Fate and Transport of Nanoscale Buckminsterfullerene Aggregates (Nc60) in Heterogenous Porous Media

Fate and Transport of Nanoscale Buckminsterfullerene Aggregates (nC60) in Heterogenous Porous Media

An Honors Thesis for the Department of Civil and Environmental Engineering Committee Members: Dr. Kurt Pennell, Dr. C. Andrew Ramsburg, Dr. Linda Abriola

Jordyn Wolfand Tufts University May 6, 2011

ABSTRACT

Buckminsterfullerene (C60), an allotrope of carbon, has unique physical and chemical properties that have led to its use in a variety of commercial applications, ranging from organic photovoltaics to biopharmaceuticals. It is inevitable that C60 will be released into the environment during manufacture, transportation, application, and disposal. C60 is nearly insoluble in water, though several methods have been developed to disperse C60 aggregates in water without the aid of stabilizing agents. These C60 aggregates have diameters of 5–200 nm, are negatively charged, and are referred to as nC60. It has been suggested that nC60 is the most environmentally relevant form of C60, but understanding of the retention and transport of these particles in heterogeneous is limited. This research sought to characterize the transport and retention behavior of nC60 in a two-dimensional (2-d) aquifer cell packed with heterogeneous porous media. It was hypothesized that nC60 would accumulate at the interfaces between high and low permeability sands.

Initially, two sets of one-dimensional (1-d) column experiments were completed to prepare for injection into the 2-d aquifer cell, and to determine parameters needed to simulate transport of nC60 with modified filtration theory. The first series of column studies was conducted to determine a background electrolyte that would result in 75% breakthrough of nC60 after a pulse injection into the 40/50 mesh sand matrix. A second series of 1-d column experiments was completed at the optimal electrolyte concentration to determine two parameters needed for simulation by modified filtration theory—maximum retention capacity (Smax), and the particle attachment rate (Katt) of the porous medium. After determination of the relevant transport parameters in the 1-d column studies, nC60 was injected into the 2-d aquifer cell. nC60 breakthrough was found to increase with an increase in ionic strength of the background electrolyte, in accordance with previous literature. 6 mM NaCl and 0.05 mM NaHCO3 was chosen as the background electrolyte for the 2-d cell. Smax was determined to be 0.57, 3.17, and -1 0.54, 3.2, and 4.7 μg/g, and katt was found to be 0.25, 1.1, and 1.4 h for 40/50, 80/100, and 100/140 mesh natural Ottawa sand, respectively. nC60 was injected separately into three of the ports of the 2-d aquifer cell (upstream of a 80/100 mesh low-permeability lens, upstream of a 100/140 mesh low-permeability lens, and in the background matrix of 40/50 mesh sand. A significant amount of nC60 mass was retained near the port of injection. Enhanced accumulation of sorbed-phase nC60 at the textural interface did not occur as expected. Though sorbed-phase concentration of nC60 decreased after the interface, accumulation did not occur on the horizontal boundaries of the lens. Results indicate that nC60 transport at textural interfaces is controlled by changes in local flow velocities through heterogeneous media.

TABLE OF CONTENTS

Abstract ii Table of Contents iii List of Acronyms and Initialisms iv List of Symbols vi List of Figures viii List of Tables xi

1.0 Introduction 1

2.0 Literature Review 1

2.1 Properties and Applications of C60 1 2.2 Dispersion of C60 in Water 2 2.3 Toxicity of nC60 3 2.4 Fate and Transport of nC60 4

3.0 Purpose 8

4.0 Hypothesis 10

5.0 Materials and Methods 12

5.1 nC60 Preparation and Characterization 12

5.2 One-Dimensional Column Studies 14

5.2.1 1-D column set-up 15 5.2.2 Effect of background electrolyte on nC60 transport 18 5.2.3 Effect of soil particle size on nC60 transport 19

5.3 Two-Dimensional Aquifer Cell 20

5.3.1 2-D aquifer cell set-up 20 5.3.2 Non-reactive tracer tests 23 5.3.3 nC60 injection 24

5.4 Quantification of Aqueous nC60 26

5.4.1 Quantification by UV-VIS 26 5.4.2 Quantification by HPLC 28

5.5 Quantification of Solid-phase nC60 31

6.0 Results and Discussion 32

iii

6.1 1-D Column Studies 32

6.1.1 Effect of background electrolyte on nC60 transport 32 6.1.2 Effect of soil particle size on nC60 transport 34

6.2 2-D Aquifer Cell 41

6.2.1 Non-reactive breakthrough tracer tests 41 6.2.2 nC60 injection 45

7.0 Conclusions and Recommendations 54

8.0 Collaboration and Acknowledgements 56

List of References 57

LIST OF ACRONYMS AND INITIALISMS

ADR Advective-dispersive reaction

BTC Breakthrough tracer curve

CaBr2 Calcium bromide

CaCl2 Calcium chloride

C60 Buckminsterfullerene (Carbon-60)

DLS Dynamic light scattering

DSLR Digital single-lense reflex

DVLO Derjaguin-Landau-Verwey-Overbeek

HPLC High performance liquid chromatography

IC Ion chromatography

LC50 Median lethal dose

NaCl Sodium chloride

NaFlour Sodium flourescein nC60 Nano-scale buckminsterfullerene aggregates

PV Pore volume

THF Tetrahydrofuran

UV-VIS Ultraviolet-visible spectrophotometry

LIST OF SYMBOLS

α attachment efficiency factor

αL longitudinal dispersivity coefficient

αT transverse dispersivity coefficient

C concentration in the effluent aqueous phase

C0 concentration in the influent aqueous phase dc mean diameter of the porous medium d50 Mass-median-diameter

DL longitudinal hydrodynamic dispersion coefficient

* DL longitudinal molecular diffusion coefficient

η0 theoretical clean-bed single collector efficiency g acceleration of gravity

λ wavelength

K permeability katt particle attachment rate ki intrinsic permeability

L characteristic length

μ viscosity of water n porosity

ρ density of water

ρb soil bulk density

Pe Péclet number

S concentration in the solid-phase

Smax maximum retention capacity t time

θw volumetric water content vp pore-water velocity

vii

LIST OF FIGURES

Figure 1. The molecular structure of C60 2

Figure 2. Mechanisms for colloid attachment according to classical filtration theory 5

Figure 3. Illustration of strained colloids in the smallest regions of the soil pore space 7

Figure 4. Side-view of 2-d aquifer cell with low permeability lenses 9

Figure 5. Comparison of Smax as functions of sand grain size at two velocities 10

Figure 6. Vector maps for velocity flow for fully screened influent at left inlet and injection into ports A2 and A4 11

Figure 7. Preliminary modeling results 12

Figure 8. Sample outputs from DLS for particle size and zeta potential of nC60 14

Figure 9. 1-d column apparatus 15

Figure 10. Representative bromide standard curve 16

Figure 11. Column experiment apparatus and set-up 18

Figure 12. Dimensions of the 2-d aquifer cell and sampling ports 22

Figure 13. 2-d aquifer cell set-up 22

Figure 14. Schematic of nC60 injections into the 2-d aquifer cell 25

Figure 15. Grid of box-sections to determine sorbed-phase concentration distribution after nC60 injection 25

Figure 16. Representative standard curve for aqueous nC60 detection by UV-VIS 27

Figure 17. UV-VIS sample output for absorbance of nC60 28

Figure 18. Representative standard curve for nC60 detection by HPLC 29

Figure 19. Sample chromatogram from HPLC 30

Figure 20. Hubaux-Vos graph to determine the detection limit of C60 by HPLC 31

Figure 21. nC60 breakthrough curves in 40/50 mesh OS with various background electrolytes 33

viii

Figure 22. Breakthrough curve and retention profile for nC60 injection into 40/50 OS 35

Figure 23. Breakthrough curve and retention profile for nC60 injection into 80/100 OS 36

Figure 24. Breakthrough curve and retention profile for nC60 injection into 100/140 OS 37

Figure 25. Non-reactive tracer data in 40/50 mesh OS fit to the 1-d equilibrium ADR transport equation 38

Figure 26. Non-reactive tracer data in 80/100 mesh OS fit to the 1-d equilibrium ADR transport equation 39

Figure 27. Non-reactive tracer data in 100/140 mesh OS fit to the 1-d equilibrium ADR transport equation 39

Figure 28. Modified filtration theory fit to experimental breakthrough curves for various sand particle sizes 40

Figure 29. Photograph of non-reactive sodium flourescein and sodium bromide tracer through fully screened influent chamber 42

Figure 30. BTCs for fully-screened non-reactive tracer in 2-d cell fit for dispersivity coefficients 43

Figure 31. Time-series photographs of sodium flourescein injection through port A2 44

Figure 32. Time-series photographs of sodium flourescein injection through port A4 45

Figure 33. Photographs of influent ports after nC60 injection 46

Figure 34. Photograph of nC60 injection solution 47

Figure 35. Plot of solid-phase concentration of nC60 around injection port A4 48

Figure 36. Plot of solid-phase nC60 concentration around the 100/140 mesh lens 48

Figure 37. nC60 retained immediately before the 40/50 and 100/140 mesh interface 49

Figure 38. nC60 retained immediately after the 40/50 and 100/140 mesh interface 49

Figure 39. Retention profiles for horizontal transects outside of 100/140 mesh lens 50

Figure 40. Retention profiles for horizontal transects through the 100/140 mesh lens 50

