Scalable Sulfonate-Coated Cotton Fibers as Facile Recyclable and Biodegradable Adsorbents for Highly Efficient Removal of Cationic Dyes


 Adsorbents with superior adsorption capacity and facile recyclability are viewed as promising materials for dye wastewater treatment. In this work, a novel sulfonate decorated cotton fiber as a biodegradable and recyclable adsorbent was fabricated for highly efficient removal of cationic dyes. Herein, the poly(sodium p-styrenesulfonate-co-N-methylol acrylamide) (P(SSNa-co-NMAM)) with SSNa units as adsorption sites and NMAM units as thermal-crosslinking points was synthesized for modification of cotton fibers in a large scale at high temperature (160 oC). The various characterization investigations confirmed the successful construction of the P(SSNa-co-NMAM) coated cotton fibers (PCF). As expected, the as-obtained adsorbent presented outstanding adsorption performance toward cationic dyes in the both static and dynamic states, even in the synthetic effluent. The adsorption processes of cationic dyes onto the PCF were well fitted by the Langmuir isotherm model and Pseudo-second-order kinetics, respectively. The thermodynamics study showed that the adsorption reaction of the cationic dyes onto PCF was a spontaneous and endothermic process. The maximum adsorption capacities of PCF toward MEB, RhB and MG were 3976.10, 2879.80 and 3071.55 mg/g, respectively. The responsible adsorption of dyes ontothe PCF was electrostatic interaction. Moreover, the adsorption capacity of PCF toward cationic dyes was slight influenced by pH value of solution, because of the stable feature of sulfonate moiety in acid and alkali. In addition, the as-prepared PCF exhibited satisfactory recyclability and reusability. Given the aforementioned results, the as-obtained PCF is a promising adsorbent with great potential for practical application in the dye-contaminated wastewater remediation.


Introduction
Synthetic dyes are extensively employed in many elds to impart color including textile, leather tanning, cosmetics, and printing industries (Han et al. 2009;Janaki et al. 2013; Abdi et al. 2017; Guerra et al. 2017). It is reported that more than 100,000 commercially available dyes following the rate of 7 × 10 5 tons per year are generated and approximately two percent of the dyes in industrial e uents are let out into aqueous systems (Abdi et al. 2017). Notably, majority of these dyes are toxic, teratogenic and even carcinogenic, resulting in a serious threat to aquatic living organisms and human health (Janaki et  group has been widely utilized to fabricate various negatively charged adsorbents to remove cationic dyes (Xu et al. 2018;Xiang et al. 2019;Zhao et al. 2020). It can bind dyes through the strong interaction between the dye molecules and the sulfonic groups. Moreover, different from other charged groups like carboxyl, the sulfonic group is less impacts in harsh conditions (Jin et al. 2020). Therefore, sulfonate based adsorbents are promising candidates for cationic dye removal. Wei's group fabricated sulfonate functionalized nanodiamonds for ultrafast removal of methylene blue (MEB) with high e ciency (Lei et al. 2020). Hong et.al used a simple method for constructing sulfonate functionalized graphene as adsorbent with outstanding capacity and green regeneration for cationic dye removal (Hong et al. 2021).
However, such sulfonate based adsorbents faced troubles in reclamation di culty, cumbersome preparation routes and high cost, which signi cantly hider their practical applications. Fortunately, Zhao's group facilely prepared sulfonate groups decorated nano brous membranes and polyurethane sponges with excellent reusability (Xu et al. 2018;Jin et al. 2020). Nevertheless, the adsorption capacity of these adsorbents was sacri ced due to the decrease of the amount of sulfonate groups. Moreover, the reported sulfonate based adsorbents were non-biodegradability and may cause secondary pollution to the environment (Qi et al. 2019). Therefore, how to the facile fabrication and large-scale production of novel sulfonate based adsorbents with high adsorption capacity, low cost, satisfactory recyclability and good biodegradability is still highly desirable.
Cotton ber is mainly composed of cellulose, which is the most abundant renewable and biodegradable natural biopolymer (Tserki et al. 2003). Moreover, cotton bers can be produced in large-scale with low cost (Nabi Saheb and Jog 1999; Tserki et al. 2003). Most importantly, lots of wasted cotton bers are generated during in processing. Therefore, how to impart these cotton bers with higher additional value is meaningful. Recently, in view of the unique advantages of the cotton bers, numbers biosorbents derived from cotton bers were developed for dyes removal (Xiong et al. 2014;Yang et al. 2021b;Zou et al. 2021;Krishnamoorthi et al. 2022). For example, Chen's group utilized both cationic monomer and anionic monomer to modify the cotton bers to form an e cient biosorbent for dyes removal (Xiong et al. 2014). Krishnamoorthi and co-workers prepared a biodegradable caffeic acid/chitosan polymer coated cotton ber as an adsorbent for dye wastewater treatment (Krishnamoorthi et al. 2022). In addition, our group was scalable preparation of CO 2 -reposive cotton bers for removal of anionic dyes with ultrafast and selective features ). Inspiration from these successful works, the scalable development of sulfonate decorated cotton bers as adsorbents with highly e cient performance for cationic dye removal is expected.
In this work, sulfonate decorated cotton ber as a novel biosorbent is fabricated via a simple approach for application in the treatment of dye wastewater (Scheme 1). First of all, poly(sodium pstyrenesulfonate-co-N-methylol acrylamide) containing thermo-crosslinking moiety is prepared. Subsequently, the as-prepared polymer was coated on the cotton brous surface via thermo-crosslinking to form the targeted adsorbent. The adsorption properties of the as-prepared adsorbent towards dyes are studied in the both static and dynamic states. The adsorption kinetics, isotherms, and mechanism of cationic dyes by the obtained adsorbent are investigated. The effects of the initial pH, temperature and ionic strength on the adsorption performance of adsorbent are systematically analyzed. The dye adsorption behavior of the as-fabricated adsorbent in the synthetic e uent is investigated. Finally, the recyclability of the as-prepared adsorbent is also evaluated.

