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Modified kaolin hydrogel for Cu2+ adsorption

  • Jin Chen EMAIL logo , Kun Zhao , Lu Liu , Yuyu Gao , Lu Zheng and Min Liu
Published/Copyright: December 8, 2022
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e-Polymers
From the journal e-Polymers Volume 22 Issue 1

Abstract

Removal of Cu2+ ions from contaminated water is an important but challenging task. This study reports the synthesis of a composite hydrogel from two natural polysaccharides, namely, sodium alginate and chitosan, using inexpensive kaolin as a raw material and polyacrylamide as a modifier. The hydrogel had a high adsorption capacity and selectivity for Cu2+. The composite hydrogel was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. The pseudo-second-order kinetic model was the most suitable model for the kinetic results, and the Langmuir isotherm model was the most representative of the sorption system. The results revealed that the adsorption process was mainly controlled by chemisorption. The maximum adsorption capacity of the adsorbent was 106.4 mg·g−1. Therefore, this study presents a new perspective on the application of composite hydrogels as Cu2+ adsorbents.

Keywords: adsorbent; composite hydrogel; kinetic model; cooper; kaolin

1 Introduction

Rapid industrialization has increased the production of industrial waste and resulted in water, air, and soil pollution (1). This can have adverse health and environmental effects. The presence of heavy metals in water such as Cu2+ has drawn particular attention, because the long-term intake of water with a Cu2+ concentration exceeding 1 mg·L−1 can lead to a variety of diseases, such as liver and kidney damage and spleen and stomach weakness. In many developing countries, industrial sewage waste is not treated prior to discharge; consequently, heavy metal ions can persist in the environment (2).

Several methods such as chemical precipitation (3), membrane separation (4), ion exchange (5), and electrolysis are often employed for the removal of heavy metal ions from industrial wastewater. However, these methods are expensive, require harsh working conditions, and cannot be applied on a large scale. The adsorption method is the most favored due to its simple operation and the wide availability of raw materials. The efficiency of the adsorption method in the removal of heavy metal ions depends on the properties of the adsorption materials. A wide variety of adsorption materials exist, such as bentonite, kaolin, activated carbon, agricultural and forestry wastes, polymeric materials, and various metal oxides (6,7). However, optimal low-cost adsorption materials with high adsorption efficiency are yet to be fabricated. Most natural adsorbents have a low adsorption capacity or cannot be easily reused after adsorption. Biopolymer-based composite adsorbents synthesized from natural materials have been gaining attention because of their environmental friendliness, low cost, and wide variety of raw materials available (8).

Polymeric hydrogels can preserve many layers of water because of their unique structures (9,10,11). Hydrogels are gaining attention as adsorbents because of their three-dimensional cross-linked network structure, high water retention, and abundant surface functional groups (12). Current chemical hydrogel adsorbents are fabricated with toxic chemical cross-linking agents, which remain in the hydrogel or attach to its surface and are difficult to remove by conventional means (13). Conversely, physical hydrogels cross-link with each other via electrostatic interactions, hydrogen bonding, and other interactions, and do not require the addition of chemical cross-linking agents. However, unlike chemical hydrogels, which are permanent gels as their cross-linked networks are fixed by chemical bonds, physical hydrogels decompose upon heating and are called reversible gels (14). The application of physical hydrogels in the field of heavy metal adsorption has been reported, but enhancing their performance has not been extensively studied (15). Therefore, developing an environmentally friendly composite hydrogel with excellent adsorption performance is of great significance for the removal of heavy metal ions from industrial wastewater.

