Synthesis of cross-linking chitosan-PVA composite hydrogel and adsorption of Cu(II) ions

Water pollution by heavy metals is becoming a severe global issue due to the toxicity, non-degradability and cumulative nature of heavy metals through the food chain. Copper is often detected in industrial wastewaters, which originates from mining activities, metal plating, smelting, battery manufacture, tanneries, petroleum refining, paint and pigment manufacture, and so on (Karaer & Kaya 2017). The industrial waste effluents contain a large amount of copper pollutants, which are harmful to human health and ecological systems. The main methods for removing Cu(II) from an aqueous environment include ion exchange, chemical precipitation, membrane filtration and adsorption, etc (Shalla et al. 2019), and adsorption has been found to be the best technology due to its high efficiency, eco-friendliness, low operating cost and easy operation (Agarwal & Singh 2017). Numerous methods have been made by various researchers to develop cheaper and effective adsorbents to remove Cu(II) ions from waste water (Ayoub et al. 2013; Sahebjamee et al. 2019). Recently, bioadsorbents have been receiving increasing attention due to their nontoxic, biodegradable and natural availability (Muya et al. 2016). Among these, chitosan (CS) is an abundant biopolymer obtained by deacetylation of chitin and has a good adsorption ability for heavy metal ions because of its good metal-binding capacities, biodegradability and low toxicity (Salehi et al. 2016). However, pure chitosan materials also have some obvious disadvantages, such as low mechanical strength, poor acid resistance and difficult recovery (Peng et al. 2014). To overcome these disadvantages, considerable efforts have been made by chemical cross-linking or blending CS with other polymers, such as polyvinyl alcohol (PVA), cellulose and polyacrylamide (Zhang et al. 2016). Among the various polymers, PVA is one of materials used for interesting blends with chitosan due to its low cost, good mechanical properties and non-toxic nature. Chitosan blended with PVA has been reported to have good mechanical and chemical properties because of the formation of intermolecular hydrogen bonds (Karaer & Kaya 2017; Abdolmaleki et al. 2018). Also, it is necessary for composite hydrogel to enhance acid resistance ability through the cross-linking reaction.

In the past years, some researchers have employed various cross-linking agents (such as epichlorohydrin, glyoxal, glutaraldehyde and tripolyphosphate) to eliminate solubility concerns and improve the mechanical properties of chitosan composites (Salehi et al. 2016).

In the present work, a cross-linked chitosan-PVA spherical hydrogel (CSH) with good adsorption capability and low cost was synthesized by a two step method. First, chitosan/PVA composite spherical hydrogel was prepared via the phase inversion method. Then, the obtained composite hydrogel was crosslinked with epichlorohydrin in basic condition. The prepared CSH was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD) to identify the structures and morphology. Unlike traditional powder adsorbents, the CSH, with a mean diameter of 3 mm, is easily separated and regenerated in acid solution. In addition, the CSH adsorbent was completely insoluble in strong acid solution because of the formation of a three-dimensional network structure in the cross-linking reaction, which benefits industrial wastewater treatment. Finally, the adsorbent was used for adsorption of Cu(II) from aqueous solutions. The initial pH value, adsorption time and regeneration ability were investigated in the experiment. The adsorption experimental data were studied based on isotherm and kinetic models.

Chitosan (93% deacetylated) was obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). PVA (1788) was supplied by Macklin, Shanghai China. CuSO₄·5H₂O, KOH, HNO₃, HCl and epichlorohydrin were of analytical grade and were obtained from Shanghai Chemical Co, China. All the solutions were prepared with distilled water.

2.0 g chitosan was dissolved in 48 mL of 2% acetic acid until a clear solution was obtained. Meanwhile, 2.0 g PVA was dissolved in 18 mL distilled water by heating the solution at 90 °C under strong stirring for 30 min. Next, the above two solutions were mixed together for 3 h by magnetic stirring. The resultant homogeneous solution was left in the beaker to sufficiently free the air bubbles. After that, the solution was added dropwise to a 5% NaOH solution through a 16G syringe needle (1.15 mm inner diameter) and the formed hydrogel was immersed in a 5% NaOH solution to neutralize the excess acid for 2 h. Then, the chitosan/PVA spherical hydrogel was filtered and washed repeatedly with distilled water until all alkali was removed.

The prepared chitosan/PVA hydrogel was cross-linked with epichlorohydrin (4 mL) in 100 mL ethanol solvent containing 0.1 M potassium hydroxide. After reaction for 3 hours at 50 °C, the cross-linked hydrogel was filtered and washed with ethanol several times, and then freeze-dried for 24 h to obtain CSH. The average diameter of the freeze-dried CSH is about 3.0 mm and the preparation process is illustrated in Figure 1.

FTIR spectra of the samples were measured using a Fourier transform infrared spectrometer, Model EQUINOX55 (Bruker). A Hitachi S-4800 (Japan) scanning electron microscopic (VP-SEM) was used to examine the microstructure of chitosan and CSH. X-ray diffraction patterns of chitosan, PVA and CSH were measured using an X-ray diffractometer (D8-Advance, Bruker).