Synthesis of P(SSNa-co-NMAM)
The poly(sodium p-styrenesulfonate-co-N-methylol acrylamide) (P(SSNa-co-NMAM)) copolymer was prepared by the method of a free radical polymerization. Typically, SSNa (2 g, 9.670 mmol), NMAM (0.109 g, 1.078 mmol), and K 2 S 2 O 8 as a initiator (0.0844 g, 0.312 mmol) were added into 20 mL of DI water to obtain a homogeneous solution. Next, the mixture was purged nitrogen for 20 min to remove oxygen before putting the ask into pre-heating oil bath at 70°C. After 16h reaction, the mixture was diluted with DI water and washed repeatedly with excess ethanol and added to DI water three times, and then vacuum dried at 40°C for 20 h to get the targeted copolymer, the yield was 93.1%.

Preparation of P(SSNa-co-NMAM)-coated cotton bers
After successful preparation of the P(SSNa co NMAM)NMAM), a series of cotton bers with different masses of P(SSNa co NMAM) were fabricated by the simple impregnation method. Herein, taking the P (SSNa co NMAM) solution with 5.0 wt% of concentration as an example, rst of all, a dip-coating solution was formed via dissolution of a certain amount of P(SSNa-co-NMAM) in 20 mL of DI water .Subsequently, 160 mg of the cotton bers was dipped into the above mentioned solution for 20 min, and then the immersed cotton ber was treated at 160 o C for 60 min to form the stable P(SSNa-co-NMAM) layer on the cotton brous surfaces due tothe thermo-crosslinking reaction. Finally, the uncross-linked P(SSNa-co-NMAM) on the coated cotton ber surfaces were removed by excess DI water and vacuum dried at 80°C for 12 h, named as PCF-5. Additionally, the other P(SSNa-co-NMAM) coated cotton bers (PCFs) were also synthesized with varying concentrations of polymeric solutions (1.0, 3.0, and 7.0 wt %) for comparison, which were denoted as PCF-1, PCF-3, and PCF-7, respectively.

Characterization
1 H NMR was conducted on a Bruker AV III HD 400 MHz NMR in D 2 O at room temperature. The Flourier transform infrared spectra (FTIR) were operated by a Bruker Vertex 70 spectrometer about a wavenumber range of the 500-4000 cm −1 . X-ray photoelectron spectroscopy (XPS, Kratos AXIS Supra apparatus) was adopted to detect the surface chemical elements of samples. The morphologies of the samples were observed by eld-emission scanning electron microscope (SEM, JSM-7500F). The instrument was operated at an acceleration voltage of 15.0 kV after gold spraying process.The UV−Vis spectrometer (Beijing Purkinje General Instrument Co., Ltd.) was used to measure the dye solution's absorbance.

