Carboxymethylcellulose Mediates the Transport of Carbon Nanotube

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Carboxymethylcellulose Mediates the Transport of Carbon Nanotubes —Magnetite Nanohybrid Aggregates in Water-Saturated Porous Media Dengjun Wang, Chang Min Park, Arvid Masud, Nirupam Aich, and Chunming Su Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04037 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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Environmental Science & Technology

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Carboxymethylcellulose Mediates the Transport of Carbon Nanotubes—Magnetite Nanohybrid Aggregates in Water-Saturated Porous Media

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Dengjun Wang,†,* Chang Min Park,†,∆ Arvid Masud,§ Nirupam Aich,§ and Chunming Su‡,* †

National Research Council; and ‡Groundwater, Watershed, and Ecosystem Restoration Division, National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 919 Kerr Research Drive, Ada, OK 74820, United States ∆

Department of Environmental Engineering, Kyungpook National University, Buk-gu, Daegu 41566, South Korea

§

Department of Civil, Structural and Environmental Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, United States *

Corresponding Authors: Dengjun Wang E-mail: [email protected] Phone: (580) 436-8828 Fax: (580) 436-8703 and Chunming Su E-mail: [email protected] Phone: (580) 436-8638 Fax: (580) 436-8703

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ABSTRACT

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Carbon—metal oxide nanohybrids (NHs) are increasingly recognized as the next-

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generation, promising group of nanomaterials for solving emerging environmental issues and

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challenges. This research, for the first time, systematically explored the transport and retention of

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carbon nanotubes—magnetite (CNT-Fe3O4) NH aggregates in water-saturated porous media

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under environmentally relevant conditions. A macromolecule modifier, carboxymethylcellulose

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(CMC) was employed to stabilize the NHs. Our results show that transport of the magnetic CNT-

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Fe3O4 NHs was lower than that of non-magnetic CNT due to larger hydrodynamic sizes of NHs

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(induced by magnetic attraction) and size-dependent retention in porous media. Classical

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Derjaguin-Landau-Verwey-Overbeek (DLVO) theory can explain the mobility of NHs under

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varying experimental conditions. However, in contrast with colloid filtration theory, a novel

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transport feature—an initial lower and a following sharp, higher peaks occurred frequently in the

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NHs’ breakthrough curves and the magnitude and location of both transport peaks varied with

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different experimental conditions, due to the interplay between variability of fluid viscosity and

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size-selective retention of the NHs. Promisingly, the estimated maximum transport distance of

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NHs ranged between ~0.38−46 m, supporting the feasibility of employing the magnetically

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recyclable CNT-Fe3O4 NHs for in-situ nanoremediation of contaminated soil, aquifer, and

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groundwater.

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INTRODUCTION

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Nanoscience and nanotechnology are advancing our broad societal goals in various fields

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including, but not limited to: medicine (e.g., magnetic resonance imaging),1 agriculture (e.g.,

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nutrient nanocapsules and nanosensors),2 energy (e.g., solar cells and supercapacitors),3 and

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environmental remediation (e.g., photocatalysts)4, 5. They are unprecedentedly maximizing

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human potentials towards many frontiers such as addressing water-energy-agriculture-

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environment (WEAE) nexus.6 Recently, the focus of interest in nanomaterial (NM) research and

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development has shifted from singular NMs to multi-component nanohybrids (NHs), which are

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nano-/hierarchical assemblies of multiple NMs conjugated by non-covalent (hydrogen bond, van

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der Waals (vdW), and electrostatic interactions) or covalent (molecular linkage) bonds.7-9 The

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goal of developing NHs is to maximize the existing functionality (e.g., contaminant adsorption

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and degradation efficiency), and achieve novel-functionality that cannot be obtained by

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manipulating the singular NM system.7-9 Taking environmental remediation as an example, the

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carbon nanotubes (CNT) individually show limited adsorption capacity towards metoprolol.

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When hetero-hybridized with nanoscale zero-valent iron (nZVI), the conjugated CNT-nZVI NHs

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exhibit significantly higher and faster adsorption and degradation efficiency for metoprolol

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(compared to CNT) due to greater specific surface area (SSA) and Fenton-type catalytic

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reactions in the CNT-nZVI nanostructures.10 Promisingly, compared to the single-component

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NMs, the performance and functionality of NHs are optimized. This is particularly true for

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carbon-family based metal oxide NHs. Specifically, the photoelectrocatalytic reactivity and

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sensitivity of CNT/graphene oxide (GO)—FexOy/TiO2/ZnO/MnO2/SiO2/Ag for removing toxic

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gases (NOx),11 organic pollutants (dyes and phenolic compounds),10,

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(As(III)/As(V), Cd2+, Cr(VI), Pb2+, and Hg2+),12,

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13

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heavy metals

and radionuclides (U(VI)),14,


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are

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demonstrated to be greater than those of singular NMs. This is attributed to the

