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Effects of humic acid and electrolytes on photocatalytic reactivity and transport of carbon nanoparticle aggregates in water
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Effects of humic acid and electrolytes on photocatalytic reactivity and transport of carbon nanoparticle aggregates in water So-Ryong Chae a,*, Yao Xiao b, Shihong Lin b, Tahereh Noeiaghaei a, Jong-Oh Kim c, Mark R. Wiesner b a
School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia Department of Civil and Environmental Engineering, Pratt School of Engineering, Duke University, Durham, NC 27708, USA c Department of Civil Engineering, Gangneung-Wonju National University, Gangneung, Gangwon-do 210-702, South Korea b
article info
abstract
Article history:
The effects of naturally occurring macromolecules such as humic acid (HA) and electro-
Received 12 July 2011
lytes on four fullerene nanoparticle suspensions (i.e., C60, C60(OH)24, single- and multiwall
Received in revised form
carbon nanotubes) were explored with respect to: (1) characteristics of nanoparticle
7 May 2012
aggregates, (2) transport of the aggregates through a silica porous media, and (3) production
Accepted 10 May 2012
of reactive oxygen species (ROS) from the photosensitized fullerene aggregates. The
Available online 18 May 2012
presence of HA and salts increased the size of aggregates and relative hydrophobicity associated with transport through silica beads, while decreasing ROS production. These
Keywords:
data illustrate the importance that transformation of engineered nanomaterials (ENMs)
Fullerene nanoparticle
through interactions with aquatic solutes may have in altering the environmental behavior
Carbon nanotubes
of nanomaterials. ª 2012 Elsevier Ltd. All rights reserved.
Humic acid Electrolytes Reactive oxygen species Transport through silica beads
1.
Introduction
Engineered nanomaterials (ENMs) are being produced in increasing quantities for a wide range of applications (Hendren et al., 2011). In particular, fullerene nanomaterials (FNMs) including C60, C60(OH)24, C70, and carbon nanotubes (CNTs) show unique physical, chemical, and photochemical properties, which are applicable for water treatment, energy production, and biomedical drug carriers (Bosi et al., 2003; Mauter and Elimelech, 2008). The production and use of FNMs will likely lead to environmental exposures. The increasing use of FNMs has raised concerns over their photoreactivity producing reactive oxygen species (ROS) such as singlet oxygen (1O2) and superoxide (O 2 ).
ROS produced from FNMs have been shown to oxidize organic compounds (Chae et al., 2009; Hotze et al., 2008) and inactivate bacteria and viruses (Badireddy et al., 2007; Kasermann and Kempf, 1997; Sayes et al., 2005; Yamakoshi et al., 2003). Like many other nanoparticles, FNMs tend to form colloidal aggregates in water (Brant et al., 2005; Fortner et al., 2005; Hotze et al., 2010). Given that some of these aggregates possess properties that differ from the pristine nanomaterials (Auffan et al., 2009; Chae et al., 2010a), an evaluation of the fate and impacts of FNMs in the aquatic environment requires an understanding of the effects of aggregate properties such as size, fractal structure, and surface charges on transport of the nanoparticles, as well as their interactions with environmental elements such as soil, natural organic and inorganic substances, and biomass.
* Corresponding author. Tel.: þ61 2 9351 3832; fax: þ61 2 9351 2854. E-mail address: [email protected] (S.-R. Chae). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.05.018
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Recently, it was found that the production of ROS, microbial inactivation, and the mobility of the aggregates of fullerene (nC60) in a silicate porous medium all increased as the size of aggregates decreased (Chae et al., 2010a). As nanoparticles make their way through the environment, they will come into contact with, and possible interaction with, a variety of solutes including natural organic matter (NOM) and mono- and multivalent cations and ions. Soluteenanoparticle interactions at the surrounding environment have the potential of transforming nanoparticles with resultant impacts on the fate, transport, and toxicity (Chen et al., 2010). For example, the presence of humic acid (HA) has been observed to increase colloidal stability of nC60 and reduce aggregate size (Hyung et al., 2007; Terashima and Nagao, 2007), even the dispersion process is further accelerated by sunlight (Isaacson and Bouchard, 2010; Li et al., 2009) or ultraviolet (UV) irradiation (Qu et al., 2010). Also, an increase in electrolyte concentrations increased the size of nC60 and reduced electrophoretic mobility (EPM; Chen et al., 2010). For the co-effects of HA and electrolytes on the aggregation kinetics of nC60, Chen and Elimelech (2007) reported that HA significantly reduced the aggregation kinetics of nC60 at low concentrations of NaCl or MgCl2. However, it was also found that HA enhanced the destabilization of nC60 in the presence of high concentrations of CaCl2 (>10 mM) by making bridges between nC60 aggregates. Similarly, HA and electrolytes affect CNT stability, with aggregation favored by increasing salt concentration and the presence of divalent cations, while HA impedes the aggregation of single- (SW) and multiwall (MW) CNTs (Li and Huang, 2010; Saleh et al., 2008). Aggregation of fullerenes has been related through theory and observation to their reactivity through photosensitization (Hotze et al., 2010). Changes in aggregation behavior of ENMs are strongly related to changes in their relative affinity for a porous medium, since both deposition and aggregation are related to the surface chemistry of the ENMs and their aggregates. Although it has been recently reported that the aggregates of CNTs (i.e., nSWCNT and nMWCNT) produce ROS and degrade 2-chlorophenol (Chae et al., 2011), the effects of HA and electrolytes on ROS production from the aggregates of CNTs are largely unknown. In the current study, we consider the effects of HA and electrolytes on mobility of four different carbon nanoparticle aggregates (i.e., nC60, nC60(OH)24, nSWCNT, and nMWCNT) through a silica porous medium associated with stability of nanoparticle aggregates, and photosensitivity of the aggregates to produce ROS in water.
2.
Materials and methods
2.1.
Spherical and tubular fullerene aggregates
Two spherical FNMs, C60 and C60(OH)24, were purchased from MER (99.9%, Tucson, AZ) and two tubular FNMs, SWCNT and MWCNT, were obtained from BuckyUSA (þ99.5%, Huston, TX). All fullerene suspensions were prepared in deionized (DI) water that had a resistivity of 18.2 MU-cm and dissolved organic carbon concentration was less than 50 ppb (Nanopure, Barnstead, Dubuque, IA). For nC60(OH)24 stock suspension was prepared by
adding pristine powder form to DI water and stirred at 200 rpm for 24 h. Aqueous suspensions of nC60, nSWCNT, and nMWCNT were prepared by sonication with a high-energy probe (S-4000, Misonix, Qsonica, LLC, Newtown, CT) for 10 h (20 min (pulse on)/ 10 min (pulse off), applied energy into 1 L of suspension was approximately 2.2 MJ) without any organic solvent addition. The initial concentration of nC60, nC60(OH)24, nSWCNT, and nMWCNT suspensions were 5.8, 9.3 2.7, and 2.1 mg/L as total carbon (TC), respectively. All suspensions were filtered through a microfiltration membrane having nominal pore size of 800 nm (Nylaflo, Pall Life Sciences, Port Washington, NY) and stored in a refrigerator at 4 C before use.
