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Silicate coprecipitation reduces green rust crystal size and limits dissolution-precipitation during air oxidation

Geochemical Transactions volume 25, Article number: 12 (2024) Cite this article

Abstract

Green rusts (GR) are mixed-valence iron (Fe) hydroxides which form in reducing redox environments like riparian and wetland soils and shallow groundwater. In these environments, silicon (Si) can influence Fe oxides’ chemical and physical properties but its role in GR formation and subsequent oxidative transformation have not been studied starting at initial nucleation. Green rust sulfate [GR(SO4)] and green rust carbonate [GR(CO3)] were both coprecipitated from salts by base titration in increasing % mol Si (0, 1, 10, and 50). The minerals were characterized before and after rapid (24 h) aqueous air-oxidation by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fe extended x-ray absorption fine structure spectroscopy (EXAFS), and N2-BET surface area. Results showed that only GR(SO4) or GR(CO3) was formed at every tested Si concentration. Increasing % mol Si caused decreased plate size and increased surface area in GR(CO3) but not GR(SO4). GR plate basal thickness was not changed at any condition indicating a lack of Si interlayering. Air oxidation of GR(SO4) at all % mol Si contents transformed by dissolution and reprecipitation into lepidocrocite and goethite, favoring ferrihydrite with higher % Si content. Air oxidation of GR(CO3) transformed into magnetite and goethite but increasing Si caused GR to oxidize while retaining its hexagonal plate structure via solid-state oxidation. Our results indicate that Si has the potential to cause GR to form in smaller particles and upon air oxidation, Si can either stabilize the plate structure or alter transformation to ferrihydrite.

Graphical Abstract

Introduction

Green rusts (GR) are mixed-valence iron (Fe) hydroxides that form in redox reducing environments. Green rusts have a general chemical formula, [Fe(II)1-x Fe(III)x(OH)2]x+ · [(x/n)An− · mH2O]x− where "An−" represents intercalated anions like chloride, carbonate, or sulfate, and "x" represents the molar fraction of trivalent iron, typically ranging from 0.25 to 0.33. The most common intercalated anions in environmental GRs are carbonate and sulfate and are abbreviated in our study as GR(CO3) and GR(SO4), respectively. Green rusts contain the highest solid Fe(II) molar content and surface area of any environmental iron mineral [49]. Due to their reactive properties, GRs play an important role in redox and sorption interactions in anoxic soil and water environments. Green rusts form in permeable reactive barriers containing Fe(0) as corrosion products and act as sorbents and electron donors in groundwater remediation [3, 48]. However, in all these systems, dissolved components in soil and groundwater will alter GR composition [33, 36, 37, 40] and can affect their particle size [1, 34, 36] or surface charge [15, 41], which impact their ability to react with contaminants like hexavalent chromium, uranium, mercury [5, 25, 46, 53], and nitrate [10, 18, 27]. Environmental conditions that may impact GR’s ability to remediate or sequester contaminants must be considered to better account for its activity in the environment.

Silicon (Si) is an abundant element in the environment that is ubiquitous in soil solutions and natural waters. When Si is present during the precipitation of natural Fe oxides, it can alter the mineral properties compared to their pure, synthetic counterparts [12, 43]. Silicon is present in natural waters, typically ranging from 0.1 to 1.2 mM [6]. In natural aqueous fluids at pH < 8.5, dissolved silicon is in the silicic acid form (H4SiO40). Iron oxides have a high affinity for silicate sorption in natural waters and soils [7, 19]. Green rusts are like other Fe oxides and will sorb significant Si. Naturally formed GRs in groundwater have been collected and found to contain greater than 50% mol Si content [8]. Most studies quantifying GR’s reductive activity do not consider Si effects which are relevant to natural environments, especially environments connected to groundwater.

Silicon can inhibit many dissolution and crystallization processes [29, 32, 44, 30]. Dissolved silicic acid can inhibit kinetics and mineral transformations [11, 43]. Silicic acid can adsorb onto surfaces and disrupt interfacial electron transfer by blocking dissolution sites or hinder the nucleation of more crystalline secondary minerals [29, 30]. For these reasons, many studies investigated Si incorporation impacts on GR and its effects on oxidation products in diverse conditions [11, 21, 26]. However, in many publications, GR minerals are formed first and then Si introduced. Other studies have used natural conditions with Si present in initial Fe(III)hydroxides before they undergo bioreduction or abiotic Fe(II) induced transformation to GRs [20, 44]. We anticipate the presence of Si during Fe hydroxide nucleation may have different influences on GR formation and oxidation as well as at high Si loading as was observed at 50% wt Si from groundwater environments [9, 37, 40].

