Transport of Sulfide-Reduced Graphene Oxide in Saturated Quartz

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Transport of Sulfide-Reduced Graphene Oxide in Saturated Quartz Sand: Cation-Dependent Retention Mechanisms Tianjiao Xia, John D. Fortner, Dongqiang Zhu, Zhichong Qi, and Wei Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02349 • Publication Date (Web): 08 Sep 2015 Downloaded from http://pubs.acs.org on September 14, 2015

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Environmental Science & Technology

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Transport of Sulfide-Reduced Graphene Oxide

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in Saturated Quartz Sand: Cation-Dependent

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Retention Mechanisms

5 Tianjiao Xia,1 John D. Fortner,2 Dongqiang Zhu,3 Zhichong Qi,1 Wei Chen1*

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College of Environmental Science and Engineering/Ministry of Education Key

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Laboratory of Pollution Processes and Environmental Criteria/Tianjin Key Laboratory of

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Environmental Remediation and Pollution Control, Nankai University, Tianjin 300071,

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China

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Department of Energy, Environmental and Chemical Engineering, Washington

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University in St. Louis, St. Louis, MO 63130, USA 3

State Key Laboratory of Pollution Control and Resource Reuse/School of the Environment, Nanjing University, Jiangsu 210093, China

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Manuscript prepared for Environmental Science & Technology

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*To whom correspondence may be addressed: (Phone/fax) 86-22-6622-9517; (e-mail)

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[email protected]

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ABSTRACT

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We describe how the reduction of graphene oxide (GO), via environmentally

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relevant pathways, affects its transport behavior in porous media. Two sulfide-reduced

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GOs (RGOs), prepared by reducing 10 mg/L GO with 0.1 mM Na2S for 3 and 5 d,

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respectively, exhibited lower mobility than parent GO in saturated quartz sand.

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Interestingly, decreased mobility cannot simply be attributed to the increased

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hydrophobicity and aggregation upon GO reduction, as the retention mechanisms of

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RGOs were highly cation-dependent. In the presence of Na+ (a representative monovalent

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cation) the main retention mechanism was deposition in secondary energy minimum.

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However, in the presence of Ca2+ (a model divalent cation) cation bridging between RGO

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and sand grains became the most predominant retention mechanism; this was because

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sulfide reduction markedly increased the amount of hydroxyl groups—a strong

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metal-complexing moiety—on GO. When Na+ was the background cation, increasing pH

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(which increased accumulation of large hydrated Na+ ions on grain surface) and the

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presence of Suwannee River humic acid (SRHA) significantly enhanced transport of

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RGO, mainly via steric hindrance. However, pH and SRHA had little effect when Ca2+

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was the background cation, because neither affected the extent of cation bridging that

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controlled particle retention. These findings highlight the significance of abiotic

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transformations on the fate and transport of GO in aqueous systems.

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INTRODUCTION Graphene oxide (GO) is a novel carbonaceous nanomaterial that has shown great

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promise in many areas of applications, ranging from energy-related materials to

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biomedicines.1 The rapidly increasing production and use of GO increases the possibility

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of its environmental release, with unknown implications. Correspondingly, the fate,

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transport, and negative environmental effects of GO have received much attention.2,3 One

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important characteristic of GO is that it contains a significant amount of surface

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O-functional groups.4-7 The high surface O-content makes GO hydrophilic and easily

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dispersible in aqueous solutions.8-12 It has been shown that colloidal GO can be highly

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mobile in porous media.13-20 Furthermore, mobility of GO in saturated porous media is

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strongly dependent on ionic strength,13-15,17-19 but less sensitive to pH,15,18,19 and the

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presence of natural organic matter generally enhances the transport of GO.16,18,19

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Graphene oxide is likely reduction-sensitive in aquatic environments. This is

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evidenced by a large number of studies aimed to achieve chemical reduction of GO.21-25

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In our previous studies, we observed that GO can be reduced by low concentrations of

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sulfide under environmentally relevant conditions; such reduction resulted in significant

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alteration of the types and distribution of its surface O-functionality, making the reduced

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GO (RGO) relatively less hydrophilic and more prone to aggregation than parent GO.26,27

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Chemical reduction of GO in aquatic environments may significantly affect its transport

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properties in subsurface, but this has not been directly investigated and the controlling

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mechanisms will likely be complicated. For example, we found that even though sulfide

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reduction resulted in an overall loss of surface O-content of GO, effective surface charge

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was not significantly affected.26,27 It is noteworthy that when partially reduced, a portion 3 ACS Paragon Plus Environment


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of the epoxy groups of GO was converted to hydroxyl and carboxyl groups.26,27 This can

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be a key alteration affecting the nature of interactions between GO nanosheets and porous

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media, cations, and dissolved organic matters. It is possible that such chemical reduction

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may significantly affect not only the retention mechanisms of GO in saturated porous

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media, but also how transport properties of GO will respond to the changes of solution

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chemistry conditions. Additionally, RGO can form aggregates more easily than GO,26,27

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and this may affect particle−collector interactions and may enhance particle retention via

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straining.

