Graphene Oxide Adsorption Enhanced by Attapulgite to Remove Pb (II) from Aqueous Solution

Abstract

To improve the adsorption and separation efficiency of lead-containing wastewater by graphene oxide (GO), attapulgite (ATP) was used through a simple hydrothermal reaction. The prepared GO was characterized by SEM, TEM, FTIR, and XPS. The adsorption properties of the prepared GO were investigated. The maximum adsorption capacity of Pb²⁺ on as-prepared GO at pH 5 and 25 °C was 450.9 mg/g. It is concluded that the as-prepared GO can be used as a high-efficiency adsorbent for lead-containing wastewater.

1. Introduction

Pb²⁺ is a heavy metal ion found in industrial and agricultural wastewater and acidic landfill leachate [1]. Because of its high toxicity, nonbiodegradability, wide presence, and bioaccumulation, it constitutes a pollutant that threatens humans and the environment [2]. Long-term consumption of lead-ion-containing drinking water can induce serious diseases, such as anemia, reproductive diseases, genotoxicity, and nervous system effects. In particular, children bear the greatest risk of lead-induced toxicity [3]. These concerns have caused increasingly more scientists to investigate simple yet highly effective treatment methods for lead-containing wastewater [4]. Methods that have been used to date in the removal of lead ions from wastewater include chemical precipitation [5], coprecipitation [6], cloud-point extraction [7], electrochemical reduction [8], flocculation [9], ion exchange [10], membrane filtration [11,12], and adsorption [2,3,4,13,14,15,16]. Among these methods, adsorption offers the advantages of simple operation, high efficiency, and economic feasibility, and environmental friendliness [17]. However, it is essential to identify high-efficiency adsorbents that are easy to prepare, inexpensive, and easy to regenerate. Most studies have focused on the search for these high-efficiency adsorbents.

Graphene oxide (GO) is typically obtained by oxidation or sonication of graphite, and it possesses oxygen-containing functional groups, including hydroxyl groups (–OH), epoxy groups (–C=O), and carboxyl groups (–COOH). In addition, GO also possesses a high specific surface area [18]. Compared to other carbon-based materials, GO exhibits better environmental compatibility and biocompatibility [19]. Oxygen-containing functional groups can be used as the adsorption sites for heavy metal ions in wastewater [20]. Through interaction with these oxygen-containing functional groups, heavy metal ions can be removed from wastewater, thus enabling purification [21]. In addition, the presence of these functional groups enables the stable dispersion of GO in water and other polar organic solvents [22]. However, pure GO or GO-based adsorbents reported in the literature have suffered from limitations such as toxic preparation conditions, complex preparation methods, difficulty in separation, and high cost. As a result, inorganic clay minerals with excellent adsorption and separation properties are selected here to create a composite with GO to fabricate Pb²⁺ adsorbents with excellent properties at a low cost.

Attapulgite (ATP) is a water-containing magnesium aluminum phyllosilicate, which is an inorganic mineral that exhibits high cation exchange capacity, rapid hydration [23], and environmental compatibility. Research has shown that the as-prepared GO composites exhibit excellent adsorption to heavy metal ions such as Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺ [24], and Ni²⁺ [25]. ATP can be regenerated after adsorption saturation. In addition, because of its low cost and easy access [26], ATP exhibits wide application potential and practical value in the removal of heavy metal ions in wastewater due to its cheapness, abundance, and chemical stability [27].

Although there are abundant studies of GO composite materials [28], there are no reports on facile composite materials combining GO and the inorganic clay material ATP. GO exhibits a two-dimensional layer structure [29], whereas the ATP single crystal exhibits a fine fibrous morphology [30]. The present study aimed to graft ATP to the GO surface, forming a novel composite material with a “plane-needle” structure, thus achieving improved GO properties and decreased cost in the treatment of lead-containing wastewater. The already large specific surface area of GO was further increased to improve the separation performance, thus avoiding the loss of GO nanoparticles in lead-containing wastewater and achieving more efficient and economical treatment of heavy metal lead ions in wastewater.

