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Cation-Inhibited Transport of Graphene Oxide Nanomaterials in Saturated Porous Media: The Hofmeister Effects Tianjiao Xia, Yu Qi, Jing Liu, Zhichong Qi, Wei Chen, and Mark R. Wiesner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05007 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016
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Cation-Inhibited Transport of Graphene Oxide
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Nanomaterials in Saturated Porous Media: The Hofmeister
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Effects
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Tianjiao Xia,† Yu Qi,† Jing Liu,† Zhichong Qi,† Wei Chen,*† Mark R. Wiesner‡
5 6 †
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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 300350, China
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‡
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Implications of NanoTechnology, Duke University, Durham, North Carolina 27708,
13
United States
Department of Civil and Environmental Engineering, Center for the Environmental
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Manuscript prepared for Environmental Science & Technology
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* Corresponding author: (Phone/fax) 86-22-85358169; (E-mail)
[email protected].
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TOC Art
Small ionic radius (strongly hydrated)
Large ionic radius (weakly hydrated)
C/C0
20
Na+, K+, Cs+
1.0 0.8 0.6 0.4 0.2 0.0
(a) GO in 20 mM M+ Na+ K+ + Cs 0
Outer-sphere complexation
21
Inner-sphere complexation
C/C0
Mg2+, Ca2+, Ba2+
1.0 0.8 0.6 0.4 0.2 0.0
10
20 PV
30
(b) GO in 0.5 mM Me
40
2+
Mg2+ Ca2+ Ba2+ 0
10
20 PV
30
40
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ABSTRACT Transport of negatively charged nanoparticles in porous media is largely affected by
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cations. To date, little is known about how cations of the same valence may affect
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nanoparticle transport differently. We observed that the effects of cations on the transport
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of graphene oxide (GO) and sulfide-reduced GO (RGO) in saturated quartz sand obeyed
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the Hofmeister series, that is, transport-inhibition effects of alkali metal ions followed the
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order of Na+ < K+ < Cs+, and those of alkaline earth metal ions followed the order of
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Mg2+ < Ca2+ < Ba2+. With batch adsorption experiments and microscopic data, we
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verified that cations having large ionic radii (and thus weakly hydrated) interacted with
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quartz sand and GO/RGO more strongly than did cations of small ionic radii. In
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particular, the monovalent Cs+ and divalent Ca2+ and Ba2+, which can form inner-sphere
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complexes, resulted in very significant deposition of GO/RGO via cation bridging
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between quartz sand and GO/RGO, and possibly via enhanced straining, due to the
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enhanced aggregation of GO/RGO from cation bridging. The existence of the Hofmeister
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effects was further corroborated with the interesting observation that cation bridging was
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more significant for RGO, which contained greater amounts of carboxyl and phenolic
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groups (i.e., metal-complexing moieties) than did GO. The findings further demonstrate
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that transport of nanoparticles is controlled by the complex interplay between
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nanoparticle surface functionalities and solution chemistry constituents.
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INTRODUCTION Graphene-based materials, such as graphene oxide (GO) and reduced graphene oxide
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(RGO), are a class of promising nanomaterials with potential applications in the areas of
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electronic devices, energy storage, chemical catalysis, drug delivery and functional
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materials, to mention a few.1 The increasingly fast growth of production and use of these
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materials will inevitably result in their environmental release. A number of studies have
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shown that graphene-based materials may have adverse environmental impact.2-5 Thus, it
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is important to understand the fate and transport of these materials in the environment.
