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pH-Responsive Nanocomposites From Methylcelluloseand Attapulgite Nanorods: Synthesis, Swelling andAbsorption Performance on Heavy Metal IonsWenbo Wang a b , Jiang Wang a & Aiqin Wang aa R&D Center of Xuyi Attapulgite Applied Technology, Lanzhou Institute of Chemical Physics,Chinese Academy of Science, Lanzhou, Chinab Key Laboratory of Chemistry of Northwestern Plant Resources, Chinese Academy ofSciences, Lanzhou, P.R. China

Available online: 23 Mar 2012

To cite this article: Wenbo Wang, Jiang Wang & Aiqin Wang (2012): pH-Responsive Nanocomposites From Methylcelluloseand Attapulgite Nanorods: Synthesis, Swelling and Absorption Performance on Heavy Metal Ions, Journal of MacromolecularScience, Part A: Pure and Applied Chemistry, 49:4, 306-315

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Journal of Macromolecular Science, Part A: Pure and Applied Chemistry (2012) 49, 306–315Copyright C© Taylor & Francis Group, LLCISSN: 1060-1325 print / 1520-5738 onlineDOI: 10.1080/10601325.2012.662055

pH-Responsive Nanocomposites From Methylcellulose andAttapulgite Nanorods: Synthesis, Swelling and AbsorptionPerformance on Heavy Metal Ions

WENBO WANG1,2, JIANG WANG1, and AIQIN WANG1,*

1R&D Center of Xuyi Attapulgite Applied Technology, Lanzhou Institute of Chemical Physics, Chinese Academy of Science,Lanzhou, China2Key Laboratory of Chemistry of Northwestern Plant Resources, Chinese Academy of Sciences, Lanzhou, P.R. China

Received October 2011, Accepted November 2011

A pH-responsive methylcellulose-g-poly(sodium acrylate)/attapulgite (MC-g-PNaA/APT) nanocomposite superabsorbent was pre-pared by the free-radical solution polymerization of methylcellulose (MC), sodium acrylate (NaA) and nanoscale attapulgite (APT)in the presence of the crosslinker N,N′-methylene-bis-acrylamide (MBA). The structure and morphology of the nanocomposite werecharacterized by FTIR, FESEM, TEM, XRD and EDS techniques, and the effects of the amount of MBA, MC and APT nanorodson swelling behaviors were also evaluated. Results indicate that NaA has been grafted onto MC macromolecular chains and APTnanorods participated in polymerization by its active silanol groups, and APT led to a better dispersion in the MC-g-PNaA matrix.The incorporation of APT clearly enhanced the swelling capacity and rate of the superabsorbent. In addition, the nanocompositeexhibited excellent absorption capacity on heavy metal ions, and its absorption amounts on Ni2+, Cu2+ and Zn2+ ions reached9.86, 7.66 and 21.86 times greater than active carbon (AC). The biopolymer-based nanocomposite superabsorbents can be used as apotential water-saving material and candidate of AC for heavy metal ion absorption.

Keywords: Methylcellulose, attapulgite, nanocomposite superabsorbent, pH-responsive, swelling, heavy metal ion absorption

1 Introduction

Over the past decades, polymer/clay nanocompositesas a new class of promising functional materials haveattracted continuous attention in both academic researchand industrial application owing to their excellent hybridperformance superior to the individual components andlow production cost (1–3). Superabsorbent is a moderatelycrosslinked functional polymer material with unique 3Dnetwork structure and excellent water-swellable charac-teristics, it has found extensive application in variousfields such as agriculture (4, 5), hygienic products (6),wastewater decontamination (7–10), drug-delivery carrier(11, 12), dehydrating agent (13) and nano-reactors (14), etc.Traditionally, superabsorbents have been produced frompetroleum-based monomers, and thus, the increasinglygrowing market demands for such materials accelerated

∗Address correspondence to: Aiqin Wang, R&D Center of XuyiAttapulgite Applied Technology, Lanzhou Institute of Chem-ical Physics, Chinese Academy of Science, Lanzhou 730000,China. Tel: +86 931 4968118; Fax: +86 931 8277088; E-mail:aqwang@licp.cas.cn

the consumption of petroleum resources and aggravatedthe environmental problem. Thus, the development ofeco-friendly superabsorbent materials from renewableand abundant natural resources has showed greaterprospect and received considerable attention in recentyears (15–20). Among promising natural resources, therenewable and biodegradable polysaccharides, and theabundant and non-toxic clay minerals have especiallybeen focused (21–26) because the polysaccharides/claycomposite materials have been honored as the materials of“in greening 21 material world” by virtue of their excellentperformance and eco-friendly advantages (27).

