Ferrihydrite Nanoparticle Aggregation Induced by Dissolved Organic

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Ferrihydrite Nanoparticle Aggregation Induced by Dissolved Organic Matter Luigi Gentile, Tao Wang, Anders Tunlid, Ulf Olsson, and Per Persson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05622 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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The Journal of Physical Chemistry

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Ferrihydrite Nanoparticle Aggregation Induced by

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Dissolved Organic Matter

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Luigi Gentile*†, Tao Wang†, Anders Tunlid†, Ulf Olsson‡, and Per Persson†§

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†

Department of Biology, MEMEG unit, Lund University, Sölvegatan 35, 223 62 Lund, Sweden.

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‡

Department of Chemistry, Physical Chemistry Division, Lund University, Naturvetarvägen 14,

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223 62 Lund, Sweden.

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§

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62 Lund, Sweden.

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*Corresponding author: [email protected]

Centre for Environmental and Climate Research (CEC), Lund University, Sölvegatan 35, 223

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract. Ferrihydrite (Fh) nanoparticles are omnipresent in nature and often highly mobile

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because of their colloidal stability. Thus, Fh serves as a vector for iron as well as associated

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nutrients and contaminants. Here we demonstrate, using small angle X-ray scattering combined

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with cryo-transmission electron microscopy (cryo-TEM), that dissolved organic matter (DOM),

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extracted from a boreal forest soil, induce aggregation of Fh nanoparticles, of radius 3 nm, into

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fractal aggregates, having a fractal dimension D=1.7. The DOM consists of both fractal-like

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colloids (>100 nm) and small molecular DOM, but the attractive Fh interparticle interaction was

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mediated by molecular DOM alone as shown by cryo-TEM. This highlights the importance of

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using soil extracts, including all size fractions, in studies of the colloidal behavior of DOM-

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mineral aggregates. The Fh nanoparticles also self-assemble during synthesis into aggregates

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with the same fractal dimension as the DOM-Fh aggregates. We propose that both in the absence

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and presence of DOM the aggregation is controlled by the Fh particle charge and the process can

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be viewed as a linear polymerization into a self-avoiding random walk structure. The theoretical

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D value for this is 5/3, which in close agreement with our Fh and DOM-Fh results.

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INTRODUCTION

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Ferrihydrite (Fh) nanoparticles are abundant in natural environments, where their redox

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properties and large reactive surface areas make them important to a range of biogeochemical

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processes.1,2 Fh is formed by hydrolysis of Fe(III), which is often preceded by oxidation of

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Fe(II). This results in the formation of very small particles having a size range of 1–10 nm,3,4

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which display a significant interfacial tension (γ ≈ 0.19 J m−2).2 The colloidal properties of Fh

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particles are controlled by the solution conditions, in which pH as well as the presence of

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inorganic and organic anions are of particular importance.2,5–7 Previous results have identified

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formation of pH-dependent metastable Fh nanoparticle clusters that substantially change the

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transport and sedimentation of Fh.7 By lowering the pH from 5.5 to 3.5, these clusters dissolve

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into their primary particles within 30 min, as a result of increasing electrostatic repulsion. Fh has

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a point of zero charge of around 8.8

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Dissolved organic matter (DOM; operationally defined as the fraction filtered through 0.45 or

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0.2 µm) consisting of a mixture of organic compounds is ubiquitous in natural waters.9 At typical

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natural pH values of 4–7, a fraction of DOM is negatively charged, while Fh surfaces carry a net

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positive charge, creating favorable conditions for their electrostatic interaction.5,6,10 Guénet et al.

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investigated the structure of Fh–organic matter (OM) aggregates using a combination of neutron

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and X-ray scattering, extended X-ray absorption fine structure, and transmission electron

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microscopy (TEM) analyses.11 These aggregates were formed by oxidation of Fe(II) in the

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presence of a humic acid (HA) standard (leonardite), producing three distinct size fractions:

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primary particles of 0.8 nm; intermediate isolated fractal aggregates having a radius of gyration

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(R) of ca. 6 nm, which are bonded to organic molecules; and secondary Fh aggregates associated

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with large, dense HA structures of several hundred nm.11 This study also showed that HA was


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distributed within aggregated and non-aggregated fractions, with the latter forming monomeric

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Fe-HA complexes. Small angle neutron scattering studies of HA aggregates detected clusters of

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30–65 nm size,12–15 although in some cases structures were as large as 100–240 nm.16

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Recent complementary results were presented by Demangeat et al.17 showing that aggregation

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of magnetite and hematite nanoparticles is pH dependent, while HAs stabilize the primary

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particles. Moreover, they also showed that nanoparticles can coexist with aggregates at certain

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pH values.17

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Fh-OM interactions not only affect the size distribution of Fh particles but also their reactivity.

