There is increased interest in the terrestrial silicon cycle in the last decades as its different compounds and species have large implications for ecosystem performance in terms of soil nutrient and water availability, ecosystem productivity as well as ecological aspects such as plant–microbe and plant-animal feedbacks. The currently existing analytical methods are limited. Fourier-transform infrared spectroscopy (FTIR) analysis is suggested being a promising tool to differentiate between the different Si species. We report here on the differentiation of varying Si-species/Si-binding (in synthetic material) using FTIR-analyses. Therefore, we collected FTIR-spectra of five different amorphous silica, Ca-silicate, sodium silicate (all particulate), a water-soluble fraction of amorphous silica and soil affected by volcanic activity and compared their spectra with existing data. A decrease of the internal order of the materials analyzed was indicated by peak broadening of the Si–O–Si absorption band. Peak shifts at this absorption band were induced by larger ions incorporated in the Si–O–Si network. Additionally, short-range ordered aluminosilicates (SROAS) have specific IR absorption bands such as the Si–O–Al band. Hence, SROAS and Si phases containing other ions can be distinguished from pure amorphous Si species using FTIR-analyses.

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<p>Soil contamination with inorganic contaminants such as lead (Pb), copper (Cu) and cadmium (Cd) is a major environmental issue, especially concerning food and groundwater security. Various studies demonstrated positive effects of Si regarding resilience of some crops towards these inorganic contaminants. One reason could be a complexation reaction of Si and the metal cations. However, this process has not been systematically investigated yet. Thus, our research contributes to reducing the mobility of Cd, Cu and Pb in contaminated soils and to decreasing their transfer into aquifers or plants.</p><p>The main goal of this study is to elucidate the extent and the mechanisms of the interactions between Pb<sup>2+</sup>, Cd<sup>2+</sup> and Cu<sup>2+</sup> and silicic acid, including the long-term kinetics, and to investigate whether the metals are bound by silicic acid. We carried out a series of precipitation experiments in aqueous solution at room temperature to understand these processes.</p><p>We used Tetraethoxysilane (TEOS) as Si source and Pb(NO<sub>3</sub>)<sub>2</sub>, Cd(NO<sub>3</sub>)<sub>2</sub> and Cu(NO<sub>3</sub>)<sub>2</sub> with an initial concentration of 10 mmol l<sup>-1</sup> for synthesis. Selectivity of Si towards the metals was tested in an equimolar solution of all three salts and TEOS. Time-dependency of particle growth was examined at sixteen different dates using dynamic light scattering (DLS) and transmission electron microscopy (TEM). We measured the Si and metal concentrations in the dialyzed aliquots using microwave plasma-atomic emission spectrometry (MP-AES). Spectroscopic analysis of the dialyzed and freeze dried solid phase, was performed using FTIR and <sup>29</sup>Si-NMR spectroscopy.</p><p>DLS and TEM analyses showed that the metals had an accelerating effect on the polymerization reaction of silicic acid [Cu<sup>2+ </sup>> (Cu<sup>2+</sup>, Pb<sup>2+</sup>, Cd<sup>2+</sup>) > Cd<sup>2+</sup> > Pb<sup>2+</sup>]. Particle growth followed initial formation of nanoparticles through homogenous nucleation. Particle growth in the control synthesis (TEOS in aqueous solution) stopped after 124 days at a size of 34 nm (Z-Average). Particles in the syntheses with the metals kept growing until the experiment was completed after 211 days. The final particle sizes depended on the metal present, reaching a final size of 260 nm (Cu), 96 nm (Pb) and 196 nm (Cd). Final concentrations of up to 15, 10 and 13 µmol l<sup>-1</sup> of Cu, Pb and Cd, respectively, remained in the dialyzed aliquots. The Si concentrations in these aliquots increased continuously until an equilibrium was reached after 112 days at different concentrations (Cu, 7.3 mmol l<sup>-1</sup>; Pb, 6.9 mmol l<sup>-1</sup>; Cd, 4.8 mmol l<sup>-1</sup>). The FTIR spectra showed a shift of the Si-O stretching vibration by 10 to 32 cm<sup>-1</sup> towards lower wavenumbers, which could indicate an incorporation of the metals in the polymeric network of the silicic acid. <sup>29</sup>Si-NMR relaxation experiments showed a shortening effect of Cu<sup>2+</sup>-ions on the relaxation time of the Si nuclei. It appears that the proportion of the rapidly relaxing components decreases for the Si-atoms deep inside the silicate matrix. This indicates that the Cu centres are located predominantly at the huge surfaces (up to 667 m<sup>2</sup> g<sup>-1</sup>) of the Si matrix. Future extraction experiments will show how strong the metals are bound to the Si polymeric network.</p>

Rice cultivation requires high amounts of phosphorus (P). However, significant amounts of P fertilizer additions may be retained by iron (Fe) oxides and are thus unavailable for plants. At the same time, rice cultivation has a high demand for silicic acid (Si), reducing Si availability after short duration of rice cultivation. By studying a paddy chronosequence with rice cultivation up to 2000 years, we show that Si limitation, observed as early as a few decades of rice cultivation, is limiting P availability along the paddy soils chronosequence. Using near edge X-ray absorption fine structure spectroscopy (NEXAFS) in a scanning transmission (soft) X-ray microscope (STXM) we show release of available P was linked to a Si-induced change in speciation of Fe-phases in soil particles and competition of Si with P for binding sites. Hence, low Si availability is limiting P availability in paddy soils. We propose that proper management of Si availability is a promising tool to improve the P supply of paddy plants.

For the majority of higher plants, silicon (Si) is considered a beneficial element because of the various favorable effects of Si accumulation in plants that have been revealed, including the alleviation of metal(loid) toxicity. The accumulation of non-degradable metal(loid)s in the environment strongly increased in the last decades by intensified industrial and agricultural production with negative consequences for the environment and human health. Phytoremediation, i.e., the use of plants to extract and remove elemental pollutants from contaminated soils, has been commonly used for the restoration of metal(loid)-contaminated sites. In our viewpoint article, we briefly summarize the current knowledge of Si-mediated alleviation of metal(loid) toxicity in plants and the potential role of Si in the phytoremediation of soils contaminated with metal(loid)s. In this context, a special focus is on metal(loid) accumulation in (soil) phytoliths, i.e., relatively stable silica structures formed in plants. The accumulation of metal(loid)s in phytoliths might offer a promising pathway for the long-term sequestration of metal(loid)s in soils. As specific phytoliths might also represent an important carbon sink in soils, phytoliths might be a silver bullet in the mitigation of global change. Thus, the time is now to combine Si/phytolith and phytoremediation research. This will help us to merge the positive effects of Si accumulation in plants with the advantages of phytoremediation, which represents an economically feasible and environmentally friendly way to restore metal(loid)-contaminated sites.