Construction of hydrophilic covalent organic frameworks and their fast and efficient adsorption towards cationic dyes from aqueous solution

doi:10.21203/rs.3.rs-1970200/v1

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Construction of hydrophilic covalent organic frameworks and their fast and efficient adsorption towards cationic dyes from aqueous solution

https://doi.org/10.21203/rs.3.rs-1970200/v1

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Research about the functionalized covalent organic framework (COF) materials with well hydrophilic characteristics is significant on account of their application in the removal of dyes in wastewater. In this work, two anionic COF materials (TFPB-Pa-SO₃H COF and TFPB-BDSA COF) were successfully synthesized with sulfonated modification as dyestuffs sorbent to be used in the adsorption of cationic dyes. Two prepared COF materials exhibited predominant hydrophilicity with lower water contact angles (35° and 27°) compared with reported COF materials. The sulfonated groups on COFs can efficiently adsorb cationic dyes through electrostatic interaction, which can moderately ameliorate the adsorptive capacity towards hydro soluble dyes. TFPB-Pa-SO₃H COF and TFPB-BDSA COF showed superfast adsorption equilibrium to MLB (1 and 2 min) and the higher adsorption uptakes of CV (1559 and 1288 mg g⁻¹), which manifested that the adsorption rate may be closely related to the solubilities of host COFs materials as well as guest dye molecule in aqueous solution; while the adsorption stability between COFs materials and the dye molecule adsorbed on the COFs skeleton accounted for the highly effective adsorption of the aim dye in large quantity. In addition, two COFs as adsorbing materials were successfully applied to the removal of dyes molecules in real water samples.

Covalent Organic Frameworks

sulfonated modification

hydrophilicity

fast adsorption

cationic dyes

real samples

Light industry had developed rapidly, resulting in the continuous and intensified discharge of waste effluents from manufacturing and consuming industries, which inevitably arouse the serious pollution worldwide [1]. The dyeing effluents due to their resistance of degradation were ubiquitous in aquatic environment [2], where they absorb sunlight and hinder the natural photosynthesis of organisms [3]. In one study, about 2 ~ 20% of dyeing effluents have been typically liberated into aqueous systems [4], causing severe damage to organisms [5], human body [6] and the entire ecosystem [7]. Therefore, confronting the threat to environmental conservation as well as public health, it is urgently desirable to cultivate multiple and effective approaches for treating dyeing wastewater. At present, a broad variety of removal technologies have been developed, such as photodegradation [8], electrochemical degradation [9], chemical oxidation [10], biological method [11], membrane separation and adsorption [12]. Among them, adsorption method has been widely acclaimed in wastewater treatment because of its extraordinary advantages of easy operation, lower cost, good reproducibility, and no secondary pollution, etc [13–16]. However, controllable and precise functionalization of adsorbing material remains a major challenge during the treatment process of dye wastewater, despite considerable superiority.

Covalent organic frameworks (COFs), as a burgeoning class of porous organic building blocks with periodic skeleton structure, have shown great potential applications in many areas for their extensive tunability and high stability [17–20]. In recent years, COFs as adsorbent has been demonstrated to be one of highly effective materials used in treatment of dye wastewater for the large specific surface area [21–24]. However, a bulk COF as an organic polymer usually has poor hydrophilicity and dispersity in aqueous medium, which limit its absorption ability toward soluble dyestuffs at a certain extent. Considering that COFs have a predesignable characteristic through topology design [25–27], we deem that felicitously embedding the functional groups with high hydrophily into the building blocks of COFs would be an ideal approach to improve and even surmount the above issue. More importantly, it could be envisioned that the hydrophilic modification of COFs scaffolds could simultaneously expand the diversity of new structures and modify surface properties, and in turn improve the adsorption performance. Consequently, it is a crucial issue to realize a setting strategy through the adequate regulation for the introduction pattern of the hydrophilic fragments. To our knowledge, the template strategy for the preparation of hydrophilic COFs has not yet been reported.

Herein, we synthesized two sulfonate-based COFs (TFPB-Pa-SO₃H COF and TFPB-BDSA COF) through chemical modification and fabricated two negatively charged COFs to examine and evaluate their abilities of adsorption towards dye molecules in aqueous medium. A constructing strategy was envisioned for improving the hydrophily of COFs platform through introduction of multi-sulfonate groups with excellent physicochemical properties, surface active and compatibility. A combinational strategy was envisaged for ameliorating the adsorption ability. On the premise of excellent hydrophily, the negative COFs modified by sulfonate groups could be apt to trap low-molecular-weight cationic dye on premise of the size matching, while the adsorption of the larger dye molecules was hypothesized to be primarily a surface event with π-π conjugations interaction. The synthetic method of TFPB-Pa-SO₃H COF and TFPB-BDSA COF was represented in Scheme 1.