2.2. Preparation fullerene suspensions with electrolytes and HA The electrolyte stock solutions (3 M of NaCl or 1 M of MgCl2) were prepared in DI water and were filtered through 0.22 mm membrane (Nylaflo, Pall Life Sciences, Port Washington, NY) before use. The HA stock solution (1000 mg/L) was prepared by adding 100 mg of HA powder (Suwannee River Humic Acid Standard II, International Humic Substances Society) to 100 ml of DI water and stirring the solution overnight. A volume of 0.5 mL of the HA stock suspension or DI water (without HA) was added into 44.5 mL of the fullerene suspensions and stirring for 2 h. Then, the fullerene suspensions were mixed with 5 mL of NaCl or MgCl2 stock solutions to adjust final concentration of 0.3 M NaCl or 0.1 M MgCl2. The pH of the final suspensions was adjusted to 7.0e7.5 by adding NaOH before use.
2.3. Characterization of fullerene nanoparticle suspensions The TC concentrations of the fullerene suspensions were measured by a total organic carbon (TOC) analyzer (TOC5050A, Shimadzu, Columbia, MD). EPM and hydrodynamic diameter (dh) of nanoparticle aggregates were measured by Zetasizer Nano (Malvern Instrument, Bedford, MA). High magnification images of the suspensions are obtained by TEM (FEI Tecnai G2 Twin, Hillsboro, OR). Ten microliters of each sample was dropped on a lacey carbon/Cu grid (300 meshes, Electron Microscopy Sciences, Hatfield, PA) and dried in air before TEM measurement. Image analysis of TEM images was performed to evaluate mean diameter (dt) of fullerene aggregates (n ¼ 100) by using an Image-Pro version 4.5 (Media Cybermetics, Inc. Bethesda, MD). The hydrophobicity coefficient (Ktw) of the fullerenes in suspensions was determined as a basis for quantifying the relative affinity of these materials for the aqueous phase as described in the previous study (Chae et al., 2010a). The Ktw was calculated by the following equation. Ct (1) Ktw ¼ Cw where Ct is the concentration of fullerene in toluene and Cw is the concentration of fullerene in water. The concentration of fullerene nanoparticles was measured by liquideliquid extraction using toluene coupled with a high-performance liquid chromatography analysis as described in the previous studies (Chae et al., 2010b; Xiao et al., 2011).
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2.4. Transport of fullerene aggregates through a silicate porous medium Transport of fullerene aggregates through a silica porous medium was evaluated to quantify the relative affinity of each fraction of these fullerenes for the medium as described previously (Chae et al., 2010a). A chromatography column (10 mm outer diameter 40 mm length, GE Health care, Piscataway, NJ) packed with spherical glass beads (Potters Industries Inc., Berwyn, PA) having nominal diameter of approximately 360 mm was used for this purpose. The background solution in this study was ultrapure water driven by a peristaltic pump (Cole-Parmer Instrument Company, Chicago, IL) at a flow rate of 0.96 ml/min as compared to the flow rate of 0.04 ml/min of the fullerene suspension (TC concentration was normalized to 1 mg/L) driven by a syringe pump (Harvard Apparatus, Holliston, MA). To quantify the relative fullerene concentration, the UV absorbance of fullerene was continuously measured by UVevis spectrophotometer (UVevis 2810, Hitachi, Pleasanton, CA) at the wavelength of 198 nm, which is the maximum absorbance peak among three peaks at 198, 285, and 334 nm (Chae et al., 2010a; Chang and Vikesland, 2011). The influent concentration (Co) was measured without connecting the column while the effluent concentration (C ) was measured for the suspension at the outlet from the column. The attachment efficiency (a) for spherical nanoparticles was then calculated (Yao et al., 1971) as:
a¼
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2$dc C ln 3$h0 $L$ðε 1Þ C0
(2)
where h0 is the single-collector efficiency obtainable by using a correlation equation developed based on the numerical solution of the convective-diffusion equation governing the transport phenomena in the column used (Tufenkji and Elimelech, 2004), ε is the porosity of the porous media, dc the diameter of the collector, and L the length of the column. The attachment efficiency (a) indicates the probability of attachment of a particle onto a collector surface (glass bead) per collision.
2.5. ROS production of fullerene aggregates with HA and cations As described in the previous study, ROS production of fullerene suspensions (TC concentration was normalized to 1 mg/L) was studied in a glass beaker (90, O. D. 115 mm, Length) with a water jacket connected to a water circulator for temperature control at 25 C (Chae et al., 2009, 2011). The light sources was two 15-W fluorescent UV bulbs (Philips TLD 15W/ 08) in an UV/Cryo chamber (Electron Microscopy Science, Hatfield, PA). These bulbs had an output spectrum ranging from 310 to 400 nm and a total irradiance of 24.1 W/m2 with a peak at 365 nm (UV-A).
Fig. 1 e TEM images of nSWCNT in DI water with various sonication times (A: at time zero, B: after 1 h, C: after 5 h, and D: after 10 h).
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Hydrodynamic diameter (nm)
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nC60 nSWCNT nMWCNT nC60(OH)24
3000 2500 2000 1500 1000 500 0
A
B
C
D
E
F
Fig. 2 e Variation in hydrodynamic diameter (dh) of fullerene aggregates (TC [ 1 mg/L) treated with electrolytes and humic acid (A: fullerene aggregates in DI water, B: fullerene aggregates in 0.3 M NaCl, C: fullerene aggregates in 0.1 M MgCl2, D: fullerene aggregates with HA (10 mg/L) in DI water, E: fullerene aggregates with HA (10 mg/L) in 0.3 M NaCl, and F: fullerene aggregates with HA (10 mg/L) in 0.1 M MgCl2). All values are means ± 95% confidence interval (n [ 3).
Singlet oxygen sensor green (SOSG, Molecular Probes, Inc., Eugene, OR) was used to measure singlet oxygen (1O2) concentrations in the fullerene suspensions (Flors et al., 2006). The fluorescence units are sample measurements compared against
a control sample in the dark used as a background (Ex/Em ¼ 504/ 525, Modulus Single Tube 9200, Turner Biosystems, Sunnyvale, CA). XTT (2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) reduction was employed to measure the production of superoxide (O 2 ). The reduction of XTT results in an increase in optical density at 470 nm that can be used to quantify the relative amount of superoxide present (Bartosz, 2006; Ukeda et al., 1997). Water samples were place in the UV/Cryo chamber and were collected from the suspension every 5 min for 30 min for further analyses. All experiments were performed in triplicate. From a separate batch test, it was found that 10 mg/L HA produced about 0.4 a.u./s of singlet oxygen (data not shown). The net fluorescence increase rate of each fullerene suspension is calculated by subtracting the fluorescence increase rate by HA and/or electrolytes without nanoparticles from the fluorescence increase rate of each sample in the presence of HA and/or electrolytes under UV irradiation.
