Date of Award

12-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Engineering and Science

Advisor

Carraway, Elizabeth R

Committee Member

Johnson , Alan R

Committee Member

Powell , Brian A

Committee Member

Schlautman , Mark A

Abstract

Increasing applications of nanoparticles (NPs), such as metal oxide and mercuric sulfide NPs, lead to heightened environmental concern. Challenges to the study of natural and engineered NPs include the physical and chemical characterizations.
Commercially available and laboratory-synthesized metal based NPs, iron oxide (Fe2O3), copper oxide (CuO), titanium dioxide (TiO2), zinc oxide (ZnO) and mercuric sulfide (HgS) were studied by comprehensive characterizations methods. The general synthesis process was modified sol-gel method. The size and morphology of NPs could be influenced by temperature, sonication, calcination, precursor concentration, pH and types of reaction media. In the synthesis of nZnO in ethanol, the essential factors were temperature and reaction time. The concentration of the precursors impacted the aggregate size. The method to synthesize nFe2O3 was controlled by sonication and pH condition. The dialysis method was compared. Both methods generate reliable products. To synthesize nTiO2, the pH and temperature were adjusted in the presence of precursor, such as TiCl4 or Ti(OCH(CH3)2)4. Base-hydrolysis of Cu(NO3)2 was used to generate nCuO. To get nano-sized fine particle, calcined time and temperature were the most important factors. Synthesis of nano-sized HgS has been less studied due to the more highly toxic precursor. Here, HgCl2 and sulfur powder were used as precursors. The pH of the synthesized medium was adjusted by sodium hydroxide. Control factors included sonication and nitrogen purge. Under ultrasound sonication bath, the synthesis process became more uniform and smaller particles. Plus, it provided an oxygen-free atmosphere with continuous nitrogen bubbling.
All types of the laboratory-synthesized or commercially available NPs were characterized by physical and chemical processes. The chemical composition was carried out by acidification or digestion and inductively coupled plasma mass spectroscopy (ICP-MS) or inductively coupled plasma atomic emission spectroscopy (ICP-AES) or flow injection mercury system (FIMS) measurement, which showed the purity of the laboratory -synthesized NPs. Physically, X-ray diffraction tested the crystallinity and confirmed the particle purity, UV-Visible spectroscopy was used as a real-time measurement to check the particle formation, growth, agglomeration and possible dispersion by sonication. One characteristic of NP that can lead to ambiguous toxicity test results was the effect of agglomeration of primary nano-sized particles. Laser light scattering was used to measure the aggregated and particle size distribution. Aggregation effects were apparent and often extensive in some synthesis approaches. Electron microscopy (SEM and TEM) gave the images of those laboratory-synthesized particles and aggregation. The average single particle was about 5-20 nm of ZnO; 20-40 nm of CuO; 10-20 nm of TiO2; 20-35 nm of Fe2O3; 10-15 nm of HgS, while the aggregate size was in the range of a hundred nanometers or more. These five types of NPs were obtained with spherical and oblong formation and the agglomeration of ZnO, CuO, HgS and TiO2 was random, but Fe2O3 has web-like aggregation. Other measurements performed on the particles and aggregates include bandgap energies, surface composition, surface area, hydrodynamic radius, and particle surface charge.
In aqueous environment, NPs are subject to processes such as solubilization and aggregation. These processes can be controlling factors in the fate of nanomaterials in environmental settings, including bioavailability to organisms. This study has focused primarily on measurement of the solubility in aqueous media of varying composition (pH, ionic strength, and organic carbon), sedimentation and stability. The aggregate size distribution was monitored in solubility experiments. Typical results show nano-sized primary particles forming aggregates approximately microns in diameter. Microfiltration method, evaluated by UV absorbance monitoring and concentration monitoring, was used to rapidly separate the suspended and settled NP aggregates from the aqueous phase. In a culture medium (FETAX), there was negligible dissolved metal released from nZnO and nCuO suspensions, whereas commercial nZnO showed measurable release of the dissolved Zn. The release of dissolved metals from nTiO2, nFe2O3 and nHgS suspensions was detectable, which could contribute to the toxicity effects of FETAX culture. The solubility was tested in acidic, neutral and alkaline solutions. Visual Minteq was used to model the solubility for comparison. The solubility of the nanoscale particles, nZnO (pH> pHpzc), nCuO (pH>6), nTiO2, nFe2O3 and nHgS increased relative to the larger size particles. At three NOM concentrations (5; 20 and 50 mg C/L), the solubility increased as the NOM concentration increased. For nZnO and nHgS, 20 mg C/L NOM produced smaller aggregation size than DDI; whereas other NPs showed larger aggregate sizes in NOM. NPs solubility was not influenced by ionic strength (0.01, 0.1 and 1.0 M NaNO3).
Nano mercuric sulfide was used to test the photochemical reaction in the presence of various compositions. Light-induced solubilization occurred after 7 hours irradiation. HAc/NaAc, NOM as methyl donor assisted mercury methylation with or without light irradiation. In alkaline media, which facilitated the dissolution of nHgS, methylation became more significant. Photodegradation of 10 mg C/L NOM under all circumstances was confirmed, and resulted in an increasing amount of dissolved mercury. However, the methylmercury, inorganic mercury changed unrelated with the presence of NOM. Fe(III) helped the mercury methylation and dissolution when its concentration was as high as 10 μM in alkaline media.

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