Research On Enhanced Nitrate And Heavy Metals Removal From Groundwater Based On Fe-C Micro-Electrolysis Under Vertical Constructed Wetlands

Groundwater,as the most important drinking water source in arid regions of China,has been polluted seriously by accumulated nitrate and heavy metals mostly due to human activities.An economic alternative with capacity of simultaneous mitigation of nitrate and heavy metals is urgently needed.Owing to the advantages of low energy input,effective treatment performance and easy operation,Constructed Wetlands?CWs?have become increasingly popular for treating various wastewaters.However,the lack of bio-available organic carbon in the groundwater has often been regarded as the most common hindrance for nitrate removal through heterotrophic denitrification process in CWs.Meanwhile,the removal performance is always poor when treating the contaminated groundwater with nitrate and heavy metals simultaneously.Thus,it is great of significance to investigate the feasibility of simultaneous removal of nitrate and heavy metals as well as the removal mechanism in CWs for purifying polluted groundwater.In this study,Iron-carbon-based constructed wetlands?Fe-C CWs?by adding biochar and iron scraps into CWs was set up to investigate the long-term treatment performance and the N₂O release.The transformation and degradation characteristics of all contaminants during removal process were also studied.The effect of biochar and iron scraps on the transformation of nitrate and heavy metals?Cr and Pb?was systemically studied through the batch tests.The microbial communities in Fe-C CWs were characterized by using Illumina High-throughput Sequencing technique to further understand the removal mechanism of nitrate and heavy metals in Fe-C CWs.The main research conclusions are as follows:?1?Better purification performance of contaminated groundwater was obtained in Fe-C.The average removal efficiency of NO₃^--N in Fe-C CWs was 87.16%.While the Fe CWs and control CWs presented lower removal efficiencies of NO₃^--N.The average removal efficiencies of NO₃^--N in Fe CWs and control CWs were 80.99%and 11.63%,respectively.Meanwhile,the COD and TP could also be removed effectively in Fe-C CWs,and the average removal efficiencies during the whole experiments were 77.77%?85.64%and80.00%?86.67%,respectively.When Cr and Pb were introduced into the influent,the nitrogen removal performances decreased in all CWs.The average TN removal efficiency in Fe-C CWs decreased to 43.33%.However,the average removal efficiencies of Cr and Pb could reach to 75.01%?94.18%and 97.64%in Fe-C CWs,which were better than that in control CWs.In addition,the lower N₂O emission fluxes were detected in all CWs systems during the whole experiments.The mean N₂O emission fluxes in Fe-C CWs were 4.6?11.75?g m^-2 h^-1.Different parameters showed different variations during the typical cycle.The alternative aerobic and anaerobic DO concentration and decreasing ORP during the typical cycle provided favorable environment for the removal of pollutants.There was little accumulation of NH₄⁺-N and NO₂^--N in Fe-C CWs,indicating the complete denitrification process could be maintained in Fe-C CWs.The concentrions of COD,NO₃^--N,TP,Cr and Pb during the typical cycle all decreased firstly and then kept in a stable value in Fe-C CWs.?2?The effects of biochar and iron scraps on the transformation of nitrate and heavy metals?Cr and Pb?were determined by the batch tests.The results showed that the adorption iron scraps and biochar-iron scraps through chemical reduction.Meanwhile,the addition of Cr and Pb made little effect on the reaction between NO₃^--N and Fe,Fe-C.The adorption capcity of biochar to Cr?VI?was weaker but to Pb?II?was stronger;the Cr and Pb were removed completely in iron scraps batch test and biochar-iron scraps batch test mainly through reduction of iron,flocculation,and coprecipitation,etc.In addition,the DOM components released from biochar and biochar-iron scraps mainly included the humic-like and tryptophan-like fluorescence components,in which the humic-like components occupied the most.The Cr and Pb were removed mainly by combining with the humic-like than tryptophan-like components.?3?Microbial communities varied in different CWs.Compared with the Fe CWs and the control CWs,the application of biochar-iron scraps in Fe-C CWs did not increase the microbial diversity.However,the structure of microorganisms in Fe-C CWs was significantly different from other CWs.The results showed that the Proteobacteria phylum was the preponderant bacteria in all CW systems.The relative abundance of the Proteobacteria phylum in Fe-C CWs?54.49%?76.41%?was significantly higher than that in Fe CWs and control CWs.In the genus level,the Rhodobactere genus related to the Fe?II?oxidation and nitrogen removal was enriched in Fe-C CWs.The relative abundance of Rhodobactere genus in the layer filled with biochar and iron scraps of Fe-C CWs was 53.71%,which was significantly higher than that in other layers of different CWs?4.90%?7.76%?.In addition,the application of biochar and iron scraps in Fe-C CWs prominently improved the relative abundance of bacteria involed with nitogen recycle particularly for denitrifying bacteria.