Composition and structure of Fe(III)-precipitates formed by Fe(II) oxidation in water at near-neutral pH: Interdependent effects of phosphate, silicate and Ca. (1st August 2015)

• Record Type: Journal Article
• Authors: Senn, Anna-Caterina; Kaegi, Ralf; Hug, Stephan J.; Hering, Janet G.; Mangold, Stefan; Voegelin, Andreas
• Is Part Of: Geochimica et cosmochimica acta. Volume 162(2015:Aug. 01)
• Page Start: 220
• Page End: 246
• Publication Date: 2015-08-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.04.032
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: We studied the interdependent effects of phosphate, silicate and Ca on the formation of Fe(III)-precipitates by oxidation of 0.5 mM Fe(II) in near-neutral bicarbonate-buffered aqueous solutions at concentrations relevant for natural water resources. Complementary results obtained by a suite of analytical techniques including X-ray absorption spectroscopy and transmission electron microscopy showed that the ratio of initially dissolved phosphate over Fe(II) ((P/Fe)init ) had a major impact on precipitate formation. At (P/Fe)init above a critical ratio ((P/Fe)crit ) of ∼0.5 in 8 mM NaHCO3 and ∼0.8 in 4 mM Ca(HCO3 )2 electrolyte, Fe(II) oxidation led to exclusive formation of amorphous basic Fe(III)-phosphate or Ca–Fe(III)-phosphate ((Ca-)Fe(III)-phosphate) with maximum precipitate P/Fe ratios ((P/Fe)ppt ) of ∼0.7 in Na and ∼1.1 in Ca electrolyte. Enhanced phosphate uptake in the presence of Ca was due to phosphate-Ca interactions coupled to Fe precipitation, mainly formation of mitridatite-like Ca–Fe(III)-phosphate polymers and Ca-phosphate polymers. At (P/Fe)init < (P/Fe)crit, in the absence of silicate, (Ca-)Fe(III)-phosphate precipitation was followed by the formation of poorly crystalline lepidocrocite and concomitant transformation of the (Ca-)Fe(III)-phosphate into a phosphate-rich ferrihydrite-type precipitate with a (P/Fe)ppt of ∼0.25. In the presence of 0.5 mM silicate, initially formed (Ca-)Fe(III)-phosphate nanoparticles became coated with silicate-rich ferrihydrite during continuing Fe(II) oxidation and only limited transformation of the (Ca-)Fe(III)-phosphate occurred. The results from this study indicate the complexity of Fe(III)-precipitate formation in the presence of interfering solutes and its consequences for precipitate structure and phosphate sequestration. The findings provide a solid basis for further studies of the reactivity of different Fe(III)-precipitate types and for the systematic assessment of their impact on Fe, phosphate and trace elements dynamics in natural and engineered systems.