Accepted Manuscript Polyethylenimine coated bacterial cellulose nanofiber membrane and application as adsorbent and catalyst Jianqiang Wang, Xinkun Lu, Pui Fai Ng, Ka I Lee, Bin Fei, John H. Xin, Jianyong Wu PII: DOI: Reference:
S0021-9797(14)00799-1 http://dx.doi.org/10.1016/j.jcis.2014.10.035 YJCIS 19929
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
19 September 2014 18 October 2014
Please cite this article as: J. Wang, X. Lu, P.F. Ng, K.I. Lee, B. Fei, J.H. Xin, J-y. Wu, Polyethylenimine coated bacterial cellulose nanofiber membrane and application as adsorbent and catalyst, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.10.035
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Polyethylenimine coated bacterial cellulose nanofiber membrane and application as adsorbent and catalyst
Jianqiang Wanga, *, Xinkun Lua, Pui Fai Nga, Ka I Leea, Bin Feia, *, John H. Xina, Jian-yong Wub
a
Institute of Textiles & Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon,
Hong Kong b
Department of Applied Biology & Chemical Technology, The Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong
* Corresponding author
Tel: 852-27664795; Fax: 852-27664795
E-mail: [email protected] (Jianqiang Wang), [email protected] (Bin Fei)
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Abstract: Bacterial cellulose (BC) nanofiber membranes were simply aminalized by a flush-coating and post crosslinking method. Firstly, wet BC membranes were flushed through by an aqueous solution of polyethylenimine (PEI) and glycerol diglycidyl ether (GDE) under vaccum suction, then further heated up to 70 oC to crosslink the resultant coating on the surface of the nanofibers. The PEI coated bacterial cellulose (BC@PEI) nanofiber membrane presented excellent adsorption performance for Cu2+ and Pb2+ ions from aqueous solutions. Desorption of these ions was achieved using ethylene diamine tetraacetic acid treatment. This cycle of adsorption and desorption was repeated for several times with good remain adsorption performance (over 90%). Furthermore, the adsorbed Cu2+ ions can be reduced to copper nanoparticles, and showed excellent catalytic performance for methylene blue reduction in aqueous solution. The catalytic performance can remained after several times of usage. Keywords: Bacterial cellulose, nanofiber, Polyethylenimine, Adsorption, Catalyst.
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1. Introduction Environmental pollution especially the heavy metal ions pollution has already presented a serious threat to people’s daily lives. Excess intake of the heavy metal ions will be dangerous to humans and animals due to its tendency of accumulation. Therefore the removal of heavy metal ions from aqueous solution becomes necessary. Many methods have been developed for the removal of heavy metal ions, such as chemical precipitation, membrane separation, ion exchange and electrochemical process [1-3]. Also adsorption is considered as one of the most effective methods for heavy metal ions removal. It is suitable for dealing with large volumes of wastewater. Several kinds of functional materials have been used in the adsorption process, such as modified active carbon [4,5], agriculture wastes [6,7], metal oxide particles [8,9], functionalized graphene [10-12], layered double hydroxide [13], and zeolite [14] etc. Recent years, functional nanofibrous membranes were used for adsorption of heavy metal ions due to its large surface area and easy separation after the adsorption process [15-21]. Also cellulose acetate fibrous membrane obtained from electrospinning was used as adsorbent for heavy metal ions [22-23]. As the most abundant renewable biopolymer resource available worldwide, cellulose is very suitable for using as adsorbents. Many functional adsorbents have been developed based on this versatile material [24-28]. Bacterial cellulose (BC) has the molecular structure similar to plant cellulose. Because of its high crystallinity, ultrafine nanosize (usually 20-30 nm in diameter) and 3D network structure, BC has several advantages, such as high tensile strength and large surface area, and would be more suitable for the adsorption applications. So far, only a few functionalized BC nanofiber membranes have been used as adsorbents for heavy metal ions [29-33]. However, their preparing processes were relatively complex. Several steps of reactions
3
were carried out to aminalize the BC with harsh chemicals. Here we presented a facile method for preparation aminalized BC nanofiber membrane as highly efficient heavy metal ions adsorbent. A biocompatible polymer, branched polyethylenimine (PEI) was easily coated on the surface of BC nanofibers by a flush-through and post heating process. The obtained PEI coated BC nanofibrous membranes were used for the adsorption of Cu2+ and Pb2+ ions. In addition, many previous works [34-37] have proved that the BC nanofiber membrane can be used as an excellent template for catalyst and nanocomposite due to its large surface area and easy operation performance. Inspired by the above works, the copper ions loaded on the surface of BC@PEI nanofibers were further reduced by hydrazine hydrate through a simple immersion method. The obtained reduced BC@PEI nanofibers can be used as a high efficient and reusable catalyst for the reduction of methylene blue. The heavy metal ions adsorption and catalytic performances make the BC@PEI nanofibers a multi-functional platform. 2. Materials and experiments 2.1 Preparation of PEI coated BC nanofiber membrane BC nanofiber membranes were supplied by Hainan Yida food industry Co., Ltd, stored in acetic acid solution, and thoroughly washed by 0.1 M sodium hydroxide at 100 oC (in order to removal the residual cells) before use. 5.0 wt% branched PEI (Mw=25,000, Sigma-Aldrich) aqueous solution with glycerol diglycidyl ether (17% of PEI mass, Sigma-Aldrich) was firstly prepared. The BC nanofiber membrane was flushed with about 10.0 g PEI solution under vacuum suction and heated at 70 oC for 30 mins. Finally, the obtained membranes were washed with deionized water until the washing solution became neutral, and dried in vacuum.
