Preparation of amidoxime polyacrylonitrile chelating nanofibers for the adsorption of metal ions

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Preparation of amidoxime polyacrylonitrile chelating nanofibers for the adsorption of metal ions


    Preparation of amidoxime polyacrylonitrile chelating

    nanofibers for the adsorption of metal ions

    LIAO Shiqin, FENG Quan, ZHANG Ping, HUANG Fenglin, WEI Qufu

    5 (Key laboratory of Eco-textiles, Ministry of Education, Jiangnan University,

    JiangSu WuXi 214122)

    Abstract: In this paper, polyacrylonitrile (PAN) nanofibers were prepared by electrospinning and they were modified with hydroxylamine to synthesize amidoxime polyacrylonitrile (AOPAN) chelating nanofibers, which were further applied to adsorb copper and iron ions. The conversion rate of nitrile

    10 groups in PAN was calculated by the gravimetric method. The structure and surface morphology of AOPAN nanofibers were characterized by a Fourier transform infrared spectrometer (FTIR) and a 2+ 3+ scanning electron microscope (SEM), respectively. The adsorption capabilities of Cuand Feions

    onto the AOPAN nanofiber mats were evaluated. FT-IR spectra indicated that nitrile groups in PAN were partly converted into amidoxime groups. SEM examination revealed that there were no any

    15 serious cracks or sign of degradation on the surface of PAN nanofibers after chemical modification. The adsorption capacities of both copper and iron ions onto the AOPAN nanofiber mats were much 2+ 3+ higher than those onto the raw PAN nanofiber mats. The adsorption data of Cuand Feions were 2+ fitted particularly well with the Langmuir isotherm. The maximal adsorption capacities of Cuand 3+ Feions reached 215.18 and 221.37 mg/g, respectively.

    20 Keywords: polyacrylonitrile; electrospinning; adsorption

    0 Introduction

    With the rapid development of global industry and the advent of new technologies, environmental contamination has presented an increasing threat to human health, especially the

    [1]25 amounts of heavy metal ions in wastewater have shown an astonishing increase in recent years .

    Heavy metal pollution is characterized by its strong concealing ability, accumulative damage, and nonbiodegradable property. Heavy metals are very hazardous to the ecological environment, more seriously, which could cause various diseases, for instance, headache, nausea, vomiting,

    [2-4]abdominal pain, insomnia, forgetfulness, neurological disorder and liver damage . Thus, the

    30 removal and recovery of heavy metals from wastewater have become one of the predominant portions of environment research.

    Various methods have been utilized to remove and recycle heavy metals from aqueous solutions such as chemical precipitation, ion exchange, membrane separation, electrochemical [5-8]treatment, adsorption, etc. . Among them, adsorption is one of the most simple and common

    35 techniques. The adsorption of metal ions can be achieved by using polymer materials containing specific functional groups, for example, amino, carboxyl, phosphoric, tetrazine and amidoxime,

    [1]etc. , which can form strong complexes with metals ions via the coordination reaction. The adsorption abilities of these materials mainly depend on the functional groups on the adsorbent surface. Amidoxime group, in particular, has exhibited superior adsorption ability because it

    40 contains both amino and carboxyl groups.

    Recently, electrospinning technique, a simple and versatile method, has been widely applied to produce nanofibers. Electrospun nanofibers are known to possess numerous interesting characteristics such as high porosity, small interfibrous pore size, and most importantly a large

     specific surface area in comparison to conventional fibers. Nanofibers have been increasingly

    Foundations: the Innovation Project of Jiangsu Graduate Education (NO. CXLX11_0500); the Specialized Research Fund for the Doctoral Program of Higher Education (NO. 20090093110004); Central Universities (JUSRP11102 and JUSRP20903)

    Brief author introduction:Liao shiqin, (1987-), Female, Graduate Student, Main research: Nano textile materials. Correspondance author: Wei qufu, (1964-), Male, Professor, Ph. D. Candidate Tutor, Main research: Nano Textile Material. E-mail:

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     [9][10][11][12]45 applied to tissue engineering , drug delivery , sensors , protective clothing , fine

    [13, 14]filtration and adsorptive membrane . The presence of high specific surface area makes it

    [15]possible for nanofibers to serve as adsorbents , which result in high adsorption rate and capacity

    [16]as compared to other types of materials such as resins, foams, and conventional fibers, etc. .

