Date of Award

12-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Toxicology

Advisor

Dr. Stephen J. Klaine

Committee Member

Dr. Sarah A. White

Committee Member

Dr. Cindy M. Lee

Committee Member

Dr. Douglas G. Bielenberg

Committee Member

Dr. JoAn S. Hudson

Abstract

Nanomaterials (NMs) have promised lighter, stronger, smaller and more efficient products in areas such as electronics, medicines and even environmental sectors. This investigation started at the base of the food chain and characterized NMs interactions with aquatic plants. Citrate capped gold nanoparticles were used as a model nanoparticle to track fate and gain insight on factors that influence gold nanoparticle (AuNP) bioavailability and absorption. Four species of aquatic macrophytes were investigated. Azolla caroliniana, Myriophyllum simulans, Egeria densa and Myriophyllum aquaticum were selected due to growth habitat, leaf morphology and root structure. Because aquatic plants absorb the majority of their nutrients from the water column, it is logical to hypothesize that they may absorb nanomaterials in suspension, potentially facilitating trophic transfer. Azolla caroliniana, E. densa and M. simulans were exposed to 4 nm and 18 nm AuNPs at a nominal concentration of 250 μg Au/L for 24 h. Macrophytes were harvested at six different time points (1,3,6,12,18 and 24 h), dried and then analyzed for gold concentration via inductively coupled plasma mass spectrometry. Concentrations were normalized to whole plant dry tissue mass. Electron microscopy revealed that 4 nm and 18 nm AuNPs adsorbed to the roots of each species. Further, it was observed that 4 nm and 18 nm AuNPs were absorbed by A. caroliniana, however, only 4 nm AuNPs were absorbed by M. simulans; E. densa did not absorb AuNPs of either size. To further identify factors that influence the bioavailability of gold nanoparticles to aquatic macrophytes, A. caroliniana, E. densa and M. simulans were exposed to 4, 18, and 30 nm gold nanoparticles. Results indicated that particle uptake was influenced by plant species, presence or absence of plant roots, particle size and dissolved organic carbon and their interactions; this suggests that nanoparticle bioavailability is influenced by multiple parameters. Absorption of AuNP was species specific and dependent upon the presence of roots and nanoparticle size. In the presence of dissolved organic carbon, the suspension of 4 and 18 nm gold nanoparticles formed a nanoparticle/organic matter association that resulted in 1) minimized particle aggregation and 2) a decrease of nanoparticle absorption by the aquatic plants. The same effect was not observed with the 30 nm nanoparticle treatment. Multiple factors, both biotic and abiotic, must be taken into account when predicting bioavailability of nanomaterials to aquatic plants. Electron microscopy was used to further investigate the influence of AuNP size on species dependent uptake. Root micrographs of E. densa, M. simulans and A. caroliniana indicated that absorption of gold nanoparticles from suspension correlated with root microfibril density. The microfibril network defines the porous structure of the root cell wall. The cell wall porosity of A. caroliniana was 4.5 - 5.0 nm, as measured by solute exclusion. The effect of evapotranspiration on AuNP uptake was measured over 16 days for emergent species A. caroliniana and M. aquaticum using 4 nm AuNPs. Disrupting boundary layers and varying humidity around the emergent plant achieved changes in the evapotranspiration rate. Placing plant units under a fan, in a sealed system or open (control) conditions correlated with increased, decreased or control measured evapotranspiration rates. Plant root and shoot samples were separated and analyzed for gold content. Increased evapotranspiration rates correlated with an increase in AuNP root loading (mg Au/kg dry tissue/24 h). An average of 18.83 ± 3.3 mg Au/kg dry tissue/24 h was observed in the fan treatment for A. caroliniana and in M. aquaticum, an increase of 1.07 ± 0.18 mg Au/kg dry tissue/24 h was observed in the fan treatment when compared to the control treatments. While an increase in evapotranspiration rate increased root loading, shoot concentrations of Au did not correlate with evapotranspiration rate. This suggested that shoot translocation was a diffusive process, not dependent on water movement into the root tissue. Shoot tissue concentrations in A. caroliniana increased from 14.58 ± 3.29 mg Au/kg dry tissue to 140.15 ± 6.73 mg Au/kg dry tissue over the course of 16 days. This corresponded with an average AuNP translocation rate of 6.79 ± 3.26 mg Au/kg dry tissue/24 h in A. caroliniana. At the same point, day 16, root tissue concentration for A. caroliniana is the highest observed, 1265.34 ± 139.3 mg Au/kg dry tissue. Overall, my dissertation results indicate that absorption of gold nanoparticles by aquatic macrophytes from suspension is a complex interaction of plant species, nanomaterial size, levels of dissolved carbon in water, root structure, and evapotranspiration rate. Because no visual toxicity or deleterious effects were observed with the exposure of AuNPs to aquatic plants, there is potential for the use of AuNPs in tracking and fate studies within these macrophytes.

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