Adsorption properties and mechanism of sepiolite to graphene oxide in aqueous solution

2.1 Materials

The water used in the experiment was 18MΩ·cm deionized water, which was prepared by a UPW-R15 pure water machine produced by Shanghai Yidian Scientific Instrument Co., ltd. The HCl used to adjust the pH is the hydrochloric acid standard titration solution produced by Guangzhou Hewei Pharmaceutical Technology Co., ltd., and the NaOH is produced by Oulian Hongda Industry and Trade Co., ltd., both of which are of domestic analytical grade and do not need further purification. The GO aqueous solution used in this experiment was 2 mg/mL and purchased from Suzhou Tanfeng Technology Co., ltd., China. The specific surface area is 420 cm²/g, and the element composition is shown in Table 1. SEP was purchased from Shijiazhuang Yuxin Building Materials Co., ltd. Its main chemical components are Si and Mg, the chemical formula is Mg(H₂O)₄[Si₆O₁₆]₂·(OH)₄·8H₂O. The content of SiO₂ is generally from 50 % to 65 %, and the content of MgO is from 12 % to 24 %. The main chemical components is shown in Table 2. The elemental composition of GO and the chemical composition of SEP are obtained from the material manufacturer.

The specific surface area and pore size are important factors that affect the adsorption process. Table 3 shows the specific surface area, average pore size, pore volume and pore volume data of SEP used in this experiment.

Fig. 2 shows the particle size distribution diagram of GO (a) and SEP (b). It can be seen from the figure that the particle size distribution of SEP and GO is normal distribution. The particle size distribution range of GO is 0.991um ∼ 186um, most of the particles are distributed around 27.4um, and the average particle size is 27.8um. The particle size distribution range of SEP is 0.405um ∼ 127um, most of the particles are distributed at 31.1um, and the average particle size is about 14.3um. The comparison shows that the particle size of GO is larger than that of SEP.

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Fig. 2

Particle size distribution of GO(a), SEP(b).

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2.2 Test scheme

2.3 Test procedure

The specific operation of the test is:

① The appropriate amount of GO was taken with a pipette gun and added to a glass bottle. Then deionized water added to prepare 50 mL of GO aqueous solution.

② Adjust the pH of the solution by adding negligible volumes of sodium hydroxide solution and hydrochloric acid solution, then measure with a pH meter and adjust the pH to the desired value.

③ According to the experimental design, the corresponding mass of SEP powder was weighed and added to the aqueous solution.

④ The glass bottle was placed in a constant temperature vibrating shaker, and vibrated at 240 rpm for 3 h.

⑤ After vibration, put the glass bottle into the curing box and cure it for 24 h at the set temperature.

⑥ After being cured, 1 mL of the supernatant was taken with a pipette gun and placed in a test tube. Three groups were taken from the same sample and diluted to 25 mL with deionized water.

⑦ The absorbance of the sample was measured by UV–vis spectrophotometer at a wavelength of 210 nm.

The main operation process is shown in Fig. 3:

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Fig. 3

The main operation process of the test.

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2.4 Test data processing

According to the initial GO concentration C₀ (mg/L) and the equilibrium concentration C_e (mg/L), the adsorption amount Q_e (mg/g), the adsorption rate R and the partition coefficient K_d (g/L) were calculated. The calculation formulas are as follows (Liu et al., 2021):

Where, m (mg) represents the mass of SEP powder, and V (mL) represents the volume of the solution. To ensure data accuracy, all experiments were repeated three times and the average value of the three experiments was taken for subsequent data analysis.