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Several studies have been conducted to understand the transport of GO and RGO in
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porous media.6-17 It has been reported that GO generally exhibits high mobility,6-17
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whereas RGO can be less mobile;17 transport of GO is not very responsive to pH (5 to
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9),7, 10, 11 but sensitive to the increase of ionic strength;7, 8, 10, 11 and the presence of natural
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organic matter or surfactants can enhance the transport of GO, via steric hindrance or by
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competing with GO for deposition sites on grain surfaces.10, 11, 15-17 Divalent cations have
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particularly strong effects on the transport of GO/RGO, not only because they are more
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effective in compressing the electrical double layer than monovalent cations, but also
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because they can increase deposition of GO/RGO nanosheets via cation-bridging.12, 17 In
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our previous study using GO and two RGO materials obtained by reducing GO with low
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concentration of sulfide (designed to mimic environmentally relevant reduction of GO),
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we further demonstrated that the significance of cation-bridging is dependent on the type
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of surface O-functional groups.17 Specifically, even though the RGOs had much lower O-
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contents than GO, more significant cation-bridging was observed for these two materials,
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because the RGOs contained greater amounts of phenolic groups, a moiety that can 4
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complex strongly with Ca2+ than the epoxy groups of GO.17 Furthermore, studies on
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aggregation properties of GO and RGO have shown that the sizes of GO/RGO aggregates
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also depend on the interplay between divalent cations and GO/RGO surface O-
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functionalities.18, 19 For instance, Ca2+ and Mg2+ can enhance the face-to-face interactions
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between GO nanosheets,18, 19 but negligible binding was observed between nanosheets of
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a RGO material obtained by solvothermally reducing GO with N-methyl-2-pyrrolidone.19
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Such cation-induced effects on aggregation properties can in turn affect the transport
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properties of GO/RGO, for example, through physical straining.20-23
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Ion composition can be very complex in natural aquatic environments. Even though a
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large number of studies have demonstrated that divalent cations can affect the transport
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of negatively charged nanoparticles more significantly than monovalent cations (e.g.,
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Mg2+/Ca2+ vs. Na+), very little is known about how cations of the same valence (e.g.,
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alkali metal ions such as Na+ and K+, and alkaline earth metal ions such as Mg2+ and Ca2+)
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may affect nanoparticle transport differently. Note that several studies have shown that
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Ca2+ was more effective in inducing aggregation of negatively charged nanoparticles
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(including GO and RGO) than Mg2+,18, 23-28 because Ca2+ can bind to O-functional groups
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of nanoparticles more strongly than Mg2+.18, 23, 27 Such cation species-dependent effects
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on particle−particle interactions likely can be extrapolated to particle−collector
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interactions. For example, Ca2+ may result in more significant deposition of GO/RGO
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than Mg2+, via more significant cation-bridging between grain surfaces and GO/RGO
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nanosheets.
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Herein, we hypothesize that the effects of cations on the transport of GO and RGO in
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saturated porous media under unfavorable deposition conditions will obey the Hofmeister 5
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series.29, 30 Specifically, we expect that cations having larger ionic radii (and accordingly,
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being more weakly hydrated) will interact with both grain surfaces and GO/RGO
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nanosheets more strongly than cations of smaller ionic radii, and thereby, will induce
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more significant transport inhibition of GO/RGO in saturated porous media. Based on
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observations of various cations and anions to either destabilize or stabilize proteins,29 the
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Hofmeister series can be explained in large part by the extent that ions interact with water
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and in particular, their degree of hydration. The Hofmeister series has been used to
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explain the affinities of ions to clays and oxides,31-33 and is likely to be applicable also for
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cation−GO/RGO interactions. To test the hypothesis, we examined the transport of GO
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and a sulfide-reduced GO in saturated quartz sand, as affected by different monovalent
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cations (Na+, K+ and Cs+) and different divalent cations (Mg2+, Ca2+ and Ba2+). We
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intentionally chose cations from the same groups in the periodic table as the comparative
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ions to single out their differences in ionic and hydrated radii (see Supporting
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Information (SI) Table S1). The breakthrough curves and retained profiles of GO/RGO in
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the presence of different monovalent or divalent cations were compared, and the
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underlying mechanisms controlling the effects of different cations were analyzed.
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Supplemental batch adsorption experiments were conducted and microscopic evidence
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was collected to verify the proposed mechanisms.
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MATERIALS AND METHODS
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Materials. Graphene oxide (referred to as GO hereafter) was synthesized using a
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modified Hummers method.34 Reduced graphene oxide (referred to as RGO hereafter)
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was prepared by reducing GO using Na2S. The detailed procedures for the synthesis of 6
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GO and RGO are described in SI. Surface elemental compositions of GO/RGO were
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determined by X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, Japan).
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Fourier transform infrared (FTIR) transmission spectra were obtained using a 110 Bruker
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TENSOR 27 apparatus (Bruker Optics Inc., Germany).
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Quartz sand (50–70 mesh, 0.21–0.30 mm) was purchased from Sigma–Aldrich
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(USA). The average grain size of the sand was 0.26 mm. The sand was pretreated before
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use.35 It was first washed with 0.1 M HCl and then with 5% H2O2. Next, it was rinsed
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repeatedly with deionized water until neutral pH was reached. Then, it was oven-dried at
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90 °C overnight and stored for future use.
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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., USA) with 10-µm
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stainless-steel screens (Valco Instruments Inc., USA) on both ends. Each column
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contained approximately 3.6 g sand (dry-weight) with an average length of 6.8 cm. The
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columns were operated in an upward direction12, 13 using syringe pumps (KD Scientific,
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USA). The sand-packed columns were equilibrated by flushing with 100 mL deionized
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water at a flow rate of 3 mL/h followed by 180 mL background electrolyte solution.
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Column properties are given in SI Table S2.
