The Mechanism Of Degradation Of Metronidazole By Algae-Anoxygenic Phototrophic Bacteria Electroactive Biofilms In The Coexistence Of Nanomaterials

Photosynthetic microorganisms have an ability to convert solar energy into chemical energy under light conditions and photosynthetic electrons,produced from the process of energy exchange,transfer through the photosynthetic respiratory chain,which leads to ATP synthesis and activating cell metabolism.Therefore,simultaneous pollutant removal and biomass recovery based on photosynthetic microorganisms are recognized as one of the most promising advanced sewage treatment and recycling technologies.Applying photosynthetic microorganisms to an electrochemical system,bias potential can regulate cell metabolism,and promote the formation of electroactive biofilm,so that the high efficiency of electron transfers from intracellular to extracellular and the removal rate of pollutants are achieved.In this study,electrochemical methods,in-situ biofilm observation methods,three-dimensional fluorescence and ultraviolet spectroscopy and other optical techniques were used to explore the mechanism of high-efficiency removal of metronidazole（MNZ）by photosynthetic biofilm under the optimal potential.In addition,with the introduction of nanomaterials,it’s necessary to explore the role of nanoparticles when MNZ is removed,because it’s meaningful to pave the way for broadening the application range of photosynthetic bioelectrochemical technology and breaking through the technical bottleneck.The capacity of photo-electron uptake and MNZ removal by biofilm under different working potentials（-0.3 V,0 V,0.2 V）were learned.The result shows that the photo-electron from the co-culture of Chlorella vulgaris and Rhodopseudomonas Palustris in bioelectronchemical system（BES）were successfully up extracted by electrode,and the removal rate of MNZ indeed significantly improved.The maximum photosynthetic current reaches 0.44 m A at 0 V vs SCE and 0.12 m A at 0.2V,and the degradation rate were increased by 33 times and 26 times respectively compared with open circuit.Although the maximum current is 0.13 m A at 0.3 V,it takes 108 hours─almost 4 times as long as 0.2 V to reach it,and the removal rate is slowest at 0.3 V.It’s noted that the current of pure culture of R.Palustris is about an order of magnitude smaller than co-culture,and degradation rate of pure culture is one twenty-first of co-culture.It is found that the extracellular extraction of photosynthetic electrons significantly promoted the accumulation of EPS in further analysis.When the potential is 0V,the contents of polysaccharides,protein and humic acid of EPS are Increased 28,14.1 and 22.59mg·g-1 dry cell respectively in comparison with open circuit.It’s concluded that 0V significantly promotes the removal of metronidazole from water,which is mainly attributed to the adsorption and the photosensitive degradation induced by EPS.This research can provide a theoretical basis for enhancing removal technology of sewage toxic organic pollutants which relies on regulating the photosynthetic metabolism though controlling potential.Different nanomaterials（CNT,Ti O₂ NPs,Fe₃O₄ NPs）and the concentration（74μg/L,185μg/L,370μg/L）influence the performance of photosynthetic biofilm.When the water environment contains low concentration of nanomaterials,only Fe3O4 NPs shows a great help for enhancing electrons uptake of biofilm,whose maximum current increases by 42.7%compared to the control.But there is little effort on MNZ removal.When the nanomaterials in the water environment are at an intermediate concentration,compared with the control,the current increases by 42.7%（CNT）,37.7%（Ti O2 NPs）,138%（Fe3O4 NPs）;at the same time,the degradation rate of MNZ are 1.13（CNT）,1.49（Ti O2 NPs）,1.42（Fe3O4 NPs）times as rapid as control.At this concentration,the nanomaterial effectively improves the capacity of electrons transfer from the microorganism to the surface of the electron acceptor,and enhances the metabolic activity of MNZ.When the concentration of nanomaterials in the water environment is high,the current of the reactor containing Ti O2 NPs and Fe3O4 NPs shows a downward trend,while the degradation rate of MNZ still keeps an upward trend;while the current of the CNT reactor keeps increasing,an increase of 115%compared to the control,the degradation rate of MNZ has decreased compared with the addition of intermediate concentration.Through analysis of extracellular polymer and confocal laser observation,lower concentration of CNT promotes the growth of biofilm,and when exceeded a certain threshold,it is more toxic to biofilm;regardless of the concentration,Ti O2 NPs showed inhibition of biofilm Growth;Fe3O4 NPs are relatively mild stimulating effects on photosynthetic biofilms.