3.1. Characterization of Graphite
Graphite has a crystal structure made up of stacked graphene layers in which the separation distance of the layers is 3.35
, whereas the separation of atoms within a layer is 1.42
. At the microscale, the starting (powder) graphite shows an irregular bulk structure with a lateral size ranging from 2
m to 50
m (
Figure 1a). The XRD pattern of raw graphite is shown in
Figure 1b. The most intense peak at
corresponds to the graphite stacking crystallinity (002) [
43]. The less intense peak at
displays the long-range order of stacked graphene layers (004) [
43].
Figure 1c,d show the Raman spectrum of raw graphite. The main features observed are: (i) the absence of the D peak demonstrating a defect-free starting graphite, (ii) the G peak at 1577
is ascribed to the C-C strC stretching mode in sp
2 carbon bonds, and (iii) the 2D peak at 2720
is characterized by two bands, the intense 2D
2A band at 2720
and a shoulder 2D
1A band at 2677
. In particular, these bands originate as the effect of the splitting of
electron bands due to the interaction between stacked graphene layers. The G* peak found at 2447
is characteristic of carbon-based materials with a graphitic-like structure.
Compared to graphite, the Raman spectrum of oxidized graphenes shows a highly broadened and very-low intense 2D peak. With this in mind, the 2D band region of GO and rGO is not analyzed here, and instead, we focus on the region from 1000 to 2000 cm−1 to scrutinize the crystallinity and, most importantly, the basal/edge defects of the obtained materials after the oxidation-reduction process (discussed below).
3.2. Characterization of GO and rGO
While, in the present work, rGO is used for the adsorption of cationic pollutants, it is extremely important to discuss its transformation from GO. SEM micrographs of GO and rGO are shown in
Figure 2a,c, respectively. The surface morphology of GO indicates a face-to-face stacking of flakes as well as randomly aggregated flakes with wrinkles and folds on the surface (
Figure 2a). Instead, rGO shows a surface morphology with mesopores and micropores and randomly organized flakes (
Figure 2c). The highly distorted porous surface of rGO can avoid the face-to-face stacking of flakes, as observed in GO.
EDS measurements were carried out to determine the elemental composition of GO and rGO, considering a bombarded region large enough. Then, the carbon and oxygen content were C: 49.7% and O: 50.3% for GO (after oxidation process) and C: 62.9% and O: 37.1% for rGO (after de reduction process). The oxygen content decreased by 26.2% using CA as an alternative green-reducing agent, confirming the (partial) removal of oxygen functional groups.
Representative TEM graphs of GO and rGO are shown in
Figure 2e,f, respectively. GO looks like a semi-transparent thin nanosheet with various wrinkles and folds on the surface and edges (
Figure 2e). The observed wrinkled/folded structure is attributed to surface defects because of the deviation from sp
2 to sp
3 hybridization as the effect of a high density of oxygen-containing functional groups [
54]. After the reduction process with CA, well-defined and impurity-free nanosheets with slightly wrinkled regions are observed in rGO, suggesting the recovery of sp
2 hybridization by the removal of functional groups. The observed regular surface allows concluding that rGO did not undergo severe in-plane disruption compared to GO.
Raman analyses were performed to further corroborate the transformation of GO into rGO (
Figure 3a,b, respectively). As is typical for oxidized graphenes, two characteristic peaks are observed in GO and rGO: (i) the D peak at
1349 cm
−1 is attributed to the breathing mode of aromatic carbon rings, which is Raman active by structural defects [
32], and (ii) the G peak at
1588 cm
−1 is due to the C-C stretching mode in the sp
2 hybridized carbon structure [
46]. A detailed analysis using Lorentz functions shows the existence of three prominent bands: the D band (yellow line), the G band (green line), and the D’ band (blue line). In particular, the D’ band confirms the presence of basal/edge defects, and a decrease in the D’ band intensity is a direct indication of GO reduction, which is observed in the Raman spectrum of rGO (
Figure 3b). On the other hand, the I
D/I
G intensity ratio can be used as an indicator of the density of structural defects in the obtained oxidized graphenes [
43]. It was found that the intensity ratio of GO (2.2) is larger than that of rGO (1.65), indicating that the size of the graphene-like domains increases after the reduction process.
