Study On The Efficiency And Mechanism Of Heavy Metal Removal By Typical Permeable Pavement Systems

In this study,three common permeable pavement systems(permeable interlocking concrete paver(PICP),permeable concrete(PC),and permeable asphalt(PA))were constructed.The runoff with high metal concentration was artificially configured to simulate the long-term operation of the permeable pavement systems by multiple inflow methods,and the effects of inflow rate and dry period on the removal rate were investigated to analyze the concentration changes and migration law.In order to reveal the heavy metals accumulation characteristics,the rainfall conditions and particulate matter on the removal rate of different structural layers were investigate.Finally,the heavy metal change patterns of fugitive forms and leaching characteristics in the base and cushion media were studied,including the contamination risks.The main conclusions are as follows:(1)PA has the best efficiency for heavy metal removal.The system was the best for Pb removal and the worst for Zn removal.The overall removal rate of the system decreases as the influent flow rate increases and the fall-dry period increases.The long fall-dry period contributed to the increase in metal concentration,and the short fall-dry period contributed to the vertical migration of metals.No failure was observed for Pb and Cu removal by the three paving systems throughout the simulation years,while Zn and Cd removal by PICP and PC failed between 44 and 61 years.The removal mechanisms of dissolved heavy metals in water by the systems were dominated by adsorption and precipitation.(2)The surface layer with poor permeability and small particle size will remove more proportion of heavy metals.As the particle size and concentration increase,the percentage of surface layer removal increases and the percentage of base layer removal decreases,and as the influent flow rate increases,the percentage of surface layer removal decreases and the percentage of base layer and cushion layer removal increases.The effluent concentration gradually increases with the increase of the influent flow rate,and the increase of particle size and concentration tends to cause system blockage,which in turn affects the effluent concentration.Under the action of particulate matter,the system’s heavy metal removal mechanism is mainly based on retention and filtration.(3)The concrete base has better adsorption of exchangeable state metals and precipitation of carbonate-bound metals,and the medium sand bedding layer has better adsorption of exchangeable state metals.The proportion of active state of crushed stone base increased and then decreased,and the proportion of concrete base continued to increase,indicating that the concrete medium has sustained and good control of solid-liquid migration of heavy metals.Under the same conditions,the solid-liquid partition coefficient of medium sand bedding layer is the highest and the coefficient of crushed stone base is the lowest.The solid-liquid partition coefficient decreases with the increase of inflow concentration,rainfall return period and dry period before rain.(4)With the increase of rainfall field,the concentration of Zn in base after infiltration increased significantly,and the concentration of Cu did not change much.Zn in the base was released more into the water column,while Cu was mostly retained inside the medium.The crushed stone base showed moderate pollution and higher environmental risk than the concrete base,while both mat layers did not show pollution during the infiltration process.