2. Materials and Methods

2.1. Materials

GO in liquid form with a concentration of 2 mg/mL was purchased from Nano Technology Co., Ltd. ATP was purchased from Anhui Mingguang ATP Clay Factory. Other reagents were of analytical or guaranteed grade, and deionized (DI) water was generated in the lab. Pb²⁺ stock solution was obtained by dissolving Pb (NO₃)₂ in DI water.

2.2. Methods

2.2.1. Preparation of the As-Prepared GO Composite

GO was reacted with ATP in two steps. First, 50 g ATP was dissolved in 1000 mL DI water and immersed for 24 h. Then, 1 g of 2% sodium hexametaphosphate was added to the above solution. The resulting solution was stirred under magnetic stirring for 2 h and allowed to stand stationary for 2~3 h until a stable separation of liquid was observed. The upper suspension was removed to a volumetric flask, and water was added to a final volume of 1 L in the volumetric flask; the precipitate at the bottom of the beaker was then dried and weighed. The concentration of the 1 L suspension solution was calculated, and the suspension solution was then diluted to a concentration of 4 mg/mL. Second, 100 mL of 2 mg/mL GO and 100 mL of 4 mg/mL ATP were sonicated individually at 1200 W for 1 h, and the two solutions were mixed and sonicated at 80 W for 3 h. The sonicated mixture solution was then stirred at 120 r/min on a magnetic stirrer for 10 h at 45 °C. The chemical reaction between ATP and GO is shown in Figure 1.

2.2.2. Characterization

The sample morphology was characterized by scanning electron microscopy (SEM, Carl Zeiss group, Germany), transmission electron microscopy (TEM, TECNAIG2 TF20, Thermo Fisher Scientific, USA), and X-ray photoelectron spectroscopy (XPS, Phi5702, Al target excitation source, Physical Electronics Inc., USA). Fourier transform infrared spectroscopy (FTIR, IR Prestige-21, Shimadzu, Japan) was used to analyze the IR spectra of samples at room temperature in the wavenumber range of 4000~400 cm⁻¹ using KBr as the background.

2.2.3. Adsorption Experiment

In the adsorption experiments, the absorbent solution and Pb²⁺ stock solution were mixed, and then DI water was added to obtain 0.3 mg/mL adsorbent and 150 mg/L Pb²⁺.The pH value was 5, which was adjusted by 0.1 mol/L HCl and 0.1 mol/L NaOH. The reaction mixture was stroked 2 h at 298 K followed by centrifuged at 6000 r/min for 10 min. The concentration of Pb²⁺ in the supernatant was measured using an atomic absorption spectrophotometer (Spectra AA110/220, Varian Inc., Palo Alto, PA, USA). All experimental data were the average of three parallel experiments. The removal efficiency of Pb²⁺ was calculated using the following equation:

$$\eta =\frac{C_0-C_{\mathrm{e}}}{C_0}\times 100\%$$

(1)

where C₀ and C_e are the initial and equilibrium concentrations of Pb²⁺ in mg/L; and η is the removal efficiency in %.

To compare the adsorption capacity, the initial concentration of Pb²⁺ was 100 mg/L. To investigate the pH effects on Pb removal, the pH was adjusted from 3~9. To examine the contact time effects on Pb removal, the solution was stroked for 5 to 120 min. To evaluate the initial concentration on Pb²⁺ removal, Pb²⁺ dosage varied from 20 mg/L to 160 mg/L. The experiments ware conducted at 298 K, 308 K, and 318 K to investigate the thermodynamic models. To investigate the regeneration and recycling properties of the as-prepared GO, HCl solution was used as the regenerate; the concentration was varied from 0.2 to 0.3, 0.5, 0.8, and 1.2 mol/L.

