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. 2021 Oct 21;6(43):29243–29253. doi: 10.1021/acsomega.1c04938

Study on the Adsorption Performance of Casein/Graphene Oxide Aerogel for Methylene Blue

Wenshuo Xu †, Yanhui Li †,‡,*, Huimin Wang †, Qiuju Du ‡, Meixiu Li ‡, Yong Sun †, Mingfei Cui †, Liubo Li †
PMCID: PMC8567406  PMID: 34746612

Abstract

graphic file with name ao1c04938_0013.jpg

Casein (CS) and graphene oxide (GO) were employed for the fabrication of a casein/graphene oxide (CS/GO) aerogel by vacuum freeze drying. Fourier transform infrared spectroscopy, scanning electron microscopy, surface area and micropore analysis (BET), and thermogravimetric analysis were used to characterize the specific surface area, structure, thermal stability, and morphology of the CS/GO aerogel. The influence of experimental parameters such as the GO mass fraction in the aerogel, metering of the adsorbent, pH, contact time, and temperature on the adsorption capacity of the CS/GO aerogel on methylene blue (MB) was also investigated. According to Langmuir isotherm determination, the maximum removal rate of MB from the CS/GO aerogel was 437.29 mg/g when the temperature was 293 K and pH was 8. Through kinetic and thermodynamic studies, it is found that adsorption follows a pseudo-second-order reaction model and is also an exothermic and spontaneous process.

1. Introduction

With the continuous increase in organic dyes in the environment, organic dye pollution has become one of the major threats faced by human beings. Organic dyes from textiles, papermaking, food additives, leather coloring, cosmetics production, and other fields are very common.1 These dyes all contain aromatic structures that lead them to have good stability in the presence of light, heat, or oxidants and are very difficult to degrade. The release of dyes in the environment will involve both toxicology and aesthetics.2 Even if the water contains merely thimbleful dyes, due to the reduced light permeability, it will also cause serious water pollution problems, which is obviously undesirable.3 Therefore, it is important to separate and remove the dyes contained in the wastewater before releasing it into the environment. In recent years, many methods such as the flocculation method,4 coagulation method,5 photocatalytic degradation method,6 sedimentation method,7 biological oxidation method,8 membrane filtration method,9,10 ion exchange method,11 and adsorption method12 have been used to separate the dyes in the wastewater, which has become the main research direction of wastewater purification in domestic and overseas.13 As far as these methods are concerned, the adsorption method has the advantages of a low price, easy preparation, wide range of material sources, wide range of adaptation, and convenient operation. It is widely used by people.14 At present, many kinds of adsorption materials have been studied such as carbon nanotubes,15 zeolite,16 activated carbon,17,18 silicates,19 and graphene-based composite materials.20 In these studies, graphene has become a material of concern because of its special mechanical, thermal, and electronic properties.21 Graphene oxide (GO) is one of many derivatives of graphene that can be obtained by chemically exfoliating natural graphite. GO has a high specific surface area and rich oxygen-containing groups with good dispersion.22 Due to its good dispersibility, it is difficult to recycle after being used for adsorption. Some research has shown that GO dispersed in water is harmful to people.23 The shortcomings of GO recovery can be effectively solved by compounding GO and other polymers into a gel. At present, many biopolymers have been used to prepare GO biopolymer gels.24 For example, the agar-crosslinked GO composite aerogel prepared by Chen et al. can efficiently adsorb dyes.25 Zhuang et al. prepared gel-adsorbed ciprofloxacin by crosslinking soybean protein isolate and GO.26

Casein (CS), as an important biological polymer, can be precipitated from raw skim milk by acidification. It is nontoxic and biodegradable. It has many hydrophilic functional groups and has good surface activity and stability,27 which is very suitable for preparing gels. In addition, compared with bovine serum albumin,28 silk fibroin,29 collagen,30 and elastin-like peptide31 and other proteins used to prepare protein gels, CS materials are easier obtained, and its economy is better. In this study, the hydrophilicity of CS was used to prepare a composite aerogel of CS and GO by freeze drying, and its adsorption performance was studied.

