Interaction Between Graphene Oxide Nanoparticles and Quartz Sand

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Interaction Between Graphene Oxide Nanoparticles and Quartz Sand Nikolaos P. Sotirelis, and Constantinos V. Chrysikopoulos Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03496 • Publication Date (Web): 14 Oct 2015 Downloaded from http://pubs.acs.org on October 15, 2015

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Environmental Science & Technology

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Interaction Between Graphene Oxide Nanoparticles and

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Quartz Sand

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by

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Nikolaos P. Sotirelis, and Constantinos V. Chrysikopoulos*

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School of Environmental Engineering, Technical University of Crete, 73100 Chania, Greece.

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Manuscript submitted to the

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Key words: Nanoparticles, graphene oxide, deposition, attachment, quartz sand, DLVO.

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October 12, 2015

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*Corresponding author ([email protected], Fax: +30 28210 37847).

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ACS Paragon Plus Environment


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ABSTRACT: In this study, the influence of pH, ionic strength (IS), and

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temperature on graphene oxide (GO) nanoparticles attachment onto quartz

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sand were investigated. Batch experiments were conducted at three

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controlled temperatures (4, 12, and 25°C) in solutions with different pH values

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(pH=4, 7, and 10), and ionic strengths (IS=1.4, 6.4, and 21.4 mM), under static

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and dynamic conditions. The surface properties of GO nanoparticles and

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quartz sand were evaluated by electrophoretic mobility measurements.

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Derjaguin-Landau-Verwey-Overbeek (DLVO) potential energy profiles were

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constructed for the experimental conditions, using measured zeta potentials.

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The experimental results showed that GO nanoparticles were very stable

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under the experimental conditions. Both temperature and pH did not play a

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significant role in the attachment of GO nanoparticles onto quartz sand. In

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contrast, IS was shown to influence attachment. The attachment of GO

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particles onto quartz sand increased significantly with increasing IS. The

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experimental data were fitted nicely with a Freundlich isotherm, and the

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attachment kinetics were satisfactorily described with a pseudo-second-order

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model, which implies that the quartz sand exhibited substantial surface

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heterogeneity and that GO retention was governed by chemisorption.

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Furthermore, thermodynamic analysis revealed that the attachment process

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was non-spontaneous and endothermic, which may be associated with

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structural changes of the sand surfaces due to chemisorption. Therefore,

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secondary minimum interaction may not be the dominant mechanism for GO

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attachment onto the quartz sand under the experimental conditions.

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INTRODUCTION

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Graphene was first produced in 2004, and is a single atom-thick sheet of

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carbon packed in a hexagonal lattice.1 For this pioneering discovery, Andre K.

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Geim and Konstantin S. Novoselov, were awarded the Nobel prize in Physics

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in 2010. This award acknowledged that graphene, owing to its remarkable

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physicochemical properties (e.g. high specific surface area, high electron

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mobility, excellent mechanical strength, excellent structural transformability,

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and high thermal conductivity) is one of the fastest growing nanomaterials that 2


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has changed many fields of science, engineering, and industry.1−4

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Graphene oxide (GO) is a layered nano-material that contains graphene

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sheets and oxygen-bearing functional groups.5 GO is one of the most

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important graphene derivatives.6 Due to its excellent electrochemical

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properties, GO is used in a wide range of applications. Consequently, the

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production growth of this material is very rapid, and large quantities of GO

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particles are expected to eventually reach sensitive environmental systems,

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including subsurface formations.7 Note that the solubility of GO in water is

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very high.8 Also, the presence of mobile GO particles in surface waters, and

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groundwater may affect the fate and simultaneous transport (cotransport) of

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dissolved species and suspended particles (e.g. colloids, biocolloids).9−11

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Furthermore, various studies have reported that GO may be toxic to a variety

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of mammalian organisms, as well as to human and bacterial cells.12−17 For this

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reason, it is important to fully understand the fate of GO in environmental

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systems, and particularly in groundwater systems, because GO transport in

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porous media is significantly affected by the interaction between GO and solid

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matrix (e.g. sand). Numerous studies published in the literature have focused

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on the transport of GO in porous media.18−21 The results of these studies have

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shown that GO has great colloidal stability and the main factor that affects GO

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retention in porous media is the ionic strength (IS).

