(Photo)chlorination-Induced Physicochemical ... - ACS Publications

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(Photo)chlorination-Induced Physicochemical Transformation of Aqueous Fullerene nC60 Chao Wang,† Chii Shang,*,† Mengling Ni,† Ji Dai,† and Feng Jiang†,‡ †

Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China MOE Key Laboratory of Theoretical Chemistry of Environment, School of Chemistry and Environment, South China Normal University, Guangzhou, China

‡

S Supporting Information *

ABSTRACT: This study confirmed the physicochemical transformation of aqueous fullerene aggregates (nC60) produced via solvent exchange from toluene by chlorine in the dark and under fluorescent light (representing visible light) by comparing the changes in light absorbance at 700 nm and size distribution of nC60 and characterizing the photochlorination products of nC60 by XPS, FTIR and TOF-SIMS techniques. The (photo)chlorination of nC60 was enhanced by increasing the chlorine dosage and the salinity concentration, and the presence of fluorescent light. During (photo)chlorination, nC60 underwent surface chlorination, hydroxylation and oxidation, and was transformed into products containing carbon-chlorine, epoxy and hydroxyl functional groups. Extensive (photo)chlorination produced products that might not possess the isolated benzenoid ring structure on their cages, although they retained the 60-carbon cage structure. These findings imply the necessity of assessing the fate and toxicity of nC60 after (photo)chlorination in both engineered and natural environments and demonstrate a simple way to produce new nC60 derivatives that contain chlorine and oxygen.

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INTRODUCTION With the potential mass production and extensive use of fullerene C60 in the foreseeable future, there is a high chance that the molecules will enter our environment at some stage. C60 is virtually insoluble in water, with an estimated solubility of only 2.63 ng/L.1 Stable, nanoscaled aqueous colloidal aggregates usually referred to as nC60, however, can be prepared by two common methods: solvent exchange2 and extended mixing with water.3 The presence of stable nC60 provides another environmentally relevant route of fullerene exposure (from water) and raises concerns in its fate and transformation in the aqueous environment, and its potential adverse effects to human health and aquatic organisms. Physicochemical transformation of nC60 by chlorine may occur in both engineered and receiving water environments. The transformation of nC60 in these environments is thus an important environmental issue that deserves a better understanding. A few studies have been carried out to elucidate the fate and transport of nC60 in natural and engineered aqueous environments. Natural organic matter (NOM) in natural water disaggregated nC60 due to the steric hindrance effect and the reduced surface hydrophobicity.4 Inorganic salts aggregated nC60, in compliance with the classic Derjaguin−Landau− Verwey−Overbeek (DLVO) theory.4 The transformation of nC60 by disinfectants/oxidants has been the subject of several recent studies.5−8 nC60 is susceptible to various oxidation and © 2012 American Chemical Society

photochemical reactions when exposed to ultraviolet (UV) light, sunlight or ozone, generating a diverse range of watersoluble C60 derivatives of smaller sizes. For example, nC60 can be oxidized by ozone to form water-soluble fullerene oxides that contain a number of hydroxyl and hemiketal functionalities.5 When nC60 is exposed to monochromatic UV light (at 245 nm), UVA light (at 300−400 nm), or sunlight, it can be photochemically transformed, leading to surface oxidation and hydroxylation.6−8 The transformation is associated with the formation of more hydrophilic products, which behave different aggregation and deposition kinetics and have different bioavailability and toxicity.5,8 Surface oxidation and hydroxylation of solid C60 and C60 in organic solvents by ozonation or UV irradiation have also been reported in the literature.9−11 Owing to C60’s electron-rich cage structure with localized conjugated π-systems, the addition of electrophile, such as chlorine, to C60 is possible.12,13 However, a few studies that attempted to prepare/synthesize chlorinated C60 have found that rather extreme conditions (5-h continuous pure chlorine gas purging at 250°C) are required to synthesize chlorinated C60 in organic solvents in the dark and the products contain low chlorine content.14 Photochlorination of C60 under UV Received: Revised: Accepted: Published: 9398

March 15, 2012 August 8, 2012 August 11, 2012 August 11, 2012 dx.doi.org/10.1021/es301037f | Environ. Sci. Technol. 2012, 46, 9398−9405

