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. 2021 Oct 15;6(42):28285–28296. doi: 10.1021/acsomega.1c04433

Facile Preparation and Dye Adsorption Performance of Poly(N-isopropylacrylamide-co-acrylic acid)/Molybdenum Disulfide Composite Hydrogels

Jianping Yang †,*, Kailun Wang ‡, Zhengxiang Lv †, Wenjun Li ‡, Keming Luo ‡, Zheng Cao ‡,§,∥,*
PMCID: PMC8552478  PMID: 34723025

Abstract

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Using N-isopropylacrylamide (NIPAM) and acrylic acid (AAc) as monomers, N,N′-methylenebisacrylamide (MBA) as a cross-linking agent, and molybdenum disulfide (MoS2) as functional particles, a P(NIPAM-co-AAc)/MoS2 composite hydrogel was prepared by free radical polymerization initiated by ultraviolet light. The results of Fourier transform infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy show that MoS2 has been successfully introduced into the P(NIPAM-co-AAc) system, and the obtained composite hydrogel has a porous network structure. Studies on the swelling property and dye adsorption performance show that the addition of MoS2 can increase the swelling ratio of P(NIPAM-co-AAc) hydrogels to a certain extent and can significantly improve the ability of the P(NIPAM-co-AAc) hydrogel to adsorb methylene blue (MB). The adsorption process of MB by the composite hydrogels conforms to the pseudo-second-order kinetics and the Langmuir isotherm adsorption models. The estimated equilibrium adsorption capacity (Qm) using the Langmuir isotherm model can reach 1258 mg/g, mainly due to the electrostatic interaction between the negatively charged groups −COO– and MoS2 particles on the network structure and the positively charged dye MB. The adsorption of MB by P(NIPAM-co-AAc)/MoS2 composite hydrogels depends on the temperature during adsorption. Compared with room temperature, a high temperature of 40 °C above the poly(N-isopropylacrylamide) (PNIPAM) phase transition temperature (∼32 °C) leads to a decreased adsorption capacity of the P(NIPAM-co-AAc)/MoS2 composite hydrogel for MB due to the enhanced hydrophobic properties of the network structure and the decrease of the swelling ratio. The prepared hydrogel material can be used as a good adsorbent for dyes, which is promising in wastewater treatment.

Introduction

The discharge of dye-containing wastewater in various countries in the world is increasing. Once the water body is polluted, it is not easy to restore to the expected original state quickly.1,2 At present, adsorption technology is considered by environmental researchers to be the most effective and promising method for sewage treatment.3 Therefore, it is essential to develop an economical and efficient material for adsorbing harmful substances. The adsorption materials that have been reported are organic and inorganic materials, including poly(4,4′-diaminodiphenylmethane),4 polyelectrolyte multilayers,5 polyaniline and polypyrrole macro-/nanoparticles,6 sodium titanate,7 CoMo4,8 sandwichlike magnesium silicate/reduced graphene oxide nanocomposite,9 activated carbon,10 and carbon nanotubes.11 However, the adsorption materials that have been reported so far also have many shortcomings, such as complex preparation processes, use of toxic and hazardous substances, poor stability, and weak adsorption capacity.

Hydrogel is a polymer network with a three-dimensional structure that is porous and has a large surface area and many channels. Because of the presence of hydrophilic groups such as hydroxyl, carboxylic acid, amide, and sulfonic acid groups in the polymer forming the hydrogel structure, the hydrogel has an affinity for water, swells in water but does not dissolve, and can absorb a large amount of water. Poly(N-isopropylacrylamide) (PNIPAM) hydrogels are temperature-sensitive and are a type of intelligent hydrogels that have been studied extensively. The temperature response is corresponding to a discontinuous volume phase change, which can be found near 32 °C.12−14

Copolymerization of NIPAM and acrylic acid (AAc) can lead to a copolymer P(NIPAM-co-AAc), which can form a chelate with various metal ions to achieve good adsorption performance due to the presence of the AAc units containing ionizable and hydrophilic carboxyl groups and its function as an excellent chelating group. The obtained composite hydrogel can have better swelling, adsorption, pH response, and other properties.15−20 For example, Kureha et al.21 studied the structural changes of P(NIPAM-co-AAc) microgels and found that the hydrophobic and electrostatic interactions between cationic dye molecules and anionic microgels affected the separation of microgels and phase transition behavior. Compared with other adsorption materials, the hydrogel has good biocompatibility, environmental friendliness, higher adsorption rate, adsorption capacity, and is easy to prepare. As a fast response and high-capacity adsorption material, the hydrogel has become an important material for removing heavy metals,22,23 herbicides,24 organic dyes,25 and many other pollutants from aqueous media.

