| Nitrogen is one of the indispensable elements of life on the earth and at the same time,it is a very valuable natural resource.However,with the development of modern industry and agriculture,human activities have brought about various nitrogen pollution problems,such as increased eutrophication of water bodies,acidification of the atmosphere,and deterioration of water quality,posing a serious threat to human health and the living environment.Compared with the traditional denitrification processes,the bioelectrochemical system?BES?-based technology has the advantages of generating electric energy while degrading pollutants,saving energy,high treating efficiency,and adaptability to wastewater.In recent years,BES has attracted a lot of attention of research scholars.This study combined the advantages for the simultaneous removal and recovery of nitrogen in bioelectrochemical systems,and the possibility of using bioelectrochemical systems to recover draw solutes for forward osmosis?FO?.We creatively proposed that the microbial desalination cells?MDC? could be used to desalinate the diluted draw solution from FO and recover ammonia nitrogen in the cathode chamber as a new driving solution,hoping to simultaneously achieve the goal of electricity generation and pollutants removal in the anode chamber,ammonia nitrogen removal in the desalination chamber,and ammonia nitrogen recovery in the cathode chamber.This study provides a new idea to recover the draw solutes for FO system,possessing theoretical significance and reference value to some extent for the research of MDC-FO coupled system.The main findings are as follows:?1?During the start-up phase of BES,we inoculated the anode electrogenic microbes by lowering the load resistance step by step,and the nutrient solution was replaced according to the substrate consumption condition.After two months of domestication,the reactors' electricity production was gradually stable with an output voltage of about-0.5 V,indicating that the anode electrogenic bacteria was successfully accumulated and the BES was started with sucess.?2?In order to improve the electron transfer efficiency of the reactor,neutral red?NR? was added into the anode of BES as the electron transfer mediator.The mechanism of neutral red was verified by scanning the anolyte with cyclic voltammetry?CV?.The results showed that the addition of neutral red could increase the energy output of Microbial Fuel Cell?MFC?.The maximum power density of the three reactors with varied neutral red concentrations of 0.05mM,0.10 mM and 0.15 mM were 260.78 mW/m3,270.41 mW/m3 and 228.65 mW/m3,respectively.They were 1.8 times,1.9 times,and 1.6 times that of the control group?146.03mW/m3?,respectively.What's more,adding neutral red could also reduce the internal mass transfer resistance of MFC.Among the reactors,the internal resistance was most significantly decreased with neutral red concentration of 0.10 mM?4.33?,and the internal resistance of the control group was 7.81??.?3?The electrochemical performance of MDC during operation was great.The peak current of MDC could reach 20 mA,and the peak current density was about 80 A/m3.In addition,the maximum power density was 32 W/m3 with an estimated internal resistance of about 22.5?.In terms of pollutants removal in the MDC,the COD removal efficiency in the anode chamber was 83.1±3.2%,and the ammonia nitrogen removal efficiency in the desalting chamber was 72.1±5.5%,and the ammonia nitrogen recovery efficiency in the cathode chamber was 68.6±4.6%.After 19 reaction cycles,the ammonia nitrogen concentration in the catholyte gradually increased to 11.54 g/L,which was sufficient to provide the source of ammonia nitrogen for the FO system to recover the draw solution.?4?Optimized analysis of three key factors affecting MDC performance-external resistance,initial salt solution concentration in the desalination chamber,and concentration of buffer solution in anolyte-showed that MDC produced the highest current density of 77.8 A/m3 when the external resistance was 10?,and the highest ammonia nitrogen removal and recovery efficiency of 72.1±5.5% and 68.6±4.6%,respectively.With an initial salt concentration of 0.25M NH4HCO3 in the influent of the desalination chamber,the ammonia removal efficiency of MDC was the highest of about 84%,and the ammonia recovery efficiency was also the highest of about 78%.The HCO3- in the desalination chamber could be driven by the current into the anode chamber,alleviating the acidification of the anolyte to a certain extent,thereby providing a stable living environment for the electicity-producing bacteria.At this time,a buffer solution concentration of 0.01 M in the anolyte was sufficient to maintain the stability of the MDC system,saving chemical reagents and operating costs as well. |
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