Figure 41. Plot of solid-phase nC60 concentrations in the background matrix 51

Figure 42. Comparison of nC60 retention profiles 52

Figure 43. Scaled comparison of nC60 retention profiles 52

Figure 44. Comparison of retention profiles in the column and aquifer cell 53

LIST OF TABLES

Table 1. Experimental conditions for the first set of column experiments 19

Table 2. Experimental conditions for the second set of column experiments 20

Table 3. Intrinsic permeability of sands packed in 2-d aquifer cell 21

Table 4. Parameters for nC60 injections into 2-d aquifer cell 24

Table 5. Parameters for column studies in unwashed 40/50 mesh OS with various background electrolytes 33

Table 6. Parameters for column studies in various sand size fractions 34

Table 7. Determined modeling parameters for representative results of nC60 injection into columns of varying sand size fractions 40

Table 8. Ratio of colloid diameter to particle diameter for various sand size fractions 41

Table 9. Concentration of sodium chloride in effluent during nC60 injection 45

1.0 INTRODUCTION

Nanomaterials, or materials that range in size from 1–100 nm, have a wide range of industrial, biomedical, and electronic applications, because of their characteristic size. There are seven main categories of nanomaterials: carbon-based materials, nanocomposites, metals and alloys, biological materials, nano-polymers, nano-glasses, and nano-ceramics. Carbon-based nanomaterials are defined as materials of pure carbon, and this category includes various configurations of carbon black, fullerenes, graphite, and nanoclusters. Fullerenes are pure allotropes of carbon, and are in the form of tubes, spheres, or ellipsoids. The particle of interest in this experiment is buckminsterfullerene (C60) —a spherical fullerene of sixty carbon molecules (NanoRoad 2005).

2.0 LITERATURE REVIEW

2.1 Properties and Applications of C60

C60 was the first fullerene discovered, by Kroto et al. in 1985. C60 is a truncated icosahedron of twenty hexagons and twelve pentagons (Figure 1). Each carbon node is connected to three other carbon atoms by one double bond and two single bonds. C60 was named in honor of Richard

Buckminster Fuller because its shape resembles his geodesic dome. The diameter of each C60 molecule determined by single-crystal x-ray diffraction is 7.065 Å (Liu et al. 1991).

Fullerenes have unusual reduction-oxidation (redox) chemistry, and may be reversibly reduced by up to six electrons. This, and other unique characteristics, such as its antioxidant and antiviral properties (Jensen et al. 1996) have led to the use of fullerenes in a variety of commercial applications, including polymer electronics, organic photovoltaics, and pharmaceuticals (Nano-C 2008). Since a technique for commercial scale production of C60 was

discovered in 1990 (Krätschmer et al.), its manufacture has increased significantly, and it is expected to increase exponentially over the next few decades (Maynard et al. 2006).

Figure 1. The molecular structure of C60 (3dchem.com).

2.2 Dispersion of C60 in Water

C60 is a non-polar molecule, and therefore nearly insoluble in water, but soluble in organic solvents (ranging from 0.01 mg/mL in methanol to 50 mg/mL in 1-chloronaphthalene; Heymann

1996, Ruoff et al. 1993). Three methods have been developed to create stable suspensions of C60 in water: (1) modify C60 chemically with hydrophilic groups (Brettreich and Hirsch 1998, Arrais and Diana 2003, Yang et al. 2003), (2) disperse C60 with the help of water-soluble stabilizers

(Bensasson et al. 1994, Terashima and Nagao 2007), and (3) form stable colloidal aggregates

(Andrievsky et al. 1999, Deguchi et al. 2001, Fortner et al. 2005, Wang et al. 2008, Li et al.

2008). The third method, unlike the first two methods, delivers C60 in water without any stabilizer, allowing concentrations up to 100 mg/L, which is ~11 orders of magnitude greater than the estimated molecular solubility. The resulting C60 aggregates (nC60) have a negative surface charge (zeta potential of about -50 mV), and absorb light at 336, 407, 540, and 595 nm

(Fortner et al. 2005). In absence of background electrolyte, the hydrodynamic diameter of nanoscale C60 aggregates do not change significantly for periods up to six months (Chen and

Elimelech 2006). Recently, nC60 has been proposed as the most environmental relevant form of

C60 because unintentional introduction of C60 to water is possible, and the concentrations generated may be enough to produce ecological effects (≤100 ppm). Molecules analogous to C60 are known to have significant ecological impacts at concentrations of 1–10 ppm (Fortner et al.

2005, Lyon et al. 2006).

2.3 Toxicity of nC60

Research into the toxicology of nC60 is limited. In 2004, Oberdörster found significant lipid peroxidation in brains of largemouth bass after 48 h of exposure to 0.5 ppm uncoated nC60.

However, in 2006, Oberdörster and coauthors re-examined the acute toxicity of nC60 with contrasting results (Zhu et al. 2006). The initial 2004 study was performed using THF- solubilized nC60, although C60 can also be solubilized by stirring in water. The 2006 study investigated the differences in acute toxicity of THF-solubilized and waterstirred-nC60 to

Daphnia magna and fathead minnows (Pimephales promelas). Negative effects were observed from THF-nC60-exposure, but waterstirred-nC60 had no significant toxic effect. The daphnia-48- h median lethal dose (LC50) for THF-nC60 was at least one order of magnitude less (0.8 ppm) than that for waterstirred-nC60 (>35 ppm). There was 100% mortality in the THF-nC60-exposed fathead minnows between 6 and 18 h but the waterstirred-nC60-exposed fish showed no obvious physical effects after 48 h. In 2007, Tong et al. found that fullerenes had little impact on the soil microbial structure and function. In 2009, Spohn et al. concluded that potential side products (2- hydroxytetrahydrofuranol and γ-butyrolactone) from THF-nC60 were mainly responsible for the observed toxic effects of nC60 and that previous publications ―are not useful for an unbiased discussion of [nC60] toxicity.‖ Because of the unknown toxicity of C60, current OSHA guidelines

3 for handling and disposal of C60 follow the guidelines for simple carbon black. The inevitable release of C60 into the environment requires research into its fate and transport.

2.4 Fate and Transport of nC60

Colloid (clean-bed or classical) filtration theory (CFT; Yao et al. 1971) has typically been used to model nC60 transport. CFT is a modified version of the 1-D advective-dispersive reaction

(ADR) transport equation, which is given by

(1)

where C is the aqueous effluent concentration of the particle, S is the concentration of the sorbed-phase particle, t is time, x is the distance parallel to flow, ρb is the soil bulk density, DL is the hydrodynamic dispersion coefficient in the longitudinal direction, vp is the pore-water velocity, and θw is the volumetric water content. The longitudinal hydrodynamic dispersion coefficient is a function of molecular diffusion and mechanical dispersion:

(2)

* where αL is the longitudinal dispersivity, vp is average linear velocity, and DL is the effective molecular diffusion coefficient. Diffusion is assumed to be negligible in high velocity conditions, and therefore

(3)

Dispersion and advection of a fluid flow can be described by the dimensionless Péclet number, which relates the dispersion coefficient (DL) to velocity (vp) and characteristic length (L) as follows:

(4)

where L is characteristic length (e.g. the length of the column).

According to CFT, particle deposition is conceptualized as a two-step process: (1) transport of the particle to the media surface, and (2) attachment of the particle to the surface.

Step 1 is controlled by physical factors including grain size, particle size, porosity, and flow. The sorption term in the 1-d ADR equation is modified by

(5)

where katt is the particle attachment rate, and is given by

(6)

where dc is the mean diameter of the colloid, and η0 is the theoretical clean-bed single collector efficiency—the ratio of the rate at which particles strike the collector to the rate at which particles flow toward the collector. A colloid may come into contact with a detector and be absorbed by sedimentation, interception, or diffusion. Sedimentation refers to contact due to the buoyancy and fluid drag of the colloid, interception refers to contact due to the size of the colloid, and diffusion describes the contact attributed to the random motion of particles (Figure

2).

Figure 2. Mechanisms for colloid attachment according to classical filtration theory (Image courtesy of Dr. Yusong Li, adapted from Yao et al. 1971).

Step 2 is controlled by the fraction of collisions that lead to successful attachments and is described by the attachment efficiency factor (α). The attachment efficiency factor is governed by interaction forces between particles and media surfaces, which according to Derjaguin-

Landau-Verwey-Overbeek (DLVO) theory typically include van der Waals attraction and electrical double layer repulsion forces (Malvern Instruments Ltd. 2011). The attachment efficiency factor (α) can be described as

(7)

where C0 is the influent concentration (Yao et al. 1971).

Several column transport studies (Lecoanet et al. 2004, Brant et al. 2005, Espinasse et al.

2007) have been conducted to apply CFT to the transport of nC60 in porous media. CFT failed to accurately describe the asymmetric breakthrough curves of nC60. Wang et al. (2008) proposed an alternative modeling approach that successfully simulated nC60 transport and retention data. The model modifies CFT by adding ψ, a blocking function that accounts for the limited surface area available on the collector, into the sorption term as follows:

(8)

ψ is related to a particle retention capacity Smax by

(9)

The value of ψ decreases as more nC60 aggregates are deposited on the grain surface, resulting in a reduction of the effective attachment rate, kattψ where kattψ approaches a value of zero as S approaches Smax. This modified filtration theory also accounts for detachment of the particles from the collectors such that the final sorption kinetics are described by:

(10)

6 where kdet is the rate of detachment. It has been suggested that CFT or modified CFT do not adequately describe colloid transport, but that colloid transport is instead governed by particle straining (Bradford et al. 2005, 2006). Particle straining is similar to mechanical filtration, in that colloids are trapped in the small pore spaces between soil particles (Figure 3).

Figure 3. Illustration of strained colloids in the smallest regions of the soil pore space formed adjacent to points of grain-grain contact (Bradford et al. 2006).