Cellulose is the most abundant polysaccharide in naturewith excellent biodegradable, non-toxic and biocompatiblecharacteristics, but the poor solubility of cellulose limitedits application for development of new material. Methylcel-lulose (MC, Fig. 1) is important hydrophilic methyl etherof cellulose produced by heating the mixture of cellulose,alkaline solution and methyl chloride, and has been used inthe pharmaceutical and food industries for many years (28).The methylation of cellulose not only improved the water-soluble properties of cellulose without prejudice to its non-toxic, biodegradable and biocompatible advantages, butalso endows it with moderate hydrophobic characteristic.

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Fig. 1. Structural scheme of MC.

Attapulgite (APT) is a kind of natural nanorod-like hy-drated magnesium aluminum silicate mineral with reactive−OH groups on its surface. APT has usually been used aseffective adsorbents (29), reinforce additive of compositematerials (30) and carrier of catalyst (31). The incorpora-tion of APT into gel network may change the type andnumber of hydrophilic groups, and subsequently improvethe properties of materials. In our previous work, it wasproved that the existence of moderate hydrophobic moi-eties in the network structure may generate synergistic ef-fect with the clay component in the superabsorbent networkand thus improve the performance of superabsorbent (23).Thus, it is desired that the eco-friendly superabsorbent withimproved structure and performance could be obtained bysimultaneously introducing MC and APT.

As part of the systematic work to develop new eco-friendly superabsorbent materials, in this work, themethylcellulose-g-poly (sodium acrylate)/attapulgite(MC-g-PNaA/APT) nanocomposite was prepared bythe simultaneous graft copolymerization and nanocom-posite technology. The structure and morphology of thenanocomposite were characterized by Fourier transforminfrared spectra (FTIR), X-ray diffraction (XRD), Fieldemission scanning electron microscope (FESEM), trans-mission electron microscope (TEM) and Energy dispersivespectroscopy (EDS) techniques. The optimal reactionvariables were determined, and the swelling kinetic,pH-responsive behaviors and the absorption capacity onheavy metal ions were extensively evaluated.

2 Experimental

2.1 Materials

MC (Viscosity (20 g/L, 20◦C), 350–550 mPa·s) was fromSinopharm Chemical Reagent Co., Ltd., Shanghai, China.Acrylic acid (AA, chemically pure, Shanghai ShanpuChemical Factory, Shanghai, China) was distilled underreduce pressure before use. APT powder (Jiuchuan Nano-Materials Technology Co., Jiangsu, China) was milledand passed through a 200-mesh screen before use. N,N’-methylene-bis-acrylamide (MBA, chemically pure, Shang-hai Chemical Reagent Corp., China) as well as Ammoniumpersulfate (APS, analytical grade, Xi’an Chemical Reagent

Factory, China) was used as received. Active carbon (AC)from coconut shell was purchased from Fuzhou YihuanCarbon Co., LTD (Fuzhou, China). All solutions in theexperiment were prepared with distilled water and otherreagents were analytical grade.

2.2 Preparation of MC-g-PNaA/APT Nanocomposites

1.44 g MC powder was added in a 250 mL four-neckedflask equipped with a mechanical stirrer, thermometer,reflux condenser and nitrogen line. 30 mL distilled waterwas added and the mixture was continuously stirred untilMC powder was dissolved to form a transparent stickysolution. The solution was maintained at 60◦C in an oilbath and purged with N2 for 30 min to remove existedoxygen. Then, 5 mL aqueous solution containing 0.072g initiator APS was added to the flask dropwise and keptat 60◦C for 10 min to generate radicals. A 7.2 g of acrylicacid (AA) was neutralized by 8.2 mL 8.5 mol/L NaOHsolution and fully mixed with 18 mg crosslinker MBA andcalculated amount of APT powder (0 g for MC-g-PNaA;0.45 g for 5 wt%; 0.95 g for 10 wt%; 2.05 g for 20 wt%;3.5 g for 30% wt%; 4.7 g for 35%) under magnetic stirring.The mixture was added into the flask after cooling thereactants to 50◦C. Afterwards, the temperature was risento 70◦C and maintained for 3 h under the continuouspurge of nitrogen to complete the reaction. The obtainedgel products were washed with distilled water several timesand dried to a constant weight at 70◦C. The product wasfinally ground and sieved to 40–80 mesh (180–380 µm).

