Selective electrostatic sorption of water-soluble anionic molecules over electrospun cationic polymer fibers

Abstract

The development of cationic-decorated polymeric electrospun fibers is a promising approach for removing soluble anionic contaminants in wastewater treatment applications. Ionomers exhibit excellent performance in binding and capturing ionic contaminants, but preventing their dissolution in aqueous environments necessitates cross-linking the ionomeric chains. However, cross-linked ionomeric networks do not lend themselves well to various polymer-processing techniques, which restricts their utilization as membranes and fibers for water treatment. Herein, we report the successful formulation of dispersed cross-linked ionomer into linear polymer solution to construct a spinning matrix for fiber electrospinning. The generation of water-permeable electrospun fibers decorated with a cationic network demonstrated high adsorption capacities and a selective affinity towards anionic pollutants, including methyl orange, phenol red, eosin yellow, and Congo red. The electrostatic interactions between the cationic fibers and the anionic contaminants resulted in removal efficiencies up to 99%, with adsorption capacities reaching 500 mg g⁻¹, in contrast to cationic pollutants (≤5%). Moreover, comparative studies revealed that the fibers incorporating the cationic cross-linked polymer outperformed a control fiber membrane utilizing the linear polymer polycaprolactone, confirming the critical role of the cationic surface charges in achieving the recorded uptake. Interestingly, the adsorption isotherms highlighted the influence of the contaminant size and structure on the adsorption mechanism, where small molecules demonstrated adsorption isotherms best fitted by the Langmuir model, while bulky, long-chain contaminants, such as Congo red, followed the Freundlich model. Additionally, the nanofibers demonstrated outstanding regeneration and reusability, maintaining removal efficiencies of 99% for MO and 78% for PR after ten adsorption–desorption cycles. The data collectively highlight the potential of this approach for practical applications in sustainable water treatment.

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Introduction

Freshwater scarcity is becoming a global concern, necessitating the preservation of our natural resources through the reduction of waste discharge, recycling, and the reuse of industrial wastewater.^1–3 To achieve this goal, a growing interest has emerged in novel materials^4–7 paving the way to emerging technologies for efficient, rapid, and cost-effective wastewater treatment. Soluble organic dyes in industrial wastewater discharges are particularly concerning due to their toxic, non-biodegradable nature, which can potentially disrupt aquatic ecosystems.^8–10 Polymeric fibers have shown great promise for industrial wastewater treatment due to their high surface area, tunable composition, and compatibility with high flux applications. However, specific challenges remain to be addressed, including chemical stability, fouling, and insufficient selectivity toward specific contaminants.^11–13 Utilizing non-charged polymers¹⁴ for membranes and fiber filters in water treatment is justified by their insolubility in aqueous media, with a wide variety of usable polymers including, but not limited to, polysulfone, polyacrylonitrile, polyamides, and polyesters.¹⁴ However, the limited presentation of surface functional groups on such polymers restricted their water treatment action to physical filtration of contaminants. Utilizing ionomers, polymeric material with a high content of ionic repeat units, is expected to demonstrate greatly enhanced removal efficiency of charged or ionic contaminants.^15,16 However, highly ionic polymers are water soluble, which preclude their usage in polymeric membranes and fibers for industrial wastewater treatment. Cross-linked ionic matrices have, therefore, emerged as a plausible solution to this fundamental challenge, commonly known as ion-exchange resins.¹⁷ Due to their cross-linked nature, such resins are insoluble also in common solvents, precluding further processing into different shapes and forms like membranes or electrospun fibers.¹⁸ Herein, we demonstrate an approach by which a cationic, cross-linked polymer (CCP) can successfully be incorporated into the matrix of soluble, spinnable linear polymer to construct a permeable water treatment fiber mat with far enhanced capability for electrostatic-driven capture of soluble contaminants from the water stream.

Results and discussion

The synthesis of the cationic cross-linked polymer (CCP) is outlined in Scheme 1. The reaction of the amine groups (–NH₂) of the polyallylamine, as nucleophiles, with epichlorohydrin resulted in cross-linkage of the polyallylamine chains with pendant alcohol groups decorating the formed network. While the formed network contained primary and secondary amine groups capable of bearing cationic charges in neutral to slightly acidic medium, the cationic charge on the backbone of the solid can further be enhanced through decoration with tertiary alkylammonium groups. We achieved this goal by tethering hexyl trimethyl ammonium groups to the polymer backbone to produce a cationic cross-linked polymer (CCP), maintaining its surface charges independent of the solution pH.

Scheme 1 Synthesis scheme for the cationic cross-linked polymer (CCP).

