Adsorption of 17β-estradiol onto two polymeric adsorbents and activated carbon

Removal of 17 beta-estradiol from aqueous streams by two polymeric resins XAD-7 and XAD-16, and by Granulated Activated Carbon (GAC) was investigated. Adsorption equilibrium isotherms were measured at various ionic strengths (0.1, 20 and 100 mM NaCl) and pH (4, 5.7, 10). The adsorption equilibrium isotherms were correlated with the Langmuir and Freundlich isotherm models. It was found that the adsorption capacity was strongly dependent on the ionic strength and pH of the external phase in the case of GAC. There are no serious influences of salt concentration and pH were observed in the case of polymeric adsorbents. Higher sorption capacity was obtained in the case of the XAD-16 polymer and GAC. Thermodynamic parameters for the adsorption process were determined using adsorption equilibrium parameters calculated at three temperatures (23, 37, 50 degrees C). The negative values of Gibbs energy about -10 kJ/mol for XAD-7, -14 kJ/mol for XAD-16, and -18 kJ/mol for GAC indicate that adsorption has spontaneous nature. For the polymeric adsorbents the enthalpy of adsorption was found to be negative and relatively low -11 and -26 kJ/mol for XAD-7 and XAD-16, respectively, that indicates physical character of adsorption. The adsorption on GAC was chemical in nature according to calculated value of enthalpy (58 kJ/mol). The negative values of entropy for the polymers indicate decreased randomness at the solid/solution interface, while the positive entropy for GAC indicates an increased randomness during estradiol adsorption.