PHStat testing and geochemical modeling of inorganic compounds in recycled asphalt pavement used in highway shoulders

Abstract

Recycled asphalt pavement (RAP) has increasingly been used as a highway construction material due to its favorable physical properties. As with many industrial by-products, however, there are concerns about the impact of RAP on surface and groundwater caused by leaching of heavy metals present in the material. To determine the potential risks posed by heavy metals leaching from RAP, pHstat leaching tests were performed on seven Maryland-sourced RAPs, and were compared with two local topsoils. The pHstat leaching tests showed As leaching from RAPs between 0.3 and 68 mg/L at pHs below 9, which is above the US EPA maximum concentration limits for drinking waters (0.01 mg/L). Other analytes were largely detected below their respective EPA WQL in the drinking water pH range of 6.5–9. The topsoils leached lower concentrations for all analytes except for barium and aluminum. The geochemical model run to understand the nature and source of heavy metals leaching in the RAP samples indicated that leached metals were solubility controlled. The model results also showed Al and Fe leaching to be controlled by oxide/ hydroxide minerals, and Ba, Ca and Mg to be controlled by carbonate and/or sulfate minerals.