To prepare a proper matrix amenable to electrospinning, a dispersion of thus prepared CCP into polycaprolactone (PCL) was attempted. The CCP particles were first swelled in a polar solvent dimethylsulfoxide (DMSO), followed by mixing with a solution of PCL in dichloromethane (DCM). To achieve adequate swelling and dispersion of the CCP, the solid was maintained in DMSO at 50 °C under constant stirring for 24 h. The mixture was then cooled to ambient temperature, and dichloromethane (DCM) was added. Finally, PCL pellets were added portion-wise and left to stir for an additional 24 h. This stepwise protocol was necessary to allow for the formation of a homogenous mixture through hydrogen bonding interactions between the cross-linked polymer, with abundant secondary amine groups, and the carbonyl groups of the PCL chains, thus avoiding phase separation during the spinning of the targeted 10 wt% cationic cross-linked polymer-PCL fibers (CCP@PCL). The percentage of the integrated CCP was dictated by controlling the relative amounts of the CCP dispersed in the DMSO, and PCL dissolved in DCM.

The Fourier transform infrared (FTIR) spectroscopy was utilized to characterize the isolated fibers. The FTIR spectra, Fig. 1, demonstrated the presence of two pronounced peaks at 1726 cm⁻¹, characteristic of the C=O stretching in the PCL polymer, and the band observed at 3388 cm⁻¹, which is characteristic of O–H stretching of the cationic cross-linked polymer. This validates the inclusion of both components within the electrospun fibers. Moreover, thermogravimetric analysis was conducted for each component, as well as the isolated fibers (Fig. 2). The TGA traces revealed distinct decomposition steps corresponding to the PCL matrix and the cross-linked polymer, with the observed weight loss and remaining percentages aligning with the intended loading percentage.

Fig. 1 FTIR spectra of the CCP, PCL, and 10% CCP@PCL electrospun fibers, highlighting the C=O stretching band of PCL and O–H stretching band associated with the incorporated cationic cross-linked polymer.

Fig. 2 TGA traces of CCP, PCL, and 10% CCP@PCL electrospun fibers.

These findings validate the accurate composition and thermal stability of the electrospun fibers. The morphology of the obtained fibers was investigated using scanning electron microscopy (SEM). As shown in Fig. 3, the SEM images illustrated an entangled network of fibers with multiple fusion points, which likely is a result of the use of DMSO solvent with high boiling point.

Fig. 3 SEM images of the 10% CCP@PCL electrospun fibers showing the dimensions and morphology of the electrospun fibers and EDX mapping demonstrating the elemental composition homogeneity in the fibers.

This porous and highly connected fiber network is ideal for producing a durable filtration membrane that exhibits both sufficient structural integrity and high porosity, enabling it to sustain high flux for efficient and rapid water treatment.

Contaminant uptake capacity and selectivity

To investigate the selective removal of anionic contaminants using the CCP@PCL fibers, 10 mL of 60 ppm solution of different contaminants was prepared and tested in batch reaction setup by contacting a piece of the fiber mat (50 mg) for 4 h, Fig. 4. This setup was conducted to test the performance of the CCP@PCL towards adsorptive removal of selected dyes, namely methylene blue (MB), methyl orange (MO), phenol red (PR), eosin yellow (EY), and congo red (CR). The selected dyes included both cationic and anionic organic contaminants. The CCP@PCL fibers achieved a removal efficiency of up to 99% for anionic contaminants, while cationic contaminant was minimally removed at only 5%. This indicates a strong selectivity of the cationic fibers for anionic dyes over cationic species, attributed to the electrostatic interactions with the positively charged nitrogen species present in the cross-linked polymer. Comparatively, the control PCL fibers demonstrated negligible contaminant removal and no discernible selectivity. Measurement of zeta potential for the CCP polymer suspended in DI indicated a median value of +48 mV, further supporting the argument for electrostatic-driven sorption of anionic molecules within the CCP@PCL fibers.

Fig. 4 Selective removal of anionic contaminants utilizing the 10% CCP@PCL after 4 h of contact with either anionic or cationic contaminant solutions, in comparison to that of the PCL electrospun fibers as a control.

Moreover, the dependence of the uptake capacity of the CCP@PCL on solution pH was probed by measuring the removal efficiency of MO, as a model anionic dye, in different pH of the solution, Fig. 5. The CCP@PCL demonstrated very comparable uptake capacity toward MO, at broad pH range, namely from pH 2–12, without significant changes to the capacity of the fibers towards adsorptive removal of MO, indicating that the surface-expressed cationic groups of the CCP@PCL are maintained throughout this wide pH range. The presence of tertiary alkylammonium groups within the CCP is likely the key to maintaining this activity over a large pH window. Furthermore, a separate experiment was designed to investigate the stability of the CCP@PCL against highly acidic environments that could potentially be experienced in real industrial wastewater. Incubation of the CCP@PCL fibers in 0.1 M HCl solution with mechanical agitation for 24 h did not result in a noticeable decay of uptake capacity towards PR and EY molecules, Fig. 6. However, some decay in uptake capacity towards MO was noticed after incubation in 0.1 M HCl, which could be attributed to competitive binding of chloride ions. As the MO ions are the smallest among the group of ionic molecules investigated, and indeed the one demonstrating the highest uptake on CCP@PCL, it is reasonable to assume competitive binding of chlorides to the smaller binding sites at the CCP@PCL fibers, thus affecting its capacity after strong acid incubation.