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In the study by Guénet et al., the Fh-HA interactions were shown to influence the interactions

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between Fh and arsenate, potentially controlling the transport and availability of this

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contaminant. Moreover, Henneberry et al.18 showed that co-precipitation of Fe(III) and DOM

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yielded non-crystalline aggregates that were stable under the pH and redox conditions studied.

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This suggests that Fh-DOM interactions can promote sequestration of DOM.

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There are numerous studies of other aspects of Fh-DOM interactions,19 but the lack of a

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complete characterization at a colloidal length scale limits our ability to understand the

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involvement of Fh in biogeochemical processes. To date, only the effects of organic model

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compounds or specific HA standard samples on colloidal Fh have been determined. Therefore, a

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main objective of the present study was to investigate the effect of complex DOM on Fh, as

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representative of material present in soil solutions. This DOM was extracted by water from an

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organic boreal forest soil. An additional objective was to determine which of the DOM

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components interacted with Fh. The effects caused by DOM were also compared to those

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induced by variation in pH and aging of Fh suspensions. The systems were characterized using a

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combination of small-angle X-ray scattering (SAXS) as well as static light scattering (SLS) and


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dynamic light scattering (DLS) at probe length scales from approximately 1 nm to 1 µm. These

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scattering experiments were complemented with cryo-TEM, nuclear magnetic resonance (NMR),

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infrared (IR) and X-ray photoelectron spectroscopy (XPS), which evaluated the composition and

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spatial distribution of Fh, DOM, and Fh-DOM aggregates.

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EXPERIMENTAL SECTION

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Materials.

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Dissolved organic matter (DOM) was extracted using hot water20 from a boreal forest

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soil collected from the O horizon of a nitrogen poor site in central Sweden

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(56°42′2.47″N,13°6′57.75″W; soil pH of 4.48). A 1:5 w/V ratio in MilliQ water (Merck

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AG, Darmstadt, Germany) was used for the extraction. After extraction, DOM was

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separated from the remaining soil particles by filtration through a 0.2-µm membrane

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(Sarstedt AG & Co. KG, Nuembrecht, Germany). DOM contains the major classes of

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biomolecules that are present in soil organic matter;21 thus, it is considered to be the most

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reactive fraction, and relevant to the formation of organic matter-mineral associations.19,22

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Total organic C concentration was measured with an organic C analyzer (Shimadzu

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Corp., Kyoto, Japan). Total N content was measured with the same apparatus, equipped

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with a total nitrogen module (TNM-1). The total C in the DOM was 2060 ± 12 mg l−1,

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while total N was 123 ± 1 mg l−1. Organic matter is often estimated from organic C values

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by applying a factor of 1.724,23 because C makes up approximately 58% of the total mass

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of typical organic matter. However, this calculation does not take in account differences

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in molecular composition. Here, a factor of 1.55 was used to calculate the DOM

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concentration (3.2 g l−1), because this factor is better suited to forest soils.23,24


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6-line ferrihydrite was synthesized according to the method of Schwertmann and

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Cornell.25 A 20-g sample of Fe(NO3)3·9H2O was dissolved in 2 l of preheated distilled

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water while rapidly stirring. The solution was kept at 75°C for 10–12 min and thereafter

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rapidly cooled to room temperature. The suspension was transferred to a dialysis bag (cut-

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off: 12–14 k Da; Spectrum Laboratories Inc., Rancho Dominguez, CA, USA) and

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dialyzed against MilliQ water until the electrical conductivity of the equilibrium solution

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was < 5 µS m−1. The solid concentration of ferrihydrite in the final suspension was 2.1 g

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l−1, while the pH of the suspension was 5.7. The suspension was thoroughly purged with

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N2 to remove carbonate species in solution or adsorbed on the ferrihydrite surfaces. This

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nitrogen purge was repeated before each experiment. The 6-line ferrihydrite structural

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identity of the material has previously been confirmed by X-ray diffraction (XRD)26.

