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 nonb iodegradable toxic xylidines from industrial or military effluents is a matter of growing concern. Besides the traditional nondestructive tr eating methods to remove these substances in water bodies, the applica tion of advanced oxidation technologies such as Fenton photoassisted r eactions seems a suitable way to remove and mineralize these contamina nts and is the aim of the present study. Primary photochemical reactio ns 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. T he 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 sp ectroscopy. 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 solution s containing XYL/H2O2/FeCl3, increasing the oxidant concentration incr eased the amount of XYL+, indicating that the H2O2 competes with the C l- and XYL for the available Cl-. in solution. This was not the case o f the anion-radical Cl-2(.-). To decide if the radical Cl-2(.-) or ClO H.- prevails after photoexcitation of ferric chloride solutions, a rea ction scheme was considered for the formation of the radicals at acidi c pH through simultaneous differential equations. The reaction sequenc e could be kinetically modeled on the basis of laser spectroscopic mea surements. 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 c onstant (4.0 +/- 2.0) x 10(10) (M s)(-1). The Cl-. atoms react with H2 O2 with (1.8 +/- 0.7) x 10(10) (M s)(-1). The reaction of Cl-. atoms w ith H2O2 explains the decrease observed for XYL.+ and Cl-2(.-) radical s in solution with increasing H2O2 concentration. The latter rate cons tant was observed to be about 5 orders of magnitude higher than the ra te constant for the reaction k(Cl-2(.-) + H2O2 --> 2Cl(-) + H+ + HO2.) = (9.0 +/- 0.4) x 10(4) (M s)(-1).