Formation and properties of inorganic Si-contaminant compounds

Comparing amorphous silica, short-range-ordered silicates and silicic acid species by FTIR

“…A relative decrease of their intensities to the intensity of the Si–O–(Al) stretching region (WN 1018 and 975 cm −1 ) is caused by a decreasing Si content of the SROAS. Furthermore, Si-rich SROAS show an absorption maximum at 690 cm −1 , while the bands at WN 430–440 cm −1 are related to vibrations of Si–OH groups 19 . Thus, these spectral regions are suitable to distinguish SROAS from “pure” Si phases.…”

“…The spectra were interpreted as follows: The bands at spectral regions between WN 3500–3300 cm −1 are characteristic for hydroxyl groups (O–H; stretching vibration) that are part of water molecule and silanol groups (Si–OH), the latter mostly located at the surface of the silica particles. The adsorption band at about 1630 cm –1 is attributed to the bending 19 or deformation mode of molecular coordinated water adsorbed to the Si–O–Si structure (H 2 O band; Kaya et al 21 , Roulia et al 26 ). While the bands at 1050–1100 cm −1 are the most significant spectral region (with regard to Si) and accompanying shoulders are attributed to asymmetric stretching vibrations of Si–O–Si.…”

“…Lower internal order of the material under study may leads to peak broadening of the entire envelope 20 , denoted here as Si–O–Si band. Infrared spectra of SROAS showed an absorption band between 1018 and 975 cm −1 which shifted to lower WN with increasing Al:Si ratio and is attributed to stretching vibrations of the Si–O–Al bond (oxygen bridges formed by condensation of Si and Al hydroxides 19 , 27 . The bands at wavenumber (WN) 980 cm −1 are characteristic for Si–O stretching vibrations of Si–O–H (Si–OH band) groups, which are attributed to asymmetric vibration of Si–OH.…”

“…Fourier-transform infrared spectroscopy (FTIR) –already used e.g. by Farmer et al 18 to characterize the chemical structure of synthetic aluminoosilicate gels- was recently suggested to be a potential tool to analyze certain Si species 19 , 20 . For example, compressive stress in the subsurface regions of glasses through exchange of network modifier ions with larger ions induces a red-shift of the Si–O–Si bridging oxygen (BO) stretch band in the IR spectrum (Ref.…”

Silicon Cycling in Soils Revisited

“…Modelling suggested silicic nanocolloids as potentially being an important species under low pH and neutral conditions [70]. The presence of other elements, for example metals such as lead, copper or cadmium, in soil solution was also shown to be a driver for polymerization and precipitation [71], but this process might also happen in the presence of other elements. This finding suggests that silicic acid is a main control on ion concentration in soils.…”