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combinative/synergistic effects within the hybridized nanostructure system.7-9

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The superior performance and multi-functionality of carbon—metal oxide NHs make

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them the next-generation, promising candidates for in-situ nanoremediation of contaminated air,

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soil, aquifer, and groundwater. It is also anticipated that increasing production and use of NHs

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will result in their release to the environment. However, to date, little is known about their fate

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and transport in the subsurface, which will likely restrict their widespread applications in

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resolving issues and meeting challenges within WEAE nexus. Over the past decade, the fate and

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transport of individual NMs such as carbonaceous (CNT16-19 and graphene20-22) and iron-bearing

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NMs (nZVI23-25 and magnetite (Fe3O4)26, 27) in the subsurface have been well-explored. It is

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noted that colloid science principles, Derjaguin-Landau-Verwey-Overbeek (DLVO) theory28, 29

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and colloid filtration theory (CFT)30 can describe the colloidal stability and transport of

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individual NMs in aquatic environments. Nonetheless, deviations between experimental

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observations and theoretical predictions occur frequently under unfavorable attachment

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conditions or with varying properties (e.g., irregular shapes and surface modifications) of NMs.

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For example, under unfavorable conditions, non-exponential retention31 of NMs was frequently

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encountered in porous media (e.g., hyper-exponential retention for tubular CNT)16. Surface

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modifications32 due to surfactant and polyelectrolyte coatings impart non-DLVO interactions

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(steric hindrance) and change the rheological properties (viscosity) of fluid in porous media,33

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likely yielding unanticipated transport behaviors for NMs. CFT predicts that transport of colloids

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in porous media is controlled by random Brownian motion; and that interception and

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gravitational sedimentation start to dominate colloid deposition when particle diameter is ≥1

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µm.30, 34 Once released to the environment, the transport of individual NMs is dependent on the

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interplay of various physicochemical conditions including: (1) NM-specific properties (e.g.,

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size/shape/hydrophilicity/aggregation state/surface chemistry); (2) medium-specific properties

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(e.g., grain size/shape/surface chemistry/moisture content); (3) solution chemistries (e.g.,

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pH/ionic strength and composition (IS and IC)/presence of natural organic matter (NOM)); and

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(4) hydrodynamic conditions (e.g., flow rate/temperature/oxygen content).35, 36 However, it is

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unclear whether the colloid science principles used for describing the environmental behaviors of

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individual NMs hold true for the assembled NHs, e.g., how and to what extent (e.g., qualitatively,

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semi-quantitatively, or quantitatively) the DLVO theory and CFT explain NHs’ transport in the

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subsurface. Particularly, NHs’ new features such as altered hydrophilicity, magnetism, and

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DLVO-type (e.g., vdW attraction) interactions, and enhanced population heterogeneity (e.g., size

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distribution, morphology, surface defect and roughness, and charge heterogeneity; compared to

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individual NMs) likely impact their aggregation and transport propensities in the subsurface.9, 37

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Assembling magnetic iron oxide (e.g., Fe3O4) NMs having high redox potential and

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contaminant immobilization capacity38-40 with CNT having superlative mechanical-strength and

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large SSA41 provides a powerful strategy (e.g., synergistic adsorption and redox degradation) for

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in-situ capturing and decomposing/detoxificating both inorganic and organic pollutants. But

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knowledge of how DLVO theory and CFT describe the transport of CNT-Fe3O4 NHs in the

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subsurface is nonexistent. This study systematically investigated the transport of the

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multifunctional CNT-Fe3O4 NHs in water-saturated porous media under environmentally

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relevant conditions. An environmentally-friendly and ‘green’ (non-toxic and biodegradable)

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macromolecule carboxymethylcellulose (CMC) was employed to stabilize the CNT-Fe3O4 NHs

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as they were highly-hydrophobic and easily-aggregated. The CMC has long been used to

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stabilize individual nZVI23, 42, 43 and Fe3O4 NMs44 by imparting electrosteric repulsions arising

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from the hydrophilic, negatively charged carboxymethyl (-CH2-COOH) groups. Critical

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parameters characterizing the NHs’ mobility in water-saturated porous media were evaluated to

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shed light on the feasibility of using the recyclable CNT-Fe3O4 NHs for in-situ nanoremediation

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of contaminated sites. DLVO theory and CFT were employed in combination with electrokinetic

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property and hydrodynamic size of NHs to understand the mechanisms behind their transport and

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retention within the porous media. Significant efforts were devoted to explore how DLVO theory

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and CFT explain the variability in NHs’ transport under varying experimental conditions. Finally,

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potential limitations and future research directions of DLVO theory/CFT in describing NHs’

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transport in the subsurface were also discussed.