Environmental Science & Technology

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because they enabled higher conversion. The salinity of the nC60 suspension was mostly adjusted to 15000 mg/L with sodium chloride but occasionally to 230 mg/L to represent two water matrixes: saline water and fresh water, respectively. The latter kept the nC60 in its prepared state and the former allowed the nC60 to aggregate first due to the well-known salinity effect that salts screen the surface charge of nC60 to reduce or even eliminate the energy barrier to promote nC60 aggregation.4 Chlorination in the dark was achieved by covering the flask with aluminum foil while photochlorination under visible light was achieved by placing the reaction flask under fluorescent lamps with a light intensity of 245 μW/cm2. Figure S1 in the Supporting Information shows the spectrum of light emitted from the fluorescent lamps measured with a spectroradiometer (RPS900-R, International Light, USA). Most wavelengths were in the visible light range between 400 and 720 nm. As (photo)chlorination proceeded, sample aliquots of 5 mL were withdrawn from the flask at specific time intervals, dechlorinated with Na2SO3 at concentrations of 1.2 times of the spiked chlorine concentrations, and subjected to UV−vis absorbance scans and size distribution analysis. The experiment for characterizing the photochlorination products of nC60 at low salinity was carried out in the same manner at 400 mg/L as Cl2 and under the fluorescent lamps with a light intensity of 245 μW/cm2 for 32 h, except the water volume and the nC60 concentration were increased to 500 mL and 12 mg/L, respectively, to obtain a highly modified sample of large enough quantity for product characterization. For this experiment, Na2CO3 was not added to quench chlorine. Sample Pretreatment for Product Characterization. After photochlorination for 32 h, the sample was washed three times immediately to eliminate the interference from the spiked NaOCl by repeated centrifugation and resuspension. Several drops of the centrifugation-concentrated sample were then deposited onto a silicon substrate and evaporated at room temperature in a dust-free atmosphere. The silicon substrate used for XPS analysis was coated with Au for 2−10 min at 100 mA, whereas that for TOF-SIMS analysis was used without further manipulation. The remaining concentrated sample was frozen into a solid state and then freeze-dried to obtain powders for FTIR analysis. Analytical Methods. Size distribution and zeta potential of nC60 were determined with a dynamic light scattering (DLS) analyzer and a zeta potential analyzer (ZetaPlus Brookhaven Instrument Corporation, USA), respectively. All absorbance spectra at wavelengths of 300−900 nm were recorded with a UV−vis spectrophotometer (Lambda 25, PerkinElmer Ltd., USA) equipped with a 1-cm-path-length-matched quartz cuvette. The nC60 concentration in the stock was determined using a liquid−liquid extraction method.2 In brief, a portion of each sample, a 2% NaCl solution, and toluene were mixed at a volumetric ratio of 1:1:1 and agitated for 30 min continuously. The toluene phase was then collected for the measurement of UV−vis absorbance at 332 nm. The concentration of free chlorine in the NaOCl stock solution was periodically standardized with DPD/FAS titrimetric method.17 For surface characterization, the freeze-dried powders obtained for FTIR analysis were mixed with spectroscopicgrade KBr, grinded to fine powders with a mortar and pestle, and compressed to form pellets. The FTIR spectra of all samples were recorded with a FTIR spectrometer (Bio Rad FTS 6000) equipped with a DTGS detector scanning from 2000 to 400 cm−1. Sixteen scans, each with a resolution of 4

irradiation at the chlorine concentration of 73958 mg/L in carbon disulfide can, nevertheless, enhance the chlorine content of the chlorinated products, with an average formula of C60Cl40.13 Oxychlorinated products have also been identified by Fourier transform infrared (FTIR) spectroscopy during the photochlorination of C60, owing to the presence of oxygen during the prolonged UV and chlorine coexposure.13 All these studies have demonstrated the possibility of forming chlorinated C60 in organic solvents under specific preparation conditions. However, less is known about the transformation of nC60 by (photo)chlorination under more realistic, environmentally relevant conditions (e.g., aqueous environments and low chlorine dosages), which is likely to occur during water production, wastewater treatment and disposal, and even under visible light. Such oxidative transformation may produce products of different size, solubility, functionality and thus toxicity. This study was carried out to test the likelihood of nC60 transformation by chlorine in the dark and under fluorescent light (representing visible light). Aqueous nC60 was prepared by the solvent exchange method from toluene15 and was chlorinated at different chlorine dosages and two salinity levels under the light or in the dark. The changes in light absorbance at 700 nm and size distribution of nC60 were recorded and compared. The photochlorination products of nC60 were characterized by several techniques including X-ray photoelectron spectroscopy (XPS), FTIR spectroscopy, and time-offlight secondary ion mass spectrometry (TOF-SIMS).