2. Materials and Methods

2.1. Reagents and Materials

Expandable graphite (99.8%, 300 meshes) comes from Qingdao Henglid graphite company in China. Methylene blue (MB) was purchased from Tianjin Chibi Chemical Reagent Factory. CS (chemically pure) is provided by China National Pharmaceutical Chemical Reagent Co. Ltd. We obtained glutaraldehyde, concentrated sulfuric acid, sodium nitrate, hydrogen peroxide, potassium permanganate, and sodium hydroxide from China National Pharmaceutical Chemical Reagent Co. Ltd. The chemicals used in the experiment do not require further purification. Throughout the experiment, deionized water was used to prepare the required solution.

2.2. Preparation GO

The Hummers32 method was used to prepare GO for the experiment. First, 5 g of expanded graphite and 5 g of sodium nitrate were added into the beaker. During magnetic stirring, 230 mL of concentrated sulfuric acid was slowly added to completely disperse. After that, the solution was stirred continuously, and 5 g of potassium permanganate slowly added and dispersed completely in the mixture. The abovementioned process is realized in an ice bath at 273 K. Then, the mixture obtained by the abovementioned operation was kept in a 273 K environment for a whole day and night. After completion, the mixture was taken out and heated to 308 K under an oil bath, and stirring was continued for 30 min until the mixture became discolored. The temperature was slowly adjusted to 371 K, and 460 mL of deionized water was added into the mixture while heating. After that, the mixture was stirred to a suspension, which was stirred for 15 min at a temperature of 371 K. After the suspension cools, it was treated with 3% hydrogen peroxide until the suspension turns bright yellow. The purpose of washing the suspension with 1500 mL of 5% strong diluted hydrochloric acid is to remove the metal ions contained therein. Afterward, the suspension must be washed repeatedly with deionized water for a period to make it no longer acidic.

2.3. Preparation of the CS/GO Aerogel

The specific preparation method is shown in Figure 1. First, 1 g of CS was put into the beaker, and 10 mL of deionized water was added to make a dispersion, and its pH was adjusted to about 8 with sodium hydroxide solution (2 mol/L). Stirring was continued for 1 h until the CS was completely dissolved to form a solution. Then, different quantities of the prepared GO were added to the CS solution (varies at 0, 2, 4, 8, and 10 wt % CS weight), and the mixed solution needs to be continuously stirred on a magnetic stirrer for 1 h. Then, the abovementioned solution stirred using the magnetic stirrer is put into an ultrasonic device (KQ-300DA) at room temperature, and ultrasonic treatment was performed with a power of 300W for 2 h until a uniform solution is obtained. A total of 5% glutaraldehyde was slowly added under stirring, and the dispersion was kept at room temperature for a period until a hydrogel formed. After washing the obtained hydrogel with deionized water multiple times, it was frozen at a temperature of 253 k for 24 h, then placed in a vacuum freeze dryer for 24 h under vacuum freeze drying (FD-1B-50), and allowed to dry completely to a constant weight to obtain a CS/GO aerogel.

Figure 1.

Figure 1

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Schematic diagram of preparing CS/GO.

2.4. Characterization of the CS/GO Aerogel

Scanning electron microscopy (SEM) (Sigma-Aldrich 500, Carl-Zeiss, German) shows the surface morphology of the CA/GO aerogel; samples are all sprayed with gold prior to SEM; the changes of functional groups on the surface of the CS aerogel and CS/GO aerogel after adding GO were used by Fourier transform infrared (FT-IR) spectroscopy (Nicolet iS50, Thermo Scientific, USA). The specific surface area of the CS aerogel and CS/GO aerogel was determined by the BET equation (Quantachrome Autosorb-IQ-MP/XR); all samples have been preprocessed prior to BET, including the degassing process. The thermogravimetric analysis (TGA) of GO, the CS aerogel, and the CS/GO aerogel was performed from 313 to 1073 K using a (ASAP2460-2M) thermogravimetric analyzer. The temperature rises from 303 to 1073 K at a rate of 10 K/min.

