Aqueous Aggregation Behavior of Engineered Superparamagnetic

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Aqueous Aggregation Behavior of Engineered Superparamagnetic Iron Oxide Nanoparticles: Effects of Oxidative Surface Aging Wenlu Li, Seung Soo Lee, Anjuliee M. Mittelman, Di Liu, Jiewei Wu, Carl H Hinton, Linda M. Abriola, Kurt D Pennell, and John D. Fortner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04130 • Publication Date (Web): 04 Nov 2016 Downloaded from http://pubs.acs.org on November 7, 2016

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Environmental Science & Technology

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Aqueous Aggregation Behavior of Engineered

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Superparamagnetic Iron Oxide Nanoparticles:

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Effects of Oxidative Surface Aging

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Wenlu Li, † Seung Soo Lee, † Anjuliee M. Mittelman, §Di Liu, † Jiewei Wu, † Carl H. Hinton, †

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Linda M. Abriola, § Kurt D. Pennell, §and John D. Fortner*, †

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†

Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA §

Department of Civil and Environmental Engineering, Tufts University, Medford, MA 02155, USA

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Submitted to

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Environmental Science and Technology

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2016

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*To whom correspondence should be addressed:

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John D. Fortner: Tel: +1-314-935-9293; Fax: +1-314-935-5464; Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

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For successful aqueous-based applications, it is necessary to fundamentally understand and

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control nanoparticle dispersivity and stability over a range of dynamic conditions, including

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variable ionic strengths/types, redox chemistries, and surface ligand reactivity/degradation states

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(i.e. surface aging). Here, we quantitatively describe the behavior of artificially aged, oleic acid

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(OA) bilayer coated iron oxide nanoparticles (IONPs) under different scenarios. Hydrogen

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peroxide (H2O2), used here as a model oxidant under both dark and light ultra-violet (UVA)

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conditions, was employed to ‘age’ materials, to varying degrees, without increasing ionic

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strength. Short-term stability experiments indicate that OA-IONPs, while stable in the dark, are

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effectively de-stabilized when exposed to UVA/H2O2/OH based oxidation processes. Compared

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to bicarbonate, phosphate (1.0 mM) has a net stabilizing effect on OA-IONPs under oxidative

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conditions, which can be attributed to (surface-based) functional adsorption. Corresponding

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aggregation kinetics in the presence of monovalent (Na+) and divalent cations (Ca2+) show that

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attachment efficiencies (α) are strongly dependent on the cation concentrations/types and degree

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of surface aging. Taken together, our findings directly highlight the need to understand the

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critical role of particle surface transformation(s), via oxidative aging, among other routes, with

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regard to the ultimate stability and environmental fate of surface functionalized engineered

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nanoparticles.


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KEYWORDS

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Engineered

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aggregation, nanoparticle stability, nanoparticle aging, surface oxidation

nanomaterials,

superparamagnetic

iron

oxide

nanoparticles,

nanoparticle


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INTRODUCTION

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The development and production of engineered iron oxide nanoparticles (IONPs) are

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motivated by their broad material applications in biomedical research, high-capacity data

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storage, advanced drug delivery, magnetic resonance imaging (MRI), magnetic separations,

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environmental sensing, and remediation technologies, among others.1-11 For engineered

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nanomaterials with large surface-to-volume ratio and high surface energies in water, surface

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modification is required to avoid (or reduce) aggregation, (surface) attachment, and

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sedimentation phenomena.1,

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including ligand addition and/or ligand exchange (e.g., stabilizing agents coated onto

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nanoparticle surfaces), which can alter wettability, surface charge, and hydrophobicity, thus

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enhancing the monomeric stability of nanoparticles.3, 5, 15-17 For example, Tombácz et al. found

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that, at a moderate surface coverage (1-2 mmol/g), carboxylated (e.g., citric acid, gallic acid, and

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poly(acrylic acid)) magnetite nanoparticles have a critical coagulation concentration (CCC) of

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approximately 500 mM NaCl compared to the CCC of 1 mM NaCl for bare magnetite.18 Our

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previous work demonstrated that engineered oleic acid bilayer coated iron oxide nanoparticles

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(OA-IONPs) have relatively high CCC values of 710 mM NaCl and 10.2 mM CaCl 2 at neutral

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pH.3 For these types of particles, ligands are strongly adsorbed onto particle surfaces through

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either physisorption or chemisorption, whereby the adsorption process is irreversible and/or the

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desorption process is minor under relevant conditions.19 However, desorption or degradation of

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attached ligands may occur in the environment under more reactive or extreme conditions.6, 20

5, 6, 12-14

This can be accomplished via a number of methods,

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Upon inadvertent release to and/or intended applications within environmental systems, such

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nanoparticles may be exposed to reductive or oxidative surface reactions, including photo-

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catalyzed reactions.4,

20-24

Surface reactions leading to particle destabilization in water,


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specifically particle-particle aggregation and surface deposition processes, will likely play a

