Few studies have investigated the influence of environmental conditions on ENM uptake and toxicity, particularly throughout the entire plant life cycle. Here, soil-grown plants were exposed until maturity to TiO2, CeO2, or Cu2 ENMs under different illumination intensities, in different soils, and with different nutrient levels. Fluorescence and gas exchange measurements were recorded throughout growth and tissue samples from mature plants were analyzed for metal content. ENM uptake was observed in all plant species, but was seen to vary significantly with ENM type, light intensity, nutrient levels, and soil type. Light intensity in particular was found to be important in controlling uptake, likely as a result of plants increasing or decreasing transpiration in response to light. Significant impacts on plant transpiration, photosynthetic rate, CO2 assimilation efficiency, water use efficiency, and other parameters related to physiological fitness were seen. The impacts were highly dependent on environmental conditions as well as ENM and soil type. Notably, many of these effects were found to be mitigated in soils with limited ENM mobility due to decreased uptake. These results show that abiotic conditions play an important role in mediating the uptake and physiological impacts of ENMs in terrestrial plants.Nanotechnology has the potential to enhance or revolutionize many fields of study, including medicine, transportation, energy storage, personal care, construction, environmental remediation, military applications, manufacturing, and scientific research. Reflecting this broad applicability, nanotechnology has become a multi-billion dollar industry in spite of being in its infancy, and is expected to reach a global market value of over half a trillion U.S. dollars by the end of the decade.With this in mind, and with nanomaterials currently used in nearly 2000 consumer products and many industrial applications,plastic planting pots concerns have naturally arisen about the health and environmental impacts of the manufacture, use, and disposal of this new and extremely varied class of materials.
A nanomaterial is defined as a material with at least one dimension in the size range of approximately 1 to 100 nm.3 Here, the term “engineered nanomaterial”is used to differentiate intentionally designed and manufactured nanomaterials from those produced incidentally by natural or anthropogenic processes. The extreme size of ENMs, and the high surface area to volume ratio that comes along with it, typically results in unique properties not found in larger scale or dissolved materials of the same composition. For example, quantum dots, nanoscale particles composed of semiconducting materials , can utilize quantum band gap phenomena to fluoresce in a narrow range of wavelengths, which are highly dependent on their diameter.4 Additionally, ENMs can be extremely reactive due to their high surface area relative to their volume . These novel properties are simultaneously the source of global interest in ENMs and the main issue of concern in terms of the impacts to human health and environmental safety, as regulations for a bulk or dissolved material may not be appropriate for ENMs of the same composition. Additionally, since ENMs can have radically different behavior depending on their composition, size, shape, doping agents, coatings, and/or the characteristics of the media they are present in, a predictive framework for the fate, transport, and toxicity of ENMs in a variety of environments and organisms is needed to effectively regulate ENMs throughout their life cycles. The majority of the current production, use, and disposal of engineered nanomaterials occur in terrestrial environments, and consequently terrestrial ecosystems are and will increasingly be some of the largest receptors of ENMs at all stages of their life cycles.
In particular, soil is predicted to be one of the major receptors of ENMs due to ENM–contaminated bio–solid fertilizer and nanopesticide application to agricultural fields, runoff from landfills or ENM-bearing paints, or atmospheric deposition.Both agricultural and natural systems are at risk to ENM contamination via these release scenarios, which makes it necessary to understand the interactions between ENMs, soils, and soil organisms such as plants in order to predict their impacts in real-world scenarios.The goal of the research presented in this thesis was to uncover some of the underlying mechanisms controlling the following processes under environmentally relevant conditions: how ENMs move through unsaturated soils, the effects ENMs have on key soil properties, the uptake and distribution of ENMs in plants, and how ENMs influence plant growth and physiology. These topics were addressed using methods approximating real-world scenarios as closely as possible while maintaining reproducibility and analytical power. The holistic approach utilized here differs fundamentally from that of many studies currently published on these subjects, which use reductionist experimental design to attempt to break down the complex ENM-soil-plant system into simplified components. Reductionist methods can be powerful in providing detailed information about well-understood systems, but when addressing systems as complex and poorly-understood as these designing experiments to closely mimic real-world scenarios can give insight into key controlling mechanisms that can then be targeted for further study. An example of this can be found in Chapter 2, which shows that the main mechanism impeding ENM transport through unsaturated natural soils is physical straining of large ENM aggregates formed via interaction with ions in the soil solution, not through electrostatic attraction or repulsion as was predicted by several studies using well-dispersed ENMs in typical saturated columns of washed quartz sand.As mentioned, Chapter 2 discusses tracking the movement and characteristics of three metal oxide ENMs through three soils, with ENMs being either coated with natural organic matter or uncoated.
In contrast to studies such as those cited above, which use active pumping to push ENMs through water-saturated media, ENM transport in this study was in unsaturated soils and was driven solely by gravity. This was done in order to more closely simulate conditions likely to occur in the real world, as some of the major predicted exposure scenarios involve ENMs entering from the top layers of soil, which are typically unsaturated. Chapter 3 looks at the effects ENM contamination has on several soil properties, which is a subject that is poorly represented in the literature. Metal oxide ENMs like those used in this study have characteristics that make them likely to influence soil properties in some way, such as being similar in composition to naturally-occurring clay minerals that are important in controlling nutrient retention, soil porosity, and organic content.Additionally, they are amphoteric, that is, capable of producing both H+ and OH–ions depending on their crystal structure and the composition of the media they are in and thus potentially altering soil pH. Soil pH and nutrient availability are both critically important to plants and other soil organisms and were therefore targeted in this chapter. Chapters 4 and 5 explore two aspects of the same system: how plants grown to maturity in ENM-contaminated soils uptake and distribute ENMs throughout their tissues,plastic grow pots and how their growth and physiological processes are affected by the presence of ENMs. Keeping with the theme of designing experiments to predict ENM behavior in real-world scenarios, aspects of the environmental conditions the plants were grown under, specifically illumination intensity and soil nutrient levels, were varied in order to mimic some of the range of conditions plants growing under real conditions would experience.
