Alteration Of The Number-Averaged Molecular Weight And The Relevant Parameters Of Aquatic Dissolved Organic Matter During Its Photodegradation

Dissolved organic matter (DOM) in the ocean can be combined not only with clay particles but also with heavy metals and organic pollutants, and thus change their transformation, bioavailability and ecological toxicity. DOM is highly photochemical active. The photodegradation of DOM not only results in degradation products (such as CO, CO2, NH3, CH4, CH2O and CH3CHO, etc) which are of environmental and biological significance, but also can change its chemical composition and structure, which will cause changes in binding ability between DOM and pollutants. Photodegradation of DOM, therefore, not only plays a very important role in the biogeochemical cycle of carbon and nitrogen, but also is one of the crucial processes that control the ecological effects of itself and many other pollutants.In this study, we investigated the alteration of number-averaged molecular weight and relevant parameters (pH, A320 and DOC concentration) of Suwannee river DOM (purchased from International Humic Substances Society) during its photodegradation by irradiating in laboratory with a xenon lamp. The effects of environmental parameters such as light intensity, solution media, temperature, concentrations of dissolved oxygen and cupric ion were also studied. Major results and conclusions were summarized as follows:1. Increasing light intensity led to an enhanced photodegradation of DOM, resulting in a more pronounced decline in solution pH, A320 (absorption at wave length of 320nm), DOC concentration and number-averaged molecular weight of DOM. Some refractory DOM could only be degraded under higher intensity. The exist of this refractory part of DOM conduced to a slower decreasing of number-averaged molecular weight during the later stage of DOM photodegradation.2. Production of acidic intermediates during DOM photodegradation generally resulted in a decrease in the pH of the aquatic solution, while the pH of seawater system remained relatively stable since seawater is a natural buffer solution. As a result of medium effect and the effect of components (especially those photoactive compounds) contained in seawater itself, decrease in DOC concentration and number-averaged molecular weight of DOM during photodegradation is most profund for seawater system while decline of A320 is relatively slow.3. No much influence of temperature on DOM photodegradation was observed in the studied range. Consequently, decline in DOC concentration and number-averaged molecular weight of DOM during degradation process was not much affected by the temperature. However, the degradation pathway may be altered by temperature, leading to accumulation and thus a more difficult degradation of acidic intermediates at lower temperature. Accordingly, decreasing in medium pH and A320 is more pronounced at lower temperature.4. Decrease in medium pH, A320, DOC concentration and number-averaged molecular weight of DOM during degradation process is more profound at elevated dissolved oxygen (DO) concentration, and the effect of DO is more remarkable at the later stage of degradation. This is an indication that photo oxidation is a important approach of DOM degradation, and its rate may be higher that that of direct photodegradation.5. Since nitrate anion was added to the solution along with cupric ion at the same time, and a promotion effect of nitrate to DOM photodegradation is usually reported, a more notable decreasing in DOC concentration was observed when cupric nitrate was added to the solution. Moreover, cupric ion can be completed (or cheated) not only with DOM itself but also with organic acids produced as intermediates of DOM degradation, a more marked decrease of medium pH was usually observed while the descending tendency of number-averaged molecular weight of DOM was slowed down.