Removal of lead from aqueous solution using

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Removal of lead from aqueous solution using ... through FTIR, zeta potential analyzer, and scanning electron microscope (SEM) and were .... standard, 99.9% by volume, molecular wt. = 73.09), and ..... easy separation via magnet and manual.

Environ Sci Pollut Res https://doi.org/10.1007/s11356-017-0706-7

RESEARCH ARTICLE

Removal of lead from aqueous solution using polyacrylonitrile/magnetite nanofibers Hammad Malik 1 & Umair Ahmed Qureshi 2,3 & Muhammad Muqeet 1 & Rasool Bux Mahar 1 & Farooq Ahmed 2 & Zeeshan Khatri 2

Received: 6 September 2017 / Accepted: 6 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017

Abstract Lead is known for its toxic and non-biodegradable behavior. The consumption of lead-contaminated water is one of the major threat the world is facing nowadays. In this study, polyacrylonitrile (PAN) and magnetite (Fe3O4) composite nanofiber adsorbent was developed for Pb2+ removal in batch mode. The synthesis was done by a simple and scalable process of electrospinning followed by chemical precipitation of Fe3O4. The nanofibers thus obtained were characterized through FTIR, zeta potential analyzer, and scanning electron microscope (SEM) and were analyzed for their adsorption capability for Pb2+ ions. The amount of metal ion adsorbed was influenced by the initial metal ion concentration, the time the adsorbent was in contact, the amount of nanofiber, and the pH of the solution. The experimental data fitted well with pseudo 2nd-order and Langmuir adsorption isotherm model. The nanofibers showed high adsorption capability and could be recommended for Pb2+ removal successfully.

Responsible editor: Guilherme L. Dotto Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11356-017-0706-7) contains supplementary material, which is available to authorized users. * Rasool Bux Mahar [email protected]

1

U.S. Pakistan Center for Advanced Studies in Water (USPCASW), Mehran University of Engineering and Technology, Jamshoro 76062, Pakistan

2

Nanomaterials Research Lab, Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76062, Pakistan

3

Government Boys Degree College Qasimabad, Hyderabad 71000, Pakistan

Keywords Pollution . Lead . Magnetite . Adsorption . Electrospinning . Nanofibers

Introduction The enormous upsurge in the industrial activities and the uncontrolled population growth are one of the leading causes of pollutants in our environment. Lead is highly desirable industrial product owing to the set of physical properties it offers and being a metal that shows high malleability, low melting temperature, and good strength. It has been commonly used in manufacturing of household utensils including cooking wares. Lead is commonly used in petrol and about 50% of lead emissions in air are attributed to use of lead-containing petrol (Tchounwou et al. 2012). The use of lead in the industries cannot be permanently halted and is still taking place. In fact, the highest contributors of lead contamination to our ecosystem are industrial activities including lead smelting, lead battery plants, lead paint pigments, and the mining activities. The health problems caused by the lead include damage to central nervous system, kidney failure, effects on cellular processes, and malfunctioning of other organs including the brain (Sone et al. 2009). Numbers of technologies and systems have been introduced in the past in contest to get a breakthrough technology that can remove heavy metal ions like lead (Pb2+) out of the water to make it safe and suitable for consumption. These techniques included chemical, physical, and biological processes (Hashim et al. 2011). By far, the best-known technique for the removal of heavy metal and specifically for selective contaminant removal is adsorption as it has shown better results compared to ion exchange, chemical precipitation, and membrane processes (Morillo et al. 2016). There is a long list of Pb2+-adsorbing materials being developed and tested; these

