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Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment Indranil Chowdhury , Mathew C. Duch, Nikhita D. Manuskhani, Mark C Hersam, and Dermont Bouchard Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es400483k • Publication Date (Web): 13 May 2013 Downloaded from http://pubs.acs.org on May 16, 2013

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Environmental Science & Technology

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Colloidal Properties and Stability of Graphene

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Oxide Nanomaterials in the Aquatic

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Environment

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Indranil Chowdhury1,3, Matthew C. Duch2, Nikhita D. Mansukhani2, Mark C. Hersam2, and Dermont

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Bouchard3* 1

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National Research Council Research Associate, Athens, GA

Departments of Material Science and Engineering, Chemistry, and Medicine, Northwestern University, Evanston, IL

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National Exposure Research Laboratory, Ecosystem Research Division, United States Environmental Protection Agency, Athens, GA

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* Corresponding Author: Dermont C. Bouchard, e-mail: [email protected], Tel: 706-355-8333

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ACS Paragon Plus Environment


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Abstract

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While graphene oxide (GO) has been found to be the most toxic graphene-based nanomaterial,

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its environmental fate is still unexplored. In this study, the aggregation kinetics and stability of

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GO were investigated using time-resolved dynamic light scattering over a wide range of aquatic

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chemistries (pH, salt types (NaCl, MgCl2, CaCl2), ionic strength) relevant to natural and

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engineered systems. Although pH did not have a notable influence on GO stability from pH 4 to

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10, salt type and ionic strength had significant effects on GO stability due to electrical double

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layer compression, similar to other colloidal particles. The critical coagulation concentration

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(CCC) values of GO were determined to be 44 mM NaCl, 0.9 mM CaCl2, and 1.3 mM MgCl2.

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Aggregation and stability of GO in the aquatic environment followed colloidal theory (DLVO

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and Schulze-Hardy rule), even though GO’s shape is not spherical. CCC values of GO were

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lower than reported fullerenes CCC values and higher than reported carbon nanotube CCC

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values. CaCl2 destabilized GO more aggressively than MgCl2 and NaCl due to the binding

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capacity of Ca2+ ions with hydroxyl and carbonyl functional groups of GO. Natural organic

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matter significantly improved the stability of GO in water primarily due to steric repulsion.

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Long-term stability studies demonstrated that GO was highly stable in both natural and synthetic

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surface waters, although it settled quickly in synthetic ground water. While GO remained stable

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in synthetic influent wastewater, effluent wastewater collected from a treatment plant rapidly

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destabilized GO, indicating GO will settle out during the wastewater treatment process and likely

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accumulate in biosolids and sludge. Overall, our findings indicate that GO nanomaterials will be

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stable in the natural aquatic environment, and that significant aqueous transport of GO is

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possible.

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1. Introduction

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Graphene is an atomically thin two dimensional carbon-based nanomaterial that is

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composed of a single layer of sp2 – hybridized carbon atoms such as found in graphite.1,

2

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Graphene’s potential as a renewable resource is evidenced by studies that demonstrate graphene

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can be synthesized from inexpensive carbon sources like food, insects, and waste materials.3

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Graphene is one of the fastest growing nanomaterials in industry and consumer products, and

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graphene-based nanomaterials show tremendous potential in electronic, medical, energy, and

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environmental sector applications.4,5,6

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applications are pristine graphene, graphene oxide (GO), and reduced graphene oxide.7

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Examples of environmental applications include filtration, where GO-coated sand has been

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shown to remove five times more heavy metals than pure sand;8 and desalination, where

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theoretical calculations indicate GO’s physicochemical properties lead to water permeability an

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order of magnitude higher than conventional reverse osmosis membranes.9

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devices, graphene can extract more energy from water flow than carbon nanotubes due to

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coupling of ions present in water with free-charge carriers in graphene.10 TiO2-P25-graphene has

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been shown to be a high performance photo-catalyst, while P25-graphene is a more effective

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catalyst in photo-degradation of methyl blue under visible light than P25 alone.11 Additionally, a

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graphene coating significantly decreases corrosion rates of Ni12, Cu and a Cu/Ni alloys13, making

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graphene the thinnest known corrosion-protecting coating.