In contrast to mechanical filtration, straining occurs in only a fraction of the available pore space so that some colloid transport can still occur in larger continuous pore networks. Traditionally, straining is considered to control colloid transport when the ratio of colloid diameter to grain diameter is greater than 0.15. Recent studies, however, suggest that straining can be observed at a ratio as low as 0.002 (Li et al. 2008). Particle straining is characterized by hyper-exponential deposition profiles. Deposition rapidly decreases with increasing distance due to larger colloids being removed upgradient (Bradford et al. 2005).

3.0 PURPOSE

Natural aquifers often contain layers and lenses of differing soil permeabilities. Hydraulic conductivity (K) is defined as the ability of a porous media to permit a fluid flow. It is a function of the properties of both the porous media and the fluid, and is given by

(11)

where ki is the intrinsic permeability, ρ is the density of the fluid, g is the acceleration of gravity, and μ is the viscosity of the fluid. The intrinsic permeability is a function of only the properties of the media and is described empirically for sands by

(12)

where dc is the mean diameter of the porous media, and n is the porosity of the media (Carman

1956).

Few studies have examined the mechanisms of colloid transport and retention in heterogeneous porous media (Bradford et al. 2005), and these studies have been limited to one- dimensional (1-d) column studies. There are no studies to date on the transport and retention of nC60 in heterogeneous media. The goal of this project was to characterize nC60 transport and retention in a two-dimensional (2-d) aquifer cell to simulate subsurface heterogeneity (Figure 4).

The 2-d aquifer cell was packed with two low-permeability lenses (80/100 and 100/140 mesh) in a higher permeability background matrix (40/50 mesh sand). Specific sub goals were: (1) to determine necessary transport parameters so that experimental results could later be modeled by modified filtration theory, and (2) to describe nC60 retention behavior at the interface between the high permeability background matrix and lower permeability lenses, herein referred to as the

―textural interface.‖

FLOW 

Figure 4. Side-view of 2-d aquifer cell with low permeability lenses (80/100 and 100/140 mesh sand from top to bottom) in a high permeability background matrix (40/50 mesh sand). nC60 was injected into ports A2, A4, and C3.

Three tasks were established to address the project goals. The first task was to prepare and characterize a stable suspension of nC60. The second task consisted of two sets of 1-d column experiments. The first set of column studies was performed to screen for the necessary ionic strength needed for 25% mass retention of nC60 in 40/50 mesh sand in the 2-d aquifer cell. This percent retention was chosen such that an adequate amount of nC60 would be available for quantification in both the sorbed-phase and aqueous-phase. A second series of experiments was conducted to determine the transport parameters Smax and katt (maximum retention capacity and particle attachment rate, respectively) of the three sizes of porous media (40/50, 80/100, and

100/140 mesh sand) used in the 2-d cell. After determination of the relevant modeling parameters in the 1-d column studies, nC60 was injected into a 2-d aquifer cell, upgradient of the two low-permeability lenses, and in the background coarse sand, so that the transport and retention of nC60 at the textural interfaces could be quantified.

4.0 HYPOTHESIS

It was hypothesized that there would be accumulation of nC60 particles at the interface between the low permeability lens and high permeability matrix. This accumulation was hypothesized to occur because of the unique flow conditions at the textural interface, and because of potential particle straining. As mentioned in Section 3.0, soil particle diameter is proportional to permeability, which is proportional to velocity. Particle attachment rate and maximum retention capacity are known functions of velocity and particle size such that Smax increases with decreasing particle size and increasing flow velocity (Figure 5). Particles therefore have lower mobility in soils of higher permeability, and the 2-d aquifer cell has a non-uniform flow field, with higher flow velocities in the larger grain size sand and slower flow velocities in the lenses

(Figure 5).

Figure 5. Comparison of Smax as a function of sand grain size at two flow velocities (Li et al. 2008).

5 A

40 0 10 20 30 40 50 60 70

0 B 5

40 0 10 20 30 40 50 60 70

Figure 6. Vector maps for velocity flow for fully screened influent at left inlet (A) and injection into ports A2 and A4 (B). Figure provided by Ms. Chunmei Bai and Dr. Yusong Li (University of Nebraska, Lincoln).

It was hypothesized that the local flow velocity at the textural interface would be dramatically decreased, and that the nC60 particles would accumulate in this area of low flow velocity and be diverted around the lens. It was also hypothesized that particle straining may control some transport in the 2-d aquifer cell, and that the impact of this would enhance accumulation at the

11 textural interface. Previous studies have shown colloid deposition at the textural interface when there was flow from coarse to fine sand, but little deposition with transport from fine to coarse sand (Bradford et al. 2005). It was predicted that this effect may also be seen in the 2-d aquifer cell. This hypothesis was also supported by preliminary modified CFT modeling results (Figure

7).

Figure 7. Preliminary modified CFT modeling results showing solid-phase concentration of nC60. Results were provided by Dr. Yusong Li and Ms. Chunmei Bai (University of Nebraska, Lincoln). Smax = 0.47μg/g, α = 0.00705, αL=0.1cm, αT=0.01cm.

5.0 MATERIALS AND METHODS

5.1 nC60 Preparation and Characterization

The method for C60 preparation was adapted from Wang et al. 2008. Approximately 100 g of solid-phase C60 (99.9 % purity, Materials Electronics Research Corp., Tuscon, AZ) was dissolved in a previously unopened 4-L bottle of spectra-analyzed tetrahydrofuran

(THF; >99.99%, Fisher Scientific, Houston, TX), and sparged with N2 to remove oxygen. The

o THF-C60 mixture was mixed at room temperature (23±1 C), on a magnetic stirrer, for 72 h to ensure uniform dispersal and complete saturation of nC60 throughout the THF solution (solubility

~ 9 mg/L). Upon saturation, the THF-C60 solution was vacuum-filtered through a 0.22 μm nylon membrane (Osmotics Corp., Minnetonka, MN) to remove remaining C60 particulates. An aliquot of the pale-purple filtered THF-C60 solution (ca. 250 mL) was added to a 2-L wide mouth

Erlenmeyer flask and stirred rapidly. An equal amount of deionized water (ca. 250 mL) was mixed with the filtered THF-C60 solution at 1 L/min, while stirring, to create a pale yellow solution.

A stepwise evaporating procedure was used to consistently remove THF from the C60 solution. The solution was heated to 75°C in a rotary evaporator (Rotavapor Model R210,

BUCHI Corp., New Castle, DE) for 20 min, until approximately 100 mL of solution remained.

Water was added to a total volume of 300 mL, and the evaporation process was repeated twice at

85°C until 100 mL remained (about 20 min). The remaining solution was diluted to 250 mL, transferred to a clean glass bottle, and stored in the dark for 12 h to allow additional solid particulates to settle. The solution was then filtered through a sterile 0.22 μm cellulose acetate membrane (Corning Inc., Corning, NY), evaporated to 100 mL, and stored in a sterile container for further use. Preparation of nC60 suspension by this method results in a solution with a residual THF concentration of less than 0.15 mg/L (Wang et al. 2008).

The mean diameter and zeta potential of the nC60 suspension was determined by dynamic light scattering (DLS; Zetasizer Nano Series, Malvern Instruments, Worcestershire, UK). Sample outputs for particle size and zeta potential distribution are given in Figure 8.

Figure 8. Sample outputs from DLS for particle size (A) and zeta potential (B) of nC60.

The concentration of the prepared solution was determined by ultraviolet-visible (UV-VIS) absorbance spectroscopy and High-Performance Liquid Chromatography (HPLC; Sec. 5.4). The stock aqueous suspension contained 20.3 mg/L of nC60.

5.2 One-Dimensional Column Studies

Two sets of column experiments were performed to measure nC60 transport and retention in sieved F-60 and F-75 Ottawa Sand (OS; U.S. Silica, Berkeley Springs, WV, USA) in one- dimension. The first set was a screening process to determine the background electrolyte needed for 25% retention of injected nC60

5.2.1 1-D column set-up

The column apparatus consisted of a syringe pump (Chemyx Inc., Stafford, TX) to establish the flow field, a fraction collector (Spectrum Laboratories, Inc., Rancho Dominguez, CA) to collect effluent samples, and a 16-cm (2.5-cm inner diameter) borosilicate glass column (Kontes,

Vineland, NJ). Each column was washed with deionized water, dried, assembled, weighed, and packed with OS in 1.5 cm increments under vibration (Figure 9). Column end plates were fit with two 40-mesh nylon screens and the influent end plate was fit with an additional 70 μm nylon filter. After packing, columns were sparged with CO2 gas for 20 min, and then saturated with deaired deionized water containing the background electrolyte.

Figure 9. 1-d column apparatus.

After columns were completely saturated (> 10 pore volumes), a non-reactive tracer test was performed to assess flow field and hydrodynamic dispersion. A three pore volume (PV) pulse of

15 non-reactive tracer (NaBr) solution, with equivalent ionic strength to the background chloride electrolyte, was injected into each column followed by three PVs of the background electrolyte.

Column effluent was collected continuously in 15-mL sterile plastic centrifuge tubes (Fischer

Scientific, Hampton, NH) and analyzed for bromide concentration by an ion selective bromide electrode (Cole-Parmer Instrument Co., Vernon Hills, IL) connected to a pH meter (Accumet

Model 50, Fischer Scientific). Bromide probe readings were compared with those of a set of standards with known concentration. Standards were prepared by dissolving a known mass of sodium bromide in 10 mL of deaired water. Standards also included the sodium carbonate buffer present in the column background solution. A representative bromide calibration curve is shown in Figure 10.

