*2.6. Adsorption Capacity of MO*

The hydrogels were analyzed according to their ability to adsorb MO dye. An aqueous solution (water conductivity 3.15 μS/cm) of MO dye (150 mg L−1) was prepared, and 40 mL was used for each test. The hydrogel (50 mg) was placed in contact with 40 mL of aqueous MO dye, and the duration of the experiment was 300 min. The following hydrogels were selected for these tests: Hy01, Hy02, Hy03, Hy04, Hy07, Hy10, and Hy12.

First, the adsorption kinetics were determined using washed and unwashed hydrogel Hy12 after synthesis. Subsequently, the adsorption was studied as a function of time up to 300 min for Hy01 and Hy03; the adsorption capacity of the hydrogel was calculated every 15 min up to 60 min, then every 30 min up to 240 min, and finally at 300 min. Thereafter, the adsorption tests were performed simultaneously as mentioned above with Hy01, Hy02, and Hy03, by varying the amount of CNF in the hydrogel.

The adsorption tests were performed with Hy01 by varying the pH of the colored solution to acidic (pH 3.0) with HCl (0.1 mol L−1) and alkaline (pH 10.0) with NaOH (0.1 mol L−1). The MO dye concentration was measured using a UV-visible spectrometer (model BK-UV1800, BIOBASE) at a wavelength of 466 nm [37].

The amount of MO dye retained by the hydrogel was determined according to Equation (11):

$$q = \frac{\left(\mathbb{C}\_i - \mathbb{C}\_p\right) \times V\_i}{m\_\chi} \tag{11}$$

where *q* is the adsorption capacity or amount in milligrams of MO retained per gram of sorbent (mg g<sup>−</sup>1); *Cp* is the concentration of dye in the supernatant (mg L<sup>−</sup>1); *Ci* is the initial concentration of dye in the supernatant (mg L−1); *Vi* is the volume of the dye solution (mL); and *mx* is the mass of the xerogel (g).

The removal percentage %R of MO by the hydrogels was determined using Equation (12):

$$\%R = \frac{\mathcal{C}\_i - \mathcal{C}\_p}{\mathcal{C}\_i} \times 100\tag{12}$$

where %*R* is the removal percentage; *Cp* is the concentration of dye in the supernatant (mg L<sup>−</sup>1); and *Ci* is the initial concentration of the dye in the supernatant (mg L<sup>−</sup>1).

The effect of the initial concentration of MO was studied by varying the concentrations between 50, 100, 200, 350, 500, 750, 1000, 1500, and 2000 mg L<sup>−</sup>1.