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All the influents were prepared immediately before the column experiments by
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diluting a stock suspension of GO/RGO in an electrolyte solution and then stirring for 2 h.
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Electrophoretic mobility (EPM) and hydrodynamic diameter (Dh) of GO/RGO
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nanoparticles in the influents were measured with a ZetaSizer Nano ZS (Malvern
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Instruments, UK). Ultraviolet (UV) absorbance spectra of the suspensions were recorded
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with a UV/vis spectrophotometer (UV-2401, Shimadzu Scientific Instruments, USA). 7
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The scan was performed in the wavelength range of 200−600 nm. The slit width and
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sample interval were set at 1 and 0.2 nm, respectively. Aggregation properties of the
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RGO/GO suspensions were also examined with a JEM-2100 transmission electron
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microscope (TEM) (JEOL Ltd., Japan), and the samples were prepared by air-drying a
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drop of suspension onto a copper TEM grid (Electron Microscopy Sciences, USA).
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In a typical column experiment, the influent was pumped into the column from a
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100-mL glass syringes (SGE Analytical Science, Australia). Column effluent samples
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were collected in 4-mL glass vials every 2–3 pore volumes (PV) to determine the
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concentrations of GO/RGO. The concentrations of GO/RGO in the influent (C0) and
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effluent (C) were determined by measuring the UV absorbance at 230 nm (for GO) or
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249 nm (for RGO), based on pre-established calibration curves of RGO and GO (SI
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Figure S1).
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Batch Adsorption Experiments and Granular-Scale Visualization. Batch
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adsorption experiments were conducted to determine the binding of GO and RGO to
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quartz sand as affected by different cations. First, 5 g sand was added to each of a series
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of 20-mL amber glass vials. Then, 15 mL of a GO/RGO suspension was added to each
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vial. The vials were equilibrated for 3 d by tumbling at 8 rpm in the dark. Then, the vials
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were left undisturbed overnight, and the supernatants were withdrawn to measure the
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concentrations of GO or RGO in the suspension, using a high sensitivity total organic
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carbon analyzer (Shimadzu Scientific Instruments, USA). The concentrations of GO or
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RGO adsorbed to sand were calculated based on a mass balance approach. The
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distribution coefficients, Kd (L/kg), between sand and water is defined as Kd = q/CW,
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where q (mg/kg) and CW (mg/L) are equilibrium concentrations of GO or RGO on sand 8
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and in the suspension, respectively. The adsorption experiments were done in triplicate.
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Two sets of Kd values were obtained, by varying the initial concentrations of GO/RGO.
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The GO and RGO adsorbed to sand grains were also visualized with a laser scanning
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confocal microscope (Leica TCS SP5, Germany). The adsorbed GO or RGO was
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detected by fluorescence, while the sand surfaces were visualized by UV light.36
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DLVO Calculations and Analysis of Attachment Efficiencies. The particle–
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collector interaction energy profiles under different solution chemistry conditions were
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calculated using the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory (see detailed
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equations and parameters in SI). (Note that in theory the equations described in SI are
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only applicable for spherical particles and 1:1 electrolytes. Thus, the DLVO calculations
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are only approximate, intended to qualitatively illustrate the relative effects of different
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cations on the depth of secondary minimum well.) Additionally, theoretical attachment
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efficiencies, α, were calculated using the Maxwell model and were compared with the
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attachment efficiencies observed in the column experiments (the detailed equations are
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given in SI).
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RESULTS AND DISCUSSION Characteristics of GO and RGO. Selected physicochemical properties of GO and
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RGO are summarized in Table 1. Graphene oxide contained abundant O-functionality
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such as epoxy/hydroxyl (C-O-C/C-OH) (29.32%), carbonyl (C=O) (7.59%) and carboxyl
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(O-C=O) (3.85%), as indicated by the deconvoluted peaks of C 1s spectra that correspond
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to carbon atoms with different chemical states (SI Figure S2). In comparison, RGO had a
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much higher C/O ratio, indicating the loss of O-functional groups during reduction. 9
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Notably, a higher carboxyl content was observed for RGO than for GO, likely from the
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conversion of the epoxy groups.37 Furthermore, the FTIR spectra (SI Figure S3) show the
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increase of hydroxyl groups (O−H bending band at ~1385 cm-1) of RGO, compared with
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those of GO. The higher contents of carboxyl and phenolic groups (two metal-
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complexing moieties) of RGO likely will make it more sensitive to the effects of divalent
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cations.