In order to explore the change of adsorption capacity with adsorption time and better study the adsorption mechanism, adsorption kinetics was introduced and the pseudo-first-order kinetic model and pseudo-second-order kinetic model were used to fit and analyze the data. The formulas are as follows (Yang et al., 2015):

The pseudo-first-order adsorption kinetic equation:

The pseudo-second-order adsorption kinetic equation:

Elovich adsorption kinetics equation:

Where, Q_t represents the adsorption amount (mg/g) when the adsorption time is t, and k₁ and k₂ are pseudo-first-order and pseudo-second-order adsorption rate constants (g/(mg·min)), α is the initial adsorption rate constant (mg/g·min⁻²), β is the parameter related to the surface coverage of the adsorbent and the chemical adsorption activation energy (g/mg·min).

In order to further determine the control steps of the adsorption rate, the Intraparticle diffusion model, Boyd pseudo first model and Boyd-Ruthven-Ho model were used to fit the experimental data (Azari et al., 2021; Benjelloun et al., 2021b), the formula is as follows:

Intraparticle diffusion model:

Boyd pseudo first model:

Boyd-Ruthven-Ho model:

Where, k_d is the coefficient of diffusion rate within the particle (mg/(g•min^0.5)), C is the constant related to the boundary layer thickness (mg/g), k_i,B is the rate constant of the particle diffusion model (mg·g⁻¹·min^−0.5), and B is the model parameter.

In order to explore the adsorption model of SEP for GO, GO with different concentrations was adsorbed at three temperatures of 293 K, 303 K, and 313 K, the adsorption isotherm was drawn. Langmuir, Freundlich and Temkin models were introduced to fit the adsorption data (Febrianto et al., 2009). The models are as follows:

Langmuir adsorption isotherm model:

Freundlich adsorption isotherm model:

Temkin adsorption isotherm model:

Where, Q_e represents the equilibrium adsorption capacity (mg/g), C_e represents the equilibrium concentration (mg/L), Q_max represents the maximum adsorption capacity (mg/g), K_L represents the adsorption equilibrium constant of Langmuir (L/mg), and K_F represents the adsorption equilibrium constant of Freundlich (mg/g), n is a dimensionless constant related to temperature and adsorbent, K_T is the adsorption equilibrium constant (L/mg) of Temkin, and C is the infinitesimal number related to temperature and the properties of the adsorption system.

According to the adsorption isotherms at different temperatures, the relevant thermodynamic parameters are obtained by calculation, and the specific calculation formulas are as follows (Cortes et al., 2019):

Where, R is the ideal gas constant, generally 8.3145 J/(mol·K), ΔG⁰ is the Gibbs free energy (kJ/mol), ΔH⁰ is the enthalpy change (kJ/mol), and ΔH⁰ is the entropy change (kJ/(mol·K)), T is the absolute temperature.

3.1 Micro analysis

3.1.1

3.1.1 XRD and FTIR analysis

To further reveal the reaction mechanism, GO, SEP and SEP/GO were analyzed and compared by XRD, and the detection results are shown in Fig. 4. It can be seen that GO has a strong diffraction peak at 2θ = 9.5°, which disappears after reacting with SEP. This is mainly because GO entered the interior of the SEP and was successfully loaded on the surface of the SEP, and the residual GO content was too low to be detected by the instrument (Li et al., 2022c). XRD analysis shows that the main components of SEP used in this paper are dolomite, calcite, amphibole, etc. Referring to the ICSD standard on the PDF card, 10.5° and 12.2° are marked as amphibole (1 1 0), 28.54° as amphibole (3 1 0), 29.5° as calcite CaCO₃ (1 0 4), and 30.9° as dolomite CaMg(CO₃)₂ (1 0 4). It can be seen from Fig. 4 that the intensity of the characteristic diffraction peaks of SEP/GO is mostly lower than that of SEP, and no new diffraction peaks appear. After the adsorption reaction occurred, the front at 29.5° (1 0 4) and 30.9° (1 0 4) decreased significantly, indicating that calcite CaCO3 and dolomite CaMg(CO₃)₂ may be involved in the reaction.

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Fig. 4

XRD patterns of GO, SEP, SEP/GO.