2.5. MB Adsorption Experiments

The MB solution was prepared with a concentration of 1 g/L before the adsorption test and diluted to get the solution with the concentration required for the experiment. During the experiment, a 50 mL conical bottle containing 20 mL of MB solution was added with a mass of 10 mg of CS/GO aerogel. Next, the abovementioned conical flask containing the adsorbent was placed into a gas bath constant-temperature oscillator (SHZ-82A) for the shaking adsorption experiment. After 48 h of adsorption, the concentration of MB remaining in the Erlenmeyer flask solution after adsorption equilibrium was determined with an ultraviolet–visible spectrophotometer (TU-1810, Beijing Pujinjie General Instrument Co., Ltd.). The adsorption capacity (qe mg/g) at adsorption equilibrium is obtained by formula 1

2.5. 1

In the formula, c0 (mg/L) represents the initial MB solution concentration, ce (mg/L) represents the MB solution concentration after adsorption equilibrium, W represents the amount of the adsorbent (g), and V represents the volume of the solution (L).

The effects of different adsorbent doses were explored by adding different doses of CS/GO aerogel (5–25 mg) to a conical flask containing 20 mL of MB solution at 293 K.

The influence of the pH value of MB solution was evaluated by placing 20 mL of MB solution at pH 3–10 in different conical bottles at 293 K and then adding 10 mg of adsorbent to them.

By adding 125 mg of adsorbent to a beaker with 250 mL of MB solution and measuring the remaining MB concentration currently according to a predesigned time interval, the temperature is also 293 K, the effect of adsorption time can be evaluated. The calculation formula of the adsorption amount qt (mg/g) is as follows

2.5. 2

In the formula, the MB concentration at time t is represented by ct (mg/L).

At temperatures of 303, 323, and 343 K, by adding 10 mg of adsorbent to conical flasks containing 20 mL of MB solution (80–180 mg/L) of different concentrations, the impact of temperatures was evaluated.

The abovementioned adsorption processes are all oscillated and carried out on a gas bath thermostatic oscillator.

3. Results and Discussion

3.1. Characterizations of the CS/GO Aerogel

Figure 2a,d shows optical photos of the CS aerogel and CS/GO aerogel, respectively. As can be seen from the picture, the aerogel with GO has a rougher surface and more pores than the pure CS aerogel. Figure 2b,c shows SEM images of the CS aerogel, indicating that its surface is very smooth and has pores in the cross-section. Figure 2e,f depicts CS/GO aerogel SEM images. It can be seen that due to the specific 3D hollow hierarchical structure, the as-prepared samples owned a large specific surface area,33 which can promote the diffusion of dye molecules when adsorbing the dye, thereby improving the efficiency of the adsorbent.

Figure 2.

Figure 2

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Optical photograph of CS aerogels (a), SEM images of CS aerogels (b and c), the optical photograph of CS/GO aerogels (d), and SEM images of CS/GO aerogels (e and f).

The broadband vibration of GO (curve a) and the CS aerogel (curve c) at 3435–3454 cm–1 is −OH, which was attributed to the existence of phenolic compounds.34 For GO (Figure 3 curve a) at 1726 cm–1, the peak is due to the co-tensile vibration of the −COOH group. The bands of the CS aerogel (Figure 3 curve c) at 2924 and 1401 cm–1 are derived from the −CH tensile vibration of the −CH2 and −CH3 groups and the C–OH stretching vibration in the −COOH groups, respectively.35 The peak at 1637 cm–1 is the stretching vibration of the amide carbonyl group. The CS/GO aerogel (Figure 3 curve b) indicates that the tensile vibration general band of the amide carbonyl group moved to a higher frequency of 1654 cm–1, which is considered to form a C=N bond. There is a new peak appearing at 981 cm–1, which can be explained by the C–H trans bending vibration of the −CH=N group. Both curves have a peak at 1087–1088 cm–1, which can confirm the formation of a Schiff base in the aerogel.27 The CS/GO aerogel contains the peaks contained in GO and the CS aerogel, which indicates that the CS/GO aerogel was successfully synthesized.

