Immobilization of heavy metals in contaminated soils through ball milling with and without additives

In the present work, the use of ball milling for remediation of heavy metals from 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 with or without additives, by taking advantage of weak transformations induced on the soil through mechanical loads occurring during collisions in the milling process. When hydroxylapatite (HA) is added to the heavy metal contaminated soil, the corresponding immobilization efficiency of the mechanochemical treatment increases. The degree of metal immobilization is evaluated by analyzing the leachable fraction of heavy metal from the treated soil as obtained through the “synthetic precipitation leaching procedure”. In particular, for soils contaminated by heavy metals displaying concentration levels similar to field contaminated soils, specific ball-milling treatments without the use of hydroxylapatite were able to reduce the leachable fraction of heavy metals to levels lower than the USEPA regulatory thresholds. XRD, SEM/EDS and granulometric analyses reveal no significant alterations of the intrinsic character of sandy and bentonitic soils after milling except for a partial amorphization of the treated soil. On the other hand, the mechanical treatment causes the total amorphization of kaolinitic soil. The increase of immobilization efficiency when soils are mechanically treated is hypothesised to be due to the specific phenomena induced during ball milling such as entrapment of heavy metals into aggregates, solid diffusion into the crystalline reticulum of soil particles as well as the formation of new fresh surfaces (through particle breakage) onto which heavy metals may be irreversibly adsorbed. In addition, when HA is added to the heavy metal contaminated soils, the mechanical treatment is hypothesised to be able to provide an increase of the specific surface and reactivity of hydroxylapatite, whose metal immobilization properties are well known.