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2.2 Adsorption and desorption experiments The adsorption experiments were carried out by immersing about 0.10 g BC@PEI membranes into 30 mL aqueous solutions (containing Cu2+ or Pb2+ ions) with different initial concentrations. The pH value and temperature of the solutions were set at 5.0 and 25 oC, respectively. A calibration curve and an equation between metal ions concentration and conductivity were then used to calculate the metal ions concentrations. The adsorption amount at time t (qt) and equilibrium adsorption amount (qe) were calculated from the following equations: q
బ ౪
q
బ
V V
(1) (2)
where C0 is the initial concentration of metal ions in solution (mg/L), Ce is the equilibrium concentration (mg/L), Ct is the concentration of metal ions in solution at time t (mg/L), m is the dry mass of adsorbent (g), and V is the volume of solution (L), respectively. The adsorption isotherm experiments were carried out at 25
o
C with the initial metal ions
concentration of 136-371 mg/L (Cu2+ ions) or 151-451 mg/L (Pb2+ ions). The solution pH values were set at 5.0. Reuse ability of the BC@PEI nanofibrous membranes was tested using 0.05 M ethylene diamine tetraacetic acid (EDTA) disodium salt dehydrates as the regenerator. Typically, the BC@PEI membranes (after 12 h adsorption in 371 mg/L or 356 mg/L solutions containing Cu2+ or Pb2+ ions, respectively) were immersed into 20 mL 0.05 M EDTA salt solution for 30 mins and stirred at a 100 r/min at room temperature. Before the next adsorption, the membranes were washed by deionized water to remove the EDTA salt.
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2.3 Catalytic activity study Firstly, 0.50 g BC@PEI nanofiber membranes (after immersion in 500 mg/L Cu2+ ions aqueous solution for 12 h) with copper ions loaded were immersed into 100 mL hydrazine hydrate (50-60%, Sigma-Aldrich) solution with a concentration of approximately 10-12 wt% for 12 h to reduce the copper ions. Secondly, about 0.10 g reduced BC@PEI nanofiber membranes were immersed into 10.0 mL 30 mg/L methylene blue (MB) aqueous solution followed by adding of about 10.0 mg NaBH4. Finally, the BC@PEI nanofiber membranes were taken out and washed using deionized water and then used for the next reaction without any other treatments. 2.4 Characterizations The surface morphologies and chemical compositions of the nanofiber membrane were studied by JEOL JSM-6335F field emission scanning electron microscopy (SEM) with an energy dispersive X-ray spectroscopy (EDX) detector. The surface chemical compositions of pristine and the modified nanofibers were also analyzed by X-ray photoelectron spectroscopy (XPS) using Thermo Electron Corporation ESCALAB250 equipment with an Al Kα X-ray source (1486.6eV). The solution conductivity was monitored during the adsorption process using a conductivity meter (CMD 200, WPA Co., Ltd). The conductivity meter was calibrated using a series of prepared standard Cu2+ or Pb2+ solutions with known concentrations. UV-vis spectra were determined using a UV-vis spectrophotometer (Perkin Elmer, Lambda18). 3. Results and discussion 3.1. Membrane morphology and composition BC nanofiber membrane is suitable for using as adsorbents because of its large surface area and
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porous structure. However, the nanopores between BC nanofibers are prone to be blocked by applied polymers. Here we designed a flush-coating method and successfully avoided the blockage problem. The fibrous structure was retained very well after coating by PEI (as shown in Fig. 1(a)-(b)). Although some adhesions occurred, it does not have so much effect on the fibrous structure. Fig. 1 (c)-(d) presented the morphologies of the membranes which have been used for adsorption of Cu2+ or Pb2+ ions for 12 h. Since the adsorption occurs through the coordination between the metal ions and nitrogen atoms on adsorbent surfaces in a monolayer mechanism, the adsorbents would not show any noticeable changes under SEM (as shown in Fig. 1(c)-(d)). The embedded EDX spectrums confirmed the presence of copper and lead element on the surface of nanofibers.
Fig. 1. SEM images of BC membrane before (a) and after PEI coating (b), and BC@PEI after adsorption of Cu2+(c) and Pb2+ (d). (the inset is the EDS spectrums, scale bar is 100 nm).
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Fig. 2. XPS wide scan of the BC membrane before and after coating by PEI. As can be seen from Fig. 1 (b), there was only a very thin layer of PEI coated on the surface of the nanofibers, therefore the surface chemical composite can be better revealed using XPS method. Fig. 2 showed the XPS results of the membrane before and after coating by PEI. The results indicated that the nitrogen content increased, and both the oxygen and carbon content decreased after coating by the cross-linked PEI. The PEI can be cross-linked by GDE at an elevated temperature [38]. The difference was due to the large amount of nitrogen atoms and the absence of oxygen atoms in the PEI chains. Furthermore, the mass of GDE used in the experiments was 17 wt% of PEI, therefore the C/N ratio in the coating layer should be 2.27 in theory (supposing that all of the GDE were involved in the crosslinking reaction). However, the C/N ratio from XPS analysis (19.5) was much higher than the theory value, indicating that thickness of the coated PEI layer was extremely thin (considering the usual detectable depth of XPS for polymer materials was less than 10 nm [39]. These XPS results about the coating thickness estimation coincided with the SEM results as shown above. 3.2. Adsorption kinetics As Cu2+ and Pb2+ ions are common contaminations in water, the two ions were selected to evaluate the 8
adsorption and desorption performance of BC@PEI nanofiber membranes. The adsorption kinetics for Cu2+ and Pb2+ ions were firstly studied. The conductivity change during the adsorption was used to illustrate this process, and the results are shown in Fig. S1. The solution conductivity decreased rapidly at the initial adsorption stage, and the time needed to reach the adsorption equilibrium increased from 50 to nearly 100 minutes as the initial Cu2+ concentration increased from 136 to 547 mg/L. However, much longer time was needed as the Pb2+ concentration increased from 151 to 548 mg/L (as shown in Fig. S1b).
