*2.1. Materials*

Hen eggshells were purchased from a local organic farm. Sodium hydroxide (NaOH), acetic acid (C2H4O2), sodium phosphate monobasic monohydrate (H4NaO5P), hydrogen peroxide (H2O2), and hydrochloric acid (HCl) were obtained from Sigma-Aldrich (Madrid, Spain) and boric acid (H3BO3) was purchased from Scharlau (Madrid, Spain). All of these chemicals were used to prepare dye solutions at different pH values. Colorprint (Valencia, Spain) kindly provided Direct Blue 78 dye. Deionized water was utilized to prepare all aqueous solutions throughout the experiments.
