Document Type

Dissertation

Date of Award

12-31-2020

Degree Name

Doctor of Philosophy in Civil Engineering - (Ph.D.)

Department

Civil and Environmental Engineering

First Advisor

Jay N. Meegoda

Second Advisor

Wen Zhang

Third Advisor

Taha F. Marhaba

Fourth Advisor

Bruno M. Goncalves da Silva

Fifth Advisor

Datta, Dibakar

Sixth Advisor

David Washington

Abstract

This dissertation consists of two sections. First, nanobubbles' stability and behavior are studied using experimental and theoretical approaches. Second, nanobubbles application combined with ultrasound to remediate contaminated sediments is discussed.

The stability study consists of four sections. (i). Laboratory investigation to determine bubble size distributions and zeta potentials for different gases, pH levels, temperatures, and salt conditions. (ii). A theoretical study based on the diffused double layer theory to explain nanobubbles' behavior in different NaCl concentrations. (iii). Nanobubbles' stability in electrolyte solutions under different ion valencies using deionized water, NaCl, Na2SO4, Na3PO4, CaCl2, and FeCl3. (iv). The molecular dynamic simulation to evaluate the O2 gas nanobubbles' properties and behavior.

Test results show that the average bubble size depends on the gas solubility in water. The zeta potential depends on the gas's ability to generate OH- ions at the gas-liquid interface. Bubbles with high negative zeta potentials can be generated in high pH solutions, low temperatures, and low salt concentrations. High pH solutions produce smaller but stable nanobubbles. With time, the zeta potential of bubbles decreases while the bubble size increases. Although bubble sizes are expected to decrease with time due to gas diffusion, the increased bubble sizes are attributed to the possible bubble coalescence.

With increase NaCl concentration, bubble size, surface charge density, and the number of negative charges on the bubble surface increases. In contrast, the magnitude of zeta/surface potential, double layer thickness, internal pressure, and electrostatic repulsion force decreases. The total net energy for 0.001 M NaCl solution had a 6.99x10-20 J energy barrier, which prevents bubble coalescence. In different valency salts solutions, size and zeta potential depend on solution pH and cation valency. The cation concentration at the bubble surface is higher than that of bulk, confirming the bubbles are negatively charged. The high valency cations could neutralize or completely reverse the bubble charge. There is no significant energy barrier to overcomes the attractive van der Waals forces for all the solutions, questioning the validity of the used Hamaker constant in calculations as that nanobubble may contain exceptional interfacial properties.

High inner density O2 gas nanobubble is simulated using LAMMPS molecular dynamics code. Bubble size is stable for the simulated length 5ns. After 3ns, the diffusion coefficient is small, and the gas diffusion rate becomes nearly constant. The calculation shows that the inside pressure and surface tension at the bubble surface decrease, and the external gas partial pressure increases with time. Thereby, the bubble is metastable with high inner gas density and the supersaturate condition with slow diffusion.

An in-situ sediment remediation method is studied using ultrasound and ozone nanobubbles to remediate contaminated sediments. Two experimental setups are conducted (i). remediation of organic contaminant (p-terphenyl), (ii). remediate both organic (p-terphenyl) and inorganic (chromium) chemicals in contaminated sediments. Experiments are conducted with ultrasound and ozone nanobubbles under different operating conditions. For organic contaminant treatment, the maximum treatment efficiency of 91.50%, and the combined contaminant treatment, the average removal efficiencies of 71% for chromium and 64% for p-terphenyl are recorded.

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