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Article

Anisotropic morphological changes in goethite during Fe -catalyzed recrystallization 2+

Prachi Joshi, and Christopher A. Gorski Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00702 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on July 3, 2016

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Environmental Science & Technology

Anisotropic morphological changes in goethite during Fe2+-catalyzed recrystallization Prachi Joshi and Christopher A. Gorski∗ 1

212 Sackett Building, Department of Civil & Environmental Engineering, Pennsylvania State University, University Park, PA 16802, Phone: 814-865-5673 E-mail: [email protected]

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Abstract

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When goethite is exposed to aqueous Fe2+ , rapid and extensive Fe atom exchange

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can occur between solid-phase Fe3+ and aqueous Fe2+ in a process referred to as Fe2+ -

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catalyzed recrystallization. This process can lead to the structural incorporation or

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release of trace elements, which has important implications for contaminant remedia-

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tion and nutrient biogeochemical cycling. Prior work found that the process did not

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cause major changes to the goethite structure or morphology. Here, we further inves-

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tigated if and how goethite morphology and aggregation behavior changed temporally

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during Fe2+ -catalyzed recrystallization. Based on existing literature, we hypothesized

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that Fe2+ -catalyzed recrystallization of goethite would not result in changes to in-

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dividual particle morphology or inter-particle interactions. To test this, we reacted

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nanoparticulate goethite with aqueous Fe2+ at pH 7.5 over 30 days and used trans-

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mission electron microscopy (TEM), cryogenic TEM, and

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to observe changes in particle dimensions, aggregation, and isotopic composition over

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time. Over the course of 30 days, the goethite particle substantially recrystallized, and

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the particle dimensions changed anisotropically, resulting in a preferential increase in

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the mean particle width. The temporal changes in goethite morphology could not be

1 ACS Paragon Plus Environment

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Fe as an isotope tracer


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completely explained by a single mineral transformation mechanism, but rather indi-

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cated that multiple transformation mechanisms occurred concurrently. Collectively,

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these results demonstrate that the morphology of goethite nanoparticles does change

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during recrystallization, which is an important step towards identifying the driving

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force(s) of recrystallization.

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Introduction

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Thermodynamically stable iron (Fe) (hydr)oxides, such as goethite, hematite, and magnetite,

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are ubiquitous in soils, sediments, and rocks, 1 where they control the availability and spe-

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ciation of trace elements, nutrients, contaminants, and radionuclides by serving as redox

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buffers and sorbents. 2–17 These oxides are also important proxies in the geological record, as

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their isotopic and trace element compositions are thought to preserve information regarding

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how and under what conditions they formed and weathered. 18–26 While the importance of

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reactions that occur at the oxides’ surfaces has been studied for decades, 1,27 only recently

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have researchers recognized that the interior of stable Fe oxides can also be highly reactive,

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especially in aqueous reducing environments. 5,28–35 Recent studies have shown that when

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goethite, hematite, and magnetite are exposed to aqueous Fe2+ at circumneutral pH values,

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extensive exchange can occur between the solid and dissolved Fe atoms on time scales of

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weeks, as evidenced by changes in the solid and aqueous Fe isotopic compositions. 30,36–43

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This process is commonly referred to as “Fe2+ -catalyzed recrystallization” in the literature,

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and it occurs under apparent equilibrium conditions without altering the mineral structure or

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solution chemistry. Understanding when and how this process occurs is important for prop-

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erly interpreting the isotopic and elemental compositions of iron oxides in the rock record as

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well as using iron oxides to sequester toxic elements and radionuclides in remediation efforts.

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Currently, the driving force(s) and mechanism(s) of Fe2+ -catalyzed recrystallization are

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unclear. One proposed mechanism is a redox-driven pathway, in which oxidative Fe2+ up-

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take and reductive Fe3+ dissolution occur at different crystallographic faces. 36 This idea is 2 ACS Paragon Plus Environment

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supported by experimental observations and modeling results for hematite (α−Fe2 O3 ) indi-

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cating that potential gradients can exist between different crystal faces in the presence of

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aqueous Fe2+ , which are connected through bulk electron conduction. 44–46 However, it re-

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mains unclear if observations made for hematite can be applied to other iron oxides. In the

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case of hematite, microscopic analyses of particles revealed unambiguous preferential growth

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and dissolution at different crystal faces. 44,45 For goethite (α−FeOOH), there has been no

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experimental evidence of similar morphological changes occurring during Fe2+ -catalyzed re-

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crystallization. One study found indirect evidence that individual goethite particles grew

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when exposed to aqueous Fe2+ based on X-ray diffraction peak widths, 30 while others saw

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no significant differences in particle dimensions between recrystallized goethite and control

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samples using electron microscopy. 36,47 Note that these studies only examined the particles

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at the start and end of their experiments, but not at intermediate time points. In addition

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to the reaction of individual particles with aqueous Fe2+ , inter-particle interactions may also

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play a role in Fe2+ -catalyzed recrystallization, since aggregation can affect particle reactiv-

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ity. 48–50 Aqueous Fe2+ has been shown to affect the aggregation of Fe oxide nanoparticles over

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short time scales (36 hours), 51 but no work has investigated these changes over time frames

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relevant to Fe2+ -catalyzed recrystallization (weeks). Collectively, an analysis of these stud-

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ies reveals key uncertainties regarding how iron oxide particle dimensions and aggregation

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change over the course of Fe2+ -catalyzed recrystallization, making it difficult to conclusively

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identify how and why this process occurs.

