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Polymer 128 (2017) 12e23

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Polymer

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Porous lignin based poly (acrylic acid)/organo-montmorillonitenanocomposites: Swelling behaviors and rapid removal of Pb (II) ions

Yanli Ma a, Ling Lv a, Yuanru Guo a, Yujie Fu b, *, Qian Shao c, Tingting Wu d, Sijie Guo e,Kai Sun e, Xingkui Guo c, Evan K. Wujcik f, Zhanhu Guo e, **

a Material Science and Engineering College, Northeast Forestry University, No.26, Hexing Road Xiangfang District, Harbin, 150040, Chinab Postdoctoral Research Station of Biological Station, Northeast Forestry University, No.26, Hexing Road Xiangfang District, Harbin, 150040, Chinac College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, PR Chinad Civil and Environmental Engineering Department, The University of Alabama in Huntsville, Huntsville, AL 35899, USAe Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996 USAf Materials Engineering and Nanosensor [MEAN] Laboratory, Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa,AL, USA

a r t i c l e i n f o

Article history:Received 29 July 2017Received in revised form31 August 2017Accepted 3 September 2017Available online 6 September 2017

Keywords:PorousLignin based poly (acrylic acid)OMMTPb2þ absorption

* Corresponding author.** Corresponding author.

E-mail addresses: yujie_fu@163.com (Y. Fu), zguo1

http://dx.doi.org/10.1016/j.polymer.2017.09.0090032-3861/© 2017 Published by Elsevier Ltd.

a b s t r a c t

Lignin grafted N, N0-methylene-bisacrylamide (LM) was copolymerized with acrylic acid (AA) to fabricatelignin based poly (acrylic acid) (LBPAA) nanocomposites, and organo-montmorillonite (OrgMMT) wasuniformly dispersed in LBPAA by ultrasonic method. The water absorbency in KCl solution was 302.9 g/gfor LBPAA/OrgMMT (3 wt% OrgMMT, 43.65 wt% LM, and 53.35 wt% poly(acrylic acid) (PAA)), higher thanPAA/OrgMMT (104.5736 g/g), and PAA (131.8 g/g). The Pb2þ adsorption for LBPAA/OrgMMT wasdemonstrated to be pH-dependent and followed Freundlich multilayer adsorption, with the removalfollowing a pseudo-second-order kinetic model. The Pb2þ absorption capacity of LBPAA (PAA 48.50/48.50LM)/OrgMMT (3 wt%) was 1.0803 mmol$g�1, the highest among the composites. When the mole ratio ofNaþ and Pb2þ was less than 0.21, the Pb2þ adsorption capacity of LBPAA/OrgMMT decreased slightly. Thenegative DG0 indicated that the Pb2þ adsorption process for composite hydrogel and PAA was sponta-neous. XPS analysis data indicated that Pb2þ ions formed Pb-O bonds by hydroxyl groups and carboxylgroups for efficient lead ion adsorption. This study demonstrates that LBPAA/OrgMMT can be used as awater retention agent with salt tolerance and as an adsorbent for removing Pb2þ ions from industrialwaste water.

© 2017 Published by Elsevier Ltd.

1. Introduction

Even at very low concentration, lead ions can affect the blood, aswell as nerve, immune, renal and cardiovascular systems. Becauseit mainly influences the growth of the brain and nerve system,pregnant women and children are particularly vulnerable to thetoxicity of lead. Exposure to high amounts of lead can causegastrointestinal symptoms and may cause reproductive effects.Lead has two valence states, i.e., Pb(IV) and Pb(II). Pb(IV) ions withmore toxicity have stronger oxidbillity than Pb(II) and even po-tassium permanganate. Moreover, lead is very difficult to be

0@utk.edu (Z. Guo).

eliminated [1,2]. Once the lead enters the organism, it will soonproduce deposits in the bones, brain, kidneys and muscles, leadingto severe developmental disorders, injuries, diseases and evendeath [1]. Chemical deposit, adsorption, ionic exchange adsorption,reverse osmosis, electrochemical treatments and barrier separationare used to remove heavy metal ions [2]. Adsorption technology isconsidered as a better economic choice to purify low concentra-tions metal ion for protecting the environment against Pb2þ ionpollution.

Recently, different functional groups or fillers were incorporatedin the polymeric networks for introducing new functionalities andimproving performance and endured reusability [3,4]. For example,hydrogel with hydrophilic groups such as hydroxyl (eOH), car-boxylic (eCOOH), amino (eNH2), acryl amino (eCONH2), and sul-fonic (SO3H) can be utilized for the removal of toxic Cd(II), Co(II),Fe(II), Pb(II), Ni(II), Cu(II) and Cr(III) [5]. Superabsorbent polymer

Y. Ma et al. / Polymer 128 (2017) 12e23 13

(SAP) of polyacrylic acid was commercial products as an expand-able hydrogel system. The advantages of easy processing to use PAAcan bemade by comparing with other polymer matrix such as ethyloleate [6], polyvinylpyrrolidone [7], acrylonitrile-butadiene-styrene [8], polydimethylsiloxane [9], polyurethane [10], poly(-dimethylsilylene) diacetylenes [11], polyoxymethylene [12], poly-pyrrole [13], polyphenylene sulfide [14], poly(p-phenylenebenzobisoxazole) [15], and bisphenol A cyanate ester [16,17].However, the shortcomings of PAA, including high costs and vol-ume shrinkage caused by Pb2þ ion neutralization, not only limit thewater absorption but also reduce the removal effectiveness of Pb2þ

ion adsorption capacity [18]. Natural polymers have some advan-tages of biocompatibility, biodegradability and reducing thetoxicity and costs [19]. Cellulose [20], lignin [21], starch, chitosan[18], and nut shells are common natural materials. Lignin is abiodegradable and renewable material and at the same time theprice of lignin is low. In addition to cellulose, lignin also providesmechanical support and nutrient transport for plant cell walls[22,23]. It was reported that lignin can reinforce the hydrogenbonded/cross-linked polymer networks to increase the thermo-mechanical properties of poly (ester-amine) [24]. Lignin is anorganic pore forming agent that can help composite hydrogel toform porous structure and avoids the complicated operation fromwashing pore template. LBPAA composite hydrogel can biodegradein the soil, and the minimum cycle of characteristic index of soilsamples is only 35 days, and the biodegraded waste has little in-fluence on the soil and environments [25]. China lacks wood, about86% paper pulp comes from the crop straw, grass, and cotton stalk.Developed countries used wood as raw material in paper-making.More than 50% industrial lignin (mainly lignin sulfonate andlignin sulfate) can be effectively used as organic chemical resources.Each year, more than 90% of the wheat straw alkali lignin can onlybe treated as waste, simply burned and released directly into thenature [25].

