Browsing by Author "Plata, Desiree L"

Oil spills in marine environments often damage marine and coastal life if not remediated rapidly and efficiently. In spite of the strict enforcement of environmental legislations (i.e., Oil Pollution Act 1990) following the Exxon Valdez oil spill (June 1989; the second biggest oil spill in U.S. history), the Macondo well blowout disaster (April 2010) released 18 times more oil. Strikingly, the response methods used to contain and capture spilled oil after both accidents were nearly identical, note that more than two decades separate Exxon Valdez (1989) and Macondo well (2010) accidents.

The goal of this dissertation was to investigate new advanced materials (mechanically strong aerogel composite blankets-Cabot® Thermal Wrap™ (TW) and Aspen Aerogels® Spaceloft® (SL)), and their applications for oil capture and recovery to overcome the current material limitations in oil spill response methods. First, uptake of different solvents and oils were studied to answer the following question: do these blanket aerogel composites have competitive oil uptake compared to state-of-the-art oil sorbents (i.e., polyurethane foam-PUF)? In addition to their competitive mechanical strength (766, 380, 92 kPa for Spaceloft, Thermal Wrap, and PUF, respectively), our results showed that aerogel composites have three critical advantages over PUF: rapid (3-5 min.) and high (more than two times of PUF’s uptake) oil uptake, reusability (over 10 cycles), and oil recoverability (up to 60%) via mechanical extraction. Chemical-specific sorption experiments showed that the dominant uptake mechanism of aerogels is adsorption to the internal surface, with some contribution of absorption into the pore space.

Second, we investigated the potential environmental impacts (energy and chemical burdens) associated with manufacturing, use, and disposal of SL aerogel and PUF to remove the oil (i.e., 1 m3 oil) from a location (i.e., Macondo well). Different use (single and multiple use) and end of life (landfill, incinerator, and waste-to-energy) scenarios were assessed, and our results demonstrated that multiple use, and waste-to-energy choices minimize the energy and material use of SL aerogel. Nevertheless, using SL once and disposing via landfill still offers environmental and cost savings benefits relative to PUF, and so these benefits are preserved irrespective of the oil-spill-response operator choices.

To inform future aerogel manufacture, we investigated the different laboratory-scale aerogel fabrication technologies (rapid supercritical extraction (RSCE), CO2 supercritical extraction (CSCE), alcohol supercritical extraction (ASCE)). Our results from anticipatory LCA for laboratory-scaled aerogel fabrication demonstrated that RSCE method offers lower cumulative energy and ecotoxicity impacts compared to conventional aerogel fabrication methods (CSCE and ASCE).

The final objective of this study was to investigate different surface coating techniques to enhance oil recovery by modifying the existing aerogel surface chemistries to develop chemically responsive materials (switchable hydrophobicity in response to a CO2 stimulus). Our results showed that studied surface coating methods (drop casting, dip coating, and physical vapor deposition) were partially successful to modify surface with CO2 switchable chemical (tributylpentanamidine), likely because of the heterogeneous fiber structure of the aerogel blankets. A possible solution to these non-uniform coatings would be to include switchable chemical as a precursor during the gel preparation to chemically attach the switchable chemical to the pores of the aerogel.

Taken as a whole, the implications of this work are that mechanical deployment and recovery of aerogel composite blankets is a viable oil spill response strategy that can be deployed today. This will ultimately enable better oil uptake without the uptake of water, potential reuse of the collected oil, reduced material and energy burdens compared to competitive sorbents (e.g., PUF), and reduced occupational exposure to oiled sorbents. In addition, sorbent blankets and booms could be deployed in coastal and open-ocean settings, respectively, which was previously impossible.

As the United States slowly transitions from coal and foreign oil to more renewable energy sources, domestic natural gas is being utilized as an intermediate "bridge fuel". Advances in horizontal drilling and hydraulic fracturing have increased gas extraction and exploration over the past ten years. The massive quantities of fluids used to fracture gas-producing shale plays contain potentially toxic chemicals and the wastes are highly saline and contain naturally occurring radioactive materials. It is unclear if there are risks to shallow drinking water aquifers and if the chemical constituents of fracking fluid are inherently biodegradable if released into the environment. Monitoring the fate of these chemicals as they enter the environment is critical to understanding potential health hazards and persistence. Here, groundwater monitoring for organic compounds coupled with spatial data analysis indicates no subsurface contamination from hydraulic fracturing of the Marcellus Shale in northeastern Pennsylvania. However, a chemical fingerprint is presented suggesting increased organic compounds in shallow groundwater are due to subsurface mixing with brine containing geogenic hydrocarbons from the Marcellus Shale.

