C60 Fullerene Soil Sorption, Biodegradation, and Plant Uptake

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C60 Fullerene Soil Sorption, Biodegradation, and Plant Uptake Raghavendhran Avanasi,† William A. Jackson,‡ Brie Sherwin,†,§ Joseph F. Mudge,† and Todd A. Anderson*,† †

Department of Environmental Toxicology, The Institute of Environmental and Human Health (TIEHH), ‡Department of Civil and Environmental Engineering, and §Texas Tech School of Law, Texas Tech University, Lubbock, Texas 79409-1163, United States ABSTRACT: Assessments of potential exposure to fullerenes and their derivatives in the environment are important, given their increasing production and use. Our study focused on fate processes that determine the movement and bioavailability of fullerenes in soil. We evaluated the sorption, biodegradation, and plant uptake of C60 fullerene using 14C-labeled C60 solutions in water produced by either solvent exchange with tetrahydrofuran or sonication/extended mixing in water. Organic carbon appeared to have an important influence on C60 soil sorption. The log Koc values for 14C60 were equivalent for sandy loam and silt loam (3.55 log[mL/g]) but higher for loam (4.00 log[mL/g]), suggesting that other factors, such as pH, clay content and mineralogy, and cation exchange capacity, also influence C60 soil sorption. There was little 14CO2 production in the silt loam or the sandy loam soil after 754 and 328 days, respectively, suggesting high resistance of C60 to mineralization in soil. Plant uptake was generally low (∼7%), with most of the uptaken 14C accumulating in the roots (40−47%) and smaller amounts of accumulation in the tuber (22−23%), stem (12−16%), and leaves (18−22%). Our results indicate that C60 released to the environment will not be highly bioavailable but will likely persist in soil for extended periods.

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INTRODUCTION The discovery of C60 fullerene1 triggered tremendous interest in its unique properties and applications.2 On the basis of its structure, properties, and numerous possible derivatives, the medical,3 electronic,4 and cosmetic industries5 are currently developing applications or uses of C60. With the growing number of applications, the production and use of C60 has increased,6 increasing the likelihood of either intentional or unintentional releases into the environment.7 Nanotechnology researchers have been calling for more data on the fate and impacts of nanoparticles in the environment.8−11 Although there is now a growing body of literature on the environmental fate of nanomaterials, there also remain data gaps, especially since only a small percentage of manufacturers have been in the practice of conducting nanomaterial risk assessments.12 Ruoff et al.13 studied the solubility of C60 in a number of solvents and concluded that it has very low solubility in polar solvents like water (1500 nm. Retention of smaller sized agglomerates under tetrahydrofuran solvent exchange varied by filter type. The use of nylon filters increased the “loss” of C60 agglomerates to the filter compared to PTFE and PVDF filters, suggesting that nylon filter material inflates the apparent agglomerate size. Just 25% and 6% of C60 passed through 450 and 200 nm nylon filters, respectively, whereas 53% of the same C60 preparation was able to pass through each of the 450 nm PTFE, 450 nm PVDF, and 200 nm PTFE filters. Variability of the results among different filter materials may be explained by different properties of the filters including surface charge, variability of pore size, and extent of clogging in different filter materials. Although 53% of C60 prepared by tetrahydrofuran solvent exchange was able to pass through a 100 nm PVDF filter, agglomeration sizes in this study were greater than the 5− 500 nm agglomerate sizes reported by Fortner et al.14 Considering that fullerene agglomerates in the natural environment may be nano in size or larger, it was decided to proceed with using the aqueous C60 solution prepared by tetrahydrofuran solvent exchange for the remaining fate experiments, as this method yielded a mixture of small and large C60 agglomerates. These results also suggest that suspension techniques more analogous to those which C60 would be subjected to in the environment, that is, extended mixing, may be expected to yield primarily large C60 agglomerates >1500 nm in size. Sorption of 14C60 to Soil. A series of batch equilibrium experiments were used to determine the soil−water partition coefficients (Kd) of C60 in a sandy loam, a silt loam, a loam, and laboratory sand (Table 2). Kd values in conjunction with Table 2. Mean Log Kd (Log[mL/g]) and Log Koc (Log[mL/ g]) Values for 14C60 Fullerene Nanomaterial Sorption in Three Soil Types and Laboratory Sand.a soil type sandy loam silt loam loam sand a

log Kd log(mL/g) 1.67 1.95 2.39 0.75

(SE (SE (SE (SE

= = = =

0.05) 0.06) 0.04) 0.04)

% OC

log Koc log(mL/g)

1.3 2.5 2.5 0

3.55 (SE = 0.05) 3.55 (SE = 0.06) 4.00 (SE = 0.04)

Values are the mean and standard error of five replicates.

