Polyhexamethylene guanidine functionalized chitosan nanofiber membrane with superior adsorption and antibacterial performances

Journal Pre-proof Polyhexamethylene guanidine functionalized chitosan nanofiber membrane with superior adsorption and antibacterial performances

Sheng Chen, Chengpeng Li, Tingting Hou, Ying Cai, Limei Liang, Lanmei Chen, Mingshan Li PII:

S1381-5148(19)30744-8

DOI:

https://doi.org/10.1016/j.reactfunctpolym.2019.104379

Reference:

REACT 104379

To appear in:

Reactive and Functional Polymers

Received date:

25 July 2019

Revised date:

5 October 2019

Accepted date:

8 October 2019

Please cite this article as: S. Chen, C. Li, T. Hou, et al., Polyhexamethylene guanidine functionalized chitosan nanofiber membrane with superior adsorption and antibacterial performances, Reactive and Functional Polymers (2018), https://doi.org/10.1016/ j.reactfunctpolym.2019.104379

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© 2018 Published by Elsevier.

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Polyhexamethylene guanidine functionalized chitosan nanofiber membrane with superior adsorption and antibacterial performances Sheng Chen1, Chengpeng Li1,2*, Tingting Hou1, Ying Cai1，Limei Liang1，Lanmei Chen2, Mingshan Li1 1

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School of Chemistry and Environmental Science, Guangdong Ocean University, Zhanjiang 524088, PR China 2 Public Service Platform of South China Sea for R&D Marine Biomedicine Resources, Marine Biomedical Research Institute, Guangdong Medical University， Zhanjiang 524023, PR China

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Correspondence to: Chengpeng Li ([email protected]). Fax: + 86 0759-2383001. Telephone: +86 0759-2383133.

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Abstract: Although various nanofiber membranes have been developed for sewage treatment, only a few membranes possess antibacterial activities. Thus, poly-hexamethylene guanidine (PHMG) with broad-spectrum antimicrobial activity was used to functionalize the oxidized chitosan (OCS) nanofibers in solid state. Scanning electron microscope (SEM) investigation indicated that polyvinyl alcohol (PVA) introduced leads to the significantly improved spinnability. Fourier transform infrared spectroscopy (FTIR) and elemental analysis indicate that the as-proposed solid phase grafting strategy is successful and the graft ratio of PHMG is about 36.5%. As expected, PHMG grafted can significantly enhance the antibacterial activity of OCS-PVA nanofibers, but leads to the decrease of the specific surface area and total pore volume. Further adsorption analysis shows that the adsorption capacities of the PHMG-OCS-PVA fiber membrane for Cu(II) and Congo red are 57.0 mg g-1 and 183.3 mg g-1, respectively. Kinetic and isotherm model studies indicate that the pseudo second-order kinetic and Freundlich models can well match the adsorption process of Cu(II) and Congo red. The PHMG-OCS-PVA fiber membrane also shows excellent selectivity towards the separation of various anionic dyes from an aqueous dye mixture. In conclusion, the as-fabricated PHMG-OCS-PVA fiber membrane may be used as a new sewage treatment membrane with superior anti-biofouling performance. Key word: Chitosan; Polyhexamethylene guanidine; nanofiber membrane; adsorption; antibacterial Sheng Chen and Chengpeng Li contribute equally to this article. 1.Introduction Heavy metal ions such as Cu(Ⅱ), Cr(Ⅵ) and Cd(Ⅱ) etc. are widely used in electroplating metal, finishing and mineral processing etc. Dyes a kind of natural or synthetic colorants containing aromatic rings, are widely used in textile, printing,

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coating and leather etc. [1]. Modern industrialization, producing a large amount of wastewater containing those toxic metal ions and dyes, pose a great threat to ecosystems and biodiversity [2]. In addition, those pollutants are non-degradable and their negative effects are long-term [3-5]. Thus, elimination those substances from effluent are of great significance for environment protection. Various technologies have been applied for sewage treatment, for example adsorption [6], flocculation [7], ion exchange [8], membrane separation [9], electrochemistry [10], chemical/ozone oxidation [11, 12], etc. Among them, adsorption is considered as a promising method due to its high efficiency, low cost and simple operation [13]. Due to the importance of the adsorption, various new adsorbents including electrospinning nanofiber membranes [14], hydrogels [15], nanoparticle [16] and porous carbon [17] have been developed for sewage treatment. Among those adsorbents, electrospinning nanofibers with quick separation, high porosity and high specific surface area are regarded as the most attractive materials [18, 19]. For instance, nylon 66 [14] and polyacrylonitrile/titanium dioxide [20] nanofiber membranes have been developed and applied for nickel, cadmium and hexavalent chromium removal. However, nylon 66 and polyacrylonitrile/titanium dioxide are non-degradable and may produce so-called secondary pollution. Chitosan is the second abundant polysaccharide with nontoxicity and biodegradability. In addition, chitosan possesses abundant amino and hydroxyl groups. Under acidic conditions, its amino groups will be protonated, which can then interact with negatively charged molecules (or ions). In addition, the amino and hydroxyl groups can remove contaminants from wastewater via chelation interactions [21, 22]. Thus, more and more research work have been focused on the development of the chitosan-based adsorbents [23, 24]. Most recently, Li et al [25] used glutaraldehyde to crosslink the chitosan fiber, and the adsorption capacity of the crosslinked chitosan fiber membrane for Cr(VI) was improved twice (131.58 mg g-1,). In 2019, Mohammad et al [26] prepared a new kind Poly(vinyl alcohol)/chitosan/Silica composite nanofiber with mesoporous structure, whose optimum adsorption capacity on Direct Red 80 is as high as 322 mg g-1. On the other hand, bio-fouling is another challenge for the long-term efficacy of water treatment membrane during the practical application. Previous research indicated that only those membranes with antibacterial activity can effectively resist the microorganism accumulation [27]. Nano-silver particles with super antibacterial ability have been utilized for anti-biofouling application in membranes [28]. However, nano-silver particles will be gradually eluted from the membrane surface during the particles application, leading to the loss of the anti-fouling activity ultimately. Meanwhile, the eluted nano-silver particles are toxic, which may pose negative effects on water treatment [27]. Chitosan is a kind of non-toxic and biocompatible polysaccharides. However, chitosan only possesses very weak antibacterial activity [29]. Thus, modification or functionalization of the chitosan nano-fiber membrane for its antibacterial activity improvement is very important for practical application. Poly-hexamethylene Guanidine (PHMG), a positively charged polyelectrolyte with ionized guanidine groups, is an environmentally friendly polymer disinfectant