Adsorption experiments
Herein, the batch adsorption experiments were done by varying parameters including initial dye concentration, solution pH, contact time, ionic strength, and temperature to study the adsorption behaviors of the PCFs towards cationic dyes. About 10 mg of the adsorbent was putted into the 20 mL of dye solutions with different initial concentrations (200-2400 mg/L) to investigate the adsorption performance. The experimental contact time ranged from 10 min to 60 min. The pH values of the dye solution were adjusted in presence of 0.01 mol/L HCl and 0.01 mol/L NaOH solutions. The effect of ionic strength for adsorption of dye was evaluated by varying concentrations of NaCl solution ranging from 0.0 mol/L to 0.6 mol/L. The adsorption thermodynamic studies were performed with temperature from 298 K to 318 K. After dye adsorption equilibrium, the adsorbent was ltered off and the nal concentration of dyes in the aqueous solution was determined using a UV-Vis spectrophotometer. The batch adsorption experiments were performed three times and the corresponding average values were given. Herein, the adsorption capacity (Q t ) of cationic dyes uptake by PCFs was calculated using the following equation Where C 0 and C t are the initial dye concentration and the dye concentration in the solution at a certain time (t), V is the volume of the solution, and m is the mass of absorbent (g).

Recyclability
The adsorbent's recyclability was evaluated by the adsorption-desorption process of the spent adsorbent from batch adsorption studies. The as-prepared PCF was placed into 50 mL of 50 mg/L MEB solution for 60 min. For the desorption and regeneration, the MEB adsorbed PCF was performed via immersing it into 0.1 mol/L HCl solution for desorption, then washed with excess DI water and vaccum dried at 70 o C for 8 h to yield regenerated PCF. Finally, the regenerated PCF was used for following cycles.

Preparation and Characterization
Herein, the P(SSNa-co-NMAM) was synthesized by free radical polymerization. In the P(SSNa-co-NMAM), SSNa contains sulfonic acid moieties as active sites for cationic dye adsorption (Goswami and Phukan 2017), and the function of NMAM units are thermo-crosslinking points (Yldz et al. 2001). The successful preparation of the P(SSNa-co-NMAM) was demonstrated by the FT-IR and 1 H NMR tests. In Figure 1a. the characteristic adsorption bands of sulfonic acid groups at 1007 cm −1 and 1035 cm −1 can be observed on corresponding FTIR spectra. Further, the chemical composition of the synthesized copolymer can be con rmed by 1 H NMR spectrum ( Figure S1). It suggested that approximately 9.8 mol% of NMAM units in the compolymer after calculating the integral areas between the peak at 6.5 ppm from the SSNa units and the broad peak at 0.6 ∽ 2.2 ppm from the backbone of P(SSNa-co-NMAM), which was close to the NMAM feeding ratio of 10 mol%. These results of 1 H NMR further con rmed the target P(SSNa-co-NMAM) was successfully synthesized. Subsequently, a cotton ber with P(SSNa-co-NMAM)-coating was fabricated by sample thermo-crosslinking. Herein, NMAM was employed as the thermo-crosslinker to anchor P(SSNa-co-NMAM) on the cotton berous surface due to N-methylol moieties highly e cient reaction with -OH groups from the cellulose and themselves at high temperature ( Figure S2) (Yin et al.

