Surfactant-Wrapped Multiwalled Carbon Nanotubes in Aquatic Systems

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Surfactant-Wrapped Multiwalled Carbon Nanotubes in Aquatic Systems: Surfactant Displacement in the Presence of Humic Acid Xiaojun Chang, and Dermont C. Bouchard Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01536 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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Environmental Science & Technology

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Surfactant-Wrapped Multiwalled Carbon Nanotubes

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in Aquatic Systems: Surfactant Displacement in the

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Presence of Humic Acid

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Xiaojun Chang‡ and Dermont C. Bouchard§,* ‡

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§

National Research Council Research Associate

USEPA Office of Research and Development, National Exposure Research Laboratory,

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960 College Station Road, Athens, GA 30605, USA

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*Corresponding author E-mail: [email protected];

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Phone: 706-355-8333

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Total Word Length: 5055 + 6 figures*300 = 6855 words (not including references)

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Abstract

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Sodium dodecyl sulfate (SDS) facilitates multi-walled carbon nanotube (MWCNT) de-

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bundling and enhances nanotube stability in the aqueous environment by adsorbing on the

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nanotube surfaces, thereby increasing repulsive electrostatic forces and steric effects. The

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resulting SDS-wrapped MWCNTs are utilized in industrial applications and have been widely

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employed in environmental studies. In the present study, MWCNTs adsorbed SDS during

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ultrasonication to form stable MWCNTs suspensions. Desorption of SDS from MWCNTs

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surfaces was then investigated as a function of Suwannee River Humic Acid (SRHA) and

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background electrolyte concentrations. Due to hydrophobic effects and π-π interactions,

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MWCNTs exhibit higher affinity for SRHA than SDS. In the presence of SRHA, SDS adsorbed

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on MWCNTs was displaced. Cations (Na+, Ca2+) decreased SDS desorption from MWCNTs due

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to charge screening effects. Interestingly, the presence of the divalent calcium cation facilitated

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multi-layered SRHA adsorption on MWCNTs through bridging effects, while monovalent

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sodium reduced SRHA adsorption. Results of the present study suggest that properties of

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MWCNTs wrapped with commercial surfactants will be altered when these materials are

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released to surface waters and the surfactant coating will be displaced by natural organic matter

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(NOM). Changes on their surfaces will significantly affect MWCNTs fate in aquatic

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environments.


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1. Introduction.

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As nanotechnology develops, more engineered nanomaterials are being utilized in industry

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and studied to understand their impacts on environmental health and safety. In both industrial

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applications and environmental research, the original properties of nanomaterials (NMs) are

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likely to be altered as media constituents are adsorbed on their surface upon exposure to

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biological or environmental media.1 Since many NMs are coated with surface active materials to

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enhance suspension processing and stability, there is competition between the original coatings

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and media constituents for adsorption sites. Displacement of the original adsorbed ligand by

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other constituents on NM surfaces has been reported: bovine serum albumin (BSA), with a

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higher binding affinity to gold nanoparticles (AuNPs), has been observed to displace thiolated

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poly(ethylene glycol) on AuNP surfaces;2 Similarly, carboxylate and thiol groups displaced

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oleylamine and oleic acid, respectively, on FePt nanoparticles surfaces due to their stronger

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respective affinities for iron and platinum atoms.3 These changed surface characteristics, rather

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than their original properties, dictate NM application and their environmental fate.4-6 Although

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extensive research has been conducted on the surface properties of nanoparticles stock

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suspensions, tracking changes in surface coatings and properties as NMs are released into

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environmental media are less well studied.7 Lack of knowledge about the dynamics of

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nanomaterials surface identity in the environment greatly weakens our ability to understand their

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behaviors and to accurately predict their environmental impacts.

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Due to high van der waal interaction energies between tubes,8 carbon nanotubes (CNTs)

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cannot be readily dispersed in water as de-bundled individual nanotubes; in practical applications

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and scientific studies, methodologies to suspend CNTs in aqueous systems are necessary. Other

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than functionalization via covalent bonds,9 the most commonly used method to obtain stable 3 ACS Paragon Plus Environment


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CNT suspensions is to ultrasonicate CNTs in surfactant solutions.10-15 The resulting de-bundled

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CNTs remained stable in the aqueous environment due to electrostatic and/or steric forces

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acquired from surfactant molecules adsorbed on their surfaces. Surfactant-wrapped CNTs have

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been widely used in studies on CNTs toxicities,16,17 transport, and fate.11,18-20 The surfactant

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coatings can affect CNTs ability to adsorb other chemicals,21-23 however, they can also be

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replaced by other adsorbates with a higher affinity for CNTs. For example, while employing

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near-infrared fluorescence spectroscopy, Cherukuri et al. observed the displacement of Pluronic

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108 surfactant coating on single-walled carbon nanotubes (SWCNTs) by rabbit serum albumin.24

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Natural organic matter (NOM), which is ubiquitous in the environment, has shown an

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exceptional capability for dispersing and stabilizing CNTs.12,25,26 Numerous studies have

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investigated NOM effects on organic pollutants adsorption on CNTs surfaces. When added

