Study On Effect Of Polystyrene Microplastics On Vertical Flow Constructed Wetlands

Because of convenience and low price,plastics are widely used in our daily life.Due to improper disposal,the used plastic enters the environment and then be decomposed into"microplastics"under the external force of environmental physics,biology and chemistry force.Microplastics have the characteristics of small mass and large specific surface area,which are easy to adsorb pollutants,thus posing a potential threat to the ecosystem.Constructed wetland is a wetland system with water treatment capabilities composed of plants-microbes-substrate.Microplastics in the environment will inevitably enter the constructed wetland ecosystem and affect the operation,microbial activity and plant growth of the constructed wetland.The current research on the ecotoxicological effects of microplastics on constructed wetlands and the removal of microplastics in constructed wetlands is still in the early stage of research.In this thesis,a vertical downward flow constructed wetland was constructed.Polystyrene Microplastic（PS-MP）was used as the research object.The PS-MP solution with a particle size of 2μm and 10μm was used in a volume ratio of 1:1.Mix uniformly and add them to the constructed wetland at the concentration of 0,100μg/L and 1000μg/L intermittently.On the one hand,the feasibility of removing PS-MP from the vertical flow constructed wetland will be explored;on the other hand,the PS-MP convection will be explored.The toxicological effects of constructed wetland ecosystems and their mechanism of action are of great significance for clarifying the compound toxicological effects of PS-MP and clarifying the migration of PS-MP in vertical flow constructed wetlands.The quantitative analysis results of microplastics showed that the removal rate of 2μm PS-MP in the vertical flow constructed wetland was 93%～98%,and the removal rate of 10μm PS-MP was 95%～100%.The concentration distribution of PS-MP with 2μm and 10μm in the matrix layer of the vertical flow downward flow constructed wetland is the wetland surface layer>upper layer>middle layer>lower layer.The results of single-factor analysis of variance showed that PS-MP had no significant effect on the removal of COD,TP and TN in constructed wetlands（P>0.05）,and 1000μg/L PS-MP had a significant inhibitory effect on the removal of NH₄⁺-N（P<0.05）.The average removal rate of COD of CW-100μg/L PS-MP and CW-1000μg/L PS-MP constructed wetland decreased by 0.44%and 1.62%respectively compared with the blank group;the average removal rate of TP increased by 0.21%and decreased by 0.06%;TN The average removal rate decreased by 3.31%and 2.10%,respectively;the average removal rate of NH₄⁺-N decreased by 0.41%and 0.50%,respectively.The 16s rRNA high-throughput sequencing showed that with the addition of different concentrations of PS-MP,PS-MP changed the composition and relative abundance of the microbial communities on the surface and upper layers of the constructed wetland,and had no significant impact on the microbial community in the wet subterranean layer.When PS-MP was added on the 439th day,the relative abundance of functional microorganisms on the surface and upper layers of the wetland changed.The relative total abundance of Nitrospira and Nitrosospira of CW-100μg/L PS-MP and CW-1000μg/L PS-MP constructed wetland were lower than the blank group by 0.41%and 0.8%;Nitrospira in the upper layer was 0.50%,0.92%and respectively lower than the blank group.The relative total abundances of Bradyrhizobium and Mesorhizobium,which are related to plant nitrogen fixation,were lower than the blank group by 3.95%and 4.13%,respectively,and the upper layer was lower than the blank group by 0.95%and 1.64%,respectively.The relative total abundances of Rhodocyclaceae and Flavobactrium were lower than those of the blank group by 3.87%and 4.76%,respectively;Rhodocyclaceae in the upper layer were 3.1%and 2.15%higher than those in the blank group,respectively.The results of microbial enzyme activity studies indicate that the effect of PS-MP on the removal of COD in constructed wetlands may be related to microbial dehydrogenase.The addition of PS-MP inhibited the chlorophyll content,N content and total biomass of Cyperus alternifolius,and affected the photosynthesis parameters of Cyperus alternifolius.The results of plant enzyme activity showed that the influence mechanism of PS-MP on Cyperus alternifolius may be related to oxidative stress resistance.After PS-MP was added with 439 d,the chlorophyll content of CW-100μg/L PS-MP and CW-1000μg/L PS-MP Cyperus alternifolius decreased by 0.15 mg/g and0.82 mg/g compared with the blank group,respectively.The N content of leaves was lower than that of the blank group by 0.205%and 0.625%,and the N content of stems was lower than that of the blank group by 0.08%and 0.12%,respectively.The total biomass of leaves was lower than that of the blank group by 43.55 g and 45.75 g;the total biomass of the stem was lower than that of the blank group by 183 g and 179 g respectively.The net photosynthetic rate,intercellular carbon dioxide concentration,stomatal conductance and transpiration rate of the three groups of constructed wetlands all showed significant differences（test value P<0.05）.100μg/L PS-MP promotes the production of POD（Peroxidase,POD）enzymes in Cyperus alternifolius.The results of the cell viability of the roots of the Cyperus alternifolius proved that 2μm and 10μm PS-MP would not damage the root cells of the Cyperus alternifolius.In summary,the addition of different concentrations of PS-MP in the constructed wetland inhibited the removal efficiency of NH₄⁺-N、the related microbial community structure and enzyme activity,and the photosynthesis and nutrition of Cyperus alternifolius.The absorption of the elements was inhibited,which affected the growth of Cyperus alternifolius.It also proved the feasibility of the downward vertical flow constructed wetland water treatment technology to remove microplastics,with a removal efficiency of 93%～100%.