Effects Of Dissolved Organic Matter Extracted From Coastal Seawaters On Photodegradation Behavior Of Organic Micropollutants

Due to riverine transport,discharge from land-based sources,atmospheric deposition,mariculture activities and so on,coastal waters in many regions of the world have been polluted by organic micropollutants.It is of great significant to understand their transformation behavior in coastal waters for ecological risk assessment of these micropollutants.Previous studies proposed that photochemical transformation was an important transformation pathway for organic micropollutants in surface waters and dissolved organic matter(DOM)is the primary factor affecting their photodegradation.So far,the effects of freshwater DOM on photodegradation of organic micropollutants have been investigated.However,the role of marine DOM on photodegradation of organic micropollutants has seldom been investigated.In the present study,the impact mechanisms of coastal seawater DOM on the environmental photodegradation of some of emerging contaminants(sulfonamide antibiotics,sunscreen 2,4-dihydroxybenzophenone)were investigated.The main research contents and results are as follows:(1)A coupled reverse osmosis/electrodialysis equipment was established to extract DOM from seawaters.The constructed reverse osmosis/electrodialysis device was employed to isolate DOM from coastal seawaters in mariculture areas and in areas that less impacted by mariculture.The recovery of dissolved organic carbon(67 ± 15%)achieved the level of extraction of international peer reports.Scaled UV-vis absorption spectra,spectral slopes and the molar C:N ratios indicate that the isolated samples can represent the DOM in the source coastal seawaters for the study of photochemical property.It was believed that DOM concentrations in seawater are low and the relevant effects on photodegradation of organic micropollutants were regarded as negligible.This study found that DOM concentrations in some coastal areas are comparable to or even higher than those in freshwaters,which highlights the important of studying the effects of coastal seawater DOM on the environmental photochemical behavior of pollutants.(2)It was found that DOM from seawaters in the fish-shellfish mixed culture area of Dalian showed more promoting effects on environmental photodegradation of three sulfonamide antibiotics(SAs)than those from coastal seawaters in areas that are less impacted by mariculture.Simulated sunlight irradiation environmental photochemical experiments were performed to probe the effects of DOM extracted from mariculture impacted seawaters and from offshore areas,on photodegradation of sulfamethoxydiazine,sulfamonomethoxine,and sulfamethoxypyridazine.Results show that the three SAs are transformed mainly by indirect photodegradation induced by triplet excited DOM(3DOM*).Compared with DOM from the more pristine coastal seawaters,the DOM from mariculture impacted areas undergoes less photobleaching,contains higher percentage of humic-like materials and higher proportions of aromatic and carbonyl structures.Thus,the DOM from mariculture areas exhibits higher rates of light absorption,higher formation quantum yields of 3DOM*,higher 3DOM*steady-state concentrations and higher reactivity on photodegradation of the SAs.(3)2,4-dihydroxybenzophenone(BP-1)was adopted as a model compound to probe the disparate effects of DOM derived from coastal seawaters and freshwaters on photodegradation kinetics of organic micropollutants.Results show that DOM extracted from coastal seawaters promotes the photodegradation of BP-1 via the pathway involving 3DOM*mainly;while 3DOM*,singlet oxygen(1O2)and hydroxyl radical(·OH)were responsible for BP-1 photodegradation in the presence of freshwater DOM.Compared with the freshwater DOM,the coastal seawater DOM undergoes more photobleaching,contains lower aromatic C=C and C=O functional groups and thus has lower rates of light absorption and exhibits lower 3DOM*,1O2 and*OH steady-state concentrations.However,the photodegradation in presence of the coastal seawater DOM is faster than that in presence of the freshwater DOM,due to higher second-order reaction rate constants between BP-1 and coastal seawater 3DOM*and fewer antioxidants contained in the coastal seawater DOM.In summary,this study revealed the effects of coastal seawater DOM on photodegradation of three sulfonamide antibiotics and one benzophenone-derived sunscreen agent.This study highlights the importance of probing the effects of DOM from coastal seawaters on photodegradation of organic micropollutants.These results are helpful in deeply understanding the photochemical fate of organic micropollutants in coastal seawaters.