Microbial Electrochemical Technology And Process Mechanism Of Nitrate Removal In Electron-donor-deficient Waters

Nitrate nitrogen pollution in aquifers attracts extensive attention.Excessive nitrate and nitrite not only deteriorate the water quality,but also severely affect the health of human beings,animals,and plants.Nitrogen in aquifers was usually removed with the biological denitrification method.However,due to the lack of electron donors,complete nitrogen removal is usually limited in electron-donordeficient waters.Normally,researchers took advantage of the electron donor compensation character of the microbial electrochemical system and offered a novel resolve strategy for nitrate removal in electron-donor-deficient waters.Nitrite intermediates usually accumulated in traditional microbial electrodedependent denitrification systems,and the interaction mechanisms between bacteria and bacteria,biofilm and electrode are still unclear.In order to resolve those problems,in this investigation,biofilm microbial mutualistic growth between exoelectrogens and denitrifiers was cultivated to regulate the denitrification micro-environment.The interactions between cell and cell and the interfacial interaction mechanism between biofilm and electrode were explored comprehensively.In the investigation of applying electrode as the electron donor for microbial electrochemical denitrification,the traditional electrode-dependent denitrifying system was firstly constructed and the denitrification efficiency was investigated.Significant nitrite accumulated in the effluent.In order to relieve the nitrite accumulation in microbial electrochemical denitrification system,single electrode polarity reversal and periodic polarity reversal were carried out separately to construct bidirectional electron transfer biofilm to achieve mutualistic growth between exoelectrogens and denitrifiers.The abundance of exoelectrogens represented by Geobacter sp.in biofilm and the ability of bidirectional electron transfer were only sustained with the addition of a small amount of acetate after single electrode polarity reversal.The bidirectional electron transfer system achieved high total nitrogen removal of 98 ± 1.2%,which is significantly higher than the traditional electrode-dependent autotrophic denitrifying system(7 ±3.8%).However,in the system without acetate addition,the exoelectrogens Geobacter sp.was gradually lost in the biofilm under the long-term autotrophic circumstance with the total nitrogen removal decreased from 76 ± 8.7% to 47 ±6.6%.Electrochemical characteristics analysis revealed that Geobacter sp.lasted from anode might act as an “electron-bridge” to provide extra access for denitrification,as a result,the nitrogen removal efficiency was significantly improved.In order to avoid the addition of organic carbon,periodic polarity reversal was proposed to cultivate bidirectional electron transfer biofilm.A high nitrate removal of 93 ± 5.9% was achieved in this periodic polarity reversal system without nitrite accumulation.Exoelectrogens Geobacter sp.positively correlated with denitrifiers Chryseobacterium sp.and Acidovorax sp.,suggesting the possibility of mutualistic growth between exoelectrogens and denitrifiers.Periodic polarity reversal took advantage of “bio-pseudocapacitance”characteristics of electroactive biofilm to provide electrons for denitrification during the cathode period,which was stored in biofilm during the anode period.This investigation proposed the calculation method to quantitative the contribution ratio of direct electron transfer(DET)and mediated electron transfer(MET).In the bidirectional electron transfer biofilm obtained by periodic polarity reversal,with the scan rate increasing from 1 m V/s to 10 m V/s,DET contribution to anodic and cathodic peak current increased from 4.84% to 21.34%,and from34.34% to 100%,respectively.In the investigation of hydrogen mediated microbial electrochemical denitrification,we constructed heterotrophic hydrogen production coupled denitrification system and electrode-dependent autotrophic hydrogen production coupled denitrification system.In the system of heterotrophic hydrogen production coupled denitrification,the hydrogen mediated mutualistic growth between exoelectrogens and denitrifiers was eventually formed under the longterm stress of nitrate.The cultivated mature electroactive biofilm gradually lost the capability of transferring electrons to electrode under the initial stress of nitrate,but transfer electrons to nitrate.The biofilm partially processed dissimilatory nitrate reduction to ammonium(DNRA)transitorily with ammonia accumulation of 12 ± 4.2 mg/L,which was a transient state.After sufficient succession,the biofilm achieved complete denitrification without ammonia accumulation and started to acquire electrons from the electrode with the total nitrogen removal of 92 ± 5.4%.Finally,with nitrate disturbance,the exoelectrogens-centered biofilm dominated by Geobacter sp.preferred to cooperate with highly enriched hydrogenotrophic denitrifiers for mutualistic growth with hydrogen as the electron mediator.Aggregates formed on the electrode surface,and the extracellular polymer substrate serving as a network contributed to aggregates formation.Cyclic voltammetry results under different scan rates also suggested mediated electron transfer process.In the system of electrode-dependent autotrophic hydrogen production coupled denitrification,biohydrogen production biofilm was enriched by polarity reversal,in which electrode was served as electron donor.Although the total nitrogen was observed at 54%,nitrite accumulation didn’t occur.Hydrogenotrophic denitrifiers were also enriched in this biofilm,indicating that hydrogen producers mutualistically grew with hydrogenotrophic denitrifiers,eliminating the nitrite accumulation in the traditional electrode-dependent system,implying the electron transfer pathway of electrode-bacteria-bacteria.However,the denitrification efficiency was still low,which may be limited by the biological hydrogen production process,and further investigations to improve nitrogen removal were warranted.Based on the above results,biofilm either directly extracted electrons from the electrode for denitrification or utilized hydrogen as mediator for denitrification.In this study,the denitrification system with the electrode as electron donor(MEC)and the gas diffusion membrane integrated microbial electrochemical denitrification system(MMEC)with hydrogen as electron donor were constructed,respectively.Electrons were recovered from anode substrates and were supplied as exogenous donor for denitrification either in form of electrons(MEC)or hydrogen(MMEC).The differences in denitrification efficiency and electron utilization efficiency were investigated in different systems under different applied voltages.The total nitrogen removal was more than 90% in MMEC under different applied voltages,while that was only 40-50%in MEC because high amounts of ammonia accumulated in the system with excess electron donor under an applied voltage of 0.8 V.Decreasing applied voltages significantly improved cathodic electron utilization efficiency,achieving controllable adjustment of cathode electron utilization efficiency.In MMEC,abundant hydrogenotrophic denitrifiers were enriched.However,bacteria with the function of dissimilatory nitrate reduction to ammonium were enriched in MEC.This investigation suggested that electron donors in form of hydrogen realized micro-environment regulation and the introduction of gas diffusion membrane makes the microbial electrochemical denitrification more flexible and easier to control.