Changes in the heavy metal solubility of two contaminated soils after heavy metals phytoextraction with Noccaea caerulescens

Hyperacumulator plant species, such as Noccaea caerulescens, have been deeply studied due to their use in phytoextraction techniques, although the fate of the metals remaining in the roots at the end of the remediation process is still uncertain. Here, germination and growth, metal accumulation in plant tissues and degradation of roots remaining in the soil after harvest have been studied in two contaminated soils from an area affected by a toxic pyritic sludge spillage. Specially designed pots allowed the separation of the bulk soil and rhizosphere, where heavy metals fractionation in soil was determined at the end of the growing period. High Cd and, especially, Zn concentrations in the aerial parts of the plants were found, although the bio-concentration factors (BCF) were higher for Cd (13-34) than for Zn (2.37-4.34). For both soils, the soluble and exchangeable (CaCl2-extractable) concentrations of Fe and Mn were higher in the rhizosphere than in the bulk soil, while Zn and Cd concentrations were greater in the bulk soil. After plant harvesting, the degradation of heavy metal enriched roots in the soil was studied as well as the effect of this process on soil metal solubility. The soluble concentrations of Cu and Mn were higher in the soils with roots that in the corresponding soils without roots. Nevertheless, the amount of heavy metals released to the soil after root degradation (8-14% of their organic C was decomposed) was rather low (below 0.1 μg g−1), showing the feasibility of the use of N. caerulescens for phytoextraction.