Layer charge and heavy metals structures in hydrated 2 : 1 silicates: state of the art and new advances on cadmium

This study will discuss how layer charge can affect chemical speciation and topology of heavy metals adsorbed to 2: 1 layer silicates, by providing: i) an overview of literature data; ii) experimental data on Cd complexes adsorbed by 2: 1 layer silicates with different layer charge (montmorillonite and vermiculite); iii) a comparison between our results and literature data. This study will also be supported by several different experimental techniques such as chemical and thermal analyses, X-ray powder diffraction and X-ray absorption spectroscopy. Based on our data Cd atoms were found to complex water molecules in both clay minerals and to show fourfold coordination in montmorillonite (Cd-O distances of 2.24 angstrom) and six-fold coordination in vermiculite (Cd-O distances of 2.16 and 2.28 angstrom). Furthermore our models clearly suggest that Cd mainly bonds to interlayer water, without neglecting the more limited, but still significant, Cd multinuclear surface complexes at the octahedral broken edges. Both clay minerals show H2O/Cd ratio, as evidenced by thermal analyses, drastically higher than expected from X-ray absorption spectroscopy data, thus implying that most of the water molecules are only loosely coordinated to interlayer cations.