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In this study, we aimed to resolve if and how the morphology and aggregation behavior of

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goethite changes during Fe2+ -catalyzed recrystallization. We used nanoparticulate goethite

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in our experiments because (i) it is known to rapidly and extensively recystallize, 30,36,47 and

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(ii) the particles were sufficiently small such that we could observe changes in individual par-

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ticles and aggregates using transmission electron microscopy (TEM). Based on existing data,

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we hypothesized that Fe2+ -catalyzed recrystallization of goethite does not result in changes

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to individual particle morphology or inter-particle interactions. To test this hypothesis, we



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reacted goethite nanoparticles with aqueous Fe2+ at pH 7.5 for 30 days and examined the

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particles using TEM and cryogenic TEM (cryo-TEM). We examined the particles at multi-

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ple time points over the course of the 30 day reaction period to identify intermediate phases

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and temporal trends. In addition, we used cryogenic

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if the goethite underwent any detectable structural changes. We used a

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method to quantify the fraction of goethite that recrystallized and correlate it to the changes

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observed with microscopy and spectroscopy.

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Materials and Methods

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All experiments and dilutions were carried out with deionized (DI) water (Millipore Milli-Q

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system, >18 MΩ·cm). Acidic solutions were prepared from HCl (Sigma Aldrich, 37.3%) and

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basic solutions were prepared from NaOH and KOH (VWR, solid pellets, 98.9%). MOPS

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(3-(N-morpholino)-propanesulfonic acid, pK A=7.20 at 20◦ C; Calbiochem, 100%) was used

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as a pH buffer.

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Anoxic conditions

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All experiments were conducted inside an anoxic glovebox (MBraun Unilab Workstation,

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100% N2 atmosphere, < 0.1 ppm O2 , MB-OX-EX-PLC oxygen sensor). Aqueous solutions

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and ethanol were purged with N2 (Praxair, 99.99%) for 3 hours prior to being taken inside

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the glovebox. Glassware and plastic-ware were placed under vacuum overnight before being

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taken into the glovebox and were allowed to equilibrate with the glovebox atmosphere for

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at least 12 hours prior to use. FeCl2 (Acros, anhydrous, 99%) was purchased in a sealed

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container with headspace filled with N2 gas and opened directly inside the glovebox to prevent

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oxidation.

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Fe Mössbauer spectroscopy to test


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Fe isotope tracer

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Nanoscale goethite synthesis

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Nanoscale goethite was synthesized according to established methods (Section S1, Supporting

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information (SI)). 52 The measured BET surface area of the resulting nanoscale goethite was

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89.9±0.6 (±σ) m2 /g. Goethite purity was confirmed using Mössbauer spectroscopy (Section

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S9, SI). 1

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Tracking Fe2+ -catalyzed recrystallization of goethite using 55

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Fe (Perkin Elmer, half-life: 2.7 years, received as a solution of

Fe 55

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We used radioactive

FeCl3

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in 0.5 M HCl) as an isotope tracer to quantify the fraction of goethite that recrystallized

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in the presence of aqueous Fe2+ . The original solution was diluted and electrochemically

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reduced inside the glovebox to create an aqueous

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stock solution of Fe2+ was prepared inside the glovebox by dissolving 633 mg FeCl2 in 25

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mL DI water to achieve a final concentration of 200 mM. HCl (50 µL, 1 M) was added to

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this stock solution to prevent inadvertent oxidation.

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Fe2+ stock solution (Section S2.1, SI). A

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We set up sacrificial batch reactors in which nanoscale goethite was exposed to an aqueous

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Fe2+ solution at circumneutral pH under anaerobic conditions. Reactors were set up by

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adding 10 mL of MOPS buffer (25 mM, pH 7.5) in 20 mL glass vials. The Fe2+ stock solution

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was then added to the vials to achieve a concentration of 0.8 mM, followed by the addition

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of radioisotopic tracer (100 µL of the aqueous

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solution was then readjusted to 7.5 by adding a small volume of 5 M NaOH. The reactor

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vials were capped with rubber septa and allowed to mix for at least 30 minutes, after which

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a small aliquot was withdrawn to determine initial aqueous Fe2+ concentration. Goethite

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(20.0±0.1 mg) was then added to each reactor. Reactors were wrapped in aluminum foil and

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stirred using magnetic stir bars on a stir plate inside the glovebox during equilibration and

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reaction. Batch reactors were sacrificed at six time points (1, 3, 5, 7, 15 and 30 days) and

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the solids in the reactors were separated from the aqueous phase by filtration (PALL Life

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Sciences, GHP Polypro 200, 0.2 µm) inside the glovebox. The aqueous Fe2+ concentration

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Fe2+ stock solution). The pH of the reactor



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was measured using the 1,10-phenanthroline method. 53 The radioactivity of both phases

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was then measured using liquid scintillation counting to determine the mass of

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phase (Sections S2.2 and S2.3, SI). Using the measured mass of radioisotopic tracer

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we calculated the fraction of recrystallized goethite based on a mass balance approach,

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assuming that the recrystallized solid had the same isotopic composition as the solution at

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time of sampling (Section S2.3, SI). 47

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Transmission electron microscopy (TEM) and cryogenic TEM

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We used transmission electron microscopy (TEM) to identify changes in goethite particle

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dimensions during Fe2+ -catalyzed recrystallization. For microscopy, batch reactors were set

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up similar to those used to track the extent of recrystallization, except that no

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was added. Control reactors were set up by adding 20.0±0.1 mg nanoscale goethite to 10

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mL MOPS buffer at pH 7.5 in the absence of aqueous Fe2+ .