Fig. 3. The effect of contact time and initial concentration on the adsorption of Cu2+ (a) and Pb2+ (b) ions onto the BC@PEI nanofiber membranes. Adsorption amount for Cu2+ and Pb2+ ions was as high as 90.1 mg/g and 130 mg/g (when the initial metal ions concentration is about 550 mg/L) as shown in Fig. 3, and it is higher than the other reported [40,41]. The adsorption amount increased as either the Cu2+ or Pb2+ ions concentration increased. Two commonly used kinetic models [42], namely, the pseudo-first-order and the pseudo-second-order were used here to better understand the adsorption behaviors. These two kinetic models are used to describe the adsorption of solid/liquid systems, which can be expressed in the linear forms as equations (3) and (4), respectively: 9
logq q logq ౪
మ మ
భ .
t
(3)
t
(4)
Where K1 and K2 are the pseudo first order and second order rate constants, respectively.
Fig. 4. Pseudo-first-order kinetic model (a, c), pseudo-second-order kinetic model (b, d) for adsorption of Cu2+ and Pb2+ on BC@PEI nanofiber membranes. The adsorption kinetic plots for Cu2+ and Pb2+ ions are shown in Fig. 4, and the kinetic parameters obtained from the models were summarized in Table 1. The values of the correlation coefficients clearly indicated that the adsorption kinetics closely followed the pseudo-second-order model. The qe, cal
values obtained from pseudo-second-order kinetic model appeared to be very close to the
experimentally observed values (as shown in Table 1).
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Table 1 Kinectics parameters for Cu2+ and Pb2+ ions adsorption onto the BC@PEI nanofiber membrane. Metal ions
C0 (mg/L)
qe, exp (mg/g)
Cu2+ ions
136 371 547
Pb2+ ions
151 356 548
Pseudo-first-order model
Pseudo-second-order model
K1 (1/min)
qe, cal (mg/g)
R2
K2 (g/mg/min)
qe, cal (mg/g)
R2
40.74 60.62 90.10
0.012 0.017 0.015
11.81 23.54 43.85
0.77233 0.87833 0.88720
2.15×10-3 1.33×10-3 6.23×10-4
41.93 62.66 94.34
0.99892 0.99921 0.99853
43.01 91.20 130.47
0.017 0.011 0.013
16.09 42.94 108.58
0.93139 0.92004 0.99205
2.56×10-3 6.51×10-4 2.18×10-4
43.98 94.34 138.89
0.99977 0.99948 0.99700
3.3 Adsorption isotherm
Fig. 5. Equilibrium isotherms of Cu2+ and Pb2+ ions adsorption onto BC@PEI nanofiber membrane (a); Langmuir isotherm model (b), Freundlich isotherm model (c) for adsorption of Cu2+ and Pb2+ ions onto BC@PEI nanofiber membrane; The regeneration performance of the BC@PEI nanofiber 11
membranes.
The adsorption isotherm was also studied, and the results were shown in Fig. 5. The adsorption amount for Pb2+ ions was much higher than Cu2+ ions at the range of tested concentrations. Two well-known models (Langmuir and Freundlich isotherms) [43], were used to fit the adsorption equilibrium data, and the results were shown in Fig. 5b and Fig. 5c. The Langmuir model is based on the assumption of adsorption homogeneity, representing equally available adsorption sites, monolayer surface coverage, and no interaction between adsorbed species. The linearized Langmuir isotherm model is represented by the following equation:
ౣ
(5)
ౣ
The Freundlich isotherm describes reversible adsorption and is not restricted to the formation of the monolayer. This empirical equation takes the form: lnq lnK
(6)
Where qm (mg/g) is the maximum adsorption capacity, b (L/mg) is the Langmuir constant related to the energy of adsorption, Kf and 1/n constants are related to the adsorption capacity and intensity of adsorption. The parameters obtained from the two models were shown in Table 2. Obviously, the adsorption process can be better described by Langmuir isotherm model, which means that the adsorption occurs uniformly on the active sites (nitrogen atoms) of the BC@PEI nanofiber membrane. The adsorption removal of metal ions through the coordination between the nitrogen atoms was already known, and has been used in some previous works [20,44,45]. Furthermore, a monolayer adsorption occurred at the solid-liquid interface according to the Langmuir model, that is why the membrane morphology has
12
no change before and after the metal ions adsorption (as can be seen in Fig. 1). However, a blue color BC@PEI membrane was indeed obtained after the adsorption of Cu2+ ions. Table 2 Langmuir and Freundlich isotherm parameters for Cu2+ and Pb2+ ions adsorption onto BC@PEI nanofiber membrane. Langmuir model
Freundlich model
Metal ions
qm (mg/g)
b (L/mg)
R2
Kf (mg/g)
1/n
R2
Cu2+ ions Pb2+ ions
61.46 116.41
0.012 0.074
0.98930 0.99972
35.28 23.98
0.09569 0.32894
0.90887 0.94007