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The overall objective of this study is to understand the effects of environmental

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reduction of GO on its transport properties in saturated porous media. The specific goals

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are to identify the primary mechanisms controlling the retention of sulfide-reduced GOs

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in saturated quartz sand as affected by several important solution chemistry parameters,

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and to link observed differences in transport properties between RGOs and GO to the

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reduction-induced changes of surface chemistry of GO. Two Na2S-reduced GOs were

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prepared to represent different degrees of reduction. Column experiments of the RGOs

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and GO were conducted under varied solution chemistry conditions, including different

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concentrations of monovalent and divalent cations (using Na+ and Ca2+ as the model

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cations), different pH, and the presence of Suwannee River humic acid (SRHA, selected

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as a model dissolved organic matter). Specific mechanisms (and their relative

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contributions) controlling the retention of RGOs were analyzed and found to be highly

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dependent on the types of cations present. The environmental implications of the findings

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are discussed.

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MATERIALS AND METHODS

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Materials. Quartz sand (50–70 mesh, 0.21–0.30 mm) was purchased from

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Sigma–Aldrich (St. Louis, MO); the average grain size was 0.26 mm. The sand was

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pretreated before use.28 It was first washed with 0.1 M HCl and then with 5% H2O2. Next,

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it was rinsed repeatedly with deionized (DI) water until neutral pH was reached. Then, it

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was oven-dried at 90 °C overnight and stored for future use. Suwannee River humic acid

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was purchased from the International Humic Substance Society (St. Paul, MN), and was

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reported to be composed of 52.6% C (wt:wt), 4.3% H, 42.0% O, and 1.2% N.18 The

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distribution of functional groups, determined with 13C-nuclear magnetic resonance, was

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carboxylic (15%), aromatic (31%), aliphatic (29%), and carbonyl (6%). In this paper the

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concentrations of SRHA are expressed as mg SRHA per liter of solution.

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Preparation and characterization of GO and RGOs. Graphene oxide was

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synthesized using a modified Hummers method.29 The detailed procedures are described

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in Supporting Information (SI). To obtain sulfide-reduced GOs, an aqueous suspension of

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~10 mg/L GO in 10 mM Tris buffer was first purged with N2 for 40 min to remove

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dissolved O2. The pH of the suspension was adjusted to neutral using 0.1 M HCl. Then, a

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stock solution of Na2S was added to give a Na2S concentration of 0.1 mM in the

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suspension. Next, a full volume of the suspension was added to 200-mL glass vials, and

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the suspension was mixed on an orbital shaker incubated at 30.0 ± 0.5 °C for 3 or 5 d.

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Then the suspension was filtered through 0.22-µm membrane filters. The RGO material

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retained on the filter was collected and added to approximately 200 mL DI water, and

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was ultra-sonicated at 100 W for 30 min. The rinsing procedure was repeated three times.

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Finally, the suspension was filtered through 0.45-µm membrane filters to remove large 5 ACS Paragon Plus Environment


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RGO aggregates. The sample undertaken the 3-d reduction is termed RGO3 and that

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undertaken the 5-d reduction is referred to as RGO5.

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Surface elemental compositions of RGO/GO samples were determined by X-ray

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photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, Japan). Fourier transform

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infrared (FTIR) transmission spectra were obtained using a 110 Bruker TENSOR 27

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apparatus (Bruker Optics Inc., Germany). Raman spectra were recorded with a Renishew

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inVia Raman spectrometer (RM2000, UK). Physical dimensions of the samples were

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characterized by atomic force microscopy (AFM) (MMAFM/STM, D3100M, Digital

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Ltd.).

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Column Transport Experiments. Quartz sand was dry-packed into Omnifit

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borosilicate glass columns (10 cm × 0.66 cm, Bio-Chem Valve Inc., Boonton, NJ) with

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10-µm stainless-steel screens (Valco Instruments Inc., Houston, TX) on both ends. Each

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column contained approximately 3.6 g sand (dry-weight) with an average length of 6.8

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cm. The columns were operated in an upward direction using syringe pumps (KD

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Scientific, Holliston, MA). The sand packed columns were equilibrated by flushing with

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100 mL DI water at a flow rate of 3 mL/h followed by 180 mL background electrolyte

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solution. Column properties are given in SI Table S1.

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All the influents were prepared immediately before the column experiments by

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diluting a stock suspension of RGO/GO in an electrolyte solution and then stirring for 2 h.