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The objective of this work was to improve our understanding of the

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mechanisms responsible for deposition and attachment of GO nanoparticles

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onto quartz sand. Batch experiments were conducted, to investigate the effect

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of temperature, pH, and IS on GO attachment onto quartz sand, and GO

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aggregation, under static and dynamic conditions. The attachment kinetics,

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related attachment isotherms of GO interactions with quartz sand under

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different temperatures, and the corresponding thermodynamics were

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examined. Also, the aggregation of GO and the attachment behavior of GO

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onto quartz sand were related to theoretically determined DLVO energy

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interaction profiles. To our knowledge, no previous study has investigated the

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effect of temperature and pH on GO attachment onto quartz sand. 3



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MATERIALS AND METHODS

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Graphene oxide. High purity SP-1 graphite powder (Bay Carbon Inc,

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Bay City, MI) was used to produce graphite oxide based on the procedures

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reported by Hummers et al.22 The graphite oxide was exfoliated by sonication

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and centrifugation to obtain graphene oxide flakes.23 The GO suspensions

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were prepared by mixing 3 mg of graphene oxide flakes with 250 mL of a

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phosphate buffered solution (PBS) with low ionic strength (IS=1.4 mM). A

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transmission electron microscopy (TEM) system, JEOL (JEM-2100, operated

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at 200 kV), equiped with an Erlangshen CCD camera (Model 782 ES500W),

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was used to record images of GO nanoparticles. The morphology of

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representative GO flakes is shown in Figure SI1. The suspensions with

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different ionic strengths were adjusted with NaCl; whereas, the suspensions

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with different pH values were adjusted with either H2PO4 or NaOH.

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Subsequently, the suspensions were sonicated (Elmasonic S 30/(H), Elma

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Schmidbauer GmbH, Singen, Germany) for 2 h to ensure that the dispersion

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is thoroughly uniform. It was confirmed that prolonged sonication did not to

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have negative effects (e.g. re-aggregation) on GO particles (see Figure SI2).

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All of the solutions were prepared with distilled deionized water (ddH2O).

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Furthermore, all chemicals employed in this study were of analytical reagent

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grade, employed without any additional purification.

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Calibration curves were prepared, for each set of solution chemistry (pH

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and IS) examined in this study, in order to establish the relationship between

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absorbance, Abs [-], and GO concentration, CGO [M/L3], in the range 0 to 30

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mg/L. A series of diluted samples were prepared from an aqueous solution

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with known GO concentration, and the absorbance of each diluted sample

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was measured at the wavelength of λmax=231 nm with a UV-Visible

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spectrophotometer (Cary 400 BIO, Varian, Palo Alto, California). Also, a

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zetasizer (Nano ZS90, Malvern Instruments, Southborough, MA) was used to

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measure the zeta potential and hydrodynamic diameter of GO nanoparticles

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under the various experimental conditions of this study at 25 °C. All zeta

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potential and hydrodynamic diameter measurements were obtained in 4


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triplicates. The sizes of GO nanoparticles under the various experimental

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conditions of this study are presented in Table SI1.

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Sand. Quartz sand with grain diameter ranging from 0.425 to 0.600 mm

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(sieve no. 40) was used in this study for the GO attachment experiments.

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Following the procedures reported by Chrysikopoulos and Aravantinou,24 the

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particle-size distribution value determined by sieve analysis was used to

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calculate the coefficient of uniformity, Cu=d60/d10=1.21 (were d10, and d60 is the

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diameter of a sand grain that is barely too large to pass through a sieve that

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allows 10%, and 60%, respectively, of the material (by weight) to pass

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through). The quartz sand employed in this study was relatively uniform

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because the smaller the value of Cu the more uniform the sand. Note that

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Cu=1 corresponds to uniform sand.25 The chemical composition of the quartz

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sand as reported by the manufacturer (Filcom, Netherlands) was: 96.2% SiO2,

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0.15% Na2O, 0.11% CaO, 0.02% MgO, 1.75% Al2O3, 0.78% K2O, 0.06% SO3,

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0.46% Fe2O3, 0.03% P2O5, 0.02% BaO, 0.01% Mn3O4, and 0.28% loss on

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ignition. The quartz sand was thoroughly cleaned with 0.1 M nitric acid HNO3

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(70%) for 3 h to remove surface impurities (e.g. metal hydroxides and organic

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coatings), rinsed with distilled deionized water (ddH2O), then soaked in 0.1 M

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NaOH for 3 h, and rinsed with ddH2O again.26,27 Finally, the quartz sand was

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dried in an oven at 80 °C. The sand grains were relatively large for direct zeta

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potential measurement by a zetasizer. Consequently, the sand grains were

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crushed into fine powder. Several stable suspensions were formed with the

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crushed sand, each having a desired ionic strength, which were used for zeta

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potential measurements.28,29

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Batch experiments. Both static and dynamic batch experiments were

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conducted under various solution chemistry conditions at 4, 12 and 25 °C, in

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order to examine the effect of pH, IS, and temperature on GO aggregation and

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GO attachment on quartz sand. All batch experiments were performed in 20

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mL Pyrex glass screw-cap tubes (Fisher Scientific). Glass tubes were washed

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with detergent, rinsed thoroughly in ddH2O, autoclave sterilized, and oven 5



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dried at 80°C overnight. A PBS solution with IS=1.35 mM was prepared with

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0.001 M phosphate buffer salts in ddH2O and adjusted to a pH 7.2 with NaOH.