Most common forms of graphene used in different

In energy storage

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Recent studies have shown graphene can be toxic towards organisms including bacteria

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and humans. Liu et al.14 demonstrated that graphene is cytotoxic towards bacteria through both

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membrane and oxidative stress and that GO has the highest antibacterial capacity, followed by

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reduced graphene oxide and graphite. In another study, reduced GO showed higher cytotoxicity 3 `



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than GO, indicating antibacterial properties of GO are a function of the graphene oxidation

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state.15 Akhavan and Ghaderi16 found that reduced GO nanowalls can be more effective against

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bacteria, compared to unreduced GO, due to their sharper edges. Graphene shape can also play a

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role in its biological activity.17 Another study18 on graphene-TiO2 composite film reported a

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strong solar light-induced toxicity on Caenorhabditis elegans nematodes due to a high

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generation of reactive oxygen species. LiQiang et al.19 found moderate toxicity of GO towards

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human cell lines and zebrafish, whereas multi-walled carbon nanotubes showed acute toxicity to

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organisms. This study attributed these findings to the geometrical structure difference between

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GO and nanotubes. A recent study20 reported an interesting finding that graphene sheets can

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wrap bacteria and inactivate by disconnecting biologically from environment. This study further

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found that sonication can reactivate the trapped bacteria by removing the sheets from bacterial

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surface, while near-infrared irradiation can inactivate the trapped bacteria forever. Duch et al.21

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investigated graphene biocompatibility in the lung and found that GO caused severe and

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persistent lung injury due to chemical composition, while graphene dispersed in Pluronic block

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copolymer minimized the toxic response. Another recent study22 found that lateral dimension of

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GO sheets has notable influence on antibacterial activity with larger GO sheets showing stronger

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antibacterial activity. Besides lateral dimension, oxygen content of GO has been reported to

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influence the GO film formation at air-water interface.23 Higher oxygen content can lead to

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smaller GO sheets, which can transfer to air-water interface quicker. Size and shape dependent

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genotoxicity of graphene nanoribbons24 and nanoplatelets25 in human cells have recently been

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reported. Reduced GO nanoribbons24 showed significantly higher genotoxic effects to human

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stem cells than reduced GO alone, while smaller lateral dimension of graphene nanoplatelets25

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can lead to higher cell destructions. Recent studies26, 27 reported that bacteria can reduce GO and

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transform its properties. Salas et al.26 found heterotrophic bacteria can utilize GO as a terminal

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electron acceptor, whereas Akhavan and Ghaderi27 reported E. coli bacteria can reduce GO to

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bactericidal graphene. Taken together, these studies demonstrate that the toxicities of graphene,

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graphene oxide, and reduced graphene oxide are dependent on their chemical properties, physical

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properties, and dispersion state.

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Although significant research has been conducted on graphene nanomaterials

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applications, there is almost no published research on the fate of graphene nanomaterials in the

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environment. To date, only one study has been published on the transport of GO through porous

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media;28 it found that GO is highly mobile through saturated porous media, but GO retention in

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packed-bed columns is reversible.

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nanomaterials will be governed primarily by their stability in natural and engineered aquatic

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systems, no studies have been conducted on the aggregation kinetics and stability of GO

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nanomaterials in aquatic environments. Therefore, we investigated the aggregation kinetics and

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stability of GO in a wide range of aquatic chemistries, including pH, salt types, ion valence and

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in the presence of organic matter. In addition to the well-controlled solution chemistry studies,

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we also determined the stability of GO in natural and synthetic waters that simulated natural and

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engineered systems.

Although the fate and transport of graphene-based

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2. Materials and Methods

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2.1 Synthesis of Graphene Oxide

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A modified Hummers method was used to produce GO.29 Briefly, this method involves

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treatment of natural graphite flakes (3061 grade material from Asbury Graphite Mills) with 5 `



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concentrated sulfuric acid and other oxidizing agents followed by filtration, washing, and

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centrifugation to remove residual contaminants. A full description of the procedure is provided

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in Supporting Information (SI) in Section 2.1.

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2.2 GO Characterization

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Physical dimensions of GO were determined following an atomic force microscopy

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(AFM) procedure developed elsewhere21 (described in Supporting Information). A UV-vis plate

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reader (Enspire Multimode Reader 2300, PerkinElmer Inc, MA) was utilized to determine optical

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absorption spectra as a function of GO concentration (Figure S1).

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hydrodynamic properties of GO were also determined over a wide range of solution chemistries

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including different salt types (NaCl, CaCl2, MgCl2), varying ionic strength (IS), and natural

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organic matter (NOM) concentrations.

