Immobilisation of elements by iron and alkalinity in nine contaminated soils and mine residues without vegetation, growth of Dactylis glomerata and reduction of contaminant concentration in leaf
Résumé
Some contaminated soils and mine tailings or residues are so phytotoxic (especially sulfidic tailings or process residues) that they are bare or with a very scarce vegetation for decades. Without amendment and soil development, these materials do not evolve into soils. Nine contaminated soil or mine residues, most of them without vegetation, were treated at laboratory, in lysimeters or at field: excavated urban subsoil from Marseille (F), heavily Cu polluted soil from Bordeaux area (F), bare pyritic Pb and Zn mine tailings in sandstone from Alès (F), flotation tailing dumps, Campo Pisano, Sardinia (I), As and Pb mine residues and tailings from Marseille area (F), marsh downstream of a zone of mine tailings, Sa Masa, Sardinia (I), acid mine tailings with adapted vegetation, Saint Laurent le Minier (F), Pb and Zn mining residue in dolomite and pyritic tailings, St Félix (F), and pyritic process residues (MR3-1, F), with a control forest soil, Aix-en-Provence (F). The pH of these materials was in the 2.4 – 8.3 range, and total contaminants concentration were for some very high (As: 4 – 1300 mg/kg, Cd: 1 – 250; Cu: 1 – 10 000; Pb 10 – 20000; Zn 10 – 4000). The soil improver rich in iron and alkalinity was washed bauxite residue, treated with gypsum to lower the pH to pH 8.5 and called “modified bauxite residue” (MBR). The leachable (10 L deionised water/kg DM, EN 12457-2) fraction of 20 major and minor elements were measured one month after addition of 0.1% to 30% of MBR. Other portions of 400 g of materials were amended with 0, 10% or 30% of MBR, with NPK fertilizer at agronomic rate, with 1% compost, and sown with a common cultivated grass, Dactylis glomerata at spring 2016 in Aix-en-Provence (F). For two materials, D. glomerata was grown in lysimeters (for one year and two years), and for one material two-years field plot plant was used. In June 2016, the aerial part of D. glomerata were harvested and their contaminants measured. The leaching concentration of phytotoxic elements are immobilised (frequently up to the limits of quantification - LOQ) for inputs between 0.3% to 30 % MBR, depending of the material. Leachable cations (Cd, Cu, Zn), anions (As, Cr, Sb), and major potentially phytotoxic elements (Fe, Al, Mn) are easily immobilised up to LOQ (< 0.1 mg/kg or < 0.5 mg/kg). Among the 8 materials with leachable lead, this element is immobilised up to the LOQ (< 0.8 mg/kg) in 4 materials, and to 1 to 2 mg/kg in 4 other materials. Growth of Dactylis glomerata was poor or zero for untreated material in small pots, moderate in amended pots, and luxurious in lysimeters or field plot. The decrease of foliar concentration is correlated with the decrease of leaching concentration with MBR addition. Foliar concentrations of all elements (excepted Pb) are reduced up to a 10 to 100 factor, and up to the concentration of the uncontaminated control soil for lysimeters (St Félix, MR3-1) and field plot (St Félix). For Pb the foliar concentrations are reduced in pots by a factor 10 with 30% of MBR, and are larger than the control (3 mg/kg) for St Félix (62 mg/kg) and MR3-1 (8 mg/kg). Iron and alkalinity of MBR are effective immobilisers for contaminants.