Light-induced degradation of aquatic humic substances by simulated sunlight

Filtered (0.22 mu m) humic surface water and solutions of isolated fulvic and humic acids at pH 4 and 7 were treated with simulated sunlight. Silver nitrate was used as a bactericide in order to determine the extent of microbial degradation during exposure of the surface water sample. In most sets of samples the decrease in the dissolved organic carbon (DOC) content was found to be independent of the added silver. This reveals that the loss of carbon does not to any substantial extent result from growth of bacteria in the water. After prolonged irradiation part of the dissolved organic matter in the surface water sample was found to precipitate as iron complexes. Several aliphatic carboxylic acids of low molecular weight were quantified by gas chromatography/mass spectrometry after irradiation of the water samples for 48 h. These acids accounted for 10% of the remaining DOC in the surface water sample. The photo-induced degradation of the dissolved humic acid reference was slower than the corresponding degradation of the dissolved fulvic acid reference, and it was found to be further retarded at pH 4. Accordingly, the net concentration of the identified carboxylic acids was lowest in the humic acid solutions.