The advantages of easy accessibility and low cost to use organicmontmorillonite (OrgMMT) can be obviously observed bycomparing with other fillers such as barium titanate, graphenequantum dots, magnetic nanoparticles, and others [26,27]. Forexample, calcium alginate/organic MMT composites are effectiveand demonstrate rapid removal of anionic dyes fromwater becauseof the higher specific surface area and porosity [28,29]. OrgMMT isoften combinedwith biopolymers to improve its properties, such asOrgMMT can improve water vapor resistance of hydroxypropylstarch. OrgMMT composites have good performance to removeheavy metals, dyes, pesticides from water [30,31]. Silicate clayswith lignin/fibers composites showed imrpoved mechanical andthermal properties. OrgMMT modified chitosan composites havegood adsorption capacity [32]. ZrO2 doped chitosan hybrid materialwas used as an artificial bone [30,33]. Carboxy methyl cellulose-nano OrgMMT loaded pesticide was taken as atrazine as well as itabsorbed imidacloprid and thiamethoxam from water [30].Recently, hydrogels with OrgMMT as adsorbents have been appliedfor the removal of heavy metal ions, especially with three dimen-sional crosslinked polymeric structures and hydrophilic groups[28]. But synthetic polymers have some shortcomings of harddegradation, poor salt tolerance and slower adsorption rate [34].

In this work, alkali lignin grafted with N, N0-ethylene-bisacrylamide (LM) was used to copolymerize with acrylic acid. Inaddition, OrgMMT nanoparticles were dispersed in the matrix [26].The swelling kinetics, pH values, temperatures and concentrationsof KCl solution of the LBPAA/OrgMMTcomposites were investigatedas well as water absorbency at various lignin amounts. The effect ofpH value and ion competing ratio of Naþ to Pb2þ on the Pb2þionremoval was studied. Different swelling properties of OrgMMT andthe polymer matrix increase the porosity and efficiency of Pb2þ

adsorption. These composites were testified to increase adsorptioncapacity of Pb2þ ion and make removal rate of metal ion faster.

2. Materials and methods

2.1. Materials

Alkali lignin, from wheat straw, contained 82.69% lignin, 8.35%carbohydrates, and 8.96% ash (Tralin Paper Co., Ltd., Shandong,China). Functional groups of purified lignin via removing sugarsand ashes are as follows: phenolic hydroxyl (1.4896 mmol g�1);aliphatic hydroxyl (1.4604 mmol g�1); weight average molecularweight of purified lignin (1903 Da). SodiumMMTwas purchased byAladdin Chemistry Co., Ltd., Shanghai, China. All the reagents werepurchased analytical grade without any further purification. Allaqueous solutions were treated by deionized water.

2.2. Preparation of LBPAA/OrgMMT composites

Lignin (1.10 g) was dissolved in 85 mL NaOH (1.25 M) and mixedwith N, N0-methylene-bisacrylamide (MBAm, 1.97 g), then addedAPS (0.10 g) and the flask was sealed at 60 �C for 10 min, then gotlignin grafted with N, N0-methylene-bisacrylamide (LM). Differentweights of LM (0.70, 0.88, 1.09, 1.34, and 1.64 g) were ultrasonicdispersed with 1.56 mL acrylic acid (AA, degree of 63 mol%), APS(0.16 g), MBAm (0.97 g) and 3wt% OrgMMT, reacted 15min at 60 �C,then copolymerized 2 h with nitrogen protecting and freeze driedto get LBPAA/OrgMMT composites [27]. For example, compositesnamed as LBPAA/OrgMMT (PAA 67.90/29.10 LM) that consists with67.90 wt% PAA and 29.10 wt% LM. PAA was also prepared as acontrol sample. There are 97 wt % LBPAA and 3 wt % OrgMMT infinal product composition of the LBPAA/OrgMMT composites. Theinfluencing factors for the preparation of LBPAA/OrgMMT includethe graft reaction time of LM, the amount of MBAm, APS andOrgMMT, and hydrogel performance evaluation index is the waterswelling capacity (see in Fig. A1) [6,35].

2.3. Characterization of LBPAA/OrgMMT composites

The FTIR spectra were analyzed using Nicolet 6700 (KBr pressedpellets, Thermo Scientific Inc., USA) [36]. SEM (S-4800, Hitachi Ltd.,Japan; and Quanta 200, FEI Ltd., USA) was used for getting themorphologies of OrgMMT composites. The BrunauereEmmette-Teller (BET) specific surface area, pore size and pore volume wereobtained by ASAP 2020 (nitrogen adsorption at 77.3 K, the relativepressure ranging from 10�6 to 1.0 bar, Micromeritics Co., USA)[37,38]. Samples were freeze-dried at �60 �C for 24 h. The bindingenergy can be recorded by XPS (Thermo Fisher Scientific Co., Ltd,USA). XPS was tested at room temperature with monochromatic AlKa radiation (1486.6 eV).

2.4. Swelling experiments

Kinetic data of water swelling and salt tolerance of compositehydrogels were measured by the gravimetric method. And theLBPAA/OrgMMT composites were kept at 20 �C in the air after theabsorption equilibrium, the water absorbency of LBPAA/OrgMMTcomposites was still recorded at fixed intervals until its adjacentdifference with mass was less than 0.0002 g (1.0 g dried gel/200 mLdeionized water) [10]. All of the experiments were run in triplicate.The effect of pH on water absorption was examined in the rangefrom 3 to 11. The effects on the water absorption caused by tem-peraturewere tested in the range from 4 to 60 �C. Salt tolerancewastest by a series of KCl solution at 25 �C.

Y. Ma et al. / Polymer 128 (2017) 12e2314

2.5. Adsorption of Pb2þ ion

Atomic adsorption spectrophotometer was used for measuringthe equilibrium concentration of Pb2þ ions before and afteradsorption test (TAS-990, Purkinje General, China).

2.6. Effect of pH on Pb2þ ion adsorption

To prevent the precipitation or hydrolysis of Pb2þ ion in me-dium, the initial pH value of was investigated in the range of3.0e5.0. The pH of solutions was adjusted by 0.01 or 0.10MHNO3 toavoid the effect on the adsorption of LBPAA/OrgMMT compositescaused by the concentration of Naþ cation during the adsorptionprocess, and then 0.1 g dried sample was added to 100 mL Pb2þ

solution (1mmol L�1) and shaken in a constant temperature shaker(25 �C, 150 rpm) for 13 h.

2.7. Effect of competing Na2þ on Pb2þ ion adsorption

0.10 g dried sample and 100 mL solution with different initialNa2þ/Pb2þ ratios were mixed and the mixtures were shaken in ashaker bath (25 �C, 150 rpm) for 13 h, and fixed Pb (II) ion con-centration (1 mmol L�1).