    Thus, nanofibers, modified by introducing functional groups on their surface, could be applicable

     for the removal and recovery of heavy metals from aqueous solutions. 50

    Polyacrylonitrile (PAN) has been recognized as a highly efficient material for the removal

    [1]and enrichment of heavy metals . In the present study, PAN nanofibers were prepared by

    utilizing the electrospinning technique and they were further chemically modified with

    hydroxylamine to synthesize chelating nanofibers. The conversion rate of nitrile group in the PAN 55 molecule was calculated gravimetrically. The structure and surface morphology of amidoxime

    PAN (AOPAN) nanofibers were analyzed by a Fourier transform infrared (FTIR) spectrometer

    and a scanning electron microscope (SEM), respectively. The modified nanofibers were

    subsequently applied to adsorb copper and iron ions from aqueous solutions. The concentrations

    of copper and iron ions were measured by atomic absorption spectroscopy (AAS). The adsorption

     isotherm was constructed at a given constant temperature of 303.15 K. The equilibrium parameters 60

    were also calculated.

    1 Experimental

    1.1 Material

    The PAN powders (Mw=30,000-50,000 g/mol) were purchased from American Integrity 65 Group Ltd. The ferric chloride (FeCl?6HO), copper chloride (CuCl?2HO), and N, N-dimethyl 3222

    formamide (DMF) were obtained from Sinopharm Chemical Reagent co., Ltd (China). All the

    chemicals were analytical grade and were used without further purification.

    1.2 Electrospinning of PAN nanofibers

    The PAN powders, with the concentration of 10 wt%, were dissolved in DMF to make even 70 spinning solution by stirring at room temperature for 24 h. The prepared solution was loaded in a

    10 mL of plastic syringe with a metal needle (0.3 mm inner diameter and 0.7 mm external

    diameter). The electrospinning parameters were determined based on the previous research work

    [17]. The applied electrical voltage, distance between the needle tip and collector, and flow rate

    were respectively fixed at 18 kV, 15 cm and 0.5 mL/h. Under the high voltage, the droplet was 75 ejected and accelerated towards the collector in an external electrostatic field. Finally, PAN

    nanofibers were collected on the surface of aluminum foil and formed a nanofiber mat.

    1.3 Chemical modification of PAN nanofibers

    Electrospun PAN nanofibers were modified to form chelating fibers containing amidoxime

    groups by reacting with hydroxylamine hydrochloride, in which nitrile group was transformed into 80 amidoxime group. Scheme 1 reveals the reaction between hydroxylamine hydrochloride and

    nitrile group. Dried samples of PAN nanofiber mats were immersed in a 50 mL of aqueous

    solution of hydroxylamine hydrochloride for some time. The pH value was adjusted with

    anhydrous sodium carbonate. After reaction, the nanofiber membranes were washed several times

    and then dried in an oven at 323 K.

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     NH 2 NC OHN C CHCH 70? 2 .+ COHCl + NaCHCH OH NH232 2n n 85 Scheme 1.The reaction between hydroxylamine and nitrile group [18]The conversion rate of nitrile group in PAN was calculated as follows :

     ? WW53 0 1 (1)C (%) = × ×100W33 0 is the Where C (%) is the conversion rate of nitrile group into amidoxime group in PAN, W0

    90 dry weight of the PAN nanofiber membranes before modification (g), Wis the dry weight of the 1 PAN nanofiber membranes after modification (g), 53 and 33 are the molecular weight of acrylonitrile monomer and hydroxylamine, respectively.

     1.4 Characterization

    The surface morphologies of raw PAN and AOPAN nanofibers were analyzed using SEM,

    Quanta-200 from HITACHI (Japan). The surface chemical features were characterized by a 95

     Fourier transform infrared (FT-IR) spectrometer (Thermo Fisher Scientific, China). A Spectr AA-220 atomic absorption spectroscopy (AAS) was used to measure the concentration of copper

     and iron ions in solutions. 1.5 Adsorption behaviors

    All adsorption experiments were conducted in 250 mL of beakers at 303 K. Dried raw PAN 100

     nanofiber and modified nanofiber mats, with the same total area, were immersed in 50 mL of metal salt aqueous solutions for different duration of time, and then removed from the beakers at

     0.5, 1, 2, 3, 6, 24 h. The obtained nanofiber mats were washed with deionized water. The lotion and the remaining metal salt aqueous solutions were transferred to a 100 mL of volumetric flask,

    evenly mixed for measurement. The equilibrium isotherm time was determined, and the 105

    equilibrium isotherm was investigated at 72 h and 303 K. The concentration of metal-ions in

    solutions was measured by atomic absorption spectroscopy (AAS). The adsorption amounts were

     calculated as follows:

