The effect of extractant on the removal of heavy metal ions by thermoresponsive cellulose graft copolymer

The effect of extractant on the removal of heavy metal ions by thermoresponsive cellulose graft copolymer

Journal of Environmental Chemical Engineering 4 (2016) 1948–1954 Contents lists available at ScienceDirect Journal of Environmental Chemical Enginee...

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Journal of Environmental Chemical Engineering 4 (2016) 1948–1954

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

The effect of extractant on the removal of heavy metal ions by thermoresponsive cellulose graft copolymer Zehra Ozbas1, Canan Püren Sahin, Emine Esen, Gülten Gurdag, Hasine Kasgoz* Istanbul University, Faculty of Engineering, Department of Chemical Engineering, 34320 Avcilar, Istanbul, Turkey

A R T I C L E I N F O

A B S T R A C T

Article history: Received 14 December 2015 Received in revised form 12 February 2016 Accepted 5 March 2016 Available online 7 March 2016

The adsorption and desorption property of the thermoresponsive cellulose-graft-poly(N-isopropyl acrylamide) (PNIPAM) copolymer for Cu(II), Pb(II), Ni(II) and Cd(II) ions have been investigated by temperature swing adsorption (TSA) process. In this study, sodium lauryl sulfate (SLS) and dodecyltrimethylammonium chloride (DTAC) which interact with heavy metals were chosen as the extractants and their effect on the adsorption capacity of cellulose copolymer was investigated. The adsorption (50  C) and desorption (10  C) processes were performed by temperature swing. The kinetic data were found to follow the pseudo-second order model and intraparticle diffusion is not the only rate controlling step. In the non-competitive medium, the adsorption of heavy metal ions occurred in the order Cd(II) > Cu(II) > Ni(II) in the presence of SLS, but in the case of DTAC it followed the order Cu(II) > Cd (II)> Ni(II) > Pb(II). ã 2016 Elsevier Ltd. All rights reserved.

Keywords: Cellulose Graft Kinetics Metal removal N-isopropyl acrylamide

1. Introduction Temperature swing adsorption (TSA) is one of the most used application fields of thermosensitive polymers. The adsorption and desorption processes can be carried out by changing the temperature within a few degree across the phase transition of the thermosensitive adsorbent [1,2]. The physical properties of thermoresponsive polymers change by temperature change. The temperature at which precipitation of the linear polymer occurs is called as the lower critical solution temperature (LCST) [3]. Below the LCST, the linear polymer is in the hydrated and soluble state, while above the LCST, it is dehydrated and becomes hydrophobic, and finally precipitates [4]. The most studied thermoresponsive polymer is poly(N-isopropylacrylamide) (PNIPAM) with a LCST about 32  C [5], and the other one is poly(N,N-diethylacrylamide) (PDEAM) which has a LCST in the range of 25–35  C [6]. Kanazawa et al. prepared a molecular imprinted thermosensitive gel adsorbent composed of (N-isopropylacrylamide) (NIPAM) as thermosensitive component and N-(4-vinyl) benzyl ethylenediamine (VBEDA) as the chelating monomer. The adsorption and desorption were performed repeatedly by temperature swing and the selective adsorption of Cu(II) ion was confirmed by comparing

* Corresponding author. E-mail address: [email protected] (H. Kasgoz). 1 Present address: Çankırı Karatekin University, Faculty of Engineering, Department of Chemical Engineering, Çankırı 18100, Turkey. http://dx.doi.org/10.1016/j.jece.2016.03.006 2213-3437/ ã 2016 Elsevier Ltd. All rights reserved.

