Bioaffinity-type Nanoparticles Promote Microbial Extracellular Electron Transport And Application In Wastewater Treatment

Extracellular electron transfer（EET）is a process in which the electron donor is thoroughly oxidized in the cell and the electrons are transferred to the extracellular electron acceptor through specific respiratory chain.EET plays a vital role in wastewater treatment,environmental remediation,resource utilization and recycling.Compared with the chemical reduction methods,EET efficiency is low.In order to improve the efficiency of wastewater treatment,attempts have been made to speed up EET.Artificial nanoparticles such as reduction of graphene oxide have toxic effects on microorganisms,will produce the potential environmental risk,and easily losted with water,increasing the processing cost.Therefore,the preparation of a bioaffinity,cost-effective and environmentally friendly electron reduction mediator（RM）for biological reduction and decolorization of azo dyes has important practical significance.In this paper,EET was investigated by using azo dye-methyl orange（MO）and free divalent palladium as electron receptors.MO was chosen as a model pollutant,and the immobilized quinone nanocomposite（Fe₃O₄@EPS）was prepared to evaluate its promotion on the bio-reduction of MO.The flower-like core-shell configuration of Fe₃O₄@EPS with a 12 nm of light layer of EPS was confirmed by various analytical methods.Meanwhile,the material shows strong resistance to various anions and strong acid and strong base conditions.The application of Fe₃O₄@EPS on sustained acceleration of MO decolorization were confirmed by batch experiments and anaerobic sequenced batch reactors.The experimental results showed that Fe₃O₄@EPS showed a continuous and stable promoting effect in five cycles,the enhancement ratio was in the range of 1.45～1.83 h^-1 and the enhancement efficiency was 5.51～17.95%.The redox ability of EPS was well reserved on Fe₃O₄@EPS by FTIR and electrochemical test.Due to biocompatibility of the biogenic shell,the as-prepared Fe₃O₄@EPS exhibited low toxic to microorganisms by the Live/dead cell test.The results of study provided a new preparation method of biological intimate and environmentally friendly redox mediators and suggested a feasible way for its use on bio-reduction of pollutants.In addition,EET was investigated by analyzing the changes of EPS,Pd concentration and cell activity.TEM images showed that Chem-Pd particles were in an agglomerated state,while Bio-Pd nanoparticles were more evenly and dispersed.The Bio-pd was mainly located on the cell membrane,EPS on the cell membrane,inside the cell and the free EPS adhered to the system.A few cell surface particles were relatively large,forming black strips wrapped around the cells.Bio-Pd and Chem–pd were metals with a crystalline structure.The Live/dead cell test showed that the closer to Pd aggregation,the more dead bacteria,indicating the Bio-pd exhibited certain toxic to microorganisms.AAS was used to determine the change of palladium concentration over time in Bio-Pd and Chem-Pd synthesis.The results showed that the synthesis rate of Bio-Pd was faster and the particle dispersion was more uniform.Moreover,enzyme inhibitors（including Cu Cl₂,rotenone,dicumarol,capsaicin and BAL）were selected to block respiratory chain transfer.The results showed that the reduction efficiency of palladium was inhibited to varying degrees,indicating that the competitive inhibition or non-competitive inhibition may occur at different sites on the respiratory chain,and better understanding the mechanism of intracellular electron transport on nanosynthesis of palladium.