Silicon interaction with GR is known to affect GR oxidation rate and mechanism, which will change the resulting Fe(III)hydroxide products [11, 21, 28, 44]. Green rust can oxidize by two mechanisms, either solid-state oxidation (SSO) or dissolution-oxidation-precipitation (DOP) [23]. Silicate sorption can change the GR oxidation mechanism from DOP to SSO beyond a sorption threshold [11]. In SSO, the GR platelets remain intact as all Fe is oxidized to Fe(III), abbreviated in our study as EX-GR. Conversely, DOP transformation can yield a range of Fe oxides depending on oxidation kinetics and solution conditions like temperature and pH. Adsorbing silicon to preformed GRs slows oxidation kinetics with increasing Si additions [11], which changes the Fe(III) hydroxides formed. The oxidation rate follows the trend, lepidocrocite > goethite > magnetite [52]. These Fe(III) hydroxides have the general chemical formulas of (γ-Fe(III)O(OH)) for lepidocrocite, (α-Fe(III)O(OH)) for goethite, and Fe(II)Fe(III)2O4 for magnetite. Interlayer silication of GR is possible, as evidenced by minerals such as nontronite (Na0.3Fe(III)2(Si,Al)4O10(OH)2•n(H2O)), greenalite ((Fe(II),Fe(III))2-3Si2O5(OH)4, and minnesotaite ((Fe(II),Mg)3Si4O10(OH)2) existing in soil environments [16]. Starcher et al. [47] found Fe(II) coprecipitating with pyrophyllite (Al2Si4O10(OH)2) forms an Fe(II) phyllosilicate at pH > 8, suggesting an Fe(II) octahedral sheet with Si layering is possible.

This study investigates how Si coprecipitation with Fe salts can affect GR(CO3) or GR(SO4) formation, crystallization extent, and the products resulting from air-oxidation. Defining the extent of Si influence on the initial nucleation steps of GRs and eventual oxidation will add to our understanding of how GRs form and function in natural environments. These results will be most relevant to environments with high concentrations of dissolved Si such as groundwater systems.

Methods

Coprecipitation of GR(CO3) and GR(SO4) with Si

Green rusts (GR) were precipitated by acid to base titration from ferric, ferrous, and silicon salts following modified methods [35, 38, 39]. Reactants and conditions used are outlined in Table 1 and the methods used are discussed in detail. Syntheses were conducted utilizing a custom-built anaerobic mesocosm apparatus to maintain anoxic condition with gas sparging, base titration, suspension mixing and pH measurement datalogging simultaneously, as described in previous studies [24]. The initial solutions were prepared by dissolving ferrous sulfate (FeSO4), ferric sulfate [Fe2(SO4)3], and varying amounts of sodium meta-silicate salt (Na2SiO3) salts in 200 mL of deoxygenated MilliQ water into a 250 mL HDPE reaction vessel with constant Argon (Ar) gas sparging to mix suspension and exclude ambient air. Silicon salt was added at initial conditions with Fe salts according to total molar % of Fe moles described by \(\% \text{mol Si}= \frac{\text{n}(\text{Si})}{\text{n}(\text{Fe})} \times 100\) (Table 1).

Table 1 Green rust synthesis conditions by coprecipitation: hydroxysulfate green rust [GR(SO4)] and hydroxycarbonate green rust [GR(CO3)]

Syntheses were initiated by beginning automatic datalogging pH measurements of suspension every ten seconds followed by starting base solution addition using a peristaltic pump at a constant 0.5 mL min−1 flow rate. This method was used to create pH response diagrams [pH response vs R (moles OH added/Fe moles)] for interpretation of hydrolysis as used with GR syntheses in previous studies [35, 38, 39]. Results of pH response diagrams and discussion are provided in supplemental material (SFig 1, 2). The GR(CO3) syntheses were stopped at R = 3.0 and pH ~ 10 and GR(SO4) was stopped at pH 7 ± 0.1, both following methods by [35, 38, 39]. Reaction vessels were quickly capped and transferred into the glovebox chamber under 2% H2, and 98% N2 atmosphere (Coy glovebox, Coy gas reader). The suspensions remained capped in the glovebox for 24 h before recovering the solids by vacuum filtration onto a 0.22 μm polypropyline filter. The collected solids were used for subsequent characterization.

