3.1 Physicochemical characterization of solutions and nanomaterials
Table 3 presents the pH values of the systems in contact with the reference hydrogel PA as well as the zeta potential value of the MG solution. The MG solution does not influence the pH value after contact with the hydrogel for 24 h. The stability of the hydrogel is also observed when measuring the pH values of the deionized water when in contact with the material, with the value varying from 6.30 to 6.25. The zeta potential value measured from the MG solution confirms the presence of malachite green cation since it presented a positive value of +9.85 ± 2.95 mV. The zeta potential values for the carbon nanomaterials at pH = 4.5 show negative and positive values that are intermediate between those at pH = 6 and 3, as shown in Table 3.
Table 3. pH values of systems in contact with the hydrogel and zeta potential value of the MG solution and carbon nanomaterials at pH = 4.5.
System
|
pH*
|
Zeta Potential/mV
|
Hydrogel/Deionized water
|
6.30
|
-
|
MG Solution
|
4.51
|
+9.85 ± 2.95
|
GO
|
|
-34.99 ± 0.27
|
GOA
|
|
+28.08 ± 0.90
|
CNTO
|
|
-39.43 ± 0.74
|
CNTOA
|
|
+17.89 ± 0.19
|
* Measurements of pH gave similar results after initial contact with hydrogel and after 24 hours of contact with hydrogel
3.2 Physicochemical characterization of polyacrylamide hydrogels
TG/DTG, DSC, FTIR and SEM were performed in samples of hydrogel with 0.5% of the various nanomaterials, in addition to the reference, to access a general understanding of structure and morphology, which may help to explain the adsorption of the dye.
Figure 2a shows the FTIR spectra for samples PA, GO-0.50, GA-0.50, NO-0.50 and NA-0.50. All spectra are similar, indicating that the content of nanomaterial used in the hydrogel nanocomposites did not provoke changes associated with interactions that can be probed by the FTIR-ATR measurement. As highlighted in the figure, the broad band centered at 3335 cm-1 refers to the stretching of the N-H bond. The band present at 2935 cm-1 refers to the asymmetric vibration of the C-H bond. The band present at 1654 cm-1 is characteristic of amide carbonyl. Finally, the band present at 1416 cm-1 is assigned to C-N binding stretching [21], [22].
In the TG and DTG curves presented in Figures 2b-c, as well as in the other characterizations, there is no significant difference between the analyzed samples. Hydrogels are fully degraded at a temperature of approximately 600 °C. The DTG curves presented in Figure 2c present three peaks at approximately 280, 370 and 520 °C that can indicate the temperatures at which loss or degradation in the maximum intensity of the different functional groups present in the hydrogels occurs. Taking these peaks into account, the TG curves can be divided into four temperature ranges: from room temperature to 200 °C, from 200 to 305 °C, from 305 to 420 °C and from 420 to 800 °C. The first interval, which shows a 12% decrease in mass, is associated with water loss, the second and third intervals show a mass loss of 12% and 29%, respectively, which may be associated with the loss of ammonia and water by the imidization and dehydration reactions, respectively, the last interval presents the mass loss of the last 47%, related to the degradation of the main carbon chain of the hydrogels[23], [24].
Figure 3 shows SEM micrographs for the PA, GO-0.50, GA-0.50, NO-0.50 and NA-0.50 samples. In general, hydrogels have pores with similar shapes and orientations, differing in terms of diameter and distribution. Pores have a wide range of diameters, from hundreds of nm to dozens of μm [25], [26].
3.3 Swelling test
Figure 4 presents the swelling data for all samples. It is possible to note that, in general, the values of swelling ratios are similar for all hydrogels. After 34 h of contact, the highest swelling ratio values are those for samples NO-0.10 and GO-0.25, which are 1172 and 1162%, respectively. The lowest values are those of the GO-0.10 and GA-0.25 samples, which are 1016 and 1033%, respectively.
The obtained swelling values are within the common range found in the literature. Ismail and Kocabay [27] showed 850% swelling for a polyacrylamide hydrogel crosslinked by poly(ethylene glycol) dimethacrylate. Yürüksoy [28] obtained a swelling of 1122% for acrylamide-based hydrogels. Onder et al [29] obtained a swelling of approximately 500% for a crosslinked hydroxyethyl starch hydrogel with GO.