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Electrokinetics and Aggregation Properties of GO and RGO as Affected by
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Cation Species. The effects of cations on the electrokinetics and aggregation properties
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of GO/RGO suspensions varied significantly with the type of cation species in the
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background solution (SI Table S3, Figure 1, and SI Figure S4). Less negative EPM
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values of GO/RGO were observed at low concentrations of divalent cations (0.1 and 0.5
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mM) than at high concentrations of monovalent cations (10 and 20 mM). Similarly,
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divalent cations were also more effective in causing aggregation of GO/RGO nanosheets
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than were monovalent cations, as indicated by the Dh values (Table S3 and Figure 1) and
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the TEM images (Figure S4). The greater effects of divalent cations are consistent with
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the literature.5, 21, 23, 24, 38-40
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Interestingly, even cations of the same valence had markedly different effects on the
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electrokinetics and aggregation properties of GO/RGO. Specifically, the capability of
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cations to neutralize the negative surface charge of GO/RGO generally followed the order
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of Cs+ > K+ > Na+, and Ba2+ > Ca2+ > Mg2+, as indicated by the EPM values (Table S3
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and Figure 1). The effects of cations on aggregation also followed the same orders, as
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indicated by the Dh values (Table S3 and Figure 1) and the different extents of
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aggregation shown by the TEM images (Figure S4). These trends correlated well with the 10
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Hofmeister series, that is, the effects of cations on electrokinetics and aggregation
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increase with the ionic radii of the cations. Cations with smaller ionic radii have higher
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hydration numbers and larger hydrated radii, whereas cations with larger ionic radii have
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weaker hydration shells, and can more easily be detached from their hydration layer.21, 41
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Thus, Cs+ (with the most labile hydration sphere among the three monovalent cations)
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has the tendency to form inner-sphere complexes, whereas K+ and Na+ can only form
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outer-sphere complexes.21, 42, 43 Similarly, the poorly hydrated Ba2+ and Ca2+ can form
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inner-sphere complexes, whereas Mg2+ forms mainly outer-sphere complexes.21, 44
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Cations that form inner-sphere complexes with GO/RGO surfaces can more effectively
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neutralize the negative surface charge of the nanosheets. Furthermore, by forming
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complexes with carboxyl and phenolic groups of GO/RGO, the poorly hydrated cations
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can result in more significant aggregation of nanosheets by serving as the bridging agents.
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Note that the differences in Dh as affected by cations of the same valence were more
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profound than the differences in EPM, indicating the important role of cation bridging in
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the aggregation of GO/RGO.
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Transport of GO and RGO as Affected by Different Monovalent Cations. The
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effects of monovalent cations on the transport of both GO and RGO were strongly
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dependent on cation species (Figure 2). In particular, Cs+ exerted much greater transport
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inhibition effects than K+ and Na+. For example, at 10 mM Na+ or K+ breakthrough of
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GO reached ~90% after 17 PV, whereas at 10 mM Cs+ breakthrough only reached ~44%
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(Figure 2a). At the higher cation concentration tested (20 mM) the differences were even
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larger, in that little breakthrough was observed in the presence of Cs+, whereas
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breakthrough reached 83% and 76% in the presence of Na+ or K+ (Figure 2c). 11
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Furthermore, between Na+ and K+, the latter inhibited the transport of GO to a larger
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extent, especially at the higher cation concentration (20 mM). Similar patterns were
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observed for RGO (Figures 2b and 2d). Note that RGO exhibited lower mobility than GO
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under the same solution chemistry conditions. This was attributable to the greater surface
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hydrophobicity, less negative surface charge, and larger particle size of RGO aggregates
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than GO aggregates, as demonstrated in our previous study.17
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The observation that different monovalent cations inhibited the transport of GO and
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RGO to different extents can be understood by considering the mechanisms via which
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cations affect the transport of negatively charged nanoparticles in quartz sand. First,
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cations compress the thickness of electrostatic double layer, decrease the electrostatic
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repulsion between nanoparticles and sand, and deepen the secondary minimum energy
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well.7, 45, 46 Cations with smaller hydrated radius (e.g., Cs+) can result in larger electronic
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shielding on the negatively charged surfaces of quartz sand due to stronger electrostatic
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forces; this is evident by comparing the DLVO particle–collector interaction energy
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profiles as affected by different monovalent cations (SI Figure S5). Second, accumulation
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of large hydrated cations (e.g., Na+ and K+) on the surface of quartz sand may interfere
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with particle deposition through steric hindrance,17 whereas cations with smaller hydrated
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radius (e.g., Cs+) would have smaller effects. Third, Cs+ can serve as a bridging agent by
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forming inner-sphere complexes with surface functional groups of both GO/RGO and
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quartz sand, and therefore, significantly enhance the deposition of GO and RGO. In
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comparison, Na+ and K+ only form outer-sphere complexes, and cannot serve as bridging
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agents. Fourth, as mentioned above cations of different hydrated radii also affected the
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particle sizes differently, in that larger aggregates were formed in the presence of weakly 12
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hydrated cations (see the Dh values in Table S3 and the TEM images in Figure S4).