Figure 3.

Figure 3

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FT-IR spectra of (a) GO, (b) CS/GO aerogels, and (c) CS aerogels.

Figure 4 is the TGA curve of three aerogels. GO has obvious weight loss behavior around 500 K. As can be seen from the figure, before the temperature is 500 K, the CS/GO aerogel lost 5.078% by weight, which can be explained by the removal of a small amount of water contained in the aerogel in the form of water vapor. When the temperature exceeds 510 K, the CS/GO aerogel reaches a stage of rapid weight loss (510–900 K); at this stage, the weight loss of the sample reaches 50.646%. In the end, 26.747% of the residual mass remains. The thermal stability of the synthetic aerogel is closer to that of CS and is significantly stronger than that of pure GO.

Figure 4.

Figure 4

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TGA of GO, CS, and CS/GO aerogel.

Figure 5 is a nitrogen adsorption/desorption isotherm of CS and CS/GO aerogels. The specific surface area of the CS aerogel is 0.8798 m2/g, and the specific surface area of the CS/GO aerogel is 2.4878 m2/g. After adding GO, the specific surface area of the aerogel increased. The increase in the specific surface area can be attributed to the self-assembly of GO nanosheets and the foaming effect of CS. Generally speaking, the larger the specific surface area of materials, the more conducive to the adsorption of materials it is.

Figure 5.

Figure 5

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Nitrogen adsorption/desorption isotherms of CS aerogels and CS/GO aerogels.

3.2. MB Adsorption

3.2.1. Comparison of Adsorption Performance of CS/GO Aerogels with Different GO Mass Fractions

Figure 6 is a graph showing the change in the mass fraction of GO on the adsorption capacity of the aerogel. Figure 6 indicates that the adsorption amount of the pure CS aerogel to MB is smaller than that of the CS/GO aerogel after adding GO. This is because the specific surface area of the CS aerogel is small, and the internal pore mechanism is insufficient. With the increase in the amount of GO added, the amount of MB adsorbed by the CS/GO aerogel increased to 360 mg/g. However, with the amount of GO further increased, the amount of MB adsorbed by CS/GO decreased instead, which may be because as the amount of GO increases, the crosslinking reaction between CS and glutaraldehyde is affected, and too much GO will affect the formation of the CS/GO aerogel. Through the analysis of the adsorption amount of MB-containing aerogels with different GO mass fractions, the CS/GO aerogel with a GO mass fraction of 8% was selected for the experiment.

Figure 6.

Figure 6

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Influence of the mass fraction occupied by GO on adsorption capacity.

3.2.2. Temperature Effect

The initial concentration and temperature of the MB solution are the key factors affecting the efficiency of the adsorbent. The temperature will have a great influence on the diffusion speed of the dye molecules. The impact of the study temperature on the adsorption performance of CS/GO aerogels was obtained through a batch of studies at temperatures of 293, 313, and 333 K. Figure 7a shows the results of adsorption at different temperatures and different initial MB solution concentrations. When the temperature rises from 293 to 333 K, the adsorption amount of the CS/GO aerogel to MB decreases from 333.11 to 285.74 mg/g; therefore, the adsorption of MB by the CS/GO aerogel is exothermic. Figure 7a indicates that the greater the concentration of the MB solution added initially, the greater the amount of MB adsorbed by the CS/GO aerogel.

Figure 7.

Figure 7

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Influence of (a) temperature, (b) adsorbent dose, (c) contact time, and (d) pH on MB adsorption by CS/GO aerogels.

3.2.3. Dose Effect

Figure 7b is a graph of the influence of different adsorbent doses on adsorption. As shown in the figure, in the same amount of MB solution, the amount of CS/GO aerogel increased from 5 to 15 mg. The surface active sites of the adsorbent increased, and the removal rate of MB by the adsorbent also increased;36 they are proportional to each other. When the dosage of the adsorbent was increased to more than 15 mg, the removal rate of MB in the solution by the adsorbent did not increase significantly. Because with the adsorbent increasing, the active sites of adsorption cannot be fully utilized, the adsorption capacity will be inversely proportional to the amount of the adsorbent.