2.3 Measurement of the Equilibrium Swelling Capacityand Swelling Kinetics

The sample (0.05 g) was immersed in excessive aqueous liq-uid at room temperature for 4 h to reach swelling equilib-rium. Then, a 100-mesh sieve was used to filter the swollengels. After draining the sieve for 20 min until no residual freewater remained, the mass of the swollen gels were weighedand the equilibrium swelling capacity was calculated byEquation 1.

Qeq = (w2 − w1)/w1 (1)

Where Qeq is the swelling capacity calculated as grams ofwater absorbed by per gram of the sample and w1 and w2are the mass of the dry sample and water-swollen sample,respectively.

Swelling kinetics of the sample in distilled water wasmeasured as follows: 0.05 g of the sample was added intoa 500 mL beaker and then 300 mL of distilled water wasadded. The swollen sample was filtered by a 100-mesh sieveafter a set interval and the swelling capacity at a givenswelling time was derived from the mass changes beforeand after swelling. In all cases, three parallel samples wereused and the averages were reported in this paper.

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2.4 Evaluation of pH-Responsivity

The solutions with pHs 2.0 and 7.4 was adjusted by0.1 M HCl and NaOH solutions. The pH values ofsolution were determined by a pH meter (DELTA-320).The pH-responsive properties of the nanocompositewere investigated in terms of its swelling and deswellingbetween pHs 7.4 and 2.0. Typically, a 0.05 g samplewas placed in a 100 mesh sieve and fully contactedwith pH 2.0 solution until the swelling equilibrium wasreached. The wetted sample was then immersed in pH7.4 solution for a set period of time. Finally, the swollennanocomposite was filtered, weighed and calculatedits swelling capacity at a given moment according toEquation 1. The consecutive time interval is 15 min foreach cycle, and the same procedure was repeated for fivecycles. After each measurement, the used solution wasrenewed.

2.5 Absorption Equilibrium Experiments on Heavy MetalIons

The solutions of Ni2+, Cu2+ and Zn2+ ions (0.01 mol/L)were prepared by dissolving Ni(Ac)2, Cu(Ac)2 and Zn(Ac)2in distilled water, and the pH values were adjusted to 5.5using dilute HCl or NaOH solution (0.1 mol/L). Theequilibrium absorption experiments were carried out onthermostatic shaker (THZ-98A) with a constant speed of120 rpm. The absorption capacities of activated carbon(AC) and nanocomposites were evaluated in the 25 mLof test solutions with 0.05 g of adsorbents at 30◦C for3 h. At the end of the absorption period, the solutionwas filtered using a 200-mesh nylon screen. The initialconcentrations and final concentrations of heavy metalions in the solution were measured by an EDTA titri-metric method using 0.002 mol/L EDTA solution as thestandard solution and 0.5% xylenol orange solution as theindicator.

2.6 Characterizations

FTIR spectra were recorded on a Nicolet NEXUS FTIRspectrometer in 4000–400 cm−1 region using KBr pellets.The samples were extracted with ethanol for 24 h beforedetermination. The surface morphologies and energy dis-persive spectrometer (EDS) were examined using a JSM-6701F field emission scanning electron microscope (JEOL)after coating the sample with gold film. TEM micrographswere obtained using a JEM-2010 high-resolution transmis-sion electron microscope (JEOL, Tokyo, Japan) at an ac-celeration voltage of 200 kV, the sample was ultrasonicallydispersed in ethanol before observation. XRD analysis wasperformed using an X-ray power diffractometer with Cuanode (PAN analytical Co. X’pert PRO), running at 40 kVand 30 mA.

