Effects of Transformations of Ag and CuO Nanoparticles on Their Fate in Freshwater Wetland Sediments and Plants
Engineered nanomaterials (ENMs) are increasing becoming incorporated into consumer products to imbue remarkable physical and chemical properties. The increased use of these ENMs leads to a growing need to understand the environmental fate of ENMs after release. Many ENMs, including Ag and Cu ENMs, have the potential to undergo complex physical and chemical transformations which impact their toxicity, solubility and fate in the environment. There is a lack of research characterizing the transformation rate and understanding how these transformations affect interactions with organisms and the ultimate environmental fate. The first objective of this thesis was to understand how transformations of Ag ENMs affect the uptake, distribution and speciation of these materials in plants. Terrestrial (alfalfa, Medicago sativa) and an aquatic (duckweed, Landoltia punctate) plant species were exposed hydroponically to as manufactured (“pristine”) Ag0-NPs and more environmentally relevant (“transformed”) Ag2S NPs. The uptake, spatial distribution and speciation of Ag were analyzed using synchrotron based X-ray Absorption Spectroscopy (XAS) techniques to provide mechanistic insights into the uptake of these ENMs. The reduced solubility and reactivity of Ag2S ENMs was expected to prevent plants from solubilizing these particles and only allow for direct uptake of particles. For the more soluble Ag species, the absorption of Ag+ ions was expected to be primarily the mechanism of Ag uptake. Although the total Ag associated with the plants was similar, the Ag distribution in the roots was dramatically different. The transformed ENMs (Ag2S) appeared to be taken into the plant tissue as sulfidized ENMs. The pristine Ag0 ENMs were found to partially dissolve and incorporate into the plant tissue as both dissolved Ag and Ag0-NPs. The fact that ENMs readily attach onto plant tissue regardless of speciation and solubility suggests that exposure to ENMs may be controlled by factors affecting attachment to root surfaces. However, internalization of Ag appears to be affected by solubility. The second objective was to characterize the impact of transformations of Ag and Cu-based ENMs on the distribution, speciation and fate of these materials in subaquatic sediments and the aquatic plant, E. Densa in a simulated emergent freshwater wetland using large-scale mesocosms. The exposure of pristine (Ag0 and CuO) ENMs and their transformed analogues (Ag2S and CuS) was compared to an ionic control (Cu(NO3)2) to determine if nanoparticulate species of metals were distributed differently than their dissolved counterparts. The metal speciation was determined using XAS to elucidate relative timescales of transformations. The pristine ENMs were expected to rapidly transform into their more stable sulfidized species and the uptake of Ag and Cu were expected to depend on the solubility of the ENMs. Transformations of the pristine ENMs were found to be rapid (weeks) in the surficial sediment, but slower (months) in the aquatic plant tissue. The uptake of ENMs coupled with the slow transformation in the aquatic plant tissue suggests ENMs persist longer than the timescales measured in sediments. This knowledge enables better risk forecasting for ENMs exposed to aquatic organisms and informs toxicity testing to ensure correct forms of ENMs are examined. This thesis provided several novel contributions to the understanding of how transformations of ENMs affect their interactions with plants and their fate in real complex environments. Mechanistic insights into the attachment and uptake of ENMs into plant tissues were identified suggesting two predominant uptake pathways. Relative timescales of ENM transformations in freshwater wetland sediments and plant tissue provided suggests plants can slow transformations and allow labile ENMs to persist longer than assumed.