The hydrogel has a large surface area and numerous channels inside, providing adsorption sites, and can be used as an ideal material for adsorbing dye molecules. However, a single hydrogel has low strength and low adsorption capacity. Introducing functional components,26 loading with a certain amount of inorganic nanoparticles,27,28 and designing a double cross-linked network structure29 can greatly improve the mechanical properties, adsorption capacity, environmental stimulus–response performance, and stability of the hydrogels.30−35 Inorganic nanoparticles have unique properties in mechanical, electrical, magnetic, thermal, optical, and chemical activities due to their unique volume effect, surface effect, quantum size effect, and macroscopic quantum tunneling effect. Recently, with the development of nanoscience and nanotechnology, nanomaterials have great potential to solve environmental problems.36−38 Compared with traditional materials, nanostructured materials have a higher surface area and show higher adsorption efficiency and faster adsorption rate in water treatment. In addition, nanomaterials also have the following characteristics: they have high adsorption performance, are not harmful to the environment, and can be easily recycled from the environment. In the hydrogel system, the introduction of nanomaterials, including graphene oxide,39,40 Fe3O4,41,42 manganese oxide (MnO2),43 titanium oxide (TiO2),44,45 and zinc oxide (ZnO),46 to prepare composite hydrogels plays an increasingly important role in the wastewater treatment process. Nanomaterials can be successfully used as efficient, economical, and environmentally friendly adsorbents to remove various toxic substrates in wastewater, such as heavy metals and azo dyes. Zhao et al.47 prepared a three-dimensional hemin-functionalized graphene hydrogel (Hem/GH) by a simple self-assembly method, which showed good mechanical strength and a high adsorption capacity of 341 mg g–1 for rhodamine B (RhB). The above-reported research work provides a new way for preparing hydrogel materials with high dye adsorption capacity and excellent mechanical properties. However, it is still necessary to explore new inorganic nanomaterials to prepare composite hydrogels with low cost and high adsorption capacity, which is helpful for sewage purification and environmental protection.

Two-dimensional layered molybdenum disulfide (MoS2) is composed of a single layer or a few layers of MoS2. It is a two-dimensional nanomaterial with a sheet structure similar to the carbon material graphene. It has become a research hotspot in the field of emerging materials due to its excellent optoelectronic properties.48−51 Schneider et al.52 prepared poly(lactic acid) (PLA) by solution blow spinning (SBS) technology and then modified by spraying with MoS2 to obtain the PLA/MoS2 nanocomposite. The results show that the MoS2 nanoflakes used to alter the sub-micrometer fibers achieve an adsorption capacity of 111.2 mg g–1 for methylene blue (MB). The high adsorption capacity of MoS2 combined with the interconnected pores of the fiber membrane and simple modification strategies make PLA/MoS2 fiber composite materials particularly suitable for high-performance adsorption membranes. This study by Tian et al.53 shows that the use of MoS2/CuS nanosheet composites (NCs) as adsorbents can quickly and effectively remove different dyes in wastewater. MoS2/CuS NCs are prepared by a simple hydrothermal route. The adsorption capacity of the composite material for rhodamine B (RhB), methylene blue (MB), methyl orange (MO), and rhodamine 6G dyes (RhB 6G) is 273.23, 432.68, 98.78, and 211.18 mg/g, respectively. This is due to its high specific surface area (106.27 m2/g) and small mesopores (2.3 nm), providing numerous adsorption sites and uniform coverage for dye molecules. Based on the unique volume effect and surface effect of nanomaterials, when nanoparticles are introduced into the hydrogel to form composite materials, the nanoparticles can maintain their original characteristics and adjust the physical and chemical properties of the nanocomposite hydrogel materials in a variety of ways. For example, changing the amount of nanoparticles can make the comprehensive performance of the nanocomposite better than each single component and even obtain some new properties to meet the application requirements of high-capacity dye adsorption.

In this study, the P(NIPAM-co-AAc)/MoS2 composite hydrogel was prepared by free radical polymerization of NIPAM and AAc initiated by ultraviolet light. The influence of adsorption temperature, MoS2 loading content, and other factors on the structure and performance of hydrogels was studied. The synthesis of the composite hydrogel containing MoS2 is simple, its adsorption capacity for MB is high, and it can be more effectively used in water treatment for the removal of dyes.

Results and Discussion

Figure 1 shows the formation reaction process of the P(NIPAM-co-AAc)/MoS2 composite hydrogel prepared in this work: the temperature phase transition of the hydrogel, the dye adsorption behavior of the hydrogels, and the appearance of the prepared hydrogels. As shown in Figure 1a, NIPAM and AAc were used as monomers, and the photoinitiator 2960 was used to initiate the polymerization and cross-linking reaction of the monomers by ultraviolet light irradiation. In the presence of MoS2 particles, the P(NIPAM-co-AAc)/MoS2 composite hydrogel was obtained. The introduction of the NIPAM unit endows the composite hydrogel with temperature-responsive properties. The effects of different temperatures on the three-dimensional cross-linked network structure and swelling and shrinkage properties of the composite hydrogel can be studied. As shown in Figure 1b, the composite hydrogel undergoes a volume phase transition at 40 °C above the phase transition temperature. Because AAc contains a carboxylic group, it has a negative charge and can interact with positively charged dye molecules in water in the next step. MoS2 particles are a kind of flake-shaped granular material, which can take advantage of its superior size and surface effect and combine with AAc to improve the adsorption capacity of dyes. Figure 1c shows the dye adsorption behavior at room temperature and 40 °C.

Figure 1.

Figure 1

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Formation of P(NIPAM-co-AAc)/MoS2 composite hydrogels (a); temperature volume phase transition of hydrogels (b); dye adsorption behavior of hydrogels (c); and the appearance of the obtained composite hydrogels containing different contents of MoS2 (d).

