Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemical Engineering

Major Professor

Norma Alcantar, Ph.D.

Committee Member

Julianne Harmon, Ph.D.

Committee Member

John Kuhn, Ph.D.

Committee Member

Sylvia Thomas, Ph.D.

Committee Member

Ryan Toomey, Ph.D.

Keywords

Natural Materials, Ocean Pollution, Surfactant, UV-Vis

Abstract

Oil spills are a critical threat to the marine environment, and the spilled oil as a source of pollution can have long-term adverse effects on the marine ecosystem. Traditional mechanical clean-up operations have apparent advantages for dealing with small and medium-scale oil spills. For large-scale oil spills, dispersants are a practical and minimally influential approach to the environment. As the dispersant components are iteratively updated, dispersants become more efficient and safer. However, the toxicity of the dispersants remains a significant impediment to their application. An efficient and non-toxic dispersant is increasingly becoming a requirement.

The cactus mucilage extracts studied in this research were extracted from the leaves of the Opuntia ficus-indica cactus. The extraction process yields two types of mucilage extracts: gelling extract (GE) and non-gelling extract (NE). As natural materials, NE and GE have the advantage of being biodegradable and non-toxic. This work centered on using NE for the research of phenomena and properties on the surface and interface because NE performance as a dispersant was better than GE. Neutral sugars, carbohydrates, and proteins are the main compounds in NE. This composition has an interesting correlation with the intermolecular forces in the surfactants.

The overarching goal of this dissertation is to investigate the application of cactus-based natural dispersant (non-gelling extract, NE) to be implemented in oil spill response strategies. To complete this goal, the dispersion mechanism of the dispersant was investigated; the effectiveness and toxicity of the dispersant were evaluated and determined; and the effectiveness of the dispersant was also improved.

The evaluation to assess the efficacy of NE to disperse crude oil was based on characterizing surface and interfacial tension, dispersion efficiency, mixing effects, salinity effects, stability, and droplets size distributions. The air-water surface tension of NE solution decreased as the concentration of NE increased. There was a linear relationship between the surface tensions and the natural logarithm of the concentrations of NE. The application of NE in water was found to decrease the interfacial tension between oil and water; this is the main feature of a surfactant. Such a phenomenon is also the primary reason and mechanism by which the dispersant can disperse the oil into the water column. We found that the surface tension of the oil-in-water (O/W) emulsion was significantly reduced with the addition of NE at oil to emulsion volume ratios of 3% and 6% v/v. The salinity of the seawater and the mixing energy both affected the dispersion efficiency of the NE. It was observed that NE provided a smaller weighted average diameter to the dispersed O/W emulsion compared to that with COREXIT EC9500A.

In order to improve the dispersion effectiveness of NE, d-limonene (DL) and silica nanoparticles (SiNPs) have been applied with NE as a new dispersant system. The synergistic effect of DL and SiNPs on NE as a dispersant system was studied based on surface tension measurement, baffled flask test, shaking turbidity test, and droplet size measurement. Addition of DL can further reduce the surface tension of the O/W emulsion, which indicated the emulsion balance was pushed firmly from W/O emulsion further towards dispersion. The stability of the O/W emulsion was increased by adding of SiNPs. The synergy of both DL and SiNPs significantly improved the dispersion effectiveness of NE. The volume ratios among NE, DL, and SiNPs have been optimized through dispersion effectiveness measurement by using the baffled flask test. The shaking turbidity test was used to quantify the stability of O/W emulsion in a shaking environment. Through the droplet size analysis, O/W emulsion with NE+DL+SiNPs dispersant system had a smaller weighted average droplet size and a more stable performance. As an environment-friendly dispersant system, NE+DL+SiNPs can be used as a candidate for an efficient and stable dispersant.

Daphnia magna neonates were used to determine the acute toxicity of NE and COREXIT EC9500A. It was found that NE can be classified as practically nontoxic to daphnia magna neonates, but COREXIT EC9500A was found to be highly toxic to daphnia magna neonates. MTT cell viability assay with Neuro2a (N2a) cell line was applied to investigate the toxicity of COREXIT EC9500A and NE based dispersant system. We found that COREXIT EC9500A was significantly toxic to N2a cells after 50 ppm. However, NE had no effect on N2a cells until the concentration reached 500 ppm. It was found that the addition of d-limonene had a negative impact on the toxicity of the NE based dispersant system.

In summary, the dispersion mechanism of the cactus mucilage extract as a dispersant was evaluated the various indicators of the cactus mucilage extract as a dispersant, and studied the factors affecting the dispersion efficiency of the cactus mucilage extract. Synergy of d-limonene and silica nanoparticles with cactus mucilage extract on the dispersion process of crude oil and significantly improved the dispersion efficiency of the cactus mucilage extract dispersant system. In the end, the toxicity of the cactus mucilage extract based dispersant system was evaluated by using the acute toxicity assessment with daphnia magna and MTT cell viability assay with N2a cell line.

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