A kappa carrageenan sponge material for Marine oil spill remediation

There are significant problems associated with the methods employed to clean up oil released from non-natural sources (greater than 343 million gallons annually). Many of the cleanup methods utilized create additional waste or increase the toxicity of spilled oil. To address these problems, and sustainably and effectively remove spilled oil, an environmentally friendly, cost-effective, and biodegradable sorbent material is needed. Here, a super-oleophilic and reusable sponge material is created from a freeze dried, benzalkonium chloride modified, kappa carrageenan polymer matrix. This sponge adsorbs significant amounts of oil, is moderately reusable over multiple adsorption cycles, and is biodegradable in marine environments after use. Degradation tests indicate potential areas of improvement, and processing and material optimizations are needed to ensure the industrial applicability of the developed sponge material.

ABSTRACT There are significant problems associated with the methods employed to clean up oil released from non-natural sources (greater than 343 million gallons annually). Many of the cleanup methods utilized create additional waste or increase the toxicity of spilled oil. To address these problems, and sustainably and effectively remove spilled oil, an environmentally friendly, cost-effective, and biodegradable sorbent material is needed. Here, a super-oleophilic and reusable sponge material is created from a freeze dried, benzalkonium chloride modified, kappa carrageenan polymer matrix. This sponge adsorbs significant amounts of oil, is moderately reusable over multiple adsorption cycles, and is biodegradable in marine environments after use. Degradation tests indicate potential areas of improvement, and processing and material optimizations are needed to ensure the industrial applicability of the developed sponge material. ii TABLE OF CONTENTS ABSTRACT ii MOTIVATION 1 OIL SPILL RESPONSES: PRESENT TO FUTURE 3 MATERIALS AND METHODS 19 RESULTS AND DISCUSSION 21 CONCLUSIONS AND FUTURE WORK 34 ACKNOWLEDGEMENTS 37 REFERENCES 38 iii 1 MOTIVATION The British Petroleum Deep Horizon drilling platform failure in 2010, though highly publicized, is only one of many non-natural oil releases that occur each year. Over 185 million gallons of oil are released into marine environments annually; while coastal ecosystems can be detrimentally affected by the presence of oil that washes ashore from marine oil spills, the main problems surrounding these spills are not due only to the oil itself [1-4]. Many environmental and ecosystem problems are caused by the methods employed in an attempt to clean up spilled oil [5-7]. Cleanup methods include leaving the spill alone to naturally disperse, using booms to contain spills and specialized boats to collect surface oil, burning the oil off the water’s surface, and adding chemicals (dispersants) that cause the oil to disperse into smaller droplets in the water column, where they are left to natural oceanic processes. These methods are, at best, ineffective, and at worst, more wasteful and damaging to the ecosystem than the oil they remove; dispersants in particular are often toxic to marine and coastal animal and plant life, and yet are still widely used in oil spill remediation [6, 7]. These problems have spurred research into new ways of removing spilled oil without causing additional environmental harm. Materials currently being studied and explored include membranes, gelators, hydrogels, aerogels, and sponge-like materials, which adsorb hydrocarbons from oceanic surfaces [8]. Although there has been extensive research into naturally derived, bio-based materials for oil adsorption over the past decade, many of the foam and sponge-like materials studied are not yet environmentally and economically viable on an industrial scale [8]. To effectively deal with these problems, there is still a need to develop a reusable adsorbent sponge, created from entirely 2 sustainable materials, for use in oil spill remediation that leaves the environment free of both oil and additional contaminants. 3 OIL SPILL RESPONSES: PRESENT TO FUTURE 1. Impacts of and Responses to Marine Oil Spills The publicity given to extreme oil spills, such as the British Petroleum Deep Horizon drilling platform failure in 2010, exponentially increased public attention to marine pollution; however, the spillage of oil is not the only problem associated with such disasters. Since 1967, there have been 44 large marine oil spills (releasing over 10,000 barrels (420,000 gallons) each) affecting the waters surrounding the United States [2]. When smaller spills and spills affecting all countries globally are taken into account, the number and severity of oil spills greatly increases. Annually, more than 185 million gallons of oil are released into marine environments globally from a variety of non-natural sources [1]. These sources include the use and transportation of oil or other goods (oil tankers and pipelines, which account for approximately 70 percent of the total spilled, or commercial ships respectively), as well as offshore oil wells and smaller recreational vessels [2-4]. When large amounts of oil are released, marine ecosystems and communities are detrimentally affected in a variety of ways. In the time directly following oil spillage, lighter, more volatile hydrocarbons, such as toluene