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Fe in each 55

Fe,

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Fe tracer

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At each time point (0, 0.5, 7, 15, 20 and 30 days), one reactor was sacrificed and the

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solids were separated from the aqueous phase by filtration (Pall Life Sciences, GHP Polypro,

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0.2 µm) inside the glovebox. After filtration, the solids were scraped off the filter paper

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and mixed with O2 -free ethanol. Two drops of this suspension were deposited on a lacey

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carbon grid (Electron Microscopy Sciences, 200 mesh Cu grid) and the grid was allowed to

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dry inside the glovebox. The grid was kept anaerobic until placement in the TEM sample

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holder, and the total exposure to air was less than 30 seconds. To test if ethanol affected

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the morphology of the goethite particles, TEM samples were also prepared by suspension in

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DI water at two time points (0 and 15 days). No significant difference in measured particle

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dimension data was observed between solids suspended in ethanol and those suspended in

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DI water.

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TEM images were collected on a JEOL 2010F microscope at 200 kV. We checked for

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the beam sensitivity of goethite by exposing an area of a control sample to the beam for

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several minutes. Changes were detected in the goethite particles after focusing the beam on 6 ACS Paragon Plus Environment

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a specific spot for 5 minutes. To prevent beam damage, all images were collected within 2

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minutes of focusing. For each time point, >15 images were obtained. The length and width

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of a minimum of 400 goethite nanoparticles were manually measured for each time point.

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Further details regarding how the particles were measured, the statistical analyses used to

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compare particle distributions at different time points, and how the error associated with

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measuring the particle distributions was estimated are found in Sections S4 and S5, SI.

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For TEM, the samples were prepared by filtering the solids and re-suspending them in

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ethanol, possibly affecting the aggregation behavior of the nanoparticles. Therefore, to get

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a more accurate “snapshot” of the goethite in the reactor, we used in-situ cryogenic TEM

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(cryo-TEM) (JEOL JEM-2010 LaB6), which involved the instantaneous vitrification of the

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reactor suspension such that the arrangement of the solids was preserved. Reactors for cryo-

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TEM were set up identical to those for TEM. At each time point (1, 5, 13 and 30 days), one

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reactor was crimp-sealed and taken out of the glovebox. Just before vitrification, the reactor

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was decrimped and 2 µL of the reactor contents were deposited on a copper grid (Ted Pella,

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INC, Quantifoil R2/2, 200 mesh Cu grid), blotted with filter paper, and plunged into liquid

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ethane. The frozen grids were stored in liquid nitrogen, then transferred to the microscope

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for imaging. Details regarding the analysis of these images are in Section S7, SI.

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M¨ ossbauer Spectroscopy

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To detect changes in goethite mineral structure during Fe2+ -catalyzed recrystallization, we

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used cryogenic 57Fe transmission Mössbauer spectroscopy (SVT400 Mössbauer spectrometer;

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SEE Co., USA). We analyzed two samples at 5 K: (i) goethite reacted with aqueous Fe2+

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for 15 days and (ii) goethite mixed in buffer for 15 days without Fe2+ (control). Reactors

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were prepared identical to those for microscopy. Details of sample preparation are given in

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Section S9, SI.



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Results and discussion

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Tracking Fe2+ -catalyzed recrystallization of goethite

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To measure the extent of Fe2+ -catalyzed recrystallization of nanoparticulate goethite, we

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exposed isotopically-normal goethite to aqueous Fe2+ enriched with

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mass of

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ratio (ng

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30 days, which was coupled to an increase in the goethite

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Fe in the aqueous and solid phases over 30 days. The

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Fe and tracked the

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Fe to total Fe mass

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Fe/mg FeTotal ) in the aqueous phase dropped sharply from 7.27 to 0.37 after 55

Fe to total Fe mass ratio (ng

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Fe/mg FeTotal ) from 0.00 to 0.33 (Figure 1, tabulated values in Table S1, SI). In these

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experiments, the aqueous Fe2+ concentration decreased from 0.8 mM to 0.32 mM during the

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first day, then plateaued (Figure S2, SI). This loss was due to Fe2+ uptake by the solid, not

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inadvertent oxidation, as we recovered >92% of the total Fe2+ added by completely dissolving

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the solid phase at each time point (Section S2.8, SI). After 30 days, the final

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Fe mass ratios of both phases were nearly identical, suggesting that the system approached

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isotopic equilibrium. We estimated the fraction of the goethite Fe atoms that exchanged with

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aqueous Fe2+ atoms using the homogenous exchange model, 47,54 which indicated that 95%

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of the structural Fe atoms in goethite had exchanged over the 30 day reaction period (Figure

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S3). This finding was in good agreement with previous studies conducted on nanoparticulate

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goethite under similar experimental conditions. 36,47

