Project Overview

Summary:

     In January 2016, Dr. Vijay John at Tulane University was awarded an RFP-V grant of $1,226,358 to lead the GoMRI project entitled, “The Design of Synergistic Dispersant and Herding Systems using Tubular Clay Structures and Gel Phase Materials” consisted of consisted of 2 collaborative institutions and approximately 10 research team members (including students).

     Dispersants are typically solutions containing one or more surfactants dissolved in a solvent. They work by reducing the interfacial tension between oil and water, thereby reducing the work needed to break oil into sufficiently small droplets that are in the colloidal size range and disperse into the water column. The COREXIT class of dispersants (C9500) was used extensively in the Deepwater Horizon incident, and was considered a success in preventing significant amounts of oil from reaching the shoreline. The ecological consequences of deep-sea dispersant addition and subsequent oil dispersion are issues of intensive research efforts.

     From a technological perspective, there are significant opportunities to improve dispersant efficiency. C9500 and other commercial dispersants are not effective in the dispersion of weathered oil and high viscosity crudes. Some components of C9500, in particular the di-octyl sodium sulfosuccinate (DOSS) component, may persist for extended periods in the marine environment. C9500 also contains a significant amount of paraffin as solvent, and alternative formulations that decrease the solvent content while maintaining efficiency are desirable. Being a liquid solution, significant amounts of dispersant become wasted if encounter with oil is not rapidly realized.

     It is therefore proposed to conduct fundamental and applied research to develop dispersant systems that are synergistic with C9500, but that may alleviate many of the disadvantages of C9500 without the need for entirely different chemical components. This is motivated by the realization that many years of research have gone into the development of C9500 which is currently stockpiled along coastlines of offshore oil exploration and production. The proposed research involves fundamental concepts relevant to the stabilization oil droplets by particles (Pickering emulsions) that are relevant to the formation of oilmineral aggregates. While such particles stabilize oil droplets against coalescence, they do not lead to the generation of small droplets which require the surfactants in dispersants to significantly reduce the oil-water interfacial tension. The innovation in the proposed work lies in the use of natural tubular clays known as halloysites which are available in the large quantities necessary for oil spill remediation. When filled with surfactant, the clays not only stabilize the oil drops against coalescence, but also reduce the interfacial tension through a targeted release of surfactant to the oil-water interface. This is Specific Aim 1 of the proposal. Concomitantly, it is proposed to develop a new gel-based dispersant that adheres to the oil and is buoyant, thus encountering oil efficiently, and has the potential to disperse weathered oil. The encapsulation of these gels into the tubular lumen of halloysite and the targeted delivery to oil are the topics of Specific Aim 2.

     It is also the hypothesis that the presence of a solid phase (halloysite clay tubes) at the oil-water interface will facilitate anchoring of microbial oil degrading communities to the interface and will enhance biodegradation. Specific aim 3 therefore, is to examine the microbial degradation of oil when the interface is stabilized by halloysite. Our innovation lies in the understanding of microbial biodegradation by following the process at the nanoscale using high resolution cryogenic electron microscopy to characterize biofilm formation and the dynamics of oil droplet degradation. It is also the objective that such studies will provide insights into the formation of marine snow.

Research Highlights

     As of December 31, 2019, this project’s research resulted in 10 peer-reviewed publications and 28 scientific presentations, and 10 datasets being submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are/will be made available to the public. The project also engaged 6 PhD level students over its award period. Significant outcomes of this project’s research according to GoMRI Research Theme are highlighted below.

All outcomes relate to Research Theme 4. The highlights are essentially abstracts of our key papers.

Development of a stimulus responsive delivery of surfactant to the oil-water interface. 

The use of chemical dispersants is a well-established approach to oil spill remediation where surfactants in an appropriate solvent are contacted with the oil to reduce the oil-water interfacial tension and create small oil droplets capable of being sustained in the water column. Dispersant formulations typically include organic solvents, and to minimize environmental impacts of dispersant use and avoid surfactant wastage it is beneficial to use water-based systems and target the oil-water interface. The approach here involves the tubular clay minerals known as halloysite nanotubes (HNTs) that serve as nanosized reservoir for surfactants. Such particles generate Pickering emulsions with oil, and the release of surfactant reduces the interfacial tension to extremely low values allowing small droplets to be formed that are colloidally stable in the water column. We report new findings on engineering the surfactant-loaded halloysite nanotubes to be stimuli responsive such that the release of surfactant is triggered by contact with oil. This is achieved by forming a thin coating of wax to stopper the nanotubes to prevent the premature release of surfactant. Surfactant release only occurs when the wax dissolves upon contact with oil. The system thus represents an environmentally benign approach where the wax coated HNTs are dispersed in an aqueous solvent and delivered to an oil spill whereupon they release surfactant to the oil-water interface upon contact with oil.

Development of Clay Nanotube Marbles Enhanced with Inner Biofilm Formation for the Encapsulation and Storage of Bacteria to be used in augmenting bioremediation. 

Reversing the organization of oil-in-water Pickering emulsion formed with halloysite nanoclay, liquid marbles were created with water droplets encapsulated by a layer of clay nanotubes. After incorporating microorganisms inside the spherical liquid marble of 0.5 mm in diameter, the simple clay capsule has potential as a storage vehicle for bacterial cultures. Halloysite nanotubes are naturally formed biocompatible and widely available aluminosilicate clay. The inherently hydrophilic halloysite is rendered hydrophobic (contact angle >90°) through silane grafting of long alkane groups onto the external surface. Because of the water repellency, the nanotubes are able to trap water inside a thin shell making a stable interface between inner liquid and air, thus forming marbles with reversed emulsion architecture. The halloysite with its alkane modifications encourages the growth of selected bacteria inside the marble; Alcanivorax borkumensis is one such example. The biofilm produced at the inner walls of the halloysite shell by such bacteria strengthens the marbles’ structure and reduced evaporation, keeping the bacteria viable for a period of up to 4 days after drying. A symbiotic correlation between the halloysite external shell and bacteria creates stable liquid marble structures, paving the way for a strategy to encapsulate and store bacteria at room temperature.

