Elsevier

Water Research

Volume 43, Issue 9, May 2009, Pages 2463-2470
Water Research

Dispersion of C60 in natural water and removal by conventional drinking water treatment processes

https://doi.org/10.1016/j.watres.2009.03.011Get rights and content

Abstract

The first objective of this study is to examine the fate of C60 under two disposal scenarios through which pristine C60 is introduced to water containing natural organic matter (NOM). A method based on liquid–liquid extraction and HPLC to quantify nC60 in water containing NOM was also developed. When pristine C60 was added to water either in the form of dry C60 or in organic solvent, it formed water stable aggregates with characteristics similar to nC60 prepared by other methods reported in the literature. The second objective of this study is to examine the fate of the nC60 in water treatment processes, which are the first line of defense against ingestion from potable water – a potential route for direct human consumption. Results obtained from jar tests suggested that these colloidal aggregates of C60 were efficiently removed by a series of alum coagulation, flocculation, sedimentation and filtration processes, while the efficiency of removal dependent on various parameters such as pH, alkalinity, NOM contents and coagulant dosage. Colloidal aggregates of functionalized C60 could be well removed by the conventional water treatment processes but with lesser efficiency compared to those made of pristine C60.

Introduction

Carbon fullerenes such as C60 have been at the center of the recent prosperity in nanoscale science and engineering. C60 consists of 60 carbon atoms arranged in 20 hexagons and 12 pentagons that form a perfectly symmetrical cage structure, approximately 1 nm in size. With increasing commercial interest in its unique chemical and physical properties, the manufacture and use of fullerenes are expected to grow rapidly over the next decade (Ball, 2001, Colvin, 2003, Wiesner et al., 2006). However, as with many other engineered nanomaterials, information required to accurately assess the influence of fullerenes on the natural environment and human health is rather scarce. Such assessment requires major research efforts on multi-disciplinary, multi-scale subjects, including the physical and chemical status of C60 in the environmental media to which it is released, transport behaviors in both small and large scales, potential physical, biological and chemical transformations in both natural and engineered systems, and the ultimate interaction of pristine or transformed fullerene with various biological receptors.

Generally, C60 has not been considered a potential contaminant in aquatic systems since it is extremely hydrophobic and virtually non-wettable. However, recent findings have suggested that, upon release to water, C60 forms stable, nanoscale colloidal aggregates (commonly referred to as nano-C60 or nC60) of which the dispersion status is affected by natural water constituents (Chen and Elimelech, 2007, Deguchi et al., 2001, Fortner et al., 2005). Consequently, numerous studies have been conducted or are currently underway to assess the fate and transport characteristics of nC60 in natural and engineered environments, including chemical transformation in engineered oxidation processes (Fortner et al., 2007) and transport behavior in natural subsurface environments (Chen and Elimelech, 2007). Several recent studies (Lyon et al., 2006, Sayes et al., 2004) have also suggested that nC60 interacts with numerous types of cells (including human cells), potentially causing toxic effects.

These aggregate forms of C60 can be prepared using various laboratory procedures. In the solvent exchange method, C60 is first dissolved in a water miscible, polar organic solvent, such as tetrahydrofuran, and mixed with water. The transfer organic solvent is subsequently removed via distillation (Deguchi et al., 2001). This method has been widely applied to test the toxicity and reactivity of nC60 due to the high yield and control of aggregate size. Alternatively, nC60 can be formed by dissolving C60 in a water immiscible organic solvent such as toluene and applying ultrasound to gradually transfer C60 into water as the organic solvent is evaporated (Lee et al., 2007). Although the above widely used methods are relatively convenient and useful for the laboratory studies, they do not represent the most likely scenarios of C60 release to natural aqueous environments.

The first objective of this study is to examine the fate of C60 under two disposal and spillage scenarios through which pristine C60 is introduced to water containing natural organic matter (NOM): (1) introduction of dry phase C60 to the aqueous phase, and (2) contact of water immiscible organic solvent containing C60 (i.e., used for storage purposes) with the aqueous phase and subsequent inter-phase transfer of C60 from organic phase to the aqueous phase, both under simple and prolonged mixing (without application of unrealistic external factors, such as widely used sonication). Additionally, a method to quantify nC60 in water containing NOM was developed.

The second objective of this study is to examine the fate of these nC60 particles in water treatment processes, as these are the first line of defense against ingestion from potable water consumption – a potential route for direct human consumption. Jar tests were performed to evaluate the removal of nC60 by a conventional water treatment process, which consists of a suite of alum coagulation, flocculation, sedimentation and granular media filtration steps to remove particulate contaminants. The effects of NOM and water quality parameters such as pH and alkalinity on the removal of nC60 were evaluated.

Section snippets

Materials

C60 of over 99% purity was obtained from the MER Corporation (Tucson, AZ). The ACS grade (>99% purity) sodium dodecylsulfate (SDS, CH3(CH2)11OSO3Na) was purchased from Aldrich Chemical Company (Milwaukee, WI). The Suwannee River NOM (SRNOM) stock solution was prepared by mixing a known amount of SRNOM (International Humic Substances Society, St. Paul, MN) with ultrapure water for 24 h. Dissolution of SRNOM was facilitated by adding 1 N potassium hydroxide (KOH) to increase the solution pH to 7.0.

Dispersion of C60 in aqueous phase

Solid C60 mixed with all the aqueous phases (i.e., ultrapure water, 5 mg-C/L SRNOM solution, 50 mg-C/L SRNOM solution, and 1% SDS solution) formed stable suspensions (Fig. 1a). Characteristics of the C60 suspension were consistent with those of the nC60 reported in the literature (Deguchi et al., 2001, Fortner et al., 2005, Lyon et al., 2006). They showed characteristic orange–yellow color, while the intensity of the color varied depending on the solution composition. TEM analysis suggests that

Conclusions

The results of this study illustrate that in the event of a large scale release of pristine C60 into a natural water body – either in the form of dry C60 or in organic solvent – it will form water stable aggregates in the presence of NOM, with characteristics similar to nC60 prepared by other methods reported in the literature. NOM is expected to play critical role on the transfer of C60 in aqueous phase either hindering or facilitating dissolution of C60 depending on disposal scenario. These

Acknowledgements

All the work presented herein was performed when Hoon Hyung was at Georgia Institute of Technology. This study was supported by the United States Environmental Protection Agency (USEPA) STAR Grant #D832526. The authors thank Dr. John Fortner at Department of Chemistry, Rice University, for the assistance on DLS analysis and Dr. Jim Millette and Whitney Hill at MVA Scientific Consultants (Duluth, Georgia) for the help with TEM analysis.

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