3.
Results and discussion
3.1. Variation in characteristics of fullerene suspensions by HA and electrolytes The level of surface hydroxylation and oxidation by sonication affects the surface potential, dispersion size, and aggregation kinetics of fullerene C60 aggregates (Chae et al., 2010a). As shown in Fig. 1, as sonication time increased, pristine powder
Fig. 3 e TEM images of nC60 (TC [ 1 mg/L) (A: nC60 in DI water, B: nC60 in 0.3 M NaCl, C: nC60 in 0.1 M MgCl2, D: nC60 with HA (10 mg/L) in DI water, E: nC60 with HA (10 mg/L) in 0.3 M NaCl, and F: nC60 with HA (10 mg/L) in 0.1 M MgCl2).
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of SWCNT was broken down and formed colloidal aggregates (nSWCNT). In DI water, size (i.e., hydrodynamic diameter, dh) of four fullerene aggregates averaged between 340 (nC60) and 610 (nMWCNT) nm (Fig. 2a). As reported in the previous study (Chae et al., 2011), the MWCNT formed larger aggregates at a given level of sonication than SWCNT. The size increased when electrolytes (i.e., 0.3 M NaCl or 0.1 M MgCl2) were added to the fullerene suspensions (Fig. 2b,c). However, the size decreased to less than 400 nm in the presence of HA (Fig. 2d). Although the sonication of fullerene nanoparticles is not environmentally relevant conditions, it has been often employed to prepare aqueous suspension of ENMs. Here, we explore the effects of HA and electrolytes on physical and chemical properties on the sonicated fullerene nanoparticles in the aqueous environment. NOM is highly heterogeneous and the composition of NOM varies widely across ecosystems. Although, fulvic acid (FA) may be more representative than HA in some natural settings, Suwannee River Humic Acid (SRHA) was selected as a model HA in this study because it has been well characterized (Waples et al. 2005 ) allowing for readily reproducible studies. When the sonicated aggregates were exposed to HA with electrolytes, their size dramatically increased to over 1.5 mm (Fig. 2e,f). In particular, it was found that the fullerol nanoparticles (C60(OH)24) formed bigger aggregates than the other fullerene nanoparticles in the presence of HA and MgCl2. TEM
Electrophoretic mobility (10-8 m2/Vs)
Fig. 4 e TEM images of nC60(OH)24 (TC [ 1 mg/L) (A: nC60(OH)24 in DI water, B: nC60(OH)24 in 0.3 M NaCl, C: nC60(OH)24 in 0.1 M MgCl2, and D: nC60(OH)24 with HA (10 mg/L) in 0.1 M MgCl2).
0 -1 -2 -3 nC60 nSWCNT nMWCNT nC60(OH)24
-4 -5 A
B
C
D
E
F
Fig. 5 e Variation in electrophoretic mobility (EPM) of fullerene aggregates (TC [ 1 mg/L) treated with electrolytes and humic acid (A: fullerene aggregates in DI water, B: fullerene aggregates in 0.3 M NaCl, C: fullerene aggregates in 0.1 M MgCl2, D: fullerene aggregates with HA (10 mg/L) in DI water, E: fullerene aggregates with HA (10 mg/L) in 0.3 M NaCl, and F: fullerene aggregates with HA (10 mg/L) in 0.1 M MgCl2). All values are means ± 95% confidence interval (n [ 3).
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Table 1 e Effects of MgCl2 and humic acid on relative hydrophobicity of nC60. Sample nC60 in DI water nC60 with HA (1 mg/L) in DI water nC60 with HA (5 mg/L) in DI water nC60 with HA (10 mg/L) in DI water nC60 with HA (10 mg/L) in 0.1 M MgCl2 nC60(OH)24 in DI water
Cw (mg/L) Ct (mg/L) Ktw 5.0 5.0 5.0 5.0 5.0 5.0
3.9 3.6 3.4 3.3 3.8 n.m.
0.78 0.72 0.68 0.66 0.76 n.m.
n.m. (not measurable): the concentration of C60 in the toluene was below detection limit by HPLC (i.e., 15 mg/L).
images of both nC60 and nC60(OH)24 confirmed trends in aggregated diameter with HA and electrolytes (Figs. 3 and 4). However, the structural properties of the two nanoparticle aggregates appear to be different. The nC60 formed a more dense structure showing less surface boundary layer than nC60(OH)24 under the same condition. Variations in EPM (Fig. 5) and relative hydrophobicity (i.e., Ktw) (Table 1) of fullerene nanoparticles are also consistent with the behaviors of nanoparticle aggregates in the presence of HA and electrolytes. As shown in Fig. 5a,b, all
Fig. 6 e Effects of humic acid and electrolytes on transport of nC60 (TC [ 1 mg/L) through a silica porous medium (A: nC60 with HA (1 mg/L) in DI water, B: nC60 with HA (5 mg/L) in DI water, C: nC60 with HA (10 mg/L) in DI water, D: nC60 with HA (10 mg/L) in 0.3 M NaCl, and E: nC60 with HA (10 mg/L) in 0.1 M MgCl2). All values are means ± 95% confidence interval (n [ 3).
Fig. 7 e ROS production from fullerene aggregates (TC [ 1 mg/L) treated with electrolytes and humic acid (10 mg/L) (A: nC60, B: nC60(OH)24, C: nSWCNT, and D: nMWCNT). All values are means ± 95% confidence interval (n [ 3).
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Table 2 e Schematic diagram of aggregation states of nC60 with humic acid and MgCl2.
fullerene nanoparticles are negatively charged and HA increased stability of the aggregates. A higher absolute value of EPM is consistent with a higher degree of functionalization associated with the surface of the aggregates due to either hydroxylation in the case of the fullerol, or adsorption of HA. A greater degree of functionalization also
corresponds to decreased relative hydrophobicity (as evidenced by Ktw in Table 1). However, when the electrolytes were added to the suspensions, the EPM dramatically decreased (Fig. 5b,c,e,f). Differences between HA and FA (molecular size, surface property, aromaticity and functional groups) may influence
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the NOM-fullerene interaction. A hypothesis that remains to be tested is whether or not fullerenes will behave differently during transport and photosensitization processes in the presence of FA, relative to HA, particularly given the higher degree of aromaticity in HA which may enhance photosensitization.
3.2.
Effect of HA and electrolytes on transport of nC60
The variation in structural properties of fullerene aggregates affected transport of the nanoparticles through a silica porous medium, which is a negatively charged collector. As shown in Fig. 6, when the HA was added to the suspensions, the affinity, as represented by attachment efficiency (a) between the fullerene aggregates (nC60) and the collector, decreased slightly. However, electrolytes increased the attachment efficiency in the presence of HA (10 mg/L) and divalent ion (MgCl2) was more efficient than monovalent ion (NaCl).
