PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [WANG, WEN-BO] On: 27 March 2011 Access details: Access Details: [subscription number 935395999] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37- 41 Mortimer Street, London W1T 3JH, UK Separation Science and Technology Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713708471 Adsorption Behavior of Methylene Blue from Aqueous Solution by the Hydrogel Composites Based on Attapulgite Yi Liu ab ; Wenbo Wang a ; Yeling Jin c ; Aiqin Wang ac a Center of Eco-Material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, P. R. China b Graduate University of the Chinese Academy of Sciences, Beijing, P. R. China c Key Laboratory of Attapulgite Science and Applied Technology of Jiangsu Province, Huaiyin Institute of Technology, Huaian, P. R. China Online publication date: 26 March 2011 To cite this Article Liu, Yi , Wang, Wenbo , Jin, Yeling and Wang, Aiqin(2011) 'Adsorption Behavior of Methylene Blue from Aqueous Solution by the Hydrogel Composites Based on Attapulgite', Separation Science and Technology, 46: 5, 858 — 868 To link to this Article: DOI: 10.1080/01496395.2010.528502 URL: http://dx.doi.org/10.1080/01496395.2010.528502 Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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PLEASE SCROLL DOWN FOR ARTICLE
This article was downloaded by: [WANG, WEN-BO]On: 27 March 2011Access details: Access Details: [subscription number 935395999]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Separation Science and TechnologyPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713708471
Adsorption Behavior of Methylene Blue from Aqueous Solution by theHydrogel Composites Based on AttapulgiteYi Liuab; Wenbo Wanga; Yeling Jinc; Aiqin Wangac
a Center of Eco-Material and Green Chemistry, Lanzhou Institute of Chemical Physics, ChineseAcademy of Sciences, Lanzhou, P. R. China b Graduate University of the Chinese Academy of Sciences,Beijing, P. R. China c Key Laboratory of Attapulgite Science and Applied Technology of JiangsuProvince, Huaiyin Institute of Technology, Huaian, P. R. China
Online publication date: 26 March 2011
To cite this Article Liu, Yi , Wang, Wenbo , Jin, Yeling and Wang, Aiqin(2011) 'Adsorption Behavior of Methylene Bluefrom Aqueous Solution by the Hydrogel Composites Based on Attapulgite', Separation Science and Technology, 46: 5,858 — 868To link to this Article: DOI: 10.1080/01496395.2010.528502URL: http://dx.doi.org/10.1080/01496395.2010.528502
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.
Adsorption Behavior of Methylene Blue from AqueousSolution by the Hydrogel Composites Based on Attapulgite
Yi Liu,1,3 Wenbo Wang,1 Yeling Jin,2 and Aiqin Wang1,21Center of Eco-Material and Green Chemistry, Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences, Lanzhou, P. R. China2Key Laboratory of Attapulgite Science and Applied Technology of Jiangsu Province,Huaiyin Institute of Technology, Huaian, P. R. China3Graduate University of the Chinese Academy of Sciences, Beijing, P. R. China
A series of carboxymethyl cellulose-g-poly (acrylic acid)/attapulgite hydrogel composites were synthesized for the removalof cationic dye methylene blue. Various factors affecting the uptakebehavior were investigated. Adsorption rate of the hydrogel wasquite fast, and adsorption equilibrium could be reached within30min. Adsorption kinetics well followed the pseudo-second-orderequation for all systems. The Langmuir isotherm was found to bestrepresent the data for the dye uptake. Even when 20wt% attapulgitewas introduced into the hydrogel, the corresponding maximumadsorption capacity reached 1979.48mg/g at 30�C. The as-preparedadsorbents exhibited excellent affinity for the dye, and can beapplied to treat wastewater containing basic dyes.
