Aggregation and Stability of Reduced Graphene Oxide: Complex

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Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter Indranil Chowdhury , Nikhita D. Mansukhani, Linda M. Guiney, Mark C Hersam, and Dermont C. Bouchard Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01866 • Publication Date (Web): 17 Aug 2015 Downloaded from http://pubs.acs.org on August 23, 2015

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Environmental Science & Technology

Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter Indranil Chowdhury1*, Nikhita D. Mansukhani2, Linda M. Guiney2, Mark C. Hersam2, and Dermont Bouchard3*

1 2

Department of Civil and Environmental Engineering, Washington State University, Pullman, WA

Departments of Material Science and Engineering, Chemistry, and Medicine, Northwestern University, Evanston, IL 3

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

*Address correspondence to either author. Phone: 509-335-3721 (I.C.); 706-355-8333 (D.C.B.) E-mail: [email protected] (I.C.); [email protected] (D.C.B.)

Submitted to Environmental Science and Technology July 2015.

1 ACS Paragon Plus Environment


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Abstract

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The aggregation and stability of graphene oxide (GO) and three successively reduced GO

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(rGO) nanomaterials were investigated. Reduced GO species were partially reduced GO (rGO-

4

1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically,

5

influence of pH, ionic strength, ion valence and presence of natural organic matter (NOM) were

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studied. Results show that stability of GO in water decreases with successive reduction of

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functional groups, with pH having the greatest influence on rGO stability. Stability is also

8

dependent on ion valence and the concentration of surface functional groups. While pH did not

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noticeably affect stability of GO in the presence of 10 mM NaCl, adding 0.1 mM CaCl2 reduced

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stability of GO with increased pH. This is due to adsorption of Ca2+ ions on the surface

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functional groups of GO which reduces the surface charge of GO. As the concentration of rGO

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functional groups decreased, so did the influence of Ca2+ ions on rGO stability. Critical

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coagulation concentrations (CCC) of GO, rGO-1h and rGO-2h were determined to be ~200 mM,

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35 mM and 30 mM NaCl, respectively. In the presence of CaCl2, CCC values of GO and rGO

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are quite similar, however. Long-term studies show that a significant amount of rGO-1h and

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rGO-2h remain stable in Call’s Creek surface water, while effluent wastewater readily

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destabilizes rGO. In the presence NOM and divalent cations (Ca2+, Mg2+), GO aggregates settle

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from suspension due to GO functional group bridging with NOM and divalent ions. However,

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rGO-1h and rGO-2h remain suspended due their lower functional group concentration and

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resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations and NOM can

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play complex roles in the fate of rGO and GO.

22 23



24 25 26

1. Introduction

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Graphene-based nanomaterials have attracted significant attention due to their exceptional

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properties1-4 having potential electronic, energy, medical, and environmental applications.4-7

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Graphene-based materials include pristine graphene, graphene oxide (GO) and reduced graphene

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oxide (rGO),2 with GO being an intermediate product created during the synthesis of rGO. GO

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can be reduced to rGO to improve electronic properties since its electrical conductivity is

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significantly higher than that of GO’s. rGO is one of the most common forms of graphene-based

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nanomaterials due to its superior electrical conductivity and electron mobility. rGO is widely

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used in electronics, energy storage devices and biomedical applications.2, 8 With many promising

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applications, rGO will eventually end up in the environment which necessitates a thorough

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understanding of its environmental fate.

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Besides large production of chemically reduced GO, most graphene-based materials in the

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environment will occur in reduced form. GO is considered a metastable material which

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undergoes spontaneous reduction.9, 10 UV irradiation11, natural reductants, bacteria,12 and biota

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can accelerate its reduction in the environment. Recent studies show that GO is reduced to rGO

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by sunlight exposure13, 14 and by bacterial degradation.12 Our recent study15 showed that GO

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readily photo-reacts under simulated sunlight and photo-disproportionates to CO2-reduced

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materials similar to rGO which are more fragmented low molecular-weight species. Naturally-

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available metal-based reductants including Al16 and Fe17 can reduce GO to rGO. Sulfur-based

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inorganic reductants commonly found in anaerobic environments have also been shown to


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reduce GO.18 Besides bacteria,12, 19 other studies found thatyeast,20 mussel polymer,21 carrot

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root,22 and caffeic acid23 can transform GO to rGO.

