*3.2. E*ff*ect of Initial pH Solution*

The pH of dye solution is considered an essential factor in any adsorption process that affects the adsorption capacity and the adsorption mechanism of the eggshell. With the objective to investigate the effect of the initial dye pH in the adsorption of DB78 dye on the eggshell surface, the equilibrium was studied at different pH values. The effect of pH in the adsorption of DB78 dye was evaluated in the range of pH from 3 to 11. The pH was adjusted by adding NaOH or HCl 0.1 M. The pH experiments were conducted using 2 g eggshell, at a concentration of 25 mg/L, 90 min of contact time, constant stir (500 rpm), and room temperature (25 ◦C), and the results are shown in Figure 2. The result indicates that the adsorption capacity and removal efficiency of eggshell depend on the pH.

As shown in Figure 2, the removal efficiency of dye increased at acidic pHs (3 and 5). From this pH, the adsorption capability of the eggshell decrease from 94.5% to 63.4% at pH 11. Figure 2 shows that the optimum pH required to obtain the maximum adsorption of DB78 onto eggshell is 5. The isoelectric point for the eggshell is 5.5 [44], which is close to our optimum pH (5.0). At this pH, the adsorbent is positively charged and interacts effectively with the negative charges in DB78. This trend was similar in the removal of Direct Red 80 and Acid Blue 25 dyes from aqueous solutions on eggshell membrane, in which the adsorption capacities of both anionic dyes onto eggshell membrane increased with acidic pHs [9] and, in another study, Chojnacka carried out the biosorption of Cr (III) ions from aqueous solutions by eggshells at pH 5 [45].

**Figure 2.** Effect of pH on the adsorption of DB78 by eggshell at different pH values.

The principal component of the shell is CaCO3 in the form of the mineral calcite. In water, the carbonate species derived from calcite are H2CO3, HCO3 <sup>−</sup>, and CO3 <sup>2</sup>−, and a sufficient amount of carbonates is solubilized from the shells to buffer the mixtures to a low alkaline pH (from 7.5 to 8) after reaching the equilibrium. Therefore, at various pH, the electrostatic attraction as well as the structure of dye molecules and the eggshell could play very important roles in dye adsorption on this adsorbent. At pH 5, a significantly high electrostatic attraction exists between the positively charged surface of the adsorbent, owing to the ionization of functional groups of adsorbent and negatively charged dye. As the pH of the system increases, the number of negatively charged sites is increased. A negatively charged site on the adsorbent does not favor the adsorption of this dye owing to the electrostatic repulsion [46,47].