Keywords adsorption; attapulgite; carboxymethyl cellulose;composite; methylene blue
INTRODUCTION
Some dyes are very toxic pollutants which must bestrictly controlled in the water system because of theirthreat to human physiology and ecological systems evenat low concentrations. The presence of even a minuteamount of coloring substance in the aquatic system willmake it undesirable due to its appearance. Methylene blue(MB) has wider applications due to its good solubility, likecoating for paper stock, coloring paper, coloring leatherproducts, dyeing cottons, wools, and temporary hair color-ant, etc. Although MB is not strongly hazardous, it canrender several harmful effects. Acute exposure to MB willcause increased heart rate, shock, Heinz body formation,vomiting, quadriplegia, cyanosis, jaundice, and tissuenecrosis in humans (1,2). It is harmful when it is breathedand in contact with skin and it will be harmful when it is
swallowed (3). Accordingly, effluents containing MB andother dyes must be treated before they were discarded.Among existing techniques used for the removal of dyesand other pollutants from wastewater (4), adsorption hasbeen frequently used owing to its facility in design andapplication (5,6). In addition, the adsorption processes givethe best results as they can be used to remove differenttypes and concentrations of dyes, providing an attractivetreatment, especially if low-cost adsorbents are used. Manyadsorbents have been employed to remove dyes from efflu-ents. Activated carbon is a widely used adsorbent for thispurpose as a result of its versatility and porous structureand high adsorption capacity, with the handicaps of beingan expensive product and having difficulty in separationfrom the wastewater after use (7). Consequently, manyresearchers have diverted to search for more economicand effective adsorbents.
Very recently, the application of hydrogels as adsorbentshas been paid special attention. Hydrogels with three-dimensional crosslinked polymeric structures and hydro-philic groups can swell considerably in aqueous solutionwithout dissolution because hydrophilic chains contactone to the other by cross-links (8). Hydrogels have manypredominant properties including low interfacial tensionand a variety of functional groups which can trap metalions and ionic dyes like MB from wastewater and endowedhydrogels with high adsorption capacities, which is a favorfor the treatment of the environment (9). But pure hydro-gels often have some limitations such as low mechanicalstability and gel strength and introduction of clays materi-als into hydrogels can overcome these drawbacks becausehydrogels with clays materials combine elasticity and per-meability of the gels with high ability of the clays to adsorbdifferent substances (10). As a consequence, much researchconcerning hydrogels with clays materials adsorbing dyes(11), ammonium nitrogen (12), and metal ions (13) haveexponentially increased.
Received 10 May 2010; accepted 28 September 2010.Address correspondence to Aiqin Wang, Center for Eco-
Material and Green Chemistry, Lanzhou Institute of ChemicalPhysics, Chinese Academy of Sciences, Lanzhou, 730000, China.Tel.: þ86 931 4968118; Fax: þ86 931 8277088. E-mail: [email protected]
Separation Science and Technology, 46: 858–868, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0149-6395 print=1520-5754 online
DOI: 10.1080/01496395.2010.528502
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Attapulgite (APT), with a fibrous morphology,exchangeable cations, and reactive –OH groups on itssurface, is a crystalline hydrated magnesium silicate (14).Carboxymethyl cellulose is an important ether derivativeof cellulose and usually used as its sodium salt, which haswide applications (15). Due to its degradation, and beingwater-soluble, and possessing many functional groups likehydroxyl and carboxymethyl groups, sodium carboxy-methyl cellulose (CMC) has attracted great attention.Reactive –OH groups of CMC are used for graft polymer-ization of hydrophilic vinyl monomers to obtain hydrogelswith novel properties.
Based on the background mentioned above, to acquire anovel hydrogel composite with excellent property and lowcost, free radical polymerization between CMC, acrylicacid (AA), and APT was carried out and then correspond-ing hydrogel composites carboxymethyl cellulose-g-poly(acrylic acid)=attapulgite (CMC-g-PAA=APT) wereobtained. The morphology of the composite hydrogelwas characterized by Field emission scanning electronmicroscope (FESEM). To know whether the hydrogelcomposites have excellent adsorption behavior towarddyes, MB was chosen as representative because of its goodaffinity for solid surface to investigate the adsorptionproperties of the as-prepared hydrogel composites towarddyes from aqueous solution. The process parametersaffecting the removal amount of MB pertaining to APTcontent, pH of MB solution, contact time, initial concen-tration, temperature, and ionic strength were investigatedto determine optimum operating conditions. The experi-mental data were evaluated by applying the pseudo-first-order, pseudo-second-order, and intra-particle diffusionmodels. The Langmuir, Freundlich, and Tempkin isothermmodels were tested for their applicability. The adsorptionmechanism was discussed with the assistance of both theadsorbent and the MB loaded adsorbent.
EXPERIMENTAL
Materials
Acrylic acid (AA, chemically pure, monomer, distilledbefore use), ammonium persulfate (APS, analytical grade,initiator, recrystallized from distilled water before use),and N,N0-methylenebisacrylamide (MBA, chemical pure,crosslinker, used without further treatment) werepurchased from Shanghai Reagent Corp. (Shanghai,China). Sodium carboxymethyl cellulose (CMC, chemicalpure, 300� 800mPa � s (25 g=L, 25�C)) was supplied bySinopharm Chemical Reagent Co., Ltd. (Shanghai, China).Attapulgite (APT) was obtained from Jiangsu Xuyi JundaAotubang Material Co., Ltd. (Jiangsu, China). Methyleneblue (MB) was purchased from Alfa Aesar A JohnsonMatthey Company and used as received. The otherreagents used were all of analytical reagent grade.