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Dissolved organic matter/ferrihydrite suspensions preparation. DOM was mixed with

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ferrihydrite nanoparticles at DOM/ferrihydrite volume ratios of 9.9/0.1, 9.5/0.5, 9/1, 7/3,

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and 5/5. Herein, the notation DOM/Fh refers to these solution ratios, while Fh-DOM is

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used as a generic notation of Fh and DOM containing samples. The samples for the

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scattering measurements were prepared by mixing an appropriate volume of a ferrihydrite

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suspension into a DOM solution. This mixture was shaken for 5 minutes with a

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mechanical shaker. The SAXS measurements were performed after 30 h from preparation.

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Due to practical reasons the TEM samples were mixed for longer periods of time between

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3 and 4 weeks. However, SAXS data indicated no or only minimal changes between

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scattering profiles, of a selected sample, obtained after 30 hours and 1 month from sample

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preparation (Figure S5 in SI).

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Scattering techniques.


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Static light scattering (SLS). The setup used for SLS measurements was an ALV/DLS/SLS-

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5022F, CGF-8F-based compact goniometer system (ALV-GmbH, Langen, Germany), with a 22-

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mW He-Ne laser as the light source. The laser operates at 632.8 nm; its intensity is varied using a

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software-controlled attenuator. A vertical polarization is achieved using a Glan laser polarizer

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prism, with a polarization ratio of better than 105 in front of the temperature-controlled cell

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housing. The scattering cells were made of borosilicate glass (10-mm inner diameter) and were

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immersed in a thermostated bath filled with a refractive index matched liquid (cis-

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decahydronaphthalene). The temperature was controlled using a F32 Julabo heating circulator

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(Jubalo GmbH, Seelbach, Germany), which kept the bath at 25°C with an accuracy of ca. 0.1°C.

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The unpolarized scattered light was collected with a detection unit, comprising a near-

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monomodal optical fibre and two high-quality avalanche photodiodes placed in a pseudo-cross

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geometry. The rotary table of the goniometer covers the range of scattering angles (θ) between

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30° and 140°. The background subtracted scattering intensity I(q) was converted to an absolute

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scale, using ∆I ( q )  n  I ref ( q )  nref

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  Rref 

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I (q) =

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where n is the refractive index of the solution, while Iref(q), nref, and Rref are the scattered

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intensity, refractive index, and Rayleigh ratio of the reference, toluene. Here, q is the scattering

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vector magnitude given by q=(4πn/λ)sin{θ/2}, where λ is the laser wavelength and θ is the

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scattering angle.

(1)

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Small-angle X-ray scattering (SAXS). SAXS measurements were performed using a

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SAXSLab Ganesha 300XL instrument (SAXSLAB ApS, Skovlunde, Denmark), a pinhole

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collimated system equipped with a Genix 3D X-ray source (Xenocs SA, Sassenage, France). The


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scattering intensity I(q) was recorded with the detector placed at three sample-to-detector

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distances, yielding scattering vectors (q) of 0.004–1 Å−1. Samples were sealed at room

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temperature in a 1.5-mm diameter quartz capillary (Hilgenberg GmbH, Malsfeld, Germany). In

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all cases, the temperature was controlled by an external recirculating water bath fixed to 25°C,

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with an accuracy of ca. 0.2°C. The two-dimensional (2D) scattering pattern was recorded using a

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2D 300 k Pilatus detector (Dectris Ltd., Baden, Switzerland) and radially averaged using

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SAXSGui software to obtain I(q). The measured scattering curves were corrected for solvent

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scattering. In addition to samples at various pH, samples of Fh aged at pH of 3.7 and 5.7 were

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measured.

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Dynamic Light Scattering (DLS) and Electrophoretic Mobility Measurements. The

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Zetasizer Nano ZS instrument (Malvern Instruments, Ltd., Worcestershire, UK) was used for

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DLS measurements at θ = 173°, as well as electrophoretic mobility measurements. The

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goniometer system was equipped with a 4-mW He−Ne laser and an automatic laser attenuator,

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and the detector was an avalanche photodiode. The temperature was set to 25°C. Three

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consecutive DLS measurements were performed on the same solution. The hydrodynamic radius

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(RH) was determined using the Stokes–Einstein equation: k BT 6πη0 D

(2)

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RH =

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where kB is the Boltzmann constant, T is temperature, η0 is the solvent viscosity, and D is the

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diffusion coefficient.