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MATERIALS AND METHODS

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Preparation of CNT-Fe3O4 NH Influents and Solution Chemistries

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Carbon nanotubes (purity>99.9%, outer diameter=8−15 nm, inside diameter=3−5 nm,

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length=10−50 µm, and carboxyl group content=1.85%) were purchased from the Cheap Tubes

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Inc. (Grafton, VT). The CNT-Fe3O4 NHs were synthesized in-house, and their physicochemical

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properties including those of purchased CNT were characterized using X-ray diffraction (XRD),

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transmission electron microscopy (TEM), Raman spectroscopy, and thermogravimetric analysis

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(TGA) (see Supporting Information (SI) S1). The synthesized CNT-Fe3O4 NHs were highly-

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hydrophobic because most hydrophilic carboxyl groups on the CNT surfaces were used for

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anchoring CNT and magnetite during synthesis.

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The NHs were easily-aggregated due to strong vdW and magnetic attractions (magnetism

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was confirmed using a magnet). Various surfactants (sodium dodecyl sulfate (SDS), Triton X-

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100, and Tween 20), polyelectrolytes (polyacrylic acid (PAA), polyvinyl alcohol (PVA), and

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polyvinyl pyrrolidone (PVP)), and macromolecules (CMC; nominal molecular weight=90,000

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g/mol; Sigma-Aldrich) were tested to investigate the NHs’ colloidal stability. By monitoring the

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UV-Vis response of absorbance for the NHs under different surfactants, polyelectrolytes, and

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macromolecules, we found that only the CMC at the concentration of 2% well-stabilized the

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NHs (see SI S2). Consequently, 2% CMC was chosen as the ‘modifier’ to stabilize the CNT-

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Fe3O4 NHs for column experiments (described below).

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Environmentally-relevant solution chemistries were selected to evaluate the mobility of

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CNT-Fe3O4 NHs in porous media including: monovalent (0, 1, 10, and 50 mM NaCl) and

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divalent electrolytes (0, 0.33, 1.67, and 3.33 mM CaCl2; note IS of 0, 0.33, 1.67, and 3.33 mM

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CaCl2 is 0, 1, 5, and 10 mM, respectively); the presence of NOMs (0, 1, and 10 mg C/L

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Suwannee River humic acid (SRHA) and fulvic acid (SRFA)) (details on preparing SRHA/SRFA

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stock suspensions were given in SI S3), pHs (4.0, 7.3, and 10), and particle concentrations (10,

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25, and 50 mg/L) (Table 1).45 2% CMC-stabilized CNT-Fe3O4 NH influents (via adding a pre-

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determined volume of 5% CMC stock suspension; S3) at the desired solution chemistries were

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freshly prepared via ultrasonication at 100 W and 42 kHz for 1 h (Branson 3510R-DTH

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sonicator). Changes in concentration and average hydrodynamic radius (rh) of CNT-Fe3O4 NHs

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in the influent suspensions were monitored over the time frame of influent injection experiments

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(described below) to investigate their colloidal stability (aggregation propensity). The influent

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concentration changes of NHs were determined spectrophotometrically at 218 nm (SI Figure S1).

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Time-resolved dynamic light scattering (DLS) was used to monitor rh change of NHs in influents

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using the Zetasizer Nano-ZS ZEN3600 analyzer (Malvern Instruments Inc.) at a scattering angle

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of 173° and 25 °C (see SI S4).

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Table 1. Physicochemical properties of column transport experiments and mass recoveries of CNT-Fe3O4 NHs (in 2% CMC) under different experimental conditions. NaCl (mM) 0 1 10 50 0 0 0 1 1 1 1 1 1 0 0 1 1

CaCl2 (mM) 0 0 0 0 0.33 1.67 3.33 0 0 0 0 0 0 0 0 0 0

SRHA (mg C/L)

SRFA (mg C/L)

pH

0 0 0 0 0 0 0 1 10 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 1 10 0 0 0 0 0 0

7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 4.0 10 7.3 7.3 7.3 7.3

Particle conc (mg/L)

Sand size (µm)

Mass recovery (%) Meff

Mret

Mtot

10 10 10 10 10 10 10 10 10 10 10 10 10 25 50 10 10

360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 280 230

88.3±0.02 79.9±2.4 37.8±0.67 20.3±0.88 57.4±2.0 22.7±4.4 16.2±0.84 86.9±1.5 97.6±3.8 83.1±2.6 92.2±1.2 35.0±0.58 89.5±0.67 98.5±1.2 104±0.49 28.6±1.8 97.4±0.36

12.8±0.25 25.0±0.49 62.2±1.1 84.2±0.18 44.2±2.2 73.1±0.9 86.9±1.4 14.2±0.28 8.0±0.28 14.9±0.72 9.7±0.77 66.1±1.8 16.1±0.49 8.0±0.36 4.0±0.30 64.9±0.89 6.1±0.42

101±0.23 105±2.9 100±0.41 104±1.1 102±0.29 95.8±3.5 103±2.3 101±1.2 106±4.1 98.0±1.9 102±0.41 101±2.4 106±1.2 107±1.5 108±0.19 93.5±2.7 104±0.79

170 171 172 173 174

SRHA and SRFA are Suwannee River humic acid and Suwannee River fulvic acid, respectively. Conc is concentration. Meff, Mret, and Mtot are mass percentages of CNT-Fe3O4 NHs recovered from effluent, retentate, and total column, respectively. Other column transport parameters such as porosity, pore-water velocity, and dispersivity are shown in SI Table S1.