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Changes in goethite particle dimensions during Fe2+ -catalyzed re-

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crystallization

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To determine if and how the nanoparticulate goethite particle dimensions changed during

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Fe2+ -catalyzed recrystallization, we collected and analyzed TEM images of the solid phase

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at six time points over 30 days. The goethite particles in these TEM images appeared to be

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qualitatively similar at all time points (Figure 2), but quantitative measurements of numerous

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particles (n>400) from several images collected at each time point revealed temporal changes 8 ACS Paragon Plus Environment

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Fe to total

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Figure 1: Measured 55Fe to total Fe mass ratio (ng 55Fe/mg FeTotal ) values for aqueous Fe2+ and goethite over time. The dashed line represents the 55Fe/FeTotal ratio if complete isotopic mixing occurred. Reactors contained 2 g/L goethite, 0.8 mM initial Fe2+ , and 3.35 ng 55Fe at pH 7.5 (25 mM MOPS). 195

in both their length and width distributions. To quantitatively represent these changes and

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assess their statistical significances, we constructed frequency distribution plots, box and

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whisker plots, and mean dimension plots of the particle length and width distributions as a

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function of time (Figures 3 and 4).

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Over the course of 30 days, the goethite particle length distributions changed in the

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presence of aqueous Fe2+ (Figure 3, Table S3). The original, unreacted goethite consisted

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of long rod-like particles that had a mean length of 56.3±1.2 nm (±standard error) (Figure

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3A,H; additional representative TEM images shown in Figure S4). The length distributions

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at 0.5 days (mean length: 56.5±0.9 nm) and 7 days (56.9±0.8 nm) were similar to that of

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the initial goethite. At 15 and 20 days, however, the particles were substantially shorter

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than the original particles (mean length at 15 days: 37.9±0.6 nm, 20 days: 48.5±1.1 nm).

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At 30 days, the particle length distribution was similar to that of the original goethite (mean

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length: 55.9±1.2 nm). The same trend in mean particle length changes (Figure 3H) was

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also observed for the median lengths (Figure 3G). To determine if the differences in mean



Figure 2: Representative TEM images of goethite nanoparticles reacted with Fe2+ for (A) 0, (C) 15, and (D) 30 days. Reactors consisted of 2 g/L goethite and an initial aqueous Fe2+ concentration of 0.8 mM, buffered at pH 7.5 (25 mM MOPS), covered with aluminum foil, and stirred on a magnetic stir plate inside the glovebox. A control reactor (B) without aqueous Fe2+ was mixed for 15 days. Additional representative images for each panel are given in Figures S4-S10. 209

particle lengths at 15 and 20 days was due to the presence of aqueous Fe2+ , we analyzed

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TEM images of goethite mixed in buffer for 15 days in the absence of aqueous Fe2+ as a

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control. The mean length of the control goethite at 15 days was 49.1±0.8 nm, which was

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larger than the mean length for the reactor containing aqueous Fe2+ at 15 days, but was

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also smaller than that of the initial goethite. This comparison suggested that the differences

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in particle lengths at 15 and 20 days were due to the presence of aqueous Fe2+ , which we

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further explored with statistical analyses.

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To determine if the differences in particle length distributions represented statistically

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significant changes to the goethite, we used two lines of analyses. First, we performed a 10 ACS Paragon Plus Environment

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Figure 3: Length frequency distributions of goethite nanoparticles (2 g/L) reacted with 0.8 mM Fe2+ solution at (A) the start of the reaction, (C) 7 days, (D) 15 days, (E) 20 days and (F) 30 days. Control reactor (B) contained goethite in buffer without aqueous Fe2+ , mixed for 15 days. The blue bars denote the reacted goethite while the green bars denote the control. The length distribution of original unreacted nanoparticles is overlaid in gray on each panel. A minimum of 400 particles were sized for each time point (Table S3, SI). (G) Box and whisker plots for the length of nanoparticles at the seven time points with the boxes bound by 25th and 75th percentile, whiskers bound by 10th and 90th percentiles, and outliers shown by open circles. The blue boxes represent the reacted goethite while the green boxes denote the control. (H) The mean lengths at the seven time points with thin error bars representing standard deviations and thick error bars denoting the standard errors of mean. The blue markers denote the reacted goethite while the green markers represent the control. 218

series of control analyses to quantitatively estimate the analysis error (σanalysis ) associated

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with the mean length values. This error contained two components: the measurement error 11 ACS Paragon Plus Environment


Figure 4: Width frequency distributions of goethite nanoparticles (2 g/L) reacted with 0.8 mM Fe2+ solution at (A) the start of the reaction, (C) 7 days, (D) 15 days, (E) 20 days and (F) 30 days. Control reactor (B) contained goethite in buffer without Fe2+ , mixed for 15 days. The blue bars denote the reacted goethite while the green bars denote the control. The width distribution of original unreacted nanoparticles is overlaid in gray on each panel. A minimum of 400 particles were sized for each time point (Table S3, SI). (G) Box and whisker plots for the width of nanoparticles at the seven time points with the boxes bound by 25th and 75th percentile, whiskers bound by 10th and 90th percentiles, and outliers shown by open circles. The blue boxes represent the reacted goethite while the green boxes denote the control. (H) shows the mean width values at the seven time points with thin error bars representing standard deviations and thick error bars denoting standard errors of mean. The blue markers denote the reacted goethite while the green markers represent the control. 220

