Characterization of FCS hydrogel
The morphology of the FCS hydrogel of the fracture surface was analyzed by SEM, and the results were shown in Fig. 2. In the cross-sectional images of the FD-CS hydrogel, there are a highly porous matrix with interconnected pores extending from the core to the surface of the monolithic hydrogel, indicating the successful crosslinking of chitosan polymer chains to form chitosan composite hydrogels. Furthermore, the prepared chitosan hydrogel with freeze-thawing treatment exhibited open pores with high interconnectivity structure. In contrast with other study which presented chitosan hydrogel with rough surface but with no pores were observed on the surface [25]. This is due to the freeze-thawing method has been used to create porous chitosan hydrogel with interconnected pores [42]. During cooling the prepared chitosan hydrogel, the nucleation and the ice crystals was grown and moves into non-nucleated solution leading to change the morphology and form interconnected pores [46]. At a temperature below the freezing temperature (room temperature ), these ice crystals melt and gives rise to interconnected macroporous networks [47, 48]. This network structure can improve the specific surface area and then increase the adsorption capacity of the dye molecules from aqueous media due to promoted diffusion of the dyes molecules within the hydrogel structure and makes contact between the functional groups of the chitosan hydrogel with dye molecules more efficient [49]. The specific surface area and pore size distributions of FDCS hydrogel was determined by employing the nitrogen sorption technique. The N2 adsorption–desorption isotherms of FDCS are represented in Fig. 3A.
The N2 adsorption–desorption isotherm for FCS belongs to type H3 hysteresis loop according to IUPAC classification, which proves interconnected pore structure with a parallel plate shape [50]. The synthesized FCS in this work displayed a specific surface area of 78.25 ±8.75 m2g-1. The BET surface area of the prepared FCS chitosan compared with previous studies in the literature and it was found that the surface area for the FCS hydrogel was larger value [50–52]. These results strongly support the fact that the surface area and porosity of FCS were increased after freezing the CS hydrogel. Obviously, cryogenic treatment for the synthetic glutaraldehyde cross-linked chitosan hydrogel played an impressive role in the increasing of the surface area of this hydrogel. As expected, after freezing the CS hydrogel convert the porous structure from close pores to interconnected pores, which can allow more active sites in the hydrogel to interact with dye molecules and led to improve adsorption efficiency [53, 54]. Therefore, FCS hydrogel with larger specific surface area and more active groups are a promising adsorbent materials due to these characteristics are particularly important for adsorption purposes for wastewater treatment [55]. The BET results well proved the SEM images as the pores development can be seen obviously in the surface morphology of FCS hydrogel. For a better evaluation of the impact of the initial CS concentrations on the porosity % of FCS hydrogel, four different CS concentrations were used by fixing the crosslinking agent concentration constant. Increasing the CS concentration decreased the porosity of final FCS hydrogel as shown in Fig. 3B. The average porosity of FCS composite hydrogels decreases from 93.49 % to 80.05 % when the initial concentration of CS increases from 0.5 to 2.5% w/v. This is due to the fact that increasing the concentration of the polymer increases the cross-link densities of the network structure of the prepared hydrogel, which is lead to decrease the porosity [56].
In order to further confirm the successful crosslinking and the structural changes of composite hydrogel after crosslinking, the infrared spectra were investigated for powder chitosan and cross-linked chitosan hydrogel as shown in Fig. 3C. Comparison of the FTIR results reveals that the FCS cross-linked chitosan hydrogel shows spectra that are different from those of non-cross-linked chitosan. Non-cross-linked chitosan demonstrate
peaks at 949 and 1104 cm-1 are attributed to C–O–C bridge of chitosan unit. The absorption bands at 2911 and 2873 cm-1 can be corresponds to C–H symmetric and asymmetric stretching. A broad band in the region of 3240- 3400 cm-1 attributed to the associated of the vibrations of -OH and –NH2 groups. Band at about 1564 cm−1 are attributed to the bending of the primary amine’s N–H bond [57]. After chemical crosslinking of chitosan using glutaraldehyde, these band present a change in it is intensity and width, this is occurring due to the bond generated by overlapped the N-H, O-H, and C-O bonds of CS with the GA during the crosslinking reaction. This proven that some of the hydrogen bonds in the chitosan structure are broken and new hydrogen bonds between GA and chitosan polymer chains are created as NH2 and OH functional groups engaged in hydrogen bonds [58]. A new peak at about 1646 cm−1 was observed, which is most probably attributed to the imine bond (C=N). This bond produces and observed in the spectrum of FCS hydrogel due the chemical crosslinking reaction between GA and CS amino groups [26]. The FTIR results confirms that GA cross-linking takes place by the reaction of amine groups of chitosan with aldehyde groups of glutaraldehyde. Fig. S1 shows change of the chitosan hydrogel color from transparent light yellow to orange this was also confirms the formation of cross-links between CS polymer chains by GA. This is most likely due to the formation of a chromophore as stable crosslinking products, which is not present in the starting precursors [59]. The Z-potential of FCS hydrogel in deionized water was measured by the DLS of suspensions that were prepared by dispersing 0.05 g of the vacuum dried FCS hydrogel in 50 mL of deionized water by a digital sonicator. The zeta potential of FCS hydrogel was found to be +26.5 ± 3 mV as shown in Fig. 3D. At this point, the positive surface charge of FCS hydrogel contributed to increase the adsorption capacity towards the anionic contaminated and this result agrees with previous studies [60–62].