3 Results and Discussion

3.1 FTIR Spectra Analysis

FTIR spectra of MC, MC-g-PNaA, MC-g-PNaA/APT(20 wt%) nanocomposite and APT are presented in Fig-ure 2. In Figure 2(a), the characteristic bands of MC at3453 cm−1 (O–H stretching), 2927 cm−1 (C–H stretching of–CH3), 2833 cm−1 (C–H stretching of –CH2), 1645 cm−1

(H–O–H bending of adsorbed water molecules), 1459 (pla-nar deformation of H–C–H and O–C–H groups), 1378cm−1 (–CH3 groups), 1201 and 1160 (C–H wagging), 1116and 1062 cm−1 (C–O–C stretching of ether linkages) and946 cm−1 (C–O(H) stretching vibration) were observed. Af-ter reacting with NaA monomer, the C–O(H) absorptionband of MC at 946 cm−1 was obviously weakened, and thenew bands at 1714 and 1711 cm−1 (C O stretching of–COOH), 1575 and 1570 cm−1 (asymmetric stretching of–COO− groups) and at about 1454 and 1409 cm−1 (sym-metric stretching of –COO− groups) appeared in the spectraof MC-g-PNaA and MC-g-PNaA/APT (Fig. 2(b, c)), in-dicating that the NaA monomers were grafted onto MCmacromolecular chains.

As shown in Figure 2(d), the (Mg)O–H stretchingvibration at 3614 cm−1, (Al)O-H stretching vibration at3581 cm−1 and (Si)O–H stretching vibration of APTat 3550 cm−1 almost disappeared in the spectrum ofnanocomposite (Fig. 2(c, d)), and the absorption band ofAPT at 1029 cm−1 (≡Si–O stretching vibration) appearedin the spectrum of MC-g-PNaA/APT with a weakenedintensity and slight shift compared with that of APT (Fig-ure 2(c, d)). These analyses reveal that APT existed in thenanocomposite and participated in the graft copolymeriza-tion reaction through its active silanol groups (25, 32, 33).

It can also be observed that the –COOH band at1714 cm−1 and the –COO− band at 1575 cm−1 (MC-g-PNaA) shifted to 1711 and 1570 cm−1, respectively,after forming MC-g-PNaA/APT nanocomposite, whichimplied the incorporation of APT weakened the hydrogenbonding interaction among polymer chains.

3.2 XRD Analysis

X-ray diffraction patterns of APT and MC-g-PNaA/APT(20 wt% and 35 wt%) were shown in Figure 3. It was no-ticed that APT shows the characteristic peaks at 2θ = 8.41◦(d = 1.0505 nm, [110] reflection), 2θ = 13.75◦ (d = 0.6435nm, [200] reflection), 2θ = 16.42◦ (d = 0.5394 nm, [130]reflection), 2θ = 19.89◦ (d = 0.4460 nm, [040] reflection),2θ = 21.48◦ (d = 0.4134 nm, [310] reflection), 2θ = 24.25◦(d = 0.3667 nm, [240] reflection). After compounding withMC-g-PNaA matrix, these reflection peaks of APT stillappeared in the diffraction pattern of the nanocomposite.The peak positions have no change but the intensity wasweakened. This observation indicates that the crystallinestructure of APT has not been disrupted during the

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Fig. 2. FTIR spectra of (a) MC, (b) MC-g-PNaA, (c) MC-g-PNaA/APT and (d) APT.

polymerization reaction, and the nano-scale APT rodswere retained and compounded with the MC-g-PNaAmatrix without intercalation. This result is consistentwith the FESEM and TEM observations and proved theformation of nanocomposite.

Fig. 3. XRD patterns of (a) APT, (b) MC-g-PNaA/APT (20wt%) and (c) MC-g-PNaA/APT (35 wt%). (Color figure availableonline.)

3.3 FESEM, TEM and EDS Analyses

Figure 4 depicts the FESEM micrographs of MC-g-PNaA,APT and MC-g-PNaA/APT (20 wt%) and the TEM im-age of MC-g-PNaA/APT (20 wt%) nanocomposite. It canbe seen that MC-g-PNaA shows a slightly rough surface,and APT shows a nanorod-like structure with the diam-eter of a single rod less than 100 nm and the length of asingle rod is about several hundred of nanometers. Afterintroducing APT into the MC-g-PNaA matrix, the ran-domly arranged nano-scale rod of APT can be observed inthe nanocomposite (Fig. 4(c)), which almost embeds withinthe MC-g-PNaA matrix. Also, the surface roughness of thesuperabsorbent was increased with incorporating the APT,some pores can be observed in the FESEM micrograph ofthe nanocomposite. In addition, the nano-scale APT rodwas also observed in the TEM image of the nanocomposite,which is identical to the FESEM observation (Fig. 4(d)).The FESEM and TEM observations indicate that the APTnano-rods were uniformly dispersed in the nanocompositewithout agglomeration and formed a homogeneous com-position.

