Microbial controls on the environmental fate of carbon nanomaterials
Since the synthesis of the first carbon nanomaterials (CNM) 30 years ago, researchers and manufacturers have recognized the potential of these materials to transform the world. The unique physical properties and incredible resilience of these materials gives them countless applications and has led to significant increases in their production over the last decade. However, despite their growing prevalence, the properties and processes controlling the degradation of CNM in the environment remain poorly understood. The primary purpose of this dissertation is to examine the role of environmental microorganisms in degrading CNM and to elucidate which CNM properties and environmental processes represent important controls on this degradation. To accomplish this, a series of incubation studies were conducted in which pure fungal cultures and soils were exposed to a CNM. In pure culture studies the wood-decay fungi Trametes versicolor and Phlebia tremellosa were grown in the presence of single-walled carbon nanotubes (SWCNT) and CNT-polymer composites. By using CNT with varying purities and surface chemistries we were able to relate CNT properties to the production of degradative enzymes by the fungi. While fungal enzyme activity and growth patterns were not affected by purified and unfunctionalized SWCNT, incubation with either impure SWCNT or purified SWCNT with 3% surface carboxylation altered fungal growth morphology and the activity of the extracellular laccase and peroxidase enzymes. The role of CNM surfaces in initiating microbial responses to these materials was further highlighted by the inability of cultured fungi to increase enzyme activity when grown with CNT-polymer composites in which the CNT surface was not accessible. To further explore the role of surface chemistry in controlling the degradation of CNM, laboratory soil incubations using 13C-enriched C60 fullerenes were conducted. We observed that while pristine C60 was not degraded in the soil, C60 that had been previously abiotically transformed by prolonged photo-exposure were, with ~0.78 % of added C60 mineralized. In a follow up study using C60 fullerols, a hydroxylated analog of C60 fullerenes, were found to be rapidly mineralized in soils with a maximum mineralization of ~59.1 % of fullerol C mineralized. Analysis of microbial phospholipid fatty acids (PLFA) extracted from soils found that Gram negative PLFA contained the largest proportion of fullerol derived 13C. The works in this dissertation are the first to demonstrate that surface functionalization is necessary to induce an enzymatic response in saprotrophic fungi and first to confirm that abiotic degradative processes are able to significantly increase the rate of CNM decomposition in soils. The use of isotopically labelled CNM has allowed for sensitive quantification of microbial mineralization and uptake of CNM carbon. Together, these findings demonstrate the critical importance of surface chemistry in controlling the microbial degradation and environmental fate of CNM.
Filley, Purdue University.
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