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Removal of Cu(II) from Aqueous Solutions Using Amine-Doped Polyacrylonitrile Fibers

Removal of Cu(II) from Aqueous Solutions Using Amine-Doped Polyacrylonitrile Fibers applied sciences Article Removal of Cu(II) from Aqueous Solutions Using Amine-Doped Polyacrylonitrile Fibers 1 1 1 , 2 , Kay Thwe Aung , Seung-Hee Hong , Seong-Jik Park * and Chang-Gu Lee * Department of Bioresources and Rural System Engineering, Hankyong National University, Anseong 17579, Korea; kaythweag11@gmail.com (K.T.A.); trea1568@naver.com (S.-H.H.) Department of Environmental and Safety Engineering, Ajou University, Suwon 16499, Korea * Correspondence: parkseongjik@hknu.ac.kr (S.-J.P.); changgu@ajou.ac.kr (C.-G.L.); Tel.: +82-31-670-5131 (S.-J.P.); +82-31-219-2405 (C.-G.L.) Received: 4 February 2020; Accepted: 24 February 2020; Published: 3 March 2020 Abstract: Polyacrylonitrile (PAN) fibers were prepared via electrospinning and were modified with diethylenetriamine (DETA) to fabricate surface-modified PAN fibers. The surface-modified PAN fibers were used to evaluate their adsorption capacity for the removal of Cu(II) from aqueous solutions. Batch adsorption experiments were performed to examine the e ects of the modification process, initial concentration, initial pH, and adsorbent dose on the adsorption of Cu(II). Kinetic analysis revealed that the experimental data fitted the pseudo-second-order kinetic model better than the pseudo-first-order model. Adsorption equilibrium studies were conducted using the Freundlich and Langmuir isotherm models, and the findings indicated that the PAN fibers modified with 85% DETA presented the highest adsorption capacity for Cu(II) of all analyzed samples. Moreover, the results revealed that the Freundlich model was more appropriate than the Langmuir one for describing the adsorption of Cu(II) onto the modified fibers at various initial Cu(II) concentrations. The maximum adsorption capacity was determined to be 87.77 mg/g at pH 4, and the percent removal of Cu(II) increased as the amount of adsorbent increased. Furthermore, the surface-modified PAN fibers could be easily regenerated using NaOH solution. Therefore, surface-modified PAN fibers could be used as adsorbents for the removal of Cu(II) from aqueous solutions. Keywords: polyacrylonitrile; electrospun fibers; Cu(II) removal; batch experiments; adsorption models 1. Introduction Environmental problems a ect humans, animals, and plants worldwide. In particular, rapid industrialization has increased water pollution, and thus water sanitation has become a significant global concern. Owing to the increasing environmental and economic impacts of heavy metal ions, their removal from industrial wastewater has been studied in depth [1]. Heavy metals, such as Cr, Cu, Fe, Pb, Ag, Ni, and Zn, which are highly toxic and nonbiodegradable, could cause problems for both the environment and living organisms [2]. The presence of heavy metal pollutants in water could lead to serious health problems, such as stomach aches, nausea, dehydration, dizziness, nervous system damage in young children, skin rashes, vomiting, abdominal pain, lung irritation, eye irritation, and liver damage [3]. Cu is not only an essential nutrient but also a water contaminant. Moreover, Cu has many commercial and industrial applications; it can be used to manufacture pipes, valves, and fittings, and could also be a constituent of di erent types of alloys and coatings. Copper(II) sulfate pentahydrate (CuSO 5H O) is sometimes added to surface waters to control the growth of algae [4]. The typical 4 2 oxidation states of Cu are +1 and +2, and the corresponding compounds are referred to as cuprous and cupric, respectively [5]. Cu compounds, whether organic complexes or organometallics, could be Appl. Sci. 2020, 10, 1738; doi:10.3390/app10051738 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 1738 2 of 12 used to promote or catalyze numerous chemical and biological processes [6]. Copper(I) and copper(II) sulfides are important examples of copper sulfides. CuSO 5H O is the most frequently used Cu 4 2 compound in the laboratory. Highly toxic amounts of Cu are present in the industrial wastewaters of the metal cleaning and plating facilities, refineries, paper and pulp plants, fertilizer mills, and wood preservative factories [7]. Furthermore, exposure to environmental sources of Cu could cause vomiting, headaches, stomach cramps, nausea, liver and kidney damage, and even death [8]. Cu is found in drinking water at concentrations in the range of 0.005 to 30 mg/L, and it mainly originates from the corrosion of Cu plumbing [9]. The concentration of Cu in treated water often increases during distribution, particularly in systems where the pH of the water is acidic or basic, or where the water is high in carbonates [4]. According to the U.S. Environmental Protection Agency, the maximum allowable limits for Cu in drinking and industrial waters are 1.3 and 3.0 mg/L, respectively [10]. Several methods could be used to reduce the amounts of heavy metal from euents, including chemical precipitation, electrodialysis, reverse osmosis, ion exchange, evaporation, and adsorption [8]. Of these, adsorption is an e ective and economical method of wastewater treatment [2]. Many adsorbents, such as activated carbon, biosorbents, illite–smectite, natural zeolites, natural bentonites, oxide minerals, polymer materials, resins, and synthetic hydroxyapatite have been used for the removal of metal ions from wastewaters [11–13]. Recently, electrospinning has attracted significant attention in academic research circles and for industrial applications because it (1) could be used to fabricate continuous fibers with diameters as small as a few nanometers; (2) could be applied to a wide range of materials, such as synthetic and natural polymers, metals, and ceramics and composite systems; and (3) could be a low-cost and high-yield method for the preparation of nanofibers [14]. Modified electrospun nanofibers have been developed as adsorbents for metal ions from solutions because they have high porosity, high specific surface area, and controllable surface functionality [15]. In this study, polyacrylonitrile (PAN) fibers were prepared via electrospinning, and diethylenetriamine (DETA) was used to modify the PAN fibers. The purpose of the study was to fabricate surface-modified PAN fibers for the removal of heavy metals from aqueous solutions. The e ects of the pH, contact time, and initial concentration of the Cu(II) solution on the adsorption capacity of the surface-modified PAN fibers for Cu(II) ions were examined. Adsorption kinetics and isotherm experiments were performed to analyze the experimental data. 2. Materials and Methods 2.1. Reagents PAN powder (Sigma-Aldrich, Mw = 150,000 g/mol) and dimethylformamide (DMF, Sigma-Aldrich, Steinheim, Germany, 99.8%) were used to prepare the polymer spinning solution, and DETA (ACROS Organics Co., Geel, Belgium, 98.5%) and aluminum chloride hexahydrate (AlCl 6H O, Sigma-Aldrich, 3 2 99%) were used as the modifying agent and catalyst, respectively. Analytical grade CuSO 5H O 4 2 (Daejung Co., Siheung, Korea, 99%), NaCl (Samchun Co., Seoul, Korea, 99%), and CaCl (Samchun Co., 95%) were used for experiments. The pH of the test solution was adjusted using HCl (Samchun Co., 36%) and NaOH (Samchun Co., 99%) solutions, and deionized (DI) water was used throughout the experiments. 2.2. Preparation of Electrospun PAN Fiber PAN powder (4% (w/v)) was dissolved in DMF solution and the mixture was magnetically stirred at 60 C for 10 h to obtain a clear solution. Then, the mixture was continuously stirred overnight at room temperature to obtain a homogeneous blend. The prepared polymer solution was converted into PAN fibers using an electrospinning device that featured a 10-mL injection syringe with a stainless steel needle. PAN fibers were collected by rotating a metal drum wrapped with Al foil. The flow rate of the syringe pump was 1 mL/h and the speed of the rotating drum was 120 rpm. The distance between the rotating drum and needle tip was 12 cm, and the applied high DC electrospinning voltage was 17 kV. Appl. Sci. 2020, 10, 1738 3 of 12 2.3. Modification of PAN Fibers To prepare surface-modified PAN fibers, the PAN fibers were added to recipients that contained di erent concentrations of DETA (40%, 55%, 70%, and 85% (v/v)) solutions. Then, the mixtures were heated in a reflux system at 100 C for 1 h using Al as the catalyst until the fibers became yellow. Afterward, the fibers were washed with 0.1 M HCl, 0.1 M NaOH, and DI water in sequence to achieve neutral pH. The washed fibers were dried in a vacuum oven at 60 C for 18 h to obtain the surface-modified PAN fibers, and the resulting fibers were stored in a desiccator for further use. A field emission-scanning electron microscope (FE-SEM, S-4700, Hitachi, Japan) was used to observe the surface morphologies of the surface-modified PAN fibers. Fourier-transform infrared spectroscopy (FTIR, Nicolet iS10, Thermo Scientific, USA) was employed to identify functional groups of the samples. 