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(International Humic Substances Society, MN) was used as standard NOM and a SRHA stock

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solution was prepared with accepted procedures.30,31

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electrophoretic mobility (EPM), and zeta potential (ζ-potential) were measured with a ZetaSizer

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Nano ZS (Malvern Instruments, Worcestershire, U.K.), using well-established techniques (SI

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Section 2.3). To determine the GO isoelectric point, electrokinetic and hydrodynamic properties

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of GO were measured in 10 mM KCl over a pH range from 2 to 10, using 10 mM HCl or 10 mM

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NaOH as titrants. All solutions were filtered through a 100 nm filter (Anotop 25, Whatman,

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Middlesex, UK).

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replicates.

Suwannee River Humic Acid standard II (SRHA)

GO hydrodynamic diameter (Dh),

In our study error bars indicate one standard deviation of at least three

130 131

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Electrokinetic and


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2.3

GO Aggregation Kinetics

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Change of GO Dh as a function of IS, ion valence, and presence of organic matter was

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measured with time by time-resolved dynamic light scattering (TR-DLS)32. A GO concentration

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of 40 mg/L provided a strong DLS signal and therefore was used in all aggregation studies.

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Equal volumes (750 µL) of GO suspension and electrolyte solution (NaCl, CaCl2 or MgCl2) were

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pipetted into a DLS glass cuvette (Malvern Instruments, Worcestershire, U.K.) to achieve a

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specific electrolyte and GO concentration. The cuvette was immediately placed in the DLS

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instrument after vortexing for 1 s.

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autocorrelation function was allowed to accumulate for 15 s during aggregation study. The Dh

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measurements were conducted over periods ranging from 30 to 300 min. The initial aggregation

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period was defined as the time period from experiment initiation (t0) to the time when measured

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Dh values exceeded 1.50Dh,initial.32 The initial aggregation rate constants (ka) for the GO are

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proportional to the initial rate of increase of Dh with time:33

∝

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Intensity of scattered light was measured at 173° and

(1)

→

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where N0 is the initial particle concentration. The particle attachment efficiency α is used to

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quantify particle aggregation kinetics; it is defined as the initial aggregation rate constant (ka),

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normalized by the aggregation rate constant measured under diffusion-limited (fast) conditions:33

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,

→ , →,

(2)

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The GO concentration across all samples was identical, allowing a simplification of eq. 2 (i.e.,

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N0 drops out). Therefore, α can be determined directly by normalizing the initial slope of the 7 `



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aggregation profile for a specific background solution chemistry, by the initial slope under

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diffusion-limited (fast) conditions. Critical coagulation concentrations (CCC) of GO

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nanomaterials were determined from the intersection of extrapolated lines through the diffusion

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and reaction limited regimes.

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2.4 Long Term Stability of GO in Natural Water

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Long term stability of GO was investigated in synthetic and natural waters to relate well-

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controlled simple solution chemistries to more complex, environmentally relevant conditions.

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Nine different types of water were used. Natural surface water was collected from a tributary of

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Calls Creek, a small stream near Athens, GA. Wastewater was collected from the North Oconee

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Water Reclamation Facility, Athens, GA. In addition to natural water, stability of GO in several

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types of synthetic water was also investigated. Synthetic surface water34 and ground water35

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were prepared following recipes mentioned elsewhere, and synthetic wastewater was prepared

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following the OECD guideline.36 Detailed characteristics of these waters are provided in SI

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Section 3. 10 mL of 10 mg/L GO suspended in the treatment water were placed in a 20-mL

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borosilicate glass bottle (Fisher Scientific, PA) continuously shaken at 100 rpm; GO

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concentration and Dh were monitored for 28 days. Concentration was determined using a UV-

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vis plate reader (Enspire Multimode Reader 2300, PerkinElmer Inc, MA) at 230 nm wavelength

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(Figure S1B), and Dh was determined by a DynaPro Plate Reader II (Wyatt Technology, CA).