     VC? CV00 11 Q(mg / g ) = (2) M

    110 is the initial Where Q is the adsorption amount onto the PAN nanofibers (mg/g), C0 concentration of metal ions (mg/L), Cis the final concentration (mg/L), and M is the weight of 1

     nanofiber mats (g). 2 Results and discussion

     2.1 SEM observations

    The SEM photos of the raw PAN nanofibers and AOPAN nanofibers are shown in Fig.1. The 115

     SEM images clearly revealed the fibrous structures of the electrospun nanofiber mat. The fibers showed random orientations in the fibrous web, as indicated in Fig.1a. The surface of the

     electrospun PAN nanofibers looked rather smooth. It was observed that the modified PAN nanofibers became expanded and bent compared to the untreated electrospun PAN nanofibers, as

    illustrated in Fig.1b. However, the surface morphology of the AOPAN nanofibers looked similar 120

    to the PAN nanofibers, and the surface of AOPAN nanofiber did not appear any serious cracks or

    sign of degradation. In the electrospinning process, the electrical force at the surface of the drop

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     solution overcame the solution surface tension, and then the polymer solution was stretched and elongated into nanofibers. Consequently the polymers in the PAN nanofibers were in the

    high-energy metastable state. While in this wet heating process, the polymers relaxed to a lower 125

     energy state, which led to the swelling and contracting of nanofibers.

     (a) (b)

     Fig.1. SEM images of (a) PAN nanofibers, (b) AOPAN nanofibers (53% conversion) 130

     2.2 FT-IR analysis The FT-IR spectra of raw PAN and AOPAN nanofiber mats are shown in Fig.2. The FT-IR -1 spectrum of raw PAN presented the characteristic absorption peak at 2243 cm(-C?N) and 1735 -1 cm(C=O), which indicated that the PAN was a copolymer or the DMF solvents did not volatilize

    135 completely. The FT-IR spectrum of modified PAN exhibited correlative characteristic bands of

    -1-1-1-1-1 amidoxime at 3100 cm, 1577 cm, 1506 cm, 1161 cm, and 1000 cm, which were attributed [19] to the stretching vibration of O-H, C=N, N-H, C-N and N-O, respectively . The FT-IR spectra -1 approved that the amidoxime group was introduced onto the PAN surface. The peak at 2243 cm

    in the FT-IR spectrum of modified PAN suggested that nitrile groups in PAN were partly

    140 converted into amidoxime groups.

     Fig.2. FT-IR spectra of (a) Raw PAN, (b) Modified PAN. 2.3 Conversion of nitrile groups 145 Tab.1 shows the effects of the reaction conditions on the conversion rate of nitrile groups in

    PAN and the appearance properties of modified PAN nanofibers.

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     Tab.1 Effects of the reaction conditions on conversion and appearance properties t/h T/K NHOH content pH Conversion Appearance and properties 2 g/L rate/%

    1 343.15 40 7 21 Soft and white 2 343.15 40 7 36 Soft and white

    3 343.15 40 7 43 Brittle and white

    4 343.15 40 7 53 Brittle and light yellow

    5 343.15 40 7 56 Brittle and light yellow

    2 343.15 40 3 26 Soft and white

    2 343.15 40 5 51 Brittle and light yellow

    2 343.15 40 7 63 Brittle and light yellow

    2 343.15 40 9 31 Soft and white

    2 343.15 40 11 22 Soft and white

    2 343.15 10 7 27 Soft and white

    2 343.15 20 7 42 Soft and white

    2 343.15 30 7 59 Brittle and light yellow

    2 343.15 40 7 78 Hard and light yellow

    2 343.15 50 7 84 Hard and light yellow

    2 313.15 40 7 26 Soft and white

    2 323.15 40 7 33 Soft and white

    2 333.15 40 7 58 Brittle and light yellow

    2 343.15 40 7 79 Hard and light yellow

    2 353.15 40 7 86 Hard and light yellow 150 The conversion rate of the nitrile groups in PAN molecules increased along with the increase of reaction time, reaction temperature and concentration of hydrochloride hydroxylamine. The conversion rate achieved the maximum when the pH value of the reaction system was about 7. The conversion rate depended on the diffusion amount of hydroxylamine molecules from the

