Characterization of Chitosan
The degree of deacetylation, molar mass and moisture content are fundamental characteristics to guarantee the quality and maintain the standard of chitosan. The degree of deacetylation and molar mass have a direct relationship with the physicochemical properties of chitosan, these parameters are decisive for the purpose of the application, for example, in the formation of films or gels, and also in mechanical properties.
Chitosan produced was characterized (Table 1), showing a degree of deacetylation, according to the methodology used, within the expected range in literature, between 70 and 95% [31, 35]. The relevance of determining the degree of deacetylation is linked to the protonated amino groups available for adsorption [17]. Another parameter to consider in chitosan is the molar mass, which is related to the mechanical properties of resistance, and the value obtained was within the range of literature, in the range from 100 to 200 kDa [ 31]. The moisture content of the chitosan powder also was within the range of literature, below 10% (w w-1, wet basis) [32]. Therefore, the results shown in Table 1 were considered suitable, thus, chitosan was used as the precursor material for the development of polymeric films for potential application in adsorption.
In Table 2 is observed that the thickness determined for all films did not show significant differences between them (p>0.05). The result was considered suitable, as the same amount of chitosan mass of 2 g (dry basis) was used for all films, varying only the proportions of carbon nanotubes added to the film-forming solution, which were very low (in the range of 0.1% to 0.5% w w-1).
Table 3 shows the data obtained for the color parameters of the developed adsorbent films. These results corroborate with the analysis of the CIELAB 3D staining diagram. It was observed that the CS film (control) presented a light-yellow color (higher L*value and Hue angle next to 90º). The films containing CNTs showed an intensification in color, being significantly different (p≤0.05), becoming respectively darker with the addition of a greater proportion of CNTs (CS/0.1 dark yellow film, CS/0.3 light brown film and CS/0.5 a little darker brown film).
Characteristics of adsorbent films
Mechanical properties
Table 4 presents a comparison of the values obtained to the tensile strength and elongation of all films. It was observed that the control film (CS) presented the highest value of tensile strength. As for all CS/CNTs composite films (0.1%, 0.3%and 0.5%) tensile strength values were not significantly different (p>0.05). The proportions of CNTs added to the polymer matrix led to a significant decrease in the tensile strength of the films to the control CS film. The behavior was similar to elongation, however inversely, because there was an increase in the elongation value of all composite films (CS/CNTs) to CS film, and there was no significant difference between composite films (p>0.05).
Crystallinity
In the X-ray Diffraction (XRD) analysis, the diffractograms for all films were presented as shown in Fig, 1. When observing the diffractogram of the films, it is noted that all films maintained the profile of a semi-crystalline material. However, the pure chitosan film presents irregular intense peaks and an amorphous part, showing the semi-crystalline structure of the polymer. For chitosan-based films reinforced with CNTs, it is noted that they presented more intense peaks of approximately 26° and 45°, which are characteristic peaks of carbon nanotubes and can be attributed to the Bragg indices (002) and (101) which indicate a spacing of approximately 34 nm between the graphene layers. Comparing the reinforced carbon nanotube films, it can be noted that only the film with the lowest concentration of CNTs presented peaks of different intensities and widths. These differences are evidence in the organization of the CNTs layers. Despite this, there were no significant changes, suggesting that the cylindrical and concentric structures of the CNTs were maintained. According to Crini et al. [17], the increase in the amorphous region of the material after the incorporation of CNTs leads to relevant contributions to the adsorption of contaminants, facilitating their adherence to interaction sites.
Thermal profile
Fig. 2 shows the thermograms obtained for all prepared films. Thermogravimetric analysis was carried out to identify the thermal stability of the developed materials. The percentages of mass loss observed in the thermogram for the pure chitosan (CS) film occurred in three thermal events. The first stage occurs between 25°C and 115° C and corresponds to the evaporation of water molecules adsorbed on the material; the second event occurs between 115°C and 265°C and can be related to the decomposition of the amino and hydroxyl groups of the polysaccharide; and the third event between 265°C and 500°C refers to the carbonization of the material's organic matter (ash). By comparing the thermogram obtained for the CS film with those of chitosan films modified with carbon nanotubes (CS/CNTs), records of mass loss in three similar thermal events were observed [44].
DSC analysis results (Fig. 3) also showed the decomposition curves of the prepared materials. The thermal stability was not affected by the carbon nanotube content. However, a high filler content added to a polymer matrix can reduce the desirable properties of the polymer [45]. For this reason, carbon nanotubes are promising materials since a low content can substantially improve the efficiency of material properties.
