Research On Efficiency And Mechanism Of Nitrate And Heavy Metals Removal From Groundwater Based On Sulfur-iron Cycle Controlled Constructed Wetlands

In recent decades,with the rapid development of Chinas economy and society,the exploitation of groundwater is growing rapidly,and the over-exploitation of groundwater could lead to the deterioration of water quality.Groundwater is often characterized by complex pollution due to the control of multiple pollutants,of which nitrate and heavy metal pollution are a common type of complex pollution.The current technology still has limitations such as easy to produce secondary pollution,high input cost and unsatisfactory treatment effect.Therefore,it is of great practical significance to carry out research on green and sustainable remediation technologies for contaminated groundwater.Constructed wetlands（CWs）has received much attention because of its good purification effect,stable operation,and easy management.This study addressed the nitrate-heavy metal complex pollution in groundwater.Different autotrophic denitrification CWs were constructed,including sulfur-based CW,iron-based CW,sulfur-iron-based CW,and pyrite-based CW,to evaluate the effectiveness of different types of CWs in purifying groundwater and to assess their greenhouse gas emission characteristics.In order to investigate the effect of sulfur and iron matrix fillers on the migration and transformation of pollutants such as nitrogen and heavy metals,batch experiments were carried out.High-throughput sequencing technology was used to analyze the microbial community diversity and community characteristics of autotrophic denitrification CWs.The main findings are as follows.（1）The magnitude of denitrification capacity of CWs was sulfur-based>sulfur-iron-based>iron-based>pyrite-based>gravel-based.Only when NO₃^--N was present,the sulfur-based CW,iron-based CW,and sulfur-iron-based CW all achieved better removal effects,with NO₃^--N removal efficiency of 98.47%,65.05%and 72.03%,respectively;and total nitrogen（TN）removal efficiency of 96.96%,40.93%and 55.69%,respectively.The addition of heavy metals in the influent caused the decrease of N removal in iron-based CW and pyrite-based CW,and the removal performance of pyrite was not affected by the addition of heavy metals.However,the removal performance of sulfur-iron-based CW showed an increasing trend after the addition of heavy metals,with NO₃^--N removal increasing about 16-19%and TN removal increasing 15-30%.As for the removal of heavy metals,all CWs have better removal effect on Cu（Ⅱ）,with removal efficiency of 90.46-98.06%.However,only sulfur-based,iron-based,and sulfur-iron-based CWs could have high removal effect on Cr,with removal efficiency above 98%.In addition,the iron-based CW had the highest N₂O emission fluxes with average emission fluxes of 215.50-299.53μg m^-2 h^-1,and the sulfur-based,sulfur-iron-based,and pyrite-based CWs had lower emissions with average emission fluxes of 24.22-70.83μg m^-2 h^-1,19.10-119.40μg m^-2 h^-1,respectively.The addition of heavy metals caused different effects on autotrophic CWs:the addition of Cu（Ⅱ）increased N₂O emission fluxes in sulfur-based and sulfur-iron-based CWs and decreased in iron-based and pyrite-based CWs.Simultaneous addition of Cu（Ⅱ）and Cr resulted in elevation of N₂O emission fluxes from iron-autotrophic,sulfur-Fe-based CWs and reduction of emissions from sulfur-based,pyrite-based CWs.（2）In the reaction system without microbial action,sulfur filler and pyrite might not have redox reaction with NO₃^--N and have little adsorption effect on NO₃^--N.Iron and mixed sulfur-iron could convert NO₃^--N to N₂ and NH₄⁺-N by redox reaction.Furthermore,the addition of heavy metals had little effect on the redox reaction of iron with NO₃^--N.But for mixed sulfur-iron,the addition of heavy metals could promote the conversion of NO₃^--N to N₂,and reduce the NH₄⁺-N content,and the addition of Cr only had the most significant effect.Sulfur and pyrite batch have little removal on the removal of heavy metals Cu（Ⅱ）and Cr.Iron and mixed sulfur-iron achieved high removal of Cu（Ⅱ）and Cr by precipitation reaction,redox,neutralization reaction and flocculation co-precipitation.（3）Gravel-based and pyrite-based CWs had the highest amount of microorganisms,but sulfur-iron-based CW had the lowest amount of microorganisms.And the addition of heavy metals could reduce the abundance of microbial communities.There were differences in microbial community composition in different types of autotrophic denitrification CWs.The dominant group with the highest relative abundance of microorganisms at the phylum level in the substrate of CWs during the experimental period was Proteobacteria.The distribution pattern of Proteobacteria was sulfur-based>iron-based>sulfur-iron-based>pyrite-based>gravel,which was consistent with the results of NO₃^--N removal.At the genus level,the relative abundance of Thiobacillus as the dominant microorganism was higher in the autotrophic denitrification CWs.Especially in the sulfur-based,sulfur-iron-based,and pyrite-based CWs,and the relative abundance increased continuously with the operation of the system.Except for Thiobacillus,all other dominant microorganisms in autotrophic denitrifying CWs were associated with denitrification processes.In addition,some resistance and intolerance to heavy metal addition microorganisms were shown:Thiobacillus,Sulfurimona and Thermomonas were resistant to heavy metals,and Denitratisoma,Hydrophilus and Monococcus were intolerant to heavy metals.