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and has been widely applied in industry, agriculture, medical and daily life. Its strong positive charge can cover the negatively charged outer layer of the bacterial/fungal cell membrane effectively, leading to the failed metabolization and even death [30]. Due to its wide antimicrobial spectrum and non-toxicity, it has been received extensive attention in recent years [31]. However, to our best knowledge, there is no report on PHMG functionalized chitosan and its nanofiber membrane. Therefore, to obtain green nanofiber membrane adsorbents with good antibacterial activity, oxidized chitosan (OCS) was firstly electrospun into nano-fiber membranes and then functionalized and crosslinked via PHMG grafting. To retain the unique nanofiber structure, graft modification was realized on the surface of the solid-state fiber membranes. The chemical structure, morphology, pore size, pore diameter, adsorption performance and antibacterial activities of the as-fabricated composite fiber membrane were also discussed systematically in this article. It is anticipated that this new composite fiber membrane can combine the merits of high adsorption capacity of the chitosan fiber and the strong antibacterial activity of PHMG.

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2. Experimental

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2.1. Materials

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Chitosan (86.5% degree of deacetylation, viscosity 100~200 mPa s-1) was purchased from Qingdao BZ Oligo biotech CO (China). PHMG was purchased from Shanghai Gaoju Biological Technology Co., Ltd (China). (Mw=12 kDa). Polyvinyl Alcohol (PVA, Mw =92 kDa, hydrolysis degree 88%) was supplied by Sigma Aldrich. All other reagents were analytical grade and used without further purification.

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2.2 PHMG functionalized nanofiber membrane Accoding to Scheme 1, there are three steps involved in the fabrication of the PHMG functionalized nanofiber membrane: chitosan oxidation, chitosan electrospining and nanofiber membrane functionalization.

Scheme 1 PHMG-OCS-PVA nanofiber fabrication 2.2.1 Synthesis of OCS OCS was synthesized using our previous procedures with slight changes [32]. For details, 6.0 g CS was dissolved in 400 ml of 1% acetic acid, and then 50 ml of sodium periodate (1.993 g) was introduced in the CS solution. The mixture was reacted in the dark condition at 303 K for 1 h. The reaction was terminated by the addition of ethylene glycol (1.735 g) having a molar ratio of sodium periodate of 3:1.

Journal Pre-proof The reacted solution was purified via dialysis in a semipermeable membrane (molecular weight cut off: 8 kD-14 kD) for 3 days. According to our previous report, the newly produced aldehyde groups can react with the primary amino groups along the chitosan chains and then form the Schiff base structure. Particularly，the Schiff base reaction may lead to the slight crosslinking [32]. Thus, to avoid the negative effects of the crosslinked OSC on electrospinning, the insoluble OCS or OCS gel in the purified OCS solution was removed via filtration before lyophilization. 2.2.2 Preparation of chitosan nanofiber membrane

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OCS was dissolved in aqueous acetic acid solution (90 wt%) at 303 K under stirring to form a 30 wt% homogeneous solution. PVA was dissolved in deionized water to get 10 wt% solution, which was used as a spinning aid. OCS and PVA solutions were then uniformly mixed according to the predetermined mass ratio (Table 1) and the electrospinning was carried out under pre-set process conditions (Table 1). The electrospun nanofiber membranes were collected on a collector plate covered with aluminum foil.

OCS/PVA Mass ratio

1

1:1

2

1:1

3

1:1

4

1:1

5

1:1

7

Tip-to-collector distance (cm)

12

14

12

18

12

14

8

14

16

2:1

14

12

1:1

16

16

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10

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Voltage (kV)

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Samples

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Table 1 The process conditions for electrospinning of OCS/PVA blend solutions.

Flow rate=0.1 mm/min，humidity ≤70% 2.2.3 Nanofiber membrane functionalization 6.0 g of PHMG was accurately weighed, 60 mL of ethanol was added, and the mixture was stirred and dissolved to obtain a 0.1 g mL-1 PHMG alcohol solution. The chitosan fiber membrane was added to the PHMG solution, and 2 drops of triethylamine (TEA) was added and reacted at 328 K, 100 rpm on a shaker for 6 h. After the reaction was completed, a volume of NaCNBH3 solution was added, and the reaction was continued at 303 K for 1 h. After completion of the reaction, the fibrous membrane was washed with anhydrous ethanol for 2-3 times to remove unreacted PHMG, TEA and NaCNBH3, and then freeze-dried to obtain a polyhexamethylene guanidine-modified chitosan fiber membrane.

Journal Pre-proof 2.3 Characterizations

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2.4.1 Adsorption Kinetics

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2.4 Adsorption experiments

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For Fourier transform infrared spectroscopy (FTIR) measurement, all samples were blended with KBr powder and pressed into a transparent flake; which was then scanned using a Fourier Transform Infrared Spectrometer (TENSOR 27, Bruker, Germany) within the wavenumber range of 4000 and 450 cm-1 at a resolution of 4 cm-1. Element contents were determined using a Flash 2000 elemental analyzer. Morphology was observed by Scanning electron microscope (SEM) using JSM-7500F instrument. Before investigation, all samples were sputter-coated with gold. The average diameter of nanofibers was obtained by analyzing SEM images using the Nano Measurer software. The specific surface area and N2 adsorption/desorption isotherms were obtained with Micro 2020 instrument. Before the test, the samples were firstly degassing 2 h under 373 K. The crystalline structures of all samples were characterized with X-ray diffractometer (Bruker D8) in the range of 2θ = 5° to 90° (by steps of 0.02°). The tube voltage and tube current were kept at 40 kV and 30 mA, respectively. Water absorptivity was determined via a gravimetric method. Pre-dried fibrous membranes were immersed in distilled water at 303 K. The fibrous membrane was then weighted at predetermined time till it reached equilibrium.

