3.3.3. Thermogravimetric Analysis

Thermogravimetric analysis is a technique that allows the evaluation of the thermal stability of synthesized poly(ClAETA) hydrogels and the determination of the effect generated by the addition of CNF in the hydrogels. Figure 5 shows the thermograms of hydrogels Hy03, Hy04, Hy07, and Hy11, which show a typical sigmoidal shape, indicating the weight loss in three stages. The first stage was recorded at temperatures around 100 ◦C, corresponding to the dehydration of water in the polymer and the elimination of humidity [46]. The second stage occurred at approximately 280–330 ◦C, corresponding to the thermal decomposition of the groups that protruded from the polymer chain. Similarly, a peak was observed at 390–410 ◦C, corresponding to exothermic reactions resulting from the decomposition of the ammonium salt [43].

**Figure 5.** Thermogravimetric analysis TGA of the samples: (**a**) Hy03 and Hy04, (**b**) Hy07 and Hy11.

Upon analysis of Hy03, which did not contain CNF, a decomposition was observed from 248.0 ◦C to 330.0 ◦C, resulting in a residual mass percentage of 45.1%; in the second stage, a residual mass percentage of 18.4% was obtained with respect to the initial mass; and finally, at 550 ◦C, only 8% of the residual mass of the hydrogel remained. In the case of Hy04, a rapid decomposition was observed that generated 58% of the residual mass in the second stage; in the third stage, a residual mass percentage of 26.7% was obtained; and finally, at 550 ◦C, the resulting value of the residual mass was 21.6%. It can be observed that there was a difference in stability between the hydrogels Hy03 and Hy04 at 18 ◦C, which could be due to the presence of CNF [47]. In the thermogram of Hy07, the first stage had a temperature range of 238–317.3 ◦C, which generated a residual mass percentage of 46.7%; in the third stage, 317.3 ◦C and 426 ◦C corresponded to a residual mass percentage value of 17.8%; and finally, at 550 ◦C, only 8% of the residual mass of the hydrogel remained.

Finally, the hydrogel Hy11 presented the second range of decomposition between the temperatures 256 ◦C and 330.2 ◦C, and the residual mass was 57.2%; between 330.2 ◦C and 434.3 ◦C, the residual mass was 24.8%. For this material, a thermal stability was achieved at approximately 8 ◦C. Analysis of Hy11 with Hy07 reveals that the main chain decomposition stage had a wider temperature range, which could be caused by the higher concentration of APS and CNF. Finally, at 550 ◦C, a residual mass percentage of 20.3% was obtained. The higher residual mass may be due to the higher amount of initiator, which can accelerate the cross-linking process during polymerization [48].

#### *3.4. Adsorption Capacity of Methyl Orange Dye by Hydrogels*

The functionality of hydrogel as a water dye adsorbent was evaluated. These tests were conducted using MO as the study molecule. For all the tests, the concentration was fixed at 150 mg L<sup>−</sup>1.

First, we studied the adsorption of dyes when the polymers were washed after synthesis, as well as the effects of this process on the result. For this test, we used Hy12. Figure 6 shows dye retention and capacity of adsorption per gram of resin for the washed (Hy12W) and unwashed (Hy12NW) hydrogel.

**Figure 6.** Dye retention and capacity of adsorption per gram of resin by hydrogel, as a function of time for Hy12NW and Hy12W at pH 7.64.

In the case of the Hy12W hydrogel, the minimum concentration of MO that remained in the solution at 90 min was 8.26%, compared to the case of Hy12NW, in which MO concentration was 28.13% at 60 min. Furthermore, as shown in Figure 6, Hy12W attained an optimal MO adsorption capacity of 96% at 90 min after the start of the experiment; however, the MO adsorption capacity was 37.55% at 300 min. In Hy12NW, an optimal MO adsorption capacity of 84.77% was attained at 60 min; at 300 min, the MO adsorption capacity was 81.16%. Thus, we concluded that despite washing, the hydrogels without CNF had a greater instability, which led to the desorption of the previously adsorbed dye.

The removal of MO using two synthesized hydrogels, one with CNF and one without the reinforcement (Hy01 and Hy03, respectively), at a pH of 7.64 was evaluated with the same percentages of cross-linker and initiator. The results for the percentage of adsorption and the retention of the hydrogel are shown in Figure 7.

**Figure 7.** Dye retention and capacity of adsorption per gram of resin by hydrogel as a function of time for Hy01 and Hy03 at pH 7.64.

