Effect And Mechanism Of Improving Performance Of Constructed Wetlands In Treating Wastewater Containing Typical PAHs By Strengthening Manganese Cycle

The constructed wetlands(CWs)have been widely used due to its effectively purification,low investment and operating cost,large buffer capacity,easy maintenance,simple process and aesthetic value.However,the lack of oxygen restricts purification efficiency of CWs on aerobic pollutants.In addition,polycyclic aromatic hydrocarbons(PAHs),as a kind of persistent organic pollutants(POPs),have caused extensive attention PAHs cause great harm to animals,plants and ecosystems,and even human health.Although CWs can effectively remove organic pollutants through multi-pathway,microorganisms could not play effective role in organic pollutants removal under trace concentration.Therefore,how to enhance the removal of conventional pollutants and PAHs in CWs process has great practical significanceThis study first constructed a microbial electrolysis cell-birnessite-coated sand filled horizontal surface flow CWs(E-B-CWs),based on the strong adsorption and oxidation of Mn oxides and combined with the redox reaction at the anode and cathode of the microbial electrolysis cell to study the removal performance and microbial mechanism on conventional pollutants.Furthermore,in order to cover the shortage of the E-B-CWs,such as poor adaptability,the electrochemical bacteria(Shewanella oneidensis)was selected to construct electrochemical bacteria-birnessite-coated sand subsurface CWs(S-W-CWs)due to its low cost,easy cultivation and low energy.In addition,removal performance and microbial mechanism on conventional pollutants and typical PAHs were also involved The main conclusions of this study were as follows:(1)The removal efficiency of E-B-CWs on conventional pollutants was high.And the removal efficiency of E-B-CWs on NH4-N,NO3-N and TN was the best,with removal rates of 94.60±3.31%,95.53±1.34%and 94.82±2.59%respectively,followed by birnessite-coated sand CWs(B-CWs)and control CWs(C-CWs).Microbial action for TN removal accounted for 72.73%,while matrix storage for TP removal accounted for 42.84%,which were much higher than B-CWs and C-CWs,and showed stronger capacity in microbial action and matrix adsorption(2)MEC promoted the oxidation of Mn(?)in the deep matrix of E-B-CWs and changed the microbial action mechanism on the removal of conventional pollutants.The proportion of Mn(?)in the 10-15cm matrix of this wetland system was 60.06%,which was higher than B-CWs with MEC oxidizing Mn(?)to Mn(?)oxides.Proteobacteria was dominant bacterial community,and the higher abundance of Proteobacteria might enhance TN removal.In the C-CWs without birnessite-coated sand,the mcroorganisms related to nitrification and denitrification played a main role in pollutants removal.However,in the B-CWs and E-B-CWs filled with bimessite-coated sand,the microorganisms related to ammoniation and manganese oxidation occupied a main role.Moreover,the relative abundance of manganese oxidizing bacteria of E-B-CWs was the highest in three wetland systems.(3)The combination of birnessite-coated sand and Shewanella oneidensis promoted the removal efficiency of typical PAHs(pyrene)and the adding of pyrene affected the removal efficiency of conventional pollutants in the S-W-CWs.Compared with the control groups,about 96.42%of pyrene could be removed in S-W-CWs.After adding pyrene,the removal efficiency of NH4-N and TN in S-W-CWs decreased by 51.13%and 30.05%,respectively,while the removal rate of NO3-N increased significantly.And the birnessite-coated sand could effectively adsorb phosphorus(4)Shewanella oneidensis strengthened the manganese cycle in S-W-CWs,which affected the structure and composition of microbial community in this wetland system.In the 10-16cm matrix of this wetland system,the proportion of Mn(?)was less than Mn(?).The reason might be that Mn(?)oxides can oxidize NH4-N in the anaerobic zone,and Shewanella oneidensis reduced Mn(IV)to Mn(?)and degraded organic pollutants such as pyrene with Mn(IV)oxides being electron receptor.At the genus classification level,the presence of Shewanella oneidensis in wetland system was the most relevant environmental factor on affecting the composition and structure of microbial community,followed by birnessite-coated sand.And it was the combinition of Shewanella oneidensis and birnessite-coated sand that affected the composition and structure of microbial community much more easily.