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concurrently with organic pollutants (e.g., polycyclic aromatic hydrocarbon, perfluorinated

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compounds, pharmaceutical and personal care products, etc.) to the adsorption system, NOM

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facilitates de-bundling and dispersion of CNTs which increases their surfaces areas, leading to an

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increase in adsorption sites. At the same time, NOM competes with pollutants for available

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adsorption sites, resulting in a decrease in pollutant adsorption on CNTs. Due to these opposing

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effects, the presence of NOM in a system with de-bundled CNTs either promotes27 or inhibits

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pollutants adsorbing on CNTs.28-32 When NOM adsorb on CNT surfaces prior to pollutants

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introduction, the suppression of pollutants adsorption on CNTs is more profound since NOM

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occupies available adsorption sites.29,30,33 From the referenced studies, it is clear that NOM can

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exert significant effects on CNT surface chemistry and adsorption capacity,34 changing the

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transport, bioavailability, and toxicity of CNTs and organic pollutants adsorbed on CNTs

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surfaces.


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Upon release of surfactant-wrapped CNTs to a surface water, CNT surface chemistry may be

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substantially changed as the initial coating is replaced by NOM. While it is clear coating of

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CNTs is a major determinant of their environmental fate, there are no systematic studies on

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NOM exchange with CNT surfactant coatings. To fill this gap, the present study investigated

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displacement of surfactant on multi-walled carbon nanotubes (MWCNTs) surfaces by NOM and

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the effects of solution chemistry during this process. Sodium dodecyl sulfate (SDS), a commonly

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used surfactant, was employed to disperse MWCNTs in aqueous systems. The resultant SDS-

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wrapped MWCNTs were then exposed to solutions with varying Suwannee River Humic Acid

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(SRHA) concentration and ionic composition, and the mass and kinetics of SDS release and

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SRHA adsorption were quantified.

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2. Materials and Methods:

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2.1 Chemicals.

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The unfunctionalized MWCNTs with purity of 95 wt% were purchased from Cheap Tubes

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Inc. (Brattleboro, VT). Their inside diameters, outside diameters, and lengths are reported as 5-

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10 nm, 20-30 nm, and 10-30 µm, respectively. Detailed characterization (e.g., XPS for

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functionalization and ICP-AES for metal residue analysis) for the MWCNTs powder was

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presented in our previous study.35 Analytical grade sodium dodecyl sulfate (SDS), sodium

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chloride (NaCl), calcium chloride (CaCl2), and ScintiSafe 30% liquid scintillation cocktail for

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14

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labeled SDS (0.1 mCi/mL, 55 mCi/mM) was purchased from American Radiolabeled Chemicals

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(St. Louis, Mo). Suwanee River Humic Acid (SRHA, Standard II) was purchased from the

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International Humic Substance Society (St. Paul, MN). A representative surface water sample

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was collected from Brier Creek GA, USA.

C isotope activity measurement were purchased from Fisher Chemicals (Fremont, CA).

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C-



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2.2 Solution and Suspension Preparation.

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2.2.1 Solutions. 1 M NaCl, 0.1 M CaCl2, and 1 g/L SDS solutions were prepared in deionized

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water (DI) with a resistivity of 18.2 MΩ, from Aqua Solution Type I Water Purification System

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(Aqua Solutions, Jasper, GA). The stock SDS solution was diluted to obtain 5, 10, 20, and 40

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mg/L SDS solutions (pH ≈ 7.0).

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spiking 50 µL of the as purchased

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solutions. This added an additional 0.0952 mg/L SDS, causing a negligible [SDS] increase – less

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than 4% for 2.5 mg/L SDS solution – and even lower additional fractions for solutions with

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higher [SDS]. To prepare the SHRA stock solution, SRHA powder was first dissolved in DI

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water and then filtered through a 0.22 µm polycarbonate filter (Osmonics Inc. Trevose, PA). The

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working SRHA solutions (2.5 to 40 mg C/L and pH 4.8 to 5.5) were all diluted from this stock

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solution.

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C-labeled SDS working solutions were then prepared by 14

C-labeled SDS stock solution into 250 mL of the SDS

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2.2.2 MWCNT Suspensions. All MWCNT suspensions used in the present study were

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prepared using probe sonication.12 Four milligrams of MWCNT powders, as purchased, were

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mixed with 40 mL dispersant (i.e., 14C-SDS or SRHA) working solutions, whose concentrations

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were adjusted within 2.5 - 40 mg C/L (note: SDS concentrations used in the present study were

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2-3 orders of magnitude lower than its critical micelle concentration).36 The mixtures were then

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ultrasonicated with a probe sonicator (Sonic & Materials, Newton, CT) in an ice-water bath for

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10 min at an average input energy level of ~33 W. Here, we refer to MWCNT suspensions

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prepared in different dispersant solutions as XX mg/L YY-MWCNTs, where XX indicates

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concentration of the dispersant and YY indicates dispersant species.

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2.3 Characterization.