2.8. Adsorption kinetics experiments

0.1 g dried sample was put in 100mL Pb2þ solution at 25 �C withshaking rate of 150 rpm, and fixed Pb2þ concentration of20 mmol L�1.

2.9. Adsorption isotherm experiments

0.10 g dried sample was put in 100 mL solutions into a shakingwater bath that transformed several different initial Pb2þconcen-tration for 13 h (150 rpm, 25 �C).

Scheme 1. Synthesis of (a) PAA, (b) PAA/OrgMMT c

2.10. Adsorption thermodynamics

Adsorption thermodynamics tested by adding 0.10 g dryhydrogel samples into 100 mL solutions with an initial Pb2þcon-centration of 20 mmol L�1, and the solutions were then shaken in athermostatic shaker bath (20, 25 and 30 �C, 150 rpm) for 13 h.

3. Results and discussion

3.1. Preparation of LBPAA/OrgMMT composites

The double bond is introduced by grafting between lignin andMBAm. Ligninwas grafted with acrylic acid by free radical-initiatedreaction. Moreover, OrgMMT was uniformly dispersed underultrasonication as depicted in Scheme 1. It should be noted that theinfrared spectrum of the gel (Fig. A2) cannot be considered as thesuperposition of the FTIR spectra of the PAA, OrgMMT, lignin andMBAm. The stretching vibration of ester bonds of PAA at 1735 cm�1

(in Fig. A2, f) shifted to 1720 cm�1 (in Fig. A2, b and d), indicatignthat LM crosslinked with PAA successfully [24,39e41]. Thebreathing vibrations of aromatic ring in the LM at 1600 cm�1 alsoshifted to 1589 cm�1 [8]. The syringyl breathing vibration appearedat 1329 cm�1, guaiacyl breathing vibration appeared at 1266 cm�1,and phenolic hydroxyl groups stretching vibration appeared at1210 cm�1 (See in Fig. A2, a, b&d) [42]. Compared with Fig. A2 (c)-(e), the stretching vibration of Si�O, Si�O�Al and Si�O� Si bondsappeared at 1032, 523 and 460 cm�1, no significant changes wereobserved, indicating that OrgMMT tended to be blended withLBPAA [9,43]. OrgMMT is physically dispersed in the compositematerial rather than chemically bonded.

3.2. Morphology and textural properties

Fig. 1 shows the SEM microstructures of the freeze-driedhydrogels. The SEM photo shows that PAA (Fig. 1g) has a thickcell wall with honeycomb-like structure. Compared with PAA, thePAA/OrgMMT hydrogels revealed shrunk holes. Nevertheless, the

omposites, and (c) LBPAA/OrgMMT composites.

Fig. 1. SEM images of the freeze-dried hydrogels (a) LBPAA/OrgMMT (PAA 67.90/29.10 LM), (b) LBPAA/OrgMMT (PAA 58.20/38.8 LM), (c) LBPAA/OrgMMT (PAA 53.35/43.65 LM), (d)LBPAA/OrgMMT (PAA 63.05/33.95 LM), (e) LBPAA/OrgMMT (PAA 48.50/48.50 LM), and (f) PAA/OrgMMT composite, and (g) PAA.

Y. Ma et al. / Polymer 128 (2017) 12e23 15

LBPAA/OrgMMT composites show rougher surfaces than PAA andPAA/OrgMMT composite due to the incorporation of LM. Thehydrogel with lower cross-linker density (gel a-e) shows moreporous morphology, and the cell walls of gel a-e are observed to bethinner than that of PAA/OrgMMT (gel g) and PAA (gel h) [44]. Theresult was consistent with a rougher surface found in the xanthan/lignin hydrogels (lignin from annual herbaceous) [45e48]. FromFig. 1 and Table 1, lignin was found to fill in the pore of LBPAA/OrgMMT composites. High magnification SEM photograph of theinner surface of PAA/OrgMMT (PAA 58.20/38.8 LM) was rougherand hadmore pores than PAA/OrgMMT, as a result of grafting LM onthe surface of composite hydrogel (Fig. 2) [12]. The OrgMMT withrougher surfaces indicates the coploymerization of AA and LM (seein Fig. 2 a&b). OrgMMT was embedded in the body of hydrogels(PAA 58.20/38.8 LM), Fig. 2 c.

LBPAA/OrgMMT (PAA 53.35/43.65 LM) has the highest specificsurface area (12.2 ± 2.8 m2 g�1) and specific pore volume(1.40 � 10�3±2.00 � 10�4 cm3 g�1). The specific surface area ofthe hydrogels was 1.70 ± 0.90 m2 g�1, and its pore volume of2.30 � 10�4±1.10 � 10�5 cm3 g�1. With the crosslinking density ofLBPAA/OrgMMT decreased, the composites have developed porestructure, higher water absorption capacity and faster swellingand de-swelling rate [49]. The order of specific surface area,specific pore volume, swelling rate, and de-swelling rate of LBPAAhydrogels was c > b > a > d > e (See in Table 1). OrgMMT causedthe phase separation to form a well-distributed number of poresin the polymer matrix. In addition, the average pore size ofLBPAA/OrgMMT composites is larger than that of PAA/OrgMMTcomposites, and its specific surface area is smaller than thatof PAA/OrgMMT composites. The water absorption capacity

Table 1Textural property and swelling rate constants of porous LBPAA/OrgMMTs and PAA.

SamplePAA/LM(w/w)

SSA (m2$g�1) SPV (Vg,

PAA 53.35/43.65 LM/OrgMMT 12.2 ± 2.8 1.4 � 10PAA 58.20/38.8 LM/OrgMMT 10.2 ± 1.7 1.2 � 10PAA 63.05/33.95 LM/OrgMMT 7.2 ± 0.1 7.5 � 10PAA 67.90/29.10 LM/OrgMMT 6.5 ± 1.6 5.5 � 10PAA 48.50/48.50LM/OrgMMT 3.6 ± 0.8 3.6 � 10PAA/OrgMMT 1.7 ± 0.9 2.3 � 10PAA 0.3 ± 0.5 1.5 � 10

Note: Specific surface area (ab. SSA), Specific pore volume (ab. SPV), Swelling rate const

(302.9 g g�1), swelling rate (1.48 g g�1$min�1), and deswellingrate (0.0258 g g�1$min�1) of LBPAA/OrgMMT (PAA 53.35/43.65LM) composites are better than those of the PAA/OrgMMT com-posites (131.8 g g�1, 1.13 g g�1$min�1 and 0.0087 g g�1$min�1,respectively).

