Functionalized polyacrylonitrile fibers with durable antibacterial activity and superior Cu(II)-removal performance

Journal Pre-proof Functionalized polyacrylonitrile fibers with durable antibacterial activity and superior Cu(II)-removal performance Qian Yang, Xiaoxin Guo, Xiufang Ye, Haijin Zhu, Lingxue Kong, Tingting Hou PII:

S0254-0584(20)30134-6

DOI:

https://doi.org/10.1016/j.matchemphys.2020.122755

Reference:

MAC 122755

To appear in:

Materials Chemistry and Physics

Received Date: 22 August 2019 Revised Date:

27 January 2020

Accepted Date: 30 January 2020

Please cite this article as: Q. Yang, X. Guo, X. Ye, H. Zhu, L. Kong, T. Hou, Functionalized polyacrylonitrile fibers with durable antibacterial activity and superior Cu(II)-removal performance, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/j.matchemphys.2020.122755. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Functionalized polyacrylonitrile fibers with durable antibacterial activity and superior Cu(II)-removal performance Qian Yang1,a; Xiaoxin Guo1,a; Xiufang Ye2；Haijin Zhu3, Lingxue Kong3，Tingting Hou1,* 1

School of Chemistry and Environmental Science, Guangdong Ocean University, Zhanjiang

524088, China 2

School of Materials Science and Energy Engineering, Foshan University, Foshan 528000,

Guangdong Province, China 3

Institute for Frontier Materials and the ARC Centre of Excellence for Electromaterials Science,

Deakin University, Waurn Ponds, Victoria 3216, Australia

* Correspondence to: Tingting Hou (E-mail: [email protected]) a. These authors have made equal contribution to this work.

Abstract: To develop functionalized polyacrylonitrile (PAN) fibers with superior adsorption performance and durable antibacterial activity, adhesive polydopamine (PDA) layers were introduced for effective immobilization of silver nanoparticles (AgNPs). The results showed that the pre-treatment (acidization and oxidation) may lead to the slight degradation of PAN and the formation of unsaturated C=C double bonds, which can significantly promote the loading of PDA and AgNPs. Further scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses indicated that the average sizes of the AgNPs on sulfonated PAN fibers and p-toluenesulfonated PAN fibers were 166.7 nm and 158.6 nm respectively, and their corresponding atomic percentages were 0.65% and 0.31% respectively. As expected, two functionalized PAN fibers showed excellent antibacterial activities against both gram-negative and gram-positive bacteria. Particularly, the antimicrobial ratio of the two functionalized PAN fibers could retain more than 90% after 5 laundering cycles. On the other hand, the spontaneous Cu(II) ion adsorption process followed the pseudo-second order kinetics, Langmuir and Freundlich models, and the adsorption mechanism of the two functionalized PAN fibers is the combination of surface adsorption and intraparticle diffusion. Last but not least, the two functionalized PAN fibers can be used for multiple metal ions removal simultaneously.

Keywords: Polyacrylonitrile; Silver nanoparticles; Antibacterial activity，Heavy metal ions

1. Introduction Severe water pollution, caused by heavy metal contaminants, harmful microbes or organic waste materials, is a huge challenge for the sustainable development of mankind [1, 2]. According to the recent report of the World Health Organization (WHO), over 844 million people lack access to safe drinking water around the world. Heavy metal ions are not biodegradable, and can persist in an environment for

indefinite periods of time, imposing long-term health risk via the bioaccumulation [3]. Cu(II) ions are originated from the metal-bearing industrial effluents [4], which may lead to various health problems such as irritation of the central nervous system, mucosal irritation, kidney and liver failure [5]. Therefore, it is a technical and scientific challenge to develop and implement effective and reliable adsorbents for Cu(II) ion removal. On the other hands, microbial contamination in drinking water is another serious public safety problem, which can spread many diseases such as diarrhea, cholera, dysentery, typhoid, and polio etc.[6]. In 2011, an outbreak of E. coli was found in Germany, which eventually spread to 15 countries around the world [7]. To address those issues, some adsorbents with anti-fouling performance have been developed. For instance, polyacrylonitrile (PAN)-graft-polyvinyl alcohol hollow fibers could effectively reduce protein adhesion and show good anti-fouling performance [8]. Compared with other polymer fibers，PAN fiber possesses the advantages of easy access，high quality，low price, good chemical resistance and low flammability [9], which has been widely used for various water treatment [10, 11]. However, pristine PAN filers with hydrophobic surface can only perform simple filtration and are lacking in adsorption and antibacterial functions. Therefore, PAN fibers need to be functionalized. For instance, a novel PAN fiber adsorbent was successfully fabricated via thiourea modification, showing good adsorption ability for Cd(II) and Pb(II) in aqueous solution [12]. Meanwhile, ethylenediamine was also used to functionalize PAN fiber for improved adsorption of uranium in water [13]. In the last several years, much attention has been paid to develop antibiotic replacement using nanoparticles [14]. Among many nanoparticles, silver nanoparticles (AgNPs) have been demonstrated to be a kind of excellent antimicrobial agents with broad spectrum antimicrobial activity [15, 16]. And AgNPs have been applied for various water treatment processes [17-19] or incorporated into composite fibers to generate antibacterial activity [20]. Polydopamine (PDA), with numerous catechol groups, is a mimic of the adhesive foot proteins from mussels [21]. It not only has a strong adhesion to various surfaces,

