Study On The Mechanism Of Extracellular Degradation Of Sulfadiazine Enhanced By Photosynthetic Electron Extraction From Rhodopseudomonas Palustris

The frequent detection of antibiotics in surface water,groundwater and even effluents from municipal wastewater treatment plants has not only caused social panic,but also posed a potential threat to human health.The ability to tolerate and degrade most toxic chemicals gives R.palustris a natural advantage in wastewater treatment.In this study,electroactive microorganism R.palustris was used as the research subject.Solid electrode was used as electron acceptor to continuously extract photosynthetic electrons generated during the growth and metabolism of R.palustris,so that R.palustris could form photosynthetic electroactive biofilms on solid electrode.In addition,the removal of antibiotics in wastewater can be accelerated under the drive of electrode potential.EPS secreted by PEABs was further extracted,and its chemical composition and spatial structure were analyzed by threedimensional excitation-emission matrix and confocal laser scanning electron microscopy.The role of electrode potential in photosynthetic electron extraction promoting the secretion of EPS by PEABs was studied,and the efficiency and mechanism of EPS in promoting the indirect photodegradation of SDZ was explored.By applying different electrode potentials(-0.2,0,0.2,0.4 V)to enhance the photoheterotrophic metabolism of R.palustris to generate photosynthetic electron,the effects of different electrode potentials on the extraction of photoheterotrophic electron and its efficiency and mechanism of enhancing the degradation of SDZ were analyzed.The results showed that the electrode potential could affect the removal efficiency of SDZ in aqueous solution by regulating the photoheterotrophic metabolism metabolic activity and electrochemical performance of PEABs.The best removal efficiency was achieved at 0.2 V,and the removal rate reached 78% within 58 h.However,the removal rate was only less than40% under open circuit conditions,indicating that the electrode potential played an important role in the degradation of SDZ by PEABs.Illumination has a great influence on the degradation ability of SDZ.It was difficult to degrade SDZ under dark conditions,and the removal rate was only less than 8%.It can be seen that illumination is an essential factor for better degradation abilities of PEABs.Under light and dark conditions,the removal rate of SDZ in the sterilized water was only 0.9% and 0.47%,respectively.The SDZ removal rate was only increased to 2.8% even if the anode potential 0.2 V was applied.And the analysis results of degradation products indicated that the degradation products of SDZ were mainly hydrolysis,biological reduction and hydroxylated products,which further demonstrated the great contribution of biological effect of PEABs in the removal of SDZ.The excellent degradation efficiency of PEABs posied at 0.2 V is attributed to the catalytic role of microorganisms in this process.By comparing the removal efficiency of SDZ under different potentials and combining with the differences in photosynthetic electron extraction ability and electrochemical performance of each PEAB,it can be seen that the degradation ability of SDZ by PEABs is directly related to its own metabolism and electrochemical activity.In addition,the better electrochemical activity of PEABs may be related to the fact that the electrode potential can promote the secretion of outer membrane cytochrome(Omc A)and thus change the extracellular electron transport pathway.On the basis of PEABs formed above,the regulatory effects of electrode potentials on EPS secretion in PEABs were further investigated,and the role of EPS in promoting the indirect photodegradation of SDZ was explored.The results showed that the addition of SDZ could increase the proportion of protein and decrease the proportion of polysaccharide in EPS structure,but had no significant effect on the change of humic acid content,indicating that the addition of SDZ could inhibit the secretion of extracellular polysaccharide,and protein could be used as a weapon against the toxicity of SDZ to obtain a large amount of secretion.Different from the increase of total EPS content in PEABs acclimated by electrode potential,the addition of SDZ under the open circuit condition can reduce the contents of each component in EPS to different degrees,and the decrease of polysaccharide content is the most obvious,indicating that R.palustris has improved its ability to resist adverse external environment after the regulation of electrode potential.By comparing the contents of each component of EPS in PEABs under different electrode potential,the three-dimensional fluorescence spectra and the spatial distribution of EPS obtained by CLSM,it can be seen that the good ability to extract photosynthetic electron at 0.2 V is attributed to the maximum protein content under this electrode potential.Similarly,The lower maximum current at 0.4 V might be related to the least secretion of extracellular proteins and the most extracellular polysaccharides near the electrode which blocked electron transfer to the electrode.The fluorescence intensity of fulvic acid-like and humic acid-like substances in TB was stronger,and the measured humic acid content was higher at-0.2 V,which indicated that under the long-term regulation of-0.2 V,it was conducive to the accumulation of humic acids which are tightly bound to cells.The chemical composition analysis of EPS combined with the results of EPS induced photosensitive degradation of SDZ indicated that protein and humic acids in EPS containing chromophoric groups were the main reasons for promoting the indirect photosensitive degradation of SDZ.In this study,the bioelectrochemical system was used to promote the secretion of photosensitizer which is beneficial to the degradation of antibiotics in water for the first time,providing a theoretical basis for the development of highly efficient and environmentally friendly photosensitizer for water treatment.