40 y = -56.74x + 46.779 R² = 0.9995 30

0 0 0.2 0.4 0.6 0.8 1 1.2 Bromide Probe(mV) Reading Bromide -10

-20 Log (Conc. Br-)

Figure 10. Representative bromide standard curve.

Each non-reactive breakthrough tracer curve (BTC) was fit to the 1-D ADR transport equation using CFITM code of van Genuchten (1980) in STANMOD Ver. 2.2 (U.S. Salinity Laboratory,

Riverside, CA), assuming homogeneity and local equilibrium conditions. The CFITM code uses

16 a least-squares curve-fitting algorithm to determine Pe, which can in turn be used to find the longitudinal dispersivity (αL).

After completion of the tracer test, a three to five PV pulse of nC60 suspension (about 2.6 mg/L) was injected into each column and flushed by three PVs of the background electrolyte.

The concentration of nC60 in effluent samples was determined by either UV spectrophotometry or HPLC (Sec. 5.4). nC60 breakthrough curves, consisting of a plot of normalized effluent concentrations (C/C0) versus time expressed as dimensionless pore-volumes, were plotted for each column. The mean diameter and zeta-potential of nC60 in every third effluent sample was determined by DLS as described in Section 5.1. At the conclusion of the second series of experiments, the column was dissected to determine nC60 solid phase concentration and the retention profile of nC60 throughout the column. Sections were dried in glass vials at 95°C, and then analyzed by methods described in Section 5.5. Columns were not dissected in the first series of experiments because only effluent breakthrough data was needed to screen for the desired background electrolyte. A schematic of the column experimental set-up is shown in Figure 11.

Figure 11. Column experiment apparatus and set-up. Effluent samples are analyzed for C60 content and produce a breakthrough curve. The column is dissected after nC60 injection and elution to produce a retention profile of sorbed-phase C60 distribution throughout the column. Figure courtesy of Dr. Yonggang Wang.

5.2.2 Effect of background electrolyte on nC60 transport

The first series of one-dimensional (1-d) column experiments was performed to screen for the optimal background electrolyte in the 2-d aquifer cell experiment, so that about 25% of C60 mass would be retained in the 40/50 mesh size fraction OS. This retention percentage was chosen so that an adequate amount of C60 mass would be available for quantification in both the aqueous and sorbed phases. The column apparatus and experimental set-up followed the protocol given in

Section 5.2.1. The background electrolyte concentrations screened included 1 mM CaCl2 and 1,

3, 5, 6, 7, and 10 mM NaCl. Each electrolyte solution was buffered with NaHCO3, to keep a neutral pH. Experimental conditions for the first series of column studies are shown in Table 1.

Flow rates ranged from 0.135 to 0.147 ml/min, which correspond to a pore-water velocity of

about 1.0 to 1.1 m/d.

Table 1. Experimental conditions for the first set of column experiments. Average Average Background Aqueous Volumetric Concentration Diameter Zeta Electrolyte Phase Water Flow rate Column NaHCO of injected Potential Concentration 3 Pore Volume Content (mL/min) (mM) nC (nm) of nC (mM) (PV; mL) (θ ) 60 60 w (mV) A CaCl2 1 0.065 30.4 0.38 0.147 100.7 -19.3 B NaCl 3 0.05 30.3 0.38 0.137 98.3 -39.7 C NaCl 5 0.05 30.2 0.38 0.136 98.6 ND D NaCl 6 0.05 29.6 0.37 0.115 97.9 -47.0 E NaCl 6 0.05 29.0 0.37 0.113 99.9 -46.5 F NaCl 7 0.05 30.0 0.37 0.137 98.1 -44.5 G NaCl 10 0.05 30.0 0.38 0.135 ND* ND* *ND = not determined.

5.2.3 Effect of soil particle size on nC60 transport

A second series of 1-d experiments was performed to determine the parameters needed to model

nC60 transport with modified filtration theory (Sec. 2.4). A solution of ca. 2.60 mg/L nC60 was

injected into separate columns of unwashed 40/50, 80/100, or 100/140 mesh OS (mass-median-

diameter (d50) = 0.36, 0.16, and 0.13 mm respectively). These size fractions correspond to the

fractions of sands used in the 2-d aquifer cell. The injection solution was eluted with a

background electrolyte consisting of 6 mM NaCl and 0.05 NaHCO3 (based on the results from

the first set of column experiments). The experimental protocol in Section 5.2.1 was followed

and replicates were performed for each type of sand. Particular care was taken so that replicate

column studies were performed under controlled conditions. Replicates were run in tandem with

the same solution of background electrolyte and this solution was used for all saturation and

flushing throughout the experiment. The parameters for each column experiment are shown in

Table 2. Flow rates ranged from 0.125 to 0.137 mL/min, which correspond to a pore-water

19 velocity of about 0.69 to 1.0 m/d. The attachment parameters (katt and Smax) were determined by collaborator Mr. Matthew Becker, by fitting C60 breakthrough curve data.

Table 2. Experimental conditions for the second set of column experiments. Background electrolyte was 6 mM NaCl and 0.05 mM NaHCO3. Average Average Aqueous Volumetric OS Size Diameter Zeta Phase Water Flow rate Column Fraction of injected Potential Pore Volume Content (mL/min) (mesh) nC (nm) of nC (PV; mL) (θ ) 60 60 w (mV) D 40/50 29.6 0.37 0.135 97.9 -47.0 E 40/50 29.0 0.37 0.133 99.9 -46.5 H 80/100 31.9 0.40 0.130 99.5 -48.1 I 80/100 31.2 0.39 0.132 96.6 -48.6 J 100/140 31.8 0.39 0.134 97.7 -48.1 K 100/140 30.6 0.39 0.133 99.3 -48.4 L 100/140 30.0 0.39 0.125 98.4 -47.1

5.3 Two-Dimensional Aquifer Cell

5.3.1 2-D aquifer cell set-up

The 2-d aquifer cell was assembled and packed similarly to those completed by Taylor et al.

(2001) and Suchomel et al. (2007) by Mr. Douglas Walker. The cell was constructed of two parallel glass plates containing 18 low-volume glass sampling ports, an aluminum bottom, and fully screened aluminum influent and effluent end chambers. The resulting inside dimensions was 40 × 63.2 × 1.4 cm. Following assembly, the aquifer cell was packed under water-saturated conditions in 3-cm lifts with mild agitation and mixing to minimize media stratification. The cell contained 40/50 mesh OS as the background matrix, two rectangular lenses measuring 2.5 cm high by 10 cm long (80/100 and 100/140 mesh OS), and a 2.5 cm high low-permeability (F-70) confining layer along the bottom. The mean diameters and permeabilities of the sand grain sizes used in the 2-d aquifer cell are shown in Table 3.

Table 3. Intrinsic permeability of sands packed in 2-d aquifer cell. Values estimated by Equation 12. Intrinsic Sieve Size d (mm) 50 Permeability (cm2) 40/50 0.36 2.4 * 10-6 80/100 0.16 3.2 * 10-7 100/140 0.13 1.9 * 10-7

The total pore volume was about 1360 mL. The dimensions and packing configuration of the 2-d aquifer cell are shown in Figure 12. After packing, flow was established by connecting a 5-L constant-head Marriot bottle to the influent and effluent end chambers. The influent constant- head bottle was equipped with an aspirator to ensure constant head regardless of water level in the bottle. The height of the constant head bottle and the exit tubing leading to the effluent collection bottle were adjusted until a pore-water velocity of about 1 m/d was established

(equivalent to a flow rate of 1.3 mL/min) to correspond to the flow conditions in the column experiments. The box apparatus is shown in Figure 13. The cell was fully saturated with deaired deionized water with 6.05 mM salt content (6 mM NaCl, 0.05 mM NaHCO3; chosen because of the results of the primary column experiments, Section 6.1.1).

Figure 12. Dimensions of the 2-d aquifer cell and sampling ports.

Constant-head influent bottle

40/50 mesh background

Flow 

2.5 x 10 cm low permeability lenses (80/100 and 100/140 mesh)

Adjustable effluent container

Figure 13. 2-d aquifer cell set-up.

5.3.2 Non-reactive tracer tests

After complete saturation of the cell (8 PVs), three non-reactive tracer tests, through the fully screened influent chamber and through two of the injection ports upstream of the low- permeability lenses, were conducted to determine flow conditions.

A pulse (443 mL) of 11.2 mg/L sodium flourescein (NaFlour, Fluka Chemical), 0.05 mM

NaHCO3, and 7 mM NaCl was added through the fully screened influent chamber at constant head. 100-μL samples were taken from the middle three ports in the first transect (A2, A3, and

A4), and all ports in the second transect (B1, B2, B3, B4) and analyzed for bromide with a

Dionex ICS 2000 ion chromatograph. The chromatograph was equipped with a Dionex AS-18 4x

250 mm column and a A6-18 column guard. Samples were run isocratically with a mobile phase of 23 mM sodium hydroxide at 1 mL/min. The tracer was also monitored by photographing the fluorescent dye backlit by eight KinoFlo blue bulbs every 30 minutes with a Sony A330 DLSR camera. The average flow rate was 1.43 mL/min.

Additional tracer tests were conducted to qualitatively determine how injection through the side ports would influence the flow field, and how nC60 would be transported around the low permeability lenses. The first tracer was injected into port A2 and consisted of 40 mg/L NaFlour,

6 mM NaCl, and 0.05 mM NaHCO3. A total of 204 mL of tracer solution was injected at 0.5 ml/min through a 20-gauge needle attached to Teflon tubing connected to a calibrated peristaltic pump. The total injection time was 400 min (6.7 h). Background flow consisted of 6 mM NaCl and 0.05 mM NaHCO3 solution from a constant head apparatus at a flow rate of 1.47 mL/min.