The absorbance spectra of GO and rGO are shown in
Figure 3c,d, respectively. Using the Lorentzian function, GO has the main absorbance band at 230 nm (darker green line) and a shoulder band at 329 nm (yellow line), which are related to the
transitions of C-C bonds and
transitions of C=O bonds, respectively. To confirm the transformation of GO into rGO, two characteristics are needed: (i) a redshift of the main absorbance band and (ii) the loss of the shoulder band. After the reduction process, rGO only meets the first point when the main absorbance band shifts to 261 nm, but the second one is observed at 324 nm, suggesting a close content of oxygen-containing functional groups, particularly hydroxyl and epoxide groups.
The presence and type of oxygen functional groups are confirmed by the FTIR analysis (
Figure 4a). It is widely accepted that the hydroxyl and epoxide groups are attached to the basal in-plane of the graphene, whereas the carboxyl and carbonyl groups are located at the edges. The FTIR spectrum of GO shows the following bands: C-O-C at 1044
, C-O at 1222
, C
C at 1644 cm
−1, and C
O at 1729
. The broadband observed at
3426
is due to the presence of the hydroxyl groups (C-H) as well as adsorbed water molecules between GO flakes. The latter provides a hydrophilic characteristic in GO to be highly dispersible in water. It is worth noting that a higher hydrophilic property could interfere with the removal of pollutants from aqueous media, giving a poor adsorption process. After the reduction, these bands are significantly attenuated and weakened in the rGO spectrum, evidencing the removal of oxygen-containing functional groups [
33].
To determine the thermal stability of as-made oxidized graphenes and the effect on the oxygen-containing functional groups, we carried out TGA analyses on GO and rGO (
Figure 4b). In GO, the weight loss below 100 °C is ascribed to the loss of water molecules [
33]. The significant weight loss in the region of 200–300 °C is attributed to the pyrolysis of unstable molecules (such as CO, CO
2, and H
2O) [
33]. In the region of 300–600 °C, the weight loss is due to the removal of stable oxygen functional groups [
33]. Instead, rGO shows relative thermal stability, but the observed TGA curve follows a similar trend as GO, confirming a reduced density of oxygen functional groups.
Finally, the crystallinity changes from GO to rGO were revealed by XRD analysis (
Figure 4c). As mentioned, graphite is characterized by an intense crystalline peak at 2θ = 26.73° related to a lattice spacing of 0.334 nm, which corresponds to the (002) interplane distance [
43] (
Figure 1b). In GO, this peak is found at 2θ = 10.93° with a lattice spacing of 0.81 nm, indicating the oxidation of graphite. The increased interlayer spacing appears as an effect of the intercalation of water molecules and oxygen functional groups. Additionally, the very low width of this peak demonstrates an ordered stacking along the out-of-plane axis. After the reduction process, the peak becomes broader due to the partial breakdown of the long-range order, and it shifts towards higher angles, 2θ = 22°, showing a decrease in the lattice spacing (~0.39 nm) [
43].
All these facts and pieces of evidence demonstrate the transformation of GO into rGO, which will be used for the removal of cationic pollutants from aqueous media, i.e., MB and Hg(II).
3.3. Adsorption Kinetics
We begin analyzing the effectiveness of rGO for removing MB and Hg(II) from water by using the following expression:
where
and
are the initial pollutant concentration (mg L
−1) and the pollutant concentration at time
, respectively.
is the adsorbent mass (g), and
represents the volume of the aqueous solution (L). At the equilibrium, the equilibrium concentration is
, and the equilibrium adsorption capacity is
.