Insert Scheme 1

Materials and Instrumentation

All materials and reagents involved in this work were of analytical grade and used without further purification. 1,3,5-Tris(4-formylphenyl)benzene (TFPB), 2,5-diaminobenzenesulfonic acid (Pa-SO3H) and 4,4'-diamino-[1,1'-biphenyl]− 3,3'-disulfonic acid (BDSA) were purchased from Innochem Technology, Ltd. (Beijing, China). Rhodamine B (RhB), methylene blue (MLB), crystal violet (CV) and methyl orange (MO) were the products from Fuyu Fine Chemical Co. Ltd. (Beijing, China). 1,4-Dioxane, N,N-dimethylformamide (DMF), acetonitrile, mesitylene, o-dichlorobenzene, glacial acetic acid (AcOH) and tetrahydrofuran (THF) were received from the local reagents shop (Qiqihar, China). The tested water is deionized water.

Synthesis of sulfonated-functional COFs

TFPB (0.05 mmol, 0.02 g) and Pa-SO₃H (0.075 mmol, 0.03g) were mixed together inside a 15 mL Pyrex tube in presence of 2 mL o-dichlorobenzene and 2 mL mesitylene. Then 0.1 mL of AcOH was added. After uniform dispersion by ultrasound for 20 min, the system was flash-frozen under the environment of liquid nitrogen and degassed via three freeze − pump − thaw rounds. The reactor was sealed off and the resulting mixture was heated at 120°C for 72 h. Upon the accomplishment of the condensation reaction, the precipitate was collected by centrifugation and extracted by Soxhlet extraction for 48 h with anhydrous THF until the trapped guest molecules and solvents were completely removed. Eventually, the lavender powder denoted as TFPB-Pa-SO₃H COF was obtained after vacuum-drying overnight at 60°C. TFPB-BDSA COF was synthesized via the same process by reacting TFPB (0.05 mmol, 0.02 g) and BDSA (0.075 mmol, 0.03 g) with the same molar ratio of 1:1.5. Finally, the yellow powder was collected as the final product named as TFPB-BDSA COF.

Adsorption experiment

The typical dye adsorption performance of TFPB-Pa-SO₃H COF/TFPB-BDSA COF toward cationic dyes (MLB, CV, and RhB) was examined as below. 3 mg of TFPB-Pa-SO₃H COF/ TFPB-BDSA COF powder was added into 10 mL of the different dye solutions, and then, the solution was shaken at room temperature for 12 h. After the accomplishment of the adsorption, the solution was filtered to separate the corresponding adsorbent followed by the UV − vis absorption measurement on the filtrate. Considering the effect of pH on the adsorption performance, the pH values of the dye solution was adjusted with 5% HCl or 10% NaOH aqueous solution. The adsorption ability (q_e, mg g^− 1) of dyes on the prepared COFs was calculated at adsorption equilibrium according to the following formulas [Eq. (1)]:

$${q}_{e}=\frac{\left({C}_{o}-{C}_{e}\right)V}{m}$$

where C₀ and C_e (mg L^− 1) are the concentrations of dyes initially and at the equilibrium, respectively. V (L) is the volume of the dye solution, and m (g) is the mass of the adsorbent.

In order to obtain the adsorption isothermals data, the eleven initial concentrations of the dye solution were adopted with 5, 10, 20, 30, 40, 50, 100, 200, 300, 400, 500 mg L^− 1, respectively, in presence of 5 mg of TFPB-Pa-SO₃H COF COF/TFPB-BDSA COF. After reaction of 48 h at 25°C, the residual concentrations of the dyes in the solution before and after adsorption were measured by the UV − vis spectra. The adsorption equilibrium amounts (q_e) related with the different initial concentrations were calculated, respectively, according to the Eq. (1). The adsorption isothermal data was fitted by the Freundlich isothermal model and Langmuir isothermal model, respectively. The linear forms of two models can be expressed according to the Eq. (2) and Eq. (3).

$${q}_{e}={K}_{F}{C}_{e}^{1/n}$$

$${q}_{e}=\frac{{q}_{m}{K}_{L}{C}_{e}}{1+{K}_{L}{C}_{e}}$$

where K_F (L g^− 1) and n represent the Freundlich constant and the Freundlich parameter, respectively; q_m (mg g^− 1) is the maximum monolayer capacity of the adsorbent for Langmuir isothermal model and K_L (L mg^− 1) is the Langmuir affinity constant.