2.4. Adsorption Experiments Adsorption experiments were conducted in batch systems. The required concentrations of Cu(II) were prepared by diluting the stock Cu(II) solution (1000 mg/L). Cu(II) solutions of di erent concentrations (50, 100, 150, 200, 300, and 500 mg/L) were stirred with 1.0 g/L surface-modified PAN fibers at 100 rpm and 25 C. Then, the samples were filtered and their residual Cu(II) concentrations were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5100, Agilent Technologies, USA). Kinetics experiments were conducted on the system and were obtained by adding 0.04 g adsorbent to 40 mL of 100 mg/L Cu(II) test solution at di erent reaction times (0.25, 0.5, 1, 2, 3, 6, 12, and 24 h). To analyze the e ect of the pH on the adsorption process, the pH of the solution was adjusted to 2, 3, and 4 using 1 M HCl or 1 M NaOH solutions. For the adsorbent dosage experiments, 0.02, 0.04, 0.06, 0.08, and 0.1 g surface-modified PAN fibers was added to 40 mL of 100 mg/L Cu(II) solution and the mixtures were shaken for 24 h at 100 rpm. Regeneration experiments were also carried out to observe the removal of Cu(II) ions using the surface-modified PAN fibers after they were subjected to a few regeneration cycles. Adsorption experiments were performed using 0.04 g surface-modified PAN fibers and 40 mL of 100 mg/L Cu(II) solution at pH 4. After 2 h, the adsorbed Cu was desorbed using 50 mL of 0.1 M NaOH followed by washing with DI water. Further adsorption tests were carried out under the same conditions. The adsorption capacity (q) and removal percent of Cu (%) were calculated using the following equations: q = (C C ) (1) i f and Removal (%) = (C C )  (2) i f where q (mg/g) is the amount of Cu(II) adsorbed per unit of adsorbent, C and C (mg/L) are the initial i f and final Cu(II) concentrations, respectively, V (L) is the volume of Cu(II) solution, and w (g) is the amount of adsorbent. 2.5. Adsorption Model Analyses The kinetic data were fit using pseudo-first- and pseudo-second-order kinetic models (Equations (3) and (4), respectively), and the best fit was determined using the Sigma Plot 10 software. k t q = q 1 e (3) t e and k q t 2 e q = (4) 1 + k q t where q and q (mg Cu/g) are the amounts of Cu removed at time t and at equilibrium, respectively, and t e k (1/h) and k (g/(mg Cu h)) are the pseudo-first- and pseudo-second-order rate constants, respectively. 1 2 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 12 Appl. Sci. 2020, 10, 1738 4 of 12 where qt and qe (mg Cu/g) are the amounts of Cu removed at time t and at equilibrium, respectively, and k1 (1/h) and k2 (g/(mg Cu h)) are the pseudo-first- and pseudo-second-order rate constants, respectively. The most common mathematical models used to analyze isotherm data are the Langmuir and The most common mathematical models used to analyze isotherm data are the Langmuir and Freundlich models. The Langmuir model was derived to describe adsorption on homogeneous Freundlich models. The Langmuir model was derived to describe adsorption on homogeneous adsorbent surfaces, where each adsorptive site could be occupied only once. Mathematically, the adsorbent surfaces, where each adsorptive site could be occupied only once. Mathematically, the Langmuir model can be described as follows [16]: Langmuir model can be described as follows [16]: q K C m L e q = (5) = (5) 1 + K C L e 1+ where q (mg/g) is the maximum sorption capacity of the metal ions on the adsorbent, and K (L/mg) m L where qm (mg/g) is the maximum sorption capacity of the metal ions on the adsorbent, and KL (L/mg) is the Langmuir adsorption equilibrium constant. is the Langmuir adsorption equilibrium constant. The Freundlich model is used to describe the adsorption of an absorbate on heterogeneous The Freundlich model is used to describe the adsorption of an absorbate on heterogeneous adsorbent surfaces, and its mathematical expression could be written as follows [16]: adsorbent surfaces, and its mathematical expression could be written as follows [16]: (6) q = K C n q = K C (6) where KF and n are the Freundlich constants. where K and n are the Freundlich constants. 3. Results and Discussion 3. Results and Discussion 3.1. Effect of DETA Concentration on Surface Modification of PAN Fibers Used for Cu(II) Removal 3.1. E ect of DETA Concentration on Surface Modification of PAN Fibers Used for Cu(II) Removal The adsorption capacities of the surface-modified PAN fibers obtained using four different The adsorption capacities of the surface-modified PAN fibers obtained using four di erent DETA DETA concentrations (40%, 55%, 70%, and 85%) were analyzed. As presented in Figure 1, the concentrations (40%, 55%, 70%, and 85%) were analyzed. As presented in Figure 1, the adsorption adsorption capacity of the fibers for Cu(II) increased as the concentration of DETA increased, and the capacity of the fibers for Cu(II) increased as the concentration of DETA increased, and the highest highest adsorption capacity of 78.17 mg/g was achieved when the PAN fibers modified using 85% adsorption capacity of 78.17 mg/g was achieved when the PAN fibers modified using 85% DETA were DETA were used. This was attributed to the number of nitrile groups converted to amine groups on used. This was attributed to the number of nitrile groups converted to amine groups on the surface the surface of PAN, which increased as the amount of DETA increased [17]. These results were similar of PAN, which increased as the amount of DETA increased [17]. These results were similar to those to those previously reported in the literature, where the DETA concentration of 70–80% was reported previously reported in the literature, where the DETA concentration of 70–80% was reported to be the to be the optimal concentration for the modification of modacrylic fibers that contained 40% optimal concentration for the modification of modacrylic fibers that contained 40% acrylonitrile [18]. acrylonitrile [18]. Therefore, the surface-modified PAN fibers treated with 85% DETA were used for Therefore, the surface-modified PAN fibers treated with 85% DETA were used for further tests. further tests. 40% 55% 70% 85% DETA concentrations Figure 1. Adsorption capacities (q) of surface-modified polyacrylonitrile fibers with di erent Figure 1. Adsorption capacities (q) of surface-modified polyacrylonitrile fibers with different concentrations of diethylenetriamine (DETA) for Cu(II) removal. concentrations of diethylenetriamine (DETA) for Cu(II) removal. q (mg/g) Appl. Sci. 2020, 10, 1738 5 of 12 3.2. Characterization of Surface Modification of PAN Fibers Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12 The surface morphologies of the surface-modified PAN fibers were observed by FE-SEM (Figure 2). 3.2. Characterization of Surface Modification of PAN Fibers The surface of pure PAN fiber was smooth and uniform, while the surface roughness increased as The surface morphologies of the surface-modified PAN fibers were observed by FE-SEM (Figure it reacted with DETA. The thickness of the fibers also increased due to the reaction between DETA 2). The surface of pure PAN fiber was smooth and uniform, while the surface roughness increased as and the PAN fibers [8,19]. The reaction was also confirmed by the FTIR spectra (Figure 3). The pure it reacted with DETA. The thickness of the fibers also increased due to the reaction between DETA 1 1 PAN fiber showed adsorption peaks of stretching vibrations at 1070 cm (C-O), 1450 cm (CH ), and and the PAN fibers [8,19]. The reaction was also confirmed by the FTIR spectra (Figure 3). The pure 2936 cm (CH stretching in CH, CH , and CH groups) [1,20]. The FTIR spectra of the PAN fibers −1 −1 2 3 PAN fiber showed adsorption peaks of stretching vibrations at 1070 cm (C-O), 1450 cm (CH2), and −1 showed the most distinctive bonds of the nitrile (CN) vibration at 2242 cm , which decreased with 2936 cm (CH stretching in CH, CH2, and CH3 groups) [1,20]. The FTIR spectra of the PAN fibers −1 showed the most distinctive bonds of the nitrile (C≡N) vibration at 2242 cm , which decreased with increasing DETA concentration and almost disappeared with 85% EDTA [19,21,22]. The peaks of the increasing DETA concentration and almost disappeared with 85% EDTA [19,21,22]. The peaks of the amine modified at 1561–1571 cm (Figure 3c–e) correspond to the stretching bending vibration of NH −1 amine modified at 1561–1571 cm (Figure 3c–e) correspond to the stretching bending vibration of NH groups, demonstrating the presence of primary amine groups [20]. The broad band around 3300 cm −1 groups, demonstrating the presence of primary amine groups [20]. The broad band around 3300 cm was attributed to the O-H spectrum [23]. was attributed to the O-H spectrum [23]. Figure 2. Field emission-scanning electron microscope (FE-SEM) images of (a) unmodified Figure 2. Field emission-scanning electron microscope (FE-SEM) images of (a) unmodified polyacrylonitrile (PAN) fiber and PAN fiber with different concentrations of DETA: (b) 40% DETA, polyacrylonitrile (PAN) fiber and PAN fiber with di erent concentrations of DETA: (b) 40% DETA, (c), (c), 55% DETA, (d) 70% DETA, and (e) 85% DETA. 55% DETA, (d) 70% DETA, and (e) 85% DETA. Appl. Sci. 2020, 10, 1738 6 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 Figure 3. Fourier-transform infrared spectroscopy (FTIR) spectra of (a) unmodified PAN fiber and Figure 3. Fourier-transform infrared spectroscopy (FTIR) spectra of (a) unmodified PAN fiber and PAN fiber with di erent concentration of DETA: (b) 40% DETA, (c) 55% DETA, (d) 70% DETA, and (e) PAN fiber with different concentration of DETA: (b) 40% DETA, (c) 55% DETA, (d) 70% DETA, and Figure 3. Fourier-transform infrared spectroscopy (FTIR) spectra of (a) unmodified PAN fiber and 85% DETA. (e) 85% DETA. PAN fiber with different concentration of DETA: (b) 40% DETA, (c) 55% DETA, (d) 70% DETA, and (e) 85% DETA. 3.3. Adsorption Kinetics 3.3. Adsorption Kinetics The kinetics of the Cu(II) adsorption on the surface-modified PAN fibers were analyzed using 3.3. Adsorption Kinetics The kinetics of the Cu(II) adsorption on the surface-modified PAN fibers were analyzed using pseudo-first- and pseudo-second-order kinetic models. Figure 4 depicts the plot of the experimental The kinetics of the Cu(II) adsorption on the surface-modified PAN fibers were analyzed using pseudo-first- and pseudo-second-order kinetic models. Figure 4 depicts the plot of the experimental data for the initial Cu(II) solution concentration of 100 mg/L. The calculated kinetic parameters are pseudo-first- and pseudo-second-order kinetic models. Figure 4 depicts the plot of the experimental data for the initial Cu(II) solution concentration of 100 mg/L. The calculated kinetic parameters are summarized in Table 1. The