Michèle Bolte

Degradation of dibutyl phthalate by homogeneous photocatalysis with Fe(III) in aqueous solution

The degradation of dibutyl phthalate (DBP) photoinduced by Fe(III) in aqueous solution has been investigated under monochromatic irradiation and sunlight. Hydroxyl radicals OH, responsible of the degradation, are formed via an intramolecular photoredox process in excited Fe(III) aquacomplexes. The concentration in Fe(OH)2+ in the starting Fe(III) solution appears to be a controlling parameter of the degradation rate, as already stated in our previous works. The first step of the decomposition of DBP involves the hydrogen abstraction on the butyl chain mainly on the carbon in α-position to aromatic ring. The major primary photoproducts are hydroxy, dihydroxy and carboxylic derivatives. For prolonged irradiations, DBP and its photoproducts are completely mineralized due to the regeneration of the absorbing species and the continuous formation of OH radicals that confers a catalytic aspect to the process. Consequently, the degradation photoinduced by Fe(III) could be an efficient method of DBP removal from water.

Degradation of Diuron Photoinduced by Iron(III) in Aqueous Solution

The degradation of diuron photoinduced by iron(III) in aqueous solution has been investigated with different iron(III) species (monomeric species Fe(OH) 2+ , dimeric species Fe 2 (OH) 2 4+ and water-soluble oligomeric species) under monochromatic excitation at 365 nm and under sunlight. The rate of degradation depends on the concentration in Fe(OH) 2+ , the most reactive species in terms of . OH radical formation. The major photoproduct is 3-(3,4-dichlorophenyl)-1-formyl-1-methylurea which represents more than 60% of diuron disappearance. The mechanism only involves the attack by . OH radicals arising from iron(III) excited species. The half-lives of diuron when submitted to such a process in the environment were estimated to be 1-2 h and a few days according to the concentration of Fe(OH) 2+ (respectively 70% and <10% of total iron(III) concentration).

Iron (III) aquacomplexes as effective photocatalysts for the degradation of pesticides in homogeneous aqueous solutions.

The degradation of the herbicide asulam (4-amino-benzosulfonyl-methylcarbamate) was studied by excitation of iron (III) aquacomplexes in aqueous solutions at 365 nm as well as by exposition to solar light. Sulfanilamide was also studied as a model molecule. The initial step of asulam disappearance was shown to be due to the formation of hydroxyl radicals generated from the excitation of Fe(OH)2+, the most photoactive iron (III) monomeric species. However, when the iron (III) species was totally photoreduced to iron (II), the degradation of asulam in the presence of oxygen continued to completion. A photoreactivity of iron (II) species and/or iron (II) complexes under our experimental conditions was proposed. The experimental results indicate that the presence of iron (III), iron (II) and molecular oxygen is the condition for achieving the complete mineralization of the solution. The proposed photocatalytic cycle could provide an interesting tool for oxidations in the environment.

Photooxidation of 4-chlorophenol sensitised by iron meso-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrin in aqueous solution

The photosensitised degradation of 4-chlorophenol (4-CP) by iron meso-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrin (FeTDCPPS) has been studied in aerated aqueous solution, and is shown to lead to formation of p-benzoquinone (BQ) and p-hydroquinone (HQ) as main photoproducts. In deaerated solution no p-benzoquinone was formed. The photolysis products were identified by high performance liquid chromatography (HPLC) and UV-visible spectroscopy. The photodegradation in aerated solution was also carried out in the presence of sodium azide (NaN(3)) as a singlet oxygen [(1)O(2)((1)[capital Delta](g))] quencher, and showed a significant decrease in the rate of photolysis, suggesting under these conditions, that Type II sensitisation is one of the dominant mechanisms of 4-CP degradation. Support for this comes from laser flash photolysis and time-resolved singlet oxygen phosphorescence measurements. However, these also show direct reaction between the excited porphyrin and 4-CP, indicating that there are two mechanisms involved in the chlorophenol photodegradation.

Degradation of tributyltin chloride in water photoinduced by iron(III)

The degradation of tributyltin chloride (TBT) photoinduced by iron(III) was investigated. Upon irradiation at λexcitation >300 nm a photoredox process was observed, yielding iron(II) and ·2OH radicals. The disappearance of TBT was proved to involve only an attack by ·2OH radicals: the quantum yield of TBT disappearance was determined. A wavelength effect was observed; the shorter the excitation wavelength, the higher the rate of TBT disappearance. Most of the photoproducts were identified and the mechanism of degradation was elucidated. The main route to degradation is a stepwise debutylation of TBT to di- and mono-butyltin with final formation of inorganic tin. The complete mineralization of TBT was achieved with long irradiation times, leading to innocuous inorganic tin. Copyright

The photo-oxidation of 2,6-dimethylphenol and monophenylphenols by uranyl ion in aqueous solution