Fig. 5 Removal capacity of MO at different pH ranges (initial concentration 150 ppm) adsorbed within the 10% CCP@PCL electrospun fibers.

Fig. 6 Aging and stability test of the CCP@PCL fibers, incubation in 0.1 M HCl under constant mechanical agitation for 24 h.

Sorption kinetics and isotherms

The fibers also demonstrated rapid kinetics for the adsorption of all anionic dyes, reaching quantitative removal in 60–80 min after contacting a solution of each dye with the fibers, Fig. 7. It is evident that the CCP@PCL fibers are characterized by rapid adsorption of soluble anionic dyes from aqueous solutions, in contrast to many common examples found in the literature where several hours of contact time are required for efficient guest adsorption. To further assess the maximum uptake capacity of different anionic contaminants by CCP@PCL, adsorption isotherms were conducted at 298 K, Fig. 8. The adsorption capacity reached maximum values of 263 mg g⁻¹ for MO (0.8 mmol g⁻¹), 294 mg g⁻¹ for PR (0.83 mmol g⁻¹), 500 mg per g EY (0.77 mmol g⁻¹), and 588 mg g⁻¹ for CR (0.84 mmol g⁻¹), Table 1. Interestingly, the uptake capacity expressed in mmol of contaminant per gram of the CCP@PCL indicated very close similarity around 0.8 mmol g⁻¹ for all anionic dyes investigated, which indicated an electrostatic-driven adsorptive process for the uptake of anionic molecules by the cationic CCP@PCL fibers. For contaminants with relatively small molecular sizes, the adsorption isotherms can best be fitted by a Langmuir model. In contrast, those with relatively large molecular sizes tend to be fitted better with a Freundlich-type isotherm. This observation can potentially be explained by assuming adsorbate–adsorbate interactions through pi–pi stacking for relatively large molecules adsorbed onto the CCP@PCL fibers.

Fig. 7 Kinetic profiles for the removal of the anionic dyes by the CCP@PCL fibers in 1 h contact time with the aqueous solutions.

Fig. 8 Adsorption isotherm data for MO, PR, EY, and CR, fitted to Langmuir and Freundlich models.

Table 1 Adsorption isotherm fitting parameters of the Langmuir and Freundlich models for uptake of MO, PR, EY, and CR by 10% CCP@PCL electrospun fibers

Model	Methyl orange	Phenol red	Eosin yellow	Congo red
Langmuir
Slope	0.0038	0.0034	0.002	0.0017
q m	263.16	294.12	500	588.24
Uptake mmol g^−1	0.8	0.83	0.77	0.84
Intercept	0.0237	0.0229	0.0466	0.1392
b	0.1603	0.1484	0.0429	0.0122
R ^2	0.9998	0.9962	0.9749	0.9148
Freundlich
Slope	0.1346	0.3555	0.3175	0.3056
n	7.4294	2.8129	3.1496	3.2723
Intercept	2.0559	1.7520	1.8837	1.8368
K f	113.7365	56.4937	76.5068	68.6752
R ^2	0.8408	0.8323	0.7826	0.9741

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Kinetic model evaluation

Four kinetic models–pseudo-first-order (PFO), pseudo-second-order (PSO), intra-particle diffusion (IPD), and mass transfer models–were employed to interpret the dye removal performance for methyl orange (MO), phenol red (PR), eosin yellow (EY), and Congo red (CR). These dyes vary in molecular structure and weight, which in turn influence their interaction with the polymer matrix. The kinetic parameters and correlation coefficients (R²) for the PFO, PSO, IPD, and mass transfer models are summarized in Table 2. The results reveal that no single model alone comprehensively describes the adsorption process for all dyes, highlighting the complexity of interactions in cross-linked systems. Methyl orange (MO) showed moderate fits with the PFO and PSO models (R² = 0.7549 and R² = 0.7868, respectively) but a strong fit with the IPD model (R² = 0.9612), Fig. 9. The plot revealed two kinetic phases: an initial fast adsorption attributed to surface interaction, followed by a slower diffusion into the polymer matrix. This suggests a diffusion-controlled mechanism, where the transport of dye into the polymer matrix is the rate-limiting step. Phenol red (PR) had the highest PSO fit (R² = 0.9972), indicating that chemisorption dominates the uptake mechanism, likely through strong electrostatic interactions with the cationic sites. The IPD model fit was also good (R² = 0.9384), suggesting that while intra-particle diffusion is present, binding to active sites is rapid and efficient, with minimal steric hindrance due to its relatively moderate molecular size.