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2.3.1 General Physicochemical Characterization for MWCNTs. 6 ACS Paragon Plus Environment

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Intensity-weighted hydrodynamic diameter (Dh) and electrophoretic mobility (EPM) were

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used to represent average particle size and surface charge of dispersed MWCNTs, respectively.

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Dh and polydispersity index (PDI) were measured by dynamic light scattering (DLS) and EPM

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was measured by phase analysis light scattering using a Nano ZetaSizer (Malvern Instruments,

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Worcestershire, UK) with a helium/neon laser (λ = 633 nm).12

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2.3.2 Dispersant Concentration Determination.

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SRHA concentrations (presented as total organic carbon, TOC, mg C/L) were monitored by a

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calibration curve established using TOC concentrations and their corresponding ultraviolet

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absorbance values at 300 nm, obtained from a series of diluted stock SRHA solutions. TOC for

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the stock SRHA solution was determined using a Shimadzu carbon analyzer (TOC-VCPH,

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Columbia, MD). 14C-SDS was employed to facilitate measurement of the SDS concentration. 14C

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activity was determined using a liquid scintillation counter (PerkinElmer Tri-Carb 3100). For

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samples collected from MWCNTs suspensions prepared using a certain SDS working solution,

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there is a linear relationship between 14C activities and [SDS].

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2.4 Adsorption and Desorption Experiments

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2.4.1. Adsorption Isotherms. To determine the adsorption of SDS and SRHA on MWCNTs,

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1.6 mL of MWCNTs suspensions prepared in 2.2.2 were filtered through the 0.22 µm

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polycarbonate filters 24-hour after suspension formation via ultrasonication. Preliminary tests

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showed the [MWCNTs] in the filtrate was below the detection limit of the UV-vis method (< 0.4

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mg/L), indicating that the membrane retained more than 99.6% of the suspended MWCNTs.

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Filtration of

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both SDS and SRHA during filtration were higher than 98%. To further minimize dispersant

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adsorption, prior to the filtration of any MWCNTs suspension, 1.6 mL of the corresponding

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C-SDS and SRHA working solutions demonstrated that recovery efficiencies for



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dispersant solution was first filtered to saturate any possible adsorption sites on the filter

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membrane and housing and the residue solution was completely drained. Dispersant adsorption

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on the filter membrane and housing was therefore negligible during MWCNTs filtration and

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elution. After MWCNTs suspension filtration, dispersants that adsorbed onto MWCNTs surfaces

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were also retained on filter membranes, while SDS and SRHA concentration in the filtrates were

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determined using the aforementioned methods. These data were fitted with both Freundlich (Eqn.

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1) and Langmuir (Eqn. 2) adsorption models:

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ܳ௘ = ‫ܭ‬௙ × ‫ܥ‬௘௡

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ܳ௘ = ܳ௠ × (ଵା஻஼೐

(1)

஻஼

೐)

(2)

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where Qe (mg C/g MWCNTs) and Ce (total organic concentration, mg C/L, for both dispersant)

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are the equilibrium solid-phase and aqueous-phase concentrations, respectively; Kf [(mg C/g

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MWCNTs)/(mg C/L)n] is the Freundlich adsorption coefficient; n is the nonlinearity factor; Qm

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(mg C/g MWCNTs) is the maximum adsorption capacity; and B (L/mg C) is the affinity

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parameter.

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2.4.2 Dispersant Desorption by DI/Benchmark Elution. MWCNTs retained on filters in 2.4.1

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were eluted repeatedly (14 times) with 0.8 mL DI water to determine desorption of the adsorbed

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SDS/SRHA. Each eluate was collected and analyzed to determine the amount of desorbed

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dispersant. For SDS-MWCNTs – after all DI elutions – each filter with retained MWCNTs was

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sonicated in 10 mL of cocktail for 30 min in a bath sonicator, and the amount of SDS retained on

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MWCNTs surfaces was determined by its 14C activity. For comparison to elution experiments by

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salt and/or SRHA solutions (see 2.4.3), the DI-eluted SDS-MWCNTs experiments were referred

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to as benchmark elution experiments.


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2.4.3 SDS Desorption from MWCNTs as a function of SRHA and Salt Concentration. 5.0

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mg/L SDS-MWCNTs retained on filter in 2.4.1 were eluted by aliquots (0.8 mL) of SRHA,

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CaCl2, NaCl, or solutions with both SRHA and salts for 10 times, followed by 0.8 mL DI elution

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for 4 times. SDS desorption during each elution and SDS retained on MWCNTs after all elutions

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was determined by the method described in 2.3.2. Adsorbed SRHA during each elution was

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determined by the difference between [SRHA] in the eluting solution and eluate. The same

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experiments were conducted using Brier Creek (BC) water to investigate SDS desorption in the

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presence of an actual surface water sample. For comparison, a synthetic water, i.e., a solution

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containing the same amount of Na+ and Ca2+ as BC water but without dissolved organic carbon

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(DOC), was also employed in the elution experiments.

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3. Results and Discussion.

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3.1 Physicochemical Properties of MWCNTs.