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The PBS solution was used to stabilize the pH of GO dispersion.5 Note that

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the relatively wide temperature range (4 to 25°C), and pH range (4 to 10)

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considered in this study is representative of the conditions observed in various

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ground and surface waters.30

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For each experiment, 16 glass tubes were employed, which were divided

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into two groups. Each group consisted of 8 glass tubes. The glass tubes of the

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first group (experimental tubes) contained 14 mL of GO dispersion with 14 g

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of sand, and the glass tubes of the second group (control tubes) contained 20

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mL of GO dispersion without sand. All glass tubes were filled to the top.

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However, a small air bubble was always trapped within the tubes when the

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caps were screwed onto the tubes. Both groups were treated in the same

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manner. The experiments at 4 and 12°C were conducted in an incubator (Foc

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120E, Velp Scientifica, Italy). The dynamic batch experiments were performed

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with the tubes attached to a rotator (Selecta, Agitador orbit), operated at 12

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rpm, in order to allow the sand to mix within the GO suspension. Control

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tubes, in the absence of sand, were used to monitor GO aggregation and

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possible GO attachment onto the walls of the glass tubes. A sample of the

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PBS solution (3.0 mL) was removed from each selected glass tube at different

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preselected times (0, 5, 10, 20, 30, 60, 90, 120, 180, 240 minutes) and the

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GO concentration was measured in triplicates. All used glass tubes were

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discarded. The experimental conditions of the various batch experiments

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conducted in this study are summarized in Table SI2.

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THEORETICAL ANALYSIS OF GO-SAND INTERACTIONS

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Assuming that GO particles are practically colloids, the classical theory

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developed by Derjaguin-Landau-Verwey-Overbeek (DLVO) is applicable.

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Based on the DLVO theory, the total interaction energy between two surfaces

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(here GO and quartz sand) equals the arithmetic sum of the van der Waals

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ΦvdW, double layer, Φdl, and Born, ΦBorn, potential energies:26

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ΦDLVO (h) = Φ vdW (h) + Φdl (h) + ΦBorn (h)

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where h [m] is the separation distance between the approaching surfaces.

(1)

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Following the work by Chrysikopoulos and Syngouna,31 for the case of

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two approaching surfaces, the ΦvdW [J] interactions were calculated with the

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expression provided by Gregory,32 using λ≈10-7 m for the characteristic

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wavelength of the sphere-plate or plate-plate interactions, and A123=6.26×10-

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21

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sand.18,19 The Φdl for sphere-plate interactions were calculated with the

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expression provided by Hogg et al.,33 using NA=6.02×1023 [1/mol] for the

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Avogadro’s number, e=1.602×10−19 [C] for the elementary charge, and

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kB=1.38×10−23 [J/K] for the Boltzmann constant. The ΦBorn [J] for sphere-plate

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was estimated by the relationship provided by Ruckenstein et al.34 Note that

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ΦBorn can easily be neglected if h>1 nm. The effect of Born interaction may not

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be of great significance in aqueous systems due to the possible presence of

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hydrated ions, which prevent surface-surface separation distances to

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approach h~0.3 nm.31

[J] for the combined Hamaker constant for the system GO-water-quartz

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For the case of two approaching surfaces, both with planar geometries

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(plate-plate), the ΦvdW [J], Φdl [J], and ΦBorn [J] interactions were calculated

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with the equations provited by Gregory,32 Hogg et al.,33 and Mahmood et al.,35

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respectively. All the details associated with the DLVO calculations are

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presented in the Supporting Information section.

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RESULTS AND DISCUSSION

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Zeta potentials and DLVO interactions. Figure SI3 illustrates

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graphically the measured zeta potential values of GO and quartz sand at

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various pH values. Both GO and quartz sand are negatively charged for all the

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pH values examined in this study (2 ≤ pH ≤ 10), suggesting that GO-sand and

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GO-GO interactions are repulsive. Similar findings for GO-sand and GO-GO

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interactions have been reported in the literature by several other

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investigators.18−20 Zeta potential values can also be used to evaluate the

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stability of suspended particles. Note that the zeta potentials of both GO and 7



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quartz sand decrease with increasing pH, suggesting that the suspended

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particles become more stable with increasing pH. It is worthy to note that the

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stability of suspended particles is expected to increase with increasing

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absolute zeta potential values.36 At relatively low absolute zeta potential

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values aggregates may form, because existing attractive forces may be

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stronger than the repulsive forces.37 The commonly used threshold for

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absolute zeta potential value for stable colloidal suspensions is considered to

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be >30 mV.38,39 Therefore, based on the data of Figure SI3 and preliminary

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laboratory observations, particle suspensions with pH0.3 nm, rendering the

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effect of Born interaction as insignificant. Consequently, the interaction energy

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profiles constructed do not exhibit a Φmin₁.