Abstract The photo-oxidation of 2,6-dimethylphenol and o -, m - and p -phenylphenols by uranyl ion was investigated in aqueous solution. Steady state and dynamic luminescence quenching studies at pH 0.8 show the rapid dynamic deactivation of excited uranyl ions by the phenols. At the natural pH of uranyl salt solutions (pH 2.3), differences are observed between the steady state and dynamic quenching behaviour, and it is suggested that these differences are due to uranyl hydrolysis. Flash photolysis studies with uranyl ion in the presence of 2,6-dimethylphenol and m - and p -phenylphenols show that the initial photoreaction leads to phenoxyl radical formation. The photolysis products were identified by high performance liquid chromatography (HPLC) and UV absorption spectroscopy. With 2,6-dimethylphenol in aerated solution, both quinone and dimer formation are observed. Kinetic studies show that these processes occur concurrently. In contrast, photolysis of degassed solutions leads to dimer formation only. The quantum yields for these processes are reported. The photo-oxidation of aerated solutions of o -phenylphenol in the presence of uranyl ions leads to the production of two dimers and the quinone, whereas with degassed solutions only the dimers are observed. The photo-oxidation of these substrates by uranyl ion is contrasted with the behaviour of [Co(NH 3 ) 5 N 3 ] 2+ as photooxidant of the same substrates. With m -phenylphenol, quinone and dimers are observed in aerated solution, whereas only the dimer is observed in deoxygenated solution. With p -phenylphenol, only dimer formation is observed. Possible mechanistic origins of the differences in the selectivity of oxidation by the different metal complexes are discussed.

Deactivation processes of the lowest excited state of [UO2(H2O)5]2+ in aqueous solution

A detailed analysis of the photophysical behaviour of uranyl ion in aqueous solutions at room temperature is given using literature data, together with results of new experimental and theoretical studies to see whether the decay mechanism of the lowest excited state involves physical deactivation by energy transfer or a chemical process through hydrogen atom abstraction. Comparison of the radiative lifetimes determined from quantum yield and lifetime data with that obtained from the Einstein relationship strongly suggests that the emitting state is identical to that observed in the lowest energy absorption band. From study of the experimental rate and that calculated theoretically, from deuterium isotope effects and the activation energy for decay support is given to a deactivation mechanism of hydrogen abstraction involving water clusters to give uranium(v) and hydroxyl radicals. Support for hydroxyl radical formation comes from electron spin resonance spectra observed in the presence of the spin traps 5,5-dimethyl-1-pyrroline N-oxide and tert-butyl-N-phenylnitrone and from literature results on photoinduced uranyl oxygen exchange and photoconductivity. It has previously been suggested that the uranyl emission above pH 1.5 may involve an exciplex between excited uranyl ion and uranium(v). Evidence against this mechanism is given on the basis of quenching of uranyl luminescence by uranium(v), together with other kinetic reasoning. No overall photochemical reaction is observed on excitation of aqueous uranyl solutions, and it is suggested that this is mainly due to reoxidation of UO2+ by hydroxyl radicals in a radical pair. An alternative process involving oxidation by molecular oxygen is analysed experimentally and theoretically, and is suggested to be too slow to be a major reoxidation pathway.

3-chlorophenol elimination upon excitation of dilute iron(III) solution: evidence for the only involvement of Fe(OH)2+

The transformation of 3-chlorophenol (3CP) photoinduced by iron(II) in aqueous solution has been investigated under monochromatic irradiation (lambda(exc) = 365 nm) representative of atmospheric solar emission. Hydroxyl radicals are formed via an intramolecular photoredox process in iron(III) excited hydroxy-complexes. Fe(OH)2+ is the most active complex in terms of HO* formation and according to our experiments and calculations, it appears that Fe(OH)2+ is the only iron(III) species involved in 3CP oxidation process. Hydroxyl radicals react very rapidly with 3CP, which is eliminated from the solution. The primary intermediates do not accumulate in the medium but rapidly degraded to non-absorbing compounds by a subsequent action of hydroxyl radicals.

Nitrilotriacetic acid transformation photo-induced by complexation with iron(III) in aqueous solution

SummaryThe phototransformation of iron(III) nitrilotriacetate, Fe(NTA), was studied at 20 °C under monochromatic excitation at different pHs. The conjugation of excitation wavelength and pH gives rise to different photochemical behaviour. In acidic medium, it always results in a redox process giving rise to FeII, HCHO and CO2 but the stoichiometry of the photoproducts depends on the excitation wavelength. At long wavelength (365 nm), the FeII/HCHO ratio of unity implies a redox reaction between FeIII and the carboxylic group whereas at short wavelength (254 nm) the Fe/HCHO ratio is equal to 2 and implies a redox process between FeIII and a water ligand. In neutral solution and at 365 nm, a photosolvation is observed with NTA release; at 254 nm a subsequent redox process between OH and the hydrous ferric oxide is involved. In terms of the fate of Fe(NTA) in the environment at pH 5–6 and under sunlight, all of the above photochemical reactions can occur.

The interaction "light, Fe(III)" as a tool for pollutant removal in aqueous solution: degradation of alcohol ethoxylates.

The photoinduced degradation of an alcohol ethoxylate (AE) (Brij 30) by Fe(III) in aqueous solution has been investigated. The study was carried out with the major fraction of ethoxymers having an alkyl chain length of 12 carbon atoms and n ethoxy units E (C12En). The Fe(III) sensitised degradation of this fraction occurs efficiently at 365 nm. The rate of degradation depends on the concentration of Fe(OH)2+, the most photoreactive species in terms of .OH radical formation. Formate ethoxylates were identified as photoproducts and shortening of the ethoxylated chain all along the degradation process was observed. The mechanism of Brij 30 degradation implies a major .OH radicals attack on the polyethoxylated chain. For prolonged irradiations, the total degradation of Brij 30 and of the photoproducts is obtained. Consequently, the degradation photoinduced by iron (III) could be an efficient method of AEs removal in water.