Publication

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

Janet Gordon Hering
2015
Journal Articles

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 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.

Official source

Janet Gordon Hering
2015
Journal Articles

Abstract

Official source

Related concepts (21)

Precipitation hardening

Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels, stainless steels, and duplex stainless steel. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength. Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice.

Precipitation (chemistry)

In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a supersaturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemical reagent causing the solid to form is called the precipitant. The clear liquid remaining above the precipitated or the centrifuged solid phase is also called the supernate or supernatant.

Phosphate

In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid, aka. phosphoric acid . The phosphate or orthophosphate ion [PO4]3− is derived from phosphoric acid by the removal of three protons H+. Removal of one proton gives the dihydrogen phosphate ion [H2PO4]− while removal of two ions gives the hydrogen phosphate ion [HPO4]2−. These names are also used for salts of those anions, such as ammonium dihydrogen phosphate and trisodium phosphate.

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