Preparation of CMC-g-PAA/APT Hydrogel Composites
CMC-g-PAA=APT hydrogel composites were preparedaccording to our previous report (16). The products weremilled and sieved to 80-mesh for further experiment. FTIRspectra indicated that the graft reaction took place amongCMC, AA, and APT (16).
MB Uptake Experiments Using Batch Method
Batch sorption experiments were carried out on a ther-mostatic shaker (THZ-98A) with a constant speed of120 rpm, 0.025 g adsorbent, and 50mL tested dye solution.
Effect of APT Content
To optimize the experimental process and decide aboutthe appropriate adsorbent for further experiment, six dif-ferent APT contents of hydrogel composites were studied.The initial MB concentration was 1000mg=L, and thepH of the solution was not adjusted. The reaction tempera-ture was 30�C and the reaction duration was 180min. Thesuspension was centrifuged at 5000 rpm for 10min. Theinitial and final concentrations were analyzed using aSpecord 200UV=vis spectrophotometer by monitoring theabsorbance changes at a wavelength of maximum absor-bance (670 nm). Then the adsorption capacity was calcu-lated according to Eq. (1):
q ¼ ðCi � CfÞ � V=W ð1Þ
where Ci and Cf represent initial and final concentration ofMB solution (mg=L); V is the volume of the MB solutionused (L), andW is themass of adsorbent used in the study (g).
Effect of Initial pH of MB Solution
The chosen adsorbents (APT content of 5wt% and20wt%) with the same amount and the same concentrationof MB solution with different pH values ranging from 2 to9 adjusted by 0.1M hydrochloric acid or sodium hydroxidedilute solution were put into several conical flasks for180min at 30�C. The pH value was measured with apHS-25 pH-meter.
Adsorption Kinetics
The solution pH was adjusted to 7 and the reaction tem-perature was maintained at 30�C. The solution was takenat a specified time up to 240min. To explain the adsorptionmechanism and investigate the rate law describing MBadsorbed by the hydrogel composites, the kinetic data wereanalyzed through three kinetic equations, namely pseudo-first-order, pseudo-second-order, and intra-particle diffu-sion equations.
The pseudo-first-order model equation is presentedas (17):
logðq1e � qtÞ ¼ log q1e � k1t=2:303 ð2Þ
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The pseudo-second-order kinetic equation is given asfollows (18):
t=qt ¼ 1=ðk2q22eÞ þ t=q2e ð3Þ
The equation for the intra-particle diffusion model isdenoted as follows (19):
qt ¼ kidt0:5 þ C ð4Þ
where q1e, q2e, and qt are the amount of adsorbate removedby the adsorbent at equilibrium and any time, respectively(mg=g); k1, k2, and kid represent the pseudo-first-order(min�1), pseudo-second-order (g=(mgmin)), and intra-particle diffusion model (mg=(gmin0.5)) rate constant,respectively.
Adsorption Isotherms at Different Temperatures
Batch tests about the initial concentration and tempera-ture influencing the uptake for MB were conducted at fourtemperatures (i.e., 30, 40, 50, and 60�C). The initial concen-tration ranged from 900 to 1200mg=L for CMC-g-PAAand CMC-g-PAA=5% APT in which the APT contentwas 5wt% and 800 to 1200mg=L for CMC-g-PAA=20%APT and pH value of MB solution was set at 7. All sampleswere equilibrated for 240min.
Effect of Ionic Strength
Taking CMC-g-PAA=5% APT as representative, six dif-ferent concentrations of sodium chloride (i.e., 0, 0.01, 0.02,0.04, 0.08, and 0.16mol=L) were investigated to determinethe effect of ionic strength on adsorption. The initial MBsolution concentration was 1000mg=L and the pH wasnot adjusted. The temperature was kept at 30�C for 120min.
Characterization
Infrared spectra of the samples were recorded by aFTIR Spectrophotometer (Thermo Nicolet, NEXUS,USA) in the range of 4000� 400 cm�1 using KBr pellet.FESEM micrographs were obtained with a JSM-6701Ffield emission scanning electron microscope (JEOL) at50,000� magnifications after coating the sample with goldfilm.