3.3. Swelling kinetics

The water absorbency was calculated according to Eq. (1).

SW ¼ Wt �Wd

We �Wd(1)

where SW (g$g�1) is the swelling ratio per gram dried sample, Wd

(g) andWt (g) are the mass of dried and swollen hydrogels at time t.We (g) is the absorption capacity of samples on the equilibriumadsorption [50].

In Fig. 3a, the water absorption capacity of LBPAA/OrgMMT (PAA53.35/43.65 LM) hydrogels is the highest (302.9 g g�1), while otherLBPAA hydrogels also have higher water absorption capacity thanPAA (131.8 g g�1) (See in Table 2). The higher water absorptioncapacity in the LBPAA/OrgMMT composites than that in the PAA/OrgMMT composites is believed arising from the decreased cross-linking density of composite polymer by adding LM that also helpsLBPAA/OrgMMT composites to form the pores [49]. The grafting ofLM leads to the amorphous structure, which disrupts the regularstructure of PAA/OrgMMTcomposites, and reduces the crosslinkingof LBPAA/OrgMMT composites. Fig. 3b shows that the cumulativerelease percentage of LBPAA/OrgMMT composites is larger thanthat of the PAA/OrgMMT composites. For instance, LBPAA/OrgMMT

cm3$g�1) SRC(g$g�1$min�1)

DRC(g$g�1$min�1)

�3±2.0 � 10�4 1.48 ± 0.27 �0.0258 ± 0.0020�3±1.3 � 10�4 1.41 ± 0.20 �0.0185 ± 0.0011�4±2.4 � 10�5 1.34 ± 0.11 �0.0182 ± 0.0012�4±1.4 � 10�5 1.34 ± 0.18 �0.0180 ± 0.0014�4±1.5 � 10�5 1.23 ± 0.17 �0.0165 ± 0.0016�4±1.1 � 10�5 1.13 ± 0.16 �0.0087 ± 0.0006�4±8.2 � 10�5 0.28 ± 0.03 �0.0241 ± 0.0024

ant (ab. SRC), Deswelling rate constant (ab. DRC).

Fig. 2. SEM images (high magnification) of the freeze-dried (a) LBPAA/OrgMMT (PAA 48.50/48.50 LM), (b) PAA/OrgMMT composites, and (c) PAA.

Fig. 3. Plots of swelling ratio of LBPAA/OrgMMT composites, PAA/OrgMMT compositeand PAA as function of time for (A) swelling at deionized water and (B) deswelling inair at 20 �C. Inset: (a) LBPAA/Org MMT (PAA 53.35/43.65 LM), (b) LBPAA/OrgMMT (PAA58.20/38.8 LM), (c) LBPAA/OrgMMT (PAA 63.05/33.95 LM), (d) LBPAA/OrgMMT (PAA48.50/48.50 LM), (e) LBPAA/OrgMMT (PAA 67.90/29.10 LM), (f) PAA/OrgMMT and (g)PAA.

Table 2Related kinetic parameters of Díez-Pefia. E and Schott's second-order models.

Samples Sr(g$g�1)

Díez-Pefia. E models

10�3k1(g$min�1)

10�3k2(g$min�1

LBPAA/OrgMMT(PAA 53.35/43.65 LM) 302.8656 6.76 8.99LBPAA/OrgMMT(PAA 58.20/38.8 LM) 280.8522 6.5 16.49LBPAA/OrgMMT(PAA 63.05/33.95 LM) 269.7456 6.18 11.01LBPAA/OrgMMT(PAA 67.90/29.10 LM) 260.8302 3.99 15.04LBPAA/OrgMMT(PAA 48.50/48.50 LM) 255.834 5.55 12.88PAA/OrgMMT 104.5736 13.6 29.15PAA 131.834 6.04 2.82

Y. Ma et al. / Polymer 128 (2017) 12e2316

(PAA 53.35/43.65 LM) can release 158.0 g g�1 water, and otherLBPAA/OrgMMT composites (PAA 58.20/38.8 LM 140.8 g g�1, PAA63.05/33.95 LM 132.6 g g�1, PAA 67.90/29.10 LM 132.3 g g�1, andPAA 48.50/48.50 LM 139.2 g g�1) are also higher than PAA/OrgMMTcomposites (73.0 g g�1) and PAA (122.3 g g�1).

The swelling rate of LBPAA/OrgMMT composite was carried outvigorously within 480 min, and after that the water absorption ca-pacity of the desorption process increased slowly until 7620 min(Fig. 3 a). The swelling rate constant (1.48 ± 0.27 g g�1$min�1) anddeswelling rate constant (�0.026 ± 0.0020 g g�1$min�1) of LBPAA/OrgMMT (PPA55/45LM) were higher than PAA/OrgMMT (swelling1.13± 0.16 g g�1$min�1, de-swelling�0.0087± 0.0006 g g�1$min�1)(See in Table B1). Thus, lignin can improve water absorption ca-pacity and the amount of cumulative release rate of PAA/OrgMMTcomposites.

To further elaborate the swelling mechanism, the kinetic pa-rameters of LBPAA/OrgMMT composites were calculated usingDíez-Pefia. E. (Eq. (2)) and Schott Second-order (Eq. (3)) models(see in Table 2).

StS∞

¼

�k1k2

��1� e�ðk1þk2Þt

��k1k2

�þ e�ðk1þk2Þt

(2)

tSt

¼ 1k3S2∞

� tS∞

(3)

where k1 is the first-order rate constant, k2 is the auto-catalytic rateconstant, and k3 is the initial swelling rate of Schott swellingequation. St is water absorption capacity (g$g�1) at time t; S∞ istheoretical equilibrium water absorption capacity (g$g�1) [36]. Inthe initial 200 min, the water absorption of the LBPAA/OrgMMTcomposites follows the Díez-Pefia E. model well. (Correlation

Schott's second-order models

)Seq(theory)(g$g�1)

R2 k4(10�6 g min�1)

S∞(g$g�1)

R2

302.8656 0.9961 37.3082 325.3079 0.9880280.8522 0.9962 34.3568 305.0517 0.9894269.7456 0.9978 33.7756 293.5948 0.9887260.8302 0.99852 31.6602 286.6220 0.9857255.834 0.99636 23.3612 289.8038 0.9766104.5736 0.9751 371.423 105.8814 0.9994131.834 0.9953 63.3 144.3306 0.9957

Y. Ma et al. / Polymer 128 (2017) 12e23 17

coefficients R2 is above 0.99 higher than that of the Schott second-order model, see in Table 2). After that, the water absorption ratedeclines until reaching absorption equilibrium. This can beexplained that a proton of the carboxyl group of LBPAA/OrgMMTcomposites carries a positive charge and repels each other, leadingto their volume expansion. That results in a decrease of osmoticresistance of water molecules. As hydrogen ion gradually accu-mulates on the surface of hydrogels, causing hydrogel volumefurther expand. Thus, the autocatalytic effect would becomestronger. Water absorption rates of LBPAA/OrgMMT compositeswere almost lower than that of the PAA/OrgMMTcomposites in thebeginning of 30 min, and then the water absorption rates suddenlywere accelerated higher than PAA/OrgMMT composites. The syn-ergies of LM and OrgMMT formed porous hydrogels on the basis ofphase separation principle [25].