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The particle size distribution and ζ potential of RGO/GO nanoparticles in the influents

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were measured by dynamic light scattering (DLS) and electrophoretic mobility,

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respectively, using a ZetaPALS (Brookhaven Instruments, Holtsville, NY). Aggregation

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properties of the RGO/GO suspensions were examined with a JEM-2100 transmission 6 ACS Paragon Plus Environment

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electron microscope (TEM) (JEOL), and the samples were prepared by air-drying a drop

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of suspension onto a copper TEM grid (Electron Microscopy Sciences). UV absorbance

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spectra of the suspensions were obtained with a UV/vis spectrophotometer (UV-2401,

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Shimadzu Scientific Instruments, Columbia, MD). The scan was performed in the

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wavelength range of 200−600 nm. The slit width and sample interval were set at 1 and

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0.2 nm, respectively. No apparent changes in aggregation state was observed for any

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influents in the duration of respective column experiments.

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In a typical column experiment, the influent (containing approximately 13 mg/L of

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RGO/GO) was pumped into the column from a 100-mL glass syringes (SGE Analytical

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Science, Victoria, Australia). Column effluent samples were collected in 4-mL glass vials

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every 2–3 pore volumes (PV) to determine the concentrations of RGO/GO. The

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concentrations of RGO/GO in the influent (C0) and effluent (C) were determined by

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measuring the UV absorbance at 249 nm (for RGOs) or 230 nm (for GO) (SI Figure S1),

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based on a pre-established calibration curve of RGO/GO (SI Figure S2).30 In the presence

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of SRHA (5 mg/L), the concentrations of RGO/GO were determined using the method of

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Chen et al.,31 by obtaining the calibration curve of UV absorbance of RGO/GO as a

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function of RGO/GO concentration in the presence of 5 mg/L SRHA (SI Figure S3).

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Selected experiments were repeated to ensure data reproducibility (SI Figure S4).

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DLVO Calculations. The particle–collector interaction energy profiles under

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different solution chemistry conditions were calculated using the

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Derjaguin−Landau−Verwey−Overbeek (DLVO) theory and extended DLVO theory (see

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detailed equations and parameters in SI). The interaction energy profiles and the



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associated values of the maximum energy barriers and secondary minimum depth are

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given in SI Figures S5−S9 and Tables S2 and S3.

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RESULTS AND DISCUSSION Characteristics of RGOs and GO. Selected physicochemical properties of RGOs

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and GO are summarized in Table 1. In general, the data show varied degrees of reduction

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upon sulfide treatment of GO – the C/O ratios of RGOs are 40% and 63% higher than

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that of GO. Sulfide reduction resulted in significant changes in the distribution of surface

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O-functionality, as indicated by the deconvoluted peaks of C1s spectra that correspond to

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carbon atoms with different chemical states (Table 1 and SI Figure S10). For example, a

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decrease of the C−O carbon atoms and increase of carboxyl (O−C=O) carbon atoms were

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observed upon sulfide reduction. Reduction-induced changes in surface O-functional

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groups are also evident from the decrease of the epoxy groups (C−O−C stretching band at

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~1050 cm-1) and the increase of hydroxyl groups (O−H bending band at ~1385 cm-1) in

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FTIR spectra (SI Figure S11).32,33 It has been proposed that reduction of GO starts from

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the ring-opening of the epoxy groups that forms hydroxyl groups,34,35 and given the

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relatively mild reducing conditions in this study, dehydroxylation was likely incomplete,

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resulting the observed increase of hydroxyl groups. Furthermore, Raman spectra (SI

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Figure S12) show that the IG/ID values of RGOs are larger than that of GO (Table 1),

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indicating restoration of the sp2 network from sulfide reduction.36 The AFM images (SI

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Figure S13) show that sulfide reduction did not affect the areal dimensions and thickness

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of GO nanosheets significantly.


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In Table 2 ζ potential and average particle size (Zave) values of RGO/GO

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nanoparticles under various aqueous chemistries are presented. Aggregation state of

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RGOs and GO under selected solution chemistry conditions is illustrated with the TEM

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images (SI Figure S14). Under given aqueous chemistry conditions, the ζ potential values

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of both RGO3 and RGO5 are not significantly different from the respective value of GO,

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even though reduction of GO resulted in considerable changes in the type and abundance

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of O-functional groups. This was possibly attributable to the relatively high pKa range

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(9–10) of phenolic groups on the surfaces of carbonaceous materials.37 However, the Zave

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values follow the order of RGO5 > RGO3 > GO consistently over conditions studied.

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Enhanced aggregation of RGOs compared with GO was likely due to the increased

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hydrophobicity of nanosheets upon sulfide reduction (as evidenced by the increased C/O

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ratios and Raman spectra);26,27 in other words, RGO nanosheets with more hydrophobic

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graphitic lattice tended to undergo layer-to-layer aggregation in aqueous solutions.