Table 2 Kinetic parameters and correlation coefficients (R²) of the pseudo-first order, pseudo-second order, and intra-particle diffusion models for MO, PR, EY, and CR uptake by 10% CCP@PCL

Model	Parameter	MO	PR	EY	CR
Pseudo-first order	k 1 (min^−1)	0.0348	0.0893	0.2002	0.2726
	R ^2	0.7549	0.9163	0.8178	0.9822
Pseudo-second order	k 2 (g mg^−1 min)	2.55 × 10^−4	4.354 × 10^−3	4.36 × 10^−4	2.01 × 10^−6
	q e (mg g^−1)	47.39	31.35	38.17	370.37
	R ^2	0.7868	0.9972	0.8576	0.0249
Intra-particle diffusion model	k id (mg g^−1 min^−0.5)	2.9333	1.0927	2.4856	3.7373
	R ^2	0.9612	0.9384	0.855	0.9529
Mass transfer model	K f (min^−1)	0.007	0.0163	0.0095	0.0007
	R ^2	0.9612	0.8943	0.928	0.9833

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Fig. 9 Intra-particle diffusion plots for (a) MO, (b) PR, (c) EY, and (d) CR adsorption by the 10% CCP@PCL at 298 K.

Conversely, Eosin yellow (EY) displayed a three-stage profile in the IPD plot: the initial phase corresponds to external surface adsorption, followed by a sharper uptake phase attributed to progressive pore filling, and finally, a plateau phase indicating active site depletion. Its moderate PSO fit (R² = 0.8576) and IPD fit (R² = 0.855) suggest mixed kinetics, where both chemisorption and pore diffusion contribute. The larger molecular size of EY likely introduces steric effects, delaying deep pore penetration and necessitating reorientation within the polymer matrix. Congo red (CR) exhibited a similar three-stage diffusion profile, with strong fits for the PFO (R² = 0.9822) and IPD (R² = 0.9529) models, but an inferior PSO fit (R² = 0.0249). The early phase is dominated by rapid physisorption on the surface, facilitated by electrostatic attractions. However, the transition into slower uptake and final equilibrium reveals an apparent saturation of active sites. The plateau indicates that site availability becomes the limiting factor, particularly due to CR's large molecular size. These features inhibit its ability to penetrate deeply into the polymer network, leaving many inner active sites underutilized.

Active site depletion and diffusion-controlled mechanisms

The concept of active site depletion becomes especially relevant in the third kinetic stage observed for EY and CR. As adsorption progresses, readily accessible cationic sites on the polymer surface and within shallow pores are occupied first. Subsequently, dye molecules must navigate more tortuous paths to reach inner binding sites, which are increasingly shielded by already adsorbed dye or restricted by steric hindrance, resulting in a slowed adsorption rate. This is consistent with reports on adsorbents where the diffusion rate decreases significantly upon approaching equilibrium due to both reduced concentration gradients and physical obstruction from earlier adsorbed layers.

Influence of molecular structure and size

The molecular dimensions and geometry of the adsorbate heavily influence the number and accessibility of active sites. The adsorption mechanism transitions from a kinetically fast and site-rich (MO, PR) to a diffusion-controlled and site-limited (EY, CR) process. As the dye molecule size increases, so do steric constraints, leading to partial pore blockage and a heterogeneous distribution of accessible active sites. For these bulkier dyes, active site availability becomes a dynamic parameter, decreasing progressively as larger molecules dominate outer adsorption layers. In conclusion, film diffusion followed by surface adsorption dominates early uptake, especially for smaller to moderate dye molecules (MO and PR). However, film and intra-particle diffusion is the rate-limiting step for larger dyes (EY and CR), where penetration into the polymer matrix is hindered. Active site depletion emerges as a critical factor in the later stages, particularly for bulky dyes, such as EY and CR.

Performance comparison to similar systems

The observed capacity of CCP@PCL towards anionic dyes is remarkable when compared to recent reports of anion-exchange membranes and nanofibers, Table 3. The adsorption performance of various materials for dye removal reveals significant differences based on material structure and dye properties. Traditional anion exchange membranes (AEMs), such as AEM BI, AEM BIII, and BPPO, exhibit limited methyl orange (MO) uptake (∼17–20 mg g⁻¹), attributed to low functional site density. In contrast, functionalized AEMs (e.g., DMEA-grafted, AE-PVC) demonstrate significantly enhanced adsorption capacities (90–100 mg g⁻¹) due to increased cationic group density. Hyper-cross-linked porous polymers (HCPs) further improve performance (up to ∼275 mg g⁻¹) by offering high surface areas and tunable porosity. Notably, CCP@PCL matches HCP capacity (∼263 mg g⁻¹) while achieving superior removal efficiency (∼99%), indicating more effective active site utilization and stronger surface affinity for MO.