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3. Results and Discussion

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3.1 GO Imaging

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Figure 1A contains a representative AFM image of GO, from which the height, area, and

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perimeter distributions of the GO sample were determined. The average thickness of GO was

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determined to be 0.85 ± 0.21 nm with a range of 0.3 to 1.4 nm which is consistent with typical

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GO samples1, 2; more than 70% of flakes characterized were between 0.5 and 1.0 nm in height

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(Figure S2A). The average square root of the area was 179.2 ± 111.5 nm (Figure 1B) with an

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area range of 1225 to 350000 nm2, and a majority of GO flake areas between 2500 and 23000

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nm2. Figure 1

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3.2 Electrokinetic and Hydrodynamic Properties of GO

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3.2.1

Influence of pH

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Electrokinetic measurements (Figure 2A) showed that GO remained negatively charged

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over the pH range from 2 to 10. Absolute values of GO EPMs decreased significantly from pH 4

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to pH 2 (from (-3.09 ± 0.13) × 10-8 to (-1.52 ± 0.10) × 10-8 m2V-1s-1), while EPMs remained very

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similar from pH 5 to pH 9 (from (-3.34 ± 0.21) × 10-8 to (-3.64 ± 0.14) × 10-8 m2V-1s-1).

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Maintaining pH below 2 without sacrificing control of the IS (10 mM) was not possible, but it is

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clear that the isoelectric point of GO is < pH 2. Other carbon-based nanomaterials such as

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carbon nanotubes37, 38 and fullerenes39, 40 have shown similar behavior with pH, although the

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origin of surface charge on these carbon-based nanomaterials is still unknown. The change in

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EPM at low and high pH values, however, indicates that there may be dissociable functional 9 `



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groups on GO. A previous study21 of GO -- also produced via a modified Hummers method --

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found that synthesized GO contained hydroxyl and carbonyl functional groups. These may be

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responsible for the change in GO EPM values, as a function of pH particularly at low and high

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pH. GO Dh as a function of pH is presented in Figure 2B. The response of Dh to varying pH

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was similar to that of EPM, with GO Dh being quite constant (~250 nm) from pH 4 to pH 10,

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then increasing sharply as pH decreased from 4 to 2. This increased Dh (> 1000 nm) below pH 4

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was due to a reduction in the electrostatic repulsive forces between GO, as predicted by colloidal

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theory.41

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Figure 2

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The range of pH usually observed in the aquatic environment is from 5 to 9.42 Since we

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did not observe notable changes in electrokinetic or hydrodynamic properties of GO over this

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range, it is quite likely that pH will have minor effects on fate and transport of GO in the aquatic

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environment.

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unadjusted pH 5.5 ± 0.2 in this study.

Thus, we investigated the aggregation kinetics and stability of GO at an

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3.2.2

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Influence of Ionic Strength and Salt Types

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Figure 3 summarizes the electrokinetic and hydrodynamic characterization of GO as a

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function of IS and salt type (NaCl, CaCl2, MgCl2). GO was highly negatively charged in

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deionized (DI) water ((-4.47 ± 0.11) × 10-8 m2V-1s-1), resulting in highly stable aqueous GO

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suspensions with Dh = 272.4 ± 0.8 nm due to the large electrostatic repulsion between GO.43

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EPM values of GO became less negative from (-3.77 ± 0.05) × 10-8 m2V-1s-1 to (-1.05 ± 0.090) ×

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10-8 m2V-1s-1, as NaCl concentration increased from 1 to 300 mM. 10 `


The increased charge

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screening with increased IS is due to electrical double layer compression, as predicted by

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classical colloidal theory.41,

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including metal oxide45, 46 and carbon-based nanomaterials.38, 39 From 1 to 30 mM NaCl, GO

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EPM remained quite constant around -3.5 × 10-8 m2V-1s-1, and Dh was fairly constant around 260

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nm. Above 30 mM NaCl, GO EPM increased notably with IS, which resulted in an increase in

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Dh from 267.4 ± 31.2 nm to 2940 ± 133.8 nm. In most natural freshwater bodies, including

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surface and ground waters, the concentration of monovalent ions (Na+, K+) is less than 10 mM,42

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indicating that GO will be stable in natural aquatic environments dominated by these monovalent

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cations.

44

This trend is also regularly observed in other nanomaterials

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Figure 3

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Electrokinetic and hydrodynamic properties of GO in the presence of divalent ions

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(CaCl2 and MgCl2) are reported in Figure 3. Both Ca2+ and Mg2+ influenced the EPM and

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hydrodynamic size of GO in the aquatic suspension more aggressively than monovalent Na+.

