Study On The Purification Effect And Mechanism Of Bioretention Facilities For Typical POPs In Stormwater Runoff

The rapid development of urbanization has led to an increase in impervious area,thus causing a series of environmental problem s such as surface source pollution and urban flooding.Bioretention facilities,as a green facility with easy construction and wide applicability,are a representative technology for urban surface runoff pollution treatment.Persistent Organic Pollutants(POPs)in runoff are easily trapped by bioretention facilities and can exist inside the media for a long time,accumulating with strong ecotoxicity and eventually threatening human health.However,there are still many shortcomings in the research on the effectiveness of bioretention facilities in removing POPs and their purification mechanisms,which need further exploration and research.In this study,the bioretention facilities in Fengxi New City and Xi’an University of Technology were used as research objects to study the purification effect and removal mechanism of runoff POPs by bioretention facilities through on-site monitoring combined with stable carbon isotope tests and model simulations.The main research findings are as follows:(1)By monitoring the rainfall runoff in the pilot sponge city area of Xi’an New Area and Xi’an University of Technology,the pollution characteristics of pollutants in surface runoff were clarified and the source analysis and risk evaluation of POPs were conducted.The concentrations of polycyclic aromatic hydrocarbons(PAHs),polychlorinated biphenyls(PCBs)and organochlorine pesticides(OCPs)in the runoff ranged from 4.558 to 968.4 ng/L,1.085 to 34.013 ng/L and 3.78 to 67.27 ng/L,respectively.Most of the total phosphorus(TP)in stormwater runoff was below 2 mg/L,while chemical oxygen demand(COD)and suspended solids(SS)concentrations were the highest,with the highest COD concentration of 180.22 mg/L and the highest SS concentration of 582.2 mg/L in a single rainfall;among heavy metal pollution,the concentrations were Zn,Cu,and Cd in descending order,with lighter pollution and their ecological risks lower,overall,road runoff pollution is higher,and SS is an important carrier of heavy metals and POPs.The ecological risk of PAHs in stormwater runoff mostly comes from oil discharge,low-chlorinated PCBs may come from historical residues,and OCPs mainly comes from industrial and historical residues.the ecological risk of PAHs is higher,and direct discharge will cause greater harm;the ecological risk of PCBs is lower;the risk of γ-HCH in OCPs is medium,and the risk of DDT is second,and the relevant risk sources need to be investigated.(2)Through sampling and analysis of the completed bioretention facilities,the removal effects of carbon,nitrogen and phosphorus,heavy metals Cu,Zn and Cd were studied,the types and concentrations of POPs in the inlet and outlet water were clarified,and the purification effects of bioretention facilities on PAHs,PCBs and OCPs were evaluated.The results showed that the retention and removal effects of rain gardens on carbon,nitrogen,phosphorus,and heavy metals were somewhat different,influenced by the filler type and runoff source,among which S5(oncampus modified fill rain garden:BSM+WTR)had the best removal effect,and the reduction rates of carbon,nitrogen,phosphorus,and heavy metals Cu,Zn,and Cd were 72.8%-83.6%and 70.9%-79.1%,respectively.The average removal rate of PAHS by bioretention facilities ranged from 63.8%to 93.8%,and the highest removal efficiency was achieved for highly cyclic PAHs,among which S5(on-campus modified fill rain garden:BSM+WTR)had the best purification effect on PAHS;the removal effect of PCBS was more stable between 62.9%and 98.4%,and the removal effect of perchlorinated PCBs was better The removal rate of OCPs can reach up to 99%,but the removal rate of different components of OCPs is not stable,and the rain gardens containing improved fillers S5(on-campus improved filler rain garden:BSM+WTR)and S8(vegetation retention tank)have the best effect on the removal of OCPs,and S6(on-campus impermeable rain garden),which was built at the earliest time,has the worst removal effect.(3)The transport and transformation behavior of POPs in bioretention columns was investigated through indoor experiments based on stable carbon isotope filter column tests,and model simulations were conducted to reveal the fate behavior of POPs in bioretention facilities.The results showed that the fate modes and percentages of POPs in bioretention columns were:media adsorption(61.1%-70.0%),plant uptake(8.9%-13.2%),biodegradation(1.15%-11.41%),leaching(6.21%-8.87%),and volatilization(0.17%-0.55%),respectively.Media adsorption occurred mainly in the surface layer of media(0-20cm)and decreased with increasing depth;plant uptake relied mainly on root adsorption,which was higher than that of stems and leaves;biodegradation had the strongest effect on the degradation of pyrene,but had limited effects on the removal of PCB169 and p,p’-DDT;volatilization accounted for a relatively small percentage.The presence of heavy metals had an inhibitory effect on the removal of POPs,with media adsorption,mineralization and plant uptake decreasing by 4.3%-6.4%,1.8%-8.3%and 1.5%3.6%,respectively,while the content in leachate increased and the effect on volatilization was negligible.The predicted cumulative concentration in soil is very close to the measured value,indicating that the model simulates the accumulation of media better.