Photodegradation-induced changes in dissolved organic matter in acidic waters

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.