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Notable charge screening of GO was observed, even as low as 0.05 mM for both CaCl2 and

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MgCl2, while significant electrical double layer compression was not observed for NaCl until 50

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mM. This is because of higher-charge screening from divalent ions with respect to monovalent

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ions, as described in classical colloidal theory.44 EPMs and ζ-potential values of GO were quite

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similar as a function of IS, for both CaCl2 and MgCl2. As IS increased from 0.01 to 10 mM,

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EPM values of GO became less negative from -40 mV to -12 mV for both divalent ions. The

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trend with CaCl2 and MgCl2 was also observed in previous studies including multi-walled carbon

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nanotubes38 and silver nanoparticles47. Because both Ca2+ and Mg2+ are divalent ions, both

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should produce similar charge screening effects;44 however, the hydrodynamic size of GO was

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significantly different for CaCl2 and MgCl2 as a function of IS (Figure 3B). 11 `


No notable


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aggregation of GO was observed until 0.5 mM for both divalent ions, although significant charge

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screening was observed from 0.05 mM; after 0.5 mM, the hydrodynamic size of GO increased

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significantly with IS for both ions. Aggregates were notably larger for GO in CaCl2 than in

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MgCl2, however, indicating that Ca2+ ions had a greater effect on GO aggregation than Mg2+

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ions. Further discussion regarding aggregation of GO is provided in Section 3.3.2.

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3.2.3

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Influence of Natural Organic Matter

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The effect of NOM on the electrokinetic and hydrodynamic properties of GO for

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different salts is reported in Table S1. The SRHA concentration was varied from 1 to 10 mg/L

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total organic carbon (TOC) which captures the typical range of NOM concentration in ground

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and surface waters.42 SRHA concentration had a negligible effect on the ζ-potential of GO in all

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three salt types investigated, NaCl, CaCl2, and MgCl2. Studies with pristine and oxidized carbon

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nanotubes,37,

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matter, while NOM has been reported to increase the negativity of TiO231 and hematite

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nanomaterials46. Addition of SRHA, however, reduced aggregation of GO under all conditions

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investigated here.

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nm in 50 mM NaCl, 1.0 mM CaCl2, and 1.5 mM MgCl2 (Table S1). Since SRHA did not affect

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GO ζ-potential, steric repulsion (due to the sorption of SRHA on GO) may be the dominant

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mechanism in GO stabilization, as observed in other carbon-based nanomaterials. 37, 48

48

and fullerenes30,

49

have observed similar results in the presence of organic

With increased SRHA concentration, GO Dh decreased from ~500 to ~250

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3.3 Aggregation Kinetics of GO

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Figure S3 presents GO aggregation as a function of GO concentration in deionized water.

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There was no notable aggregation observed as a function of GO concentration. Figure S4

267

contains a typical aggregation kinetics profile of GO as a function of NaCl concentration. GO

268

attachment efficiencies were determined from the aggregation kinetics profiles for NaCl, CaCl2

269

and MgCl2. GO stability in the presence of SRHA for all salt types was also investigated. Figure 4

270

3.3.1

271

Influence of Ionic Strength

272

Figure 4 contains plots of attachment efficiency (α) as a function of salt and SRHA

273

concentrations. Aggregation of GO below 20 mM NaCl was not observed, indicating that GO is

274

highly stable in this regime.

275

observed as a function of NaCl concentration (Figure 4A), which indicates that GO aggregation

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follows Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.50 In the reaction-limited regime α

277

increased from 0.5% to 100% as NaCl concentration increased from 20 mM to 60 mM. The

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increase of α was notably higher between 30 mM and 50 mM, from 9% to 95%. IS beyond 60

279

mM did not increase α which indicates that electrostatic repulsion between GO is completely

280

suppressed and the diffusion-limited regime starts near 60 mM NaCl. DLVO interaction profiles

281

between GO-GO are generated assuming sphere-sphere geometry (SI Section 4.0) and are

282

presented in Figure S5 and Table S2. Large energy barrier (>150 kT) and negligible secondary

283

minimum (< 0.1 kT) observed at 10 mM NaCl further explains the negligible aggregation of GO

284

in this condition. Increased IS led to reduced energy barrier and greater secondary energy

285

minima, which resulted higher aggregation with IS as observed from experiments. Figure 4A

Distinct reaction-limited and diffusion-limited regimes were

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indicates the CCC of GO is around 44 mM NaCl. Since our study is the first to report the CCC

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of GO, no direct comparison to previously published studies can be made. However, Chen and

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Elimelech reported the CCC of C60 fullerenes to be 120 mM NaCl33, and later showed that the

289

CCC of fullerenes is dependent on their synthesis methods.40

290

significantly higher CCC value (260 mM) for fullerenes, and higher values for fullerene

291

derivatives. It is clear, therefore, that the CCC value of GO (44 mM NaCl) is notably lower than

292

those reported for fullerenes. The reported CCC value of carbon nanotubes is 20 mM NaCl for

293

single-walled48 and 25 mM NaCl for multi-walled38, however, surface oxidation can play a

294

significant role in the CCC of carbon nanotubes with more oxidized nanotubes yielding higher

295

CCC values.51

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dimensional (1D) carbon nanotubes and lower than three dimensional (3 D) nC60.

