Labile Fe(III) supersaturation controls nucleation and properties of product phases from Fe(II)-catalyzed ferrihydrite transformation
Introduction
Because the nanomineral ferrihydrite (Fh) is typically the first iron (oxyhydr)oxide to precipitate during Fe(III) hydrolysis or rapid Fe(II) oxidation, it is commonly found where strong chemical gradients pervade in soils, sediments and various aquatic environments including acid mine drainage settings (Childs, 1992, Jambor and Dutrizac, 1998). Because of its high specific surface area and reactivity, Fh plays critical roles as a source of bioavailable iron and as a sorbent that can strongly impact the cycling of carbon, nutrients, and contaminants (Childs, 1992, Michel et al., 2007, Tang et al., 2016, Shi et al., 2020). Given its poor crystallinity, Fh tends to spontaneously transform, typically slowly over months to years, into more stable crystalline forms of Fe(III) such as goethite (Gt) and hematite (Hm) (Schwertmann and Murad, 1983, Cornell and Schneider, 1989). In anaerobic environments, coexisting aqueous Fe(II) greatly accelerates these transformations to near completion within hours, at the same time opening pathways to lepidocrocite (Lp) and magnetite (Mt) (Hansel et al., 2003, Yee et al., 2006). In addition to redox potential, the strong dependence of the transformation rate and products on other common system components such as ligands, organic matter, and metal impurities in Fh remain poorly understood in molecular detail. Advances are ongoing and necessary to ultimately improve predictive models of contaminant and nutrient mobilization (Burton et al., 2019, Shi et al., 2020) and the bioavailability of iron and co-associated elements (Aeppli et al., 2019, Hu et al., 2020).
In the absence of a reducing agent, the slow spontaneous transformation of Fh to, for example, Gt or Hm has been explained in various ways including dissolution-reprecipitation, solid-state topotaxy, and particle-mediated crystallization (Schwertmann and Murad, 1983, Burleson and Penn, 2006, Zhu et al., 2021). Unambiguously tracking Fe mass transfer from source to sink is an underlying challenge; pathways tend to be obscured as extremely fine-grained and often aggregated Fh particles give way to nucleation and growth of product crystallites. A critical limiting factor is the very low aqueous solubility of Fe(III), typically on the order of 10−17 M at near-neutral conditions (Hiemstra, 2015). The dramatic accelerating effect of Fe(II) (Hansel et al., 2003, Yee et al., 2006, Boland et al., 2014b, Qafoku et al., 2020), which is vastly more soluble, is curious from the standpoint of having no obvious role in determining the saturation state with respect to iron (oxyhydr)oxide product phases (e.g., Lp, Gt, Hm) (Jones et al., 2015, Gorski et al., 2016). It is however well established that electron transfer occurs between sorbed Fe(II) and Fh (Williams and Scherer, 2004). Conceptually such a process can enhance the chemical lability and flux of Fe(III) in two ways, on the one hand by destabilizing the already metastable Fh structure, and on the other through the prospect that Fe(II) oxidized at the Fh surface is poorly incorporated. Recent studies using Fe(III)-selective complexing agents and stable iron isotopic tracers strongly reinforce this picture by showing that chemically labile Fe(III) accumulates much faster and to greater extent when Fh is contacted by Fe(II) (Sheng et al., 2020a, Sheng et al., 2020b). The Fe(II)-catalytic effect thus appears to be based primarily in its ability to facilitate more rapid release of Fe(III), enabling rapid saturation with respect to more crystalline product phases and their more immediate nucleation and growth (Sheng et al., 2020b). In this light, dissolution-reprecipitation now features more prominently as a unifying mechanistic explanation across cases with and without Fe(II), consistent with early assertions (Schwertmann and Murad, 1983), a conclusion recently further verified by ex situ (Qafoku et al., 2020) and quasi in situ (Gomez et al., 2020) electron microscopy studies.
These mechanistic findings enable a set of testable expectations regarding relationships between relative amounts of aqueous Fe(II) and Fh, the consequential amount of labile Fe(III) and the rate of emergence of product crystallites. Exploring such relationships could help understanding the half-life of Fh in natural environments, where processes such as groundwater table fluctuation or soil aggregate drying-wetting cycles can change the ratio of Fh/solution. It could also help reconcile previous work. Although the ratio of Fe(II)/Fh has long been considered a critical parameter controlling Fh transformation kinetics and secondary mineral phases (Hansel et al., 2003, Hansel et al., 2005, Boland et al., 2014b, Chen et al., 2015, Xiao et al., 2018), studies to date apparently remain contradictory in this regard. For example, during Fh reduction by Shewanella oneidensis MR-1, Mt formation was only observed at low initial Fe(II):Fh ratios (<2.8 mmol Fe(II)/g Fh) and high Fh loadings (>0.53 g/L) at pH 7.0 (Piepenbrock et al., 2011), which opposed widely recognized Mt precipitation at high Fe(II):Fh ratios (>1.0 mmol Fe(II)/g Fh at pH = 7.2) (Hansel et al., 2005). Using kinetics modelling for Fh transformation in different Fh/solution ratios, similar Gt formation rate constants were fit at different Fh/solution ratios in fixed 1 mM Fe(II) suspensions (i.e., altering the Fe(II)/Fh ratio). However, Gt formation rate constants were evidently enhanced with increasing Fe(II)/Fh ratio at fixed Fh/solution ratios (Boland et al., 2014b, Xiao et al., 2018). Hence it is apparent that the phases formed and their rate of formation do not have a singular dependence on Fe(II)/Fh ratio alone. To resolve such issues, quantifying labile Fe(III) intermediate concentrations and correlating its concentration with nucleation and growth rates of secondary mineral phases under different conditions is essential.
