High-valent iron (FeVI, FeV, and FeIV) species in water: characterization and oxidative transformation of estrogenic hormones

Abstract

This paper presents solid state synthesis and characterization of tetra-oxy iron(IV) and iron(V) species in their salt forms (Na₄FeO₄–Fe^IV and K₃FeO₄–Fe^V). Stability of the synthesized salts, commonly called ferrates, in water was determined by applying the ⁵⁷Fe Mössbauer spectroscopy technique. Within 2 s in water, Fe^IV converted into Fe^III while Fe^V transformed into Fe^VI and Fe^III at pH = 8.2. Comparatively, Fe^VI (bought as K₂FeO₄) remained stable in aqueous solution during the short time period. The oxidative removal efficiency of the high-valent iron species was then tested against five environmentally important estrogenic hormones (estron (E1), 17-β-estradiol (E2), estriol (E3), 17-α-ethinylestradiol (EE2), and diethylstibestrol (DES)) in effluent water of a wastewater treatment plant. Three dosages of iron species (1, 10, and 100 mg L⁻¹) were applied to the effluent water. An increase in the concentration of dosages enhanced the removal of estrogens. Both Fe^V and Fe^VI were effective in degrading estrogens, but Fe^IV showed limited oxidation capacity to transform estrogens. The oxidized products of the estrogens were analyzed using Raman spectroscopy and high-performance liquid chromatography-mass spectrometry (HPLC-MS) techniques. Results demonstrated the transformation of estrogens into low molecular weight oxygenated compounds such as quinone-like and opened-aromatic ring species. A detailed study on E1 by using excess Fe^VI showed the mineralization of the parent compound. The results demonstrate great potential of high-valent iron species in the degradation of endocrine disruptor chemicals like estrogens with several superior aspects including fast reactions, complete degradation and/or formation of benign organic species, and environmentally-acceptable iron oxide by-products.