Trattamento meccanico per l’immobilizzazione di metalli pesanti in suoli contaminati

In the present work, the use of ball milling for remediation of heavy metals contaminated soils of sandy, bentonitic and kaolinitic type is investigated. Immobilization of heavy metals (Cd(II), Pb(II), Zn(II)) is achieved by mechanically treating the contaminated soil without additives, thus taking advantage of weak transformations induced on the soil by mechanical loads occurring during collisions among the milling spheres. The degree of heavy metal immobilization is evaluated by analyzing the corresponding leachable fraction from the treated soil as obtained through the “synthetic precipitation leaching procedure” SPLP. The reduction of leachable fraction of heavy metals from the synthetic soils investigated, was obtained under specific milling regimes. XRD, SEM/EDS and granulometric analyses do not reveal alterations of the intrinsic character of sandy and bentonitic soils after milling. The increase of immobilization efficiency when soils are mechanically treated may be due to specific phenomena induced during ball milling such as the formation of new fresh surfaces (through particle breakage) onto which heavy metals may be irreversibly adsorbed as well as entrapment of heavy metals into aggregates. Also solid diffusion into the crystalline reticulum of soil particles determined by mechanical action may contribute to the increase of immobilization capacity.