Table 3 Removal capacity of anionic dyes (MO, CR, EY, and PR) by polymer-based membranes

	Composite material	Removal capacity (mg g^−1)	Removal (%)	Ref.
Methyl orange	AEM BI	19.95	—	19
	AEM BIII	19.85	—
	BPPO-based AEM	17.85	—	20
	Chitosan	15.75	—	21
	(DMEA)-grafted AEM	100	—	22
	Modified-AE-PVC membrane	90.67	—	23
	Triethanolamine functionalized-AEM	19.42	—	24
	Glutaraldehyde-polyvinyl alcohol cross-linked cellulose membrane (P-G-RC)	—	93.5	25
	Carbazole-based hyper-cross-linked porous polymer (HCP-CP)	274.73	92	26
	Bi-heteroatom functionalized hyper-cross-linked porous polymers (HCP-CT)	212.77	80	27
	Bi-heteroatom functionalized hyper-cross-linked porous polymers (HCP-CF)	131.75	80
	CCP@PCL	263.16	99	This work
Congo red	Chitosan-sulfonated polyphenyl sulfone electrospun nanofibers	531.56	70	28
	AFGM	92.69	86	29
	Nitrogen-doped polyvinyl alcohol (PVA) cross-linked graphene sponges (N-PGA)	112	89.6	30
	Cross-linked poly(vinyl alcohol)/ZnO-vitamin M (PVA/ZnO-VM)	56.49	92	31
	PVA/PEI/rGO membrane	226.36	—	32
	CCP@PCL	588.24	95	This work
Eosin yellow	Ethylenediamine modified chitosan	294.12	—	33
	CCP@PCL	500	93	This work
Phenol red	CCP@PCL	294.12	78	This work

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For Congo red (CR), a bulky anionic dye, effective removal necessitates high surface area and strong electrostatic interactions. While chitosan composites exhibit moderate performance due to swelling and limited charge density, graphene-based materials demonstrate improved uptake (112–226 mg g⁻¹) through π–π stacking and hydrogen bonding. CCP@PCL exhibits exceptional CR removal (588.24 mg g⁻¹, ∼95% efficiency) attributed to optimized porosity and strong electrostatic affinity that facilitate access for large dye molecules.

Eosin yellow (EY) removal benefits from amine-functionalization, as evidenced by ethylene-diamine-modified chitosan. CCP@PCL significantly outperforms these systems, achieving a capacity of 500 mg g⁻¹, likely due to its multivalent cationic binding sites and minimal steric hindrance from its open porous structure. Although limited data are available for phenol red (PR), CCP@PCL shows promising performance, which is expected given PR's moderate size and sulfonation level and the material's favorable pore architecture.

Overall, CCP@PCL consistently outperforms conventional and advanced adsorbents across multiple dyes, particularly with bulky and sterically hindered molecules like CR and EY. While hyper-cross-linked polymers set a high standard for adsorption capacity, CCP@PCL achieves comparable or superior performance with potentially simpler synthesis and added benefits such as tunable surface chemistry and biodegradability. Dye adsorption efficiency is governed not only by capacity but also by the nature of dye–adsorbent interactions, including electrostatics, π–π stacking, hydrogen bonding, and steric accessibility.

The recyclability of the CCP@PCL fibers was investigated through several adsorption–regeneration cycles, revealing an exceptional ability for regeneration, making them suitable for long-term applications in wastewater treatment. The fibers demonstrated exceptional stability, maintaining a 99% removal efficiency toward methyl orange for up to 10 consecutive cycles without detectable loss of performance. For phenol red, the removal efficiency was initially around 95% during the first cycle, dropping to 77% for cycles 2–10. This observation can be rationalized by assuming irreversible trapping of PR within tight binding sites at the CCP. Simultaneously, the membrane efficiency is still highly maintained beyond this initial first drop in capacity (Fig. 10). This decline, although noticeable, still reflects a significant retention of functionality over repeated cycles. After each adsorption cycle, the fibers were regenerated using an acid wash followed by deionized water rinsing, effectively flushing away the adsorbed organic molecules and regenerating the matrix for subsequent use. These findings validate the facile regeneration of the material, making it a viable option for the sustainable removal of dyes from wastewater. Finally, the utilization of PCL as the polymer matrix in this work was selected not only due to its excellent mechanical properties and processability for the electrospinning process but also due to its biodegradability, which adds value and minimizes the environmental impact of the produced membranes.

Fig. 10 A batch regeneration study was conducted on (a) 10 mL of 15 ppm MO and (b) 10 mL of 30 ppm PR, stirred with 10 wt% CCP@PCL electrospun fibers for 1 h. Regeneration after each run was conducted by washing with 10 mL of acid, followed by 10 mL of deionized water.