(σmeasurement ) (i.e., the error associated with manually measuring particle lengths from a set

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of images) and the sampling error (σsampling ) (i.e., the error due to the analysis of only a 12 ACS Paragon Plus Environment

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sample of the whole population of particles). The measurement error of the mean particle

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length, which was quantified by measuring particle lengths for the same sets of images

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three times and comparing the means, was determined to be 1.9 nm (Section S5.2, SI). The

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sampling error of the mean particle length, which was quantified by preparing two TEM

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grids from a single reactor after 15 days and determining the particle size distributions, was

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1.2 nm (Section S5.2, SI). Together, these two analyses generated a σanalysis value of 2.2 q 2 2 ), which was comparable to the range of standard + σsampling nm (σanalysis = (σmeasurement

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error values (0.6-1.2 nm) determined from the distributions themselves. Based on these

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calculations, the differences between the mean particle lengths of the initial goethite (56.3

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nm) and the goethite at 15 days (37.9 nm) and 20 days (48.5 nm) were substantially larger

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than the analysis error. Likewise, the mean particle lengths of the initial goethite (56.3 nm)

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and the reacted goethite at 0.5 (56.5 nm), 7 (56.9 nm), and 30 (55.9 nm) days were similar

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within error.

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The second line of analysis we used to compare the differences among goethite particle

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length distributions was statistical hypothesis testing (Sections S5, SI). Initial analyses re-

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vealed that the length distributions were not normal (Section S3, Figure S12 and S13, SI);

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consequently, we used non-parametric tests, which do not assume a given probability distri-

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bution. Analyses of the particle distributions using (1) the Kolmogorov-Smirnov test and (2)

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2-sample t-tests revealed that the length distribution of original, 0.5 day, 7 day, and 30 day

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samples were statistically similar (α=0.005, where the significance level α is the acceptable

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probability of rejecting a null hypothesis when it is actually true; Section S5.4, SI). The anal-

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yses also all revealed that the length distributions at 15 days and 20 days were significantly

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shorter than that of the initial goethite (Section S5.4, SI). The length of nanoparticles in

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the absence of aqueous Fe2+ (control, 15 days) was significantly different than that of the

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original goethite as well as the goethite reacted with Fe2+ for 15 days. Possible explanations

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for why the control goethite particle length distribution was different from that of the origi-

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nal goethite are discussed below. Collectively, these two lines of evidence indicated that the



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goethite particles had shortened at 15 and 20 days in the presence of aqueous Fe2+ over the

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course of the reaction, but had increased back to their original lengths at the end of 30 days.

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The mean widths of the goethite particles also changed over the 30 day time period

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(Figure 4, Table S3). The mean width initially increased from 6.3±0.1 nm (±standard

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error) to 7.5±0.1 nm over the first 0.5 days, which may have been due to the uptake of

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aqueous Fe2+ preferentially along the edges during the initial Fe2+ uptake period (Figure

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S2, SI). The mean width then remained fairly stable from 0.5 days to 20 days (7.6±0.1 nm),

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except for the 15 day sample which had a smaller mean width (6.8±0.1 nm). From 20 to

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30 days, the mean width increased to 9.1±0.1 nm. The same trend observed for the mean

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widths (Figure 4H) was also observed for the median widths (Figure 4G). In the control

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reactor without aqueous Fe2+ analyzed after 15 days of reaction, the mean width was lower

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(5.1±0.1 nm) than that of the initial goethite (6.3±0.1 nm) (Figure 4B).

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As we did for the length distributions, we performed error analyses and hypothesis tests

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on the width distributions. The analysis error (σanalysis ) for the width measurements was

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estimated to be 0.06 nm, which was similar to the standard error range (0.08-0.13 nm)

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calculated for the distributions at each time point (Section S5, SI). The changes in mean

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width values over the course of the reaction were therefore much larger than σanalysis . Testing

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each width distribution using normality testing revealed that none of the distributions were

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normally distributed (Figure S13, SI). Hypothesis testing using the Kolmogorov-Smirnov

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and 2-sample t-tests indicated that the particle width distributions at each time point were

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statistically different from that of the initial goethite (α = 0.005; Section S5.4, SI). These

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data collectively indicated that the particle widths significantly increased over the 30 day

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period in the presence of aqueous Fe2+ .

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An apparent question that arose from these measurements and analyses was: Why did

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the length and width distributions of the control goethite change in the absence of aqueous

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Fe2+ ? Addressing this question was the primary motivating factor for quantifying σanalysis

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for the length and width distributions, which clearly indicated that the differences between


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the initial and control goethite were larger than could be explained by error associated

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with our analyses. The finding that the control length and width distributions changed was

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unexpected because goethite is very insoluble at this pH and is thought to be stable. The

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changes may have been due to reactions with water (despite the insolubility of goethite) or

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the pH buffer (MOPS). Prior work has found that commonly used pH buffers for this pH

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range, including MOPS as well as HEPES and TEA, can affect the aggregation behavior of

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goethite over time frames far shorter than the 15 days examined here (i.e., minutes). 51 To the

283

best of our knowledge, no prior work has investigated how these buffers may alter particle

284

lengths or widths. While we cannot identify the cause for the change in the control goethite

285

length and width distributions from the available data, we can draw two conclusions: (1)

286

the changes were significant and were not a result of analysis error and (2) the changes were

287

different from what was observed in the case of aqueous Fe2+ , as the mean particle width

288

decreased in the control reactor, but increased in the presence of aqueous Fe2+ .

