Electrically accelerated removal of organic pollutants by a three-dimensional graphene aerogel

Materials

Graphite powder, natural briquetting grade, ∼100 mesh, 99.9995% (metals basis) was purchased from Alfa Aesar, USA. Analytical grade NaNO₃, KMnO₄, 98% H₂SO₄, and 30% H₂O₂ aqueous solution were commercially available and purchased from Beijing Chemical Reagent Factory, China. AR88, O-II, and sodium ascorbate were purchased from Sinopharm Chemical Reagent Co., China. MB was purchased from Aladdin Industrial Corporation, China. Table S1 presents the chemical structures of these three dyes.

Preparation of graphene oxide (GO)

GO was prepared by oxidation of graphite powder according to a modified Hummers’ method.[20, 21] Briefly, graphite powder (0.5 g) was first oxidized using NaNO₃ (0.5 g) and KMnO₄ (3 g) in sulfuric acid (23 mL) at 0°C, then the temperature was raised to 35°C and held for 1 h. Subsequently, 40 mL deionized water was added and the mixture was further heated at 90°C for 30 min. After that, the water bath system was removed, and deionized water (100 mL) and H₂O₂ (5 mL) were added. The above processes were performed under magnetic stirring and the temperature was controlled by water bath. Finally, the solution was cooled to room temperature and centrifuged at 5000 r/min for 15 min. After centrifugation, the solid was washed with deionized water several times, until the pH of the supernatant was neutral.

Fabrication of 3DG

The as-prepared GO formed a stable suspension in water by ultrasonication for 30 min. Subsequently, the GO aqueous dispersion (25 mL, 1 mg/mL) and sodium ascorbate (200 mg) were placed in a 50 mL glass vial. After ultrasonication for about 5 min to dissolve the reducing agent, the as-prepared suspension was heated at 90°C for 1.5 h to prepare the graphene hydrogel. The whole synthesis process is described in Figure 1a.

The excess sodium ascorbate was further removed by dialysis. Finally, the prepared graphene hydrogel was freeze-dried (FD-1B-50 Boyikang Co., Beijing, China). Then, the 3DG aerogel was obtained, and a titanium wire was fixed in the 3DG aerogel.

Characterization of 3DG electrodes

The morphology and structure of 3DG were characterized by digital camera (SONY NEX5C, Japan), scanning electron microscopy (SEM with gold coating, Hitachi S-570, Japan), x-ray diffraction (igaku D/MAX-Ra x-ray diffractometer with Cu K_α radiation), and x-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA. Al K_α as the x-ray source). The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution of 3DG were determined by N₂ adsorption–desorption (Quantachrome Instruments). The pH_pzc (the point of zero charge) was measured by pH meter (PB-10, Sartorius Group). To explore the electrochemical characteristics of the electrosorption system, cyclic voltammetry (CV) was performed on an electrochemical workstation (CHI1030B). Fourier transform infrared (FTIR) spectra were recorded using FTIR spectrophotometer (Aratar, ThermoNico-Let, USA).

Adsorption and electrosorption experiments

All of the adsorption experiments were performed in a 150 mL conventional two electrode system at room temperature with stirring (Figure S1). To study the effect of pH on the adsorption, the solution pH was adjusted to specific values using 1 M H₂SO₄ and NaOH solutions. Based on the results of pH effect study (Figure S2), the pH value was controlled at about six for all experiments. The concentrations of AR88, O-II, and MB were determined based on the UV-Vis spectra at wavelengths of 504, 484, and 664 nm using a UV-Vis spectrophotometer (UV-2450, Shimadzu, Kyoto, Japan).

Isotherm experiments of electrosorption were performed in batch equilibrium experiments with AR88 and O-II at 0.6 V, and MB at −0.6 V. At pH 6, 0.07 g of the 3DG was added into 100 mL solutions of dyes with initial concentrations of 40–200 µM. After stirring for 72 h, the dye solution was sampled and the concentration was analyzed. As a comparison, isotherm experiments of open circuit (OC) adsorption were conducted at the same conditions as electrosorption.

Adsorption kinetics of dyes (5 µM) were investigated in 100 mL aqueous solution containing 1 mM Na₂SO₄ with stirring, and the concentrations of dyes at different times were analyzed to determine maximum adsorption capacity. The electrosorption kinetics of these three dyes (5 µM) were carried out in the same solution as above. The electrode potential was kept at 0.6, 0.3, and −0.6 V, and kinetic data was recorded in 20 min intervals during this process.