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critical role in the material’s behavior over time and, in the case of technical applications, will

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also affect performance regimes.16,

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surface aging processes of nanomaterials is key to predicting the ultimate material fate and

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transport behavior, and thus the environmental volume/endpoints affected.4,

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Typically, for charge stabilized particles, elevated ionic strengths can compress the (particle

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surface) electrical double layer, thus lowering the energy barrier required for aggregation and/or

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surface deposition. Noting that the critical coagulation concentration (CCC) of our

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aforementioned OA-IONPs is much higher than the concentration of common cations found in

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typical surface waters, it may be expected that these (and other similar) nanoparticles can remain

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stable for extended periods of time.3 In such scenarios, particle surface reactions, including

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oxidation processes and potential sunlight irradiation (photo-enhanced chemistries), are likely to

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affect ultimate particle stabilities. Sun et al. observed that the stability of TiO2 nanoparticles

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decreased more than 27-fold, as measured by aggregation rates, following 50 h of UV irradiation,

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which was attributed to the changes in surface hydroxyl groups.25 Conversely, when nC60

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nanoparticles were subjected to extensive UVA irradiation (up to 7 days), their stability (in the

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presence of NaCl) increased significantly due to an effective increase in surface charge.30 A

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recent study by Li et al. demonstrated that surfactant coated (PVP and citrate) Ag NPs show

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enhanced colloidal stability compared to bare Ag NPs under natural solar irradiation and UV

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irradiation, highlighting the importance of surface passivation with regard to stability,

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dissolution, and photo-transformation.31

20, 24-27

In the case of inadvertent release, understanding

6, 20, 26, 28, 29

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The objective of this study is to elucidate the effect of (UVA) light irradiation and chemical

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oxidation on the stability of OA-IONPs under varied water chemistries. Short-term stabilities of


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these nanoparticles was quantitatively investigated in the presence of two ubiquitous aqueous

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buffers, bicarbonate and phosphate, as a function of irradiation time, cation type, and ionic

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strength using time-resolved dynamic light scattering (TR-DLS). The organic aging products

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were further probed via gas chromatography–mass spectrometry (GC-MS) and Fourier transform

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infrared spectroscopy (FTIR) analyses. Lastly, we explore and propose a simplified aging

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mechanism/pathway through (artificially) synthesizing and evaluating a model product (mixed

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bilayer coatings) for comparison. To our knowledge, this is the first study to describe the short-

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term stability and aggregation behaviors of engineered OA-IONPs upon exposure to

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environmentally relevant oxidative aging conditions. Findings highlight the importance of

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recognizing and understanding particle surface aging as it relates to the environmental fate and

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transport of engineered nanoparticles.

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MATERIALS AND METHODS

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Materials: Iron(III) oxide (hydrated, catalyst grade), 1-octadecene (technical grade, 90%),

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oleic acid (technical grade, 90%), sodium chloride (ACS reagent, ≥99.0%), calcium chloride

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dihydrate (ACS reagent, ≥99%), sodium hydroxide (ACS reagent, ≥97.0%), sodium bicarbonate

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(ACS reagent, 99.7-100.3%), sodium phosphate dibasic (ACS reagent, ≥99%), sodium phosphate

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monobasic (ReagentPlus, ≥99.0%), hydrogen peroxide solution (TraceSELECT, ≥30%) and

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nitric acid (trace metal grade) were purchased from Sigma-Aldrich. Reagent grade ethanol,

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acetone and hexane were used as received without further purification.

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Preparation and Characterization of OA-IONPs: Monodisperse IONPs were first

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synthesized by high temperature pyrolysis of iron carboxylate salts in the presence of organic

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solvent.2 To prepare 8 nm IONPs, 2 mmol FeO(OH) fine powder, 2 mmol oleic acid, and 5 g 1-

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octadecene (1-ODE) were mixed intensively in a three-necks flask under ultra pure argon. The


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mixture was then heated gradually (5-10 °C/min) to 240 °C to dissolve the FeO(OH) and further

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heated to 320 °C to decompose the iron carboxylate salts formed in the previous procedure. The

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reaction was maintained at 320 °C for 1-3 h before removing the heating mantle. After cooling

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down to room temperature, the black colloid was purified with a standard hexane/acetone

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procedure five times.2 Purified IONPs were transferred and stored in hexane. In order to disperse

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the synthesized IONPs into water, an oleic acid bilayer (surface) structure was employed, as

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described previously.3, 32 Approximately 20 µL of pure oleic acid was added to 1 mL of IONPs

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in hexane and the mixture was sonicated for 1 min using a bath sonicator (Branson 2510,

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Danbury, CT), followed by the addition of 10 mL of ultrapure water (Millipore, 18.2 MΩcm). A

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probe sonicator (Qsonica, Q-700, Taperd microtip, 2 mm) was then utilized for extensive mixing

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of the hexane and water phases at 50% amplitude for 5-10 min. Through a fast centrifugation

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(6,000 g) step, the sonicated emulsion was separated into two phases with nanoparticles in the

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bottom water phase. The aqueous phase was then collected and the nanoparticles were purified

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via centrifugation, redispersion, and filtration through a syringe filer (polyethersulfone, pore size

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of 0.2 µm, Millipore).