3.3. Effect of HA and electrolytes on ROS production of fullerene aggregates We explored ROS production of fullerene aggregates using dye agents such as XTT and SOSG. By measuring the reduction of XTT, it was confirmed that superoxide production in the suspension was negligible (data not shown) due to the lack of an electron donor to produce the intermediate radical in the system. However, singlet oxygen was produced in all fullerene suspensions as measured by fluorescence of SOSG (Fig. 7). As shown in Fig. 7, all samples produced the highest level of fluorescence without HA and electrolytes. The highest level of ROS was produced by nC60(OH)24 followed by nC60, nSWCNT and nMWCNT. But, when electrolytes were introduced, the ROS production of the four nanoparticles significantly decreased (again, MgCl2 had greater effect than NaCl). Singlet oxygen production of nC60 and nC60(OH)24 in DI water without HA and electrolytes was approximately 7 and 15 times greater than that of nC60 and nC60(OH)24 in the presence of HA and electrolytes. The hypothesized states of fullerene aggregates for different behaviors and photocatalytic ability are summarized in Table 2. In DI water without HA and electrolytes, all nanoparticles (types A and B in Table 2) produced the highest level of ROS. In our previous study (Chae et al., 2010a), it was found that as the size of nC60 decreased, the fraction of C60 on the surface of the aggregates increased, and the density of nC60 aggregates as expressed by D2 decreased. These factors lead to a decrease in tripletetriplet annihilation and quenching by ground state C60 in the aggregate that allow for greater ROS production. When the HA (10 mg/L) was introduced, the size of nanoparticle aggregates diminished. This was probably due to static repulsion induced by the HA having aromatic backbones as reported in the previous study (Chen and Elimelech, 2007). Although the fullerene nanoparticles formed the smallest aggregates in the presence of HA, the ROS production from these aggregates was smaller than that of the aggregates in DI water without HA. Again, the fluorescence increase rate of HA by 1O2 production was about 0.4 a.u./s, implying that the HA
competed with fullerene nanoparticles for electrons to produce ROS, and that it could retard the photocatalytic ability of fullerene nanoparticles. In the presence of electrolytes, the size and EPM of fullerene aggregates increased and they became less negative. Both of which are consistent with the observations made in previous studies (Brant et al., 2005; Chen and Elimelech, 2009), where there was a decrease in ROS production and increase in the attachment efficiency of the aggregates on silica beads. Divalent ions were more effective compared with monovalent ions. When fullerene nanoparticles were exposed to HA along with cations, the nanoparticles formed the biggest aggregates observed likely due to the electrolyte binding to the carboxylic functional groups of HA adsorbed on the nanoparticles (Chen and Elimelech, 2007) and the ROS production was significantly reduced (almost negligible). This effect was more significant with nC60(OH)24 (type B in Table 2) than the other aggregates (i.e., nC60, nSWCNT and nMWCNT) due to the initial structural properties of nC60(OH)24. These aggregates are less dense and have more surface boundary layer than the others, resulting in formation of bigger aggregates than the Type A aggregates in the presence of HA and electrolytes (Fig. 2). In summary, the smaller aggregates of fullerene nanoparticles were more hydrophilic than their larger counterparts, thus had lower attachment efficiency on silica, which contributed to higher production of ROS in DI water. In the presence of HA and electrolytes, however, the ROS production from four different fullerene aggregates significantly decreased (Table 2) and it likely leads to less toxicity to microorganisms in the environments.
4.
Conclusions
Fullerene-based nanomaterials are emerging in a variety of potential applications, including cosmetics, energy production, semiconductors, and medical treatments. However, while their applications are beneficial to society, it is clear that the substantial production of these nanomaterials will likely lead to environmental exposure with unknown consequences and potential health risk to humans. In this study, alteration of physical and chemical characteristics of carbon nanoparticle aggregates is studied in relation to transport and photosensitivity of the aggregates in the presence of HA and/or electrolytes. As a result, it was found that the presence of HA and salts increased the size of aggregates reducing transport of fullerene aggregates through a silica porous media, and decreasing ROS production. Contrary to the common notion that humic materials prevent aggregation of nanoparticles, an abundance and variety of divalent ions in aquatic environments may lead to an increase in the size of nanoparticle aggregates in the presence of NOM. While the final fate of these aggregates and long-term interactions with environmental factors such as NOM and microorganisms introduce additional uncertainties, this work underscores the complex web of transformations that nanomaterials may undergo in the environment and the impacts of these transformations on nanoparticle reactivity.
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Acknowledgments This study was supported by Korea Ministry of Environment as "The Eco-Innovation project (Global Top project, Project no. GT-SWS-11-01-002-0) and the Basic Science Research Program through National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110013792). This material is based upon work also supported by the National Science Foundation (NSF) and the Environmental Protection Agency (EPA) under NSF Cooperative Agreement EF-0830093, Center for the Environmental Implications of NanoTechnology (CEINT). Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF or the EPA. This work has not been subjected to EPA review and no official endorsement should be inferred.