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In the presence of SDS or SRHA, ultrasonication successfully de-bundled and stabilized

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MWCNTs.12 MWCNTs suspended in both dispersants were negatively charged with

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electrophoretic mobilities (EPMs) < -4.5 × 10-8 m2/V-s. Average particles sizes (Dh) for all

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MWCNTs suspensions prepared via ultrasonication were ~220 nm; Dh was also observed to be

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insensitive to dispersant species and concentration (Figure S1).

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3.2 Dispersants Adsorption and Desorption.

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During the formation of stable MWCNTs suspensions using ultrasonication, dispersants

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adsorbed on MWCNTs reached adsorption equilibrium by 24 hours (Figure S2). For both SDS

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and SRHA, solid-phase equilibrium concentration on MWCNTs (Qe, mg C/g MWCNTs) first

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increased with aqueous-phase equilibrium concentration (Ce, mg C/L), then reached a plateau



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(Figure 1A). Fitting results for the nonlinear Langmuir and Freundlich models are presented in

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the table inset in Figure 1A. At the same Ce, Qe for SRHA was at least four times higher than Qe

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for SDS. The greater SRHA adsorption on MWCNTs surfaces is likely caused by structural

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differences between SRHA and SDS: previous experimental37 and theoretical38 studies have

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reported that linear-shaped SDS molecules lay randomly on CNTs surfaces with little protrusion

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of the SDS hydrophobic tail into surrounding water. SRHA, with its more three-dimensional,

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nonlinear structure,39 likely forms a thicker wrap layer on MWCNTs surfaces, which is indicated

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by the greater SRHA adsorption. Multi-layer adsorption (i.e., subsequent SRHA adsorption on

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SRHA layers directly adsorbed on MWCNTs) is enabled by hydrogen bond and π-π interactions

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and is also likely to account for this higher SRHA adsorption. Multi-layer adsorption on CNTs

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has been observed in tannic acid40 and lysozyme41 adsorption. Multi-layer adsorption of SRHA

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on α-Fe2O3 nanoparticles surfaces has been reported by Mudunkotuwa and Grassian:1 SRHA

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exhibited two different ATR-FTIR spectra on α-Fe2O3 nanoparticles, suggesting one type of

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SRHA is directly adsorbed on α-Fe2O3 surfaces and a second type is adsorbed onto subsequent

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SRHA layers.

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Maximum adsorption capacities (Qm) from the Langmuir model fitting are 54.8 and 12.2 mg

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C/g MWCNTs for SRHA and SDS, respectively. The two values can be interpreted as that each

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MWCNT carbon atom can adsorb, on average, 0.054 and 0.012 carbon atoms of SRHA and

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SDS, respectively. Such low values do not necessarily mean that only a small portion of the

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MWCNTs surfaces was covered by these dispersants. Based upon parameters provided by the

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manufacturer and some basic assumptions,42 a straightforward calculation (details in Supporting

210

Information) demonstrates that the carbon atoms on the outermost wall are ~5.7% of the total

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carbon atoms in MWCNTs used in the present study. Therefore, each outermost wall carbon 10 ACS Paragon Plus Environment

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atom adsorbed roughly 0.96 and 0.21 carbon atoms of SRHA and SDS, respectively, suggesting

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substantial coverage of the MWCNTs surface by either dispersant. The Freundlich model-fitting

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shows that the nonlinear factors, n, for both dispersants were quite similar (0.326 and 0.304 for

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SDS and SRHA, respectively), which is consistent with this parameter being mainly dependent

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upon adsorbent surface heterogeneity and independent of adsorbate species.43

217 218 219 220 221 222

Figure 1 (A) Solid-phase equilibrium concentration (Qe) as a function of aqueous equilibrium concentration (Ce) for SRHA and SDS adsorption on MWCNTs; Table: Adsorption isotherm fitting results for Langmuir and Freundlich adsorption models. [Ce is presented as total organic carbon concentration (mg C/L) for both dispersants.] (B) [SDS] in eluate for SDS-MWCNTs eluted by DI; Inset: [SRHA] in eluate for 10 mg C/L SRHA-MWCNTs eluted by DI.

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The higher affinity parameter B for SRHA in the Langmuir model (Table in Figure 1A)

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suggests a stronger bonding between SRHA and MWCNTs than between SDS and MWCNTs.43

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One approach to quantify adsorption affinity is to measure desorption of adsorbate from

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adsorbent.1 In the present study, the SRHA- or SDS-wrapped MWCNTs retained on filter

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membranes were eluted with DI and desorption was quantified by measuring released dispersant

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concentration in the eluate. Sequential elution of 2.5 mg/L SDS-MWCNTs resulted in [SDS] in

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the eluate gradually decreasing from 0.30 to 0.03 mg/L; and [SDS] in the eluate decreased from



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0.58 to 0.03 mg/L for the sequential elution of 5.0 mg/L SDS-MWCNTs (Figure 1B). After all

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14 times of DI elution, 62.9 ± 0.4% and 65.1 ± 0.7% of the adsorbed SDS desorbed from 2.5 and

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5.0 mg/L SDS-MWCNTs (Figure S3), respectively. The desorption profile (Figure S4) of 5.0