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EXPERIMENTAL SECTION Materials and Chemicals. C60 (purity >99.9%) was purchased from MER Corporation (Tucson, AZ, USA). A 5% sodium hypochlorite (NaOCl) stock solution was obtained from Allied Signal (Morristown, NJ, USA). Toluene and ethanol (of HPLC grade) were obtained from Mallinckrodt Baker (USA) and Merck KGaA (Germany), respectively. All other chemicals used in this study, such as Na2HPO4 and NaH2PO4, were of reagent grade or higher. All solutions, without further purification, were prepared with water (18.2 MΩ/cm) purified by a NANOpure system (Barnstead, IA, USA) with an organic free cartridge. Preparation of Aqueous nC60 Suspension. An aqueous nC60 stock suspension was prepared with toluene following a solvent exchange procedure.15 Specifically, 100 mg of C60 was dissolved in 100 mL of toluene completely using a magnetic stirrer. The purple C60-toluene suspension was transferred to a beaker containing 250 mL of ultrapure water and 7.5 mL of ethanol. The mixture was then sonicated using a sonicator horn operated at approximately 70 W until the toluene disappeared. The resulting yellow-brown suspension was filtered through a 0.45-μm membrane (Advantec, Japan) and then stored in an amber glass container at 4°C until use. The mean hydrodynamic size of the aqueous nC60 in the stock was about 123 nm and the zeta potential was approximately −40 mV. Chlorination and Photochlorination Protocols. (Photo)chlorination experiments were carried out in 50-mL volumetric flasks by mixing different aliquots of the NaOCl stock solution with 5-mg/L nC60 suspension buffered at pH 7 (with phosphate) to achieve 0 (as controls), 20, 100, and 400 mg/L as Cl2 under fluorescent light or in the dark. The chlorine dosage of 20 mg/L was on the upper limit of typical free chlorine dosages in wastewater disinfection16 and the chlorine dosages of 100 or 400 mg/L were used to illustrate the effects 9399

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cm−1, were averaged to obtain each FTIR spectrum. The XPS analysis of all samples was performed on a PHI 5600 multitechnique photoelectron spectrometer (Physical Electronics) and the core level spectra were measured using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The analyzer was operated at 23.5-eV pass energy and the analyzed area was 800 μm in diameter. The SIMS spectra of all samples in positive and negative modes were obtained using a TOFSIMS instrument (TOF SIMS V, ION-TOF GmbH, Germany).