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In addition, changes of functional groups before and after adsorption were observed by FTIR. It can be clearly seen in Fig. 5 the characteristic absorption peaks before and after the adsorption of GO by SEP. By observing the FTIR spectrum of GO, it can be seen that the absorption peaks at 3191 cm⁻¹ and 1734 cm⁻¹ represent the stretching vibration adsorption of —OH and C⚌O on the carboxyl group, respectively (Singh, 2021). The adsorption peak at 1622 cm⁻¹ is the vibrational adsorption of C⚌C (Boukhoubza et al., 2020). The absorption peak at 1122 cm⁻¹ is due to adsorption of C—O—C (Depan et al., 2011). The spectra of SEP and SEP/GO are similar, no new characteristic peaks appear, but the intensities are weakened. The absorption peak at 3676 cm⁻¹ is generated by —OH combined with Mg⁺ (Qiu et al., 2013). The absorption peak at 3448 cm⁻¹ is caused by the stretching vibration adsorption of zeolite water in the pores of SEP (Sabah and Çelik, 2002). The deformation of the water level H—O—H bending corresponds to 1428 cm⁻¹ in the spectrum (Li et al., 2020). The characteristic peak at 993 cm⁻¹ is attributed to the antisymmetric stretching vibration adsorption of the Si-O bond (Soheilmoghaddam et al., 2014). The symmetric stretching vibration of Si—O—Si and the vibrational absorption peak of octahedral Mg-O at 685 cm⁻¹ and 758 cm⁻¹ (Wang et al., 2010). The disappearance of the 1734 cm⁻¹ front may be caused by the combination of Ca²⁺ in SEP and O—C⚌O in GO (Chowdhury et al., 2015), which is consistent with the change rule of CaCO₃ and CaMg(CO₃)₂ in XRD. New characteristic peaks appeared at 685 cm⁻¹ and 758 cm⁻¹, indicating that GO was successfully loaded on the SEP surface.

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Fig. 5

FTIR spectra of GO, SEP, SEP/GO.

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3.1.2

3.1.2 SEM and TEM analysis

SEM and TEM were used to observe the changes of the surface morphology of SEP before and after GO adsorption, as shown in Fig. 6. In Fig. 6(a) and (b), it can be observed that SEP is mainly rod-shaped and has a fibrous coarse surface structure with some impurity particles attached to the surface. It can be seen from Fig. 6(c) and (d) that GO is in the form of a smooth surface flake with slight wrinkles on the surface. Fig. 6(e) and (f) show the images of SEP/GO, it can be observed that the fibrous structure is attached to the surface of the tulle with obvious agglomeration and stacking, indicating that GO is attached to the surface of SEP.

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Fig. 6

SEM(a) and TEM(b) of SEP, SEM (c) and TEM(d) of GO, SEM(e) and TEM(f) of SEP/GO.

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3.1.3

3.1.3 XPS and AFM analysis

The types of elements contained in the material can be observed by XPS. It can be seen from Fig. 7(a) that there are O1s and C1s adsorption peaks in the XPS spectra of GO and SEP/GO, but the intensity of the peaks decreases after adsorption, and new energy peaks of Mg1s, Ca2p, and Si2p appear, which is consistent with the results described by XRD. Fig. 7(b) shows the C1s peak splitting results of GO. After peak splitting, three satellite peaks with binding energies of 284.8, 286.92, and 288.43 eV are obtained, and the corresponding chemical bonds are C—C, C—O, and O-C⚌O (Yang et al., 2009). Comparing the positions of the center fronts of SEP/GO in Fig. 7(c), the satellite front corresponding to the C—C bond changes from 284.8 eV to 284.77 eV, and the front corresponding to the O—C⚌O bond changes from 288.43 eV to 288.51 eV. It can be seen from the figure that both the intensity and position of the C1s front have changed after adsorption. The surface area ratio of C—O⚌C peak increases from 8.1 % to 11.2 %, that of C—O peak decreases from 47.4 % to 41.4 %, and that of C—C peak increases from 44.4 % to 47.4 %. The change of peak surface area ratio indicates that C—C, C—O, and O—C⚌O bonds are involved in the interaction between SEP and GO, but there is little damage to oxygen-containing functional groups, which was consistent with the results of FTIR.

