rigorously controlled.Most heavy metals exist in a cationic form, but Cr (VI)combined with oxygen is anionic in natural aquatic system.The maximum permissible limit of chromium in naturalwater is only 0.05mg/L by the US Environmental Protec-tion Agency [2]. Acute exposure to Cr (VI) causes nausea,diarrhea, liver, and kidney damage, internal hemorrhage,and respiratory problems [3, 4]. Chromium compounds arewidely used in electroplating,metal finishing,magnetic tapes,pigments, leather tanning, wood protection, chemical manu-facturing, electrical and electronic equipment, and catalysis[5]. Several treatment technologies have been developed toremove chromium from aqueous phase. Common methods

include biosorption [6], ion exchange [7], chemical reduc-tion, ultrafiltration [8], and adsorption [9]. The adsorptionprocess is an effective technique for the treatment of anionicmetals. Activated carbon, a representative adsorbent, hasbeen widely used to remove diverse pollutants, includingorganic pollutants, reactive dyes, cationic heavy metals, andanionic metals from the aqueous phase. Although it is apreferred adsorbent, and its widespread use, it is restricteddue to its high cost production. In order to decrease thecost of treatment, attempts also have been made to findinexpensive alternative adsorbents [10]. Recently, there is agreat interest to improve the adsorption capacity of low-costadsorbents for remediation of wastewater [11–13]. Nowadays,surface modification technique of diverse biosorbents hasbeen implied to increase the surface charge and to enhancetheir adsorption capacity for ionic pollutants [11, 13]. Sur-factants are chemicals that have an amphipathic structurealong with a hydrophobic tail and a hydrophilic head. Thesurfactants are adsorbed onto solid surface and change thesurface properties of solids, especially at the interface. Up todate, the performance of surfactants in water and wastewatertreatment has been regarded for the separation of metal

2 Advances in Physical Chemistry

Table 1: Physicochemical properties of the biosorbents.

Properties PB COPB SOPBpHPZC 6 7.3 7.5Surface area, m2/g 2.8 6.3 5.9Total pore volume, cm3/g 0.004 0.012 0.012Average pore diameter, A 9.9 12.5 19.5

ions and other toxic substance [14, 15]. Also, the use ofchitosan over commercially available agricultural byproductsfor the removal of heavy metals is becoming popular dueto its low cost, abundant availability, non toxicity, potentialfor ion exchange, and it has been recently reported insome papers that chitosan adsorbent has high adsorptioncapacities towards many classes of heavy metals [16], dyes[17], and other ionic molecules [18]. Chitosan, a naturallyoccurring linear polymer of glucosamine, can be obtainedfrom crustacean chitin on an industrial scale due to a largeamount of crustacean’s exoskeleton is available as a byproductof food processing industries.

Palm branches (PB) are produced in great quantitiesin our country, especially in Siwa Oasis. Palm tree wastesinclude trunks, fronds, fruit waste, and empty fruit branches.The farmers trim about 12 branches yearly from each tree,so the authors have decided that a fraction of this wood isbelieved to be enough to supply the growing demand forpreparing potential biosorbents.The palm tree branches havethe characteristic of exchange sorption properties due tothe presence of some functional groups such as carboxylic,hydroxyl, and lactone which have a high affinity for metalions [19]. In the present investigation an attempt was made toexplore the effect of both the chitosan and cationic surfactantmodifying agents on the adsorption performance of palmbranches for removing Cr (VI) ions by using batch andcolumn systems. In order to better understand the adsorptioncharacteristics, isotherms and kinetics were employed toevaluate the sorption process.

2. Material and Method

2.1. Adsorbate Solution. The stock solution of Cr (VI) wasmade by dissolving 0.2827 g K

2Cr2O7(AR grade fromMerck,

Germany) in 1 L double distilled water. The solutions ofdifferent concentrations used in various experiments wereobtained by dilution of the stock solutions.

2.2. Adsorbent Materials. The palm tree branches (PB) wereprovided from the farm of our institute. The precursor wasfirst washed to remove dirt from its surface and then driedin an oven overnight at 100∘C. The dried PB were crushedand sieved to desired mesh size of 2-3mm.Then the dried PBwere treated with 2% H

2SO4(v/v) in an incubator at 100∘C

for 24 h and soaked with deionized water until the solutionpH was stable. Afterwards, the adsorbent was soaked in 3%NaHCO

3(w/v) till any residual acid leftwas removed. Finally,

the sample was dried overnight in an oven at 100∘C, then

cooled at room temperature, and stored in a desiccator [16].Hereafter, it was called OPB (oxidized palm tree branches).