    155 reaction solution into the PAN nanofibers. The increase of reaction temperature and

     hydroxylamine concentration effectively promoted the molecular diffusion of hydroxylamine into the PAN nanofibers. The extension of reaction time, which provided more opportunities for the

     molecular diffusion of hydroxylamine from the solution into the nanofibers, also improved the reaction probability between hydroxylamine and nitrile groups. Hydrochloride hydroxylamine in

    the solution predominantly exited in the form of free Hydroxyl amine molecules at pH 7, which 160

     accelerated the conversion of nitrile groups. The color of the PAN nanofiber mats changed from white to light yellow and the mats

     became brittle or hard with increasing conversion rate of nitrile groups in PAN. This color change [1]. The decrease of the softness may probably caused by the long heating time in reaction process

    be due to the high conversion rate of nitrile groups into amidoxime groups, because the larger 165

     molecules contributed more significantly to strength and toughness compared to the shorter [18]molecules . 2.4 Adsorption behavior 2+ 3+ Fig.3 shows the adsorption amount of Cuand Feions onto the PAN nanofiber mats and

    AOPAN nanofiber mats (57% conversion) in a 1 mg/L of solution as a function of time (for 72 h). 170

     It was evident that the chemical modification of PAN nanofibers had a significant effect on the 2+ 3+ adsorption capability. The adsorption capacities of Cuand Feions onto PAN nanofiber were

     198.46 and 278 mg/g, respectively. However, the adsorption capacities onto AOPAN nanofibers reached 320 and 380 mg/g, respectively. The capacities adsorbed on the PAN nanofibers were obviously lower than those onto the AOPAN nanofibers. The results in Fig.3 confirmed that the 175

    introduction of amidoxime group on the AOPAN significantly strengthened the adsorption

    capability of metal ions.

    3+ It was also observed that the adsorption capacity of Feion onto nanofiber was higher than

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     2+ ion in Fig.3. The difference of adsorption capacity of metal ions depended on their that of Cu

    special affinities with the active adsorption sites on PAN nanofibers at same initial concentration. 180 3+ Feion was hard acids, with small size and strong electropositive, which more easily accepted 2+ electrons from the ligand. However, Cuion behaved weaker electron-accepting nature compared 3+ to Feion. PAN and AOPAN, containing hydroxyl and amine group, had the structure characteristics of hard acid, with electron-donating nature and strong electro-negativity. According [20]3+ to HSAB (Hard-Soft Acid Base) theory and Lewis acid-base theory , Feion was expected to 185

     give stronger complexes with PAN and AOPAN, which led to the higher adsorption capacity of 3+ 2+ Feion than that of Cuion.

     Fig.3 Adsorption capacities of copper and iron ions onto the PAN and AOPAN nanofiber 190 Fig.4 illustrates the adsorption of copper and iron ions onto the AOPAN nanofiber mats (44% conversion) as functions of time (24 h). The amount of adsorption increased rapidly until 3 h and then leveled off. The rapid increase in the beginning of first 3 h was due to the abundant available

     chelating oxime sites on the surface of AOPAN nanofibers and the high concentration of metal [21]ions . The adsorption rates decreased and finally reached equilibrium because of the depletion 195 of the adsorptive sites as well as the decrease of metal-ion concentrations in the solution.

     According to Fig.4, the adsorption time of 72 h was determined to study the adsorption equilibrium amounts of copper and iron ions. 2+ 3+ Fig.4 Adsorption of Cuand Feions on the AOPAN (44% conversion) nanofiber mat in a 500 ppm solution as a 200 function of time 2+ 3+ Fig.5 illustrates the equilibrium adsorption amounts (Q) of Cuand Feions onto AOPAN e

    nanofiber mats after the equilibrium time (72 h) as a function of equilibrium concentration (C). e

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205 Qincreased sharply and then gradually with an increase in C. At equilibrium, an adsorption e e

    isotherm can be constructed at a given constant temperature. Adsorption data should accurately fit

    into different isotherm models. The most common ones are the Langmuir and the Freundlich

     models. These equations are as follows, respectively:

     bCQm e (3) Q = e 1 + bCe

    210 Where Qis the equilibrium amount of the metal adsorbed onto the AOPAN nanofiber mat e

    (mg/g); Qis the maximum adsorption capacities (mg/g); b is the Langmuir constant related to m

    binging energy (L/mg); Cis the equilibrium concentration (mg/L). The values of Qand b were e m

    calculated, as shown in Tab.2. 1 / n(4) Q= KC e e

    215 Where K and n are the Freundlich constants. The values of these parameters were also

     analyzed from the plots shown in Fig.5, given in Tab.2. As shown in Tab.2, the experimental adsorption data of copper and iron ions onto the