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Furthermore, increasing cation concentration resulted in more significant aggregation of

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RGO3 and RGO5 than GO (SI Figure S15). This was particularly evident for Ca2+,

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because RGOs contained greater amounts of surface –OH groups, allowing aggregation

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of nanosheets via a cation bridging mechanism as reported for negatively charged

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particles.3,8,38-42

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Effect of Monovalent Cation on Transport. Sodium (used as a model monovalent

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cation) had more significant effects on the transport of RGOs than GO. The breakthrough

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profiles of the two RGOs and GO over four different NaCl concentrations (Figure 1)

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show that at 5 mM Na+ transport of the two RGOs were only slightly weaker than that of

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GO. With the increase of Na+ concentration, the transport of RGOs became increasingly 9 ACS Paragon Plus Environment


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more inhibited compared with the transport of GO. At 35 mM Na+, essentially no

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breakthrough of RGOs was observed, even though the transport of GO was still

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significant (C/C0, the ratio of effluent concentration to influent concentration, reached

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76% after 28 PV).

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Increasing cation concentration compresses double layer thickness and reduces

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double layer repulsion between nanoparticles and grain surfaces;8,43,44 additionally, the

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secondary energy minimum between particles and collector becomes deeper.15,45-48 The

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DLVO profiles of RGOs and GO (SI Figures S5 and S8) show that in this study

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increasing Na+ concentration had much larger effects on the depth of secondary minimum

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(Φsec) of RGOs than GO. At 5 mM Na+ the differences in Φsec value between the two

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RGOs and GO are very small (also see Table S2). However, the differences become

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increasingly larger with the increase of Na+ concentration (SI Figure S15). This trend is

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consistent with the much more inhibited transport of RGOs than GO at a Na+

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concentration of 10 mM and above. Moreover, because of the relatively larger particle

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sizes of RGOs compared to GO (refer to the Zave values in Table 2 and Figure S15) at

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elevated Na+ concentrations (e.g., 20 and 35 mM), straining could have played an

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important role in the retention of RGOs. It is commonly assumed that particles may be

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intercepted if the ratio of diameters of particle to collector (dp/dc, where dp and dc are

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diameters of particle and collector, respectively) is above 0.002–0.003.15,49-51 Based on

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the dp/dc values in Table 2, straining was likely a viable retention mechanism for RGOs at

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35 mM Na+ (or even 20 mM Na+). Furthermore, the retained particle concentration

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profiles of RGO3 and RGO5 are more hyper-exponential in shape than that of GO (SI

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Figure S16), which is consistent with the more significant straining for the RGOs.52 The 10 ACS Paragon Plus Environment

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considerably smaller particle sizes in effluents, in comparison to the respective sizes in

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influents, may also be indicative of straining (SI Figure S17).

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To understand the relative contributions of different retention mechanisms at high

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Na+ concentrations, step-wise flushing using electrolyte solutions of decreasing ionic

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strength was carried out for Column 11 (RGO3 at 35 mM Na+) and Column 12 (RGO5 at

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35 mM Na+). The observed release of retained RGO during the flushing (SI Figure S18)

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reflects the contribution of the secondary minimum.44 Results show that even at 35 mM

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Na+ deposition at the secondary minimum energy well was still the most important

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retention mechanism for RGOs—accounting for 76% overall retention of RGO3 and 75%

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of RGO5—and the contribution of straining due to increased aggregation was relatively

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small.

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Effects of Divalent Cation on Transport. Calcium (a model divalent cation) also

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influenced the transport of RGO much more significantly than that of GO, which became

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more significant with the increase of Ca2+ concentration (Figure 2). At 0.1 mM Ca2+, the

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breakthrough profile of RGO5 overlapped with that of GO in the initial phase and over

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93% breakthrough was reached after approximately 10 PV; at 0.3 mM Ca2+ the maximum

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breakthrough of RGO5 only reached 55%, much lower than the 93% of GO; at 0.5 mM

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Ca2+ essentially no breakthrough of RGO5 was observed, whereas the breakthrough of

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GO reached near 86%.

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Interestingly, the much more remarkable transport inhibition of Ca2+ on RGO5 than

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on GO cannot be explained with the DLVO theory. The differences in the DLVO profiles

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between RGO5 and GO—in particular, deepened secondary minimum well—are small

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(SI Figure S6), even at 0.5 mM Ca2+ (note that the Φsec value of RGO5 at 0.5 mM Ca2+ is 11 ACS Paragon Plus Environment


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only -0.0327 KBT, even smaller than the value of RGO5 at 5 mM Na+). More importantly,

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the contribution of secondary minimum to the overall retention of RGO5—determined by

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flushing the columns with low-ionic strength solution and DI water—was only 8.0% for

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Column 16 (0.3 mM Ca2+) and 7.5% for Column 18 (0.5 mM Ca2+). This was largely in

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contrast with the important contribution of secondary minimum when Na+ was the

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background cation. Likewise, straining was probably not an important cause for the more

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significant transport inhibition of RGO by Ca2+. The Zave values of RGO5 are 585.7 nm at

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0.3 mM Ca2+ and 908.8 nm at 0.5 mM Ca2+ (Table 2), both are smaller than the Zave value

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of GO at 35 mM Na+ (1113 nm). Since the much larger GO could pass through the

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column relatively easily (Figure 1d), it was unlikely the degree of straining was

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significant for the transport of RGO5 in the presence of 0.3 or 0.5 mM Ca2+.

