The Removal Mechanism Of Microcystin-LR By Constructed Wetlands With Bioaugmentation

The problem of toxic cyanobacteria blooms caused by eutrophication and climate change become increasingly prominent,toxic cyanobacteria blooms not only seriously damage the water ecosystem,but also the release of algal toxins threaten aquatic animals and plants and human health.Microcystin-LR(MC-LR)is one of the most widely distributed and most toxic algae metabolites.It is often detected in various surface water bodies,especially drinking water sources.However,traditional detection methods and water treatment techniques are inadequate in rapid detection and efficient control of microcystins in water bodies.Constructed wetlands as an ecological treatment technology,has been proven to be effective in the treatment of microcystins in water,but its removal mechanism is not clear.Toxic cyanobacteria blooms and algal toxins released often outbreak in specific seasons,the general constructed wetlands due to the lack of specific functional microbes or microbial abundance can not be an emergency treatment of microcystin contamination.Augmentation by adding microcystin-degrading bacteria to constructed wetlands,which are screened from water with serious microcystis blooms,to increase the abundance of microcystin-degrading bacteria and enhance the removal of microcystins in the water.The emergent and efficient treatment of the outbreak of toxic cyanobacteria bloom and the release of microcystins is of great significance to the protection of residents' water use.In this paper,enzyme-linked immunosorbent assay(ELISA)combined with realtime quantitative PCR(q PCR)method was used to rapidly assess the pollution status of toxic microcystis and microcystins in water in Erhai Lake,and to investigate the rule of microcystis release,for its further use of providing background data reference to microcytins' control by constructed wetlands.In the laboratory,twelve microcosm subsurface flow constructed wetlands were used to study the effect of the treatment of nitrogen and phosphorus nutrients and toxic algae cells and microcystins in the continuous and stable operation.Then,the wetlands restart to treat with wastewater with microcystins after a month,to simulate the emergency treatment of microcystis VI bloom and release of microcystins.The effect of adding compound microcystin degrading bacteria to constructed welands on the formation of biofilm and rapid degradation of microcytins is also examined.In addition,the microcystin-degrading bacteria was quanlitatively and quantitatively detected by molecular biology techniques at different periods to investigate the response of its abundance during microcystin-LR removal.And the microbial degradation of microcystin-degrading bacteria was determined by the method of material balance.The removal mechanism of microcystins from the wetland system was studied by means of quantitative analysis of matrix adsorption,microbial degradation and plant uptake.The main findings are as follows:(1)Fluorescence quantitative PCR method can quickly quantify the abundance of toixc microcystis in Erhai Lake,and the highest abundance of toxic microcystis occurred in Erhai Lake in September.The detection rate of microcystin LR in the Erhai Lake was higher in October than that,and the total microcystin was close to or exceeded the limit of MC-LR(1.0 ?g/L)of the centralized drinking water surface water source.The results showed that in order to ensure the safety of drinking water during water blooms of Erhai Lake,effective control measures should be taken to control the microcystis cells and their toxins in water.(2)The removal of nitrogen and phosphorus nutrient by constructed wetland system was stable.The average removal rate of NH4+-N is 52.6%~66.3%,the average removal rate of NO3-N is 91.6%~94.2%,the average removal rate of TP is 76.0%~92.8%,the average removal rate of TN is 67.5% ~ 75.3%,and the presence of wetland plants can improve the effect of nitrogen and phosphorus on the system.The average degradation rate constant of the extracellular microcystin(EMC-LR)in the wetland system was 0.022 h-1 and the average half-life was 31 h.The decay process could well fit the first-order kinetics equation(R2>0.9).The optimal hydraulic retention time of the wetland system was 3d.Under this hydraulic retention time,the removal rate of toxic algal cells(intracellular IMC-LR)was 73.5% ~95.4%,and the MC-LR content of the extracellular microcystin(EMC-LR)was 87.5%~98.4%.The wetland system can effectively reduce the MC-LR content to below the drinking water safety standards 1.0 ?g/L.If hydraulic retention time of the wetland system was further extended,the MC-LR content could be degraded to less than the detection limit(0.10 ?g/L).Adding microcystins degrading bacteria could signifcantly enhance the removal rate of EMC-LR in constructed welands and reduced the half-life to 22h(P<0.05),which played an important role in the emergent treatment of microcystins.(3)Based on the principle of material balance,the most important way to remove microcystins in wetland system is microbial degradation,and its contribution rate is 88.75%.Secondly,the contribution rate of matrix absorption is 11.08%.The direct uptake of wetland plants is very small and its contribution rate is only 0.17%.Bioaugmentation by adding microcystin degrading bacteria could significantly improve its abundance in constructed wetlands(P<0.05),and further enhance the microbial degradation,and increased its contribution rate to 99.23%.The abundance of mlr A was significantly increased with the increase of the time of exposure to microcystins in the wetland system,and the abundance of the microcystins degrading bacteria gene mlr A was negatively correlated with the content of MC-LR in wetland effluent(P<0.01).The results showed that the microbial degradation of microcystins in wetland played the most important role in the degradation of microcystins,and the addition of exogenous compound degrading bacteria could further enhance the effect of microorganisms.In summary,the constructed wetlands with bioaugmentation can effectively control microcystin contamination.This study systematically revealed the mechanism of the removal of typical microcystin LR in constructed wetlands,and elucidated the role of microcystin-degrading bacteria in the removal of microcystin.Which provides the theoretical and practical basis to the optimal design of on constructed wetlands technology for the control of toxic algae and algal toxins.