3.2.4. Contact Time Effect

Figure 7c shows the effect of the contact time on adsorption efficiency. Figure 7c indicates that at the beginning of the experiment (before 700 min), the adsorption capacity increased rapidly; it can be explained that there are a lot of surface active adsorption sites available for adsorption on the surface of MB when the CS/GO aerogel starts to adsorb MB.37 As the time continued to increase, the increase in the adsorption capacity slowed down, and the growth rate was no longer obvious and gradually approached the adsorption equilibrium (1600 min).

3.2.5. pH Effect

The pH of the MB solution will affect the adsorption efficiency of the adsorbent. As shown in Figure 7d, when the pH increases from 3 to 5, the MB removal rate at adsorption equilibrium continues to increase, but the adsorption amount is relatively low, which can be analyzed by the molecular charge of MB and the adsorbent. At a lower pH environment, there are lots of H+ in the solution, which will compete with the same cation MB(=N+(CH3)Cl–) for the same adsorption site on the adsorbent, resulting in steric hindrance, at the same time promoting the protonation of carbonyl, amine, hydroxyl, and carboxyl groups of the CS/GO aerogel to form −OH2+ groups; this will cause electrostatic repulsion with positively charged MB ions, which will reduce the adsorption of the CS/GO aerogel.38 When the pH is greater than 7.0, the adsorption efficiency of the adsorbent is relatively high. At this time, the CS/GO aerogel will be deprotonated, which will enhance its electrostatic attraction with MB ions, thereby enhancing its adsorption capacity.39

3.2.6. Comparison of CA/GO with Other Adsorption Materials

Comparing CA/GO with other GO-based adsorbents, the specific parameters are shown in Table 1 below, and it can be seen that CA/GO has better adsorption performance.

Table 1. GO-based Adsorbent.
adsorbent qmax (mg/g) reference
3D graphene aerogel 221.77 (40)
β-cyclodextrin/GO 76.4 (41)
GO/nanoscale cellulose aerogels 111.2 (42)
titania nanotube graphene aerogel 380.7 (43)
metal ferrite-enabled GO 76.34 (44)
reduced GO/ferrite nanoparticles 105 (45)
CA/GO 437.29 this research
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3.3. Adsorption Isotherms

The interaction between the CS/GO aerogel and MB can be described by adsorption isotherms. Langmuir and Freundlich models can be used to study equilibrium data. The Langmuir model presumes that adsorption happens at the same adsorption site on the surface with homogeneous medium, and no chemical reaction will occur. The Langmuir expression is as follows46

3.3. 3

where ce (mg/L) represents the concentration when the adsorption equilibrium is reached, where kL is a constant related to the adsorption strength between the adsorption site and the adsorbent, and qe (mg/g) represents the adsorption amount when the adsorption equilibrium is reached.

According to eq 3, the nonlinear Langmuir model is shown in Figure 8a; the adsorbent had the best adsorption performance at 293 K of about 437.29 mg/g. The Langmuir constants kL (L/g) and qmax (mg/g) are listed in Table 2:

Figure 8.

Figure 8

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Equilibrium isotherms obtained by (a) Langmuir model and (b) Freundlich model.

Table 2. Adsorption Isotherm Model Parameters of CS/GO Aerogels Adsorbing MB.

  Langmuir
 Freundlich
T (K) qmax (mg/g) kL (L/mg) R2 RL kF (L/mg) 1/n R2
293 437.293 0.468 0.992 0.010–0.026 173.083 0.316 0.927
313 406.279 0.224 0.989 0.022–0.052 129.967 0.316 0.919
333 374.878 0.196 0.985 0.024–0.059 122.341 0.285 0.913
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From Table 2, the maximum adsorption amount qmax is 437.29 mg/g, and R2 values are all in line; therefore, the fitting is relatively good. It is appropriate to use the nonlinear Langmuir model to describe the adsorption of MB to the CS/GO aerogel.