but also possess the high adsorption capacity for heavy metal ions [22, 23]. In addition, previous reports showed that PDA can act as a mild reducing agent for Ag+ reduction on different substrates [24]. It was also found that the O- and N- atoms in PDA can chelate with AgNPs and improve their durability [25]. To impart PAN fiber with high adsorption capacity and good antibacterial activity, PAN fibers were functionalized with adhesive PDA and antibacterial AgNPs, respectively, where the PDA layer introduced acts as the in-situ reducing reagent for AgNPs growth. Considering the hydrophobic properties of PAN fiber, acid and pre-oxidation treatments were used to increase the hydrophilia of PAN fiber in advance and make the subsequent functionalization easier. The chemical structure, morphology, antibacterial activity and adsorption ability of the as-functionalized fibers were also systematically investigated in this article.

2. Experimental Section 2.1 Materials PAN fibers were purchased from Huimin Tai Li chemical fiber products Co., Ltd. All other chemicals were of analytical pure and used as received. 2.2 Fabrication of AgNPs-functionalized PAN fiber Nano silver-functionalized PAN fibers were prepared in following four steps: sulfonation, pre-oxidation, PDA loading and AgNPs loading (Fig.1). To enhance the loading efficiency of the AgNPs and PDA, the loading process was repeated twice.

Fig.1 Schematic of preparation procedures of AgNPs-functionalized PAN fiber 2.2.1 Fiber Sulfonation and Pre-oxidation The original PAN fibers were firstly soaked in sulfuric acid (30w%) or p-toluensulfonic acid (30w%) under ultrasonic treatment for half an hour, which was then heated at 110 ºC for 2 h for sulfonation. To remove the residual reactants, the modified fibers were rinsed with purified water for four times. After vacuum drying at 70 ºC for four hours, the fibers were collected and named as sulfonated PAN (PAN-SA) or p-toluenesulfonated PAN (PAN-PA) fibers.

For further functionalization, PAN-SA and PAN-PA fibers were oxidized in a tube furnace (SK-G06123K. Zhonghuan, Tianjin, China) under air atmosphere. All samples were heated to 230 ºC at a heating rate of 5 ºC min-1 and then kept isothermally for 30 min. After that, all samples were cooled down at a cooling rate of 5 ºC min-1. The oxidized PAN-SA and PAN-PA fibers were named as PAN-S and PAN-P fibers respectively.

2.2.2 In Situ loading of PDA & AgNPs A dopamine solution of 1.0 g L-1 was prepared by dissolving DA-HCl in 1.0 g L-1 Tris-HCl buffer solution and the pH value was adjusted to 8.5. The as-obtained PAN-S (or PAN-P) was firstly soaked in dopamine solution under ultrasonic bath for 1 h. After that, the muddy solutions were put into a shaking table (THZ-103B, Shanghai Yiheng Instrument Ltd.) at 120 rpm at 30 ºC for 12 h to initiate the dopamine polymerization. The fibers were washed three times using ultrapure water and dried, which were then immersed in 1 g L-1 AgNO3 solution (pH 8.5) for further modification at 30 ºC with a shaking speed of 120 rpm for 12 h. The fibers were then washed and dried using the same procedure before. To enhance the AgNPs loading, PDA and AgNO3 modification cycles were repeated twice, and the corresponding samples obtained were labeled as PAN-S-Ag and PAN-P-Ag, respectively. For comparison, PAN fibers were also functionalized by PDA and AgNPs under the same procedures using the pristine PAN as the raw materials, which were named as PAN-Ag.