3.4. Effect of pH and temperature on swelling

The effect of LBPAA/OrgMMT composites on pH is divided intothree segments (See in Fig. 4 a). At pH < 5, the water absorbencyincreased with increasing the pH value. This phenomenon may bedue to the carboxylate groups protonated (COO�/COOH) that in-creases the molecular interactions and forms extra crosslinking[39,51]. But when 6 < pH < 7, the molecular interactions wereweakened because of the lack of Hþ (pKa of carboxyl group is at4.75) and the network shrunk. When 7 < pH < 10, the carboxylgroups of LBPAA/OMMT are converted to sodium carboxylates, andwith the increase of pH in the solution, the increase of the Naþ

Fig. 4. Effects of (A) pH and (B) temperature on the water absorbency of LBPAA/OrgMMT composites, PAA/OrgMMT composites and PAA. Inset: (a) LBPAA/Org MMT(PAA 53.35/43.65 LM), (b) LBPAA/OrgMMT (PAA 58.20/38.8 LM), (c) LBPAA/OrgMMT(PAA 63.05/33.95 LM), (d) LBPAA/OrgMMT (PAA 48.50/48.50 LM), (e) LBPAA/OrgMMT(PAA 67.90/29.10 LM), (f) PAA/OrgMMT and (g) PAA.

concentration can help LBPAA/OMMT to rebuilb a new chargerepulsion balance by Naþ. That caused the volume of LBPAA/OMMTswell again. Finally, for the pH of the aqueous solution exceeding10, the LBPAA/OrgMMT hydrogel shrinks. The reasons were thatphenolic hydroxyl of lignin became phenol sodium (pKa of phenolichydroxyl group is at 9.95) when pHwas 10. It destroyed the positivecharge repulsion balance, and led to the volume shrinkage ofhydrogel and decrease of water absorption capacity. Meanwhile,the collapsed LBPAA/OrgMMT (PAA 48.50/48.50 LM) hydrogel wasbrought by partial dissolution of LM.

Fig. 4(B) shows the water absorption capacities of the hydrogelsat fixed temperatures, the water absorption capacity decreasedalmost linearly with increasing the temperature in the range of4e30 �C, and then increased almost linearly in the range of30e60 �C. The water absorption capacities of composite hydrogelsat 4 �C are 376.7 g g�1 of LBPAA/OrgMMT (PAA 53.35/43.65 LM),363.7 g g�1 of LBPAA/OrgMMT (PAA 58.20/38.8 LM), 344.3 g$g�1ofLBPAA/OrgMMT (PAA 63.05/33.95 LM), 325.7 g$g�1of LBPAA/OrgMMT (PAA 67.90/29.10 LM), and 312.5 g g�1 of LBPAA/OrgMMT(PAA 48.50/48.50 LM). Those water absorption capacites were allhigher than PAA/OrgMMT (166.3 g g�1) and PAA hydrogels(191.5 g g�1).

3.5. Salt tolerance on swelling

Salt tolerance of LBPAA/OrgMMT composite hydrogels wasperformed by using a series of different concentrations of KCl so-lutions. KCl solution helps to prevent sodium ions of LBPAA/OrgMMT from disturbing the swelling process of composites [52].In Fig. 5, the LBPAA/OrgMMT composites can release more waterwhen the concentration of KCl solution is in the range of0.02e0.34 mM. The water absorbency of LBPAA (PAA 53.35/43.65LM, 285.3 g g�1) was higher than PAA/OrgMMT (89.9 g g�1) whenthe concentration of KCl solution was 0.09 mM. The reason for thephenomenon was the porous structure formed by lignin andmontmorillonite via the pore-forming technology of phase sepa-ration that decreased the crosslinking density of composites [49].Furthermore, the water absorbency of LBPAA/OrgMMT (PAA 48.50/48.50 LM, 178.9 g g�1) was higher than PAA/OrgMMT composites(75.8 g g�1), but it was lower than PAA (187.6 g g�1) at 0.34 mM KClsolution. The strong interfacial force generated betweenintercalated-structure of OrgMMT nanoparticle and polymer chainis the main reason to cause the enhanced salt tolerance [21].

Fig. 5. Effects of varying KCl concentration on water absorbency of LBPAA/OrgMMTcomposites, PAA/OrgMMT composites and PAA. Inset: (a) LBPAA/Org MMT (PAA 53.35/43.65 LM), (b) LBPAA/OrgMMT (PAA 58.20/38.8 LM), (c) LBPAA/OrgMMT (PAA 63.05/33.95 LM), (d) LBPAA/OrgMMT (PAA 48.50/48.50 LM), (e) LBPAA/OrgMMT (PAA 67.90/29.10 LM), (f) PAA/OrgMMT and (g) PAA.

Y. Ma et al. / Polymer 128 (2017) 12e2318

3.6. Effect of pH and salt tolerance for Pb (II) adsorption

The charge of the functional groups on the surface of the com-posite hydrogel and the complex form of heavy metal is affected bypH value (see in Fig. 6 A). The equilibrium adsorption capacity (Qe,mmol$g�1) was calculated following Eqn (4) [53]:

Qe ¼ ðC0 � CeÞVm

(4)

where C0 and Ce (mmol$L�1) are the concentrations of metal ionsbefore and after the adsorption, respectively, V (L) is the volume ofsolution, and m (g) is the weight of the dried hydrogel.

When the pH of the solution was lower than 4.5, the adsorptioncapacity of the LBPAA/OrgMMT composite hydrogel did not changesignificantly with decreasing the Hþ concentration, the carboxylateanions presented in the polymeric networks are converted to theprotonated form. That hindered the interaction of LBPAA/OrgMMTcomposites with Pb2þ ions. When the pH of the solution wasgreater than 5, the Pb2þ adsorption capacity of LBPAA/OrgMMTcomposites increased slightly. This phenomenon is due to somecarboxyl change into carboxylate to help adsorb lead ion with thedecrease of Hþ concentration in Pb2þ ion solution because the pKaof the carboxyl group is at 4.75.

