The main objective of this study was to evaluate the adsorption efficiency of two adsorbent materials, Iraqi chicken eggshells (ESh) and activated carbon (AC) derived from ESh powder for the removal of a cationic dye (Janus green B; JGD) from aqueous solution. Activated carbon was synthesised from ESh using a simple chemical activation method using phosphoric acid as the activating agent. The physicochemical properties of the adsorbents were characterised by the Brunauer–Emmett–Teller (BET) method, FT-IR spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), and point of zero charge (pHpzc). The results of BET analysis confirmed that AC has a higher specific surface area (4.146 m2/g) compared to ESh (1.561 m2/g). The effects of operational parameters including contact time (5–60 min for ESh and 5–30 min for AC), adsorbent dose (0.05–1 g/10 mL), temperature (298–318 K), and pH (3.72–11.36) were systematically investigated. Optimal adsorption occurred at pH 11.36, where JGD removal efficiencies reached 90.13% with 0.2 g/10 mL of ESh after 60 min and 92.89% with 0.1 g/10 mL of AC after 30 min at 298 K. Equilibrium data were best fitted by the Freundlich isotherm model, yielding adsorption capacities of 0.09 mg/g for ESh and 1.85 mg/g for AC at 318 K and pH 5.5. The high correlation coefficient (R2 = 0.99) confirmed favourable heterogeneous adsorption. Kinetic data followed the pseudo-second-order model (R2 = 0.99). Thermodynamic parameters (ΔG°, ΔH°, ΔS°) indicated that JGD adsorption onto ESh was spontaneous (ΔG°<0), exothermic (ΔH°<0), and associated with decreased randomness (ΔS°<0), while adsorption onto AC was spontaneous (ΔG°<0), endothermic (ΔH°>0), and accompanied by increased randomness (ΔS°>0). The adsorption mechanism was attributed to electrostatic interactions, hydrogen bonding, and π–π interactions. Desorption experiments demonstrated that 0.2 mol/L HNO₃ effectively regenerated both adsorbents. After seven adsorption–desorption cycles, AC exhibited superior stability and reusability compared to ESh.
