Determination of the effects of carbon nanotubes on plants, soil microorganisms and phytoremediation of arsenic and polycyclic aromatic hydrocarbons



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Tremendous growth in nanotechnology research has been observed in recent years because of its potential application in diverse fields. Because of the unique physicochemical, mechanical, and electrical properties, carbon nanotubes (CNTs) are the most widely used nanomaterials. There are two main types of CNTs: single walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). Ecotoxicology studies on nanomaterials and CNTs have increased in recent years, however, there is still very limited literature available on the environmental impact of manufactured nanomaterials. Understanding the ecotoxicological effects of nanomaterials and CNTs is deemed necessary to prevent the possible risk to humans and organisms. The Environmental Protection Agency has been emphasizing the need for research leading to the application of nanomaterials for environmental protection as well as the assessment of risk associated with nanomaterial applications. The objective of this dissertation study was to determine the effects of MWNTs on plants, soil microorganisms, and phytoremediation of arsenic and polycyclic aromatic hydrocarbons (PAHs) in soil. It was observed during the study that MWNTs at concentrations up to 1000 mg/ kg have no effects on soil metabolic functioning and microbial community composition. However, pyrosequencing demonstrated a shift in soil microbial community structure to more tolerant genera like Rhodococcus, Cellulomonas and Nocardioides (Actinobacteria) and Pseudomonas (Gammaproteobacteria) in the presence of extremely high MWNT concentrations (10000 mg/kg). There were no negative effects of MWNTs on seedling growth and germination of corn, cotton, alfalfa and sorghum. Uptake of MWNTs in roots of 8 d old corn and cotton seedlings was low (< 20 mg MWNT/kg dry biomass) in most MWNT treatments except MWNT accumulation was high (between 20 - 40 mg/kg) at the highest treatment (10000 mg/kg) in both corn and cotton. Similarly, uptake in 40 d old corn was also low (< 20 mg MWNT/kg dry biomass) comparable to 8 d old seedlings. However, translocation to above ground parts was very limited for both 8 d old (corn and cotton) and 40 d old corn, and MWNTs were detected in only a few samples. There were no negative effects of MWNTs on plant photosynthetic response in corn, however maximum photosysnthesis rate and light saturated photosysnthesis were slightly stimulated in in the presence of the two highest concentrations of MWNTs. There were also no negative effects of MWNTs on photosysnthetic pigment content and total percent nitrogen in corn leaves. Similarly, MWNTs did not show oxidative stress on corn roots. The results from these studies suggest that there are no negative effects of MWNTs on soil microorganisms, germination and seedling growth and, physiological and biochemical responses in later stages of plant growth. However, plants can uptake CNTs. Hence, there is possibility of transfer of CNTs to higher organisms through food chain transfer. Also, the presence of extremely high concentration of MWNTs (10000 mg/kg) in a worst case scenario might cause changes in soil microbial community composition. A study conducted on the influence of MWNTs on PAH rhizodegration found pyrene mineralization in a sandy clay loam soil significantly increased by 21 % in the highest MWNT treatment group (100 mg/kg). Microbial community composition was not influenced by the MWNT treatments in this sandy clay loam soil (3.4 % organic carbon content). However, microbial community structure in both the control and MWNT treatments showed a dramatic shift in the presence of MWNTs in the sandy loam soil (0.5 % organic carbon content). Many microbial FAMEs (i15:0, 16:1ω5c, 10 Me 17:0, 10Me 16:0) were missing in the control soil and these FAMEs showed a lower abundance in the 25 mg/kg MWNT treatment (except 10Me 17:0) while the presence of these FAMEs was higher in the 50 mg/kg and 100 mg/kg MWNT treatments. This study was able to show that MWNTs can influence the different factors of PAH rhizodegradation which will depend on different soil types with different organic carbon content in soil. The results from the 28 d study on arsenic hyperaccumulation in Chinese brake fern (Pteris vittata) showed that arsenic hyperaccumulation decreased in the presence of MWNTs and functionalized MWNTs (FMWNTs) in soil after both 21 and 28 d of exposure. However, the decrease was only statistically significant in MWNT treated soil at 28 d. The study indicates that MWNTs and FMWNTs might sequester arsenic in soil making it less bioavailable for uptake and bioaccumulation in plants. These studies have provided useful data on the effects of MWNTs on contaminant bioavailability which will be useful in the evaluation of nanomaterials as well as possible applications in various remediation processes. Although nanomaterials are considered beneficial for use in various sectors, this study indicates the need for regulation of their application to minimize the future risk to humans and the ecosystem



Nanomaterials, MWNTs, Risk assessment, Plants, Soil microorganisms, Bioavailability, Polycyclic aromatic hydrocarbons, Arsenic