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To the best of our knowledge, there are no studies systematically investigating the

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environmental fate and transport of rGO. Most recent work on environmental implications of

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graphene-based nanomaterials primarily focused on GO24-27 and found that aggregation of GO25

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follows colloidal theory including Derjaguin-Landau-Verwey-Overbeek (DLVO) theory28 and

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the Schulze-Hardy rule.29

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stability of GO due to steric repulsion and long-term stability studies showed GO is highly stable

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in natural surface water.25 Studies on GO transport through porous media27, 30, 31 found that GO

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is highly mobile in saturated porous media, that retention is reversible, that increased ionic

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strength (IS) resulted in greater GO retention primarily due to straining, and that observed

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transport trends can be explained by DLVO theory. It is still unknown whether findings from

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GO studies will be applicable to rGO.

Presence of natural organic matter (NOM) noticeably increased the

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Since most graphene-based nanomaterials in the environment are expected to be in reduced

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form, understanding fate and transport of rGO is essential. Ours is the first study systematically

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investigating the fate of reduced GO species in the aquatic environment. We studied the role of

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surface oxidation state on the fate of rGO as well as long-term stability of rGO in natural and

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engineered aquatic environments and found in particular that pH, ion valence and NOM play

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significantly complex roles with rGO species. The overall mechanisms observed in this study

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have been summarized in Scheme 1 and are described throughout the article.

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2. Materials and Methods 2.1 Synthesis and Preparation of rGO 5 ACS Paragon Plus Environment


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In this study, GO and three differently-reduced GO (partially reduced GO (rGO-1h),

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intermediate reduced GO (rGO-2h) and fully reduced GO (rGO-5h)) were used. GO was

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synthesized using a modified Hummers method described previously and mentioned in the

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Supporting Information (SI).23 In this study, rGO reduced by the Solvothermal process was used.

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Chemically reduced GO, rather than rGO reduced via environmental processes, was used to

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obtain similarly sized rGO sheets with varying oxidation states so that the fundamental role of

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surface oxidations on the fate of rGO could be investigated. The reduction was achieved by

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heating a purified GO suspension in N-Methyl-2-pyrrolidone (NMP) to 150°C with constant

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stirring in a silicone oil bath. The heat reflux was performed for 1, 2, or 5 hours to achieve

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varying levels of reduction. After solvothermal reduction, the rGO was separated from the NMP

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using vacuum filtration with 0.1µm alumina filters (Millipore), rinsed heavily with DI water, and

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re-dispersed in DI water at an approximate concentration of 1 mg/mL. Concentrations of GO and

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rGO were determined by measuring mass following vacuum filtration. For AFM, and FTIR

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characterization, the original dispersions in NMP were analyzed.

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85 86

2.2 Characterization of rGO

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Physical dimensions of rGO were measured by AFM; AFM samples were prepared as 32

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described previously.

XPS and FTIR were utilized to determine surface functionalities of rGO.

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The area under peaks was used for quantifying the relative concentration of functional groups as

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shown in Table S1. Details regarding these methods are included in SI.


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Electrokinetic and hydrodynamic properties of rGO were determined over a range of solution

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chemistries including different salt types (NaCl, CaCl2), varying ionic strength (IS), and NOM

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concentrations.

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Substances Society, MN) was used as standard NOM, and an SRHA stock solution was prepared

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with accepted procedures.33,

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(EPM), and zeta potential were measured with a ZetaSizer Nano ZS (Malvern Instrument,

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Worcestershire, U.K.), following well-established techniques.

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physicochemical properties of rGO was investigated over a pH range from 4 to 10 in the

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presence of NaCl, CaCl2 and MgCl2.

Suwannee River Humic Acid Standard II (SRHA) (International Humic

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rGO hydrodynamic diameter (Dh), electrophoretic mobility

Influence of pH on the

This titration experiment was conducted using an

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autotitrator (MPT-2, Malvern Instrument, Worcestershire, U.K.); NaOH and HCl were the

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titrants for controlling pH.

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2.3 Aggregation Kinetics and Long Term Stability of rGO

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

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measured by time-resolved dynamic light scattering (TR-DLS).35 An rGO concentration of 10

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

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and CaCl2 were background electrolytes. Details on the aggregation study are included in SI.