2.3 Characterizations Fiber morphologies were observed using a scanning electron microscopy (SEM model MARA 3LMU, TESCAN) under an accelerating voltage of 10 kV. The silver contents in fibers were analyzed using an energy-dispersive X-ray spectroscopy (EDX) analyses under a voltage of 15.0 kV. The chemical structure of the fibers was analyzed using Fourier transform infrared (FT-IR) spectroscopy under KBr model. The thermal stability of fibers was investigated by thermogravimetric analysis (NETZSCH STA

449F3 TGA) under air atmosphere, and the heating rate was controlled as 10ºC/min. The X-ray powder diffraction (XRD) profiles of the fibers were studied via a Bruker AXS D8 Advance X-ray diffractometer (Bruker D8, Germany) in the range of 10 and 90° using a Vantec-1 position sensitive detector (Cu-Kα radiation at λ = 1.54 Å), and the step size was set as 0.01°. The element content of the fibers was determined by an elemental analyzer (Flash EA 1112, Thermo Electron). 2.4 Antibacterial testing Shaking flask method is a kind of quantitative assessments for the antimicrobial activity of fibers [26, 27]. Two typical bacteria G− E. coli (ATCC 8739) and G+ S. aureus (ATCC 6538) were selected as models in the assay. Briefly, a bacterial suspension that had been incubated in the broth with a shaking speed of 120 rpm at 37 ºC for 24 h was diluted to approximately 10-6~10-8 CFU mL-1 for further experiment. A flask with 0.15g fiber specimen and 15 mL diluted bacterial suspension was cultured at 37 ºC under a shaking speed of 180 rpm for 18 h. The diluted suspension was then spread onto a Luria–Bertani agar plate and cultured in an incubator for 24 h at 37 ºC. The number of viable colonies was counted visually and the bacteriostatic reduction rate (BR) was calculated as follows: BR= (B－A / B) ×100%

(1)

where B and A are the survived bacterium numbers of the fiber control (i.e. PAN) and functionalized fiber samples (i.e. PAN-S-Ag and PAN-P-Ag), respectively [28]. 2.5 Laundering durability test of antimicrobial Fibers Laundering durability is very important for fiber reusability. Here, the laundering durability test was carried out according to AATCC (American Association of Textile Chemists and Colorists) Test method 61-2006. And the laundering durability was evaluated by measuring the antimicrobial ability of the PAN-S-Ag and PAN-P-Ag after the stringent washing treatment. For details, the fibers (1.5g) were washed with 10 mL aqueous solution of sodium dodecanesulphonate (2.0%, w/w) in a beaker under stirring (260 rpm) at 25 °C for 10 min, Afterwards, the fibers were washed four times using deionized water and dried at 60 °C. The antimicrobial function of the laundered fibers was then evaluated using the same procedure in 2.4 [29].

2.6 Adsorption Experiment Copper ions adsorption experiments were performed triplicate in a centrifuge tube containing 50 mL of the copper ions solution. The copper ions solution was prepared via diluting Copper acetate hydrate in diluted water at different concentrations, ranging from 200 to 4000 mg/L. Previous report showed that the adsorption capacity of PAN fiber for Cu(II) reached a high plateau after pH 5.0 [12]. Thus, Cu(II) solution were adjusted to pH 5.0 using acetic acid solution in advance. Before adsorption experiment, a series of Cu(II) solutions with different concentrations

were

analyzed

by

a

V-5000

spectrophotometer

using

2,9-Dimethyl-1,10-phenanthroline hemihydrate as a chromogenic agent at 455 nm. The copper ions adsorption capacity (q) was calculated by the following equation: q = (C0－Ce) × V / M

(2)

where C0 (g L-1) and Ce (g L-1) are the initial and equilibrium concentration of copper ions solution, respectively; V (L) is the volume of copper solution, and M (g) is the weight of adsorbents.

2.6.1 Adsorption kinetics For kinetics studies, 50 mg PAN, PAN-S, PAN-P, PAN-S-Ag, PAN-P-Ag were added into 50 mL 1 g L-1 copper acetate monohydrate solution (pH 5.0) at room temperature. The concentrations of different standing times (20, 40, 60, 90, 120, 240, 330, 480, 720, 1080 and 1440 min) were determined to calculate the adsorption capacity. The supernatant was analyzed for the residual copper ions concentration. 2.6.2 Adsorption isotherm For isotherm studies, 50 mg PAN, PAN-S, PAN-P, PAN-S-Ag, PAN-P-Ag were respectively added into 50 mL copper acetate monohydrate solution (pH 5.0) of different concentrations (200, 500, 800, 2000 and 4000 mg L-1) at 313 K. for 6 hours. The concentrations of the residual copper ions were determined to calculate the adsorption capacity. To detect the AgNPs leaking during the adsorption, the residual solution after adsorption was analyzed using an Inductively Coupled Plasma Optical