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The solutions were filled into disposable folded capillary cells (Malvern Instruments), and

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measurements were performed at a fixed scattering angle of 173° using a laser interferometric

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technique (laser Doppler electrophoresis). This technique facilitates determination of the

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electrophoretic mobility.27 The electrophoretic mobility can be expressed using Henry’s


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equation28: ue = (2εrε0ζ/3η0)f(κR), where ζ is the zeta potential at the particle surface, εr is the

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dielectric constant of the medium, ε0 is the permittivity of the vacuum, and η0 denotes the solvent

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viscosity. The measured electrophoretic mobility values were averaged over three consecutive

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measurements. The ζ values were calculated using the Smoluchowski approximation for aqueous

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solutions having moderate electrolyte concentrations.

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Spectroscopic techniques.

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X-ray photoelectron spectroscopy (XPS). XPS measurements were performed with a PHI X-

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Tool scanning XPS microprobe (Physical Electronics Inc., Chanhassen, MN, USA). A

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monochromatic Al Kα X-ray source (hν = 1486.7 eV) with a spot size of 100 µm2 was used to

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scan each sample, while the photoelectrons were collected at a 45° take-off angle. The

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calibration was made using adventitious carbon C1s XPS peak at 284.6 eV as a reference.

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Analysis of the spectra was carried out using PHI MultiPak 8.2 C software. The ferrihydrite-

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DOM samples were prepared by centrifugation (13000 g for 15 min; Biofuge 13 Centrifuge,

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Heraeus, Hanau, Germany) and were subsequently rinsed with MilliQ water. The residue was

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dried on pre-burned (400°C for 3 h) glass fibre filters (Grade GF/F; Whatman, Maidstone, UK),

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before being introduced into the spectrometer. The pure ferrihydrite suspension was dried on pre-

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burned (400°C for 3 h) Al foil and analyzed using the same protocol.

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Nuclear magnetic resonance (NMR) and infrared spectroscopy (IR) techniques. The 1H

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spectrum was recorded on a Bruker Avance II 200 MHz spectrometer (Bruker, Billerica,

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MA, USA), equipped with 25-mm broadband probe, optimized for 1H observation. The

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freeze-dried DOM was dissolved in D2O, and the residual HDO peak was used as a

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reference to calibrate chemical shifts (see supporting information for more details).


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IR spectra were collected under vacuum with a Bruker IR spectrometer (VERTEX 80v;

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Bruker), equipped with an attenuated total reflectance (ATR) accessory comprising a diamond

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crystal. All IR spectra represent the average of 128 scans, recorded at a resolution of 4 cm−1. The

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DOM sample was prepared as an aliquot of acidified DOM solution (10 µl; pH 2), which was

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applied to the ATR crystal and dried under N2 to generate a film. To obtain separate information

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on the large aggregates present in the DOM solution, low molecular weight organic compounds

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were removed via dialysis against MilliQ water using a dialysis tube with a cut-off of 12–14 kDa

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(Spectrum Laboratories). Subsequently, the remaining large size fraction of DOM was subjected

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to IR analysis according to the procedure described above. Pure ferrihydrite was analyzed in a

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similar manner but without adjusting the pH. The Fh-DOM sample was prepared by

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centrifugation (13000 g for 15 min; Biofuge 13 Centrifuge) and rinsed once with MilliQ water to

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separate it from non-adsorbed DOM. The solid residue was re-suspended in 10 µl MilliQ water,

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transferred to and dried onto the ATR crystal, as described above. Figure S1 in the supporting

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information (SI) shows 1H NMR and FTIR spectra.

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Cryogenic-Transmission Electron Microscopy (cryo-TEM).

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Samples for cryo-TEM were prepared using a Leica EM GP immersion freezer, where the

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environmental chamber was kept at 20°C and 80% RH. A 4-µl drop of sample was placed onto a

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hydrophilized (oxygen plasma treated using Balzers SCD 004; Optics Balzers AG, Balzers,

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Lichtenstein) lacey carbon coated copper grid (Ted Pella, Inc., Redding, CA, USA) and blotted

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(one-sided, back side blotting) with No.1 Whatman filter paper before being plunged into liquid

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ethane (−184°C). Samples were stored in liquid nitrogen until further use. A Fischione Model

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2550 cryo transfer tomography holder was used to transfer the specimen into the transmission

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electron microscope (JEM 2200FS; JEOL Ltd., Tokyo, Japan), operated at 200 kV and equipped


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with an in-column energy filter (Omega filter). Zero-loss images were digitally recorded with a

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TVIPS F416 camera (TVIPS GmbH, Gauting, Germany), using SerialEM software29 under low

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dose conditions with a 10 eV energy selecting slit in place.