Functional Groups (FTIR)
Infrared spectra analysis was carried out to identify the functional groups present in the adsorbent precursor materials based on characteristic bands, as well as to analyze the chemical modifications in the composite films obtained. Fig. 4 presents the FTIR spectrum for CS films and CS/CNTs composite films. In the spectra, the particular absorption bands at 1020 and 1450 cm-1 appear highlighted due to the stretching of the C–O bond. These bands are characteristic of chitosan polymer chains. The FTIR spectra for composite films reinforced with functionalized CNTs (-OH) present bands characteristic of the functionalized CNTs, in which an increase in the intensities of vibration bands associated with alcohol is observed at 1140 cm-1, and carbonyl bands at 1620 cm-1, which can be related to carboxylic acids or amides, this increase in intensity as the proportion of CNTs in the material increases, can be mainly the result of the induction of dipole moments by the creation of defects, observed both in CNTs. Furthermore, bands relating to the vibrations of the bonds C=C at 1572 cm-1, C-H at 2930 cm-1 and N-H at 667 cm-1 are observed. The vibration at 2850 cm-1 refers to the symmetric stretching of CH3, which also indicates that groups were added to the CNTs structure.
SEM Images
The scanning electron microscopy (SEM) images of the films show the surfaces of the adsorbents before (Fig. 5(a)) and after (Fig. 5(b, c, d)) the addition of nanotubes to the polymer matrix. Morphological images are shown at magnifications of × 5,000 and × 60,000, respectively CS polymer films (Fig. 5 (a, e)), CS/CNT0.1% composite films (Fig. 5 (b, f)), CS/CNT0.3% (Fig. 5 (c, g)) and CS/CNT0.5% (Fig. 5 (d, h)). For the CS film, Fig 5(a) shows the appearance of aligned lumps; however, in the higher magnification, Fig.5(e), small grooves can be seen on the surface of the lumps. For composite films reinforced with the CS/CNT0.1% nanomaterial (Fig.5 (b, f)), morphological changes that are almost imperceptible to the naked eye are shown, such as the formation of agglomerates in the film structure, indicating that the CNTs are deposited on the surface. For the CS/CNT0.3% and CS/CNT0.5% films, the alignment of the CNTs and their lengths becomes more evident and apparent, through the formation of clusters in the morphology. All films showed roughness in the surface morphology, which attenuated as the proportion of CNTs increased.
Adsorption studies
Point of zero charge (PCZ)
The point of zero charge (PCZ) as a characterization analysis is relevant information to understand how the material adsorption process occurs. When determining the PCZ, the point at which the pH remains constant after the system reaches equilibrium is given. As shown in Fig. 6, it is possible to observe all the values found for the pH of the adsorbent films in the study.
In Fig. 6, for chitosan films (Fig. 6(a)) the PCZ was equal to 5.8, which means that for pH values < 5.8 the surface of the material has a positive charge and for pH > 5.8 it will be negatively charged. For the other chitosan-based films (Fig. 6 (b, c, d)) containing different proportions of CNTs, the PCZ pH value determined was close to 6.0, meaning that at pH < 6 the surface will be positively charged and for pH > 6 it will be negatively charged. The results corroborate with other studies in this same line of research. When a solid adsorbent into contact with a liquid solution with a pH lower than the PCZ, the surface is positively charged and many anions are adsorbed to balance the positive charges, making adsorption more effective on anionic materials [19]. However, in aqueous solutions with a pH higher than PCZ, the surface is negatively charged and preferentially adsorbs cations. In this case, the application of adsorbents is recommended to remove cationic materials [45].
Assessment of adsorption potential in dyes.
In the adsorption process, the pH is an important parameter, since its variations can affect the surface charge of the adsorbent and also the degree of ionization of the adsorbate [35]. Fig. 7 shows the influence of pH in cationic and anionic dye solutions on the adsorption capacity of the films obtained. The qe values were analyzed from the experiment carried out in the pH range from 2 to 8.
According to the data shown in Fig.7(a), it is evident that the adsorption of the crystal violet dye in the films obtained presents a behavior dependent on the pH of the medium. Comparing the CS film and the CS/CNTs films, it is noted that in all pH conditions tested, the composite films presented higher qe values than the film without CNTs. This evidence is justified due to the increased availability of the hydroxyl group from functionalized CNTs, considering that these functional groups are important sites for adsorption. Despite this, the variation in adsorption capacity as a function of the pH of the medium was similar for films containing a higher proportion of CNTs. It is also observed that in acidic conditions (pH <4) the adsorption of the dye is disfavored, as the functional groups present in CS and CNTs are positively charged and result in the repulsion of the VC dye molecules. However, when the pH increases, the amount of dye adsorbed on the surface of the films increases due to the greater availability of hydroxyl groups. Under basic conditions (pH>8) no significant changes were observed. Based on the information presented, the adsorption of the cationic dye is favored at pH above 6. For the yellow anionic dye tartrazine (Fig. 7(b)), the adsorption capacity in the films was favored by the decrease in pH for all films and the reduction in the CS/CNTs ratio. This occurred because, under these conditions, H+ ions in solution tend to protonate the NH3+ groups of chitosan, facilitating their interaction with the sulfonated groups of tartrazine yellow (negative charge).