This was done in a series of three experiments. First, the model plant Clarkia unguiculata was grown to maturity under two illumination intensities in a potting soil with and without receiving additional fertilizer in order to determine the effects of nutrient and light stress. Second, C. unguiculata was again grown to maturity under two illumination intensities, but this time in two natural soils, a grassland soil and an agricultural soil. This was done to see how these plants respond to ENM exposure in soils with different properties beyond nutrient levels. Finally, two crop plants, wheat and radishes, were grown to maturity under two illumination intensities in the grassland and agricultural soils, respectively. This was done in order to see the effects of ENM exposure on plants from different taxonomic groups that are also economically important. By varying one condition throughout this set of experiments, information can be passed from one to the next that could provide additional insight into the key factors at play. ENM mobility in the subsurface is governed by several processes of varying influence, including dissolved ion and pH-induced aggregation, coating by organic and inorganic molecules, sorption to organisms and other media components, and physical straining through soil pore spaces. In particular, chemical and electrostatic interactions with soil clay particles have been implicated as key factors in the subsurface movement of raw or coated ENMs. This has been demonstrated for TiO2 1 and uncoated, citrate-coated, and phosphate–coated CeO2 ENMs2 in soil and implied as the method of retention in other studies.Sorption can occur via electrostatic attraction between charged clay surfaces and oppositely charged ENMs5 or chemically through a dehydration reaction similar to the binding of phosphate or iron oxides to clays. Sorption to organic matter and organisms in soil may also take place through similar mechanisms. The specific organic compounds present in subsurface waters will also differ over geographic area with soil and vegetation type due to the presence of plant root exudates and bacterial communities, which will result in different coatings being available to ENMs in different areas. There is also the possibility of physical straining and collection at air–water-soil interfaces when flowing through porous media like soil. Physical straining of high aspect ratio ENMs in soil has been demonstrated with single-walled carbon nanotubes and implicated as a primary retention mechanism for nanoscale Fe0 in a sandy loam soil.
As aggregation caused by high ionic strength, pHs near the PZC, or coatings increases, physical straining becomes more likely, particularly in soils like Vertisols or Ultisols that are characterized by small pore sizes. Two hypotheses were addressed in these series of experiments. The first hypothesis was that ENM transport would be limited to the upper layers of soil, but particles coated with NOM would penetrate further into the soil due to increased electrostatic repulsive forces as a result of their more negative surface charge.The second hypothesis was that particles would be transported further through potting soil than agricultural or grassland soils due to the greater density and clay contents of the two natural soils causing increased physical straining and electrostatic/chemical sorption. Stock suspensions of CeO2, Cu2, and TiO2 ENMs were prepared by suspending dry ENM powders in 18.2 MΩ cm Nanopure water and sonicating for 30 min in a bath sonicator . Stock suspensions were sonicated for 10 min after dilution to the desired concentration and used within 24 hr. Suwannee River NOM stock solutions were prepared as described in Zhou and Keller 18 . Hydrodynamic diameter and ζ-potential of TiO2, CeO2, and Cu2 ENMs with and without NOM were measured via dynamic light scattering at 20oC by preparing 10 mg L-1 ENM suspensions with and without the addition of 1 mg L-1 NOM in Nanopure water and in soil solution extracts through dilution of a 100 mg L-1 stock, probe sonicating for 2 sec at 20% amplitude with a Misonix Sonicator S- 4000 .ENM transport through the three soils was tested by loosely packing 2.5 cm diameter x 16.34 cm long cylindrical plastic columns with air-dried soil. Due to their different densities, 17.5 ± 0.1 g potting soil, 136 ± 1 g grass soil, or 167 ± 1 g farm soil were needed to completely fill the columns. To simulate gravity-driven transport of ENMs in suspension, 50 mL of 100 mg L-1 TiO2, CeO2, or Cu2 ENM suspensions with or without the addition of 10 mg L-1 NOM were slowly applied to the top of the column. The resulting soil ENM concentrations were on the high end of those currently predicted for metal oxides in soil,but were well within the concentrations predicted for biosolids.Hence, the soil ENM concentrations used in this experiment may be indicative of those found in soils repeatedly amended with biosolids. After ENM application, columns were allowed to drain overnight, oven dried at 60°C for 72 hours, and split into 3 cm segments, ~0.3 g sub–samples of which were weighed, digested, in 10 mL 1:3 HNO3:HCl at 200°C for 1.5 hours in a microwave digestion system followed by analysis via inductively coupled plasma atomic emission spectroscopy . This technique was sufficient to dissolve the soil and ≥90% of TiO2, CeO2, and Cu2. Detection limits for all elements tested were approximately 5 μg L-1 . Standard solutions and blanks were measured every 15-20 samples for quality assurance.