experimental data fitted the pseudo-second-order kinetic model better data for the initial Cu(II) solution concentration of 100 mg/L. The calculated kinetic parameters are summarized in Table 1. The experimental data fitted the pseudo-second-order kinetic model better than the pseudo-first-order one. These results indicated that the adsorption rate of Cu(II) on the summarized in Table 1. The experimental data fitted the pseudo-second-order kinetic model better than the pseudo-first-order one. These results indicated that the adsorption rate of Cu(II) on the surface-modified than the pseu Pd AN o-fifibers rst-orddepended er one. Thes on e r the esu adsorbate lts indicated tha concentration t the adsorpti and the on ra number te of Cof u(II) active on the sites of surface-modified PAN fibers depended on the adsorbate concentration and the number of active sites surface-modified PAN fibers depended on the adsorbate concentration and the number of active sites the adsorbent [24]. Previous studies have also revealed that the adsorption of Cu(II) on amine-modified of the adsorbent [24]. Previous studies have also revealed that the adsorption of Cu(II) on amine- of the adsorbent [24]. Previous studies have also revealed that the adsorption of Cu(II) on amine- PAN fibers followed the pseudo-second-order kinetic model [2,8,25]. modified PAN fibers followed the pseudo-second-order kinetic model [2,8,25]. modified PAN fibers followed the pseudo-second-order kinetic model [2,8,25]. Exp Exp 1st 1st 2nd 2nd 0 4 8 12162024 Time (h) 0 4 8 12162024 Figure 4. Pseudo-first-order and pseudo-second-order kinetic models (1st and 2nd, respectively) for Time (h) adsorption of Cu(II) ions onto surface-modified polyacrylonitrile fibers. Here, q and t denote the adsorption capacity and time, respectively. q (mg/g) q (mg/g) Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 12 Figure 4. Pseudo-first-order and pseudo-second-order kinetic models (1st and 2nd, respectively) for Appl. Sci. 2020, 10, 1738 7 of 12 adsorption of Cu(II) ions onto surface-modified polyacrylonitrile fibers. Here, q and t denote the adsorption capacity and time, respectively. Table 1. Kinetics parameters for adsorption of Cu(II) ions onto surface-modified polyacrylonitrile fibers. Table 1. Here, k , Kinetics parameters k , q , and R denotefor adsorption the pseudo-first- of Cu(II) and pseudo-second-or ions onto surface- derm rate odified constants, polyacry amount lonitrile of 1 2 e fibers. Here, Cu(II) removed k1, k at equilibrium, 2, qe, and R and denote the determination pseudo-first- a coecient, nd pseu respectively do-se.cond-order rate constants, amount of Cu(II) removed at equilibrium, and determination coefficient, respectively. Pseudo-First-Order Kinetic Model Pseudo-Second-Order Kinetic Model Pseudo-First-Order Kinetic Model Pseudo-Second-Order Kinetic Model k q k q 1 e 2 e 2 2 R R k1 qe k2 qe (1/h) (mg/g) (g/(mg h)) (mg/g) 2 2 R R (1/h) (mg/g) (g/(mg h)) (mg/g) 0.500 80.06 0.968 0.007 89.36 0.984 0.500 80.06 0.968 0.007 89.36 0.984 3.4. Adsorption Isotherms 3.4. Adsorption Isotherms The e ect of the equilibrium concentrations of Cu(II) ions in the testing solutions on the adsorption The effect of the equilibrium concentrations of Cu(II) ions in the testing solutions on the capacities of the surface-modified PAN fibers is illustrated in Figure 5. The figure presents the fitting of adsorption capacities of the surface-modified PAN fibers is illustrated in Figure 5. The figure presents the equilibrium data, and the obtained parameters of the adsorption isotherms are listed in Table 2. the fitting of the equilibrium data, and the obtained parameters of the adsorption isotherms are listed The adsorption capacity of the fibers increased gradually as C increased. This was attributed to in Table 2. The adsorption capacity of the fibers increased gradually as Ce increased. This was the mass transfer being the driving force of the process at higher C values [26]. The determination attributed to the mass transfer being the driving force of the process at higher Ci values [26]. The coecient of the Freundlich model was higher than that of the Langmuir model, which indicated that determination coefficient of the Freundlich model was higher than that of the Langmuir model, which the Freundlich model fitted the adsorption data better than the Langmuir one. It was inferred that the indicated that the Freundlich model fitted the adsorption data better than the Langmuir one. It was Cu(II) ions were collected in multiple layers on the heterogeneous surface of the surface-modified PAN inferred that the Cu(II) ions were collected in multiple layers on the heterogeneous surface of the fibers [24]. The maximum experimental adsorption capacity on the surface-modified PAN fibers was surface-modified PAN fibers [24]. The maximum experimental adsorption capacity on the surface- 112.3 mg/g, which was in the range with the values reported in the literature (45–151 mg/g) for the modified PAN fibers was 112.3 mg/g, which was in the range with the values reported in the literature Cu(II) adsorption capacity on amine-modified PAN fibers (Table 3) [2,8,25]. (45–151 mg/g) for the Cu(II) adsorption capacity on amine-modified PAN fibers (Table 3) [2,8,25]. Exp Freundlich Langmuir 0 100 200 300 400 C (mg/L) Figure 5. Adsorption isotherms of Cu(II) ions onto surface-modified PAN fibers. Here, q and C denote the adsorption capacity and Cu(II) equilibrium concentration, respectively. Figure 5. Adsorption isotherms of Cu(II) ions onto surface-modified PAN fibers. Here, q and Ce denote the adsorption capacity and Cu(II) equilibrium concentration, respectively. Table 2. Parameters of Langmuir and Freundlich models for adsorption of Cu(II) on surface-modified PAN fibers. Here, KL, KF, qm, n, and R denote the Langmuir and Freundlich equilibrium adsorption constants, maximum sorption capacity of metal ions on adsorbent, Freundlich constant, and determination coefficient, respectively. q (mg/g) Appl. Sci. 2020, 10, 1738 8 of 12 Table 2. Parameters of Langmuir and Freundlich models for adsorption of Cu(II) on surface-modified PAN fibers. Here, K , K , q , n, and R denote the Langmuir and Freundlich equilibrium L F m adsorption constants, maximum sorption capacity of metal ions on adsorbent, Freundlich constant, and determination coecient, respectively. Langmuir Model Freundlich Model K q K L 2 F 2 1/n R R (L/mg) (mg/g) (L/g) Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 12 0.703 99.06 0.679 47.06 0.153 0.928 Langmuir Model Freundlich Model Table 3. Comparison of the adsorption capacity (q) of di erent fibrous ion exchangers and adsorbents KL qm KF 2 2 R 1/n R for Cu(II) adsorption. (L/mg) (mg/g) (L/g) 0.703 99.06 0.679 47.06 0.153 0.928 Materials Conditions q (mg/g) Reference Table 3. Comparison of the adsorption capacity (q) of different fibrous ion exchangers and adsorbents Synthetic hydroxyapatite Initial concentration = 6355 mg/L; pH = 3 50.2 [12] for Cu(II) adsorption. Bentonite Initial concentration = 3177 mg/L; pH = 5 27.6 [13] Lime sand brick Initial concentration = 30 mg/L; pH = 5 7.0 [27] Materials Conditions q (mg/g) Reference Synthetic hydroxyapatite Initial concentration = 6355 mg/L; pH = 3 50.2 [12] Initial concentration = 1000 mg/L; pH = 6 143.5 [25] Bentonite Initial concentration = 3177 mg/L; pH = 5 27.6 [13] Initial concentration = 10 mg/L; pH = 6 45.0 [8] Surface-doped PAN fiber Lime sand brick Initial concentration = 30 mg/L; pH = 5 7.0 [27] Initial concentration = 1000 mg/L; pH = 4 150.6 [2] Initial concentration = 1000 mg/L; pH = 6 143.5 [25] Initial concentration = 500 mg/L; pH = 4 112.3 This study Initial concentration = 10 mg/L; pH = 6 45.0 [8] Surface-doped PAN fiber Initial concentration = 1000 mg/L; pH = 4 150.6 [2] 3.5. E ect of Solution pH on Cu(II) Adsorption Initial concentration = 500 mg/L; pH = 4 112.3 This study The pH value is considered one of the most important parameters during metal removal processes. 3.5. Effect of Solution pH on Cu(II) Adsorption The dependence of the adsorption of Cu(II) on surface-modified PAN fibers on the initial pH of the The pH value is considered one of the most important parameters during metal removal solution is presented in Figure 6. At low solution pH levels, the modified amine groups are protonated processes. The dependence of the adsorption of Cu(II) on surface-modified PAN fibers on the initial pH of the solution is presented in Figure 6. At low solution pH levels, the modified amine groups are and the surface of the adsorbent is more positively charged, which is not favorable for the adsorption protonated and the surface of the adsorbent is more positively charged, which is not favorable for the of metal cations [19,25]. The amount of Cu(II) ions adsorbed on the surface-modified PAN fibers adsorption of metal cations [19,25]. The amount of Cu(II) ions adsorbed on the surface-modified PAN increased significantly as the number of protonated amine groups decreased, owing to the increase fibers increased significantly as the number of protonated amine groups decreased, owing to the in the solution pH [2]. The optimum solution pH was determined to be 4, and the corresponding increase in the solution pH [2]. The optimum solution pH was determined to be 4, and the corresponding adsorption amount was 87.767 mg/g for the initial Cu(II) solution concentration of 100 adsorption amount was 87.767 mg/g for the initial Cu(II) solution concentration of 100 mg/L. mg/L. Solution pH Figure 6. E ect of solution pH on adsorption of Cu(II) ions. Here, q denotes the adsorption capacity. Figure 6. Effect of solution pH on adsorption of Cu(II) ions. Here, q denotes the adsorption capacity. 