The third tracer was injected into port A4 and also consisted of 40 mg/L NaFlour, 6 mM NaCl, and 0.05 mM NaHCO3. A total of 201 mL of tracer solution was injected at 0.5 mL/min. Total injection time was 400 min (6.7 h). Background flow consisted of 6 mM NaCl and 0.05 mM

NaHCO3 solution from a constant head apparatus at a flow rate of 1.52 mL/min. Every hour, for the first 12 h after the start of injection, the background flow rate was monitored, and a photograph of the fluorescent dye (backlit by eight KinoFlo blue bulbs) was taken with a Sony

A330 DLSR camera.

5.3.3 nC60 injection

A 2.6 mg/L nC60 suspension was injected through side ports A2, A4, and C3, separately at a flow rate of 0.5 mL/min using a syringe pump and then eluted with the background electrolyte (Table

4, Figure 14). Aqueous effluent and side-port samples were collected hourly during injection for analysis by HPLC (Sec. 5.4). Effluent samples were also analyzed for chloride content by IC with the method described in Section 5.3.2. Flow rate was monitored throughout the pulse injection and elution. At the conclusion of the transport experiment, the 2-d cell was drained partially, and the front panel of the glass was removed such that the contained sand could be dissected, and the soil samples analyzed for nC60 content. Both the front panel and back panel were sectioned into 1 x1 cm and 2 x 2 cm squares (Figure 15), and the sand masses were combined and added to pre-weighed glass vials (20-mL or 40-mL respectively). The soil samples were analyzed for nC60 as in Section 4.5, except that 20 mL of toluene was added to the 2 x 2 cm sections to ensure adequate extraction.

Table 4. Parameters for nC60 injections into 2-d aquifer cell. Injection # Injection Sampling Volume Average Background Flow Port Ports Injected (mL) Rate (mL/min) 1 A2 B1, B2 200 1.57 2 A4 B3, B4 400 1.31 3 C3 D2, D3 410 1.41

B1 A2 B2 D2 C3 B3 D3 A4 B4

Figure 14. Schematic of nC60 injections into the 2-d aquifer cell. C60 was injected in ports A2, A4, and C3 (red circles), and aqueous effluent samples were taken from ports B1, B2, B3, B4, D2, and D3 (blue circles).

Figure 15. Grid of box sections to determine solid-phase concentration distribution after nC60 injection.

5.4 Quantification of Aqueous nC60

Two methods were used to quantify aqueous-phase nC60: (1) HPLC and (2) UV-VIS spectroscopy. HPLC quantification requires nC60 dissolved in THF whereas UV-VIS requires

C60 aggregates suspended in water. UV-VIS is advantageous because of its rapid quantification, while HPLC is preferred for higher-resolution quantification and separation of constituents. For the column experiments, nC60 in effluent samples were measured by UV-VIS, and the concentration of the injected nC60 solution was verified by HPLC. All samples taken from the 2- d aquifer cell were quantified by HPLC.

5.4.1 Quantification by UV-VIS

To quantify nC60 by UV-VIS, a 1-mL aliquot of sample was analyzed by a UV-1800 Shimadzu

UV Spectrophotometer. Quantification was at an absorption wavelength of 344.5 nm, with a slit width of 1.0 nm, by comparison of the unknown sample absorbance to a calibration curve generated by five nC60 standards of known concentration. Standards were prepared by serial dilutions of an nC60 stock solution with an absolute concentration verified by HPLC (4.4.2). A representative calibration curve for nC60 quantification by UV-VIS is shown in Figure 16. An example output from the UV-VIS spectrophotometer for absorbance of nC60 from 300–400 nm is shown in Figure 17.

0.5 0.45 y = 0.1346x R² = 0.9998 0.4 0.35 0.3 0.25 0.2 0.15

0.1 Absorbance at 344.5 nm at Absorbance 0.05 0 0 0.5 1 1.5 2 2.5 3 3.5 4

Conc. C60 (mg/L)

Figure 16. Representative standard curve for aqueous nC60 detection by UV-VIS.

Absorbance

Wavelength (nm) Figure 17. UV-VIS sample output for absorbance of nC60. Peak absorbance is at about 344 nm.

5.4.2 Quantification by HPLC

To quantify nC60 by HPLC, a 1-mL aliquot of sample was dried at 95°C to which 5 mL of neat toluene (>99.99% purity, Fischer Scientific) was added to extract nC60 from the aqueous phase.

The extraction vials were ultrasonicated for 4 h and then laid flat and mixed at 250 rpm for 12 h.

After mixing, 1.3 mL of the toluene phase was transferred to a 1.5-mL centrifuge tube (VWR

International, West Chester, PA) and spun at 5000 rpm for 10 min. An aliquot (0.3 mL) of the supernatant was combined with 1.2 mL methanol in a 2-mL chromatography vial and vortexed for 1 min. The concentration of the C60 in toluene was determined with an Agilent model 1200-

Series HPLC (Agilent Technologies, Santa Clara, CA) equipped with an Alltima C18 Column

(150 x 4.6 mm; Alltech Associates Inc., Deerfield, IL). After separation by the column, C60 was quantified with a diode array detector (DAD) at an absorbance of 344 nm. The 750-μL injection was eluted with a flow rate of 1 mL/min at 30°C. The isocratic mobile phase consisted of 55% toluene and 45% methanol, and was prepared daily. The integrated area of the eluted peak (with a retention time around 5.9 min) was compared to those of a set of five standards (0.02–3 mg/L).

To prepare C60 standards, a known mass of C60 was dissolved in toluene, and this concentrated stock solution was serially diluted. A representative calibration curve for C60 quantification by

HPLC is shown in Figure 18. A sample output from the HPLC is shown in Figure 18.

1400

1200 y = 386.4x + 13.773 R² = 0.9996 1000

800

600

400 Integrated Peak Area Peak Integrated 200

0 0 0.5 1 1.5 2 2.5 3 3.5 Conc. C60 (mg/L)

Figure 18. Representative standard curve for nC60 detection by HPLC.

Absorbance

Time (min) Figure 19. Sample chromatogram from HPLC. Quantification was at λ = 334 nm. Note that the peak is eluted at about 5.6 minutes.

The detection limit (LD) for nC60 quantification on the HPLC was found to be 0.07 mg/L by the

Hubaux-Vos detection limit procedure (Figure 20, Hubaux and Vos 1970). A prediction interval of 99% was used so that LD is the concentration that can be detected without confusion with blanks, with a probability of error less than 0.005.

0.10

0.08

0.06

Y Data

0.04

Concentration(mg/L)

0.02

Measured

0.00 0.00 0.02 LC 0.04 0.06 L 0.08 0.10 D True ConcentrationX Data (mg/L)

Figure 20. HubauxCol 1 -vsVos Col graph2 to determine the detection limit of C60 by HPLC. yc is the signal level, that correspondsPlot 1 Regr to LC, the critical concentration, which is defined as the value that can be measured that Plotis statistically 1 Pred1 significant from zero with a probability of less than or equal to 0.005.

5.5 Quantification of Solid-phase nC60

Dried soil samples were dissolved in 10 mL toluene and then ultrasonicated for at least 4 h,

followed by lateral agitation at 250 rpm for 12 h. After mixing, 1.3 mL of the toluene phase was

transferred to a 1.5-mL centrifuge tube (VWR International, West Chester, PA) and spun at 5000

rpm for 10 min. An aliquot (0.3 mL) of the supernatant was combined with 1.3 mL methanol in a

2-mL chromatography vial and vortexed for 1 min. The concentration of the prepared effluent

samples were then measured against prepared standards by HPLC with the method described in

Section 5.4.2.

6.0 RESULTS AND DISCUSSION

6.1 1-D Column Studies

6.1.1 Effect of background electrolyte on nC60 transport

Experimental parameters for the seven columns with various background electrolytes are shown in Table 5. Aqueous-phase breakthrough curves show that retention of nC60 increases with an increase in ionic strength of the background electrolyte (Figure 21). This supports previous findings and the concept that nC60 retention and transport is governed by electrostatic interactions between the particles and aquifer solids (Wang et al. 2008b). Retention profiles of sorbed-phase nC60 were not determined.

Prior studies (Wang et al. 2008b) report greater percentages of mass breakthrough of nC60 for identical background electrolytes and experimental conditions to those in this experiment.

This is likely attributed to the use of sands that were acid washed prior to use to remove fine particles that adhere to the sand grain surface. It is hypothesized that the fine particulates and clays present in natural OS may interact electrostatically with the negatively charged nC60 aggregates, increasing the affinity of nC60 for the solid-phase.

6 mM NaCl (and 0.05 NaHCO3) was chosen as the background electrolyte for the 2-d aquifer cell because the percent mass breakthrough (71%) of nC60 in this column matched the approximate breakthrough of 75% desired in the 2-d aquifer cell. Results from this first series of column studies show that replicated experiments do not yield identical results (e.g. Columns D and E). Replicates were more easily reproduced in columns packed with acid-washed sand in prior studies (Wang et al. 2008). It is hypothesized that the differences in surface coatings and fine particulates in natural OS result in variation in nC60 retention due to the non-homogenous coatings of these particulates on the grain surface.

