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Photodegradation-induced changes in dissolved organic matter in acidic waters

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Abstract:

Photodegradation of dissolved organic matter (DOM) from stream waters was investigated using dissolved organic carbon (DOC) analysis, ultraviolet-visible absorbance, three-dimensional excitational emission matrix fluorescence, and high-performance size exclusion chromatography. The effects of altering pH and various iron concentrations on DOM characteristics during irradiation were examined. DOC concentration, absorbance, and fluorescence all decreased with increasing irradiation. These decreases were accompanied by a decrease in absorbance spectral slope and average molecular size and a blue-shift in fluorescence maximum; decreasing pH enhanced these changes. The photooxidation rate constants were wavelength dependent. For the ratio of the photooxidation rate constant at pH 4 to that at pH 8 under ultraviolet irradiation, there were two maxima at wavelengths of approximately 280 and 320 nm, respectively, indicating that aromatic fractions were most pH photosensitive. The isolated humic acid (HA) and fulvic acid (FA) fractions had different photodegradation characteristics in terms of the photooxidation rate constant and the effects of pH and iron. The results suggest that iron played a more significant role in the photodegradation of the HA fraction than that of the FA fraction and that the HA fraction was mainly responsible for the observed DOM photodegradation. The results indicate that DOM photodegradation in stream waters is strongly influenced by iron and acidity.

Nous avons étudié la photodégradation de la matière organique dissoute (DOM) dans les eaux courantes par analyse du carbone organique dissous (DOC), l'absorbance à l'ultraviolet et au visible, la fluorescence avec matrice tridimensionnelle d'excitation/émission et la chromatographie d'exclusion de taille de haute performance. Nous avons examiné les effets du changement de pH et des diverses concentrations de fer durant l'irradiation sur les caractéristiques de la DOM. Les concentrations de DOC, l'absorbance et la fluorescence diminuent toutes avec l'accroissement de l'irradiation. Ces réductions s'accompagnent d'une diminution de la pente spectrale d'absorbance et de la taille moléculaire moyenne, ainsi que d'un glissement vers le bleu du maximum de fluorescence; la réduction du pH accentue ces changements. Les constantes du taux de photo-oxydation sont fonction de la longueur d'onde. Sous irradiation ultraviolette, il existe deux maximums du rapport de la constante du taux de photo-oxydation à pH 4 sur celle à pH 8, respectivement aux longueurs d'onde d'environ 280 nm et 320 nm, ce qui indique que les fractions aromatiques sont les plus photosensibles en fonction du pH. Les fractions isolées d'acide humique (HA) et d'acide fulvique (FA) possèdent des caractéristiques différentes de photodégradation, en ce qui a trait à la constante du taux de photo-oxydation et des effets du pH et du fer. Nos résultats indiquent que le fer joue un rôle plus significatif dans la dégradation des fractions HA que FA et que la fraction HA est en grande partie responsable de la photodégradation de la DOM observée; ils montrent que la photodégradation de la DOM en eau courante est fortement influencée par le fer et l'acidité.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: May 1, 2005

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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