the adsorption amounts of Ni(II), Zn(II) and Mn(II) [7]. In their other study, Kanazawa et al. prepared a thermosensitive NIPAM microgel adsorbent by the emulsion polymerization using an anionic polymerizable surfactant and the molecular imprinted technique [8]. They found that the adsorption rate for Cu(II) ion was very quickly compared to the their previous study [7]. Tokuyama and Iwama were suggested a new technique combined with TSA and solid phase extraction [9]. In this method, a metal ion in an aqueous solution is complexed with an extractant and the metal-surfactant complexes are adsorbed onto the NIPAM gel at the temperatures above the LCST and desorbed at those under the LCST of polymer. Anionic surfactants, sodium n-dodecylbenzenesulfonate (SDBS) and n-dodecylbenzenesulfonic acid (DBS) which have an anionic group and a hydrophobic group in their structure were used as the extractants. Consequently, the applicability of this new technique for Cu(II) ion seemed to be possible because of the hydrophilic/hydrophobic (phase) transition of PNIPAM. Tokuyama et al. developed a molecular imprinted thermosensitive microgel grafted on a supporting substrate (polypropylene film) by plasmainitiated graft polymerization, and VBEDA was used as the chelating agent. The grafted imprinted-NIPAM–VBEDA gel repeatedly adsorbs and desorbs Cu(II) ions by the swing in temperature [10]. Tokuyama and Iwama [11] prepared a model system consisting of In(III) ions, n-octyl phosphate as an organophosphorus extractant, and PNIPAM. Approximately 90% of the In(III) ions were extracted from the aqueous solution, and In(III) and Zn(II) ions were separated via solid phase extraction. The extractant played two roles of a separator and a mediator to adsorb a target

Z. Ozbas et al. / Journal of Environmental Chemical Engineering 4 (2016) 1948–1954

metal selectively onto the NIPA polymer [11]. In our earlier study, a thermosensitive copolymer was obtained via graft copolymerization of NIPAM onto cellulose and the extractant, SDBS, was complexed with Cu(II) ion before the adsorption process. It was found that the copolymer with high adsorption capacity could be used effectively for the adsorption of the Cu-SDBS complex for the removal of Cu(II) ion [12]. The present work aims to compare the effect of surfactant on the removal of heavy metal ions, i.e., Cu(II), Ni(II), Cd(II) and Pb(II), in non-competitive and competitive conditions by the synthesized graft polymer. The anionic surfactant sodium lauryl sulfate (SLS) and the cationic surfactant dodecyltrimethylammonium chloride (DTAC) interacting with heavy metal ions were used as the extractant. SLS has a hydrophilic sulfate head group (negatively charged) and a hydrophobic alkyl tail group in the molecule, while DTAC has a hydrophilic quaternized ammonium head group (positively charged) and a hydrophobic alkyl tail group in the molecule. The metal-surfactant complexes are adsorbed on the copolymer through a hydrophobic interaction above the lower critical solution temperature (LCST) of the copolymer. The adsorption and desorption processes were performed by temperature swing. The pseudo-first and -second order adsorption and intra-particle diffusion kinetic models were applied to experimental data to evaluate the adsorption mechanism. In addition, the selective removal of heavy metal ions was also investigated. 2. Materials and methods 2.1. Chemicals

a-Cellulose (Cell) and sodium lauryl sulfate (SLS) were purchased from Sigma. N-isopropyl acrylamide (NIPAM) was provided from Acros. Nitric acid, dodecyltrimethylammonium chloride (DTAC) and cerium (IV) ammonium nitrate (CAN) were Merck products. Analytical grade copper(II) nitrate dihydrate (Merck), nickel(II) acetate tetrahydrate (Merck), cadmium(II) acetate dihydrate (Aldrich) and lead(II)acetate trihydrate (Merck)

