film-pore diffusion modeling for sorption of azo dye on to exfoliated graphitic nanoplatelets

8/13/2019 Film-Pore Diffusion Modeling for Sorption of Azo Dye on to Exfoliated Graphitic Nanoplatelets

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Indian Journal of Chemical TechnologyVol. 20, January 2013, pp. 7-14

Film-pore diffusion modeling for sorption of azo dye on to exfoliatedgraphitic nanoplatelets

T Maiyalagan1 & S Karthikeyan2,*1School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639 798

2Department of Chemistry, Chikkanna Government Arts College, Tirupur 641 602, India

Received 8 September 2011; accepted 8 May 2012

Exfoliated graphitic nanoplatelets (xGnPs) have been utilized as a potential adsorbent for toxic textile dye Acid Orange7 (acid dye). The effects of major variables governing the efficiency of the process, such as temperature, initial dye

concentration and pH are studied. The kinetic measurements have been used for determining the specific rate constant,confirming the applicability of pseudo first-order rate expression. Plausible mechanism of ongoing adsorption processinvolved is obtained by carrying out kinetic measurements. To identify whether the ongoing process is particle diffusion orfilm diffusion, the treatments given by Boyd and Reichenberg have been employed. The influence of different factors on theadsorption of Acid Orange 7 from solution is explained in terms of electrostatic interaction by considering the dye speciesand the surface character of the xGnPs. The developed system for the removal of acid dye is found to be very useful,economic, rapid and reproducible.

Keywords: Acid Orange 7, Adsorption, Film diffusion, Graphitic Nanoplatelets, Particle diffusion

The adverse effects of discharge of organic pollutants ofdyeing industry waste on health have been alreadyproved. Textile effluents are known toxicants, whichinflict acute disorders in aquatic organisms. Uptake oftextile effluents through food chain in aquatic organismsmay cause various physiological disorders likehypertension, sporadic fever, renal damage and cramps1.The release of colored waste water into the eco-systemis a dramatic source of the aesthetic pollution,eutrophication and perturbation in aquatic life.

Brightly coloured and water soluble acid dyes, beingsodium salts of organic sulphonic acids, are composedof ionisable anionic groups such as sulphonates,carboxylates or sulphates. They have direct affinity forpolyamide and protein in an acidic bath and hence arecommonly used for dying polyamide, as well as nylon,

silk, wool and modified acrylics; also used to someextent for paper , leather and cosmetics. Acid dyeswith higher molecular weight are one of the mostproblematic groups of dyes which tend to pass throughconventional treatment system unaffected. Variousphysical and chemical methods of treatment ofindustrial waste water have been suggested. Theseinclude adsorption method, coagulation process, photo

catalytic degradation and hypo chloride treatment ofdye waste effluents2,3. Among these approaches,adsorption is regarded as an easy and economicprocess. This is attributed to its easy availability,simplicity of design, ease of operation, variousmaterials, such as commercial activated carbon, naturalmaterials, bio adsorbents and wastes from agriculture,have been used for such processes4.

The rapid development in nanotechnology shedslight on the waste water treatment. Nano materialshave been studied for the adsorptions of metalions5, dyes6, and antibiotics7. Exfoliated graphiticnanoplatelets (xGnPs) and graphite nano sheets8havebeen successfully utilized as sorbents to extract oils9and dyes10from their aqueous solutions. In the presentwork, exfoliated graphitic nanoplatelets (xGnPs)

have been used as an adsorbent for Acid Orange7 (AO7) removals and the adsorption capacity ofxGnPs is regulated by many influencing factors,such as temperature, pH variations and initialdye concentrations.

Experimental ProcedurexGnPs (with average diameter of 15 m and

average length of


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INDIAN J. CHEM TECHNOL., JANUARY 20138

of this material is already available11. The textiledye (AO7) was purchased from Sigma - Aldrich(Germany), and characterization of the dye are

summarized in Table 1. All the chemicals used wereobtained as research grade chemicals and usedwithout purification.

Characterization

Morphological structure of as-received xGnPs wascharacterized by the scanning electron microscopy.The sample was directly coated on the conductivesurface and SEM images were obtained witha field-emission scanning electron microscope(FESEM, JEOL JSM-6700F). BET measurementswere performed by using ASAP 2020 volumetric

adsorption analyzer (Micrometrics, USA). Thesurface functional groups on the adsorbents werequantitatively measured by Boehms titrationmethod12. The Boehm titration is based on theprinciple that oxygen groups on graphite surface havedifferent acidifies being neutralized by bases ofdifferent strengths. In our procedure, 20 mg of xGnPs(as-received) were stirred in 10 mL of 0.05 M basesolution (NaOH, Na2CO3, and NaHCO3), aqueoussolution under Ar for 48 h, (in order to equilibratewith the NaHCO3 solution). The mixtures werefiltered (on 0.20 m pore size membrane filters),

