Role of solids in heavy metals buildup on urban road surfaces

Abstract

Solids are widely identified as a carrier of harmful pollutants in stormwater runoff exerting a significant risk to receiving waters. This paper outlines the findings of an in-depth investigation on heavy metal adsorption to solids surfaces. Pollutant build-up samples collected from 16 road sites in residential, industrial, and commercial land uses were separated into four particle size ranges and analyzed for a range of physicochemical parameters and nine heavy metals including Iron (Fe), Aluminum (Al), Lead (Pb), Zinc (Zn), Cadmium (Cd), Chromium (Cr), Manganese (Mn), Nickel (Ni), and Copper (Cu). High specific surface area (SSA) and total organic carbon (TOC) content in finer particle size ranges were noted, thus confirming strong correlations with heavy metals. On the basis of their physicochemical characteristics, two different types of solids originating from traffic and soil sources were identified. Solids generated by traffic were associated with high loads of heavy metals, such as Cd and Cr, with strong correlation with SSA. This suggested the existence of surface dependent bonds, such as cation exchange between heavy metals and solids. In contrast, Fe, Al, and Mn, which can be attributed to soil inputs, showed strong correlation with TOC suggesting strong bonds such as chemsorption. Zn was found to be primarily attached to solids by bonding with the oxides of Fe, Al, and Mn. The data analysis also confirmed the predominance of the finer fraction, with 70% of the solids being finer than 150 µm and containing 60% of the heavy metal pollutant load.