All syntheses were performed in nitric acid-washed plastic containers and the MilliQ water was deoxygenated by sparging with pure Argon gas for at least 4 h. Oxygen was evacuated from the reaction vessel by continuously sparging Ar gas during titration.

Air oxidation of GR

Subsamples of the synthesis suspensions were transferred to centrifuge tubes and mixed with laboratory air for rapid air-oxidation. The anoxic GR(CO3) and GR(SO4) in this oxidation experiment were used directly after synthesis. Specifically, 30 mL subsamples of GR synthesis suspension were transferred to 50 mL centrifuge tubes, brought into ambient air from the glovebox and mixed on a rotational shaker for 24 h. The oxidized solid was separated by vacuum filtration through a 0.22 μm filter. This solid was air-dried and ground by agate mortar and pestle for characterization.

Characterizing GR Si coprecipitates and oxidized products

Total element concentrations in samples were measured by wavelength-dispersive X-ray fluorescence (WD-XRF) (Bruker). Surface area was measured on anoxically-dried GR powders via a BET N2 isotherm. Samples for XRD measurement were prepared in anoxic glovebox by mixing the freshly precipitated GR wet paste with glycerol to protect GR from air oxidation during the XRD measurement [17]. Mixed sample and glycerol pastes were smeared onto a Si low-background slide for low angle measurements. A Bruker XRD with Cu Kα1 X-ray source (1.5405980 Å) measured all samples. Scans were measured from 3 to 45 degrees 2Theta, with 0.05 o intervals and one second dwell time. All processed XRD data was peak matched by HighScore Plus software utilizing the crystallographic open database (COD) version COD2021 [51].

Scanning electron microscopy (SEM) was measured on GR(CO3) and GR(SO4) anoxic samples as well as after air-oxidation to quantify particle size and qualitatively describe morphology. Anoxic samples were placed in individual containers and sealed with three parafilm layers while transported to the SEM lab. The total time between removal from the anoxic chamber and loading into the SEM sample chamber was less than five minutes, with less than one minute outside of the parafilm-wrapped container. Dry samples were deposited onto double-sided carbon tape on steel stubs in a six position sample holder. SEM images were collected on a JSM-7400F high resolution scanning electron microscope operating at 3 kV with cold field emitter filament and approximate resolution of 1.5 nm. SEM images were analyzed using ImageJ software for calculating plate width by measuring distances between opposite basal plate edges and quantifying by the image scale.

Synchrotron-source small-angle XRD was collected at XFM 4-BM beamline, on air oxidized GR(SO4) and GR(CO3) and anoxic GR(CO3) samples. Samples were prepared as finely-ground, air-dried samples, packed into a plexiglas well, and sealed with one layer of Kapton tape on either end. Measurements were taken perpendicular to incident micro focused beam at 17 keV (0.7293 Å). The detector plate was positioned 120 cm behind the irradiated sample to detect scattering. Calibration was performed by measuring a LaB6 standard. The software Dioptas was used to process all synchrotron-source XRD data (Prescher et al., 2015).

Analysis of synchrotron-sourced XRD data to calculate crystallite size from the Scherrer equation utilizing Origin software was conducted using the relationship below,

$${\text{L}}_{{{\text{hkl}}}} = \frac{{\kappa \lambda }}{{\left( {\beta _{{{\text{hkl}}}} \cos \theta } \right)}}$$

where L is the crystallite size perpendicular to the hkl crystallographic plane, l = the X-ray wavelength, b = the full-width at half maximum of the XRD peak in radians, and q is the Bragg diffraction angle [22, 42]. The Scherrer constant or shape factor, k, varies around unity. A value of 0.998 was used, assuming the particles’ shape is a right cylinder [50]. The Scherrer equation is only valid for crystallites less than about 100 nm [22], so this was applied to the crystallographic plane for plate thickness, which should be < 100 nm.

Results and discussion

Green rust formation with coprecipitated Si

Elemental composition was measured with WD-XRF for Fe and Si mass percentages (Table 2). Measured % mol Si in solids were generally higher than calculated values despite laboratory quality controls to exclude background Si. GR(SO4) had 0.03 mol g−1 in the Si free sample, indicating some amount of background Si contamination. More Si was present at all levels of GR(SO4) than GR(CO3). This may have been caused by Si contamination of the Na(OH) used as the titrating base as this was used for GR(SO4) but not GR(CO3) syntheses. GR(CO3) had no Si detectable in the Si-free sample, and increasing Si content was near the calculated value.