10 mL volume from each mixture being furthertitrated with 0.05 M hydrochloric acid. Three samplesof each base solution were titrated; a blank samplewithout xGnPs is being titrated with the sameprocedure. NaOH solution neutralizes all acidic sites(carboxyl, lactonic and phenols) from the surface ofxGnPs; NaHCO3 neutralizes only carboxyl groups,Na2CO3 reacts with carboxyl, lactonic groups.The quantity of the possible surface groups isestimated through the difference between thecalculated amount of surface functionality. The pHof the point of zero charge (pHpzc) was determined

using the pH drift method13

. The Acid Orange 7 dyewas used without purification. The characteristicsof dye are shown below:

C.I. Number : 15510NaturalpH : 6.1Molecular formula : C16H11N2NaO4S

Molecular weight, g mol-1

: 350.33pKa : 8.86Molecular volume, 3molecule-1 : 231.95Molecular dimension, nm : 1.240.680.22

Batch adsorption studies were carried out in250 mL tight lid glass bottle (Borosil R). Standardstock solution (1000 mg/L) containing Acid Orange 7was prepared by dissolving appropriate amount of itin water. 50 mg of adsorbent was added to 100 mL ofaqueous dye solution, initial concentration of AO7ranging from 20 mg/L to 60 mg/L. The contents of theflasks were agitated by placing them in temperaturecontrolled orbital shaker. The mixture was withdrawnat specified intervals then centrifuged using electricalcentrifuge (universal make) at 3000 rpm for 10 minand un adsorbed supernatant liquid was analyzedfor residual dye concentration using Elico make BioUV-Visible spectrometer (BL-198) at a wave lengthof 484 nm. All the experiments were conducted induplicate and mean of the two values were taken forcalculation. Maximum deviation is 4%. The amountof AO7 adsorbed in mg/L at time twas computed byusing the following equation:

0 tt

s

C Cq Vm

= (1)

where C0 and Ct are the AO7 concentration in mg/Linitially and at a given time t respectively; V, thevolume of the AO7 solutions in mL; and ms, theweight of the xGnPs. The removed AO7 (%) insolution was calculated using the following equation:

0

0

% Removal = 100tC C

C

(2)

Adsorption dynamics and equilibrium studies

The study of adsorption dynamics describes thesolute uptake rate, and evidently this rate controls theresidence time of adsorbate uptake at the solid-

Table 1 Kinetic parameters for the adsorption of AO7 onto xGnPs

Pseudo first-order values Elovich values Pseudo second-order valuesConcentrationmg/L

kLager 10-2

min-1R2

mg/g min

g/mgR2 qe k2 10

-2g/mg min

h R2

20 1.7306 0.986 0.869 0.606 0.826 18.712 7.458 0.804 0.96240 1.7186 0.982 0.768 0.271 0.899 19.038 1.438 0.558 0.98060 1.7094 0.985 0.667 0.199 0.923 46.283 0.656 0.655 0.975


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MAIYALAGAN & KARTHIKEYAN: FILM-PORE DIFFUSION MODELING FOR SORPTION OF AZO DYE 11

The values of RL obtained in this study lie withinthe range 0.127-0.151 indicating the favorable caseof adsorption for the present adsorbent-adsorbate

system. The Freundlich adsorption isotherm obtainedin 160 min of agitation is shown in Fig. 7. The valuesof absorption intensity 1/n


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the rate of an adsorption process is controlled eitherby external diffusion, internal diffusion or by bothtypes of diffusion.

The adsorption of adsorbent on graphene layer isremarkably different from other conventional porouscarbons in several aspects. First, due to their two-dimensional nano structure, the external surfaceavailable for adsorption is considerably larger than thesurface area arising from inner cavities. Thepredominance of outer cavity surface area to inner cavitysurface area determines the adsorption characteristics ofdyes on xGnPs. The adsorption on the external surfaceof graphene nanoplatelets is more important than theadsorption inside micro/mesoporous cavities. Anothernoteworthy difference should be ascribed to the

interstitial space between individual graphene sheets.The dimension of this space is determined by therelative positions among individual graphene sheets.

In the batch mode contact time adsorptionexperiments, rapid stirring is maintained. This inducesAO7 from the solution to the external surface ofthe adsorbent material and this step may control therate of the adsorption process26. To interpret theexperimental data it is necessary to recognize thesteps involved in the process of adsorption thatgovern the overall rate of removal of dye. Theingenious mathematical treatments recommended by

Boyd et al.

27

have been applied. These mathematicaltreatments are found to be useful to distinguishbetween particles diffusion and film diffusion.The successive steps in the adsorption dyes byadsorbents are:(i) transport of adsorbates to the external surface of

adsorbent (film diffusion);(ii) transport of adsorbates within the pores of the

adsorbent, except for a small amount ofadsorption, which occurs on the external surface(particle diffusion); and

(iii) adsorptions of the ingoing ion (adsorbate) on theinterior surface of adsorbent.