For the adsorption kinetics experiment, 3 mg of TFPB-Pa-SO₃H COF/ TFPB-BDSA COF was mixed with the initial dye concentration of 20 mg L^− 1 and shaken at 25°C. The UV − vis spectra of the dye solutions wiped off the sorbents were recorded at different intervals (from 0 min to 180 min), respectively. The kinetic parameters for pseudo-first-order model and pseudo-second-order model were imitated by plotting q_t against and the linear equations [Eq. (4) and (5)] for two models are defined as follows:

$$\text{ln}\left({q}_{e}-{q}_{t}\right)=ln{q}_{e}-{K}_{1}t$$

$$\frac{t}{{q}_{t}}=\frac{1}{{K}_{2}{q}_{e}^{2}}+\frac{t}{{q}_{e}}$$

where q_t is the adsorption amount at time t (min); K₁ and K₂ are the adsorption rate constants for pseudo-first-order kinetic model and pseudo-second-order kinetic model, respectively.

Characterizations

The chemical composition of two materials were first analyzed by FT-IR spectra and the results are illustrated in Fig. 1 (a, b). The − HC = O stretching bands at 1699 cm^− 1 in free TFPB disappeared obviously. Correspondingly, the characteristic peaks at 1602 (Fig. 1a) and 1605 cm^− 1 (Fig. 1b) attributed to C = N stretching vibrations appeared in TFPB-Pa-SO₃H COF and TFPB-BDSA COF, which provided the evidence of the condensation of aldehyde and amine [28]. Moreover, the strong peaks of C–O bond (1017 and 1030 cm^− 1) and S–O bond (1088 and 1087 cm^− 1) demonstrated the presence of the sulfonic group in the materials. Above FT-IR spectra data means that the two materials have been successfully prepared and the sulfonic acid groups have been triumphantly grafted. XPS was performed to further ascertain the chemical compositions and bonding configurations of the two materials and the corresponding XPS full spectra were displayed in Fig. S1. Two similar element compositions of TFPB-Pa-SO₃H COF and TFPB-BDSA COF were observed in XPS survey spectra ensuring that all samples mainly contained C, N, O and S elements from the wide scan region. The binding energies and relative element contents for two materials are listed in Table S1. Remarkably, the relative content of S species (5.31%) in TFPB-BDSA COF was much higher than that (3.32%) in TFPB-Pa-SO₃H COF, hinting that there are more sulfonic acid groups stably embedded in channel of TFPB-BDSA COF. The high-resolution C 1s spectra of TFPB-Pa-SO₃H COF and TFPB-BDSA COF (Fig. 1c) could be deconvoluted into two major peaks centered at 284.1/283.3 eV and 283.5/283.0 eV respectively assigned to the unsaturated C-C and C = S groups. Meanwhile, the high-resolution N 1s spectra (Fig. 1d) can be fitted into two peaks at 397.6/399.2 and 401.9/403.2 eV, which derived from C–N and C = N group, respectively. Thus, it can be seen that the C = N bond had been successful formed through the condensation of TFPB and corresponding diamino sulfonic acid. In addition, the S–O and S = O characteristic peaks at 167.5/166.5 eV and 166.1/167.2 eV have been observed in the high-resolution S spectra (Fig. 1e), proving the presence of sulfonic groups in TFPB-Pa-SO₃H COF and TFPB-BDSA COF.