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include carbonaceous materials (Taraba and Bulavová, 2017), phosphate activated carbons (Liu et al. 2013), metallic oxides (Zhang et al. 2017), iminoacetate-functionalized polymeric resin (Atia et al. 2005), zeolite-polymer composites (Elwakeel et al. 2017), and modified polymers (Badawi et al. 2017). Other than these, there are other adsorbing materials for lead as well. The most promising adsorbent materials among the series of materials being tested earlier are the metallic oxides that show high adsorption capability with commercial availability and low cost, becoming potential Pb2+adsorbent materials. Compared to bulk metallic oxides, the nanostructured metallic oxide is far more effective (Mohan et al. 2017). Although there is a list of metallic oxides being used as an adsorbent for Pb2+ removal but still, there is a need for finding more effective adsorbent. These nanostructure adsorbents such as the highly desirable ferric oxide adsorbent that initially show high tendencies for Pb removal, later lower its adsorption capability within limited period (Abdullah et al. 2016). These limitations are because these metallic nanoparticles agglomerate and set at the bottom and a very few of them disperse with the flowing water. These conditions make them inefficient and hazardous as they add up to the sludge and are difficult to separate. Our research group has previously worked on different applications of nanofibers such as dye removal (Qureshi et al. 2017), delivery of natural drugs (Khan et al. 2017), and printing of cellulose nanofibers (Khatri et al. 2017).This research is a step forward where an adsorbent is synthesized and checked for adsorption of Lead (Pb2+ ion). This adsorbent is essentially a nanofiber composite made up of magnetite and polyacrylonitrile polymer. The polymer acts as a base on which magnetite particles are embedded, restricting the agglomeration or leaching of particles during operation.

Experimental Materials and methods Polyacrylonitrile PAN [Average molecular wt. = 150,000 g/ mol, powder], FeCl3·6H2O, and FeSO4·7H2O were purchased from Sigma-Aldrich and used without any further treatment. Lead nitrate, N-N-dimethyl formamide (DMF) (analytical standard, 99.9% by volume, molecular wt. = 73.09), and other chemicals of analytical grade were used. Synthesis of Fe3O4-loaded PAN nanofibers The synthesis of PAN and magnetite (Fe3O4) is a two-stage process where the first stage consists of electrospinning followed by chemical treatment for in situ growth of the magnetite on the PAN-loaded FeCl 3 nanofibers. The electrospinning was done by preparing an electrospinning

solution. The electrospinning solution was prepared by dissolving 2% FeCl3 with 10% PAN in DMF solvent (Natraj et al. 2010). The solution was heated at 90 °C with stirring for around 1 hour till a homogenous mixture was obtained. The electrospinning solution was poured into multiple 5-ml syringes with the tip having internal diameter of 0.6 mm. The solution was electrospun providing a high voltage using power supply (Har-100*12, Matsusada Co., Tokyo, Japan). Copper wires were used as anodes and cathodes. The anode wire was attached to the syringe and the cathode was attached to the metallic drum. The voltage was set at 15 kV and the tip to collector distance was maintained at 20 cm. The second stage of the synthesis process consists of introduction of a piece of FeCl3-loaded PAN nanofiber weighing 75 mg into a solution containing 65 mg of FeSO4·7H2O in 25 ml of deionized water. The solution with the nanofibers was kept in suspension for 3 h. After that, 1.5 M NaOH (250 mL) was dropwise added to the solution and the temperature was raised to 90 °C. This treatment turned yellowish solution into black precipitates suggesting formation of Fe3O4 inside PAN nanofibers. The nanofibers were filtered out of the solution and washed with deionized water till neutral pH was achieved. After washing nanofibers and adjusting the pH, the nanofibers were ultrasonicated for 30 min. Characterization The field emission electron microscope (FE-SEM) was used to analyze the morphology of the nanofiber and to estimate the diameter of the nanofibers (Wang et al. 2010). The diameter of Fe3O4-loaded PAN nanofibers was analyzed using a J image software. For the analysis of the nanofibers diameter, multiple nanofibers were taken and the percentage distribution gave us the average nanofibers diameter. The FE-SEM images were taken at an accelerating voltage of 10 and 5 kV with magnifications around 30,000 and × 16,000. Zeta potential of the nanofiber was also examined. Zeta potential of the particles indicates the nature of charge present on the particles. Zeta potential represents the surface electrical potential of the colloidal particles in the aqueous solution. The zeta potential of Fe3O4-loaded PAN nanofibers was evaluated using SurPASS 3 electrokinetic analyzer, equipped with automatic titration unit, at Anton Paar USA. The experiment was conducted for evaluation of the surface charge and the change in charge with change in pH. The electrokinetic analyzer supplements in determination of the electrokinetic effects occurring at the solid and liquid interface. The zeta potential analysis is one of the important factors when studying the liquid on solid adsorption processes as it highlights many basic components on which adsorption capability of any material depends. For this study, 1-mM KCl solution was used as an electrolyte. The nanofiber samples were run on an automatic pH sweep using 0.05-M

