Multi-step sequential chemical extraction of heavy metals from urban soils

About 90 Glasglow surface soil samples (0–5 cm) were subjected to a six-step sequential chemical extraction procedure designed to establish the partitioning, mobility and availability of heavy metals lead, zinc, copper and cadmium in soils from a typical contaminated urban environment. The six fractions corresponding to the selected extractants were termed exchangeable, carbonate-bound, easily reducible, moderately reducible, organic and residual. Significant features of the average partitioning patterns were the association of 32% of cadmium (for total concentrations ≥ 0·6 mg kg⁻¹) with the exchangeable + carbonate fractions, 51% of lead with the moderately reducible fraction and 41% of copper and 29% of zinc with the organic fraction. Only 17% of lead, compared with 42–46% of the other three elements, remained in the residual fraction. While total concentrations for the heavy metals suggested an order of contamination, lead > zinc > copper > cadmium, in Glasgow soils, the detailed sequential extraction data indicated an order of release or mobility of cadmium ⪢ lead > zinc > copper and an order of environmental concern, with respect to availability, of lead > cadmium > copper > zinc.