Table 5. Parameters for column studies in unwashed 40/50 mesh OS with various background electrolytes. Background Influent nC Pulse % Mass Concentration 60 Column Electrolyte concentration width Breakthrough NaHCO3 (mM) Concentration (mM) (C0; mg/L) (PVs) A CaCl2 1 0.065 ND 5.0 <1 B NaCl 3 0.05 2.97 5.0 97.6 C NaCl 5 0.05 3.01 5.1 94.2 D NaCl 6 0.05 1.55 3.5 71.1 E NaCl 6 0.05 1.47 3.6 81.6 F NaCl 7 0.05 2.12 5.1 45.9 G NaCl 10 0.05 ND 5.0 <1

Figure 21. nC60 breakthrough curves in 40/50 mesh OS with various background electrolytes. Flow and injection conditions are shown in Table 1.

6.1.2 Effect of soil particle size on nC60 transport

Experimental parameters for nC60 injection into columns of various sand size fractions are shown in Table 5. At least one replicate was performed for nC60 injection in each OS size fraction.

Effluent breakthrough curves and retention profiles are shown in Figures 23–24.

Table 6. Parameters for column studies in various sand size fractions. Background electrolyte is 6 mM NaCl and 0.05 mM NaHCO3. Influent nC Pulse % Mass % Mass Size Fraction 60 Pulse width Column concentration elution Breakthrough Recovered Unwashed OS (PVs) (C0; mg/L) (PVs) D 40/50 2.46 3.54 3.02 71.1 93.0 E 40/50 2.46 3.64 3.11 81.6 91.4 H 80/100 2.58 2.73 2.71 5.1 91.4 I 80/100 2.61 2.75 2.78 16.5 78.4 J 100/140 2.60 2.79 3.19 3.1 96.2 K 100/140 2.60 2.88 3.29 14.4 82.5 L 100/140 2.62 2.71 2.96 8.1 77.6

1.2

0.8 0

0.6 Tracer C/C 0.4 Column D Column E 0.2

0 0 2 4 6 8 PVs

1.4 1.2

g/g) μ

( 0.8 60

0.6 Column D

0.4 Column E Conc. C Conc. 0.2 0 0 5 10 15 Distance from Inlet (cm)

Figure 22. Breakthrough curve and retention profile for nC60 injection into 40/50 OS. Columns D and E are replicates.

1.2

0.8 0

0.6 Tracer C/C 0.4 Column H Column I 0.2

0 0 2 4 6 PVs

10 9 8

g/g) μ ( 6

60 5 4 Column H 3 Column I Conc. C Conc. 2 1 0 0 5 10 15 Distance from Inlet (cm)

Figure 23. Breakthrough curve and retention profile for nC60 injection into 80/100 OS. Columns H and I are replicates.

1.2

0.8 0 0.6 Tracer C/C Column J 0.4 Column K 0.2 Column L

0 0 2 4 6 8 PVs

g/g) 8

( 60 6 Column J 4 Column K

Conc. C Conc. Column L 2

0 0 5 10 15 Distance from Inlet (cm)

Figure 24. Breakthrough curve and retention profile for nC60 injection into 100/140 OS. Columns J, K, and L are replicates.

Retention of nC60 varied significantly among replicates, as in the first series of column studies.

Percent mass recovery varied from 78 to 96%. A possible cause of the low mass recoveries is incomplete extraction of nC60 from the dried samples. Because of the variable results of replicates, experiments with the best percent mass-recovery were chosen for modeling (Columns

D, H, and J). For these columns, Péclet numbers, and subsequently dispersivity (αL) were found

to increase with a decrease in permeability of the porous medium (Figures 25–27). Smax and katt,

were determined by Mr. Matthew Becker by inverse fitting modified filtration theory to

experimental values (Figure 28). Both Smax and katt increased with a decrease in permeability of

medium (Table 7). Concentration vs Pore Volume

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0 1 2 3 4 5 6 7 Pore Volume

Figure 25. Non-reactive tracer data in 40/50 mesh OS fit to the 1-d equilibrium ADR transport equation.

Concentration vs Pore Volume

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0 1 2 3 4 5 6 Pore Volume

Figure 26. Non-reactive tracer data in 80/100 mesh OS fit to the 1-d equilibrium ADR transport Concentrationequation. vs Pore Volume

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0 1 2 3 4 5 6 7

Pore Volume Figure 27. Non-reactive tracer data in 100/140 mesh OS fit to the 1-d equilibrium ADR transport equation.

Table 7. Determined modeling parameters for representative results of nC60 injection into columns of varying sand size fractions. Sand Size Percent Mass Percent Mass S k Column Pe α (cm) max att Fraction Recovery Breakthrough L (μg/g) (1/h) D 40/50 93.0 71.1 3174 0.00488 0.536 0.249 H 80/100 91.4 5.1 271.2 0.0572 3.17 1.11 J 100/140 96.2 3.1 177.8 0.0872 4.74 1.37

1.2

40/50 Tracer 80/100 Tracer 1 100/140 Tracer Column D (40/50) Column H (80/100) 0.8 Column J (100/140) Modified CFT Fit 40/50 Modified CFT Fit 80/100 0 Modified CFT Fit 100/140

0.6 C/C

0.4

0.2

0 0 2 4 6 Pore Volumes

Figure 28. Modified filtration theory fit to experimental breakthrough curves for various sand particle sizes. Simulated fits were provided by Mr. Matthew Becker.

Modified filtration theory was unable to fit both the steep increase and plateau of the nC60 injection breakthrough curve in 40/50 mesh OS. This suggests that there is another mechanism beyond those incorporated into modified CFT that controls nC60 transport and retention. nC60 deposition distribution was found to be hyper-exponential near the inlet for all columns, so there is a possibility that particle straining was responsible for nC60 retention behavior in these

40 studies. However, the ratios of colloid diameter to particle diameter (Table 8) are below the threshold of 0.002 reported by Bradford et al. (2006) for particle straining. d50 used in these ratios were based on clean sand, and do not account for the likely additional fine particulate matter attached to the unwashed sand. The presence of fine particulates may also block pore throats and additionally contribute to particle straining.

Table 8. Ratio of colloid diameter to particle diameter for various sand size fractions. dp (nC60) = 100 nm. Sieve Size d50 dp/d50 40/50 0.36 0.00028 80/100 0.16 0.00063 100/140 0.13 0.00077

6.2 2-D Aquifer Cell

6.2.1 Non-reactive breakthrough tracer tests

A photograph of the sodium flourescein and sodium bromide tracer through the fully-screened influent chamber is shown in Figure 29. Flow is visually uniform across the high permeability background matrix, and reduced but still uniform throughout the low-permeability lenses.

Figure 29. Photograph of non-reactive sodium flourescein and sodium bromide tracer through fully screened influent chamber.

Effluent breakthrough curves from the side ports monitoring during the fully-screened non- reactive sodium bromide tracer test were fit to determine the transverse and longitudinal dispersivity in the 2-d aquifer cell (Figure 30). αT and αL were found to be 0.01 and 0.1 cm, respectively.

Figure 30. BTCs for fully-screened non-reactive tracer in 2-d cell fit for dispersivity coefficients.

Photographs from the injected sodium flourescein tracer indicate a relatively symmetrical flow field around both lenses, with a lower flow velocity through the lenses (Figures 31–32), as seen in the simulated velocity field and the fully-screened influent tracer (Figure 6 and 28). The injected tracers do show a slightly greater flow velocity above the midpoint of the lenses. This could be because the midpoints of the lenses were not packed directly adjacent to the injection ports, or because the influent needle was slightly angled during injection.

Figure 31. Time-series photographs of sodium flourescein injection through port A2.

Figure 32. Time-series photographs of sodium flourescein injection through port A4.

6.2.2 nC60 injection

As seen previously, nC60 transport is sensitive to small changes in background electrolyte.

Because of this, chloride content of the 2-d cell effluent was monitored throughout nC60 injections. Chloride content was constant throughout each injection and elution as seen in Table

9, so variations in the ionic strength of the background solution are unlikely to have an effect on the observed retention of nC60.

Table 9. Concentration of sodium chloride in effluent during nC60 injection. Average Sodium Chloride Injection Port Concentration (mM) A2 6.19 ± 0.1 A4 6.02 ± 0.14 C3 6.06 ± 0.13

All nC60 pulse injections resulted in a visible brown ring around the injection port (Figure 33), presumed to be the nC60 suspension, which is originally dark yellow in color (Figure 34). The

45 visible ring is darker above the injection port than below it. It was hypothesized that the exaggerated asymmetrical distribution of nC60 mass was due to a density difference between the injected solution and the background solution. However, a series of density measurements indicated that the densities of the two solutions were identical (specific gravity of nC60 injection solution = 1.0004). The asymmetry is therefore most likely due to minimal differences in the flow field attributed to box-packing differences above and below the low-permeability lenses.

80/100 mesh low- A2 permeability lens

100/140 mesh low- A4 permeability lens

40/50 mesh background matrix C3

Figure 33. Photos of influent ports after nC60 injection.

Figure 34. Photograph of nC60 injection solution.

No aqueous nC60 was measured in any of the side-port samples downgradient of the injections into ports A2 and A4. This contrasted with the injection into the higher permeability background matrix, in which there was breakthrough in port D1 at 800 and 870 minutes after injection (0.31 and 0.17 mg/L, respectively). This is as expected as the velocity flow decreases significantly in the lenses, which increases retention, and decreases the concentration of nC60 in the aqueous phase.

Dissection around the 100/140 mesh lens showed that nC60 solid-phase concentrations matched the observed brown ring around the injection port. nC60 retention was greatest within centimeters of the injection port (Figure 35). The concentrations of solid-phase nC60 are greater upgradient of the lens, as seen in Figures 36–38. Retention profiles of transects around the injection port are similar to those in the column studies where nC60 concentration decreases as distance from the injection port increases (Figure 39 and 40). Transects adjacent to the injection port (Figure 40) have hyper-exponential retention profiles, which were also seen in fine-sand column studies. A comparison of retention profiles for transects above and below the lens

(Figure 39) with retention profiles for transects through the lens (Figure 40) show a decrease in solid-phase nC60 after the textural interface.

