Characteristics of a polluted artificial soil located along a motorway and effects of acidification on the leaching behavior of heavy metals (Pb, Zn, Cd)

Polluted artificial soils (average concentrations: Pb: 1460 ppm; Zn: 2830 ppm and Cd: 2.8 ppm) have been collected at the edge (less than 1 m) of the A-71 motorway in Sologne. These soils are characterized by XRD and a procedure of sequential extraction of Pb, Zn, Cd, Mn and Fe (Tessier et al., 1979). Excepting for Fe, the “labile” fractions F I, F II and F III, respectively “exchangeable”, “carbonate-related” and “Fe-hydroxides-related” are dominant on the “residual” fraction F V. Fraction F IV “related to organic matter” is mostly significant for Pb. A study of the different size fractions indicates a strong metal enrichment in the finest one (<20 μm) while sequential extractions suggest that small organic and Fe-hydroxide particles play a significant role in the fixation of Pb. Artificial soils and, for comparison, suspensions from a nearby retention pond (Lee et al., 1996) have been submitted to 24 h leaching tests in HNO3 solutions of increasing acidity (10 to 100 mmol/L). Differences between both types of samples result mainly from the buffering effect of carbonates, present in artificial soils and lacking from suspensions. Acid leaching of the suspensions is equivalent to the sum F I+F II+F III while, probably for kinetic reasons, the amounts leached from artificial soils are lower. As deduced from both sequential extractions and leaching runs, the relative mobility of heavy metals is found to be: Cd>Zn Pb, suggesting that in carbonate-free systems moderately acid rainstorms may leach Cd and a part of Zn from the border of the motorway, while Pb would remain fixed. A similar mobility scale has been derived from the study of other highly polluted materials related to motorways (Lebreton and Thévenot, 1992; Colandini et al., 1995; Lee and Touray, 1997). When in addition Fe and Mn are considered, the mobility scale becomes Mn Cd>Zn Pb>Fe, for artificial soils as well as for suspensions, in spite of large differences in heavy metal contents and localizations. This is in agreement with the idea that most of the “acid labile” heavy metals from suspensions are localized within a highly polluted material, comparable to artificial soils and diluted by a large proportion of local Sologne soil.