Does silica addition affect translocation and leaching of cadmium and copper in soil?

Soil and groundwater contamination with potentially toxic elements (PTEs) including cadmium (Cd) and copper (Cu) has become a serious problem for ecosystem functioning. Silicon (Si) may precipitate these metals as silicates, and may also form, at undersaturation of silicates, 'Si-contaminant compounds', i.e. particles of polymerized silica with PTEs incorporated or adsorbed by inner-sphere complexes. While the formation of these compounds in aqueous solution has been proven, their formation in soil remains unclear yet. Therefore, we conducted column experiments with a topsoil horizon artificially contaminated with Cd or Cu solutions (10 mM) in the presence (10 mM) and absence of monomeric Si, and monitored the elemental composition of the eluates during 12 irrigation steps with artificial rainwater by microwave-plasma atomic emission spectrometry, the size and charge of the particles eluted by dynamic light scattering and phase analysis light scattering, and determined the spatial distribution of total and exchangeable Cd and Cu in soil after the experiments. When Si was previously applied to soil, significantly larger particles (up to > 200 nm) in the eluates indicated Si polymerization and formation of Si-contaminant compounds. However, Cd and Cu concentrations were very low (<0.4 µM), pointing to efficient retardation in soil. In any variant, the particles formed were slightly negatively charged (-11 mV). The molar metal:Si ratios in the eluates and significant correlations between the amounts of Si and metals in soil extracted by NH4NO3 pointed to the formation of Si-contaminant compounds, too. More Cu than Cd was retained in soil, and significantly more in the presence of Si, but less Cu than Cd was in exchangeable form. While particularly Cu formed Si-contaminant compounds, which reduced the concentration of Cu ions, the Si-contaminant-compound particles in the eluates remained very small, thus potentially susceptible to particulate export from soil into the groundwater.

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.