and benzenes, readily evaporate off the surface of the ocean [9]. These types of compounds negatively affect air quality and can lead to adverse health effects upon inhalation by all organisms including humans [10]. Oil compounds that do not evaporate are dispersed into the water column and may become incorporated into ocean sediments. Once incorporated, natural degradation processes are limited and toxic compounds present adversely affect benthic organisms [11]. Oil that remains on the surface, as an easily recognizable oil slick, is dispersed by waves and carried 4 away from the spill site by ocean currents and winds. As these oil compounds spread, they are often pushed towards shore, where they cause severe ecological damage. Shorebirds, otters, plants, and marshy and wetland ecosystems experience the highest casualties of all organisms affected by oceanic oil spills [9]. These organisms are often poisoned by ingesting or inhaling oil constituents or become too covered in oil to perform daily tasks such as preening, staying warm, identifying and communicating with young, as well as photosynthesis [2, 12]. If left uncollected, shoreline oil and its effects can last more than 25 years [9]. While some ecosystems are able to deal with spilled oil and recover within a few years, sensitive ecosystems with extreme biodiversity, such as the Galapagos Islands and marshy ecosystems, would not be able to regenerate and recover after an oil spill, even over long periods of time [5, 9]. Due to the wide range of adverse effects caused when oil is released into the environment, different cleanup methods are employed industrially to deal with these spills. The cleanup method utilized is dependent on the type of hydrocarbon released, the location of the spill, the severity of the spill (quantity released), the political and social environment at the time and location of the spill, and cleanup cost [3, 6, 8]. Booms, floating barriers made of a variety of materials, are often used to contain spills and keep them from spreading, after which specialized boats called skimmers are used to collect surface oil or oil is burned off the water’s surface [2]. As was the case for the British Petroleum Deep Horizon spill, chemicals called dispersants are often added to break oil up into smaller droplets in the water column. These dispersants hide the visible effects of spilled oil, but spread the oil underwater and increase its toxicity as oil is broken down into harmful byproducts [7]. Fish, corals, sea turtles, benthic organisms, and humans are then negatively 5 affected by this dispersed oil [7]. Though used less frequently, biological materials are added to assist in natural oil biodegradation near shore, or oil is simply left alone to naturally disperse through regular ocean processes, where accumulation in sediments still poses ecosystem problems [6, 13]. Sorbent materials are currently used to adsorb spilled oil on small scales; however, collection of sorbents and oil disposal after collection are still unsolved problems. Land based cleanup methods for dealing with oil that has been brought to shorelines often cause more ecological problems than the oil itself: collecting oil, burning or removing affected plant-life, and dispersant use can all cause additional damage, limit ecosystem recovery, and lead to loss of biodiversity [7, 9]. None of the cleanup methods currently employed for oil spills completely remove the spilled oil from marine environments. Additionally, cleanup methods can increase the toxicity of the oil and can leave additional debris behind [7, 8]. If marine oil spills are contained and thoroughly removed before they reach sensitive coastal ecosystems, many of these problems can be avoided; however, effective and sustainable cleanup of large oil spills is not currently achievable. The materials used often result in additional environmental pollution, take too long to employ, and do not effectively remove spilled oil [5, 8, 14]. Many sorption-based materials still contain toxic or environmentally harmful materials, degrade in water, and require clean up and disposal of the materials themselves after use. Because of these issues, there has been increased interest and demand for more environmentally friendly and sustainable materials to deal with oil spills in marine environments. Methods currently being studied and explored, but yet to reach industrial scale, rely on a variety of materials to collect or degrade marine oil spills. Research into these materials has focused on a variety of formulations and approaches; these include 6 membranes, gelators, hydrogels, aerogels, and sponge-like materials, which act as sorbents of hydrocarbons from oceanic surfaces [8]. Many of these materials show reusable characteristics and have extended lifetimes, further increasing their sustainability. Research into these materials has greatly increased over the past ten years, and future use seems promising; however, significant advancements are still needed for many configurations to be environmentally and economically viable on an industrial scale [8]. 