Role of iron species in the photo-transformation of phenol in artificial and natural seawater

Abstract

The role played by iron oxides (goethite and akaganeite) and iron(II)/(III) species as photo-sensitizers toward the transformation of organic matter was examined in saline water using phenol as a model molecule. The study was carried out in NaCl 0.7M solution at pH 8, artificial (ASW) and natural (NSW) seawater, in a device simulating solar light spectrum and intensity. Under illumination phenol decomposition occurs in all the investigated cases. Conversely, dark experiments show that no reaction takes place, implying that phenol transformation is a light- activated process. Following the addition of Fe(II) ions to aerated solutions, Fe(II) is easily oxidized to Fe(III) and hydrogen peroxide is formed. Regardless of the addition of Fe(II) or Fe(III) ions, photo-activated degradation is mediated by Fe(III) species.Several (and different) hydroxylated and halogenated intermediates were identified. In ASW, akaganeite promotes the formation of ortho and para chloro derivatives (2- and 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol), while goethite induces the formation of 3-chlorophenol and bromophenols. Conversely, Fe(II) or Fe(III) addition causes the formation of 3- and 4-chlorophenol and 2,3- or 3,4-dichlorophenol. 4-Bromophenol was only identified when irradiating Fe(II) spiked solutions.Natural seawater sampled in the Gulf of Trieste, Italy, has been spiked with phenol and irradiated. Phenol photo-induced transformation in NSW mediated by natural photosensitizers occurs and leads to the formation of numerous halophenols, condensed products and nitrophenols. When NSW is spiked with phenol and iron oxides, Fe(II) or Fe(III), halophenols production is enhanced. A close analogy exists between Fe(III), Fe(II)/goethite in ASW and NSW products. Different halophenols production in the natural seawater samples depends on Fe(II)/goethite (above all for 3-chlorophenol, 2,3-dichlorophenol and 4-bromophenol formation) and on Fe(III) colloidal species (3-chlorophenol).