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

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well-controlled simple solution chemistries to more complex, environmentally relevant

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conditions; eight types of water were used. Details on water types have been described in our

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previous study25 and also in SI. 6 mL of 10 mg/L rGO suspended in the treatment water were

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placed in a 20-mL borosilicate glass bottle (Fisher Scientific, PA) and continuously shaken at

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100 rpm; rGO concentration and Dh were monitored for 28 days. Concentration was determined 7 ACS Paragon Plus Environment


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using a UV-vis plate reader (Enspire Multimode Reader 2300, PerkinElmer Inc, MA) at 230 nm

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

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Calibration curves of GO and rGO using a UV-Vis plate reader at 230 nm are shown in Figure

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S1. Samples were collected 2 cm below the top of sample for the long term study.

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3. Results and Discussion 3.1 Characterization of rGO

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AFM images (Figure 1) show that average vertical and lateral size of the flakes remained

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similar from GO to rGO-5h, indicating the reduction process did not significantly change

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physical dimensions of the graphene flakes. XPS results confirm the reduction of oxygen-

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containing functional groups since GO was reduced to rGO-5h (Figure 1); FT-IR results show

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functional groups on GO decreased with the reduction process (Figure S2) and the relative

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amount of functional groups on GO and rGO, as determined from XPS spectra, is summarized in

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Table S1. Progressing from unreduced GO to rGO-5h, the relative presence of carbon increases

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from 40% to 64% while the presence of various functional groups correspondingly decreases.

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We note that the percent of carboxyl functional groups increases as the reduction process

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proceeds due to the higher thermal stability of carboxyl functional groups relative to carbonyl

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and epoxide functional groups. Overall oxygen-to-carbon ratio also decreases as GO is reduced.

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These results are qualitatively corroborated by FTIR.

134 135 136 137

3.2 Electrokinetic and Hydrodynamic Properties of rGO


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EPM increased from (-5.49 ± 0.12) × 10-8 m2V-1s-1 to (-3.89 ± 0.19) × 10-8 m2V-1s-1 as GO

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was successively reduced (Figure 2). Interestingly, EPM decreased initially from GO to rGO-1h,

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then remained similar from rGO-1h to rGO-5h, indicating that further reduction of GO does not

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influence surface potential of rGO although the hydrodynamic size of graphene changed

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noticeably. Dh of rGO increased from 373 nm to 1049 nm as GO was reduced fully to rGO-5h

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(Figure 2). Even though EPM remained quite similar from rGO-1h to rGO-5h, increased Dh with

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rGO indicates surface functional groups are playing major roles in aggregation. XPS results

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show that functional groups of GO decrease as GO is reduced to rGO-5h which leads to higher

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hydrophobicity of the materials and increased aggregation.

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EPM of all rGO increases with greater IS due to charge screening, and remains quite similar

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across NaCl concentrations (Figure S3). The influence of NaCl concentration on hydrodynamic

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size of GO and rGO is significantly different, however. For GO, Dh increased from 489.9 ± 82.9

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nm in 0.1 mM NaCl to 669.4 ± 94.5 nm in 100 mM NaCl (Figure S3); for both rGO-1h and rGO-

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2h, hydrodynamic size increased above 2000 nm in 100 mM NaCl. Higher aggregation of GO

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and rGO was observed in the presence of CaCl2 although EPM was significantly less than in

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NaCl.

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concentration on hydrodynamic size was similar for GO and rGO.

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3.3 Influence of pH on Physicochemical Properties of rGO

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Increased CaCl2 concentration resulted greater EPM, but the influence of CaCl2

3.3.1 pH Effect on GO In the presence of 10 mM NaCl, the EPM of GO decreased from (-3.44 ± 0.03) × 10-8 m2V-

159 160

1 -1

s to (-5.46 ± 0.15) × 10-8 m2V-1s-1 since pH increased from 4 to 10 (Figure 3) due to the

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deprotonation of surface functional groups of GO with increased pH.25, 26 GO Dh was relatively 9 ACS Paragon Plus Environment


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unchanged (~500 nm) throughout the pH range, however, which has been observed earlier. 10, 11

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In the presence of 10 mM NaCl with 0.1 mM CaCl2, the EPM of GO increased from (-2.61 ±

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0.06) × 10-8 m2V-1s-1 to (-1.72 ± 0.22) × 10-8 m2V-1s-1 as pH increased from 4 to 10, which is

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completely opposite to the trend observed in the presence of NaCl alone. GO Dh increased from