It is important to highlight that the positive value of ZP for FCS hydrogel indicating that the high protonation of the NH2 groups, which are did not take part in the crosslinking reaction and suggests that the interactions between the FCS hydrogel and anionic dyes molecules may occur by hydrogen bonding and/or electrostatic interactions [63].
The swelling behavior of the FCS hydrogel was analyzed via gravimetric method. In detail, a small piece of vacuum dried hydrogel (~0.05 g) was immersing in D.I. water for 24 hours at room temperature (25±3 °C) and neutral pH. The FCS hydrogel shows remarkable and quick volume change due to the water pass through the hydrogel via capillary forces. This was owing to its interconnected porous structure and the presence of abundance hydrophilic functional groups on backbone [64]. Swelling behavior of FCS hydrogels as a function of pH was evaluated as shown in Fig. 4. As can be seen in Fig. 4A the swelling ratio of FCS hydrogels with decreasing the pH value from 12 to 3 was determined. The swelling ratio of FCS hydrogels in pH 3 was 26.3, which doubled compared to the same sample in pH 12 (11.7). It is worth noting that, FCS hydrogel in acidic environment showed higher swelling ratio than the same sample in basic and neutral environment. In order to explain this phenomenon, it was due to in acidic media ionized the –NH2 groups in the FCS hydrogel to ammonium ion (NH3+) and these cationic charges made repulsive forces between polymer chains [65]. Fig. 4B shows the swelling deswelling reversibility of FCS hydrogel between pH 3 and pH 10. The results demonstrated that the reversibility toward the pH changes was retained even after more than ten cycles. FCS hydrogel water uptake were studied by measuring the degree of swelling with time. Fig. 4C depicts the degree of swelling as a function of time for FCS hydrogels in acidic media. Fig. 4D shows deswelling kinetics of the FCS hydrogel at pH 10. FCS hydrogels exhibited quicker loss most of water after change the pH value from 3 to 10 in comparison with water uptake of the same FCS sample. This results are reasonable since both swelling and deswelling processes start from the surface of the hydrogels to inside the hydrogel through its porous structure. During swelling, water will take longer time to penetrate inside the sample, while it will take a shorter time for the transportation from the hydrogel during deswelling. Fig. 4E demonstrated schematic represented the swelling and deswelling properties of FCS hydrogel in acidic and alkaline conditions respectively.
