Influence of the excitation wavelength on the photoinductive properties of humic substances

We studied the efficiency of soil-extracted humic and fulvic acids to photoinduce the transformation of phenylurea derivatives and chloroaromatic compounds when irradiated by polychromatic light within the wavelength range 300-450 nm. The results were compared with those obtained upon monochromatic irradiations at 253.7 and 365 nm. In polychromatic light, solvated electrons are produced, as observed at 253.7 nm, and hydroxyl radicals are formed in oxygenated solutions. Reactions between the substrates and excited triplet states also take place leading to hydrogen atom or energy transfer as at 365 nm. With the humic acid used, the oxidation of the substrates by hydroxyl radicals is the main route of degradation. With the fulvic acid, both the oxidation by hydroxyl radicals and the reactions involving the triplet states occur. (C) 1999 Elsevier Science Ltd. All rights reserved.