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Based on the discussion above, we propose that the much stronger transport

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inhibition effects of Ca2+ on RGO5 than GO was likely attributable to the stronger

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particle−collector bridging effects of Ca2+ with RGO5. Cation bridging has been

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recognized as an important retention mechanism for negatively charged

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nanoparticles.8,40,43,53 Compared with GO, RGO5 contained a greater amount of surface

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–OH group, which is a strong metal-complexing moiety, both in its neutral and

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deprotonated forms.54 Thus, in the presence of Ca2+, RGO5 could bind to the sand grains

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through Ca2+ bridging (i.e., Ca2+ serves as the bridging agent between surface

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O-functional groups of sand grains and –OH groups of RGO5) more strongly than GO.

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The stronger binding of RGOs than GO to sand grains in the presence of Ca2+ was

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confirmed with adsorption experiments (see SI for detailed procedures), which showed


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that adsorption coefficients of GO, RGO3, and RGO5 to sand were 7.59, 11.5, and 13.8

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L/kg, respectively.

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Effects of pH on Transport. The effects of pH on the transport of RGOs (tested

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with RGO5) were largely dependent on the type of cations present in the background

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solution. Over a pH range of 5 to 9, transport of RGO5 increased drastically with

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increasing pH when Na+ was the predominant cation; but counterintuitively, increasing

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pH had very little effect on the transport of RGO5 when Ca2+ was the predominant cation

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(Figure 3). For example, at 20 mM Na+, the maximum breakthrough of RGO5 only

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reached 11% at pH 5, but at pH 7 and 9 the maximum breakthrough reached 61% and

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76%, respectively. At 5 mM Na+, the effect of pH was less remarkable, but still exhibited

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the same trend. In comparison, at 0.3 mM Ca2+ the breakthrough curves of RGO5 at pH

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4.5, 5.5 and 6.5 nearly overlap.

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Interestingly, at any given Na+ concentration ζ potential did not change appreciably

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with pH (Table 2), and the DLVO profiles (SI Figures S7 and S9) show that pH had very

283

small effects on the depth of secondary minimum. Even though small decrease in Zave

284

with increasing pH was observed at 5 and 20 mM Na+ (likely due to the deprotonation of

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O-functional groups), it likely had negligible effect on transport because straining was

286

not a major retention mechanism. As Na+ is a weak complexing agent that does not

287

induce strong bridging (as can Ca2+), varying pH could not have affected the deposition

288

of RGO5 through bridging. Based on the fact that the pH effect became more significant

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with the increase of Na+ concentration (Figures 3a−c), we propose that pH affected the

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transport of RGO5 mainly by affecting the electrostatic double layer properties of sand

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grains. That is, the surface charge of sand grains became more negative with increasing 13 ACS Paragon Plus Environment


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pH (the ζ potential values are -38 mV, -50 mV, and -55 mV at pH 5, 7, and 9,

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respectively55), and this resulted in increased accumulation of Na+ near the grain surface.

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In aqueous solutions, Na+ ions exist as densely hydrated ions, and accumulate on grain

295

surfaces via outer-sphere complexation. The accumulation of large hydrated Na+ ions

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likely hindered the interaction of RGO5 with grain surface (mainly by steric hindrance),

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and therefore, facilitated the transport of RGO. The higher the Na+ concentration, the

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more significant the accumulation of hydrated Na+ ions on grain surface, and

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consequently, the more pronounced transport enhancement.

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The fact that pH had little effect on the transport of RGO5 when Ca2+ was the

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predominant cation is consistent with the above mentioned cation-bridging mechanism by

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Ca2+. Increasing pH could also enhance the accumulation of Ca2+ near the grain surface,

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in a similar manner as for Na+. However, the increased accumulation of Ca2+ (in

304

particular, those adsorbed on the grain surface) might also enhance the deposition of

305

RGO5 through enhanced cation bridging.