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Porous Media

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Ottawa sands (U.S. Silica, Berkeley Springs, WV) were chosen as representative aquifer

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materials for column experiments. The sands were pre-sifted through 40-, 50-, 60-, and 70-mesh

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sieves (U.S. Standard Testing Sieves), and the fractions captured by 40−50, 50−60, and 60−70

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mesh sieves (average grain sizes of 360-, 280-, and 230-µm, respectively) were used. Before use,

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the sieved sands were cleaned using 1 M HCl and washed thoroughly with deionized (DI) water

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(Nanopure Diamond D11911, Barnstead International, Dubuque, IA). The colloidal particles

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from pulverized, cleaned sands were used as surrogates of sand grains for electrophoretic

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mobility (EPM) measurements in triplicate on the Zetasizer analyzer at 25 °C. Experimentally

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determined EPM values were converted to apparent zeta (ζ)-potentials for the sand grains using

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the Smoluchowski equation.46

186 187

Column Experiments

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Column experiments were conducted using glass chromatography columns (1.7-cm i.d. ×

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10-cm long).47 Each column was dry-packed using the cleaned 360-, 280-, or 230-µm sands. The

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packed-column was then purged with CO2 gas to remove any air remained during column

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packing processes, and to maximize the water accessibility of the column. After purging, the

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column was immediately saturated with DI water slowly in an upward mode. Following the

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saturation step, a nonreactive tracer experiment was performed to assess the hydrodynamic

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properties of column (see SI S5 and Table S1).

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After the completion of tracer experiment, the column was pre-equilibrated with desired

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background solution (Table 1). 2% CMC was not co-injected during the pre-equilibration

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procedure because CMC alters column hydromechanical (rheological) properties due to its high

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viscosity33 (described below). A two-step transport experiment was then initiated by injecting 3

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PVs of CNT-Fe3O4 NH influents (in 2% CMC) at the desired solution chemistries (designated as

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phase 1), followed by elution with 7 PVs of NH-free background electrolyte solution (without

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CMC; phase 2). Column experiments with different collector sizes (360-, 280-, and 230-µm,

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respectively) were also performed to examine the role of straining48,

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Previous studies suggest that, under unfavorable conditions, secondary minimum starts to

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capture colloids when solution IS is ≥10 mM NaCl50 or ≥1 mM CaCl245. To identify whether

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on NHs’ retention.

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secondary minimum is also involved in NHs’ retention, two three-step transport experiments

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were run at the highest NaCl (50 mM) and CaCl2 concentrations (3.33 mM) investigated in this

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study, i.e., eluting the column with 8 PVs of DI water in phase 3 after completing the two-step

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transport experiments. Darcy velocity was maintained at 0.44 cm/min for all experiments,

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mimicking typical fluid velocities in coarse aquifers51 or in forced-gradient in-situ remediation52.

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Column effluents were collected via a fraction collector. Following completion of each transport

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experiment, the spatial distribution of CNT-Fe3O4 NHs retained in the column (dissection

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experiments) was determined (SI S6). Column transport and dissection experiments were also

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performed for CNT-alone (in 2% CMC) to compare the mobility between NHs and parent NMs.

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The concentrations of CNT-Fe3O4 NHs and CNT in the effluents and retentates (retained

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colloids collected from dissection experiments) were determined spectrophotometrically at 218

216

nm (SI Figure S1). To eliminate the interferences of CMC in NHs (and CNT) measurements,

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two-step column transport experiments were also performed for 2% CMC-alone under the

218

experimental conditions identical to those for the transport of CNT-Fe3O4 NHs (Table 1). The

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actual concentration of CNT-Fe3O4 NHs (and CNT) in the effluents was obtained by subtracting

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the spectrophotometric response (at 218 nm) of CMC from that of the NHs (and CNT) at the

221

identical PV. Furthermore, to minimize the potential interferences of particle aggregation and

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sedimentation, spectrophotometric measurements for all NHs, CNT, and CMC in the effluents

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were controlled to be completed within 1.5 h after collecting from the fraction collector.