RESULTS AND DISCUSSION
FESEM Analysis of the Adsorbents
The morphologies of APT and CMC-g-PAA=10% APTwere examined and shown in Fig. 1. It is found that APTshows a randomly oriented nano-scale fibril. The diameterof a single fibril is less than 100 nm but its length is aboutseveral hundreds of nanometers (Fig. 1(a)). The APT fibrilis also observed in the FESEM micrograph of CMC-g-PAA=10% APT, suggesting that APT fibril disperses in
continuous phase of the hydrogel composite. Meanwhile,the introduction of APT into the hydrogel leads to thecoarse surface of the hydrogel, which may benefit fromthe adsorption.
Effect of APT Content
The content of clay in the hydrogel was an importantparameter which significantly influenced the removalamount of dyes (20). Consequently, the variation of APTcontent affecting the uptake of MB on the hydrogel com-posites was investigated, as shown in Fig. 2. As can be seen
FIG. 1. FESEM micrographs of (a) APT and (b) CMC-g-PAA=APT.
FIG. 2. Variable of APT content on adsorption.
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the APT content did influence the adsorption capacities ofthe adsorbents, and the uptake reduced with increasingAPT content. It is known that carboxyl groups on thehydrogel composites dominate the removal of MB fromaqueous solution. The introduction of CMC with plentyof groups of –CH2COO– into the hydrogel composites,could significantly improve the adsorption properties ofthe hydrogel composites. Accordingly, even if the hydrogelcomposite was incorporated into 20wt% APT, the adsorp-tion capacity of the adsorbent for MB reached 1862.31mg=g, which distinctly reduced the cost of the adsorbent andenhanced adsorbability for MB and additionally, validatedthe feasibility of the introduction of CMC into the hydro-gel composites.
The change tendency of adsorption capacity along withthe APT content resulted from the following reasons. AsAPT content increased, more APT acted as cross-linkingpoints, and more hydroxyl groups on the APT reacted withcarboxyl groups on the CMC or AA, which would con-sume more carboxyl groups, thus increasing gel intensity(21), correspondingly reducing the uptake of MB. Takingeconomic cost into account, CMC-g-PAA, CMC-g-PAA=5% APT, and CMC-g-PAA=20% APT were taken as repre-sentatives to explore the adsorption properties for MB indetail.
Effect of Initial pH
The initial pH of the solution plays a significant role inthe chemistry of the adsorbate and the adsorbent becausethe change of pH influences the adsorption process throughdissociation of functional groups on the active sites of theadsorbent surface and adsorbate (22). It is known thatthere are lots of –COOH on the backbone of as-preparedhydrogel composites and –OH on the APT, which maybe affected by the change of the solution pH. In this case,the effect of pH on the adsorption was performed usingvarious initial solution pH values (pH¼ 2� 9), as shownin Fig. 3. As expected, the uptake of MB increased withthe increase of solution pH as the surface became progress-ively more negatively charged due to deprotonation of thesurface, which indicated the pH-dependent adsorptionmechanism. However, it seemed that the uptake of MBremained constant in the pH range 4� 9 for CMC-g-PAA and CMC-g-PAA=5% APT. When pH was lowerthan 4, the removal amount of MB showed a significantdecrease. But for CMC-g-PAA=20% APT, the adsorptioncapacity appeared to be a constant in the pH range of7� 9. When the pH value was lower than 4 the adsorptioncapacity sharply decreased but when the pH value waswithin 4� 6, there was a slight decrease in adsorptioncapacity. The phenomena possibly attributed to the dis-sociation constant (pKa) of poly (acrylic acid). The pKaof poly (acrylic acid) is about 4.7 (23), and then the group–COOH can be facilely ionized above the pH value of 4.7,
which favors the removal of cationic dye MB. Due to thebuffer action of –COOH and –COO– groups, it may beconcluded that the equilibrium adsorption capacity wouldremain a constant to a certain extent, which was validatedin Fig. 3 and mentioned above. At lower pH values, agreater part of the carboxyl groups exists in the form of–COOH, resulting in the decrease of affinity of adsorbentfor MB. Accordingly, with the adsorption capacitydecreasing, while at high pH, it favors the transformationof –COOH to –COO–, promoting the uptake of MB.However, at lower pH, a part of –COOH is ionized, so aconsiderable removal amount of MB can be obtained.The optimum pH range for MB removal can be foundfrom Fig. 3, from which it can conclude that the hydrogelcomposites can be used in a wide pH range.