Emission Spectrometry (ICP-OES). Before ICP test, the residual solution was homogenized via ultrasound and then filtered using a polysulfone membrane (0.45µm). 2.6.3 Adsorption-desorption recycle experiments. The adsorption-desorption recycle experiments were carried out by add 0.1 g of PAN, PAN-S, PAN-P, PAN-S-Ag and PAN-P-Ag fibers into 100 mL of 1.0 g/L copper acetate monohydrate solution (pH 5.0) at 298 K for 12h. The concentration of Cu(II) ions of supernatant liquid was calculated. Subsequently, 100 mL 0.1 M HCl solution added into the fibers sediments at 298 K for 120min to desorb the Cu(II) ions away from the fibers. Then, the fibers were washed with deionized twice and drying in oven. And the fresh Cu(II) ions solution was added into the final fibers with the same volume-mass rate. 2.6.4 Adsorption selectivity To evaluate the adsorption selectivity, PAN-S-Ag or PAN-P-Ag was immersed in a mixture of Cu(II) and other ions (i.e. iron(III) and Ni(Ⅱ)) under same concentration. For details, 0.100g PAN-S-Ag (or PAN-P-Ag) was added into 100ml copper acetate monohydrate solution (1.0g/L, pH5.0) mixed with 1.0g/L Iron(III) chloride hexahydrate (or 1.0g/L Nickel(Ⅱ) chloride hexahydrate). The residual concentration of Cu(II), iron(III) or Ni(Ⅱ) ions were quantified at different adsorption time (30, 60, 90 and 120min) using 2,9-Dimethyl-1,10-phenanthroline hemihydrate (455 nm), 1,10-phenanthroline (510nm) and Dimethylglyoxime (469nm) as a chromogenic agent, respectively.

3

Results and discussion

3.1 Characterization To find the ideal pre-oxidation temperature, the thermal degradation behaviors of the PAN, PAN-SA and PAN-PA under the air atmosphere were evaluated using TGA. As can be seen from the Fig.S1, the main degradation was in the range of 280 and 470 °C. To avoid the serious degradation, pre-oxidation should be selected below 280°C. In our experiment, 230 ºC was chosen as the pre-oxidation temperature. It is worth

mentioning that the residual weight of PAN-SA (40.1%) and PAN-PA (40.2%) at 800°C is much higher than that of the untreated PAN (37.7%) at 800°C, indicating that the pre-treatment (i.e. sulfonation and oxidation) can lead to the slight structure changes and the improved thermal stability. Thus, the as-proposed pre-treatment strategy is helpful for the functionalization.

(a)

(b)

Fig.2. (a) Appearance comparison of PAN, PAN-S, PAN-P, PAN-S-Ag and PAN-P-Ag fibers;（b） ）FT-IR spectra of untreated PAN, PAN-S, PAN-P, PAN-S-Ag and PAN-P-Ag fibers. Fig.2 (a) shows the photograph of the PAN, PAN-S, PAN-P, PAN-S-Ag and PAN-P-Ag fibers. The untreated PAN fibers are characteristic cream color. After pre-oxidation, PAN-S and PAN-P were turned into kermesinus, which is due to the degradation and formation of ladder ring structure during the pre-oxidation[30]. Compared to PAN-S and PAN-P fiber, PAN-S-Ag and PAN-P-Ag fibers are much darker. To further clarify the mechanism of the color change above, FTIR spectra of untreated PAN, PAN-S, PAN-P, PAN-S-Ag, PAN-P-Ag fibers was presented in Fig.2(b). The peaks at 2937 cm-1 and 1454 cm-1 are the stretching and bending vibration of the methylene (-CH2-), respectively [31, 32]. The sharp band at about 2240 cm-1 is assigned to nitrile groups C≡N stretching vibration. The peak at 1733 cm-1 is the characteristic carbonyl group originated from the ester groups in methyl acrylate comonomer [33, 34]. After the acid-treatment and pre-oxidation, the original carbonyl signals become much broader. This broadening is due to the overlapping of the conjugated C=O stretching emerged, indicating that the degradation involved in the acid-treatment and pre-oxidation leads to the formation of unsaturated C=C double bond [35]. However, there is no detectable change after the loading of PDA and AgNPs.