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Nanoparticle Characterization: The size and morphology of IONPs and OA-IONPs were

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characterized by transmission electron microscopy (TEM, FEI Tecnai G2 Spirit) operated at 120

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kV. A small drop (10-20 µL) of suspension was placed on a carbon-coated copper grid (300

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mesh, Electron Microscopy Sciences) and dried under N2 flow. In order to calculate the size

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distribution of IONPs, randomly chosen nanoparticles (> 2,000) from several different TEM

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micrographs were selected and analyzed using ImageJ software (National Institutes of Health).

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To determine the iron concentration in samples, the IONPs were digested using concentrated

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nitric acid and analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Agilent


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7500ce). The concentration of the water stable OA-IONPs stock suspension was measured and

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set to 100 mg/L as Fe. The hydrodynamic diameter and zeta potential of OA-IONPs were

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measured by dynamic light scattering (DLS) with a He-Ne laser operated at the wavelength of

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633 nm (Zetasizer, Malvern Nano ZS, Malvern, UK). All size and zeta potential measurements

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were conducted in triplicate at room temperature (22 °C).

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UV Initiated Reactions: UVA light was used to mimic the actual sunlight exposure in real

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environmental systems. A custom build UV-reactor equipped with two UV lamps (351 nm, BHK

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Inc. CA) was used to achieve a total irradiation intensity of 2000 ± 50 mW/cm2 in the middle of

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the reactor, as measured by a radiometer (UVP, Inc. CA). Ambient air was circulated through the

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photo-reactor to maintain the temperature at 22 °C. For each experiment, 50 mL of 8nm OA-

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IONPs solution was added to a 100 mL quartz vial (Technical Glass Products, OH). The final

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concentration of OA-IONPs was 50 mg/L as Fe. Varied amounts of H2O2 were then added to

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initiate oxidative aging reactions. H2O2 was used as a model oxidant as it does not increase the

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ionic strength of the system and thus eliminates potential aggregation of nanoparticles by the

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oxidant (which also acts as an ion) itself. Bicarbonate and phosphate buffers were used to adjust

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the initial pH to 7.5 and the reactions were allowed to run continuously for 7 days. Samples were

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collected periodically and evaluated for size, zeta potential, and particle stability. Control

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experiments were conducted by wrapping samples with aluminum foil to avoid exposure to light

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irradiation. UV-Vis spectra (200-800 nm) of the samples containing H2O2 were analyzed by

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using a Varian Cary Bio50 UV-Visible spectrometer. The concentration of H2O2 was determined

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by TiSO4 spectrophotometry at low concentrations following methods described previously.33, 34

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Solvated products were analyzed using attenuated total reflection Fourier transform infrared

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spectroscopy (Nexus 470 FTIR, Thermo Nicolet, NC) with a ZnSe trough. Liquid samples in the


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trough were dried under vacuum at room temperature for one day. The transmittance spectra

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were recorded from 800-3600 cm-1 with spectral resolution of 1 cm-1. Aging products were

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extracted and analyzed by GC-MS (Hewlett Packard model 7890A, Agilent Technologies)

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equipped with a DB5-MS column (J&W Scientific) and a mass spectrometer (5975C, Agilent

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Technologies). The GC program was as follows, the column was equilibrated at 80 °C for 1 min,

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then ramped to 280 °C at 30 °C/min, and was held at constant temperature for 3 min. The mass

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spectra of the products were analyzed using Enhanced Data Analysis software (Agilent

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Technologies).

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Aggregation Kinetics: DLS measurements were carried out using a Nano ZS Zetasizer

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(Malvern Instruments, UK) equipped with a photodetector at a fixed scattering angle of 173°.

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The OA-IONPs stock solution was diluted to 1 mg/L for the DLS experiments and pH was

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adjusted to 7.5. For each measurement, a predetermined volume of diluted nanoparticle

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suspension was added into a polystyrene vial. After that, a certain amount of electrolyte stock

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solution was injected into the vial to a total sample volume of 1 mL. After vortexing (1.5 s),

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samples were immediately transferred to the DLS chamber and measured. Size measurements

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were collected every 15 s and recorded continuously from 20 to 60 min, depending on the

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aggregation rate of each sample.

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The attachment efficiency (α) of the OA-IONP aggregates in the presence of various

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electrolyte concentrations was calculated by the following equation35-37 where k is the initial

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aggregation rate constant at any salt concentration, kfast is the aggregation rate constant under

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diffusion-limited (fast) aggregation conditions, N0 is the initial particle concentration and Dh(t) is

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the intensity-weighted hydrodynamic diameter of nanoparticles at time t.