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Effects of SRHA on Transport. The presence of SRHA enhanced the transport of

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RGO5, even under solution chemistry conditions extremely unfavorable for transport;

308

however, the significance of transport enhancement by SRHA was also largely dependent

309

on the type of cations present in the background solution (Figure 4). When Na+ was the

310

background cation, transport-enhancement effect of SRHA was very significant. For

311

example, in the absence of SRHA the transport of RGO5 at 35 mM Na+ was completely

312

inhibited, but in the presence of 5 mg/L SRHA, breakthrough of RGO5 reached 85%

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after 18 PV (Figure 4b). Because the presence of SRHA had negligible effects on ζ

314

potential and sizes of RGO5 nanoparticles (Table 2), SRHA likely enhanced the transport 14 ACS Paragon Plus Environment

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of RGO5 via steric hindrance.3,38,56-59 That is, adsorption of SRHA to RGO5 and to the

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surfaces of quartz sand interfered with the interaction between RGO5 and quartz sand,

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and thus, inhibiting the deposition of RGO5. To further understand the relative

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importance of SRHA coating on quartz sand versus SRHA adsorption on RGO5,

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additional experiments were conducted by first saturating the sand with SRHA solution

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before flushing RGO5 suspension through the column.60 In this case, only the sand was

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coated with SRHA, whereas adsorption of SRHA to RGO5 was negligible. The transport

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of RGO5 through the SRHA-saturated columns was significantly enhanced compared

323

with the transport of RGO5 in the absence of SRHA but still considerably lower than the

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transport of RGO5 in the presence of SRHA (Figures 4b−c). Judging from the relative

325

positions of the breakthrough curves, it was likely that both SRHA adsorption on sand

326

grains and adsorption to nanoparticles contributed to the transport-enhancement effects of

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SRHA.

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An interesting (and surprising) observation in this set of experiments was that SRHA

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was much less effective in mitigating the transport-inhibiting effects of divalent cation

330

(i.e., Ca2+) than those of monovalent cation (i.e., Na+). This appears to be related to the

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different transport-inhibiting mechanisms between Ca2+ and Na+. As discussed earlier,

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Na+ enhanced the deposition of RGO mainly by deepening the secondary minimum well,

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and adsorption of SRHA could greatly weaken this deposition mechanism through steric

334

hindrance. Ca2+, however, enhanced the retention of RGO primarily by serving as a

335

bridging agent between RGO and sand grains. Thus, even when quartz sands were coated

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with SRHA, binding of RGO could still occur through bridging. In this case, the bridging

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would be in the form of RGO−Ca−SRHA (instead of RGO−Ca−sand), because SRHA is 15 ACS Paragon Plus Environment


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also rich in surface O-functional groups and able to complex with divalent

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cations.39,40,56,61,62

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Environmental Implications. Given its reactive nature, GO likely can be reduced

341

in aquatic environments by naturally occurring reductants such as sulfide, among others,

342

resulting in increased hydrophobicity with corresponding changes in surface O-functional

343

groups. As demonstrated here and in our previous study, reduction of GO will likely

344

reduce its colloidal stability and consequently, mobility in the subsurface. While the

345

reduced mobility is more or less expected, the underlying mechanisms are not

346

straightforward and in some cases counterintuitive. The most important observation in

347

this study was that because RGOs contained relatively higher amounts of surface

348

hydroxyl groups than GO, their transport behaviors deviated more significantly from the

349

DLVO theory in the presence of Ca2+ (i.e., 0.3 mM and above). Additionally, the sulfide

350

reduction-induced changes in the types and distribution of GO surface O-functional

351

groups significantly altered the underlying mechanisms via which solution chemistry

352

(e.g., pH and dissolved organic matter) influenced the transport of GO, leading to

353

strikingly different effects depending on the type of predominant cations in the solution.

354

Overall, these findings clearly highlight the significance of identifying, understanding,

355

and delineating relevant GO transformation pathways as they relate to the transport, fate,

356

and effects, thus ultimate sustainability, of this novel class of carbon nanomaterials. Note

357

that the GO products used in this study were of relatively uniform sizes, and more

358

research is needed to understand how size effects of GO may interplay with GO surface

359

O-functionalities to affect the overall transport properties.

360 16 ACS Paragon Plus Environment

Page 17 of 33


361

Supporting Information. Procedures used to synthesize GO and prepare SRHA

362

solutions, calculation of DLVO and extended DLVO interaction energy, determination of

363

binding affinities of RGO/GO to sand, column properties, determination of RGO/GO

364

concentrations with UV/Vis spectrometry, data reproducibility results, particle–collector

365

interaction energy profiles, XPS, FTIR and Raman spectra of RGOs and GO, AFM and

366

TEM images, retained particle profiles, changes of particle sizes during transport,

367

column-flushing results and mass balance. This material is available free of charge via

368

the Internet at http://pubs.acs.org.

369 370

Acknowledgments. This project was supported by the Ministry of Science and

371

Technology of China (Grant 2014CB932001), and the National Natural Science

372

Foundation of China (Grants 21425729 and 21237002).