289

When aqueous Fe2+ was present, the changes in goethite particle dimensions were anisotropic:

290

the particle lengths at the end of 30 days was similar to those of the original goethite, but the

291

particle widths were larger than that of the original goethite. To track the temporal changes

292

in relative particle dimensions, we calculated the mean aspect ratio (i.e., mean length/mean

293

width) over time (Figure 5, Table S3). Over the course of the reaction, the mean aspect

294

ratio decreased from 9.0±1.5 (±standard error) for the original goethite to 6.1±1.0 (30 days)

295

(Table S3, SI). To visualize these changes, we plotted the goethite particles’ mean length and

296

mean width at each time point, drawn to scale. Initially, the decrease in aspect ratio was

297

primarily due to the shortening of the particles, reaching a minimum of 5.0±0.6 at 15 days.

298

After 15 days, the mean length increased, causing the aspect ratio to increase to 6.5±1.1

299

(20 days), reaching a final value of 6.1±1.0 (30 days). In the 15 day control reactor without

300

aqueous Fe2+ , the aspect ratio was much larger (9.7±1.1) than any of the time points for

301

the reactors with aqueous Fe2+ , primarily due to the decrease in particle width. Overall,

302

these trends show that the goethite particles became preferentially wider over time in the



Figure 5: Scaled representation of aspect ratio (mean length:mean width) of goethite nanoparticles (2 g/L) reacted with 0.8 mM Fe2+ over 30 days. The mean aspect ratio of goethite nanoparticles mixed in buffer without Fe2+ for 15 days is represented in green. Aspect ratio of the nanoparticles decreased in the presence of Fe2+ , while it increased in absence of Fe2+ . 303

presence of aqueous Fe2+ .

304

A clear and puzzling observation from Figure 5 is that, on average, the reacted particles at

305

15 days appear to be smaller than those at 0 and 30 days. To determine if the particles were

306

indeed smaller at some time points than others, we performed simple volumetric calculations

307

using the mean length and width values, with two different assumptions for how the measured

308

widths should be interpreted with respect to the goethite particle geometry (Section S6,

309

SI). The mean particle volume should be conserved if the goethite density and number of

310

particles remained constant, even if some particles grew at the expense of others. When

311

the measured width is assumed to represent the diagonal of a goethite rod cross-section, the

312

mean particle volume increased over the first seven days from 1120±130 nm3 (±standard

313

error) to 1570±130 nm3 , decreased to a value that was smaller than that of the initial

314

goethite (880±80 nm3 ) at 15 days, then increased to a value of 2320±230 nm3 at 30 days.

315

The same trend was seen in volumetric values calculated based on the assumption that the

316

measured width represents one side of the cross section of a nanoparticle (Table S8, SI).

317

The smaller value for mean particle volume at 15 days relative to the initial goethite could

318

not be explained by any known particle transformation processes such as Ostwald ripening, 16 ACS Paragon Plus Environment

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319

in which one particle grows at the expense of others, 55 which would result in no change

320

or an increase to the mean particle volume. Instead, the decrease in mean particle volume

321

indicated that either the particles partially dissolved or that there was an increase in the

322

number of particles.

323

To test if net dissolution was occurring, we measured the concentration of Fe2+ in the

324

aqueous phase at each time point, along with the mass of Fe2+ and Fe3+ in the solids. The

325

aqueous Fe3+ concentration was below detection limits at all time points. The aqueous

326

Fe2+ decreased during the first 24 hours, and then remained constant over the course of the

327

reaction. The total masses of Fe2+ and Fe3+ were conserved over the course of the reaction

328

(recovery = 93-103%, Section S2.8, SI). As no change was observed in aqueous or solid phase

329

Fe pools corresponding to the calculated particle volume decrease, we conclude that no

330

net dissolution of goethite occurred. Thus the most plausible explanation for the decrease

331

in particle volume at 15 days was that the number of particles increased. We could not

332

directly test this hypothesis because we could not measure the particle concentration with

333

commonly employed techniques, such as dynamic light scattering (DLS), since the particles

334

were aggregated. While an increase in the number of particles seemed unlikely, it should

335

qualified by the fact isotopic measurements indicated that virtually all the Fe atoms in the

336

goethite (>100%) had exchanged with Fe atoms in solution at the 15 day time point (Figure

337

S3, SI), demonstrating that extensive dissolution and precipitation must have occurred by

338

this point in the reaction and that the potential for new particle nucleation should not be

339

ruled out.

