Simulated Photochemistry Process Of Dissolved Organic Matter (DOM) And Its Effects On Binding Of Environmental Organic Pollutants

In this study, we investigated kinetics of O₂ consumption, pH changes, photobleaching, and more importantly, MW and distribution changes under simulated solar irradiation in different O₂ and Fe levels and the wavelength of incident light of the DOMs prepared from a German soil humic acid, Suwannee River humic and fulvic acids, and natural water from the Saguenay Fjord in Quebec, Canada. For the first time the impact of photo-oxidation of DOM on its affinity for binding organic pollutants (two model compounds, benzo(a)pyrene (BaP) and pentachlorophenol (PCP)) were investigated using above DOM samples plus Rimouski river fulvic acid. We also examined the effects of O₂ abundance and the wavelength of incident light on photo-alteration of bingding coefficient (K_oc) and discussed the effects of pH, molecular weight and aromaticity of DOM on K_oc.In generally, high dissolved oxygen and total iron concentration can accelerate the photooxidation of DOM and caused rapid decreases in the dissolved oxygen, absorptivity and pH. Adding 20 nM DFOM in SRHA decreased the oxygen consumption rate by 20%, while adding FeCl₃ to reach total 10 μ M iron accelerated the oxygen consumption rate by 22%, 94%, 45% for SFW, SRFA, FSHA respectively. The oxygen consumption of UV-A plus visible or only visible light irradiations were 61 ±25% and 20 ± 19% of that of full spectrum. The pH of SRHA, SRFA and FSHA increased 0.1 unit after 72-h N₂-saturated irradiations (initial pH5.0), while SFW decreased to 7.18 from initial 8.18. UV-irradiation is the key factor to cause pH decreases during irradiation. Both dissolved oxygen and a₃₂₀ were found a good exponential correlation in the course of irradiation under different conditions.The production of DIC were linear with irradiation time and were found the quickest in the O₂-saturated and total Fe 10 μ M irradiations, followed by air-saturated,UV-A+visible and visible irradiations.MW decreased continuously in time-series irradiations in most cases, and found a well inversely correlated with E2/E3. Wavelength dependence irradiation showed that the each part of light region, UV-A, UV-B, and visible, played an important role to reduce the MW, their contributions were 32.2±19.4%, 31.4±23.2% and 36.5±10.6% respectively. Addition of Fe increased the initial MW of DOM, but also can accelerated MW decrease combined with dissolved O2; while addition of DFOM retarded the MW decline rate. The polydispersity also decreased during irradiations and the decrease well matched the decrease extent of MW.Solar-simulated irradiations substantially decreased the BaP and PCP binding affinity to DOM. After 24-h irradiations, KBap decreased 33%, 33%, 23%, 14 % for SFW, SRFA, SRHA, and FSHA respectively, RRFA incresed 6%; KPCP decrased 48% and 39% for SRHA and FSHA respectively. The decreases of KoC (corrected for photo-induced pH changes) were positively correlated with the decreases of the molecular weight and aromaticity of DOM in the time-course of irradiation. The binding capacity, R, defined as KoC * [DOC], is more useful to evaluate the relative importance of DOC loss and structural change of DOM. The contribution of ARdoc to AR was generally lower but significant compared to the contribution of ARk. Decreases in pH due to photo-production of acidic products often partly offset the reduction of the binding affinity (7%) caused by direct photo-alteration of DOM's chemical structure, suggesting that the binding affinity of DOM in natural waters with high acid-neutralizing capacity is expected to be more severely affected than in natural waters with low acid-neutralizing capacity. Increasing O2 abundance accelerated the decreases of the binding affinities as a result of enhanced photo-degradation of DOM. Over a 72-h exposure period, full-spectrum irradiation reduced Kb3p of SRHA, among which UV-B (i.e. full spectrum minus (UV-A plus visible)) contributed 7%, UV-A (i.e. (UV-A plus visible) minus visible) 42% and visible 51%. UV-B, UV-A and visible wavelengths accounted for 8%, 38% and 54%, respectively, of the reduction of BaP binding by SRFA. For the decrease of Kpcp in SRHA, the relative contribution in ascending order was UV-B (18%), visible (35%),and UV-A (47%). Visible light played a more important role in reducing the molecular weight and aromaticity of DOM (and hence its binding affinities) than in reducing the content of dissolved organic carbon (DOC) while vice versa for UV radiation, indicating that photochemical reduction of the binding affinities may occur in natural waters at depths deeper than UV radiation can reach.The decreases of the capacities of DOM for binding BaP and PCP, resulting from a combination of photo-remineralization (DOC loss) and photo-alteration of DOM's chemical characteristics, will increase the freely dissolved fractions of the two compounds and thus their availability and toxicity to aquatic organisms. The results from this study may have similar implications for organic pollutants other than BaP and PCP.