*2.6. Adsorption of Dyes in Binary Systems*

MB and CR were added into 20 mL DI water with various mass ratios at a total concentration of 200 mg L<sup>−</sup>1. The mass ratio of MB to CR was 3:1, 1:1 or 1:3. An adsorbent of 5 mg was added, and the mixture was agitated at 25 ◦C and 120 rpm for 16 h. Regarding a binary system, dye A (MB) and dye B (CR) concentrations were calculated as follows:

$$C\_{\rm A} = \frac{k\_{\rm B2}d\_1 - k\_{\rm B1}d\_2}{k\_{\rm A1}k\_{\rm B2} - k\_{\rm A2}k\_{\rm B1}}\tag{3}$$

$$\mathbf{C}\_{\rm B} = \frac{k\_{\rm A1}d\_{2} - k\_{\rm A2}d\_{1}}{k\_{\rm A1}k\_{\rm B2} - k\_{\rm A2}k\_{\rm B1}} \tag{4}$$

where *d*<sup>1</sup> and *d*<sup>2</sup> are the mixture optical densities measured at λ<sup>1</sup> (664 nm) and λ<sup>2</sup> (498 nm), respectively. *k*A1, *k*B1, *k*A2, and *k*B2 are the calibration constants for components A and B at wavelengths of λ<sup>1</sup> and λ2, respectively [35,36].