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Chen, K.L., Smith, B.A., Ball, W.P., Fairbrother, D.H., 2010. Assessing the colloidal properties of engineered nanoparticles in water: case studies from fullerene C-60 nanoparticles and carbon nanotubes. Environmental Chemistry 7, 10e27. Flors, C., Fryer, M.J., Waring, J., Reeder, B., Bechtold, U., Mullineaux, P.M., Nonell, S., Wilson, M.T., Baker, N.R., 2006. Imaging the production of singlet oxygen in vivo using a new fluorescent sensor, Singlet Oxygen Sensor Green. Journal of Experimental Botany 57, 1725e1734. 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., 2005. C-60 in water: Nanocrystal formation and microbial response. Environmental Science & Technology 39, 4307e4316. Hendren, C.O., Mesnard, X., Droge, J., Wiesner, M.R., 2011. Estimating production data for five engineered nanomaterials as a basis for exposure assessment. Environmental Science & Technology 45, 2562e2569. Hotze, E.M., Bottero, J.-Y., Wiesner, M.R., 2010. Theoretical framework for nanoparticle reactivity as a function of aggregation state. Langmuir 26, 11170e11175. Hotze, E.M., Labille, J., Alvarez, P., Wiesner, M.R., 2008. Mechanisms of photochemistry and reactive oxygen production by fullerene suspensions in water. Environmental Science & Technology 42, 4175e4180. Hyung, H., Fortner, J.D., Hughes, J.B., Kim, J.H., 2007. Natural organic matter stabilizes carbon nanotubes in the aqueous phase. Environmental Science & Technology 41, 179e184. Isaacson, C.W., Bouchard, D.C., 2010. Effects of humic acid and sunlight on the generation and aggregation state of Aqu/C(60) nanoparticles. Environmental Science & Technology 44, 8971e8976. Kasermann, F., Kempf, C., 1997. Photodynamic inactivation of enveloped viruses by buckminsterfullerene. Antiviral Research 34, 65e70. Li, M.H., Huang, C.P., 2010. Stability of oxidized single-walled carbon nanotubes in the presence of simple electrolytes and humic acid. Carbon 48, 4527e4534. Li, Q.L., Xie, B., Hwang, Y.S., Xu, Y.J., 2009. Kinetics of C(60) fullerene dispersion in water enhanced by natural organic matter and sunlight. Environmental Science & Technology 43, 3574e3579. Mauter, M.S., Elimelech, M., 2008. Environmental applications of carbon-based nanomaterials. Environmental Science & Technology 42, 5843e5859. Qu, X.L., Hwang, Y.S., Alvarez, P.J.J., Bouchard, D., Li, Q.L., 2010. UV irradiation and humic acid mediate aggregation of aqueous fullerene (nC(60)) nanoparticles. Environmental Science & Technology 44, 7821e7826. Saleh, N.B., Pfefferle, L.D., Elimelech, M., 2008. Aggregation kinetics of multiwalled carbon nanotubes in aquatic systems: measurements and environmental implications. Environmental Science & Technology 42, 7963e7969. Sayes, C.M., Gobin, A.M., Ausman, K.D., Mendez, J., West, J.L., Colvin, V.L., 2005. Nano-C-60 cytotoxicity is due to lipid peroxidation. Biomaterials 26, 7587e7595. Terashima, M., Nagao, S., 2007. Solubilization of 60 fullerene in water by aquatic humic substances. Chemistry Letters 36, 302e303. Tufenkji, N., Elimelech, M., 2004. Correlation equation for predicting single-collector efficiency in physicochemical filtration in saturated porous media. Environmental Science & Technology 38, 529e536. Ukeda, H.H., Maeda, S.S., Ishii, T.T., Sawamura, M.M., 1997. Spectrophotometric assay for superoxide dismutase based on tetrazolium salt 30 -1-(phenylamino)-carbonyl-3,4tetrazolium]-bis(4-methoxy-6-nitro)benzenesulfonic acid
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Effects of humic acid and electrolytes on photocatalytic reactivity and transport of carbon nanoparticle aggregates in water So-Ryong Chae a,*, Yao Xiao b, Shihong Lin b, Tahereh Noeiaghaei a, Jong-Oh Kim c, Mark R. Wiesner b a
School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia Department of Civil and Environmental Engineering, Pratt School of Engineering, Duke University, Durham, NC 27708, USA c Department of Civil Engineering, Gangneung-Wonju National University, Gangneung, Gangwon-do 210-702, South Korea b
article info
abstract
Article history:
The effects of naturally occurring macromolecules such as humic acid (HA) and electro-
Received 12 July 2011
lytes on four fullerene nanoparticle suspensions (i.e., C60, C60(OH)24, single- and multiwall
Received in revised form
carbon nanotubes) were explored with respect to: (1) characteristics of nanoparticle
7 May 2012
aggregates, (2) transport of the aggregates through a silica porous media, and (3) production
Accepted 10 May 2012
of reactive oxygen species (ROS) from the photosensitized fullerene aggregates. The
Available online 18 May 2012
presence of HA and salts increased the size of aggregates and relative hydrophobicity associated with transport through silica beads, while decreasing ROS production. These
Keywords:
data illustrate the importance that transformation of engineered nanomaterials (ENMs)
Fullerene nanoparticle
through interactions with aquatic solutes may have in altering the environmental behavior
Carbon nanotubes
of nanomaterials. ª 2012 Elsevier Ltd. All rights reserved.
Humic acid Electrolytes Reactive oxygen species Transport through silica beads
1.
Introduction
Engineered nanomaterials (ENMs) are being produced in increasing quantities for a wide range of applications (Hendren et al., 2011). In particular, fullerene nanomaterials (FNMs) including C60, C60(OH)24, C70, and carbon nanotubes (CNTs) show unique physical, chemical, and photochemical properties, which are applicable for water treatment, energy production, and biomedical drug carriers (Bosi et al., 2003; Mauter and Elimelech, 2008). The production and use of FNMs will likely lead to environmental exposures. The increasing use of FNMs has raised concerns over their photoreactivity producing reactive oxygen species (ROS) such as singlet oxygen (1O2) and superoxide (O 2 ).
ROS produced from FNMs have been shown to oxidize organic compounds (Chae et al., 2009; Hotze et al., 2008) and inactivate bacteria and viruses (Badireddy et al., 2007; Kasermann and Kempf, 1997; Sayes et al., 2005; Yamakoshi et al., 2003). Like many other nanoparticles, FNMs tend to form colloidal aggregates in water (Brant et al., 2005; Fortner et al., 2005; Hotze et al., 2010). Given that some of these aggregates possess properties that differ from the pristine nanomaterials (Auffan et al., 2009; Chae et al., 2010a), an evaluation of the fate and impacts of FNMs in the aquatic environment requires an understanding of the effects of aggregate properties such as size, fractal structure, and surface charges on transport of the nanoparticles, as well as their interactions with environmental elements such as soil, natural organic and inorganic substances, and biomass.
* Corresponding author. Tel.: þ61 2 9351 3832; fax: þ61 2 9351 2854. E-mail address: [email protected] (S.-R. Chae). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.05.018
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Recently, it was found that the production of ROS, microbial inactivation, and the mobility of the aggregates of fullerene (nC60) in a silicate porous medium all increased as the size of aggregates decreased (Chae et al., 2010a). As nanoparticles make their way through the environment, they will come into contact with, and possible interaction with, a variety of solutes including natural organic matter (NOM) and mono- and multivalent cations and ions. Soluteenanoparticle interactions at the surrounding environment have the potential of transforming nanoparticles with resultant impacts on the fate, transport, and toxicity (Chen et al., 2010). For example, the presence of humic acid (HA) has been observed to increase colloidal stability of nC60 and reduce aggregate size (Hyung et al., 2007; Terashima and Nagao, 2007), even the dispersion process is further accelerated by sunlight (Isaacson and Bouchard, 2010; Li et al., 2009) or ultraviolet (UV) irradiation (Qu et al., 2010). Also, an increase in electrolyte concentrations increased the size of nC60 and reduced electrophoretic mobility (EPM; Chen et al., 2010). For the co-effects of HA and electrolytes on the aggregation kinetics of nC60, Chen and Elimelech (2007) reported that HA significantly reduced the aggregation kinetics of nC60 at low concentrations of NaCl or MgCl2. However, it was also found that HA enhanced the destabilization of nC60 in the presence of high concentrations of CaCl2 (>10 mM) by making bridges between nC60 aggregates. Similarly, HA and electrolytes affect CNT stability, with aggregation favored by increasing salt concentration and the presence of divalent cations, while HA impedes the aggregation of single- (SW) and multiwall (MW) CNTs (Li and Huang, 2010; Saleh et al., 2008). Aggregation of fullerenes has been related through theory and observation to their reactivity through photosensitization (Hotze et al., 2010). Changes in aggregation behavior of ENMs are strongly related to changes in their relative affinity for a porous medium, since both deposition and aggregation are related to the surface chemistry of the ENMs and their aggregates. Although it has been recently reported that the aggregates of CNTs (i.e., nSWCNT and nMWCNT) produce ROS and degrade 2-chlorophenol (Chae et al., 2011), the effects of HA and electrolytes on ROS production from the aggregates of CNTs are largely unknown. In the current study, we consider the effects of HA and electrolytes on mobility of four different carbon nanoparticle aggregates (i.e., nC60, nC60(OH)24, nSWCNT, and nMWCNT) through a silica porous medium associated with stability of nanoparticle aggregates, and photosensitivity of the aggregates to produce ROS in water.
