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This study aimed to evaluate the influence of Eisenia fetida (Savigny), added to an acidic soil contaminated with potentially toxic elements (PTEs; As, Sb, Cd, Pb, Zn) and amended with a softwood-derived biochar (2 and 5% w/w), on the mobility of PTEs and soil health (i.e. nutrient availability, enzyme activity and soil basal respiration). The PTEs bioaccumulation by E. fetida and the acute ecotoxicity effects of the amended soils were also evaluated. The interaction between earthworms and biochar led to a significant increase in soil pH, organic matter, dissolved organic carbon content, cation exchange capacity, and exchangeable Ca compared to the untreated soil. Moreover, the water-soluble and readily exchangeable PTE fraction decreased (with the exception of Sb) between 1.2- and 3.0-fold in the presence of biochar and earthworms. Earthworms, biochar, and their combination, led to a reduction of phosphomonoesterase activity which in soils amended with biochar and earthworms decreased between 2.2- and 2.5-fold with respect to the untreated soil. On the other hand, biochar and earthworms also enhanced soil basal respiration and protease activity. Although the survival rate and the weight loss of E. fetida did not change significantly with the addition of 2% biochar, adding the highest biochar percentage (5%) resulted in a survival rate that was ~2-fold lower and a weight loss that was 2.5-fold higher than the other treatments. The PTE bioaccumulation factors for E. fetida, which were less than 1 for all elements (except Cd), followed the order Cd > As>Zn > Cu > Pb > Sb and were further decreased by biochar addition. Overall, these results highlight that E. fetida and biochar, especially at 2% rate, could be used for the restoration of soil functionality in PTE-polluted environments, reducing at the same time the environmental risks posed by PTEs, at least in the short time.

The effect of long-term ageing (up to 700 days) on the mobility, potential bioavailability and bioaccessibility of antimony (Sb) was investigated in two soils (S1: pH 8.2; S2: pH 4.9) spiked with two Sb concentrations (100 and 1000 mg·kg−1). The Sb mobility decreased with ageing as highlighted by sequential extraction, while its residual fraction significantly increased. The concentration of Sb (CDGT), as determined by diffusive gradients in thin films (DGT), showed a reduction in potential contaminant bioavailability during ageing. The DGT analysis also showed that Sb-CDGT after 700 days ageing was significantly higher in S1–1000 compared to S2–1000, suggesting soil pH plays a key role in Sb potential bioavailability. In-vitro tests also revealed that Sb bioaccessibility (and Hazard Quotient) decreased over time. Linear combination fitting of Sb K-edge XANES derivative spectra showed, as a general trend, an increase in Sb(V) sorption to inorganic oxides with ageing as well as Sb(V) bound to organic matter (e.g. up to 27 and 37% respectively for S2–100). The results indicated that ageing can alleviate Sb ecotoxicity in soil and that the effectiveness of such processes can be increased at acidic pH. However, substantial risks due to Sb mobility, potential bioavailability and bioaccessibility remained in contaminated soils even after 700 days ageing. Refereed/Peer-reviewed

Softwood-derived biochar (5% w/w) was added to two mining soils (S1 and S2) contaminated with Cd (4.8–74 mg kg−1), Pb (318–1899 mg kg−1) and Zn (622–3803 mg kg−1), to evaluate its immobilization capabilities towards such potentially toxic elements (PTEs). Biochar addition (S + B) increased soil pH, organic carbon content, extractable phosphorous and calcium. Sequential extractions showed that biochar reduced the labile pools of PTEs (e.g. −29, 55 and 79% of water-soluble and exchangeable Cd, Zn and Pb respectively in S1 + B compared to S1) and at the same time increased their most stable and less mobile fractions. Leaching experiments revealed a significant decrease of DOC, N-NO3−, P and PTEs in biochar-treated soils, and an increase of leached K. Kinetic equations derived from leaching data showed that PTEs in control soils were quickly mobilized, while those in biochar-treated soils needed longer time to leachate. In vitro tests showed that biochar was effective at reducing the bioaccessibility of Cd and Pb in the gastric phase of S2 and that of Zn and Pb in the intestinal phase of S1. The results obtained showed that biochar could be used as alternative amendment for the recovery of PTEs-contaminated soils.

The use of organic-based amendments for gentle remediation options (GRO), i.e. the stabilization of trace elements (TE) in polluted soils and the reduction of their impact on soil microbial and biochemical features, has been constantly growing in last 10 years. To verify the effectiveness of biochar and compost in such context, biochar (1 and 3% w/w), compost (3% w/w) and their combination (compost 2% + biochar 2% w/w) were added to two sub-alkaline soils (FS and MS) contaminated with Sb (41–99 mg kg−1 respectively), As (~18 mg kg−1), and trace metals such as Ni (103–172 mg kg−1 respectively) and Cr (165–132 mg kg−1 respectively). Most of the treatments (especially 3% biochar) reduced labile TE pools (water-soluble and exchangeable) and increased their residual (non-extractable) fractions (e.g. +48, 56, 66, and 68% of residual Sb, As, Cr and Ni in MS-treated soil compared to the untreated control). The amendments addition had both stimulating and inhibiting effects on the activity of soil microbial communities, as shown by the Biolog community level physiological profiles. However, in both soils, 3% biochar produced the highest increase of metabolic potential as well as the use of carboxylic acids and polymers by the soil microbial communities. Likewise, soil dehydrogenase (DHG), β-glucosidase (β-GLU) and urease (URE) activities were significantly enhanced in FS and MS soils treated with 3% biochar (e.g. +77, 48, and 17% for DHG, URE and β-GLU in FS-3% biochar with respect to untreated FS). Overall, the results from this study showed that the amendments investigated (particularly 3% biochar) can be effectively used for GRO of sub-alkaline soils, being able to reduce labile TE and to increase the metabolic potential and actual biochemical activities of the respective soil microbial communities. The manifold environmental implications of such effects are discussed.