Table 2 XRF total Fe and Si of GR(SO4) and GR(CO3) Si series

X-ray diffraction on freshly-precipitated wet pastes verified the precipitation of GR(SO4) and GR(CO3). The only identified peaks in GR(CO3) were type one GR [4] and in GR(SO4) were type two GR [45] reflecting the interlayer spacing specific to carbonate and sulfate anions, respectively (Fig. 1). In the 10% mol Si GR(SO4) and GR(CO3) samples, peaks were broadened for basal plane peaks (at either 8 or 11.5°2Ɵ, respectively) was found which may indicate decreased crystal size (Fig. 1). Difference in peak intensity may also be caused by preferential orientation of crystallites, however all samples being compared were prepared with identical methods. Peak broadening may also indicate lattice strain typically caused by substitution. In our samples, Si substitution for intercalated anions would have caused interlayer collapse and dehydration, resulting in a shift of basal plane peak which was not observed making this causation unlikely [49]. GR(CO3) samples, which were rinsed in anoxic DI and air-dried in a glovebox, were measured by synchrotron-source XRD for all four Si concentrations (Fig. 2). Dried GR(CO3) XRD indicates GR carbonate with minor transformation to magnetite (Fig. 2). In glycerol suspended preparations of the same samples, the lack of magnetite indicates the rinsing and drying sample preparation may destabilize and oxidize GR to magnetite and should be avoided. However, it is notable that the washed and dried 50% mol Si GR(CO3) did not contain magnetite, suggesting at the high Si content the GR is more stable.

Fig. 1

Bruker XRD Cu Kα (1.5418 Å) of 0 and 10% Si GRSO4. The addition of 10% mol Si causes peak broadening at low angles (2Ɵ < 15°), miller index (001)

Fig. 2

Synchrotron-source XRD GR(CO3) anoxic Si series. Small amount of magnetite was detected in all but the 50% mol Si sample

Performing SEM microscopy (Fig. 3) shows pseudo-hexagonal plates in anoxic GR(CO3) samples decrease in size with increasing Si content. The plates did not have clear hexagonal edges, even when Si is absent, and GR crystal growth should theoretically be greatest. Samples were washed and dried with evidence of nanosized magnetite as spherical particles on the hexagonal plates in 0, 1, and 10% mol Si (Fig. 3 a, b, and c). This partial oxidation to magnetite may be an artifact caused by washing and drying.

Fig. 3

SEM images of anoxic GR(CO3) with increasing % mol Si a Si absent; b 1%; c 10%; d 50%

Crystal size in the plate width and thickness dimensions and synthesized GR surface area measurements are presented in Table 3. Interpretation involved a combination of the Scherer equation from XRD measurements and SEM image analysis for crystallite faces larger than 100 nm. Average distance is shown with standard deviation over 10 particles for each sample (Table 3). Green rust basal planes were estimated by SEM photographs because the GR basal planes can commonly exceed 100 nm [49]. In the absence of Si, the average plate width was 178 nm with standard deviation of 27 nm, which is small compared with many other published GR plate sizes, which range from hundreds of nanometers to a few microns [13, 14, 37, 40]. When GR(CO3) contained 1 or 10% mol Si, the plate width significantly decreased from Si free GR(CO3). There was no difference in plate width between the 1 and 10% Si concentrations. At the highest content of Si (50% mol Si), GR plate width was approximately half of that found in 1 or 10% mol Si contents. This indicates that GR coprecipitation with Si reduces basal plane dimension with increasing Si content from 178 to 64 nm, keeping crystal size to nanoparticulates.

Table 3 Green rust crystal size and surface area with Si coprecipitation

Green rust plate thickness was quantified using the fresh precipitate XRD spectra of GR(CO3) and GR(SO4) 0 and 10% mol Si samples. Using the hkl planes 001, 002 for GR(SO4) and 003, 006 for GR(CO3) in the Scherer equation, significant differences were observed between GR(SO4) and GR(CO3) but not between Si content for each mineral (Table 3). GR(SO4) and GR(CO3) thickness were calculated as 16 nm (1.37) and 14 nm (0.56) respectively. Green rust sulfate and GR(CO3) were each found to have interlayer spacing in XRD for containing purely sulfate or carbonate, respectively. These data also confirm the interlayer anion was not replaced by Si, as this would cause interlayer dehydration and collapse, evidenced by the difference from the interlayer d-spacing of greenalite (GRN; 002 hkl, d = 7.083; or 001 hkl, d = 14.166) and the interlayer observed for GR(CO3) 0–50% mol Si (GR1; 003 hkl, 7.565) [2].