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Additionally, selected column effluents of NHs were digested in 2.6 M HNO3 for 3 d at 25 °C,

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and then filtered through 0.2-µm cellulose acetate membranes. Total Fe concentrations in the

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filtered digestion solutions were determined using inductively-coupled-plasma optical-emission-

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spectroscopy (ICP-OES; PerkinElmer Optima 3300DV, Norwalk, CT) to verify the reliability of

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spectrophotometric measurements of the NHs. Selected column effluents of NHs were also

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measured using ICP-OES (effluents after HNO3 digestion for total Fe measurements) to identify

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whether Fe3O4 NMs detach from the NHs or dissolve under acidic conditions (pH 4.0). Our

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measurements (Fe content < detection limit, meaning no Fe3O4 NMs detach or dissolve)

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substantiate the integrity of the NHs during porous media transport.

233 234

Electrokinetic Properties and Hydrodynamic Radii of CNT-Fe3O4 NHs in the

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Influents

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Electrophoretic mobility (EPM) and ζ-potential of CNT-Fe3O4 NHs (and CNT) in various

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influents (Table 1) were measured using the Zetasizer analyzer at 25 °C. The ζ-potentials of

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CNT-Fe3O4 NHs and sand grains were then used to calculate their interaction energy using

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DLVO theory that includes vdW and electrostatic double layer (EDL) interactions. It is

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impossible to determine Hamaker constant (A) of the hydrophobic CNT-Fe3O4 NHs (without

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CMC modification) in aqueous solutions experimentally. Thus, three Hamaker constants, i.e.,

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Hamaker constants of CNT (ACNT=6.00×10−20 J)53, 54 and Fe3O4 NMs (AFe3O4=33.0×10−20 J)55,

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and an estimated Hamaker constant of the NHs (ACNT−Fe3O4=17.6×10−20 J) based on the Hamaker

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constants of parent NMs and NHs’ elemental (C and Fe) contents, were employed to calculate

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the interaction energy between NHs and sand grains.56,

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energy calculation including inherent limitation of this approach were given in SI S7.

57

Details on the DLVO interaction

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The average hydrodynamic radius (rh) of CNT-Fe3O4 NHs (and CNT) in various influents

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and hydrodynamic particle size distribution of CNT-Fe3O4 NHs in the influents at different NaCl

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and CaCl2 concentrations were determined using DLS (described above). Particle size

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distribution of 2% CMC was also determined to investigate the role of CMC on the transport of

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CNT-Fe3O4 NHs. The CONTIN algorithm was used to convert intensity autocorrelation

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functions to intensity-weighted rh using the Stokes-Einstein equation for spherical particles.58

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Given that the NHs, CNT, and CMC are non-spherical, DLS provides the diameter of a sphere

254

that has the same average translational diffusion coefficient as the particle being measured. For

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selected column experiments, the rh of NHs and CNT in the effluents and retentates (retained in

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the column inlet 0−1 cm) was measured using DLS to understand size-dependent retention in

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saturated sand.

258 259

Data Analyses

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Parameters characterizing the mobility of CNT-Fe3O4 NHs in water-saturated porous

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media including single-collector contact efficiency (η), attachment efficiency (α), deposition rate

262

coefficient (kd), and maximum transport distance (Lmax) were calculated using the well-defined

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Tufenkji-Elimelech (TE) equation (SI S8).34

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RESULTS AND DISCUSSION

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Electrokinetic Properties and Hydrodynamic Radii of CNT-Fe3O4 NHs in the

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Influents and Electrokinetic Properties of Sand Grains

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Physicochemical properties of CNT-Fe3O4 NHs and CNT were characterized using XRD,

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TEM, Raman spectroscopy, and TGA (Figure 1). A strong peak occurred at 26.5° for both NHs

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and CNT in the XRD spectra, which corresponds to the d-spacing of [002] plane of graphite

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(CNT; Figure 1a).59 In contrast, the characteristic reflections of 30.0° [220], 35.6° [311], and

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43.4° [400] of Fe3O4 (JCPDS No. 19-0629) occurred only for the NHs, confirming the successful

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hybridization of CNT and Fe3O4 NMs. TEM images showed that the Fe3O4 NMs (20−30 nm size)

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heterogeneously-deposited on CNT’s surface as dense aggregates (Figure 1b). This is due to the

275

large SSA and surface reactivity, and strong magnetic attraction of the Fe3O4 NMs that promote

276

partial aggregation on the CNT’s surface (see SI Figure S5). The Fe3O4 NMs were strongly

277

anchored on the CNT’s surface even after ultrasonication treatment (100 W and 42 kHz for 1 h)

278

since no Fe was released from the NHs (via ICP-OES measurements). This could be due to the

279

strong covalent interaction between CNT (e.g., carboxyl (1.85% in the pristine CNT) and

280

carbonyl groups) and Fe3O4 NMs.7 The Raman spectrum of CNT exhibited three absorption

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bands at 1343, 1571, and 2688 cm−1 (Figure 1c), which is assigned to the D-, G’-, and G’-bands,