Chitosan (CTS) is a natural polymer polysaccharide with amino, hydroxyl, and other functional groups. It is formed by the deacetylation of chitin and is non-toxic, biodegradable, and biocompatible (16). Sodium alginate (SA) is another natural polymer. Its molecular skeleton contains a large number of functional groups, which render it an excellent heavy metal cationic adsorbent. It is used in the biotechnology industry as a thickener and stabilizer. Notably, SA and Ca ions can cross-link via non-covalent bonds to generate hydrogels without the need for toxic chemical cross-linking agents (17,18,19). Consequently, CTS and SA have both been gaining attention in the field of heavy metal adsorption (20). However, under strong electrostatic action, the anions and cations of natural bio-based polymers usually form precipitates rather than hydrogels according to the semi-dissolved acidified-sol–gel transformation method (21). Furthermore, the adsorption capacity and mechanical strength of SA hydrogels with simple structures are not ideal. In order to optimize the properties of existing hydrogels, the synthesis of modified and composite gels has been gaining attention. Zidan et al. (22) modified CTS with N-aminorhodanine and then reacted N-aminorhodanine with glutaraldehyde with low toxicity before modifying CTS. The maximum adsorption capacity of Cu2+ of the resultant hydrogel was 62.5 mg·g−1. Jiang et al. (23) synthesized an SA-polyacrylamide (PAM)/graphene oxide hydrogel composite adsorbent by free radical polymerization. Under the optimal conditions (pH 5), the maximum Cu2+ adsorption capacity was 68.76 mg·g−1. After several cycles of adsorption, the adsorption capacity remained above 80%.

Composites with inexpensive inorganic materials such as clay are promising adsorbents. Resources of kaolin, which is considered a low-cost natural adsorbent and has demonstrated outstanding pollutant-removal capabilities, are abundant on the Earth’s surface (24). Most kaolinites are close to the ideal formula 2SiO2·Al2O3·2H2O, where the surface of Al3+ has a net negative charge due to the isomorphic substitution of Si4+. Kaolinite also has a large specific surface area. These two characteristics favor the adsorption of heavy metal ions (25). Nevertheless, the adsorption capacity of kaolin to metal ions is poor and the adsorption efficiency is weak. It has been reported that the Cu2+ and Fe3+ adsorption capacities of kaolin are 11.0 and 11.2 mg·g−1, respectively (26). Further, removing the adsorbed kaolin from the solution completely with the available separation methods is difficult (27). Currently, most modifications of natural clay require high-temperature activation and the addition of toxic reagents, which increase both environmental pollution and production cost. Xia et al. (28) reported that the heavy metal adsorption ability of ligand/PAM composite materials was excellent. Huang et al. (29) prepared SA/polyvinyl alcohol/kaolin composite hydrogels by ion osmosis. Under the optimal conditions, the theoretical maximum Cu2+ adsorption capacity of the hydrogel with a high water content was 5.061 mg·g−1. Li et al. (30) prepared a fibrous calcium alginate-immobilized kaolin hydrogel by a sol–gel method using SA to fix powdered kaolin, which overcame the issues with recovering powdered kaolin from solution. Notably, the Cu2+ adsorption capacity reached 53.63 mg·g−1.

This study reports the synthesis of a novel, environmentally friendly, and low-cost composite hydrogel adsorbent. First, kaolin was physically modified with PAM to improve its adsorption performance. Then, the modified kaolin was mixed with CTS and SA, followed by cross-linking with Ca2+ under acidic conditions, to prepare an SA/CTS/P-kaolin composite physical hydrogel. Notably, no toxic reagents are required to fabricate this hydrogel. Therefore, this study provides a reference for the development of new low-cost hydrogel adsorbents based on inorganic materials. Furthermore, the prepared hydrogel has good application prospects in the field of environmental protection.

2 Materials and methods

2.1 Materials

CTS was purchased from Beijing Chemical Industry. SA and CuCl2 were purchased from Tianjin Kemeiou Chemical Reagent Co., Ltd. Kaolin was obtained from Guangzhou Chemical Reagent Factory. Glacial acetic acid, CaCl2, and sodium hydroxide were purchased from Shanghai MacLean Biochemical Technology Co., Ltd. All reagents except kaolin were of analytical grade. Deionized water was used in all experiments.

2.2 Preparation of composite hydrogel

2.2.1 Preparation of the modified kaolin

PAM (1 g) and deionized water (50 mL) were mechanically stirred to form a uniform solution. Kaolin (10 g) was dissolved in distilled water (100 mL). The two solutions were then combined and the mixture was magnetically stirred for 6 h. The resulting solution was filtered and dried at 60°C for 3 h to obtain the modified kaolin.

