Sperimentazione di materiali organici come possibili ammendanti per scarti minerari e sviluppo di una procedura di rimozione di metalli da drenaggi circa-neutri tramite la precipitazione indotta di LDH (Layered Double Hydroxides)

Previous mining activity in the Sulcis-Iglesiente area (Sardinia, Italy) has left environmental problems related to the release of heavy metals from mining wastes. The addition of organic matter as an amendment was applied to a neutral-reactive mining waste, suitably chosen and characterized, in order to modify the solid speciation of polluting metals (Zn, Mn, Cd and Pb) from a "mobile" to an "immobile" form. Four different types of organic wastes, easily available in Sardinia, were chosen as potential amendments: grape marc, grape stalk, olive pomace and coffee grounds. After chemical characterization and various leaching tests and batch sorption experiments with single metal and binary/quaternary metal mixtures, different amounts of each amendment (0, 1, 2.5, 5, 10 g) were mixed with the mining waste (50 g) in filtering funnels and leached for 7 weeks with distilled water. Grape marc and grape stalk, causing a slight pH decrease, produced a release of Zn and Mn greater than the mining waste without amendment, but Pb decreased by 75% with grape stalk, and Cd and Pb decreased respectively by 78% and 64% with grape marc. Olive pomace and coffee grounds respectively produced a decreased release for Zn (30% and 53%), Cd (71% and 33%) and Pb (96.5% and 41%). The efficiency and selectivity of the used organic amendments in inhibiting the release of metals from the mining waste seem to depend on: type, amount and properties of biomaterial (e.g. acid-generating or acid-neutralizing capacity), competition among metals in sorption processes, and physico-chemical characteristics of bulk and microenvironment establishing in biomaterial-mining waste interaction. Layered double hydroxides (LDH) are lamellar compounds with general formula [M2+1-xM3+x(OH)2](An-)x/n • mH2O, where M2+ are divalent metals (e.g. Mg2+, Zn2+, Fe2+), M3+ are trivalent metals (e.g. Al3+, Fe3+) and An- are anions (e.g. CO32-, SO42-, Cl-). These compounds are widely studied for the removal of anionic pollutants from water, but they can also be effective removers of cationic contaminants by (co)precipitation. Since LDH precipitation generally presupposes neutral-alkaline conditions and the presence of dissolved divalent and trivalent metals in adequate ratios, the M3+-poor drainage from the impoundment of “Red Muds” (metallurgical wastes from an electrolytic plant used in processing oxidized Zn-ores) in the Monteponi mine area (Iglesias, Sardinia, Italy) was treated by inducing the precipitation of LDH and the consequent removal of metals. The “Red Muds” drainage is characterized by neutral pH and high levels of sulphate (4000 mg/L), Mg (600 mg/L), Zn (200 mg/L) and other divalent metals (Mn, Pb, Cd, Ni), but very low concentrations of trivalent metals such as Fe3+ and Al3+. The precipitation of a sulphate Zn-Mg/Al LDH was tested through different laboratory batch experiments by adding to the drainage an adequate amount of a salt of Al, in order to induce the LDH precipitation, and NaOH to maintain or bring again the pH at neutral values. The main parameter controlling the removal of metals and the type of precipitate appeared to be the pH. As a function of pH variations during the batch experiments, XRD analysis of precipitates and chemical analysis of solutions (before and after each experiment) and precipitates showed either the formation of poor crystalline LDH combined with almost complete removal (96-99%) of Zn, Mn, Ni and Pb, and less Cd (43%), or the formation of more crystalline LDH combined with lower removal of Zn (62%), Mn (43%), Ni (88%), Pb (64%) and Cd (1%). The precipitates were also characterized by HRTEM-EDS-SAED; the amorphous Al-hydroxysulphate precipitating at acidic pH was identified as a fundamental precursor of the poor crystalline LDH forming as pH was increased and removal of metals was more efficient. These results indicate that the induced precipitation of LDH can be a simple and effective method for the removal of metals.