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were used without any purification. All other chemicals were in extra pure grade. 2.2. Preparation of graft polymer The procedure for graft copolymerization of NIPAM onto Cell was given in our previous study [12]. Briefly, Cell (3 g) and NIPAM (12.7 g) were placed in a three-necked, round-bottom flask with 250 mL capacity, fitted with a mechanical stirrer at 30.0  0.1  C. Then, aqueous HNO3 (2.5  103 M) solution was added into the flask, and the solution was stirred at a constant rate. The mixture was purged with nitrogen gas for 30 min to remove oxygen gas. After that, the initiator CAN (12  103 M) was added in the four step with equal time intervals to initiate the graft copolymerization. After the desired reaction time (10 h), the reaction was stopped and the reaction mixture was washed with a large amount of distilled water to remove the unreacted monomer, linear homopolymer and impurities. The graft copolymer was dried at 40  C under vacuum until a constant weight was reached. The nitrogen content, the graft yield (G%) and the grafting efficiency (GE %) of the copolymer were calculated within our previous study [12]. The nitrogen content was 8.60%, G% which was the PNIPAM content per 100 g of total product, was 69.5% and GE % was 85.9%. 2.3. Adsorption and desorption studies For adsorption and desorption studies, the dry copolymer (0.5 g) and an aqueous solution of surfactant (20 mL) and a metal solution (20 mL) were mixed in a vial. Cu(II), Ni(II), Cd(II) and Pb(II) were used as heavy metal ions and the ratio of metal ion/surfactant was adjusted to 1/2 (25 mmol/L/50 mmol/L). To swell the copolymer, it was kept in a shaking water bath at 20  C for 1 day. Then, the vial was held at 50  C for 48 h, which was a sufficient period to obtain equilibrium state. After the adsorption, the copolymer was filtered and distilled water (40 mL) was added for desorption studies at 10  C for 48 h. The initial and equilibrium concentrations of metal ions in the solutions were measured by a Perkin Elmer

Fig. 1. Effect of time on the adsorption capacities of heavy metal ions in the presence of (a) SLS and (b) DTAC.

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2000 series atomic absorption spectrometer. The q (mmol/g) values of the graft copolymer were calculated by the Eq. (1): qðmmol=gÞ ¼ ðC i  C e Þ 

V m

ð1Þ

where q is the adsorption capacity of the copolymer (mol g1); Ci and Ce are the concentrations of the metal ions in the initial solution and after adsorption for different periods of time (mmol/ L), respectively, V is the volume of the solution added (L); and m is the amount of polymer (g). For competitive removal, because of the precipitation of Pb(SLS)2, the stock solution with SLS was prepared with Cu(II), Ni(II) or Cd(II) ions by adjusting the ratio of metal ion/SLS to 25 mmol/50 mmol for each metal ion. On the other hand, the stock solution with DTAC was prepared for Cu(II), Ni(II), Cd(II) or Pb(II) ions by adjusting metal ion/ DTAC concentration to 25 mmol/50 mmol. 3. Results and discussion 3.1. Non-competitive adsorption The adsorption capacity of copolymer for heavy metal ions in the single metal ion solution was performed at 50  C. The effect of time on the adsorption capacity for heavy metal ions was shown in Fig. 1a and b for SLS and DTAC, respectively. It is clearly seen that increasing adsorption time led to an increase in the adsorption capacity. At the solid–aqueous interface, hydrophobic interactions may exist between the surfactant and the solid surface, and also laterally between adsorbed surfactants [13]. As known, surfactants could complex with metal ions through hydrophilic head groups and the metal-surfactant complexes can occur. On the other hand, above the LCST, hydrophobic interactions between the alkyl chain of the surfactant and the hydrophobic methyl groups on the solid graft copolymer due to PNIPAM could induce the adsorption [12,14]. The graft copolymer was not efficient in the removal of metal ions without surfactants. For instance, in preliminary experiments, q of the polymer was found as 2.4  102 mmol/g for Cu(II) ions [12]. The adsorption amount of heavy metal ions was in the order Cd (II) (1.39 mmol g1) > Cu(II) (1.06 mmol g1) > Ni(II) (1.03 mmol g1) in the presence of SLS, while the order changed to Cu(II) (1.01 mmol g1) > Cd(II) (0.85 mmol g1) > Ni(II) (0.43 mmol g1) > Pb(II) (0.36 mmol g1) for the adsorption by DTAC. It was supposed that the stability of metal-surfactant complex may important role on the adsorption capacities. In our earlier study, a thermoresponsive polymer by grafting of NIPAM monomer onto cellulose were synthesized, and it was used for the removal of Cu(II) ion in the presence of sodium dodecyl benzene sulfonate (SDBS). The adsorption capacity of the copolymer was 1.18 mmol g1 [12]. Denizli et al. synthesized poly(hydroxyethyl methacrylate-Nmethacryloyl-(L)-glutamic acid) copolymer beads for the removal of lead ions and the lead adsorption capacity was 348 mg g1 [15]. Thermosensitive copolymers consisting of (dimethoxyphosphoryl) methyl 2-methylacrylate (MAPC1) and N-n-propylacrylamide (NnPAAm) were synthesized and used for sorption studies towards Ni(II) ions by Graillot et al. [16]. The highest Ni(II) ion removal capacity of the copolymer was around 0.52 mmol/g polymer. Takeshita et al. investigated thermal-swing extraction of Cd(II) ion by a thermosensitive gel obtained by crosslinking of PNIPAM with an encapsulating ligand, N,N,N0 ,N0 -tetrakis(4-propenyloxy-2-pyridylmethyl) ethylenediamine (TPPEN) [17]. Its extraction capacity of Cd(II) ion with NIPAM–TPPEN gel at 5  C attained as about 11 mg g1 dry gel. It is seen that our values are compatible with those in literature.