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Two PHMG-OCS-PVA nanofiber membranes (0.100 g) were put into two plastic beakers with 100 mL Cu(II) (200 mg L-1) and 100 mL Congo Red (CR, 200 mg L-1) aqueous solution, respectively. The plastic beakers were stabilized in a thermostatic water bath at 303 K in advance. When the pre-determined time interval was reach, PHMG-OCS-PVA nanofiber membranes were separated and the solution concentration changes were recorded using an ultraviolet-visible spectrophotometer (UV5000, Shanghai Element Instrument Co., Ltd., China). For detection of Cu(II), phenanthroline was used as chromogenic agent [33]. The wavelengths for Cu(II) and CR was used as 455 nm and 499 nm, respectively. 2.4.2 Adsorption Isotherm

For isotherm adsorption study, 0.100 g PHMG-OCS-PVA was transferred into a 100 mL Cu(II) or CR solution at 303 K. The adsorption was allowed to reach the equilibrium for consecutive 10 hours. After reaching the equilibrium time (10 hours), the residual concentrations of the Cu(II) or CR was quantified using the same procedures mentioned in 2.4.1. 2.4.3 Adsorption Mechanism & Adsorption Selectivity To investigate the salt effect, 0.100 g PHMG-OCS-PVA membrane was immersed into 100 mL Cu(II) or CR solution (200 mg L-1), where the predetermined ionic salt Na2SO4 or NaCl was introduced. The adsorption was evaluated at 303 K and the adsorption capacity was monitored by UV-vis spectrometer with respect to contact time.

Journal Pre-proof In order to evaluate the selectivity, 0.100 g PHMG-OCS-PVA membrane was transferred into a mixture of cationic Methyl Blue (MB) and anionic dyes (i.e. CR or Sunset Yellow (SY)) under same concentration. 2.4.4 Adsorption-desorption cycles

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Initially, 0.100 g PHMG-OCS-PVA fiber membrane was put into 100 ml copper acetate monohydrate solution (100 mg L-1, pH 5.0) at 303 K for 6 h. The residual Cu(II) ion concentration in the bulk solution was analyzed using a UV-vis spectrophotometer, 50 ml EDTA solution (0.05 mol L-1) was introduced into the fibers sediments at 303 K for 60 min 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 for another adsorption treatment. CR adsorption-desorption cycles were carried out using the similar procedures of Cu(II) ions mentioned above, where the diluted NaOH solution (5×10-5 mol L-1) and HCl solution (1×10-5 mol L-1) were used as de-sorbent and regeneration agent for CR adsorption-desorption cycles, respectively.

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2.5 Antibacterial assay

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Gram-negative P. aeruginosa and Gram-positive bacteria S. aureus were selected to study the antibacterial properties of PHMG-OCS-PVA nanofiber membranes. The bacteria were inoculated into nutrient broth and then incubated for 18 h in a rotary shaker at 310 K, where the rotational speed was set as 120 rpm. The cultured bacterial suspension was then diluted 100-fold for further experiments. The mixture of 1 mL diluted bacterial solution and 10 mL AGAR was poured into the culture dish (90 mm) till cooling and coagulation. Fiber membranes with a diameter of 6 mm were placed in the coagulated medium above. The size of the inhibition zone was measured after 24 h culture. 3. Results and discussion 3.1 Characterizations

Voltage, tip-to-collector distance and polymer dosage, are important electrospinning parameters for nanofibers fabrication [34]. When the voltage is too small or the receiving distance is too large, the electric field strength is insufficient to overcome the surface tension, resulting in poor formability of the fiber such as microspheres or molten fibers. On the other hand, when the voltage is high or the tip-to-collector distance is short, the electric field and pulling force is high, which may lead to instability and excessive speed of the solution jet flow [34], producing discontinuous fibers. Therefore, fine nanofibers can only be obtained under certain voltage and tip-to-collector distance. In addition, higher polymer dosage can facilitate the nanofibers with larger diameter [35] due to the higher viscosity and lower bending jet instability [36]. To get homogenous nanofiber, polymer dosage, voltage and tip-to-collector were preliminarily explored using optical microscopic investigation (optical images are not

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supplied here). And the processing parameters (Table 1) were then further optimized using SEM investigation (Fig. 1). Though those processing parameters in Table 1 can all produce nanofibers successfully, there are still various defects existed. For instance, microsphere defects were presented in samples 1(Fig. 1(a)), 3(Fig. 1(c)) and 5(Fig. 1(e)). Molten fibers were produced in samples 1(Fig. 1(a)), 2(Fig. 1(b)) and 6(Fig. 1(f)). Discontinuous fibers were found in samples 1(Fig. 1(a)), 4 (Fig. 1(d)) and 5(Fig. 1(e)). Compared with other samples, sample 7 is the most homogenous, which was named as OCS-PVA and chosen for further functionalization. Although the graft modification was realized in solid state, the functionalized nanofiber (Fig. 1(i)) was swelled significantly from 220.95 nm (Fig. 1(h)) to 327.81 nm (Fig. 1(j)), where most of the pores were blocked. The functionalized OSC-PVA was named as PHMG-OCS-PVA.

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Fig. 1 (a) SEM image of sample 1, (b) SEM image of sample 2, (c) SEM image of sample 3, (d) SEM image of sample 4, (e) SEM image of sample 5, (f) SEM image of sample 6, (g) SEM image of sample 7, (h) statistical average size of sample 7, (i) SEM image of PHMG-OCS-PVA and (j) statistical average size of PHMG-OCS-PVA.

