3.4.3. Kinetic Models

A study of adsorption kinetics is desirable because it provides information on the progress of adsorption and whether physical or chemical interactions predominate the process. The pseudo-first and pseudo-second-order kinetic parameter values for MO adsorption are presented in Table 4. The correlation coefficient criterion (highest value of R2) was used to describe the most suitable kinetic adsorption model [52]. According to the described criteria, MO adsorption for hydrogel Hy01 conforms to the pseudo-second-order kinetic model with an R2 value greater than 0.9733 (see Figure 11). Similar kinetic results were obtained in previously reported MO adsorption studies [7,19,53].

**Table 4.** The MO sorption data for the pseudo-first and -second-order kinetic model.

**Figure 11.** MO sorption kinetics: (**a**) pseudo-first-order and (**b**) pseudo-second-order kinetic model.

From a systematic study of the literature, it is clear that the textile industry is the main industry that generates large volumes of wastewater containing dyes, consuming about 100 L of water to process approximately 1 kg of textile material [54]. These are highly recalcitrant and biocompatible synthetic chemical compounds, considered as potential threats to human and environmental health [4]. About 3500 different types of synthetic dyes are used in the textile industry. The most commonly used dyes are anthraquinone and azo dyes and more than 60% of these dyes are reactive [55]. These chemical species are released through the industrial processes of dyeing and washing, among others [56], resulting in wastewater with high concentrations fluctuating between 350–1000 mg L−<sup>1</sup> of dyes [4,57,58]. Most synthetic dyes are soluble in water, thanks to the ionizable groups that compose them, such as: -OH, -COOH, and -SO3H in acid dyes, and -NH2, -NHR, and -NR2 in basic dyes. It is estimated between 50% and 70% of the world production of 10,000 synthetic dyes (dyes and distinctive dyes used in the textile industry) corresponds to azo dyes, which represent the class of compounds most used in textile and food processes [10,11]. The pH value of the aqueous medium favors the ionization of the groups, depending on the pKa value of the chemical species in the solution. It is known that the average pH value of wastewater is 8.75 ± 1.29 [4], where the vast majority of dyes are in an ionic state. Regarding the applicability of hydrogels, they are materials that possess a number of functional groups suitable for dyes, and also offer the possibility of reuse/regeneration in sorption–desorption cycles by washing processes with acidic solutions for the anionic hydrogel and basic brine/NaCl for the cationic hydrogel. If it is not possible to regenerate the structures, these materials should be disposed of in the solid waste landfill, following the usual route for hazardous solid waste [59].

Table 5 shows a comparison of adsorption capacities of MO by biopolymer composites, highlighting the possibility of further developing this type of adsorbent materials with natural polymers, such as CNF, which was corroborated to improve the stability at the time of adsorption, giving the possibility of reusing the hydrogel in desorption–adsorption cycles.


**Table 5.** Comparative table of maximum adsorption results.

Is important to advance in the development of bio-based materials to be tested in real applications in industry. The adsorption is inexpensive, simple, and easy to adapt. In addition, its treatment period is short, causes no pollution to the environment, and has been confirmed as one of the most promising technologies for removing dyes from wastewaters [50].

#### **4. Conclusions**

Nanocomposite hydrogels based on ClAETA were successfully synthesized by varying the concentrations of CNF, MBA, and APS. From the ANOVA analysis, it was observed that the concentration of APS significantly affects the performance of the hydrogel synthesis compared to the other factors. It was determined that the combination of the three factors significantly affected the degree of cross-linking because the APS affects the length of the polymeric chains formed, the MBA maintains the solidity and porosity of the hydrogel, and the APS provides stability and rigidity, and an increase in hydrogel swelling is observed when the concentration of CNF is increased, which may be explained by the increased number of carboxyl groups in the hydrogel. In contrast, all individual factors, in double or triple combination with each other, did not significantly affect the water absorption capacity.

In addition, in the microstructural analysis, the texture of the hydrogels was determined, and the CNF fibers were individually identified. The functional groups of the structures of the hydrogels can be determined by FTIR spectroscopic analysis. From TGA it was verified that the hydrogels containing CNF generated greater thermal stability compared to hydrogels with only poly(ClAETA). The surface morphology of the obtained hydrogels was observed by SEM and the incorporated CNF was observed.

In the application of the hydrogels to the absorption of the dye, it was observed that the hydrogels containing only poly(ClAETA) achieved removal values above 80% and then decreased, but these were unstable after reaching the maximum swelling capacity and tended to destabilize. In contrast, hydrogels with CNF, such as Hy01, had lower removal rates than those without CNF but were chemically and mechanically more stable, capturing 1379 mg of MO per gram of resin after 300 min. The reuse/regenerative hydrogel was tested and was found to be satisfactory in up to three cycles. Tests with pH variations indicated that the adsorption of MO was favored under neutral pH. Therefore, it can be concluded that the incorporation of CNF improves the MO adsorption as a function of time.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/polym13142265/s1, Table S1: ANOVA results, where the significance is 0.05: (a) yield of the reaction, (b) crosslinking degree, and (c) water absorption capacity. DF (degree of freedom of the data), SS (the sum of the squares of the data), MS (mean sum of the squares of the data), F (F-statistic), value-*p* (*p*-value) and Fcritical (F-statistic critical value).

**Author Contributions:** K.R., conceptualization, methodology, writing; Y.T., visualization, software, data curation; M.O.T., original draft preparation, visualization, investigation; J.S., writing, reviewing, editing, supervision, project administration. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by FONDECYT, grant 1191336.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Exclude this statement.

**Acknowledgments:** The authors thank project FONDECYT n◦ 1191336.

**Conflicts of Interest:** The authors declare no conflict of interest.