Out of these three processes the third process is

considered to be not the limiting step in the uptake ofdyes on to xGnPs28. The remaining two steps impartthe following three possibilities:Case I External transport < internal transport, whererate is governed by particle diffusion.Case II External transport > internal transport,where rate is governed by external diffusion.Case III External transport internal transport,where the transport of the adsorbate ions to theboundary may not be possible with significant rate,this may result into a possibility of formation of aliquid film surrounded by the adsorbent particles with

a proper concentration gradient.In the present study, the quantitative treatment of

the sorption dynamic is found in accordance with theobservation of Reichenberge29, as described by thefollowing equation:

22 2

1

6 11 exp[ n ]

n tNF

= (3)

where Fis the fractional attainment of equilibrium attime t; and n, the constant30.

tQFQ

= (4)

where Q1and Qare the amounts adsorbed after timet and after infinite time respectively.

2

20

time constantiD

Br

= = (5)

where Di is the effective diffusion coefficient ofadsorbate in the adsorbent phase; and ro, the radius ofadsorbent particles.

For energy observed values of F, correspondingvalues ofBtare derived from Reichenbergs table

38. In

each case the plot ofBtvs time distinguishes betweenthe processes involved film diffusion and particles-diffusion controlled rate of adsorption.

Typical Bt vs time plots at the concentration20 mg/L of AO7 adsorbed on xGnPs at differenttemperature are represented in Fig. 8. It is found to benon-linear throughout the temperature 30, 45 and60C, thus the process involved can be represented asfilm diffusion. At 30C the adsorbent exhibits linearityin Bt vs time plots in the entire concentration range,but the straight lines obtained do not pass through

Table 3 Comparison of maximum adsorption capacity forAcid orange 7 on other different adsorbents.

Adsorbent Adsorption capacity

mg/g

Reference

Canola stalks 25.06 20Beech wood sawdust 5.06 21Spent brewery grains 30.5 22Soil 3.47 23Waste Brewerys yeast 3.56 24Untreated S. marginatum 35.62 25Exfoliated graphiticnanoplatelets

85.172 Present study


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MAIYALAGAN & KARTHIKEYAN: FILM-PORE DIFFUSION MODELING FOR SORPTION OF AZO DYE 13

origin, revealing thereby that the rate-determining

process is film diffusion at this temperature forchosen adsorbent.The Di values were also calculated for each

adsorbent material at the three different temperatures(30, 45 and 60C) using Eq. (6), and the valuesobserved specify that Di increases within increasingtemperature. This may be due to the increasedmobility of ions and decreased retarding forces actingon diffusing ion. The energy of activation (Ea),entropy of activation (S#), and pre-exponentialconstant (Do) analogous to the Arrhenius frequencyfactor are evaluated indicating no significant change

in the internal structure of xGnPs during theadsorption, as shown below:

[ ]RTEDD aoi /exp = (6)

2 #(2.72 / )exp / oD d kT h S R = (7)

where d is the average distance between thesuccessive exchange sites and is taken as 5; and R,h and k are the gas, plank and Boltzmann constantsrespectively. The values of Ea,Di,Do, S

#and otherparameters are given in the Table 4. The negative

values of S# reflect that no significant changeoccurs in the internal structure of chosen adsorbentusing the adsorption process.

ConclusionThe study shows that xGnPs is an effective

adsorbent for the removal of AO7 from aqueoussolution. The adsorption of AO7 is dependent on theinitial concentration and agitation time. Equilibriumof AO7 adsorption reaches at 160 min.

The pseudo first- and second-order equationsprovide a best fit description for the sorption ofAO7 onto xGnPs related to Elovich model, but thepseudo first-order correlation coefficient has bettercorrelation value than pseudo second-order equation,Pseudo first-order equation is consider to be themost appropriate due to high correlation coefficientwhen compared to pseudo second-order equation, andadsorption takes place via film diffusion process.Langmuir and Freundlich adsorption isothermscorrelate the equilibrium adsorption data. Theadsorption of AO7 onto xGnPs is an exothermicreaction based on enthalpy change values.

Acknowledgement

The authors acknowledge with thanks the supportof Department of Chemistry, Chikkanna Govt.Arts College, Tirupur and Sophisticated Analytical

Instrument Facility, Indian Institute of Technology,Madras for characterization process.

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Fig. 8 Time vsBtplots at different temperature of AO7 - xGnPsadsorption

Table 4 Values of energy of activation (Ea), entropy ofactivation (S#),effective diffusion coefficient (Di) and

pre-exponential factor (Do)

Parameter Value

Di, cm2s-1

30 C 1.4687 10-11

45 C 1.313 10-1160 C 1.093 10-11

Ea, kJmol-1 -9.7153

S#,JK-1mol-1 -179.53Do,cm2s-1 9.4932 10-12


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film-pore diffusion modeling for sorption of azo dye on to exfoliated graphitic nanoplatelets

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