Modeling the cationic surface

To assess the hypothesis for charge-assisted adsorption of the anionic molecules at the cationic sites of the CCP, a model cationic surface of trimethylcetylammonium molecules was constructed. A molecule of MO was allowed to probe all possible conformations near this cationic surface, among which the most favorable orientation is depicted in Fig. 11. The model demonstrated that charge-assisted adsorption is much favored as compared to weaker van der Waals attractive forces, outlining the successful design principle for cationic CCP immobilization within the PCL matrix for adsorptive removal of anionic dyes from wastewater. The molecular surface is constructed by placing 100 (10 × 10) trimethylcetylammonium in a periodic structure using the Atomic Simulation Environment (ASE) Python package.³⁴ The nitrogen-to-nitrogen distance for two adjacent molecules is 5.9 Å. This distance is obtained using geometry optimization of 3 molecules using Density Functional Theory. DFT is also used to obtain the partial charge distribution using B3LYP functional and 6-31G+(d,p) basis sets for both trimethyl cetyl ammonium and methyl orange. The methyl orange is placed on the top of the surface, and 100 rotational conformers are generated using a randomly generated rotation matrix. The pairwise electrostatic interactions between the different fragments in the simulations are calculated using DelPhi.³⁵ The surface is assigned a dielectric constant of 4, while the surrounding and the cavities are filled with implicit water (dielectric of 80). The probe radius for placing water is 1.4 Å, and 0.15 M salt concentration is used. MCCE³⁶ was used to generate Boltzmann occupancies for the different rotational conformers based on the calculated electrostatic energies. The most selected conformer is the one in which the sulfate group points vertically toward the surface, as shown in Fig. 11. This results in a strong, favorable electrostatic interaction between the negatively charged oxygen and the positively charged hydrogen atoms. The difference in energies between the selected conformer, which is pointing down to the surface, and the conformer that is pointing up, is around 37 kcal mol⁻¹ (Fig. 11).

Fig. 11 Model of the methyl orange interaction with trimethylcetylammonium cationic surface where (a) 100 possible orientations of the MO were sampled demonstrating (b) the most energetically stable conformation for charge-assisted surface interactions, (c) and (d) are magnified side and top view of the system, respectively.

Conclusion

We presented a facile approach to construct electrospun fibers of a cationic cross-linked polymer uniformly dispersed within a matrix of a linear spinnable polymer, enabling the selective uptake of anionic model compounds from aqueous solutions. The produced fibers demonstrated a high uptake capacity towards anionic dyes, including methyl orange, phenol red, eosin yellow, and congo red, with a notable ability to regenerate the fibers for subsequent use in dye removal over 10 cycles without significant loss of removal efficiency. Moreover, it is demonstrated in carefully designed control experiments that the cationic nature of the fibers did not allow for the uptake and removal of cationic dye. In contrast, the fibers prepared from the non-charged PCL demonstrated minimal and non-specific uptake of the investigated probe molecules. This approach paves the way for expanding the horizon of anion-exchange resins into anion-exchange fibers and membranes, thereby circumventing the need for tedious preparative cross-linking steps previously required to generate ionomer-based membranes for water purification applications.

Experimental

Materials

All reagents were used as received without further purification. Polycaprolactone (PCL, average Mn 80 000) was purchased from Sigma-Aldrich. Dichloromethane (DCM, HPLC grade) and dimethyl sulfoxide (DMSO, >99%) were purchased from Fisher Scientific, UK. Methyl orange (MO) and methylene blue (MB) were purchased from Cambrian Chemicals. Eosin yellow (EY) (water soluble) was purchased from SRL-India, and Congo red (CR) from ACG-Egypt. Poly(allyl-amine hydrochloride), 97% average MW ∼15 000, and 6-bromo-N,N,N-trimethylhexan-1-ammonium bromide (95%) were purchased from BLD Pharm, sodium hydroxide (pellets, extra pure, 98%) from PioChem, and methanol (HPLC grade) from Merck. Deionized water was used to prepare the solutions (MilliQ).

Characterization

Fourier Transform Infrared (FTIR) spectroscopy was conducted on a ThermoScientific iQ-10. Thermogravimetric analyses were conducted on a Thermal Analysis Q50 under a nitrogen atmosphere. Scanning electron microscopy (SEM) was conducted on Thermofisher (USA) Quattro S Field Emission Gun, Environmental SEM (FEGSEM), and Ultraviolet-visible spectroscopy (UV-vis) was conducted on ThermoScientific Evolution 600 to determine the uptake of colored contaminants.

