Sunlight-driven photo-transformation of bisphenol A by Fe(III) in aqueous solution: Photochemical activity and mechanistic aspects

Iron is one of the most abundant elements in aquatic environments, and plays important roles in the fate and transport of environmental contaminants. Previous studies on the photochemical properties of Fe(III) species have largely focused on complexes formed between Fe(III) and environmental ligands such as natural organic matter (NOM) under UV irradiation, whereas the potentially important roles of hydrolysis species of Fe(III) in Fe(III)-mediated photo-transformation of environmental contaminants under solar light are not fully understood. In this study, the solar light-driven photochemical activities of hydrolysis species of Fe(III) were further explored, using a system containing only 0.5 mM Fe₂(SO₄)₃ and bisphenol A. The important role of colloidal [Fe(OH)₃]_m, formed from the hydrolysis of Fe³⁺, as a core photochemical species of Fe(III) was proposed and verified. Interestingly, O₂^-, rather than OH, was identified (via electron spin resonance) as the key active radical responsible for the degradation of bisphenol A. We propose that unlike Fe(OH)²⁺, which under UV irradiation can yield OH (Fe(OH)²⁺ + hv → Fe²⁺ + OH), colloidal [Fe(OH)₃]_m produces O₂^- even in sunlight ([Fe(OH)₃]_m + 2O₂ + hv → Fe(II) + 2O₂^- + H₂O). The fact that Fe(III) can produce strong radicals in sunlight may have important environmental implications.