Surface modification of OPB by the chitosan and cationicsurfactant was carried out as follows. About 20 g of chitosanflakes (98% acetylation degree, Figure 1(a)) were immersedin 10% of acetic acid (v/v) with constant stirring for 24 h toform a homogenized whitish viscous gel. Afterwards, OPBwas dipped into the gel (dose ratio of chitosan to OPB is1 : 5) and shaken overnight at 120 rpm. This sample was thenwashed with deionized water and dried at 100∘C. It was calledCOPB. The process was repeated for four times to form athick coating of chitosan onto oxidized palm branches. Incase of modification with a cationic surfactant, a 10 g of OPBwas added to a solution of hexadecyltrimethyl ammoniumbromide (HDTMA) of 0.197 g/L (Figure 1(b)). This concen-tration corresponds to the 0.5 critical micelle concentration(CMC), which is a measure of surfactant efficiency. Abovethe CMC, a surfactant forms a micelle that is very stableand remains in the bulk zone. For that reason, 0.5 CMC wasselected tomaximize the amount of surfactant adsorbed ontothe OPB. The mixture was agitated for 24 h and then filtered.The filtered OPB was dried at 100∘C for 4 h. The surfactantmodified OPB was called SOPB. The characteristics of thethree raw PB, COPB, and SOPB biosorbents are summarizedin Table 1.

2.3. Characterization of the Biosorbents. The determinationof pHPZC for the biosorbents was performed in 50mL of0.01M NaCL solution and placed in a closed Erlenmeyerflask. The pH was adjusted to a value between 2 and 12 byadding 0.1M NaOH or HCl solutions. Then, 0.1 g of biosor-bents was added and the final pH was measured after 48 hunder agitation at 30∘C. The pHPZC is the point where thecurve pHfinal versus pHinitial crosses the line pHinitial = pHfinal[20].

Themorphology of biosorbents was analyzed by scanningelectron microscope using JEOL 560LV SEM at 20KeV withbackground subtraction and a summation of 60 scans. Thefunctional surface groups present on the prepared biosor-bents were determined by FTIR analysis. The FTIR spectraof the biosorbents were recorded using JASCO-410 FTIRinstrument in the region 4000–400 cm−1.The specific surfacearea and total pore volume of the biosorbents were calculatedfrom the adsorption data of nitrogen at 77K by BET analyzer(Quantachrome AS1Win, Version 2.01, USA).

2.4. Batch Studies. All batch experiments were carried outwith biosorbent samples in 250mL conical flasks with 100mLCr (VI) aqueous solutions on a rotary shaker at 250 rpm/min.The concentration of chromium ion was determined spec-trophotometrically at 540 nm using 1,5-diphenyl carbazide(Sigma-Aldrich, USA) as the a complexing agent [21]. Thestudy of the pH (2–12) dependency of Cr (VI) adsorptiononto PB, COPB, and SOPB was carried out in 100mL of Cr(VI) solution with initial concentration of 100mg/L at roomtemperature.The pH value was adjusted by 1MNaOHorHClsolution.

Advances in Physical Chemistry 3

HOHO

HOHO OH

OHOHOH

OOOOO

NH2NH2NH2

n

(a)

H3C(H2C)15 N+ CH3

CH3

CH3 Br−

(b)

Figure 1: Chemical structures of (a) chitosan and (b) hexadecyltrimethyl ammonium bromide surfactant.

Isotherm studies were conducted by contacting 0.1 gof PB, COPB, and SOPB samples with 50mL of Cr (VI)solution at different initial concentrations (20, 40, 60, 80,and 100mg/L) shaking for 24 h at room temperature (30∘C).The maximum removal of Cr (VI) solution was found atpH 2 for PB while for COPB and SOPB adsorbents at pH6, respectively. The percentage dye adsorbed (%𝑅) and theamount of residual metal ions adsorbed on biosorbents (𝑞

𝑒,

mg/g) was calculated by the following equations:

%𝑅 =(𝐶0− 𝐶𝑒) 100

𝐶0

,

𝑞𝑒=

(𝐶0− 𝐶𝑒) 𝑉

𝑊

,

(1)

where 𝑞𝑒is the equilibrium adsorption capacity (mg/g) and

𝐶0and 𝐶

𝑒are the initial and equilibrium liquid-phase con-

centrations of solute in aqueous solution (mg/L), respectively.𝑉 is the liquid phase volume (L) and 𝑊 is the mass ofbiosorbent used (g).