L of NaCl after 40 min.4 After 1 h, the nC60 continued to aggregate, though the aggregation rate became much slower than that in the first hour. At a chlorine dosage of 20 mg/L, the mean particle hydrodynamic size of nC60 at 50 h was slightly smaller than it was at 1 h, which means that chlorination at 20 mg/L prevented the further aggregation of nC60 aggregates between 1 and 50 h in the dark. This observation suggests that some transformation of nC60 aggregates took place at this environmentally relevant chlorine dosage. When the chlorine dosage was increased to 100 mg/L and further to 400 mg/L, the salinity effect on the mean particle hydrodynamic size of nC60 also overrode the effect of chlorination within the first hour. Nevertheless, after 1 h, the sizes of nC60 obviously decreased because of the disaggregation of the salt-aggregated nC60 by chlorination. These results clearly show that saltinduced aggregation is exceptionally rapid, whereas chlorination is a slower process. As shown in Figure 1B, under the 245-μW/cm2 fluorescent light, the disaggregation of nC60 was strongly enhanced by the photochlorination. Even at a low chlorine dosage (Figure 1B, 20 mg/L as Cl2), the mean particle hydrodynamic size of nC60 decreased significantly. This could not be observed in samples with the same chlorine dosage in the dark but it was observed in samples with dosages of 100 and 400 mg/L as Cl2 in the dark. At a chlorine dosage of 400 mg/L, the photochlorination reduced the size of the salt-aggregated nC60 back to approximately 120 nm, a size similar to its initial size, after 50 h. At low salinity of 230 mg/L representing fresh water, which came from the use of phosphate salts for pH buffering, saltinduced aggregation of nC60 and disaggregation of nC60 by (photo)chlorination at chlorine dosages up to 400 mg/L and reaction times up to 50 h were not statistically significant (see Figure S2 in the Supporting Information). The results displayed in Figure 1 and Figure S2 in the Supporting Information suggest that the oxidation delivered by the (photo)chlorination was weak so that only the salt-aggregated nC60 could be disaggregated. It has been reported in the literature that, without further salt-induced aggregation, the size of discrete nC60 remained fairly constant within the 21-day UVA irradiation simulating sunlight irradiation.8 On the other hand, strong oxidation such as ozonation causes dissolution of discrete nC60 aggregates.5 Aqueous nC60 shows a distinct light absorbance peak at 350 nm, a broadband absorbance from 450 to 550 nm and a tailing absorbance from 550 to 900 nm (see Figure S3 in the Supporting Information). The broadband and tailing absorbances from 450 to 900 nm are known to represent the characteristic absorbance of aggregated C60 (ref 2) and, as shown in Figure S3 in the Supporting Information, they increased with time after the prepared nC60 was put into the high-salinity solution without (photo)chlorination. Therefore, the absorbance at 700 nm was selected to provide additional information on the change of the aggregation status of nC60. However, it should be noted that the absorbance provides only qualitative information but not the true concentrations of nC60 in water in quantitative terms, because the transformed intermediates and final products in the aqueous phase may also display absorbance at 700 nm and their molar absorptivities may differ from that of the untransformed nC60. Figures 2 displays the effects of chlorine dosage, reaction time and the fluorescent light on the changes in absorbance of nC60 at 700 nm in saline phosphate buffer solution. As shown in

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RESULTS AND DISCUSSION Transformation of nC60 by (Photo)chlorination. Panels A and B in Figure 1 show the changes in the mean particle

Figure 1. Mean particle hydrodynamic sizes of nC60 as a function of reaction time in the saline phosphate buffer solution at salinity of 15000 mg/L after chlorination at different dosages of chlorine in the dark (A) and under fluorescent light (B).

hydrodynamic size of nC60 as a result of chlorination and photochlorination, respectively, at different chlorine dosages and reaction times. As shown in Figure 1A, at high salinity and no chlorine addition, the mean particle hydrodynamic size of nC60 changed from the original 123 to 346 nm within 1 h, demonstrating salt-induced nC60 aggregation. The rapid saltinduced aggregation has also been observed in the literature where the size of nC60 increased by 50 nm (83%) at 38025 mg/ 9400

dx.doi.org/10.1021/es301037f | Environ. Sci. Technol. 2012, 46, 9398−9405

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Figure 3. Absorbance of nC60 at 700 nm as a function of reaction time in the fresh phosphate buffer solution at salinity of 230 mg/L after chlorination at 0 and 400 mg/L as Cl2 in the dark and under fluorescent light.

A few plausible reasons provide some explanations of the transformation of nC60 by (photo)chlorination at different salinity concentrations and chlorine dosages. Hypochlorous acid (HOCl) is a weak acid that dissociates to form hypochlorite (OCl−)18 HOCl ↔ OCl− + H+ pK a = 7.6 at 20°C

(1)

There exists an equilibrium between HOCl and aqueous chlorine (Cl2) and the equilibrium is dependent on pH and the chloride concentration19 HOCl + Cl− + H+ ↔ Cl 2 + H 2O Figure 2. Absorbance of nC60 at 700 nm as a function of reaction time in the saline phosphate buffer solution at salinity of 15000 mg/L after chlorination at different dosages of chlorine (A) in the dark and (B) under fluorescent light.