Figure 5 exhibits the EDS spectrograms of MC-g-PNaA,APT and MC-g-PNaA/APT (20 wt%). It can be observed

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Fig. 4. The FESEM micrographs of (a) MC-g-PNaA, (b) APT and (c) MC-g-PNaA/APT (20 wt%); the TEM image of (d) MC-g-PNaA/APT (20 wt%).

that the characteristic peaks of C, O and Na elements wereobserved in the EDS spectrogram of MC-g-PNaA, and thepeaks of Si, Al, K, Ca, Fe, Na and Mg elements were ob-served in the EDS spectrogram of APT. After compound-ing MC-g-PNaA with APT, the main elements ascribed toAPT and MC-g-PNaA appeared in the EDS spectrogramof the composite (Fig. 5(b)), indicating that APT was dis-tributed in the polymer matrix. This result is consistentwith the FESEM observations and proved the formationof a composite structure.

3.4 Effects of Reaction Variables

3.4.1 Effect of the Mass Ratio of AA to MC on theSwelling Capacity

As shown in Figure 6, the swelling capacity clearly en-hanced with increasing the mass ratio of AA to MC untilan optimum value (535 g/g) was reached at the ratio of 5.In the reaction system, the dosage of AA, APS and MBA isfixed. Thus, the concentration of MC reduced with increas-ing the mass ratio of AA to MC. Due to the definite dosageof APS, the number of radicals generated by the decom-position of APS is invariable. Thus, the active radical siteson MC chains were distributed thick at low MC concen-tration, but sparse at high MC concentration. After somemonomers being grafted on the active sites, the formed

graft chains generated a steric hindrance and prevent thegrafting of other monomers on adjacent grafting sites. Asa result, these superfluous NaA monomers tend to copoly-merize. For one thing the copolymerization of monomersaccelerated the reaction rate and increased the crosslinkingdensity; for another, the copolymerization also form manylow molecular weight water-soluble homopolymers, whichcontributes little to the 3D water-absorbing network (34,35). These factors lead to the decrease of swelling capacitywith increasing the ratio of AA/MC from 5 to 8. How-ever, when the AA/MC ratio decreased from 5 to 3, theviscosity of reaction system increased and the initiation ef-ficiency decreased. Consequently, both grafting ratio andthe molecular weight of the grafted polymeric chains de-creased, and the swelling capacity decreased (36). Theseresults indicate that introduction of moderate amounts ofMC formulated a new biopolymer-based superabsorbentswith better swelling properties.

3.4.2 Effect of the Concentration of Crosslinker MBAAs shown in Figure 7, the swelling capacity clearly de-creased with increasing the concentration of crosslinkerMBA from 2.34 to 11.21 mmol/L. This is because thatthe increase of MBA concentration led to the increase ofcrosslinking density. As a result, the network voids forholding water were diminished and the swelling capacity

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Fig. 5. The EDS curves of (a) MC-g-PNaA, (b) APT and (c)MC-g-PNaA/APT (20 wt%). (Color figure available online.)

Fig. 6. Effect of mass ratio of AA to MC on the swelling capacity.The other reaction variables: APS, 1 wt%; AA, 7.2 g; MBA con-centration, 2.34 mmol/l; APT, 20 wt%. (Color figure availableonline.)

Fig. 7. Effect of the concentration of MBA on the swelling capac-ity. The inset is the plots of Ln(Qeq) against Ln(1/CMBA). Theother reaction variables: APS, 1 wt%; AA, 7.2 g; mass ratio ofAA to MC, 5; APT, 20 wt%. (Color figure available online.)

was reduced (37). This directly induces the rapid shrinkageof swelling capacity. The relationship between equilibriumswelling capacity and MBA concentration can be definedas Equation 2 and its logarithmic form (Eq. (3)) (38).