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Fig. 7

XPS spectrum of GO and SEP/GO (a), high-resolution photoelectron spectrum of C1s of GO (b) and SEP/GO (c).

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AFM was used to characterize the size and thickness of the material sheets before and after adsorption. Fig. 8 and Fig. 9 show the AFM test results of GO and SEP/GO. It can be seen that the maximum thickness of GO is 1.13 nm, while that of SEP/GO is 1.69 nm. The thickness of SEP/GO is larger than that of GO. The initial GO is relatively flat, and the surface thickness increases after adsorption. This is due to the attachment of GO to the surface of the SEP, which is consistent with the results of the TEM (Fig. 6(f)), indicating that SEP can effectively remove GO in aqueous solution.

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Fig. 8

3D height topographic image (a), AFM image (b) and height profile (c) of GO.

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Fig. 9

3D height topographic image (a), AFM image (b) and height profile (c) of SEP/GO.

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3.2 Effect of pH

pH is one of the important parameters affecting the adsorption and ion exchange performance of clay minerals (Doğan et al., 2009). SEP naturally has good acid and alkali resistance. When it is in a medium with a pH value of < 3 or > 10, the internal structure of SEP will be corroded. When the pH value is between 3 and 10, SEP will show strong stability performance (Komadel and Madejová, 2006). The adsorption results at different pH are shown in Fig. 10. It can be seen that, with the increase in pH, the adsorption capacity Q_e and adsorption rate R of SEP for GO basically show a downward trend. When pH = 3, the adsorption rate is the highest, reaching 93.1 %. The good adsorption effect under acidic conditions may be due to the following reasons: firstly, GO has strong self-cohesion under acidic conditions, and large-scale aggregation will occur (Shih et al., 2012). Secondly, the melting of Mg²⁺, Ca²⁺ and other cations in SEP weakens the bonding force between layers, makes the interlayer lattice crack, expands the interlayer spacing, and expends the micropores and voids, thus increasing the specific surface area and enhance the adsorption capacity of SEP (Huang et al., 2017). Thirdly, under acidic conditions, the positively charged adsorption sites on the adsorbent surface increase, thus improving the electrostatic attraction to the negatively charged GO solution (Badi et al., 2019). Fourth, when the pH is high, the carboxyl groups on the surface of GO act to protonate, which inhibits the binding between cations and GO (Zhao et al., 2017).

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Fig. 10

Effect of pH on GO adsorption. (Operating conditions: pH = 3,4,5,6,7,8,9,10, C₀ = 80 mg/L, m = 50 mg, T = 303 K).

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Considering the buffering properties of SEP, the final pH of the solution after the reaction was determined, resulting in Fig. 11. It can be seen from the figure that when the initial pH of the prepared solution is between 4 and 9, the pH after the reaction between SEP and GO is maintained between 7.73 and 8.58. This is due to the buffering properties of clay minerals (SEP), whose surface O^2– and H⁺ form —OH. These hydroxyl groups have both acid-base characteristics and generate variable charge (Jeon and Nam, 2019), which also makes SEP have strong buffering capacity when removing GO from aqueous solution. Combining with Fig. 10, it can be seen that when the initial pH of the solution is between 4 and 9, the adsorption rate of SEP on GO is 85.1 %-91.2 %, indicating that the initial pH of the solution within this range has little effect on the adsorption of GO by SEP. In the case of pH = 3, the final pH of the solution was maintained at 7.33, and the adsorption rate reached the highest. When pH reached 10, the buffering capacity of SEP decreased and the solution was maintained at a higher pH of 9.88, which may be because the hydroxyl groups on the surface of SEP were protonated, which inhibited the progress of the adsorption reaction.