Dispersion corrected DFT calculations

Hybrid DFT calculations, which include DFT-D correction, were used to describe the interactions of different dyes with graphene.[22] Spin polarized DFT calculations were performed for non-periodic clusters using G09 program. AR88–3DG nanosheet, O-II–3DG nanosheet, and MB–3DG nanosheet systems were individually studied at the wB97XD/6-31G (d, p) level of calculations. The adsorption energies, the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap, and the dipole moments for the interactions were also studied.

Characterization of 3DG aerosol electrode

GO is hydrophilic and can be well dispersed into water to form a stable suspension. The reduction of GO by sodium ascorbate under mild conditions results in the formation of hydrophobic graphene. Subsequently, the process of self-assembly occurs, likely driven by the hydrophobic and π–π stacking interactions of graphene. The resulting 3DG was obtained with ∼3 cm length and 0.5 cm diameter, and the SEM images show that it has porous structure (Figures 1a,b).

To probe the transformation process from GO to the final 3DG, Raman spectroscopy and XPS were used. Raman spectra of GO, 3DG, and graphite show that there are two prominent peaks corresponding to the D peak and the G peak, respectively (Figure 1c). The D peak originates from the stretching vibration of sp³ carbon atoms, which indicates defects and disorders, whereas the G peak originates from the stretching vibration of sp² carbon atoms. The spectra of graphite shows the G peak located at 1583 cm⁻¹. For GO, the D and G peaks are located at 1367 and 1580 cm⁻¹, while for 3DG, the D and G peaks shift to 1359 and 1588 cm⁻¹, respectively. From GO to 3DG, the intensity ratio of D/G increased, indicating a decrease in the average size of the sp² carbon domains, which is caused by the increased number of smaller graphitic domains formed during the chemical reduction process.[15] However, the existence of D band predicts the surface defect of 3DG.[23] The reduction of GO could also be confirmed directly by XPS measurements. GO shows C/O molar ratio of 1.3 and this value increases to 3.8 after the reduction process, indicating that most of the functional groups are eliminated (Figure 1d). The peaks of oxygen-containing functional groups at CO (hydroxyl and epoxy, 286.5 eV) and CO (carbonyl, 287.9 eV) decreased (Figures 1e,f), confirming that GO was reduced during the formation of 3DG.[21, 23, 24]

The specific surface area of the 3DG was calculated to be 211.5 m²/g based on N₂ adsorption–desorption isotherms. This large specific surface area of 3DG was crucial for its high adsorption capacity. The density of the obtained 3DG was found to be 22 mg/cm³. The average pore diameter of 3DG from pore size distribution curve (Figure S3) was 4.039 nm, which is in the mesoporous range. According to the electrical double-layer model, micropores could increase electrical double-layer overlapping effect and macropores could reduce surface area of adsorbent. Therefore, the presence of both micropores and macropores would decrease electrosorption capacity,[8] while the mesoporous 3DG possesses advantages in the electrosorption of pollutants.

The point of zero charge (pH_pzc) of 3DG electrode, which is the pH of the solution when the net charge on 3DG electrode surface is zero, was 3.63 (Figure S3). When pH > pH_pzc, the surface charge of the electrode is negative, and when pH < pH_pzc, the surface charge of the electrode is positive.[8]

To investigate the electrochemical stability of the three dyes, CV measurement was carried out. CVs of different dye molecules in 1 mM Na₂SO₄ aqueous solution was performed at a scanning rate of 5 mV/s in the scanning range of −0.65 V to 0.65 V (Figure 2). No differences were observed from the CV, indicating that the three dyes were electrochemically stable in the electrochemical window tested.

Adsorption and electrosorption isotherms

Adsorption and electrosorption capacities and behaviors of dyes on the 3DG electrode can be illustrated by adsorption and electrosorption isotherms. These isotherms were described by the well-known Langmuir and Freundlich models (see Supporting Information for details), and the correlation parameters are shown in Table 1.

By fitting the data, we found that the R² values of AR88, O-II, and MB were greater for Freundlich model than Langmuir model, suggesting the multilayer coverage of 3DG surface on OC adsorption. The adsorption isotherms of AR88, O-II, and MB on 3DG at OC conditions clearly show that 3DG had the highest adsorption capacity toward MB (Figure 3a). At an equilibrium concentration of 20 µM, adsorption capacities of 375.01, 380.12, and 591.16 µmol/g were determined for AR88, O-II, and MB, respectively.