Qeq = kCMBA(−n) (2)

Ln(Qeq) = Lnk + nLn(1/CMBA) (3)

CMBA is the concentration of crosslinker MBA; k andn are law constant for an individual hydrogel. The plot ofLn(Qeq) against Ln(1/CMBA) gives perfect straight line withbetter linear correlation coefficient (R2 >0.9907). By fittingthe experimental data, the k and n values can be calculatedthrough the slope and intercept of the straight line. ForMC-g-PNaA/APT (20 wt%) nanocomposite, the relationof MBA concentration to equilibrium swelling capacity indistilled water follows the equation Qeq = 3.52 CMBA

(–0.8143)

in distilled water and Qeq = 2.28 CMBA(–0.5545) in 0.9 wt%

NaCl solution.

3.4.3 Effect of APT Content on Swelling CapacityAs can be seen from Figure 8, the swelling capacitysharply increased with incorporating APT until a max-imum swelling capacity (535 g/g in distilled water and71 g/g in 0.9 wt% NaCl solution) was reached at 20 wt%.Compared with the sample without APT, the addition of20 wt% APT enhanced the swelling capacity by 98.15% indistilled water and 24.56% in 0.9 wt% NaCl solution. Thegreat improvement of swelling capacity is closely relatedto the special structure of APT and its action in the gelnetwork. APT is a rigid nanoscale rod containing largeamounts of active –OH groups on its surface, which canuniformly disperse in the polymerization solution andparticipate in copolymerization reaction. Thus, APT can

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Fig. 8. Effect of APT content on the swelling capacity. Theother reaction variables: APS, 1 wt%; MBA concentration,2.34 mmol/l; AA, 7.2 g; mass ratio of AA to MC, 5. (Colorfigure available online.)

combine with the gel network as a composition unit. Theexistence of nanoscale rods in the network takes a “sup-porting” action and prevents the collapse of gel networkduring oven-drying, and the network voids for holdingwater can be retained to a greater degree. In addition, therigid APT in the network may restrict the intertwiningof polymer chains and weaken the hydrogen-bondinginteraction between hydrophilic groups such as –COOH,–COO− and –OH, etc, which decreased the physicalcrosslinking density. These factors are extremely favorableto the improvement of swelling capacity.

However, the swelling capacity decreased when thedosage of APT exceeding 20 wt%. This can be ascribedto the additional crosslinking of APT and its block tothe network voids. As reported previously (39), highlydispersed clay powder may act as additional crosslinkingpoints in polymer networks. Thus, the incorporation ofexcess APT may increase the crosslinking density andreduce the network voids for absorbing and holding water,which lead to the decrease of swelling capacity. In addition,the excess APT may physically stack in the network voids,which not only plug up the network voids, but also decreasethe hydrophilicity of the nanocomposite. As a result, theswelling capacity decreased with increasing the APTcontent above 20 wt%. A similar tendency was also ob-served for poly(sodium acrylate)/sepiolite nanocomposite(40).

3.5 pH-Responsive Characteristics

Besides swelling properties, the pH-responsive characteris-tics of a hydrogel are essential to its some application suchas drug-delivery carrier. In this section, the pH-responsiveproperties of the MC-g-PNaA/APT nanocomposite wereevaluated in pHs 2.0 and 7.4 solutions. As shown in Fig-

Fig. 9. On-Off swelling curves of the nanocomposite between pH7.4 and 2.0. The time interval for each cycle is 15 min, and fiveswelling-deswelling cycles were carried out. (Color figure availableonline.)

ure 9, the swelling capacity of the nanocomposite is almostequal to zero at pH = 2.0, but it rapidly swells when con-tacting with pH 7.4 solution. The change of swelling ca-pacity with altering external pH value was characterizedas pH-responsive behavior, and is attributed to the follow-ing reasons. The MC-g-PNaA/APT nanocomposite is ananionic polymer containing large amount of hydrophilic–COO− and –COOH groups. As altering external pH val-ues, the –COO− and –COOH groups may convert with eachother. In acid medium (pH = 2.0), the –COO− groups trans-formed into –COOH groups, which strengthen the hydro-gen bonding interaction among –COOH groups and gener-ate additional physical crosslinking. Also, the reduction ofnumber of –COO− groups weakens the electrostatic repul-sion among –COO− groups, and thus the swelling capacityis decreased. As increasing the external pH to 7.4, the num-ber of –COO− groups in polymer network increased. As aresult, the hydrogen bonding interaction was broken andthe electrostatic repulsion among negatively charged poly-mer chains increased, and so the polymer network tends toswell more.