Synthesis of the poly(allylamine) hydrochloride cross-linked polymer

To a solution of polyallylamine (1 g) in deionized water (10 mL), 150 mg of sodium hydroxide was added. The mixture was cooled to 10 °C, and 200 μL of epichlorohydrin was added dropwise while stirring. The reaction was allowed to proceed at room temperature for 16 h, resulting in a colorless, gelatinous mass. The mass was diluted with an additional 10 mL of deionized water and stirred for 1 h. The product was then isolated by centrifugation, followed by air drying, yielding the desired compound as a granular, colorless solid. (Yield = 880 mg).

Hexyltrimethylammonium-grafted cationic cross-linked polymer

250 mg of sodium hydroxide solution in 15 mL deionized water was added in portions to a solution containing 1 g of poly(allylamine) hydrochloride cross-linked polymer and 1.5 g of 6-bromohexyltrimethyl ammonium bromide in 20 mL of methanol. The reaction mixture was stirred at 65 °C for 20 h. After completion, methanol was evaporated under reduced pressure, yielding a foam-like gel. The gel was washed three times with deionized water (20 mL) and subsequently dried by lyophilization. The resulting product was a white, fluffy powder. (Yield = 1.4 g).

Electrospinning of poly-caprolactone fibers

In a clean glass vial, 1 g of dry polycaprolactone (PCL) was left to stir at room temperature in a mixture of 7.0 mL of dichloromethane (DCM) and 4.0 mL of dimethyl sulfoxide (DMSO). After complete dissolution, the solution was loaded into a 20 mL syringe fitted with a 21-gauge needle. A home-made electrospinner was used to prepare the electrospun fibers at a potential difference of 23 kV with a flow rate of 5.0 mL h⁻¹ at a distance of 5 cm between the needle tip and the collection drum.

Electrospinning of 10 wt% CCP@PCL fibers

To ensure the homogenous distribution of the cationic cross-linked polymer within the electrospun PCL fibers, 100 mg of CCP was left to swell in 4 mL of DMSO at 50 °C for 24 h under stirring prior to the addition of 7 mL of DCM and 900 mg of PCL, portion-wise, at room temperature. The mixture was then stirred at room temperature for an additional 24 h before electrospinning.

Calibration curves for UV-vis spectroscopy

Methyl orange (MO). Different MO concentrations (2.5, 5, 10, 15, 20, and 25 ppm) were prepared. First, a solution of 100 ppm MO was prepared by dissolving 25 mg of MO in 250 mL of DI water, and subsequent dilutions were prepared with desired concentrations. Finally, UV-vis spectrophotometry was used to determine the absorbance of each of the prepared concentrations at λ_max of 464 nm to construct the calibration curve.

Methylene blue (MB). Different MB concentrations (2.5, 5, 10, 15, 20, and 25 ppm) were prepared. First, a solution of 100 ppm MB was prepared by dissolving 25 mg of MB in 250 mL of DI water, and subsequent dilutions were prepared with desired concentrations. Finally, UV-vis spectrophotometry was used to determine the absorbance of each prepared concentration at λ_max of 665 nm to construct the calibration curve.

Phenol red (PR). Different PR concentrations (2.5, 5, 7.5, 12.5, 15, and 17.5 ppm) were prepared. First, a solution of 100 ppm PR was prepared by dissolving 25 mg of PR in 250 mL of DI water, and subsequent dilutions were prepared with desired concentrations. Finally, UV-vis spectrophotometry was used to determine the absorbance of each prepared concentration at λ_max of 430 nm to construct the calibration curve.

Eosin yellow (EY). Different EY concentrations (2.5, 5, 10, 15, 20, 25, 30, 25 and 40 ppm) were prepared. First, a solution of 100 ppm EY was prepared by dissolving 25 mg of EY in 250 mL of DI water, and subsequent dilutions were prepared with desired concentrations. Finally, UV-vis spectrophotometry was used to determine the absorbance of each prepared concentration at λ_max of 514 nm to construct the calibration curve.

Congo red (CR). Different CR concentrations (4, 8, 12, 16, 20, 24 ppm) were prepared. First, a solution of 100 ppm CR was prepared by dissolving 25 mg of CR in 250 mL of DI water, and subsequent dilutions were prepared with desired concentrations. Finally, UV-vis spectrophotometry was used to determine the absorbance of each prepared concentration at λ_max of 499 nm to construct the calibration curve.

Determining the uptake selectivity using batch studies

Methylene blue. In a sealed glass vial, ∼50 mg of 10% CCP@PCL fiber was left to stir at room temperature with 10 mL of 72 ppm MB for 4 h, after which the supernatant was collected for UV-vis acquisition.

Methyl orange. In a sealed glass vial, ∼50 mg of 10%CCP@PCL fiber was left to stir at room temperature with 10 mL of 63 ppm MO for 4 h, after which the supernatant was collected for UV-vis acquisition.