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460.2 ± 50.9 nm to 1354.5 ± 275.6 nm as pH increased from 4 to 10 in the presence of 0.1 mM

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CaCl2 and 10 mM NaCl. This trend indicates a small amount of background CaCl2 (0.1 mM) can

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significantly affect the physicochemical properties of GO as a function of pH. Since CaCl2

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concentrations in natural aquatic environments often exceed 0.1 mM, the presence of Ca2+ ions

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will play a major role in GO stability, an effect that may be due to binding capacity of Ca2+ ions

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to functional groups of GO (Scheme 1).36, 37 Higher pH causes increased deprotonation of GO

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surface functional groups, resulting in higher binding of Ca2+ ions with GO functional groups

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and increased surface charge of GO (Scheme 1). Increased EPM also results in greater Dh due to

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charge screening (Figure 3D). In the presence of 10 mM NaCl with 0.1 mM MgCl2 background,

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EPM of GO decreased slightly from (-2.92 ± 0.06) × 10-8 m2V-1s-1 to (-3.20 ± 0.46) × 10-8 m2V-

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1 -1

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observed in the presence of 10 mM NaCl alone, indicating Mg2+ ions are also binding with GO

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surface functional groups, but not as strongly as Ca2+ ions. GO Dh values remained similar across

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the pH range in the presence of background 0.1 mM MgCl2.

s as pH increased from 4 to 10. This decrease in EPM is not as significant (p > 0.05) as that

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3.3.2 pH Effect on rGO

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In the presence of 10 mM NaCl only, EPM of rGO-1h decreased from (-3.28 ± 0.08) × 10-8

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m2V-1s-1 to (-4.37 ± 0.08) × 10-8 m2V-1s-1 as pH increased from 4 to 10 (Figure 3B). The

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decrease for rGO-1h as a function of pH is not as significant as that observed for GO because

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rGO-1h contains fewer surface functional groups (Table S1), and deprotonation of surface 10 ACS Paragon Plus Environment

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functional groups with increased pH is lower for rGO-1h (Scheme 1). rGO-1h Dh was also

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relatively insensitive to pH changes in the presence of 10 mM NaCl. The EPM of rGO-1h

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remained almost constant (~-2 × 10-8 m2V-1s-1) from pH 4 to 10 in the presence of 10 mM NaCl

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with 0.1 mM CaCl2, and was significantly higher than in NaCl alone, indicating that binding of

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Ca2+ ions with surface functional groups of rGO-1h influences electrokinetic properties of rGO-

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1h. Increase of EPM due to binding of Ca2+ ions with rGO-1h is not as significant as that

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observed for GO because rGO-1h has fewer surface functional groups (Scheme 1). Similarly, the

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Dh of rGO-1h increased slightly from pH 4 to 10 in the presence of background 0.1 mM CaCl2,

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but is not as significant as that observed for GO. In the presence of 10 mM NaCl and 0.1 mM

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MgCl2, EPM of rGO-1h decreases as pH increased from 4 to 10. In the presence of background

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0.1 mM MgCl2, Dh of rGO-1h remained fairly constant (~500 nm) throughout the pH range

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investigated in this study.

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In the presence of 10 mM NaCl only, both EPM and Dh of rGO-2h decreased from as pH

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increased from 4 to 10 (Figure 3C, F). In the presence of 10 mM NaCl and 0.1 mM CaCl2, EPM

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of rGO-2h decreased from (-1.29 ± 0.15) × 10-8 m2V-1s-1 to (-3.20 ± 0.17) × 10-8 m2V-1s-1 as pH

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increased from 4 to 10, which is the opposite of the trend observed for GO in the presence of

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background 0.1 mM CaCl2. Since rGO-2h contains a very low number of surface functional

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groups (Table S1), binding capacity of Ca2+ ions with them is minimal. Hence, increased pH

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does not result in further binding of Ca2+ ions with rGO-2h, and an increase in EPM as a function

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of pH was not observed (Scheme 1). Dh of rGO-2h decreased from 2072.7 ± 310.8 nm to 741.7 ±

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154.1 nm as a function of pH, which is also completely opposite to the trend for GO. In the

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presence of 10 mM NaCl with 0.1 mM MgCl2, EPM of rGO-2h decreased from (-2.32 ± 0.03) ×

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10-8 m2V-1s-1 to (-3.55 ± 0.14) × 10-8 m2V-1s-1 as pH increased from 4 to 10. Dh of rGO-2h 11 ACS Paragon Plus Environment


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decreased from 1071.1 ± 74.4 nm to 726.6 ± 43.2 nm as a function of pH in the presence of 0.1

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mM MgCl2.

