Binding of phenanthrene to extracellular polymeric substances (EPS) from aerobic activated sludge: A fluorescence study

Extracellular polymeric substances (EPS) are the essential components of activated sludge for removal of pollutants from wastewater. Limited information is available on the binding constants and binding mode of organic pollutants to EPS. In the present study, binding of phenanthrene (PHE) to extracellular polymeric substances (EPS) from aerobic activated sludge was investigated using fluorescence spectroscopy. Two protein-like fluorescence peaks (Ex/Em = 225 nm/340 nm for peak A; Ex/Em = 275–280 nm/338 nm for peak B) were identified in the three-dimensional excitation-emission matrix (3DEEM) fluorescence spectroscopy of the EPS. The fluorophores in EPS were clearly quenched by PHE and the quenching processes were static. The quenching constants (ln Ka) were in the range of 11.27–13.82 M−1 and the binding constants (log Kb) in the range of 6.11–8.98 M−1. The binding site number increased with increasing temperature. The corresponding thermodynamic parameters ΔG, ΔH and ΔS were calculated. The interaction of the fluorophores in EPS and PHE is spontaneous and exothermic. The binding of EPS with PHE is dominated by the hydrophobic interactions. Fluorophore A has stronger hydrophobic interaction with PHE than fluorophore B.