2.2.2 Preparation of the composite hydrogel

The SA/CTS/P-kaolin composite physical hydrogel was prepared by partial optimization based on the semi-dissolved acidified-sol–gel transformation method reported by Lin et al. (21). SA was dissolved in distilled water, followed by adding CTS and stirring for 2 h to form a slurry solution. Calcium chloride and the modified kaolin were then added successively, and the solution was stirred for 3 h. The resulting solution was poured into a petri dish and left for 30 min to remove bubbles. The petri dish was then acidified for 24 h in a confined space filled with glacial acetic acid (50 mL). The mixture was cross-linked in acetic acid vapor to form the composite hydrogel. The final product was rinsed several times with distilled water to remove any residual surface impurities.

2.3 Characterization

The lyophilized hydrogels were characterized using Fourier-transform infrared (FTIR) spectrometry (Tianjin Gang Dong Technology Co., Ltd.) at 500–4,000 cm−1. Scanning electron microscopy (SEM) was carried out on a Zeiss Sigma300 microscope at an accelerating voltage of 5 kV. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with a Cu Kα anode (λ = 0.1540 nm) over an angular range of 10–80°.

2.4 Adsorption/desorption experiments

A standard solution of Cu2+ (1,000 mg·L−1) was prepared by diluting copper chloride dehydrate. This solution was then used to prepare Cu2+ solutions with various concentrations. As the Cu2+ concentration of industrial wastewater is in the range of 150–250 mg·L−1, the standard solution was diluted to ten different solutions with concentrations in the range of 0–500 mg·L−1.

Glacial acetic acid and sodium hydroxide solution (5 wt%) were used to prepare a buffer solution. Xylenol orange (2.5 mL) and the buffer (1 mL) were then added to the Cu2+ solutions (10 mL). After 20 min, the absorbance of the mixture was measured using ultraviolet-visible spectrophotometry at 575 nm.

Adsorption tests were carried out at a Cu2+ concentration of 100 mg·L−1 with an adsorbent dosage of 0.2 g for 24 h. The initial solution pH was 7. The Cu2+ concentration was then adjusted by controlling the adsorption time and pH. To determine the selectivity of the composite hydrogel, the change in the adsorption capacity was observed by controlling the concentrations of Cu2+ and Na+ and the pH.

To demonstrate the repeatability of the adsorption performance, the adsorbent was immersed in a hydrochloric acid solution (0.3 mol·L−1) for 1 h after Cu2+ adsorption, after which the adsorption process was repeated. The adsorption–desorption process was repeated for five cycles.

3 Results

3.1 FTIR spectroscopy

The FTIR spectra of the kaolin and composite hydrogel are shown in Figure 1.

Figure 1 FTIR spectra of kaolin and the SA/CTS/P-kaolin composite hydrogel.
Figure 1

FTIR spectra of kaolin and the SA/CTS/P-kaolin composite hydrogel.

3.2 Morphological observation of the composite hydrogel and kaolin

Figure 2 shows SEM images of the kaolin and SA/CTS/P-kaolin composite hydrogel at different magnifications. The composite hydrogel had a non-uniform surface with many voids.

Figure 2 SEM images of (a and b) the composite hydrogel and (c and d) kaolin.
Figure 2

SEM images of (a and b) the composite hydrogel and (c and d) kaolin.

The XRD results revealed that the diffraction peaks were weaker for the SA/CTS/P-kaolin composite hydrogel than those for kaolin, while the half-slit widths were wider (Figure 3).

Figure 3 XRD images of SA/CTS/P-kaolin and kaolin.
Figure 3

XRD images of SA/CTS/P-kaolin and kaolin.

3.3 Water content

The water contents of the SA/CTS, SA/CTS/kaolin, and SA/CTS/P-kaolin hydrogels were measured to be 42.39%, 33.84%, and 41.53%, respectively (Table 1).

