Speciation and Crystal Chemistry of Iron(III) Chloride Hydrolyzed in the Presence of SiO₄ Ligands. 1. An Fe K-Edge EXAFS Study

Abstract

The hydrolysis of Fe−Si systems with Si/Fe ratios between 0 and 4 leads to the formation of poorly crystalline or, more frequently, of long-range disorganized precipitates. The increase of Si/Fe molar ratios results in an dramatic change of Fe polymerization. The formation of double and single corner-sharing Fe linkages is reduced compared to pure Fe hydrolysis products. The growth regime depends on the Si concentration in the system. Three-dimensional and two-dimensional growth of Fe colloids occurs at low and high Si/Fe ratios, respectively, systems with Si/Fe ratios around 1 representing a crossover between these two regimes. Though Si neighbors cannot be detected unequivocally by Fe K-edge EXAFS, their presence in the close environment of Fe atoms is evident from the change in Fe speciation.

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.…”

Silicon and Phosphorus Linkage with Iron via Oxygen in the Amorphous Matrix of Gallionella ferruginea Stalks

“…As was reported earlier, inorganic ligands such as SiO 4 or PO 4 in Fe(III) hydrolysates could interfere with the polymerization and crystallization of Fe oxyhydroxide precipitates (9,11,46). Furthermore, organic materials are known to similarly affect the crystallization of Fe oxyhydroxide precipitates during hydrolysis (17,39,40).…”

“…S1 in the supplemental material) (52). Commonly, in the structure of Fe(III) oxyhydroxide, which occurs through the Fe(III) hydrolysis reaction in the presence of SiO 4 or PO 4 anions, Fe exists as Fe 3ϩ (O, OH) 6 octahedral units, while Si and P are present as Si 4ϩ O 4 and P 5ϩ O 4 tetrahedral units, respectively (11,12,36,43,45,46). This information and the current HRTEM and EDX data led us to suspect that the putative four elements were distributed amorphously as these respective units in the primary particles of the fiber.…”

“…As far as we know, the only published information available is the hypothesis by Kennedy et al (32) for silicified iron oxides in Gallionella stalks, which states that adsorbed water might be replaced by adsorbed Si in the form of Fe-O-Si bonds. We believe that chemical linkages of these minor elements such as Si and P with Fe and O must be closely associated with the degree of crystallinity of the stalk and sheath matrices because it is known that inorganic ligands such as SiO 4 and PO 4 in Fe(III) hydrolysates interfere with the polymerization and crystallization of Fe oxyhydroxide precipitates (9,11,46).…”

Fractal Mechanisms in Coagulation/Flocculation Processes in Environmental Systems

“…As a consequence, the speciation and structure of fresh Fe-Si phases, for which no organized structure is expected, have long been neglected. Although synchrotron-based SAXS and especially XAS techniques have gained popularity since the mid 1980s, the structure of freshly prepared Fe-Si systems has been studied only recently [28,[80][81][82]. These studies cover Fe hydrolysis over a large range of pH, Fe concentrations and Si/Fe molar ratios.…”