3.6. E ect of Adsorbent Dosage 3.6. Effect of Adsorbent Dosage The e ect of the surface-modified PAN fiber dose on the adsorption capacity of the surface-modified The effect of the surface-modified PAN fiber dose on the adsorption capacity of the surface- PAN fibersmodi for fi Cu(II) ed PAN fi was bers evaluated, for Cu(II) wa and s the evalu results ated, an ar d eth illustrated e results are in illu Figur stratee d in F 7. As igu the re 7surface-modified . As the q (mg/g) Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 12 Appl. Sci. 2020, 10, 1738 9 of 12 surface-modified PAN fibers were added to the Cu(II) solution, the mass of adsorbed Cu(II) per unit mass of fiber gradually decreased and the removal percentage increased. The decrease in adsorption Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 12 capacity per unit mass was attributed to the concentration of Cu(II) being insufficient to match the PAN fibers were added to the Cu(II) solution, the mass of adsorbed Cu(II) per unit mass of fiber increasing number of adsorption sites caused by the addition of fibers, and therefore some adsorption gradually decreased and the removal percentage increased. The decrease in adsorption capacity per surface-modified PAN fibers were added to the Cu(II) solution, the mass of adsorbed Cu(II) per unit sites were unsaturated [28]. The maximum percent removal reached 97.63% when the dose of 0.1 g unit mass was attributed to the concentration of Cu(II) being insucient to match the increasing mass of fiber gradually decreased and the removal percentage increased. The decrease in adsorption adsorbent was added to 40 mL of 100 mg/L Cu(II) solution. capacity per unit mass was attributed to the concentration of Cu(II) being insufficient to match the number of adsorption sites caused by the addition of fibers, and therefore some adsorption sites were increasing number of adsorption sites caused by the addition of fibers, and therefore some adsorption unsaturated [28]. The maximum percent removal reached 97.63% when the dose of 0.1 g adsorbent 120 120 sites were unsaturated [28]. The maximum percent removal reached 97.63% when the dose of 0.1 g was added to 40 mL of 100 mg/L Cu(II) solution. q (mg/g) % Removal adsorbent was added to 40 mL of 100 mg/L Cu(II) solution. 100 100 120 120 q (mg/g) % Removal 80 80 100 100 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0.02 0.04 0.06 0.08 0.1 0.02 0.04 0.06 0.08 0.1 Adsorbent dose (g) Adsorbent dose (g) Figure 7. E ect of adsorbent dose on Cu(II) adsorption. Here, q denotes the adsorption capacity. Figure 7. Effect of adsorbent dose on Cu(II) adsorption. Here, q denotes the adsorption capacity. Figure 7. Effect of adsorbent dose on Cu(II) adsorption. Here, q denotes the adsorption capacity. 3.7. E ect of Coexisting Cations 3.7. Effect of Coexisting Cations 3.7. Effect of Coexisting Cations The e ects of NaCl and CaCl as coexisting cations on the Cu(II) adsorption with the The effects of NaCl and CaCl2 as coexisting cations on the Cu(II) adsorption with the surface- surface-modified The effects o PAfN NaCl fiber and C are shown aCl2 as coexisti in Figur ng ca e 8. tThe ions on the Cu( amounts of II)adsorbed adsorption wi Cu(II) th the surfa in the pr ce- esence modified PAN fiber are shown in Figure 8. The amounts of adsorbed Cu(II) in the presence of 1 mM modified PAN fiber are shown in Figure 8. The amounts of adsorbed Cu(II) in the presence of 1 mM of 1 mM of NaCl and CaCl were 92.42 and 91.89 mg/g, respectively. When the concentrations of of NaCl and CaCl2 were 92.42 and 91.89 mg/g, respectively. When the concentrations of coexisting of NaCl and CaCl2 were 92.42 and 91.89 mg/g, respectively. When the concentrations of coexisting coexisting cations increased to 10 mM, the amounts of adsorbed Cu(II) were 92.80 and 92.26 mg/g, cations increased to 10 mM, the amounts of adsorbed Cu(II) were 92.80 and 92.26 mg/g, respectively. cations increased to 10 mM, the amounts of adsorbed Cu(II) were 92.80 and 92.26 mg/g, respectively. respectively. These increases in the adsorption capacity were due to the salting-out e ect of NaCl and These inc These inc reases in reases in the adso the adso rption cap rption cap acit acit y y were were due to due to the sal the sal tit n ig- ng- out ef out ef fect of fect of NaCl NaCl and Ca and Ca Cl2 [2 C 9,30] l2 [2 , 9,30], CaCl [29,30], causing Cu(II) ions to move into the interface of the surface-modified PAN fiber. The causing Cu(II) ions to move into the interface of the surface-modified PAN fiber. The adsorption causing Cu(II) ions to move into the interface of the surface-modified PAN fiber. The adsorption adsorption capacity increased with increasing salt concentration, but there was no di erence between capacity increased with increasing salt concentration, but there was no difference between types of capacity increased with increasing salt concentration, but there was no difference between types of types of salts at the same molar concentration. salts at the same molar concentration. salts at the same molar concentration. 0 mM 1 mM 10 mM 0 mM 1 mM 10 mM NaCl CaCl2 NaCl CaCl2 Figure 8. E ect of coexisting cations on Cu(II) adsorption. Here, q denotes the adsorption capacity. q (mg/g) q (mg/g) q (mg/g) q (mg/g) % Removal % Removal Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 12 Appl. Sci. 2020, 10, 1738 10 of 12 Figure 8. Effect of coexisting cations on Cu(II) adsorption. Here, q denotes the adsorption capacity. 3.8. Regeneration Study 3.8. Regeneration Study The reusability of the surface-modified PAN fibers was tested during three consecutive regeneration The reusability of the surface-modified PAN fibers was tested during three consecutive cycles using 0.1 M NaOH and DI water, in sequence. The adsorption capacity of the surface-modified regeneration cycles using 0.1 M NaOH and DI water, in sequence. The adsorption capacity of the PAN fibers decreased as the number of regeneration cycles increased but remained above 51 mg/g surface-modified PAN fibers decreased as the number of regeneration cycles increased but remained after three cycles (Figure 9). The decrease in adsorption capacity was associated with the degradation above 51 mg/g after three cycles (Figure 9). The decrease in adsorption capacity was associated with of the surface-modified PAN fibers during the regeneration process [19,26]. Nevertheless, the the degradation of the surface-modified PAN fibers during the regeneration process [19,26]. surface-modified PAN fibers could be easily reused for the removal of Cu(II) from aqueous solutions Nevertheless, the surface-modified PAN fibers could be easily reused for the removal of Cu(II) from after they were regenerated with NaOH. aqueous solutions after they were regenerated with NaOH. Regeneration cycles Figure 9. Cu(II) adsorption dependence on number of regeneration cycles. Here, q denotes the Figure 9. Cu(II) adsorption dependence on number of regeneration cycles. Here, q denotes the adsorption capacity. adsorption capacity. 4. Conclusions 4. Conclusions PAN fibers modified with DETA were used as adsorbents for the removal of Cu(II) from aqueous solutions. The surface-modified PAN fibers that were treated with 85% DETA presented the highest PAN fibers modified with DETA were used as adsorbents for the removal of Cu(II) from aqueous Cu(II) adsorption capacity. The adsorption of Cu(II) followed the pseudo-second-order kinetic model, solutions. The surface-modified PAN fibers that were treated with 85% DETA presented the highest and the equilibrium data fit the Freundlich isotherm model, which indicated that the adsorption was Cu(II) adsorption capacity. The adsorption of Cu(II) followed the pseudo-second-order kinetic model, a multilayer process that occurred on a heterogeneous surface. The maximum adsorption capacity and the equilibrium data fit the Freundlich isotherm model, which indicated that the adsorption was of 87.77 mg/g was achieved at pH 4. The percent removal of Cu(II) was increased from 49.63% to a multilayer process that occurred on a heterogeneous surface. The maximum adsorption capacity of 97.63% as the adsorbent dosage increased from 0.02 to 0.1 g. The surface-modified PAN fibers could be 87.77 mg/g was achieved at pH 4. The percent removal of Cu(II) was increased from 49.63% to 97.63% regenerated with NaOH solution. These results indicated that the surface-modified PAN fibers could as the adsorbent dosage increased from 0.02 to 0.1 g. The surface-modified PAN fibers could be be used as adsorbent for the removal of Cu(II) from aqueous solutions. regenerated with NaOH solution. These results indicated that the surface-modified PAN fibers could be used as adsorbent for the removal of Cu(II) from aqueous solutions. Author Contributions: Conceptualization, methodology, formal analysis, writing—original draft preparation, K.T.A. and C.-G.L.; formal analysis, validation, visualization, S.-H.H.; conceptualization, supervision, Author Contributions: Conceptualization, methodology, formal analysis, writing—original draft preparation, writing—review and editing, S.-J.P. All authors have read and agreed to the published version of the manuscript. K.T.A. and C.-G.L.; formal analysis, validation, visualization, S.-H.H.; conceptualization, supervision, writing— review and Funding: This editring, S.-J.P. esearch was All supp auth orted ors have read and by the Basic ag Science reed to the Resear pu chblish Program ed ver thr sion of ough th the e m National anuscript. Resear ch Foundation (NRF) of Korea funded by the Ministry of Education (Grant no. NRF-2017R1D1A1B03030649) and by Funding: This research was supported by the Basic Science Research Program through the National Research the National Research Foundation (NRF) of Korea (Grant no. NRF-2018R1C1B5044937). Foundation (NRF) of Korea funded by the Ministry of Education (Grant no. NRF-2017R1D1A1B03030649) and Conflicts of Interest: The authors declare no conflict of interest. by the National Research Foundation (NRF) of Korea (Grant no. NRF-2018R1C1B5044937). Conflicts of Interest: The authors declare no conflict of interest. References 1. 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Removal of Cu(II) from Aqueous Solutions Using Amine-Doped Polyacrylonitrile Fibers