To investigate the fate and extent of the chemicals used in hydraulic fracturing and the associated wastes if released into the environment, bench-scale reactor experiments were performed. These reactors contained a synthetic hydraulic fracturing fluid and activated sludge from a wastewater treatment plant. Bulk organic carbon decreased upwards of 76 ± 2% in a freshwater mixture and 69 ± 2% in a 10,000 mg L-1 NaCl saline solution. Furthermore, gasoline range organic compounds degraded over 99 ± 1% for both solutions while diesel range organic compounds recorded significantly less degradation in the saline solution than the freshwater solution (68 ± 2% and 92 ± 1%, respectively). Recalcitrant compounds of interest proved to be in the higher-molecular weight range of diesel range organic constituents. Additionally, salinity decreased the initial concentrations of both gasoline range and diesel range organic compounds in the synthetic frack solution. To explain the concentration differences in the saline reactors, the compound losses due to volatilization were quantified and further loss mechanisms were suspected to be sorption to particle surfaces.

Manufacturing of carbon nanotubes (CNTs) via chemical vapor deposition (CVD) calls for thermal treatment associated with gas-phase rearrangement and catalyst deposition to achieve high cost efficiency and limited influence on environmental impact. Taking advantage of higher degree of structure control and economical efficiency, catalytic chemical vapor deposition (CCVD) has currently become the most prevailing synthesis approach for the synthesis of large-scale pure CNTs in past years. Because the synthesis process of CNTs dominates the potential ecotoxic impacts, materials consumption, energy consumption and greenhouse gas emissions should be further limited to efficiently reduce life cycle ecotoxicity of carbon naotubes. However, efforts to reduce energy and material requirements in synthesis of CNTs by CCVD are hindered by a lack of mechanistic understanding. In this thesis, the effect of operating parameters, especially the temperature, carbon source concentration, and residence time on the synthesis were studied to improve the production efficiency in a different angle. Thus, implications on the choice of operating parameters could be provided to help the synthesis of carbon nanotubes.

Here, we investigated the typical operating parameters in conditions that have yielded successful CNT production in the published academic literature of over seventy articles. The data were filtered by quality of the resultant product and deemed either "successful" or "unsuccessful" according to the authors. Furthermore, growth rate data were tabulated and used as performance metric for the process whenever possible. The data provided us an opportunity to prompt possible and common methods for practioners in the synthesis of CNTs and motivate routes to achieve energy and material minimization.

The statistical analysis revealed that methane and ethylene often rely on thermal conversion process to form direct carbon precursor; further, methane and ethylene could not be the direct CNT precursors by themselves. Acetylene does not show an additional energy demand or thermal conversion in the synthesis, and it could be the direct CNT precursors by itself; or at least, it would be most easily to get access to carbon nanotube growth while minimizing synthesis temperature. In detail, methane employs more energy demand (Tavg=883℃)than ethylene (Tavg=766℃), which in turn demands more energy than acetylene (Tavg=710℃) to successfully synthesize carbon nanotubes. The distinction in energy demand could be the result of kinetic energy requirements by the thermal conversion process of methane and ethylene to form direct CNT precursors, and methane employs the highest activation demand among three hydrocarbons. Thus, these results support the hypothesis that methane and ethylene could be thermally converted to form acetylene before CNT incorporation.

In addition, methane and ethylene show the demand for hydrogen in thermal conversion process before CNT incorporation; whereas, hydrogen does not contribute to the synthesis via acetylene before CNT incorporation, except the reduction of catalyst. At relatively low hydrogen concentration, this work suggests that hydrogen prompts growth of carbon nanotubes via methane and ethylene, probably by reducing the catalysts or participating thermal reactions. In addition, "polymerization-like formation mechanism" could be supported by the higher growth rate of CNTs via ethylene than acetylene.

There could be an optimum residence time to maintain a relatively higher growth rate. At too low residence time, carbon source could not be accumulated, causing a waste of material; while too high residence time may cause the limitation of carbon source supplement and accumulation of byproducts.

At last, high concentration of carbon source and hydrogen could cause more energy consumption, while it helps to achieve a high growth rate, due to the more presence of direct carbon precursor.

Advanced electronics and the clean energy industries increasingly rely on rare earth and specialty metals (e.g., yttrium, osmium, and indium; RESE). This may cause a bottleneck effect where demand for these metals cannot keep up with supply from primary extraction alone. Furthermore, few management strategies exist across the life cycle, leading to low recycling rates of less than 1% and little to no reuse of materials. These factors have led the U.S. Department of Energy to label these metals as “critical”, with respect to their importance to clean energy.

The goal of this thesis was to target the problem of material criticality and provide specific scientific advancements needed to mitigate this issue to enable a circular economy within the electronics industry. After identifying these needs across the life cycle, it was apparent that the manufacturing and end-of-life stages were where technological contributions could have the biggest impact. In response to this need, I then focused on building a device capable of recycling RESE for direct reuse in industrial manufacturing.

First, I outlined the technical objectives and advancements needed across the electronics life cycle to enable a closed-loop system. Here, I focused on detailing tasks including: (1) design devices for disassembly, (2) materials for substitution, (3) manufacturing processes that enable the use of recycled materials, (4) fabrication efficiency, (5) technology interventions to enable e-waste recovery, (6) methods to collect and separate e-waste components, (7) technologies to digest and recover RESE, and (8) technologies to separate commercially desirable, high-purity outputs.

