Back to overview

Dynamic Fe-precipitate formation induced by Fe(II) oxidation in aerated phosphate-containing water

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Voegelin Andreas, Senn Anna-Caterina, Kaegi Ralf, Hug Stephan J., Mangold S.,
Project Iron(III) precipitates formed by iron(II) oxidation in natural waters: Structure, reactivity and impact on arsenic
Show all

Original article (peer-reviewed)

Journal Geochimica et Cosmochimica Acta
Title of proceedings Geochimica et Cosmochimica Acta


We studied the effect of phosphate on the precipitation of Fe during the oxidation of 1 mM Fe(II) in aerated 8 mM NaHCO3-CO2 buffered aqueous solutions at near-neutral pH. The structure and morphology of the precipitates were analyzed by X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS) at the Fe K-edge, and transmission electron microscopy (TEM). Most phosphate was incorporated into the Fe(III)-precipitates up to an initial dissolved P/Fe ratio of ~0.55. At dissolved P/Fe ratios from 0.55 to 1.91, the precipitate P/Fe ratios only exhibited a minor increase from 0.56 to 0.72. XRD patterns and Fe EXAFS spectra indicated a shift in precipitate type from mostly poorly-crystalline lepidocrocite in the absence of phosphate to amorphous Fe(III)-phosphate at dissolved P/Fe ratios >0.55. A time-resolved oxidation experiment at an initial dissolved P/Fe ratio of 0.29 revealed that amorphous Fe(III)-phosphate formed during Fe(II) oxidation until phosphate was nearly depleted from solution. During continuing Fe(II) oxidation, about half of the newly formed Fe(III) contributed to the polymerization of Fe-phosphate (mostly monomeric and oligomeric Fe(III)) into phosphate-rich hydrous ferric oxide with a maximum P/Fe ratio of 0.25 (HFO-P; edge-sharing linkage of Fe(III) octahedra) and about half precipitated as poorly-crystalline lepidocrocite in the phosphate-depleted solution. At initial P/Fe ratios <0.2, initially formed Fe(III)-phosphate was fully transformed into HFO-P during continuing Fe(II) oxidation. The dynamic interactions between phosphate and Fe described in this study impact the structure of fresh Fe(III)-precipitates at redox transitions in environmental and technical systems. The modulating effects of other dissolved species such as silicate and Ca on Fe precipitate formation and implications for co-transformed trace elements require further study.