To investigate the kinetic characteristics of the adsorp-tion, 0.1 g of biosorbents was added to 100mL of heavymetal solution with initial concentration of 100mg/L and thesamples were agitated for designated time periods (5, 10, 20,30, 40, 50, and 60min).

2.5. Column Studies. The fixed-bed column was made ofPyrex glass tube of 2 cm inner diameter and 30 cmheight.Thecolumn performance of Cr (VI) adsorption onto biosorbentswas studied at initial concentration of 100mg/L, bed height60mm, and flow rate 2mL//min. The Cr (VI) solutionwas pumped to the column in a down-flow direction by aperistaltic pump. Samples were collected at regular intervalsat 30∘C and the initial pH was 2.

2.6. Column Regeneration Studies. Regeneration studies helpto elucidate the recycling nature of the spent biosorbent. Theregeneration method was performed in the same columnpacked with 1 g of each of the three different biosorbents.Then the columnwas loadedwith the 100mg/L concentrationof Cr (VI) solution at the flow rate of 2mL/min. The columnwas operated until the effluent concentration matched theconcentration of the loaded chromium (VI). After com-plete saturation of the adsorbent, distilled water was passedthrough the column to remove traces of undesorbed Cr (VI).

Afterwards, the efficiency of 1M HCl was tested as an eluentto desorb Cr (VI) from the three biosorbents.

3. Results and Discussion

3.1. Surface Features of the Prepared Biosorbents. Figures2(a), 2(b), and 2(c) show the SEM micrographs for the rawpalm branches (PB) and its modified biosorbents (COPBand SOPB). In Figure 2(a), the surface of PB was rough anddented, besides small pores are also evident. For COPB andSOPB biosorbents, Figures 2(b) and 2(c) show highly definedpores and cavities resulted in the modification process. Thisexplainswhy theBET surface area value for PBwas lower thanthat of COPB and SOPB (cf. Table 1).

To confirm the type of functional groups, FTIR spectraof the three biosorbents are shown in Figure 3. Whencomparing the FTIR spectra of palm tree branches precursorwith the modified samples, significant shifts in absorptionpeaks were obtained which may be assigned to the effectof modification process on the surface of PB. As shown inFigure 3, characteristic peaks obtained for PB are 3452 cm−1(stretching vibration of the –OHgroups), 2926 and 2874 cm−1(C–H sp3 asymmetric and symmetric stretching), 1633 cm−1(C=O stretching of acetyl groups), 1446 cm−1 (O–H bend-ing), and 1174 and 1090 cm−1 (alcoholic C–O) [22]. Aftermodification, the peaks obtained for COPB biosorbent arefound to be 3423 cm−1 (wide peak of the –NH groupsstretching), 2926 cm−1 (C–H stretching of polymer back-bone), 1628 cm−1 (N–Hbending andC=O stretching of acetylgroups), 1320 cm−1 (C–N stretching), and 617–560 cm−1(nitro compounds) [23]. This confirms that the chitosancopolymer is successfully loaded onto palm tree branches.For SOPB (Figure 3(c)), their absorption peakswere observedat 3751 cm−1 (N–H groups stretching of secondary amides).Depending on the configuration of surfactant molecules, thevibration bands of HDTMA micelles possess vibration at2927 cm−1 that have been assigned to the symmetric andasymmetric stretching vibration of methyl and methylenegroups. Bands at about 1600–1200 cm−1 have been ascribedto vibration of trimethyl ammonium quaternary group CN(CH)3

+ [24].This result indicates that the HDTMAmodifiedthe surface properties of palm tree branches.

3.2. Effect of pH. The effect of initial solution pH on Cr (VI)removal by three biosorbents was performed over a range 2–12 and the studies were carried out for 24 h. Figure 4 indicates


80 𝜇m ×600

(a)

50 𝜇m ×1000

(b)

50 𝜇m ×1000

(c)

Figure 2: SEM images of (a) PB, (b) COPB, and (c) SOPB biosorbents.

