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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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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Environmental Science & Technology

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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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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

293  

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

298  

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

304  

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

307  

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

308  

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

309  

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

310  

presented in Figure 2, and the corresponding Freundlich isotherm parameters

311  

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

312  

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

313  

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

315  

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

320  

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

321  

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

322  

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

323  

(m