By controlling the content of MoS2 inorganic particles, five groups of hydrogels were prepared by free radical copolymerization in this experiment. Table 1 shows the formula and product information of the P(NIPAM-co-AAc)/MoS2 composite hydrogels, and their appearance is shown in Figure 1d. Among them, NG-1 is the blank control sample P(NIPAM-co-AAc) hydrogel without added MoS2. In addition, NG-2, NG-3, NG-4, and NG-5 are P(NIPAM-co-AAc)/MoS2 composite hydrogels with different MoS2 loading amounts, including 1, 5, 10, and 25 mg, accounting for 0.1, 0.5, 1, and 2.5% of the NIPAM content, respectively. It can be seen from Table 1 and Figure 1d that the appearance of the hydrogel without added MoS2 in NG-1 is white. As the amount of added MoS2 increases, the color of the prepared hydrogel becomes darker and gradually turns black from gray. This is because as MoS2 particles appear black, they make the hydrogel appear black. In the next step, a series of tests and characterization will be conducted on the synthesized hydrogel to study its composition, swelling, and dye adsorption properties.

Table 1. Formula and Appearance of P(NIPAM-co-AAc)/MoS2 Composite Hydrogels.

label name NIPAM (g) AAc (g) MBA (mg) 2960 (g) MoS2 (mg) H2O (mL) appearance
NG-1 1 0.1 2.72 0.02 0 6 white
NG-2 1 0.1 2.72 0.02 1 6 gray
NG-3 1 0.1 2.72 0.02 5 6 dark
NG-4 1 0.1 2.72 0.02 10 6 black
NG-5 1 0.1 2.72 0.02 25 6 black
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Figure 2 shows Fourier transform infrared (FT-IR) and Raman spectra of P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 composite hydrogels. As seen in Figure 2a, there are two strong absorbance bands located at 3440 and 1640 cm–1, corresponding to the stretching vibration of OH in AAc and NH in NIPAM, and the stretching vibration of C=O in AAc and NIPAM,54 respectively. The characteristic absorbance of the two peaks at 1380 and 1108 cm–1 is assigned to the stretching vibration of CO in AAc, indicating that the composite hydrogel containing monomer units including NIPAM and AAc was successfully synthesized. According to the infrared spectra of P(NIPAM-co-AAc)/MoS2 composite hydrogels with different contents of MoS2, it can be seen that the characteristic absorbance bands of the P(NIPAM-co-AAc) hydrogel also appears at around 3437 and 1636 cm–1. Since the FT-IR spectra were usually used for qualitative analysis, it is impossible to confirm whether there is any introduction of inorganic MoS2 particles in the P(NIPAM-co-AAc) hydrogel. Therefore, in the following analysis, Raman spectroscopy was used to determine the presence of MoS2.

Figure 2.

Figure 2

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(a) FT-IR and (b) Raman spectra of P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 composite hydrogels.

Figure 2b shows the Raman spectra of MoS2, P(NIPAM-co-AAc) hydrogels, and P(NIPAM-co-AAc)/MoS2 composite hydrogels. In the Raman spectrum of MoS2, it was found that there are two strong characteristic peaks at about 403 and 374 cm–1, which are attributed to the out-of-plane vibration mode (A1g) and the in-plane vibration mode (E2g1), respectively.55 In the Raman spectrum of the P(NIPAM-co-AAc) hydrogel without MoS2, a wide band was found at 1000–2000 cm–1, indicating the amorphous dried hydrogel, and no MoS2 characteristic peak was found. When the MoS2 loading content is low, there are rather weak peaks belonging to MoS2 in the Raman spectrum of P(NIPAM-co-AAc)/MoS2. When the MoS2 content reaches more than 10 mg, the characteristic peak of MoS2 appears, indicating that MoS2 has been successfully incorporated into the P(NIPAM-co-AAc) hydrogel. As seen in Figure 2b, compared to NG-4, NG-5 with a higher MoS2 content should have an increased intensity of the characteristic Raman absorbance bands. However, the intensity of the characteristic MoS2 peaks at about 403 and 374 cm–1 in NG-5 was slightly lower than that in NG-4. It is most likely that at a very high concentration (2.5%) of MoS2 dispersion during the preparation of NG-5, the inorganic MoS2 particles are becoming unstable and prone to form aggregates, resulting in an uneven composition of the P(NIPAM-co-AAc)/MoS2 composite hydrogels. At a lower MoS2 content, because the hydrogel encapsulates and shields MoS2, the characteristic peaks of MoS2 are not obvious in the spectrum.

Thermogravimetric analysis (TGA) should be used for detecting the MoS2 content in the P(NIPAM-co-AAc)/MoS2 composite hydrogels. Figure S1a shows the TGA curves of P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 composite hydrogels. Figure S1b shows an enlarged part of the indicated area with a green frame in Figure S1a. From Figure S1, it can be seen that the char residual mass ratio of the pure P(NIPAM-co-AAc) hydrogel NG-1 is about 5.54%. The char residual mass ratio of P(NIPAM-co-AAc)/MoS2 composite hydrogels including NG-3, NG-4, and NG-5 are 5.97, 6.61, and 8.11%, respectively. The MoS2 contents of NG-3, NG-4, and NG-5 were calculated to be 0.43, 1.07, and 2.57%, respectively. The calculated values are very consistent with the theoretical values.