70

75

80

85

90

95

100

105

400900140019002400290034003900

PBCOPBSOPB

(a)

(b)

(c)

T(%

)

Wavelength (cm−1)

Figure 3: FTIR spectra of (a) PB, (b) COPB, and (c) SOPB biosor-bents.

that the Cr (VI) uptakes were much higher in acidic solutionthan those in alkaline conditions. Optimal metal removalefficiency occurs at pH 6 for COPB and SOPB while for PBthe optimal is pH 2. It can be explained that the pH of theaqueous solution affects the speciation of chromium and thesurface charge of the biosorbent [24, 25].

Cr (VI) exists in different forms in aqueous solution andthe stability of these forms is dependent on the pH of thesystem. In the pH range of 1 to 6 different forms of chromiumions such as Cr

2O7

2−, HCrO4

2−, Cr3O10

2−, and Cr4O13

2−,while HCrO

4

2− predominates in the solution [25]. Thus Cr(VI) ions exist predominantly as HCrO

4

2− in acidic mediumand the amino groups (–NH

2) of chitosan (COPB) would

be in protonated cationic form (–NH3

+) to a higher extentin acidic solution which cause an electrostatic interactionbetween the biosorbent and HCrO

4

2− resulting in highchromium removal [26]. The same trend was also observedfor SOPB as depending on the configuration of surfactantmolecules, the surfactant head (trimethyl ammonium group)has a lot of positive charge in acidic medium [27]. Withrespect for PB, originally the palm branches have a lot of

0102030405060708090

100

0 2 4 6 8 10 12 14pH

PBCOPBSOPB

Rem

oval

of C

r (V

I) (%

)

Figure 4: Effect of pH on the removal of Cr (VI) by the biosorbents.

positively charged groups on the surface as depicted fromFTIR, for this reason its adsorption capacity is relatively highin acidic medium.

At higher pH, the decrease in removal of chromiummay be explained by the dual competition of both anions[HCrO

4

2− and OH−] to be adsorbed on the surface of thebiosorbents of which OH− predominates in the solution [27,28].The pHPZC obtained for the three biosorbents were in therange 6–7.5. This is consistent with results reported in ourstudy [27]. Accordingly, the biosorption of chromium ionsperformed at pH 2 for raw PB while for COPB and SOPBsamples occurred at pH 6.

3.3. Adsorption Isotherms. Adsorption isotherm studies areimportant to determine the efficacy of adsorption. Thisprovides a relationship between the concentration of Cr (VI)in the liquid phase and the amount of Cr (VI) adsorbed on thesolid-phase when the two phases are at equilibrium. Lang-muir, Freundlich, and Flory-Huggins adsorption isothermswere used in this study [29].


0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60

PBCOPBSOPB

Ce (mg/L)

qe

(mg/

g)

Figure 5: Adsorption isotherms of Cr (VI) on the biosorbents.

The Langmuir model assumes that a monomolecularlayer is formedwhen adsorption takes part without any inter-action between the adsorbed molecules [30]. The Langmuirmodel can be represented as linear form:

𝐶𝑒

𝑞𝑒

=

1

𝐾𝐿𝑞max+

1

𝑞max𝐶𝑒, (2)

where 𝑞max is the maximum monolayer adsorption capacity(mg/g), and 𝐾

𝐿is the Langmuir constant (g/L) related to

the affinity of the binding sites. The experimental data wereplotted as 𝐶

𝑒/𝑞𝑒versus 𝐶

𝑒(figure not shown). The values

of Langmuir constants 𝑞max and 𝐾𝐿 were obtained by linearregression method and are listed in Table 2.