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(1)

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Fast aggregation rate constants in diffusion-limited regimes can be calculated under relatively

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high ionic strengths (above the CCC point). In reaction-limited regimes (i.e., below the CCC), an

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increase in electrolyte concentration will further compress the electric double layer, thus

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decreasing the electrostatic repulsion forces between nanoparticles and promoting particle

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aggregation. The CCC refers to the point at which the energy barrier to aggregation is effectively

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screened. Above the CCC, the addition of electrolyte will fully destabilize the nanoparticles and

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the aggregation becomes solely dependent on the number of particle collisions: this is referred to

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as the diffusion-limited regime. At points above the CCC, the aggregation rate becomes

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independent of salt concentration and the attachment efficiency is equal to 1. Nanoparticles with

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a lower CCC are more prone to aggregation and thus exhibit a low degree of stability compared

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to nanoparticles with a high CCC in the same electrolyte solutions. CCC values are often used to

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directly compare and predict the stability of nanoparticles in real or simulated environmental

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scenarios.4

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Phosphate Sorption: OA-IONPs (50 ppm as Fe) were analyzed for phosphate sorption at

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phosphate concentrations ranging from 0 to 1200 mg/L as PO43-. The solution pH was adjusted

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with HNO3 and/or NaOH to pH 7.5. After 48 h of equilibrium, OA-IONPs were separated using

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ultracentrifugation at 60,000 g for 2 h and the remaining concentrations of phosphate in the

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supernatant solution were analyzed by an inductively coupled plasma optical emission

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spectroscopy (ICP-OES, Perkin Elmer Optima 7300DV). All measurements were conducted in

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triplicate. The measured phosphate sorption capacity as a function of equilibrium concentration

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of phosphate (mg/L) was fit using the Langmuir isotherm equation:


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(2)

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where qe is the amount of adsorbed phosphate at equilibrium concentration (mg/g), k is the

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sorption constant, qmax is the maximum sorption density (mg/g, mass of the sorbed phosphate per

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mass of Fe), and Ce is the equilibrium concentration of phosphate.

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RESULTS AND DISCUSSION

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Characterization of IONPs

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Highly monodispersed, spherical IONPs were synthesized by thermal decomposition of iron

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precursor (iron carboxylate) in an organic solvent at 320 °C, as reported previously.2, 8 TEM

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micrographs of 8 nm (8.1 ± 0.6 nm) IONPs are shown in Figure 1a. These engineered

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nanoparticles are highly reproducible while remaining single domain, single crystalline, and

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highly monodispersed with a very narrow size distribution (Figure 1b).3 As described by our lab

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and others, a highly effective oleic acid bilayer stabilizing strategy was incorporated to facilitate

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the transfer of IONPs from the organic phase (hexane) into water.3, 14, 32 Corresponding TEM

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micrographs (Figure 1c) show aqueous transferred IONPs remain monodisperse with no core

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size and shape change.32 The number-weighted hydrodynamic diameter of the OA-IONPs was

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measured to be 16.2 ± 1.8 nm for the 8 nm IONPs cores dispersed in hexane, which is similar to

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the value we have reported before for these materials (15.6 ± 1.3 nm).3 OA-IONPs aqueous

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suspensions are colloidally stable for over 6 months with negligible change in hydrodynamic

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diameter.14

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Short-term Colloidal Stability of OA-IONPs

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The average hydrodynamic diameter of OA-IONPs as a function of time (7 days) in the

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presence of bicarbonate or phosphate and H2O2 is shown in Figure 2. Bicarbonate and phosphate

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are two common anions in both surface and ground waters, albeit at different concentrations (10-


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to 10 mN for HCO3- and < 10-5 mN for HPO42-).38 Their concentrations can be found up to 1.5

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mM in wastewater.39 H2O2, a naturally occurring oxidant,40 was used as a model oxidant to avoid

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introducing additional ionic strength into the system (upon redox reactions). For (dark) control

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experiments (Figure 2a), OA-IONPs remain highly stable with negligible size change over the

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entire 7 days except in the presence of 10 mM phosphate, whereby the hydrodynamic diameter

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slightly increased to ca. 30 nm due to the reduced energy barrier to aggregation at elevated ionic

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strength, consistent with classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory.22

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Colloidal stability of parallel OA-IONPs solution under UVA irradiation is shown in Figure 1b.

