Cation-Inhibited Transport of Graphene Oxide Nanomaterials in

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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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Environmental Science & Technology

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Cation-Inhibited Transport of Graphene Oxide

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Nanomaterials in Saturated Porous Media: The Hofmeister

3

Effects

4

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

10 11

‡

12

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].

19

1

ACS Paragon Plus Environment


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

22

2


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ABSTRACT Transport of negatively charged nanoparticles in porous media is largely affected by

25

cations. To date, little is known about how cations of the same valence may affect

26

nanoparticle transport differently. We observed that the effects of cations on the transport

27

of graphene oxide (GO) and sulfide-reduced GO (RGO) in saturated quartz sand obeyed

28

the Hofmeister series, that is, transport-inhibition effects of alkali metal ions followed the

29

order of Na+ < K+ < Cs+, and those of alkaline earth metal ions followed the order of

30

Mg2+ < Ca2+ < Ba2+. With batch adsorption experiments and microscopic data, we

31

verified that cations having large ionic radii (and thus weakly hydrated) interacted with

32

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

34

complexes, resulted in very significant deposition of GO/RGO via cation bridging

35

between quartz sand and GO/RGO, and possibly via enhanced straining, due to the

36

enhanced aggregation of GO/RGO from cation bridging. The existence of the Hofmeister

37

effects was further corroborated with the interesting observation that cation bridging was

38

more significant for RGO, which contained greater amounts of carboxyl and phenolic

39

groups (i.e., metal-complexing moieties) than did GO. The findings further demonstrate

40

that transport of nanoparticles is controlled by the complex interplay between

41

nanoparticle surface functionalities and solution chemistry constituents.

42 43

3



44 45

INTRODUCTION Graphene-based materials, such as graphene oxide (GO) and reduced graphene oxide

46

(RGO), are a class of promising nanomaterials with potential applications in the areas of

47

electronic devices, energy storage, chemical catalysis, drug delivery and functional

48

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

50

shown that graphene-based materials may have adverse environmental impact.2-5 Thus, it

51

is important to understand the fate and transport of these materials in the environment.

52

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

55

9),7, 10, 11 but sensitive to the increase of ionic strength;7, 8, 10, 11 and the presence of natural

56

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

60

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-

65

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

75

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+)

80

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

82

(including GO and RGO) than Mg2+,18, 23-28 because Ca2+ can bind to O-functional groups

83

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

85

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.

88

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



90

series.29, 30 Specifically, we expect that cations having larger ionic radii (and accordingly,

91

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

98

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

105

underlying mechanisms controlling the effects of different cations were analyzed.

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Supplemental batch adsorption experiments were conducted and microscopic evidence

107

was collected to verify the proposed mechanisms.

108 109

MATERIALS AND METHODS

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Materials. Graphene oxide (referred to as GO hereafter) was synthesized using a

111

modified Hummers method.34 Reduced graphene oxide (referred to as RGO hereafter)

112

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).

115

Fourier transform infrared (FTIR) transmission spectra were obtained using a 110 Bruker

116

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.

122

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,

127

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

131

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

132

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

134

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

139

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).

148

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–

165

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).

174 175 176

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

178

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

180

to carbon atoms with different chemical states (SI Figure S2). In comparison, RGO had a

181

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

184

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

187

cations.

188

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

195

than were monovalent cations, as indicated by the Dh values (Table S3 and Figure 1) and

196

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

200

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

202

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)

210

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.

217

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

226

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

229

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

232

hydrophobicity, less negative surface charge, and larger particle size of RGO aggregates

233

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

236

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

238

repulsion between nanoparticles and sand, and deepen the secondary minimum energy

239

well.7, 45, 46 Cations with smaller hydrated radius (e.g., Cs+) can result in larger electronic

240

shielding on the negatively charged surfaces of quartz sand due to stronger electrostatic

241

forces; this is evident by comparing the DLVO particle–collector interaction energy

242

profiles as affected by different monovalent cations (SI Figure S5). Second, accumulation

243

of large hydrated cations (e.g., Na+ and K+) on the surface of quartz sand may interfere

244

with particle deposition through steric hindrance,17 whereas cations with smaller hydrated

245

radius (e.g., Cs+) would have smaller effects. Third, Cs+ can serve as a bridging agent by

246

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

248

comparison, Na+ and K+ only form outer-sphere complexes, and cannot serve as bridging

249

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

254

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

255

diameters of particle and collector, respectively) is above 0.002–0.003.7, 50 Based on the

256

dp/dc values in Table S3, straining was likely a viable retention mechanism for GO/RGO

257

in the presence of Cs+.