2.
Materials and methods
2.1.
Spherical and tubular fullerene aggregates
Two spherical FNMs, C60 and C60(OH)24, were purchased from MER (99.9%, Tucson, AZ) and two tubular FNMs, SWCNT and MWCNT, were obtained from BuckyUSA (þ99.5%, Huston, TX). All fullerene suspensions were prepared in deionized (DI) water that had a resistivity of 18.2 MU-cm and dissolved organic carbon concentration was less than 50 ppb (Nanopure, Barnstead, Dubuque, IA). For nC60(OH)24 stock suspension was prepared by
adding pristine powder form to DI water and stirred at 200 rpm for 24 h. Aqueous suspensions of nC60, nSWCNT, and nMWCNT were prepared by sonication with a high-energy probe (S-4000, Misonix, Qsonica, LLC, Newtown, CT) for 10 h (20 min (pulse on)/ 10 min (pulse off), applied energy into 1 L of suspension was approximately 2.2 MJ) without any organic solvent addition. The initial concentration of nC60, nC60(OH)24, nSWCNT, and nMWCNT suspensions were 5.8, 9.3 2.7, and 2.1 mg/L as total carbon (TC), respectively. All suspensions were filtered through a microfiltration membrane having nominal pore size of 800 nm (Nylaflo, Pall Life Sciences, Port Washington, NY) and stored in a refrigerator at 4 C before use.
2.2. Preparation fullerene suspensions with electrolytes and HA The electrolyte stock solutions (3 M of NaCl or 1 M of MgCl2) were prepared in DI water and were filtered through 0.22 mm membrane (Nylaflo, Pall Life Sciences, Port Washington, NY) before use. The HA stock solution (1000 mg/L) was prepared by adding 100 mg of HA powder (Suwannee River Humic Acid Standard II, International Humic Substances Society) to 100 ml of DI water and stirring the solution overnight. A volume of 0.5 mL of the HA stock suspension or DI water (without HA) was added into 44.5 mL of the fullerene suspensions and stirring for 2 h. Then, the fullerene suspensions were mixed with 5 mL of NaCl or MgCl2 stock solutions to adjust final concentration of 0.3 M NaCl or 0.1 M MgCl2. The pH of the final suspensions was adjusted to 7.0e7.5 by adding NaOH before use.
2.3. Characterization of fullerene nanoparticle suspensions The TC concentrations of the fullerene suspensions were measured by a total organic carbon (TOC) analyzer (TOC5050A, Shimadzu, Columbia, MD). EPM and hydrodynamic diameter (dh) of nanoparticle aggregates were measured by Zetasizer Nano (Malvern Instrument, Bedford, MA). High magnification images of the suspensions are obtained by TEM (FEI Tecnai G2 Twin, Hillsboro, OR). Ten microliters of each sample was dropped on a lacey carbon/Cu grid (300 meshes, Electron Microscopy Sciences, Hatfield, PA) and dried in air before TEM measurement. Image analysis of TEM images was performed to evaluate mean diameter (dt) of fullerene aggregates (n ¼ 100) by using an Image-Pro version 4.5 (Media Cybermetics, Inc. Bethesda, MD). The hydrophobicity coefficient (Ktw) of the fullerenes in suspensions was determined as a basis for quantifying the relative affinity of these materials for the aqueous phase as described in the previous study (Chae et al., 2010a). The Ktw was calculated by the following equation. Ct (1) Ktw ¼ Cw where Ct is the concentration of fullerene in toluene and Cw is the concentration of fullerene in water. The concentration of fullerene nanoparticles was measured by liquideliquid extraction using toluene coupled with a high-performance liquid chromatography analysis as described in the previous studies (Chae et al., 2010b; Xiao et al., 2011).
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 4 0 5 3 e4 0 6 2
2.4. Transport of fullerene aggregates through a silicate porous medium Transport of fullerene aggregates through a silica porous medium was evaluated to quantify the relative affinity of each fraction of these fullerenes for the medium as described previously (Chae et al., 2010a). A chromatography column (10 mm outer diameter 40 mm length, GE Health care, Piscataway, NJ) packed with spherical glass beads (Potters Industries Inc., Berwyn, PA) having nominal diameter of approximately 360 mm was used for this purpose. The background solution in this study was ultrapure water driven by a peristaltic pump (Cole-Parmer Instrument Company, Chicago, IL) at a flow rate of 0.96 ml/min as compared to the flow rate of 0.04 ml/min of the fullerene suspension (TC concentration was normalized to 1 mg/L) driven by a syringe pump (Harvard Apparatus, Holliston, MA). To quantify the relative fullerene concentration, the UV absorbance of fullerene was continuously measured by UVevis spectrophotometer (UVevis 2810, Hitachi, Pleasanton, CA) at the wavelength of 198 nm, which is the maximum absorbance peak among three peaks at 198, 285, and 334 nm (Chae et al., 2010a; Chang and Vikesland, 2011). The influent concentration (Co) was measured without connecting the column while the effluent concentration (C ) was measured for the suspension at the outlet from the column. The attachment efficiency (a) for spherical nanoparticles was then calculated (Yao et al., 1971) as:
a¼
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2$dc C ln 3$h0 $L$ðε 1Þ C0
(2)
where h0 is the single-collector efficiency obtainable by using a correlation equation developed based on the numerical solution of the convective-diffusion equation governing the transport phenomena in the column used (Tufenkji and Elimelech, 2004), ε is the porosity of the porous media, dc the diameter of the collector, and L the length of the column. The attachment efficiency (a) indicates the probability of attachment of a particle onto a collector surface (glass bead) per collision.
2.5. ROS production of fullerene aggregates with HA and cations As described in the previous study, ROS production of fullerene suspensions (TC concentration was normalized to 1 mg/L) was studied in a glass beaker (90, O. D. 115 mm, Length) with a water jacket connected to a water circulator for temperature control at 25 C (Chae et al., 2009, 2011). The light sources was two 15-W fluorescent UV bulbs (Philips TLD 15W/ 08) in an UV/Cryo chamber (Electron Microscopy Science, Hatfield, PA). These bulbs had an output spectrum ranging from 310 to 400 nm and a total irradiance of 24.1 W/m2 with a peak at 365 nm (UV-A).
