The structure of Fe(iii) ions in strongly alkaline aqueous solutions from EXAFS and Mössbauer spectroscopy

Abstract

To establish the structure of ferric ions in strongly alkaline (pH > 13) environments, aqueous NaOH solutions supersaturated with respect to Fe(III) and the solid ferric-hydroxo complex salts precipitating from them have been characterized with a variety of experimental techniques. From UV measurements, in solutions of pH > 13, only one kind of Fe(III)-hydroxo complex species was found to be present. The micro crystals obtained from such solutions were proven to be a new, so far unidentified solid phase. Mössbauer spectra of the quick-frozen solution and that of the complex salt indicated a highly symmetrical ferric environment in both systems From the EXAFS and XANES spectra, the environment of the ferric ion in these solutions (both native and quick-frozen) and in the complex salt was found to be different. In the complex salt, the bond lengths are consistent with an octahedral coordination around the ferric centres. In solution, the coordination geometry of Fe(III) is most probably tetrahedral. Our results demonstrate that in strongly alkaline aqueous solutions, ferric ions behave very similarly to other structurally related tervalent ions, like Al(III) or Ga(III).

Fe(III) uptake by calcium silicate hydrates

“…In strongly alkaline solution, Fe(III) is known to have a tetrahedral coordination geometry (Sipos et al, 2008). At the same conditions, in complex salts, Fe(III) is found to be bound preferentially in octahedral coordination (Sipos et al, 2008).…”

“…In strongly alkaline solution, Fe(III) is known to have a tetrahedral coordination geometry (Sipos et al, 2008). At the same conditions, in complex salts, Fe(III) is found to be bound preferentially in octahedral coordination (Sipos et al, 2008). Ferric iron is known to replace Al(III) in Al(III)-bearing cement phases, such as Fe-ettringite, Fe-monosulphate, Fehemicarbonate, and Fe-monocarbonate (Dilnesa et al, 2011;Dilnesa et al, 2012;Möschner et al, 2008;Möschner et al, 2009).…”

Speciation and structure of tin(ii) in hyper-alkaline aqueous solution

“…For obvious reasons, metal ions with reasonable solubility (e.g., amphoteric ones) are most 50 intensely studied (e.g., Al(III) [7][8][9] , Ga(III) 10,11 , Cr(III) 12, 13 , Pb(II) 14 , Tl(I) 15,16 , etc.). Beside these, data for metal ions that are hardly soluble in alkaline conditions also emerge (e.g., Cu(II) 17 , Fe(III) 18 , actinides 19 , etc.) In the current paper, we will focus on the behaviour of the 55 amphoteric Sn(II) ion in hyperalkaline media.…”

Preparation and Structural Characterization of Alkaline Earth−Iron(III) Layered Double Hydroxides and their Acrylate-Intercalated Derivatives

“…These pieces of information completely coincide with the observation that the pre-edge peaks at ~7115 eV appeared for the materials precipitated at the two lower NaOH concentrations indicating distorted octahedral environment around the Fe(III) ion. The disappearance of the small maximum in the pre-edge region gives clear evidence of the close to perfect octahedral environment of the Fe(III) [118] at the two highest NaOH concentrations. This is another indication that LDH formation only occurred when 10 M or 20 M NaOH was used for precipitation.…”

“…It is worth noting that Pyrex (as opposed to soda glass) shows very little reactivity towards alkaline solutions at ambient temperature. No visible signs of deterioration were observed for highly concentrated base solutions stored in Pyrex vessels at ambient temperature over long periods [118]. The density of the solution was determined by a picnometer.…”