K = 2.3 × 103 M−2 at 25°C

(2)

Increasing chloride concentration further pushes the reaction toward to aqueous Cl2. There also exists an equilibrium between HOCl and Cl2O20 2HOCl ↔ Cl 2O + H 2O K = 8.74 × 10−3 M−1 at 25°C

Figure 2, the absorbance at 700 nm, representing aggregated nC60, increased rapidly in the first hour and then continued to rise for the rest 49 h in the dark control and the control with fluorescent light only, due to the salt-induced aggregation. During (photo)chlorination, the absorbance rapidly increased in the first hour and then decreased gradually with time in all cases. The increase was caused by salt-induced aggregation. The decrease could be attributed to the disaggregation of saltaggregated nC60 by (photo)chlorination (Figure 1). It is also possible that the extinction coefficient at 700 nm for the (photo)chlorinated products might be less than that for the untransformed nC60. The decreases in absorbance at 700 nm were also enlarged by increasing the chlorine dosage and the presence of fluorescent light. Figure 3 displays the time-dependent changes in absorbance of nC60 at 700 nm in the low salinity phosphate buffer solutions due to (photo)chlorination at a chlorine dosage of 400 mg/L. At low salinity, the absorbance of the dark and light controls and that treated with a chlorine dosage of 400 mg/L in the dark remained unchanged after 50 h. The absorbance of the photochlorinated nC60 monotonically decreased with time but the decrease was much smaller than that observed at high salinity (Figure 2B).

(3)

At pH 7 and chloride concentrations up to 9100 mg/L (salinity of 15000 mg/L used in the current study), the major chlorine species are HOCl and OCl− while the concentrations of aqueous Cl2 and Cl2O are 5 and 4 orders of magnitude smaller than the concentrations of HOCl and OCl−. Nevertheless, it has been reported that aqueous Cl2 and Cl2O are much more reactive, with rate constants over 5 orders of magnitude higher than that of HOCl in chlorination of aromatic ethers and dimethenamid.20,21 Aqueous Cl2 and Cl2O have also been reported to be more reactive than HOCl in oxidation of pxylene and PAH.22,23 These highly reactive chlorine species should contribute to the transformation of nC60, particularly at high salinity. Direct photolysis of nC60 by UV and visible light is known to produce reactive oxygen species (ROS), such as singlet oxygen (1O2) and superoxide (O2•−).24 However, it has been reported that rapid quenching of the triplet state C60 (3C60) occurs in nC60 aggregates, resulting in a very low concentration of 3C60 to generate 1O2 after exposed to light at 300−400 nm.25 Therefore, under the fluorescent light used in the current study, the generation of 1O2 via direct photolysis of nC60 was 9401

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unlikely to be significant. In the photochlorination process, it is also well-known that photolysis of aqueous chlorine at wavelengths less than 511 nm forms chlorine radicals (Cl•) and hydroxyl radicals (OH•)12,26−28 hv

HOCl(OCl−) → OH •(O•−) + Cl •

(4)

Cl• and OH• further react with aqueous chlorine to form oxychlorine radicals (ClO•)26,28 Cl • + HOCl(OCl−) → ClO • + HCl(Cl−)

(5)

OH • + HOCl(OCl−) → ClO • + H 2O(OH−)

(6)

In the presence of chloride, Cl• reacts with chloride to generate Cl2•−29 Cl • + Cl− → Cl •− 2

(7) 3

Other reactive oxygen species, such as O( P), ozone (O3), hydrogen peroxide (H2O2), and O2•− can also form during photolysis of chlorine.26,30 In addition, photolysis of Cl2O generates ClO• and O(3P)31

Figure 4. C(ls) XPS spectra of untreated nC60, photochlorinated nC60, and photochlorinated nC60 after Ar etching.

chlorination products. Figure 5 shows the FTIR spectra of the untreated nC60 and the photochlorinated nC60. The untreated

hv

Cl 2O → Cl • + ClO •(λthresh = 840 nm)

(8)

Cl • + Cl 2O → Cl 2 + ClO •

(9)

hv

Cl 2O → Cl 2 + O(3P) (λthresh = 710 nm)

(10)