Table 1

Water content of the adsorbents

Adsorbent SA/CTS SA/CTS/kaolin SA/CTS/P-kaolin
Water content (100%) 42.39 (±0.87) 33.84 (±0.82) 41.53 (±1.28)

Abbreviations: SA – sodium alginate, CTS – chitosan, P – PAM.

3.4 Adsorption performance

3.4.1 Influence of the pH on copper ion adsorption capacity

The Cu2+ adsorption capacities of the kaolin and SA/CTS/P-kaolin adsorbents were studied in the pH range of 2–5. As shown in Figure 4, as the pH decreased, the Cu2+ adsorption capacities of the hydrogel and kaolin decreased. The adsorption capacity of the hydrogel decreased significantly at pH 2.

Figure 4 Effect of pH on Cu2+ adsorption.
Figure 4

Effect of pH on Cu2+ adsorption.

3.4.2 Influence of hydrogel weight on Cu2+ adsorption capacity

The Cu2+ adsorption capacities of kaolin and SA/CTS/P-kaolin adsorbents with different weights are shown in Figure 5. When the weight of the hydrogel adsorbent increased, the adsorption capacity per unit weight gradually decreased to half of the maximum adsorption capacity. However, because the kaolin adsorbent was a powder, it was evenly dispersed in the Cu2+ solution during the adsorption process; therefore, the adsorption capacity per unit weight did not decrease significantly with increasing weight.

Figure 5 Effect of adsorbent weight on Cu2+ adsorption.
Figure 5

Effect of adsorbent weight on Cu2+ adsorption.

3.4.3 Adsorption isotherm and kinetics study

The adsorption capacity over time was plotted, as shown in Figure 6. The adsorption capacity increased rapidly in the first 3 h and then gradually slowed down after 5 h. As the adsorption process progressed, the change in adsorption capacity gradually decreased. The adsorption process was completed in approximately 20 h, and 57.6% of the total adsorption occurred within the first 3 h. The pseudo-first-order and pseudo-second-order dynamic models were fitted to the adsorption curves. The kinetic parameters and correlation coefficients (R 2) of the models are listed in Table 2.

Figure 6 Kinetic models of Cu2+ removal by SA/CTS/P-kaolin and kaolin adsorbents.
Figure 6

Kinetic models of Cu2+ removal by SA/CTS/P-kaolin and kaolin adsorbents.

Table 2

Kinetic parameters of adsorbent removal of Cu2+

Pseudo-first-order model Pseudo-second-order model
K 1 q e R 2 K 2 q e R 2
Kaolin 0.16063 10.13546 0.95566 0.0124 12.80335 0.96662
SA/CTS/P-kaolin 0.32014 102.86586 0.99073 0.00382 115.94513 0.99162

The adsorption capacity and mechanism of the adsorbent are usually described by an adsorption isotherm. Figure 7 shows the isotherms of the composite hydrogel and kaolin adsorbents at room temperature. In the studied concentration range, the adsorption capacity of the composite hydrogel increased with an increase in concentration. At Cu2+ concentrations greater than 350 mg·L−1, the rate of growth of the adsorption capacity gradually decreased. The adsorption capacities of both the kaolin and composite hydrogel adsorbents increased significantly at concentrations lower than 100 mg·L−1. Notably, the adsorption capacity of the composite hydrogel adsorbent increased in the concentration range of 100–300 mg·L−1. It is speculated that this occurs because the gel structure formed by SA and CTS and the functional groups carried by them favor the adsorption of Cu2+ ions over natural clay. The kinetic parameters and correlation coefficients (R 2) of the Langmuir and Freundlich models are listed in Table 3.

Figure 7 Langmuir and Freundlich isotherms of Cu2+ removal by SA/CTS/P-kaolin and kaolin adsorbents.
Figure 7

Langmuir and Freundlich isotherms of Cu2+ removal by SA/CTS/P-kaolin and kaolin adsorbents.

