Formation and properties of inorganic Si-contaminant compounds

Abstract 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.