| Since the last century,endocrine disrupting compounds have emerged as a new type of pollutants in the natural water environment and have attracted more and more attention among researchers.The steroidal estrogens are among the major contributors to estrogenic effect in natural water environment,because Ⅰ)they are widely distributed,Ⅱ)the tranditional water treatment processes are not capable of effectively removing them and Ⅲ)all of them have strong endocrine disrupting effects.In addition to the physical adsorption process,the fate of estrogens in natural water environment can be divided into two aspects,degradation and humification.Photodegradation is a class of degradation process of EE2 and plays a significant role on the fate of EE2 in water bodies,while the oxidative polymerization catalyzed by extracellular peroxidase(POD)is an important humification process.Apparently,these two environmental process are uaually coexistent and simultaneous in natural water bodies,and the ubiquitous natural organic matter(NOM)has significant impacts on both processes.In the sunlit surface water,the photochemical process of NOM is the joint of photodegradation and humification process of estrogens;under conditions without sunlight,NOM with varying structures may also have different influence on oxidative polymerization catalyzed by POD.Systematic study on the aforementioned influence could aid in the understanding of the transformation of organic pollutants in natural water bodies,and further provide theoretical support for ecological risk assessment of organic contaminants.In this paper,17α-ethinylestradiol(EE2)and horseradish peroxidase(HRP)were selected as typical organic contaminants and representative extracellular peroxidase,and NOM from Suwannee River was used to study the enzymatic transformation of EE2 in the simulated sunlit surface water,and the respective effects of NOM with varying concentration on the photodegradation and enzymatic transformation of EE2 were further discussed.Meanwhile,NOM fractions with different molecular weight(MW)were obtained by ultrafiltration(UF),and the mechanism by which varying MW NOMs impacted the catalytical oxidation of EE2 without sunlight was also investigated.The main findings of this paper are displayed as follows:(1)Under simulated sunlight irradiation,the addition of HRP can significantly enhance the removal rate of EE2 in the presence of NOM,and further experiments proved that this was because HRP was capable of utilizing H2O2 produced by the photochemical process of NOM and thus catalyzing the oxidation of EE2.NOM had important impacts on both photodegradation and enzymatic transformation process of EE2.With NOM concentration increased from 2 to 20 mgC L-1,the direct photolysis rate constant decreased substantially,the apparent photodegradation rate constants were however increased from 0.028 to 0.048 h-1,and a strong linear relationship between the apparent photodegradation rate constants and the concentration of NOM was observed.The attenuating rate of H2O2 indirectly reflected that the enzymatic transformation rate of EE2 progressively decreased with increasing NOM concentration,which may be due to the higher concentration of the photoproduced ROS in the presence of higher concentration NOM.Besides,the presence of HRP in real natural water samples could also enhance the removal rate of EE2.We also studied the removal rate of ambient concentration EE2 in different reaction systems under sunlight exposure.The results showed that compared to the system containing NOM only,the addition of HRP did promote the removal rate of EE2.These two validation experiments further confirmed the importance of HRP-catalyzed oxidative polymerization on the fate of steroidal estrogens in natural water bodies.(2)Spectroscopic characterization of the MW-frantionated NOMs(Mf-NOMs)showed that small Mf-NOMs had a higher maximum fluorescence instensity,indicating that the phenolic structure was higher in small Mf-NOMs;the large Mf-NOMs had higher SUVA254,indicating that they contain more aromatic ring structures.All NOM fractions inhibited the transformation of EE2 induced by HRP,but the inhibitory mechanisms varied as the molecular weight of NOM changed.The small Mf-NOMs inhibited the enzytmatic reactions by competing with EE2 for HRP,whereas the large Mf-NOMs by sequestrating free EE2 from the attack of HRP.These two inhibitory mechanisms were coexisted in the system containing pristine NOM,but the importance of the two inhibitory mechanisms was highly detrermined by the reaction conditions.Competitive inhibition contributed more with lower HRP enzymatic activities,whereas the situation was exactly reversed with low EE2 concentration.In addition,the maximum fluorescence intensity and the SUVA254 of NOM had strong negative correlation with its competitive and sequestrating inhibition,respectively. |
PDF Full Text Request