Construction Of Sediment-Groundwater Self-driving Remediation System For Nitrate Pollution

The excessive application of agricultural nitrogen fertilizer leads to the aggravation of nitrate pollution in groundwater in China,which directly threats to the safety of drinking water.Bio-electrochemical remediation technology is one of the promising methods to treat nitrate pollution in groundwater,but there are some unsolved problems on practical application,such as high-power consumption,anode sterilization and so on.To solve the problems,the Sediment Microbial Fuel Cell(SMFC)principle was adopted in this study.The chemical energy in the high organic matter content sediment was converted into electric energy to supply power to the bio-electrochemical denitrification system in groundwater.A Sediment-Groundwater Self-driving Remediation(SGSR)system was proposed and constructed,which can remove nitrate in groundwater and simultaneously reduce the content of organic matter in the sediment,which provides a new idea for simultaneous remediation of groundwater and sediment pollution.This research mainly includes the following four aspects:(1)Based on the principles of SMFC and bio-electrochemical denitrification,a SGSR system was proposed and constructed.The self-driving experiment was carried out by four control groups.It was found that the SGSR system equipped with electricity and biofilm had the highest removal efficiency of nitrate in groundwater,which could be removed below the standard limit of 10 mg/L in 10 h,and there was no accumulation of nitrite and ammonia nitrogen at the end of a reaction cycle.Nitrate was removed below the standard limit in 18 h,but nitrite accumulation exceeded the standard limit in“have biofilm but no electricity(CB)”.However,the other two groups of“have electricity but no biofilm(CE)”and“no electricity no biofilm(C)”did not show any effciency of nitrate removal.The closed circuit of SGSR system generates micro current,which stimulates the metabolism of biofilm,promotes the removal of nitrate in groundwater and the consumption of organic matter in sediment.(2)On the basis of the construction of SGSR system,graphene oxide(GO)with good hydrophilicity and large specific surface area was selected as the modified electrode,and the optimal modification scheme was proposed to enhance the performance of SGSR system.Considering the system performance and cost factors,the optimal electrode configuration mode of“GO modified air cathode is arranged at the water air interface and unmodified anode is embedded in anaerobic sediment”is determined.The maximum power density was 14.21 W/m~2.Compared with the unmodified control group,the maximum output power increased by 1.5 times,and the nitrate nitrogen removal rate increased from 89.13±1.40%to 99.9±0.10%within 10h.At the same time,the equivalent circuit of different modified electrode systems was fitted by electrochemical impedance spectroscopy(EIS),and it was found that the air cathode was the limiting factor affecting the power generation performance of GO-SGSR system in the current configuration mode.(3)On the basis of GO-SGSR system,through Box-Benhnken experimental Design(BBD),Response Surface methodology(RSM),satisfaction function and 3D response surface model,the external factors and optimized operation conditions of SGSR system are explored.The initial nitrate load,COD concentration and hydraulic retention time(HRT)were selected to carry out dynamic continuous flow experiment.The response surface multiple regression analysis was used to optimize the nitrate concentration and nitrite accumulation after reaction.The satisfaction function optimization model was used to carry out numerical simulation.The optimal operation conditions were obtained as follows:the initial nitrate concentration was 33.0 mg/L,when COD concentration is 35.0 mg/L,HRT is 8 h,outflow nitrate concentration is 3.5mg/L,outflow nitrite concentration is 0.01 mg/L,and it is verified by experiment the predicted value is consistent with the experimental value.(4)The effect of long-term operation of GO-SGSR system on indigenous microbial community structure was explored.During the 270 days operation of GO-SGSR system,the biofilm electrodeγ-Proteobacteria in the groundwater was enriched.The relative abundance ofγ-Proteobacteria accounted for more than 50%of the total abundance,which was 20%higher than the initial state.That means denitrification function gradually dominated.Also,the Geobacter in the sediment is enriched at the anode,which converts chemical energy into electrical energy,and stimulates the enrichment of denitrifying bacteria in groundwater by using micro current through closed circuit.The GO-SGSR system constructed and enhanced in this study can effectively remove nitrate and its by-products in groundwater,meanwhile,consume organic matter in sediment.The results provide a new idea for the development of sediment reuse and nitrate removal technology in groundwater.