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HCL solution. Fourier transform infrared (FTIR) spectroscopy was used to characterize the magnetite-loaded PAN nanofiber before and after adsorption. The samples were tested in the range 400–4000 cm−1. The chemical interactions and the chemical bonds breaking and formation responsible for adsorption can be examined using the FTIR spectrum. Analyzing these spectra, we can determine the dominating chemical species and thoughtfully establish facts about the mechanism of adsorption happening. Batch adsorption experiment The adsorption study was conducted in batch mode. The study consisted of varying different parameters to get the optimum conditions. For the adsorption study, the first study conducted was the selectivity study. The selectivity study was conducted by making the stock solutions and dilutions of different metals namely copper, manganese, lead, and zinc. The adsorption capability of the magnetiteloaded PAN nanofiber was tested for each of these metals under similar experimental conditions. Through the selectivity study, lead was chosen due to the highest adsorption using magnetite-loaded PAN nanofibers. Lead adsorption was tested further for getting optimum adsorption parameters. The concentration, time, dosage, and pH study was done for getting the optimum adsorption conditions. The stock solution was obtained by dissolving known concentrations of lead nitrate in deionized water to get the stock solution of lead ions (Pb2+) with initial concentration of 1000 ppm. Different concentration solutions were made from the stock solution by dilution process. The batch adsorption experiments consisted of adding known quantity of the nanofibers into 45-ml centrifuge tubes each containing 10 ml of the Pb2+ solution. The tubes were mounted onto a stirrer for stirring at a constant speed of 300 rpm. After stirring was over, the tubes were detached and the adsorbent was separated manually. The adsorption efficiency (AE%) was calculated by analyzing initial and residual Pb2+ concentrations in solution before and after contact with nanofibers through the following equation: AE% ¼

ðC i −C f Þ  100 Ci

ð1Þ

where Ci and Cf (mg/L) are the initial and final residual concentrations of Pb2+ in aqueous solutions. The adsorption capacity of nanofibers was calculated through the following equation: qe ¼

ðC i −C e Þ V m

ð2Þ

where qe (mg/g) is the maximum adsorption capacity of magnetite-loaded PAN nanofibers; Ci and Ce (mg/L) are

the initial and equilibrium Pb2+ concentrations; m and V are the mass of nanofibers and volume of solution.