To determine the kinetics performances, 100 mg of CSH was added to 50 mL of Cu(II) solution (100 mg/L or 200 mg/L). The mixture was then stirred, samples were filtered at different time intervals and the filtrate solution was analyzed. Equilibrium adsorption isotherm experiments were conducted over a range of initial Cu(II) concentrations from 50 to 200 mg/L. 100 mg of CSH was added into 50 mL of the solutions. The mixtures were stirred at 25 °C for 24 h. After adsorption, the solution was filtered and the final concentration of Cu(II) was determined.

After the Cu(II) was adsorbed, the CSH was added into 100 mL 0.1 mol/L HCl for 3 h to ensure the desorption of Cu(II) from the adsorbent. To use the adsorbent in the next cycle, it was washed several times with distilled water. Then, the treated CSH was applied in the recycled adsorption study. The adsorption/desorption measurements were repeated five times.

To investigate the interaction between the chitosan and PVA molecules, chitosan, PVA, and CSH were characterized by FTIR. FTIR spectra of chitosan, PVA and CSH before and after Cu(II) adsorption are shown in Figure 2. In the spectrum of chitosan, the major bands can be assigned as follows: 3,423 cm⁻¹ (–OH and –NH₂ stretching vibrations), 2,883 cm⁻¹ (–CH stretching vibration), 1,639 cm⁻¹ (amide I), 1,600 cm⁻¹ (–NH₂ bending vibration), 1,325 cm⁻¹ (stretching vibration of C–N), 1,153 cm − 1 (bridge-O-stretch), 1,035 and 1,078 cm⁻¹ (primary and secondary stretching vibration of C–OH) (Song et al. 2017; Yu et al. 2017). For PVA, the broad characteristic peak appeared at 3,427 cm⁻¹ relating to the stretching -O-H group. Peaks at around 2,978 cm⁻¹ and at 1,402 cm⁻¹ corresponded to the C-H stretching and -CH₂- bending from alkyl groups, respectively. The strong peak at 1,635 cm⁻¹ indicates the presence of acetate groups of PVA. This is related to the partial (87% − 89%) hydrolyzation of the PVA. In addition, the peak at 1,093 cm⁻¹ is attributed to the C–OH stretching of PVA (Jamnongkan et al. 2014). Compared with the spectra of chitosan and PVA, the wide peak at 3,433 cm⁻¹ of CSH corresponding to the stretching vibration of the –NH₂ group and –OH group shifted to a higher frequency. The strong band at 1,600 cm⁻¹ assigned to the free amino group disappeared and shifted to 1,639 cm⁻¹ (C = O stretching vibration of PVA and N − H bending vibration of chitosan), indicating –NH₂ groups take part in the crosslinking reaction. Furthermore, the peaks of primary and second C − OH stretching vibration were also overlapped and shifted to 1,082 cm⁻¹. All of these results confirmed that intermolecular hydrogen bonding and crosslinking reaction formed among –NH₂ and -OH groups in CSH. After adsorption of Cu(II), significant changes in the FTIR spectrum were found at the wavenumbers of 3,407 cm⁻¹, 1,635 cm⁻¹ and 1,111 cm⁻¹. Therefore, it can be concluded that the adsorption mainly occurred at the -NH₂ and -OH groups of CSH.

The morphology of chitosan (a1, a2) and CSH (b1, b2) was assessed through SEM. As is depicted in Figure 3, the pure chitosan appears to have a smooth and homogeneous surface. On the contrary, the surface of CSH shows a highly porous structure, these structures will provide a convenient diffusion channel benefit to adsorption of Cu(II). The porous structure of CSH was probably caused by the presence of extensive hydrogen bonding between the functional groups of the two polymers and chemical crosslinking processes (Hu et al. 2018).

Figure 4 shows the XRD patterns for chitosan, PVA and CSH. It can be seen that there are some differences in peak heights between them. Chitosan reveals a significant crystalline peak near 20°, and the characteristic peak of PVA was at around 19.5° of 2θ (Kalantari & Afifi 2018). However, CSH has a relatively characteristic peak that was significantly weakest. It is well known that the XRD peak is related to the size of crystallite, a weaker peak usually results from small crystallites (Song et al. 2011). Hence, the crystalline structure of CSH was destroyed due to forming a new interpenetrating network structure by the chemical crosslinking reaction (Ahmad et al. 2017).