Table 3

Langmuir and Freundlich isotherm parameters for Cu2+ removal by adsorbent

Langmuir isotherm Freundlich isotherm
K 1 q m R 2 K 2 N R 2
Kaolin 0.02862 11.36831 0.99442 3.80455 5.89185 0.97235
SA/CTS/P-kaolin 0.01271 130.514 0.99399 18.73079 3.39051 0.96591

3.4.4 Coexisting ion interference

Cu2+ adsorption experiments were conducted using the kaolin and SA/CTS/P-kaolin adsorbents in the presence of Na+ ions to determine the effect of coexisting ions on the adsorption capacity. As shown in Figure 8, competitive adsorption was not observed at low Na+ concentrations.

Figure 8 Effect of Na+ concentration on Cu2+ adsorption.
Figure 8

Effect of Na+ concentration on Cu2+ adsorption.

3.4.5 Desorption and regeneration experiment

The repeatability of the adsorption performance was evaluated by measuring the adsorption capacity of the hydrogel after five adsorption–desorption cycles. As shown in Figure 9, the Cu2+ adsorption capacity of the composite hydrogel decreased with cycle number. The adsorption capacity formula is represented by the following equation:

(1) q t = ( C t − C 0 ) V m

where q t (mg·g−1) is the adsorption capacity at time t; C 0 (mg·L−1) is the initial concentration of Cu2+ ions in solution; C t (mg·L−1) is the concentration of Cu2+ ions at time t; V (mL) is the volume of the Cu2+ solution; and m (mg) is the weight of the adsorbent.

Figure 9 Adsorption capacity of the SA/CTS/P-kaolin hydrogel over five adsorption–desorption cycles.
Figure 9

Adsorption capacity of the SA/CTS/P-kaolin hydrogel over five adsorption–desorption cycles.

4 Discussion

The FTIR spectra of CTS typically contain two peaks at 1,650 and 1,608 cm−1, which are attributed to the stretching vibration of C═O and the bending vibration of –NH, respectively (31). In the SA/CTS/P-kaolin hydrogel, a further absorption peak was observed at 1,637 cm−1 (Figure 1), indicating the protonation of the amino group on CTS. The characteristic peaks of SA (–COO−) in the hydrogel adsorbent shifted from 1,603 and 1,410 cm−1 to 1,421 and 1,091 cm−1, respectively, indicating that –COO− reacted with the –NH3+ groups of CTS. The peak observed at 3,762 cm–1 is attributed to the stretching and bending vibration of –OH during hydration. Peaks corresponding to the amide bonds of the modified kaolin were observed at 1,637 and 3,000–3,700 cm−1 (32). Thus, the FTIR results indicate an interaction between SA, CTS, Ca2+, and the modified kaolin.

The SEM images showed that the surface of the modified kaolin composite hydrogel was rougher than the kaolin powder, and the pores were more diverse (Figure 2). The addition of PAM made the surface pore size of kaolin uneven, thus improving the surface structure of the hydrogel (33). The non-uniform pore sizes in the composite hydrogel favor the swelling and adsorption of metal ions. The composite product of kaolin with alginate and CTS by cross-linking had a rough surface and can thus form Cu2+ adsorbing nanoparticles. The XRD results (Figure 3) revealed that due to the cross-linking process in acetic acid gas, CTS and PAM damaged the original dense structure of kaolin and exposed more active sites. Consequently, the adsorption capacity of the composite hydrogel was improved.

Water content is an important property of hydrogel adsorbents. A higher water content increases the specific surface area of the hydrogel and exposes more adsorption sites, which improves the adsorption of heavy metals by the hydrogel (34). The water content of the SA/CTS/kaolin hydrogel was lower than that of the SA/CTS hydrogel (Table 1) due to the low density of kaolin. Furthermore, the hydroxide groups of kaolin can form hydrogen bonds with SA and interfere with the cross-linking process of SA and CTS during hydrogel formation (35). In contrast, the SA/CTS/P-kaolin and SA/CTS hydrogels had similar water contents (Table 1). This indicates that the modified kaolin had little effect on the water content of the adsorbent, as the presence of PAM improved the environment for SA and CTS cross-linking and reduced the probability of hydrogen bond formation between kaolin and SA.