3.4 Desorption study Another important property of adsorbents was the regeneration possibility. A good adsorbent must have high regeneration performance. EDTA is a most used regenerator for desorption of heavy metal ions due to its high affinity to metal ions [20,29,46]. As shown in Fig. 5d, the BC@PEI nanofibrous membranes have a wonderful regeneration performance. Their adsorption amount remained more than 90% for both Cu2+ and Pb2+ ions after three regenerations, which makes the membranes a potential candidate as practical adsorbents for metal ions. 3.5 Catalytic activity study Usually, the adsorbents were regenerated after adsorption of metal ions and finally discarded after several times of usage. Because the adsorption capacity of the recycled adsorbents decreased a lot after several recycles. The discarded adsorbents are another threatens to the environment. The treatment to the discarded adsorbents will need a high cost. Several previously works have reported that copper nanoparticles can be used as high efficiency catalyst for many reactions [47-49]. Inspired by the above research, the adsorbed Cu2+ ions were reduced through a simple method by hydrazine hydrate. As shown in Fig. 6, after reduction, many nanoparticles with a diameter about 400 -500 nm can be found 13
on the surface of the BC@PEI nanofiber membrane. The similar results have been reported by the previously researchers [47]. Then we can hope that the reduced BC@PEI nanofiber membrane have the potential for using as catalyst for degradation or decoloration of organic pollutant.
Fig. 6. SEM images of BC@PEI nanofiber membrane after adsorption of Cu2+ ions and reduction by hydrazine hydrate. (a) low magnification, (b) high mangnification. A reaction between the methylene blue (MB) and NaBH4 was selected to confirm the catalytic performance of the obtained BC@PEI nanofiber membrane. As shown in Fig. S2, after adding the reduced BC@PEI nanofiber membrane into the mixture of MB and NaBH4 solution, the color of the solution disappeared within 15 seconds (Fig. S2c), which suggested that the reaction rate was significantly improved. However, without the BC@PEI nanofiber membrane, the color of the solution (the mixture of MB and NaBH4) nearly unchanged (almost the same as the pure MB solution as shown in Fig. S2a) even after 5 hours (Fig. S2b). The dynamic process of this reaction can be found in Fig. 7.
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F 7. The Fig. T e dynnam mic proc p cesss of the collor chan c nge durringg thee reaactioon. UV-viss speectrra ass shhow UVwn in n Fiig. 8 cllearlly in ndiccateed thhe chan c nge befforee annd affter thee reaaction. Aftter a ding thee NaaBH add H4 too thee MB sooluttionn forr 5 h, h thhe abbsorrbannce slig ghtlyy deecreeasedd co omppareed too thee puure MB so MB olutiion.. Thhe absorbance decr d reaseed rapiidlyy affter addingg thhe reduucedd BC@ B @PE EI nano n ofibber m mbrranee. Thhe catal mem c lyticc mech m hanissm has h alreeadyy beeen cleaar. BH B 4- ions w worrkedd as elecctroon doonoor, and t metthyllenee bluue aas thhe ellectrron accceptor. The the T e reaactioon rrate wass im mproovedd thrroug gh an a eelecttronn rellay m chan mec nism m of thhe copp c per nannopaarticcless [50]. Thee reeusaabiliity of the t redduceed BC@ B @PE EI nano n ofibber m mbrranee waas also moonito mem oredd, ass shhownn in n Figg. 8, 8 thhe caatallyticc peerforrmaancee cann bee well w rem r mainned e n after even a r fivve timees of o usag u ge. Thhe eexceellennt rreusabillity sug ggestedd thhe stron s ng stabbilitty of o the t B @P BC PEI@ @Cuu naano--com mpoositee annd make m es itt a ppoteentiaal caatalyyst carrrier for reaal wasteewatter trea t atmeent a licaationn. app
15 5
Fig. 8. UV-vis spectra of the MB solutions after reaction under different conditions and its reuse performance. 4. Conclusions BC@PEI nanofiber membrane was prepared via a facile flush-coating and post crosslinking method. XPS results confirmed the presence of a thin layer of PEI on the surface of BC nanofibers. The obtained BC@PEI membranes showed excellent adsorption performance for Cu2+ and Pb2+ ions. Adsorption process was better described by pseudo-second-order rather than pseudo-first-order model. Adsorption isotherm data fitted well to Langmuir model, supporting a monolayer adsorption mechanism. The adsorption capacity can remain more than 90% for both Cu2+ and Pb2+ after three cycles. The adsorbed Cu2+ ions can be reduced to copper nanoparticle by hydrazine hydrate and showed excellent catalytic performance for the reduction of methylene blue. The catalytic performance remained well after 5 times of usage. This facile preparation process and excellent muli-functional performance make it a potential candidate for the application of wastewater treatment.
Acknowledgements 16
We sincerely acknowledge the funding supports of PolyU A-PK90, A-PL17, 1-ZV7L, and ITF 112/11.