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The chemical structure changes were analyzed using FTIR spectrum in Fig. 2. The peak located at 3293 cm-1 is assigned to O-H and/or N-H vibration in all samples [37, 38]. Signal at 1596 cm-1 is belonged to the bending vibration peak of -NH, and peaks at 1654 cm-1 and 1560 cm-1 are the stretching of amide Ⅰ band and amide Ⅱ band [39], respectively. The bond at 1028 cm-1 is a stretching vibration peak of C-O-C of a pyran ring, and the bond at 1072 cm-1 is attributed to stretching vibration of -OH. Peaks at 1157 cm-1 and 897 cm-1 are characteristic peaks of chitosan β(1→4) glycosidic bond. After the oxidation, the peak at 1596 cm-1 is disappeared, and the peak intensity at 1157 cm-1 and 897 cm-1 became weak, indicating that -NH2 may be lost, and some glycosidic bonds are broken in oxidized chitosan. As for PVA, the peak at 1736 cm-1 is assigned as the signal of C=O in residual acetate. And this peak was also observed in OCS-PVA and PHMG-OCS-PVA. As for PHMG, the characteristic peaks of 1637 cm-1 and 3189 cm-1 are attributed to the stretching vibration of C=N and –NH [40], respectively. 3313 cm-1 is attributed to the stretching vibration of -OH, and 1355 cm-1 is attributed to stretching vibration of C-N on the guanidine group [41]. As for the PHMG-OCS-PVA, a strong peak at 1650 cm-1 was attributed to the overlapping peak of the C=N characteristic peak in the PHMG and the amide 1 band of the OCS. The sharp peak at 1375 cm-1 may be the signal of C-N in PHMG. Thus, it can be concluded that PHMG had been successfully grafted to OCS [42].

Fig. 2 FTIR spectrum of (a) OCS, (b) OCS, (c) PHMG, (d) PVA, (e) OCS-PVA and (f) PHMG-OCS-PVA.

Journal Pre-proof The chemical structure changes were also evaluated using element analysis (Table 2). Compared to the CS, the N/C of OCS is decreased clearly, showing that the N element was released from CS in the form of NH3 during oxidation [32]. This conclusion coincides well with FTIR analysis above. PHMG possesses the highest N/C (0.526). As expected, graft of PHMG leads to the improved N/C for PHMG-OCS-PVA nanofiber (0.267). Base on the N/C differences among PHMG, OCS-PVA and PHMG-OCS-PVA, the graft rate of PHMG is around 36.47%. Table 2 Mass percentages of C,N and H in CS, OCS,PHMG,OCS-PVA and PHMG-OCS-PVA C%

N%

H%

N/C

CS

40.17

7.39

7.257

0.184

OCS

34.82

5.76

PHMG

44.78

23.58

OCS-PVA

40.57

PHMG-OCS-PVA

42.43

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0.165

9.645

0.526

4.16

6.996

0.103

11.33

7.693

0.267

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Fig. 3 X-ray diffraction diagrams

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The X-ray diffraction diagram of the raw materials and modified fiber membrane samples are shown in Fig. 3. The original chitosan has a weak peak at 11.7° and a strong peak at 19.8° (Fig. 3(a)), which are due to the diffraction of the lattice (020) and (110) planes [43], respectively. However, no peak at 2θ = 11.7° was found in the oxidized chitosan, and the intensity of the peak at 2θ = 19.8° was also greatly weakened, indicating that the crystal region is partly destroyed after the oxidation [44]. According to Fig. 3(b), PHMG is characteristic of amorphous materials with a broad peak between 20 and 30°. OCS-PVA shows characteristic PVA signals at13.1°and 19.7° [45]. After the graft modification，no detectable changes were found, indicating that the graft reaction do not change the crystal structure of OSC.

Fig. 4 N2 adsorption-desorption isotherms Fig. 4. shows that both the N2 isotherms of OCS-PVA and PHMG-OCS-PVA are type III. In the rising phase of high P/P 0, the desorption isotherms are above the adsorption isotherms, indicating that the mesopores are H 3 type hysteresis [46]. The specific surface areas of the OCS-PVA and PHMG-OCS-PVA composite fiber membranes are 20.51 m2 g-1 and 18.24 m2 g-1, respectively. According to the single

Journal Pre-proof point adsorption of P/P0=0.99, the total pore volume of OCS-PVA and PHMG-OCS-PVA are 0.241 and 0.0199 cm3 g-1, respectively. Those results indicate that PHMG grafting leads to the decrease in specific surface area and total pore volume, which coincides well with the SEM investigation (Fig. 1 (j-k)). Water uptake ratio of PHMG-OCS-PVA fiber membrane was also shown in Fig. 5. It is found that the PHMG-OCS-PVA fiber membrane could absorb water quick initially, which reached the swelling equilibrium slowly after 2 hours. The maximum water absorptivity is about 1200%.

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Fig. 5 Water uptake ratio of PHMG-OCS-PVA fiber membrane

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3.2 Adsorption Kinetics

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Fig. 6 shows the effect of contact time on CR and Cu(II) adsorption. As can be seen form Fig. 6 (a), the whole adsorption process can be divided into two stages. The first stage is from the starting point to 120 min, where the adsorption capacity of CR and Cu(II) increases rapidly. The second stage lasts from 120 to 480 min, where the adsorption tends to be balanced. This may be explained by the fact that the pollutants fill the adsorbent receptor site by increasing the reaction time interval [5]. And the maximum adsorption capacities investigated for CR and Cu(II) are 183.3 mg g-1 and 57.0 mg g-1, respectively.

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Fig. 6 (a) Effect of contact time on adsorption, (b) linear dependence based on pseudo-first-order model, (c) linear dependence based on pseudo-second-order model, (d) linear dependence based on Elovich kinetic model, and (e) linear dependence based on intra-particle diffusion model. To further explore the adsorption mechanism, pseudo first order kinetic model, pseudo second kinetic model, Elovich kinetic model and intra-particle diffusion model were used to study the adsorption kinetics. The pseudo first-order kinetic model is expressed as follows, lg (qe - qt) =lg qe - K1 t/2.303

(1)

where qe (mg g-1) is the adsorption capacity at the time of equilibrium, qt (mg g-1) is the adsorption quantity of CR and Cu(II) at time t, and K1 (min-1) is the pseudo first-order kinetic rate constant. And pseudo second order kinetic rate equation is described as follows, t/qt = 1/K2 qe2 + t/qe

(2)

where K2 (g mg-1 min-1) is the rate constant of pseudo second-order kinetic. Equilibrium adsorption quantity qe in Eq. (1) is different from Eq. (2). for Eq. (1) qe

