*2.5. Adsorption of Dyes in Single Systems*

Anionic dye CR and cationic dye MB were used in the adsorption experiment. CNF, CNF–CNT, CNF–GnP were aerogels, while pristine CNT and GnP were in the form of powder. Aqueous phase adsorption studies were conducted at 25 ◦C by submerging 5 mg of a specific nano adsorbent into 20 mL of each dye solution at neutral pH. The mixture was continuously agitated on an orbital shaker at 120 rpm and the amount of residual dye in the solution was determined by ultraviolet-visible spectroscopy (UV-Vis, Thermo Scientific, Evolution 201, Waltham, MA, USA) at the maximum absorption wavelength using a measured extinction coefficient from a Beer's law analysis for each solution. The adsorption capacity of MB, CR on each adsorbent was calculated using Equation (2).

$$\eta\_{\rm t} = \frac{(\mathcal{C}\_0 - \mathcal{C}\_{\rm t})V}{m} \tag{2}$$

where *q*<sup>t</sup> is the amount of dye adsorbed at a given time (mg g−1), *C*<sup>0</sup> is the initial dye concentration (mg L<sup>−</sup>1), *C*<sup>t</sup> is the residual dye concentration at a given time (mg L<sup>−</sup>1), *V* is the solution volume (L), and *m* is the mass of the adsorbent (mg).

The effect of contact time (0–240 min) was examined using 5 mg of adsorbent and an initial dye concentration of 10, 250 and 500 mg L−<sup>1</sup> for MB and 100, 600 and 2000 mg L−<sup>1</sup> for CR. The effect of initial dye concentration on the final adsorption capacity was investigated in a range of dye concentrations (MB: 10, 50, 100, 150, 200, 250, 300, 400, 500, 600, 800 and 1000 mg L−1; CR: 10, 50, 100, 150, 200, 250, 300, 400, 500, 600, 800, 1000, 1500 and 2000 mg L<sup>−</sup>1) using an adsorbent of 5 mg at 25 ◦C and 120 rpm for 16 h.