Table 1 Element analysis results for fibers Samples PAN PAN-S PAN-P PAN-S-Ag PAN-P-Ag

N (%) 24.9 24.2 24.4 23.5 23.8

C (%) 65.5 64.8 65.6 63.6 64.6

C/N 2.63 2.67 2.69 2.71 2.71

The elemental analysis results of PAN, PAN-S, PAN-P, PAN-S-Ag, PAN-P-Ag fibers were illustrated in Table 1. Compared to the untreated PAN fiber, the nitrogen content of the PAN-S (24.2%) and PAN-P (24.4%) is slightly decreased, showing the loss of nitrogen element during the pre-oxidation. According to the previous report， the nitrogen element is due to the denitrification process occurred during the pre-oxidation [36]. The further decreased nitrogen content as shown in PAN-S-Ag (23.5%) and PAN-P-Ag (23.8%) indicated that the PDA and AgNPs were successfully loaded onto the surface of the PAN-S and PAN-P fibers [37].

Fig.3. Wide-angle XRD patterns of the PAN fibers To further confirm the successful loading of AgNPs, XRD analysis was shown in Fig.3. It can be seen clearly that all fibers possess two characteristic peak centered at 17°and 29° respectively, which are contributed to the {100} and {101} crystallite planes of PAN [38]. After the introduction of AgNPs, four peaks emerged at 38°,44°, 64° and 77° respectively，assigning to the {111}, {200}, {220} and {311} crystallite planes of cubic Ag, respectively [39]. Therefore, it is confirmed that AgNPs were successfully introduced.

Fig.4. (a), (b), (c), (d) and (e) show the SEM image for PAN-S fiber, PAN-P fiber, PAN-Ag fiber, PAN-S-Ag fiber and PAN-P-Ag fiber, respectively; (f) and (g) present the particle size analysis for PAN-P-Ag and PAN-S-Ag; (h) and (i)

indicate the quantitative EDX analysis results for PAN-P-Ag and PAN-S-Ag, respectively.

Surface morphology of the fibers was subsequently investigated and compared in Fig.4(a-e). As can be seen in Fig.4 (a-b), PAN-S, and PAN-P fibers possess a smooth surface. After the functionalization of PDA and AgNPs, there are small particles emerged on the fiber surface (Fig.4 (c-e)), which may be the PDA and AgNPs introduced. As expected, the particle density in PAN-Ag (Fig.4 (c)) is much lower than that of PAN-S-Ag (Fig.4 (d)) and PAN-P-Ag (Fig.4(e)), indicating that the pre-treatment is helpful for the functionalization of PDA and AgNPs. This may be due to the improved hydrophilicity of the treated fibers [40]. Based on the statistical analysis of the inset image (Fig.4 (f-g)), the average particle sizes on PAN-S-Ag and PAN-P-Ag fibers are 166.7 nm (Fig.4(f)) and 158.6 nm (Fig.4(g)) respectively.

The

surface element composition of the particles observed in Fig.4 (d-e) was further analyzed using EDX (Fig.S2). According to the results in Fig.4 (h-i), the atomic percentages of silver are 0.65% (PAN-P-Ag) and 0.31% (PAN-S-Ag) respectively, confirming that those particles observed in Fig. 4(d) and Fig. 4(e) are AgNPs. The successful loading of AgNPs shows that PDA coating is an effective reducing reagent for silver nitrate [41]. 3.2 Antimicrobial test and laundering durability The bacteriostatic properties of PAN-P-Ag and PAN-S-Ag were evaluated using G− E. coli and G+ S. aureus as models (Fig.5 (a-b)). It was found that those fibers without AgNPs loaded (i.e. PAN, PAN-S and PAN-P) showed no antibacterial activities. As expected, AgNPs loading significantly improve the antimicrobial activity of PAN-P-Ag and PAN-S-Ag. Particularly, both the BR values of PAN-P-Ag and PAN-S-Ag are as high as 100%. This is because AgNPs can attach to the cell membrane surface, disrupting the permeability and respiratory function of the microorganism [42]. On the other hand, the laundering durability is another key parameter for the practical application of the antimicrobial fiber. In this work, the AgNPs-coated fibers

were washed through a certain number of laundering cycles and antibacterial performances were investigated. As shown in Fig.5 (b), the antimicrobial ratio of PAN-S-Ag and PAN-P-Ag for S. aureus are over 97% after 5 washing cycles, which are slightly higher than that of E. coli (94%). However, the antimicrobial rate of PAN-S-Ag and PAN-P-Ag for S. aureus is decreased greatly to 35% and 31% respectively after 10 laundering cycles, which are still higher than that of E. coli (24% for PAN-S-Ag and 20% for PAN-P-Ag). Thus, the durability of PAN-S-Ag and PAN-P-Ag is only acceptable when the laundering cycle is less than 10. During the laundering tests, the loss of AgNPs caused by abrasion and dissolution is the main reason for the decreased antimicrobial rate. According to the investigation in Fig.6, clear reduction in the number of AgNPs is found after 5 washing cycles and AgNPs are nearly invisible after 10 washing cycles. On the other hand, our results clearly show that AgNPs-loaded fibers are more resistant to Gram-positive S. aureus than Gram-negative E. coli, which may be due to the different mechanisms. The cell walls of gram-negative bacteria contain a complex of phospholipids and lipopolysaccharides with highly asymmetric biological barriers, which are less permeable to most antimicrobial agents than Gram-positive bacteria [43]. Therefore，gram-negative bacteria are more difficult to be killed by AgNPs than gram-positive bacteria. In addition, the cell wall of Gram-positive bacteria can bind more metal atoms than that of Gram-negative bacteria [44].