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mg/L SDS-MWCNTs lies above the SDS-MWCNTs adsorption isotherm suggesting that

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although desorption equilibrium was not reached during the relatively short contact time between

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eluent (i.e., DI) and MWCNTs, a significant amount of adsorbed SDS was still released from the

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MWCNTs during DI elution. The first DI elution of 10 mg C/L SRHA-MWCNTs yielded an

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eluate concentration of 0.2 mg C/L. SRHA concentrations in later DI elutions were all below

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detection limits (Figure 1B inset). The SRHA released from MWCNTs surfaces after fourteen DI

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elutions accounted for only 3.5% of the adsorbed SRHA. The stronger SRHA-MWCNTs

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association suggested by more facile SDS desorption than SRHA desorption is most likely due to

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greater hydrophobic effects44 and π-π interactions45 of the more chemically heterogeneous

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SRHA.

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3.3 Displacement of SDS by SRHA on MWCNTs

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Higher affinity of SRHA for MWCNTs than SDS, and the resulting slower SRHA desorption

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from MWCNT surfaces suggest that SRHA will be a more efficient competitor for MWCNT

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adsorption sites. Consequently, when SDS-wrapped MWCNTs are exposed to SRHA, SDS

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coating will be displaced by SRHA.

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To test the above hypothesis, 5.0 mg/L SDS-MWCNTs retained on polycarbonate membranes

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were eluted by solutions with varying [SRHA]. SRHA was very effective at promoting SDS

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release from MWCNTs surfaces, as depicted in Figure 2A. A significant [SRHA] effect was

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observed in the first elution: SDS concentration in first eluate increased four-fold (from 0.579 ±

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0.07 to 2.274 ± 0.013 mg/L, Figure S5) with [SRHA] from 0 to 40 mg C/L. During the second 12 ACS Paragon Plus Environment

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SRHA elution, desorbed SDS first increased with [SRHA], reaching the maximum at [SRHA] =

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10 mg C/L, then decreased. The SDS desorption maxima in later elutions occurred at even lower

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SHRA levels, e.g., 5 and 2.5 mg C/L for the third and fourth elutions, respectively (Figure S5).

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After 10-0.8 mL SRHA elutions, the total SDS desorption in the presence of SRHA was much

257

higher than that desorbed in the benchmark DI elutions (59.9 ± 0.7%, Figure 2A). They were

258

insensitive to [SRHA], however, as SRHA concentration in eluent increased from 2.5 to 40 mg

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C/L, the total desorbed SDS increased slightly – from 83.0 ± 0.6% to 91.1 ± 0.9%. A similar

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trend was observed by Shen et al.46 in their study of lindane displacement by atrazine on

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MWCNTs: lindane desorption spiked at a low atrazine level, then slowly increased as [atrazine]

262

further increased. The observation that most of the adsorbed SDS was released from the

263

MWCNTs regardless of [SRHA] is of great environmental importance: it indicates that even in

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surface waters with low dissolved organic carbon (DOC) concentrations, DOC will significantly

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enhance SDS release from MWCNTs surfaces and therefore alter the MWCNTs surface

266

properties that dictate their environmental fate.

267 268 269

Figure 2 (A) The fraction of released SDS to total SDS adsorbed on SDS-MWCNTs when 5.0 mg/L SDSMWCNTs was first eluted by SRHA solutions (0.8 mL × 10 times) and then DI (0.8 mL × 4 times). (Note: the



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[SRHA] in the top x-axis label indicates the SRHA concentration used in the previous SRHA elutions.) (B) SRHA adsorbed on MWCNTs when SRHA solution eluted 5.0 mg/L SDS-MWCNTs.

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SRHA adsorption on MWCNTs occurred simultaneously with SDS desorption during SRHA

273

elution. In contrast to total SDS release, which was virtually independent of [SRHA], total

274

adsorbed SRHA increased significantly with [SRHA] in the eluent (Figure 2B). After 10-0.8 mL

275

elutions with 2.5 mg C/L SRHA solution, each gram of MWCNTs adsorbed 75 mg SRHA (total

276

carbon mass), while adsorbed SRHA concentration was as high as 230 mg C/g MWCNTs when

277

eluted by 40 mg C/L SRHA.

278

It is worth noting that the total adsorbed SRHA on SDS-MWCNTs in all SRHA elution

279

experiments (Figure 2B) was higher than the maximum adsorption capacity determined in the

280

previous adsorption experiment (55 mg C SRHA/g MWCNTs, see table in Figure 1A), meaning

281

that each carbon atom on the outermost layer of the MWCNTs associated with more than one

282

SRHA carbon atom during SRHA elution. Although straining effect has been reported to be an

283

important mechanism for colloids/nanoparticles retention in porous media,47 it is unlikely the

284

reason for the higher retention of SRHA observed here due to the low particle/pore size ratio48,49

285

in the system and the extremely thin MWCNTs layer formed on the filter membrane (Detailed

286

calculation and explanation is presented in Supporting Information). In addition, the filter elution

287

systems contained both SDS and SRHA, as opposed to the batch adsorption systems that

288

contained only one dispersant, and SDS-SRHA hydrophobic interactions50,51 may have resulted

289

in an increase in total SRHA retained on the SDS-MWCNT surfaces. Finally, the increase in

290

SRHA adsorption is also likely due to the inherent differences between the batch adsorption

291

system and elution system: the former is at equilibrium with a fixed amount of adsorbate while

292

the latter is at non-equilibrium with a continuous adsorbate supply.