Surface area was measured by BET on the washed and dried anoxic GR(CO3) samples. Surface areas were equal for Si contents 0, 1, and 10% mol Si but much greater for 50% mol Si (Table 3). This reduction in surface area is likely a combination of magnetite oxidation in low Si GR and the smaller particle sizes in higher Si GR, as seen in SEM images and broader XRD patterns. Magnetite is a much more dense and low surface area mineral than GR [49].

Green rust air oxidation and the effect of coprecipitated Si

Air oxidation caused GR(SO4) to transform by dissolution-oxidation precipitation (DOP) at all % mol Si contents as shown in XRD fitting results (Fig. 4) but increasing Si content resulted in more poorly-crystalline ferric hydroxides such as 2-line and 6-line ferrihydrites (Fe5HO8·4H2O). The Si absent GR(SO4) oxidized and transformed into lepidocrocite and goethite. Loading of 1% mol Si contained the same lepidocrocite and goethite but with less intense peaks, which can be caused by lower mineral crystallinities (Fig. 4 a). At 10% mol Si, the only Fe(III) hydroxide formed was a 6-line ferrihydrite and a 2-line ferrihydrite at the highest Si concentration.. These transformations to Fe hydroxides identified by XRD were also observed in Fe K-edge EXAFS measurements and linear combination fitting analysis with standards (Figure S4, Table S2). The dissolution of GR(SO4) precedes the Fe(III) oxidation. As Fe(III) precipitates, the Si-hydroxide may be complexing with Fe(III) and maintaining small crystal sizes [30].

Fig. 4

Synchrotron-source XRD patterns following rapid air-oxidation (24 h) of a green rust sulfate and b green rust carbonate, each at 4 concentrations of Si. Peak identifications are indicated as G = goethite, L = Lepidocrocite, 6LF = 6-line ferrihydrite, 2LF = 2-Line ferrihydrite, M = magnetite, and EXGR = oxidized green rust plates

However, air oxidation of each GR(CO3) showed some amount of GR plate preservation during oxidation which increased with % mol Si. When Si was absent, GR(CO3) transformed to goethite and minor amounts of magnetite. The conversion to magnetite and goethite from GR(CO3) oxidation has been observed when the oxidation rate was lower [35, 38, 39]. The presence of magnetite may be caused by the basic pH (pH 10), which can cause GR(CO3) transformation to magnetite and Fe(OH)2 [37, 40]. The Fe(OH)2 would be unstable and oxidize easily while the magnetite would be relatively stable and could remain in the fully air oxidized product.

The lowest Si concentration (1% mol Si) caused some of the GR plate structure to be preserved upon oxidation (EXGR) (Fig. 4b). SOP is the only oxidation mechanism present above 10% mol Si, as the plate structure is preserved fully after oxidation. The Fe K-edge EXAFS of these same samples was not able to identify particle structure as platelets (EX-GR) and instead was best modelled as a mixture of 6-line ferrihydrite and Fe(III) citrate with a trace amount of goethite in GRCO3 1% mol Si (Figure S4, Table S2). In environmental samples, use of bulk XRD is limited by the presence of quartz and other interferences and Fe mineral identification would be ideally performed by Fe K-edge XAS. However, our findings indicate that the use of Fe XAS would be unable to identify the EX-GR product that is possible with XRD which may cause an underreporting of EX-GR phases in natural media if only Fe XAS is used.