(a)

(b) Fig.5. The antibacterial effect of the modified fiber samples, the optical images of the antibacterial tests (a) and the durability results of the antibacterial properties against E. coli and S. aureus (b).

Fig.6. (a), (b) and (c) refer to PAN-P-Ag morphology changes after 2, 5 and 10 washing cycles, respectively; (d), (e) and (f) show PAN-S-Ag surface after 2, 5 and 10 washing cycles. 3.3 Adsorption Analysis Contact time is an important parameter for Cu(II) ion removal. As can be seen from Fig.7 (a), Cu(II) ion adsorption capacity increases sharply initially, and then reach an equilibrium. In addition, it shows that PAN-P-Ag and PAN-S-Ag possess the maximum equilibrium adsorption capacity for Cu(II) ions, followed by PAN-S and PAN-P. The original PAN, however, possess the lowest adsorption capacity. The greatly enhanced adsorption capacity of PAN-P-Ag and PAN-S-Ag may be due to the functional groups such as amines in PDA [45, 46]. Meanwhile，hydrophilic groups such as -COO- and -SO3H produced during the sulfonation and pre-oxidation [47, 48] may be responsible for the improved adsorption capacity as well. To get the insight view

of

the

entire

adsorption

kinetics,

the

pseudo-first-order

and

the

pseudo-second-order [49] were adopted to analyze the whole adsorption process. The linear pseudo-first-order model is expressed as follows: ln(qe－qt) = ln qe－k1 t

(3)

where t is the time (min), qe is the adsorption capacity at equilibrium (mg g-1), qt was adsorption capacity at t (mg g-1), k1 was the pseudo-first-order rate kinetic constant (min-1). And the pseudo-second-order kinetic equation can be presented as： t /qt = k2 / qe2 + t / qe

(4)

where k2 is the pseudo-second-order kinetic constant (g min-1 mg-1).

(a)

(b)

Fig.7. (a) Effect of contact time on the adsorption of Cu(II) ions, (b) linear dependence based on pseudo-second-order kinetic equation.

Table 2 Parameters of the pseudo-second-order kinetic model The pseudo-second-order -3

Sample

k2×10 g min-1 mg-1

PAN PAN-S PAN-P PAN-S-Ag PAN-P-Ag

0.978 0.389 0.428 8.184 2.304

qe mg g-1 13.8 30.6 29.1 47.9 52.3

R2 0.998 0.997 0.997 1.000 0.997

The linear log(qe - qt) vs. t and t qt-1 vs. t were plotted in Fig. S2 and Fig.7(b), and the linear fitting results were presented in Table S1 and Table 2. According to the results in Table S1 and Table 2, the correlation coefficient values of the pseudo-second order kinetic model (R2 ≥ 0.997) are all higher than the pseudo-first order kinetic model, Thus the pseudo-second order kinetic model is more applicable for the adsorption of Cu(II) ions. The adsorption capacity of PAN-S-Ag and PAN-P-Ag is also compared with other absorbents. In our case, adsorbent dosages are 1.0 mg mL-1, which are higher than those of the PD nanoparticles, MWCNTs and s-MWCNTs used. However, the adsorption capacity of PAN-S-Ag and PAN-P-Ag is higher than that of PD nanoparticles, MWCNTs and s-MWCNTs under the same conditions [50-52], showing that PAN-S-Ag and PAN-P-Ag are as good as PD nanoparticles, MWCNTs and s-MWCNTs. Table 3 Adsorption capacity comparison for various adsorbents Adsorbents PD nanoparticles MWCNTsa s-MWCNTsb PAN-S-Ag PAN-P-Ag a

pH T(℃) 5.0 5.0 5.0 5.0 5.0

25 25 25 25 25

Adsorption capacity (mg g-1) 37.0 28.5 43.2 47.9 52.3

Dosage (mg mL-1) 0.8 0.5 0.5 1.0 1.0

MWCNTs represent multi-walled carbon nanotubes.

b

s-MWCNTs mean sulfonated multi-walled carbon nanotubes

Refs [51] [50] [52] This study This study

During the adsorption process, Cu(II) ion diffusion plays a vital importance role [53]. Thus, intra-particle diffusion model was also used to clarify the whole adsorption mechanism [53], which is expressed as following: qt = Kd×t 0.5+ C

(5)

where Kd is the intra-particle diffusion rate constant (mg g-1 min-0.5) and C is the thickness between boundary layers (mg g-1).