2011).
To explore the chemical structures of as-prepared P(SSNa-co-NMAM) coated cotton bers, the FTIR measurement was employed. The results were shown in Figure 1a. From the spectrum of PCFs, the adsorption bands at 1497 cm −1 and 1640 cm −1 were attributed to the skeletal vibration of the aromatic ring (C=C) from SSNa units, and the characteristic adsorption bands at 1007 cm −1 and 1035 cm −1 belonged to the sulfonic sodium groups from SSNa units. Meanwhile, the characteristic adsorption band at 833 cm −1 and 1408 cm −1 were assigned to C=O stretching and the C-N stretching vibration of the secondary amide for NMAM units. Therefore, these results imply successful formation of P(SSNa-co-NMAM) polymeric layers on the cotton brous surfaces. As expected, the intensity of these characteristic absorption bands from the P(SSNa-co-NMAM) increased when cotton bers were introduced into the higher concentration coating solution (Figure 1a), further indicating that the coated polymer amount on the cotton bers increased as polymeric souliton concentration increased. Moreover, the mass change of the PCF before and after treatment were summarized in Table S1, which suggested the coated weight of the P(SSNa-co-NMAM) on the cotton bers.
Afterward, the surface chemical composition of the PCF was analyzed by the XPS. As shown in Figure 1b, the main peaks at 288.5, 535.5 and 981.5 eV in cotton bers were ascribed to the binding energy of C 1s, O 1s and O (A), respectively. However, new peaks appeared at 1075.5 eV, 500.6 eV, 233.4 eV, 172.5 eV and 402.5 eV can be observed after immobilization of P(SSNa-co-NMAM) on the cotton bers, which were assigned to the S 2s, S 2p, Na 1s, Na (A) and N 1, respectively.
Next, the micromorphology of the PCFs was visualized through SEM measurement, and the corresponding SEM images were presented in Figure 2. As shown in Figure 2a-c, the pristine cotton ber has a smooth surface. However, the P(SSNa-co-NMAM) coated cotton brous surfaces could clearly observe polymeric layer (Figure 2d-f). Moreover, the SEM images can verify that as-prepared samples with different loading amounts of the P(SSNa-co-NMAM) could be easily adjusted by the concentration of the polymeric solution ( Figure S3). However, the concentration of the polymeric solution was 7%, the intact polymeric layer could be formed among the cotton bers, which would signi cantly decrease the speci c surface area of the samples. Furthermore, the energy-dispersive spectroscopy (EDS) of the sample was performed. The corresponding EDS spectrum (Figure 2g) and EDS mapping images (Figure 2h-k) were presented. The elemental mapping demonstrated a homogeneous distribution of Na and S elements derived from SSNa units on the PCF, indicating that P(SSNa-co-NMAM) are homogeneously coated on the entire cotton brous matrix. Until now, it can be concluded from the FTIR, XPS and SEM results that the P(SSNa-co-NMAM) coated cotton bers were successfully prepared.

Adsorption isotherm
Next, the adsorption capacities of the as-prepared PCFs were investigated. As depicted in Figure S5, the adsorption capacity of the PCFs toward cationic dyes signi cantly increased with increment of the coated mass of P(SSNa-co-NMAM) on the cotton bers. Notably, the adsorption capacity of the PCF-7 only slightly enhanced compared with that of PCF-5. The reason can be attributed to that some polymeric lms were formed on the PCF-7 surface, resulting in decrease of its speci c surface area compared to that of PCF-5. As a result, the active sites on the PCF-7 without signi cantly increased to adsorb dyes compared with that of PCF-5, implying the optimum composition ratio of the PCF-5. Thus, the PCF-5 was selected for the following experiments.
The excellent adsorption capacities are signi cant for absorbents which affect the practical application of materials. Hence, for investigation of adsorption isotherms of the PCF-5 toward cationic dyes, MEB, RhB, and MG were employed as the cationic dye models, and Figure S6 displays chemical structures of the dyes. As presented in Figure 3a, it could be observed that the adsorption capacity of the PCF-5 toward MEB showed an escalation tendency at the beginning of dye concentration. However, the plateau region appeared when a certain equilibrium concentration was attained. Meanwhile, the adsorption isotherms of PCF-5 toward MEB, RhB, and MG were presented in Figure 3b, which displaed the maximum adsorption capacites of 3976.10 mg/g for MEB, 2879.80 mg/g for RhB and 3071.55 mg/g for MG respectively. The difference of Q m values may be ascribed to the varying chemical structures of dyes resulting in the different interactions between the sulfonate moieties of PCF-5 and dye molecules (Zhan et al. 2021).
Furthermore, the adsorption isotherm is beni cal to reveal the mechanism of cationic dyes onto the PCF-5. Herein, the adsorption data were tted by the Langmuir and Freundlich empirical isotherm models. The Langmuir model proposes a monolayer sorption onto a homogeneous surface and and there is no interaction between the adsorbed particles, while the Freundlich model assumes that multilayer adsorption occurs resulting in the heterogeneous adsorption systems (Song et al. 2020). The linear form of abovementioned two isotherm models are able to be described in the following equations (Zhao et al. 2021a): The Langmuir model: In the two equations, q e (mg/g) is absorption capacity (mg/g) at equilibrium and q m (mg/g) is the maximum adsorption, C e (mg/L) is the concentration of the dye solution at equilibrium; K L (L/mg) is the Langmuir isotherm constant related to the free energy of adsorption; K F (mg/g) and n are the constant of the Freundlich model and heterogeneity factor, respectively. The Langmuir and Freundlich tting plots of PCF-5 toward MEB, RhB and MG dyes were presented in Figure 3c-d. The correlation coe cients (R 2 ) and model parameters were exhibited in Table 1. By the judgment on the values of the R 2 , the Langmuir model was more suitable in elucidating the adsorption process of cationic dyes onto the PCF-5 in comparison with Freundlich isotherm model, indicating that monolayer adsorption occurs. Noteworthyly, the dye adsorption capacity of PCF-5 is more competitive compared with numerous similar adsorbents (Table 2), which further con rms that the PCF-5 may be a promising adsorbent andcan be used in wastewater treatment.

