Visible-light-mediated photodegradation of 17β-estradiol: Kinetics, mechanism and photoproducts

The endocrine disrupting compounds 17β-estradiol (ESD) and its derivatives – major constituents of contraceptives – are incorporated to waste waters mainly through human excretion and anthropogenic industrial activities. In this paper, the kinetic and mechanistic aspects of the Riboflavin (Rf)- and Rose Bengal (RB)-photosensitized degradation of ESD and the model compound 5,6,7,8-tetrahydro-2-naphthol (THN), irradiating with visible light (>400 nm), have been studied in methanolic solution by stationary photolysis, polarographic detection of dissolved oxygen, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Rf (Vitamin B2) is a known human endogenous photosensitizer present as traces in most natural waters. The irradiation of Rf in the presence of ESD triggers complex competitive reactions that finally produce the photodegradation of both ESD and, to a lesser extent, Rf. ESD quenches singlet and triplet excited states of Rf, 1Rf* and 3Rf*, respectively. The quenching of 3Rf* with ESD gives rise, through an electron transfer process, to the semireduced form of the pigment, Rf−, a species that generates superoxide radical anion, O2−. Moreover, energy transfer from 3Rf* to dissolved molecular oxygen generates the oxidative species singlet molecular oxygen, O2(1Δg). Both O2− and O2(1Δg) produce the photodegradation of ESD. The exclusively O2(1Δg)-mediated photooxidation mechanism was studied employing the artificial dye RB as a sensitizer, a very efficient process especially in alkaline medium. Several photooxidation products, likely generated through the primary oxidation of the aromatic ring of ESD, have been detected under this experimental condition by means of HPLC/MS techniques.