All these reactive species could take parts in enhancing the transformation of nC60 under the fluorescent light. Characterization of Photochlorination Product(s) by XPS, FTIR, and TOF-SIMS. XPS, FTIR, and TOF-SIMS techniques were used to provide further information on the microscopic chemical properties of the products. To clearly illustrate the chemical transformation of nC60, only the photochlorinated products with higher conversion obtained after 32 h of coexposure to chlorine at 400 mg/L as Cl2 and the fluorescent light at an intensity of 245 μW/cm2 were examined. It should be reiterated that the sample was repeatedly centrifuged and suspended in order to eliminate the interference from any remaining NaOCl. Therefore, some small photochlorinated products that might have been retained in the supernatant could have been washed away and were not collected. Thus, only the collected products were characterized. XPS Analysis. The XPS spectrum of the untreated nC60 showed a single C(1s) peak at binding energy of 285 eV on the surface of nC60 (Figure 4), which has been assigned to sp2 hybridized carbon in C60.32 After the photochlorination, two broad shoulders at higher binding energies were observed (Figure 4), indicating the presence of highly oxidized carbon.8 The peaks at 289 and 287 eV can be attributed to the C−Cl bond and the C−O bond (from hydroxyl and epoxy functional groups), respectively.33,34 The XPS analyzes the surface chemistry of a material in its ″as received″ state and can probe the outer surface of the material with a penetration depth of 3−5 nm.8 For further penetration, the sample was etched to a depth of approximately 3 nm using argon ion etching (3 kV) for 1 min. The etched sample did not show the shoulder peaks at 289 and 287 eV (Figure 4), indicating that the transformation of nC60 caused by the photochlorination occurred only on the outer surface of the nC60 aggregates and the core of the aggregates remained intact. FTIR Analysis. FTIR supplies information on the identities of the functional groups present on the surface of the photo-

Figure 5. FTIR spectra of untreated nC60 and photochlorinated nC60.

nC60 displayed four active vibrational modes at 1429, 1182, 577, and 527 cm−1, which have been attributed to be the infraredallowed, dipole active vibrational modes of F1U symmetry of C60.35 The FTIR spectrum of the photochlorinated nC60 still retained the four characteristic bands of nC60, indicating that there was underivatized nC60 in the photochlorinated nC60, consistent with the XPS observation. In addition, strong bands at 1630, 1161−1036, and 900−500 cm−1 and shoulders at 1726, 1462, and 1383 cm−1 were observed in the FTIR spectrum of the photochlorinated nC60. The broad band at 1630 cm−1 with a shoulder at about 1726 cm−1 points to carbonyl groups such as carboxyl groups and ketones.36 The bands at 1462 and 1383 cm−1 are due to C−OH in plane bending and C−O stretching.5 The strong absorbance in the range of 1161−1036 cm−1 is attributed to the C−Cl bond due to the formation of highly chlorinated C60 or aryl C−Cl stretching and that in the range of 900−500 cm−1 is due to C− Cl stretching.12,37 The FTIR analysis confirmed that the transformation of nC60 by photochlorination involved the oxidation, hydroxylation and chlorination of nC60. The 9402

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oxidation of nC60 was partially owing to the presence of dissolved oxygen in the aqueous solution. The oxidation and hydroxylation of nC60 was also possibly attributable to the attack of the conjugated double bonds of C60 by ROS (e.g., 1O2, O2•−, OH•, O3, O(3P), and ClO•) produced from, as discussed earlier, photolysis of chlorine, Cl2O and nC60. The chlorination of nC60 could be attributed to the attack of the conjugated double bonds of C60 by Cl2, Cl2O, ClO•, Cl• and Cl2•− that, as discussed earlier, were available in the system. It has been reported that during the chlorine radical attack, chlorine adds preferentially and directly to a double bond of C60, without breaking the simple C−C bonds and rearranging the other double bonds of the cage.12 TOF-SIMS Analysis. Figure 6 displays the SIMS spectrum of the photochlorinated nC60 in negative mode, which is