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Accordingly, retention via straining23, 47-49 can be more significant in the presence of
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poorly hydrated cations such as Cs+. 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 0.002–0.003.7, 50 Based on the
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dp/dc values in Table S3, straining was likely a viable retention mechanism for GO/RGO
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in the presence of Cs+.
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To understand the relative contributions of different retention mechanisms, step-
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wise flushing using electrolyte solutions of decreasing ionic strength was carried out. The
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observed release of retained GO/RGO during the flushing (SI Figure S6) reflects the
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contribution of the secondary minimum.51 The results indicate that when the background
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cation was Na+ or K+ deposition at the secondary minimum energy well was the most
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important retention mechanism for both GO and RGO, accounting for 64−69% of the
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overall retention in the presence of Na+, and 44−70% in the presence of K+ (Table 2). In
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comparison, deposition at the secondary minimum was a minor deposition mechanism in
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the presence of Cs+, accounting for only 6−7% of the overall deposition. The fact that Cs+
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affected the transport of RGO more profoundly than that of GO was consistent with its
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cation-bridging effects. As mentioned earlier, RGO contained greater amounts of surface
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carboxyl and phenolic groups than did GO. These metal-complexing moieties can
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magnify the particle−collector and particle−particle bridging effects of Cs+, leading to
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more significant deposition through cation-bridging and possibly through straining. Note
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that the experimentally observed attachment efficiencies in the presence of Cs+ are
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significantly greater than the respective theoretical values calculated with the Maxwell 13
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model (based on the DLVO interaction energies) (SI Table S4). This further corroborates
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the cation-bridging role of Cs+.
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Transport of GO and RGO as Affected by Different Divalent Cations. The three
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divalent cations also influenced the transport of GO/RGO differently. Overall, the
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transport inhibition effects followed the order of Ba2+ > Ca2+ > Mg2+, however, the
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specific differences between different cations depended on the concentrations of cations,
280
and varied between GO and RGO (Figure 3). At a cation concentration of 0.1 mM the
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transport-inhibition effects of the three cations were not too significant – even in the
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presence of Ba2+ breakthrough of GO/RGO reached ~80%. Much more significant
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inhibition was observed at the higher cation concentration tested (0.5 mM), wherein little
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breakthrough of GO was observed in the presence of Ba2+ and little breakthrough of RGO
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was observed in the presence of Ca2+ or Ba2+. It is noteworthy that for GO the transport
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inhibition effects of Ca2+ were comparable to those of Mg2+, and both were substantially
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weaker than those of Ba2+ (Figure 3c); nonetheless, for RGO Ca2+ inhibited the transport
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to a similar extent as did Ba2+, and much more significantly than did Mg2+ (Figure 3d).
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In our previous study, we demonstrated with column flushing experiments that in the
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presence of Ca2+ deposition at the secondary minimum energy well is an insignificant
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mechanism for the retention of GO/RGO under experimental conditions similar to those
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involved in this study, and cation bridging between GO/RGO and sand grains becomes
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the most predominant retention mechanism.17 (The shallow secondary minimum wells
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(Table S4 and Figure S5) and the significant underestimation of attachment efficiencies
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using the Maxwell model and DLVO theory (Table S4) are consistent with this argument.)
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Thus, we propose that the significantly different effects on the transport of GO/RGO 14
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among Mg2+, Ca2+, and Ba2+ stemmed from their different complexing strength. As
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mentioned above, cations with small ionic radii have large hydrated radii, and can only
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form outer-sphere complexes, whereas cations with large ionic radii tend to form inner-
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sphere complexes.21 It has been reported that Ba2+ has a strong propensity for forming
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inner-sphere complexes.43 Even though Ca2+ is smaller in ionic radius than Ba2+, it can
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also form inner-sphere complexes.21 The smallest cation of the group, Mg2+, however,
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forms mainly outer-sphere complexes.21 Accordingly, Ba2+ could result in stronger
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particle−collector bridging (by forming complexes with surface functional groups of both
305
GO/RGO and quartz sand) than Ca2+ and Mg2+, and Ca2+ would be a stronger bridging
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agent than Mg2+. Similarly, the particle−particle bridging effects of the cations would
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also follow the order of Ba2+ > Ca2+ > Mg2+, leading to more significant straining in the
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presence of Ba2+ than Ca2+ than Mg2+. The comparable effects of Ca2+ and Mg2+ on the
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transport of GO, as well as the comparable effects of Ba2+ and Ca2+ on the transport of
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RGO, further corroborate this argument – because RGO contained greater amounts of
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carboxyl and phenolic groups (i.e., metal-complexing moieties) than GO, its transport
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was affected more remarkably by the stronger complexing cations (i.e., Ba2+ and Ca2+).