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applied sciences Article Removal of Cu(II) from Aqueous Solutions Using Amine-Doped Polyacrylonitrile Fibers 1 1 1 , 2 , Kay Thwe Aung , Seung-Hee Hong , Seong-Jik Park * and Chang-Gu Lee * Department of Bioresources and Rural System Engineering, Hankyong National University, Anseong 17579, Korea; kaythweag11@gmail.com (K.T.A.); trea1568@naver.com (S.-H.H.) Department of Environmental and Safety Engineering, Ajou University, Suwon 16499, Korea * Correspondence: parkseongjik@hknu.ac.kr (S.-J.P.); changgu@ajou.ac.kr (C.-G.L.); Tel.: +82-31-670-5131 (S.-J.P.); +82-31-219-2405 (C.-G.L.) Received: 4 February 2020; Accepted: 24 February 2020; Published: 3 March 2020 Abstract: Polyacrylonitrile (PAN) fibers were prepared via electrospinning and were modified with diethylenetriamine (DETA) to fabricate surface-modified PAN fibers. The surface-modified PAN fibers were used to evaluate their adsorption capacity for the removal of Cu(II) from aqueous solutions. Batch adsorption experiments were performed to examine the e ects of the modification process, initial concentration, initial pH, and adsorbent dose on the adsorption of Cu(II). Kinetic analysis revealed that the experimental data fitted the pseudo-second-order kinetic model better than the pseudo-first-order model. Adsorption equilibrium studies were conducted using the Freundlich and Langmuir isotherm models, and the findings indicated that the PAN fibers modified with 85% DETA presented the highest adsorption capacity for Cu(II) of all analyzed samples. Moreover, the results revealed that the Freundlich model was more appropriate than the Langmuir one for describing the adsorption of Cu(II) onto the modified fibers at various initial Cu(II) concentrations. The maximum adsorption capacity was determined to be 87.77 mg/g at pH 4, and the percent removal of Cu(II) increased as the amount of adsorbent increased. Furthermore, the surface-modified PAN fibers could be easily regenerated using NaOH solution. Therefore, surface-modified PAN fibers could be used as adsorbents for the removal of Cu(II) from aqueous solutions. Keywords: polyacrylonitrile; electrospun fibers; Cu(II) removal; batch experiments; adsorption models 1. Introduction Environmental problems a ect humans, animals, and plants worldwide. In particular, rapid industrialization has increased water pollution, and thus water sanitation has become a significant global concern. Owing to the increasing environmental and economic impacts of heavy metal ions, their removal from industrial wastewater has been studied in depth [1]. Heavy metals, such as Cr, Cu, Fe, Pb, Ag, Ni, and Zn, which are highly toxic and nonbiodegradable, could cause problems for both the environment and living organisms [2]. The presence of heavy metal pollutants in water could lead to serious health problems, such as stomach aches, nausea, dehydration, dizziness, nervous system damage in young children, skin rashes, vomiting, abdominal pain, lung irritation, eye irritation, and liver damage [3]. Cu is not only an essential nutrient but also a water contaminant. Moreover, Cu has many commercial and industrial applications; it can be used to manufacture pipes, valves, and fittings, and could also be a constituent of di erent types of alloys and coatings. Copper(II) sulfate pentahydrate (CuSO 5H O) is sometimes added to surface waters to control the growth of algae [4]. The typical 4 2 oxidation states of Cu are +1 and +2, and the corresponding compounds are referred to as cuprous and cupric, respectively [5]. Cu compounds, whether organic complexes or organometallics, could be Appl. Sci. 2020, 10, 1738; doi:10.3390/app10051738 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 1738 2 of 12 used to promote or catalyze numerous chemical and biological processes [6]. Copper(I) and copper(II) sulfides are important examples of copper sulfides. CuSO 5H O is the most frequently used Cu 4 2 compound in the laboratory. Highly toxic amounts of Cu are present in the industrial wastewaters of the metal cleaning and plating facilities, refineries, paper and pulp plants, fertilizer mills, and wood preservative factories [7]. Furthermore, exposure to environmental sources of Cu could cause vomiting, headaches, stomach cramps, nausea, liver and kidney damage, and even death [8]. Cu is found in drinking water at concentrations in the range of 0.005 to 30 mg/L, and it mainly originates from the corrosion of Cu plumbing [9]. The concentration of Cu in treated water often increases during distribution, particularly in systems where the pH of the water is acidic or basic, or where the water is high in carbonates [4]. According to the U.S. Environmental Protection Agency, the maximum allowable limits for Cu in drinking and industrial waters are 1.3 and 3.0 mg/L, respectively [10]. Several methods could be used to reduce the amounts of heavy metal from euents, including chemical precipitation, electrodialysis, reverse osmosis, ion exchange, evaporation, and adsorption [8]. Of these, adsorption is an e ective and economical method of wastewater treatment [2]. Many adsorbents, such as activated carbon, biosorbents, illite–smectite, natural zeolites, natural bentonites, oxide minerals, polymer materials, resins, and synthetic hydroxyapatite have been used for the removal of metal ions from wastewaters [11–13]. Recently, electrospinning has attracted significant attention in academic research circles and for industrial applications because it (1) could be used to fabricate continuous fibers with diameters as small as a few nanometers; (2) could be applied to a wide range of materials, such as synthetic and natural polymers, metals, and ceramics and composite systems; and (3) could be a low-cost and high-yield method for the preparation of nanofibers [14]. Modified electrospun nanofibers have been developed as adsorbents for metal ions from solutions because they have high porosity, high specific surface area, and controllable surface functionality [15]. In this study, polyacrylonitrile (PAN) fibers were prepared via electrospinning, and diethylenetriamine (DETA) was used to modify the PAN fibers. The purpose of the study was to fabricate surface-modified PAN fibers for the removal of heavy metals from aqueous solutions. The e ects of the pH, contact time, and initial concentration of the Cu(II) solution on the adsorption capacity of the surface-modified PAN fibers for Cu(II) ions were examined. Adsorption kinetics and isotherm experiments were performed to analyze the experimental data. 2. Materials and Methods 2.1. Reagents PAN powder (Sigma-Aldrich, Mw = 150,000 g/mol) and dimethylformamide (DMF, Sigma-Aldrich, Steinheim, Germany, 99.8%) were used to prepare the polymer spinning solution, and DETA (ACROS Organics Co., Geel, Belgium, 98.5%) and aluminum chloride hexahydrate (AlCl 6H O, Sigma-Aldrich, 3 2 99%) were used as the modifying agent and catalyst, respectively. Analytical grade CuSO 5H O 4 2 (Daejung Co., Siheung, Korea, 99%), NaCl (Samchun Co., Seoul, Korea, 99%), and CaCl (Samchun Co., 95%) were used for experiments. The pH of the test solution was adjusted using HCl (Samchun Co., 36%) and NaOH (Samchun Co., 99%) solutions, and deionized (DI) water was used throughout the experiments. 2.2. Preparation of Electrospun PAN Fiber PAN powder (4% (w/v)) was dissolved in DMF solution and the mixture was magnetically stirred at 60 C for 10 h to obtain a clear solution. Then, the mixture was continuously stirred overnight at room temperature to obtain a homogeneous blend. The prepared polymer solution was converted into PAN fibers using an electrospinning device that featured a 10-mL injection syringe with a stainless steel needle. PAN fibers were collected by rotating a metal drum wrapped with Al foil. The flow rate of the syringe pump was 1 mL/h and the speed of the rotating drum was 120 rpm. The distance between the rotating drum and needle tip was 12 cm, and the applied high DC electrospinning voltage was 17 kV. Appl. Sci. 2020, 10, 1738 3 of 12 2.3. Modification of PAN Fibers To prepare surface-modified PAN fibers, the PAN fibers were added to recipients that contained di erent concentrations of DETA (40%, 55%, 70%, and 85% (v/v)) solutions. Then, the mixtures were heated in a reflux system at 100 C for 1 h using Al as the catalyst until the fibers became yellow. Afterward, the fibers were washed with 0.1 M HCl, 0.1 M NaOH, and DI water in sequence to achieve neutral pH. The washed fibers were dried in a vacuum oven at 60 C for 18 h to obtain the surface-modified PAN fibers, and the resulting fibers were stored in a desiccator for further use. A field emission-scanning electron microscope (FE-SEM, S-4700, Hitachi, Japan) was used to observe the surface morphologies of the surface-modified PAN fibers. Fourier-transform infrared spectroscopy (FTIR, Nicolet iS10, Thermo Scientific, USA) was employed to identify functional groups of the samples. 