It can be noticed that the nanocomposite still has bet-ter sensitivity after five On-Off cycles, indicating that thenanocomposite exhibits excellent pH reversibility.

3.6 Swelling Kinetics

Introducing APT to construct 3D network structure cannot only affect the swelling capacity, but also influencethe kinetic swelling behaviors. As can be seen from Fig-ure 10, the swelling rate of the superabsorbents with var-ious amounts of APT is faster at initial 1200 s, and thenreached equilibrium. The Schott’s pseudo second order ki-netics model was adopted to evaluate the dynamic swellingbehaviors (see Eq. (4)) (41).

t/Qt = 1/Kis + (1/Q∞)t (4)

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Fig. 10. (a) swelling kinetic curves of the superabsorbents, and (b)the plots of t/Qt against time t. (Color figure available online.)

Qt (g/g) is the swelling capacity corresponding toswelling time t (s); Q∞ (g/g) is the power parameter,denoting the theoretical equilibrium swelling capacity; K isis the initial swelling rate constant (g/g•s). By plotting t/Qtvs. t, the straight lines with better linear correlation coeffi-cient (Fig. 10 (b)) were obtained, indicating that the swellingkinetics of the superabsorbent follows the pseudo secondorder model. The K is and Q∞ values can be calculated bythe slope and intercept of above straight lines (Table 1). Itis obvious that the initial swelling rate decreased in the or-der: MC-g-PNaA/APT (5 wt%) > MC-g-PNaA/APT (20wt%) > MC-g-PNaA/APT (30 wt%) > MC-g-PNaA. Thisresult indicates that the introduction of moderate amountof APT is favorable to enhance the swelling rate.

Table 1. Swelling kinetic parameters for the superabsorbents indistilled water

Samples Q∞ (g/g) Kis (g/g·s) R

MC-g-PNaA 345 3.0537 0.9999MC-g-PNaA/APT (5 wt%) 364 5.0615 0.9999MC-g-PNaA/APT (20 wt%) 585 3.9089 0.9998MC-g-PNaA/APT (35 wt%) 373 3.4535 0.9999

Fig. 11. Effect of external saline solutions on the swelling capacity.(Color figure available online.)

3.7 Effect of Saline Solutions on Swelling Behaviors

Ionic hydrogel is usually responsive to external saline me-dia. As shown in Figure 11, the swelling capacity of thenanocomposite degreased with increasing the concentra-tion of external saline solutions, and multivalent salts havemore obvious effect on the swelling capacity than monova-lent one. At low concentration (0.5 mmol/L), the swellingcapacities in NaCl, CaCl2 and AlCl3 solutions are 445, 329and 272 g/g, respectively. With increasing the concentra-tion to 2 mmol/L, the swelling capacity in CaCl2 and AlCl3solutions sharply decreased to about 20 g/g, but sill reach326 g/g in NaCl solution. Above 5 mmol/L, the swelling ca-pacity in multivalent saline (CaCl2 and AlCl3) solution is al-most close to zero, but can reach 118 g/g in 20 mmol/l NaClsolution. Two reasons are responsible for the shrinkage ofswelling capacity in saline solution: (1) the osmotic pressuredifference (driving force of swelling) decreased with increas-ing the concentration of saline solutions; (2) the shieldingeffect of cations on negatively charged hydrophilic −COO−groups increased with increasing the concentration of salinesolutions. In multivalent saline solution, the ionic sites ofthe nanocomposite can complex with multivalent cations.The negative charges in polymeric chains were weakenedand the denser network by ionic crosslinking interactionwas formed. This restricts the expanding of polymeric net-work and decreased the swelling capacity more remarkably.