Adsorption study
The synthesized three-dimensional FCS hydrogel with unique pore structure were evaluated for the removal of some organic dyes. In the present work, Congo Red and Sodium Fluorescent dyes as an anionic dye model and Methylene Blue and Rodamine 6G as cationic dye model to test the ability of FCS hydrogel to remove organic dyes. It is also important to note that the preparation process of our FCS hydrogel requires only mixing of chitosan and the chemical cross linker in acidic media and then freeze-thaw treatment to generate the pores, with no further drying step required to form the hydrogel absorbent. This low energy fabrication method, together with the use of the green solvent, water, renders our approach relatively environmentally friendly. Further, we described the dye adsorption process utilizes monolithic hydrogel, with no required for the additional steps of centrifugation or adsorbent separation, unlike methods that utilized particulate-based absorbents. Fig. S2 show a photograph of the adsorption process of CR and SFL by FCS monolithic hydrogels. After immersing the FCS monolithic hydrogel by 24 hours into CR and SFL dyes solutions separately, the red and yellow solutions containing CR and SFL dyes become very light and almost clear and transparent, whereas the hydrogels turned red and dark yellow due to the adsorption of CR and SFL dyes respectively. These images indicate that the composite FCS pores hydrogels possess highly efficient of anionic dyes adsorption. In contrast, FCS hydrogel demonstrates very low removal towards MB and R6G as shown in Fig. S3. However, the monolithic FCS hydrogel turned to blue and dark orange after immersing these hydrogels into MB and R6G solutions by 24 hours. The reason for this is due to the interconnected porous structure of FCS hydrogel make easily filled by these dyes solutions without being adsorbed [66]. The adsorption capacities of the FCS hydrogel toward CR and SFL dyes were evaluated by using the UV−Vis spectra to measure the concentrations of the dye solution before and after adsorption as shown in Fig. 5 A and B. Removal and adsorption of CR and SFL dyes onto the FCS monolithic hydrogels from the aqueous solution was observed. UV-Vis spectroscopy shows low absorption bands for these two dyes in the solution after adsorption. This phenomenon may be explained by electrostatic attraction between NH3+ group protonated in the acid medium of FCS hydrogel and anionic dye molecules formation (Dye-NH3+), which is cause the captured of this type of dyes on FCS hydrogel [3]. We discovered that the hydrogen bonding and electrostatic interactions played a significant role in the adsorption of the anionic dyes [67]. Similarly, FCS hydrogel adsorption capacity were tested towards MB and R6G dyes aqueous solutions as cationic dyes. In contrast to CR and SFL dyes, no significant adsorption was observed toward MB and R6G (less than 10%), with little change in the UV-Vis spectrum of the dyes solutions after adsorption as shown in Fig. 5C and D.
This confirmed that FCS monolithic hydrogel has no obvious adsorption of cationic dyes on its surface due to the electrostatic repulsion interactions between the dyes and the cationic FCS hydrogel [67]. However, it should be noted that the poor removal efficiency of cationic dye due to the hydroxyl groups in the FCS may also participate to chelation with cationic dyes [68]. In order to study the adsorption behavior of the FCS porous monolithic hydrogel towards CR and SFL dyes batch adsorption method was also performed. The batch adsorption of these two anionic dyes were determined at different initial concentrations with constant weight of FCS hydrogel (0.05 g) for different adsorption times intervals at a pH value of 6. As shown in Fig. 6A, demonstrates the adsorption process of both CR and SFL dyes (the concentrations were 60 mg/L) onto the FCS monolithic hydrogel were progressed rapidly in the initial stage and the adsorption rate decreased gradually moving toward equilibrium after 12 hours. The best possible explanation for this phenomenon is that at the beginning of the reaction, the higher available active site with the higher driving force on the FCS hydrogels surfaces provides better opportunity of collision between anionic dyes molecules and the adsorbent [69]. As the adsorption reaction reached equilibrium the adsorption capacity was remained constant, which is may be attributed to saturated the available adsorption sites of the FCS hydrogel by dyes molecules [70]. The adsorption capacity at equilibrium of CR and SFL dyes were calculated and it were found 104.9 and 99.6 mg. g-1 respectively. The removal percentage of CR and SFL dyes were up to 96.73% and 95 % at equilibrium as shown in Fig. 6B. The study of the adsorption isotherm play a significant role to describe how adsorbates molecules interact with adsorbents surface [71]. In the present work, to investigate the equilibrium condition of anionic dyes (CR and SFL) onto FCS hydrogel, two types of isotherms including Freundlich and Langmuir isotherm models were evaluated. The Langmuir isotherm model can be regarded as a single layer adsorption system onto a homogeneous surface, while the Freundlich model are considered as a heterogeneous multi layers adsorption system [72]. As we can see in Fig. 6 C and D the plots of Langmuir and Freundlich isotherm for CR and SFL dyes. The fitted parameters and data are shown in Table S1. The results indicate that the Freundlich adsorption isotherm showed a better value of correlation coefficient (R2). Therefore, the adsorption of both CR and SFL dyes onto FCS hydrogel obeyed with the Freundlich model, and is mainly multilayer adsorption. Through the Langmuir model fitting, the maximum adsorption capacity qm values for CR and SFL dyes onto FCS hydrogel were obtained and it were found 136.69 and 142.857 mg.g−1 respectively. The small difference in maximum adsorption capacities of these dyes was owing to their different affinities for adsorbance.
