Effect of organic amendments on the mobility of trace elements in phytoremediated techno-soils: role of the humic substances.

The efficiency of aided phytostabilization using organic amendments such as ramial chipped wood (RCW) and composted sewage sludge (CSS) was studied on contaminated techno-soils, on nine experimental plots. The objective was to characterize the role of fulvic (FA) and humic acids (HA) on the mobilization of trace elements, specifically As, Cu, Mo, Pb and Zn. Results showed that the addition of CSS increased the total organic carbon and nitrogen content more than with RCW and as a result, the C/N ratio in the CSS soil was higher than in the RCW and non-amended (NE) soil, reflecting the high decomposition of soil organic matter in the CSS soil compared with the other soils. The RCW and CSS amendments increased the hydrogen index (HI) values and the oxygen index (OI) values compared with the NE soil, especially for the soil treated with CSS which contained more aliphatic than aromatic compounds. The addition of CSS to the techno-soil significantly increased the percentage of C org associated with the HA fractions compared with the RCW and NE soils. The soil amended with CSS showed the highest E 4/E 6 ratio and the lowest E 2/E 3 ratio of FA. Zn and As were more abundant in the FA fraction than in the HA fraction, whereas Pb, Cu and Mo were more associated to HA than to FA in the treated and untreated soils, which may explain the difference in their mobility and availability.

Are plants useful as accumulation indicators of metal bioavailability?

The use of accumulation bioindicator to assess metal bioavailability has mainly concerned individual species. This work addresses this issue at the plant community level. Metal content within different species from plant communities found at three contaminated and one uncontaminated site was compared. Results showed that for two contaminated sites, leaf metals concentrations were comparable to those in plants from control site, i.e. approx (mg/kg) 0.1 Cd, 0.2 Cr, 9.2 Cu, 1.8 Ni, 0.5 Pb and 42 Zn. Only plants from the third site showed higher metal contents, ranging from 1.5- to 8-fold those of the control community. This contrasted with ammonium acetate-EDTA extractions, which indicated a very high "availability" of metals at the three sites, as compared to the control site. Thus, metal content in plant communities provided accurate information on actual transfer toward the ensemble of vegetation, which could be used to establish site-specific "fingerprints" of metal bioavailability.

Multi-tolerance to heavy metals in Plantago arenaria Waldst. & Kit.: adaptative versus constitutive characters.

Tolerance to Cu, Cd, Ni and Zn was investigated in a population of the pioneer species Plantago arenaria growing in a metallurgical landfill. Tolerance levels were compared with those of two other pioneer species (Coniza sumatrensis and Verbascum densiflorum) growing in the same location, and with a control population taken from an uncontaminated site. Results showed that the metalliferous population of P. arenaria was more tolerant to metal toxicity than C. sumatrensis and V. densiflorum. Comparisons with literature data confirmed that the metalliferous population of P. arenaria was highly tolerant to Cu, moderately tolerant to Cd and Ni, but not particularly tolerant to Zn. The control population of P. arenaria responded the same as the metalliferous one excepted for Cu, for which it was much more sensitive. This suggested that multi-metal tolerances in the metalliferous population of P. arenaria resulted both from constitutive and adaptative traits, depending on the metal. To check whether P. arenaria was able to cope with high internal metal levels, accumulation patterns were evaluated in pot experiments. Results showed that metals accumulated in roots and leaves, at levels proportional to soil content. Metal content was much higher in roots than in leaves and the leaf:root concentration ratio was kept constant over a wide range of soil metal contents. This suggested that metal tolerance was related to the ability to retain metal ions in roots and to tightly control their translocation to leaves. Finally metal tolerance in P. arenaria is discussed in relation to its pioneer and xerophytic characteristics.

Soil characteristics, heavy metal availability and vegetation recovery at a former metallurgical landfill: Implications in risk assessment and site restoration.

Pedological and botanical characteristics of a former metallurgical landfill were examined to assess the risks of heavy metals mobility and to evaluate remediation feasibility. In addition to very high heavy metals levels (Cu, Cr, Mn, Ni, Pb, Zn), the soil was characterized by a lack of clear horizonation, a relatively high pH, a high mineral and organic carbon contents, a low nitrogen level and a high C/N ratio. A two step sequential extraction showed that heavy metals were poorly labile (i.e. not soluble in diluted CaCl2), indicating that their leaching under natural conditions was probably very low. However, extraction with DTPA generated significant amounts of metals (mainly Pb and Cu), suggesting they were potentially mobilizable. A botanical survey of the area showed a biodiverse plant community (28 species and 11 families), with no obvious toxicity symptoms. Measurements of metal contents in dominant species confirmed that they were closely similar to those reported for species growing in unpolluted environments. Consequently, for an effective site restoration, indigenous species could be well suited to cope with local conditions in a phytostabilization strategy.