*3.7. Desorption and Regeneration of the Adsorbent*

Prior to the desorption measurements, it was mandatory to carry out adsorption experiments. After this first adsorption cycle, the adsorbent was put in an alkaline medium (pH 12). In the first desorption cycle, 21% of dye was released from the eggshell; this could explain the electrostatic repulsion between the anionic dye and the negatively charged surface on the adsorbent. The ability of the eggshell to desorb more dye molecules after four consecutive cycles could be attributed to the distribution of charges on the adsorbent surface. After exposing the adsorbent to the same pH solution four times, the adsorbent releases the dye more easily. Arami et al. observed similar results in the adsorption of two different dyes onto the microporous and mesoporous eggshell membrane [37].

This adsorption/desorption cycle was repeated four times (Figure 8); according to the results observed, the adsorption abilities of the eggshell decreased with the increasing the number of cycles. This trend was the opposite for the desorption experiments. The results confirm that the eggshell can be reused in different cycles of adsorption/desorption.

**Figure 8.** Adsorbed/desorbed dye (%) onto eggshell for four consecutive adsorption (-)/desorption (-) cycles.
