Photochemical Transformation of Aqueous C 60 Clusters in Sunlight WEN-CHE HOU AND CHAD T. JAFVERT* Purdue University, School of Civil Engineering, West Lafayette, Indiana 47907 Received September 1, 2008. Revised manuscript received November 6, 2008. Accepted November 13, 2008. C 60 is emerging in a variety of potential applications; however, its environmental fate remains largely unknown. Photochemical transformation may be an important fate process of C 60 in the aquatic environment due to its strong light absorption within the solar spectrum. In this study, the photochemical transformation of aqueous C 60 clusters (nC 60 ) in sunlight (West Lafayette, IN, 86° 55′ W, 40° 26′ N) and in lamp light (300-400 nm wavelengths) was investigated. When exposed to light, the brown to yellow color of nC 60 was lost gradually, and the cluster size decreased as the irradiation time increased. TOC analysis on the water phase of centrifuged samples indicated that water soluble products formed and that with continued light exposure, these intermediates eventually mineralized, volatilized, or were converted to other products not quantified by TOC after centrifugation and filtration. In sunlight at ∼1 mg/L C 60 , the decay rate of C 60 in small clusters (diameter ) 150 nm) was greater than for C 60 in larger (500 nm) clusters, with half-lives of 19 and 41 h, respectively. The presence of fulvic acid, changes in pH, and the preparation method of the clusters had minimal effects on the phototransformation rate. Deoxygenated samples resulted in negligible loss after 17 h of lamp exposure, indicating O 2 played a role in the phototransformation mechanism. These findings suggested that release of nC 60 into surface waters will result in photochemical production of currently unknown products. Introduction The production and application of carbonaceous nanoma- terials, including C 60 and its derivatives, is developing rapidly. Considering its potential widespread use, C 60 and its deriva- tives will eventually find their way into the aquatic environ- ment. This has raised concerns over the potential risks to the environment. Oberdorster et al. (1) reported nC 60 , a stable colloidal suspension of C 60 “clusters” formed by solvent exchange, induced brain damage of juvenile largemouth bass. Recently, the toxicity of nC 60 has been complicated by the fact that trace amounts of organic solvent used in the preparation of the clusters is found in the nC 60 solution, and that this solvent might contribute to any observed toxicity (2). Other studies, however, indicated that nC 60 formed by simply mixing C 60 and water showed potential toxicity to microorganisms (3). C 60 ’s aqueous solubility has been reported at 2.6-8.0 ng/L (4). The extremely low water solubility clearly impedes the mass transport of molecular C 60 via water; however, it is well- known that colloidal suspensions of nC 60 are very stable in water (5-7). Through solvent exchange or by other means, nanosized C 60 aggregates can be dispersed in water, with ionic strength generally controlling aggregate size and stability (6). Colloidal stability has been attributed to the negative charge on the nC 60 particles, resulting in electrostatic repulsion (5, 6); however, the mechanistic cause for the negative charge is unknown. In fresh surface waters, nC 60 can be expected to remain suspended for some time due to its stability at low ionic strength, indicating that if pho- totransformation occurs in sunlight, it may be an important fate process for any C 60 discharged to the environment in cluster form. Due to C 60 ’s strong light absorption, the photochemistry of C 60 has been researched intensively, however not under environmentally relevant conditions. Arbogast et al. (8) reported the generation of singlet oxygen ( 1 O 2 ) in laser pulse photolysis of C 60 dissolved in benzene with high quantum yields of 0.76 ( 0.05 and 0.96 ( 0.04 at wavelengths λ ) 355 and 532 nm, respectively. The suggested mechanism (eq 1) involves the photoexcitation of ground-state C 60 to singlet C 60 ( 1 C 60 ), followed by the fast conversion of 1 C 60 to triplet C 60 ( 3 C 60 ) via intersystem crossing (ISC). Subsequently, 3 C 60 is quenched by O 2 through energy transfer from 3 C 60 to O 2 , leading to the formation of 1 O 2 . C 60 9 8 hv 1 C 60 9 8 ISC 3 C 60 9 8 O 2 C 60 + 1 O 2 (1) The photo-oxidation of C 60 in organic solvents has been reported (9-11) and C 60 epoxides (C 60 O x , x ) 1-5) were shown as the products (10, 11). Juha et al. (12) provided evidence that the formation of C 60 O required the reaction of 1 O 2 with 3 C 60 (eq 2). 3 C 60 + x 2 1 O 2 f C 60 O x x ) 1-5 (2) Extended irradiation can result in the formation of polar products that are insoluble in nonpolar solvents (9). In the oxygen-limiting condition, the photopolymerization of C 60 (eq 3) has been reported in organic solvents (13) via the mechanism of [2 + 2] cycloaddition of neighboring 3 C 60 . m 3 C 60 f(C 60 ) m (3) The number of studies reporting photochemistry of C 60 in aqueous solutions is limited, with essentially no previous studies reporting reactivity under sunlight. Recently, Lee et al. (14) investigated the photochemical production of reactive oxygen species (ROS) from nC 60 after 1-2 h of UV irradiation: It was claimed that C 60 loses its intrinsic photoreactivity when in aggregate form due to the rapid quenching of 3 C 60 , a key transient species in mediating ROS formation and C 60 photodegradation, by ground-state C 60 or by triplet-triplet annihilation. Using a femtosecond laser flash photolysis technique, it was shown that the 3 C 60 lifetime in nC 60 clusters was considerably reduced to picoseconds, as opposed to microseconds when C 60 is dispersed in aqueous surfactant systems (15). In these short- term experiments, no attempt was made to measure chemical loss or product formation. In this study, we sought to answer questions regarding if and how fast C 60 phototransforms in aqueous nC 60 solutions in sunlight under different environmentally relevant condi- tions at long irradiation times (i.e., up to 110 h). The effects of the C 60 cluster size, water quality parameters, and the * Corresponding author phone: (765) 494-2196; fax: (765) 496- 1107; e-mail: jafvert@ecn.purdue.edu. Environ. Sci. Technol. 2009, 43, 362–367 362 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 2, 2009 10.1021/es802465z CCC: $40.75  2009 American Chemical Society Published on Web 12/18/2008