258

To understand the relative contributions of different retention mechanisms, step-

259

wise flushing using electrolyte solutions of decreasing ionic strength was carried out. The

260

observed release of retained GO/RGO during the flushing (SI Figure S6) reflects the

261

contribution of the secondary minimum.51 The results indicate that when the background

262

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

264

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

266

the presence of Cs+, accounting for only 6−7% of the overall deposition. The fact that Cs+

267

affected the transport of RGO more profoundly than that of GO was consistent with its

268

cation-bridging effects. As mentioned earlier, RGO contained greater amounts of surface

269

carboxyl and phenolic groups than did GO. These metal-complexing moieties can

270

magnify the particle−collector and particle−particle bridging effects of Cs+, leading to

271

more significant deposition through cation-bridging and possibly through straining. Note

272

that the experimentally observed attachment efficiencies in the presence of Cs+ are

273

significantly greater than the respective theoretical values calculated with the Maxwell 13



274

model (based on the DLVO interaction energies) (SI Table S4). This further corroborates

275

the cation-bridging role of Cs+.

276

Transport of GO and RGO as Affected by Different Divalent Cations. The three

277

divalent cations also influenced the transport of GO/RGO differently. Overall, the

278

transport inhibition effects followed the order of Ba2+ > Ca2+ > Mg2+, however, the

279

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

281

transport-inhibition effects of the three cations were not too significant – even in the

282

presence of Ba2+ breakthrough of GO/RGO reached ~80%. Much more significant

283

inhibition was observed at the higher cation concentration tested (0.5 mM), wherein little

284

breakthrough of GO was observed in the presence of Ba2+ and little breakthrough of RGO

285

was observed in the presence of Ca2+ or Ba2+. It is noteworthy that for GO the transport

286

inhibition effects of Ca2+ were comparable to those of Mg2+, and both were substantially

287

weaker than those of Ba2+ (Figure 3c); nonetheless, for RGO Ca2+ inhibited the transport

288

to a similar extent as did Ba2+, and much more significantly than did Mg2+ (Figure 3d).

289

In our previous study, we demonstrated with column flushing experiments that in the

290

presence of Ca2+ deposition at the secondary minimum energy well is an insignificant

291

mechanism for the retention of GO/RGO under experimental conditions similar to those

292

involved in this study, and cation bridging between GO/RGO and sand grains becomes

293

the most predominant retention mechanism.17 (The shallow secondary minimum wells

294

(Table S4 and Figure S5) and the significant underestimation of attachment efficiencies

295

using the Maxwell model and DLVO theory (Table S4) are consistent with this argument.)

296

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

298

mentioned above, cations with small ionic radii have large hydrated radii, and can only

299

form outer-sphere complexes, whereas cations with large ionic radii tend to form inner-

300

sphere complexes.21 It has been reported that Ba2+ has a strong propensity for forming

301

inner-sphere complexes.43 Even though Ca2+ is smaller in ionic radius than Ba2+, it can

302

also form inner-sphere complexes.21 The smallest cation of the group, Mg2+, however,

303

forms mainly outer-sphere complexes.21 Accordingly, Ba2+ could result in stronger

304

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

306

agent than Mg2+. Similarly, the particle−particle bridging effects of the cations would

307

also follow the order of Ba2+ > Ca2+ > Mg2+, leading to more significant straining in the

308

presence of Ba2+ than Ca2+ than Mg2+. The comparable effects of Ca2+ and Mg2+ on the

309

transport of GO, as well as the comparable effects of Ba2+ and Ca2+ on the transport of

310

RGO, further corroborate this argument – because RGO contained greater amounts of

311

carboxyl and phenolic groups (i.e., metal-complexing moieties) than GO, its transport

312

was affected more remarkably by the stronger complexing cations (i.e., Ba2+ and Ca2+).

313

To further understand the relative significance of particle−collector bridging versus

314

particle−particle bridging by the three cations, we analyzed the retained particle profiles

315

of the columns used in the transport experiments in the presence of divalent cations.50, 52

316

Interestingly, hyper-exponential retained profiles were only observed for RGO in the

317

presence of 0.5 mM Ba2+ or Ca2+ (Figure 4), an observation consistent with the large

318

particle sizes of RGO under these conditions (see the Dh values in Table S3). Thus, the

319

retained particle profiles indicate that significant straining only occurred for RGO in the 15



320

presence of relatively high concentrations of cations capable of forming inner-sphere

321

complexes, and under the experimental conditions of this study the significant effects of

322

divalent cations (in particular, Ba2+ and Ca2+) on the transport of GO and RGO were

323

mainly exerted through particle−collector bridging.

324

Further Evidence for Cation-Dependent Effects of Particle− −Collector Bridging.

325

To further verify that the extents of particle−collector bridging by different cations are

326

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


Page 16 of 34

Page 17 of 34

343


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



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


Page 18 of 34

Page 19 of 34


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25



Page 26 of 34

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

26


total Ob (wt%) 28.39 21.35

C/O ratio 2.47 3.55

Page 27 of 34


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).

27



(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).

28


Page 29 of 34


(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).

29



(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

Page 30 of 34

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).

30


Page 31 of 34


(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)


300

2

200 150 100

200 150 100 50

50

0

0 0

2

4

0

6

2

4

6



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).

31



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.

32


Page 32 of 34

Page 33 of 34


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



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.

34


Page 34 of 34

Cation-Inhibited Transport of Graphene Oxide Nanomaterials in

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