Mercury(Ⅱ) Removal From Wastewaterby Biochar-based Constructed Wetlands

Mercury（Hg）is a highly toxic heavy metal,which can be transformed into highly toxic methylmercury（MeHg）in water environment,thus causing serious harm to all kinds of organisms and human beings through the food chain/web.Therefore,it is extremely necessary to reduce the pollution level of Hg in water and prevent Hg in wastewater from entering water environment.In this study,granular biochar was used as the substrate to construct an intermittently aerated constructed wetlands（CWs）to treat wastewater containing Hg（Ⅱ）,the Hg removal pathways in the CWs were explored by analyzing the Hg accumulation capacity of wetland plants,the total amount and speciation of Hg in wetland matrix.The removal efficiencies of COD,N and P in wastewater by the biochar-based CWs was investigated,and the response of microbial community structure in different substrates to the presence of Hg（Ⅱ）was analyzed.The potential role of microorganisms in Hg removal was explored.The aims of this study are to provide theoretical and technical basis for the application of CW technology in Hg removal from wastewater.The main conclusions are as follows:（1）The removal rate of Hg（Ⅱ）in biochar-based CWs（B-CWs）was significantly higher than that in gravel-based CWs（G-CWs）（p<0.05）,with the removal rate being relatively stable and always>96%during the experimental period.The Hg removal rate of G-CWs decreased after 50 days of operation,and decreased to 46.1%after 100days.（2）In the B-CWs,81.0%of Hg was removed by chemical and microbial reduction and following volatilization,while in the G-CWs,only 19.0%of Hg was removed by this route,indicating that biochar substrate can effectively remove Hg from wastewater by volatilization.In the G-CW systems,54.8%of Hg was retained in the matrix,while only 9.0%in the B-CW systems.Most of the influent Hg was not retained in the matrix of the B-CW systems.The plant biomass and Hg contents in the B-CW systems were both higher than those in the G-CW systems,with the proportion of Hg removed by plants being 7.0%in the B-CWs,while only accounting for 0.7%in the G-CWs.The Hg translocation factor（TF）for plants in the B-CWs was lower than that in the G-CWs,indicating that biochar substrate was beneficial for plant growth and Hg uptake Hg was mainly distributed in plant roots,alleviating the stress of Hg to the plant aboveground parts.（3）The removal rates of COD,NH₄⁺and TP in biochar CWs were significantly higher than those in the gravel systems,indicating that biochar substrate also had the advantages of effectively removing conventional pollutants in wastewater.Hg presence significantly reduced the removal efficiencies of NH₄⁺and TP in CWs（p<0.05）.（4）Biochar substrate provided a good environment for the growth of microorganisms.Microbial species in the B-CW systems are richer than those in the G-CWs,among which the abundances of Arenimonas,Lysobacter,Micropruina and Hydrogenophaga were higher.They might play an important role in Hg removal.Hg presence affected microbial community structure in CWs,but the effect was lower than that of substrate type.The results showed that the intermittently aerated biochar-based CWs had great application advantages and potential in the treatment of Hg（Ⅱ）containing wastewater,and its efficacy,influencing factors and mechanism deserve further exploration.