Figure 3a shows the scanning electron microscope (SEM) image of MoS2 particles. It can be seen from Figure 3a that MoS2 exists in the form of flakes. Through statistical average, the average size of the flake diameter of MoS2 is about 3.2 μm. Using transmission electron microscope (TEM) to analyze the single irregularly shaped particle of MoS2 (see Figure 3b), the size of the MoS2 flake is about 3.4 μm. Figure 3c shows the ζ-potential distribution of MoS2 particles. The ζ-potential of particles is considered to be an important parameter for characterizing the stability of the colloidal dispersions and gives information on the effective surface charge. As shown in Figure 3c, it can be seen that the ζ-potential of MoS2 particles is about −27.5 mV, indicating that the dispersion is relatively stable and the electrostatic force is large. Since MoS2 is negatively charged, it is beneficial to adsorb oppositely charged dye molecules through an electrostatic force. The P(NIPAM-co-AAc) hydrogel system was introduced with MoS2, and the resulting composite hydrogel (NG-4) was freeze-dried and fractured with liquid nitrogen. The cross-sectional SEM image is shown in Figure 3d. It can be seen from Figure 3d that the cross-sectional morphology of the composite hydrogel is a three-dimensional porous network structure, and the pore walls are relatively thin. The pore size is about 100 μm. The porous structure has different morphologies, indicating that the area is filled with water.

Figure 3.

Figure 3

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SEM (a) and TEM (b) images; the ζ-potential distribution (c) of MoS2 particles and the cross-sectional SEM image (d) of the P(NIPAM-co-AAc)/MoS2 composite hydrogel.

Figure 4 shows the swelling ratio of the P(NIPAM-co-AAc) hydrogel (NG-1) and the P(NIPAM-co-AAc)/MoS2 composite hydrogel (NG-4) with time, and the relationship between the equilibrium swelling ratio and the amount of MoS2 added. As shown in Figure 4a, the swelling ratio of the P(NIPAM-co-AAc) hydrogel without MoS2 is first increased with time from 0 to 12 h. It then changes slowly after 12 h, and the final swelling ratio reaches about 95 g/g. The P(NIPAM-co-AAc)/MoS2 composite hydrogel obtained after the addition of MoS2 shows a similar change trend. The final equilibrium swelling ratio is higher than that of the pure P(NIPAM-co-AAc) hydrogel. Figure 4b shows the relationship curve between the equilibrium swelling ratio of P(NIPAM-co-AAc)/MoS2 composite hydrogels and the amount of the added MoS2. It can be seen that the equilibrium swelling ratio began to increase sharply with the increase of the amount of MoS2 and tended to be stable when the amount of addition was 10 and 25 mg. When 25 mg is added, the equilibrium swelling ratio of the P(NIPAM-co-AAc)/MoS2 composite hydrogel can reach 176.5 g/g. This is mainly because MoS2 itself has oxygen-containing groups on the surface and is negatively charged. As the amount of the added MoS2 increases, the hydrophilicity of the composite hydrogel improves, leading to the expansion of the networks. At the same time, because MoS2 itself is negatively charged, the increase of MoS2 caused the electrostatic repulsion of the same charged counterparts in the network structure, resulting in an increase in the volume and swelling ratio. The excellent swelling performance indicates that the composite hydrogel may have a high dye adsorption capacity.

Figure 4.

Figure 4

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Swelling ratio of the P(NIPAM-co-AAc) hydrogel and the P(NIPAM-co-AAc)/MoS2 composite hydrogel as a function of time (a), and the equilibrium swelling ratio of hydrogels as a function of the MoS2 loading content (b).

Figure 5 shows the relationship between the amount of dye adsorption of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels as a function of time at room temperature and 40 °C, as well as the appearance of the hydrogel before and after adsorption, and the adsorption kinetic models of MB by the hydrogel. Note that the initial concentration of MB for the adsorption experiment is 64 mg/L. It can be seen from Figure 5a that with the extension of time, the adsorption capacity of both P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels for MB both gradually increases. After 6 h, the adsorption kinetic process reaches equilibrium and the adsorption kinetic curve attains a plateau. At equilibrium, the adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels is 227 and 202 mg/g, respectively. The P(NIPAM-co-AAc) hydrogel has a porous network structure, a large area, and numerous channels, which is conducive to the diffusion and adsorption of the positively charged dye molecules. Moreover, the cross-linked network structure has ionizable carboxyl groups −COO–, which is negatively charged and can absorb the positively charged MB dye molecules through electrostatic attraction. In addition, due to the presence of the −COOH group on the network structure, the cationic organic dye molecule MB can also interact with the hydrogel through hydrogen bonds.56 Compared with the P(NIPAM-co-AAc) hydrogel, the P(NIPAM-co-AAc)/MoS2 composite hydrogel has MoS2 particles, which have a negative charge and can also adsorb the cationic dye MB through electrostatic interactions, leading to an increased adsorption capacity.

Figure 5.

Figure 5

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Dye adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels as a function of time at room temperature (a) and 40 °C (b); (c) appearance of the hydrogels before and after adsorption; and (d) adsorption curve of MB by the composite hydrogel and the adsorption kinetic models at room temperature.

Figure 5b shows the relationship between the adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels as a function of time at 40 °C. The initial concentration of the MB solution is 64 mg/L. It can be seen from Figure 5b that with the increase of time, the adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels to MB is gradually increased and begins to reach adsorption equilibrium at about 6 h. The adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels at equilibrium is 91 and 35 mg/g, respectively. By comparing the results from Figure 5a,b, the adsorption temperature greatly influences the adsorption capacity of both hydrogels. With the increase of the adsorption temperature, when the dye adsorption is performed above the phase transition temperature near 32 °C, the adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels at equilibrium was both also decreased. This shows that the hydrogel swells in the solution at room temperature, the molecular chain stretches, the three-dimensional porous cross-linked network structure, and numerous channels are beneficial to the diffusion and adsorption of dye molecules. Above the phase transition temperature of 32 °C, the macromolecular chain collapses and water is discharged. The three-dimensional cross-linked network structure shrinks, which is not conducive to the diffusion and adsorption of dye molecules, resulting in lower dye adsorption quantity.