Salt has two functions, one is that the increased salt concen-tration can stress adsorbent to releases water; another is enhancingthe dissociation degree of the adsorbate molecules and facilitatesthe amount of pollutants adsorbed. The ionic strength of the me-dium is an important factor for affecting the electrostatic repulsion

Fig. 6. Effects of (A) pH and (B) competing Naþ on the Pb (II) ion absorbency of LBPAA/OrgMMT composites, PAA/OrgMMT composites and PAA. Inset: (a) LBPAA/Org MMT(PAA 48.50/48.50 LM), (b) PAA/OrgMMT, (c) LBPAA/OrgMMT (PAA 58.20/38.80 LM), (d)LBPAA/OrgMMT (PAA 67.90/29.10 LM), (e) LBPAA/OrgMMT (PAA 53.35/43.65 LM), (f)PAA and (g) LBPAA/OrgMMT (PAA 63.05/33.95 LM).

balance of the LBPAA/OrgMMTcomposite hydrogels. In this section,the effects of Naþ/Pb2þ ratio on the adsorption process were dis-cussed, Fig. 6(B). The Pb2þ adsorption capacity of LBPAAA/OrgMMTdecreases slightly with the increase of the molar ratio of Naþ/Pb2þ

in the range of 0e0.21. When Naþ/Pb2þ(mol/mol) exceeded 0.21,Pb2þ adsorption capacity of all the samples decreased rapidly. Thatis due to the shrinkage of composite hydrogel (volume and pore)caused by the interference of Naþ ions. The Pb2þ adsorption ca-pacity of the LBPAA/OrgMMTs and PAA/OrgMMT composites ishigher than that of PAA. The intercalated structure of OrgMMT canhelp LBPAA to resist Naþ adsorption in the process of Pb2þ

adsorption.

3.7. Adsorption kinetics for Pb (II) adsorption

Eqs. (5)e(7) equations are pseudo-first, pseudo-second order,and intraparticle diffusion models [53,54]:

lnðQe � QtÞ ¼ lnQe � k1t (5)

tQt

¼ tQe

þ 1Q2e k2

(6)

Qt ¼ k3t12 (7)

where Qe (mmol$g�1) and Qt (mmol$g�1) are adsorption capacity inequilibrium and at time t, respectively; k1 (min�1), k2(g$mmol�1$min�1) and k3 (mmol�1$min�0.5) are the rate constants.

From Fig. 7(A), the Pb2þ adsorption capacity of LBPAA/OrgMMTcomposites increased sharply during the first 120 min and then

Fig. 7. Effects of (A) time and (B) initial concentration on the Pb(II) absorbency ofLBPAA/OrgMMT composites, PAA/OrgMMT composites and PAA. Inset: (a) LBPAA/OrgMMT (PAA 48.50/48.50 LM), (b) PAA/OrgMMT, (c) LBPAA/OrgMMT (PAA 58.20/38.8LM), (d) LBPAA/OrgMMT (PAA 67.90/29.10 LM), (e) LBPAA/OrgMMT (PAA 53.35/43.65LM), (f) PAA and (g) LBPAA/OrgMMT (PAA 63.05/33.95 LM).

Y. Ma et al. / Polymer 128 (2017) 12e23 19

tended to be equilibrium after 180 min, and then those adsorptioncapacities gradually increased until 6 h. As a control, the adsorptionrate of PAA was only higher than that of LBPAA/OrgMMT (PAA63.05/33.95 LM). The LBPAA/OrgMMT (PAA 48.50/48.50 LM) com-posites (1.08 mmol g�1) and PAA/OrgMMT composites(0.98 mmol g�1) have higher adsorption capacities than PAA(0.19 mmol g�1). The adsorption rate was related to the amount ofactive adsorption sites on composite hydrogel. Apparently, onlyLBPAA/OrgMMT (PAA 48.50/48.50 LM) (1.08 mmol g�1) has ahigher Pb2þ absorption rate and Pb2þ absorption capacity thanPAA/OrgMMT composites (0.98 mmol g�1). This reveals that thegrafting of lignin in the LBPAA/OrgMMT composites enhanced thePb2þ absorption capacity.

Fig. 7(B) shows that the equilibrium adsorption capacity of theLBPAA/OrgMMT composites has been significantly enhanced withincreasing the initial Pb2þ concentration. Until the maximumadsorbed amount is obtained, the removal efficiency decreaseswith increasing the Pb2þ concentration. When the Pb2þ concen-tration was below 1 mmol L�1, the Pb2þ ion removal rate wasalmost unaffected by its initial concentration.

High correlation coefficients (R2 > 0.979) and the proximitybetween the experimental adsorption capacity (Qt) and the calcu-lated equilibrium adsorption capacity (Qe) indicated that the Pb (II)ion adsorption onto LBPAA/OrgMMT composites could be approx-imated by the pseudo-second-order model (See in Table 3, Table B1

and Fig. A3). Therefore, the adsorption on LBPAA/OrgMMT com-posites is the chemical bonding through sharing or exchange ofelectrons between LBPAA/OrgMMT composites and Pb (II) ions[55,56].

3.8. Adsorption isotherm for Pb (II) adsorption

Table 3 shows the comparison of the Pb2þ adsorption capacitiesof LBPAA/OrgMMT composites, PAA/OrgMMT composites and PAA.The Langmuir model assumes a monolayer sorption, Eqn (8) [55]:

CeQe

¼ 1Q∞

Ce þ 1KLQ∞

(8)

where Ce (mmol$L�1) is the equilibrium concentration of Pb2þ ionsin solution, Qe (mmol g�1) is the adsorption capacity of Pb2þ ion atadsorption equilibrium, Q∞ (mmol g�1) represents the saturatedmonolayer adsorption capacity, and KL (L$mmol�1) is a Langmuirconstant. The separation factor RL (RL ¼ 1

1þkLC0) can predict the

availability of adsorption system, “unfavorable” (RL > 1), “linear”(RL ¼ 1), “favorable” (0<RL < 1), or “irreversible” (RL ¼ 0) [57]. The

Table 3Parameters of Pseudo-second-order and Freundlich equation for the absorption of Pb2þ

Sample Qe

(mmol$g�1)Pseudo-second-order

Qt

(mmol$g�1)k2(g$mmo

LBPAA/OrgMMT(PAA 48.50/48.50 LM)

1.1920 1.0806 0.0196

PAA/OrgMMT 1.0663 0.9797 0.0234LBPAA/OrgMMT(PAA 58.20/38.8 LM)

0.9508 0.8600 0.0220

LBPAA/OrgMMT(PAA 67.90/29.10 LM)

0.5572 0.5041 0.0367

LBPAA/OrgMMT(PAA 53.35/43.65 LM)

0.2457 0.2242 0.0906

PAA 0.2090 0.1925 0.1044LBPAA/OrgMMT(PAA 63.05/33.95 LM)

0.1095 0.1006 0.1916

Freundlich isotherm model assumes adsorption on heterogeneoussurfaces and possibly in a multilayer adsorption. The Freundlichequation is shown as Eqn (9):

lgQe ¼ 1nlgCe � lgKF (9)

where Ce (mmol L�1) and Qe (mmol g�1) have the same definitionsas before, the KF and n are constants indicating the adsorption ca-pacity and adsorption intensity, respectively.