Results and discussion Characterization Figure 1a is the SEM image of FeCl3-loaded PAN and Fig. 1b is the SEM image of Fe3O4-loaded PAN. SEM images show that the FeCl3-containing PAN nanofibers had a smooth, continuous, and cylindrical morphology. Contrastingly, the SEM images of Fe3O4-loaded PAN nanofibers show rough surface and thicker nanofibers compared to FeCl3-containing PAN nanofibers. The nanofibers in both the cases were almost beadless. The roughness in Fe3O4-loaded PAN is due to the chemical treatment of FeCl3-containing PAN nanofibers. The treatment changed the smooth nanofibers into more wavier and rough. The increased thickness is attributed to the swelling that occurred due to dipping of the nanofibers in iron salts solution during chemical treatment (Fig. 2). The nanofibers take up the solution and swell. The enhanced roughness of Fe3O4 loaded PAN nanofiber supplement in adsorption. Fe3O4 particles over the PAN nanofiber add up in entanglement of lead ions from the lead solution during adsorption. The zeta potential of Fe3O4-loaded PAN nanofiber is pH dependent. The zeta potential values varied from −18.8 mV at pH 2.6 to −62.3 mV at pH 7.2. The PAN nanofiber had negative surface charge regardless of the pH (Cho et al. 2012). Figure 3 indicates the results obtained for zeta potential of Fe3O4-loaded PAN. The pH dependence of the samples infers that the Fe3O4 has a shear effect on the surface charge as PAN nanofiber itself is essentially pH independent (Cho et al. 2012). At pH 6 and above, the zeta potential value does not change much and levels out. The zeta potential analyzer also gives us the isoelectric point (IEP). The IEP for Fe3O4-loaded PAN was extrapolated and it was obtained at pH 2; this clearly shows the presence of acidic surface charge. Lead adsorption tests Selectivity study Firstly, the most imperative step in adsorption study is the compatibility of adsorbent with target pollutant. Therefore, it was realized to confirm the selective adsorption potential of magnetite-loaded PAN nanofibers. For that purpose, lead (Pb), copper (Cu), zinc (Zn), and manganese (Mn) were used. Stock solutions of each of the metals were made followed by dilution to get 50-ppm concentration solution of each of the metal. Ten milliliters of each of diluted samples were taken into centrifuge tubes and 4 mg of the adsorbent was introduced into these tubes. The tubes were stirred for a span of 80 min each

Environ Sci Pollut Res Fig. 1 SEM image of FeCl3containing PAN. b SEM image of magnetite-loaded PAN

a

b 10 m

20 m

20 m

by mounting them on a stirrer. The adsorption was checked using the flame atomic absorption spectroscopy. The nanofibers showed highest adsorption capability for lead ions and least for manganese ions (shown in Fig. 4). The adsorption of Cu and Zn was the average but the adsorbent demonstrates highest potential for Pb2+ ions. Thereby, this adsorption study is focused on lead (Pb2+) ions. The adsorption results of the different metals demonstrate the following order: Mn < Zn < Cu < Pb. One of the factors that is impacting the selectivity is the size of hydrated ionic radii. The least adsorption was for Mn ions that have the maximum hydrated ionic radii in all ions examined. Contrastingly, adsorption was maximum for Pb ions that have the minimum hydrated ionic radii. The higher the hydrated ionic radii, the greater will be the distance from the adsorbing surface (Giraldo et al. 2013). Wastewater contains number of interfering species; the most frequent among them being dissolved or colloidal organic matter. Since the dissolved organic matter such as humic acids or fulvic acid consists of multiple functional groups, therefore, these may have some interactions with magnetite particles on studied adsorbents. In a study reported by Elwakeel (Elwakeel et al. 2017), humic acid up to 20 mg/L suppresses adsorption of lead on zeolite/PAN composite but with further increase in concentration, the adsorption of lead was increased due to binding of humic acid on adsorbent and thereby promoting lead transfer from aqueous phase to solid phase and consequently leading to significant Pb2+ removal. Fig. 2 a Nanofiber diameter distribution of FeCl3-containing PAN. b Nanofiber diameter distribution of Fe3O4-loaded PAN