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RESULTS AND DISCUSSION

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Dissolved organic matter. Typically, DOM is a complex mixture of different chemical

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compounds. This chemical complexity was evident in our DOM spectra from IR and NMR,

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presented in Figure 1 and ESI Figure S1. These spectroscopic data indicate the presence of

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saturated and unsaturated fatty acids, amino acids, alcohols, carbonyl and carboxylate groups as

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well as aromatics and carbohydrate structures.

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The cryo-TEM images revealed colloidal DOM consisting of ca. 100 nm structures (Figure 1).

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These objects appear to be elongated in these 2D projections, but it is not possible to determine

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their precise shape. In addition to the colloidal DOM, resonances in the high resolution 1H NMR

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spectrum (SI Figure S1) indicate that part of the organic matter existed as smaller aggregates or

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dissolved molecules, since colloidal DOM is supposed to have short spin-spin relaxation time,

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T2, resulting in broad peaks; this fraction is denoted as molecular DOM herein. This fraction of

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DOM should consists of compounds having a molecular weight less than 12.5 kDa accordingly

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to Wang et al. 2017.30 Comparison between the IR spectra before and after dialysis (Figure 1B)

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showed that all bands decreased in intensity relative to those between 1100 and 1000 cm−1 after

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dialysis. The main origin of bands in this region is vibrational modes of carbohydrates,

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suggesting this class of compounds dominated the colloidal DOM fraction.

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The scattering experiments report on colloidal structures averaged over a much larger volume

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than that observed in cryo-TEM. In the case of SAXS, the data cover a q-range of 0.004–1 Å−1


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(Figure 2), probing real space length scales from approximately 1 to 100 nm. At q-values

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between 0.004 and 0.05 Å−1, a scattering power law, I(q)~q-m, with m ≈ 1.7, was observed. The

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power law exponent, i.e., the Porod exponent, indicates the dimensionality of these objects.31

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Thin elongated objects, as observed under 2D cryo-TEM, have m = 1, whereas sheets would

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scatter as q−2 and homogeneous three-dimensional objects would show asymptotic q−4 decay of

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the scattering intensity.

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Based on these SAXS results, we concluded that colloidal DOM has an open fractal structure,

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with a fractal dimension of 1.7. This is similar to what has been reported in other studies of soil

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organic matter.12,13,32,33 A value of 1.7 is consistent with an open network structure, and is close

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to the value for flexible polymer coils.31

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Figure 1. Cryo-transmission electron microscopy images of dissolved organic matter (DOM) at

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pH 3.7. Images (A) and (C) are different regions of the same sample, while (D) is a magnified

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area of (C). Infrared spectra of the same DOM and dialyzed products are shown in (B).


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Although a minor feature, the small peak around q = 0.1 Å−1 indicates the presence of objects with a size of around 1 nm.

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The size of the colloidal DOM was estimated from DLS measurements. This was

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accomplished by analyzing the initial slope of the correlation function (evaluated at t = 0), which

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yielded an average hydrodynamic radius of ca. 90 nm (Figure 2, bottom panel; see SI for

249

further details). To confirm this value, we determined its size using SLS by applying the Guinier

250

approximation, which states that the scattered intensity can be written as I(q) = I(0)exp{-

251

2q2/3}, where Rg is the radius of gyration.31 From a Guinier plot, lnI(q) vs. q2, was

252

evaluated to be ca. 90 nm (Figure 2, inset), similar to the RH value obtained from DLS. Thus, we

253

deduced that the colloidal DOM has an average size of 90 nm.

254 255

Figure 2. Small-angle X-ray scattering (SAXS) profile of dissolved organic matter (DOM),

256

where the red line represents a power-law fitting. The top right inset shows the Guinier plot for

257

the static light scattering data, in which the red line is a linear fitting to obtain the gyration radius

258

using the Guinier approximation31. The bottom left inset shows the dynamic light scattering data,

259

in which the red line is a linear fitting of the initial decay of the correlation function to obtain the


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average diffusion coefficient and subsequently the average hydrodynamic radius from the

261

Stokes-Einstein equation.