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must be determined experimentally, while the qe of Eq. (2) is the theoretical value, which can be directly calculated. By linearly plotting the data using Eq. (1) and Eq. (2), the corresponding characteristic kinetic parameters were shown in Table 3. It can be seen that the pseudo-second-kinetic model of CR and Cu(II) has great correlation coefficient values (0.998 and 0.999), which is much higher than that of the pseudo-first-order kinetic models (0.904 and 0.672). In addition, the calculated qe values (qe,cal) obtained from the pseudo second-order kinetic model, are 200.0 mg g-1 and 58.8 mg g-1 for CR and Cu(II), respectively, which are approaching the experimental qe values (qe,exp) (183.3 mg g-1 and 57.0 mg g-1). However, the qe,cal obtained by the pseudo first-order kinetics model are 101.0 mg g-1 and 19.0 mg g-1, respectively, which were much lower than qe,exp. Based on the analyses above, it can be concluded that the pseudo second-order kinetic model is more suitable for describing the adsorption of CR and Cu(II) than the first-order kinetic model. Since that pseudo-second-order kinetic model cannot determine the diffusion mechanism, Elovich and intra-particle diffusion models was used to analyze the adsorption mechanism. Elovich kinetic model is used to describe the second order kinetics of adsorbent surface energy inhomogeneity. Elovich equation is expressed as follows [1], (3)

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qt = ln(αβ)/β + ln(t)/β

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where, α (mg min-1) is the initial adsorption rate, and β (g mg-1) is the degree of surface coverage and activation energy. The linear curve and kinetic parameters showed that CR and Cu(II) adsorption had a linear fitting degree of 0.933 and 0.898 (Fig. 6(d)), respectively. It is worth mentioning that both the parameters α and β of CR are much higher than Cu(II), indicating that the adsorption of CR is more effective. Intra-particle diffusion model can explain the transport mechanism of pollutants from media to adsorbents in water. Its linear equation is expressed as follows [23]: qt =Kp t1/2+ci

(4)

where Kp (g mg-1 min-1) is the intraparticle diffusion rate constant and ci is the constant associated with the thickness of the boundary layer. According to the linear fit of qt vs t1/2, the curve does not pass the origin (Fig. 6(e)), which indicates adsorption is not controlled by the intraparticle diffusion step, but is controlled jointly by inner and outer diffusions. Table 3 The calculated parameters of the kinetic models for CR and Cu(Ⅱ) adsorption Parameters Models Pseudo first order

qe,exp (mg g-1)

Congo Red

Cu(II) ions

183.3

57.0

k1 (min-1)

0.0069

0.0023

R2

0.9040

0.6720

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19.0

2.8×10-5

1.8×10-3

qe,cal (mg g-1)

200.0

58.8

R2

0.9980

0.9990

β (g mg-1)

0.1812

0.0274

α (mg g-1 min-1)

542.390

2.166

R2

0.9330

0.8980

Kip (g mg-1 min-1)

6.626

1.363

ci

54.121

f

38.675

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qe,cal (mg g-1)

0.7640

0.7330

k2 (g mg-1 min-1)

Pseudo second order

Elovich

Intra-particle diffusion

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To compare the adsorption performance among various adsorbents, the adsorption capacities based on the adsorption isotherm experiment of the different adsorbents were presented in Table 4. It is found that the maximum adsorption capacity of PHMG-OCS-PVA for Cu(Ⅱ) is higher than chitosan, Dop-TMSNs and porous cellulose, but lower than CTSg-P (AA-co-AT) hydrogel. Previous reports shown that allylthiourea is a strong ligand and can form stable complexes with various ions, including Cu(Ⅱ) [33]. Compared to PHMG-OCS-PVA fiber membranes, CTSg-P (AA-co-AT) hydrogels possess much lower surface areas. Thus, the capability of coordination is key factors for heavy ions removal. As for anion CR removal, the maximum adsorption capacity of PHMG-OCS-PVA is higher than V-CDP, BE/ CH@Co, chitosan–alginate sponge and attapulgite. As discussed, both the amine groups in OCS and Guanidine groups in PHMG are positively charged, which can interact with the anion CR via electrostatic attractions. Although the chemical compositions of V-CDP, BE/CH@Co, chitosan–alginate sponge and attapulgite are different from PHMG-OCS-PVA, they are all positive charged [5, 47-49]. Thus, the interaction mechanism for the five adsorbents should all be electrostatic attractions and the positive charge density of PHMG-OCS-PVA fiber membrane may be the highest. On the other hand, the unique there-dimensional structure and high surface area of PHMG-OCS-PVA fiber membrane may also improve its adsorption efficiency and capacity. Table 4 The comparison of maximum adsorption capacity reported from the literatures Adsorbents PHMG-OCS-PVA Chitosan CTSg-P(AA-co-AT) hydrogela

Adsorption capacity (mg g-1) Cu(Ⅱ) 81.52 Cu(Ⅱ) 16.80 Cu(Ⅱ) 98.92

Reference Present study [50] [33]

Journal Pre-proof Dop-TMSNsb Cu(Ⅱ) 58.70 [6] Porous cellulose Cu(Ⅱ) 60.00 [51] c V-CDP CR 288.00 [47] d BE/ CH@Co CR 211.80 [5] Chitosan–alginate sponge CR 121.95 [48] Attapulgite CR 189.00 [49] PHMG-OCS-PVA CR 289.00 Present study a CTSg-P(AA-co-AT) represent Chitosan-P(Acrylic acid-co-allylthiourea) b Dop-TMSNs mean dopamine tannic-acid-templated mesoporous silica nanoparticles c V-CDP represent viologen-based β-cyclodextrin polymer d BE/ CH@Co mean bentonite/chitosan@cobalt oxide composite

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3.3. Adsorption Isotherm

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Fig. 7 (a) Experimental sorption isotherms, (b) Langmuir model, (c) Freundlich model and (d) Temkin model.

