Photochemical aspects related to humic substances: Fid-Bau Portal

Photochemical aspects related to humic substances

Frimmel, Fritz H.

AbstractDissolved humic substances (HS) show yellow color and relatively strong absorption in the UV range [a(254 nm) ca. 0.04 cm⁻¹ for c (DOC)= 1 mg/L]. This is the basis for photochemical reactions in the photic zone of aquatic systems and in water treatment using UV sources. Even though understanding the mechanisms involved in the energy transfer and the resulting reactions is hampered by the poorly defined structure of HS, reliable information has been gathered on some typical aspects of their photochemistry. The luminescence of HS can be influenced and partly quenched by molecular interactions with other water constituents (e. g., heavy metals and organic micropollutants). The presence of oxygen may lead to the sensitized production of singlet oxygen (¹O2), that can react specifically with substances containing diene structures or low valent sulfur. Because of the presence of these structures in HS, humic molecules will also react with the sensitized products. As a consequence, their biological, chemical, and physical properties are influenced. In addition, HS have a significant impact on the photochemical treatment of organic micropollutants in water. This has to be kept in mind when using photochemical steps for water treatment. The results from model experiments reflecting the conditions in surface water and in water treatment are given and discussed. In the presence of H2O2, irradiation led to a transformation and partial degradation of HS. The rate of photochemical degradation of pesticides (e. g., atrazine) was decreased in the presence of HS. Fe and Mn quenched the luminescence. From this, a decrease of excited states of HS for sensitizing reactions can be deduced. The results suggest the manyfold and significant influences of HS on the photochemistry of aquatic systems.