The expression of another dimensionless parameter RL is as follows47

3.3. 4

The parameter RL is introduced to determine the type of Langmuir isotherm: (RL = 0) irreversible, (0 < RL<1) favorable, (RL = 1) linear, or (RL > 1) unfavorable. The data of RL in the table are all greater than 0 and less than 1, which indicates that the CS/GO aerogel is an adsorbent conducive to the adsorption of MB.

The Freundlich model simulates multilayer adsorption and assumes that adsorption occurs at unevenly distributed adsorption points on the surface of the medium. The nonlinear Freundlich equation is as follows48

3.3. 5

where kF (L/g) and 1/n are Freundlich constants. Figure 8b shows the nonlinear Freundlich model. The results are shown in the table, and other parameters are also shown in Table 2.

Table 2 indicates that the measurement coefficient R2 is low, and it is concluded that the adsorption of MB by the CS/GO aerogel does not conform to the Freundlich model.

3.4. Kinetic Studies

Adsorption kinetics can describe the specific process of the adsorption reaction completely. The adsorption process of MB to the CA/GO aerogel was studied by a pseudo-first-order kinetic model, pseudo-second-order kinetic model, and intraparticle diffusion equation. The expression of the pseudo first-order model can be expressed by a formula49

3.4. 6

where k1 (1/min) is the adsorption rate constant. k1 and qe are determined by plotting a fitted linear graph of log(qe – qt) versus t (Figure 9a). The parameters are shown in Table 3.

Figure 9.

Figure 9

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(a) Pseudo-first-order kinetic model, (b) pseudo-second-order kinetic model, and (c) intraparticle diffusion model for MB adsorption by CS/GO aerogels.

Table 3. Kinetic Constants of Adsorption of MB Onto CS/GO Aerogels.

kinetic model parameters values
pseudo-first-order K1 (min–1) 2.01 × 10–3
  qe (mg/g) 225.66
  R2 0.96394
pseudo-second-order K2 (g/mg min) 3.45 × 10–5
  qe (mg/g) 311.53
  R2 0.99885
intraparticle diffusion model Kid1 14.80838
  C1 –3.08769
  R12 0.98767
  Kid2 3.46878
  C2 167.88546
  R22 0.9581
  Kid3 1.02825
  C3 249.21278
  R32 0.99431
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From Table 3, the R2 is 0.96394, and the adsorption of MB on CS/GO aerogels does not conform to the pseudo-first-order kinetic model.

The expression of the pseudo-second-order model can be expressed by the equation50

3.4. 7

where k2 (g/mg min) represents a pseudo-second-order rate constant and k2 and qe can be obtained by plotting a fitted straight line graph with t as the X axis and t/qt as the Y axis (Figure 9b). The obtained kinetic parameters are listed in Table 3.

Table 3 shows that R2 is 0.99885, which is greater than the measurement coefficient of the pseudo-first-order model, and the experiment-obtained qe (304.44 mg/g) is consistent with the data in the table (311.53 mg/g). The pseudo-second-order kinetic model perfectly describes the adsorption behavior of the CS/GO aerogel on MB.

A study on the diffusion mechanism of CS/GO aerogels adsorbing MB was conducted by an intraparticle diffusion model. The mathematical expression of the intragranular diffusion model is as follows51

3.4. 8

In the formula, the intragranular diffusion constant is kid (mg/g min1/2). It can be obtained from the fitted line graph with t1/2 as the X axis and qt as the Y axis (Figure 9c). The parameters are listed in Table 3. According to the adsorption characteristics of the CS/GO aerogel of MB, the scattered points in Figure 9c were fitted in three stages. In the first stage, the slope of the straight line is steep, and the slope is large, which means the fast outer surface adsorption of the adsorbent boundary layer diffusion.52 The slope of the second stage decreases and is progressive adsorption, corresponding to the adsorption generated by intragranular diffusion. The reduction of the MB solution concentration in the third stage causes the intraparticle diffusion to slow down, which is the equilibrium phase. Figure 9c indicates that the three linear fitting straight lines do not pass the origin point to indicate that the adsorption of MB by the CS/GO aerogel, in addition to the particle diffusion rate control, will also be controlled by other steps.