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Fig. 11

Initial pH and final pH of the solution. (Operating conditions: pH = 3,4,5,6,7,8,9,10, C₀ = 80 mg/L, m = 50 mg, T = 303 K).

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Zeta potential is an important parameter for the quantitative characterization of particle dispersibility and stability. The larger the absolute value is, the more stable the colloidal particles are in the sol system and the better the dispersibility is (Dembek et al., 2022). For non-metallic ore particles, acid groups are generally formed due to the existence of broken bonds on the surface, which makes the surface negatively charged (Yu et al., 2019). Fig. 12 shows the effect of different pH on the zeta potential for GO adsorption by SEP. It can be seen that the pH value directly affects the type and amount of charge on the material surface. Zeta potentials at different pH are all negative, and with the increase of pH, the hydroxyl group on the surface of SEP is protonated or deprotonated, leading to the decrease of the positive charge of SEP (Yang et al., 2016). When pH = 3, the zeta potential values of SEP/GO and GO are the largest, which are −11.9 mV and −12.7 mV respectively; when pH = 10, the zeta potential values of SEP/GO and GO are the smallest, which are −41.6 mV and −46.4 mV respectively. It shows that the solution system of SEP/GO is more active at pH = 3. However, when pH = 10, the amount of negative charge on the surface is the largest. Due to the protonation of oxygen-containing functional groups on the surface of GO at high pH, the surface of GO is also negatively charged, generating strong electrostatic repulsion with the negative charge on the surface of SEP (Jiang et al., 2022), making it difficult to aggregate, resulting in poor adsorption of GO by SEP. Therefore, the increase of pH is not conducive to GO adsorption, but it is beneficial to the adsorption under weak acidic conditions.

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Fig. 12

Zeta potential of GO and SEP/GO at different pH. (Operating conditions: pH = 3,4,5,6,7,8,9,10, C₀ = 80 mg/L, m = 50 mg, T = 303 K).

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3.3 Effect of adsorbent quality

The effect of adsorbent quality on GO adsorption is further studied, and the results are shown in Fig. 13. It can be seen that when the SEP quality is 30 mg, the adsorption rate is the highest, which is 94.4 %. With the increase of SEP quality, the adsorption capacity Q_e decreases continuously, the adsorption rate R first increases, and then decreases when SEP quality exceeds 30 mg, and then tends to be flat. Therefore, 30 mg of SEP was selected for subsequent experiments. This is mainly because the number of effective adsorption sites of SEP increases with the increase of SEP quality, thereby improving the adsorption rate and partition coefficient. However, as the quality of the SEP increases, the number of particles per unit volume increases, which is prone to collision and agglomeration, resulting in a decrease in the number of effective active adsorption sites per unit quality of the SEP and a decrease in the specific surface area of the SEP, which ultimately leads to a decrease in the adsorption effect (Benjelloun et al., 2021a).

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Fig. 13

Effect of SEP quality on GO adsorption. (Operating conditions: pH = 3, C₀ = 80 mg/L, m = 10,20,30,40,50 mg, T = 303 K).

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3.4 Effect of GO concentration

Under conditions of SEP quality of 30 mg and pH = 3, the effect of initial GO on the adsorption capacity of SEP is explored. The results are shown in Fig. 14. It can be seen that the adsorption rate R first increases and then decreases with increasing GO concentration. The adsorption capacity Q_e increases continuously. This may be because: SEP quality is constant, when GO concentration is low, SEP provides more adsorption sites and the removal rate is high; when GO concentration is too high, the particles are prone to collision and agglomeration, the adsorption sites on the surface of SEP are limited, and the adsorption process is also restricted (Zazouli et al., 2016). Then, with the increase of GO concentration, the adsorption capacity Q_e increases, but the adsorption rate decreases, which may be because the increase of GO concentration inhibits the electrostatic interaction between SEP and GO (RezaeiKalantary et al., 2014). It can be seen from Fig. 14 that when GO concentration is 100 mg/L, the adsorption rate is the highest, reaching 94.8 %.