Effect of Contact Time and Adsorption Kinetics
The adsorption of MB onto the three adsorbents wasinvestigated as a function of contact time to determinethe required time for maximum adsorption, and theresults were illustrated in Fig. 4. It was clear that the
FIG. 3. Effect of initial pH of MB solution on adsorption.
FIG. 4. Effect of contact time on adsorption.
ADSORPTION BEHAVIOR OF METHYLENE BLUE FROM AQUEOUS 861
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adsorption was quite fast with the increased time from 0to 20min, and then progressively slowly increased as timeprogressed, and reached equilibrium in 30min. Simul-taneously, more than 90% of the equilibrium adsorptioncapacity occurred within 20min, which indicated thatmany vacant activated sites were available for adsorptionduring the initial stage, thereby making MB prone tocontact with the adsorption sites, and thus the removalamount rapidly increasing.
The investigation of adsorption kinetics describes theadsorbate adsorption rate, which dominates the time ofadsorbate adsorption at the solid-liquid surface (24). More-over, adsorption kinetics can provide valuable informationon the mechanism of the adsorption process. In this regard,adsorption data were analyzed in terms of pseudo-first-order, pseudo-second-order, and intra-particle diffusionequations, and the related parameters and correlationcoefficients obtained from Eqs. (2) and (3) tabulated inTable 1. It can be seen that the relatively higher values ofcorrelation coefficient for pseudo-second-order kineticsthan those for pseudo-first-order kinetics indicated thatpseudo-second-order expression is the best fit kineticexpression for the entire adsorption process. Though thevalues of the correlation coefficient in the range of 0.9706to 0.9864 were acceptable to argue as a best fit, this modelfails to explain the sorption saturation (qexp). According tothe pseudo-first-order kinetics, the intercept of the plot(q1e) should theoretically represent the sorption saturationfor the given condition. In the present study, from Table 1,it can be seen that the values of q1e failed to predicate thevalues of qexp, while the values of q2e could successfullydo so, which indicated the adsorption process of MB ontothe adsorbents fitted the pseudo-second-order kineticmodel.
For the intra-particle diffusion model Eq. (4) the depen-dencies of qt against t
0.5 were only shown in Fig. 5. It canbe seen that the plots consisted of two linear sections withdifferent slopes. The multi-linearity suggested that two ormore steps occurred in the sorption process (25). The twolinear sections in the root time plots were separately eval-uated through Eq. (4), and the corresponding parameterswere summarized in Table 2. It was reported that the firststraight section was ascribed to macro-pore diffusion andthe second linear section was attributed to micro-porediffusion (26). The value of intercept C reflects thicknessof the boundary layer, and the larger the intercept, thegreater the thickness of the boundary layer (27). It wasclear that the linear line did not pass through the origin,indicating that the intra-particle diffusion was not theonly rate-controlling step. The values of the correlationcoefficient were lower than those obtained from thepseudo-second-order kinetic model, ascertaining thatthe pseudo-second-order model reasonably described thedye sorption more accurately.
Effect of Initial Concentration and Adsorption Isotherms
The effect of initial concentration of MB solution on theremoval amount of MB by the adsorbents was investigated,as shown in Fig. 6. It was evident that the initial concen-tration played a significant role in the adsorption processof MB onto the adsorbents. It can be seen that the uptakesharply increased with increasing initial concentrationwhen the concentration was lower than 1050mg=L, andthereafter gradually increased until it remained unchangedby further increasing the concentration to 1200mg=L. Theresults may be ascribed to the following facts. The momen-tum of the mass transfer would increase with increasinginitial concentration, thus bringing on a greater uptake ofMB (28). The higher the initial concentration is, the greaterthe driving force to overcome mass transfer resistance atthe solid-liquid surface is. Vacant sites were depleted,which resulted in the adsorption capacity being kept con-stant when the initial concentration reached a certain limit.Moreover, from Table 3 (1), it was clear that the tempera-ture had a negative effect on the adsorption.