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To further understand the relative significance of particle−collector bridging versus
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particle−particle bridging by the three cations, we analyzed the retained particle profiles
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of the columns used in the transport experiments in the presence of divalent cations.50, 52
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Interestingly, hyper-exponential retained profiles were only observed for RGO in the
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presence of 0.5 mM Ba2+ or Ca2+ (Figure 4), an observation consistent with the large
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particle sizes of RGO under these conditions (see the Dh values in Table S3). Thus, the
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retained particle profiles indicate that significant straining only occurred for RGO in the 15
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presence of relatively high concentrations of cations capable of forming inner-sphere
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complexes, and under the experimental conditions of this study the significant effects of
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divalent cations (in particular, Ba2+ and Ca2+) on the transport of GO and RGO were
323
mainly exerted through particle−collector bridging.
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Further Evidence for Cation-Dependent Effects of Particle− −Collector Bridging.
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To further verify that the extents of particle−collector bridging by different cations are
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correlated to the ionic radii of the cations (and thus their capability of forming inner-
327
sphere complexes), we examined the adsorption of GO and RGO to quartz sand as
328
affected by the presence of different cations. Figure 5 clearly shows that for a given
329
nanomaterial (i.e., GO or RGO) and a given cation concentration, the adsorption affinity
330
of GO/RGO—as indicated by the sand–water distribution coefficients (Kd)—was highly
331
dependent on the type of cations present in the solution (the Kd values are summarized in
332
SI Table S5). For the reaction systems containing monovalent cations, the strongest
333
adsorption was observed in the presence of Cs+, followed by K+ and then by Na+. The
334
order is consistent with the Homeister effects mentioned above (and the subtitle
335
difference between GO and RGO might also be related to the size-dependent affinity of
336
cations to hydrophobic vs. hydrophilic surfaces53). Similarly, for the reaction systems
337
containing divalent cations, the strongest adsorption occurred when the cation present
338
was Ba2+, followed by Ca2+ and then by Mg2+. The confocal images of GO/RGO
339
adsorbed to quartz sand (Figure 6) further corroborate the Kd data, in that for both the
340
monovalent cations and divalent cations greater amounts of GO/RGO (the fluorescent
341
green areas on the images) were observed on quartz sand (the dark areas on the images)
342
in the presence of cations with larger ionic radii. 16
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The cation-dependent effects on the binding of GO/RGO to quartz sand were highly
344
consistent with the cation-inhibition effects on transport. For example, Figure 5 shows
345
that much stronger adsorption of GO was observed in the presence of 20 mM Cs+ than in
346
the presence of the same amount of K+ or Na+, correlating well with Figure 2, which
347
shows that the maximum breakthrough of GO in the presence of 20 mM Cs+ reached only
348
10%, much lower than that in the presence of K+ (76%) or Na+ (84%). Similarly, much
349
stronger adsorption of RGO was observed in the presence of 0.5 mM Ba2+ or Ca2+ than in
350
the presence of Mg2+ (Figure 5), consistent with the transport data in Figure 2 showing
351
very low breakthrough of RGO in the presence of 0.5 mM Ba2+ or Ca2+ in comparison to
352
the 68% maximum breakthrough of RGO in the presence of 0.5 mM Mg2+. The striking
353
similarities between the effects of cations on adsorption and on transport provide
354
convincing evidence that cations capable of forming inner-sphere complexes can
355
significantly influence the transport of GO/RGO via cation-bridging, and the extents of
356
the effects correlate well with the ionic radii of the cations, as described by the
357
Hofmeister series. Note that the Hofmeister series might be (partially) reversed at high
358
pH.22, 53 Further studies are needed to understand how cations of the same valence may
359
affect transport of GO/RGO under more basic solution chemistry conditions.
360
Environmental Implications. Natural aquatic environments contain many different
361
ions. The findings of this study further demonstrate that these ions—which may vary
362
significantly in charge density, size, and complexing capability— can affect transport of
363
nanoparticles very differently. An important observation of this study was that the
364
specific mechanisms through which cations inhibit transport of negatively charged
365
GO/RGO nanosheets and the extents of the effects depended on the complex interplay 17
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366
between the properties of the cations and the type and distribution of surface functional
367
groups of the nanomaterials. It is probably reasonable to expect that more intriguing
368
effects will be observed for more complex porous media, as besides of quartz sand,
369
cations can also interact strongly with other constituents of porous materials (e.g., clay
370
minerals and oxides). More studies are needed to fully understand the transport of
371
nanoparticles as a function of nanomaterial physicochemical properties and solution
372
chemistry parameters.