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Graaph G hicall ab bstraact
23 3
Highlights 1 Aminalized bacterial cellulose nanofiber membranes 2 Flush-coating and post crosslinking method 3 Adsorption capacity remained up to 90% within 3 regenerations 4 High efficient catalysts for methylene blue degradation
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S0021-9797(14)00799-1 http://dx.doi.org/10.1016/j.jcis.2014.10.035 YJCIS 19929
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
19 September 2014 18 October 2014
Please cite this article as: J. Wang, X. Lu, P.F. Ng, K.I. Lee, B. Fei, J.H. Xin, J-y. Wu, Polyethylenimine coated bacterial cellulose nanofiber membrane and application as adsorbent and catalyst, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.10.035
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Polyethylenimine coated bacterial cellulose nanofiber membrane and application as adsorbent and catalyst
Jianqiang Wanga, *, Xinkun Lua, Pui Fai Nga, Ka I Leea, Bin Feia, *, John H. Xina, Jian-yong Wub
a
Institute of Textiles & Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon,
Hong Kong b
Department of Applied Biology & Chemical Technology, The Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong
* Corresponding author
Tel: 852-27664795; Fax: 852-27664795
E-mail: [email protected] (Jianqiang Wang), [email protected] (Bin Fei)
1
Abstract: Bacterial cellulose (BC) nanofiber membranes were simply aminalized by a flush-coating and post crosslinking method. Firstly, wet BC membranes were flushed through by an aqueous solution of polyethylenimine (PEI) and glycerol diglycidyl ether (GDE) under vaccum suction, then further heated up to 70 oC to crosslink the resultant coating on the surface of the nanofibers. The PEI coated bacterial cellulose (BC@PEI) nanofiber membrane presented excellent adsorption performance for Cu2+ and Pb2+ ions from aqueous solutions. Desorption of these ions was achieved using ethylene diamine tetraacetic acid treatment. This cycle of adsorption and desorption was repeated for several times with good remain adsorption performance (over 90%). Furthermore, the adsorbed Cu2+ ions can be reduced to copper nanoparticles, and showed excellent catalytic performance for methylene blue reduction in aqueous solution. The catalytic performance can remained after several times of usage. Keywords: Bacterial cellulose, nanofiber, Polyethylenimine, Adsorption, Catalyst.
2
1. Introduction Environmental pollution especially the heavy metal ions pollution has already presented a serious threat to people’s daily lives. Excess intake of the heavy metal ions will be dangerous to humans and animals due to its tendency of accumulation. Therefore the removal of heavy metal ions from aqueous solution becomes necessary. Many methods have been developed for the removal of heavy metal ions, such as chemical precipitation, membrane separation, ion exchange and electrochemical process [1-3]. Also adsorption is considered as one of the most effective methods for heavy metal ions removal. It is suitable for dealing with large volumes of wastewater. Several kinds of functional materials have been used in the adsorption process, such as modified active carbon [4,5], agriculture wastes [6,7], metal oxide particles [8,9], functionalized graphene [10-12], layered double hydroxide [13], and zeolite [14] etc. Recent years, functional nanofibrous membranes were used for adsorption of heavy metal ions due to its large surface area and easy separation after the adsorption process [15-21]. Also cellulose acetate fibrous membrane obtained from electrospinning was used as adsorbent for heavy metal ions [22-23]. As the most abundant renewable biopolymer resource available worldwide, cellulose is very suitable for using as adsorbents. Many functional adsorbents have been developed based on this versatile material [24-28]. Bacterial cellulose (BC) has the molecular structure similar to plant cellulose. Because of its high crystallinity, ultrafine nanosize (usually 20-30 nm in diameter) and 3D network structure, BC has several advantages, such as high tensile strength and large surface area, and would be more suitable for the adsorption applications. So far, only a few functionalized BC nanofiber membranes have been used as adsorbents for heavy metal ions [29-33]. However, their preparing processes were relatively complex. Several steps of reactions
3
were carried out to aminalize the BC with harsh chemicals. Here we presented a facile method for preparation aminalized BC nanofiber membrane as highly efficient heavy metal ions adsorbent. A biocompatible polymer, branched polyethylenimine (PEI) was easily coated on the surface of BC nanofibers by a flush-through and post heating process. The obtained PEI coated BC nanofibrous membranes were used for the adsorption of Cu2+ and Pb2+ ions. In addition, many previous works [34-37] have proved that the BC nanofiber membrane can be used as an excellent template for catalyst and nanocomposite due to its large surface area and easy operation performance. Inspired by the above works, the copper ions loaded on the surface of BC@PEI nanofibers were further reduced by hydrazine hydrate through a simple immersion method. The obtained reduced BC@PEI nanofibers can be used as a high efficient and reusable catalyst for the reduction of methylene blue. The heavy metal ions adsorption and catalytic performances make the BC@PEI nanofibers a multi-functional platform. 2. Materials and experiments 2.1 Preparation of PEI coated BC nanofiber membrane BC nanofiber membranes were supplied by Hainan Yida food industry Co., Ltd, stored in acetic acid solution, and thoroughly washed by 0.1 M sodium hydroxide at 100 oC (in order to removal the residual cells) before use. 5.0 wt% branched PEI (Mw=25,000, Sigma-Aldrich) aqueous solution with glycerol diglycidyl ether (17% of PEI mass, Sigma-Aldrich) was firstly prepared. The BC nanofiber membrane was flushed with about 10.0 g PEI solution under vacuum suction and heated at 70 oC for 30 mins. Finally, the obtained membranes were washed with deionized water until the washing solution became neutral, and dried in vacuum.