3. Results and Discussion

3.1. Characterization

The SEM and TEM images of ATP, GO, and the prepared GO are shown in Figure 2. Figure 2a shows that the purified ATP exhibited a rod-like morphology and a typical fibrous structure and easily aggregated to form clusters. Figure 2b indicates that the GO exhibited a layer structure that was folded at the edges. In addition, many layers were stacked together because the chemical bond between GO layers easily led to aggregation. Figure 2c shows that ATP was uniformly intercalated on the GO surface. The presence of ATP changed the stacking behavior of pure GO, and the presence of GO resulted in the uniform distribution of ATP. The weakening of aggregate stacking and the appearance of homogenization and uniformity increased the surface area of the as-prepared GO considerably, thus producing more adsorption sites to aid in the removal of heavy metals. The results indicated that the ATP and GO were very well mixed. Figure 2d shows that GO exhibited a transparent, thin film structure and that folds were found on the edges, in agreement with the SEM results. It can be observed from the comparison of the TEM images of GO in Figure 2e that ATP was present and uniformly distributed on the GO surface, further indicating successful fabrication of the as-prepared GO composite. That is, a simple hydrothermal reaction can be used to prepare the as-prepared GO composite adsorbent.

Figure 3 shows the FTIR spectra of GO, ATP, and the as-prepared GO. In the FTIR spectrum of GO, the peaks at 1110 cm⁻¹, 1400 cm⁻¹, and 1629 cm⁻¹ correspond to the C–O–C vibration, C–OH vibration, and C–C vibration, respectively, in the sp² hybridized carbon skeleton; the peaks at 1732 cm⁻¹ and 3149 cm⁻¹ correspond to the C–O and O–H vibrations in –COOH, further revealing the presence of carboxyl, epoxide, and hydroxyl groups in GO [31]. The FTIR spectrum of ATP indicates that the appearance of the broad peak at 3550 cm⁻¹~3616 cm⁻¹ was due to the stretching vibrations of (Al)O–H and (Mg)O–H, which, however, shifted to lower wavenumbers in the FTIR spectrum of the as-prepared GO, suggesting the existence of hydrogen bonding between ATP and GO. The absorption peaks at 729 cm⁻¹ and 796 cm⁻¹ in the FTIR spectra of ATP and the as-prepared GO are attributed to the stretching vibration of Si–O. Evidently, the Si–O in ATP was transferred to the as-prepared GO [32]. Thus, ATP was successfully intercalated onto the GO surface, forming a novel adsorbent for heavy metals.

The main chemical compositions are SiO₂, MgO, Al₂O₃, and contain a certain amount of Fe₂O₃, CaO, K₂O, MnO, Na₂O, and TiO₂, which vary from places of origin [33]. Figure 4 shows the XPS spectra of GO and the prepared GO. By comparing the XPS spectrum of GO with that of the as-prepared GO, the peaks of Mg 2p, Al 2p, and Si 2p appeared at 54 eV, 78 eV, and 104 eV, respectively, in the spectrum of the as-prepared GO, indicating that ATP, a material rich in magnesium aluminum phyllosilicate, can form a composite adsorbent with GO. The elemental compositions of GO revealed the presence of C and O; however, for the as-prepared GO, in addition to C and O, the presence of Mg, Al, Si, Ca, and Fe further suggests that the reaction combined ATP and GO, thus forming a novel composite adsorption material.

3.2. Comparison of Adsorbents

To compare the adsorption properties of as-prepared GO with those of pure GO and ATP on adsorption, GO, ATP, and as-prepared GO were chosen in the adsorption experiments, as shown in Figure 5. ATP was obtained by purification; GO was used as received. Figure 5 indicates that the removal of Pb²⁺ was 308.7 mg/g as-prepared GO, compare to 124.7 mg/g GO and 72.7 mg/g ATP. GO has high adsorption capacity—it is difficult to separate from water by centrifugation—and other researchers rely mainly on filtration for separation. The adsorption capacity of ATP is far less than GO, while it easy separate by centrifugation due to its very high density. for the as-prepared GO, the high adsorption capacity of GO and the ease of separation of ATP were brought into play.

The adsorption capacity of Pb²⁺ on as-prepared GO is greater than the 62.1 mg/g adsorption on attapulgite [33], and far greater than 23~27 mg/g adsorption obtained on zeolite [34] and zeolite-type [35] absorbents.

