[Adsorption of Congo red from aqueous solution on hydroxyapatite]

Area of Science:

• Environmental Science
• Materials Science
• Chemistry

Background:

• Congo red (CR) is a widely used dye in the textile industry, posing environmental risks due to its persistence and potential toxicity.
• Effective and sustainable methods for removing CR from industrial wastewater are crucial to mitigate environmental pollution.

Purpose of the Study:

• To investigate the efficacy of hydroxyapatite as an adsorbent for removing Congo red from aqueous solutions.
• To elucidate the adsorption mechanisms, kinetics, and thermodynamics of CR onto hydroxyapatite.

Main Methods:

• Batch adsorption experiments were conducted to study CR removal by hydroxyapatite.
• Adsorption kinetics were analyzed using the pseudo-second-order model.
• Equilibrium data were fitted to Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models.
• Thermodynamic parameters and the effect of pH were evaluated.
• Regeneration studies were performed using thermal methods.

Main Results:

• Hydroxyapatite demonstrated high efficiency in removing CR from aqueous solutions.
• The adsorption process followed pseudo-second-order kinetics and was well-described by Langmuir, Freundlich, and D-R isotherms.
• Adsorption was spontaneous and exothermic, with capacity decreasing at higher pH (8-10).
• Hydroxyapatite maintained high removal efficiency over six regeneration cycles.
• Adsorption mechanisms involved electrostatic attraction, hydrogen bonding, and Lewis acid-base interactions, varying with pH relative to the point of zero charge.

Conclusions:

• Hydroxyapatite is a highly effective and regenerable adsorbent for Congo red removal.
• The study provides insights into the adsorption mechanisms and thermodynamic favorability.
• Hydroxyapatite presents a promising, sustainable solution for treating dye-contaminated wastewater.