Removal Of Mechanism Of Typical Pollutants From Livestock And Poultry Wastewater On Constructed Wetland Mediated By Microplastics

Microplastics(MPs)can adsorb antibiotics to form complex pollutants,which seriously threaten the health of aquatic ecosystems.The concentration of antibiotics in livestock and poultry wastewater is high and there are a lot of microplastics,which will interfere with the normal operation of the system,the growth and development of plants and the activity of microorganisms in the process of constructed wetland(CW)treatment.At present,the discussion on the removal of typical pollutants from livestock and poultry wastewater by constructed wetlands and the mechanism of microplastics synergistic removal is still in the initial stage.In this thesis,firstly,the common substrates of constructed wetlands were screened,and the influence of microplastics on the adsorption of antibiotics by substrates was investigated.A submerged plant-type constructed wetland was constructed.The typical plastic polystyrene/microplastics(NPs/MPs)was used as the research object.0.7 μm(NPs)and 90-110 μm(MPs)of polystyrene were added into the constructed wetland system according to the concentrations of 0,10 μg/L and 1mg/L,and the influence and mechanism of polystyrene on the treatment of livestock and poultry wastewater by the constructed wetland system were explored.The main conclusions are as follows:(1)The natural zeolite(NZ)of six kinds of constructed wetland substrates(activated carbon,quartz sand,medical stone,ceramsite,sea sand and natural zeolite)has the best adsorption performance for antibiotics(OTC),and the removal rate can reach 75.76%.In addition,NPs/MPs can adsorb a small amount of antibiotics,and 25 mg/L is the best concentration for NPs/MPs to adsorb antibiotics.Scanning electron microscopy showed that NPs were adsorbed on the surface of NZ,while MPs were not.The p H value can change the surface charge of the adsorbent and the existing form of OTC in the solution which has a great influence on the adsorption performance.The ionic strength also has a significant influence on the adsorption performance of OTC by NZ.When the concentration of KCl increases from 0 mol/L to 0.2 mol/L the removal rate of OTC by NZ decreases from 79.90% to 70.63% and the adsorption capacity decreases by 14.35%.The results of X-ray photoelectron spectroscopy show that the binding energy of C1 s spectrum changes,the ratio of C-C decreases from 73.57% to 71.85% after OTC adsorption,and the functional group C=O-OH disappears.The adsorption of OTC coexisting with MPs on NZ is more consistent with Langmuir and pseudo-first-order kinetic equations,while for NPs,Freundlich and pseudo-first-order kinetic models are more consistent.It is speculated that NZ successfully adsorbs OTC mainly through electrostatic attraction,hydrogen bond and π-π bond,and MPs and NPs act as additional adsorbents and competitive adsorbents in the adsorption process.(2)The occurrence of NPs/MPs inhibited the removal of COD,TP and TN in constructed wetlands.The average removal rates of TP and TN in NPL,NPH,MPL and MPH groups were 16.39%,14.23%,9.65% and 4.28% lower than those in the blank group respectively,and 4.81%,2.18%,1.81% and 3.29% lower than those in the blank group respectively.The average removal rate of COD in NPL,NPH and MPL treatment groups was 1.41%,1.36% and 2.20% lower than that in blank group,respectively.The average removal rate of COD in MPH treatment group was 3.70% higher than that in blank group.The existence of NPs/MPs significantly promoted the uptake of TP and TN by Myriophyllum leaves,and the uptake of TP and TN by high concentration of NPs/MPs was more obvious than that by low concentration of NPs/MPs.The total chlorophyll content of Myriophyllum leaves in NPH and MPH treatment groups were 90.1% and 94.6%of the control group,respectively,and there was no significant difference.However,low concentration of NPs/MPs exposure would inhibit chlorophyll synthesis to a certain extent.In addition,exposure to NPs/MPs could induce stress on Myriophyllum and stimulate the synthesis of SOD,MDA and POD enzymes in Myriophyllum.(3)Microbial community of CK,NPL,NPH,MPL and MPH were different.Microbial diversity analysis showed that MPs had a great influence on microbial community.The relative abundance of Proteobacteria in NPL and NPH groups was 1.76%and 1.35% lower than that in CK group,while the relative abundance of Proteobacteria in MPL and MPH groups was 4.95% and 12.22% higher than that in CK group,respectively.The selected ARGs and MRG(cop A and cop B)were detected in five constructed wetland systems.The total relative abundance of genes in CK group was higher(0.57769),and the total relative abundance of genes in MPL group was the lowest(0.10016).The involvement of polystyrene microplastics also limited the accumulation of tet gene in constructed wetland system to a certain extent,and the absolute number of MRGs was also related to the particle size and concentration of polystyrene,and increased with the increase of microplastics concentration.The addition of polystyrene microplastics decreased the number of 16 s r RNA genes,which indicated that they could inhibit the growth of microorganisms in the system.This study clarified the NPs/MPs complex impact on constructed wetlands,and provided an important theoretical basis for the actual treatment of livestock and poultry wastewater.