Figure 5c shows the appearance of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels before adsorption. Both freeze-dried samples with the same mass of 10 mg are much smaller. The freeze-dried hydrogel containing MoS2 is black granules, while the one without MoS2 is white. After MB dye molecules are adsorbed, the hydrogel volume is much larger than the original one, and the color is blue, indicating that MB dye molecules are absorbed on the surface and bound to the hydrogel. Considering that the P(NIPAM-co-AAc)/MoS2 composite hydrogel has a high dye adsorption capacity, Figure 5d shows the adsorption kinetics and simulation curves of MB adsorption on the P(NIPAM-co-AAc)/MoS2 composite hydrogels at room temperature. The experimental data are fitted with two kinetic models, pseudo-first-order and pseudo-second-order models (see eqs 1 and 2).

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where qe and qt (mg/g) represent the equilibrium adsorption capacity and the adsorption capacity at contacting time t, and k1 and k2 are pseudo-first-order constant and pseudo-second-order constant. Table 2 shows the kinetic parameters for the adsorption of MB in P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 hydrogels at room temperature. From Figure 5d and Table 2, it can be seen that at room temperature, the correlation coefficient value (R2) of the pseudo-second-order model reaches 0.982, which is higher than that for the pseudo-first-order model. This indicates that the adsorption of MB dye molecules onto P(NIPAM-co-AAc)/MoS2 composite hydrogels follows the pseudo-second-order model well.

Table 2. Kinetic Parameters for the Adsorption of MB in P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 Hydrogels at Room Temperature.

  pseudo-first-order constants
 pseudo-second-order constants
sample qe (mg/g) K1 (min–1) R2 qe (mg/g) K2 (g/mg × min) R2
P(NIPAM-co-AAc)/MoS2 205.5 0.354 0.963 212.5 3.503 0.982
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As indicated in the literature,57 the kinetics of adsorption study in the regions with constant adsorption acceleration (abbreviated as KASRA) model was also used to calculate the kinetics of the adsorption process. According to the KASRA model, the adsorption kinetics curve is composed of two regions (1 and 2). Regions 1 and 2 like regions I and II in adsorption isotherms are belonging to the most active and rest of sites, respectively. Several first data and then remaining data were fitted in a quadratic KASRA equation (see eqs S1–S3 in the Supporting Information). The adsorption kinetic experiment was performed at a MB initial concentration of 64 mg/L. The obtained data were analyzed by the KASRA model, as shown in Figure S1 and Table S1. As shown in Figure S1 and Table S1, the kinetic curves are composed of two regions (1 and 2). Regions 1 and 2 like regions I and II in adsorption isotherms pay to the most active and rest of sites, respectively. We fit several first data and then the remaining data in a quadratic equation. In region 1, the coefficients including A, B, and R2 of the KASRA equation are calculated to be −0.613, 24.12, and 0.706, respectively. In region 2, A, B, C, and R2 are estimated to be −4.28 × 10–5, 0.085, 194.17, and 0.89, respectively. As indicated in the results above, the experimental data were fitted with two kinetic models, pseudo-first-order and pseudo-second-order models (see eqs 1 and 2). It can be seen that at room temperature, the correlation coefficient values (R2) of pseudo-second-order and pseudo-first-order models reach 0.982 and 0.963, which are both higher than the two values of 0.706 and 0.890 at 40 °C. This indicates that the adsorption of MB dye molecules onto P(NIPAM-co-AAc)/MoS2 composite hydrogels follows the pseudo-second-order model well. Also, in most of the literature studies28,58−63 (see Table 5), the adsorption kinetics of the hydrogels for MB is well described with the pseudo-second-order or pseudo-first-order models. To compare our results with the data reported in the literature, we would keep the results and discussion on the kinetics study using pseudo-second-order and pseudo-first-order models.

Table 5. Adsorption Isotherm Constants of Some Adsorbents for MB.

adsorbents qm (mg/g) references
β-cyclodextrin/poly(acrylic acid) grafted onto graphene oxide (β-CD/PAA/GO) hydrogels 248 (64)
polydopamine microspheres 90.7 (58)
the modified banana pseudo-stem cellulose backbone with sodium acrylate (NaAc) and acrylamide (AM) onto (BPCMC-g-poly(NaAc-co-AM)) 333 (59)
mesoporous carbon material from fishery waste 184.4 (60)
cellulose nanocrystal-alginate hydrogel beads 255.5 (65)
core@double-shell-structured magnetic halloysite nanotube 714.29 (61)
polyacrylamide/chitosan/Fe3O4 composite hydrogels 1603 (28)
poly(acrylic acid)/laponite hydrogel 3846 (62)
villilike poly(acrylic acid)-based hydrogel 2249 (63)
P(NIPAM-co-AAc)/MoS2 hydrogels 1258 this work
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Figure 6a,b shows the adsorption isotherms of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels for MB at room temperature and 40 °C. Figure 6c indicates the plotting curves of Langmuir and Freundlich isotherm adsorption models. It can be seen from Figure 6a that with the increase of the equilibrium concentration, the adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels to MB dye increases, and the P(NIPAM-co-AAc)/MoS2 composite hydrogel has better adsorption performance than the P(NIPAM-co-AAc) hydrogel. The P(NIPAM-co-AAc) hydrogel has an adsorption effect, which is due to the AAc component in the hydrogel. Polyacrylic acid is a polyelectrolyte that can ionize protons in the aqueous solution to render the polymer negatively charged, while MB is the positively charged organic dye molecule. Therefore, the two can be combined through electrostatic interactions so that the P(NIPAM-co-AAc) hydrogel has an adsorption effect. The addition of MoS2 makes the adsorption performance of hydrogel better because MoS2 has a hexagonal sheet structure similar to graphene, has a large specific surface area, and is negatively charged, which can adsorb a large amount of MB through the electrostatic attraction and van der Waals forces. This shows that the addition of MoS2 can improve the adsorption performance of the pure P(NIPAM-co-AAc) hydrogel. The adsorption capacity of the P(NIPAM-co-AAc)/MoS2 composite hydrogel can reach 916 mg/g when the MB initial concentration is 640 mg/L.

