Study On Treatment Of Acid Mine Drainage Using Constructed Wetlands Filled With Composite Matrix And Strengthened By Exogenous Carbon

Acid mine drainage(AMD)can cause great harms to the surrounding environment and human health,so its effective treatment is highly necessary.In this paper,waste plant biomass of walnut shells and its biochar materials were used as composite substrates of constructed wetlands(CWs)to treat AMD,and simulated domestic wastewater(DW)and plant litter broth(PLB)were added as exogenous carbon to enhance the efficiency of sulfate reduction.The efficacy of the CWs with reinforced composite substrates in the treatment of AMD and the influence of influent p H on the treatment efficiency were investigated.The removal pathways of metals in the wetlands were explored through analyzing content and fractionation of heavy metals in the wetland substrates and the accumulation of metals in plants.Furthermore,the mechanism of AMD treatment by the CW systems was further explored by combining the characteristics of functional microorganisms in the wetland systems.The purposes of this study are to provide theoretical and technological bases for the application and optimization of CW system for AMD treatment.The main conclusions are as follows:(1)The CW filled with composite substrates and added with DW and PLB achieved a good performance in AMD treatment.The p H of AMD increased from 4.0 to6.4 on average after treatment.The removal efficiencies for Cu²⁺,Cd²⁺,Zn²⁺and Cr in the wetland system were all more than 95%,and for sulfate and Fe the efficiencies were26.6%and 75.9%,respectively.The metals in AMD were mainly retained in wetlands in the forms of oxides,hydroxides and sulfides.The capacity of biochar in retaining metals was stronger than those of gravel and walnut shells.Cd,Zn,Cr and Fe were predominantly present in Fe-Mn oxide-bound fraction within the substrates,and Cu was predominantly present in organic-and sulfide-bound fractions.Plants only marginally contributed to metal removal.The relative abundances of key genera such as Geobacter,Acinetobacter,Propioniciclava and sulfate-reducing bacteria(SRB)in the wetland system were high,indicating that they played a key role in the treatment of AMD in the CW.The CW filled with composite substrates and added with DW and PLB is promising for AMD remediation.(2)The influent p H had a significant effect on AMD treatment efficiency of the CWs.Higher p H was conducive to the formation of anaerobic environment and the establishment and strengthening of sulfate reduction process in wetland systems,thus good sulfate and metal removal performance was obtained.However,when the influent p H was less than 2,the removal performance of major pollutants is poor.(3)The influent p H affected the retention capacity of the substrates and the fractionation of heavy metals.The contents of heavy metals retained in sediments of CWs feeding with influent of p H 2(2-CWs)were lower than those of other wetlands,while the CWs feeding with influent of p H 5(5-CWs)retained the largest amount of metals.Cd and Zn were predominantly presented in Fe-Mn oxide-bound and organic-and sulfide-bound fractions in the CWs feeding with influent of p H 3(3-CWs),4(4-CWs)and 5(5-CWs).Cu was predominantly presented in organic and sulfide fractions,while Fe was mainly retained in wetlands in the residual form.(4)Influent p H significantly affected the microbial diversity,community structure and abundance of functional groups in the wetland systems.In 2-CWs,the abundance of eosinophilic bacteria was higher,while the abundances of SRB and organic decomposition bacteria were low.Thus,the bacterial community structure in 2-CWs was significantly different from that in other systems,which was not conducive to the AMD treatment.The higher abundance of SRB and dissimilatory sulfate reduction genes in 4-CWs and 5-CWs indicated that higher p H was beneficial for the growth of SRB and occurrence of biological sulfate reduction,so that these two systems achieved better AMD treatment effect.