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In the presence of bicarbonate, OA-IONPs are quasi-stable under direct UVA irradiation for up

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to 4 days in the absence of H2O2, with the hydrodynamic diameter increasing from ca. 20 nm to

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ca. 50 nm at the end of 7 days. Corresponding TEM micrographs of the aged nanoparticles (7

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day UVA exposure) also show the formation of small clusters (SI Figure S1b). The addition of

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H2O2 was found to promote the destabilization of the nanoparticles. With 0.03 wt. % of H2O2,

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OA-IONPs are severely destabilized, forming larger flocs, and settle out from the solution after 5

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days (reaching an average size of > 500 nm). The addition of a higher concentration of H2O2

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(0.12 wt. %) accelerates the destabilization process, with nanoparticles completely settling out

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from solution upon ca. 4 days of UVA irradiation. After 7 days of UVA irradiation, the OA-

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IONPs were no longer monodispersed and formed aggregate flocs visible to the naked eye under

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both conditions, as depicted in SI Figure S1. The shape and size of OA-IONPs were evaluated

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before and after aging via TEM micrographs; results indicate the existence of large OA-IONPS

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aggregates, while primary nanoparticle shape and size does not change. This observation was

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further confirmed by the ICP-OES results, as no substantial dissolved (released) iron was


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observed for any of the aging scenarios under neutral pH conditions (SI Figure S2), indicating

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minimal (particle core) dissolution during surface aging processes.21

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The colloidal stability of OA-IONPs under UVA irradiation was investigated in the presence

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phosphate concentrations of 0.1 mM, 1.0 mM, and 10 mM. Results indicate that 1.0 mM

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phosphate effectively stabilizes OA-IONPs upon 7 days of UVA irradiation without the addition

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of H2O2, while the hydrodynamic diameter of OA-IONPs only moderately increases in the

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presence of 0.1 mM and 10 mM phosphate under the same conditions (Figure 2b). Similar to

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observations from parallel bicarbonate experiments, the addition of H2O2 also promotes the

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destabilization of OA-IONPs under UVA irradiation, but to a relatively lesser extent. The

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effective rate of particle size increase (or rate of destabilization) is slower in the presence of

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phosphate buffer when compared to bicarbonate buffered systems. The presence of phosphate

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ions inhibits the decomposition of H2O2 and thus enhancing the stabilizing effect (SI Figure

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S3).33 Additionally, in the presence of 10 mM tert-butanol (t-BuOH), a highly efficient hydroxyl

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radical scavenger,29, 41, 42 the size of OA-IONPs modestly increased from ca. 20 nm to ca. 30 nm

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under UVA irradiation for 7 days. This final particle hydrodynamic diameter after 7 days was

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relatively small compared to that observed in the absence of t-BuOH, indicating hydroxyl

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radicals generated from UVA (photo)oxidation may be involved in the NPs destabilization

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process(es). It is expected that other oxygen reactive species (e.g., peroxides, superoxide, and

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singlet oxygen) may also possibly contribute to this process as well; however, the exact

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combination of oxidation/degradation mechanisms remain unclear and will be further

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investigated in future reports. Taken together, results show that a phosphate buffer concentration

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of 1.0 mM (higher than typical surface water background concentrations) provided the greatest


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stabilizing effect for OA-IONPs during UVA/H2O2 irradiation when compared to bicarbonate

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and t-BuOH under neutral pH condition.

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Aggregation Kinetics of Pristine and Aged OA-IONPs

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Aggregation kinetics of OA-IONPs before and after aging were evaluated as a function of

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ionic strength and composition using TR-DLS. The hydrodynamic diameter increase (through

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aggregation processes) of pristine OA-IONPs over time in the presence of varied NaCl and

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CaCl2 concentrations is shown in the SI Figure S4. We observe aggregation rates increase from

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0.89 nm/min to 12.69 nm/min when NaCl concentration was correspondingly increased from 100

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mM to 700 mM. However, at higher NaCl concentrations (1000 and 1400 mM), the increase in

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electrolyte concentration did not affect the aggregation rate, as the energy barrier was effectively

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eliminated, indicating a maximum aggregation rate has been reached. Similar behavior was

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observed in the presence of CaCl2 (SI Figure S4b), albeit at lower concentrations (1-50 mM).

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Calculated attachment efficiencies (α) of OA-IONPs, before and after aging, in the presence of

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NaCl and CaCl2 are presented in Figure 3. For these systems, aggregation kinetics can be

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classically divided into two regimes: reaction-limited (where α is less than 1) and diffusion-

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limited aggregation (where α is equal to 1). By extrapolating the reaction limited and diffusion

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limited regimes, the critical coagulation concentration (CCC) can be derived. For these systems,

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distinct reaction-limited and diffusion-limited regime observations indicate that particle

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aggregation before and after aging follows classical DLVO theory. In each of the experiments,

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the CCC decreases as UVA irradiation time was increased. CCC values of OA-IONPs were 763

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mM, 507 mM, 381 mM, and 174 mM NaCl for samples irradiated for 0h (pristine OA-IONPs),

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10 h, 1 d, and 3d, respectively. Similar trends were observed with CaCl2; however, due to charge

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neutralization and surface complexation effects, divalent cations, such as Ca2+, are more effective


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in reducing the surface charge, resulting in lower CCC values compared to monovalent cations

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(Na+).35, 43 CCC values of OA-IONPs in CaCl2 were 10.4 mM, 9.3 mM, 8.2 mM, and 5.8 mM for

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samples irradiated for 0 h, 10 h, 1 d, and 3 d, respectively. Decreasing CCC values indicate