The Pb (II) ions adsorption on LBPAA/OrgMMT composites,PAA/OrgMMT composites and PAA could be represented by theFreundlich and Langmuir model. Its results are summarized inTable 3 and Table B1). The adsorption capacity of LBPAA/OrgMMT(PAA 48.50/48.50 LM) (1.08mmol g�1) for Pb2þwas comparativelyhigher than other adsorbents (PAA/OrgMMT, 0.98 mmol g�1,LBPAA/OrgMMT (PAA 58.20/38.8 LM, 0.86 mmol g�1) and PAA(0.19 mmol g�1). The adsorption on lead ions of LBPAA/OrgMMTfollows the Freundlich model that indicates a multilayer absorp-tion [55].

Lignin was found to improve water absorbency and adsorptioncapacity of Pb(II), Figs. 7(A) and 4(A), but the OrgMMT (additionamount 3 wt%) decreased the water absorbency, but it could in-crease the swelling rate in water and adsorption rate of Pb (II) ofLBPAA/OrgMMT composites. The grafting of lignin not only helpsthe network of LBPAA/OrgMMT to form holes, but also fills andsupports the network, and the latter obviously increases thepenetration resistance of the lead ions into the system, which re-duces the adsorption capacity of lead ions, Fig. 6(A) and (B).

Another interesting phenomenon is that the swelling ratio(104.57 g g�1) of PAA/OrgMMTcomposites is lower than that of PAA(131.83 g g�1). Moreover, the adsorption capacity of Pb2þ (PAA/OrgMMT composite, 0.98 mmol g�1) was 0.79 mmol g�1 greaterthan PAA (0.19 mmol g�1). This is because the multivalent ions mayincrease the crosslinking density slightly and strengthen theintramolecular and intermolecular complex formation ability. Thus,the Pb (II) adsorption on the surface of PAA/OrgMMTand PAA led toa higher crosslinking density, that in turn stimulated the rapidshrinkage of those hydrogels, and decreased the pore size sharply.After constriction, some lead ions could not pass through the poresof composite hydrogels that caused by insufficient contact of leadions and PAA. However, in this research, when the PAA/OrgMMThydrogel shrunk because of the Pb (II) adsorption, OrgMMTbrought more pores in the PAA/OrgMMT, which helped Pb (II) ionsto enter into the internal of PAA/OrgMMT, and thus improvedPb (II) adsorption capacity in the PAA/OrgMMTand LBPAA/OrgMMTcomposites.

onto LBPAA/OrgMMT composites at 25 �C.

Freundlich equation

l�1$min�1)R2 kF

(L$g�1)n R2

0.9795 1.0353 1.7431 0.9965

0.9884 0.8586 1.6356 0.98470.9924 0.8176 1.6784 0.9984

0.9943 0.4374 1.4646 0.9834

0.9912 0.21 0.994 0.998

0.9911 0.1947 1.0167 0.99750.9963 0.1281 0.9296 0.9891

Y. Ma et al. / Polymer 128 (2017) 12e2320

3.9. Adsorption thermodynamics for Pb (II) adsorption

The distribution coefficient (Kd: L/g, calculation equation wasshown in Eq. (10)) of Pb2þ ions adsorption capacity versus 10�3/T atseveral temperatures are given in plots for the thermodynamicparameters (see in Fig. A4) and the calculation formula is illustratedas Eq. (11):

Kd ¼ Qe

Ce¼ ðC0 � CeÞV

mCe(10)

lgKd ¼ DS02:303R

� DH0

2:303RT(11)

The thermodynamic parameters including free energy change(DG0), enthalpy change (DH0) and entropy change (DS0) arerespectively estimated from Eq. (11), where R is the gas constant(J$mol�1$K�1), Qe is the equilibrium adsorption capacity(mmol$g�1), and Ce is the equilibrium concentration of Pb2þ ions inthe residual liquid (mmol$L�1).

The thermodynamic parameters of samples are tabulated inTable 4. Positive values of DH0 represent that the adsorption for Pb(II) ions adsorption of LBPAA/OrgMMT is endothermic pro-cesses. The DH0 values are �31.9782, �42.3581, �46.0689, �82.0546, �135.9721, �158.7193, and �177.4264 kJ/mol for Pb (II)ions adsorption on LBPAA/OrgMMT(PAA 48.50/48.50 LM), PAA/OrgMMT, LBPAA/OrgMMT (PAA 58.20/38.80 LM), LBPAA/OrgMMT(PAA 67.90/29.10 LM), LBPAA/OrgMMT(PAA 53.35/43.65 LM), PAAand LBPAA/OrgMMT (PAA 63.05/33.95 LM) composites, respec-tively, indicating that both physical adsorption and chemicaladsorption coexist. The observed negative DG0 indicates that theabsorption reactions can happen spontaneously. Low temperaturebenefits the adsorption process. Both the negligible entropychanges and the Freundlich adsorption isotherm model indicatethe multilayered adsorption of LBPAA/OrgMMT composites forPb2þ.

3.10. XPS analysis of Pb (II) adsorption

The wide-scan spectra, Fig. 8 a, b&c, clearly show the existenceof Pb 4f peak, confirming the absorption of Pb2þ ions. The test resultshowed that C/O atom molar ratio of PAA was 1.076, higher than

Table 4Thermodynamic parameters of Pb2þ on for LBPAA/OrgMMT composites, PAA/OrgMMT co

Sample T(K) C0(mmol$L �1)

DH(k

LBPAA/OrgMMT(PAA 48.50/48.50 LM)

293 1 mmol/L �3298303

PAA/OrgMMT 293 �4298303

LBPAA/OrgMMT(PAA 58.20/38.8 LM)

293 �4298303

LBPAA/OrgMMT(PAA 67.90/29.10 LM)

293 �8298303

LBPAA/OrgMMT(PAA 53.35/43.65 LM)

293 �1298303

PAA 293 �1298303

LBPAA/OrgMMT(PAA 63.05/33.95 LM)