Understanding Biofilm Formation by Hydrocarbon-Degrading Marine Bacteria and its Effects on Oil Dispersion

Biodegradation of oil by marine bacteria is a significant pathway to oil spill remediation. Marine hydrocarbon degrading bacteria are known to form biofilms consisting of exopolymer and interconnected bacterial cells. This work indicates that microbial biofilm aids in the stabilization of dispersed oil droplets through the formation of biofilm at the oil-water interface and is therefore an environmentally benign and sustainable method to aid dispersion of spilled oil. Using a model hydrocarbon degrading organism Alcanivorax borkumensis, we show, through a combination of optical and high-resolution cryogenic scanning electron microscopy, that these microbes sequester into biofilm at the oil-water interface. We show that the bacterial culture incubated for 3 days and containing biofilm can disperse oil slicks moderately well (40-50%) as estimated by the baffled flask test and can thus be used as an environmentally benign response to oil spills. The dispersion occurs through bacterial adsorption at the oil-water interface together with the aid of naturally secreted biosurfactants that lower the oil-water interfacial tension by a factor of 2 to around 23 mN/m. When the bacterial culture is incubated for a week, the presence of biofilm at the interface can have a hindering effect at oil dispersion through formation of a rigid interfacial layer of biofilm. We show that the dispersion effectiveness of the commercial dispersant Corexit 9500A decreased approximately 25% in the presence of a mature microbial biofilm at the interface. Hexadecane biodegradation by the microbial culture was estimated, and it was found that approximately 90% of hexadecane was degraded in the period of 5 days. This work provides a comprehensive view on marine microbial biofilm from a detailed characterization at the formation stage to the overall role in the context of oil spill dispersion and further biodegradation. Bacterial biofilm and biosurfactants represent fully environmentally sustainable and natural materials for oil spill dispersion.

 The use of Stoppers and Skins on Clay Nanotubes to help Stabilize Oil-in-Water Emulsions and Modulate the Release of Encapsulated Surfactants

This work develops the concepts of particle-stabilized emulsions using tubular natural clays known as halloysites to attach to the oil-water interface and stabilize oil-in-water emulsions. Such halloysite nanotubes (HNT) serve as reservoirs for surfactants and can deliver surfactants to the oil-water interface and thus lower the oil-water interfacial tension. This two-step concept of surfactant delivery and droplet stabilization by particles has significant implications to oil spill remediation. However, to deliver surfactant loaded HNTs in a water-based solvent slurry, it is important to stopper the nanotubes to prevent premature release of the surfactant. This work focuses on the use of an environmentally benign two-dimensional metal-organic framework formed by coordinating Fe(III) with a polyphenolic as a stoppering agent. Such metal-phenolic networks (MPN) form a skin around the HNTs, thus providing a way to effectively sequester surfactant cargo for controlled release. Cryo-scanning electron microscopy (Cryo-SEM) shows that these HNTs and HNT bundles attach to the oil-water interface with side-on orientation. Inverted drop tensiometry was used to characterize the dynamic interfacial tension resulting from the release of a model surfactant (Tween 80) from the HNTs and indicates that the MPN stoppers are effective in sequestering the surfactant cargo for extended periods at neutral pH values. Release triggered by MPN disassembly at acidic pH values can be performed just prior to delivery to oil spills. The concepts and scalability of this process have significant implications for oil spill remediation, enhanced oil recovery, and biomedical and pharmaceutical applications.

 The Development of Amphiphilic Polypeptoid-Functionalized Halloysite Nanotubes as Emulsion Stabilizers for Oil Spill Remediation

Halloysite nanotubes (HNTs), naturally occurring and environmental benign clay nanoparticles, have been successfully functionalized with amphiphilic polypeptoid polymers by surface-initiated polymerization methods and investigated as emulsion stabilizers toward oil spill remediation. The hydrophilicity and lipophilicity balance (HLB) of the grafted polypeptoids was shown to affect the wettability of functionalized HNTs and their performance as stabilizers for oil-in-water emulsions. The functionalized HNTs having relatively high hydrophobic content (HLB = 12.0–15.0) afforded the most stable oil-in-water emulsions containing the smallest oil droplet sizes. This has been attributed to the augmented interfacial activities of polypeptoid-functionalized HNTs, resulting in more effective reduction of interfacial tension, enhancement of thermodynamic propensity of the HNT particles to partition at the oil–water interface, and increased emulsion viscosity relative to the pristine HNTs. Cell culture studies have revealed that polypeptoid-functionalized HNTs are noncytotoxic toward Alcanivorax borkumensis, a dominant alkane degrading bacterium found in the ocean after oil spill. Notably, the functionalized HNTs with higher hydrophobic polypeptoid content (HLB = 12.0–14.3) were shown to induce more cell proliferation than either pristine HNTs or those functionalized with less hydrophobic polypeptoids. It was postulated that the functionalized HNTs with higher hydrophobic polypeptoid content may promote the bacterial proliferation by providing larger oil–water interfacial area and better anchoring of bacteria at the interface.

  Proposal Abstract - RFP-V PI Vijay T. John