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Additional DI elutions were conducted following SRHA elution to further measure SDS

294

release and the reversibility of SRHA adsorption when solution chemistry changes. Although

295

most of the adsorbed SDS (~90%) was released during SRHA elution, the amount of released

296

SDS during the following DI elution was quantifiable, and decreased from 3.6 ± 0.3 to ~1.0% as

297

[SRHA] used in previous elutions increased from 2.5 to 40 mg C/L (Figure 2A). At the same

298

time, a small fraction of SRHA that adsorbed on MWCNTs during SRHA elution desorbed

299

during DI elution: the amount of released SRHA increased from 1 to 24 mg C/g MWCNTs as

300

adsorbed SRHA increased from 75 to 232 mg C/g MWCNTs (i.e., [SRHA] increased from 2.5 to

301

40 mg C/L in eluent). The fraction of released SRHA to total adsorbed SRHA also increased

302

(Figure 3A). Higher SRHA desorption ratios in this step (e.g., 6.7% and 10.1 % for 20 and 40

303

mg/L SRHA eluted SDS-MWCNTs samples, respectively) suggest that the later adsorbed SRHA

304

during elution is more readily desorbed during DI elution than the SRHA adsorbed on MWCNTs

305

during ultrasonication.

306

Less than 5% of the initially adsorbed SDS was retained on the MWCNTs surfaces after

307

SRHA and DI elutions, while more than 90% of the adsorbed SRHA during SRHA elution

308

remained associated with MWCNTs after DI elution. For 5.0 mg/L SDS-MWCNTs eluted by

309

various SRHA solutions, final adsorbed SDS concentrations were at least one order of magnitude

310

lower than SRHA finally adsorbed on MWCNTs (Figure 3B, on carbon mass basis). In the

311

present study, concurrent SDS desorption and SRHA adsorption during elution indicates that the

312

original SDS surfactant coating on MWCNTs was displaced by SRHA, resulting in a SRHA-

313

predominant heterogeneous coating on MWCNTs. In surface waters, NOM, instead of the

314

original surfactant, will therefore be the primary material coating MWCNTs and it is the



Page 16 of 31

315

chemical and physical properties of these NOM-coated MWCNTs that will determine nanotube

316

transport, fate, and interaction with the biota.12

317 318 319 320

Figure 3 (A) The fraction of released SRHA during DI elution to total SRHA adsorbed on MWCNTs during previous SRHA elution. (B) SDS and SRHA retained on MWCNTs surfaces after all SRHA and DI elution as a function of [SRHA] used in SRHA elution.

321

3.4 Cation Effects on SDS Desorption

322

To investigate the effects of cations on SDS desorption, CaCl2 and NaCl solutions were used

323

to elute 5.0 mg/L SDS-MWCNTs. Divalent calcium cation was highly efficient at inhibiting

324

adsorbed SDS release from MWCNTs surfaces: as [Ca2+] in the eluent increased from 0 to 8

325

mM, the fraction of desorbed SDS decreased from 59.9 ± 0.7% to 28.9 ± 0.3% (Figure 4A).

326

Even at a very low [Ca2+] level (1 mM), the released fraction (39.4 ± 0.2%) dropped to 2/3 of

327

that released in the benchmark DI elution experiment. Cation electronic screening effect, leading

328

to less negatively charged MWCNTs surfaces, is likely to be the reason for this reduced SDS

329

desorption. Interestingly, Ca2+ inhibition effects were not permanent and the subsequent DI

330

elution readily released ~15% to 30% of the adsorbed SDS (Figure 4A). Despite the fact that the


Page 17 of 31


331

SDS release during calcium salt elution varied with [CaCl2], total SDS desorption during all

332

CaCl2 and DI elutions was not statistically different – 53.4% with the 95% confidence level of

333

1.8% – and was still ~20% lower than total released SDS in the benchmark experiment.

334

Consistent with the amount of desorbed SDS, SDS retained on MWCNTs surfaces after CaCl2

335

and DI elution was also independent of [Ca2+] (3.40 mg C/g MWCNTs with the 95% confidence

336

level of 0.19 mg C/g MWCNTs, Figure 4B) and ~20% higher than that obtained in benchmark

337

elutions (2.84 ± 0.05 mg C/g MWCNTs). The above results suggest that reducing ionic strength

338

partially mitigates Ca2+’s repression of SDS release from MWCNTs surfaces.

339

Only a slight decrease in SDS desorption was observed when using 20 mM NaCl to elute 5.0

340

mg/L SDS-MWCNTs (53.2 ± 0.6% vs. 59.9 ± 0.7% in the benchmark DI elutions, Figure 4A and

341

Table S3), which reveals that the mono-valent Na+ is much less effective at inhibiting SDS

342

desorption from MWCNTs than di-valent Ca2+, likely due to Na+’s weaker screening effect.

