Accumulation And Release Characteristics Of Heavy Metals By Bioretention Medias And Pollution Risk Assessment

The researches currently are focused on heavy metals' removal effect,mechanism and influence factors,ignoring the long-term accumulation characteristics and trend of heavy metals in different kinds of bioretention media,and there are few reports about heavy metals release from bioretention media,which can not provide effective theoretical support for potential ecological risk in engineering application and total heavy metals' control in the environment.Therefore,in this study,the cumulative characteristics of heavy metals from road runoff in differet media were investigated,accumulation trend of heavy metals was predicted,and the release regulation and influence factors were analyzed,and the risk assessment of heavy metals' accumulation and release in media was performed,which could help to predict and control the second-pollution of heavy metals in bioretention,and to keep the sustainable operation of the bioretention system.As the heavy metals accumulation trend and ecological pollution risk is not clear,the bioretention systems with different medias(e.g.,roungh sand,Fe-Mn bio-oxide particles and water plant sludge particles)were set up to investigate the removal effect and accumulation trend of heavy metals from road runoff by bioretention,the risk assessment was alos performed.Aiming at understanding the heavy metals release from the bioretention media and the possible ecological risks,the effects of different temperatures,pH values,salinity and humic acids on the release of accumulated heavy metals in bioretention system were investigated through static test,and the release rules of accumulated heavy metals in the biological retention system were investigated through water leaching.To understand the the accumulation and release of heavy metals properties in the pratical bioretention project,bioretention media were sampled in the bioretention systems which have run for years(kechuang 17 th street and the park of Boda mansion which is located in the Yizhuang economic and technological development zone).The accumulation of heavy metals was investigated,the correlation between the pH value,organic matter content and metal accumulation – release was analyzed,and the heavy metals pollution risk was assessed.The results are as follows:(1)With the years increasing,the removal rate decreased of three bioretention systems;with the media depth increasing,the removal rate increased.On the contrary,the cumulative heavy metals contents increased with the years increasing,and decreased with the depth increased.(2)The removal effect of heavy metals by improved media(Fe-Mn bio-oxide and water plant sludge)performed better than inert media(rough sand);It is predicted that a thickness of 0~20cm media of the bioretention system need to be replaced within 20~30 years of operation,otherwise the secondary pollution of Cd caused by overflow may be generated.(3)There is no pollution risk in the effluent by bioretention systems;the pollution risk decreased with the increase of media depth.(4)Saturated bioretention media can release heavy metals quickly.The high temperature,low pH and high salinity was beneficial to the release of heavy metals accumulated in the bioretention media.(5)The maximum and minimum heavy metal contents in the soil of the biological retention system of Kechuang 17 th street and Boda mensin are both Zn and Cd.The risk of surface soil pollution is higher at Kechuang 17 th street,and the risk of soil pollution at a depth of 10 cm in the park of Boda mension biological retention system is higher.Total organic carbon in soil can release synchronously with heavy metals from the soil.