In order to find out the adsorption kinetics, three kinetic models were used, including the pseudo-first and -second orders and intra-particle diffusion kinetic models. The pseudo-first order kinetic model of Lagergren is linearized as presented in Eq. (2) [18]: Inðqe  qt Þ ¼ Inqe  ðk1 tÞ

ð2Þ

where qe (mmol/g) is the amount of adsorbed metal ions at equilibrium time, qt (mmol/g) is the amount of adsorbed metal ions at time t (h) and k1 (h1) is the rate constant of adsorption. The plots of ln (qe  qt) against t give straight lines, k1 and qe values for each ion were obtained from the slop and the intercept of the straight lines, respectively. The pseudo-second order kinetic model of Ho and McKay is presented by the following Eq. (3) [19]: t=qt ¼ ð1=k2 q2e Þ þ ðt=qe Þ 1

ð3Þ

1

where k2 (g mol h ) is the rate constant of pseudo-second order adsorption. The plots of t/qt against t give straight lines and from the slop and the intercept of the plots, the qe and k2 values were determined, respectively. The intra-particle diffusion equation of Weber and Morris is formulated as the Eq. (4) [20]: qt ¼ kpi t1=2 þ C i

ð4Þ

where kpi (mol g1 h1/2) is intra-particle diffusion rate constant of stage i and Ci (mol g1) is the adsorption constant of stage i. The kpi values were estimated from the slope of the plot of qt against t1/2 and Ci values were estimated from the intercept of the plot of qt against t1/2. Kinetic model constants and the correlation coefficients associated with heavy metal adsorption on the graft copolymer for SLS and DTAC were given in Tables 1 and 2, respectively, and the plots were presented for each model in Figs. 2 and 3 for SLS and DTAC, respectively. As can be seen from tables and Figs. 2a and 3a, the correlation coefficients for the pseudo first-order kinetic model were low ranged between 0.7985 and 0.9761, and the difference between the calculated qe,cal values and the experimental qe values were high for each ion. In the case of pseudo second-order kinetic model, the higher correlation coefficient values were indicated that the experimental kinetic data tended to fit well of this kinetic equation (Figs. 2b and 3b), and the experimental qe values agreed with the calculated qe,cal values.

Table 1 Kinetic parameters for the adsorption of Cu(II), Ni(II) and Cd(II) ions in the presence of SLS. Kinetic model

Heavy metal ions Cu(II)

Ni(II)

Cd(II)

Pseudo-first order qe,cal, mmol g1 k1, h1 r12

0.5813 0.1122 0.7988

0.6054 0.1092 0.9622

0.5624 0.1199 0.9529

Pseudo-second order qe,cal, mmol g1 k2, g mol1 h1 r22

1.0654 0.6217 0.9956

1.0570 0.5675 0.9978

1.4077 0.9488 0.9997

Intra-particle diffusion kp1, mmol g1 h1/2 C1, mmol g1 rp12 kp2, mmol g1 h1/2 C2, mmol g1 rp22