373 374
Acknowledgments. This project was supported by the National Natural Science
375
Foundation of China (Grant 21237002), the Ministry of Science and Technology of
376
China (Grant 2014CB932001), and the National Science Fund for Distinguished Young
377
Scholars (Grant 21425729).
378 379
Supporting Information Available: Procedures used to prepare GO and RGO, and
380
calculations of DLVO interaction energies and attachment efficiencies; tables
381
summarizing ionic and hydrated radii of cations studied, column and influent properties,
382
DLVO energy profiles and attachment efficiencies, and distribution coefficients of
383
GO/RGO to quartz sand; figures showing UV/Vis calibration curves of GO/RGO, XPS
384
and FTIR spectra of GO and RGO, TEM images of GO/RGO suspensions, DLVO energy
385
profiles, and column-flushing results. This information is available free of charge via the
386
Internet at http://pubs.acs.org.
387 388
Notes—The authors declare no competing financial interest. 18
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Table 1. Selected Physicochemical Properties of GO and RGO a C b (wt%) aromatic rings
epoxy/ hydroxyl
carbonyl
GO 29.23 29.32 7.59 RGO 39.71 28.28 3.53 a RGO represents sulfide-reduced GO. b Analyzed with X-ray photoelectron spectroscopy.
carboxyl 3.85 4.30
total Cb (wt%) 70.00 75.82
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total Ob (wt%) 28.39 21.35
C/O ratio 2.47 3.55
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Table 2. Mass Balance Expressed as Percentage of Eluted Mass during Each Flushing Step, and Mass Recovered from Column Eluted mass a (%)
Column No.
GO/ RGO
Effluent mass (%)
1
GO
75.06
3.91
1.50
12.33
7.20
Mass recovered from column (%) 5.86
98.66
Contribution of secondary minimum d (%) 66
2
GO
71.33
5.66
2.74
7.40
12.87
11.07
98.20
44
3
GO
32.04
2.69
1.99
2.54
60.73
46.39
85.66
6.9
4
RGO
68.28
3.86
3.16
15.96
8.74
8.46
99.72
69
5
RGO
66.71
3.72
1.65
19.19
8.73
8.13
99.39
70
6
RGO
26.51
2.77
2.03
2.50
66.21
48.24
82.04
6.4
7
GO
67.03
3.34
2.48
16.59
10.56
9.03
98.48
64
8
GO
53.98
7.34
4.35
17.40
16.94
15.24
98.30
56
9
GO
6.15
1.21
2.47
3.59
86.58
68.46
81.89
6.5
10
RGO
27.64
4.45
3.46
42.23
22.22
21.30
99.08
67
11
RGO
19.44
4.25
3.41
38.02
34.87
32.42
97.54
54
12
RGO
3.23
1.65
2.40
3.62
89.10
69.31
80.21
6.3
Flushing #1
Flushing #2
Flushing #3
Mass retained in column b (%)
a
Mass balance c (%)
Columns were flushed with the GO/RGO-free background solutions (flushing #1), lower concentration of background solutions (flushing #2), and deionized water (flushing #3). b Mass retained in column = 100 – effluent mass – eluted mass. c Mass balance was calculated as: effluent mass + eluted mass + mass recovered from column. d Contribution of secondary minimum to overall deposition was calculated as: (eluted mass in Flushing #2 + eluted mass in Flushing #3)/(100 – effluent mass – eluted mass in Flushing #1).
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(a) EPM of GO/RGO in M
+
0
-1
EPM (10-8 m2/Vs)
EPM (10-8 m2/Vs)
0
-2
Na+ + K + Cs
-3
-4
-1
-2
GO in 20 mM M+
(c) Dh of GO/RGO in M
RGO in 10 mM M+
GO in 2+ 0.1 mM Me
RGO in 20 mM M+
+
GO in 2+ 0.5 mM Me
RGO in RGO in 2+ 2+ 0.1 mM Me 0.5 mM Me
2+
1400
(d) Dh of GO/RGO in Me 2+
Na K+ Cs+
1200 1000
Dh (nm)
Dh (nm)
1000
2+
Mg 2+ Ca Ba2+
-3
+
1200
(b) EPM of GO/RGO in Me2+
-4 GO in 10 mM M+
1400
Page 28 of 34
800 600
Mg 2+ Ca Ba2+
800 600
400
400
200
200 0
0 GO in 10 mM M+
GO in 20 mM M+
RGO in 10 mM M+
RGO in RGO in GO in GO in 2+ 2+ 0.1 mM Me2+ 0.5 mM Me2+ 0.1 mM Me 0.5 mM Me
RGO in 20 mM M+
Figure 1. Electrophoretic mobility (EPM) and hydrodynamic diameter (Dh) of GO/RGO as affected by different monovalent cations (M+) (plots a and c) and divalent cations (Me2+) (plots b and d).
