Study On Adsorption-Leaching Mechanism Of Pollutants For Bioretention System Modified Media

With the rapid development of urbanization,the increase of impervious land area and the intensified interference of human activities,urban rainwater runoff pollutants,especially road runoff pollutants,have attracted wide attention.Stormwater runoff carries excessive hydrocarbons,nutrients（nitrates and phosphates）and heavy metals（such as Cu,Zn and Cd）,threatening aquatic ecosystems and people’s health through the food chain,leading to eutrophication and various diseases.As a sustainable and eco-friendly sponge for urban rainwater management,bioretention system plays an important role in alleviating urban waterlogging and improving urban water environment and water ecology.It has been proved that the modification and ratio of media can effectively improve the pollutant purification capacity of bioretention system and the infiltration capacity of rainwater.Due to the impact of intermittent storm runoff on facilities,the water/pollution load has strong temporal and spatial variability.Meanwhile,the improved bioretention media of solid wastes（improve the infiltration capacity,pollutant purification capacity,enrich biodiversity,etc.）and long-term operation will also increase the risk of pollutant leaching.In addition,there is a lack of systematic test and simulation research on the leaching behavior,leaching mechanism and environmental risk assessment of pollutants in bioretention system.In this study,static desorption-adsorption experiment and dynamic desorption-adsorptiondesorption three-stage mini-column field pulse test were used,combined with modern detection technology,to study the leaching mechanism and time-varying characteristics of solid waste modified biological detention media.The migration and transformation law of runoff water and pollutants in the modified media of bioretention system was studied by outdoor simulated water distribution test.The HYDRUS-3D soil physics model was used to simulate the mechanism of water and solute transport in bioretention system under different rainfall conditions,and to determine the impact of pollutant leaching on surface water and groundwater quality.The main research results are as follows:（1）Based on the principles of high adsorption capacity,low cost and local accessibility,six representative solid wastes were selected as improvers,namely leaf compost（LC）,corn stover biochar（Bio-char）,naturally air-dried water treatment residual（WTR-1）,medium temperature thermal decomposition of water plant sludge（WTR-2）,fly ash（FA）and recycled aggregates from construction waste（RACW）.Six bioretention media were prepared based on the physical and chemical properties of the modified media,soil and sand.The natural air-dried and mediumtemperature heat-treated WTR showed better adsorption capacity for NH₄⁺-N and PO₄³-under the interaction of the background pollutant content,influent pollutant concentration（ammonia nitrogen and orthophosphorus）and operation time.In contrast,NH₄⁺-N leaching from different modified media was higher,and Bio-char was more obvious.（2）The leaching mechanism and time-varying characteristics of the modified bioretention media of solid waste were studied through 74 mini-column field pulse tests.The results showed that the pollutant concentrations in the effluent showed a decreasing-increasing-decreasing trend,and the leaching of NO₃^--N and TP were the most obvious in the leaching process.The average pollutant concentration removal rates of TN,TP and heavy metals（Cu,Zn and Cd）were 54.1%,71.1%,95.1%,91.8%and 94.2%,respectively.In addition,the adsorption capacity of modified media with 5%WTR-1、5%WTR-2 and 10%RACW was higher than that of traditional bioretention system media and other modified media.（3）BSM+5%WTR-1 and BSM+10%RACW,which have better overall effect,were selected as modified media,and four sets of infiltration and impermeable bioretention systems were constructed for soil column simulated water distribution tests.The infiltration performance 1^#>2^#≈4^#>3^#,and the bioretention system with 5%WTR-1 had better water retention performance and total runoff control than the bioretention system with 10%RACW;The overflow rate and water reduction rate of the infiltration system were higher than those of the impermeable system.The bioretention system had better purification effects on NH₄⁺-N,TP,Cu and Zn,with average pollutant concentration removal rates of 69.4%,76.4%,79.5%and 84.7%,respectively.During the test,NO₃^--N,TN,COD and Cd were leached,and the leaching of NO₃^--N was the most obvious.The average pollutant concentration removal effects of each system were as follows:modified media BSM+5%WTR-1>BSM+10%RACW,impermeable system>infiltration system.（4）Through the comparison of the test results of single factors（inflow pollutant concentration,rainfall intensity,dry period days before rain）,the average load reduction effect of media on pollutants is Zn>Cu>TP>COD>Cd>TN.High inflow pollutant concentration,low rainfall intensity and long dry period before rain showed better pollutant purification effect.During the experiment,the content of organic matter in the media first decreased and then increased with the increase of soil depth,and the contents of nutrients TN,TP and heavy metals（Cu,Zn,Cd）showed an upper>middle>lower layer.（5）A HYDRUS-3D soil physical model was established for the modified media bioretention system,and reliable hydraulic and water quality characteristics parameters（R2>0.61）of the media layer of the bioretention system were obtained based on the above simulated rainfall test for rate determination and verification.The outflow concentration increases with the increase of the inflow pollutant concentration,and the time when the outflow concentration appears to rise and level off is also earlier;The higher the rainfall intensity,the earlier the transport time of the wetting front,the faster the rise rate,and the larger the peak when it reaches stability.The greater the intensity of rainfall,the more obvious the phenomenon of pollutant leaching;the dry period before the rain has no obvious influence on the pollutant outflow concentration,but has a greater influence on the time when the water content starts to change.The greater the initial water content of the soil,the earlier the time when the water content starts to rise.As the dry period before rain increased,the lag of moisture change between the two observation points was more obvious.（6）The HYDRUS-3D model was used to simulate the water and solute transport processes in the undersoil layer for 24h.The water content decreased and the migration rate slowed down with increasing soil depth.1^#migrated to the bottom of the system（470cm below the surface layer）at 74min,and 2^#migrated between 370cm and 42 cm below the surface layer during the 24h simulation period.The groundwater recharge amounts of 1^#and 2^#were 54.66L and 43.87L,with recharge rates of 90.33%and 72.5%,respectively.and 72.5%,respectively.At the end of the simulation,TN and TP of 1^#migrated to 470cm and 370cm,and TN and TP of 2^#migrated to 370cm and 220cm,respectively.Because NO₃^--N is prone to a large amount of leaching,the longitudinal migration rate of TN is relatively fast during rainfall.