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Fig. 14

Effect of initial GO concentration. (Operating conditions: pH = 3, C₀ = 60,80,100,120,140 mg/L, m = 30 mg, T = 303 K).

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3.5 Adsorption kinetics

In order to explore the change of SEP adsorption GO with time, the adsorption kinetics was introduced. The adsorption kinetics characteristics were observed under the optimal conditions that the mass of adsorbent was 30 mg, pH = 3, and the initial concentration of GO was 100 mg/L. The relevant kinetic parameters were calculated according to formulas (4)–(6). The calculation results are shown in Table 5. The change of adsorption capacity with time is shown in Fig. 15.

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Fig. 15

Curves of adsorption capacity changing with time and fitting curves of pseudo-first-order model and pseudo-second-order model (a), fitting curve of pseudo-second-order kinetic model (b), fitting curve of Elovich kinetic model (c). (Operating conditions: pH = 3, C₀ = 100 mg/L, m = 30 mg, T = 303 K).

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It can be seen from Fig. 15 that GO adsorption of GO by SEP reaches equilibrium at 2160 min. In the early stage of adsorption, the adsorption capacity increases rapidly. When the adsorption time is 100 min, the adsorption capacity increases sharply, and then slowly increases until equilibrium, which may be due to the gradual saturation of the adsorption potential on the surface of SEP. It can be seen in Table 5 that the correlation coefficient R² fitted by the pseudo-first-order kinetic equation is 0.872, and the adsorption capacity Q_e is 152.7 mg/g. The correlation coefficient R² adapted to the pseudo-second-order kinetic equation is 0.918, and the adsorption capacity Q_e is 162.7 mg/g. The R² of the Elovich model is only 0.829. Compared with the pseudo-second-order kinetic model, the Elovich model is not suitable for describing the data obtained in this experiment. The correlation coefficient R² of the pseudo-second-order kinetics is higher, and the adsorption capacity Q_e is closer to the experimental result. Therefore, the pseudo-second-order kinetic equation can better describe the adsorption of GO by SEP, which indicates that the adsorption behavior of GO by SEP is controlled by chemical effects (Gong et al., 2008).

Considering that the pseudo-second-order kinetic model cannot identify the role of intraparticle diffusion in the adsorption process, the Intraparticle diffusion model, Boyd pseudo first model and Boyd-Ruthven-Ho model are introduced to identify whether intraparticle diffusion is the adsorption kinetics The rate-limiting step in (Eyni et al., 2019). Fig. 16 shows the fitting curves of the three models, and the relevant parameters calculated according to the formulas (7)–(9) are shown in Table 6 and Table 7:

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Fig. 16

Intraparticle diffusion model fitting curve (a), Boyd pseudo first model and Boyd-Ruthven-Ho model fitting curve (b). (Operating conditions: pH = 3, C₀ = 100 mg/L, m = 30 mg, T = 303 K).

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Intraparticle diffusion model is divided into 3 sections. The first straight line is steeper, which is the rapid adsorption stage. At this time, there are a large number of active adsorption sites on the surface of SEP; the second section is relatively slow, which is the slow adsorption stage. The fitting degree of this stage is relatively high, and the R² is 0.964, indicating that the adsorption rate is affected by the diffusion in the particles after entering this stage. The third stage is the adsorption equilibrium stage, which is mainly because after a long time of adsorption reaction, there are few active adsorption sites on the SEP surface, or the concentration of the residual GO solution is low, so the adsorption reaction is no longer carried out. It can also be seen from Fig. 16 that the fitting curve does not pass through the origin, indicating that internal diffusion is not the only speed limiting step (Jaafari et al., 2018).