a

Since the current adsorbent and the previously reported adsorbent are structurally interrelated, therefore, we expect similar behavior of PAN/magnetite like zeolite/PAN on lead removal in the presence of dissolved organic matter. Effect of pH The pH is an important parameter during adsorption experiments. The pH affects both the adsorption process and the removal capacity of the adsorbent material. The removal capacity is altered by varying pH because the varying pH directly alters the surface charge of the adsorbent material and changes the solution characteristics (Patel and Hota. 2016). Figure 5 illustrates the adsorption of lead ions at varying pH. The graph shows the rise in the adsorption with the pH approaching from low to high, i.e., 2–10. The increasing trend can be concluded to be a consequence of protonation of hydroxyl group present on the surface of the nanofibers at low pH. The protonation results in the electrostatic repulsion between the lead ions and the surface-active sites hindering the adsorption and decreasing the adsorption capacity. The adsorption capacity is also lowered at higher pH. This trend is an outcome of competition between ions at higher pH. It has been observed from the pH study that the maximum adsorption capacity of the nanofibers is in the neutral range that is between 6 and 8. Thereby, the pH chosen for the further study was pH 6. Elwakeel (Elwakeel et al. 2017) also noticed similar optimum pH for lead removal using zeolite-based PAN

Average Diameter 400 nm

b

Average Diameter 458 nm

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-20

Removal Efficiency, %

Zeta potential, mV

85

-40

-60

80 75 70 65 60 55

2

3

4

5

6

7

50

8

2

4

pH

Fig. 3 Zeta potential of PAN-loaded magnetite

6 pH

8

10

Fig. 5 Effect of aqueous pH on adsorption efficiency of Pb on magnetiteloaded PAN nanofibers

functionalized with amidoxime. It is evident from the experimental data that along with the facts discussed earlier, the physical adsorption has a due contribution. The greater surface area and abundant vacant sites on the nanofiber surface as well as inter fibrous structure of the nanofibers are causing large-scale physical adsorption of lead ions. Effect of nanofiber dosage The adsorption efficiency depends heavily upon the mass of the adsorbent used. The dosage study was conducted at laboratory temperature and at optimum pH with varying mass of adsorbent (dose). The different masses of adsorbent introduced into the lead solution (50 mg/L) were 2, 4, 6, 8, 10, and 12 mg at optimum pH for 80 min. Figure 6 illustrates that continuous increase in amount of adsorbent causes slight increase in lead removal efficiency. The adsorption efficiency (90%) was much greater when 2 mg of nanofiber dosage was taken. There was a slight increase in adsorption efficiency with further rise in nanofiber mass and with 8 mg, 95% removal was noticed. However, this minute amount (8 mg) with excellent performance favors its application in treating leadcontaminated water, but in order to avoid sludge generation, 4 mg was selected optimum amount for further testings.

Effect of initial Pb2+ concentration and isotherm study The concentration study was conducted (10–100 mg/L, Pb2+) to gather the adsorption data to be fitted into different isotherm models to understand the mechanism and achieve maximum adsorption capacity. Figure 7 shows that the adsorption efficiency of magnetite-loaded PAN decreases with increase in Pb2+ concentration. This behavior can be attributed to the availability of limited number of vacant sites for adsorption. The adsorption data was fitted to two different isotherm models namely, Langmuir and Freundlich isotherms (Eqs. 3 and 4, respectively). qe ¼

qm K L C e 1 þ K LCe

ð3Þ

1

qe ¼ K f C en

ð4Þ

where Ce (mg/L) and qe (mg/g) are the equilibrium Pb2+ concentrations in solution and in nanofiber surface respectively. qm (mg/g) and KL (L/g) are the maximum adsorption capacity and Langmuir affinity constant, respectively. n and Kf (mg/g) are the heterogeneity factor and Freundlich

100

97 96 Removal Efficiency

Removal Efficiency,%

80

60

40

95 94 93 92 91

20

90 89

0 Pb

Cu

Zn

Mn

Types of metal ions

Fig. 4 Selective adsorption of Pb2+by magnetite-loaded PAN nanofibers

2

4

6 8 10 Mass of nanofibers, mg

12

Fig. 6 Effect of nanofiber mass on adsorption efficiency for Pb2+

Environ Sci Pollut Res 95

Table 2 Comparison of adsorption capacities of reported materials for Pb2+ removal

Adsorption Efficiency, %

90 85

Material

Adsorption capacity

References

Magnetite-loaded PAN

156.25 mg/g 44.05 mg/g

Present work (Shalaby et al. 2016)

