Annali di Chimica, 97, 2007, by Società Chimica Italiana 135 Short Communication PHOTODEGRADATION OF XENOBIOTIC COMPOUNDS RELEVANT TO ESTUARINE WATERS Serge CHIRON a , Claudio MINERO b , Davide VIONE b (°) a, Laboratoire Chimie et Environnement, Université de Provence, 3 place Victor Hugo, 13331 Marseille cedex 3, France. b, Dipartimento di Chimica Analitica, Università di Torino, Via Pietro Giuria 5, 10125 Torino, Italy. http://www.environmentalchemistry.unito.it INTRODUCTION Photochemical processes are the main removal pathway for a number of organic compounds in natural waters. 1-7 They include direct photolysis, photosensitised transformation by dissolved organic matter (DOM), and reaction with transient species generated by irradiation of photoactive compounds (DOM, nitrate, nitrite, Fe(III)). 8-11 The described processes are affected by the chemical composition of natural waters, which influences the concentration values of the photoactive species and, therefore, the transformation processes involving a given substrate. A substantial change in the chemical composition of surface waters takes place in deltas and estuaries, where the chloride build-up from saltwater deeply alters the ionic strength of river water and causes the precipitation of colloids. Furthermore, a sharp increase of DOM is usually observed. 12,13 A recent study into the degradation of carbamazepine under conditions relevant to estuarine systems has shown that the phototransformation kinetics of the substrate is substantially enhanced by chloride ions. 14 The most likely reason is that the interaction between Cl − and illuminated Fe(III) colloids, yielding the chlorinating and oxidising radical species Cl 2 − • , 15,16 causes faster substrate degradation compared to irradiation of Fe(III) alone, in which case degradation would mainly be accounted for by charge- transfer processes on the colloid surface. 17,18 Such an effect could more than compensate the [Fe(III)] decrease in estuaries. The present work is aimed at extending the results obtained on carbamazepine to other substrates, in order to find general indications on the behaviour of organic compounds, and in particular the xenobiotic ones, in estuaries. Diuron and 4-chloro-2- methylphenoxyacetic acid (MCPA) were chosen as substrates because they are commonly found in the Rhône river delta at µg L −1 levels. EXPERIMENTAL MCPA (purity grade > 95%) and humic acid sodium salt were purchased from Sigma-Aldrich. Diuron (>99%) from Cluzeau Info Labo (Puteaux-La Defense, France). Methanol was LiChrosolv (°) Corresponding author; fax: (++39)-011-6707615; e-mail: davide.vione@unito.it; www.abcrg.it