The adsorption isotherms which reflect the interactionbetween the adsorbate and the adsorbent until a state ofequilibrium are important to design and optimize theadsorption system and examine the effectiveness of adsorb-ent, so it is essential to search for an optimum isothermmodel for depicting MB onto applied hydrogel composites.Therefore, the correlation of equilibrium data using eithera theoretical or empirical equation is essential for theadsorption interpretation and prediction of the extent ofadsorption. In this study, the Langmuir, Freundlich, andTempkin isotherm models were employed to analyze theadsorption equilibrium data. The Langmuir isothermmodel assumes that the adsorption sites on the surface ofthe adsorbent are identical and the molecules adsorbeddo not react with each other. It indicates monolayer cover-age of the adsorbate on the adsorbent. The Langmuirequation is given as (29):
Ce=qe ¼ 1=ðb� qmÞ þ Ce=qm ð5Þ
where Ce is the concentration of adsorbate solution atequilibrium (mg=L); b is the Langmuir constant relatedto the binding energy of the adsorbent (L=mg); and qm isthe theoretical monolayer saturation adsorption capacityof the adsorbent (mg=g). The values of qm and b can bedetermined from the slope and intercept of the plot ofCe=qe versus Ce.
The essential characteristic of the Langmuir isothermis expressed in terms of a dimensionless constantcalled separation factor (RL), defined by the followingequation (30):
RL ¼ 1=ð1þ bC0Þ ð6Þ
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TABLE
1Parameter
constants
calculatedfrom
pseudo-first-order
andpseudo-second-order
modelsforMB
onto
theadsorbents
Pseudo-first-order
model
Pseudo-second-order
model
qexp
q1e
k1
q2e
k2�10�4
Samples
Equation
mg=g
mg=g
min
�1
R2
Equation
mg=g
g=(m
gmin)
R2
CMC-g-PAA
log(q
1e�qt)¼
3.3072–0.0793t
2001.53
2028.76
0.1827
0.9782
t=qt¼1.1900�10�11
þ4.9305�10�4t
2028.20
2.0428
0.9998
CMC-g-PAA=
5%
APT
log(q
1e�qt)¼
3.3169–0.0849t
1952.46
2074.58
0.1956
0.9706
t=qt¼1.2500�10�11
þ5.0540�10�4t
1978.62
2.0435
0.9998
CMC-g-PAA=
20%
APT
log(q
1e�qt)¼
3.2763�0.0962t
1918.95
1889.25
0.2215
0.9864
t=qt¼9.6890�10�12
þ5.1723�10�4t
1933.39
2.7611
0.9999
863
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where C0 is the initial concentration of the adsorbatesolution (mg=L). The value of RL is an indicator of thetype of the isotherm to be either favorable (0<RL< 1),unfavorable (RL> 1), linear (RL¼ 1), or irreversible(RL¼ 0).
In comparison to the Langmuir isotherm, the Freundlichmodel can be applied to nonideal adsorption on a hetero-geneous surface and is generally found to be better suitedfor the characterization of the multilayer adsorption pro-cess indicating that the molecules adsorbed are dependenton each other. The Freundlich equation is empirical. Theequation can be written as (31):
ln qe ¼ lnKf þ 1=n lnCe ð7Þ
where Kf represents Freundlich constant ((mg=g)(L=mg)1=n), which indicates the adsorption capacity andthe strength of the adsorptive bond; 1=n is an indicatorsuggesting the adsorption intensity of adsorbent.
The Tempkin isotherm assumes that the heat ofadsorption of all molecules should decrease linearly withthe surface coverage due to the existence of the adsorbent-adsorbate interactions. In addition, adsorption is
characterized by a uniform distribution of binding energies,up to some maximum binding energy (32). The linear formof the Tempkin isotherm is given as
qe ¼ v lnðmÞ þ v lnCe ð8Þ
where v is corresponding to the heat of adsorption (L=g),and m is the dimensionless constant.
By virtue of the Langmuir, Freundlich, and Tempkinisotherm Eqs. (5), (7), and (8) the adsorption equilibriumdata were analyzed, and the related parameters were listedin Table 3 (1) and (2), respectively. It can be seen that thevalues of the correlation coefficient obtained from theLangmuir equation were much higher than those fromthe Freundlich equation and Tempkin equation at differenttemperatures. According to the values of the correlationcoefficient, the Tempkin isotherm equation represents thepoorest fit of experimental data than the other isothermequations among the analyzed isotherm equations. Inaddition, it was evident that the calculated qm values fromthe Langmuir equation (Eq. (5)) were in accordance withthose experimentally determined. Furthermore, constantsRL (Eq. (6)) and 1=n lied within the favorable rangebetween 0 and 1. Based on the above analysis, the experi-mental data were reasonably described by the Langmuir
FIG. 5. Intra-particle diffusion plots for MB onto adsorbents. The equa-
tions of the first stage are qt¼ 427.74 t0.5þ 92.72, qt¼ 421.72 t0.5þ 75.00,
FIG. 6. Effect of initial concentration of MB solution on adsorption.
