| Using porous asphalt pavement as an effective tool to mitigate road runoff heavy metalspollution is one of the core developing tendencies of porous asphalt. However, there is still littlestudy in China, investigating the removal capacity of porous asphalt on heavy metals in roadrunoff, where there is a lack of data support. Therefore, the aim of this thesis was to investigatethe removal mechanism based on the removal behaviour of porous asphalt to heavy metals fromroad runoff from the infiltration path of rainwater, the pavement material and its structureperspective.In the thesis, the rainwater infiltration process was firstly analysed. Based on the principleof water balance, a “rainwater infiltration-storage” calculation model for porous asphalt wasestablished. The expression of objective air void conformed considering the porous asphaltmixture reducing the volume of road runoff and the evaluation method of the efficiency ofrainwater storage and infiltration for porous asphalt were put forward. Then, the design ofporous asphalt mixture was studied through the objective air void. The performance of porousasphalt mixture was evaluated through experiments. The reasonable cross section of porousasphalt was studied from the reducing the volume of road runoff perspective. Meanwhile, theefficiency of rainwater storage and infiltration for a typical structure of the porous asphalt wasanalysed by use of different duration of rainfall and design return period. Thirdly, an artificialrainfall was implemented directly to the porous asphalt models with the typical structure. Theremoval capacity of porous asphalt on Pb and Zn from road runoff was analysed, and theremoval mechanisms were analysed based on the nature of reservoir structure aggregates.Finally, the adsorption of dissolved Pb and Zn in road runoff by asphalt on pore-wall withinporous asphalt was investigated using a period-soak simulated experiment. In this work, thesurface chemical state information of asphalt films before and after the experiment wasanalysed using X-ray photoelectron spectroscopy (XPS). The adsorption mechanisms werediscussed according to the fitting results based on adsorption isotherms and kinetics.Several results were mainly obtained from above studies:1. The expression of objective air void for porous asphalt mixture and the evaluationmethod of the efficiency of rainwater storage and infiltration for porous asphalt were putforward according to the “water infiltration-storage” model for porous asphalt which built basedon the analysis of water infiltration process and runoff producing mechanism considering theprinciple of water balance.2. Porous asphalt had been shown to cause a significant decrease in lead and zincconcentrations from initial flush road runoff. However, during the storage period, the capacityof porous asphalt to remove Pb from road runoff differed significantly from Zn. It is shown thatthe Pb removal rate was high, but the Zn removal rate was moved from negative to positive.3. The removal of Pb and Zn from initial road runoff mainly depended on the retentionaction by the micro-pores within porous asphalt and the adsorption of the sediment or cloggingmaterials. The removal of Pb and Zn from road runoff in a long term was mainly related to thenature of reservoir structure aggregates and the storage time. The mechanism of Pb2+and Zn2+adsorption by limestone aggregates was mainly “precipitation by metal carbonate complexes” or "hydroxides precipitation". And the mechanism of Pb2+and Zn2+adsorption using basalt aggregates was mainly "hydroxides precipitation"4. Analysis through XPS confirmed the adsorption of Pb2+and Zn2+from aqueous solution by asphalt film as adsorbent.5. The complexity of chemical composition in asphalt has naturally led to important effects on the adsorption of Pb2+and Zn2+from aqueous solution by asphalt film. Firstly, the number of H+and R-COO was increased tremendously through the hydrolysis and ionisation of some polar components. Secondly, R-COO reacted with Pb2+and Zn2+to form Pb(RCOO)2and Zn(RCOO)2. Thirdly, some coordination complexes were formed through coordination bond between O‖R-S-R*and metal ions, while such sulfoxide could react with water. Fourthly, new asphaltogenic acid would be formed due to the replacement reaction between H+and Pb(RCOO)2or Zn(RCOO)2in solution.6. Parabolic diffusion equation was found to best describe the initial adsorption under a given dosage of asphalt with higher initial concentrations of Pb2+and Zn2+in solution by asphalt film. Both double constant rate equation and Elovich equation were found best to describe the initial adsorption under a given dosage of asphalt with lower initial concentrations of Pb2+and Zn2+. And in some given conditions, the second-order equation was found to best describe the kinetics of adsorption in the entire time series.7. The Freundlich isotherm model was found to best describe the adsorption of Pb2+and Zn2+by asphalt film under a given dosage of asphalt with different initial concentrations of Pb2+and Zn2+in solution. |
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