Adsorption kinetics
In order to better inquire into the adsorption behaviors of the PCF-5, the effect of contact time on the adsorption of cationic dyes on absorbent was investigated. In Figure 4a, it was clearly observed that the adsorption equilibrium time of dyes increased with the increment of molecular weight of dyes. This result can be explained that dye with higher molecular weight has weaker thermal motion ability and greater steric hindrance, thus it is not easy to reach the adsorption site (Hussain et al. 2022). Generally, the PCF-5 toward cationic dyes exhibited rapid adsorption rate, especially in the rst 10 min, due to the presence of abundant active adsorption sites, and then gradually reached to adsorption equilibriums within 50 min. Selecting MEB as an example, a rapid adsorption rate of the PCF-5 was veri ed by the UV−vis spectra. As we can see from Figure 4b, as the prolongation of adsorption time the absorbance intensity rapidly disappeared, suggesting the rapidly decreased MEB concentrations in the solution within the short time, the corresponding optical picture also implying this phenomenon (Figure 4c).
Adsorption kinetic studies provide important information to reveal the mechanism of the dye adsorption process. Herein, the adsorption processes of PCF-5 toward MEB, RhB and MG were descired via the pseudo-rst-order (Eq. (2)) and pseudo-second-order (Eq. (3)) kinetic models, respectively, as expressed in the following equations (Yang et al. 2021a): The pseudo-rst-order model: Where q e is the amount of adsorbed dye (mg/g) at equilibrium state; q t is the adsorption amount at time t (min); k 1 (min −1 ) and k 2 (g/mg/min) denote the rate constant of the pseudo-rst-order and the pseudosecond-order adsorption kinetics, respectively. The related kinetic model parameters were listed in Table   3. It could be found that the correlation coe cients (R 2 ) of the pseudo-rst-order kinetic model were very closer 1 than that of the the pseudo-second-order model. Moreover, q e,cal calculated by pseudo-secondorder model was much closer to the experimental q e,exp as compared with the pseudo-rst-order model.
These results proved that the adsorption toward MEB of the PCF-5 obeys the pseudo-second-order model. Hence, these kinetic results veri ed that the chemical interaction between the cationic dye molecules and the sulfonic acid groups of the PCF-5 promoted the adsorption process.

Effect of pH value
The pH value of the water sample acts a very important role in this adsorption system. The changes of the ionic or neutrality state of the target compounds and the charge of the adsorbent with pH would promote or inhibit the interaction between the adsorbates and the adsorbents, and thus affect the adsorption performance (Zhang et al. 2013;Zhao et al. 2015). In Figure 5a, the removal rate of the PCF-5 slightly decreased at pH=2 that because of the protonation of the sulfonic acid group in a highly acidic environment. However, the PCF-5 presented an insigni cant effect of pH=2-11 on the adsorption performance. These results demonstrated that the PCF-5 could maintain stable adsorption capabilityin a wide range of pH.