3.5. Thermodynamic Study

It is obtained in Figure 7 that the adsorption capacity of the CS/GO aerogel for MB adsorption decreases with increasing temperature. The parameters Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were obtained through the Van’t Hoff equation. The expression is as follows53

3.5. 9
3.5. 10

In the formula, T (K) and R (8.314 j/-mol k) are the absolute temperature and universal gas constant, respectively. The abovementioned parameters were obtained by drawing a fitted straight line graph of ln(qe/ce) relative to 1/T. The parameters are shown in Table 4.

Table 4. Thermodynamic Parameters for MB Adsorbed by CS/GO Aerogels.

T/K ΔG (kJ/mol) ΔH (kJ/mol) ΔS (J/mol K)
293 –10.93 –25.22 –48.79 mn
313 –9.95    
333 –8.98    
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Table 4 shows that at temperatures of 293, 313, and 333 K, ΔG is −10.93 (kJ/mol), −9.95 (kJ/mol), and −8.98 (kJ/mol), respectively, all less than zero, indicating that the adsorption of MB by the GO aerogel is feasible and spontaneous. The negative change of enthalpy ΔH indicates that the adsorption of MB by the CS/GO aerogel is an exothermic process, which is consistent with the results obtained from the graph. The negative value of the change in entropy ΔS indicates that the adsorption on the solid–liquid interface is disorderly reduced.

3.6. Adsorption Mechanism of CS/GO

Figure 10 shows the adsorption mechanism of CS/GO on MB. According to the electrical properties of CS/GO and MB after hydrolysis, both contain a carboxyl group and benzene ring. It is speculated that the adsorption of MB on CS/GO may be due to three reasons: CS/GO and MB molecules are electrostatically attracted; the carboxyl group of CS/GO forms a hydrogen bond with MB; and a π–π stacking action is present between the benzene ring of CS/GO and MB.

Figure 10.

Figure 10

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Adsorption mechanism of CS/GO for MB.

3.7. Regeneration and Reuse of the CS/GO Aerogel

Reusability is one of the important indicators when evaluating the adsorption performance of the adsorbent. A total of 100 mg of CS/GO aerogel was added to 200 mL of MB solution for oscillatory adsorption. After 48 h, the adsorbent was filtered and soaked in 1 mol/L HCl for 6 h. The residual HCl in the adsorbent was washed with deionized water, and the sample was dried to constant weight. Then, the abovementioned steps were repeated. As Figure 11 shows, after five cycles, the ratio of the nth and 0th adsorption capacity was calculated from qe, n/qe, 0,54 where is the nth cycle, and the data obtained are 89.06%. The adsorbents regenerated even after several cycles always performed better than the native ones.55

Figure 11.

Figure 11

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Regeneration of CS/GO.

4. Conclusions

The CS/GO aerogel was prepared by the vacuum freeze drying method and characterized; the adsorption mechanism was researched through a large number of experiments. The effects of the mass fraction of GO, pH value, adsorbent dosage, contact time, and temperature on the adsorption of MB by the CS/GO aerogel were studied. It was found that the CS/GO aerogel had the best adsorption performance when the mass fraction of GO in the aerogel was 8 wt %. The results of the adsorption experiment indicate that the nonlinear Langmuir model is described. After kinetic research, the adsorption process conforms to the pseudo-second-order kinetic model, and the adsorption of MB by CS/GO aerogels is controlled by the diffusion rate of the particles and other steps. The thermodynamic parameters ΔG, ΔH, and ΔS all have negative values, indicating that the adsorption of MB on the CS/GO aerogel is an exothermic spontaneous process, and the disorder of the solid–liquid interface decreases.

Acknowledgments

This work was supported by the Taishan Scholar Program of Shandong Province (201511029).

The authors declare no competing financial interest.

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