2. Cleanly cleaning up oil: materials research to fix the problem New materials are being explored as potential alternatives to the environmentally hazardous cleanup methods currently used. Three main classes of sorbent materials have been extensively studied: carbon nanomaterial sorbents, synthetic sorbents, and natural sorbents [15]. Research within all three of these material classes strives for a similar set of optimal sorbent properties, defined by inherent hydrocarbon properties and marine specific application constraints. Throughout this field of research, there are several sorbent characteristics used to judge and evaluate the effectiveness of created materials and assess how their properties compare to idealized marine oil spill sorbents. Adsorption capacity, the most used figure of merit to report sorbent efficiencies and selectivities, is measured as grams adsorbed per gram of starting sorbent material (g/g). It is also important to note that adsorption capacities depend heavily on the type and density of oil adsorbed, giving rise to a range of adsorption values for various configurations created. Hydrophobic and oleophilic sorbent properties, especially important for use in marine oil spills, are assessed using contact angle measurements: hydrophilic/oleophilic materials show a contact angle below 90°, 7 hydrophobic/oleophobic materials show a contact angle between 90° and 150°, and superhydrophobic/super-oleophobic materials show a contact angle greater than 150°, using water and oil droplets on the sorbent surface, respectively [8]. Along with sorption properties, oil collection and reusability of sorbent materials are also important considerations for clean oil spill cleanups. Four main methods of reusability are reported, listed from most environmentally friendly to most environmentally taxing: mechanical extrusion/compression (squeezing), distillation (selective heating to collect oils and reuse sorbents), combustion (burning of saturated sorbents), and extraction (addition of solvents to remove oils from sorbents prior to heating) [8, 16]. Sorbent materials and physical properties determine which method is employed to collect oil; reusability studies based on retained adsorption capacity over a number of cycles are often included in characterization assessments of new sorbent materials. A general comparison of these characteristics between products and classes can assess which materials may perform better under future industrial oil spill remediation applications and can indicate which materials are in need of further development. 2.1 Carbon Nano-Material Sorbents Carbon-based nanomaterials are one of the recently studied and most promising oil sorbent materials. Research has covered theoretical studies, molecular simulations, and experimental syntheses of carbon materials [17]. These materials possess nearly all the necessary characteristics for an effective and useful sorbent; carbon nanomaterials have extremely high aspect ratios (surface area to volume), high structural/mechanical stability, high chemical stability, high adsorption capacity and efficiency, high porosity, low density 8 and therefore high buoyancy, and are potentially reusable [8, 17-19]. Researchers have explored many different configurations to optimize these features: carbon-based sorbent material designs include hydrogels, aerogels, separators or membranes, and sponge-like materials [8]. Of the explored carbon nanomaterial sorbents, aerogels are one of the most widely studied configurations. An aerogel is defined as a “gel comprised of a microporous solid in which the dispersed phase is a gas” [20]. Aerogels show promise as sorbent materials because of their extremely low density and highly porous, interconnected structures, typically created through a freeze-drying process. Several research groups have recently applied carbon in the form of graphene and carbon nanotubes (CNTs) or carbon nanofibers (CNFs) to create superhydrophobic and super-oleophilic aerogels [18, 21, 22]. Graphene, particularly when synthesized as a 3D network, is used because of its inherent hydrophobicity, macro-porosity and low density. CNTs or CNFs are then added to increase surface roughness, and therefore further increase hydrophobicity, of these graphene-based materials [21]. Additional treatment with nitrogen has been shown to increase fire resistance, particularly for combustion-based re-use, as well as aerogel efficiencies [23]. A wide variety of useful sorption properties have been shown with carbon-based aerogels; examples of some configurations and their sorption properties are shown in Table 1. CNTs have also been used to create sponge-based (as opposed to aerogel-based) sorbent materials [24]. CNT sponges have been synthesized with 3D interconnected frameworks, low density, and highly porous structures. One specific configuration, unlike its aerogel counter parts, swells when in contact with organic liquids, can adsorb oil from an area 800 times its own size, shows an adsorption capacity of 180 g/g, a 156° water 9 contact angle, and oil can be recollected after adsorption by either mechanical squeezing or combustion [24]. Table 1: Carbon-based aerogel sorbent materials and associated properties. Material configuration Adsorption capacity Reusability method and results 6 heating cycles, ~ constant adsorption Graphene/CNT Mechanical >100 g/g hybrid by MWIcompression, (reached within mediated approach 70-90% oil 20 sec) [22] recovery Graphene/CNT by Distillation, natural graphite 80% adsorption 30 g/g rock reduction capacity after 8 [18] cycles Distillation: 100 cycles, negligible adsorption Graphene/CNT 75.5 - 95.6 changes, with nitrogen [23] volume % Combustion: 100 cycles, >61% adsorption retained Distillation: 10 Bulk CNF by