3.3. Effect of pH

The pH affects not only the charge and state of functional groups on the surface of the as-prepared GO, but also the ionic state of Pb in aqueous solution [36]. The optimal pH is different for the adsorption of different heavy metals. The pH in the adsorption of Pb²⁺ on the as-prepared GO varied from 3 to 9; the results are shown in Figure 6. The contact time was 2 h to ensure that adsorption reached equilibrium. Figure 6 indicates that at a low pH value of 3, the oxygen-containing functional groups on the as-prepared GO surface were protonated. Consequently, because of electrostatic repulsion, the positively charged surface of the adsorbent did not complex well with Pb²⁺, thus resulting in a low removal efficiency. When the pH was increased to 5, the degree of protonation of the oxygen-containing functional groups was weak, directly resulting in an increase in removal efficiency to 90%. When the pH was increased to 6, a precipitation reaction was initiated, thus maintaining the removal efficiency at approximately 99%. In fact, the main states of lead in aqueous solution include Pb²⁺, Pb(OH)⁺, Pb(OH)₂, Pb(OH)₃⁻, and Pb(OH)₄²⁻ [37]. When pH < 6, Pb²⁺ is the primary form. Therefore, a pH value of 5 was chosen for the subsequent adsorption experiments of Pb²⁺.

3.4. Adsorption Kinetics

The effects of adsorption time on the removal efficiency of Pb²⁺ at pH 5.0 are shown in Figure 7. The adsorption of Pb²⁺ on the as-prepared GO increased significantly with an increasing reaction time of 100 min. Different adsorption kinetic models exist to describe adsorption mechanisms, including the pseudo-first-order kinetics model [38,39] and the pseudo-second-order kinetics model [40], which can be represented as follows:

$$\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}=1-e^{-k_{1}t}$$

(2)

$$\frac{t}{q_{\mathrm{t}}}=\frac{1}{k_2{q}_{\mathrm{e}}^2}+\frac{1}{q_{\mathrm{e}}}t$$

(3)

where q_e is the equilibrium adsorption capacity in mg/g, q_t is the adsorption capacity at time t in mg/g, and k₁ (min⁻¹) and k₂ (g/(mg·min)) are the rate constants of the pseudo-first-order and pseudo-second-order kinetics models, respectively.

By nonlinear fitting, the fitting parameters of the pseudo-first-order model are $k_1$ = 0.5398 min⁻¹, $q_{\mathrm{e}}$ is 358.40 mg/g and R² is 0.5202, and the fitting parameters of the pseudo-second-order model are $k_2$ = 0.001438 g/(mg·min), $q_{\mathrm{e}}$ is 371.54 mg/g, and R² is 0.8312. It can be observed that the pseudo-second-order model is a better fit to the adsorption results of Pb²⁺ by the as-prepared GO, further suggesting that the adsorption rate depends primarily on chemisorption [4]. This result reveals that in addition to the formation of chemical bonds, the transfer and exchange of electrons occur in the adsorption reaction. In the initial 5 min, chemisorption was rapid, thus leading to a rapid increase in the adsorption reaction rate.

3.5. Adsorption Isotherm

The Langmuir isotherm equation [41] and the Freundlich isotherm equation [42] were used to fit the equilibrium of the experimental data, as shown in Figure 8 and

Table 1. Parameters of Langmuir and Freundlich adsorption isotherm models for Pb²⁺ on as-prepared GO.

Langmuir			Freundlich
qmax (mg/g)	KL (L/mg)	R2	KF (mg/g)	n	R2
450.9	0.147	0.9925	116.5	3.005	0.9127

:

$$q_{\mathrm{e}}=\frac{q_{\max }{K}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}$$

(4)

$$q_{\mathrm{e}}=K_{\mathrm{f}}{C_{\mathrm{e}}}^{\frac{1}{n}}$$

(5)

where C_e is the equilibrium concentration in mg/L, q_e is the equilibrium adsorption capacity in mg/g, k_L is the constant of the Langmuir model in L/mg, q_max is the maximum theoretical adsorption capacity in mg/g, and K_f and n are the adsorption capacity and adsorption strength of the Freundlich model, respectively.