As a results, the high adsorption capacity of FCS hydrogel due to the adsorption mechanism is affected by the interconnected porous structure of the hydrogel, which is allow better access of dye molecules through the internal structure of the hydrogel and increase the available active adsorption sites [73].
Additionally, the strong electrostatic force between the positively charged amine groups (-NH3+) in the hydrogel FCS interact with the negatively charged groups on anionic dyes and the like hydrogen bonds interaction encourage anionic dyes adsorption on hydrogels resulting high adsorption capacity [16, 74]. To preform, the adsorption behavior of the FCS hydrogel, kinetic models containing pseudo-first order (PFO) and pseudo-second-order (PSO) models, which are widely used in the fitting of experimental kinetics data as shown in Table S2 and S3 for CR and SFL dyes respectively. The fitting plots of the adsorption kinetic model is shown in Fig. 7. It was observed that the correlation coefficient (R2) corresponding to the PSO model was significantly higher than that corresponding to the PFO model. Furthermore, the results showed that the simulation adsorption capacity from the plot of PSO model was found to be well fit with the experimental results than that calculated from PFO. Therefore, PSO model better describe the adsorption mechanism for both CR and SFL dyes on FCS hydrogel. It is worth to note that the adsorption reaction of CR and SFL dyes on to FCS hydrogel is more tending towards physical adsorption. Basically, the adsorption mechanism of dye molecules on a porous hydrogel adsorbent material cannot be described by only the PFO and PSO models due to the adsorption on these materials may include a multi-stage process [75]. As a result, an intraparticle diffusion model was employed for further interpretation to show the movement of the dye molecules from the aqueous solution into the interconnected pores of the FCS hydrogel. Fig. 7E and F show that the linear plote do not pass through the origin for both CR and SFL dyes, indicating that the intraparticle diffusion models is based on the migration of the adsorbates into the hydrogel pores and controlling the rate adsorption process and the results of adsorbent parameters for intraparticle diffusion model were listed Table S2 and S3 for CR and SFL dyes respectively. This model represents two stages, the first stage where there is a rapid adsorption of dye molecules on the external surface of the hydrogel and the second stage where the intraparticle diffusion is the rate-limiting step [76]. Therefore, in addition to PSO, the intraparticle diffusion model plays a significant role to interpret the adsorption mechanism.
Reusability and desorption studies
In order to evaluate the reusability and regeneration of FCS monolithic hydrogel as adsorbents to be a cost effective material, desorption experiments were carried out using FCS hydrogel as adsorbent and CR and SFL dyes as adsorbates. So the desorption of adsorbed CR and SFL dyes from FCS monolithic hydrogel was investigated in a batch desorption system. Because the FCS is a monolithic hydrogel, the collection of the adsorbent was fast and easy. The FCS hydrogel was recovered by immersing in a 0.01 M NaOH solution for 6 h to desorb CR and SFL dyes that had adsorbed on the hydrogel. Then the hydrogel was protonated in excess 0.01 M HCl for 1 h to regenerate cationic active sites to use in the next adsorption cycle. The regenerated FCS was further washed by deionization water to reach neutrality. The reusability of FCS hydrogel was evaluated over five cycles of adsorption/desorption by using NaOH solution as the desorption agents with 0.05 g FCS monolithic hydrogel in 100 mL dye solution (10 ppm) at 25°C for 24 h (each cycle), and the results are shown in Fig. 8. As seen, no significant losses in the dye adsorption performance was detected even after five cycles of reusability. It was observed that about 9% and 3% reduction in removal efficiency of FCS hydrogel for CR and SFL dyes respectively.
Thus, the results depict that the electrostatic attraction is dominating driving force in the adsorption process in this study. It is important to highlight that the positively charged amino group in FCS hydrogel was deprotonated in alkaline conditions, which is affecting the electrostatic interactions between FCS hydrogel and anionic dye and then desorbed the loaded dye from adsorption active site [77]. Therefore, it can be concluded that FCS monolithic hydrogel promising ecofriendly adsorbent material due to can be regenerated easily and used repeatedly as an efficient adsorbent for water purification.