*3.3. E*ff*ect of Contact Time*

The following step in the adsorption experiments is to elucidate the effect of contact time using different concentrations of dye (from 25 to 300 mg/L). All the experiments were carried out at pH 5.0, constant stir (500 rpm), and room temperature (25 ◦C), with a fixed amount of adsorbent (0.5 g), and the results obtained are presented in Figure 3.

As can be observed in Figure 3, the adsorption capacity increased in each concentration until the equilibrium was achieved. In the equilibrium, the amount of adsorbed dye inside the adsorbent and the amount of dye desorbed were in a dynamic equilibrium. The time required for the adsorption of the dye onto the adsorbent to attain the equilibrium state is called the equilibrium time and the amount of dye removed by the adsorbent at that time indicates the maximum adsorption capacity of the adsorbent under these conditions [48].

Different adsorption stages can be differentiated in the range of concentrations analyzed (25–300 mg/L). At a low concentration (25 mg/L), the equilibrium was reached after 40 min of contact. The adsorption was fast at the concentration of 100 mg/L, however, it was slower than at low concentrations, reaching the equilibrium after 80 min of contact time. At high concentrations of dye (>100 mg/L), the curves did not present the asymptotic form. Hence, the equilibrium time increased with increasing concentrations of DB78 (from 150 to 300 mg/L).

**Figure 3.** Effect of contact time between eggshell at different concentrations of Direct Blue 78 of 25 mg/L (•), 50 mg/L (-), 100 mg/L (-), 150 mg/L (), 200 mg/L (), 250 mg/L (Δ), and 300 mg/L ().