To measure the electrosorption capacities of dyes on 3DG electrode, the electrosorption isotherms were obtained for negatively charged dyes (AR88 and O-II) at 0.6 V, and the other conditions were the same as OC. Table 1 shows the correlation fitting parameters. The q_m value for O-II was 598 µmol/g, about 1.2 times higher than that under OC conditions. Moreover, the q_m value for AR88 was estimated from the plateau reached on Freundlich curve as 492 µmol/g, about 1.1 times higher than that under OC conditions. Therefore, the electrosorption capacities of negatively charged dyes on 3DG surface were greater than adsorption by electrosorptive attraction. To further investigate whether positively charged dyes exhibited the same phenomenon, the electrosorption isotherm for MB was obtained at −0.6 V. The q_m value of adsorption was estimated from the plateau reached on Freundlich curve as 771 µmol/g, which was higher than that under OC conditions, as expected. Therefore, electrosorption could indeed improve adsorption capacity. In addition, we found that 3DG had the highest adsorption capacity for MB, by either OC adsorption or electrosorption (Figure 3), followed by O-II and AR88, while the sizes of dye molecules were in the order of AR88 > O-II > MB. The smaller molecules would find more pores accessible for adsorption, which explains the highest adsorption capacity for the smallest dye, MB, by 3DG.[6]

The Gibb's free energy (ΔG⁰), entropy (ΔS⁰), and enthalpy (ΔH⁰) changes for AR88, O-II, and MB adsorption have also been determined. All three thermodynamic parameters are tabulated in Table S2. The negative amounts of ΔG⁰ at different temperatures and the positive amount of ΔH⁰ for the three dyes revealed the spontaneous and endothermic nature of adsorption. The positive value of ΔS⁰ suggests increased randomness at the solid solution interface during the adsorption of dyes onto 3DG. A similar trend was observed for chlorophenol adsorption onto activated carbon fiber and resin.[25, 26]

The maximum adsorption capacities of 3DG obtained in this work, compared against various other adsorbents previously studied for the adsorption of AR88, O-II, and MB, are listed in Table 2. Although it is not based on a comprehensive survey, Table 2 shows that 3DG has a higher capacity for AR88, O-II, and MB sorption than many other materials reported previously, including activated carbon. This clearly indicates that the 3DG-based sorption process is an efficient and cost effective technology for the treatment of dye wastewater.

Adsorption and electrosorption kinetics

Adsorption kinetics were investigated to determine the time necessary for reaching equilibrium, and to elucidate the mechanism of the adsorption process. As shown in Figure 3, both rate and extent of adsorption of the dyes increase in the order of AR88 < O-II < MB.

The electrosorption kinetics were measured at 0.6 V, 0.3 V, and −0.6 V for AR88 and O-II to investigate the effects of different potentials toward adsorption. Fast transfer of AR88 and O-II to the surface of 3DG electrode was observed in the first 100 min either by OC adsorption or electrosorption. This was followed by a slow diffusion of the adsorbed dye molecules to internal sites of 3DG electrode (Figure 4). Compared with the initial dye solution, we could clearly observe that the color of the solution changed after the first 100 min of adsorption (Figure S4). Almost complete removal of the dye from solution was achieved at 0.6 V within 200 min for AR88, and 150 min for O-II, shorter than the OC adsorptions. The kinetics plot shows two main stages. The first portion (stage 1) can be attributed to the diffusion of dyes through the solution to the external surface of 3DG. The external surface sorption (stage 1) shows influence of the applied voltage on the process rate. While for longer times, >100 min, the second portion (stage 2) describes the gradual adsorption stage, where intraparticle diffusion is rate limiting. At this stage, the transfer is much less intense due to lower driving force, that is, the equilibrium concentration on the adsorbent surface.

To examine the controlling mechanism of adsorption processes, the pseudo-first-order equation and pseudo-second-order equation were used to simulate the electrosorption kinetics (the equations are described in Supporting Information).[27] The kinetic parameters (Table S3) revealed that they can be simulated well by the pseudo-second-order model (R² > 0.99), indicating the coexistence of chemisorption and physisorption.[28] The initial electrosorption rates were found to be 2.35 times (AR88) and 3.68 times (O-II) higher at 0.6 V than that for OC adsorption, and 1.94 times (AR88) and 2.28 times (O-II) of 0.3 V compared to OC adsorption. This indicates that the fast initial transfer can be improved under a low positive potential and further increase with increasing positive potential from 0 to 0.6 V. Conversely, for negative potential of −0.6 V, the initial sorption rates were 4.69 µmol/h/g (AR88) and 8.67 µmol/h/g (O-II) (Table S3), lower than for OC adsorption, indicating that there is some electrostatic repulsion between the two dyes and 3DG electrode.