343

Total desorbed SDS and SDS retained on MWCNTs, after all NaCl and DI elution, were both

344

comparable to their corresponding values in benchmark DI elutions (Figure 4B and Table S3).

345

Unlike the partially reversible Ca2+ inhibition on SDS desorption, inhibition of SDS desorption

346

by Na+ can be totally reversed by removing Na+.



Page 18 of 31

347 348 349 350

Figure 4 (A) The fraction of released SDS to total SDS adsorbed on MWCNTs when 5.0 mg/L SDS-MWCNTs eluted by salt solution and DI. (Note: the top x-axis label indicates salt type and concentration used in the previous salt elutions.) (B) SDS retained on MWCNTs surfaces after salt and DI elutions.

351

3.5 Ionic Effects on the Displacement of SDS by SRHA

352

A series of solutions containing SRHA and CaCl2/NaCl were used to elute 5.0 mg/L SDS-

353

MWCNTs. At a fixed SRHA level, the presence of Ca2+ reduced total SDS desorption (Figure

354

5A, Tables S1 and S2). Calcium cation’s inhibition of SDS release in the presence of SRHA,

355

however, was insensitive to [Ca2+] and less profound than in the absence of SRHA (Figure 5A).

356

The amount of retained SDS on MWCNTs surfaces after Ca2+-SRHA/DI elutions shared a

357

similar, but reversed trend (Figure S6). Compared to Ca2+, mono-valent Na+ was less efficient at

358

inhibiting SDS desorption in the presence of SRHA: 82.2 ± 0.4% adsorbed SDS was released by

359

NaCl-SRHA elution (20 mM and 5 mg C/L, respectively), which was only ~5% less than that

360

eluted by 5 mg C/L SRHA (Table S3). These results collectively demonstrate that SDS

361

desorption by NOM is relatively insensitive to cation concentration and type and that, in a

362

cation-NOM system, NOM controls displacement of adsorbed SDS on MWCNTs surfaces.


Page 19 of 31


363

In benchmark DI elution and all experiments with eluents that contained one single

364

component (e.g., SRHA, CaCl2, or NaCl), SDS release decreased as elution progressed. When

365

eluted with solution containing 5 mg C/L SRHA and CaCl2, however, desorbed SDS did not

366

follow this trend – the highest desorption occurred during the second or third elution (Figure S7).

367

Co-effects of Ca2+ and SRHA on SDS desorption from MWCNTs should not be treated as a

368

simple combination of the Ca2+-inhibition and SRHA-enhancement. It is more likely that there

369

are complex adsorption, desorption, competition, and replacement interactions among

370

MWCNTs, SDS, SRHA, and Ca2+.29 And this is environmentally important since many surface

371

waters contain significant concentrations of both cations and various DOC species.

372

Similar to SRHA-only elution experiments, SRHA simultaneously adsorbed on MWCNTs as

373

SDS released from MWCNTs surfaces during SRHA-salt elution. The presence of Na+ reduced

374

SRHA adsorption on MWCNTs: the total adsorbed SRHA after ten SRHA-NaCl elutions were

375

66.9 and 59.2 mg C/g MWCNTs when eluted by 5 mg C/L SRHA-20 mM NaCl and 5 mg C/L

376

SRHA-80 mM NaCl, respectively, which were lower than the amount of adsorbed SDS after

377

salt-free 5 mg/L SRHA elution (89.7 mg C/g MWCNTs). In contrast, divalent calcium cation

378

facilitated SRHA adsorption (Figure 5B): after ten SRHA-CaCl2 elutions using 5 mg C/L SRHA

379

with 1, 4, and 8 mM CaCl2, each gram of eluted SDS-MWCNTs retained 105.5, 120.4, and 126.3

380

mg C SRHA, respectively. These values were 10%, 34%, and 41% higher than those for SRHA

381

adsorbed when eluted by the same SRHA solution in the absence of Ca2+. Interestingly, no

382

statistical difference in SRHA adsorption was observed in the first elution when compared to the

383

salt-free 5 mg C/L SRHA elution. The Ca2+-caused SRHA adsorption enhancement became

384

more profound after the first two elutions (Figure 5B), suggesting that Ca2+ promoted SRHA-

385

MWCNTs association mainly through multi-layered SRHA adsorption by bridging SRHA



Page 20 of 31

386

molecules with each other, rather than by increasing direct association between SRHA and

387

MWCNTs surfaces.

388 389 390 391

Figure 5 (A) The fraction of released SDS to total SDS adsorbed on MWCNTs when 5.0 mg/L SDS-MWCNTs eluted by solutions containing CaCl2 and/or SRHA (0.8 mL × 10 times). (B) SRHA adsorbed onto MWCNTs surfaces at each elution step when eluted by 5.0 mg C/L SRHA with varying CaCl2 and NaCl levels.

