Optical spectra and kinetic characteristics of radicals formed upon the photolysis of aqueous solutions of a FeOHaq 2+ complex and phenol

Abstract

The nature, optical spectra, and kinetic characteristics were determined for intermediate radicals formed upon the photolysis of aqueous solutions of a FeOH aq 2+ complex with phenol additives. The primary radical • ОН reacts with phenol to form ortho and para isomers of the Ph(OH) 2 • radical. The Ph(OH) 2 • radical eliminates a water molecule to form a phenoxyl radical PhO • . The latter disappears in the reactions with Fe III complexes, recombination, and disproportionation. The final products of photochemical transformations were determined. Among them, o quinone and diphenoquinones were identified.Phenol is a raw materials widely used in the chemical industry for syntheses of plastics, drugs, dyes, pesticides, preservatives, and surfactants. 1 Its world production ex ceeds 6 millions tons per year. 2 The active use of phenol contaminates the environment. The concentration of phenols in sewage can reach 100 µg L -1 ≈ 10 -6 mol L -1 . 3 Phenol is very toxic (maximum allowable concentra tion ≈ 1 µg L -1 ≈ 10 -8 mol L -1 4 ); therefore, the study of mechanisms of its degradation in natural water is of con siderable interest from the ecological point of view. Pho tochemical processes can play an important role in the removal of contaminants from natural aqueous sys tems. 5-7 A special attention is given to photoprocesses involving hydroxo complexes of Fe III , whose photolysis affords an • OH radical, being one of the most active species in chemistry and capable of oxidizing almost all organic admixtures dissolved in water, including such haz ardous substances as derivatives of the phenol series. 8-10 The concentration of iron ions in natural water can ap proach ∼10 -3 mol L -1 . 11,12 In the absence of coordinat ing organic ligands, hydroxo complexes are the main forms of Fe III in water with рН < 5, and the photochemistry of these complexes can exert a substantial effect on the bal ance of organic admixtures. 6,13Among the Fe III hydroxo complexes, the highest pho tochemical activity belongs to a complex FeOH aq 2+ , whose excitation is accompanied by the generation of an • OH radical with a high quantum yield (ϕ = 0.2, λ = 308 nm). 14,15 The activation energy of formation of the • OH radical is only ~10 kJ mol -1 , 15 which explains its formation by the electron phototransfer to the Fe III ion from both the outer sphere hydroxide ion (reaction (1)) and outer sphere water molecule (reaction (2)). 15(1)The formation of the • OH radical by the excitation of the FeOH aq 2+ complex was confirmed in several studies on steady state 6,7,14 and flash photolyses 10,15,16 using ac ceptors of the • OH radical. At least three mechanisms of degradation of phenol and its derivatives photoinduced by the FeOH aq 2+ complex were proposed in the literature. These mechanisms include reactions (3)-(9).