Coprecipitation of Phosphate and Silicate Affects Environmental Iron (Oxyhydr)Oxide Transformations: A Gel-Based Diffusive Sampler Approach

Abstract

Sorption of nutrients such as phosphate (P) and silicate (Si) by ferric iron (oxyhydr)oxides (FeOx) modulates nutrient mobility and alters the structure and reactivity of the FeOx. We investigated the impact of these interactions on FeOx transformations using a novel approach with samplers containing synthetic FeOx embedded in diffusive hydrogels. The FeOx were prepared by Fe(III) hydrolysis and Fe(II) oxidation, in the absence and presence of P or Si. Coprecipitation of P or Si during synthesis altered the structure of Fe precipitates and, in the case of Fe(II) oxidation, lepidocrocite was (partly) substituted by poorly ordered FeOx. The pure and P-or Si-bearing FeOx were deployed in (i) freshwater sediment rich in dissolved Fe(II) and P and (ii) marine sediment with sulfidic pore water. Iron(II)-catalyzed crystallization of poorly ordered FeOx was negligible, likely due to surface passivation by adsorption of dissolved P. Reaction with dissolved sulfide was modulated by diffusion limitations and therefore the extent of sulfidation was the lowest for poorly ordered FeOx with high reactivity toward sulfide that created temporary, local sulfide depletion (Fh < Lp). We show that coprecipitation-induced changes in the FeOx structure affect coupled iron-nutrient cycling in aquatic ecosystems. The gel-based method enriches our geochemical toolbox by enabling detailed characterization of target phases under natural conditions.

Biological Reduction of Ferrihydrite with Silica Addition: Rates and Controlling Mechanisms

“…The data also indicated that Si incorporation into Fh disrupts Fe–O–Fe bonds, forms Fe–O–Si bonds, and results in both shorter length scales of crystal order, reduced surface area, and the presence of silica tetrahedra. These data were complemented by X-ray adsorption fine structure (XAFS) results from the literature indicating that Si inhibited Fe­(III) polymerization during Fh formation by preventing corner sharing of distorted Fe­(O/OH) 6 octahedra, with silica tetrahedra bridging Fe octahedra at these corners. ,,, This results in the formation of smaller aggregates of Fe­(III) oligomers, which was in agreement with our HRTEM images and XRD data.…”

“…This procedure was shown to cause Fh particle aggregation, a decrease in Fh surface area, and slower rates of Fh reduction, ,, e.g., slower by up to 80 times in one study and almost 8 times in another . Another possible explanation was inhibition of pure Fh transformation by inorganic and organic ligands present in media and possibly as cell exudates. ,, For example, increasing concentrations of lactate were found to decrease Fh reduction rates by S. oneidensis MR-1 …”

“…For impurities that can substitute directly for Fe in the Fh lattice, e.g., Al 3+ , there is little similarity to Si incorporation into Fh and trends in this work cannot be extended for interpretation. It is also difficult to extend results in this work to Fh with NOM impurities, as mechanisms of NOM incorporation into the Fh lattice are less clear. ,, However, for oxyanion impurities that form tetrahedral complexes and disrupt edge and/or corner binding between Fe octrahedral structures in Fh, e.g., AsO 4 3– , PO 4 2– , ,,, then the similarities with Si are more clear, and more impurities may result in faster rates of Fh reduction.…”

“…64 Another possible explanation was inhibition of pure Fh transformation by inorganic and organic ligands present in media and possibly as cell exudates. 2,36,65 For example, increasing concentrations of lactate were found to decrease Fh reduction rates by S. oneidensis MR-1. 66 FTIR results are shown in Figure 1c.…”

“…A number of studies have evaluated the impact of Si addition to iron (oxyhydr)­oxides (including Fh) on transformation from less to more stable crystalline phases. ,,− In general, Si incorporation into Fh has been shown to inhibit transformation to more stable goethite (Gt) and lepidocrocite (Lp) minerals. , This transformation has been attributed to reductive dissolution of Fe­(III) at the Fh surface by sorbed Fe­(II), resulting in recrystallization of a more stable mineral phase; , Sheng et al presented evidence that this was related to the formation of a more labile and reactive intermediate-Fe­(III) species. Isotope exchange studies indicated that the exchange of sorbed Fe­(II) with structural Fe­(III) occurred at similar or faster rates for Si-containing versus pure Fh, suggesting that Si inhibited Fh polymerization during recrystallization.…”