It can be seen from Table 7 that the correlation coefficient R² of Boyd-pseudo-first model fitting is 0.887, and the correlation coefficient R² of Boyd-Ruthven-Ho model is 0.930, so Boyd-Ruthven-Ho model is more suitable for describing the experimental data obtained in this study. The Boyd-Ruthven-Ho model fitting curve is a straight line and the intercept is not 0, which indicates that the rate of the slow adsorption phase is not solely controlled by the intra particle diffusion, which is consistent with the fitting result of the Intraparticle diffusion model. The adsorption may be controlled by the liquid film diffusion or chemical reaction at the same time (Cáceres-Jensen et al., 2013).

3.6 Adsorption isotherm and thermodynamic analysis

At different temperatures (293, 303, 313 K) and different initial concentrations of GO (60, 80, 100, 120, 140 mg/L), the equilibrium adsorption experiment was carried out, and the adsorption effect was further observed by drawing the adsorption isotherm. The experimental results are shown in Fig. 17. It can be seen that: ① Under the same GO concentration, the adsorption capacity of SEP increases with the increase of temperature, indicating that the increase of temperature is beneficial to the adsorption, but has little effect; ② At the same temperature, the adsorption capacity of SEP increases with the increase of GO concentration; ③ The higher the concentration, the more obvious the effect of temperature on SEP adsorption of GO.

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Fig. 17

Adsorption isotherm. (Operating conditions: pH = 3, m = 30 mg, C₀ = 60,80,100,120,140 mg/L, T = 293,303,313 K).

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Therefore, the optimal temperature for GO adsorption by SEP is at 303 K with better adsorption effect. This is mainly because SEP loses part of free water and surface bound water when a certain temperature rises, and the adsorption resistance decreases, which is conducive to the diffusion of adsorbate molecules and improves the adsorption performance of SEP (Raya et al., 2022). The number of exposed broken bonds increases, leading to an increase in the specific surface area and an increase in activity; at the same time, it makes the molecules in GO diffuse faster from the solution to SEP (Foo and Hameed, 2013). However, adsorption at high temperature will increase the operating cost, so this study chose to conduct experiments at 303 K.

In order to further understand the adsorption mechanism, Langmuir, Freundlich and Temkin models were used to fit the adsorption process of GO by SEP, the relevant parameters were calculated according to formulas (10) ∼ (12), as shown in Table 8. The fitting curve is shown in Fig. 18.

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Fig. 18

Langmuir isothermal model fitting curve (a), Freundlich isothermal model fitting curve (b), Temkin isothermal model fitting curve (c). (Operating conditions: pH = 3, m = 30 mg, C₀ = 60,80,100,120,140 mg/L, T = 293,303,313 K).

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It can be seen from Table 8 that the correlation coefficient R² of Langmuir model is higher than that of Freundlich model, so Langmuir model can better describe the adsorption of GO by SEP, and the adsorption process is mainly uniform monolayer adsorption (Sari et al., 2009), which is consistent with the SEM results. Compared with the Freundlich model, the Temkin model is more suitable for describing the adsorption of the studied adsorbent. It assumes that the decrease of the heat of adsorption is linear, and the C in the table are all positive values, indicating that the adsorption process is endothermic in nature (Hameed et al., 2008).The maximum adsorption capacity Q_max in the Langmuir model increases with increasing temperature, indicating that the increase of temperature has a certain promotion effect on the adsorption effect. In Freundlich model, it is generally considered that n > 1, which means that the adsorption is easy to carry out. It can be seen from the table that n is>1, indicating that the adsorption is a favorable preferential adsorption (Dwivedi et al., 2008).

In order to further describe the relationship between adsorption and temperature, according to the experimental data of adsorption at different temperatures, formulas (13) to (15) are substituted. Relevant thermodynamic parameters can be calculated as shown in Table 9, and the linear relationship between lnk_d and 1/T was obtained as shown in Fig. 19.

