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Abstract. The effects of size and origin of natural dissolved organic matter compounds (DOM) on the redox cycling of iron were investigated in water samples from a marine system (Strait of Gibraltar), from a freshwater system (Lake of Murten, i.e., Murtensee, Switzerland), and from a "river-to-ocean" system (Rivers Scheldt and Rhine and corresponding estuaries). In the irradiated seawater samples, Fe(II) steady-state concentrations ([Fe(II)] ss ), both as percentage of total dissolved iron and normalized to 1 mg C L -1 , were higher in the low-molecular-weight (LMW) than in the high-molecular-weight (HMW) DOM fractions, despite higher net rates of H 2 O 2 formation in the HMW DOM fractions. Also in the samples from Murtensee, [Fe(II)] ss , both as percentage of total dissolved iron and normalized to the decadic absorption coefficient at 350 nm (a 350 , unit m -1 ), were higher in the LMW than in the HMW DOM fractions. Unlike in the seawater samAquatic Sciences ples, however, net rates of H 2 O 2 production, normalized to a 350 , were higher in the LMW DOM fractions, as compared to the HMW DOM fractions, indicating a higher efficiency of O 2• -formation with these LMW compounds from Murtensee. Based on the simulation of experimental data with mathematical kinetic modeling, we hypothesize that Fe(II) is formed through photolysis of Fe(III) complexes in the LMW DOM fractions from the Strait of Gibraltar and from Murtensee and that Fe(II) is stabilized by a ligand in the water samples from Murtensee. In the irradiated HMW DOM fractions from the "river-toocean" system, [Fe(II)] ss , normalized to a 350 , were generally higher in the water samples from the Rivers Scheldt and Rhine than in the water samples from their estuaries, suggesting that terrestrially derived DOM has a higher photochemical reactivity towards Fe(II) formation than autochthonous DOM.

The photochemical behaviour of many aromatic pesticides (mainly herbicides) are compared and the main reactions are separated in three different classes: 1- reactions involving carbon-halogen bond; 2 - other reactions involving the aromatic ring; 3 - reactions of the aliphatic moiety. It appears that the nature of the substituents and their relative positions on the ring play a major role in the orientation of the reaction. The molecular and ionic forms of ionisable molecules may have different photochemical behaviour. A wavelength effect is observed with some compounds. The case of mecoprop [2-(4-chloro-2-methylphenoxy)-propanoic acid] is presented as an example.The irradiation of herbicides in aqueous solution may lead to the formation of photochemical intermediates more toxic to microorganisms than the initial substrate.

Mecoprop was irradiated under various conditions of pH, oxygenation and wavelengths in order to study the reactions involved in the phototransformation. Four main photoproducts were identi®ed: 2-(4-hydroxy-2-methylphenoxy)propionic acid (I), o-cresol (II), 2-(5-chloro-2-hydroxy-3-methylphenyl)propionic acid (III) and 4-chloro-o-cresol (IV). When the anionic form of mecoprop was irradiated between 254 nm and 310 nm (UV-C or UV-B), I was the main photoproduct. At 254 nm its formation initially accounted for more than 80% of the transformation. It has not previously been reported in the literature. The reaction results from a heterolytic photohydrolysis. Product II accounted for only a low percentage of the transformation. The stoichiometry was different with the molecular form: the main photoproduct, III, resulted from a rearrangement after a homolytic scission. Products I, II and IV were also formed as minor photoproducts. Some other minor photoproducts were also identi®ed. In contrast, IV was the main photoproduct under sunlight irradiation or when solutions were irradiated in near-UV light (UV-A). This wavelength effect is attributed to the involvement of an induced phototransformation; IV is also the main photoproduct when the phototransformation is induced by Fe(III) perchlorate or nitrite ions. In usual environmental conditions the excitation of the molecular form is negligible and the phototransformation is mainly due to induced photoreactions.