17alpha-Ethinylestradiol sorption to activated sludge biomass: thermodynamic properties and reaction mechanisms

Sorption is an important mechanism for removal of 17alpha-ethinylestradiol (EE(2)) in biological wastewater treatment. In an effort to deepen our understanding of this process, this work investigated the underlying thermodynamic parameters. Biomass was harvested from a membrane bioreactor (MBR) and a sequencing batch reactor (SBR), and sorption experiments were conducted over a range of temperatures. Sorption of EE(2) to activated sludge was spontaneous (DeltaG values were between -16 and -11 KJ/mol), enthalpy-driven (DeltaH values were -37 KJ/mol (MBR) and -48 KJ/mol (SBR)), and entropy-retarded (DeltaS values were -74 (MBR) and -119 J/mol/K (SBR)). Although EE(2) is nonpolar, hydrophobic interactions were not dominant driving forces. The thermodynamic data also suggested that EE(2) sorption to biomass was primarily physisorption, but it also included low-level chemisorption. The FT-IR results suggested that chemical reactions were not significant enough to shift the detectable chemical bonding characteristics of the biomass functional groups.