Insert Fig. 1

The crystalline natures of two materials were cognized by powder X-ray diffraction (PXRD). As shown in Fig. 2a, the strong diffraction peaks of TFPB-Pa-SO₃H COF at 5.46° in the low angle corresponded to its (210) plane and the slightly broad peaks around 22.54° assigns to its reflection from the (001) plane, which might be ascribed to its π − π stacked structure [29, 30]. By comparison, the PXRD of TFPB-BDSA COF exhibited an extra weaker peak at 2θ value of 5.76°, which is corresponding to (210) diffraction of COFs’ material [31]. The PXRD patterns of TFPB-Pa-SO₃H COF and TFPB-BDSA COF indicate both the two materials exist in a highly crystalline state. Furthermore, the two-dimensional (2D) models were simulated through Materials Studio software, and the stacking structures of two materials were depicted as shown in Fig. 2b and 2c. The eclipsed AA stacking was the possible structures for both TFPB-Pa-SO₃H COF and TFPB-BDSA COF, where the interlayer spaces between the 2D plane was of 4.124 and 3.780 Å, respectively. Generally, the crystallinity of framework material reflects the structural regularity, which is of great importance for enhancing their application ability [32]. Subsequently, the PXRD patterns of the treated TFPB-Pa-SO₃H COF and TFPB-BDSA COF by strong acid/basic and the different solvents (DMF and hot water) were also collected, and the results were shown in Fig. S2. It was clear that there were no significant changes to be found in PXRD diffraction patterns of all the treated materials compared with those of the untreated materials, implying that both TFPB-Pa-SO₃H COF and TFPB-BDSA COF maintained the construction integrity and chemical stability.

TGA was also implemented to further assess the thermal stability of two sulfonic materials. As described in TGA curves (Fig. 2d), they had a small mass loss (1% and 2%, respectively) below 100℃, which were attributed to the evaporation of absorbed water and residual solvents [33]. A key signal of the pyrolysis of TFPB-Pa-SO₃H COF arose at around 300°C; and then, the weight decreased slowly until a temperature of 600°C, which confirmed their closely similar thermal stabilities along with maintaining upper 60% of the initial mass. Interestingly, the TGA curve of TFPB-BDSA COF emerged two obvious mass losses appearing at around 285°C and 365°C, respectively. Only 5% weight loss was observed from 285°C and 365°C, which might be due to the evaporation of adsorbed water and solvents. And then, the most obvious decline occurred from 365°C to 440°C with about 43% weight loss. It was obvious that TFPB-BDSA COF owned higher decomposition temperature for the framework of COFs than that of TFPB-Pa-SO₃H COF, which indicated that TFPB-BDSA COF performed much excellent thermal stability than TFPB-Pa-SO₃H COF. The specific surface area and porous structure of the two materials were examined via N₂ adsorption/desorption measurements. Figure 2e showed that all materials belong to a typical type IV sorption isotherm profile, verifying that two material had the characteristics of the mesoporous structures [34, 35]. The BET surface area of TFPB-Pa-SO₃H COF and TFPB-BDSA COF was calculated to be 105.4 m² g^− 1 and 127.8 m² g^− 1, respectively. According to Fig. 2f, the average pore sizes of the two materials were computed to be 3.2 nm and 3.3 nm, respectively. These results implied that TFPB-Pa-SO₃H COF and TFPB-BDSA COF were seized of large specific surface area and the uniform mesoporous structure.

Insert Fig. 2

The microstructures of TFPB-Pa-SO₃H COF and TFPB-BDSA COF were visually portrayed by SEM and TEM. As shown in Fig. 3a, the surface of the two materials is glabrate and their morphologies were analogous on the whole. The regular geometric shapes with the width of ~ 2 µm and the length of ~ 10 µm were observed, which are strictly and tightly held into the packed layers in the final aggregate. SEM images elucidate that two materials had two-dimensional (2D) layered regular patterns of ~ 0.5 µm thickness which are clustered with each other. Additionally, the EDS spectra (Fig. 3b) of two materials had four bands at ~ 0.3, ~ 0.4, ~ 0.5 and ~ 2.3 KeV, which belong to C, N, O and S peaks, respectively, matching well with XPS analysis results. The deeper examination into the topological structure of the two materials was carried out by TEM. The geometric shapes were consistent with the results of the SEM images, which further confirmed the flake shapes, the smooth surfaces and the tightly aggregated morphology of the layers for both COFs (Fig. 3c). Meanwhile, the EDS elemental mapping (Fig. 3d and 3e) of two material displayed the close contact of, C, N, O and S, signifying the chemical compositions of two material are uniformly and regularly arranged throughout the materials without any random accumulation. Besides, the well-identified lattice fringes (~ 0.033 for TFPB-Pa-SO₃H COF and 0.950 nm for TFPB-BDSA COF) are clearly visible from the COF layer (Fig. 3f), indicating a highly crystalline nature for two synthesized COFs.