In summary, based on the results obtained in the XPS, FTIR and TOF-SIMS analyses, the fluorescent light-induced photochlorinated nC60 collected by centrifugation is proposed to consist of a 60-carbon cage structure with carbon-chlorine, epoxy and hydroxyl functional groups. These groups have also been found in those generated by the UV-induced photochlorination of C60 in organic solvents.13 Environmental Implications. This work indicates that nC60 in an aqueous suspension can be transformed after chlorine exposure, both in the dark and under visible light. During (photo)chlorination, nC60 underwent surface chlorination, hydroxylation and oxidation, and was transformed into products containing various oxygen- and chlorine-containing functional groups. It should be reemphasized that the nC60 using in this paper was produced via solvent exchange from toluene. It is known that nC60 produced via solvent exchange is chemically and colloidally quite different from nC60 produced by extended stirring in water.3 Although the transformation of nC60 by chlorination in the dark was found to be relatively less significant and required much higher chlorine dosages to be obvious within 50 h, chlorination in the dark for a week or two at low chlorine dosages is common in drinking water supply practices and is expected to lead to notable transformation. The transformation of nC60 by photochlorination was found to be quite significant even at low chlorine dosages under fluorescent light (representing visible light). In practice, simultaneous visible light and chlorine exposure is possible. For example, when water is prechlorinated in water treatment, the chlorine-containing water is commonly exposed to sunlight for hours before it goes to a filter bed. Chlorinated tap water can be stored in transparent containers under sunlight for months on end. In some countries and regions, wastewater effluents are chlorinated without subsequent dechlorination. The chlorine-containing wastewater is discharged into the receiving natural aqueous environment and is subjected to sunlight exposure for days to months. Free chlorine residuals are commonly maintained in the water of outdoor swimming pools worldwide, and so this water is simultaneously exposed to chlorine and visible light. Also, although not studied in this paper, the transformation of nC60 by UV and chlorine is expected to be more significant. UV can be operated together with chlorine to achieve multiple-barrier disinfection, oxidation and residual protection in drinking water production by either prechlorination followed by UV disinfection or simultaneous UV and chlorine coexposure. Consequently, the transformation of nC60 in UV-chlorine coexposure should be assessed. The (photo)chlorinated products of nC60 are expected to behave significantly differently in the aqueous environment, compared with the underivatized nC60, because of the expected decreases in size and hydrophobicity. The fates of these (photo)chlorinated products and the associated environmental and health risks, however, are unknown and deserve further study. In addition, this work demonstrated the possibility of producing new nC60 derivatives that contain chlorine and oxygen, giving rise to alternative synthesis routes of such compounds.

Figure 6. TOF-SIMS spectrum of photochlorinated nC60 in negative mode.

characterized by a peak at 720 m/z representing C60 and multiple peaks at intervals of 24 m/z below 720 m/z. These multiple peaks implied stepwise losses of C2 fragments during ionization,7 which occurred easily on the C60 cage in the presence of epoxy functionality,38 but not on underivatized C60. Formation of C60Ox with epoxy functionality has been reported via the attacks of 1O2 and O2•− when C60 was irradiated by light with appropriate wavelengths including UV light and visible light.39 In photochlorination of nC60, as discussed earlier, these ROS can be formed by photolysis of chlorine and nC60. However, other possible formation pathways of C60Ox with epoxy functionality during the photochlorination of nC60 cannot be excluded. The intensity ratio of 720 m/z in positive mode to that in negative mode of the photochlorinated nC60 was higher than that of the underivatized nC60 (data not shown), showing that the photochlorinated products were more easily to be ionized into C60+ than the underivatized nC60. This finding is likely attributable to the presence of Cl and OH functional groups on the cages of the photochlorinated products, which lends further support to the FTIR analysis results. In addition, the generation of C60+ after the ionization of the photochlorinated products suggests that some of the products retained the 60-carbon cage structure.

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ASSOCIATED CONTENT

S Supporting Information *

Three supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org/. 9403

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AUTHOR INFORMATION

Corresponding Author

*Phone: (852)2358 7885; fax: (852)2358 1534; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported partially by the Hong Kong University of Science and Technology (HKUST) under Grant RPC06/ 07.EG03. Mengling Ni is partially supported by the Postgraduate Scholarship through the Nanoscience and Nanotechnology Program of the School of Engineering, HKUST.

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