4
2.2 Adsorption and desorption experiments The adsorption experiments were carried out by immersing about 0.10 g BC@PEI membranes into 30 mL aqueous solutions (containing Cu2+ or Pb2+ ions) with different initial concentrations. The pH value and temperature of the solutions were set at 5.0 and 25 oC, respectively. A calibration curve and an equation between metal ions concentration and conductivity were then used to calculate the metal ions concentrations. The adsorption amount at time t (qt) and equilibrium adsorption amount (qe) were calculated from the following equations: q
బ ౪
q
బ
V V
(1) (2)
where C0 is the initial concentration of metal ions in solution (mg/L), Ce is the equilibrium concentration (mg/L), Ct is the concentration of metal ions in solution at time t (mg/L), m is the dry mass of adsorbent (g), and V is the volume of solution (L), respectively. The adsorption isotherm experiments were carried out at 25
o
C with the initial metal ions
concentration of 136-371 mg/L (Cu2+ ions) or 151-451 mg/L (Pb2+ ions). The solution pH values were set at 5.0. Reuse ability of the BC@PEI nanofibrous membranes was tested using 0.05 M ethylene diamine tetraacetic acid (EDTA) disodium salt dehydrates as the regenerator. Typically, the BC@PEI membranes (after 12 h adsorption in 371 mg/L or 356 mg/L solutions containing Cu2+ or Pb2+ ions, respectively) were immersed into 20 mL 0.05 M EDTA salt solution for 30 mins and stirred at a 100 r/min at room temperature. Before the next adsorption, the membranes were washed by deionized water to remove the EDTA salt.
5
2.3 Catalytic activity study Firstly, 0.50 g BC@PEI nanofiber membranes (after immersion in 500 mg/L Cu2+ ions aqueous solution for 12 h) with copper ions loaded were immersed into 100 mL hydrazine hydrate (50-60%, Sigma-Aldrich) solution with a concentration of approximately 10-12 wt% for 12 h to reduce the copper ions. Secondly, about 0.10 g reduced BC@PEI nanofiber membranes were immersed into 10.0 mL 30 mg/L methylene blue (MB) aqueous solution followed by adding of about 10.0 mg NaBH4. Finally, the BC@PEI nanofiber membranes were taken out and washed using deionized water and then used for the next reaction without any other treatments. 2.4 Characterizations The surface morphologies and chemical compositions of the nanofiber membrane were studied by JEOL JSM-6335F field emission scanning electron microscopy (SEM) with an energy dispersive X-ray spectroscopy (EDX) detector. The surface chemical compositions of pristine and the modified nanofibers were also analyzed by X-ray photoelectron spectroscopy (XPS) using Thermo Electron Corporation ESCALAB250 equipment with an Al Kα X-ray source (1486.6eV). The solution conductivity was monitored during the adsorption process using a conductivity meter (CMD 200, WPA Co., Ltd). The conductivity meter was calibrated using a series of prepared standard Cu2+ or Pb2+ solutions with known concentrations. UV-vis spectra were determined using a UV-vis spectrophotometer (Perkin Elmer, Lambda18). 3. Results and discussion 3.1. Membrane morphology and composition BC nanofiber membrane is suitable for using as adsorbents because of its large surface area and
6
porous structure. However, the nanopores between BC nanofibers are prone to be blocked by applied polymers. Here we designed a flush-coating method and successfully avoided the blockage problem. The fibrous structure was retained very well after coating by PEI (as shown in Fig. 1(a)-(b)). Although some adhesions occurred, it does not have so much effect on the fibrous structure. Fig. 1 (c)-(d) presented the morphologies of the membranes which have been used for adsorption of Cu2+ or Pb2+ ions for 12 h. Since the adsorption occurs through the coordination between the metal ions and nitrogen atoms on adsorbent surfaces in a monolayer mechanism, the adsorbents would not show any noticeable changes under SEM (as shown in Fig. 1(c)-(d)). The embedded EDX spectrums confirmed the presence of copper and lead element on the surface of nanofibers.
Fig. 1. SEM images of BC membrane before (a) and after PEI coating (b), and BC@PEI after adsorption of Cu2+(c) and Pb2+ (d). (the inset is the EDS spectrums, scale bar is 100 nm).
7
Fig. 2. XPS wide scan of the BC membrane before and after coating by PEI. As can be seen from Fig. 1 (b), there was only a very thin layer of PEI coated on the surface of the nanofibers, therefore the surface chemical composite can be better revealed using XPS method. Fig. 2 showed the XPS results of the membrane before and after coating by PEI. The results indicated that the nitrogen content increased, and both the oxygen and carbon content decreased after coating by the cross-linked PEI. The PEI can be cross-linked by GDE at an elevated temperature [38]. The difference was due to the large amount of nitrogen atoms and the absence of oxygen atoms in the PEI chains. Furthermore, the mass of GDE used in the experiments was 17 wt% of PEI, therefore the C/N ratio in the coating layer should be 2.27 in theory (supposing that all of the GDE were involved in the crosslinking reaction). However, the C/N ratio from XPS analysis (19.5) was much higher than the theory value, indicating that thickness of the coated PEI layer was extremely thin (considering the usual detectable depth of XPS for polymer materials was less than 10 nm [39]. These XPS results about the coating thickness estimation coincided with the SEM results as shown above. 3.2. Adsorption kinetics As Cu2+ and Pb2+ ions are common contaminations in water, the two ions were selected to evaluate the 8
adsorption and desorption performance of BC@PEI nanofiber membranes. The adsorption kinetics for Cu2+ and Pb2+ ions were firstly studied. The conductivity change during the adsorption was used to illustrate this process, and the results are shown in Fig. S1. The solution conductivity decreased rapidly at the initial adsorption stage, and the time needed to reach the adsorption equilibrium increased from 50 to nearly 100 minutes as the initial Cu2+ concentration increased from 136 to 547 mg/L. However, much longer time was needed as the Pb2+ concentration increased from 151 to 548 mg/L (as shown in Fig. S1b).
