Recovery Of Heavy Metals Cr(Ⅵ) From Wastewater And Simultaneous Power Generation By Constructed Wetland-microbial Fuel Cell

Heavy metals have the characteristics of high toxicity,bioaccumulated,and inability to be decomposed by microorganisms,which would pose long-term potential harm to ecological balance and human health if it discharged without reasonable treatment.High valence heavy metal ions can as electron acceptors to be removed by reduction reaction at the cathode of Microbial fuel cell.The heavy metals in wastewater can be absorbed and transferred to wetland plants in the Constructed wetlands,and then achieve heavy metals’ recovery effectively by harvesting the plants.In this study,hexavalent chromium was used as the target pollutant,a single chamber constructed wetland microbial fuel cell(CW-MFC)was constructed to investigate the Cr(Ⅵ)removal ability and power generation performance.Furthermore,the factors that influence the Cr(Ⅵ)removal efficiencies and the power generation were optimized.Then,the removal pathways of Cr(Ⅵ)in CW-MFC were explored,and the microbial community structure in CW-MFCs’ system were analyzed.Finally,the heavy metal recovery strategies were proposed.Through the research of the thesis,the main results were obtained as follows:1)The planted CW-MFC and unplanted CW-MFC were constructed to study the Cr(Ⅵ)removal effect and the power generation performance after the Cr(Ⅵ)was added.The results showed that since the Cr(Ⅵ)ions provides more electron acceptors for the system,the maximum power densities of unplanted CW-MFC and the planted CW-MFC were increased by 17.05%and 67.63%,up to 24.52 mW/m~2 and 36.44 mW/m~2,respectively.Compared with the unplanted CW-MFC,planted CW-MFC with a better power generation performance and Cr(Ⅵ)removal performance.The maximum power density was 48.58%higher than unplanted CW-MFC,After 9 h,the planted CW-MFC achieved Cr(Ⅵ)removal efficiency of 78.88%,while only 61.99%of the Cr(Ⅵ)was eliminated in the unplanted CW-MFC.And the NH4+-N removal rate was 24.89%higher than the unplanted CW-MFC,reached to 80.56%.2)With the increase of Cr(Ⅵ)concentration in the influent,the power generation,the Cr(Ⅵ)removal performance and the NH4+-N removal rate of the CW-MFC all were inhibited.The influent COD concentration increased within a certain range could promote the power generation performance and the wastewater treatment efficiency.When influent COD concentration increased to 1000 mg/L,CW-MFC obtained the highest Cr(Ⅵ)removal efficiency(Removal of 99%of Cr(Ⅵ)in the shortest time)and the highest power density(95.23 mW/mi2),but the NH4+-N removal efficiency was inhibited.So the optimal COD concentration was 800 mg/L.The electrode spacing influenced the power generation performance by changing the total internal resistance of the CW-MFC system.The power density obtained at 10 cm was the largest,which was 37.83 mW/m~2.The total internal resistance of the 10 cm system was lower than the other two.The Cr(Ⅵ)removal efficiency and the NH4+-N removal efficiency at the 10 cm system were all the highest.With the increasing of cathode area,the power generation of CW-MFC increased first and then decreased,and the optimal cathode area was 75 cm~2.3)The Cr(Ⅵ)was removed by the action of bioelectrochemical activities and wetland plants in the CW-MFC.The Cr(Ⅵ)removal efficiency of the system was greatly improved when the wetland plants and bioelectrochemical activities synergistically acted on the Cr(Ⅵ).And when the CW-MFC removed 94%-99.9%of Cr(Ⅵ),the MFC system contributed 61%-67.84%to Cr(Ⅵ)removal,while the CW contributed 28%-36.68%to Cr(Ⅵ)removal.Electrode materials and sediments can be used as chrome ore for heavy metal recovery,and also can be recovered directly by leaching,while wetland plants can be reduced by incineration,and achieve the purpose of recovering chromium at the same time.4)The microbial communities on the electrodes of the planted CW-MFC and the unplanted CW-MFC had certain differences.The richness and uniformity of the microbial community between the planted CW-MFC and unplanted CW-MFC showed the planted CW-MFC>unplanted CW-MFC,and cathode>anode.At the phylum level,the bacterial groups Proteobacteria,Actinobacteria,and Bacteroidetes are the dominant flora on the cathode of the planted CW-MFC and unplanted CW-MFC;the bacterial groups Proteobacteria,Firmicutes,and Bacteroidetes on the anode of the planted CW-MFC and unplanted CW-MFC are the dominant flora.At the genus level,the bacterial groups Nitrospira and Saccharimonadales on the cathode of the planted CW-MFC are the dominant flora;the bacterial groups Simplicispira and Nitrospira on the cathode of the unplanted CW-MFC are the dominant flora;the bacterial groups Lactococcus and Streptococcus on the anode of the planted CW-MFC are the dominant flora;the bacterial groups Trichococcus and Lactococcus on the anode of the unplanted CW-MFC are the dominant flora.Since a best power generation performance and wastewater treatment performance were obtained at the planted CW-MFC,therefore,Proteobacteria and Firmicutes at the phylum level,Lactococcus and Streptococcus at the genus level are the most beneficial flora for promoting the performance of single-chamber CW-MFC.