340

Note that the observation that the mean width of the goethite particles increased over 30

341

days was partially inconsistent with a previous study that observed no difference between the

342

mean lengths and widths of nanoparticulate goethite reacted for 30 days with and without

343

the presence of aqueous Fe2+ . 36 These studies varied in potentially significant ways, which

344

may explained the difference. First, our work looked at changes over time, while the previous

345

study compared particles with and without the presence of aqueous Fe2+ . Since we did not



346

measure the control reactor at 30 days, we cannot directly compare the present results with

347

theirs. Second, the synthesized goethites may have been different. While we used the same

348

synthesis procedure, the mean particle dimensions of our goethite (56.3 nm × 6.3 nm) were

349

smaller than those of the previous work (81 nm × 11 nm). Third, the solution chemistry was

350

slightly different. We did not use a background electrolyte (although buffer was present),

351

while the previous work used 25 mM KBr. Finally, we measured the width of more particles

352

after 30 days (n = 574) than in the previous work (n = 91). It is possible, although unlikely,

353

that the lower number of particles measured resulted in a skewed value. Currently, we cannot

354

identify a single reason for this contrast in the results of Handler (2009) 36 and this study, but

355

these factors should be considered in designing experiments and evaluating data in future

356

studies.

357

Aqueous Fe2+ -induced aggregation of goethite

358

In addition to the changes in dimensions of individual goethite particles, we also investigated

359

inter-particle interactions during Fe2+ -catalyzed recrystallization of goethite. Our interest

360

in the aggregation behavior of goethite nanoparticles during Fe2+ -catalyzed recrystalliza-

361

tion was motivated by (1) observations in our TEM images (Figure 2, Figures S4-S9, SI)

362

and a previous study 36 suggesting that goethite nanoparticles may preferentially aggregate

363

along the long edge in the presence of Fe2+ and (2) a recent study that hypothesized Fe2+

364

taken up from solution by Fe oxide particles affects aggregation by altering the oxide’s sur-

365

face charge. 51 To test whether recrystallization led to preferential aggregation, we examined

366

nanoparticulate goethite reacted with aqueous Fe2+ over 30 days using cryo-TEM, an electron

367

microscopy technique that preserved the arrangement of particles in the reactor suspension

368

(Figures S16-S23, SI). From these images, we quantified the percentage of goethite particles

369

that were preferentially aggregated along their long edge. We defined goethite nanoparticles

370

that underwent preferential aggregation as groups of two or more individual particles that

371

were attached along the longitudinal axis, and refer to this aggregated unit as a “bundle”. 18 ACS Paragon Plus Environment

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372

Negligible preferential aggregation was observed along the short edges of the particles. By

373

manually counting the number of particles in bundles as well as the total number of par-

374

ticles in each image, we compared the preferential attachment of goethite in the presence

375

and absence of aqueous Fe2+ . The percentage of bundled goethite remained constant over

376

30 days (Figure S24) when exposed to aqueous Fe2+ , between 49% and 45% of total imaged

377

particles. The percentage of bundled particles in control reactors (without aqueous Fe2+ )

378

remained stable between 59% and 64%. These results indicated that the preferential aggre-

379

gation of goethite nanoparticles was not actively influenced by aqueous Fe2+ , and therefore

380

not directly related to Fe2+ -catalyzed recrystallization.

381

Structural characterization of goethite using M¨ ossbauer spectroscopy

382

To determine if detectable changes in the mineral structure occurred during Fe2+ -catalyzed

383

recrystallization, we characterized goethite reacted with aqueous Fe2+ for 15 days and com-

384

pared it to goethite mixed in buffer without aqueous Fe2+ for 15 days using cryogenic

385

Mössbauer spectroscopy at 5 K (Figure S25). Mössbauer spectra of both reacted and control

386

(no aqueous Fe2+ ) goethite were fit with a single sextet with hyperfine parameters consistent

387

with reported values for goethite (Table S9). 1,56 The hyperfine parameters of both sextets

388

were indistinguishable, suggesting that no structural transformation had taken place due to

389

reaction with aqueous Fe2+ . We saw no indication of Fe2+ in the goethite sample reacted

390

with aqueous Fe2+ , consistent with previous studies. 57,58 The similarity between the sam-

391

ples was unexpected because our microscopy data showed that in the reacted sample, the

392

particles were different in size from those in the control sample. Also, exchange calculations

393

suggest that at 15 days of reaction with aqueous Fe2+ , almost all (>95%) of the structural

394

Fe in goethite had recrystallized. A possible explanation for the lack of difference between

395

spectra was that the goethite particles were aggregated and inter-particle interactions oc-

396

curred. Currently, the effect of aggregation on Mössbauer spectra of goethite is unknown.

397

Mössbauer spectroscopy data therefore suggested that although the particles underwent mor19 ACS Paragon Plus Environment

57

Fe


398

phological changes, these changes did not affect the local binding environment of Fe atoms

399

in the goethite during Fe2+ -catalyzed recrystallization.

400

Insights into mechanisms involved in Fe2+ -catalyzed goethite re-

401

crystallization

402

In this study, we tested the hypothesis that Fe2+ -catalyzed recrystallization of goethite does

403

not result in changes to inter-particle interactions (specifically preferential aggregation) or

404

individual particle morphology. Based on the cryo-TEM images collected over time, there was

405

no indication that the presence of aqueous Fe2+ caused preferential aggregation, consistent

406

with our hypothesis. In contrast, analysis of the goethite particle length and width over time

407

indicated that the morphology of individual particles did change significantly, thus disproving

408

our hypothesis. Specifically, the mean particle widths increased over time, leading to particles

409

with decreased length:width aspect ratios. Interpretation of these results was complicated by

410

the fact that the length and width distributions were wide and that the distributions of the

411

control goethite, which was present in buffer without aqueous Fe2+ , significantly differed from

412

the original goethite, resulting in particles that had smaller widths. Therefore, the changes

413

in goethite particle dimensions were likely not driven solely by the presence of aqueous Fe2+ ,

414

but also by the presence of water, and possibly pH buffer.

