Primary photochemical reactions in the photo-Fenton system with ferric chloride. 1. A case study of xylidine oxidation as a model compound

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 H2O2 were examined by laser photolysis (lambda = 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 Cl- anion. The oxidation of xylidine by Cl-. or Cl-2(.-) lead to the formation of the XYL+ radical-cation [C8H9NH2](.+), having an absorption maximum at lambda = 420 nm which was unambiguously identified by pulsed laser spectroscopy. The decay of XYL+ radicals in solution takes place within 2 ms in a second-order reaction with 2k = 10(9) (M s)(-1). In solutions containing XYL/H2O2/FeCl3, increasing the oxidant concentration increased the amount of XYL+, indicating that the H2O2 competes with the Cl- and 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) x 10(7) (M s)(-1). Cl-. atoms oxidize XYL in the reaction with a constant (4.0 +/- 2.0) x 10(10) (M s)(-1). The Cl-. atoms react with H2O2 with (1.8 +/- 0.7) x 10(10) (M s)(-1). The reaction of Cl-. atoms with H2O2 explains the decrease observed for XYL.+ and Cl-2(.-) radicals in solution with increasing H2O2 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(.-) + H2O2 --> 2Cl(-) + H+ + HO2.) = (9.0 +/- 0.4) x 10(4) (M s)(-1).