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(a) GO in 10 mM M+
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 0
10
20
30
(b) RGO in 10 mM M+
1.0
C/C0
C/C0
1.0
40
0
10
PV
0.6
0.6
C/C0
C/C0
0.8
0.4
0.4
0.2
0.2
0.0
0.0 10
20 PV Na+
40
30
(d) RGO in 20 mM M+
1.0
0.8
0
30
PV
(c) GO in 20 mM M+
1.0
20
0
40
10
20
30
40
PV K+
Cs
+
Figure 2. Effects of monovalent cations (M+) on transport of GO and RGO: (a) GO in 10 mM M+ (Columns 1–3); (b) RGO in 10 mM M+ (Columns 4–6); (c) GO in 20 mM M+ GO(Columns 7–9); and (d) RGO in 20 mM M+ (Columns 10–12).
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(b) RGO in 0.1 mM Me2+
2+
(a) GO in 0.1 mM Me
1.0
0.8
0.8
0.6
0.6
C/C0
C/C0
1.0
0.4
0.4
0.2
0.2
0.0
0.0 0
10
20
30
40
0
10
PV
0.6
0.6
C/C0
C/C0
1.0 0.8
0.4
40
0.4
0.2
0.2
0.0
0.0 10
30
20 PV
(d) RGO in 0.5 mM Me2+
2+
0.8
0
20 PV
(c) GO in 0.5 mM Me
1.0
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30
0
40
10
20
30
40
PV 2+
Mg2+
Ca
Ba2+
Figure 3. Effects of divalent cations (Me2+) on transport of GO and RGO: (a) GO in 0.1 mM Me2+ (Columns 13–15); (b) RGO in 0.1 mM Me2+ (Columns 16–18); (c) GO in 0.5 mM Me2+ (Columns 19–21); and (d) RGO in 0.5 mM Me2+ (Columns 22–24).
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(a) GO in 0.1 mM Me
300
2+
2+
250
250
µg-RGO/g-Sand
µg-RGO/g-Sand
(b) RGO in 0.1 mM Me
300
200 150 100 50
200 150 100 50
0
0 0
2
4
0
6
(c) GO in 0.5 mM Me
6
2+
2+
(d) RGO in 0.5 mM Me
300 250
µg-RGO/g-Sand
250
µg-RGO/g-Sand
4
Distance from Inlet (cm)
Distance from Inlet (cm)
300
2
200 150 100
200 150 100 50
50
0
0 0
2
4
0
6
2
4
6
Distance from Inlet (cm)
Distance from Inlet (cm)
2+
Ca2+
Mg
2+
Ba
Figure 4. Retained profiles of GO and RGO for transport in the presence of divalent cations (Me2+): (a) GO in 0.1 mM Me2+ (Columns 13–15); (b) RGO in 0.1 mM Me2+ (Columns 16–18); (c) GO in 0.5 mM Me2+ (Columns 19–21); and (d) RGO in 0.5 mM Me2+ (Columns 22–24).
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20
Kd (L/kg)
15
10
5
C m a 2+ M R B G a 2+ O 0. 5 R m G M O 0. M 5 g 2+ R m G O M 0. C 5 a 2+ m M B a 2+
g m M
G
O
0. 5
0. 5 O
G
O G
m M
M
C 0. 5
20 O
G R
2+
+
s
+
m M
K
a N 20
m M R
G
O
20
R
G
O
20 O
m M
s C
K
m M
m M 20 G
O G
G
O
20
m M
N
a
+
+
+
+
0
Figure 5. Adsorption coefficients (Kd) of GO/RGO to sand as affected by cation species.
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Environmental Science & Technology
Figure 6. Confocal images of GO and RGO (fluorescent green areas) adsorbed to sand surface (dark areas) showing different extents of adsorption as affected by different cations: (a) GO in 20 mM NaCl; (b) GO in 20 mM KCl; (c) GO in 20 mM CsCl; (d) 33
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RGO in 20 mM NaCl; (e) RGO in 20 mM KCl; (f) RGO in 20 mM CsCl; (g) GO in 0.5 mM MgCl2; (h) GO in 0.5 mM CaCl2; (i) GO in 0.5 mM BaCl2; (j) RGO in 0.5 mM MgCl2; (k) RGO in 0.5 mM CaCl2; and (l) RGO in 0.5 mM BaCl2.
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