Another study39 reported a

Hence, the CCC value of two dimensional (2D) GO is higher than one

297

3.3.2

298

Influence of Salt Types

299

Like NaCl, distinct reaction and diffusion-limited regimes were observed for CaCl2

300

(Figure 4B). Negligible GO aggregation was observed until 0.3 mM CaCl2 indicating that GO

301

will be stable in aquatic suspensions 5 µm due to

368

high IS (conductivity 881 µS / cm), even though a substantial amount of organic matter was

369

present (6.5 ± 0.4 mg/L TOC). According to North Oconee wastewater reclamation facility

370

operators, the effluent wastewater they supplied may have had residual coagulant (primarily

371

alum), which would have caused rapid GO aggregation.

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In synthetic surface water, GO Dh remained stable at 250 nm in the absence and presence

373

of SRHA. Synthetic surface water contained only 0.33 mM Ca2+ and 0.15 mM Mg2+, which are

374

well below the CCC values of GO in CaCl2 (0.9 mM ) and MgCl2 (1.3 mM). The low IS (1.83

375

mM) and high EPM values of GO ((-2.23 ± 0.05) × 10-8 m2V-1s-1) resulted in negligible

376

aggregation of GO in synthetic surface water. In synthetic ground water, on the other hand, GO

377

aggregated rapidly in the absence and presence of SRHA (Figure 5A). The difference between

378

synthetic surface and ground waters may be explained by the high content of divalent ions (0.68

379

mM Ca2+ and 0.24 mM Mg2+) in the synthetic ground water, compared to synthetic surface

380

water. As reported earlier, the CCC values of GO are 0.9 mM CaCl2 and 1.3 mM MgCl2, so the

381

combined effect of the Ca2+ and Mg2+ cations in the synthetic ground water was sufficient to

382

destabilize the GO. Addition of 1 mg/L SRHA to the synthetic ground water measurably

383

reduced the GO aggregation rate, although GO was still unstable in synthetic ground water in the

384

presence of SRHA. Electrokinetic measurements show that absolute value of GO EPM in

385

synthetic ground water ((-1.82 ± 0.04) × 10-8 m2V-1s-1) was significantly lower than synthetic

386

surface water ((-2.23 ± 0.05) × 10-8 m2V-1s-1) due to electrical double layer compression in the

387

high IS ground water.44

388

In synthetic wastewater (as per OECD guideline)36, negligible aggregation was observed

389

with and without organic content, indicating that GO will be highly stable in model wastewater,

390

meant to simulate wastewater entering a wastewater treatment facility. Synthetic wastewater

391

contains small amounts of salts with high organic matter content (100 mg/L TOC) which led to

392

large EPM values of GO ((-3.43 ± 0.13) × 10-8 m2V-1s-1 without organic matter and (-2,21 ±

393

0.08) × 10-8 m2V-1s-1 with organic matter), stabilizing the GO. We also did not observe any

394

notable aggregation in effluent synthetic wastewater over a one hour kinetics experiment. 18 `


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395

396

3.4.2 Long Term Stability of GO in Natural and Synthetic Water

397

Stability of GO over 28 days, presented in Figure 5B and Table S5, demonstrated a

398

notable dependence on water type. Oconee wastewater destabilized GO rapidly, with more than

399

30% of GO settling immediately, due to the rapid formation of large aggregates as also observed

400

in the initial aggregation kinetics study (Figure 5A). After one day, 100% of GO had settled,

401

indicating most of GO will deposit during the wastewater treatment process and will likely end

402

up in biosolids and sludge. On the other hand, more than 90% of GO remained suspended in

403

Calls Creek water (with a fairly stable Dh of ~250-300 nm) after one month, which indicates

404

potential for long-term transport in the environment. Images in Figure S7 demonstrate the

405

stability of GO in Calls Creek water over the course of a month, while GO was destabilized in

406

Oconee wastewater within a day.