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destabilization effects from extended periods of UVA irradiation: calculated CCC ratios for Ca2+

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to Na+ are 2-6.2 for pristine OA-IONPs and 2-5.8, 2-5.5, and 2-4.9 for samples irradiated for 10 h, 1 d,

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and 3d, respectively. These power terms generally follow the theoretical Schulze-Hardy rule,

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where the ratio of CCC values is proportional to z-6 to z-2 depending on the zeta potential value

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of the material (z is the valence of counterion).44

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Attachment efficiencies (α) of OA-IONPs as a function of electrolyte type upon 3 day (UVA)

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aging in the presence of phosphate and carbonate buffers are shown in Figure 4. In the presence

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of monovalent electrolyte (NaCl), OA-IONPs aged in bicarbonate buffer solution have a

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relatively low CCC value of 174 mM compared to the CCC value (249 mM) of OA-IONPs aged

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in phosphate buffer (under parallel conditions). However, in the presence of a divalent

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electrolyte (CaCl2), OA-IONPs aged in bicarbonate buffer have a relatively higher CCC value

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(5.8 mM) compared to OA-IONPs aged in phosphate buffer (3.0 mM). At neutral pH,

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bicarbonate and phosphate ions exist as HCO3- and H2PO4-, respectively. While the adsorption of

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bicarbonate anions onto the OA-IONPs was not anticipated, phosphate anions demonstrate

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favorable sorption onto the surface of OA-IONPs (SI Figure S5).45 The adsorption of phosphate

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onto OA-IONPs likely alters surface charge/access, leading to relatively higher CCC values

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(more stable).46 Ren et al. has reported the enhanced stabilization of graphene oxide (GO) in the

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presence of strongly adsorbing phosphate anion (H2PO4-) due to increased electrostatic repulsion

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and charge screening of monovalent cation (Na+).46 In the case of calcium, we hypothesize that

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the additional phosphate sorbed onto OA-IONPs forms inner sphere complexes with calcium


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cations, further promoting particle-particle aggregation, as described by others.47, 48 The CCC

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ratio of Ca2+ to Na+ is 2-6.4 for OA-IONPs aged in phosphate buffer for 3 days. The power term is

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larger than that for pristine OA-IONPs (6.2) and the sample aged in bicarbonate buffer (4.9).

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This discrepancy in aggregation behavior for OA-IONPs aged in phosphate buffer can be

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attributed to the interactions between Ca2+ and the nanoparticle surface, presumably with either

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carboxyl or phosphate groups, as discussed above.27, 30

316

Zeta potentials of pristine and aged OA-IONPs (3 days UVA irradiation) are plotted as a

317

function of NaCl and CaCl2 in Figure 5. As the electrolyte concentration is increased, the zeta

318

potential of all three samples becomes less negative, due to charge screening effects. Comparison

319

of the zeta potentials for pristine and aged OA-IONPs shows that pristine OA-IONPs exhibits

320

higher absolute zeta potential values in the presence of NaCl or CaCl2, which is consistent with

321

higher CCC values. The reduction of apparent zeta potential for aged particles suggests a surface

322

potential change during aging processes. Further, zeta potential values indicate that the divalent

323

calcium cation is more effective at neutralizing surface charge, which is in agreement with

324

results from prior aggregation kinetic studies.49

325

Proposed Aging Mechanisms/Pathway

326

Alterations in the surface coating chemistry of OA-IONPs before and after aging were further

327

analyzed using ATR-FTIR spectroscopy. The spectra of pure oleic acid, IONPs in hexane,

328

pristine OA-IONPs, aged OA-IONPs in bicarbonate, and aged OA-IONPS in phosphate are

329

presented in Figure 6. Pure oleic acid exhibited strong symmetric and asymmetric –CH2 stretch

330

bands at 2923 and 2854 cm-1 (Figure 6a).50-52 The intense peak shown at 1710 cm-1 is derived

331

from the asymmetric –C=O stretch and the band at 1285 cm-1 is attributed to C–O stretch.50, 51, 53

332

The O–H in plane and out-of-plane bands appear at 1460 and 935 cm-1, respectively.50, 51 For


16

Page 17 of 34


333

IONPs dispersed in hexane, similar symmetric and asymmetric –CH2 stretch bands at 2923 and

334

2854 cm-1 are observed, respectively.50 The characteristic band at 1710 cm-1 for oleic acid is

335

almost absent in the spectrum for IONPs dispersed in hexane, possibly from trace residue of free

336

oleic acid in the hexane.50 Compared with pure oleic acid, two new bands at 1590 and 1545 cm-1

337

in the spectrum of IONPs in hexane were characteristics of the asymmetric –COO- and the

338

symmetric –COO- stretches, respectively. The presence of these bands indicates the

339

complexation between the carboxylate group from oleic acid and the iron oxide surface.51, 52 This

340

confirms the attachment of first oleic acid layer on the surface of IONPs, as reported by others.50

341

As the peak of –C=O at 1710 cm-1 is a characteristic peak of free oleic acid, the resulting sharp

342

peak of –C=O for OA-IONPs was attributed to the free oleic acid from the outer (second) layer

343

of OA-IONPs.52, 53 The intensity of this peak, however, decreased when the OA-IONPs were

344

subjected to different aging conditions as seen in Figure 6d and 6e, indicating transformation of

345

the outer oleic acid layer during aging process. No new peaks were observed after the samples

346

were aged, and the only discernible functional group in the FTIR spectra was attributed to

347

carboxylate moiety. Taken together, FTIR results support an aging mechanism whereby the inner

348

coating layer was still fully/partially attached to the IONPs surface while the outer layer

349

underwent transformations during (photo)chemical oxidation processes.