Second, I developed a technology to tackle one of these necessary advancements, which was developing a device to recycle RESE from industrial waste streams. The objective was to build a technology capable of selectively recovering and separating RESE on individual filters to enable direct reuse. Here, I successfully built a carbon nanotube (CNT)-enabled filter device that electrochemically recovered and separated RESE from bulk metals, recovering them as metal oxides. I also detailed the mechanism of recovery, which was determined to be an oxygen reduction mechanism, and separation was based on hydroxide stability.

To increase the selectivity of the device, I then tested redox mediators (i.e., organic molecules that facilitate electron transfer and redox equilibrium) in the system to obtain direct metal reduction to reclaim zero-valent metal instead of metal oxides. Here, the goal was to develop a method to separate these metals based on their reduction potentials instead of their hydroxide stabilities. Results indicated that these mediators did not transfer electrons to the metals as anticipated, but instead were binding to the metals themselves, or were transferring the electrons to the oxygen, providing another pathway for enhanced oxide deposition.

Next, a variety of metal recovery techniques were tested to reclaim the metals off the CNT filters to provide metals for reuse in industrial manufacturing. Here, solution-based and solid-based methods were utilized, with acid washing and full filter combustion giving the highest recovery values, near 100%. Metal selectivity between acids indicated potential for further separation capability. Finally, my observations from the other chapters identified many key design modifications needed to enable the successful scale up of this technology. Here, I detail the specific designs modifications needed and provide rationale for each.

The contributions from my work include providing specific scientific advancements needed to enable a circular economy in the electronics industry, as well as providing a technology to recover and separate RESE from mixed metal streams. I also detail other strategies to further advance this technology, as well provide a detailed outline of design modifications needed for scale up. This set of work provides motivation and an outline of research needed to encourage academia and industry to pursue work in this critically important field.

Overall, this technology could offer several advantages including: enhanced recovery of high-value specialty minerals using low-cost CNT filters, reduced need for mining Earth-rare minerals in politically unstable or environmentally undesirable locations, and reduced emissions of toxic elements or nascent industrial minerals that have yet unknown toxicities or environmental impacts.

Chemical vapor deposition (CVD) has been recognized as one of the most promising methods to produce carbon nanotubes (CNTs) industrially. Oxygen is important in CNT high-volume production, but few of the studies propose mechanistic details for how oxygen exerts these effects. Since reported optimization conditions to generate CNTs are based on empirical results, several gray areas still exist in the CNT growth mechanism. Uncovering the CNT growth mechanism, especially for the oxygen-related CNT synthesis, is necessary to promote CNT production with atomistic control.

Here, in order to separate gas and catalyst thermal effects and allow for uniform transformation of gas as it approaches substrate, a specifically designed CVD reactor was assembled for determining the CNT growth mechanism. Two typical and promising oxygen-related CVD processes (equimolar C2H2-CO2 reaction and water-assisted CNT growth) were studied to analyze the role of oxygen. The reported equimolar C2H2-CO2 reaction process could deposit CNTs on various substrates and at relatively low temperature. The water-assisted CVD process could obtain dense, high-purity, and millimeter-scale CNTs, which is promising for mass production of CNTs.

Firstly, native CO2 (12C CO2) and 13C-labeled CO2 were individually used as feedstock with C2H2 to grow CNTs. A statistical study using the Raman spectra of the yielded CNTs in both conditions indicated that the C in CO2 was not incorporated into CNTs. Based on this conclusion, an electron-pushing CNT growth mechanism was proposed. The role of oxygen in CO2 was indicated as grasping hydrogen atom in the raw CNT lattice.

Secondly, CNTs were synthesized with various H2O concentrations ranging from 10 parts per million (ppm) to 220 ppm. Quantitative study of the CNT outer diameters by TEM imaging indicated that the outer diameter tended to increase with H2O concentration (the average outer diameter ranges from 4.8 nm at 10 ppm H2O to 6.4 nm at 220 ppm H2O). Raman spectra revealed that the dominant CNTs went from SWCNTs at 10 ppm H2O to MWCNTs at high H2O concentration, consistent with the diameter increase trend. The formed catalyst might explain the CNT quality change. The AFM images of the catalyst demonstrated that the height, size and spacing of the iron nanoparticles on the substrate increased with water concentration. The alignment property was tested by SEM imaging. The yielded CNTs at 120 ppm H2O got the best alignment. The gas composition analysis results indicated that the H2O might promote the decomposition of the main carbon precursor. The oxygen in H2O might influence the catalyst activation and carbon precursor decomposition.

Through the CNT characterization and gas composition analysis, the role of oxygen could be categorized into three areas: 1) absorb hydrogen in immature CNT lattice; 2) influence catalyst formation; 3) promote carbon precursor decomposition.