Comparing AR88 with O-II, we found that the initial rates of adsorption and electrosorption were higher for O-II than AR88. AR88 and O-II have the same charge, but the molecular size of AR88 is greater than that of O-II. This indicates that the adsorption and electrosorption rates are associated with the adsorbate molecular size. Thus, the experimental results showed that the smaller molecules would have higher adsorption and electrosorption rates under the same conditions. A similar argument was found in the works of Ocampo-Pérez et al.[30]

Mass transfer analysis

For a solid–liquid sorption process, the solute transfer is usually characterized by external mass transfer or intraparticle diffusion, or both. The adsorbate molecules first move from bulk of the liquid to the external surface of adsorbent (film diffusion) and then from surface of adsorbent into the interior of the adsorbent (particle diffusion); and finally get adsorbed on the interior of the porous adsorbent.[29] Generally, the last step is the equilibrium reaction and it is very rapid; the resistance is hence assumed to be negligible. The slowest step determines the rate-controlling parameter in the adsorption system. However, the rate-controlling parameter might be distributed between intraparticle and film diffusion mechanisms.

To identify the mechanism involved, the kinetic data were analyzed using two mass transfer diffusion models, namely, external mass transfer diffusion and intraparticle diffusion models (the related equations are described in Supporting Information). The calculated diffusion parameters obtained from the two mass transfer diffusion models are given in Table 3. The values of the intraparticle diffusion rate constant K_t (mg/[g min]) are found to be improved under a slight increase in the positive potential for AR88 and O-II (Table 3). The C value was obtained from the intercept and it gives an indication of the thickness of the boundary layer. This value increased as the potential increased from 0 to 0.6 V, suggesting that the electrochemical assistance promoted the boundary layer diffusion effect. Furthermore, the intraparticle diffusion plot does not pass through origin, which indicates that both film diffusion and intraparticle diffusion partially affect mass transfer.[30] According to Table 3, ß_L at 0.6 V is 3.46 × 10⁻⁵ cm/s for AR88 and 4.28 × 10⁻⁵ cm/s for O-II. This value was found to be 1.6 times (AR88) and 2.2 times (O-II) higher than that for OC adsorption. However, the values of correlation coefficient were lower than 0.82. This implies that film diffusion did not dominantly control the overall adsorption process, confirming that both the film diffusion and intraparticle diffusion were important for the removal of dyes from solution.

Electrosorptive behavior of the dyes with different charges

As the AR88 and O-II dyes are negatively charged, we can infer from the above analysis that electrosorption affects negatively charged dyes. To determine whether the electrosorption of positively charged dyes had the same results under different potential, we chose a positively charged dye, MB, for comparison. Since MB is positively charged, the negative potential was more advantageous for electrosorption. Obviously, the initial adsorption rate of MB was 1.07 times of −0.6 V than OC adsorption, and 1.78 times of OC than 0.6 V adsorption (Figure 4c). The results indicated that electrosorption could improve the adsorptions of either negatively or positively charged dyes, depending on the potential applied. The UV-Vis spectra confirm that the structures of dyes are intact and there was no shift in the adsorption bands during the electrosorption process (Figure S4).

To confirm the effect of potential on the adsorption of dyes onto 3DG, the adsorption experiments with AR88, O-II, and MB were conducted at OC for the first 15 min, followed by negative and positive potentials for the next 20 min. Obviously, when applying the positive potential for AR88 and O-II or the negative potential for MB after 15 min, the dyes are adsorbed on the surface of 3DG at a fast rate (Figure S6). Electrostatic attraction can change surface charge-density on the 3DG surface, and consequently increase the initial adsorption rates and adsorption extent.[6, 31] However, when applying the negative potential for AR88 and O-II or the positive potential for MB, the dyes did not desorb from the electrode. On the contrary, the dyes were further adsorbed on the surface of electrode at slow rates. The reason may be that the electrostatic repulsive force between the surface of the 3DG electrode and dyes was not sufficient to overcome the attractive forces between them.