373 374

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Page 26 of 33

Table 1. Selected Physicochemical Properties of RGOs and GO C b (wt%) RGO/ GO a

aromatic rings

epoxy/ hydroxyl

carbonyl

carboxyl

GO RGO3 RGO5

32.61 40.12 43.56

23.78 21.14 21.47

8.52 7.83 5.31

1.22 2.98 4.38

total Cb (wt%) 66.13 72.06 74.72

total Ob (wt%) 31.47 24.49 21.87

C/O ratio

IG/ID c

2.10 2.94 3.42

1.03 1.11 1.20

a

RGO represents sulfide-reduced GO; the suffix “3” or “5” indicates reduction time (d). Analyzed with X-ray photoelectron spectroscopy. c Analyzed with Raman spectrometry.

b


Page 27 of 33


Table 2. Selected Properties of RGO/GO Suspensions under Different Solution Chemistry Conditions (mV)

EPM b (10 m2/V·s)

Zave c (nm)

dp/dc d

-43.2 ± 1.3 -42.1± 0.4 -42.2 ± 0.5 -34.8 ± 0.7 -33.6 ± 0.8 -31.8 ± 1.1 -28.0 ± 0.6 -26.0 ± 0.4 -24.1 ± 1.0 -26.9 ± 0.7 -25.4 ± 0.6 -22.5 ± 1.0 -23.3 ± 1.4 -21.3 ± 0.6 -18.8 ± 0.7 -16.6 ± 0.8 -10.3 ± 0.7 -10.6 ± 1.2 -47.6 ± 1.1 -49.3 ± 0.9 -51.5 ± 0.7 -42.8 ± 0.7 -43.8 ± 2.0 -45.9 ± 1.6 -23.5 ± 0.9 -24.7 ± 0.4 -25.8 ± 0.7 -16.0 ± 1.0 -15.4 ± 0.9 -16.9 ± 1.2

-3.37 ± 0.10 -3.29± 0.03 -3.30 ± 0.04 -2.72 ± 0.05 -2.62 ± 0.06 -2.48 ± 0.09 -2.19 ± 0.05 -2.03 ± 0.03 -1.88 ± 0.08 -2.10 ± 0.05 -1.98 ± 0.05 -1.76 ± 0.08 -1.82 ± 0.11 -1.66 ± 0.05 -1.47 ± 0.05 -1.30 ± 0.06 -0.80 ± 0.05 -0.83 ± 0.09 -3.72 ± 0.09 -3.85 ± 0.07 -4.02 ± 0.05 -3.34 ± 0.05 -3.42 ± 0.16 -3.59 ± 0.12 -1.84 ± 0.07 -1.93 ± 0.03 -2.02 ± 0.05 -1.25 ± 0.08 -1.20 ± 0.07 -1.32 ± 0.09

238.5 254.0 261.6 376.6 400.2 439.6 557.4 656.6 778.2 1112.5 1377.4 1652.6 237.3 307.1 350.5 585.7 609.3 908.8 236.8 237.9 231.4 259.6 238.5 241.9 733.1 634.3 612.7 644.5 634.5 641.5

0.0009 0.0009 0.0010 0.0014 0.0015 0.0017 0.0021 0.0025 0.0030 0.0043 0.0053 0.0064 0.0009 0.0012 0.0013 0.0023 0.0023 0.0035 0.0009 0.0009 0.0009 0.0010 0.0009 0.0009 0.0028 0.0024 0.0024 0.0025 0.0024 0.0025

-25.3 ± 0.6

-1.98 ± 0.05

676.6

0.0026

35 mM NaCl+5 mg/L SRHA pH 5.7

-21.3 ± 0.9

-1.66 ± 0.07

1545.2

0.0059

RGO5

0.5 mM CaCl2+5 mg/L SRHA pH 5.3

-11.5 ± 1.0

-0.90 ± 0.08

904.2

0.0035

34 e

RGO5

35 mM NaCl SRHA-saturated column

-21.4 ± 1.1

-1.67 ± 0.09

1612.7

0.0062

35 e

RGO5

0.5 mM CaCl2 SRHA-saturated column

-11.4 ± 1.0

-0.89 ± 0.08

888.9

0.0034

Column No.

RGO/ GO a

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

GO RGO3 RGO5 GO RGO3 RGO5 GO RGO3 RGO5 GO RGO3 RGO5 GO RGO5 GO RGO5 GO RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5 RGO5