2.4. Adsorption Experiments Adsorption experiments were conducted in batch systems. The required concentrations of Cu(II) were prepared by diluting the stock Cu(II) solution (1000 mg/L). Cu(II) solutions of di erent concentrations (50, 100, 150, 200, 300, and 500 mg/L) were stirred with 1.0 g/L surface-modified PAN fibers at 100 rpm and 25 C. Then, the samples were filtered and their residual Cu(II) concentrations were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5100, Agilent Technologies, USA). Kinetics experiments were conducted on the system and were obtained by adding 0.04 g adsorbent to 40 mL of 100 mg/L Cu(II) test solution at di erent reaction times (0.25, 0.5, 1, 2, 3, 6, 12, and 24 h). To analyze the e ect of the pH on the adsorption process, the pH of the solution was adjusted to 2, 3, and 4 using 1 M HCl or 1 M NaOH solutions. For the adsorbent dosage experiments, 0.02, 0.04, 0.06, 0.08, and 0.1 g surface-modified PAN fibers was added to 40 mL of 100 mg/L Cu(II) solution and the mixtures were shaken for 24 h at 100 rpm. Regeneration experiments were also carried out to observe the removal of Cu(II) ions using the surface-modified PAN fibers after they were subjected to a few regeneration cycles. Adsorption experiments were performed using 0.04 g surface-modified PAN fibers and 40 mL of 100 mg/L Cu(II) solution at pH 4. After 2 h, the adsorbed Cu was desorbed using 50 mL of 0.1 M NaOH followed by washing with DI water. Further adsorption tests were carried out under the same conditions. The adsorption capacity (q) and removal percent of Cu (%) were calculated using the following equations: q = (C C ) (1) i f and Removal (%) = (C C )  (2) i f where q (mg/g) is the amount of Cu(II) adsorbed per unit of adsorbent, C and C (mg/L) are the initial i f and final Cu(II) concentrations, respectively, V (L) is the volume of Cu(II) solution, and w (g) is the amount of adsorbent. 2.5. Adsorption Model Analyses The kinetic data were fit using pseudo-first- and pseudo-second-order kinetic models (Equations (3) and (4), respectively), and the best fit was determined using the Sigma Plot 10 software. k t q = q 1 e (3) t e and k q t 2 e q = (4) 1 + k q t where q and q (mg Cu/g) are the amounts of Cu removed at time t and at equilibrium, respectively, and t e k (1/h) and k (g/(mg Cu h)) are the pseudo-first- and pseudo-second-order rate constants, respectively. 1 2 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 12 Appl. Sci. 2020, 10, 1738 4 of 12 where qt and qe (mg Cu/g) are the amounts of Cu removed at time t and at equilibrium, respectively, and k1 (1/h) and k2 (g/(mg Cu h)) are the pseudo-first- and pseudo-second-order rate constants, respectively. The most common mathematical models used to analyze isotherm data are the Langmuir and The most common mathematical models used to analyze isotherm data are the Langmuir and Freundlich models. The Langmuir model was derived to describe adsorption on homogeneous Freundlich models. The Langmuir model was derived to describe adsorption on homogeneous adsorbent surfaces, where each adsorptive site could be occupied only once. Mathematically, the adsorbent surfaces, where each adsorptive site could be occupied only once. Mathematically, the Langmuir model can be described as follows [16]: Langmuir model can be described as follows [16]: q K C m L e q = (5) = (5) 1 + K C L e 1+ where q (mg/g) is the maximum sorption capacity of the metal ions on the adsorbent, and K (L/mg) m L where qm (mg/g) is the maximum sorption capacity of the metal ions on the adsorbent, and KL (L/mg) is the Langmuir adsorption equilibrium constant. is the Langmuir adsorption equilibrium constant. The Freundlich model is used to describe the adsorption of an absorbate on heterogeneous The Freundlich model is used to describe the adsorption of an absorbate on heterogeneous adsorbent surfaces, and its mathematical expression could be written as follows [16]: adsorbent surfaces, and its mathematical expression could be written as follows [16]: (6) q = K C n q = K C (6) where KF and n are the Freundlich constants. where K and n are the Freundlich constants. 3. Results and Discussion 3. Results and Discussion 3.1. Effect of DETA Concentration on Surface Modification of PAN Fibers Used for Cu(II) Removal 3.1. E ect of DETA Concentration on Surface Modification of PAN Fibers Used for Cu(II) Removal The adsorption capacities of the surface-modified PAN fibers obtained using four different The adsorption capacities of the surface-modified PAN fibers obtained using four di erent DETA DETA concentrations (40%, 55%, 70%, and 85%) were analyzed. As presented in Figure 1, the concentrations (40%, 55%, 70%, and 85%) were analyzed. As presented in Figure 1, the adsorption adsorption capacity of the fibers for Cu(II) increased as the concentration of DETA increased, and the capacity of the fibers for Cu(II) increased as the concentration of DETA increased, and the highest highest adsorption capacity of 78.17 mg/g was achieved when the PAN fibers modified using 85% adsorption capacity of 78.17 mg/g was achieved when the PAN fibers modified using 85% DETA were DETA were used. This was attributed to the number of nitrile groups converted to amine groups on used. This was attributed to the number of nitrile groups converted to amine groups on the surface the surface of PAN, which increased as the amount of DETA increased [17]. These results were similar of PAN, which increased as the amount of DETA increased [17]. These results were similar to those to those previously reported in the literature, where the DETA concentration of 70–80% was reported previously reported in the literature, where the DETA concentration of 70–80% was reported to be the to be the optimal concentration for the modification of modacrylic fibers that contained 40% optimal concentration for the modification of modacrylic fibers that contained 40% acrylonitrile [18]. acrylonitrile [18]. Therefore, the surface-modified PAN fibers treated with 85% DETA were used for Therefore, the surface-modified PAN fibers treated with 85% DETA were used for further tests. further tests. 40% 55% 70% 85% DETA concentrations Figure 1. Adsorption capacities (q) of surface-modified polyacrylonitrile fibers with di erent Figure 1. Adsorption capacities (q) of surface-modified polyacrylonitrile fibers with different concentrations of diethylenetriamine (DETA) for Cu(II) removal. concentrations of diethylenetriamine (DETA) for Cu(II) removal. q (mg/g) Appl. Sci. 2020, 10, 1738 5 of 12 3.2. Characterization of Surface Modification of PAN Fibers Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12 The surface morphologies of the surface-modified PAN fibers were observed by FE-SEM (Figure 2). 3.2. Characterization of Surface Modification of PAN Fibers The surface of pure PAN fiber was smooth and uniform, while the surface roughness increased as The surface morphologies of the surface-modified PAN fibers were observed by FE-SEM (Figure it reacted with DETA. The thickness of the fibers also increased due to the reaction between DETA 2). The surface of pure PAN fiber was smooth and uniform, while the surface roughness increased as and the PAN fibers [8,19]. The reaction was also confirmed by the FTIR spectra (Figure 3). The pure it reacted with DETA. The thickness of the fibers also increased due to the reaction between DETA 1 1 PAN fiber showed adsorption peaks of stretching vibrations at 1070 cm (C-O), 1450 cm (CH ), and and the PAN fibers [8,19]. The reaction was also confirmed by the FTIR spectra (Figure 3). The pure 2936 cm (CH stretching in CH, CH , and CH groups) [1,20]. The FTIR spectra of the PAN fibers −1 −1 2 3 PAN fiber showed adsorption peaks of stretching vibrations at 1070 cm (C-O), 1450 cm (CH2), and −1 showed the most distinctive bonds of the nitrile (CN) vibration at 2242 cm , which decreased with 2936 cm (CH stretching in CH, CH2, and CH3 groups) [1,20]. The FTIR spectra of the PAN fibers −1 showed the most distinctive bonds of the nitrile (C≡N) vibration at 2242 cm , which decreased with increasing DETA concentration and almost disappeared with 85% EDTA [19,21,22]. The peaks of the increasing DETA concentration and almost disappeared with 85% EDTA [19,21,22]. The peaks of the amine modified at 1561–1571 cm (Figure 3c–e) correspond to the stretching bending vibration of NH −1 amine modified at 1561–1571 cm (Figure 3c–e) correspond to the stretching bending vibration of NH groups, demonstrating the presence of primary amine groups [20]. The broad band around 3300 cm −1 groups, demonstrating the presence of primary amine groups [20]. The broad band around 3300 cm was attributed to the O-H spectrum [23]. was attributed to the O-H spectrum [23]. Figure 2. Field emission-scanning electron microscope (FE-SEM) images of (a) unmodified Figure 2. Field emission-scanning electron microscope (FE-SEM) images of (a) unmodified polyacrylonitrile (PAN) fiber and PAN fiber with different concentrations of DETA: (b) 40% DETA, polyacrylonitrile (PAN) fiber and PAN fiber with di erent concentrations of DETA: (b) 40% DETA, (c), (c), 55% DETA, (d) 70% DETA, and (e) 85% DETA. 55% DETA, (d) 70% DETA, and (e) 85% DETA. Appl. Sci. 2020, 10, 1738 6 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 Figure 3. Fourier-transform infrared spectroscopy (FTIR) spectra of (a) unmodified PAN fiber and Figure 3. Fourier-transform infrared spectroscopy (FTIR) spectra of (a) unmodified PAN fiber and PAN fiber with di erent concentration of DETA: (b) 40% DETA, (c) 55% DETA, (d) 70% DETA, and (e) PAN fiber with different concentration of DETA: (b) 40% DETA, (c) 55% DETA, (d) 70% DETA, and Figure 3. Fourier-transform infrared spectroscopy (FTIR) spectra of (a) unmodified PAN fiber and 85% DETA. (e) 85% DETA. PAN fiber with different concentration of DETA: (b) 40% DETA, (c) 55% DETA, (d) 70% DETA, and (e) 85% DETA. 3.3. Adsorption Kinetics 3.3. Adsorption Kinetics The kinetics of the Cu(II) adsorption on the surface-modified PAN fibers were analyzed using 3.3. Adsorption Kinetics The kinetics of the Cu(II) adsorption on the surface-modified PAN fibers were analyzed using pseudo-first- and pseudo-second-order kinetic models. Figure 4 depicts the plot of the experimental The kinetics of the Cu(II) adsorption on the surface-modified PAN fibers were analyzed using pseudo-first- and pseudo-second-order kinetic models. Figure 4 depicts the plot of the experimental data for the initial Cu(II) solution concentration of 100 mg/L. The calculated kinetic parameters are pseudo-first- and pseudo-second-order kinetic models. Figure 4 depicts the plot of the experimental data for the initial Cu(II) solution concentration of 100 mg/L. The calculated kinetic parameters are summarized in Table 1. The experimental data fitted the pseudo-second-order kinetic model better data for the initial Cu(II) solution