Figure 6.

Figure 6

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Adsorption isotherms of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels for MB at room temperature (a) and 40 °C (b), and the plotting curves of Langmuir and Freundlich isotherm adsorption models (c) and the ARIAN model (d) for the MB adsorption of P(NIPAM-co-AAc)/MoS2 at room temperature.

It can be seen from Figure 6b that at 40 °C, the adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels show a trend similar to that at room temperature with the increase of the MB concentration. Compared to room temperature, the increased adsorption temperature of 40 °C leads to the decreased adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels at different concentrations. This is because the PNIPAM hydrogel is temperature-sensitive and has a characteristic phase transition temperature. At the adsorption temperature of 40 °C above the phase transition temperature (32 °C), the volume of hydrogels shrinks sharply. At room temperature, water molecules interact strongly with amide groups through hydrogen bonds, resulting in the PNIPAM hydrogels having a good affinity with water molecules. When the temperature is higher than the phase transition temperature, the hydrogen bonds between water molecules and amide groups are destroyed, and the hydrophilic force is weakened. The polymer chains are collapsed, and the volume of hydrogels shrinks significantly. At the same time, the chance of MB entering the hydrogel is reduced, and the amount of MB binding with the hydrogel is decreased. Therefore, as the temperature increases, the adsorption capacity of P(NIPAM-co-AAc)/MoS2 and P(NIPAM-co-AAc) hydrogels are both reduced. When the initial concentration of MB is 640 mg/L, the adsorption capacity of the P(NIPAM-co-AAc)/MoS2 composite hydrogel decreases from 916 to 620 mg/g, and the adsorption capacity of the P(NIPAM-co-AAc) hydrogel is reduced from 598 to 402 mg/g.

Figure 6c shows the plotting curves of Langmuir and Freundlich isotherm adsorption models for the MB adsorption of P(NIPAM-co-AAc)/MoS2 at room temperature. As shown in Figure 6c, as the equilibrium concentration of MB increases, the equilibrium adsorption capacity of the P(NIPAM-co-AAc)/MoS2 composite hydrogel for MB increases. Langmuir (eq 3) and Freundlich (eq 4) isotherm models were used to evaluate the relationship between the equilibrium adsorption capacity of the P(NIPAM-co-AAc)/MoS2 composite hydrogel for MB and the equilibrium MB concentration.

graphic file with name ao1c04433_m003.jpg 3
graphic file with name ao1c04433_m004.jpg 4

where qm (mg/g), 1/n, qe (mg/g), Ce (mg/L), KL (L/mg), and KF (mg/g) represent maximum adsorption capacity, adsorption intensity, adsorption capacity, the equilibrium concentration of MB dyes, the Langmuir constant, and the Freundlich constant, respectively. Table 3 shows Langmuir and Freundlich isotherm parameters for the adsorption of MB onto P(NIPAM-co-AAc)/MoS2 hydrogels at room temperature. From Figure 6c and Table 3, it can be seen that the adsorption isotherm of the P(NIPAM-co-AAc)/MoS2 composite hydrogel for MB at room temperature is more consistent with the Langmuir adsorption isotherm model, and R2 = 0.994. According to the Langmuir isotherm adsorption model simulation, the maximum adsorption capacity of the P(NIPAM-co-AAc)/MoS2 composite hydrogel can reach 1258 mg/g.

Table 3. Langmuir and Freundlich Isotherm Parameters for the Adsorption of MB onto P(NIPAM-co-AAc)/MoS2 Hydrogels at Room Temperature.