The biosorption isotherm of Cr (VI) on the threebiosorbents is presented in Figure 5. This figure shows thatthe biosorption of Cr (VI) within the three biosorbentscorresponds to type-S adsorption isotherm in the Gilesclassification system [31]. In this type, the initial curvatureshows that, asmore sites in the adsorbent are filled, it becomesincreasingly difficult for a bombarding solutemolecule to finda vacant site available. This implies either that the adsorbedsolutemolecule is vertically oriented or that there is no strongcompetition from the solvent.The types of systemswhich givethis curve do in fact fulfill these calculations. Thus they haveone of the following characteristics: (a) are mono-functional,(b) have moderate intermolecular attraction, causing it topack vertically in regular array in the adsorbed layer, and (c)meet strong competition for substrate sites from moleculesof the solvent or of another species [31, 32]. The weaknessof the adsorbate-adsorbent forces will cause the uptakeat low concentrations to be small, but once a moleculehas become adsorbed, the adsorbate-adsorbent forces willpromote the adsorption of further molecules—a cooperativeprocess. Therefore, the isotherm will become a convex to theconcentration axis. A polar adsorbate of particular interest inthis context is water because the dispersion contribution to itsoverall interaction energy is usually small compared with the

polar contribution. Not surprisingly, water provided manyexamples of type-S isotherms. One of the palm branchesis covered with a layer of adsorbed water; however, theadsorbate-adsorbent interaction would be virtually reducedto the weak dispersion energy of water with Cr (VI), so thata type-S isotherm is obtained. In a system that gives rise to atype-S isotherm, however,multiple layers are built upon someparts on the surface while a monomer is still incomplete onother parts.

The Freundlich isotherm is an empirical equation thatassumes that the adsorption process takes place on hetero-geneous surfaces and adsorption capacity is related to theconcentration of Cr (VI) at equilibrium.This isothermmodelis defined by the following linear form:

log 𝑞𝑒= log𝐾

𝐹+

1

𝑛

log𝐶𝑒, (3)

where 𝐾𝐹is the Freundlich constant (mg/g)/(L/mg)1/𝑛 and

also referred to adsorption capacity, while 𝑛 is the hetero-geneity factor and related to adsorption intensity. The valueof 𝑛 can be used also to describe the adsorption whether islinear (𝑛 = 1) or a physical process (𝑛 > 1) is favorable,or a chemical process (𝑛 < 1). On the other hand, thevalue of 1/𝑛 < 1 or 1/𝑛 > 1 indicates a normal Langmuirisotherm and cooperative adsorption, respectively. A plot ofln 𝑞𝑒versus ln𝐶

𝑒gives a straight line, where the values of

𝐾𝐹and 1/𝑛 are determined from the intercept and the slope,

respectively (figure not shown). As depicted from Table 2,the Cr (VI) adsorption characteristics of the three adsorbentsare better explained by the Langmuir isotherm model. Also,the adsorption capacity of Cr (VI) onto COPB shows ahigher amount, and it can be implied that the amine groups(NH3

+) of chitosan are largely responsible for the adsorptionof chromium ions from the solution, and the interaction ischiefly electrostatic attraction in nature [33].

The Florry-Huggins accounts for the degree of surfacecoverage characteristics of adsorbate on the biosorbent andthe linear form of the equation is expressed as [34]

log( 𝜃𝐶0

) = log𝐾FH + 𝑛FH log (1 − 𝜃) , (4)

where 𝜃 is the surface coverage of the adsorbent by adsorbateand 𝐾FH and 𝑛FH are the Florry-Huggins constants. Theseconstants can be obtained from the plot of log (𝜃/𝐶

0) versus

log(1−𝜃) (figure not shown). Table 2 showed that the apparentnumber 𝑛FH of metal ionoccupying sorption sites is greaterfor COPB than that of SOPB and PB biosorbents. The valuesof correlation coefficient (𝑅2) were found to be ≤0.98. Thevalue of 𝐾FH was used for the calculation of the spontaneityGibbs free energy (Δ𝐺0 = −𝑅𝑇 ln(𝐾FH), J/mol). It was foundthat theΔ𝐺0 valuewas negative that pointed out the feasibilityand spontaneity of the process.

3.4. Adsorption Kinetics. Figure 6 shows the effect of contacttime on the adsorbed amount of Cr (VI) by the raw PB,COPB, and SOPB samples, respectively. It can be seen thatthe kinetics of Cr (VI) removal by modified palm branches


Table 2: Langmuir, Freundlich, and Flory-Huggins constants for the uptake of Cr (VI) on the biosorbents.