407

Synthetic surface water exhibited interesting GO stability trends. In the absence of

408

SRHA, GO remained stable in synthetic surface water for two days, with more than 90% of GO

409

suspended. After two days, however, GO settled rapidly, despite initial aggregation kinetics data

410

(Figure 5A) that indicated GO was highly stable in synthetic surface water. DLS measurements

411

(Table S5) showed that, in fact, GO was slowly aggregating over two days. In the presence of

412

SRHA, GO remained stable for seven days (more than 90% GO suspended), however, a slight

413

settling was observed by Day 10 (82% GO suspended); more settling by Week 2 (25% GO

414

suspended); and by Week 3, GO was no longer measurable in the suspended phase. DLS results

415

indicate that GO aggregated extremely slowly over a two-week period, with a Dh shift from 280

416

nm to 1180 nm which ultimately caused the settling of GO. While both initial aggregation

19 `



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Page 20 of 36

417

kinetics and one-week stability results indicate apparent stability of GO in synthetic surface

418

water in the presence of SRHA, we identified at least one condition that results in extremely

419

slow aggregation that eventually destabilizes GO.

420

One-hour aggregation studies indicate that GO in synthetic ground water was unstable in

421

the absence and presence of SRHA (Figure 5A). Long-term GO stability data for synthetic

422

ground water shows that GO rapidly formed larger aggregates (Table S5) and that 100% of GO

423

settled within one day (Figure 5B). Although the IS of synthetic ground water (1.55 mM) was

424

comparable to synthetic surface water (1.8 mM), the higher amount of divalent ions (0.68 mM

425

Ca2+, 0.24 mM Mg2+, 0.39 mM SO4-) in ground water led to greater aggregation of GO, which is

426

consistent with earlier results demonstrating Ca2+ ions were more aggressive in destabilizing GO

427

than Mg2+ or Na+ ions.

428

Overall, more than 90% of GO remained suspended in OECD synthetic wastewater over

429

a 28-day period, which indicates GO will be highly stable in this model influent wastewater

430

(Figure 5). Although GO was slightly more stable in wastewater with added organic content,

431

less than 10% of GO settled down in all types of synthetic wastewater during the long-term

432

stability study. Although synthetic wastewater contains some divalent cations (CaCl2, Mg2SO4),

433

the salt concentration was too low to cause significant charge screening and destabilization of

434

GO. The EPM of GO in synthetic wastewater without organic content was (-3.43± 0.13) × 10-8

435

m2V-1s-1 which is very close to GO EPM in DI water. GO Dh remained between 250-300 nm in

436

synthetic wastewater without organic content, although the high organic content impaired our

437

ability to get reliable DLS results in synthetic wastewater with organic content and effluent

438

wastewater.

439

wastewater, while it remained stable in synthetic wastewater (Figure 5).

As discussed earlier, GO was found to be highly unstable in North Oconee

20 `


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440

441

4. Environmental Implications

442

In this study we found that pH does not notably affect the physicochemical properties of GO,

443

indicating pH will have minor influence on the fate of GO in the aquatic environment. The

444

stability of GO in natural surface water for about a month indicates potential for long-term

445

transport of GO in these systems exists. Effluent from a wastewater treatment plant destabilized

446

GO immediately, showing that most of GO will settle out during the wastewater treatment

447

process and end up in biosolids, sludge and ultimately in landfills and as fertilizer. While useful

448

for comparison, initial aggregation kinetics data cannot predict long-term stability of

449

nanomaterials in the aquatic environment. Therefore, long-term nanomaterial stability studies

450

should be conducted to complement more common initial aggregation kinetics studies.

451

452

Acknowledgments

453

Funding was provided by EPA to the NRC and by the University of California Center for the

454

Environmental Implications of Nanotechnology (NSF-EPA under Cooperative Agreement #

455

DBI-0830117). We thank Caroline Stevens of EPA for TOC analysis and the North Oconee

456

Water Reclamation Facility for providing wastewater. This paper has been reviewed in

457

accordance with the USEPA’s peer and administrative review policies and approved for

458

publication. Mention of trade names or commercial products does not constitute endorsement or

459

recommendation for use.

460

461

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462

Supporting Information Available

463

Additional details on physical characterization, AFM imaging, water recipes for long term

464

stability study, characterization, stability results, and DLVO theory are provided in the

465

Supporting Information.

466

http://pubs.acs.org.