  Langmuir isotherm parameters
 Freundlich isotherm parameters
sample qm (mg/g) KL (L/mg) R2 KF (mg/g) 1/n R2
P(NIPAM-co-AAc)/MoS2 1258 0.0051 0.994 34.7 0.523 0.950
Open in a new tab

As reported in the literature,57 the “adsorption isotherm regional analysis model” (abbreviated as ARIAN model) should be used to analyze the adsorption isotherm of hydrogels. Figure 6d shows the ARIAN model for the MB adsorption of P(NIPAM-co-AAc)/MoS2 at room temperature. First, several data (region I) were fit in the Henry equation (eq 5), and the rest of them (region II) were fit in the Temkin equation (eq 6). Because the adsorbent surface in our work is nonideal, it is not appropriate to use the Langmuir isotherm to fit. The Temkin equation is a suitable equation for the nonideal surface. The adsorption equilibrium constants can be obtained using them. Table 4 shows the ARIAN model isotherm parameters for the adsorption of MB onto P(NIPAM-co-AAc)/MoS2 hydrogels at room temperature. As shown in Table 4, the parameters of K and R2 for Henry’s Law are 4.927 and 0.983, respectively. In addition, C1, C2, and R2 for the Temkin equation are 247.5, 0.068, and 0.955, respectively. The maximum experimental qe is equal to the experimental qe of 916 mg/g (initial MB concentration of 640 mg/L) after 650 min but not values calculated by the Langmuir model. Actually, Langmuir and Freundlich isotherm parameters for the adsorption of MB onto P(NIPAM-co-AAc)/MoS2 hydrogels at room temperature were calculated and their R2 values were 0.994 and 0.950, respectively, which is much higher than that of 0.955 obtained by the ARIAN model. It can be seen that the adsorption isotherm of the P(NIPAM-co-AAc)/MoS2 composite hydrogel for MB at room temperature is more consistent with the Langmuir adsorption isotherm model. According to the Langmuir isotherm adsorption model simulation, the maximum adsorption capacity of the P(NIPAM-co-AAc)/MoS2 composite hydrogel can reach 1258 mg/g. Actually, in most literature studies58,60,62−64 (see Table 5), the adsorption isotherm was well fitted with the Langmuir model. To better compare our results with the reported data in the literature, we would keep the results and discussion on the modeling of Langmuir and Freundlich isotherm.

graphic file with name ao1c04433_m005.jpg 5

where K is the binding constant of the adsorbate on the surface and adsorption increases linearly with the concentration.

graphic file with name ao1c04433_m006.jpg 6

where C1 is a constant and C2 is the adsorption equilibrium constant.

Table 4. ARIAN Model Isotherm Parameters for the Adsorption of MB onto P(NIPAM-co-AAc)/MoS2 Hydrogels at Room Temperature.

  Henry’s Law
 Temkin
sample K R2 C1 C2 R2
P(NIPAM-co-Ac)/MoS2 4.927 0.983 247.5 0.068 0.955
Open in a new tab

At room temperature and 40 °C, there is one kind of adsorption site on the surface of the hydrogel. Due to changes in the structure of the hydrogel, the most active sites located on the wider channels interact with MB first. After sufficiently increasing the concentration of MB in the adsorbent, they interact with adsorption sites in a narrower space. MoS2 stabilizes the hydrogel structure and makes its channels or internal space wider. MoS2 acts in the adsorbent like a scaffolding for hydrogel. Thus, in spite of the fact that the amount of MoS2 in NG-4 is more than that in NG-1, more numbers of adsorption sites of NG-4 are exposed to MB molecules.

Nowadays, a large number of adsorbents have been developed to treat MB dyes in sewage. Table 5 lists the maximum adsorption capacity of some adsorbents. In Table 5, most of the work reported show the adsorption capacity from 90 to 715 mg/g for MB. Some representative work including polyacrylamide/chitosan/Fe3O4 composite hydrogels, poly(acrylic acid)/laponite hydrogel, and villilike poly(acrylic acid)-based hydrogel has also been listed in Table 5. It can be seen that the maximum adsorption capacity of 1603, 3846, and 2249 mg/g for MB is reported, respectively. Compared with the adsorbents reported in the literature, the P(NIPAM-co-AAc)/MoS2 composite hydrogel in this work still has an advantage and shows a high adsorption capacity of 1258 mg/g for MB.

Conclusions

Through the analysis of FT-IR, Raman, and SEM, it is proved that the P(NIPAM-co-AAc)/MoS2 composite hydrogel was successfully prepared by simple free radical polymerization. SEM images of the cross section of the P(NIPAM-co-AAc)/MoS2 composite hydrogel showed that the composite hydrogel had a porous network structure. Hydrogels are hydrophilic and can absorb large amounts of water. By studying the swelling properties of P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 hydrogels, the results show that MoS2 can increase the swelling ratio of P(NIPAM-co-AAc) to a certain extent due to its negatively charged surface. The P(NIPAM-co-AAc)/MoS2 composite hydrogel with 25 mg of MoS2 can have a swelling ratio of nearly 176.50. The addition of MoS2 can effectively improve the adsorption performance of the P(NIPAM-co-AAc) hydrogel. When the initial concentration of MB is 640 mg/L, the maximum adsorption capacity calculated by the Langmuir isotherm adsorption model can reach about 1258 mg/g. By studying the adsorption performance of the P(NIPAM-co-AAc)/MoS2 composite hydrogel at different temperatures, it is observed that as the temperature increases to 40 °C, the adsorption performance of the P(NIPAM-co-AAc)/MoS2 composite hydrogel is weakened. The adsorption isotherm of the P(NIPAM-co-AAc)/MoS2 composite hydrogel for MB at room temperature conforms to the Langmuir isotherm adsorption model. The adsorption process of MB by the P(NIPAM-co-AAc)/MoS2 composite hydrogel at room temperature fit well the pseudo-second-order kinetics model. This research provides a facile method for developing dye adsorption materials with high adsorption capacity.