Fig.8. Linear dependence based on intra-particle diffusion kinetic curves. Data fitting of the intra-particle diffusion model was presented in Fig.8 and the characteristic parameters were listed in Table S2, which indicates the adsorption process can be divided into two stages and only the first stage can match the intra-particle diffusion model [54]. The diffusion rate constant Kd in Table S2 clearly shows that the adsorption rate follow the rules: PAN-S-Ag>PAN-P-Ag> PAN-S>PAN-P>PAN [55]. In addition, all the boundary layer thicknesses are not zero, suggesting that surface adsorption may also play a vital role for copper ion uptake except for the intraparticle diffusion [56]. On the other hand, an adsorption isotherm, referring to the relationship between the adsorption capacity and the equilibrium sorbate concentration, also plays an irreplaceable role in revealing the surface properties and affinity of the adsorbent. The famous Langmuir (Eq(6)) and Freundlich isotherms (Eq(7)) are widely used to describe an adsorption process [57, 58], which are represented as follows: Ce / qe = 1/ bqm +Ce / qm

(6)

ln qe = ln Kf + ln Ce / n

(7)

where qe was the amount of Cu(II) ions adsorbed away from the liquid-phase environment at equilibrium time (mg g-1). Ce was the equilibrium concentrations of Cu(II) ions (mg L-1). n represented the adsorption intensity between absorbents and adsorbates [59, 60]; Kf was the adsorption capacity of fibers, qm and b are the monolayer maximum adsorption capacity and sorption equilibrium constant, respectively.

The linear fitting for Langmuir and Freundlich isotherm models were shown in Fig. S3 and the fitting results were presented in Table S3. According to the Table S3, all correlation coefficients are over 0.9, indicating that the adsorption can match both Langmuir and Freundlich models well under the experimental conditions. For Langmuir model, the highest monolayer adsorption capacity qm listed in Table S3 is PAN-P-Ag and PAN-S-Ag, followed by PAN-P, PAN-S and PAN, which coincides well with adsorption kinetic analysis. As for Freundlich model, all n calculated in Table S3 are greater than 1, indicating that it is a favorable sorption process. Since all adsorption processes match both Langmuir and Freundlich models, it is assumed that multiple interactions are existed in the interaction sites and adsorption sites are heterogeneous [53]. During the adsorption process, the thermodynamic function Gibbs free energy (∆G) can well provide the criterion for deciding whether or not it will occur [61]. ∆G can be expressed as the following: ∆G =－RT ln Ke =－RT ln (qe /Ce)

(8)

where Ke was the adsorption equilibrium constant, R was the gas constant (8.314 J K-1 mol-1), and T was the absolute temperature (kelvin), respectively. Table 4 shows the detailed ∆G values for Cu(II) ion adsorption at different initial concentrations. As all values of the Gibbs free energy are negative, it can be inferred that all adsorption processes are spontaneous [58].

Table 4 Gibbs free energy values at 313K C0 (mg L-1) 0.2 0.5 0.8 2.0 4.0

PAN -13.30 -11.65 -10.72 -9.06 -8.14

PAN-S -14.04 -12.29 -11.17 -9.53 -8.33

∆G (kJ mol-1) PAN-P PAN-S-Ag -12.97 -13.22 -11.85 -12.29 -11.23 -11.44 -9.41 -10.00 -8.02 -9.23

PAN-P-Ag -14.72 -13.29 -12.28 -10.31 -9.50

In spite of the spontaneous removal of the Cu(II) ions, AgNPs may also diffuse from PAN-S-Ag and PAN-P-Ag fibers and enter into the aqueous solution during adsorption process. This leaking process may potentially lead to the secondary pollution. Based on the ICP analysis, the AgNPs concentrations are 0.26±0.02µg/mL and 0.12 ± 0.01µg/mL after PAN-S-Ag and PAN-P-Ag treatment for 6 hours, respectively. According to previous report, AgNPs only shown cytotoxicity and genotoxicity when their concentrations are higher than 25µg/mL [62], indicating that PAN-S-Ag and PAN-P-Ag are very safe for sewage treatment.