     AOPAN nanofiber mats were fitted more particularly with the Langmuir model than the 2). The Freundlich model, as indicated by the very high values of the correlation coefficient (r

    220 Langmuir model was used to describe the adsorption taking place at specific homogeneous sites.

     Once an adsorptive site was occupied, no further adsorption could occur at this site. Thus, the adsorption of copper and iron ions followed the formation of a monolayer. The reported value for

     2+ 3+ the maximum adsorption of Cuion onto AOPAN nanofibers was approximately that of Feion. However, this value was not inconsistent with the results in Fig.3 because the conversion rates of

    nitrile group in AOPAN were significantly different in this study. 225

     (a) (b)

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     (c) (d) 230 Fig.5 Adsorption isotherms of iron ions on the (a) AOPAN (31% conversion) fitting models, (b) Langmuir model

    at 303 K and adsorption isotherms of copper ions on the (c) AOPAN (36% conversion) fitting models,

    (d) Langmuir model at 303 K.

     Tab.2 Langmuir and Freundlich constants for metal-ion adsorption on AOPAN nanofiber mats

     Metal ionsLangmuir model Freundlich model2 (1-1/n)1/n-1 2 Q/mg/g b/L/mg rK/mgLg1/n r m2+ 215.1783 0.00488 0.99693 9.03894 0.45573 0.96072 Cu3+ Fe 221.3681 0.00321 0.99337 7.29263 0.46002 0.97463 235

     3 Conclusion In this paper, the AOPAN nanofiber mats were produced by using electrospinning technique

     and chemical modification of nitrile group. The conversion rate of the nitrile group in PAN increased along with the increase of reaction time, temperature and concentration of hydrochloride

    hydroxylamine. When the pH value of the reaction system was 7, the conversion rate was 240

     maximized. The PAN nanofiber mats became light yellow and brittle above 50% conversion. The SEM photos suggested that the AOPAN nanofiber did not appear any serious cracks or

     degradation. The adsorption capacity of copper and iron ions onto the AOPAN nanofiber mats was 2+ 3+ higher than that onto the PAN nanofiber mats. The adsorption of both Cuand Feions on the

    AOPAN nanofiber mat increased with increase in adsorption time, and then leveled off at 245 2+ 3+ approximately 6 h. The adsorption data of both Cuand Feions were fitted particularly well

     with the Langmuir isotherm, indicating that adsorption took place via the formation of a monolayer. Acknowledgements

    This work was supported by the Innovation Project of Jiangsu Graduate Education (NO. 250

     CXLX11_0500), the Specialized Research Fund for the Doctoral Program of Higher Education (NO. 20090093110004), Central Universities (JUSRP11102 and JUSRP20903).

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    偕胺肟基 PAN 螯合纳米纤维的制备及其对金属离子 305

     的吸附性能 廖师琴,凤权,张平,黄锋林,魏取福

     ?江南大学生态纺织教育部重点实验室,江苏 无锡 214122 摘要!本文采 用静电纺丝技术制备聚丙烯腈(PAN)纳米纤维,并用盐酸羟胺对其改性后得到 偕胺肟基聚丙 烯腈(AOPAN)螯合纳米纤维,然后用来吸附铜和铁两种金属离子。采用重量 分析法计算 310 PAN 分子中腈基的转化率,并分别用 SEM FTIR 观察分析 AOPAN 螯合纳米 纤维的形貌 和结构变化。研究了 AOPAN 螯合纳米纤维对铜、铁金属离子的吸附容量。实验 结果表明 PAN 分子中有部分腈基转化成了偕胺肟基团,化学改性后 PAN 纳米纤维的形貌未 出现明显

     的裂解现象,AOPAN 螯合纳米纤维对铜、铁离子的吸附容量均大于 PAN 纳米纤 维,且两2+3+ 种金属离子的吸附平衡数据非常拟合 Langmuir 模型,由 Langmuir 模型得到的 Cu Fe 饱和吸附量分别为 215.18 mg/g 221.37 mg/g 关键词!纺织工程,聚丙烯腈,静电纺丝,315 吸附


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