Insert Fig. 3

Hydrophily of sulfonated-functional COFs

In view of the hydrophily of sulfonic acid groups, the water contact angles (CA) of the two materials were inspected and evaluated to prove the hydrophilic abilities of the synthesized COFs. The water contact angle of TFPB-Pa-SO₃H COF and TFPB-BDSA COF were 35° (Fig. 4a) and 27°(Fig. 4b), respectively, which was relatively lower than most of those of reported COFs materials [36 − 40], promulgating that the introduction of sulfonic acid groups onto frameworks structures can significantly ameliorate their hydrophilicity. In addition, it is obvious that the hydrophily of TFPB-BDSA COF is superior to that of TFPB-Pa-SO₃H COF, which alleged that the percentage of –SO₃H groups in materials might play a crucial role in liquid spreading on a material surface. The data of water contact angles further proved that the prepared TFPB-Pa-SO₃H COF and TFPB-BDSA COF have a better degree of hydrophilicity and can be well dispersed in the sample water solution.

Insert Fig. 4

Adsorption Performances

Adsorption selectivity and regeneration

In order to inspect the adsorption selectivity of the prepared materials to the cationic dyestuffs, three common cationic dyes [methylene blue (MLB), crystal violet (CV) and rhodamine B (RhB)] were selected to perform the contrast experiments by adding adsorbents 3 mg TFPB-Pa-SO₃H COF or TFPB-BDSA COF to 10 mL dyes aqueous solutions (20 mg·L^− 1, pH = 9). Firstly, the color changes of dye solution before and after adsorption were recorded by photographing. Figure 5a intuitively depicted that the colors of the three dye solutions gradually faded into achromaticity along with the extending of adsorption time, implying that both TFPB-Pa-SO₃H COF and TFPB-BDSA COF had preferential abilities to capture cationic dyes. Thereinto, it can be clearly seen that the color of MLB solutions quickly became nearly colorless. Relatively, the color changes of CV and RhB solutions was rather sluggish. These results initially validated that TFPB-Pa-SO₃H COF and TFPB-BDSA COF possessed best uptake efficiency towards MLB. Based on this fact, the UV − vis spectra of the dyes aqueous solutions were measured when the adsorptions were proceeding for the period of the time. A falling tendency of the absorbance intensities Fig. 5c for both three dyes was observed meaning the decrease of the dye’s concentration in the solutions. This phenomenon owed to the adsorption function of COFs materials towards the dye molecules in water solutions.

In addition, MO was used as a kind of typical anionic dyes to check the adsorption selectivity of TFPB-Pa-SO₃H COF and TFPB-BDSA COF. As shown in Fig. 5b, the initial colors for the binary mixed dye solutions composing MLB/MO, CV/MO, and RhB/MO were cyan, purple, and orange [41], respectively. After the complete adsorption, the colors of the six mixed dye solutions almost displayed yellow, nearly the same as the color of single MO dye solution, which meant that three cationic dyes were caught by TFPB-Pa-SO₃H COF and TFPB-BDSA COF, respectively while the anionic dye MO was still retained in the aqueous solutions. Similarly, in the UV−vis spectra of the binary mixed dye solutions Fig. 5d, there were two strong absorption bands for the three initial mixed solutions; one is attributed to the absorption of MO (467 nm) and the others are for those of MLB (665 nm), CV (585 nm) and RhB (550 nm), respectively. After the accomplishment of absorption, the absorption peaks belonged to the MLB, CV and RhB almost vanished while that for the MO only hardly changes. This suggested that TFPB-Pa-SO₃H COF and TFPB-BDSA COF can selectively trap cationic dye from the mixed dye solution without the disturbance of anionic dye.

Insert Fig. 5

Adsorption isotherm and kinetic

To determine the equilibrium adsorption isotherms of between surface of adsorbent and adsorbate at equilibrium, TFPB-Pa-SO₃H COF and TFPB-BDSA COF (5 mg) were blended into three mentioned dyes solution (10 mL, pH = 9) with different initial concentrations ranging from 5 to 500 mg L^− 1. As depicted in Fig. 6a, with the enhancement of the initial concentrations of the dye solutions, the adsorption capacities of three dyes on two materials first rapidly increased and then trended to the adsorption equilibrium. These phenomena were resulted from the fact that the adsorption sites were not completely saturated at lower initial concentration, and the adsorption of dyes reached the equilibrium at higher initial concentration. The equilibrium adsorption amounts of TFPB-Pa-SO₃H COF/TFPB-BDSA COF was 1174/1166 mg g^− 1 for MLB, 1559/1288 mg g^− 1 for CV, 1062/1054 mg g^− 1 for RhB, respectively. To improve our understanding of the adsorption process, two models of Langmuir isotherm and Freundlich isotherm were adopted to simulate the dye adsorption process on TFPB-Pa-SO₃H COF and TFPB-BDSA COF (Fig. 6b and 6c). The relevant parameters were enumerated in Table 1. Based on the obtained regression constants, the Langmuir models more favorably matches the adsorption characteristics of dyes, inferring a single-layer adsorption dominated by π–π stacking [42].