3.4 Study of adsorption capacity
Removal efficiency and adsorption capacity with respect to contact time for the MG dye were investigated for all synthesized hydrogels. Figure 5 shows the MG removal efficiency data after 24 h of contact. The reference sample of neat PA showed a small removal efficiency, (5 ± 1)%, and samples of hydrogel with GO-0.25 and GA-0.10 showed the highest removal rates: 76 and 71%, respectively. The samples containing GO and CNTO do not follow a tendency with respect to the nanomaterial content. The hydrogel with GA is the best adsorbent with the lowest concentration of 0.10. It is worth noting that the removal efficiency obtained by the GO-0.25 and GA-0.10 samples are lower than the rates observed in other works in the literature that contain similar adsorbents, as shown in Table 4. However, the adsorption capacity of MG in this work is very competitive with the literature, as will be reported below.
Figure 6 shows the adsorption capacity with respect to contact time of the hydrogels for the MG dye. All hydrogels showed superior performance compared to the reference. It is possible to observe that most of the dye is removed in the first 6 h of contact, as seen in the curves. The amount of dye removed increases until it stabilizes, with values in the range from 17 to 274 mg.g-1 after 24 h of contact. The largest adsorption capacity is for the GO-0.25 hydrogel, and the smallest is for the reference sample. The GA-0.10 sample also presented a high value of adsorption capacity when compared to the other samples, i.e., 257 mg.g-1. Importantly, there was an increase of approximately 1500% in the adsorption capacity for GO-0.25 in comparison with the reference PA hydrogel.
Figure 7 presents the results of the adsorption capacity of hydrogels with greater efficiency for each nanomaterial. It should be noted that the GO-0.25 and GA-0.10 samples have considerably higher adsorption capacities than the other hydrogels. The physico-chemical properties of the carbon-based nanomaterials are reported in Table 1 and Table 3. Figure 7 b shows simplified schemes of the functionalized nanomaterials, indicating only one typical functional group. It is possible to affirm that the chemical groups with heteroatoms (O and N) are an important factor in favoring the adsorption mechanism. MG is a cationic aromatic dye that can adsorb to anionic functional groups by hydrogen bonding, ion-dipole interactions and p-p interactions of the aromatic ring. GO and GOA present degrees of functionalization of 39% and 29%, respectively, while CNTO and CNTOA present 3 to 4 times lower levels of oxygenated and amino groups.
Zeta potential is frequently considered to understand the adsorption mechanism of ionic dyes. However, the fact that the oxidized nanomaterials present a negative zeta potential and the aminated nanomaterials present a positive zeta potential (Figure 7) does not correlate with the adsorption behavior observed in the results. Both GO- and GOA-based hydrogels showed high values of adsorption, which demonstrates that the charges present in the nanomaterials are not an influential factor for the adsorption efficiency. Therefore, it is possible to affirm that the factors with the greatest impact on the adsorption efficiency of MG for the studied systems are the dimensionality of the nanomaterials (layer 2D or tube 1D) and the percentage of functionalization. Most likely, the hydrogen bond, ion-dipole and p-p interactions through the aromatic structures are the relevant interactions that drive the adsorption of MG on the nanocomposite hydrogels.
Table 4 gathers some data on the adsorption capacity of different adsorbents with systems similar to those studied in this work. The results obtained herein approach or even surpass those obtained in other works. The adsorption capacity obtained by the GO-0.25 hydrogel is 274 mg.g-1, which is a value greater than double or even quadruple the values obtained by the nanocomposites of poly(acrylic acid), graphene oxide and monoacryloyl tetramonium tiacalix[4]arene [19] and by graphene oxide and aminated lignin aerogels [30]. The adsorption capacities of 289.10 and 265.87 mg.g-1 obtained in the works of Zhang et al [31] and Chen et al [32] are values closer to those obtained in this work.
Table 4. Comparative literature data on the adsorption removal efficiency and adsorption capacity of malachite green by different adsorbents.