concentration of 100 mg/L. The calculated kinetic parameters are summarized in Table 1. The experimental data fitted the pseudo-second-order kinetic model better than the pseudo-first-order one. These results indicated that the adsorption rate of Cu(II) on the summarized in Table 1. The experimental data fitted the pseudo-second-order kinetic model better than the pseudo-first-order one. These results indicated that the adsorption rate of Cu(II) on the surface-modified than the pseu Pd AN o-fifibers rst-orddepended er one. Thes on e r the esu adsorbate lts indicated tha concentration t the adsorpti and the on ra number te of Cof u(II) active on the sites of surface-modified PAN fibers depended on the adsorbate concentration and the number of active sites surface-modified PAN fibers depended on the adsorbate concentration and the number of active sites the adsorbent [24]. Previous studies have also revealed that the adsorption of Cu(II) on amine-modified of the adsorbent [24]. Previous studies have also revealed that the adsorption of Cu(II) on amine- of the adsorbent [24]. Previous studies have also revealed that the adsorption of Cu(II) on amine- PAN fibers followed the pseudo-second-order kinetic model [2,8,25]. modified PAN fibers followed the pseudo-second-order kinetic model [2,8,25]. modified PAN fibers followed the pseudo-second-order kinetic model [2,8,25]. Exp Exp 1st 1st 2nd 2nd 0 4 8 12162024 Time (h) 0 4 8 12162024 Figure 4. Pseudo-first-order and pseudo-second-order kinetic models (1st and 2nd, respectively) for Time (h) adsorption of Cu(II) ions onto surface-modified polyacrylonitrile fibers. Here, q and t denote the adsorption capacity and time, respectively. q (mg/g) q (mg/g) Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 12 Figure 4. Pseudo-first-order and pseudo-second-order kinetic models (1st and 2nd, respectively) for Appl. Sci. 2020, 10, 1738 7 of 12 adsorption of Cu(II) ions onto surface-modified polyacrylonitrile fibers. Here, q and t denote the adsorption capacity and time, respectively. Table 1. Kinetics parameters for adsorption of Cu(II) ions onto surface-modified polyacrylonitrile fibers. Table 1. Here, k , Kinetics parameters k , q , and R denotefor adsorption the pseudo-first- of Cu(II) and pseudo-second-or ions onto surface- derm rate odified constants, polyacry amount lonitrile of 1 2 e fibers. Here, Cu(II) removed k1, k at equilibrium, 2, qe, and R and denote the determination pseudo-first- a coecient, nd pseu respectively do-se.cond-order rate constants, amount of Cu(II) removed at equilibrium, and determination coefficient, respectively. Pseudo-First-Order Kinetic Model Pseudo-Second-Order Kinetic Model Pseudo-First-Order Kinetic Model Pseudo-Second-Order Kinetic Model k q k q 1 e 2 e 2 2 R R k1 qe k2 qe (1/h) (mg/g) (g/(mg h)) (mg/g) 2 2 R R (1/h) (mg/g) (g/(mg h)) (mg/g) 0.500 80.06 0.968 0.007 89.36 0.984 0.500 80.06 0.968 0.007 89.36 0.984 3.4. Adsorption Isotherms 3.4. Adsorption Isotherms The e ect of the equilibrium concentrations of Cu(II) ions in the testing solutions on the adsorption The effect of the equilibrium concentrations of Cu(II) ions in the testing solutions on the capacities of the surface-modified PAN fibers is illustrated in Figure 5. The figure presents the fitting of adsorption capacities of the surface-modified PAN fibers is illustrated in Figure 5. The figure presents the equilibrium data, and the obtained parameters of the adsorption isotherms are listed in Table 2. the fitting of the equilibrium data, and the obtained parameters of the adsorption isotherms are listed The adsorption capacity of the fibers increased gradually as C increased. This was attributed to in Table 2. The adsorption capacity of the fibers increased gradually as Ce increased. This was the mass transfer being the driving force of the process at higher C values [26]. The determination attributed to the mass transfer being the driving force of the process at higher Ci values [26]. The coecient of the Freundlich model was higher than that of the Langmuir model, which indicated that determination coefficient of the Freundlich model was higher than that of the Langmuir model, which the Freundlich model fitted the adsorption data better than the Langmuir one. It was inferred that the indicated that the Freundlich model fitted the adsorption data better than the Langmuir one. It was Cu(II) ions were collected in multiple layers on the heterogeneous surface of the surface-modified PAN inferred that the Cu(II) ions were collected in multiple layers on the heterogeneous surface of the fibers [24]. The maximum experimental adsorption capacity on the surface-modified PAN fibers was surface-modified PAN fibers [24]. The maximum experimental adsorption capacity on the surface- 112.3 mg/g, which was in the range with the values reported in the literature (45–151 mg/g) for the modified PAN fibers was 112.3 mg/g, which was in the range with the values reported in the literature Cu(II) adsorption capacity on amine-modified PAN fibers (Table 3) [2,8,25]. (45–151 mg/g) for the Cu(II) adsorption capacity on amine-modified PAN fibers (Table 3) [2,8,25]. Exp Freundlich Langmuir 0 100 200 300 400 C (mg/L) Figure 5. Adsorption isotherms of Cu(II) ions onto surface-modified PAN fibers. Here, q and C denote the adsorption capacity and Cu(II) equilibrium concentration, respectively. Figure 5. Adsorption isotherms of Cu(II) ions onto surface-modified PAN fibers. Here, q and Ce denote the adsorption capacity and Cu(II) equilibrium concentration, respectively. Table 2. Parameters of Langmuir and Freundlich models for adsorption of Cu(II) on surface-modified PAN fibers. Here, KL, KF, qm, n, and R denote the Langmuir and Freundlich equilibrium adsorption constants, maximum sorption capacity of metal ions on adsorbent, Freundlich constant, and determination coefficient, respectively. q (mg/g) Appl. Sci. 2020, 10, 1738 8 of 12 Table 2. Parameters of Langmuir and Freundlich models for adsorption of Cu(II) on surface-modified PAN fibers. Here, K , K , q , n, and R denote the Langmuir and Freundlich equilibrium L F m adsorption constants, maximum sorption capacity of metal ions on adsorbent, Freundlich constant, and determination coecient, respectively. Langmuir Model Freundlich Model K q K L 2 F 2 1/n R R (L/mg) (mg/g) (L/g) Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 12 0.703 99.06 0.679 47.06 0.153 0.928 Langmuir Model Freundlich Model Table 3. Comparison of the adsorption capacity (q) of di erent fibrous ion exchangers and adsorbents KL qm KF 2 2 R 1/n R for Cu(II) adsorption. (L/mg) (mg/g) (L/g) 0.703 99.06 0.679 47.06 0.153 0.928 Materials Conditions q (mg/g) Reference Table 3. Comparison of the adsorption capacity (q) of different fibrous ion exchangers and adsorbents Synthetic hydroxyapatite Initial concentration = 6355 mg/L; pH = 3 50.2 [12] for Cu(II) adsorption. Bentonite Initial concentration = 3177 mg/L; pH = 5 27.6 [13] Lime sand brick Initial concentration = 30 mg/L; pH = 5 7.0 [27] Materials Conditions q (mg/g) Reference Synthetic hydroxyapatite Initial concentration = 6355 mg/L; pH = 3 50.2 [12] Initial concentration = 1000 mg/L; pH = 6 143.5 [25] Bentonite Initial concentration = 3177 mg/L; pH = 5 27.6 [13] Initial concentration = 10 mg/L; pH = 6 45.0 [8] Surface-doped PAN fiber Lime sand brick Initial concentration = 30 mg/L; pH = 5 7.0 [27] Initial concentration = 1000 mg/L; pH = 4 150.6 [2] Initial concentration = 1000 mg/L; pH = 6 143.5 [25] Initial concentration = 500 mg/L; pH = 4 112.3 This study Initial concentration = 10 mg/L; pH = 6 45.0 [8] Surface-doped PAN fiber Initial concentration = 1000 mg/L; pH = 4 150.6 [2] 3.5. E ect of Solution pH on Cu(II) Adsorption Initial concentration = 500 mg/L; pH = 4 112.3 This study The pH value is considered one of the most important parameters during metal removal processes. 3.5. Effect of Solution pH on Cu(II) Adsorption The dependence of the adsorption of Cu(II) on surface-modified PAN fibers on the initial pH of the The pH value is considered one of the most important parameters during metal removal solution is presented in Figure 6. At low solution pH levels, the modified amine groups are protonated processes. The dependence of the adsorption of Cu(II) on surface-modified PAN fibers on the initial pH of the solution is presented in Figure 6. At low solution pH levels, the modified amine groups are and the surface of the adsorbent is more positively charged, which is not favorable for the adsorption protonated and the surface of the adsorbent is more positively charged, which is not favorable for the of metal cations [19,25]. The amount of Cu(II) ions adsorbed on the surface-modified PAN fibers adsorption of metal cations [19,25]. The amount of Cu(II) ions adsorbed on the surface-modified PAN increased significantly as the number of protonated amine groups decreased, owing to the increase fibers increased significantly as the number of protonated amine groups decreased, owing to the in the solution pH [2]. The optimum solution pH was determined to be 4, and the corresponding increase in the solution pH [2]. The optimum solution pH was determined to be 4, and the corresponding adsorption amount was 87.767 mg/g for the initial Cu(II) solution concentration of 100 adsorption amount was 87.767 mg/g for the initial Cu(II) solution concentration of 100 mg/L. mg/L. Solution pH Figure 6. E ect of solution pH on adsorption of Cu(II) ions. Here, q denotes the adsorption capacity. Figure 6. Effect of solution pH on adsorption of Cu(II) ions. Here, q denotes the adsorption capacity. 