Photochemical Degradation Of Aquatic Dissolved Organic Matter And Its Effect On Binding Of Cu²⁺ And Pb²⁺

Dissolved organic matter (DOM) plays an important role in regulating the chemical, biological and physical characteristics in aquatic systems. The photodegradation of DOM not only results in degradation products (such as CO, CO₂, CH₄, CH2O and CH3CHO, etc) which are of environmental and biological significance, but also can change its chemical composition, structure and physicochemical properties. And this will cause the changes of binding capacity between DOM and pollutants, thereby changing their bioavailability and biological toxicity.In this study, we investigated the photochemical transformation of Suwannee river DOM (purchased from International Humic Substances Society) irradiated by xenon lamp in aqueous solution, and its impact on binding ability of DOM to Cu²⁺and Pb²⁺. The effects of solution media, pH, temperature, salinity, concentrations of nitrate and nitrite, metallic ions (Cu²⁺, Pb²⁺), and light intensity on the photodegradation of DOM under xenon lamp were also studied. The change of binding coefficient between DOM and metallic ions (Cu²⁺, Pb²⁺) during the photodegradation process at different dissolved oxygen concentrations was also studied. The main results and conclusions were summarized as follows:1. The photodegradation process of DOM consists of two phases: in the first 72 hours, the concentration of DOC in solution declined rapidly over time, and the degradation depressed greatly in the second phase after 72 hours.2. The rate constant of the photodegradation of DOM decreased in the order of pH 11.41 > pH 10.83 > pH 4.64. It was also observed that the pH value of solution tends to decline in the process of photodegradation, apparently owing to the production of acidic compounds.3. The degradation rate was enhanced by increasing the solution temperature.4. The photodegradation of DOM was greatly promoted by increasing light intensity. Moreover, the decreasing ratio of DOC was proportional to the light intensity over the entire irradiation process. Under the same irradiation intensity, the degradation rate was inversely proportional to the initial DOC concentration.5. The degradation rate of DOM was the highest in natural seawater and the lowest in deionized water, decreasing in the order of natural seawater > synthetic seawater > tap water > deionized water. The salinity obviously played a key role.6. Both nitrate and nitrite showed promoting effect on degradation of DOM, while the effect declined with increasing concentrations of nitrate and nitrite in the dosage range studied.7. Both Pb²⁺ and Cu²⁺ exhibited photoquenching effect on DOM degradation and the effect was proportional to the dosage ratio.8. a320 (absorbency at wavelength of 320nm) was in a good correlation with DOC concentration in any case. Dissolved oxygen greatly enhanced the degradation of DOM, as manifested by the rapid decrease of a320 when the solution was saturaed with air, or oxygen especially.9. The solution pH gradually lowered down along with the degradation of DOM under different conditions. It was more significant while the solution was saturated with oxygen, indicating that the existence of oxygen promoted DOM photodegradation and generation of acidic compounds.10. The binding coefficients of DOM to Cu²⁺ and Pb²⁺ steadily declined with the irradiation, suggesting a photo-alteration of DOM's chemical and structural characteristics. The decrease in the capacity of DOM for binding Cu²⁺ and Pb²⁺, resulting from a combination of photo-mineralization (DOC loss) and photo-alteration of DOM, increases the freely dissolved fraction of the two metal ions and their availability and toxicity to aquatic organisms. The results from this study may have similar implications for heavy metal ions other than Cu²⁺ and Pb²⁺.