Another important factor that determines the application and selectivity of adsorbents is pH. An increase in pH leads to the formation of Cu(OH)2 precipitates from Cu2+ and affects the accuracy of the adsorption data. This is not discussed in this study. However, the adsorption capacity of the hydrogel was greatly reduced when the pH decreased (36). The adsorption capacity of the hydrogel was greatly decreased at pH 2 due to the competitive adsorption between H3O+ and Cu2+ in the solution, which decreases the binding of Cu2+ to the adsorption sites on the composite hydrogel (37). The presence of H+ and H3O+ at low pH was not beneficial for the adsorption of Cu2+. Thus, the adsorption capacity of the hydrogel was enhanced with an increase in pH. At pH 4, the adsorption capacity of the composite hydrogel changed significantly as compared to that of CTS hydrogel and kaolin. In fact, the oxygen atoms in PAM can react with the aluminum hydroxide groups in kaolin, thus generating multiple strong hydrogen bonds, which improved the total amount and adsorption capacity of the SA/CTS/P-kaolin composite hydrogel (38). The pH of the adsorption experiments was then set to 5.

The adsorption kinetics is important for describing the adsorption performance by relating the instantaneous adsorption amount to the adsorption time. Nonlinear kinetic simulations were used to study the Cu2+ adsorption mechanism of the adsorbents. The pseudo-first-order and pseudo-second-order kinetic formulas are represented by the following equations, respectively:

(2) q t = q e ( 1 − e k 1 t )

(3) q t = k 2 q e 2 t ( 1 − k 2 q e t )

where q e (mg·g−1) is the adsorption capacity at equilibrium, q t is the adsorption capacity at time t, and k 1 and k 2 are the pseudo-first-order and pseudo-second-order rate constants, respectively. The pseudo-first-order and pseudo-second-order kinetic models (Table 2) matched well within 25 h of adsorption (R 2 > 0.9). Therefore, the adsorption process of the composite hydrogel adsorbent included both physical and chemical adsorption.

The adsorption capacity of the SA/CTS/P-kaolin composite gel was significantly larger than that of kaolin. This is mainly attributed to the increase in the number of adsorption sites, which facilitate the binding of Cu2+ to the composite gel (39). The parameters of the kinetic model revealed a poor fit for the pseudo-first-order kinetic model compared with the pseudo-second-order kinetic model. Therefore, it was concluded that the adsorption process was mainly controlled by chemisorption.

The Langmuir and Freundlich isotherm models were used to discuss the interaction between the composite hydrogel and Cu2+, calculated according to the following equations, respectively:

(4) q e = k 1 q m C e 1 + k C e

(5) q e = k 2 C e n

where q e (mg·g−1) is the adsorption capacity at equilibrium; C e (mg·L−1) is the concentration of Cu2+ adsorbed on the hydrogel at equilibrium; q m is the maximum Cu2+ adsorption capacity; k 1 is the Langmuir constant; and N and k are the empirical parameter and binding constant of the Freundlich model, respectively.

The parameters of the Langmuir and Freundlich isotherms are shown in Table 3. The results revealed that the Langmuir model had a higher fitting degree (R 2 = 0.99442, 0.99399) than the Freundlich model (R 2 = 0.97235, 0.96591) for both the kaolin and composite hydrogel adsorbents. The Langmuir isotherm model describes the monolayer adsorption on a homogeneous surface where adsorption molecules do not interfere with each other (40), while the Freundlich isotherm model shows reversible adsorption on heterogeneous surfaces of the multimolecular layer. Therefore, the composite hydrogel adsorbent followed the monolayer adsorption mechanism (41).

The maximum adsorption capacity of the SA/CTS/P-kaolin composite hydrogel adsorbent was 106.426 mg·g−1. Notably, this adsorption capacity is significantly better than those of acid-activated kaolin (41.109 mg·g−1) (42) and kaolinite–SA graphene nanoplates (16.66 mg·g−1) (43). Wu et al. (44) reported that a polyvinyl alcohol-enhanced CTS hybrid adsorbent had a Cu2+ adsorption capacity of 38.7 mg·g−1. Thus, our results demonstrate that the SA/CTS/P-kaolin composite hydrogel adsorbent can improve the adsorption capacity of Cu2+ ions. A comparison of the adsorption properties of common adsorbents with the SA/CTS/P-kaolin composite hydrogel is presented in Table 4.