282

respectively.60 However, these three bands shifted to higher wavenumbers of 1357, 1582, and

283

2710 cm−1, respectively, for the NHs, yielding a larger D/G-band intensity ratio of ID/IG=0.326

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(ID/IG=0.316 for CNT). This indicates a smaller average size of the sp2 domains of carbon for the

285

NHs,60 likely due to the enhanced aggregation of NHs (more compressed structure) induced by

286

magnetic attraction (see S1 for more discussion). The TGA profiles showed that Fe accounted

287

for 42.8% of the total elemental mass because CNT was decomposed completely when the

288

temperature was >930 °C (Figure 1d). The C and Fe elemental composition information was

289

used to calculate the Hamaker constant and density of the NHs for DLVO interaction energy and

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transport parameter calculations (described below).

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a)

CNT

c)

CNT-Fe3O4

CNT-Fe3O4 CNT

Intensity (a.u.)

Intensity (a.u.)

CNT

Fe3O4

10

20

30

40

50

60

70

80

90

ID/IG=0.326

ID/IG=0.316

400

800 1200 1600 2000 2400 2800 3200

Raman shift (cm-1)

o

2Theta ( )

CNT-Fe3O4

Retained weight (%)

b)

CNT

100 80 60

42.8% 40 20

d)

0 0

0 200

400

600

800

1000

o

T ( C)

50 nm

291 292 293 294 295

Figure 1. XRD patterns (a), TEM images (b), Raman spectra (c), and TGA profiles (d) of the synthesized carbon nanotubes-magnetite nanohybrids (CNT-Fe3O4 NHs) and parent CNT component.

296

The EPMs and ζ-potentials of CNT-Fe3O4 NHs in the influents (in 2% CMC) and sand

297

grains under different experimental conditions are shown in Table S2. Both NHs and sand grains

298

were negatively charged, suggesting unfavorable conditions for column experiments. As

299

mentioned above, the NHs without CMC modification were highly-hydrophobic, making EPM

300

(and DLS) measurements impossible in aqueous solutions. However, in 2% CMC, the ζ-

301

potentials of NHs in the influents were highly negative (−39.4 to −58.7 mV) due mainly to the

15



302

adsorption of hydrophilic, negatively charged carboxymethyl groups of CMC (e.g., ζ-potential of

303

2% CMC-alone = −49.3 mV at 1 mM NaCl and pH 7.3) onto NHs’ surface. The ζ-potential of

304

NHs in the influents (in 2% CMC) was more negative than that of 2% CMC-alone at a specified

305

solution chemistry, e.g., −53.4 > −49.3 mV at 1 mM NaCl and pH 7.3 (Table S2). This

306

discrepancy could be due to the deprotonation of carboxyl and carbonyl groups of the NHs,

307

although most carboxyl groups are expected to be used for anchoring CNT and magnetite NMs

308

(described above). Consistent with reported results for individual NMs and sand grains,61-63

309

decreasing IS, and increasing NOM concentration and pH increased (more negative) the ζ-

310

potential of both NHs and sand grains. The divalent Ca2+ decreased the ζ-potentials of NHs and

311

sand grains more than monovalent Na+ at equivalent ISs by greater charge screening (both Na+

312

and Ca2+) and neutralization (Ca2+ only) effects.46

313

Table S2 also shows the variability in rh of CNT-Fe3O4 NHs in the influents (2% CMC)

314

with varying experimental conditions. Greater aggregation (larger rh) of the NHs was associated

315

with the lower ζ-potentials due to less electrostatic repulsions according to the DLVO interaction

316

prediction (described below). Similar findings have been documented for individual NMs.61, 62

317

The measured hydrodynamic diameter (2rh) of NHs and CNT (Table S2) lied within the diameter

318

(8−15 nm) and length (10−50 µm) of the CNT, consistent with the results reported previously.19

319

This is likely because, for non-spherical NHs and CNT, DLS gives the diameter of a sphere that

320

has the same average translational diffusion coefficient as the particle (NHs and CNT) being

321

measured (described above).

322 323

Mobility Comparison between CNT-Fe3O4 NHs and Individual CNT

16


Page 16 of 44

Page 17 of 44


324

Breakthrough curves (BTCs) and retention profiles (RPs) of CNT-Fe3O4 NHs and CNT

325

(both in 2% CMC) under the identical transport condition (1 mM NaCl and pH 7.3) are presented

326

in Figure 2. Total mass recoveries (Mtot) of NHs in the effluents (Meff) and retentates (Mret) under

327

different experimental conditions are given in Table 1, which confirms a high degree of

328

confidence in our experimental measurements because virtually all NHs were recovered from

329

column experiments (Mtot−NHs=93.5−108% and Mtot−CNT=99.4%). The ICP-OES results for

330

effluent Fe analyses matched (standard errors 1526 > 3.4

368

nm) (Table S2 and Figure S4). This is due primarily to the greater interception efficiency (ηI) and