31

RGO5

32

RGO5

33

Background solution 5 mM NaCl pH 5.8 5 mM NaCl pH 5.8 5 mM NaCl pH 5.7 10 mM NaCl pH 5.6 10 mM NaCl pH 5.7 10 mM NaCl pH 5.7 20 mM NaCl pH 5.4 20 mM NaCl pH 5.5 20 mM NaCl pH 5.6 35 mM NaCl pH 5.6 35 mM NaCl pH 5.5 35 mM NaCl pH 5.5 0.1 mM CaCl2 pH 5.3 0.1 mM CaCl2 pH 5.2 0.3 mM CaCl2 pH 5.0 0.3 mM CaCl2 pH 5.1 0.5 mM CaCl2 pH 5.1 0.5 mM CaCl2 pH 5.0 DI water pH 5.0 DI water pH 7.0 DI water pH 9.0 5 mM NaCl pH 5.0 5 mM NaCl pH 7.0 5 mM NaCl pH 9.0 20 mM NaCl pH 5.0 20 mM NaCl pH 7.0 20 mM NaCl pH 9.0 0.3 mM CaCl2 pH 4.5 0.3 mM CaCl2 pH 5.5 0.3 mM CaCl2 pH 6.5 20 mM NaCl+5 mg/L SRHA pH 5.6

ζ potentialb

-8



Page 28 of 33

a

RGO represents sulfide-reduced GO; the suffix “3” or “5” indicates reduction time (d). Values after ± sign represent standard deviation of five replicates. c Hydrodynamic diameter of RGO/GO nanoparticles based on dynamic light scattering analysis. d dp/dc represent ratio of Zave of RGO/GO aggregates to average diameter of sand grains. e Column was pre-saturated with SRHA before injecting RGO suspension. b


Page 29 of 33


(a) 5 mM NaCl

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

5

(b) 10 mM NaCl

1.0

C/C0

C/C0

1.0

10 15 20

25

30

0

5

10

PV

0.6

0.6

C/C0

C/C0

0.8

0.4

0.2

0.0

0.0 10

15 PV

30

25

30

0.4

0.2

5

25

20

(d) 35 mM NaCl

1.0

0.8

0

20

PV

(c) 20 mM NaCl

1.0

15

25

30

0

5

10

15

20

PV

GO

RGO3

RGO5

Figure 1. Effects of Na+ on transport of RGOs and GO: (a) 5 mM NaCl (Columns 1–3); (b) 10 mM NaCl (Columns 4–6); (c) 20 mM NaCl (Columns 7–9); (d) 35 mM NaCl (Columns 10–12).



(a) 0.1 mM CaCl2

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

5

10 15 20 25 30 35

(b) 0.3 mM CaCl2

1.0

C/C0

C/C0

1.0

Page 30 of 33

0

5 10 15 20 25 30 35 PV

PV (c) 0.5 mM CaCl2

1.0

C/C0

0.8 0.6 0.4 GO RGO5

0.2 0.0 0

5

10 15 20 25 30 35 PV

Figure 2. Effects of Ca2+ on transport of RGO5 and GO: (a) 0.1 mM CaCl2 (Columns 13 and 14); (b) 0.3 mM CaCl2 (Columns 15 and 16); (c) 0.5 mM CaCl2 (Columns 17 and 18).


Page 31 of 33


(a) DI water

1.0

(b) 5 mM NaCl

1.0 0.8

C/C0

0.8 C/C0

0.6 0.4

0.6 0.4

pH 5.0 pH 7.0 pH 9.0

0.2 0.0 0

5

pH 5.0 pH 7.0 pH 9.0

0.2 0.0

10 15 20 25 30 35

0

5

PV (c) 20 mM NaCl

1.0

pH 4.5 pH 5.5 pH 6.5

0.8 C/C0

C/C0

0.8

(d) 0.3 mM CaCl2

1.0

pH 5.0 pH 7.0 pH 9.0

0.6

0.6

0.4

0.4

0.2

0.2

0.0

10 15 20 25 30 35 PV

0.0 0

5

10 15 20 25 30 35

0

5

PV

10 15 20 25 30 35 PV

Figure 3. Effects of pH on transport of RGO5: (a) DI water (Columns 19–21); (b) 5 mM NaCl (Columns 22–24); (c) 20 mM NaCl (Columns 25–27); (d) 0.3 mM CaCl2 (Columns 28–30).



(a) 20 mM NaCl

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

5

10

15

20

(b) 35 mM NaCl

1.0

C/C0

C/C0

1.0

25

30

Page 32 of 33

0

5

PV

10

15

20

25

30

PV

(c) 0.5 mM CaCl2

1.0

C/C0

0.8 0.6 RGO5 RGO5 + SRHA RGO5 (SRHA-saturated column)

0.4 0.2 0.0 0

5

10

15

20

25

30

PV Figure 4. Effects of 5 mg/L Suwannee River humic acid (SRHA) on transport of RGO5: (a) 20 mM NaCl (Columns 9 and 31); (b) 35 mM NaCl (Columns 12, 32, and 34); (c) 0.5 mM CaCl2 (Columns 18, 33, and 35).


Page 33 of 33


TOC Art GO

Deepened secondary minimum RGO C/O

GO

S2-

RGO

OH

H

H O

O

Ca

O-

O

Enhanced cation-bridging

Ca O H

OH


Transport of Sulfide-Reduced Graphene Oxide in Saturated Quartz

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