392

3.6 DOC Displacement of SDS in a Surface Water.

393

Brier Creek (BC) is a coastal plain river in the Savannah River basin in Georgia.52 BC water,

394

containing various cations and DOC species (detailed information presented in Table S4), was

395

used to investigate desorption of SDS from MWCNTs when exposed to an actual surface water.

396

Compared to synthetic water that contained similar cation components to BC water (but without

397

DOC) and resulted in 61.7 ± 0.3% of the adsorbed SDS release, BC water elutions led to more

398

SDS desorption (82.0 ± 0.5%, Figure 6). Similarly to SRHA, the DOC species in BC water,

399

quantified by their UV absorbance at 254 nm, adsorbed on MWCNTs surfaces during BC water

400

elution (inset of Figure 6) which suggests displacement of SDS by DOC in the surface water.

401

Results of the elution experiment using a SRHA-CaCl2 solution with similar TOC and ionic

402

strength levels to BC water are presented as green bars in Figure 6 for comparison. The two sets


Page 21 of 31


403

of experiments share a similar SDS desorption trend, indicating SRHA is a good model for

404

naturally-occurring organic materials in the water column that can adsorb on MWCNTs surfaces.

405

Taken together, these data indicate that MWCNT coatings like SDS will be replaced by DOC in

406

surface waters.

407 408 409 410 411

Figure 6 Released SDS from MWCNTs when eluted with synthetic water (containing same Ca2+ and Na+ as Brier Creek water but without DOC, red bars), Brier Creek (BC) water (blue bars), and SRHA-CaCl2 solution (green bars). Inset: Adsorbed DOC during BC water elution (represented as the difference of UV absorbance at 254 nm between each eluent and eluate).

412

4. Environmental Implications.

413

SDS is a commercial surfactant commonly used to disperse CNTs in aqueous media. SRHA,

414

with higher affinity towards MWCNTs due to hydrophobic and π-π interactions, can displace the

415

SDS coating and most of the adsorbed SDS will be released in the presence of SRHA. At the

416

same time, SRHA adsorption occurs on MWCNTs, resulting in a heterogeneous coating

417

containing SDS and SRHA directly adsorbed on MWCNTs, and a secondary coating composed

418

of weakly associated SRHA that is more easily desorbed. Although cations can inhibit SDS from

419

desorbing from MWCNTs surfaces in the absence of SRHA, their effects are significantly

420

lessened in the presence of SRHA. Di-valent and mono-valent cations affect SRHA adsorption

421

during displacement quite differently: Ca2+ enhances it by promoting multi-layered adsorption

422

through bridging effects, while Na+ reduces it. 21 ACS Paragon Plus Environment


Page 22 of 31

423

When surfactant-wrapped CNTs are released to surface waters, they encounter humic

424

materials, proteins, extracellular polymeric substance, organic pollutants, and various

425

components present in the water column. Many of these have strong affinities toward CNT

426

surfaces,53 resulting in displacement of the original surfactant coating. According to results from

427

the present study, a homogeneous surfactant coating on CNTs will be replaced by a

428

heterogeneous coating after contact with various constituents in environmental waters.

429

Formation of this complex coating on CNTs is of significant environmental importance: CNTs

430

surface identity controls their interactions with other environmental surfaces, alters CNTs

431

transport and fate in the environment, and determines what the cell “sees” prior to and during

432

NM uptake54,55 as well as organisms’ response to CNTs exposure. In addition, displacement of

433

CNTs coatings in the water column influence their ability to further adsorb other contaminants,

434

changing their uptake pathways, bioavailability, and toxicities. Since these heterogeneous

435

coatings of naturally occurring materials play a major role in determining CNTs fate in the

436

aquatic environment, their nature must be considered in experimental design and conceptual

437

models developed in CNT environmental studies.

438

Supporting Information

439

Calculation of the ratio of carbon atoms of the outermost layer to the total carbon atoms

440

contained by the employed MWCNTs. Hydrodynamic diameter and size distributions of

441

MWCNTs determined by dynamic light scattering. Monitoring of SDS concentration in the

442

filtrate during the experimental period. The released SDS as the fraction of adsorbed SDS on

443

MWCNTs during each DI elution. Comparison between the SDS-MWCNTs adsorption isotherm

444

and desorption profile during DI elution of 5.0 mg/L SDS-MWCNTs. Size exclusion and

445

straining effects are excluded for higher SRHA adsorption during SRHA elution. Released SDS 22 ACS Paragon Plus Environment

Page 23 of 31


446

as a function of [SRHA] in eluent in the first to sixth SRHA elution. Released SDS during each

447

elution from 5 mg/L SDS-MWCNTs eluted by various solution. SDS retained on MWCNTs

448

surfaces after SRHA/CaCl2 and DI elutions. SDS desorption as elution progressed.

449

Characterization of Brier Creek Surface water. This material is available free of charge via the

450

internet at http://pubs.acs.org.

451

Disclaimer

452

This paper has been reviewed in accordance with the USEPA’s peer and administrative

453

review policies and approved for publication. Mention of trade names or commercial products

454

does not constitute endorsement or recommendation for use.

455

References.

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ACS Paragon Plus Environment

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