Acid resistance is an important property for adsorbent materials used in water treatment. Because most Cu(II) ion-containing industrial wastewater is acidic and pure chitosan and PVA dissolve in acid conditions, the ability of the CSH to resist acid was investigated. Here, the structural stability of the CSH was evaluated by directly observing the weight loss in the acid solutions after 24 h. The results are presented in Table 1. Dissolution testing has shown that CSH is completely insoluble in acidic solutions and there is no change in the mass loss. These findings demonstrated that the cross-linking significantly improved the chemical and structural stability of the hydrogel in our experimental conditions. This is very significant for the application of the adsorbents in real engineering and adsorbent regeneration. In contrast, the acid resistance of uncross-linked chitosan/PVA adsorbent was tested in the experiment. It was found that uncross-linked chitosan/PVA hydrogel was completely dissolved in an acid solution after about 5 min. Therefore, the cross-linking reaction was successful in our study. The swelling degree of the composite was also investigated. As can be seen, the swelling ratios of CSH varied from 433–663%.

The effect of initial solution pH on Cu(II) removal by CSH is shown in Figure 5. As indicated in Figure 5, the pH of the aqueous solution was clearly an important parameter that controlled the adsorption process. Within the range of pH values, which varied from 2.0 to 3.0, the adsorption capacity significantly increased from 13.3 to 56.7 mg/g. With further increase of pH (3.0–5.5), the adsorption capacity of Cu(II) gradually reached the maximum of 62.1 mg/g at pH 5.5. At pH values higher than 5.5, the adsorption capacity of CSH decreased to 59.5 mg/g. Therefore, pH 5.5 was selected as the optimal value for Cu(II) adsorption on the CSH. This can be explained by the fact that at low pH the amino groups on CSH can easily protonate, inducing an electrostatic repulsion of Cu(II) (Yang et al. 2019).

Adsorption time is an important parameter because it can reflect the adsorption kinetics of an adsorbent. The effect of contact time on the adsorption of CSH for Cu(II) was investigated at different initial solute concentrations and the results are shown in Figure 6. It was found that the absorbed amount of Cu(II) increased with increasing Cu(II) concentration. This might be because, at higher concentrations, the driving force for ion migration between the aqueous phase and solid phase enhanced (Zhang et al. 2018). In addition, the rates of adsorption of Cu(II) on CSH were seen to increase markedly during the first 480 min and gradually approach the limiting adsorption after 1,440 min (24 h). The explanation may be that a large number of free adsorption sites on the surface were void and that Cu(II) easily interacted with these sites during the initial stage. With increasing time, the remaining vacant sites are not easily occupied due to the reduction of the adsorption sites (Peng et al. 2014). Then, the adsorption rate slows down and gradually reaches equilibrium.

Then, the two kinetic models were applied to fit the experimental data, respectively. The nonlinear fitting results are also shown in Figure 6 and their parameters are all listed in Table 2. Based on the analysis of correlation coefficients (R²), the pseudo second-order model gives an R² mean value (0.995) higher than that of the pseudo-first-order model (0.986). The adsorption behavior of Cu(II) on CSH obeyed the pseudo-second-order kinetic model, suggesting the adsorption of Cu(II) is controlled mainly by the chemisorption mechanism (Pu et al. 2018).

Recycle studies were performed to explore the sustainability and cost effectiveness of the CSH. Most adsorbents are in the form of powder to increase the contact area of the adsorbent and the solution, which clearly leads to difficulty in collecting the adsorbent after sorption (Yu et al. 2017). In contrast, spherical CSH can easily desorb the bound metals under acid conditions and be collected after adsorption. In addition, during the adsorption and desorption process, the CSH adsorbent showed good mechanical stability.

In the desorption experiment (see Figure 7), five adsorption-desorption cycles were conducted, and 0.1 M HCl solution was found to be effective to desorb Cu(II) from the adsorbent. The adsorption capacity of Cu(II) on CSH decreased slowly with increasing cycle numbers over three cycles, and at the end of five regeneration cycles, the sorption capacity maintained 60% of the initial value. The results indicated that CSH could be used repeatedly in Cu(II) adsorption.

Figure 8 displays the non-linear plots of fitting models. The parameters calculated are listed in Table 3. It is clear that the adsorption isotherm results can be well described by the Langmuir model (R² > 0.99), with q_m on CSH being 100.5 mg/g. Hence, the adsorption behavior of Cu(II) on CSH suggested a monomolecular layer adsorption (Hu et al. 2018).

In the present study, cross-linked chitosan-PVA spherical hydrogel (CSH) was prepared and characterized. The adsorption parameters were evaluated by using the batch adsorption method, and adsorption equilibrium was obtained in 24 h at pH 5.5 with adsorption capacity of 62.1 mg/g. The kinetics of the experimental data were fitted with the pseudo-second-order model and the Langmuir model fitted the adsorption data better. Adsorption–desorption studies showed that CSH was completely regenerated for up to five cycles using 0.01 M HCl with the loss of adsorption capacity. In summary, a low-cost CSH exhibited good adsorption ability, excellent anti-acid property in strong acid and good regeneration ability. Thus, the composite hydrogels can potentially be used for the removal of Cu(II) from wastewater.

This work was financially supported by the Natural Science Foundation of Anhui Province (No. 1908085QB63).