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TABLE
3(1)Langmuir
andFreundlich
modelsparametersforMB
onto
theadsorbents
Langmuir
model
Freundlich
model
tqe
qm
bK
f
Samples
� CEquation
mg=g
mg=g
L=mg
R2
RL�104
Equation
(mg=g)
(L=mg)1=n
1=n
R2
CMC-g-PAA
30
Ce=qe¼4.99�10�4
þ4.64�10�4Ce
2116.22
2153.50
0.9306
0.9995
8.9468�
11.9255
lnqe¼7.46
þ0.0497ln
Ce
1735.59
0.0497
0.2864
40
Ce=qe¼3.94�10�4
þ4.72�10�4Ce
2093.34
2120.32
1.1965
0.9998
6.9599�
9.2777
lnqe¼7.44
þ0.0540ln
Ce
1706.38
0.0540
0.4834
50
Ce=qe¼4.48�10�4
þ4.72�10�4Ce
2090.09
2119.33
1.0525
0.9999
7.9114�
10.5457
lnqe¼7.42
þ0.0571ln
Ce
1676.33
0.0571
0.6454
60
Ce=qe¼4.65�10�4
þ4.75�10�4Ce
2074.52
2103.32
1.0232
0.9999
8.1378�
10.8474
lnqe¼7.41
þ0.0584ln
Ce
1652.23
0.0584
0.6724
CMC-g-PAA=
5%
APT
30
Ce=qe¼4.98�10�4
þ4.77�10�4Ce
2076.11
2094.80
0.9580
0.9991
8.6911�
11.5848
lnqe¼7.48
þ0.0361ln
Ce
1768.65
0.0361
0.2072
40
Ce=qe¼4.73�10�4
þ4.75�10�4Ce
2073.62
2103.45
1.0060
0.9994
8.2768�
11.0327
lnqe¼7.44
þ0.0486ln
Ce
1708.14
0.0486
0.3577
50
Ce=qe¼4.51�10�4
þ4.80�10�4Ce
2063.00
2084.92
1.0633
0.9995
7.8311�
10.4387
lnqe¼7.44
þ0.0466ln
Ce
1709.46
0.0466
0.3811
60
Ce=qe¼7.32�10�4
þ4.76�10�4Ce
2060.53
2101.21
0.6505
0.9991
12.7943�
17.0518
lnqe¼7.37
þ0.0646ln
Ce
1587.30
0.0646
0.4603
CMC-g-PAA=
20%
APT
30
Ce=qe¼1.01�10�3
þ5.05�10�4Ce
1956.50
1979.48
0.5002
0.9994
16.6323�
24.9277
lnqe¼7.32
þ0.0551ln
Ce
1504.05
0.0551
0.4785
40
Ce=qe¼6.47�10�4
þ5.21�10�4Ce
1932.70
1919.08
0.8057
0.9992
10.3323�
15.4904
lnqe¼7.33
þ0.0474ln
Ce
1520.43
0.0474
0.3955
50
Ce=qe¼4.88�10�4
þ5.30�10�4Ce
1927.35
1886.52
1.0859
0.9993
7.6682�
11.4980
lnqe¼7.32
þ0.0451ln
Ce
1515.74
0.0451
0.3824
60
Ce=qe¼9.99�10�4
þ5.23�10�4Ce
1918.24
1912.99
0.5231
0.9997
15.9053�
23.8390
lnqe¼7.31
þ0.0480ln
Ce
1496.54
0.0480
0.4740
865
Downloaded By: [WANG, WEN-BO] At: 02:07 27 March 2011
model for the entire adsorption process, indicating a mono-layer adsorption of MB onto the adsorbents and also thehomogeneous distribution of active sites on the adsorbents.Furthermore, the as-prepared adsorbents have highermaximum adsorption capacities qm than other adsorbentsreported (Table 4), which indicated the effectiveness ofthe adsorbents used in the present work.
Effect of Ionic Strength
Dye adsorption strongly affected by electrostatic para-meters such as oxide surface charge, pH, and ionic
strength. Consequently, taking CMC-g-PAA=5% APT asrepresentative, effect of ionic strength on adsorption ofMB onto the adsorbents was investigated, as illustratedin Fig. 7. The uptake of MB decreased as sodium chlorideconcentration increased. It was largely because of thescreening effect that Naþ partially neutralized the negativesurface charge, and then resulted in a compression of theelectrical double layer, which led to a reduction in theattractive forces between the surfaces of the adsorbentand the MB cationic species (38).
