*3.4. Adsorption of Dyes in Binary Systems*

Regarding industrial wastewater, different types of dyes could be found and they compete for the adsorption sites on the surface of the adsorbent. To investigate the adsorption capacity of CNF–GnP in a more practical setting, an MB and CR binary system was prepared and investigated. The mass ratio of MB to CR in the solution was designed to be 3:1, 1:1 and 1:3. The total initial dye concentration was set to be 200 mg L−1. The adsorption of dye in single and binary systems was compared (Figure 7). Shown in Figure 7a, when the mass ratio of MB to CR was 3:1 (i.e., MB = 150 mg L<sup>−</sup>1, CR = 50 mg L<sup>−</sup>1), the adsorption of MB onto pure CNF in the binary system was lower than that in the single system, indicating MB and CR competed for adsorption sites on the CNF surface. However, the adsorption of MB onto pure GnP and CNF–GnP was similar in both the single and binary systems. The adsorption of CR onto all adsorbents increased in the binary system compared to that in the single system. This may be attributed to the Coulombic attraction between cationic MB adsorbed on the material surface and anionic CR in solution. Similarly, when the mass ratio of MB to CR was 1:1 (Figure 7b), the adsorption of MB was lower and the CR uptake for pure CNF and CNF–GnP was higher in the binary system than in the single system. However, the adsorption of CR onto pure GnP was strongly affected by the presence of MB when the CR concentration increased. Moreover, when CR became the dominant molecule in the binary system (i.e., MB to CR ratio was 1:3, Figure 7c), the CNF–GnP hybrid was able to adsorb even more CR than pure GnP. Regarding all binary systems, the adsorption capacity of CNF–GnP was the best among the different nano adsorbents, and the presence of one dye had no negative impact on the adsorption of the other dye. The hybrid CNF–GnP adsorbed the highest amount of MB and CR combined per unit mass of adsorbent. Perhaps the cationic MB and anionic CR adsorbed onto different types of adsorption sites on the CNF–GnP surface. The cationic MB mainly adsorbed onto the negatively charged CNF portion, while most of the CR adsorbed onto the GnP portion. MB adsorbed on the material surface also may attract anionic CR in solution via Coulombic attraction.

**Figure 7.** Adsorption of MB and CR in single and binary dye solution. (**a**) mass ratios of MB to CR are 3:1; (**b**) mass ratio of MB to CR are 1:1; (**c**) mass ratio of MB to CR are 1:3. Colored bars represent adsorption in a single dye system. Colorless bars represent adsorption of one dye in a binary dye system.
