Environmental contamination in groundwater involving a variety of nonbiodegradable toxic xylidines from industrial or military effluents is a matter of growing concern. Besides the traditional nondestructive treating methods to remove these substances in water bodies, the application of advanced oxidation technologies such as Fenton photoassisted reactions seems a suitable way to remove and mineralize these contaminants and is the aim of the present study. Primary photochemical reactions in the water solutions of ferric chloride complexes in the absence and in the presence of H 2 O 2 were examined by laser photolysis (λ ) 347 nm) using xylidine (2,4-dimethylaniline, XYL) as probe molecule. The Cl 2 •radicals are formed as a result of the reaction of Cl • atoms and OH • radicals produced during the photodissociation of ferric chloride and ferric hydroxy complexes in the presence of Clanion. The oxidation of xylidine by Cl • or Cl 2 •lead to the formation of the XYL + radical-cation [C 8 H 9 NH 2 ] •+ , having an absorption maximum at λ ) 420 nm which was unambiguously identified by pulsed laser spectroscopy. The decay of XYL + radicals in solution takes place within 2 ms in a secondorder reaction with 2k ) 10 9 (M s) -1 . In solutions containing XYL/H 2 O 2 /FeCl 3 , increasing the oxidant concentration increased the amount of XYL + , indicating that the H 2 O 2 competes with the Cland XYL for the available Cl • in solution. This was not the case of the anion-radical Cl 2 •-. To decide if the radical Cl 2 •or ClOH •prevails after photoexcitation of ferric chloride solutions, a reaction scheme was considered for the formation of the radicals at acidic pH through simultaneous differential equations. The reaction sequence could be kinetically modeled on the basis of laser spectroscopic measurements. The rate constant of Cl 2 •with XYL was found (3.7 ( 0.3) × 10 7 (M s) -1 . Cl • atoms oxidize XYL in the reaction with a constant (4.0 ( 2.0) × 10 10 (M s) -1 . The Cl • atoms react with H 2 O 2 with (1.8 ( 0.7) × 10 10 (M s) -1 . The reaction of Cl • atoms with H 2 O 2 explains the decrease observed for XYL •+ and Cl 2 •radicals in solution with increasing H 2 O 2 concentration. The latter rate constant was observed to be about 5 orders of magnitude higher than the rate constant for the reaction k(Cl 2 •-+ H 2 O 2 f 2Cl -+ H + + HO 2 • ) ) (9.0 ( 0.4) × 10 4 (M s) -1 .
This study presents the detailed nature of iron clusters formed on Fe 3+ -Nafion membranes. The catalytic nature of these clusters during immobilized Fenton processes was observed to be a function of the deposition method of Fe ions on the Nafion. The nonbiodegradable azo-dye Orange II and 2-propanol were utilized as convenient organic model compounds in photoassisted Fenton degradation processes. The highest photocatalytic activity was observed when samples were prepared by ion exchange between iron(III) aquacomplexes and H + or Na + as counterions of the Nafion SO3group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of [Fe(H2O)6] 3+ and binuclear complexes [Fe(H3O2)Fe] 5+ and [Fe-O-Fe] 4+ . If NaOH or ammonia was added to the former samples prepared by ion exchange, Nafion-Fe membranes with low photocatalytic activity were obtained showing R-Fe2O3 and [Fe-O-Fe] 4+ . Detailed high-resolution transmission electron microscopy was carried out for the Nafion-Fe ion-exchanged and also base-treated membranes showing R-Fe2O3 nanocrystallites of 3.5-5 nm. Spectral bands were found for iron oxides in the Fe 3+ -Nafion by femtosecond laser spectroscopy. The R-Fe2O3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with R-Fe2O3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of R-Fe2O3 in SO3 --water clusters was observed with time constants close to 320 fs, 1.5 ps, and 31 ps after the excitation pulse. Samples of Fe 3+ -Nafion membranes with high activity show different transient dynamics relative to the Fe 3+ -Nafion with low activity. Correlation of the photocatalytic activity of Fe 3+ -Nafion with UV-vis, Fourier transform infrared, Mo ¨ssbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear [Fe(H2O)6 ] 3+ , binuclear complexes [Fe(H3O2)Fe] 5+ and oxo-bridged [Fe-O-Fe] 4+ found in the membranes.
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