369

overall single-collector contact efficiency (ηo) at larger rh, because ηI and ηo increase

18


Page 19 of 44


370

monotonically with colloid size when the diameter is ≥1 µm.34 To better understand the two-peak

371

transport feature, rh of NHs and CNT in the influents, effluents, and retentates was determined

372

(Figure 2d and Table 2). The rh of NHs was in the order: effluent < influent < retentate (e.g.,

373

1432 < 1632 < 2001 nm in 1 mM NaCl and pH 7.3; Table 2), indicating size-selective retention

374

in porous media. The size-selective retention that larger particles preferentially retain in the

375

column (retentates) due likely to greater ηI and ηo; and smaller ones elute out (effluents) has been

376

reported in colloid transport studies.47 Furthermore, for the effluents, the rh of both NHs and

377

CNT increased progressively with PV (Figure 2d), again strongly substantiating the size-

378

selective retention. It is thus logical to anticipate that smaller NHs/CNT associated with lower

379

viscous CMCs breakthrough earlier (yielding ‘peak 1’); whereas, larger NHs/CNT coated with

380

higher viscous CMCs are eluted out more retarded (i.e., ‘peak 2’). The peak location of ‘peak 2’

381

was earlier for CNT than that of NHs (4.5 vs. 5 PV; Figure 2a) further supports that larger

382

cumbersome NHs (effluent rh: CNT < NHs; Figure 2d) have a lower transport velocity (i.e., more

383

retarded breakthrough). Greater retention of NHs observed in Figure 2b is due to greater rh

384

(greater ηI and ηo) in the influents and effluents (Figure 2d). To sum up, the above results

385

underscore the interplay of viscosity variability of fluid and size-selective retention in the

386

transport of NMs (in 2% CMC) in water-saturated porous media.

19



Peak 2

1.0

Page 20 of 44

1.2

a)

c)

CNT-Fe3O4

0.8

2% CMC

1.0

CNT

0.8

C/Co [-]

C/Co [-]

0.6 Peak 1 0.4 0.2

0.6 0.4 0.2

0.0

0.0 0

1

2

3

4

5

6

7

8

9

10

0

1

2

3

4

PV 0.5

5

6

7

8

9

10

PV

b)

1600

CNT-Fe3O4

d)

CNT-Fe3O4

CNT

0.4

CNT

rh (nm)

0.3

3

S/Co (cm /g)

1200

0.2

400

0.1 0.0

800

0

1

2

3

4

5

6

7

8

9

10

0

0

1

2

3

4

5

6

7

8

9

10

PV

Depth (cm)

387 388 389 390 391 392 393 394 395 396 397 398 399 400 401

Figure 2. Breakthrough curves (BTCs; a) and retention profiles (RPs; b) of CNT-Fe3O4 NHs and individual CNT, respectively, in water-saturated sand columns at 1 mM NaCl and pH 7.3 (in 2% CMC). Breakthrough curve describes the normalized effluent concentration of NMs, C/Co (where Co is the initial influent concentration of CNT-Fe3O4 NHs or CNT) as a function of pore volume (PV); and retention profile shows the normalized solid-phase retention concentration of NMs, S/Co (where S is retention amount of CNT-Fe3O4 NHs or CNT per gram dry sand) as a function of distance from the column inlet. For column transport experiments, 3 PVs (marked by dash line in a) of CNT-Fe3O4 NH or CNT influent were injected in the column followed by elution with 7 PVs of NM-free background electrolyte solution (without CMC). Other column transport parameters were the same including: CNT-Fe3O4 NH or CNT influent concentration = 10 mg/L, influent pH = 7.3; background electrolyte = 1 mM NaCl; average sand grain size = 360 µm; and Darcy velocity = 0.44 cm/min. For comparison, the breakthrough curve of 2% CMCalone under the experimental condition (1 mM NaCl and pH 7.3) identical to that of NHs/CNT is shown in (c). Figure d shows the average hydrodynamic radius (rh) of CNT-Fe3O4 NHs and CNT

20


Page 21 of 44

402 403


in the effluents (collected from a). The error bars represent the standard deviations from duplicate experiments. Note different y-axis scales in figure.