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The equilibrium concentrations (CR and Cu(II)) and adsorption capacities at 303K were recorded in Fig. 7a.. The most commonly used Langmuir isotherm adsorption model, Freundlich isotherm adsorption model, and Temkin isotherm adsorption model were used to study the adsorption behavior of synthetic material PHMG-OCS on dyes and heavy metal ions. The Langmuir isotherm adsorption model assumes that the adsorption is a single layer adsorption, and the adsorption sites are evenly distributed on the surface of the adsorbent and have the same adsorption capacity. In contrast to the Langmuir isotherm model, the Freundlich isotherm model hypothesises that the adsorbent surface is non-uniform and allows multiple layers of adsorption of contaminants. Their linear equations Langmuir isotherm adsorption model (5) and Freundlich isotherm adsorption model (6) are as follows: ce/qe = ce/qm + 1/qmaxb

(5)

ln qe = ln Kf + ln Ce/n

(6)

where qe (mg g-1) is the sorption capacity at equilibrium, qmax (mg g-1) is the maximum sorption capacity at full monolayer coverage theoretically, Ce (mg L-1) is the pollutant concentration at the equilibrium, b (L mg-1) is the Langmuir adsorption rate constant; Kf ((mg g-1) (L mg-1)1/n) is the Freundlich adsorption rate constant related to the adsorption energy. 1/n indicates the adsorption strength. When 1/n is between 0 and 0.5, the adsorption is easy to proceed. When 1/n is higher than 2. the adsorption is difficult to be realized. The linear fitting correlation coefficient (>0.95) of the Freundlich isotherm adsorption model is higher than that of the Langmuir isotherm adsorption model (<0.9), indicating that the adsorption of CR and Cu(Ⅱ) by PHMG-OCS is multi-layer adsorption and the surface of the adsorbent is heterogenous. However, the 1/n value indicates that the adsorption of CR is difficult

Journal Pre-proof to carry out, which contradicts the experimental results, which may be caused by weak chemisorption in the adsorption process, as shown in Fig. 7 (b, c) and Table 5. In addition, according to the Langmuir isotherm adsorption model, an important dimensionless separation factor RL can be divided into unfavorable adsorption (RL>1), linear adsorption (RL=1) and favorable adsorption (0
(7)

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Qe = BT lnKT + BT lnCe

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where c0 (mg L-1) is the maximum initial concentration of pollutants. The calculated values of the coefficient RL are shown in Table 5, the RL value of the CR adsorption is 0.042 (0 < RL < 1), which is a favorable adsorption. The RL value of Cu(II) is 2.70>1, which is a unfavorable adsorption. Besides, Temkin isotherm model describes the chemical adsorption during adsorption process, and it considers interaction force between the absorbent and pollutant. Its linear relationship is given by Eq. (8), (8)

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Where BT is constant related to the heat of adsorption, and KT（L g-1）and BT are constants of Temkin isotherm model. As can be seen from Fig. 7 and Table 5, both CR and Cu(II) adsorption have high Temkin model correlation values (0.99 and 0.89). Compared to Langmuir and Freundlich models, Temkin model possesses the highest correlation coefficient for CR adsorption. Thus, CR adsorption match the Temkin model best under the experimental conditions. On the other hand, Cu(Ⅱ) adsorption can be best described by Freundlich model.

Langmuir

Freundlich

Temkin

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Isotherm models

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Table 5 The calculated parameters of the isotherm models Parameters

Congo red

Cu(II) ions

qmax (mg g-1)

-76.92

-90.91

b (L mg-1)

0.0756

-0.0021

RL

0.042

2.700

R2

0.8780

0.6640

1/n

3.1030

1.4440

Kf

0.1803

0.0320

R2

0.9840

0.9700

KT (L g-1)

0.1550

0.0156

R2

0.9900

0.8900

3.4 Adsorption mechanism & selectivity

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To clarify the adsorption mechanism of CR, the effect of ionic salt concentration on the adsorption capacity of anionic CR is systematically study. As can be seen from the Fig. 8a, the improved ionic strength leads to the enhanced adsorption capacity of PHMG-OCS-PVA clearly, which is diametrically opposite to our previous investigation on positive charged poly(N,N-dimethyl-amino ethyl-methacrylate) functionalized graphene oxide composite (PDMAEMA-GO) [52]. The electrostatic attractions between positively-charged PDMAEMA-GO and anionic ions (Cl- or SO42ions) may produce shielding effect on PDMAEMA-GO, leading to the declined charge density and adsorption capacity for PDMAEMA-GO. In addition, the outer Na+ ion atmosphere may also produce shielding effect on anionic dye, which will retard the adsorption rate [53]. However, PHMG-OCS-PVA fiber membrane is totally different from PDMAEMA-GO. This is because PHMG-OCS-PVA fiber membrane is composed by the positive charged OSC, polyelectrolyte inner salt PHMG (guanidinium cations and Cl- anions) and uncharged PVA components. Under the existence of ionic salt, the interactions between negative charged Cl- ions and positive charged guanidine cations will be weakened within PHMG molecules due to the shielding effects of the outer ionic salt. Therefore, the positive-charged guanidine groups in PHMG may possess higher mobility, which can also be understood with the improved content of the free guanidine groups. The increased content of the free guanidine group ultimately results in the improved adsorption capacity of CR. To explore the interactions between Cu(II) and PHMG-OCS-PVA nanofiber membranes, the FTIR spectra of PHMG-OCS-PVA nanofiber membranes with and without Cu(II) ions adsorbed were compared in Fig. 8b, where a couple of characteristic bond shifts were found. For instance, the stretching vibration peaks of -OH and -NH2 of PHMG-OCS-PVA were shifted to 3119 cm-1. The C-N bond at 1375 cm-1 was shifted to 1415 cm-1 and its adsorption became stronger. In addition, the amide II band was found to became stronger at 1561 cm-1. All these changes confirm that there are interactions involved between Cu(II) ions and PHMG-OCS-PVA fiber membrane [54]. Selective removal of dye pollutants is very important for practical application. Fig. 8 (c, d) shows UV-vis spectrums of the mixed solution of anionic dyes (CR and SY) and cationic dyes (MB) absorbed at different time intervals. The characteristic peaks of CR (499 nm) and SY (481 nm) decreased after adsorption, CR was substantially removed, and the absorbance of SY decreased significantly with time, while the change in absorbance of MB (664 nm) was small. Selective adsorption is caused by the electrostatic interaction of positive and negative charges. In this research, both amino groups in OCS and guanidine groups in PHMG are positive charged, which can form stable electrostatic attractions with the anion dyes [55], which can interact with anions dyes. Therefore, PHMG-OCS-PVA has a good removal effect on anion CR and SY, and adsorption on cationic MB is limited. Thus, this result confirmed that PHMG-OCS-PVA can be used as an adsorbent for selectively removing anionic dyes from sewage.