Our previous studies have shown that increasing the Fe(II)/Fh ratio at constant solution volumes and Fh loadings leads to a higher accumulation rate of labile Fe(III), accelerating Fh transformation (Sheng et al., 2020b). The present study is focused on the effect of Fh/solution ratios in controlling labile Fe(III) accumulation to the critical concentration needed to nucleate product phases. We use classical nucleation theory (CNT) as a framework for testing concentration effects on the appearance rate of product phases, which relates the energy barrier for the formation of stable crystalline nuclei to the supersaturation in solution and the interfacial free energy of nuclei. In the study of Fe(II)-catalyzed Fh transformation, nascent product phases appear within intragranular spaces of Fh within minutes (Qafoku et al., 2020), so an obvious unknown in this approach is the potentially important role of surfaces. Hydrolysis and polymerization of Fe(III) into iron (oxyhydr)oxides is well known to occur through both homogeneous and heterogeneous pathways (Hu et al., 2013, Boland et al., 2014b, Wu et al., 2020, Chen and Thompson, 2021). CNT encompasses such phenomena qualitatively by treating the heterogeneous pathway as one in which the effective surface free energy term is reduced, lowering the energy barrier governing the nucleation rate.
In order to directly examine how changing the Fh/solution ratio affects Fh transformation kinetics and secondary mineral outcomes, we performed batch experiments of a constant mass of Fh (15 mg) in constant Fe(II) concentration solution (1 mM FeSO4 solution at pH 7.2) at varying solution volumes of 30 mL, 150 mL, and 450 mL. Fh transformation kinetics and secondary mineral phases were determined by using attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) and Mössbauer spectroscopy. Concentrations of solid-associated Fe(II) and the labile Fe(III) intermediate over the course of Fh transformation were quantified through aqueous analytics and use of xylenol orange as an Fe(III)-specific extractant as established in our previous work (Sheng et al., 2020a, Sheng et al., 2020b). Moreover, dissolution of Fh aggregates and the distribution of secondary mineral particles were observed at nanoscale using quasi-in situ identical location transmission electron microscopy (IL-TEM), enabling visual evidence and quantitation of both Fh dissolution and product phase growth rates. All results obtained are consistent with the dissolution/reprecipitation pathway, and show how the emergence rate of iron (oxyhydr)oxide products is directly correlated to the accumulation rate of the labile Fe(III) intermediate to its critical concentration for product phase nucleation, which depends both on its formation rate at the Fh surface controlled by Fe(II) loading, and the available solution volume into which it is emerging.
Section snippets
Iron (oxyhydr)oxide synthesis and characterization
2-line Fh was synthesized according to the procedures reported by Cornell and Schwertmann (2003). Briefly, 0.1 M FeCl3·6H2O was titrated by dropwise addition of 1 M NaOH until solution pH of 7–7.5. After washed three times with Milli-Q water (≥18.2 MΩ·cm−1), the resulting paste were transferred into an anaerobic glovebox (N2 atmosphere, O2 < 0.1 ppm) and resuspended in degassed and deionized Milli-Q water (DDW). The stock suspension was then sparged with N2 for more than 2 h to remove dissolved
Kinetics and products of batch Fh transformation
The time evolution of solid phases for reaction of 15 mg Fh in 1 mM FeSO4 at different solution volumes of 30 mL, 150 mL and 450 mL was quantified up to 12 h reaction using ATR-FTIR. Fig. 1 shows the measured proportions of Fh and product minerals as a function of time. In all cases an ‘induction period’ of about 2 h is apparent, consistent with prior work using similar conditions and bulk solids characterization techniques with similar detection limits (Boland et al., 2014b, Sheng et al., 2020b
Effects of Fh/solution ratio on labile Fe(III) concentration and transformation kinetics
Previous studies have demonstrated that interfacial electron transfer (IET) between surface-associated Fe(II) and iron (oxyhydr)oxides occurs and that it appears to be an essential process underlying the catalytic effect of Fe(II) on Fh transformation (Williams and Scherer, 2004, Jones et al., 2009). This has been extended into arguments that it is the loading amount of Fe(II) sorbed on Fh surfaces, e.g., the mol Fe(II)/g Fh, that dominates Fh transformation kinetics and pathways (Hansel et
Conclusions
Previous studies of Fe(II)-catalyzed Fh transformation have emphasized the effect of Fe(II)/Fh ratio on the rate and relative proportion of product mineral phases by changing Fe2+ concentration but with a constant Fh loading in solution. Fe(II)-catalyzed Fh transformation at different Fh/solution ratios has been seldom studied, despite its obvious relevance to natural settings, such as groundwater table fluctuation or soil aggregate drying-wetting cycles. The results of this study reveal that
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This material is based upon work supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences & Biosciences Division through its Geosciences Program at Pacific Northwest National Laboratory (PNNL). AS and JL acknowledge support from National Natural Science Foundation of China (41820104003 and 91751105) and China National Postdoctoral Program for Innovative Talents (BX20200006). A portion of the research was performed using
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