404 Table 2. Average hydrodynamic radius (rh) of CNT-Fe3O4 NHs in the influents (in 2% CMC), effluents, and retentates at different NaCl and CaCl2 concentrations. NaCl (mM)

CaCl2 (mM)

Sand grain diameter (µm)

rh (nm) Influent

Effluent a

Retentate b

∆rh c

0 1 10 50 0 0 0

0 0 0 0 0.33 1.67 3.33

360 360 360 360 360 360 360

994±226 1632±143 1687±167 2025±290 1988±237 2238±399 2728±349

836±48 1432±152 1455±182 1734±324 1691±182 1825±297 1958±350

1096±41 2001±182 2285±234 2789±188 2338±146 2875±221 3568±438

260 569 830 1055 647 1050 1610

405 406 407 408 409 410

a

411

Mobility of CNT-Fe3O4 NHs under Varying Physicochemical Conditions

rh/r50 d (−) 5.52×10−3 9.07×10−3 9.37×10−3 1.13×10−2 1.10×10−2 1.24×10−2 1.52×10−2

Effluents collected at 1.5, 2, 2.5, and 3 PVs (Figures 2−3) were used for rh measurements. Retentates retained at the column inlet (0−1 cm) were used for rh measurements. c rh difference of NHs in the effluents and retentates. d r50 is the average sand grain radius (180 µm). The rh/r50 values were calculated to test the effect of straining on the retention of CNT-Fe3O4 NHs in column experiments. b

412

Environmentally-relevant physicochemical conditions were chosen to investigate the

413

mobility of CNT-Fe3O4 NHs (in 2% CMC) including: NaCl (Figure 3a−b), CaCl2 (Figure 3c−d),

414

SRHA (Figure 3e−f), and SRFA concentrations (Figure 3g−h), pHs (Figure S6a−b), particle

415

concentrations (Figure S6c−d), and collector sizes (Figure S6e−f). The DLVO interaction

416

energies between CNT-Fe3O4 NHs and sand grains under different experimental conditions are

417

shown in SI Figure S7 and Table S3. Before interpreting the mobility of NHs under different

418

experimental conditions, it is important to understand the colloidal stability of NH influents. In 2%

419

CMC, the NH influents were stable (influent concentration and rh stayed unchanged; Figure S2)

420

over the time frame of influent injection experiments. Recalling that without CMC, partial

21



Page 22 of 44

421

aggregation of NHs was pronounced due to magnetic attraction (Figure S5), our findings

422

emphasize the critical role of 2% CMC in stabilizing the NH aggregates (e.g., NHs exist as stable

423

aggregates because rh varies with solution chemistries). No further aggregation and gravitational

424

sedimentation of NHs occurred in the influents during influent injection experiments, again

425

confirming the strong stabilizing effect of 2% CMC. Nonetheless, both CFT and TE equation

426

predict that gravitational sedimentation is appreciable when colloid diameter is ≥1 µm.30, 34 This

427

is likely because high viscous and negatively charged CMCs (described above) counterbalance

428

the role of gravity in causing particle sedimentation.

429

Similarly, the two-peak transport feature occurred frequently in the NHs’ BTCs with

430

altered C/Co values and peak locations (Figures 3 and S6). Decreasing IS, and increasing NOM

431

concentration, pH, particle concentration, and collector size (with the exception of 230-µm sand;

432

Figure S6e−f) increased the mobility of NHs (see Meff and Mret in Table 1), consistent with

433

DLVO theory prediction (Figure S7 and Table S3). Lower mobility of NHs under different

434

experimental conditions was again associated with less negative ζ-potential and larger rh due to

435

greater ηI and ηo. The size-selective retention also occurred during NHs transport (rh order:

436

effluent < influent < retentate; Table 2); and this effect became more pronounced (increased ∆rh)

437

at higher NaCl/CaCl2 concentrations. To better unravel the size-selective retention of NHs in

438

porous media, hydrodynamic particle size distribution of NHs in the influents (in 2% CMC) was

439

determined (SI Figure S8). Our results indicate a greater degree of particle size heterogeneity

440

(wider particle size distribution) among NHs populations, likely accounting for the greater size-

441

selective retention of NHs at higher NaCl/CaCl2 concentrations. Other potential explanations for

442

the greater size-selective retention at higher NaCl/CaCl2 concentrations include deeper attractive

443

secondary minimum (Table S3) and more pronounced straining (Table 2) given that hyper-

22


Page 23 of 44


444

exponential RPs occurred at high NaCl/CaCl2 concentrations (Figure 3b, d). The fact that

445

retained NHs were released upon injecting DI water during the three-step transport experiments

446

(SI Figure S9a) substantiates the important role of secondary minimum in NHs’ retention.

447

However, the extent of NHs’ release was much less in Ca2+ than in Na+ due likely to more

448

pronounced straining due to larger rh of NHs in the influents, effluents, and retentates (Table 2

449

and Figure S9b), i.e., Ca2+ bridges the -CH2COOH and -COOH groups of CMC-coated CNT-

450

Fe3O4 NHs. Straining is demonstrated to play a critical role in colloid retention and cause hyper-

451

exponential RPs when the diameter ratio of colloid vs. collector is ≥0.008.49 Our calculations

452

show that when the size ratio of NHs vs. collector was ≥0.00937 (≥10 mM NaCl and ≥1.67 mM

453

CaCl2; Table 2), straining started to yield the hyper-exponential RPs (Figure 3b, d). The slight

454

difference (0.008

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