3.8 Heavy Metal Absorption Performance of theNanocomposites

Because the nanocomposites have a particular 3D networkstructure and numerous functional groups, they showedsatisfactory absorption performance on heavy metal ions.Figure 12 shows the equilibrium absorption capacities ofactive carbon (AC) and the nanocomposites with 5 wt%,10 wt%, 20 wt% and 30 wt% APT on Ni2+, Cu2+ and Zn2+ions from aqueous solution. It was noticed that the equi-librium absorption capacity of each absorbent follows the

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Fig. 12. Absorption capacities of active carbon and the nanocom-posites with 5 wt%, 10 wt%, 20 wt% and 30 wt% APT for Ni2+,Cu2+ and Zn2+ heavy metal ions, respectively. (Color figure avail-able online.)

order: MC-g-PNaA/APT (5 wt%) (149.78 mg/g for Ni2+,253.55 mg/g for Cu2+, 178.65 mg/g for Zn2+) > MC-g-PNaA/APT (10 wt%) (126.43 mg/g for Ni2+, 242.06 mg/gfor Cu2+, 162.34 mg/g for Zn2+) > MC-g-PNaA/APT (20wt%) (116.51 mmol/g for Ni2+, 231.55 mg/g for Cu2+,150.98 mg/g for Zn2+) > MC-g-PNaA/APT (30 wt%)(100.08 mg/g for Ni2+, 166.45 mg/g for Cu2+, 134.28 mg/gfor Zn2+)� AC (15.19 mg/g for Ni2+, 33.12 mg/g for Cu2+,8.17 mg/g for Zn2+). This trend can be ascribed to the factthat the absorption of the nanocomposites on heavy metalions is mainly a chemically chelating process, and the num-ber of functional –COOH and –COO− groups decided theabsorption capacities (42). With increasing the amount ofAPT, the number of –COOH and –COO− groups in unitmass of absorbents was decreased, and so the absorptionamount was decreased. However, the absorption capaci-ties of all the nanocomposites are significantly higher thanAC. The absorption capacities of the nanocomposite with5 wt% APT on Ni2+, Cu2+ and Zn2+ ions reached 9.86,7.66 and 21.86 times greater than AC, respectively. Thisconfirmed that the chemically complexation absorption ofthe nanocomposite is advantageous over the physical ad-sorption interaction of AC. It deserves to be pointed outthat the absorption capacity of nanocomposite still reached6.7 (for Ni2+), 5.01 (for Cu2+) and 16.4 (for Zn2+) times ofAC even the amount of APT reached 30 wt%, which isbeneficial to reduce the production cost.

4 Conclusions

For improving the eco-friendly properties and enhanc-ing the performance of superabsorbent materials, thebiodegradable methylcellulose (MC) was used as matrixpolymer and attapulgite (APT) nanorod was used as

inorganic components for producing a novel nanocom-posite. The optimum synthesis conditions were optimizedas follows: the mass ratio of AA to MC, 5; crosslinkerconcentration, 2.34 mmol/l; APT content, 20 wt%. Theoptimal nanocomposite gives the best swelling capacity of535 g/g in distilled water and 71 g/g in 0.9 wt% NaCl solu-tion. It was proved that NaA monomers grafted onto MCbackbone and APT nanorod takes part in polymerization.APT nanorods led to a better dispersion in the polymericmatrix and combined with the matrix to form a homoge-neous nanocomposite structure. After compounding, thecrystal structure of APT was retained well as shown byXRD analysis. The nanocomposite shows excellent on-offswitching swelling characteristics between pH 2.0 and 7.4,and the reversible pH-responsive behaviors render thesuperabsorbent potential as a drug-delivery carrier. Be-sides satisfactory swelling properties, the nanocompositesshowed excellent absorption capacities on Ni2+, Cu2+ andZn2+ heavy metal ions. The absorption capacity of thenanocomposite with 5 wt% APT on Ni2+, Cu2+ and Zn2+ions reached 9.86, 7.66 and 21.86 folds greater than AC.As a whole, the nanocomposites based on renewable andbiodegradable natural MC and natural nanoscale APTresources showed enhanced swelling capacity, swelling rateand satisfactory absorption capacity for heavy metal ions,which can be used as promising water-manageable mate-rials and as excellent absorbents for wastewater treatment.

Acknowledgments

The authors thank for joint support, the Science and Tech-nology Support Project of Jiangsu Provincial Science andTechnology Department (No. BY2010012) and the OpenFund of the Key Laboratory of Chemistry of Northwest-ern Plant Resources, Chinese Academy of Sciences (No.CNPR2010kfkt01).

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