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The Application of Raman Spectroscopy to Probe the Association of H4SiO4 with Iron Oxides

Ferras, Yohan, Robertson, John, Swedlund, Peter J.
Aquatic geochemistry 2017 v.23 no.1 pp. 21-31
Raman spectroscopy, adsorption, coprecipitation, ferrihydrite, geochemistry, infrared spectroscopy, iron oxides, polymerization, silicates, silicic acid, trace elements, weathering
The aquatic geochemistry of many trace elements is influenced by two important products from the weathering of minerals: iron oxide solid phases and silicic acid (H₄SiO₄) in the aqueous phase. The chemistry of H₄SiO₄ on iron oxide surfaces is an interesting mix of sorption and polymerization, and this has been shown to affect the sorption, coprecipitation and transport of many trace elements. Infrared spectroscopy is a valuable probe of H₄SiO₄ chemistry on iron oxide surfaces, and in this study, we assess the utility of Raman spectroscopy for studying H₄SiO₄ chemistry on the poorly ordered iron oxide ferrihydrite. This was undertaken because Raman spectroscopy provides complimentary information to IR, in addition to often having narrower bands than IR spectra and less interference from water during in situ measurements. The IR spectra of H₄SiO₄ adsorbed on ferrihydrite showed the expected strong Si–O stretching feature, termed ν(Si–O), which had a central band and shoulders on either side. As the surface concentration of H₄SiO₄ increased, there was a clear shift in the position of the central band of the ν(Si–O) feature from ~950 to 1060 cm⁻¹ reflecting the degree of silicate polymerization. The Raman spectra of the same samples had a very broad and weak ν(Si–O) feature which had a poor signal-to-noise ratio even after accumulating spectra over 1 h. The ν(Si–O) in the Raman spectra did not have discernable shoulders, as observed in the IR spectra, and there was only a fairly subtle shift in the position of this feature from 950 to 970 cm⁻¹ as the degree of silicate polymerization increased. Overall, the results indicate that Raman spectroscopy can be used to study H₄SiO₄ adsorption and polymerization on iron oxides, but its utility is constrained by a weak signal combined with a subtle shift in peak position with H₄SiO₄ polymerization.