Impact of Organic Matter on Iron(II)-Catalyzed Mineral Transformations in Ferrihydrite–Organic Matter Coprecipitates

Abstract

Poorly crystalline Fe(III) (oxyhydr)oxides like ferrihydrite are abundant in soils and sediments and are often associated with organic matter (OM) in the form of mineralorganic aggregates. Under anoxic conditions, interactions between aqueous Fe(II) and ferrihydrite lead to the formation of crystalline secondary minerals, like lepidocrocite, goethite, or magnetite. However, the extent to which Fe(II)-catalyzed mineral transformations are influenced by ferrihydrite-associated OM is not well understood. We therefore reacted ferrihydrite-PGA coprecipitates (PGA = polygalacturonic acid, C:Fe molar ratios = 0−2.5) and natural Fe-rich organic flocs (C:Fe molar ratio = 2.2) with 0.5−5.0 mM isotopically labeled 57 Fe(II) at pH 7 for 5 weeks. Relying on the combination of stable Fe isotope tracers, a novel application of the PONKCS method to Rietveld fitting of X-ray diffraction (XRD) patterns, and 57 Fe Mossbauer spectroscopy, we sought to follow the temporal evolution in Fe mineralogy and elucidate the fate of adsorbed 57 Fe(II). At low C:Fe molar ratios (0−0.05), rapid oxidation of surface-adsorbed 57 Fe(II) resulted in 57 Feenriched crystalline minerals and nearly complete mineral transformation within days. With increasing OM content, the atom exchange between the added aqueous 57 Fe(II) and Fe in the organic-rich solids still occurred; however, XRD analysis showed that crystalline mineral precipitation was strongly inhibited. For high OM-content materials (C:Fe ≥ 1.2), Mossbauer spectroscopy revealed up to 39% lepidocrocite in the final Fe(II)-reacted samples. Because lepidocrocite was not detectable by XRD, we suggest that the Mossbauer-detected lepidocrocite consisted of nanosized clusters with lepidocrocite-like local structure, similar to the lepidocrocite found in natural flocs. Collectively, our results demonstrate that the C content of ferrihydrite−OM coprecipitates strongly impacts the degree and pathways of Fe mineral transformations and iron atom exchange during reactions with aqueous Fe(II).

Organic matter influences transformation products of ferrihydrite exposed to sulfide

“…This is in agreement with reports of X-ray amorphous, nanometersized clusters of secondary mineral phases forming during reactions between dissolved Fe(II) and ferrihydrite in the presence of OM at circumneutral pH. [37][38][39]50,63 The importance of non-sulfur-associated Fe(II) during sulfidization reactions was also noted by Peiffer et al, 15 who demonstrated that conditions which favored its formation in excess hindered FeS precipitation. Similarly, Kumar et al 16 suggested that a minimum S(−II)/Fe molar ratio of 0.5 was required for mackinawite accumulation.…”

“…Moreover, the fraction of crystalline minerals (lepidocrocite + magnetite + goethite + pyrite) at 12 months was higher in the absence of OM (x _ = 65% vs. x _ = 16%). Collectively, these findings support Henneberry et al 40 and suggest that, similar to Fe(II)catalyzed reactions, [37][38][39]50,63 OM inhibits ferrihydrite mineral transformations and the formation of secondary crystalline phases in the presence of reduced S species.…”

“…8,13,14 However, an excess of Fe(II) may also induce secondary Fe(II)-catalyzed mineral transformations or recrystallization in ferrihydrite at circumneutral pH. 37,50,[62][63][64][65] Depending on the Fe(II)/Fe(III) molar ratio, such reactions may result in magnetite (Fe 3 O 4 ) accumulation, 63,66 which may also account for the change in solution color to black/ brown. After 3 days, the black coloration of the Fh sample reacted with 1 mM S(−II) disappeared, while all other samples retained a dark brown/black coloration.…”

“…16 It is likely that the excess Fe(II) then interacts with the mineral surface via iron atom exchange, thereby catalyzing the transformation of pure ferrihydrite to lepidocrocite and goethite; 62,64,66 a process which can be severely inhibited in the presence of organic matter. [37][38][39]50,63,65 Because XRD targets primarily the crystalline mineral fraction, samples were additionally investigated by Fe K-edge XAS and LCF including a suite of reference spectra deemed suitable through PCA-TT (see ESI †). For the 1 mM S(−II) spike samples, the pre-edge features, energy position, and intensities of the white-lines of Fe K-edge XANES spectra match those of the ferrihydrite reference, suggesting that no change in solid-phase Fe oxidation state occurred for (co) precipitates reacted with 1 mM S(−II) within 1 week (Fig.…”

“…29 In the presence of Fe(II), increasing C/Fe molar ratios result in decreasing ferrihydrite mineral transformations. 37,63 Therefore, it is possible that, at higher C/Fe molar ratios, the impact of OM on sulfidization and pyritization kinetics and pathways may similarly be increasingly inhibited. Moreover, in the current study, simple organic ligands were chosen which are not expected to interact directly or significantly with S(−II).…”