262

Ferrihydrite. The SAXS profiles of Fh dispersions aged for 1 or 9 months after synthesis at

263

pH 5.7 were distinctly different (Figure 3). The 1-month aged sample displayed high scattering

264

intensity at low q-values below 0.04 Å−1, demonstrating the presence of large clusters of

265

nanoparticles, i.e., Fh aggregates, with sizes larger than hundreds of nm. In contrast, the sample

266

aged for 9 months had lost this low-q scattering peak, showing that the large aggregates had been

267

dispersed as individual nanoparticles. This dispersion was also confirmed by cryo-TEM (Figure

268

3). Moreover, the scattering curves of the 1- and 9-month aged samples perfectly overlap for q >

269

0.04 Å−1, indicating that the same primary Fh nanoparticles occurred in both samples. Clearly,

270

the larger aggregates disaggregated into primary particles over time.

271

A transition from ferrihydrite to goethite was not detected in any of the samples. Goethite

272

would have been easily detected in SAXS profiles, because it generally presents as long needle-

273

like particles,34 clearly distinguishable from the small quasi-spherical Fh particles.

274

To further analyze the scattering intensity, the approximation in which the intensity can be

275

written as a product of the average form factor, i.e., average single particle scattering function

276

and an effective structure factor Seff(q) was made. This yields information on the average

277

relative positions of the particles, and on interparticle interactions.31 c

278

I (q) =

279

Here, c/ρ = ϕ is the particle volume fraction, where c is the concentration (e.g. in g cm−3) and ρ

280

is the mass density, given in the same units. In addition, ∆b is the scattering length density

281

difference between particles and solvent, and v is the single particle volume. In this case,

282

describes the primary nanoparticles, modeled here as polydisperse spheres having an average

ρ

v∆b 2 Seff ( q ) P ( q )

(3)


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radius of 3 nm. The polydispersity, described by the relative standard deviation of a

284

Gaussian distribution, is estimated to be 35%. The cryo-TEM images clearly show that the

285

particles have an irregular shape, and this shape polydispersity contributes significantly to the

286

effective distribution in radius. Most particles have a radius of 3–4 nm in the cryo-TEM images.

287

The clusters observed in the 1-month aged sample can be described in terms of the Teixeira

288

fractal structure factor.35 The model calculation is shown as a solid line in Figure 3. The fractal

289

dimension obtained was 1.7, similar to what was observed in a previous study.36 We return to a

290

discussion of this value below.

291

In the fully dispersed state after 9 months, I(q) decreases at lower q vectors (S(0) < 1),

292

indicating repulsive inter-particle interactions. We attribute this long-range repulsion to particle

293

charge.2,37 The z-potential of the dispersion was 40 mV at pH 3.7 (SI). The I(q) also had a small

294

peak at qmax ≈ 0.01 Å−1 that can be interpreted as Seff(q), suggesting that the average nearest

295

neighbor distance is 2π/qmax ≈ 60 nm between particles. This is consistent with the average

296

particle separation observed in the cryo-TEM images (insert, Figure 3).

297

For charged particle dispersions, the Hayter and Penfold structure factor,38,39 based on the

298

mean spherical approximation, is often used to describe the solution structure for charged

299

colloids. However, the effective particle charge within this model should often be considered

300

mainly as a fitting parameter.38 Applying this model to Seff(q), we fitted the SAXS profiles

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obtained after 9 months, using SASView.39 Our results, together with the cryo-TEM images and

302

SAXS profiles collected after 9 months aging, reveal that single nanoparticles dominate the

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solution (Figure 3).


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Figure 3. Normalized small-angle X-ray scattering (SAXS) profiles of ferrihydrite (Fh) at pH

306

5.7 aged for 1 month and 9 months following synthesis. The 1-month aged sample was also

307

analyzed with static light scattering and these data were adjusted with the KSAXS/KSLS constant

308

(see electronic supplementary information). The red line is a fractal model fit, assuming a

309

polydispersity of 0.35 for the primary unit (i.e., a Fh sphere of average radius ≈ 3 nm). The

310

blue line is a model based on the sphere form factor with a polydispersity of 0.35 incorporating

311

the Hayter-Penfold38,39 structure factor. In the fitting procedure, the charge was 9e and dielectric

312

constant was 80. The bottom left inset shows a cryo-transmission electron microscopy image of

313

the 9-month aged Fh sample.