(a)

(b)

Journal Pre-proof

(c)

(d)

Fig. 8 (a) Effects of salt concentration on PHMG-OCS-PVA adsorption of CR at 303K, (b) FTIR spectra of PHMG-OCS-PVA and PHMG-OCS-PVA with Cu(Ⅱ) absorbed, (c) selective removal of the mixture of CR and MB, (d) selective removal of the mixture of SY and MB.

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Based on the discussion above, it can be concluded that Cu (II) was adsorbed on PHMG-OCS-PVA surface mainly via the chelating interactions, while CR was via electrostatic interactions (Scheme 2).

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Scheme 2. Interaction mechanism 3.5 Reusability evaluation

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Reusability is very important for adsorbent application. During the practical usage, weight loss and performance degradation of adsorbents may spontaneously lead to the decreased adsorption capacity. According to the results in Fig. 9, the adsorption capacity of CR is found to decrease sharply for the first cycle. This is due to the low desorption rate initially. The adsorption capacity for Cu(Ⅱ), however, shows a stable decline with the enhanced cycling. After three adsorption-desorption cycles, the adsorption capacity for Cu(Ⅱ) can still retain around 85% of the initial capacity, much higher than that for CR 28.28%. Thus, reusability of the PHMG-OCS-PVA is only reliable for Cu(Ⅱ) removal.

Fig. 9 Effects of the cycle use on the adsorption capacity 3.6 Antibacterial Activities

PHMG and PVA solutions with a concentration of 10% were mixed according to the grafting ratio of PHMG and OCS and made into PVA-PHMG membrane. The antibacterial effect of PHMG-OCS-PVA membrane on S. aureus and P. aeruginosa was compared with PVA-PHMG membrane and OCS-PVA membrane. As can be seen from Fig. 10, the inhibition zone of OCS for Staphylococcus aureus and Pseudomonas aeruginosa is 6.87±0.10 mm and 6.80±0.15 mm, respectively. On the other hand, the corresponding inhibition zone of PHMG-OCS-PVA is 21.63±0.78 mm and 23.27±0.94 mm respectively, which is more than three times the length of the OCS. However, the inhibition zone of PHMG-OCS-PVA is still a bit smaller than that of PHMG (25.15±0.61 mm and 27.89±0.27 mm). The antibacterial mechanism is due to the electrostatic attractions between the positively charged guanidine groups on the PHMG chains (or amino groups in OCS) and the negatively charged cell membrane of the bacteria, which will ultimately lead to the disruption of bacterial cell membranes

Journal Pre-proof [30, 56].

Fig. 10 Inhibition zone of S. aureus (a) and P. aeruginosa (b) 4. Conclusions

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Acknowledgement

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Porous chitosan nanofiber membrane was successfully electrospun using PVA as spinning aid under voltage 16 kV and tip-to-collector distance 16 cm. The functionalization and cross-linking of the OCS-PVA fibers were then realized under in-situ solid phase graft, leading to the decrease of the specific surface area and total pore volume, but significantly improved inhibitory effect on Gram-positive bacteria S. aureus and Gram-negative bacteria P. aeruginosa. The as-obtained PHMG-OCS-PVA fiber membrane shows superior adsorption ability for Congo red and Cu(II) ions. Under the experimental conditions, the adsorption of Congo red and Cu(II) ions can be well described by pseudo-second-order kinetic and Freundlich isotherm adsorption models. In addition, the adsorption process was found to be controlled jointly by inner and outer diffusions, where the chemical adsorption was also involved. Particularly, the as-obtained PHMG-OCS-PVA fiber membrane can remove the anionic dyes selectively under the aqueous dye mixtures. In conclusion, this article develops a new in-situ strategy for electrospinning fiber functionalization, the resultant green fiber membrane combines the merits of strong antibacterial activity and high adsorption capacity, which can be exploited in medical materials such as wound dressing and artificial skin in addition to sewage treatment.

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), Innovation and Entrepreneurship Training Team Program of Guangdong Ocean University (521201004029), Special Funds for the Cultivation of Guangdong College Students' Scientific and Technological Innovation (pdjhb0237， pdjhb2019b0236) and the Public Service Platform of South China Sea for R&D Marine Biomedicine Resources, Marine Biomedical Research Institute, Guangdong Medical University，Zhanjiang，China. References: [1] M.R. Abukhadra, M. Rabia, M. Shaban, F. Verpoort, Heulandite/polyaniline hybrid composite for efficient removal of acidic dye from water; kinetic, equilibrium studies and statistical optimization, J. Adv. Powder Technol. 29(10) (2018) 2501-2511. https://doi.org/10.1016/j.apt.2018.06.030. [2] C.J. Vorosmarty, P.B. McIntyre, M.O. Gessner, D. Dudgeon, A. Prusevich, P. Green, S. Glidden, S.E.