Adsorption Mechanism
To better understand the adsorption mechanism, FTIRspectra of MB and CMC-g-PAA=5% APT before and afteradsorption were investigated, as depicted in Fig. 8. Theabsorption bands at 1609, 1487, and 1453 cm�1 (Fig. 8a)were ascribed to the stretching vibration of the C–C bond
TABLE 3(2) Tempkin model parameters for MB onto the adsorbents
Tempkin model
t vSamples �C Equation L=g m� 10�6 R2
CMC-g-PAA 30 qe¼ 1722.91þ 98.37 ln Ce 98.37 40.44 0.295840 qe¼ 1692.24þ 105.64 ln Ce 105.64 9.06 0.491750 qe¼ 1657.19þ 111.81 ln Ce 111.81 2.74 0.657960 qe¼ 1631.02þ 113.54 ln Ce 113.54 1.73 0.6855
CMC-g-PAA=5% APT 30 qe¼ 1762.39þ 70.67 ln Ce 70.67 67622 0.212040 qe¼ 1696.59þ 94.45 ln Ce 94.45 63.28 0.362950 qe¼ 11697.92þ 90.34 ln Ce 90.34 145.23 0.383760 qe¼ 1556.13þ 124.77 ln Ce 124.77 0.26 0.4658
CMC-g-PAA=20% APT 30 qe¼ 1478.76þ 100.06 ln Ce 100.06 2.62 0.495240 qe¼ 1503.97þ 84.55 ln Ce 84.55 53.12 0.404450 qe¼ 1502.87þ 79.07 ln Ce 79.07 179.79 0.380260 qe¼ 1478.32þ 84.64 ln Ce 84.64 38.50 0.4765
TABLE 4Comparison of the maximum adsorption capacity of MB
onto different adsorbents
Adsorbents t (�C) qm (mg=g) References
Raw date pits 20 281 (7)H-mag 25 173 (33)Na-mag 25 331 (33)HA-Am-PAA-B 30 242.4 (34)CTS-g-PAA=10% VMT
30 1682.18 (35)
Polymer modifiedbiomass
25 869.6 (36)
MWS 40 450.0�14.4 (37)CMC-g-PAA 30 2153.50 this workCMC-g-PAA=5% APT
30 2094.80 this work
CMC-g-PAA=20% APT
30 1979.48 this work
FIG. 7. Effect of ionic strength on adsorption.
866 Y. LIU ET AL.
Downloaded By: [WANG, WEN-BO] At: 02:07 27 March 2011
in aromatic structure. The bands at 884, 833, and 796 cm�1
were corresponding to the characteristic absorption bandsof aromatic skeletal groups, which overlapped the absorp-tion band of Si–O bond (817 cm�1) after being adsorbed.All these absorption bands appeared and shifted in thespectra of MB loaded CMC-g-PAA=5% APT. The bandat 1714 cm�1, related to the stretching vibration of –COOHwas shifted to a high wave number after adsorption of MB.The bands at 1457 and 1413 cm�1 assigned to –COO– sym-metric stretching and the band at 1579 cm�1 ascribed to–COO– asymmetric stretching were shifted after adsorption(21), which overlapped the absorption bands of the C–Cbond in aromatic structure. Based on the above analysis,groups –COOH, –COO– were involved in the adsorptionprocess of MB onto the adsorbent, which probably indi-cated that ion exchange and electrostatic attraction weretwo of the major adsorption mechanisms for binding MBto the hydrogel composite.
CONCLUSIONS
CMC-g-PAA=APT hydrogel composites were synthe-sized and MB was taken as representative to evaluate theadsorption property of the as-prepared adsorbents towarddye. Various factors affecting the uptake behavior weresystematically investigated. The adsorption kinetics wasin good accordance with the pseudo-second-order equationfor all systems studied. The equilibrium data were analyzedby the Langmuir, Freundlich, and Tempkin isothermequations, and the results showed that the Langmuirisotherm could well describe the adsorption process, andthe maximum adsorption capacities calculated from theLangmuir equation showed excellent adsorption propertiesof the adsorbents. Feasible improvements in the uptakebehaviors encourage efforts for the hydrogel compositesto be used in wastewater treatment.
ACKNOWLEDGEMENTS
The authors thank the joint support by the NationalNatural Science Foundation of China (No. 20877077)and Science and the Foundation of Key Laboratory forAttapulgite Science and Applied Technology of JiangsuProvince (No. HPK200901).
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