The removal efficiency (
) of the as-made rGO material can be defined by the following simple equation:
Figure 5 shows the adsorption kinetics of MB and Hg (II) onto rGO at 298 K considering a contact time of up to 60 min. It can be seen that rGO rapidly captures MB molecules after 30 min (
Figure 5a), while for Hg (II), the equilibrium time of adsorption is 20 min (
Figure 5b). These results highlight the effectiveness of rGO for removing cationic pollutants from aqueous solutions compared with conventional benchmark sorbents [
35,
36,
37,
38,
39,
40]. In particular, the effectiveness of rGO can be attributed to the recovered surface area after the reduction process as well as the presence of oxygen functional groups.
The parameters of the adsorption kinetic process were determined by the pseudo-first-order model and pseudo-second-order model. Specifically, Tene et al. stated that the first model assumes that the rate of change of the adsorption capacity is proportional to the concentration of available active sites per unit mass of adsorbent material [
25], whereas Arias et al. stated that the second model assumes that the rate of change of the concentration of occupied active sites per unit mass of the adsorbent material is proportional to the square of the concentration of free active sites per unit mass of sorbent [
13].
The pseudo-first-order model (red line) and pseudo-second-order model (blue line) can be described as follows:
and
where
and
are the pseudo-first-order and pseudo-second-order rate constants, respectively. The estimated parameters of the adsorption kinetics are summarized in
Table 1.
A close picture of the pseudo-first-order and pseudo-second-order parameters shows that, in the case of MB, the calculated values of the equilibrium adsorption capacity ( mg g−1 and mg g−1, respectively) are very close to the experimental value ( mg g−1). In the case of Hg(II), the calculated adsorption capacity ( mg g−1) from the pseudo-first-model is close enough to the experimental value ( mg g−1). However, the pseudo-second-order model overestimates the equilibrium adsorption capacity ( mg g−1). By the comparison of SSE and R2 metrics, the adsorption kinetics of MB onto rGO are more in line with the pseudo-second-order model (SSE = 5.24, R2 = 0.999), whereas the adsorption kinetics of Hg(II) onto rGO are more in line with the pseudo-first-order model (SSE = 1826, R2 = 0.949).
3.4. Intraparticle Diffusion Study
The diffusion process of any pollutant into porous solid materials, such as our rGO (
Figure 2c), mostly involves several steps characterized by different rates. This fact can be calculated by the intraparticle diffusion (IPD) model [
13,
25], which is given by the following expression:
where
is the intraparticle diffusion rate constant (g mg
−1 min
−1), and the intercept
reflects the boundary layer or surface adsorption. The respective plot and estimated parameters of the IPD model are shown in
Figure 6 and
Table 2.
As Ofomaja et al. [
55] stated, the larger the intercept value, the greater the contribution of the surface in the adsorption process. Indeed, the values observed in MB (
) and Hg(II) (
) indicate that a greater amount of surface adsorption occurred, leading to a decrease in the rate of diffusion of MB molecules and Hg(II) ions from the adsorbent external surface to the adsorbent internal structure. From the linearized plot of the IPD model, different regions are observed: (i) the initial region (faster stage) is related to the movement of the pollutant from the solution to the rGO surface, (ii) the second region (intermediate stage) is related to the gradual diffusion of the pollutant into the large pores of the rGO structure, and (iii) the final region (lower stage) involves a very slow diffusion of the pollutant from larger pores to smaller ones.
Interestingly, the adsorption mechanism of MB on rGO is characterized by only two regions, regions I and II (
Figure 6a), while all three regions are observed when Hg(II) becomes adsorbed onto rGO. In light of understanding this fact, we hypothesize that the size of pollutants plays a significant role in the diffusion procedure, i.e., as the MB molecules exhibit larger sizes compared to Hg(II) ions, MB cannot reach region III, particularly from larger to smaller pores.
The initial adsorption factor (
) can be estimated to further understand the above-mentioned regions (
Table 2) as follows:
where
is the final adsorption amount at the longest time. In the MB-rGO system, the estimated
value is much less than 0.5, which confirms that most of the adsorption of MB occurs on the surface of rGO. In contrast, for the Hg(II)-rGO system, the value of
0.49 indicates a limit between the strong initial adsorption (related to region I) and intermediate initial adsorption (related to region II), which means that the adsorption process of Hg(II) ions could occur at almost the same time in both regions.