Fig.9. Effects of the repetitive use on the adsorption capacity To evaluate the reusability, adsorption capacity at different recycle was compared in Fig.9, which shows that the adsorption capacity declines slowly with the enhanced recycling. The reasons may be due to the decreased performance and absorbent loss during the rinsing and centrifugation operation. However, the adsorption capacity of PAN-S-Ag and PAN-P-Ag can still retain around 90% after two adsorption-desorption cycles, much higher than that of PAN-P (85.2%), PAN-S (85.1%) and PAN (77.0%). Therefore, the as-proposed functionalization cannot only improve the adsorption capacity but also the reusability. For practical effluent treatment, other heavy metal ions (i.e. Ni(II) and iron(III) etc.) may co-exist in the system. Therefore, it is important to evaluate the removal selectivity of the fibers. Fig.10 shows the adsorption capacity of PAN-S-Ag and PAN-P-Ag fibers in mixed ion solution. As can be seen from the Fig.10 (a-b), with the prolonging treatment time, both the adsorption capacity of Cu(II) and Fe(III) increases quickly. And it is clearly that the adsorption capacity of Cu(II) is much lower than Fe(III) under the experimental conditions. Meanwhile, Fig.10 (c-d) show that the adsorption capacity of Cu(II) is much higher than Ni(II) under the experimental conditions. Thus, the removal selectivity follows this order: Fe(III) > Cu(II) > Ni(II). According to the previous report, removal selectivity is controlled by many factors (i.e., ion radii, charge density and metal electronegativity etc.) [63]. The ionic radius of Fe(III), Cu(II) and Ni(II) is 64, 69 and 72 p.m., respectively. And their charge

density is inversely proportional to their corresponding ionic radius. Thus, the selectivity may probably be governed by their ionic radii. It is supposed that the electrostatic attractions between Fe(III) and the isolated electron pairs at N atom in PDA may be the strongest, leading to the highest adsorption capacity. Therefore, Fe(III) will be preferentially removed compared to Cu(II) and Ni(II), and PAN-S-Ag or PAN-P-Ag can be used for removal of the multiple metal ions simultaneously. Fig.10. (a) and (b) show the selective removal in copper acetate monohydrate (1.0g/L) and iron(III) chloride hexahydrate (1.0g/L) for PAN-S-Ag and PAN-P-Ag respectively; (c) and (d) show the selective removal in copper acetate monohydrate (1.0g/L) and nickel(II) chloride hexahydrate (1.0g/L) for PAN-S-Ag and PAN-P-Ag respectively.

Conclusions

In this article, AgNPs were in-situ loaded on PAN fiber surface using PDA as adhesive glue and in-situ reductant. It is found that acidization and oxidation of PAN fiber are necessary for the successful loading of PDA and AgNPs. Based on the investigation, average sizes of the AgNPs on PAN-S-Ag and PAN-P-Ag fibers are 166.7 nm and 158.6 nm respectively, and their corresponding atomic percentages are 0.65% and 0.31% respectively. Compared with the original PAN, the antimicrobial activity of PAN-P-Ag and PAN-S-Ag was enhanced greatly. Particularly, their antimicrobial rates can maintain more than 90% after 5 laundering cycles. Model Cu(II) ion adsorption shows that the adsorption capacity of PAN-P-Ag and PAN-S-Ag is much higher than the original PAN, and their adsorption process are spontaneous under the experimental conditions. It is worth mentioning that both PAN-S-Ag and PAN-P-Ag can retain high adsorption capacity after recycle usage, and can be applied for real effluent treatment. In conclusions, high adsorption capacity, superior antibacterial activity and good reusability are realized in terms of PAN-S-Ag and PAN-P-Ag.

Acknowledgement We acknowledge the financial support from the Project of Zhanjiang Science & Technology Plan (2018A206)，PhD Start-up Fund of Guangdong Ocean University (R17003 & R17074), Enhancing School with Innovation of Guangdong Ocean University (230419108)，financial support from Foshan University and the Public Service Platform of South China Sea for R&D Marine Biomedicine Resources, Marine Biomedical Research Institute, Guangdong Medical University，Zhanjiang， China.

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Highlights The antimicrobial rates of the functionalized polyacrylonitrile can retain more than 90% after 5 laundering cycles. The spontaneous Cu(II) ion adsorption process match the pseudo-second order kinetic model well. The functionalized polyacrylonitrile can be used for multiple metal ions removal simultaneously.

Declaration of interest We declare that there is no conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all authors for publication on Materials Chemistry and Physics. We declare that the work described is original and has not been published or submitted for publication in any other journals.

Dr. Tingting Hou School of Chemistry and Environment Science Guangdong Ocean University Zhanjiang 524088, PR China