Polymeric Coatings for Targeted Nanoparticle Delivery to Subsurface Contaminants

Abstract

Terrestrial oil spills account for the majority of oil spills world wide and present a challenging remediation problem owing to the inaccessibility of subsurface petroleum hydrocarbons (PHC). Contaminants such as crude oil demonstrate acute and chronic toxicity, necessitating remediation activity which is applied in the form of ex situ and in situ treatments. Among in situ remediation techniques, nanomaterial-based treatment strategies have been developed over the past decade to take advantage of improved subsurface mobility and reaction kinetics due to particle size. Increased use of nanoremediation has led to development of coating strategies to improve efficiency of use, and such techniques have raised concerns over the release of mobile nanoparticles into the wider environment. This thesis focuses on the development of a nanoparticle coating to facilitate nanoparticle (NP) aqueous stability, mobility in porous media, and preferential adsorption to target contaminants in porous media. The concept of targeted delivery is borrowed from nano-medicine, where chemotherapeutic drugs are encapsulated by nanomaterials which target accumulation in diseased material through active or passive means. In this thesis, an amphiphilic polymer coating allows NP binding to a hydrophobic interface to localize the NP at the site of contamination and reduce NP migration past contaminated zones. The targeted binding, realized through hydrophobic interactions between the nanoparticle coating and the crude oil model contaminant, is an example of an active targeting technique. The NP surface was modified by oleic acid and Pluronic deposited in layers to produce an externally amphiphilic coating capable of stabilizing the NP in water and interacting with crude oil. By modifying the Pluronic coating concentration and Pluronic molecule hydrophobicity, we were able to tune the recovery of NP transport through clean porous media and NP binding to oil-impacted porous media. It was also found that Pluronic coating concentration influenced the morphology of the NP, producing larger aggregates of nanoparticles or individually stabilized nanoparticles. The effect of environmental factors such as oil concentration in porous media, oil type, temperature, and pH on nanoparticle transport and binding in flow-through sand packed columns was investigated. It was found that higher oil concentrations, longer crude oil molecules, and higher temperatures resulted in higher NP binding. pH was found to have no effect on nanoparticle attachment to clean or oil-impacted sands within the pH range of 5 – 9. High temperature was used to demonstrate complete NP retention in oil-impacted natural aquifer sand packed columns flow-through experiments, and solute transport simulation software was used to model NP transport and binding using an advection-dispersion equation with single-site attachment limited by Langmuirian blocking (1D-USAT). These parameters were used to predict the NP attachment profile within sand packed columns and how it might change under different conditions such as higher flow rate or oil concentration. NP attachment to clean sand was found to be in the range of 2 – 13 mg/kg and attachment was found to increase in the presence of oily sand in the range of 8 – 32 mg/kg, depending on the nanoparticle formulation and environmental factors selected. The attachment rate (kattach) for nanoparticles in oil-impacted sand exceeded the kattach for nanoparticles in clean sand by approximately one order of magnitude (10x). The attachment rates varied on the order of 10-5 - 10-4 s-1 in clean sand, while attachment rates varied on the order of 10-4 - 10-3 s-1 in oily sand. Detachment rates (kdetach) in clean sand flow-through were determined to be approximately equal based on 1D-USAT modelling of experimental data – approximately 10-6 s-1. The NP coating strategy was applied to multiple NP core materials, including iron oxide, silver, and cobalt ferrite, all produced using different synthetic methods. The coated nanoparticles all demonstrated preferential binding to crude oil-impacted sands in binding batch tests, as well as breakthrough in clean sand transport experiments and retention in oil-impacted sand transport experiments. This showed that the NP coating could be applied to various types of NPs and conferred targeted delivery behaviour on each. Finally, potential application of targeted NP delivery to oil-impacted porous media was explored through the investigation of X-Ray computed tomography (X-Ray CT) as a sensing technique for detecting NP bound to oil-impacted sand. The oil-impacted sand exposed to Pluronic-coated NPs generated a CT signal sufficient to differentiate it from oil-impacted sand which was not exposed to NP. Conversely, clean sand exposed to Pluronic-coated NPs did not generate a substantial CT signal. This indicates that targeted NP binding to oil-impacted porous media may have use as a contrast enhancer for detecting contaminated zones at sites of concern. This thesis summarizes the development process of a nanoparticle coating facilitating transport through porous media and targeted binding to crude oil emplaced therein. The Pluronic-coated nanoparticles demonstrated preferential attachment to oil-impacted sediments, transport through clean sand packed columns, and retention in oil-impacted sand packed columns. This nanoparticle coating-strategy shows promise as a versatile technique for enhancing nanoparticle accumulation in contaminated subsurface areas which may enable contaminant detection and enhanced remediation, as well as reduce uncertain nanoparticle environmental fate in future applications.