title: [Adsorption of Congo red from aqueous solution on hydroxyapatite].
abstract: The adsorption of Congo red (CR) from aqueous solution on hydroxyapatite was investigated using batch experiments. The hydroxyapatite was effective for CR removal from aqueous solution. The adsorption kinetics of CR on hydroxyapatite well followed a pseudo-second-order model. The equilibrium adsorption data of CR on hydroxyapatite could be described by the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. Thermodynamic parameters such as Gibbs free energy change, enthalpy change and entropy change were calculated and showed that the adsorption of CR on hydroxyapatite was spontaneous and exothermic in nature. The CR adsorption capacity for hydroxyapatite decreased significantly with increasing pH from 8 to 10. Thermal regeneration showed that hydroxyapatite could be used for six desorption-adsorption cycles with high removal efficiency for CR in each cycle. The mechanisms for CR adsorption on hydroxyapatite with pH value below the pH at point of zero charge (pH(PZC)) include electrostatic attraction, hydrogen bonding and Lewis acid-base interaction. The mechanisms for CR adsorption on hydroxyapatite with pH value above its pH(PZC) include hydrogen bonding and Lewis acid-base interaction. Results of this work indicate that hydroxyapatite is a promising adsorbent for CR removal from aqueous solution.
authors.affiliation: College of Marine Science, Shanghai Ocean University, Shanghai 201306, China. yhzhan@shou.edu.cn
authors.forename: Yan-Hui
authors.initials: YH
authors.lastname: Zhan
authors.forename: Jian-Wei
authors.initials: JW
authors.lastname: Lin
mesh_terms: D000327:Adsorption; D003224:Congo Red / Q000737:chemistry*; D017886:Durapatite / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D013816:Thermodynamics
chemical_list: D003224:Congo Red; D017886:Durapatite