60.6 mg/g

(Kampalanonwat and Supaphol 2010)

80 75 70

Magnetic nanoparticles immobilized cellulose acetate nanofibers Aminated PAN nanofibers

65 60 55 20

40

60

80

100

Initial Pb2+ concnetration, mg/L

Fig. 7 Effect of initial Pb2+ concentration on adsorption efficiency

equilibrium constant, respectively. The non-linear plots of both isotherms are provided in supplementary material (S1). On the basis of lower sum of squared errors (SSE) values and higher R2 values, Langmuir isotherm was found to fit well to experimental data. The fitting of adsorption data with the Langmuir adsorption isotherm model illustrates that the adsorption is monolayer, that is, the Pb2+ is attached just to the surface of the nanofibers, while the Freundlich isotherm model assumes that the adsorbent has heterogeneous active sites. The fitting of experimental data to these isotherm data depicted that adsorption of Pb2+ can be favorably explained by Langmuir isotherm with monolayer surface coverage. The correlation coefficients (R2), sum of squared errors (SSE), and the isotherm parameters are summarized in Table 1. The adsorption capacity of magnetite-loaded PAN nanofibers was compared with previously published work. Table 2 shows that the proposed nanofibers were of excellent adsorption capacity compared to other reported materials. Effect of time and adsorption kinetic study Time is a crucial factor in determining the efficiency of adsorption system. The influence of time was monitored by placing 50 mg/L Pb2+ solution with 4-mg magnetite-loaded PAN nanofibers. Figure 8 shows that the adsorption efficiency increases rapidly after every interval and attains the maximum adsorption at 80 min. After this time, no further change in concentration of Pb 2+ was noticed that confirms the

Table 1

Langmuir and Freundlich isotherm parameters

Alpha Fe2O3 PAN

81.97 mg/g

(Chang et al. 2016)

TiO2-loaded chitosan nanofibers

579.1 mg/g

(Razzaz et al. 2016)

attainment of equilibrium within 80 min. Compared to other cheap materials that offer prolonged processing time (Liu et al. 2017; Singh and Gautam 2017; Yao-Jen et al. 2017), magnetite-loaded PAN nanofibers decontaminate Pb2+ in 80 min that could be due to its relatively higher surface area and abundant active sites. To further interrogate the rate-limiting factors, we analyzed the adsorption data on the two most common adsorption kinetic models that are pseudo 1st and pseudo 2nd order shown in Eqs. 4 and 5, respectively. lnðqe −qt Þ ¼ lnqe −k 1 t

ð4Þ

t 1 t ¼ þ qt k 2 q2e qe

ð5Þ

where qt and qe (mg/g) are the adsorption capacities of magnetite-loaded PAN nanofibers at time t and equilibrium. k1 (min−1) and k2 (g mg−1 min−1) correspond to the rate constants related to the pseudo 1st- and 2nd-order models, respectively. In simplified terms, the adsorption kinetic data will help understand the removal rate of lead ions form the synthesized water. According to the pseudo 1st-order adsorption kinetic 95.5

Adsorption Efficiency (%)

0

95.0 94.5 94.0 93.5 93.0 92.5

Langmuir isotherm model

Freundlich isotherm model

92.0 0

KL 0.0142

qm 156.25

R2 0.98

SSE 0.013

KF 22.04

n 1.91

R2 0.97

SSE 0.145

20

40

60

80

100

120

Time (min)

Fig. 8 Effect of time on adsorption of Pb2+ by magnetite-loaded PAN nanofibers

Environ Sci Pollut Res Magnetite loaded PAN before adsorption Magnetite loaded PAN after adsorption

Table 3 Kinetic parameters for the adsorption of Pb2+ on magnetiteloaded PAN nanofibers

