Retention and exchange behaviour of vehicular lead in street dusts from major roads

Five collections of street dust were made from each of three major sites in the urban area of Brisbane, Australia. Each street dust sample (< 600 mu m) was subdivided into two portions. One portion was used for chemical and physical analyses and the other portion was size fractionated. Both total and size fractionated street dusts were subjected to a sequential extraction procedure to determine lead association with different chemical and physical fractions. Further chemical analyses were also made on the size fractionated street dust. Results showed that most of the lead was found to be associated with the carbonate fraction. With respect to street dust particle size, lead was primarily associated with smaller particles (< 90 mu m). However, at sites with active street dust resuspension, lead was found to be associated more with the medium size fraction. This was attributed to the existence of an aggregation process by which magnesium cations play an important role in aggregate formation. Upon ageing of deposited lead in street dust, a process of ion exchange between lead and basic cations (particularly calcium) was indicated. This process leads to a lower exchange capacity, exchangeable cations and pH but notably higher soluble cations.