The influence of Si on GR(CO3) oxidation by SSO is much more potent in our coprecipitation study compared to others where Si is adsorbed following GR(CO3) formation [11]. In Feng et al. [11], the authors only observed full SSO of GR(CO3) at 33% mol Si, and all other Si concentrations (2.08, 4.16, 8.33% mol Si) showed kinetic inhibition of oxidation with increasing Si with eventual DOP oxidation. Conversely, our study found some SSO occurring in 1% mol Si. To the authors’ knowledge, this is the first study where Si is introduced while Fe is still an acidic soluble salt before neutralization to GR and subsequent direct air-oxidization. The SSO at 1% mol Si found in this study may be caused by Si coprecipitation with GR from soluble salts. Because of the different synthesis methods for GR(SO4) and GR(CO3), there were differences in the pHs. The final GR(SO4) synthesis pH was 7 ± 0.1 and 10 ± 0.1 in GR(CO3). Silicic acid will deprotonate at pH > 8.5 to H3SiO4− and increase bond attraction to Fe–O sites. Silicic acid is also more soluble at higher pH, so the Si content trending with stabilized GR sheets may be due to pH effects.

The XRD identification of SSO at every Si content level is corroborated by oxidized GR(CO3) SEM images (Fig. 5). Aggregates of goethite and magnetite are visible in the GR(CO3) without Si (Fig. 5a). The 1% Si/Fe MR has generally the same aggregate particle shapes but few plates are observed that remain from SSO which are roughly 175 nm in size. Both the 10 and 50% Si content samples have oxidized entirely as plates (Fig. 5c, d).

Fig. 5

SEM images of air-oxidized GR(CO3) with Si mol % being a 0%, b 1%, c 10%, d 50%. Platy particle shape is partially preserved when Si is present (b) and completely preserved above 10% (c, d)

Conclusions

Green rusts form in soils and natural waters where in-solution Si is ubiquitous. However, this is the first study where Si concentrations were manipulated in the starting solutions of a coprecipitation reaction, more closely representing natural environments with high Si salts during redox. Diagrams also indicate that this Si inclusion lowered the pH at which GR forms above the threshold of 10% Si and increased the pH at which GR persists before transformation. Si content also inhibited the growth of GR plate width causing GR to precipitate as smaller particles with higher surface area, most observable at 50% mol Si.

Air oxidation of these coprecipitated Si GRs formed more poorly-crystalline solids (ferrihydrites) in GR(SO4) at > 10% mol Si and in GR(CO3) caused stabilization of ferric plates isostructural to GR with increasing Si % mol. The difference in Si interaction with each GR is most likely caused by changes in the equilibrium pH due to differences in synthesis methods, with ~ pH 7.0 for GR(SO4) and ~ pH 10 for GR(CO3). At neutral pH, Si did not alter the oxidative mechanism from DOP to SSO at any loading but caused oxidation to more poorly-crystalline Fe(III) hydroxides. At a much higher pH (10), Si caused a SSO mechanism of oxidation at every Si concentration and stabilized GR plate structure in fully ferric EX-GR after oxidation. When compared to similar studies adsorbing Si to GRs, our coprecipitates indicated a much stronger effect from Si starting at contents as low as 1% mol Si and particularly the restriction to 6-line ferrihydrite at 10% mol Si and 2-line ferrihydrite at 50% mol Si.

These novel findings add to our understanding of the fate of Fe after forming GRs in wetland and riparian environments and have implications for GR efficiency as a reductant for organic matter, organic pollutants (e.g., pesticides, solvents), and redox-sensitive trace metals (e.g., Cr, Se, Hg) [49]. The increase in GR surface area with Si content may increase reduction rate or sorption capacity but the presence of Si may also inhibit trace metal sorption and should be directly tested. The ability of GR to coprecipitate with trace metals upon reduction–oxidation reactions is favorable for removing contaminants from solution, so the inhibition of DOP mechanism with Si content may result in less well sequestered trace metals. Paired reduction of Cr(VI) and oxidation of GR results in a Fe(III) hydroxide coprecipitated with Cr(III), which is less soluble than Cr(III) surface precipitates [46]. The effects of Si on GR can also aid performance predictions of zero-valent iron permeable reactive barriers for removing contaminants from deep soils and groundwaters which can contain higher Si content. Our results indicate that Si has the potential to cause GR to form in smaller particles and upon air oxidation, Si can either stabilize the plate structure or alter transformation to ferrihydrite.

Availability of data and materials

The datasets used for this manuscript are displayed in the figures in the manuscript and the additional file. The data in tabulated form are available upon request.