This material is available free of charge via the Internet at

467

468

469

470

471

472

473

474

475

476

477

478

479

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480

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List of Tables and Figures

Title

No. Figure 1.

A) Representative Atomic Force Microscopy (AFM) image of GO. B) Distribution of square root of area of GO.

Thickness, and square root area of GO were

determined from at least five different AFM images and more than 150 GO flakes. Additional details on AFM imaging may be found in the Materials and Methods section and in Supporting Information. Figure 2.

Electrokinetic and hydrodynamic characterization of graphene oxide (GO) as a function of pH. A) Electrophoretic mobilities (EPMs) of GO as a function of pH. B) Hydrodynamic diameter of GO as a function of pH. GO concentration was maintained at 10 mg/L in 10 mM NaCl. pH was controlled with 10 mM NaOH or HCl. Error bars indicate one standard deviation of at least three measurements.

Figure 3.

Electrokinetic and hydrodynamic characterization of graphene oxide (GO) in different electrolytes (NaCl, CaCl2, MgCl2) as a function of ionic strength. A) Electrophoretic mobilities (EPMs) of GO. B) Hydrodynamic diameter of GO. GO concentration was maintained at 10 mg/L. pH was unadjusted at pH 5.5 ± 0.2. Error bars indicate one standard deviation of at least three measurements.

Figure 4.

Attachment efficiencies (α) of GO as a function of A) NaCl concentration, B) CaCl2 concentration, and C) MgCl2 concentration in the absence and presence of 5 mg/L SRHA. The dotted lines were used to determine the CCC value of GO from the intersection of reaction limited and diffusion limited regimes. CCC values of GO were determined from these figures as 44 mM NaCl, 0.9 mM CaCl2 and 1.3 mM

29 `



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

MgCl2 in the absence of SRHA. In the presence of 5 mg/L SRHA, CCC values of GO were 125 mM NaCl, 2.20 mM CaCl2, and 3.90 mM MgCl2.

Figure 5.

Long term stability of GO in natural and synthetic waters. Nine different water types were investigated in this study. Stability of GO was monitored for 28 days. Calls Creek surface water and Oconee wastewater were collected from a small stream near Athens, GA, and North Oconee Water Reclamation Facility, Athens, GA, respectively. Artificial surface water (ASW) and artificial ground water (AGW) were prepared following the recipe mentioned in the Supporting Information. Synthetic wastewater (SWW) was made following OECD guideline. Detailed water characteristics are mentioned in Supporting Information.

30 `


Page 30 of 36

Page 31 of 36

A

B

GO Nanomaterials (%)

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


35 30 25 20 15 10 5 0 0

100

200

300

400

500

Square root of area (nm) Figure 1.

31 `


600

0

A -1 -2

-8

2

-1 -1

EPM (10 m V s )

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

-3 -4 -5

2

4

6

8

10

Hydrodynamic Diameter (nm)


3000 2500

B

2000 1500 300 200 100 0

2

4

6

pH

pH Figure 2.

32 `

Page 32 of 36


8

10

-8

2 -1 -1 m V s )

0

EPM (10

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


-1 -2 -3 -4

A -5 0.01

0.1

1

10


Page 33 of 36

4000 NaCl CaCl2 MgCl2

3000

2000

1000

B 0 0.01

100

Concentration (mM)

1

10

Concentration (mM)

Figure 3.

33 `

0.1


100

1

No SRHA 5 mg/L SRHA

0.1 0.01 A 0.001 10

100

Attachment Efficiency (α)

NaCl Concentration (mM)

1

No SRHA 5 mg/L SRHA

0.1 0.01 B 0.001 0.1

1

CaCl2 Concentration (mM)


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



1

No SRHA 5 mg/L SRHA

0.1 0.01 0.001 0.1

C 1

MgCl2 Concentration (mM) Figure 4. 34 `


Page 34 of 36

10000

A

1.2 B

8000 0

6000

0.8

C/C

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



Page 35 of 36

2000

0.4 1000

0.0

0 0

10

20

30

40

50

0

60

10

15

20

Time (Days)

Time (min) Calls Creek

ASW

AGW

Oconee Wastewater

ASW + SRHA

AGW + SRHA

Figure 5.

35 `

5


SWW no organic SWW + Organic SWW Effluent

25

30


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

Table of Contents (TOC) Graphic

36 `


Page 36 of 36

Colloidal Properties and Stability of Graphene ... - ACS Publications

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