| Antibiotics are widely used in human medical treatment,animal husbandry,and aquaculture,and disorderly discharged into surface water in the form of a prototype or metabolites,causing serious antibiotic contamination of lakes and rivers and harm of non-target aqutic organisms.Moreover,the characteristic of"pseudo-persistence"of antibiotics may cause serious consequences,such as the generation of antibiotic resistance genes in the environment.Microalgae are regard as a potential biological resource to alleviate and eliminate antibiotic pollution,due to they are the primary producers in natural water bodies and their features of high species diversity and large biomass.In order to further solve the difficulty of low efficiency of antibiotic degradation in traditional process,a technique combining micro-green algae and advanced oxidation process?AOP?was proposed in the present study to degrade sulfonamides?SAs?,which is a representative class of antibiotics.Meanwhile,the degradation pathway and mechanism of SAs by this technique were investigated.This study provides theoretical basis and technical support for the efficient removal of antibiotic contamination in water environment.1. The physiological and biochemical changes of green algae Chlorella vulgaris in the presence of three SAs?Sulfadiazine?SD?,sulfamerazine?SM1?,and sulfamethazine?SM2??with varying numbers of-CH3 groups and its SA-removal efficiency were investigated following a 7-day exposure experiment.The results showed that the growth inhibitory effect of SD?7.9-22.6%?,SM1?7.2-45.9%?,and SM2?10.3-44%?at concentrations of 10,30,90,and 270 mg/L resulted in increased proteins and decreased soluble sugars.Oxidative stress caused an increase in superoxide dismutase and glutathione reductase levels but decreased catalase level.The antioxidant responses were insufficient to cope-up with reactive oxygen species?hydrogen peroxide and superoxide anion?·O2-??levels and prevent oxidative damage?malondialdehyde level?.The ultrastructure and DNA of SA-treated algal cells were affected,as evident from the considerable changes in the cell wall,chloroplast,and mitochondrion,and DNA migration.C.vulgaris-mediated environment was able to remove up to 29%of SD,16%of SM1,and 15%of SM2 on day 7.2. The effect of three SAs?SD,SM1,and SM2 at concentrations of 1,5,20,and 50 mg/L,respectively?on the physiological and biochemical changes of green algae Dictyosphaerium sp.and its SA-removal efficiency were investigated following a16-day exposure experiment.The results showed that SAs had slight inhibitory effect on the biomass of the alga.The chlorophyll a content in algal cells decreased significantly on day 7 and then showed a“compensation phenomenon”.Moreover,the production of·O2-in the treatment goups was relatively higher than that in the control group on day 7.The increase in protein and polysaccharide contents played a defensive role in Dictyosphaerium sp.against SAs stresses.The contents of polysaccharides including capsular polysaccharide and water-soluble released polysaccharides were positively correlated with the concentrations of antibiotics.On day 16,Dictyosphaerium sp.-mediated environment exhibited 35%-45%,30%-42%,and 26%-51%removal of SD,SM1,and SM2,respectively.3. The degradation efficiencies of C. vulgaris and Dictyosphaerium sp.,respectively,combined with AOP on 5 mg/L of SD/SM2?1:1,v/v?was observed in 7days.Four kinds of photocatalysts were selected,including graphitic carbon nitride?g-C3N4?,humic acid?HA?,bismuth vanadate?BiVO4?,and copper-deposited titanium dioxide?Cu-Ti O2?.The removal efficiencies of SD and SM2 by C.vulgaris were 20.23%and 23.27%,respectively,and that by Dictyosphaerium sp.were 26.59%and 42.71%,respectively.The order of removal efficiency of SAs by four kinds of photocatalysts was g-C3N4>BiVO4>HA>Cu-Ti O2.The order of removal efficiency of SAs by two green algae combined with four kinds of photocatalysts was BiVO4>g-C3N4>Cu-Ti O2>HA,among which the photocatalysts with positive effects were g-C3N4,BiVO4 and Cu-Ti O2.The photocatalysts played a positive role in removing SM2 and SD by C.vulgaris were BiVO4,and g-C3N4 and BiVO4,and that by Dictyosphaerium sp.were BiVO4 and Cu-Ti O2,and g-C3N4,BiVO4 and Cu-Ti O2.In the groups of two green algae combined with BiVO4,the degradation efficiencies were the highest on day 4;the contents of hydroxyl radical?·OH?and·O2-both(SO42--S)all increased;and the contents of dissolved organic carbon and inorganic carbon basically unchanged.Based on the results of this experiment,it is advisable to explore the removal pathway and mechanism of SAs by the combined action of Dictyosphaerium sp.and BiVO4 in the next chapter.4. The photocatalytic degradation mechanism of SM2 by Dictyosphaerium sp. and BiVO4 was investigated by queching experiment and algal metabolomics analysis.The hydrothermally synthesized sample was characterized by X-ray powder diffraction,X-ray photoelectron spectroscopy,ultra-violet-visible diffuse reflectance spectroscopy,transmission electron microscopy,Brunauer-Emmett-Teller surface area,and Fourier transform infrared techniques.The results demonstrated that the prepared photocatalyst corresponded to phase-pure monoclinic scheelite BiVO4.The synthesized BiVO4 showed superior photocatalytic properties under the irradiation of visible light at 4000 lux,and more than 80%of photocatalytic degradation efficiency was obtained by algae-BiVO4 system.Based on queching experiments,the photocatalytic degradation of SM2 in the algae-BiVO4 system was primarily accomplished via the generation of triplet state dissolved organic matter?3DOM*?,and·OH played a small role in the degrading process.Metabolomics data showed that a total of 91 metabolites were significantly changed between the two comparison groups?algae-SM2 group vs algae group;algae-BiVO4-SM2 group vs algae-BiVO4group?.The glycometabolism pathways were increased and the tricarboxylic acid cycle was actived when BiVO4 was present.Based on the results of this experiment,the efficient degradation of SM2 was achieved by the combined action of 3DOM*,·OH,and algae in the algae-BiVO4 system. |
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