350

During (photo)oxidation processes, oleic acid bilayer surface coatings on IONPs are likely

351

susceptible to transformation/degradation.54 Reactive oxygen species (ROS), such as hydroxyl

352

radicals, generated from UVA light and H2O2, can attack electron-rich double bonds (oxidative

353

cleavage of C=C double bond),12,

354

nonanal, 9-oxononanoic acid, nonanoic acid and azelaic acid.55,

355

process, molecular (organic) aging products were analyzed by GC-MS using Enhanced Data

55-58

resulting in a potential suite of products that include 59-65

To explore this aging


17


Page 18 of 34

356

Analysis software (Agilent Technologies). As shown in Figure 7, nonanal was detected as a

357

major product with other identified fragments, such as heptanal, decane, octanal, 1-octanol, and

358

octanoic acid, confirming the degradation of oleic acid coatings during the oxidative aging

359

process.59, 62, 63, 66 No degradation products (from the surface coatings) were observed in the dark

360

control experiments.

361

As GC-MS and ATR-FTIR analyses along with previous TEM and ICP results indicate that

362

the aging process took place on the surface of the nanoparticles, a simplified reaction pathway

363

was proposed to mimic and evaluate, thus confirm, the aging process and resulting

364

materials/surfaces. For this, one of the major (identified) aging products from oleic acid,

365

nonanoic acid, was successfully used (in place of oleic acid) for the phase transfer of IONPs

366

from hexane into water. Thus, instead of an oleic acid bilayer structure, a layer of nonanoic acid

367

was added to the original layer of oleic acid on the nanoparticle surface, forming a mixed bilayer

368

structure, which is depicted in Figure 8). Aggregation kinetics studies were performed for these

369

model materials with CCC values observed at 47.4 mM for NaCl and 3.3 mM for CaCl2 (SI

370

Figure S6), which are considerably lower when compared CCC values of aged OA-IONPs in

371

bicarbonate (174 mM of NaCl and 5.8 mM of CaCl2) and phosphate (249 mM of NaCl and 3.0

372

mM of CaCl2) buffer, supporting the proposed aging mechanism. In this simplified model, we

373

only take into account the situation where the second layer first undergoes oxidative

374

transformation and converts to an ideal layer of nonanoic acid. However, we certainly recognize

375

that actual aging processes and resulting surface complexes will likely be more complicated, as

376

surface reactions could happen to both outer and inner layer(s), with remaining coating layers

377

existing as mixtures of different short-chain molecules with varying functional groups. It should


18

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378

be expected that such alterations of surface coating structure/chemistry will likely subsequently

379

affect particle stabilities.

380

Environmental Implications

381

This study demonstrates that (organic) surfactant-coated IONPs can undergo dynamic surface

382

(photo)chemical oxidative transformations upon simple UVA irradiation and/or with ubiquitous

383

oxidants. Upon chemical aging, surface-functionalized nanoparticles exhibit considerably

384

different environmental behavior(s) compared to parent materials. OA-IONP stability is highest

385

in the presence of phosphate buffer, due to strong adsorption behavior of phosphate anions. The

386

degradation/transformations of surface coating(s) result in significant destabilization of

387

engineered particles, highlighting the sensitivity of nanoparticle behavior/transport to surface

388

coating type and stability (reactivity). These results also suggest that intentional exposure to

389

specific chemical conditions (e.g., oxidation) may be useful for controlled (mitigated) migration

390

of nanoparticles after their application lifespan is fulfilled. Further, natural organic matter

391

(NOM), which is ever-present in natural aquatic environments, will also likely alter surface

392

reaction/aggregation behavior of these and similar engineered nanoparticles, leading to varied

393

stability and environmental fate of these materials. Evaluating the (photo)oxidation process of

394

surfactant-coated engineered nanoparticles with real world waters, including NOM, is ongoing.


19


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395 396

Figure 1. (a) TEM micrograph of the as-prepared 8 nm IONPs in hexane, scale bar is 50 nm, (b)

397

Histograms of as-prepared 8 nm IONPs in hexane, (c) TEM micrograph of the as-prepared 8 nm

398

OA-IONPs in water, and (d) number mean average size distribution of OA-IONPs in water as

399

measured by DLS.