3.5. Adsorption Isotherms
Adsorption isotherms were carried out to analyze the interaction between MB molecules or Hg(II) ions and rGO considering a contact time of 30 min for MB and 20 min for Hg(II). The experimental data can be fitted using the Langmuir model and Freundlich model using the following equations, respectively:
and
where
represents the maximum adsorption capacity (mg g
−1),
is the Langmuir constant (L g
−1),
is Freundlich constant (mg L
−1), and
is the surface heterogeneity of adsorbent material. The corresponding results and estimated parameters at different temperatures (298, 313, 333 K) are shown in
Figure 7 and
Figure 8 and
Table 3 and
Table 4.
Taking the high correlation R
2 values (
Table 3 and
Table 4), it can be seen that the measured points are more in line with the Langmuir model. Although the temperature does not dramatically modify the chemical composition of rGO at temperatures below 100 °C (
Figure 4b), it seems to be an important parameter in the adsorption process because when the temperature increases, in the case of MB on rGO, a slight decrease in the maximum adsorption capacity is observed from 121.95 to 107.53 mg g
−1. In contrast, in the case of Hg (II) on rGO, a significant increase in the maximum adsorption capacity is detected from 109.49 to 255.04 mg g
−1. The temperature is a key point to be considered if rGO is used to treat water or wastewater at an industrial scale.
From the Freundlich model, the estimated values of
for MB-rGO (
Table 3) or Hg(II)-rGO (
Table 4) systems indicate that the adsorbent heterogeneity tends to be homogeneous as the temperature rises. Indeed, values of
close to zero (<0.1) indicate strong surface heterogeneity. The affinity of the tested cationic pollutants for rGO can also be determined by the
parameter, where the estimated values were found to be much less than 0.1, suggesting a good affinity of rGO to capture cationic pollutants, i.e., MB molecules and Hg(II) ions. However, this statement motivates more extended work for testing more cationic and non-cationic pollutants.
3.6. Effect of pH and Initial Concentration
To scrutinize the effect of the pH on the process of cationic pollutant removal, the experiments were carried out at different pH values ranging from 2 to 12 and setting the temperature at 298 K.
For MB on rGO (
Figure 9a), the adsorption increases, starting from a removal percentage of about 76% at pH = 3 up to 92% at pH = 6. The removal percentage remains relatively constant from pH = 6 to pH = 8. The removal percentage decreases down to 83% for pH
10. To understand this fact, the effect of pH can be divided into three different regions: (i) from pH = 2 to pH = 4, the acid region is rich in cations which are captured together with the cationic MB molecules; (ii) the (relatively) neutral region from pH = 6 to pH = 8 is free from cations in the medium, and therefore, only the cationic dye molecules are captured by rGO; and (iii) the basis region (pH
10) is characterized by an excess of OH
− ions that interact with the cationic dye molecules, remaining suspended in the aqueous media.
For Hg(II) on rGO (
Figure 9b), a removal percentage of about 39% at pH = 2 is observed, and the maximum removal percentage is found at pH = 10 (
). The removal percentage remains relatively constant for pH
6, with an average value of 76.58%. To understand these results, a similar description can be given: (i) for pH
4, the cations in the acidic medium fight with the mercury cations for the active sites of rGO; (ii) in the neutral region, mercury cations easily reach the active sites of rGO; and, interestingly, (iii) for pH
8, mercury cations sometimes prefer to interact with the active sites of rGO rather than OH
− ions due to the variation of the removal percentage when the pH increases.
The adsorption capacity (
) of rGO increases quite linearly with the initial concentration of MB in solution (
), almost in the range from 10 to 80 mg L
−1; however, at high concentrations (
90 mg L
−1), a deviation from linearity does occur (
Figure 10a). Similarly, the
values of rGO increase linearly with the initial concentration of Hg(II) in the solution, from 10 to 50 mg L
−1, and at concentrations
50 mg L
−1, a deviation from linearity is also observed (
Figure 10b). These results suggest that rGO has a finite amount of active adsorbent sites, which is fixed by its quality and the experimental conditions, such as temperature, pH, and solution volume/adsorbent mass ratio. To further emphasize, at the beginning of the adsorption process, rGO has a vast number of active sites, increasing the
value as long as free active sites are available on rGO. Then, if all the active sites are involved, the saturation, and therefore the maximum adsorbent capacity (
), is attained [
13,
25].