S (μg/g)

22 ND ND ND ND 2 0.11 ND 0.02 20

ND ND 0.99 4.40 4.95 4.31 2.65 1.34 18 4 0.13 7.71 10.7 11.5 13.3 8.90 3.62 1.61 1.12 0.62 0.51 0.41 0.25 0.17 16 2.29 18.9 6.21 17.6 11.4 6.31 1.13 0.13 14 6 0.39 23.3 21.5 15.7 6.95 6.07 2.28 0.52 0.52 0.16 0.03 0.05 0.04 0.09 12 ND 1.66 9.64 8.85 5.56 5.06 2.22 2.84 0.98 0.52 0.60 0.28 0.19 0.15 10 8 ND ND 0.67 2.35 2.47 1.63 1.09 0.84 8 0.17 0.08 0.05 ND 0.15 0.03 0.06 6 10 4

ND ND ND ND ND ND ND 2 12

2 4 6 8 10 12 14

Figure 35. Plot of solid-phase concentration (μg/g) of nC60 around injection port A4. The white cross shows the location of the injection port, and the black outline shows the location of the 100/140 mesh OS lens. ND = concentration is below the detection limit of 0.07 mg/L.

S (μg/g)

8.9 3.62 1.61 1.12 0.62 0.51 0.41 0.25 0.17 8

6 6.31 1.13 ND ND ND ND ND ND ND 5

4 6.07 2.27 0.52 0.52 0.16 0.03 0.05 0.04 0.09 3

2 5.06 2.21 2.84 0.98 0.52 0.60 0.28 0.19 0.15 1

Figure 36. Plot of solid-phase nC60 concentration (μg/g) around the 100/140 mesh OS lens (outlined in black). ND = concentration is below the detection limit of 0.07 mg/L.

13.2

11.6

10.4

9.2

6.8

5.6

Vertical Distance Verticalfrom ConfiningLayer (cm) 2

0 1 2 3 4 5 6 7 8 9 10 C60 Solid Phase Concentration (ug/g)

Figure 37. Mass of nC60 retained immediately before the interface of 40/50 and 100/140 mesh OS low-permeability lens. Samples directly upgradient of the lens are shown in orange, samples outside of the lens are shown in blue.

13.2

11.6

10.4

9.2

6.8

5.6

VerticalDistance from ConfiningLayer (cm) 2

0 1 2 3 4 5 6 7 8 9 10 C60 Solid Phase Concentration (ug/g)

Figure 38. Mass of nC60 retained immediately after the interface of 40/50 and 100/140 mesh OS low-permeability lens. Samples inside the lens are shown in orange. Samples outside of the lens are shown in blue.

Injection Location 25

15 Distance from confining layer 100/140 Lens 10.4 10

6.8 C60 Mass Retained (ug/g)RetainedMassC60 5

0 -4 -2 0 2 4 6 8 10 12 14 Horizontal Distance from Injection Location

Figure 39. Retention profiles for horizontal transects above and below the 100/140 mesh lens.

Injection Location 25

15 Distance from confining layer 9.2 10

100/140 Lens 8 C60 Mass Retained(ug/g)MassC60 5

0 -4 -2 0 2 4 6 8 10 12 14 Horizontal Distance from Injection Location

Figure 40. Retention profiles for horizontal transects through the 100/140 mesh lens.

Dissection of the aquifer soil around the third injection into the higher permeability background matrix (40/50 mesh OS) resulted in a concentration-plot similar to that around the 100/140 mesh lens (Figure 41). The greatest concentrations of solid-phase nC60 were observed directly around the injection port.

S (μg/g)

1 0.81 3.66 4.20 2.66 1.71 1.00 25 2

3 0.65 0.71 0.64 0.69 0.64 2.44 6.76 6.76 8.88 8.28 2.91 1.38 20 4

5 15 7.24 15.5 14.5 26.9 17.3 2.71 0.42 6

7 10 1.76 19.8 19.2 28.1 9.52 2.16 8 0.72 0.49 0.43 0.29 0.17 5 9 0.05 9.39 15.0 9.74 2.91 1.69 0.95 10

2 4 6 8 10 12 14 16

Figure 41. Plot of solid-phase nC60 concentrations (μg/g) in the 40/50 mesh OS background matrix. The white cross shows the location of the injection port.

nC60 retention profiles for transects downgradient of injection port C3 and A2 (into the background matrix and upgradient of a 100/140 mesh low-permeability lens, respectively) are similar (Figure 42). These retention profiles are hypo-exponential near the inlet and decrease quickly, similar to the column experiments. Retention of nC60 in the 100/140 mesh lens is slightly less than that found in the background higher permeability matrix when magnified

(Figure 43), showing that there is a difference in retention behavior when nC60 encounters low- permeability soil. Retention profiles for nC60 injected in to the background 40/50 mesh in the 2-d aquifer cell were similar to those found in the 40/50 mesh 1-d column studies (Figure 44).

Despite different injection conditions, nC60 retention decreased as distance from the injection port increased in both the 2-d cell and the 1-d columns.

Injection Location 30 Injection into C3 - Upper Transect Injection into C3 - Lower Transect 25 Injection into A2 - Upper Transect Injection into A2 - Lower Transect 20

S (ug/g) S 100/140 mesh lens 10

0 -5 0 5 10 15 20

Distance from Injection Port (cm)

Figure 42. Comparison of nC60 retention profiles for injection upgradient of a low-permeability lens (100/140 mesh) and in a higher-permeability background matrix (40/50 mesh).

Injection Location 2.5

Injection into C3 - Upper Transect 2 Injection into C3 - Lower Transect 100/140 mesh lens Injection into A2 - Upper Transect 1.5

Injection into A2 - Lower Transect S (ug/g) S 1

0.5

0 0 2 4 6 8 10 12 14

Distance from Injection Port (cm)

Figure 43. Scaled comparison of nC60 retention profiles for injection upgradient of a low- permeability lens (100/140 mesh) and in a higher-permeability background matrix (40/50 mesh).

100

1-D Column 2-D Aquifer Cell

g/g)

μ S ( S 1 0 2 4 6 8 10 12 14 16

0.1 Distance from Inlet (cm)

Figure 44. Comparison of nC60 retention profiles in the 1-d column and the 2-d aquifer cell.

The hypothesized accumulation at the textural interface was not observed. There was a decrease in nC60 solid-phase concentration immediately after contact with the low-permeability 100/140 mesh lens, but this was not enhanced around the horizontal edges of the lens as expected. Instead, results support that nC60 transport at textural interfaces is related to changes in local flow conditions. The flow velocity is known to decrease at the low-permeability lens as shown in modeled flow fields (Figure 6). nC60 retention appears to be directly correlated with this decrease in flow.

To date, results for solid-phase nC60 concentrations around the injection upgradient of the

80/100 mesh lens have not been processed and are beyond the scope of this document.

7.0 CONCLUSIONS AND RECOMMENDATIONS

This research sought to investigate the transport and retention behavior of nC60 in a heterogeneous two-dimensional fully saturated aquifer cell. Column studies were conducted to determine the effects of background electrolyte ionic strength on nC60 retention and to determine relevant transport parameters for various size fractions of OS for future model simulations of nC60 using modified filtration theory.

Ionic strength was found to be inversely correlated with percent mass breakthrough, as reported by Wang et al. (2008b). Retention in unwashed/natural OS was much greater than for studies with acid-washed OS (Wang et al. 2008b). This is possibly due to the electrostatic interactions between nC60 and the silt and clay size fraction present in natural OS, or because of the potential for particle straining because of the additional silt and clay fractions.

Retention was found to increase with a decrease in permeability. The Smax and katt values for 40/50, 80/100, and 100/140 mesh OS were found to be 0.54, 3.2, and 4.7 μg/g, and 0.25, 1.1, and 1.4 h-1, respectively. Modified filtration theory was unable to accurately simulate the experimental breakthrough curves and retention profiles for nC60 injection. This discrepancy was attributed to additional mechanisms responsible for retention of nC60 aggregates in the Ottawa sands employed in these experiments. One possible mechanism is nanoparticle straining at pore throats, which could result from the presence of silt and clay size materials in the unwashed sands.

Non-reactive tracer tests in the 2-d aquifer cell were used to determine the dispersivity coefficients and confirmed that flow is nearly symmetrical around the low-permeability lenses. nC60 was injected into three of the side ports in the 2-d aquifer cell. No breakthrough of aqueous nC60 was found directly downgradient of the injection into the side ports upgradient of low-

54 permeability lenses, but there was some breakthrough of aqueous nC60 in the background matrix of higher permeability. This was expected because the decrease in flow velocity through the lens due to lower permeability of the lens media results in an increased maximum retention capacity. nC60 aggregates were strongly retained around the injection ports used in the aquifer cells, consistent with the mechanism for particle straining. These experiments suggest that particle straining may occur when the ratio of the diameter of the colloid to the diameter of the soil particle (i.e. collector) is 70% less than the previously reported threshold of 0.002 (Wang et al.

2008).

The original hypothesis that nC60 would accumulate at textural interfaces was not supported by the experimental results. Higher concentrations of nC60 were observed in the coarse sand size fraction than in the fine sand 100/140 mesh lens at the textural interface, but there was no enhanced accumulation around the horizontal edges of the interface as expected. Comparing retention data from injection into 100/140 mesh lens to retention from injection into the higher permeability background matrix (40/50 mesh) shows that the decrease in retention after textural interface is not significant.