Insert Fig. 6

Table 1

Isotherms parameters in removal of MLB, CV and RhB at 298K by TFPB-Pa-SO₃H COF and TFPB-BDSA COF.
COFs	Dyes	Langmuir model			Freundlich model
COFs	Dyes	q_max (mg g^− 1)	K_L (L mg^− 1)	R²	K_F (mg L^− 1)	n	R²
TFPB-Pa-SO₃H COF	CV	1559	0.0043	0.9913	109.9823	1.4948	0.9367
	MLB	1174	0.0047	0.9957	110.4021	1.8543	0.9607
	RhB	1062	0.0048	0.9979	121.2809	2.3299	0.9430
TFPB-BDSA COF	CV	1288	0.0088	0.9981	164.2188	2.0652	0.8942
	MLB	1166	0.0051	0.9959	128.1037	2.0886	0.9628
	RhB	1054	0.0086	0.9998	175.5915	2.7208	0.8504

The adsorption time as a significant parameter to affect the efficiency of adsorption performance was first investigated to meticulously appraise the adsorption capacities of TFPB-Pa-SO₃H COF and TFPB-BDSA COF to three cationic dyes. Figure 5c have displayed the results of the UV − vis spectra of the dyes aqueous solutions depending on the adsorption times of TFPB-Pa-SO₃H COF. The absorbance intensities of three dyes at the corresponding maximum absorption wavelengths descended gradually with the increasing of adsorption time. Significantly, the adsorption process of MLB rapidly accomplished and reached the adsorption equilibrium within only 1 min, which validated the capture of TFPB-Pa-SO₃H COF adsorbent to MLB was most efficient among three dyes. Nevertheless, the adsorption equilibrium for CV and RhB required 120 min and 90 min at least, respectively, hinting that TFPB-Pa-SO₃H COF had a poor adsorption capacity towards CV and RhB. Analogously, the nearly complete adsorption of MLB can be also realized within 2 min upon using TFPB-BDSA COF as absorbent; while the complete times for CV and RhB exceeded 30 min and 90 min, respectively. Above results preliminarily proved that TFPB-Pa-SO₃H COF and TFPB-BDSA COF can exhibit superfast adsorption rate to MLB.

Based on the above results, the particular relationship between the adsorption capacity and adsorption time during the adsorption process was evaluated through the adsorption kinetics study. Figure 7a recorded the effect of adsorption time on the adsorption capacities of MLB, CV and RhB by two COFs. The change trend of the adsorption amount plot for both TFPB-Pa-SO₃H COF and TFPB-BDSA COF is the same. The adsorption efficiency of two materials to three dyes keep a rapid increasing during the initial phase, which intimated that the adsorption was still in a primary stage of surface adsorption. After that, the adsorption followed the trend with a slight reduction of capture efficiency, illustrating that the surface of materials was already saturated, and later the adsorption gradually reached plateaus. Especially, the fast adsorption of MLB on TFPB-Pa-SO₃H COF and TFPB-BDSA COF was much easier to reach the equilibrium (1 and 2 min) than those of CV and RhB, which the adsorption rate may be related to the smallest size and the best solubility of MLB among the mentioned cationic dyes. Nevertheless, the adsorption uptakes of CV on two materials (1559 and 1288 mg g^− 1) was the highest, which mainly endorsed to the adsorption stability on the premise of the size matching between the channels of two materials and the molecular volume of CV. The adsorption kinetics of two materials were fitted by pseudo-first-order and pseudo-second-order models to further interpret the kinetic nature of the adsorption process. The related parameters of adsorption kinetic models in Table 2 as well as Fig. 7b and 7c demonstrated that the adsorption process was more suitable for the pseudo-second-order model, indicating that the adsorption process was the surface adsorption and the chemical adsorption was a possible rate-controlling step. It is worth noting that under the pseudo second-order kinetic model, the adsorption kinetic constants (k₂) of MLB on TFPB-Pa-SO₃H COF (0.031 min^-1) ranks as one of the top values, which was almost 5.96 times faster than that of TFPB-BDSA COF (0.185 min^-1). This high-rate performance can be ascribed to its well hydrophily and larger channel size that are both sufficiently predominant to facilitate the its interaction with MLB promoted by electrostatic attraction.