Abbreviations

GR:

Green rust

GR(SO4):

Sulphate green rust

GR(CO3):

Carbonate green rust

SSO:

Solid-state oxidation

DOP:

Dissolution-oxidation-precipitation

EX-GR:

Ferric hexagonal platelets, formerly green rust

WD-XRF:

Wavelength-dispersive X-ray fluorescence

BET:

Brunauer–Emmett–Teller surface area analysis

XRD:

X-ray diffraction

SEM:

Scanning electron microscopy

EXAFS:

Extended x-ray absorption fine structure

LCF:

Linear combination fitting

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Acknowledgements

We thank Evert J. Elzinga for his helpful discussions on green rust synthesis and handling. We would also like to thank Gerald Poirier for his assistance with green rust sample preparation for x-ray diffraction measurements. We utilized the excellent facilities at the Advanced Material Characterization Lab (AMCL) at the University of Delaware (UD) and want to thank the AMCL staff for hosting instrumentation used in this paper. Parts of this research used the XFM (4-BM) Beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. This work is not a product of the U.S. Government or the U.S. Environmental Protection Agency, and the author is not doing this work in any governmental capacity. The views expressed are those of the author/editor/speaker only and do not necessarily represent those of the U.S. Government or the EPA. Mention of trade names or commercial products in the publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. The USDA is an equal opportunity provider and employer.

Author information

ARB, MHHF, AE and DLS conducted all experiments while at the Department of Plant and Soil Science, University of Delaware. The current affiliation of ARB is: Center for Environmental Solutions & Emergency Response, Office of Research & Development, U.S. Environmental Protection Agency, Cincinnati, OH, USA.

Funding

Funding for this research was provided by the National Institute for Food and Agriculture Project NC 1187.

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Authors and Affiliations

  1. Department of Plant and Soil Science, University of Delaware, 221 Academy St., Newark, DE, 19716, USA

    Aaron R. Betts, Anna Evers & Donald L. Sparks

  2. Sustainable Agricultural Systems Laboratory, USDA-Agricultural Research Service, Beltsville, MD, 20705, USA

    Matthew H. H. Fischel

  3. NSLS-II, Brookhaven National Laboratory, Upton, NY, 11973, USA

    Ryan Tappero

Authors
  1. Aaron R. Betts
  2. Matthew H. H. Fischel
  3. Anna Evers
  4. Ryan Tappero
  5. Donald L. Sparks

Contributions

ARB contributed conceptualization, design, data collection, analysis, decision to publish and preparation of the manuscript; MHHF contributed conceptualization, data collection, analysis, and preparation of the manuscript; AE contributed data collection and analysis; RT contributed data collection and analysis; DLS contributed conceptualization, design, and preparation of the manuscript. All authors reviewed the manuscript.

Corresponding author

Correspondence to Aaron R. Betts.

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The author(s) declare(s) that they have no competing interests.

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Supplementary Information

12932_2024_93_MOESM1_ESM.docx

Additional file 1: Section S1 presents results of pH response and discussion. Figure S1. pH response diagram during base titration for synthesis of GR(SO4) with % mol Si ranging from 0–100% mol Si. Figure S2. pH response diagram during base titration for synthesis of GR(CO3) coprecipitated with 0–50 % mol Si. Figure S3. X-ray diffraction (XRD) of freshly precipitated GR(CO3) (A) and GR(SO4) (B) by Bruker XRD. Diffractograms verify the formation of type 1 and type 2 green rusts, respectively. Additional characterization by Fe K-edge x-ray absorption spectroscopy. Section S2 presents Characterization by Fe K-edge x-ray absorption spectroscopy (XAS) of dried GR(CO3) and air-oxidized GR(SO4). Table S1. Fe K-edge EXAFS LCF of green rust carbonate (GR(CO3)) dried in anoxic glove box. Figure S4. Fe K-edge EXAFS of dried, anoxic green rust carbonate (GR(CO3)). Sample spectra in solid black line overlaid by LCF model in dashed line. Model parameters in Table S1. Table S1. Fe K-edge EXAFS LCF of green rust carbonate (GR(CO3)) dried in anoxic glove box. Figure S4. Fe K-edge EXAFS of air-oxidized green rust sulfate (GR(SO4)) and green rust carbonate (GR(CO3)). Model parameters in Table S2. Table S2. Fe K-edge EXAFS LCF results and fitting parameters. References cited for supplemental discussion.

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Betts, A.R., Fischel, M.H.H., Evers, A. et al. Silicate coprecipitation reduces green rust crystal size and limits dissolution-precipitation during air oxidation. Geochem Trans 25, 12 (2024). https://doi.org/10.1186/s12932-024-00093-9

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Geochemical Transactions

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