Experimental Section

Chemicals and Materials

N-Isopropylacrylamide (NIPAM, 99%), acrylic acid (AAc, 98%), and methylene blue (MB) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Molybdenum disulfide (MoS2) was obtained from Jiangsu XFNANO Materials Tech Co., Ltd. (Nanjing, China). Photoinitiator 2960 (99%) was obtained from Shanghai Yinchang New Material Co., Ltd. (Shanghai, China). N,N′-Methylenebisacrylamide (MBA, 98%) was purchased from J&K Scientific Ltd. (Beijing, China). Methylene blue (MB) was purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were of analytical grade, and the water used in all experiments was deionized water.

Preparation of P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 Composite Hydrogels

NIPAM (1 g) and 0.1 g of AAc were weighed and dissolved in an appropriate amount of deionized water. MBA (2.72 mg) and 20 mg of photoinitiator 2960 were added to the previous solution, and the mixture was shaken until an aqueous solution was obtained. A certain amount of MoS2 required was added to the deionized water and then mixed in an ultrasonic cell pulverizer for 30 min. The MoS2 dispersion was combined with the solution containing monomers, cross-linker, and initiator and placed in an ice–water bath. The reaction was allowed to take place in a 365 nm low-pressure mercury lamp catalytic reaction device for 4 h under ultraviolet light irradiation. Changing the MoS2 mass from 0 to 25 mg, a series of P(NIPAM-co-AAc/MoS2) composite hydrogels and the P(NIPAM-co-AAc) hydrogel without MoS2 can be prepared.

Characterization of Hydrogels

Fourier Transform Infrared Spectroscopy (FT-IR)

The freeze-dried P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 hydrogel samples were ground into powder in a mortar and mixed with KBr to form tablets. Then, a Fourier infrared spectrometer (Nicolet, Avatar370) was used for the test.

Raman Spectroscopy (Raman)

The freeze-dried P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 hydrogel samples were ground into powder. A DXR Laser Raman microscope from Thermo Fisher Scientific (United States) was employed for testing. The laser wavelength of the Raman spectrometer used was 532 nm.

Scanning Electron Microscope (SEM)

The freeze-dried hydrogel sample was brittle fractured in liquid nitrogen, and then the cross section of the hydrogel was sprayed with gold. Then, the hydrogel was observed using the JSM-6360LA scanning electron microscope (JEOL Ltd., Japan).

Transmission Electron Microscope (TEM)

The ethanol dispersion of MoS2 particles was dropped onto a copper mesh, and its morphology and size were observed with a Japanese JEOL JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV.

ζ-Potential Analysis

The low-concentration MoS2 dispersion was sonicated for 30 min. The ζ-potential of MoS2 particles in aqueous solutions was measured at 25 °C with a Malvern Zetasizer Nano ZS.

Swelling Ratio

At room temperature, 10 mg of the freeze-dried P(NIPAM-co-AAc) and P(NIPAM-co-AAc)/MoS2 composite hydrogel samples were added into 50 mL of water. The sample was taken out of the water at regular intervals, and the surface water was wiped with filter paper; its mass was weighed with a precision electronic balance. Equation 7 was used to calculate the swelling ratio WSR (g/g) of the hydrogel.

graphic file with name ao1c04433_m007.jpg 7

where W0 and Wt represent the initial dried sample weight and the weight of the swollen sample at time t, respectively.

Dye Adsorption Experiment

Drawing the Standard Curve

First, 100 mL of MB solution at a concentration of 640 mg/L was prepared and then diluted with deionized water to different concentrations (64, 32, 16, 6.4, 3.2, 1.6, 0.8, and 0.64 mg/L). A UV–vis spectrophotometer (UV-1800, Shimadzu Corporation, Japan) was used to measure the absorbance at a wavelength of 664 nm. A standard curve (absorbance vs MB concentration) was created. The value for the unknown sample was determined by comparing it with the standard curve.

Adsorption Process

First, 100 mL of MB solution at a concentration of 640 mg/L was prepared and diluted with deionized water to different concentrations for adsorption experiments. The freeze-dried hydrogel samples were added into a series of prepared MB solutions with different concentrations to perform dye adsorption experiments at room temperature or 40 °C. The absorbance of the solution was measured at regular intervals; its concentration was obtained from the standard curve, and the corresponding adsorption capacity was calculated according to eq 8

graphic file with name ao1c04433_m008.jpg 8

where Qt (mg/g), C0 (mg/L), Ct (mg/L), V (L), and W0 (g) represent the adsorption capacity at time t, the initial MB concentration, residual MB concentration at time t, the volume of MB solution, and the weight of the dried hydrogel sample.

Similarly, to study the effect of the initial dye solution concentration on adsorption, the adsorption equilibrium of the MB dye on the hydrogel was studied at a concentration of 6.4–640.0 mg/L at room temperature and 40 °C.

Acknowledgments

This project was supported by the National Natural Science Foundation of China (Grant No. 21704008), Major Science and Technology Project of Changzhou Health Commission (Project No: ZD201917), Natural Science Foundation of Jiangsu Province, China (Grant No. BK20201449), and Natural Science Foundation of the Jiangsu Higher Institutions of China (Grant No. 20KJA430011). Financial support provided for this project by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), and financial support from the Young Elite Scientist Sponsorship Program of the Jiangsu Province Association of Science and Technology, Postgraduate Research & Practice Innovation Program of Jiangsu Province are also gratefully acknowledged.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04433.

  • Thermogravimetric analysis of hydrogels; coefficients of the KASRA equation; and adsorption kinetic KASRA model (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c04433_si_001.pdf (298.1KB, pdf)

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