Role of residence time on the transformation of Zn, Cu, Pb and Cd attached to road dust in different land uses

Abstract

The adsorption behaviour of metals deposited on road surfaces undergo changes during dry weather periods, with the bioavailability varying compared to the original species, prior to incorporation in stormwater runoff. This study investigated the role of antecedent dry days on the transformation characteristics of Zn, Cu, Pb and Cd attached to different geochemical forms of road dust, in order to predict potential stormwater quality impacts. The study outcomes showed that the exchangeable fraction generally decreased with the increase in antecedent dry days, but the reducible, oxidisable and residual fractions increased. This implies that there is a time-dependent transformation of weakly bound metals in road dust to more persistent chemical forms. The transformation rate of metals was found to be in the order of Pb > Cu > Zn > Cd. Significant changes in metal distribution among the geochemical fractions were observed up to seven dry days, suggesting that during the initial antecedent dry days, the transformations can be relatively more significant. Among the analysed metals, Cd present on road surfaces has a higher potential for being bioavailable during the antecedent dry days.