It is recommended that more research be pursued in order to accurately describe the fate of nC60 aggregates in heterogeneous porous media. Such experimental results can be used to modify and validate 2-d models for simulation of nC60 in the subsurface. In addition, future work should examine the effects of particle straining on nC60 transport and retention as the results presented here indicate that this mechanism contributed to nC60 retention. Future work could also focus on the behavior of other nanomaterials in heterogeneous media, possibly the transport of quantum dots, which are fluorescent, so that retention can be observed visually because high resolution dissection and extraction of nC60 from the aquifer-cell dissection is difficult.

8.0 COLLABORATION AND ACKNOWLEDGEMENTS

This research was conducted under the supervision of Dr. Kurt Pennell, Dr. Yonggang Wang, and Mr. Doug Walker. One-dimensional mathematical modeling was performed by Mr. Matthew

Becker and 2-d simulations were provided by collaborators Dr. Yusong Li and Ms. Chunmei Bai

(Department of Civil Engineering, University of Nebraska) in consultation with Dr. Linda

Abriola.

REFERENCES

Andrievsky, G. V.; Kosevich, M. V.; Vovk, O. M.; Shelkovsky, V. S.; Vashchenko, L. A. On the production of an aqueous colloidal solution of fullerenes. J. Chem. Soc. Commun. 1995, 12, 1281–1282.

Arrais, A.; Diana, E. Highly water soluble C-60 derivatives: A new synthesis. Fullerenes Nanotubes and Carbon Nanostructures 2003, 11, 35–46.

Bensasson, R. V.; Bienvenue, E.; Dellinger, M.; Leach, S.; Seta, P. C60 in model biological systems – a visible UV absorption study of solvent-dependent parameters and solute aggregation. J. Phys. Chem 1994, 98, 3492–3500.

Bradford, S. A.; Simunek, J.; Bettahar, M.; Tadassa, Y. F.; van Genuchten, M. T.; Yates, S.R. Straining of colloids at textural interfaces. Water Resources Research 2005, 41, W10404.

Bradford, S. A.; Simunek, J.; Bettaharm, M.; van Genuchten, M. T.; Yates, S. R. Significance of straining in colloid deposition: Evidence and implications. Water Resources Research 2006, 42, W12S15.

Brant, J.; Lecoanet, H.; Wiesner,M.R. Aggregationanddeposition characteristics of fullerene nanoparticles in aqueous systems. J. Nanopart. Res. 2005, 7, 545–553.

Brettreich, M.; Hirsch, A. A highly water-soluble dendro[60]fullerene. Tetrahedron Letters 1998, 39, 2731–2734.

Carman, P. C. Flow of gases through porous media 1956. Butterworths, London.

Chen, K. L.; Elimelech, M. Aggregation and deposition kinetics of fullerene (C60) nanoparticles. Langmuir 2006, 22, 10994–11001.

Cheng, X.; Kan, A. T.; Tomson, M. B. Naphthalene adsorption and desorption from aqueous C60 fullerene. J. Chem. Eng. Data 2004, 49, 675–683.

Deguchi, S.; Alargova, R. G.; Tsujii, K. Stable dispersions of fullerenes, C60 and C70, in water. Preparation and characterization. Langmuir 2001, 17, 6013–6017.

Espinasse, B.; Hotze, E. M.; Wiesner, M. R. Transport and retention of colloidal aggregates of C60 in porous media: Effects of organic macromolecules, ionic composition, and preparation method. Environ. Sci. Technol. 2007, 41, 7396–7402.

Fortner, J. D.; Lyon, D. Y.; Sayes, C. M.; Boyd, A. M.; Falkner, J. C.; Hotze, E. M.; Alemany, L. B.; Tao, Y. J.; Guo, W.; Ausman, K. D.; Colvin, V. L.; Hughes, J. B., C60 in water: Nanocrystal formation and microbial response. Environmental Science and Technology 2005, 39, (11), 4307

Heymann, D. Solubility of C60 in Alcohols and Alkanes. Carbon 1996. 34, 5.

Hubaux, A.; Vos, G., Decision and detection limits for linear calibration curves. Analytical Chemistry 1970, 42, (8), 849–855.

Jensen, A. W.; Wilson, S. R.; Schuster, D. I. Biological applications of fullerenes. Bioorganic & Medicinal Chemistry 1996, 4, (6), 767–779.

Krätschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Solid C60: a new form of carbon. Nature 1990, 347, 354–358.

Kroto, H.W., J.R. Heath, S.C. Obrien, R.F. Curl, and R.E. Smalley. 1985. C60 Buckminsterfullerene. Nature. 318, 162-163.

Lecoanet, H. F.; Bottero, J.-Y.; Wiesner, M. R. Laboratory assessment of the mobility of nanomaterials in porous media. Environ. Sci. Technol. 2004, 38, 5164–5169.

Li, Y.; Wang, Y.; Pennell, K. D.; Abriola, L. M. Investigation of the transport and deposition of fullerene (C60) nanoparticles in quartz sands under varying flow conditions. Environ. Sci. Technol. 2008, 42, 7174–7180.

Liu, S.; Lu, Y-J.; Kappes, M. M.; Ibers, J. A. The structure of the C60 molecule: x-ray crystal structure determination of a twin at 110 K. Science 1991, 254, 408–410.

Lyon, D. Y.; Fortner, J. D.; Sayes, C. M.; Colvin, V. L.; Hughes, J. B. Bacterial cell association and antimicrobial activity of a C60 water suspension. Environ. Toxicol. Chem. 2005, 24, 2757–2762.

[Malvern Instruments Ltd.]. Derjaguin, Landau, Verwey and Overbeek theory (DLVO theory). 2011. http://www.malvern.com/LabEng/industry/colloids/dlvo_theory.htm.

Maynard, A. D.; Aitken, R. J.; Butz, T.; Colvin, V.; Donaldson, K.; Oberdörster, G.; Philbert, M. A.; Ryan, J.; Seaton, A.; Stone, V.; Tinkle, S. S.; Tran, L.; Walker, N. J.; Warheit, D. B. 2006. Safe handling of nanotechnology. Nature 2006, 444, 267–269.

[Nano-C]. Nano-C: Fullerene Applications. 2008, http://www.nano-c.com/fullereneapp.html.

[NanoRoad]. Overview on Promising Nanomaterials for Industrial Applications. 2005. http://www.nanoroad.net/download/overview_nanomaterials.pdf

Oberdörster, E. Manufactured nanomaterials (Fullerenes, C60) induce oxidative stress in the brain of juvenile largemouth bass. Environ. Health Perspect. 2004, 112, 1058–1062.

Ruoff, R. S.; Tse, D. S; Malhotra, R.; Lorents, D. C. Solubility of C60 in a variety of solvents. J. Phys. Chem. 1993, 97, 3379–3383.

Sharma, H. D.; Reddy, K. R. Site Remediation, Waste Containment, and Emerging Waste Management Technologies 2004. Hoboken, New Jersey.

Spohn, S.; Hirsch, C.; Hasler, F.; Bruinink, A.; Krug, H. F.; Wick, P. C60 fullerene: A powerful antioxidant or damaging agent? The importance of an in-depth material characterization prior to toxicity assays. Environ. Poll. 2009, 157, 1134–1139.

Suchomel, E. J.; Ramsburg, C. A.; Pennell, K. D. Evaluation of trichloroethene recovery processes in heterogeneous aquifer cells flushed with biodegradable surfactants. Journal of Contaminant Hydrology 2007, 94, 195–214.

Terashima, M.; Nagao, S. Solubilization of [60]fullerene in water by aquatic humic substances. Chem. Lett. 2007, 36, 302–303.

Taylor, T. P.; Pennell, K. D.; Abriola, L. M.; Dane, J. H. Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses 1. Experimental studies. Journal of Contaminant Hydrology 2001, 48, 325–350.

Tong, Z.; Bischoff, M.; Nies, L.; Applegate, B.; Turco, R. F. Impact of fullerene (C60) on a soil microbial community. Environ. Sci. Technol. 2007, 41, 2985–2991.

Wang, Y.; Li, Y.; Fortner, J. D.; Hughes, J. B.; Abriola, L. M.; Pennell, K. D. Transport and retention of nanoscale C60 aggregates in water-saturated porous media. Environ. Sci. Technol. 2008, 42, 3588–3594.

Wang, Y.; Li, Y.; Kim, H.; Walker, S. L.; Abriola, L. M.; Pennell, K. D. Transport and retention of fullerene nanoparticles in natural soils. Journal of Environmental Quality. 2009, 39, 6, 1925–1933.

Wang, Y.; Li, Y.; Pennell, K. D. Influence of electrolyte species and concentration on the aggregation and transport of fullerene nanoparticles in quartz sands. Environ. Toxicol. Chem. 2008, 9, 1860–1867.

Yang, J. Z.; Alemany, L. B.; Driver, J; Hartgerink, J. D.; Barron, A. R. Fullerene-derivatized amino acids: Synthesis, characterization, antioxidant properties, and solid-phase peptide synthesis. Chemistry-a European Journal 2007, 13, 2530–2545.

Yao, K. M.; Habibian, M. M.; Omelia, C. R. Water and Waste Water Filtration - Concepts and Applications. Environ. Sci. Technol. 1971, 5, 1105–1112.

Zhu, S.; Oberdorster, E.; Haasch, M. L. Toxicity of an engineered nanoparticle (fullerene, C60) in two aquatic species, Daphnia and fathead minnow. Marine Environ. Research 2006, 62, S5– S9.