20

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400 401

Figure 2. Hydrodynamic diameters of OA-IONPs as a function of time: (a) control in the dark,

402

(b) under UVA irradiation. 50 ppm OA-IONP were present in 1.0 mM NaHCO3, 1.0 mM

403

NaHCO3 with 0.03 wt. % H2O2, 1.0 mM NaHCO3 with 0.12 wt. % H2O2, 0.1 mM phosphate, 1.0

404

mM phosphate, 10 mM phosphate, 1.0 mM phosphate with 0.03 wt. % H2O2, 1.0 mM phosphate

405

with 0.12 wt. % H2O2, and 10 mM t-BuOH, respectively.


21


Page 22 of 34

406 407

Figure 3. Attachment efficiencies of pristine and aged OA-IONPs in bicarbonate buffer as

408

functions of (a) NaCl and (b) CaCl2 concentration. The critical coagulation concentration (CCC)

409

for pristine OA-IONPs, and OA-IONPs under UVA irradiation for 10h, 1 day, and 3 days were

410

763 mM, 507 mM, 381 mM, 174 mM for NaCl, and 10.4 mM, 9.3 mM, 8.2 mM, 5.8 mM for

411

CaCl2, respectively. CCC values are derived from the intersection between lines extrapolated

412

from the reaction-limited and diffusion-limited regimes.


22

Page 23 of 34


413 414

Figure 4. Attachment efficiencies of pristine and aged OA-IONPs as functions of (a) NaCl and

415

(b) CaCl2 concentration. The critical coagulation concentration (CCC) for pristine OA-IONPs,

416

and OA-IONPs aged in bicarbonate and phosphate buffer were 763 mM, 174 mM, and 249 mM

417

for NaCl, and 10.4 mM, 5,8 mM, and 3.0 mM for CaCl2, respectively. CCC values are derived

418

from the intersection betweens lines extrapolated from the reaction-limited and diffusion-limited

419

regimes.


23


Page 24 of 34

420 421

Figure 5. Zeta potentials of pristine and aged OA-IONPs as a function of (a) NaCl and (b) CaCl2

422

concentrations. Each data point was the average of 30 measurements from triplicate samples and

423

the error bars represent standard deviations.


24

Page 25 of 34


424 425

Figure 6. FTIR spectra of (a) pure oleic acid; (b) IONPs in hexane; (c) pristine OA-IONPs; (d)

426

aged OA-IONPs in 1.0 mM bicarbonate buffer; and (e) aged OA-IONPs in 1.0 mM phosphate

427

buffer.


25


Page 26 of 34

428 429

Figure 7. GC-MS analysis of organic aging products: (a) dark control in bicarbonate buffer, (b)

430

aged sample in 1.0 mM NaHCO3, (c) aged sample in 1.0 mM NaHCO3 with 0.03 wt. % H2O2,

431

(d) aged sample in 1.0 mM NaHCO3 with 0.12 wt. % H2O2, (e) dark control in phosphate buffer,

432

(f) aged sample in 1.0 mM phosphate, (g) aged sample in 1.0 mM phosphate with 0.03 wt. %

433

H2O2, and (h) aged sample in 1.0 mM phosphate with 0.12 wt. % H2O2.


26

Page 27 of 34


434 435

Figure 8. Proposed (photo)oxidation-induced aging mechanism of OA-IONPs under UVA/H2O2

436

oxidation process. Oleic acid is oxidized by ROS and yield aldehyde and carboxylic acid

437

formations. In a simplified model, the oleic acid bilayer coated IONPs were transformed to

438

nonanoic acid-oleic acid mixed bilayer coated IONPs during this process.


27


439

Page 28 of 34

ASSOCIATED CONTENT

440

Supporting Information. TEM micrographs and digital images of pristine and aged OA-

441

IONPs, soluble Fe concentration, H2O2 degradation process, aggregation profiles of OA-IONPs,

442

phosphate sorption onto OA-IONPs, and CCC values of nonanoic acid coated IONPs. These 6

443

figures are available free of charge via the Internet at http://pubs.acs.org.

444

AUTHOR INFORMATION

445

Corresponding Author

446

*To whom correspondence should be addressed:

447

John D. Fortner: Tel: +1-314-935-9293; Fax: +1-314-935-5464; Email: [email protected]

448

Funding Sources

449

The authors would like to thank the American Chemical Society’s Petroleum Research Fund

450

(#52640-DNI10) and the National Science Foundation (CBET, #1236653) for supporting this

451

work.

452

ACKNOWLEDGMENT

453

This work was supported by American Chemical Society’s Petroleum Research Fund (#52640-

454

DNI10) and the National Science Foundation (CBET, #1236653). TEM, DLS facilities were

455

provided by the Nano Research Facility (NRF) at Washington University in St. Louis, a member

456

of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the

457

National Science Foundation under Grant No. ECS-0335765.


28

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458

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Aqueous Aggregation Behavior of Engineered Superparamagnetic

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