314

Guénet et al.11 have recently described Fh aggregation and interaction with organic matter.

315

Their SAXS profiles for aggregated Fh particles are qualitatively very similar to the SAXS

316

profiles obtained for the 1-month aged sample (Figure 3). However, Guénet et al. suggested a

317

slightly different model to explain their data. They proposed a smaller primary particle of 0.8

318

nm, with hierarchical aggregation of small and large clusters. In contrast, we have here identified


16

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3 nm particles as the primary Fh unit, which aggregates into clusters. These 3 nm particles are

320

the ones detected, when the larger clusters have been dispersed.

321

Previously, the formation of Fh aggregates has been shown to be pH-dependent,43 and because

322

our DOM had a pH of 3.7, we were motivated to compare DOM-Fh behavior with pure Fh at the

323

same pH values. SAXS profiles showed that lowering the pH to 3.7 produced rapid dispersion of

324

Fh aggregates into primary nanoparticles (Figure 4). As can be seen from loss of scattering

325

intensity below q = 0.02 Å−1, only a very small number of the large aggregates remained one day

326

after pH adjustment. Six days after the pH adjustment, the Fh aggregates appeared to have

327

completely dispersed into 3-nm primary Fh particles. These observations are supported by cryo-

328

TEM images, which only show primary Fh nanoparticles after pH adjustment (Figure 4). The

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SAXS profile of the Fh samples at pH 3.7 exhibited a structure factor peak, although this was

330

less pronounced than the Fh samples at pH 5.7 (Figure 3). This difference is most likely caused

331

by the higher ionic strength at pH 3.7, which screens long-range interactions.

332 333

Figure 4. Small-angle X-ray scattering (SAXS) profiles of ferrihydrite (Fh) adjusted to pH 3.7;

334

just after pH adjustment, and 30 min, 1 d and 6 d after pH adjustment. The red line is a model

335

based on the sphere form factor31 with a polydispersity of 0.35 along with the Hayter-Penfold


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Page 18 of 34

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structure factor for the ferrihydrite after six days from pH adjustment to 3.7 (steady state). The

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inset at the at the top right is a linear-linear plot of the same data at steady state, while the inset at

338

the bottom left is the corresponding cryo-transmission electron microscopy image.

339

Ferrihydrite-dissolved organic matter mixtures. SAXS profiles of a suspension containing

340

equal volumes of dispersed Fh at pH 3.7 and DOM showed an increased scattering intensity at q-

341

values below 0.15 Å−1 (Figure 5A). Similar results were obtained at DOM/Fh ratios of 9.9/0.1,

342

9.5/0.5, 9/1, and 7/3 (SI, Figure S4). This increased intensity indicates that DOM induced re-

343

aggregation of the primary Fh nanoparticles into larger aggregates.

344

We also investigated DOM/Fh suspensions prepared from the 1-month aged Fh sample at pH

345

5.7, which still contained large Fh aggregates (Figure 5B). The final pH of these suspensions was

346

close to the initial pH of 3.7 of the DOM. Comparison of the scattering intensity normalized to

347

the Fh concentration of the 1-month aged Fh sample in the absence and presence of DOM shows

348

perfectly overlapping scattering curves at q < 0.1 Å−1. This demonstrates that the larger fractal

349

aggregates were retained, after mixing with the DOM. Thus, the presence of DOM did not lead

350

to any rearrangement or change of the packing of Fh nanoparticles in these solutions. For q > 0.1

351

Å−1, the DOM contribution to the scattering dominates, producing the differences between the

352

spectra for the pure Fh and Fh-DOM mixtures (Figure 5B).

353

In Figure 5B, we can compare both pH 5.7 Fh and Fh-DOM samples containing fractal

354

aggregates and primary nanoparticles to the pH 3.7 Fh-DOM sample, in which aggregation was

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induced by adding DOM, as discussed above. There is a total agreement between the scattering

356

curves of both Fh-DOM samples over the whole q-range, and among all three curves below q

Ferrihydrite Nanoparticle Aggregation Induced by Dissolved Organic

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