Formation and properties of inorganic Si-contaminant compounds

Soil contamination with inorganic contaminants such as lead (Pb), copper (Cu) and cadmium (Cd) is a major environmental issue. Silicon (Si) may reduce the mobility of the contaminants in the environment so that we studied the extent and the mechanisms of the interactions between Pb²⁺, Cd²⁺ and Cu²⁺ and silicic acid during its polymerization. We used tetraethyl orthosilicate as Si source and separately Pb(NO₃)₂, Cd(NO₃)₂ or Cu(NO₃)₂. Selectivity of Si towards the metals was tested in an equimolar solution of all three salts and the polymerizing Si source. Time-dependency of particle growth was examined using dynamic light scattering. Transmission electron microscopy was used for visualizing the particles. We characterized the solid phases by Fourier transform infrared (FTIR) and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy. Polymerized silica bound relative to the initial concentrations (10 mmol L^-1) up to 2.1‰ Cd²⁺, 2‰ Cu²⁺ and 1.4‰ Pb²⁺. The FTIR spectra indicated an incorporation of the metals in the polymeric network. ²⁹Si-NMR relaxation experiments showed an accelerating effect of Cu²⁺ on the ²⁹Si longitudinal relaxation time. It appears that the proportion of the rapidly relaxing components decreases with increasing distance to the surface. This points to a predominant location of Cu centers close to the surface of the Si matrix. Thus, polymerizing silica may contribute to reduced metal mobility in the environment.