Insert Fig. 7

Table 2

Kinetic parameters in removal of MLB, CV and RhB at 298K by TFPB-Pa-SO₃H COF and TFPB-BDSA COF.
COFs	Dyes	Pseudo-first order		Pseudo-second order
COFs	Dyes	K₁ (min^− 1)	R²	K₂ (min^− 1)	R²	q_e,exp (mg·g^− 1)	q_{e, cal} (mg·g^− 1)
TFPB-Pa-SO₃H COF	CV	0.385	0.9500	0.085	0.9916	1559	1558
	MLB	0.573	0.9882	0.031	0.9938	1174	1172
	RhB	0.109	0.8995	0.104	0.9942	1062	1060
TFPB-BDSA COF	CV	0.312	0.9711	0.094	0.9956	1288	1285
	MLB	0.367	0.9760	0.185	0.9983	1166	1164
	RhB	0.127	0.8901	0.056	0.9992	1054	1052

Effect of the initial solution pH

The influence of pH on adsorbent properties of the materials must be considered in practical application due to the electrostatic interaction. Therefore, the initial pH values of the dye aqueous solutions were adjusted in the range of 4 ~ 12 by adding a small amount of 5% HCl or 10% NaOH solution. Figure 7d depicted the influence of initial solution pH on the removal efficiency and the surface zeta potential of materials. Obviously, when the pH value was equal to about 9, the removal efficiencies of three dye nearly reached to the maximum values (~ 95%). Lower than this pH environment (pH = 4 ~ 9), the adsorption capacities of TFPB-Pa-SO₃H COF and TFPB-BDSA COF showed an upward trend with the decrease of pH value. Higher than this pH value, the uptake of two materials for dyes is almost constant irrespective of the pH value. Above result implied that the weak alkaline condition (pH = 9) was most conductive to the cationic dyes adsorption onto sulfonated-functional material TFPB-Pa-SO₃H COF and TFPB-BDSA COF, which might be explained by the stronger electrostatic attractive force between two materials and dyes. The zeta potentials depending on pH values was measured to assess the surface charges of materials. TFPB-Pa-SO₃H COF and TFPB-BDSA COF are in a state of negative charge on the surfaces at a wide pH range on account of the presence of − SO₃H groups, which supported the electrostatic attraction between negatively charged adsorbents TFPB-Pa-SO₃H COF and TFPB-BDSA COF and cationic dyes. Furthermore, an initial zeta potential of two materials greatly decreased with the increasing pH values, accompanying with the increasing the removal efficiency of cationic dyes. Of note, the lower zeta potential of TFPB-BDSA COF was observed than that of TFPB-Pa-SO₃H COF, obviously meaning that the surface of TFPB-BDSA COF carried more negative charges, which was probably ascribed to the existence of more sulfonic acid groups. Correspondingly, the removal efficiencies of TFPB-BDSA COF were higher than those for TFPB-Pa-SO₃H COF. On the basis of the above results, it can be conferred that the more the negative charges on the surface of prepared COFs, the stronger the electrostatic attraction between negative-charged COFs and positive-charged dyes.

Adsorption mechanism and adsorbent reusability

According the above results of adsorption performance, it is clear that the electrostatic interaction promoted by the well hydrophilic effect of COFs materials between the negative COFs and positive dyes was mainly responsible for the host-guest function. Based on the electrostatic interaction, the π-π interaction was further assessed through investigating the FT-IR spectra of the TFPB-Pa-SO₃H COF and TFPB-BDSA COF treated with three cationic dyes solution and the results are shown in Fig. 8. Evidently, contrasting for the FT-IR spectra of TFPB-Pa-SO₃H COF before and after MLB adsorption (Fig. 8a), the characteristic peaks at 1330 and 1390 cm^-1 belonging to the − CH₃ groups of MLB appeared, hinting that MLB molecule have been tightly trapped by TFPB-Pa-SO₃H COF. More importantly, the characteristic absorption peak of the aromatic ring (around 1611 cm^-1) for free MLB almost disappeared in the FT-IR spectra of the TFPB-Pa-SO₃H COF, which was probably prone to the formation of π-π stacking interaction between aromatic fused-rings of MLB and the planar benzene rings of TFPB-Pa-SO₃H COF. At the same time, the vibration peak (889 cm^-1) in free MLB have slightly shifted toward a lower wavelength (883 cm^-1) after MLB was captured on the surface of TFPB-Pa-SO₃H COF, further demonstrating the formation of π-π conjugations interaction. The similar phenomena were noticed both in the FT-IR spectra of the TFPB-Pa-SO3H COF with CV (Fig. 8b) and RhB (Fig. 8c) as well as TFPB-BDSA COF with MLB (Fig. 8d), CV (Fig. 8e) and RhB (Fig. 8f). Combined with all the above factors, the highly efficient adsorption of cationic dyes molecules on TFPB-Pa-SO₃H COF and TFPB-BDSA COF can be realized smoothly. Furthermore, to ascertain that TFPB-Pa-SO₃H COF and TFPB-BDSA COF were recyclable, the reusabilities of two COFs materials were tested for adsorption of cationic dyes. As indicated in Fig. 8g and 8h, the similar removal rates of cationic dyes in aqueous solution were achieved after five consecutive cycles, meaning that two materials maintained the favorable stabilities of structure as well as adsorption properties.