percent organic carbon of the soils were used to calculate the normalized organic carbon partition coefficient (Koc). In theory, Koc should be consistent for all soil types, if C60 primarily partitions to organic carbon. Although Koc values were well within an order of magnitude among soil types, we did observe a higher mean log Koc value for loam (4.00, SE = 0.04) than for silt loam (3.55, SE = 0.06) or sandy loam (3.55, SE = 0.05) (Table 2). These results are consistent with organic carbon acting as the primary driver for C60 sorption to soil, but they also suggest that other soil factors also influence sorption of fullerene nanomaterials. 2795

dx.doi.org/10.1021/es405306w | Environ. Sci. Technol. 2014, 48, 2792−2797

Environmental Science & Technology

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Table 3. Mean Uptake of 14C60 into Plants and Distribution of the 14C Taken up among the Different Plant Parts from Plant Uptake Experiments Involving Sand and Hydroponic Substrates mean % 14C uptake into plants

plant structure

sand (n = 9)

6.8% (SE = 1.7%)

hydroponic (n = 7)

7.0% (SE = 1.0%)

root tuber stem leaf root tuber stem leaf

substrate type

of 14C60 to less hydrophobic products may not have been extracted by the tetrahydrofuran and could have contributed to the unclosed mass balances for these experiments. Plant Uptake of 14C60 from Sand and Hydroponic Substrates. Of the C60 dosed to plants, approximately 7% was uptaken into plant tissues (Table 3), with no significant difference in the uptake percentages between the sand and hydroponic treatments (two-tailed, two-sample t test, n(sand) = 9, n(hydroponic) = 7, critical effect size (Cohen’s d) = 2, α = 0.05, β = 0.04, p = 0.932). The 14C60 uptaken by plants was distributed among all plant structures examined with the following relative partitioning: root > tuber ≈ leaf > stem. Efficiency of 14C60 Recovery. Despite the use of 14C60 for the biodegradation and plant uptake studies, the mass balance of 14C60 in studies involving sandy loam, silt loam, and sand could not be closed. In the biodegradation experiment, a mean of 53% (SE = 1.2%) of the total initial 14C was recovered from the sandy loam replicates and a mean of 51% (SE = 5.6%) was recovered from the silt loam replicates. In the plant uptake experiment, the mean total recovery in sand was 61% (SE = 3.1%), whereas the mean recovery was quantitative (110%, SE = 4.9%) for the hydroponic experiment. Extraction of C60 using solvents does not appear to be a completely efficient process. Isaacson et al.39 suggested that the presence of a polar cloak around the hydrophobic fullerene particles in aqueous solutions could cause low efficiency in the recovery and extraction of fullerenes from samples using organic solvents. Inefficiency of the extraction methodology could produce uncertainty in the determination of the 14C60 mass balance. There is a need for more information and a better understanding of the properties of fullerenes in polar environments and, subsequently, better extraction methods. Environmental Fate of C60. Our results describing basic environmental fate parameters for C60 have helped to fill in three key data gaps related to the potential for fullerene nanomaterials to impact the environment. This research shows the following: (1) Measured Koc values for C60 are lower than extrapolated Koc values that have been previously published, suggesting that C60 may be more mobile in the environment than previously assumed. (2) C60 is resistant to mineralization in soil over periods of at least 1 to 2 years. (3) 14C from 14C60 can be uptaken by plants and is transported to both above and below ground plant parts. With respect to the possibility of trophic transfer of fullerenes from soil to higher organisms through plants, the more likely exposure is for organisms that consume below-ground plant parts, although some 14C is translocated to above-ground vegetation. The high lipophilicity of C60 fullerene also suggests the possibility of biomagnification, but to date, this has not been evaluated.

mean distribution of 14C within plants 40% 22% 16% 22% 47% 23% 12% 18%

(SE (SE (SE (SE (SE (SE (SE (SE

= = = = = = = =

2.1%) 1.0%) 1.5%) 0.9%) 3.8%) 4.0%) 1.3%) 3.0%)

This experiment also highlights the need to continually develop and refine methods to assess nanoparticle fate in ecosystems (as discussed by Simonet and Valcárcel33). Despite elaborate attempts at minimizing agglomerate sizes, we found C60 exhibited a strong tendency to form large agglomerates. C60 appears to have a strong tendency to associate with soil types based on their texture and other properties. Desorption of C60 from soil (data not shown) also varied based on the soil type and the strength of sorption. The unusual physiochemical properties of nanomaterials make their study prone to uncertainties. Agglomeration, solubility in different solvents, and extraction efficiencies need to be studied in greater detail before more certain conclusions can be obtained. All of these factors affect bioavailability in the environment, and refinement of this data is critical in developing more precise modeling for risk assessment purposes. In turn, this will promote appropriate regulation and ecologically responsible development of engineered nanoparticles.

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AUTHOR INFORMATION

Corresponding Author

*T. A. Anderson. E-mail: [email protected]. Phone: (806) 834-1587. Notes

The authors declare no competing financial interest.

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REFERENCES

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