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Fig. 19

Relationship between lnK_d and 1/T. (Operating conditions: pH = 3, m = 30 mg, C₀ = 60,80,100,120,140 mg/L, T = 293,303,313 K).

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It can be seen from Table 9 that the ΔG values at different temperatures are all negative values. With the same concentration, the absolute value of ΔG increases with the increase of temperature, which indicates that the adsorption reaction is a spontaneous reaction. With the increase of temperature, the reaction is more likely to proceed spontaneously. Under different concentrations, ΔH is all positive, indicating that the adsorption reaction is an endothermic process (Heshmati et al., 2014). The higher the temperature is, the more favorable the adsorption is, which is consistent with the fitting results of the Temkin model. ΔS > 0 indicates that the disorder of the solid–liquid interface increases after adsorption (Mata et al., 2009), the system becomes more disordered, and the process is generally realized as an increase process (Xu et al., 2022).

4.1 Conclusion

In this study, the factors affecting the performance of SEP adsorption on GO were investigated, and the data were analyzed. Combining microscopic experiments to characterize the samples before and after adsorption, the adsorption mechanism was deeply explored. The following conclusions can be drawn:

• The adsorption capacity of SEP for GO was studied by changing the pH, adsorbent quality, initial GO concentration, and temperature. It can be found that SEP has the best adsorption performance for GO under the condition of 30℃, pH = 3, 30 mg of SEP quality and 100 mg/L of GO concentration, the adsorption rate R is 94.8 %. The increase in temperature is helpful to improve the adsorption effect. The higher the concentration is, the more obvious the effect of temperature on GO adsorption is.

• Comparing the fitting parameters of the pseudo-first-order, pseudo-second-order kinetic models and the Elovich kinetic model, it can be seen that the adsorption of SEP on GO is more in line with the pseudo-second-order kinetic fitting equation. The adsorption process is controlled by chemical action, and the adsorption capacity increases rapidly with the increase of time at 0 ∼ 100 min, and reaches equilibrium at 2160 min. According to the Intraparticle diffusion model and Boyd-Ruthven-Ho model, the rate in the slow adsorption stage is jointly controlled by the intraparticle diffusion and liquid film diffusion.

• By comparing the parameters of Langmuir, Freundlich and Temkin isothermal models, it can be seen that the adsorption of GO by SEP is more consistent with the Langmuir and Temkin isothermal models, and the adsorption process is mainly monolayer adsorption and endothermic. From the perspective of thermodynamic parameters, the adsorption of SEP to GO is a spontaneous, endothermic and entropy-increasing process.

• Through microscopic experiments, it can be known that the adsorbed GO aggregates and stacks on the surface of SEP, and GO adheres to the surface of SEP. From FTIR, it can be known that the adsorption process is accompanied by the vibration deformation of chemical bonds and the interaction of functional groups. The XPS results further indicated that the adsorption of GO by SEP is mainly due to the coordination reaction of Mg²⁺ and Ca²⁺ dissolved in the solution with O-C⚌O.

In summary, SEP is a material that can effectively adsorb GO in aqueous solution with a strong adsorption capacity. Through experiments, the adsorption performance of SEP is deeply understood, which provides a reference for the removal of GO in aqueous solution, and helps to improve the ecosystem and promote sustainable development.

4.2 Foresight

Table 10 summarizes other relevant research results for comparison with this study. Due to the lack of theoretical knowledge and the limitation of experimental conditions, the following research can be carried out in the future:

• The changes in the adsorption performance of SEP for heavy metal ions under different pH, solution concentration, adsorbent quality and temperature can be further explored.

• Comparing the adsorption of GO by SEP with other materials, especially with the adsorption properties on different types of clay minerals, it is helpful to deepen the understanding of the adsorption of GO by materials.