Table 4

Comparison of the properties of common adsorbents and the products in this study; all adsorbents followed the Langmuir isotherm and pseudo-second-order kinetics

Adsorbent Cu2+ adsorption capacity (mg·g−1) Process conditions Ref.
pH Time (h) Temp (°C)
SA/CTS/P-kaolin 106.42 5 20 25 This study
GO–CTS aerogel 25.4 7 8 51 (45)
Microporous spongy CTS monoliths doped with GO 53.69 5 15 27 (46)
Nanochitin-contained CTS hydrogel beads 38.7 4.2 0.17 (10 min) 25 (47)
CTS/PVA adsorptive membrane 86.08 6 20 25 (48)

Abbreviations: SA – sodium alginate, CTS – chitosan, P – PAM, GO – graphene oxide, PVA – polyvinyl alcohol.

In actual water treatment plants, other metal ions, such as Na+, are also present. This can interfere with the adsorption process. Therefore, it is necessary to test the selectivity of the adsorbents in solutions with different Na+ concentrations (49). Competitive adsorption was not observed at low Na+ concentrations. However, the Cu2+ adsorption capacity of the adsorbent significantly decreased as the Na+ concentration increased to 150 mg·L−1. The hydroxyl and carboxyl groups of SA are conducive to the adsorption of Cu2+; however, the carboxyl groups can also adsorb Na+ ions, which reduces the Cu2+ adsorption capacity. In the kaolin-containing hydrogel, the Al-containing hydroxides on the surface of kaolin react with the carboxyl groups to form hydrogen bonds, which also reduces the adsorption capacity of Cu2+ (50). In contrast, in the composite hydrogel, the ═O groups of PAM combine with the Al-containing hydroxides of kaolin to form strong hydrogen bonds, which avoids the interference between kaolin and SA. Therefore, the modified kaolin can significantly improve the adsorption capacity, selectivity, and mechanical stability of the composite hydrogel.

In industry, reusability is an extremely important standard for adsorbents (51). To test the reusability, five repeated adsorption–desorption cycles were carried out, with an acid solution used to dehydrate the absorbent gel during the desorption steps. The adsorption capacity remained stable and the regeneration efficiency was above 80% after five cycles, indicating the good reusability of the hydrogel adsorbent (52). In the adsorption–desorption cycles, the adsorption capacity of the composite hydrogel for Cu2+ continued to decrease. This may be attributed to the degradation of SA in the acidic solution, which would affect the stability of the adsorbent and deteriorate its performance.

5 Conclusion

In this study, SA/CTS/P-kaolin composite hydrogels were prepared by a semi-solubilized acidified sol–gel transformation method using CTS and SA as raw materials. This synthetic procedure is both safe and environmentally friendly, because toxic cross-linking agents are not employed and the reaction is carried out at room temperature and pressure. The characteristics of the hydrogel adsorbent were investigated, and the Cu2+ adsorption capacity was measured. The results revealed that the optimal ratio of CTS, SA, and modified kaolin for Cu2+ adsorption was 3:8:12, and the adsorption capacity of the hydrogel was 106.426 mg·g−1. The SA/CTS/P-kaolin composite hydrogel provides insights into the application of natural bio-based adsorbents in wastewater treatment.

  1. Funding information: The authors state no funding involved.

  2. Author contributions: Kun Zhao: writing – original draft, writing – review and editing, methodology, formal analysis; Jin Chen: writing – original draft, visualization; Yuyu Gao: resources, formal analysis; Lu Liu: project administration; Lu Zheng: supervision, validation; Min Liu: software.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2022-09-06
Revised: 2022-10-15
Accepted: 2022-10-24
Published Online: 2022-12-08

© 2022 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2022-0085
Keywords for this article
adsorbent; composite hydrogel; kinetic model; cooper; kaolin
Creative Commons
BY 4.0

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