Study On Emissions And Mechanisms Of Greenhouse Gases From Constructed Wetland Systems Containing Iron/Manganese Matrix

Constructed wetlands(CWs)have been widely used as an advanced sewage treatment process because of their low energy consumption,low construction cost,and easy maintenance.Although CWs can well remove various pollutants in sewage due to the simultaneous presence of aerobic and anaerobic conditions,the impact of greenhouse gases(GHGs,N2O,CH4,and CO2)released by them on the environment has also attracted widespread attention.Due to their wide variety,large specific surface area,and the ability to participate in various chemical and biological reactions,iron and manganese materials have been used in CW matrix in recent years to improve water purification performance,but the effects of iron and manganese materials on GHGs in CWs have been neglected.In this study,laboratory-scale vertical subsurface flow constructed wetland(VSSFCW)systems with quartz sand,iron ore,and manganese ore as matrix were constructed,to explore the effects of different matrixes on the removal performance of conventional pollutants and the emissions of N2O,CH4,and CO2.On this basis,in order to explore the mechanism of the iron-containing CW matrix systems on CH4 and N2O emissions,in subsequent experiments,CW matrix systems simulated by feed bottles were established.The CW matrix systems were supplemented with quartz sand and iron ore as matrix,respectively,and were added with the typical dissimilated metal-reducing bacteria(DMRB)Shewanella oneidensis MR-1 to explore their effects on the short-term concentration changes of CH4 and N2O and the mechanism of microbial action.This research could provide theoretical support for selecting better metal matrix and achieving better ecological and environmental benefits.The main conclusions obtained were as follows:(1)Both iron ore and manganese ore-based CW systems improved the removal of conventional pollutants such as COD,TN,and TP,but had different effects on GHG fluxes and global warming potential(GWP).Compared with the control groups iron ore and manganese ore both reduced the CH4 fluxes in the CW systems,and manganese ore completely suppressed CH4 emission;iron ore increased the N2O fluxes,while manganese ore slightly reduced the N2O fluxes compared with the control group;combining the three GHGs,manganese ore reduced GWP and iron ore increased GWP,so manganese ore was a better choice for the metal matrix of CWs.(2)The presence of iron oxides and Shewanella oneidensis MR-1 changed the microbial community structure and metabolic pathways in CH4 production and oxidation.Iron oxides acted as an electron mediator in the process of symbiotic degradation of organics,and could increase the amount of partial enzymes in the process of acetic acid fermentation,methyl cleavage and CO2 reduction,thereby promoting CH4 production.However,iron oxides could also directly inhibit methanogens and reduce the amount of another part of the enzymes in the CO2 reduction pathway,thereby inhibiting CH4 production.In addition,DMRB could promote the reduction of iron oxides,and then competed with methanogens for organic substrates,thereby inhibiting CH4 production.DMRB also promoted anaerobic oxidation of CH4(AOM)by promoting the reduction of iron oxides.Combining the above effects,iron oxides had greater promotion effects on CH4 production than inhibitory effects.However,due to the existence of DMRB,the promotion effect of iron oxides on CH4 production was weakened,AOM was strengthened,and finally the CH4 emissions in the CW matrix systems were reduced.(3)The presence of iron oxides and Shewanella oneidensis MR-1 changed the microbial community structure and metabolic pathways in nitrification and denitrification.Under anaerobic conditions,iron oxides could act as electron acceptors to oxidize NH4'-N to NO2-/N2O/N2,while Fe3+in iron oxides was reduced to Fe2+.This electron transfer would be affected by the pH of systems.In addition,Fe2+could also provide electrons for denitrification through oxidation reactions.This coupling process was called nitrate-dependent iron oxidation(NDFO).Most nitrate-dependent iron oxiding bacteria(IOB)could reduce NO3-to NO2-and further denitrification intermediates and products,including NO,N2O,and N2.Combining the above effects,the N2O concentrations in Fe-CWMs and FeB-CWMs increased rapidly at the beginning of the experimental period,and were quickly converted to N2 in the later period.Finally,the N2O concentrations in Fe-CWMs were still higher than C-CWMs,and the N2O concentrations in FeB-CWMs were significantly lower than the other two CW matrix systems.