It was observed that for Hy03, the removal percentage at 30 min after being immersed in the MO solution was 80.92%, and adsorption decreased thereafter. Concurrently, it was observed that the concentration of dissolved dye in the solution increased, which could be due to the low stability of hydrogel. In general, it is observed that hydrogels absorb the contaminant until a plateau of stability or equilibrium is attained in the adsorption curve, which is maintained until the hydrogel reaches its maximum swelling capacity, whereupon the sample sorption starts decreasing [30]. In the case of the hydrogel with CNF, Hy01, a slower response to adsorption is observed, but this response is sustained when nearing 300 min with 76.41 mg of MO per gram of resin.

Comparing the amount of MO retained per gram of resin (q = mg MO g−1) and considering different percentages of CNF (see Figure 8), it can be ascertained that with CNF, the adsorption was slower but sustained with time, except in the case of Hy03 (control sample, without CNF), which retained a greater amount of MO in less time but lost the retained dye and returned it to the solution. In Hy01, containing 1% CNF, and Hy02, containing 2% CNF, we observed 76.41 mg of MO per gram of resin and 72.42 mg of MO per gram of resin after 300 min, respectively, which could be caused by the stability provided by the incorporation of CNF into the matrix.

**Figure 8.** Removal of MO as a function of time when containing different concentrations of CNF at pH 7.64.

#### 3.4.1. Effect of pH on MO Removal

The performance in terms of retention of the MO dye in the acidic and basic environment of Hy01 was analyzed. Figure 9 shows the results of the q response (concentration of MO dye absorbed in the hydrogel structure). There was a higher MO load at pH 7.64, with 76.41 mg of MO for each gram of resin; the MO load at pH 3 and 10 was 59.92 and 65.70 mg for each gram of resin, respectively. MO dye is a weak acid that is widely used as an indicator of pH change. It has a pKa value of 3.47; therefore, at pH 3.0, the sulfonate molecules in the dye are neutralized and the amino groups are protonated, generating a positive charge [28]. The positive charge of the quaternary ammonium group exerts electrostatic repulsion with the positive charge of the hydrogel. In contrast, in a basic medium, the molecules of MO have a completely ionized sulfonate group, and the amino component is uncharged; thus, the molecules in the dyes and the fixed quaternary ammonium groups in poly(ClAETA) can completely interact with each other electrostatically. In addition, CNF has carboxyl groups on its surface and a pKa of approximately 4.6. In an alkaline environment, the carboxylic acid groups in CNFs gradually change to carboxylic anions, leading to the interaction of weakened hydrogen bonds and increased electrostatic repulsion in the hydrogels [40].

**Figure 9.** Removal of MO dye at pH 3, 7.64, and 10. *y*-axis: right—MO retention (%R); left adsorption capacity (q).

#### 3.4.2. Adsorption as a Function of MO Concentration

The initial concentration of the dye affects adsorption because it affects the mass transfer between the aqueous phase and the solid phase. For this reason studies were carried out varying the concentration between 50–2000 mg L−<sup>1</sup> at pH 7.64 with an initial hydrogel mass of 50 mg (see Figure 10a). The adsorption capacity increased with increasing initial concentration; a similar situation was observed in previous investigations with this dye [49,50]. With respect to the retention percentage, it also increase as the concentration of MO increases. Equilibrium is achieved between 1500 and 2000 mg L−1, obtaining an maximum adsorption capacity of 1379 mg g−<sup>1</sup> when the initial concentration is 2000 mg L−1. This high dye concentration was also studied by Onder et al., who using their hydrogel of [(2-(acryloyloxy)ethyl]trimethylammonium chloride-*co*-1-vinyl-2-pyrrolidone] hydrogel reached 905.6 mg g−<sup>1</sup> [21]. Regenerability and reusability of the adsorbent are also very important, as they make the adsorption process economical. Figure 10b shows the adsorption–desorption cycles when hydrochloric acid was used as an eluent [51]. As the number of cycles increases, the adsorption capacity gradually decreases to 5% by the fifth cycle. In general, in our experimental conditions, the reuse is recommended up to the third cycle, since after this adsorption capacity decreases significantly. It is also noted that the hydrogel delivers low concentrations of MO up to the third desorption process.

**Figure 10.** (**a**) Effect of MO concentration on the adsorption capacity of Hy01 at pH 7.64 with different concentrations of MO (mg L<sup>−</sup>1). (**b**) Effect of adsorption–desorption cycles on adsorption capacity (pH = 7.64; initial concentration: 2000 mg/L; and 50 mg adsorbent).