0.0677 0.6195 0.9683 0.0601 0.6423 0.9930

0.1671 0.3473 0.9092 0.0420 0.7477 0.9601

0.2786 0.5419 0.9904 0.0336 1.1692 0.9535

Z. Ozbas et al. / Journal of Environmental Chemical Engineering 4 (2016) 1948–1954 Table 2 Kinetic parameters for the adsorption of Cu(II), Ni(II), Cd(II) and Pb(II) ions in the presence of DTAC. Kinetic model

Heavy metal ions Cu(II)

Ni(II)

Cd(II)

Pb(II)

Pseudo-first order qe,cal, mmol g1 k1, h1 r12

0.8132 0.0883 0.9451

0.0948 0.2528 0.8447

0.0872 0.2802 0.9425

0.0915 0.2902 0.9761

Pseudo-second order qe,cal, mmol g1 k2, g mol1 h1 r2 2

1.0537 0.2656 0.9905

0.4360 1.2195 0.9927

0.8505 1.2963 0.9986

0.3849 0.7283 0.9938

Intra-particle diffusion kp1, mmol g1 h1/2 C1, mmol g1 rp12 kp2, mmol g1 h1/2 C2, mmol g1 rp22

0.1328 0.2231 0.9673 0.0954 0.3645 0.9787

0.0420 0.2045 0.9665 0.0264 0.2472 0.9494

0.0683 0.5362 0.9472 0.0273 0.6568 0.9387

0.0787 0.0246 0.9309 0.0266 0.1850 0.9780

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To identify the adsorption mechanism affecting the adsorption kinetics, the Weber’s intra-particle diffusion model (external mass diffusion and intra-particle diffusion) was fitted to the kinetic data obtained from experimental data. If the plot of q versus t1/2 gives a straight line, the adsorption process is controlled by intra-particle diffusion only [21–23]. As presented in Figs. 2c and 3c of this study, the plots were multi-linear and implying that two steps influence the adsorption process. The first stage was the external surface adsorption, and the second stage was the gradual adsorption due to the intra-particle or pore diffusion [24–27]. The calculated qe,cal values for intra-particle diffusion model were relatively close to the experimental ones. In the presence of SLS, qe,cal = 1.06, 1.03 and 1.39 at t = 48 h for Cu(II), Ni(II) and Cd(II), respectively and in the presence of DTAC, qe,cal = 1.01, 0.43, 0.85 and 0.36 at t = 48 h for Cu (II), Ni(II), Cd(II) and Pb(II) ions, respectively. The intercept values (C) were not zero but had high values (C2 was higher than C1), and the external mass diffusion rates kp1 was higher than the values of the intra-particle diffusion rates kp2. The whole process had different adsorption stages, and to determine the rate-controlling stage, the following Eq. (5) can be employed for the adsorption

Fig. 2. (a) The pseudo-first order kinetic, (b) the pseudo-second order kinetic and (c) intra-particle diffusion kinetic model plots for Cu(II), Ni(II) and Cd(II) ions for SLS.

Fig. 3. (a) The pseudo-first order kinetic, (b) the pseudo-second order kinetic and (c) intra-particle diffusion kinetic model plots for Cu(II), Ni(II), Cd(II) and Pb(II) ions for DTAC.

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Fig. 4. The linearity between Bt against t for metal ions in the presence of (a) SLS and (b) DTAC.

dynamics given by Boyd et al. [28]. F¼

qt 6 ¼1 2 q1 p

1  X

   1=n2 exp n2 Bt

ð5Þ

1

where, qt and q1 are the amount of adsorbed ions at time t and infinite time, respectively, n is the Freundlich constant of the adsorbate and Bt is a mathematical function of F. The kinetic expression can be arranged as the following Eqs. (6) and (7) [29]:   ð6Þ Bt ¼ 2p  p2 F=3  2pð1  pF=3Þ1=2 for 0  F  0:85 and Bt ¼ Inð1  F Þ  0:4977 for