Effect of ionic strength
Ions can be found commonly in industry wastewater including textile and aquaculture wastewater.
Therefore, the existence of salts in wastewater may affect the ionic strength between the adsorbent with the dye molecules ). Thus, the effect of ions strength was examined by addition of NaCl into the MEB solution. The result shown that the removal e ciency of PCF-5 was signi cantly inhibited when the salt concentration increased from 0 to 0.6 mol/L (Figure 5b). This phenomenon was probably attributed to the salt ions (Na + ) competition with MEB molecules to occupy the active adsorptive site with sulfonic acid groups from the PCF-5. Hence,the increase of salt concentration in MEB molecules will lead to the enhancement of shielding effect of salt ions on positively charged MEB molecules, thus reducing the removal e ciency of adsorbent.. Above phenomenon can be concluded that electrostatic interaction is a signi cant derive forcein the adsorption process of cationic dyes onto the PCFs.

Effect of Temperature and Adsorption Thermodynamic Analysis
Another important parameter in adsorption process is the e cacy of temperature on the adsorption rate.
Herein, the effect of temperature on the adsorption of dyes on the PCF-5 was investigated under different temperatures (298, 308, and 318 K). As depicted in Figure 6a, the removal rate of PCF-5 toward cationic dyes increased with the increment of temperature. It would be considered that the increment of the temperature facilitates the movement of cationic dye molecules and provides additional energy to enhance the interactions between the cationic dye molecules and the adsorption sites of the PCF-5.
To explore the spontaneous completion of the adsorption process and further to lookinto the adsorption mechanism of the PCF-5. The thermodynamic parameters of dyes adsorption onto PCF-5, including Gibbs free energy (ΔG 0 ), ntropy (ΔS 0 ), and enthalpy (ΔH 0 ), were calculated by the following equations  Table 4. The negative values of ΔG° con rmed that the adsorption process toward MEB, RhB, and MG onto the PCF-5 was spontaneous nature, indicating that the e ciency of adsorption is more desirable at higher temperatures. Additionally, considering the positive values of ΔH° for MEB onto PCF-5 revealed that an endothermic reaction was in the adsorption process (Far et al. 2020). Furthermore, the fact that the positive value of ΔS° also revealed a randomness increase on the solid-liquid interface between PCF-5 and dye solution. These analyses were consistent with the result that higher temperatures could promote the sorption process of cationic dyes on the PCF-5.

Selective Adsorption and Dynamic Filtration Performance
Based on the negatively-charged property owing to the sulfonic acid groups (-SO 3 H), the as-prepared PCF-5 was supposed to be bene cial to adsorb cationic dyes due to the electrostatic interaction (Zhou et al. 2018;Zheng et al. 2020). Three cationic dyes (MEB, MG and RhB) and three anionic dyes (MO, CR and NGB) were selected to study the adsorption performance of PCF-5, and Figure S6 shown their corresponding structures. The PCF-5 exhibited an excellent adsorption capacity for MEB, MG and RhB dyes, compared with that of MO, CR and NGB dyes ( Figure S7). Based on these conclusions, we further explored the selective adsorption performance of the PCF. As observed from Figure 7a, the color of the MEB/MO mixture solution switched from dark green to yellow after being fully adsorbed by PCF-5. Moreover, the UV-vis spectra suggested that the peak at 664 nm of MEB almost disappeared after the adsorption, while the peaks at 464 nm of MO was only a slight decrease, verifying that MEB was selectively adsorbed from the MEB/MO mixture dyes (Figure 7b).
Further, the PCF-5 was expected to have a highly e cient wastewater puri cation through the ltration process. Therefore, the dynamic ltration performance of the PCF-5 toward dyes was investigated. As shown in Figure S8, the 20 mL of dark blue MEB solution can be puri ed by PCF-5 membrane with a gravity-driven force. Besides, the dynamic selective separation measurement of the PCF-5 toward mixed dyes can also be carried out. As we can see, MEB could be successful separated by PCF-5 from MEB/MO mixture dyes during the ltration−separation process (Figure 7c). Meanwhile, the completely disappearing of the adsorption peak from MEB in UV−vis spectrum (Figure 7d) after dynamic ltration further proved that the cationic dyes can be highly effective adsorbed onto the PCF-5 from the mixture under dynamic state owing to the high adsorption e ciency of the PCF-5. Therefore, aforementioned adsorption experiments demonstrated that the PCF-5 possessed outstanding adsorption capacity toward cationic dyes.

