Copper accumulation and changes in soil physical–chemical properties promoted by native plants in ​​an abandoned mine site in northeastern Brazil: Implications for restoration of mine sites

In this study, the copper-accumulating capacity of plants growing spontaneously in copper-contaminated soils in an ​​abandoned mine site in northeastern Brazil was evaluated by calculating enrichment (EF) and translocation (TF) factors. The effects of physical and chemical changes in the rhizosphere soil on copper mobility were determined by using different compounds (Mehlich3/MgCl2) to extract Cu from different types of soil samples (bulk/rhizosphere soil). Finally, the possible implications for the use of these plant species in restoring the area were assessed by calculating the balance between the Cu mobilized in the rhizosphere and the Cu absorbed by the plants. On the basis of the EF and TF values obtained (all <1), none of the species under study (Ruellia paniculata, Bidens pilosa, Pityrogramma calomelanos and Combretum leprosum) were classified as hyperaccumulators. However, consideration of readily bioavailable levels (extracted with MgCl2) and the rhizosphere soils (rather than total levels and bulk soils) yielded higher correlations with the levels of metal in plant tissues. This approach therefore appears more appropriate for determining the capacity of the plants to accumulate copper. The different characteristics of the bulk and rhizosphere soils have direct effects on the concentrations of copper, which were much lower in the rhizosphere soil. In general, each species responded differently to the high concentration of Cu in soils (range 3604–9601 mg kg−1). By calculating the balance between the amounts of Cu mobilized in the rhizosphere and uptake by plants, we found that the presence of such plants in the field may have antagonistic effects. Two of the species (B. pilosa and P. calomelanos) contained more Cu in their tissues than mobilized in the rhizosphere. This is a desirable characteristic for restoration purposes, as the plants can reduce the bioavailable Cu content in soils and thus act as facilitators for regeneration of the site. By contrast, the other two species (R. paniculata and C. leprosum) mobilized more Cu in the rhizosphere than they were able to take up, which may led to transfer of bioavailable Cu to the ecosystem, which is undesirable in terms of site restoration.