Removal Mechanism Of Phthalic Acid Esters In Constructed Wetland System

As a common environmental hormone pollutant in the environment,phthalic acid eaters(PAEs)has the characteristics of bioaccumulation,refractory degradation,and influence on the secretion of organisms.With the rapid development of modern industry and the rapid improvement of people's living standards,the detection rate in the environment is also higher.The harm to the ecological environment and human health should not be underestimated.Constructed wetland sewage treatment system is an effective sewage treatment method with low operation cost and beautification environment.Rizhao City Second Wastewater Treatment Plant Constructed Wetland System was used as the test site,and six kinds of phthalic acid esters(DMP,DEP,DBP,BBP,DEHP,DOP)selected by the EPA as priority pollutants were selected as research objects in this study.The adsorption characteristics of different kinds of substrate fillers in constructed wetlands were compared.The removal effects of six kinds of PAEs by constructed wetland systems were analyzed and the removal rules of PAEs by wetland systems were also discussed in this study.The distribution of PAEs in subsurface flow constructed wetland was analyzed by collecting the substrate and plant samples of the subsurface wetland.The removal mechanisms of phthalic acid esters in the subsurface flow constructed wetland system were also discussed in this study.The main study contents and results are as follows:(1)The small-diameter substrate fillers(particle size is about 1cm)at the site of the subsurface wetland was collected.The biofilm-covered volcanic rocks,the biofilm-covered gravels,and the clean volcanic rock were selected as the research objects to study and analyze the adsorption characteristics of three substrate fillers with phenol as the target.The adsorption kinetics experiments showed that the adsorption rate of phenol in solution by three substrate fillers was slow and reached the equilibrium state within 24 hours.The adsorptionthermodynamics experiments show that under the normal temperature 20 ?,the three kinds of substrate fillers on phenol adsorptions conform to the Langmuir adsorption isotherm equation(r > 0.99),showed that adsorption is monolayer adsorption on the surface of substrate fillers.The adsorption equilibrium amount of phenol on the three substrate fillers increased with the increase of the initial phenol concentration in the solution,and the removal rate of phenol in the adsorption system decreased accordingly.The saturated adsorption capacity of the three substrate fillers on phenol was from large to small: the biofilm-covered volcanic rocks>the clean volcanic rocks > the biofilm-covered gravels.Comparing the specific surface area of the three substrate fillers,the specific surface area of the biofilm-covered volcanic rocks is about 1.5 times that of the clean volcanic rocks and about four times that of the biofilm-covered gravels.(2)In summer(August)and winter(December),sewage samples from four sampling points of the construted wetland system,including inlet,secondary distribution channel,catchment channel and outlet water.The composition of PAEs in the water samples was analyzed and detected by solid phase extraction(spe)/gas chromatography/mass spectrometry.The results showed that six kinds of PAEs were detected at each sampling point of the constructed wetland system,and the average removal rate of six kinds of PAEs in the wetland system were: DMP(47.1%),DEP(44.05%),DBP(39.25%),BBP(36.7%),DEHP(28.65%)and DOP(29.7%),respectively.The average removal rate of phthalic acid esters in constructed wetland system was 34.1%,indicating that the constructed wetland had certain removal effect on phthalate pollutants.The removal rate of PAEs in constructed wetland system is higher in summer than in winter.The removal rate of six kinds of PAEs in wetland system gradually decreases with the increase of molecular weight and side chain number of PAEs.There are three treatment stages in the constructed wetland system: primary underflow wetland stage,secondary underflow wetland stage and surface flow wetland stage.Secondary underflow wetland has the highest removal rate of PAEs and the lowest removal rate of PAEs in surface flow wetland.The total concentration of phthalic acid esters in the effluent treated by constructed wetlands was 10.23?g/L ~11.443 ?g/L,and the highest concentration of DEHP was 5.884?g/L among the six kinds of PAEs,while the concentration of other PAEs was less than 3?g/L.The DEHP content of the treated effluent meets the limit of theenvironmental quality standard of surface water.(3)Two subsurface samples(gravel and volcanic rock)and four plant samples(Scirpus validus Vahl Iris pseudacorus L.Thalia dealbata Fraser Lythrum salicaria L.)were collected from the subsurface flow wetland,and the characteristic composition and pollution distribution of PAEs in the collected wetland samples were detected and analyzed by ultrasonic extraction/extraction column purification/gas chromatography/mass spectrometry.The results showed that six kinds of PAEs were detected in the substrate and plant samples of the subsurface flow constructed wetland system,and the contents of DEHP and DBP were the highest.The content of PAEs on the surface of gravels in the first and second order subsurface flow wetlands were not significantly different,and the content of PAEs on the surface of volcanic rocks in the second order subsurface flow wetlands(volcanic matrix was only distributed in the second order subsurface flow wetlands)was higher than that of gravels.The content of PAEs on the substrate of subsurface flow varies with the seasons.The content of PAEs on the substrate of subsurface flow in winter(December)is higher than that in summer(August).Thalia dealbata Fraser was the plant with the highest absorption of PAEs among the four plants.The content of PAEs in the secondary subsurface wetland plants was higher than that in the first-grade wetland plants.Comparing the content of PAEs in the substrate and plant samples at the same time period,it was found that the absorption of PAEs in wetland plants was higher than that on the substrate surface.(4)Taking the 8# wetland unit in the north subsurface flow area as the analysis object,combining the research results obtained in the previous chapter,and investigating the design parameters of constructed wetland,the annual removal amount of PAEs in the subsurface flow wetland system,the removal amount of PAEs in the substrate and plant parts were predicted and estimated.It is estimated that the annual removal amount of PAEs by subsurface flow 8# wetland unit is about 3150.8g,the removal amount of PAEs by substrate adsorption is about 41.9g,and the removal amount of PAEs by plant absorption is about455 mg.The adsorption amount of wetland substrate plays a major role in the removal of PAEs.