Adsorption mechanism
To gain a deeper understanding on the adsorption behavior of the PCFs toward cationic dyes, adsorption mechanism is discussed in detail. It is not only for potential practical application but also for further exploitation of advanced adsorbents.
As mentioned above, the adsorption process of the PCFs conformed to the Langmuir model and pseudosecond-order kinetic model indicated that was considered to greatly involve the chemical interaction between cationic dyes and PCFs. Taking MEB as an example, the potential interactions between PCFs and MEB were comprehensively investigated. Firstly, the effect of ionic strength of the solution revealed that the adsorption capacity of the PCFs toward cationic dyes signi cantly decreased with increasing ionic strength, implying that the main driving force of MEB adsorption onto PCFs is the electrostatic interaction (Xu et al. 2018). Furthermore, the π-π interaction might have a role in the MEB adsorption since both the MEB molecules and the PCFs have the aromatic rings (Lei et al. 2020). Thus, these possible interactions between the PCFs and cationic dyes were illustrated in Figure 8. In short, the cationic dyes adsorption of PCFs could be summarized that the major mechanism for the adsorption of dyes is electrostatic interaction, while the π-π interaction also contribution of another adsorption mechanism.
3.9. Adsorption performance in simulated dye e uent The practical potentials of as-proposed PCF-5 was evaluated for treating a simulated dye e uent with containing multiple components (Chen et al. 2020). Table 5 listed the ingredients of the simulated dye e uent. The adsorption performance of PCF-5 on simulated dye e uent was demonstrated by the UV-vis spectra and corresponding photographs (Figure 9). In Figure 9a, the removal e ciency of PCF-5 could reach to 99.9% in the simulated dye e uent. Furthermore, Figure 9b shown that the original color of the simulated dye e uent changes from purple blue to transparency after treatment by the PCF-5. These results can further manifest that the PCF-5 showed excellent adsorption performance in contaminated wastewater remediation.

Recyclability
There is no doubt that the recyclability of adsorbents which is directly related to the cost-effectiveness of an adsorption process is also important indicators for its practical applications. In this work, the recyclability of the PCF-5 was evaluated via using successive cycles of adsorption-desorption. As presented in Figure 10, PCF-5 exhibited a quite stable property with a nearly constant removal e ciency during the recycling experiments. Its removal e ciency still maintained above 90% after 5 successive cycles. Meanwhile, the SEM image of the regenerated PCF-5 was presented in Figure S9a. It displayed that the cotton brous surfaces still coated by the P(SSNa-co-NMAM) copolymer well even after 5 successive cycles. Moreover, the S, Na and N peaks of the regenerated PCF-5 still can be obviously observed in the XPS spectra ( Figure S9b). Hence, from the results of the cyclic adsorption experiments and the characterization of regenerated PCF-5, it can be concluded that the PCF-5 possessed outstanding durable feature.

Conclusion
In summary, a novel P(SSNa-co-NMAM) coated cotton ber was fabricated by a simple thermocrosslinking and successfully applied them for the adsorption removal of cationic dyes from wastewater.The maximum adsorption capacity of the as-prepared PCFs for cationic dyes was 3965.1 mg/g for MEB, 2897.8 mg/g for RhB and 3074.5 mg/g for MG, respectively. Most importantly, the PCFs exhibited the e ciently separation capabilitytoward cationic dyes from the mixture under the both static and dynamicconditions. Interestingly, the PCFs could highly adsorb the cationic dyes from simulated wastewater. Furthermore, the PCFs exhibited excellent reusability, since the removal e ciency of the asprepared PCFs could be maintained even above 90% after 5 cycles. Given the combined advantages of low cost, high adsorption capacity, selected adsorption and excellent durability of the PCFs, we can foresee that PCFs would become an alternative adsorbent to the commercial adsorbents for wastewater treatment or other environmental remediation applications. <p>the UV−vis spectra <strong>(a)</strong> and the related photographs <strong>(b)</strong> of the simulated dye e uent before and after adsorption by the PCF-5 (PCF-5 dose=10 mg; concentration of dye solution=100 mg/L; volume of sample=20 mL).</p> Figure 10 <p>The removal e ciency of the regenerated PCF-5 toward MEB after 5 successive adsorption−desorption cycles. (PCF-5 dose=50 mg; concentration of dye solution=100 mg/L; volume of sample=50 mL).</p>