By nonlinear fitting, the fitting parameter q_max of the Langmuir isothermal adsorption equation is 450.9 mg/g with k_L = 0.147 L/mg and R² = 0.9925, while the R² of Freundlich is 0.9127 with k_F = 116.5 mg/g and n = 3.005. This result indicates that the adsorption process is in better agreement with the Langmuir isothermal adsorption equation than with the Freundlich equation and that the adsorption occurs through homogeneous monolayer adsorption. The maximum adsorption capacity is 450.9 mg/g.

3.6. Thermodynamic Parameters

The thermodynamic parameters ΔH⁰, ΔG⁰, and ΔS⁰ of Pb²⁺ adsorption on the as-prepared GO can be calculated according to the temperature adsorption isotherms. The standard free energy, ΔG⁰, can be obtained by the following equation [43]:

$$\Delta G^0=-R\mathrm{Tln}{K}_0=\Delta H^0-T\Delta S^0$$

(6)

where R is the universal gas constant, 8.314 J/(mol·K), and T is the absolute temperature in K. K₀ is the thermodynamic equilibrium constant in mg/L, which can be calculated by $q_{\mathrm{e}}/C_{\mathrm{e}}$. Based on Equation (6), the standard enthalpy change, ΔH⁰, and standard entropy change, ΔS⁰, can be obtained from the slope and intercept of the plot of lnK₀ vs. 1/T by the following equation:

$$\ln K_0=\frac{\Delta S^0}{R}-\frac{\Delta H^0}{RT}$$

(7)

By calculation, the negative values of $\Delta G^0$ are −4.19 kJ/mol at 298 K, −4.68 kJ/mol at 308 K, and −5.78 kJ/mol at 318 K, indicating that adsorption of Pb²⁺ by the as-prepared GO is a spontaneous process [44,45]. ΔH⁰ is 19.40 kJ/mol, and ΔS⁰ is 78.17 J/(mol·K), indicating that the adsorption of Pb²⁺ by the as-prepared GO is an exothermic process [46].

3.7. Regeneration and Recycling

The HCl concentration effect on the desorption of Pb²⁺ from the adsorbed as-prepared GO is shown in Figure 9. After selecting the optimum concentration, this concentration was used to investigate the adsorption and desorption rates of the adsorbents in recycling.

Figure 9 indicates that the desorption rate of Pb²⁺ by the HCl solution with a concentration of 0.2, 0.3, 0.5, 0.8, and 1.2 mol/L was greater than 91% in all cases, indicating that HCl can be used as a regenerator for the desorption of Pb²⁺ from the as-prepared GO. The desorption rate increased gradually with increasing HCl concentration; 0.5 mol/L of HCl solution exhibited the best desorption effect. When the concentration of the HCl was 0.2 mol/L, the concentration of H⁺ was low, and it could not fully exchange Pb²⁺ adsorbed on the prepared GO surface. When the concentration of the HCl solution reached 0.3 mol/L, the concentration of H⁺ increased; consequently, the exchange amount of Pb²⁺ increased, and the desorption rate improved. However, when the concentration of HCl increased to between 0.8 and 1.2 mol/L, the desorption rate was not as high, maybe because a high acid concentration resulted in destruction of the structure of the composite material, thus leading to a decrease in the desorption rate. The adsorption rate was 74.8% of the initial rate after six adsorption–desorption cycles, indicating that the prepared GO exhibits a high recycling efficiency and offers practical value for use in wastewater treatment areas.

4. Conclusions

The as-prepared GO is an easy-to-produce and low-cost heavy metal adsorbent that can be prepared by a simple hydrothermal intercalation reaction of ATP and GO. Compared to other types of adsorbents and GO-based adsorbents reported in the literature, the prepared GO exhibits excellent adsorption and separation properties. The maximum adsorption capacity of Pb²⁺ on as-prepared GO at pH 5 and 25 °C was 450.9 mg/g. The adsorption–desorption experiments indicate that high concentrations of HCl solution can destroy the structure of the composite adsorption material. Low concentrations of HCl, however, can improve the desorption rates. The composite adsorbent retains a high removal efficiency of lead ions after six adsorption–desorption cycles and can be used in the treatment and purification of heavy metal-containing wastewater.