Adsorbents Langmuir Freundlich Florry-Huggins𝑞max (mg/g) 𝐾

𝐿(L/g) 𝑅

2𝐾𝐹

𝑛 𝑅

2𝐾FH 𝑛FH Δ𝐺

0 (kJ/mol) 𝑅

2

PB 25 0.023 0.982 0.230 1.0 0.883 0.018 3.4 −32.2 0.969COPB 55 0.047 0.988 10 1.3 0.912 0.139 4.0 −31.8 0.978SOPB 41.7 0.660 0.990 6.3 3.5 0.887 0.103 2.4 −31.7 0.982

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70Time (min)

PBCOPBSOPB

qt

(mg/

g)

Figure 6: Effect of contact time on the adsorbed amount of Cr (VI)by the biosorbents.

presents a shape characterized by a strong increase of thecapacity of chromium removal during the first fewminutes ofcontact between the solution and biosorbents, and followupof a slow increase until a state of equilibrium is reached.

In order to examine the mechanism of adsorptionprocess, pseudo-first-order (Lagergren) and pseudo-second-order kinetic models were used, respectively, as follows:

log (𝑞𝑒− 𝑞𝑡) = log 𝑞

𝑒−

𝑘1𝑡

2.303

,

𝑡

𝑞𝑡

=

1

𝑘2𝑞𝑒

2+

𝑡

𝑞𝑒

,

(5)

where 𝑞𝑒and 𝑞𝑡are the amounts of Cr (VI) adsorbed (mg/g)

onto the three biosorbents at equilibrium and at time 𝑡 (min),respectively. 𝑘

1(min−1) and 𝑘

2(g/mg⋅min) are the adsorption

rate constants of both pseudo-first and pseudo-second ordermodels. A plot of (5) gave straight lines with very promisingcorrelation. However, data was more linked when examinedaccording to pseudo-second-ordermodel as depicted by highvalue correlation coefficient (𝑅2) as shown in Table 3 andalso Figure 7. In addition the calculated values from thepseudo-second order model (𝑞

𝑒,cal) were also found to bevery closed to experimental values (𝑞

𝑒,exp).Thus experimentalresults support the assumption behind themodel that the ratelimiting step in adsorption of heavy metals is chemisorptioninvolving valences forces through the sharing or exchange of

00.10.20.30.40.50.60.70.80.9

1

0 10 20 30 40 50 60 70Time (min)

PBCOPBSOPB

t/qt

Figure 7: Pseudo-second-order plots for the biosorbents.

electrons between biosorbent and metal ions. Some studieson the kinetic of Cr (VI) adsorption onto various adsorbentshave also reported higher correlations for pseudo-second-order model [35].

Further, the intraparticle diffusion model can be addedto determine the rate-determining step in the adsorptionprocess of Cr (VI) by the prepared adsorbents. The equationis given as follows:

𝑞𝑡= 𝑘𝑝𝑡

1/2+ 𝐶, (6)

where 𝑘𝑝, 𝑡1/2, and 𝐶 are the intraparticle diffusion rate

constant (mg/g⋅min1/2), square root of the time (min)1/2, andthe intercept from the plot of 𝑞

𝑡versus 𝑡1/2 (not shown here),

respectively. Intraparticle diffusion plots show multilinear,implying that more than one process affected the adsorptionof chromium ions.The initial portion of the plot indicated anexternal mass transfer, whereas the second linear portion isdue to the intraparticle or pore diffusion [36].

3.5. Column Adsorption Studies. The time for breakthroughappearance and the shape of the breakthrough curve veryimportant characteristics for determining the operation andthe dynamic response of an adsorption column [37–39]. Thebreakthrough curves show the loading behavior of metalto be removed from solution in a fixed bed and is usuallyexpressed in terms of adsorbed metal concentration [𝐶ad =inlet metal concentration (𝐶

0)− outlet metal concentration


Table 3: Kinetic parameters for Cr (VI) adsorption on the biosorbents.

Adsorbents 𝑞𝑒,exp (mg/g) Pseudo-first order Pseudo-second order Intraparticle diffusion

𝑘1𝑞𝑒,cal (mg/g) 𝑅

2𝑘2

𝑞𝑒,cal (mg/g) 𝑅

2𝐾𝑝

𝐶

PB 80.5 0.013 38 0.9251 6.4 × 10

−3 82 0.997 7.2 19COPB 96.2 0.017 79 0.7294 1.7 × 10

−4 98 0.999 6.0 20SOPB 89.5 0.014 76 0.9225 7.1 × 10

−4 90 0.998 7.6 24

Table 4: Parameters of breakthrough curve for Cr (VI) adsorptionon the biosorbents.

