Study On The Efficiency And Microbial Community Characteristics Of Constructed Wetland For The Treatment Of Acid Mine Drainage

Acid mine drainage(AMD)that accompanies mine mining is one of the most serious environmental pollution sources.Using constructed wetland technology to repair AMD has the advantages of low cost,simple operation,and suitable for remote mining areas.In this study,four groups of pilot scale vertical-flow constructed wetlands were constructed with sand gravel,walnut shell and biochar as substrates and Iris pseudacorus L.as wetland plants.The treatment efficiency of each wetland system to AMD was investigated under the condition of mixed domestic sewage and fermentation liquid of plant straw.Then analyzed the distribution of heavy metals in constructed wetland substrates and plants.Lastly,the community structure of microorganisms in wetland system was analyzed by high-throughput sequencing,and the relationship between functional microorganisms and AMD treatment efficiency was discussed.The results could provide scientific reference and technical basis for the low-cost and efficient purification and remediation of AMD in this kind of constructed wetland.The main conclusions are as follows:(1)In the early stage of wetland system,the pH value of effluent was higher and the removal effect of heavy metals was better,but with the increase of time,the pH of effluent decreased and the concentration of heavy metals increased gradually,and there was little removal effect on SO₄^2-;When AMD and domestic sewage were treated together and plant straw fermentation liquor was added,the treatment effect of each wetland system on heavy metals in AMD was greatly improved.The effluent concentration of Cu²⁺and Cr⁶⁺was basically close to zero.The Cd²⁺concentration in walnut shell-based constructed wetlands(W-CWs)and the effluent Cu²⁺,Zn²⁺,Cr⁶⁺,and total Cr concentrations were within the allowable discharge range specified in integrated wastewater discharge standard GB8978-1996;The removal load of Cu,Cd,Cr and Fe was the highest in W-CWs,while Zn was better in B-CWs;The pH value of the effluent of each system was stable above 6.0,Eh was reduced to below-100mV,and DO concentration was less than 1.0 mg/L in most operation time;In W-CWs system,SO₄^2-removal rate was the highest(37.35±6.44%),while COD removal rate was the best(74.75±2.95%)in biochar based constructed wetland without plants(BN-CWs);In addition to Fe and Cr,the removal rates of Cd,Zn and Cu in B-CWs system planted with Iris pseudacorus L.were slightly higher than those in BN-CWs system without plants,and the removal effects of TN,NH₄⁺-N and TP were also better in B-CWs.(2)The content of heavy metals adsorbed by walnut shell substrate was higher than that of biochar substrate,and much higher than that of gravel substrate;Cd,Zn,Cr and Fe mainly existed as the iron-manganese oxidation combined state in each substrate,while Cu mostly existed as the organic combined state;The heavy metal content of plants in B-CWs system was significantly higher than that in sand gravel based constructed wetland(S-CWs)and W-CWs system;The absorption capacity of plants in each system to Cr and Zn was greater than that of Cu and Cd;In addition,biochar could significantly promote plant growth and root extension,and could reduce the damage of heavy metal stress on plants and inhibit the transport of heavy metals from roots to the upper part of the plant.(3)The results of diversity analysis showed that microbial species are the most abundant in biochar,and the number of nutrients and carbon sources would affect the?diversity;NMDS and cluster analysis showed that substrate and influent composition could significantly affect the composition and structure of wetland bacterial community;Analysis of the key microorganisms in AMD treatment showed that the relative abundance of sulfate-reducing bacteria(SRB)significantly affected the efficiency and operation stability of the constructed wetland treatment of AMD.The relative abundance of SRB flora(including desulfobulbus,desulfatirhabdium,desulfosporosinuss,desulfatirhabdium,sulfobacterium,desulforhabdus)in each system increased significantly when the fermentation broth was added into the influent,and the relative abundance of S-CWs,W-CWs,B-CWs and BN-CWs systems were5.70%-6.06%,5.90%-6.53%,4.15-6.36%,6.80-9.78%;In addition,the oxygen secretion of plant roots reduced the growth of SRB to a certain extent.