Fig. 1 e TEM images of nSWCNT in DI water with various sonication times (A: at time zero, B: after 1 h, C: after 5 h, and D: after 10 h).
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Hydrodynamic diameter (nm)
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nC60 nSWCNT nMWCNT nC60(OH)24
3000 2500 2000 1500 1000 500 0
A
B
C
D
E
F
Fig. 2 e Variation in hydrodynamic diameter (dh) of fullerene aggregates (TC [ 1 mg/L) treated with electrolytes and humic acid (A: fullerene aggregates in DI water, B: fullerene aggregates in 0.3 M NaCl, C: fullerene aggregates in 0.1 M MgCl2, D: fullerene aggregates with HA (10 mg/L) in DI water, E: fullerene aggregates with HA (10 mg/L) in 0.3 M NaCl, and F: fullerene aggregates with HA (10 mg/L) in 0.1 M MgCl2). All values are means ± 95% confidence interval (n [ 3).
Singlet oxygen sensor green (SOSG, Molecular Probes, Inc., Eugene, OR) was used to measure singlet oxygen (1O2) concentrations in the fullerene suspensions (Flors et al., 2006). The fluorescence units are sample measurements compared against
a control sample in the dark used as a background (Ex/Em ¼ 504/ 525, Modulus Single Tube 9200, Turner Biosystems, Sunnyvale, CA). XTT (2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) reduction was employed to measure the production of superoxide (O 2 ). The reduction of XTT results in an increase in optical density at 470 nm that can be used to quantify the relative amount of superoxide present (Bartosz, 2006; Ukeda et al., 1997). Water samples were place in the UV/Cryo chamber and were collected from the suspension every 5 min for 30 min for further analyses. All experiments were performed in triplicate. From a separate batch test, it was found that 10 mg/L HA produced about 0.4 a.u./s of singlet oxygen (data not shown). The net fluorescence increase rate of each fullerene suspension is calculated by subtracting the fluorescence increase rate by HA and/or electrolytes without nanoparticles from the fluorescence increase rate of each sample in the presence of HA and/or electrolytes under UV irradiation.
3.
Results and discussion
3.1. Variation in characteristics of fullerene suspensions by HA and electrolytes The level of surface hydroxylation and oxidation by sonication affects the surface potential, dispersion size, and aggregation kinetics of fullerene C60 aggregates (Chae et al., 2010a). As shown in Fig. 1, as sonication time increased, pristine powder
Fig. 3 e TEM images of nC60 (TC [ 1 mg/L) (A: nC60 in DI water, B: nC60 in 0.3 M NaCl, C: nC60 in 0.1 M MgCl2, D: nC60 with HA (10 mg/L) in DI water, E: nC60 with HA (10 mg/L) in 0.3 M NaCl, and F: nC60 with HA (10 mg/L) in 0.1 M MgCl2).
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of SWCNT was broken down and formed colloidal aggregates (nSWCNT). In DI water, size (i.e., hydrodynamic diameter, dh) of four fullerene aggregates averaged between 340 (nC60) and 610 (nMWCNT) nm (Fig. 2a). As reported in the previous study (Chae et al., 2011), the MWCNT formed larger aggregates at a given level of sonication than SWCNT. The size increased when electrolytes (i.e., 0.3 M NaCl or 0.1 M MgCl2) were added to the fullerene suspensions (Fig. 2b,c). However, the size decreased to less than 400 nm in the presence of HA (Fig. 2d). Although the sonication of fullerene nanoparticles is not environmentally relevant conditions, it has been often employed to prepare aqueous suspension of ENMs. Here, we explore the effects of HA and electrolytes on physical and chemical properties on the sonicated fullerene nanoparticles in the aqueous environment. NOM is highly heterogeneous and the composition of NOM varies widely across ecosystems. Although, fulvic acid (FA) may be more representative than HA in some natural settings, Suwannee River Humic Acid (SRHA) was selected as a model HA in this study because it has been well characterized (Waples et al. 2005 ) allowing for readily reproducible studies. When the sonicated aggregates were exposed to HA with electrolytes, their size dramatically increased to over 1.5 mm (Fig. 2e,f). In particular, it was found that the fullerol nanoparticles (C60(OH)24) formed bigger aggregates than the other fullerene nanoparticles in the presence of HA and MgCl2. TEM
Electrophoretic mobility (10-8 m2/Vs)
Fig. 4 e TEM images of nC60(OH)24 (TC [ 1 mg/L) (A: nC60(OH)24 in DI water, B: nC60(OH)24 in 0.3 M NaCl, C: nC60(OH)24 in 0.1 M MgCl2, and D: nC60(OH)24 with HA (10 mg/L) in 0.1 M MgCl2).
0 -1 -2 -3 nC60 nSWCNT nMWCNT nC60(OH)24
-4 -5 A
B
C
D
E
F
Fig. 5 e Variation in electrophoretic mobility (EPM) of fullerene aggregates (TC [ 1 mg/L) treated with electrolytes and humic acid (A: fullerene aggregates in DI water, B: fullerene aggregates in 0.3 M NaCl, C: fullerene aggregates in 0.1 M MgCl2, D: fullerene aggregates with HA (10 mg/L) in DI water, E: fullerene aggregates with HA (10 mg/L) in 0.3 M NaCl, and F: fullerene aggregates with HA (10 mg/L) in 0.1 M MgCl2). All values are means ± 95% confidence interval (n [ 3).
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Table 1 e Effects of MgCl2 and humic acid on relative hydrophobicity of nC60. Sample nC60 in DI water nC60 with HA (1 mg/L) in DI water nC60 with HA (5 mg/L) in DI water nC60 with HA (10 mg/L) in DI water nC60 with HA (10 mg/L) in 0.1 M MgCl2 nC60(OH)24 in DI water
Cw (mg/L) Ct (mg/L) Ktw 5.0 5.0 5.0 5.0 5.0 5.0
3.9 3.6 3.4 3.3 3.8 n.m.
0.78 0.72 0.68 0.66 0.76 n.m.
n.m. (not measurable): the concentration of C60 in the toluene was below detection limit by HPLC (i.e., 15 mg/L).
images of both nC60 and nC60(OH)24 confirmed trends in aggregated diameter with HA and electrolytes (Figs. 3 and 4). However, the structural properties of the two nanoparticle aggregates appear to be different. The nC60 formed a more dense structure showing less surface boundary layer than nC60(OH)24 under the same condition. Variations in EPM (Fig. 5) and relative hydrophobicity (i.e., Ktw) (Table 1) of fullerene nanoparticles are also consistent with the behaviors of nanoparticle aggregates in the presence of HA and electrolytes. As shown in Fig. 5a,b, all
Fig. 6 e Effects of humic acid and electrolytes on transport of nC60 (TC [ 1 mg/L) through a silica porous medium (A: nC60 with HA (1 mg/L) in DI water, B: nC60 with HA (5 mg/L) in DI water, C: nC60 with HA (10 mg/L) in DI water, D: nC60 with HA (10 mg/L) in 0.3 M NaCl, and E: nC60 with HA (10 mg/L) in 0.1 M MgCl2). All values are means ± 95% confidence interval (n [ 3).