K1 0.0074

Pseudo 2nd-order kinetic model

qe

R2

SSE

K2

qe

R2

SSE

0.382

0.74

0.4

0.0208

7.668

0.99

0.36

CN Intensity

Pseudo 1st-order kinetic model

Fe-O CH CH2 CN

model, the adsorption rate relates to the number of the unoccupied adsorptive sites offered by the adsorbent material being used. The plot between the ln (qe−qt) and time (t) represents pseudo 1st order. Adsorption rate constant is calculated by the slope of the pseudo 1st-order equation (Eq. 4) and capacity can be calculated from intercept. The adsorption data was also analyzed for pseudo 2nd order. The plot between the t/q and time represents pseudo 2nd order. The coefficient of determination (R2) and the constants were calculated and summarized in Table 3. From the results obtained, it can be clearly interpreted that adsorption of Pb2+ ions follows pseudo 2ndorder model preferentially compared to pseudo 1st-order model.

Desorption studies Nanofiber feasibility for large-scale applications such as in water treatment and domestic water purification systems largely depends upon their reusability (Liu et al. 2015). For the reusability study, the nanofiber obtained after adsorption study through filtration was rinsed with water followed by introducing the nanofiber into a centrifuge tube containing 20 ml of 0.1-M HCL solution. The tube was shaken for 2 h, respectively, and then the solution was filtered to get the nanofibers out. The solution was tested for the desorption of lead ions. High desorption was obtained followed by excellent reusability by introducing regenerated nanofibers into the lead solutions again. The nanofibers showed high reusability for up to three cycles (shown in Fig. 9).

Fe-O CH 500

1000

CH2

1500

2000

2500

3000

3500

-1

Wavenumber cm

Fig. 10 FTIR spectra of magnetite-loaded PAN nanofiber before and after adsorption

Chemical stability of magnetite-loaded PAN nanofiber FTIR analysis of the magnetite-loaded PAN nanofiber samples before and after adsorption was conducted (Fig. 10). The prominent peaks in both the samples remain the same. The peak at 575 cm−1is assigned to Fe–O stretching while the peaks around 1235 and 1360 cm−1 are assigned to the aliphatic CH group vibrations of different modes in CH and CH2, respectively (Pan et al. 2010). The peak at 1719 cm−1 suggests typical stretching of the carboxyl group that may be due to the oxidation of as-received nitrile in the presence of air (Pan et al. 2010). The peaks at 2242 and 2932 cm−1 are attributed to the nitrile group and C–H stretching vibrations. The similarity in the spectrum of both the samples illustrates the negligible chemical modification occurring during adsorption of Pb2+that could be due to either presence of Pb2+ below the detection limit of instrument or physical interactions between the Pb2+ and nanofiber surface. This also suggests the chemical stability of synthesized material since there is no any extra emergence or disappearance of any peak. The main difference between the spectrum obtained from nanofibers before and after adsorption is the increased intensity during adsorption.

90

Adsorption efficiency (%)

80

Conclusion

70 60 50 40 30 20 10 0 1st

2nd

3rd

No. of cycles

Fig. 9 Reusability profile of magnetite-loaded PAN nanofibers for Pb2+ removal

This work illustrates the in situ formation of magnetite particles in the PAN nanofibers. The nanofibers were smooth and continuous in morphology. Magnetite-loaded PAN nanofibers exhibit desirable properties for lead removal. The series of adsorption experiments were conducted for optimizing adsorption conditions to get best results. The nanofibers showed high adsorption capability and promising results. The nanofibers were tested for their reusability where the nanofibers showed effective adsorption for up to three cycles. This supports that magnetite-loaded PAN has many benefits such as easy separation via magnet and manual. In addition to this, the

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adsorbent possessed good adsorption capacity compared to other materials. The greater surface area and the abundant vacant sites on the nanofiber surface as well as interfibrous structure of the nanofibers are causing large-scale physical adsorption of lead ions. Funding information The work was supported by Mehran University of Engineering and Technology Jamshoro Pakistan and University of Utah, Salt Lake City, USA.

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