Fig. 3. The effect of contact time and initial concentration on the adsorption of Cu2+ (a) and Pb2+ (b) ions onto the BC@PEI nanofiber membranes. Adsorption amount for Cu2+ and Pb2+ ions was as high as 90.1 mg/g and 130 mg/g (when the initial metal ions concentration is about 550 mg/L) as shown in Fig. 3, and it is higher than the other reported [40,41]. The adsorption amount increased as either the Cu2+ or Pb2+ ions concentration increased. Two commonly used kinetic models [42], namely, the pseudo-first-order and the pseudo-second-order were used here to better understand the adsorption behaviors. These two kinetic models are used to describe the adsorption of solid/liquid systems, which can be expressed in the linear forms as equations (3) and (4), respectively: 9
logq q logq ౪
మ మ
భ .
t
(3)
t
(4)
Where K1 and K2 are the pseudo first order and second order rate constants, respectively.
Fig. 4. Pseudo-first-order kinetic model (a, c), pseudo-second-order kinetic model (b, d) for adsorption of Cu2+ and Pb2+ on BC@PEI nanofiber membranes. The adsorption kinetic plots for Cu2+ and Pb2+ ions are shown in Fig. 4, and the kinetic parameters obtained from the models were summarized in Table 1. The values of the correlation coefficients clearly indicated that the adsorption kinetics closely followed the pseudo-second-order model. The qe, cal
values obtained from pseudo-second-order kinetic model appeared to be very close to the
experimentally observed values (as shown in Table 1).
10
Table 1 Kinectics parameters for Cu2+ and Pb2+ ions adsorption onto the BC@PEI nanofiber membrane. Metal ions
C0 (mg/L)
qe, exp (mg/g)
Cu2+ ions
136 371 547
Pb2+ ions
151 356 548
Pseudo-first-order model
Pseudo-second-order model
K1 (1/min)
qe, cal (mg/g)
R2
K2 (g/mg/min)
qe, cal (mg/g)
R2
40.74 60.62 90.10
0.012 0.017 0.015
11.81 23.54 43.85
0.77233 0.87833 0.88720
2.15×10-3 1.33×10-3 6.23×10-4
41.93 62.66 94.34
0.99892 0.99921 0.99853
43.01 91.20 130.47
0.017 0.011 0.013
16.09 42.94 108.58
0.93139 0.92004 0.99205
2.56×10-3 6.51×10-4 2.18×10-4
43.98 94.34 138.89
0.99977 0.99948 0.99700
3.3 Adsorption isotherm
Fig. 5. Equilibrium isotherms of Cu2+ and Pb2+ ions adsorption onto BC@PEI nanofiber membrane (a); Langmuir isotherm model (b), Freundlich isotherm model (c) for adsorption of Cu2+ and Pb2+ ions onto BC@PEI nanofiber membrane; The regeneration performance of the BC@PEI nanofiber 11
membranes.
The adsorption isotherm was also studied, and the results were shown in Fig. 5. The adsorption amount for Pb2+ ions was much higher than Cu2+ ions at the range of tested concentrations. Two well-known models (Langmuir and Freundlich isotherms) [43], were used to fit the adsorption equilibrium data, and the results were shown in Fig. 5b and Fig. 5c. The Langmuir model is based on the assumption of adsorption homogeneity, representing equally available adsorption sites, monolayer surface coverage, and no interaction between adsorbed species. The linearized Langmuir isotherm model is represented by the following equation:
ౣ
(5)
ౣ
The Freundlich isotherm describes reversible adsorption and is not restricted to the formation of the monolayer. This empirical equation takes the form: lnq lnK
(6)
Where qm (mg/g) is the maximum adsorption capacity, b (L/mg) is the Langmuir constant related to the energy of adsorption, Kf and 1/n constants are related to the adsorption capacity and intensity of adsorption. The parameters obtained from the two models were shown in Table 2. Obviously, the adsorption process can be better described by Langmuir isotherm model, which means that the adsorption occurs uniformly on the active sites (nitrogen atoms) of the BC@PEI nanofiber membrane. The adsorption removal of metal ions through the coordination between the nitrogen atoms was already known, and has been used in some previous works [20,44,45]. Furthermore, a monolayer adsorption occurred at the solid-liquid interface according to the Langmuir model, that is why the membrane morphology has
12
no change before and after the metal ions adsorption (as can be seen in Fig. 1). However, a blue color BC@PEI membrane was indeed obtained after the adsorption of Cu2+ ions. Table 2 Langmuir and Freundlich isotherm parameters for Cu2+ and Pb2+ ions adsorption onto BC@PEI nanofiber membrane. Langmuir model
Freundlich model
Metal ions
qm (mg/g)
b (L/mg)
R2
Kf (mg/g)
1/n
R2
Cu2+ ions Pb2+ ions
61.46 116.41
0.012 0.074
0.98930 0.99972
35.28 23.98
0.09569 0.32894
0.90887 0.94007
3.4 Desorption study Another important property of adsorbents was the regeneration possibility. A good adsorbent must have high regeneration performance. EDTA is a most used regenerator for desorption of heavy metal ions due to its high affinity to metal ions [20,29,46]. As shown in Fig. 5d, the BC@PEI nanofibrous membranes have a wonderful regeneration performance. Their adsorption amount remained more than 90% for both Cu2+ and Pb2+ ions after three regenerations, which makes the membranes a potential candidate as practical adsorbents for metal ions. 3.5 Catalytic activity study Usually, the adsorbents were regenerated after adsorption of metal ions and finally discarded after several times of usage. Because the adsorption capacity of the recycled adsorbents decreased a lot after several recycles. The discarded adsorbents are another threatens to the environment. The treatment to the discarded adsorbents will need a high cost. Several previously works have reported that copper nanoparticles can be used as high efficiency catalyst for many reactions [47-49]. Inspired by the above research, the adsorbed Cu2+ ions were reduced through a simple method by hydrazine hydrate. As shown in Fig. 6, after reduction, many nanoparticles with a diameter about 400 -500 nm can be found 13
on the surface of the BC@PEI nanofiber membrane. The similar results have been reported by the previously researchers [47]. Then we can hope that the reduced BC@PEI nanofiber membrane have the potential for using as catalyst for degradation or decoloration of organic pollutant.