Adsorption mechanism of the dyes on 3DG surface

To probe the interactions of dyes with the 3DG nanomaterial, the FTIR spectra of 3DG after adsorption of dyes were analyzed (Figure S7). The peaks located at 1048 and 1408 cm⁻¹ corresponding to SO₃Na and NN,[32] gave direct evidence that AR88 and O-II were adsorbed on 3DG surface. The peaks for CC, CN, and CO were located in the range of 1500 to 1690 cm⁻¹. The peaks corresponding to the skeletal vibration of CC bonds shifted from 1637 to 1641, 1647, and 1641 cm⁻¹ after adsorption of AR88, O-II, and MB onto 3DG surface, respectively. This confirms that π–π interactions play an important role in the adsorption process between graphene and aromatic organic pollutants.[17, 33] Additionally, oxygen-containing functional groups still remained in 3DG, as evident by the peaks of the carbonyl CO stretching vibration at 1726 cm⁻¹ and CO stretching of ether groups.[33, 34] These results are in accordance with the XPS analysis.

The surface chemistry of the adsorbents is an important factor that controls the adsorption of adsorbate. The surface of 3DG was negatively charged when the pH of the solution was higher than the pH_pzc. The AR88 and O-II dyes are also negatively charged, thus, a slight electrostatic repulsion between the two dyes and 3DG surface may occur. However, the experimental results showed that there was a strong interaction between the surface of the 3DG and the dyes. Given the aromatic structure, the main type of attractive force involved during the adsorption is the π–π bonding between polycyclic aromatic rings and the 3DG surface.[15] Therefore, this π–π bonding attractive force together with van der Waals’ force and possible H-bonding interactions between dyes and 3DG overcome the unfavorable electrostatic interactions.[31] Although the electrostatic interactions between AR88, O-II, and the 3DG surface were not expected at pH 6, they were present in the case of MB, which is a cationic dye.[6] Thus, the electrostatic attraction may also contribute to the adsorption of MB onto 3DG surface in addition to π–π bonding, van der Waals’ force and possible H-bonding between them.

The adsorption of AR88, O-II, and MB on the 3DG sheets can be theoretically modeled by DFT-D calculations. The calculations can shed light on the nature of interactions between the graphene network and adsorbates. Figure 5 describes the geometries of AR88, O-II, MB, and dyes adsorbed onto 3DG nanosheet. After dyes were adsorbed onto 3DG nanosheet, the flat structure of graphene sheet became bent, and the aromatic rings of AR88 and O-II were almost in the same plane, illustrating that there were strong π–π interactions between the adsorbates and 3DG surface. Considering the main interactions are π–π interactions, the conditions of inclined and vertical adsorption were not considered in our calculations. The main goal of the calculations was to estimate the face-to-face interactions between the adsorbates and 3DG surface.[15] The calculated parameters are displayed in Table 4.

The adsorption energy values were 55.5, 49.5, and 79.4 kcal/mol for AR88, O-II, and MB, respectively, indicating that there are strong interactions between MB and 3DG. The adsorption energies are governed by the atomic correlation and the effective contact area between adsorbates and 3DG. The dye AR88 has four aromatic rings, one more than O-II. Thus, the π–π bonding interaction between AR88 and 3DG is stronger than that between O-II and 3DG. These results are in good agreement with the calculated adsorption energies. However, AR88 has more number of aromatic rings than MB, and the adsorption energy of MB is higher than that of AR88; therefore other strong interactions between them, besides π–π interaction, may exist. The HOMO and LUMO before and after adsorption of dyes were changed (Table 4). The HOMO–LOMO energy gap is related to reactivity, and a smaller HOMO–LOMO energy gap leads to a higher reactivity.[35] Therefore, the reactivity of the three dyes follows the order: AR88 < O-II < MB. After dyes were adsorbed on 3DG, the HOMO–LOMO energy gap of MB–3DG was higher than those of AR88–3DG and O-II–3DG, which further validates that there were strong interactions between MB and 3DG. This calculation provided us a better understanding of why MB could be quickly adsorbed onto the surface of 3DG from microscale view of point. The dipole moment correlated well with polarizability.[36] The 3DG sheet exhibited zero dipole moment, and after adsorption of dyes, the dipole moment changed by different degrees. Thus, the adsorption of dyes on 3DG induced significant polarizability, which in turn affected the charge transfer and conducting properties.[22]

Another possible mechanism involved in this variation of electrosorption behavior, specifically in the kinetics, may be the electrical effect on the mass transfer. In the smaller molecule MB, the voltage has a much lower effect on the adsorption rate, but a larger effect on the isotherm. These observations indicate that the mass transfer phenomenon within the porous structure of 3DG limits the whole process. MB exhibits faster adsorption rate, less enhanced by voltage, due to more advantageous mass transfer. Conversely, transfer of larger molecules can be more enhanced by the voltage applied.