Date of Award
Doctor of Philosophy (PhD)
Environmental Engineering and Earth Sciences
Cerium oxide nanoparticles (CeO2 NPs) are being increasingly used in industrial and consumer products and the release of CeO2 NPs into the air, water, and soil is an inevitable consequence. Once released into the environment, CeO2 NPs can have adverse impacts on the environment and human health. Thus, characterizing the environmental behavior and toxic effects of CeO2 NPs is important to protect the environment and human health. To assure such protection, the stability and phytotoxicity of CeO2 NPs need to be known because they are key to understanding their transport, toxicity and bioavailability in water and soil. The objective of this dissertation is to characterize the stability and phytotoxicity of CeO2 NPs in water-soil-plant systems. The approach was to analyze the colloidal stability and aggregation kinetics of CeO2 NPs under the influence of pH, ionic composition (monovalent NaCl and divalent CaCl2 salts), and Suwannee river humic and fulvic acids. The root system architecture and plant growth of seedlings of three different sorghum cultivars (BTx 623, Grassl, and Rio) as well as composition traits, plant growth, and Ce accumulation of sweet sorghum Grassl, were studied to evaluate the phytotoxicity of CeO2 NPs to plants.
To analyze the stability of CeO2 NPs, the hydrodynamic diameter and zeta potential were measured over time at three different electrostatic scenarios related to pHPZC (pH > pHPZC, pH = pHPZC, and pH < pHPZC) in the first study and at different concentrations of NaCl and CaCl2 and different concentrations of humic acid (HA) and fulvic acid (FA) in the second study.
To quantify the phytotoxicity of CeO2 NPs to sorghum, root system architecture, seedling size, and biomass of three sorghum cultivars (BTx 623, Grassl, and Rio) at four CeO2 NP treatment levels (0, 100, 500, and 1000 mg/kg CeO2 NPs) were determined in the third study. In addition, composition traits, Ce accumulation, plant size, and biomass of sweet sorghum Grassl at four CeO2 NP treatment levels (0, 100, 500, and 1000 mg/kg CeO2 NPs) were determined in the fourth study.
Results of the first study show that (1) the zeta potential of CeO2 NPs, with a point of zero charge (pHPZC) of 10.2, decreased (from positive to negative) with increasing solution pH; and (2) the impacts of Na+ and Ca2+ cations and HA and FA on the levels and rates of aggregation were pH-dependent. Furthermore, in the presence of salts, CeO2 NPs were stable at pH < pHPZC (except 1 mM of NaCl/CaCl2) and pH > pHPZC (except 0.5 mM CaCl2), but aggregation was enhanced at pH = pHPZC, with the diameter of CeO2 NPs in the 1300 to 3600 nm range. The study also showed that (3) HA stabilized CeO2 NPs under pH > pHPZC, but aggregation was enhanced at pH = pHPZC with the diameter of CeO2 NPs in the 1500 to 1900 nm range (in the presence of 0 and 1 mM of NaCl/CaCl2 at pH < pHPZC); and (4) FA (0.14 mg/L) showed more efficiency in stabilizing the CeO2 NPs than HA (5 mg/L) at three pH levels (8.2, 10.2, and 12.2) and under all different electrolyte concentrations (0 – 1 mM of NaCl or CaCl2).
Results of the second study show that (1) homoaggregation of CeO2 NPs occured in the presence of Na+ (> 1 mM NaCl) and in the presence of Ca2+ (> 5 mM CaCl2); (2) the critical coagulation concentration (CCC) of CeO2 NPs in the presence of monovalent Na+ (30 mM) was twice as large as the CCC in the presence of the divalent Ca2+ (15 mM) at pH 5; (3) the influence of the divalent cation Ca2+ was more efficient than the monovalent cation Na+ in enhancing the aggregation of CeO2 NPs; (4) heteroaggregation of CeO2 NPs and HA was enhanced at higher NaCl concentrations (> 100 mM NaCl) due to electrostatic attraction and at higher CaCl2 concentrations (> 1 mM CaCl2) due to the bridging effect; and (5) when compared to FA, HA was not only more reactive in inhibiting the heteroaggregation of CeO2 NPs in the presence of NaCl, but also more efficient in enhancing the heteroaggregation of CeO2 NPs in the presence of CaCl2 (> 10 mM CaCl2).
Results of the third study show that (1) when the CeO2 NP treatment levels increased, a threshold of 500 mg/kg CeO2 NPs and a decreasing trend were found in values of primary root length, primary root surface area, number of lateral roots, surface area/volume ratios of lateral root, total root length, and total root surface area in BTx 623 and primary root length, surface area/volume ratios of lateral root, and total leaves weight in Grassl; and (2) the parameters, significantly greater at 100 mg/kg CeO2 NPs than the control, were a ratio of wet root weight/total wet weight in BTx 623; total wet root weight, total wet weight, and number of lateral roots in Grassl; and top leaf length and water content in Rio. Results also show that (3) a dose-response phenomenon—low CeO2 NP treatment level (100 mg/kg CeO2 NPs) stimulations and higher CeO2 NP treatment level (500 and 1000 mg/kg CeO2 NPs) reductions—was observed for number of lateral roots, total wet root weight, and total wet weight; and (4) Rio was more CeO2 NP tolerant, and BTx 623 and Grassl were more CeO2 NP sensitive at 500 mg/kg CeO2 NPs threshold.
Results of the fourth study show that (1) the growth parameters were inhibited at 1000 mg/kg CeO2 NP treatment; (2) Ce accumulation was promoted at 1000 mg/kg CeO2 NPs in Noncut; (3) CeO2 NPs, at some level, can improve the forage quality of sweet sorghum Grassl in terms of digestibility, energy, and minerals; Furthermore, the study shows that (4) the growth stage and cultivation period also showed positive impacts on the bioenergy quality of sorghum in terms of starch and sugar.
Mu, Linlin, "Stability and Toxicity of Cerium Oxide Nanoparticles in Water-Soil-Plant Systems" (2019). All Dissertations. 2487.