Phenol red. In a sealed glass vial, ∼50 mg of 10%CCP@PCL fiber was left to stir at room temperature with 10 mL of 40 ppm PR for 4 h, after which the supernatant was collected for UV-vis acquisition.

Eosin yellow. In a sealed glass vial, ∼50 mg of 10% CCP@PCL fiber was left to stir at room temperature with 10 mL of 52 ppm EY for 4 h, after which the supernatant was collected for UV-vis acquisition.

Congo red. In a sealed glass vial, ∼54 mg of 10% CCP@PCL fiber was left to stir at room temperature with 10 mL of 52 ppm CR for 4 h, after which the supernatant was collected for UV-vis acquisition.

Five control samples having ∼50 mg of PCL fiber and 10 mL of each dye separately were conducted to validate the effect of the cross-linked cationic polymer on the adsorption affinity and capacity of fibers.

Effect of pH

Methyl orange. Three sealed glass vials were prepared, each containing 10 mL of 150 ppm MO stock solution and 113 mg of the 10%CCP@PCL fiber, and the pH was adjusted to the desired pH through the addition of 0.1 M HCl or NaOH solution. After stirring at 560 rpm for 4 h, 1 mL of the supernatant was collected and diluted prior to UV-vis spectrum acquisition.

Adsorption isotherm studies

Methyl orange. In sealed glass vials, eight separate samples of 48.5 mg of 10%CCP@PCL fiber were added into 10 mL of MO solutions of different concentrations (36, 63, 124, 241, 501, 686, 947, and 1204 ppm) and stirred for 4 h (pH 7.2). Then, 1 mL of the supernatant was collected and diluted prior to acquiring the UV-vis spectrum.

Phenol red. In sealed glass vials, eight separate samples of ∼50 mg of 10%CCP@PCL fiber were added into 10 mL of PR solutions of different concentrations (24, 50, 106, 153, 197, 236, 265, and 353 ppm) and stirred for 4 h. Then, 1 mL of the supernatant was collected and diluted prior to acquiring the UV-vis spectrum.

Eosin yellow (EY). In sealed glass vials, seven separate samples of ∼50 mg of 10% CCP@PCL fiber were added into 10 mL of EY solutions of different concentrations (26, 53, 116, 213, 439, 640, and 842 ppm) and stirred for 4 h. Then, 1 mL of the supernatant was collected and diluted prior to acquiring the UV-vis spectrum.

Congo red (CR). In sealed glass vials, eight separate samples of ∼50 mg of 10%CCP@PCL fiber were added into 10 mL of CR solutions of different concentrations (32, 57, 120, 228, 445, 668, 911, and 1066 ppm) and stirred for 4 h. Then, 1 mL of the supernatant was collected and diluted prior to acquiring the UV-vis spectrum.

For all the adsorption isotherm studies, the uptake capacity at equilibrium, q_e (mg g⁻¹), was calculated using eqn (1):

where C₀ and C_e (ppm) are the initial and remaining dye concentration at equilibrium, respectively, V is the volume of the dye solution (mL), and W is the mass (mg) of CCP present in the 10% CCP@PCL fibers. Duplicate measurements were obtained for all data. The batch equilibrium data were fitted to the Langmuir and Freundlich isotherm models, from which the fitting parameters and correlation coefficients (R²) were calculated (Table 1).

Kinetics and regeneration study

Kinetics study. In a glass vial, a 50 mg sample of the PCC@PCL was cut into 3 pieces and left to stir at RT in contact with 10 mL of 15 ppm dye (MO, PR, CR, and EY) for different time intervals (10, 20, 40, 60, and 80 min). Then, the absorbance of the residual dye was measured using a UV instrument to calculate the percentage removal of each dye as a function of the contact time interval.

Methyl orange. In a sealed glass vial, 51 mg of 10% CCP@PCL fiber was cut into 3 pieces and left to stir at room temperature with 10 mL of ∼15 ppm MO (pH = 7.2) for 1 h and 30 min, after which the supernatant was collected for UV-vis acquisition. The sample was regenerated by stirring it in 10 mL of 0.1 M HCl for 30 min, followed by 10 mL of deionized water. These uptake and washing steps were repeated for 10 cycles.

Phenol red. In a sealed glass vial, 102 mg of 10% CCP@PCL fiber was cut into 5 pieces and left to stir at room temperature with 10 mL of ∼30 ppm PR for 1 h, after which the supernatant was collected for UV-vis acquisition. The sample was regenerated by stirring it in 10 mL of 1 M HCl for 30 min, followed by 10 mL of deionized water. This uptake and washing steps were repeated for 10 cycles.

Data availability

The datasets supporting this article have been uploaded as part of the ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We acknowledge the Academy of Scientific Research and Technology (ASRT) and L'Oreal-UNESCO For Women in Science (FWIS), Egypt Young Talents grant for the funding of this work.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta00290g

‡ These authors contributed equally to the study.

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