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[47] X.M. Li, M.J. Zhou, J.X. Jia, Q. Jia, A water-insoluble viologen-based beta-cyclodextrin polymer for selective adsorption toward anionic dyes, J. React. Funct. Polym. 126 (2018) 20-26. https://doi.org/10.1016/j.reactfunctpolym.2018.03.004. [48] Q.Y. Zhang, M.J. Xie, X.M. Guo, L.X. Zeng, J.W. Luo, Fabrication and Adsorption Behavior for Congo Red of Chitosan and Alginate Sponge, J. Integr. Ferroelectr. 151(1) (2014) 61-75. https://doi.org/10.1080/10584587.2014.899458 [49] H. Chen, J. Zhao, Adsorption study for removal of Congo red anionic dye using organo-attapulgite, J. Adsorpt.-J. Int. Adsorpt. Soc. 15(4) (2009) 381-389. https://doi.org/10.1007/s10450-009-9155-z. [50] Y.-C.C. Chihpin Huang *, Ming-Ren Liou, Adsorption of Cu(I1) and Ni(I1) by pelletized biopolymer, J. Hazard. Mater. 45 (1996) 265-277 . https://doi.org/10.1016/0304-3894(95)00096-8. [51] K.S. R.R. Navarro, M. Matsumura,, Improved metal affinity of chelating adsorbents through graft polymerization, J.Water Res. 33(9) (1999) 2037-2044. https://doi.org/10.1016/S0043-1354(98)00421-7. [52] C.P. Li, H.J. Zhu, X.D. She, T. Wang, F.H. She, L.X. Kong, Selective removal of anionic dyes using poly(N,N-dimethyl amino ethylmethacrylate) functionalized graphene oxide, J. RSC Adv. 6(71) (2016) 67242-67251. https://doi.org/67242-67251 10.1039/C6RA09049D. [53] J. Lutzenkirchen, Ionic strength effects on cation sorption to oxides: Macroscopic observations and their significance in microscopic interpretation, J. Colloid Interface Sci. 195(1) (1997) 149-149. https://doi.org/.10.1006/jcis.1997.5160. [54] S.Y. Jia, Z. Yang, W.B. Yang, T.T. Zhang, S.P. Zhang, X.Z. Yang, Y.Y. Dong, J.Q. Wu, Y.P. Wang, Removal of Cu(II) and tetracycline using an aromatic rings-functionalized chitosan-based flocculant: Enhanced interaction between the flocculant and the antibiotic, J. Chem. Eng. J. 283 (2016) 495-503. http://dx.doi.org/10.1016/j.cej.2015.08.003. [55] H.L. Liu, R.X. Sun, S.Y. Feng, D.X. Wang, H.Z. Liu, Rapid synthesis of a silsesquioxane-based disulfide-linked polymer for selective removal of cationic dyes from aqueous solutions, J. Chem. Eng. J. 359 (2019) 436-445. https://doi.org/10.1016/j.cej.2018.11.148. [56] J. Budhathoki-Uprety, L. Peng, C. Melander, B.M. Novak, Synthesis of Guanidinium Functionalized Polycarbodiimides and Their Antibacterial Activities, J. ACS Macro Lett. 1(3) (2012) 370-374. http://doi:10.1021/mz200116k.

Journal Pre-proof Table 1 The process conditions for electrospinning of OCS/PVA blend solutions. OCS/PVA Mass ratio

Voltage (kV)

Tip-to-collector distance (cm)

1

1:1

10

12

2

1:1

14

12

3

1:1

18

12

4

1:1

14

8

5

1:1

14

16

6

2:1

14

12

7

1:1

16

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pr

Flow rate=0.1mm/min，humidity ≤ 70%

f

Samples

16

Journal Pre-proof Table 2 Mass percentages of C, N and H in CS, OCS, PHMG, OCS-PVA and PHMG-OCS-PVA C%

N%

H%

N/C

CS

40.17

7.39

7.257

0.184

OCS

34.82

5.76

6.087

0.165

PHMG

44.78

23.58

9.645

0.526

OCS-PVA

40.57

4.16

6.996

0.103

PHMG-OCS-PVA

42.43

11.33

7.693

0.267

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Pr

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Samples

Journal Pre-proof Table 3 The calculated parameters of the kinetic models for Congo Red and Cu(Ⅱ) adsorption Parameters Models

Congo Red

qe,exp (mg g-1)

Pseudo first order

183.0

57.0

k1 (min-1)

0.0069

0.0023

R2

0.9040

0.6720

qe,cal (mg g-1)

101.0

19.0

2.8×10-5

1.8×10-3

f

k2 (g mg-1 min-1)

Pseudo second order

0.9990 0.0274

542.390

2.166

0.9330

0.8980

Kip (g mg-1 min-1)

6.626

1.363

ci

54.121

38.675

0.7640

0.7330

rn

e-

al

R2

Pr

R2

Jo u

0.9980 0.1812

α (mg g-1 min-1)

Intra-particle diffusion

58.8

pr

R2 Β (g mg-1)

200.0

oo

qe,cal (mg g-1)

Elovich

Cu(II) ions

Journal Pre-proof Table 4 The comparison of maximum adsorption capacity reported from the literatures

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Pr

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Adsorbents Adsorption capacity (mg g-1) Reference PHMG-OCS-PVA Cu(Ⅱ) 81.52 Present study Chitosan Cu(Ⅱ) 16.80 [50] a CTSg-P(AA-co-AT) hydrogel Cu(Ⅱ) 98.92 [33] b Dop-TMSNs Cu(Ⅱ) 58.70 [6] Porous cellulose Cu(Ⅱ) 60.00 [51] c V-CDP CR 288.00 [47] BE/ CH@Cod CR 211.80 [5] Chitosan–alginate sponge CR 121.95 [48] Attapulgite CR 189.00 [49] PHMG-OCS-PVA CR 289.00 Present study a CTSg-P(AA-co-AT) represent Chitosan-P(Acrylic acid-co-allylthiourea) b Dop-TMSNs mean dopamine tannic-acid-templated mesoporous silica nanoparticles c V-CDP represent viologen-based β-cyclodextrin polymer d BE/ CH@Co mean bentonite/chitosan@cobalt oxide composite

Journal Pre-proof Table 5 The calculated parameters of the isotherm models Parameters

Langmuir

qmax (mg g-1)

-76.92

-90.91

b (L mg-1)

0.0756

-0.0021

RL

0.042

2.700

R2

0.8780

0.6640

1/n

3.1030

1.4440

Kf

0.1803

0.0320

R2

0.9840

0.9700

0.1550

0.0156

oo

Freundlich

Congo Red

f

Isotherm models

KT (L g-1)

Temkin

pr

R2

Jo u

rn

al

Pr

e-

0.9900

Cu(II) ions

0.8900

Journal Pre-proof

Conflict of interest statement

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A/Professor School of Chemistry and Environment Science Guangdong Ocean University Zhanjiang 524088, PR China [email protected]

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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 Reactive and functional polymer. We declare that the work described is original and has not been published or submitted for publication in any other journals.

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