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3.4 Aggregation Kinetics of rGO

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Aggregation profiles of rGO in 50 mM NaCl in the presence and absence of SRHA are

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presented in Figure S4. As a function of time in 50 mM NaCl, the increase in GO Dh is quite

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low, indicating GO is highly stable at this IS. rGO Dh values increase significantly as a function

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of time, however, which indicates all rGOs are highly unstable at this IS. Aggregation rates

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increase from rGO-1h to rGO-5h due to the loss of surface functional groups, as observed from

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XPS and FTIR data. Adding SRHA reduces the aggregation of GO and all rGO (Figure S4)

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although fully reduced rGO-5h is highly unstable even in the presence of SRHA. A similar trend

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is observed in 1 mM CaCl2 (Figure S5).

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Aggregation attachment efficiency (α) of GO and rGO increases with NaCl concentration

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(Figure 4A) due to charge screening, however, GO α requires higher NaCl concentration to reach

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1 (diffusion-limited region) than does rGO. α of rGO-1h and rGO-2h overlap as a function of

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NaCl concentration, yielding critical coagulation concentrations (CCC) of around ~30-35 mM

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NaCl. The GO CCC is near 200 mM NaCl and GO and rGO CCC’s are very similar (~0.7 mM

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CaCl2) in the presence of CaCl2. A previous study38 on multi-walled carbon nanotubes

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(MWNTs) observed a similar trend in the presence of CaCl2 for highly- and lowly-oxidized

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MWNTs which the authors attributed to bidentate versus monodentate carboxylate complex

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formation.

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3.5 Long-Term Fate of rGO in the Natural Environment

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Long-term stability of rGO in natural and artificial waters is summarized in Figure 5. In

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Call’s Creek water, GO remains highly stable over a month-long study, which is likely due to the

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low IS (conductivity 26 µS/cm) and presence of NOM (2.8 ± 0.57 mg/L). Moreover, Call’s

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Creek primarily contains monovalent ions (Na+, K+) and a negligible amount of divalent ions

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(Mg2+, Ca2+) (Table S2). Additionally, all rGO samples were less stable than GO in Call’s Creek

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water which is consistent with the aggregation study. The suspended concentration of fully-

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reduced GO (rGO-5h) decreases below 25% within one day, and below 10% within five days.

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After one day, about 60% of both rGO-1h and rGO-2h were suspended in Call’s Creek water,

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and after almost one month, ~15% of rGO-1h, ~15% rGO-2h and ~8% rGO-5h remain

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suspended which indicates reduced GO may not be completely removed from the water column

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in natural surface waters.

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Long-term stability of rGO in synthetic surface water in the absence of NOM is shown in

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Figure 5B. After one day, GO and all types of rGO concentrations decrease to less than 10% of

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the initial concentration which indicates that graphene-based materials are not stable in surface

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water in the absence of NOM. The addition of 5 mg/ L SRHA to the synthetic surface water

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increased stability of all graphene-based materials investigated in this study (Figure 5C); after

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one day, 80% GO, 53% rGO-1h and 45% rGO-2h remained suspended in synthetic surface water

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in the presence of SRHA, but 90% suspended) in synthetic surface water in the

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presence of SRHA. Stability of GO and rGO in synthetic surface water, with and without

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divalent ions in the presence of SRHA after 28 days appears in Figure S6. It is obvious that

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removing divalent ions significantly increases the stability of GO, confirming the role of divalent

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ions in GO’s fate. rGO-1h and rGO-2h, on the other hand,

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Ca2+ ions with SRHA than without, as observed from Figure S6. In fact, after just one day,

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almost 50% of rGO-1h and rGO-2h remained suspended in synthetic surface water with SRHA,

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while removing divalent ions drastically reduced the stability of rGO-1h and rGO-2h to ~10%.

are more stable in the presence of


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This indicates that divalent ions can increase stability of rGO in the presence of NOM. For rGO,

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Ca2+ ions can help binding of rGO with SRHA and enhance stability via steric repulsion. Similar

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effects were not observed Call’s Creek water since the sample has a very low concentration of

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divalent ions (

Aggregation and Stability of Reduced Graphene Oxide: Complex

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