3.6. E ect of Adsorbent Dosage 3.6. Effect of Adsorbent Dosage The e ect of the surface-modified PAN fiber dose on the adsorption capacity of the surface-modified The effect of the surface-modified PAN fiber dose on the adsorption capacity of the surface- PAN fibersmodi for fi Cu(II) ed PAN fi was bers evaluated, for Cu(II) wa and s the evalu results ated, an ar d eth illustrated e results are in illu Figur stratee d in F 7. As igu the re 7surface-modified . As the q (mg/g) Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 12 Appl. Sci. 2020, 10, 1738 9 of 12 surface-modified PAN fibers were added to the Cu(II) solution, the mass of adsorbed Cu(II) per unit mass of fiber gradually decreased and the removal percentage increased. The decrease in adsorption Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 12 capacity per unit mass was attributed to the concentration of Cu(II) being insufficient to match the PAN fibers were added to the Cu(II) solution, the mass of adsorbed Cu(II) per unit mass of fiber increasing number of adsorption sites caused by the addition of fibers, and therefore some adsorption gradually decreased and the removal percentage increased. The decrease in adsorption capacity per surface-modified PAN fibers were added to the Cu(II) solution, the mass of adsorbed Cu(II) per unit sites were unsaturated [28]. The maximum percent removal reached 97.63% when the dose of 0.1 g unit mass was attributed to the concentration of Cu(II) being insucient to match the increasing mass of fiber gradually decreased and the removal percentage increased. The decrease in adsorption adsorbent was added to 40 mL of 100 mg/L Cu(II) solution. capacity per unit mass was attributed to the concentration of Cu(II) being insufficient to match the number of adsorption sites caused by the addition of fibers, and therefore some adsorption sites were increasing number of adsorption sites caused by the addition of fibers, and therefore some adsorption unsaturated [28]. The maximum percent removal reached 97.63% when the dose of 0.1 g adsorbent 120 120 sites were unsaturated [28]. The maximum percent removal reached 97.63% when the dose of 0.1 g was added to 40 mL of 100 mg/L Cu(II) solution. q (mg/g) % Removal adsorbent was added to 40 mL of 100 mg/L Cu(II) solution. 100 100 120 120 q (mg/g) % Removal 80 80 100 100 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0.02 0.04 0.06 0.08 0.1 0.02 0.04 0.06 0.08 0.1 Adsorbent dose (g) Adsorbent dose (g) Figure 7. E ect of adsorbent dose on Cu(II) adsorption. Here, q denotes the adsorption capacity. Figure 7. Effect of adsorbent dose on Cu(II) adsorption. Here, q denotes the adsorption capacity. Figure 7. Effect of adsorbent dose on Cu(II) adsorption. Here, q denotes the adsorption capacity. 3.7. E ect of Coexisting Cations 3.7. Effect of Coexisting Cations 3.7. Effect of Coexisting Cations The e ects of NaCl and CaCl as coexisting cations on the Cu(II) adsorption with the The effects of NaCl and CaCl2 as coexisting cations on the Cu(II) adsorption with the surface- surface-modified The effects o PAfN NaCl fiber and C are shown aCl2 as coexisti in Figur ng ca e 8. tThe ions on the Cu( amounts of II)adsorbed adsorption wi Cu(II) th the surfa in the pr ce- esence modified PAN fiber are shown in Figure 8. The amounts of adsorbed Cu(II) in the presence of 1 mM modified PAN fiber are shown in Figure 8. The amounts of adsorbed Cu(II) in the presence of 1 mM of 1 mM of NaCl and CaCl were 92.42 and 91.89 mg/g, respectively. When the concentrations of of NaCl and CaCl2 were 92.42 and 91.89 mg/g, respectively. When the concentrations of coexisting of NaCl and CaCl2 were 92.42 and 91.89 mg/g, respectively. When the concentrations of coexisting coexisting cations increased to 10 mM, the amounts of adsorbed Cu(II) were 92.80 and 92.26 mg/g, cations increased to 10 mM, the amounts of adsorbed Cu(II) were 92.80 and 92.26 mg/g, respectively. cations increased to 10 mM, the amounts of adsorbed Cu(II) were 92.80 and 92.26 mg/g, respectively. respectively. These increases in the adsorption capacity were due to the salting-out e ect of NaCl and These inc These inc reases in reases in the adso the adso rption cap rption cap acit acit y y were were due to due to the sal the sal tit n ig- ng- out ef out ef fect of fect of NaCl NaCl and Ca and Ca Cl2 [2 C 9,30] l2 [2 , 9,30], CaCl [29,30], causing Cu(II) ions to move into the interface of the surface-modified PAN fiber. The causing Cu(II) ions to move into the interface of the surface-modified PAN fiber. The adsorption causing Cu(II) ions to move into the interface of the surface-modified PAN fiber. The adsorption adsorption capacity increased with increasing salt concentration, but there was no di erence between capacity increased with increasing salt concentration, but there was no difference between types of capacity increased with increasing salt concentration, but there was no difference between types of types of salts at the same molar concentration. salts at the same molar concentration. salts at the same molar concentration. 0 mM 1 mM 10 mM 0 mM 1 mM 10 mM NaCl CaCl2 NaCl CaCl2 Figure 8. E ect of coexisting cations on Cu(II) adsorption. Here, q denotes the adsorption capacity. q (mg/g) q (mg/g) q (mg/g) q (mg/g) % Removal % Removal Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 12 Appl. Sci. 2020, 10, 1738 10 of 12 Figure 8. Effect of coexisting cations on Cu(II) adsorption. Here, q denotes the adsorption capacity. 3.8. Regeneration Study 3.8. Regeneration Study The reusability of the surface-modified PAN fibers was tested during three consecutive regeneration The reusability of the surface-modified PAN fibers was tested during three consecutive cycles using 0.1 M NaOH and DI water, in sequence. The adsorption capacity of the surface-modified regeneration cycles using 0.1 M NaOH and DI water, in sequence. The adsorption capacity of the PAN fibers decreased as the number of regeneration cycles increased but remained above 51 mg/g surface-modified PAN fibers decreased as the number of regeneration cycles increased but remained after three cycles (Figure 9). The decrease in adsorption capacity was associated with the degradation above 51 mg/g after three cycles (Figure 9). The decrease in adsorption capacity was associated with of the surface-modified PAN fibers during the regeneration process [19,26]. Nevertheless, the the degradation of the surface-modified PAN fibers during the regeneration process [19,26]. surface-modified PAN fibers could be easily reused for the removal of Cu(II) from aqueous solutions Nevertheless, the surface-modified PAN fibers could be easily reused for the removal of Cu(II) from after they were regenerated with NaOH. aqueous solutions after they were regenerated with NaOH. Regeneration cycles Figure 9. Cu(II) adsorption dependence on number of regeneration cycles. Here, q denotes the Figure 9. Cu(II) adsorption dependence on number of regeneration cycles. Here, q denotes the adsorption capacity. adsorption capacity. 4. Conclusions 4. Conclusions PAN fibers modified with DETA were used as adsorbents for the removal of Cu(II) from aqueous solutions. The surface-modified PAN fibers that were treated with 85% DETA presented the highest PAN fibers modified with DETA were used as adsorbents for the removal of Cu(II) from aqueous Cu(II) adsorption capacity. The adsorption of Cu(II) followed the pseudo-second-order kinetic model, solutions. The surface-modified PAN fibers that were treated with 85% DETA presented the highest and the equilibrium data fit the Freundlich isotherm model, which indicated that the adsorption was Cu(II) adsorption capacity. The adsorption of Cu(II) followed the pseudo-second-order kinetic model, a multilayer process that occurred on a heterogeneous surface. The maximum adsorption capacity and the equilibrium data fit the Freundlich isotherm model, which indicated that the adsorption was of 87.77 mg/g was achieved at pH 4. The percent removal of Cu(II) was increased from 49.63% to a multilayer process that occurred on a heterogeneous surface. The maximum adsorption capacity of 97.63% as the adsorbent dosage increased from 0.02 to 0.1 g. The surface-modified PAN fibers could be 87.77 mg/g was achieved at pH 4. The percent removal of Cu(II) was increased from 49.63% to 97.63% regenerated with NaOH solution. These results indicated that the surface-modified PAN fibers could as the adsorbent dosage increased from 0.02 to 0.1 g. The surface-modified PAN fibers could be be used as adsorbent for the removal of Cu(II) from aqueous solutions. regenerated with NaOH solution. These results indicated that the surface-modified PAN fibers could be used as adsorbent for the removal of Cu(II) from aqueous solutions. Author Contributions: Conceptualization, methodology, formal analysis, writing—original draft preparation, K.T.A. and C.-G.L.; formal analysis, validation, visualization, S.-H.H.; conceptualization, supervision, Author Contributions: Conceptualization, methodology, formal analysis, writing—original draft preparation, writing—review and editing, S.-J.P. All authors have read and agreed to the published version of the manuscript. K.T.A. and C.-G.L.; formal analysis, validation, visualization, S.-H.H.; conceptualization, supervision, writing— review and Funding: This editring, S.-J.P. esearch was All supp auth orted ors have read and by the Basic ag Science reed to the Resear pu chblish Program ed ver thr sion of ough th the e m National anuscript. Resear ch Foundation (NRF) of Korea funded by the Ministry of Education (Grant no. NRF-2017R1D1A1B03030649) and by Funding: This research was supported by the Basic Science Research Program through the National Research the National Research Foundation (NRF) of Korea (Grant no. NRF-2018R1C1B5044937). Foundation (NRF) of Korea funded by the Ministry of Education (Grant no. NRF-2017R1D1A1B03030649) and Conflicts of Interest: The authors declare no conflict of interest. by the National Research Foundation (NRF) of Korea (Grant no. NRF-2018R1C1B5044937). Conflicts of Interest: The authors declare no conflict of interest. References 1. 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Applied Sciences – Multidisciplinary Digital Publishing Institute

Published: Mar 3, 2020

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