The Study Of Tetracycline Degradation By Photosynthetic Electron Extraction From Chlorella Vulgaris

Microalgae-based wastewater technologies demonstrate a great potential in wastewater recycling.Antibiotics have strong antimicrobial activity and drug resistance.They always coexist with nutrients(such as nitrogen and phosphorus)in microalgae-treated wastewater,which induces the production of drug resistance genes.Thus,antibiotics take high risks for environment.With the increasing concern on water security and ecological environment health,the efficient removal of antibiotics is a new technological challenge to microalgae-based wastewater treatment.In this study,a novel technological approach was developed to promote the photosensitive degradation of antibiotics.A poised electrode was used to build an electrochemical redox gradient difference between the two sides of microalgae cell membrane for photosynthetic electrons extraction.The effect of bias potentials and exogenous mediators on the antibiotic degradation,and the effect of photosynthetic electrons extraction on the extracellular polymeric substance metabolism of microalgae were explored.As a microalgaeoriginated photosensitizer,the extracellular polymeric substance were extracted and directly applied for photolytic removal of antibiotic in wastewater to avoid the production of antibiotic resistance genes.This research provides a theoretical basis for the development of the new energy-saving and environmentally-friendly technological approach,strengthening the removal efficiency of organic and toxic micro-pollutants in wastewater and further exploring the recovery of microalgae extracellular polymeric substance.The main results were presented as follows:1.A poised electrode with bias potentials was used as an extracellular electron acceptor to establish an electrochemical redox gradient difference,aiming to extract Chlorella vulgaris photosynthetic electrons and promote the degradation of tetracycline(TC).The results showed that the degradation of TC exhibited a good fit with the first-order kinetic model,and the degradation rate constant at 0.8 V were 1.8-folds higher than that at potential free.At 0.8 V,the competitiveness of working electrode for photosynthetic electrons was enhanced due to the appearance of the extracellular photosynthetic electron transport mediator(cytochrome),thereby generating the largest photosynthetic current intensity and promoting TC degradation greatly.The analysis of degradation contribution(algae-induced,electrochemical oxidation,direct photolysis and hydrolysis)showed that the removal of TC was mainly caused by algaeinduced degradation in the process of photosynthetic electron extraction.Appling this technological approach,the complex heterocyclic structure of TC was broken down into simple monocyclic aromatic compounds,and the biological toxicity of TC was greatly reduced.Moreover,the effect of different exogenous mediators on the promotion of TC degradation was studied.The results showed that nano-iron oxide,humic acid and sodium anthraquinone-2,6-disulfonate could promote the Chlorella vulgaris photosynthetic electron extraction and TC degradation.However,the promotion degree of different exogenous mediators on the photosynthetic electron extraction was not proportional to that on the TC degradation.Thus,the enhancement of TC degradation via exogenous mediators was depended on the synergistic effect of exogenous mediators on the biological toxicity and photosynthetic electron transfer from Chlorella vulgaris.2.The effect of photosynthetic electron extraction on the extracellular polymeric substance(EPS)of Chlorella vulgaris was studied.The results showed that photosynthetic electron extraction significantly promoted the accumulation of protein,polysaccharides and humic acid in the EPS of Chlorella vulgaris.The proportion of protein in the EPS extracted from Chlorella vulgaris applied with bias potentials increased by 11-19 %,compared with that in the EPS extracted from potential free.In addition,the fluorescence intensity of humic acid and fulvic acid substances in the EPS extracted from chlorella vulgaris grown at 0.6 V and 0.8V increased by 297 % and 189 % respectively compared with that in the EPS extracted from chlorella vulgaris produced at potential free.Photosynthetic electrons extraction promoted the extracellular secretion of humic and fulvic acid-like substances,which have the feature of photosensitization.Moreover,the EPS(TOC=8 mg/L)extracted from chlorella vulgaris grown at 0.6 V and 0.8 V was directly used for TC photolysis,and the TC removal rate increased by 1.34 times and 1.53 times respectively compared with the control(EPS free).Therefore,using microalgae EPS as a photosensitizer for removing antibiotics,the lowconcentration photosensitizer can still achieve better results and this technological approach can effectively avoid the generation of drug resistance genes.