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

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 similar to 0.5 in 8 mM NaHCO3 and similar to 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 similar to 0.7 in Na and similar to 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 similar to 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. (C) 2015 Elsevier Ltd. All rights reserved.

Published in:
Geochimica Et Cosmochimica Acta, 162, 220-246
Oxford, Pergamon-Elsevier Science Ltd

 Record created 2015-09-28, last modified 2018-09-13

Rate this document:

Rate this document:
(Not yet reviewed)