Insert Fig. 8

Two anionic COF materials named TFPB-Pa-SO₃H COF and TFPB-BDSA COF were successfully synthesized based on two different sulfonated building blocks and then their high crystallization structures were characterized in detail by the PXRD and simulation. TFPB-Pa-SO₃H COF and TFPB-BDSA COF displayed predominant hydrophilicity, which effectively stimulated the adsorption capacities to cationic dyes through electrostatic interaction. The adsorption equilibrium to MLB was as fast as 1 ~ 2 min and the adsorption uptakes to CV was as high as 1559 and 1288 mg g^− 1. The high adsorption capacity can be ascribed to the combined effect of hydrophilic interaction of two COF materials, the electrostatic interaction and π-π interaction between COF materials and dyes molecules. Furthermore, two COFs as adsorbing materials were successfully used to the removal of dyes molecules in real water samples, which promoted the application of COF materials in environmental remediation.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (No. 21978140), the Heilongjiang Provincial Natural Science Foundation of China (No. GZ20210064), the Heilongjiang Provincial Leading Talent Echelon Reserve Leader Fund of China, and the Qiqihar University Graduate Innovation Foundation of China (No. YJSCX2021010 and YJSCX2021005). The authors also gratefully acknowledge the financial support by the Heilongjiang Provincial Key Laboratory of Surface Active Agent and Auxiliary (Nos. BMHXJKF004 and BMHXJKF005).

Ethics approval and consent to participate: Not applicable.

Consent for publication: Manuscript is approved by all authors for publication.

Availability of data and materials: The data sets supporting the results of this article are included within the article and its additional files.

Competing interests: The authors declare that they have no conflict of interest.

Funding: The work was supported by the National Natural Science Foundation of China (21978140 and 21506106), the Natural Science Foundation of Heilongjiang Province (GZ20210064 and LC2017004) and the Qiqihar University Graduate Innovative Scientific Research Project, China (YJSCX2021010 and YJSCX2021005).

Authors' contributions: Bing Zhao, Wei Kan, Liyan Wang, Li Sun, Xiuwen Wang and Guangming Yin contributed significantly to write the manuscript and perform the analysis with constructive discussions; Wang Zheng, Anran Li, and Zhigang Li performed the experiments and the data analyses.

Acknowledgements: The authors also gratefully acknowledge the financial support by the Heilongjiang Provincial Key Laboratory of Surface Active Agent and Auxiliary (BMHXJKF004 and BMHXJKF005).

Authors' information: Not applicable.

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Scheme 1 is available in the Supplementary Files section.

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Scheme 1 Synthesis of TFPB-Pa-SO₃H COF and TFPB-BDSA COF.

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Construction of hydrophilic covalent organic frameworks and their fast and efficient adsorption towards cationic dyes from aqueous solution

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Abstract

Figures

Introduction

Insert Scheme 1

Experimental

Materials and Instrumentation

Synthesis of sulfonated-functional COFs

Adsorption experiment

Results And Discussion

Characterizations

Insert Fig. 1

Insert Fig. 2

Insert Fig. 3

Hydrophily of sulfonated-functional COFs

Insert Fig. 4

Adsorption Performances

Adsorption selectivity and regeneration

Insert Fig. 5

Adsorption isotherm and kinetic

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Insert Fig. 7

Effect of the initial solution pH

Adsorption mechanism and adsorbent reusability

Insert Fig. 8

Conclusion

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References

Scheme

Additional Declarations

Supplementary Files

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