293 �1298303

that of the LBPAA/OrgMMT composites (0.651), PAA/OrgMMTcomposite (0.353). This proved that the increase of OrgMMT andalkali lignin can decrease the carboxyl and hydroxyl groups ofLBPAA/OrgMMT composite hydrogel. The spectra for Pb(NO3)2 arecentered at 144.3 eV for Pb 4f5/2 and 139.4 eV for Pb 4f7/2, while forthe Pb-LBPAA/OrgMMT (PAA 48.50/48.50 LM) composites, anobvious shift of 0.92 eV to lower binding energy of both Pb 4fspectra is observed. Compare both Pb 4f5/2 and Pb 4f7/2 data ofPb(NO3) 2, a similar shift of PAA of 1.02 eV to low regions wasobserved. The characteristic peaks at Pb 4f5/2 143.28 eV and Pb 4f7/2138.38 eV of lead ions adsorbed on the PAA are in good agreementwith the reported values, and the peak of Pb 4f7/2 shows a bindingenergy corresponding to the formation of Pb‧‧‧O bond, indicating aspecial interaction between Pb2þ and samples (LBPAA/OrgMMT,PAA/OrgMMTcomposites and PAA, Fig. 8 d, e and f) [58]. The Pb 4f7/2 binding energy of PAA/OrgMMT composites becomes 138.58 eVfrom the original 138.38 eV, and that for Pb 4f5/2 changes from143.28 to 143.38 eV. The slight increase in Pb 4f binding energy ofPAA/OrgMMT composites suggests a weak interaction between thePAA and Pb2þ by OrgMMT composites. Moreover, the slightdecrease in Pb 4f binding energy of LBPAA/OrgMMT compositereveals that lignin gave more interaction between PAA/OrgMMTcomposite and Pb2þ.

The C1s spectra of the LBPAA/OrgMMT and PAA/OrgMMT com-posites could be curve fitted by three peaks at the binding energiesof 284.4 (C�C), 285.4 (C�O‧‧‧H) and 287.8 eV (C¼O), and the spectraof PAA were fit by three peaks at the binding energies of284.4 eV (C�C), 285.4 eV (C�O‧‧‧H) and 289.3 (MC‧‧‧O) eV (Fig. 8 g,h&i). This further indicated that OrgMMT could weaken thehydrogen bonds interaction between carboxyl groups and hydroxylgroups, while the LBPAA/OrgMMT composites could rebuild somehydrogen bonds by phenolic hydroxyl of alkali lignin. In Fig. 8 j, k&l,the O1s spectra of LBPAA/Org MMT (PAA 48.50/48.50 LM), PAA/OrgMMT composites and PAA could be fitted by 5 peaks at thebinding energies of 530.5, 531.1, 531.8, 532.8 and 535.2 eV, corre-sponding to the bonds from -OH, M-O, OH�, Si-O and CH3COO�,respectively. The new peaks at binding energies of 532.8 and535.2 eV are due to Si-O and CH3COO�. The CH3COO� peak area ofLBPAA/OrgMMT (PAA 48.50/48.50 LM) is less than that of the AA/OrgMMT composites, also indicating that the lignin increased thePb2þ adsorption capacity.

mposite and PAA.

0

J$mol�1)DS0(J$kmol�1)

DG0

(kJ$mol�1)R2

1.98 �104.43 �1.3639 0.9972�0.8417�0.3196

2.36 �136.21 �2.4271 0.9998�1.7461�1.065

6.07 �154.45 �0.7931 0.9997�0.02090.7513

2.05 �275.26 �1.3609 0.99940.01541.3917

35.97 �462.28 �1.3639 0.9992�0.8417�0.3196

58.72 �549.32 2.3146 0.99755.0612

77.44 �604.60 �0.1876 0.99982.83545.8584

Fig. 8. XPS curve fitting spectra of the survey, Pb 4f, C1s and O1s signals for LBPAA/OrgMMT composites, PAA/OrgMMT composites and PAA after Pb2þ ion absorption. Inset: (a)survey spectrum of LBPAA/OrgMMT (PAA 48.50/48.50 LM), (b) survey spectrum of PAA/OrgMMT, (c) survey spectrum of PAA, (d) Pb 4f spectrum of LBPAA/Org MMT (PAA 48.50/48.50 LM), (e) Pb 4f spectrum of PAA/OrgMMT, (f) Pb 4f spectrum of PAA, (g) C1s spectrum of LBPAA/Org MMT (PAA 48.50/48.50 LM), (h) C1s spectrum of PAA/OrgMMT, (i) C1sspectrum of PAA, (j) O1s spectrum of LBPAA/Org MMT (PAA 48.50/48.50 LM), (k) O1s spectrum of PAA/OrgMMT and (l) O1s spectrum of PAA.

Y. Ma et al. / Polymer 128 (2017) 12e23 21

4. Conclusions

LBPAA composite hydrogels modified with OrgMMT were pre-pared successfully. The swelling properties were found to stronglydepend on the pore of hydrogels and composition of lignin graftedMBAm. The swelling and de-swelling rate constants of LBPAA/OrgMMT (PAA 53.35/43.65 LM) were higher than those of PAA. Thewater absorbency of LBPAA (PAA 53.35/43.65 LM, 285.3 g g�1) washigher than PAA/OrgMMT (89.9 g g�1) when the concentration ofKCl solution is 0.09 mM. LBPAA/OrgMMT composites showed rapidadsorption performance over 60 min. The Pb2þ adsorption capacityof LBPAA/OrgMMT (PAA 48.50/48.50 LM) was 1.08 mmol g�1,higher than others. The fitted pseudo-second order adsorption ki-netics indicated the existence of chemical bonding of Pb2þ to theLBPAA/OrgMMT composites, which was later confirmed by XPSanalysis, proving the involvement of hydroxyl groups and carboxylgroups of LBPAA/OrgMMT composites in the adsorption process toform Pb…O bond. The adsorption mechanism of LBPAA/OrgMMTcomposites was a multilayer adsorption follwowing Freundlichisotherm. The grafting of lignin and OrgMMT not only helped toform holes, but also filled and supported the composite hydrogels.The LBPAA/OrgMMTcomposites have potential applications for fastand feasible and rapid removal of Pb2þ ions from polluted waterwith good salt tolerance and low cost.

Acknowledgments

The authors gratefully acknowledge financial supported byHeilongjiang Province Science Foundation for Youths (No.QC2014C011), the Fundamental Research Funds for the Central

Universities (No. 2572017CB14), Harbin Special Funds to Innova-tive Talents on Science and Technology Research Project (No.RC2016QN017026). Z. Guo appreciates the start-up fund fromUniversity of Tennessee and ACS Petroleum Research Fund(53930-ND6).

Appendix A. Supplementary data

Supplementary data related to this article can be found athttps://doi.org/10.1016/j.polymer.2017.09.009.

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