Improving the mining soil quality for a vegetation cover after addition of sewage sludges: inorganic ions and low-molecular-weight organic acids in the soil solution

Potentially Toxic Elements’ Contamination of Soils Affected by Mining Activities in the Portuguese Sector of the Iberian Pyrite Belt and Optional Remediation Actions: A Review

Both sectors of the Iberian Pyrite Belt, Portuguese and Spanish, have been exploited since ancient times, but more intensively during and after the second half of the 19th century. Large volumes of polymetallic sulfide ore were extracted in open pits or in underground works, processed without environmental concerns, and the generated waste rocks and tailings were simply deposited in the area. Many of these mining sites were abandoned for years under the action of erosive agents, leading to the spread of trace elements and the contamination of soils, waters and sediments. Some of these mine sites have been submitted to rehabilitation actions, mostly using constructive techniques to dig and contain the contaminated tailings and other waste materials, but the remaining soil still needs to be treated with the best available techniques to recover its ecosystem functions. Besides the degraded physical structure and poor nutritional status of these soils, they have common characteristics, as a consequence of the pyrite oxidation and acid drainage produced, such as a high concentration of trace elements and low pH, which must be considered in the remediation plans. This manuscript aims to review the results from studies which have already covered these topics in the Iberian Pyrite Belt, especially in its Portuguese sector, considering: (i) soils’ physicochemical characteristics; (ii) potentially toxic trace elements’ concentration; and (iii) sustainable remediation technologies to cope with this type of soil contamination. Phytostabilization, after the amelioration of the soil’s properties with organic and inorganic amendments, was investigated at the lab and field scale by several authors, and their results were also considered.

Use of wastes from the pulp and paper industry for the remediation of soils degraded by mining activities: Chemical, biochemical and ecotoxicological effects

Fly ash (FA) from biomass combustion and biological sludge (S), both wastes from the pulp and paper industry, were granulated in different proportions (90% FA+10% S, and 70% FA+30% S w/w, dry weight basis, dw) and used to recover the functionality of soils affected by mining activities (Aljustrel, Iberian Pyrite Belt), with and without the application of municipal solid waste compost (MSWC). Application doses of both mixtures were 2.5, 5.0 and 10% (w/w, dw). These materials corrected soil acidity to circumneutral values and increased extractable P and K concentrations. A significant increase in soil organic matter (from 0.6 to 0.8-1.5% w/w, dw) and N content (from 0.04 to 0.09-0.12% w/w, dw) was also observed, but only when MSWC was applied. The soil was already heavily contaminated with Cu, Pb and Zn and the application of amendments did not increase their pseudo-total concentrations. The CaCl₂ extractable fractions of both Cu and Zn decreased to very low values. The improvement in soil quality, compared to fertilizer only treatment, was further evidenced by the increase in some soil enzymatic activities (dehydrogenase, β-glucosidase and cellulase), with a better response for the granules with the higher proportion of biological sludge, as well as by the decrease in the soil-water extract toxicity towards different organisms (Daphnia magna, Thamnocephalus platyurus, and Pseudokirchneriella subcapitata). Agrostis tenuis germinated and grew during the first month only in the amended pots, but, after that, a considerable phytotoxic effect was evident. This was mainly attributed to salt stress or to some specific ionic toxicity. In conclusion, to establish a long-term plant cover in mining soils amended with biomass ash-based materials, the selection of plants with higher resistance to salinity and/or the stabilization of the amendments, to reduce their soluble salt content, is recommended.

Chemical and ecotoxicological effects of the use of drinking-water treatment residuals for the remediation of soils degraded by mining activities

The aim of this study was to evaluate the use of drinking-water treatment residuals (DWTR) in the amendment of a soil affected by mining activities (Aljustrel mine, Portuguese sector of the Iberian Pyrite Belt), considering the effects on its chemical, biochemical and ecotoxicological characteristics. The DWTR had neutral characteristics (pH 6.7) and an organic matter (OM) content of 575 g kg^-1 dry matter (DM), which makes them a potential amendment for the remediation of mine degraded soils, as they may correct soil acidity and reduce the extractable metal fraction. An incubation assay, with soil and DWTR, with or without lime, was carried out to test the doses to be used in the assisted-phytostabilization experiment. Based on the results obtained, the doses of DWTR used were the equivalent to 48, 96, and 144 t DM ha^-1, with and without lime application (CaCO₃ 11 t DM ha^-1). Agrostis tenuis Sibth was used as the test plant. Some amendments doses were able to improve soil characteristics (pH and OM content), to decrease metal extractability by 0.01 M CaCl₂ (especially for Cu and Zn), and to allow plant growth, that did not occur in the non-amended soil. Copper, Pb and Zn concentrations in the plant material were lower than the maximum tolerable level for cattle feed, used as an indicator of risk of entry of those metals into the human food chain. The simultaneous application of DWTR (96 and 144 t ha^-1), with lime, allowed a reduction in the mine soil ecotoxicity, as evaluated by some lethal and sub-lethal bioassays, including luminescence inhibition of Vibrio fischeri, Daphnia magna acute immobilization test, mortality of Thamnocephalus platyurus, and 72-h growth inhibition of the green microalgae Pseudokirchneriella subcapitata. However, DWTR were unable to increase soil microbial activity, evaluated by dehydrogenase activity, an important soil-health indicator. Also, OM content and N_Kjeldahl, concentrations increased slightly but remained low or very low (P and K extractable concentrations were not affected). In general, the bioassays highlighted a decrease in soil ecotoxicity with the presence of lime and DWTR (144 t DM ha^-1). In conclusion, DWTR are recommended to amend acidic soils, with high concentrations of trace elements, but an additional application of organic or mineral fertilizers should be considered.

Screening Arabidopsis mutants in genes useful for phytoremediation

Emissions of heavy metals have risen over the past 200 years and significantly exceed those from natural sources. Phytoremediation strategies may be able to recover soil productivity in self-sustaining ecosystems; however, our knowledge of the molecular mechanisms involved in plant heavy-metal perception and signalling is scarce. The aim of this study was to assemble a "molecular tool box" of genes useful for phytoremediation. To identify mutants with different heavy-metal-tolerance, we first selected a medium from mixtures containing three metals based on their presence in two Spanish mining areas and then screened about 7000 lines of Arabidopsis T-DNA mutants and found 74 lines more resistant and 56 more susceptible than the wild type (WT). Classification of the genes showed that they were mainly linked to transport, protein modification and signalling, with RNA metabolism being the most representative category in the resistant phenotypes and protein metabolism in the sensitive ones. We have characterized one resistant mutant, Athpp9 and one sensitive, Atala4. These mutants showed differences in growth and metal translocation. Additionally, we found that these mutants keep their phenotype in amended former soils, suggesting that these genes may be useful for phytoremediation and the recovery of contaminated soils.