Adsorbent
|
Removal effiency / %
|
Adsorption capacity / mg.g-1
|
Reference
|
Poly(acrylic acid), graphene oxide and monoacryloyl tetraammonium thiacalix[4]arene nanocomposite
|
96.25
|
67.5
|
[19]
|
Compound of hydroxyethyl starch and graphene oxide
|
91.1
|
89.3
|
[29]
|
Graphene Oxide/Amine Lignin Aerogels
|
91,72
|
113.5
|
[32]
|
Magnetic carboxymethyl functionalized chitosan-graphene oxide compound
|
over than 90
|
289.1
|
[30]
|
Fe3O4 @SiO2 – Graphene oxides core-shell magnetic microspheres
|
approximately 70
|
265.87
|
[31]
|
Polyacrylamide graft copolymer and potato starch
|
less than 20
|
5.9
|
[33]
|
Amine guar gum/graphene oxide nanocomposite
|
98
|
230.56
|
[35]
|
Polyacrylamide and graphene oxide nanocomposite
|
76
|
274
|
This work
|
3.5 Study of dye adsorption kinetics
Raval, U. Shah and K. Shah [14] reviewed a wide variety of adsorbents used in various works for the removal of malachite green (MG) dye. In this review, several optimal experimental conditions were evaluated, such as solution pH, equilibrium contact time, amount of adsorbent and temperature, as well as adsorption isotherms and kinetic and thermodynamic data, which were analyzed and tabulated. In these tables, it is possible to observe that the great majority of the studied adsorbents show adsorption behavior that follows a kinetics adjusted to a pseudo-second-order model, mainly the systems containing nanomaterials. Thus, the kinetic study of the current work was focused on the adjustment to a pseudo-second-order kinetic model. The pseudofirst-order kinetic model was also tested, but the adjustments for the PA hydrogel and two of the nanocomposites were not possible, and for others, the fitting quality was not satisfactory.
Figure 8 presents the graphs of t/qt as a function of t, as proposed by Equation 4 for pseudo-second-order kinetics. The linear adjustments provided the kinetic parameters shown in Table 5.
Table 5. Kinect parameters for MG adsorption for adjustment of pseudo-second-order adsorption kinetics of the hydrogels studied (experimental data from Fig. 7)
Samples
|
k/g. min-1 . mg-1
|
qe/mg. g-1
|
R2
|
PA
|
0.00144739
|
19.72
|
0.89244
|
GO-0.10
|
0.00000872
|
334.4
|
0.99818
|
GO-0.25
|
0.00001499
|
336.7
|
0.98932
|
GO-0.50
|
0.00000781
|
384.6
|
0.98737
|
GA-0.10
|
0.00001473
|
314.5
|
0.99088
|
GA-0.25
|
0.00001572
|
257.1
|
0.96262
|
GA-0.50
|
0.00004333
|
157.7
|
0.95408
|
NO-0.10
|
0.00048986
|
50.5
|
0.99411
|
NO-0.25
|
0.00088496
|
38.9
|
0.99438
|
NO-0.50
|
0.00040822
|
54.3
|
0.98189
|
NA-0.10
|
0.00081944
|
85.0
|
0.99251
|
NA-0.25
|
0.00639478
|
80.4
|
0.99999
|
NA-0.50
|
0.00156401
|
81.0
|
0.99707
|
As seen in Table 5 and Figure 8, the values of R2 indicate that the adsorption kinetics of the hydrogels studied for the MG dye approach a pseudo-second-order kinetic model. Only for the reference sample, whose R2 value was 0.89244, was the fit not a perfect match. However, as this model was very well adjusted for all nanocomposite hydrogels, it was decided to work with it in this discussion. The values of qe, i.e., the adsorption capacity at equilibrium, did not follow the same order as the experimental data at 24 h. Sample GO-0.50 has the highest value, 384.6 mg.g-1, while sample GO-0.25 comes in second with 336.7 mg.g-1. The adsorption rate constant (k) values indicate that the adsorption processes of MG in the GO and GOA hydrogels are slower than those in the CNTO and CNTOA hydrogels. Moreover, the much higher adsorption capacity of graphene-modified hydrogels indicates that the high content of functional groups probably acts as active sites for adsorption in this case. Therefore, a slower but more extensive mechanism of adsorption occurs in the layered highly functionalized structure.