Polyethylene glycol-modified hydroxyapa- tite nanoparticle was employed as a new and effec- tive adsorbent for the removal of indigo carmine dye from an aqueous solution. The synthesized adsorbent was characterized by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), energy- dispersive X-ray spectroscopy (EDX), and field-emis- sion scanning electron microscopy (FESEM). In addi- tion, the effects of factors such as pH, temperature, dye concentration, time, and amount of adsorbent on the removal efficiency or adsorbent capacity were investigated. The pH, temperature, and concentration (C) of dye solution positively affect q while the tem- perature and adsorbent dosage (m) negatively affects q. For R%, the m and C effects are inverse. With the increase of pH, the amount of electrostatic attraction between the adsorbent surface and dye increases. It will lead to an increase in dye adsorption. The opti- mum conditions were obtained as t = 60 min, pH= 7, T = 25 ̊C, m = 0.06 mg, and C = 700 mg˖L −1 . To study the equilibrium behavior of adsorbent-adsorbate inter- action, Langmuir, Freundlich, Sips, Dubinin-Radu- shkevich, and Temkin isotherms were considered. Experimental data were well described by Sips with R2 = 0.9927. The maximum adsorption capacity (qmax) of the Sips model was 129.3 (mg.g−1 ). The kinetic experimental data obey the pseudo-second-order model (R2 = 0.9989). Based on the negative value of thermodynamic parameters including ∆H°, ∆S°, and ∆G°, the adsorption process is exothermic and spon- taneous chemisorption at various temperatures (298, 308, 318, and 328 K). Based on the effect study of the presence of Mg2+ , Ca2+ , K+ , and Na+ ions on the removal efficiency of indigo carmine dye, the adsorp- tion capacity decreases with increasing salt concentra- tion, especialy for the + 2 charged ions. Furthermore, the desorption of indigo carmine dye from the modi- fied hydroxyapatite nanoparticles was evaluated with NaOH, HCl, HNO3, and CH3COOH. Also, indigo carmine removal by this adsorbent from tap water was investigated in comparison with laboratory conditions