0:86  F  1

ð7Þ

Fig. 4a and b shows the Boyd plots, for SLS and DTAC, respectively and the plots are linear. The linearity of the plots indicates the type of diffusion controlling the adsorption process which can be external mass diffusion or intra-particle diffusion. If the plot is linear and passes through the origin, the adsorption is governed by intra-particle diffusion, but if it is not linear and not passing from the origin, the process is controlled by external mass diffusion. Due to the low linearity of the plots in Fig. 4a and b and their not passing through the origin, it can be said that the external mass diffusion is dominant than the intra-particle diffusion for the adsorption processes of heavy metal ions and the former (external mass diffusion) is the rate controlling step for both surfactant. 3.2. Adsorption-desorption studies To reveal the reusability of the graft copolymer, the adsorptiondesorption cycles for heavy metal ions were applied at 50  C and 10  C, respectively. The desorption experiments were performed by changing back the metal ion-surfactant solution to pure water. The change in adsorption capacities with the decrease in temperature from 50  C to 10  C from high to low values is seen in Fig. 5. The

adsorption capacity decreased with the subsequent cycles for both surfactants. The adsorption capacities were found to be in the order Cd(II) > Cu(II) > Ni(II) with SLS, but it was in the order Cu (II) > Cd(II) > Ni(II) > Pb(II) with DTAC. 3.3. Competitive adsorption The stock solution consisting of Cu(II), Ni(II) and Cd(II) ions were prepared for SLS and the stock solution consisting of Cu(II), Ni (II), Cd(II) and Pb(II) ions were prepared for DTAC under competitive adsorption conditions. The concentration of each metal ion was adjusted to 25 mmol/L and the total metal ion concentration was 100 mmol/L. This solution was mixed with the surfactant solution, and treated with the copolymer at 50  C for 48 h. The adsorption capacities of the copolymer for each metal ion were shown in Fig. 6a and b under competitive conditions for SLS and DTAC, respectively. In the case of competitive adsorption, the metal affinity order followed the order Ni(II) (0.55 mg g1) > Cu(II) (0.35 mg g1) > Cd(II) (0.22 mg g1) for SLS, while it were Cu(II) (1.28 mg g1) > Pb(II) (0.85 mg g1) > Cd(II) (0.34 mg g1) > Ni(II) (0.20 mg g1) for DTAC. The percentage distribution (the ratio of adsorbed metal ion for each ion to the total adsorbed metal ions) of the ions with SLS: Ni(II), 49.1%; Cu(II), 31.3% and Cd(II), 19.6%, and in the case of DTAC, it was Cu(II), 47.9%; Pb(II), 31.8%; Cd(II), 12.7%; and Ni(II), 7.5%. In addition, as can be seen from the results, the adsorption capacities decreased compared to the non-competitive conditions (Fig. 1a) with SLS. The reason in this decrease is can be ascribed to the increased competition between the metal ions for the thermoresponsive graft copolymer [30]. On the other hand, the adsorption capacities increased compared to the non-competitive conditions (Fig. 1b) with DTAC, and the reason in this increase can be attributed to the highest affinity of Cu(II) ion over the other ions because of the interactions between quaternized ammonium groups in DTAC and Cu(II) ion [31].

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Fig. 5. Adsorption-desorption cycles of the copolymer in the presence of (a) SLS and (b) DTAC at 50  C and 10  C during the temperature swing.

Fig. 6. Competitive removal of heavy metals by the graft copolymer in the presence of SLS and DTAC.

4. Conclusion The thermoresponsive cellulose-graft-poly(N-isopropyl acrylamide) (PNIPAM) copolymer was investigated in the heavy metal removal by TSA process. SLS and DTAC were used as the extractants. The order of the removal of heavy metal ions was Cd(II) > Cu(II) > Ni(II) for SLS, and Cu(II) > Cd(II) > Ni(II) > Pb(II) for DTAC in the non-competitive conditions. In competitive conditions, the highest adsorption capacity was observed for Cu(II) ion in presence of DTAC. In addition, the non-competitive adsorption

kinetics was investigated, and it can be said that the adsorption of heavy metal ions onto the thermoresponsive cellulose graft copolymer takes place according to second-order kinetic model. Acknowledgement This work was supported by The Scientific and Technical Research Council of Turkey (TUBITAK) (contract grant number MAG/110M153).

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