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The simulated GO-sand (sphere-plate) interactions energy profiles (see

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Figures 1a,b) exhibit a shallow Φmin₂, and a very high Φmax₁. Note that the

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lowest Φmax₁ and deepest Φmin₂ were observed for pH=4 (see Figure 1a). Note

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that, Φmax₁ was smallest for the highest Is examined in this study (see Figure

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3b), similar trend has been observed by other investigators.19 Also, as

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expected, Φmin₂ was deepest for the highest Is. Furthermore, the simulated

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GO-GO (plate-plate) interaction energy profiles (see Figures 1c,d) suggest

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that there is a very shallow Φmin₂, and very high Φmax₁, suggesting the

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presence of strong repulsive forces between GO nanoparticles. Clearly,

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fluctuations in both pH and Is do not play a significant role in the GO-GO

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interactions energy profiles. This is in agreement with previous findings that

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for typical environmental systems (pH=5-9), GO aggregation is not

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significantly affected by pH.19 It is worthy to note that Φmin₂ is very shallow for

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all cases examined here (see Figure 1). Therefore, secondary minimum

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interaction is an unlike mechanism for GO attachment onto the quartz sand

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and GO aggregation. However, DLVO interaction energy profiles do not

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account for surface roughness, angularity, chemical impurities, and surface

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charge heterogeneity of the sand, which are known to produce local areas of

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favorable interaction that lead to attachment even under unfavorable

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conditions.43-45 Although in this study the quartz sand was thoroughly cleaned

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to eliminate surface charge heterogeneity, charge heterogeneities from the

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sand surfaces cannot be ruled out.46,47

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Isotherm batch experiments. The experimental data from the GO

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equilibrium attachment onto quartz sand at three different temperatures where

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fitted nicely with a Freundlich isotherm (the linear and Langmuir isotherm

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models were also tested, see Table SI3), which has been employed in various

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contaminant, colloid and biocolloid attachment studies of environmental 9



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significance.24,31,48,49 The Freundlich isotherm is a non-linear relationship

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between the aqueous phase GO concentration at equilibrium, Ceq [Mn/L3], in

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units of [mg GO/Liter of solution], and the GO concentration adsorbed onto

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the quartz sand at equilibrium, C∗eq [Mn/Ms], in units of [mg GO/g sand]:50

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Ceq = K f Ceq

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where Kf [L3+m/Ms Mm−1 n ] is the Freundlich constant in units of [(Liter of

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solution)m/(g sand)(mg GO)m-1], m [-] is the Freundlich exponent, which is

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equal to one for linear deposition and attachment. Also, for notational

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convenience Mn was introduced for the mass of nanoparticles (GO), and Ms

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for the mass of solids (quartz sand). Note that the Freundlich isotherm

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describes equilibrium attachment onto heterogeneous sorbent surfaces, and

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contrary to Langmuir isotherm, does not assume monolayer deposition.24 The

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Freundlich parameter m is a measure of the surface heterogeneity of the

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quartz sand (the smaller the value of m the higher the surface heterogeneity

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of the quartz sand), and Kf is directly proportional to the deposition and

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attachment capacity of the quartz sand. For the estimation of the Freundlich

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parameters the linearized form of the Freundlich isotherm was used:

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log C∗eq = log K f + m log Ceq

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The parameters m and logKf were estimated by the slope and ordinate

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(vertical axis intercept), respectively, of the linear plot of the experimental data

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in the form of log C∗eq versus logCeq. The static and dynamic attachment data

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for the experiments conducted in this study at three different temperatures are

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presented in Figure 2, and the corresponding Freundlich isotherm parameters

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are listed in Table 1. The model fittings of the experimental data were

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obtained with the graphical statistical software “IGOR-Pro” (WaveMetrics Inc.).

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Based on the calculated R2 values (see Figure 2 and Table 1), the

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Freundlich isotherm model fits very well the experimental data under static as

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well as dynamic conditions. The number of accessible attachment sites is

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much higher in dynamic than static experiments due to agitation, which

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improves the contact of quartz sand grains with the liquid and decreases the

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resistance to mass transfer.51–53 Therefore, more GO particles were adsorbed

∗

m

(2)

(3)

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onto the quartz sand under dynamic than static conditions (compare the Kf

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values listed in Table 1). This finding is in agreement with other attachment

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studies.24,31 The attachment of GO nanoparticles onto quartz sand was a

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favorable process because all of the estimated m values were less than unity

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(m

Interaction Between Graphene Oxide Nanoparticles and Quartz Sand

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