415

The dynamic changes in particle lengths and widths were inconsistent with any single

416

known crystal growth or transformation mechanism; instead, they suggest three concur-

417

rent, potentially competing mechanisms. Handler et al. 36 hypothesized that Fe2+ -catalyzed

418

goethite recrystallization may be driven by a “redox-driven conveyor belt” mechanism, in

419

which (1) oxidative Fe2+ uptake preferentially occurs at one crystal face, (2) transferred

420

electrons migrate within the goethite structure, and (3) reductive Fe3+ dissolution occurs at

421

a different face. 36,44,45 The driving force behind this mechanism was proposed to be poten-

422

tial gradients existing among faces, making preferential uptake and dissolution at different

423

crystal faces energetically favorable. In subsequent work, molecular simulations found that 20 ACS Paragon Plus Environment

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Figure 6: Schematic representation of morphological changes in goethite nanoparticles in the presence of aqueous Fe2+ . Nanoparticles preferentially grow on the edges ((101) and (100) faces) and dissolve at the tips ((210) and (234) faces) as Fe2+ -catalyzed recrystallization progresses. Figure is not drawn to scale. 424

crystal faces along goethite edges are more likely to take up Fe2+ and transferred electrons

425

travel parallel to the edges, 46 suggesting that crystal faces on the goethite tips are more

426

likely to act as sites of reductive dissolution. Interestingly, these predictions are generally

427

consistent with the results observed here (represented in Figure 6, space group Pnma). Fe2+

428

adsorbs preferentially on the edges of goethite nanoparticles (i.e., the (100) and (101) crys-

429

tal faces) leading to an increase in particle width, and the goethite tips (i.e., the (210) and

430

(234) crystal faces) reductively dissolve leading to a decrease in particle length. While our

431

results are consistent with this mechanism, the conveyor belt model cannot explain all of

432

our observations.

433

Specifically, this mechanism cannot explain how the particles at 15 days are on average

434

smaller than the initial goethite. This observation was puzzling from a thermodynamical

435

standpoint, since decreasing particle dimensions increases the surface area and hence surface

436

energy. 48 Based on our estimation of the analysis error, we ruled out the possibility that

437

the decrease was due to analytical imprecision. We also ruled out the possibility that net

438

dissolution of goethite occurred by performing a mass balance on the Fe2+ and Fe3+ over

439

time. Although we could not measure the number of particles in the suspension, we believe

440

that the only plausible explanations for this observation were that larger particles broke apart



441

or new particles nucleated. While both cases appear unlikely, virtually all the Fe atoms in

442

the solid exchanged with the aqueous Fe atoms, which can only be explained by the particles

443

extensively dissolving and reforming through precipitation reactions. We speculate that

444

synthesized nanoparticulate goethite used in this study likely contained crystalline defects, 59

445

which were being annealed through dissolution and reprecipitation reactions and potentially

446

driving the observed decrease in mean particle size.

447

A final point to consider is that the mean particle length originally decreased over the

448

first 15 days of the reaction, but then increased over the remainder. This observation was

449

inconsistent with the two mechanisms described thus far, which both predict that the particle

450

length would decrease. We suspect that this was due to a concurrent particle coarsening (i.e.,

451

Ostwald ripening) mechanism, 55 coupled to the dissolution-precipitation process mentioned

452

above. During coarsening, relatively large particles grow at the expense of relatively small

453

particles, resulting in an increase in mean particle size driven by a reduction in the goethite

454

surface energy. We therefore suggest that these three mechanisms – redox-driven preferential

455

growth, defect annealing, and coarsening – occur concurrently and influence the particle

456

dimensions over time, with the mechanisms sharing a common driving force of reducing

457

the goethite’s free energy. While we cannot evaluate the relative contributions of these

458

three transformation process towards the observed morphological changes, we speculate that

459

they each played an important role. Experiments are now underway to assess the relative

460

importance of these mechanisms over longer time scales as well as determine how goethite

461

particle morphology temporally changes in the absence of aqueous Fe2+ .

462

Acknowledgement

463

This work was supported by the U.S. National Science Foundation Division of Earth Sciences

464

(award GEO-1451593). The authors thank Jennifer Gray and Ke Wang (Materials Charac-

465

terization Lab, PSU) for help with electron microscopy, Patrick C. Duggan (PSU) for help


Page 22 of 30

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55

466

with

467

tope measurements. This manuscript was greatly improved by the valuable and constructive

468

feedback provided by the anonymous reviewers and associate editor.

469

Supporting Information Available

470

Nanogoethite synthesis, radioisotopic tracer experiments, atom exchange calculations, TEM

471

and cryo-TEM images collection and analyses, statistical analyses, volumetric calculations

472

and tabulated Mössbauer fit parameters are explained in detail.

473

474

475

476

Fe experiments, and Jeffrey A. Leavey and the EHS team (PSU) for iron radioiso-

This material is available free of charge via the Internet at http://pubs.acs.org/.

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Anisotropic Morphological Changes in Goethite ... - ACS Publications

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