Fig. 7 e ROS production from fullerene aggregates (TC [ 1 mg/L) treated with electrolytes and humic acid (10 mg/L) (A: nC60, B: nC60(OH)24, C: nSWCNT, and D: nMWCNT). All values are means ± 95% confidence interval (n [ 3).
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Table 2 e Schematic diagram of aggregation states of nC60 with humic acid and MgCl2.
fullerene nanoparticles are negatively charged and HA increased stability of the aggregates. A higher absolute value of EPM is consistent with a higher degree of functionalization associated with the surface of the aggregates due to either hydroxylation in the case of the fullerol, or adsorption of HA. A greater degree of functionalization also
corresponds to decreased relative hydrophobicity (as evidenced by Ktw in Table 1). However, when the electrolytes were added to the suspensions, the EPM dramatically decreased (Fig. 5b,c,e,f). Differences between HA and FA (molecular size, surface property, aromaticity and functional groups) may influence
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the NOM-fullerene interaction. A hypothesis that remains to be tested is whether or not fullerenes will behave differently during transport and photosensitization processes in the presence of FA, relative to HA, particularly given the higher degree of aromaticity in HA which may enhance photosensitization.
3.2.
Effect of HA and electrolytes on transport of nC60
The variation in structural properties of fullerene aggregates affected transport of the nanoparticles through a silica porous medium, which is a negatively charged collector. As shown in Fig. 6, when the HA was added to the suspensions, the affinity, as represented by attachment efficiency (a) between the fullerene aggregates (nC60) and the collector, decreased slightly. However, electrolytes increased the attachment efficiency in the presence of HA (10 mg/L) and divalent ion (MgCl2) was more efficient than monovalent ion (NaCl).
3.3. Effect of HA and electrolytes on ROS production of fullerene aggregates We explored ROS production of fullerene aggregates using dye agents such as XTT and SOSG. By measuring the reduction of XTT, it was confirmed that superoxide production in the suspension was negligible (data not shown) due to the lack of an electron donor to produce the intermediate radical in the system. However, singlet oxygen was produced in all fullerene suspensions as measured by fluorescence of SOSG (Fig. 7). As shown in Fig. 7, all samples produced the highest level of fluorescence without HA and electrolytes. The highest level of ROS was produced by nC60(OH)24 followed by nC60, nSWCNT and nMWCNT. But, when electrolytes were introduced, the ROS production of the four nanoparticles significantly decreased (again, MgCl2 had greater effect than NaCl). Singlet oxygen production of nC60 and nC60(OH)24 in DI water without HA and electrolytes was approximately 7 and 15 times greater than that of nC60 and nC60(OH)24 in the presence of HA and electrolytes. The hypothesized states of fullerene aggregates for different behaviors and photocatalytic ability are summarized in Table 2. In DI water without HA and electrolytes, all nanoparticles (types A and B in Table 2) produced the highest level of ROS. In our previous study (Chae et al., 2010a), it was found that as the size of nC60 decreased, the fraction of C60 on the surface of the aggregates increased, and the density of nC60 aggregates as expressed by D2 decreased. These factors lead to a decrease in tripletetriplet annihilation and quenching by ground state C60 in the aggregate that allow for greater ROS production. When the HA (10 mg/L) was introduced, the size of nanoparticle aggregates diminished. This was probably due to static repulsion induced by the HA having aromatic backbones as reported in the previous study (Chen and Elimelech, 2007). Although the fullerene nanoparticles formed the smallest aggregates in the presence of HA, the ROS production from these aggregates was smaller than that of the aggregates in DI water without HA. Again, the fluorescence increase rate of HA by 1O2 production was about 0.4 a.u./s, implying that the HA
competed with fullerene nanoparticles for electrons to produce ROS, and that it could retard the photocatalytic ability of fullerene nanoparticles. In the presence of electrolytes, the size and EPM of fullerene aggregates increased and they became less negative. Both of which are consistent with the observations made in previous studies (Brant et al., 2005; Chen and Elimelech, 2009), where there was a decrease in ROS production and increase in the attachment efficiency of the aggregates on silica beads. Divalent ions were more effective compared with monovalent ions. When fullerene nanoparticles were exposed to HA along with cations, the nanoparticles formed the biggest aggregates observed likely due to the electrolyte binding to the carboxylic functional groups of HA adsorbed on the nanoparticles (Chen and Elimelech, 2007) and the ROS production was significantly reduced (almost negligible). This effect was more significant with nC60(OH)24 (type B in Table 2) than the other aggregates (i.e., nC60, nSWCNT and nMWCNT) due to the initial structural properties of nC60(OH)24. These aggregates are less dense and have more surface boundary layer than the others, resulting in formation of bigger aggregates than the Type A aggregates in the presence of HA and electrolytes (Fig. 2). In summary, the smaller aggregates of fullerene nanoparticles were more hydrophilic than their larger counterparts, thus had lower attachment efficiency on silica, which contributed to higher production of ROS in DI water. In the presence of HA and electrolytes, however, the ROS production from four different fullerene aggregates significantly decreased (Table 2) and it likely leads to less toxicity to microorganisms in the environments.
4.
Conclusions
Fullerene-based nanomaterials are emerging in a variety of potential applications, including cosmetics, energy production, semiconductors, and medical treatments. However, while their applications are beneficial to society, it is clear that the substantial production of these nanomaterials will likely lead to environmental exposure with unknown consequences and potential health risk to humans. In this study, alteration of physical and chemical characteristics of carbon nanoparticle aggregates is studied in relation to transport and photosensitivity of the aggregates in the presence of HA and/or electrolytes. As a result, it was found that the presence of HA and salts increased the size of aggregates reducing transport of fullerene aggregates through a silica porous media, and decreasing ROS production. Contrary to the common notion that humic materials prevent aggregation of nanoparticles, an abundance and variety of divalent ions in aquatic environments may lead to an increase in the size of nanoparticle aggregates in the presence of NOM. While the final fate of these aggregates and long-term interactions with environmental factors such as NOM and microorganisms introduce additional uncertainties, this work underscores the complex web of transformations that nanomaterials may undergo in the environment and the impacts of these transformations on nanoparticle reactivity.
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 4 0 5 3 e4 0 6 2
Acknowledgments This study was supported by Korea Ministry of Environment as "The Eco-Innovation project (Global Top project, Project no. GT-SWS-11-01-002-0) and the Basic Science Research Program through National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110013792). This material is based upon work also supported by the National Science Foundation (NSF) and the Environmental Protection Agency (EPA) under NSF Cooperative Agreement EF-0830093, Center for the Environmental Implications of NanoTechnology (CEINT). Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF or the EPA. This work has not been subjected to EPA review and no official endorsement should be inferred.
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