Speciation and Crystal Chemistry of Fe(III) Chloride Hydrolyzed in the Presence of SiO₄ Ligands. 2. Characterization of Si−Fe Aggregates by FTIR and ²⁹Si Solid-State NMR

Abstract

This work aims at a better understanding of the interactions existing between Fe and Si in freshly precipitated Fe(III)/Si phases (prepared at pH 3, 5, 7, and 10 with Si/Fe molar ratios in the range 0.25-4). By coupling the results of two spectroscopic techniques, FTIR and 29 Si NMR, interesting structural information emerges. We show that Si and Fe atoms do not form separate silica and FeOOH particles and that the presence of Si-O-Fe bonds hinders the formation of Fe oxyhydroxides. The ratio Si/Fe ) 1 constitutes a transition point between Si-O-Fe and Si-O-Si bond formation at pH ) 3 and 5, where Si-clusters appear once the maximum amount of Si-O-Fe bonds are formed. This is confirmed by 29 Si NMR which demonstrates the presence of Si pockets in three of the eight examined samples. The return to equilibrium of the 29 Si magnetization leads to a value for both dimensionality of the silica-rich pockets (D ) 2.2) and length over which dimensionality is observed (2 nm). By using both FTIR and 29 Si solid-state NMR, we clearly demonstrate how the pH of synthesis determines the structural properties of the formed samples. The results obtained are in good agreement with our previous study conducted by EXAFS.

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…PercakDennett et al (2011) and Wu et al (2012) studied dissolved Fe(II) oxidation in the presence of dissolved silica, which produced an Fe(III)-Si gel that was homogenous on the nm scale, indicating the presence of a single phase in which Si and Fe are tightly bonded and distinct from, for example, Si sorbed to ferrihydrite, as has been previously described from terrestrial hot springs (e.g., Lalonde et al, 2007). Previous spectroscopic studies indicate that co-precipitation of Fe(III) and Si from a mixed solution tends to form an intimately bonded Fe(III)-Si phase, rather than separate Fe(III) and Si phases (Doelsch et al, 2000(Doelsch et al, , 2001(Doelsch et al, , 2003. Identifying the specific primary Fe(III) precipitate in IFs is important, because when exposed to dissolved Fe(II), Fe(III)-Si gels are resistant to phase transformations that may occur in dissolved Fe(II)-ferrihydrite experiments (e.g., Johnson et al, 2005), and uptake of Fe(II) into the solid is inhibited under abiological conditions (Fig.…”

Structure and composition of Fe–OM co-precipitates that form in soil-derived solutions

“…Oligomeric Si species, which develop at molar Si/Fe ratios >0.01 and pH = 3 (Swedlund et al, 2009) could not be traced due to superposing bands from adsorbed sulfate. Although the molar Si/Fe ratio was 0.02 -0.06 in our study, we assume the predominance of monomeric Si species owing to the effluent pH ~ 7.5 (Table 1), which effectively inhibits Si polymerization at the Fe oxide surface (Doelsch et al, 2001). For the Fe-OM coprecipitates from Ap-derived effluent, the Si-O-Fe band was only indicated in the spectrum of material with the highest Si/Fe ratio (replicate Ap_1).…”

“…; Carlson and Schwertmann (1981); Doelsch et al (2001)). This band may be indicative for monomeric silicate, which bonds to Fe polymers and ferrihydrite particles at Si contents as low as observed in our study (~5 mol%; Cismasu et al (2014)).…”

A study of sodium silicate in aqueous solution and sorbed by synthetic magnetite using in situ ATR-FTIR spectroscopy

“…Attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy has been proven to be a powerful in situ technique to study solid-liquid interface reactions, where various inorganic anions (such as arsenate, sulfate, phosphate) and organic compounds interacts with metal oxides and minerals [23][24][25][26]. Although there are some IR, XPS and EXAFS studies on the interaction of silicate with iron oxides or polymers [27,28], no in situ spectroscopy study monitoring the sorption of silicate from dilute solutions, viz. the concentrations used in mineral flotation (<10 mmol L −1 ), onto the surface of magnetite could be found in the literature.…”