Fig. 6. SEM images of BC@PEI nanofiber membrane after adsorption of Cu2+ ions and reduction by hydrazine hydrate. (a) low magnification, (b) high mangnification. A reaction between the methylene blue (MB) and NaBH4 was selected to confirm the catalytic performance of the obtained BC@PEI nanofiber membrane. As shown in Fig. S2, after adding the reduced BC@PEI nanofiber membrane into the mixture of MB and NaBH4 solution, the color of the solution disappeared within 15 seconds (Fig. S2c), which suggested that the reaction rate was significantly improved. However, without the BC@PEI nanofiber membrane, the color of the solution (the mixture of MB and NaBH4) nearly unchanged (almost the same as the pure MB solution as shown in Fig. S2a) even after 5 hours (Fig. S2b). The dynamic process of this reaction can be found in Fig. 7.
14
F 7. The Fig. T e dynnam mic proc p cesss of the collor chan c nge durringg thee reaactioon. UV-viss speectrra ass shhow UVwn in n Fiig. 8 cllearlly in ndiccateed thhe chan c nge befforee annd affter thee reaaction. Aftter a ding thee NaaBH add H4 too thee MB sooluttionn forr 5 h, h thhe abbsorrbannce slig ghtlyy deecreeasedd co omppareed too thee puure MB so MB olutiion.. Thhe absorbance decr d reaseed rapiidlyy affter addingg thhe reduucedd BC@ B @PE EI nano n ofibber m mbrranee. Thhe catal mem c lyticc mech m hanissm has h alreeadyy beeen cleaar. BH B 4- ions w worrkedd as elecctroon doonoor, and t metthyllenee bluue aas thhe ellectrron accceptor. The the T e reaactioon rrate wass im mproovedd thrroug gh an a eelecttronn rellay m chan mec nism m of thhe copp c per nannopaarticcless [50]. Thee reeusaabiliity of the t redduceed BC@ B @PE EI nano n ofibber m mbrranee waas also moonito mem oredd, ass shhownn in n Figg. 8, 8 thhe caatallyticc peerforrmaancee cann bee well w rem r mainned e n after even a r fivve timees of o usag u ge. Thhe eexceellennt rreusabillity sug ggestedd thhe stron s ng stabbilitty of o the t B @P BC PEI@ @Cuu naano--com mpoositee annd make m es itt a ppoteentiaal caatalyyst carrrier for reaal wasteewatter trea t atmeent a licaationn. app
15 5
Fig. 8. UV-vis spectra of the MB solutions after reaction under different conditions and its reuse performance. 4. Conclusions BC@PEI nanofiber membrane was prepared via a facile flush-coating and post crosslinking method. XPS results confirmed the presence of a thin layer of PEI on the surface of BC nanofibers. The obtained BC@PEI membranes showed excellent adsorption performance for Cu2+ and Pb2+ ions. Adsorption process was better described by pseudo-second-order rather than pseudo-first-order model. Adsorption isotherm data fitted well to Langmuir model, supporting a monolayer adsorption mechanism. The adsorption capacity can remain more than 90% for both Cu2+ and Pb2+ after three cycles. The adsorbed Cu2+ ions can be reduced to copper nanoparticle by hydrazine hydrate and showed excellent catalytic performance for the reduction of methylene blue. The catalytic performance remained well after 5 times of usage. This facile preparation process and excellent muli-functional performance make it a potential candidate for the application of wastewater treatment.
Acknowledgements 16
We sincerely acknowledge the funding supports of PolyU A-PK90, A-PL17, 1-ZV7L, and ITF 112/11.
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Graaph G hicall ab bstraact
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Highlights 1 Aminalized bacterial cellulose nanofiber membranes 2 Flush-coating and post crosslinking method 3 Adsorption capacity remained up to 90% within 3 regenerations 4 High efficient catalysts for methylene blue degradation
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