Control Effects Of Reverse Solute Flux Inhibition And Utilization By Draw Solution In An Osmotic Microbial Fuel Cell

Osmotic microbial fuel cells?OsMFCs?combine the merits of microbial fuel cell?MFC?and forward osmosis?FO?for simultaneous contaminant removal,electricity generation,and high-quality water extraction.An ideal draw solution?DS?should possess desirable properties for both water extraction and electricity generation,such as high osmotic pressure,high solubility,high conductivity,nonbiotoxicity and low cost to ensure economic viability.As an FO based technology,reverse solute flux?RSF?is one of the key challenges for its operation.The detrimental effects of RSF included membrane fouling,draw solute loss,reduction of water flux,salinity accumulation in the feed solution and reduction of effluent quality.In this study,the FO membrane was used as the intermediate membrane to construct the two-compartment OsMFC system and examine the effects of inorganic-based draw solutes in OsMFC.The RSF controlling was the main goal of this study.The bioelectricity inhibited RSF reduction is of particular interest to OsMFC operation.It will be of strong interest to turn this“negative effect”?i.e.draw solute intrusion?into a“beneficial effect”.The self-buffering OsMFC system was established with reverse-fluxed sodium bicarbonate when using sodium bicarbonate draw solution/catholyte.The RSF was inhibited and utilizated as electronic mediator in OsMFC by using EDTA-Na₂as a p H-responsive recoverable draw solute.The main research conclusions are as follows:1.Examination of inorganic-based draw solutes in OsMFCThis study has examined ten inorganic-based DS and investigated the effect on OsMFC operation.The OsMFC exhibited stable electricity generation with Na⁺-series and K⁺-series DS.For NH₄⁺-series DS,the ammonia-nitrogen loss percentage,which varied from 12.5±0.4%to 47.7±2.4%,was significant due to the high p H and aeration in the cathode.Inorganic scaling/precipitation was observed with Mg²⁺-series and Ca²⁺-series DS with DS loss percentage from 13.5±2.8%to 28.0±0.3%.The Na⁺-series and K⁺-series DS was identified as the ideal DS for the OsMFC because of stable electricity generation and water flux.NH₄⁺-series,Mg²⁺-series and Ca²⁺-series DS were not appropriate due to significant loss.2.Reduction of RSF and specific RSF by current generationRSF is the reverse flux of draw solute ions,including cations such as Na⁺and K⁺and anions such as Cl^-,HCO₃^-and SO₄^2-in the Na⁺-series and K⁺-series DS.Bioelectricity generation could reduce both RSF and specific RSF.The RSF and sRSF of various ions in closed circuit were much smaller than in open circuit.Electricity generation could reduce the specific RSF by 70.3±4.9%and 56.7±13.4%for the Na⁺and K⁺DS,respectively.3.Self-buffering system with reverse-fluxed sodium bicarbonateThis study proposed a new OsMFC system that took advantage of reverse-fluxed draw solute to achieve anolyte buffer for enhanced electricity generation and simultaneously saved the cost of buffer in the anolyte.Na HCO₃ was selected as a draw solute due to its high buffer capacity and solubility to secure high-quality/quantity water reclamation,efficient electricity production,and p H stabilization of both the catholyte and the anolyte.The reverse-fluxed Na HCO3helped buffer the anolyte p H and thus enhance electricity generation,compared to the OsMFC using the Na Cl DS/catholyte.At the same concentration,the Na HCO₃DS/catholyte achieved a higher Coulomb production of 1349.2±80.3 C and higher anolyte p H of 6.48±0.19 than those of the Na Cl DS/catholyte.At the same conductivity,the Na HCO₃ DS/catholyte exhibited better electricity generation performance with a comparable recovered water volume of 417.7±13.7 m L to that of the Na Cl DS/catholyte.As the Na HCO3 concentration increased from 0.1 M to 0.75M,the OsMFC electricity generation was enhanced due to the increased RSF from19.2±2.3 mmol m^-2 h^-1to 210.8±17.5 mmol m^-2 h^-1.In the anode,92.0±0.8%to97.1±0.9%of reverse-fluxed Na HCO₃ was used to neutralize protons.These results have demonstrated a new strategy that uses the bicarbonate migration driven by both a concentration gradient and electricity generation to successfully raise the alkalinity of the anolyte towards enhancing electricity generation.4.RSF inhibition and utilization by using EDTA-Na₂as a p H-responsive recoverable draw soluteThis study has investigated a promising DS-EDTA-Na₂,a widely used chelating agent.Highly protonated EDTA speciation can have strong hydrophobicity and will precipitate under an acidic condition.This p H-responsive feature makes it possible to separate EDTA-Na₂ from a solution by a simple acid-base adjustment method.The OsMFC with the EDTA DS achieved 779.6±18.5 C?electricity production?and 1.22±0.02 LMH?water flux?,both of which were comparable to that with the Na Cl DS at the same conductivity.However,the EDTA DS resulted in a significantly lower reverse solute flux?RSF?of 0.36±0.08 g m^-2 h^-1 and a lower catholyte p H that could ensure healthy operation of the tested FO membrane.The OsMFC with the EDTA DS exhibited a positive forward flux for Na⁺ions,likely related to the effect of EDTA-Na complexion.Due to the lumping effects of EDTA dissociation equilibrium and membrane surface chemistry,a higher catholyte p H led to a higher water flux and reduced RSF,but lower electricity production.In the reuse test,>90%of EDTA DS could be recovered and then successfully reused in the subsequent OsMFC operation.It was found that the addition of EDTA with acetate in MFC resulting in 22%increase in the production of total Coulomb,indicating that the EDTA might have acted as electron mediators to enhance electrochemical reactions of the anode.Similarly as the reverse-fluxed EDTA in OsMFC.These results of this study would encourage further exploration of using EDTA-based compounds as a draw solute for OsMFC applications.