The adsorption effectiveness of rGO—defined as the percentage of cationic pollutant removal from water—is almost independent of in the adsorption of MB onto rGO, assuming an average value of 89.21%. Interestingly, a clear dependence on is observed for the adsorption of Hg(II) onto rGO, i.e., an abrupt drop from 92.89% ( mg L−1) to 48.85% ( mg L−1), giving an average value of mercury removal of 72.93%.
3.7. Adsorption Thermodynamics
To acquire information about the energy changes due to the involved adsorption process [
13], the Gibbs free energy (
), enthalpy change (
), and entropy change (
) were calculated by the following expressions:
where
represent the distribution coefficient [
13].
and
were calculated from the slope and intercept of Van’t Hoff plot of
as a function of
[
25]. The Van’t Hoff plot and estimated parameters are shown in
Figure 11 and
Table 5, respectively.
The negative
values observed at different temperatures indicate spontaneous adsorption of MB molecules and Hg(II) ions onto the rGO surface. It is worth noting that, for
values in the range from 0 to −20 kJ mol
−1, the adsorption process is assigned to physisorption or multilayer adsorption [
25], while in the range from −80 to −400 kJ mol
−1, the adsorption is assigned to chemisorption or monolayer adsorption [
25]. The region from −20 to −80 kJ mol
−1 remains unclear, and a combined adsorption process can be assumed. With this in mind, the estimated
values for MB onto rGO (−22.75, −23.81, and −25.16 kJ mol
−1) and Hg(II) onto rGO (−39.84, 31.55, 32.97 kJ mol
−1) suggest that the adsorption process of tested cationic pollutants on rGO is governed by a mixed physisorption–chemisorption process. Interestingly, for MB on rGO, the
value increases by 5% at 313 K and by 10% at 333 K. In contrast, an inversely proportional relationship is observed for Hg(II) on rGO; say, the
value decreases by 21% at 313 k and by 17% at 333 K.
The negative
values indicate the exothermic nature of the adsorption process, i.e., a negative enthalpy implies that the temperature increase had a negative impact, particularly on the adsorption of MB (
kJ mol
−1). However, in the adsorption of Hg(II) on rGO, the value observed (
kJ mol
−1) is very small and could be considered negligible since increasing the temperature significantly increases the maximum adsorption capacity of rGO, as evidenced by the Langmuir model (
Figure 8a and
Table 4). The positive values of
kJ mol
−1 · K
−1 and
kJ mol
−1 · K
−1 corroborate the affinity of MB molecules and Hg(II) ions toward the rGO surface.
3.8. Final Remarks
Table 6 shows the estimated
values for MB (
mg g
−1) and Hg(II) (
mg g
−1) at 298 K, which are compared to those of recent studies.
The estimated value of the dye adsorption is higher than those previously reported and only surpassed by GO reduced by Citrus hystrix ( mg g−1), suggesting that as-made (non-extra functionalized) rGO are excellent platforms to replace conventional adsorbent materials. In the case of heavy metal adsorption, the estimated value is higher than some functionalized/decorated GO and rGO. However, S-GO seems to be more profitable to be used for the removal of mercury ( mg g−1), but this is due to the fact that, obviously, the presence of sulfur improves the affinity and specificity for Hg (II) ions in any adsorbent material.
Although, in the present work, the regeneration of rGO was not studied, which motivates more extended work, we propose the following well-known techniques or processes: (i) the adsorbed rGO-pollutant system can be separated from aqueous media by filtration using filters with a pore size less than 1 µm since rGO is within the order of few micrometers, (ii) pollutant can be released from rGO by applying the concept of ionic force, i.e., by applying buffer solutions, and (iii) the isolated MB molecules or Hg(II) ions can be extracted by sulfide precipitation.