Cadmium decontamination through ball milling using an expandable clay mineral

Mechanochemical treatments have been widely used for the remediation of soils and sediments polluted by organic and inorganic pollutants. However, there has been still limited knowledge about the molecular mechanisms underlying the mechanochemical transformations responsible of heavy metals immobilization on clay minerals. In the present study, the ability of a dioctahedral smectite to retain cadmium (Cd) as induced by mechanochemical treatments was investigated. The smectite was ground with different amounts of Cd (from 0.3 to 5.2% w/w). Solid-state characterizations (XRF, XRD, XPS, FT-IR, NMR) as well as desorption isotherm experiments were carried out to understand the critical metal sorption mechanisms onto dioctahedral smectite occurring in mechanochemical interactions. The "entrapping efficiency" and the type of interaction between clay surfaces and cadmium were evaluated. Cd immobilization degree was assessed by extract analyses and expressed as leachable fraction of cations. Leaching was both performed with deionized water and 1 M MgCl2 solution. Spectroscopic analyses evidenced that long time grinding (24 h) allowed Cd sorption on two sites: Type I, exhibiting characteristic of an outer-sphere complex likely localized in the montmorillonite interlayers and coordinated by two water molecule shells, i.e. in an exchangeable position; Type II sites, where Cd was more tightly bound to the TOT layers, either onto broken edges via the new OHs formed during the mechanochemical treatments or onto montmorillonite surfaces likely bridged via a water molecule. Desorption isotherms also confirmed a two-adsorption site model. This information could be useful to understand, develop and manage new remediation technologies based on mechanochemical treatments.