Heavy metal distribution in mine-soils and plants growing in a Pb/Zn-mining area in NW Spain

The aim of this study was to identify metal-tolerant plants with potential application in phytoremediation strategies. For this we evaluated the distribution and chemical fractionation of heavy metals in soils and their accumulation or exclusion by native plant species growing in an abandoned Pb/Zn mine in NW Spain. Mine-soils (I = 0–20 cm, II = 20–40 cm) and shoots of the dominant plants were sampled at 39 sampling points in a (100 m × 100 m) regular grid at two sites (mine zone (MZ) and tailing ponds (TP)). metal contents in mine-soils presented a high heterogeneity and the principal contaminants were Cd, Hg, Pb and Zn (in agreement with the mineralogy of the site). Total concentrations varied from 2 to 95 mg Cd kg−1, 0.2 to 24.8 mg Hg kg−1, 46 to 6100 mg Pb kg−1, and 340 to 52,000 mg Zn kg−1. Soil metal distribution did not vary significantly with depth, but total and EDTA-extractable metal concentrations were higher in MZ than TP. On the other hand, the relative extractability of soil metals (EDTA-extractable concentration expressed as a % of total concentration) was higher in TP. The residual fraction was dominant for Co, Cr, Cu, and Ni, representing generally more than 50% of total concentration. Pb was preferentially associated with the oxidisable fraction, followed by the acid-extractable fraction. Both Cd and Zn were amply represented by the oxidisable, reducible and acid-extractable fractions. All the studied plant species were metal-tolerant (pseudometallophytes), although differences were found between species. Significant differences were found in plant accumulation of Cd and primarily, Zn. Cytisus scoparius and C. multiflorus were efficient metal-excluding species, and could be interesting candidates for phytostabilisation techniques and/or revegetation of heavily contaminated soils. Salix atrocinerea, presented significantly higher concentrations of Zn in its aerial biomass (543 ± 108 mg Zn kg−1) than the remaining species and bioconcentration factors (BCF) of up to 2.35 for Zn and 4.17 for Cd, respectively. This population could be interesting in the phytoextraction of low to moderately-contaminated soils.