AdsorbentsBreakthrough parameters

𝐶𝑜

(mg/g)𝑡tot

(min) 𝑚tot𝑞tot

(mg/g)Total

% removal𝑞𝑒

(mg/g)PB 100 355 50 24 62.4 12COPB 100 288 58 31 55.1 19.5SOPB 100 250 71 39 41 15.6

00.10.20.30.40.50.60.70.80.9

0 50 100 150 200 250 300 350Time (min)

PBCOPBSOPB

Ct/C0

Figure 8: Breakthrough curves for Cr (VI) adsorption onto thebiosorbents (flow rate = 2mL/min, initial concentration = 100mg/L,and pH = 2).

(𝐶𝑡)] or normalized concentration defined as the ratio of

effluent metal concentration to inlet metal concentration(𝐶𝑡/𝐶0) as a function of time or volume of effluent for a given

bed height [37]. Effluent volume (𝑉eff) can be calculated fromthe following equation:

𝑉eff = 𝑄𝑡, (7)

where 𝑡 and 𝑄𝑡are the total flow time (min) and vol-

umetric flow rate (mL/min), respectively. The area underthe breakthrough curve (𝐴) is obtained by integrating theadsorbed concentration (𝐶ad) versus time (𝑡), and plot canbe used to find the total adsorbed metal quantity (maximumcolumn capacity). Total adsorbed metal quantity (𝑞tot, mg/g)

in the column for a given feed concentration and flow rate iscalculated from the following equation:

𝑞tot =𝑄𝑡𝐴

1000

=

𝑄𝑡

1000

∫

𝑡=total

𝑡=0

𝐶ad𝑑𝑡. (8)

The total amount of adsorbed metal ions sent to column(𝑚tot) is calculated from the following equation [38]:

𝑚tot =𝐶0𝑄𝑡

1000

. (9)

Total removal is calculated from the following equation[36]:

Total removal% =𝑞tot𝑚tot× 100. (10)

Equilibrium metal uptake (𝑞eq) in the column is definedby (11) as the total amount of metal adsorbed per gram ofadsorbent (𝑋) at the end of the total flow time:

𝑞eq =𝑞tot𝑋

. (11)

The breakthrough curves of Cr (VI) adsorption by PB,COPB, and SOPB at flow rate (2mL/min), fixed bed heightof 6 cm, and initial Cr (VI) concentration of 100mg/L areshown in Figure 8. At the interval time of 80min, thevalue of 𝐶

𝑡/𝐶0reached 0.29, 0.39, and 0.42 for PB, SOPB

and COPB, respectively. The breakthrough curve of COPBbecomes steeper than the other sorbents.The probable reasonbehind this is the presence of deriving force which representsthe amine groups, these groups are responsible for interactionbetweenCOPB andCr (VI) ions, which leads to an increase inthe total adsorbed quantities and maximum Cr (VI) uptakesand removal percent [39] (cf. Table 4).

3.6. Desorption Column Studies. The possibility of adsorbentregeneration (desorption) and metal recovery was primarilystudied based on the general assumption that the regenera-tion of adsorbent promotes economic adsorption treatment.The results showed 50% desorption for raw palm branches(PB), while 95% desorption for both COPB and SOPB with1M HCl solution. From this data it may be implied that theadsorbed chromium forms a film of Cr (OH)

3on the external

surface of modified palm branches and this film can be easilydissolved in 1M HCl [1].

4. Conclusions

This study investigated the biosorption characteristics of Cr(VI) on raw and modified palm branches (PB). Adsorption


of hexavalent chromium ion is found to be effective in thelower pH range. Equilibrium isotherm data were fitted usingLangmuir and Flory-Huggins. Kinetic study showed thatthe pseudo-second-order kinetic model is appropriate andfilm diffusion might be involved in the sorption process.The chitosan coated palm branches (COPB) appear to betechnically feasible and eco-friendly and with high efficacycompared to the other adsorbents. The adsorbents could beregenerated by using hydrochloric acid and therefore canbe reused. These adsorbents could be good candidate foradsorption of Cr (VI) ions.

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