cycles, hydrothermal 51-139 g/g negligible carbonization and adsorption pyrolysis [25] changes Graphene/CNT foam by CVD [21] 80-130 g/g Contact angle BET surface area results (m2/g) Water: 152.3° Oil: 0° - - 20-30 Water: 147.6° 315 Water: 127.2° - 547 While carbon nanomaterial-based sorbents show desirable, and in some cases almost optimal, oil sorption properties, there are still additional questions associated with their production and eventual industrial use. For many of the configurations studied, processing techniques used are energy intensive and rely on harmful and expensive 10 precursor materials and expensive and complicated processing and equipment [19]. The environmental and health impacts of carbon nanomaterials are also still widely understudied and largely unknown. Despite the drive of some researchers to create carbon nanomaterial sorbents from bio-based sources, these materials result in a similar product made of carbon nanomaterials. Some publications have noted that carbon nanoparticles are highly toxic and have been called the “least biodegradable man-made materials ever devised” [26]. In the case of introducing these materials for large scale environmental cleanup, these questions may warrant additional studies, even for bio-based sorbents. 2.2 Synthetic Sorbents There are a variety of synthetic polymer materials that have been synthesized and studied for oil spill adsorption, ranging from membranes to sponges to surface coatings for bulk materials [27]. Many of the polymeric materials used make use of relatively inexpensive, commercially utilized polymer materials and are mainly of interest due to their inherent chemical resistance, low density, and high hydrophobicity [28]. Some specific polymer materials utilized include poly-urethanes (PU), polyethylenes (PE), polystyrenes (PS), polyvinylchlorides (PVC), polypropylenes (PP), and a large range of copolymer materials [8, 18, 29-31]. Within the synthetic sorbent field, polymer nanofibers have received research attention because of properties similar to those of carbon nanomaterials: high oleophilicity and hydrophobicity, high surface areas, which are further increased by interconnected porous networks of porous polymer fibers, and high re-use potential. These materials also show high potential for recycling after use. Polymers such as PP and PU are currently 11 commercially utilized as sorbent materials such as booms for marine oil spills [32]. Despite the commercial use of some versions of polymer sorbents, research is ongoing to improve various sorbent properties and processing. In one path of this research, polymeric sorbent materials are being created from recycled waste plastics such as polyethylene terephthalate, high and low density polyethylene, and polystyrene [31]. Various methods have been employed to synthesize these polymeric sorbents, including chemical conversion, grafting, gamma radiation treatment, and the creation of porous sponges, aerogels, and films (which have yet to see large success). While these show promise for two environmental remediation applications (plastic recycling and oil spill cleanup), processing can be energy intensive and waste plastic composition is not homogenous nor well enough defined to repeat recycling processes on a large scale [31]. Adsorption results for various synthetic polymer sorbent materials are summarized in Table 2. Many studies involving these polymeric materials have focused on enhancing surface properties to further decrease hydrophobicity and increase oleophilicity. One such study involved the creation of CNT modified polyurethane foam (PUF) [33]. The CNTs improved hydrophobicity, increased pore size (allowing more oil to reach internal surface), and improved thermal and mechanical resistance to the original PUF material. This configuration shows an adsorption capacity of 24.75 g/g, 85.45% of which is retained after 4 cycles of petroleum washing and heating to remove adsorbed oil (extraction based reuse) [33]. Although surface modified polymers have shown desirable sorption properties, bulk structure of these materials has been shown to be just as important as the surface 12 characteristics to overall oil adsorption capacity [34]. Porosity, interconnectivity of pores, and pore size are all important parameters for oil adsorption; pores must be large enough to allow oil molecules to pass into the structure, but also small enough to retain oil after adsorption. Highly interconnected, porous poly-urethane foams have shown adsorption values as high as 30 g/g, similar to adsorption values of surface modified PUFs [34]. These structures have optimal pore sizes between 400 and 500 μm and obtain full oil saturation in under 20 seconds. Table 2: Synthetic polymer sorbent materials and associated adsorption properties. Material Adsorption (g/g) PU foam from recycled PET [35] 3 - 41 PU foams recycled from mattresses, modified by polystyrene grafting [36] PVC-PS copolymer fibers by electrospinning [37] PS fibers by electrospinning [38] PU foams (range over many different samples and methods) [31] Polystyrene [39] PP nonwoven [29] PE film [40] Methacrylate-based copolymer [41] Reusability Mechanical Squeezing, 98% constant adsorption over 5 cycles 58.25 (±2.81) 51.65 (±2.57) 46.87 (±1.17) ~ constant adsorption for 3 cycles 38 - 146 - 84.41 - 25-100 - 7.13 – 112.13 8.46 - 9.12 42 - 290 Extraction, ~ 20 g/g constant over 10 cycles 23 While many of these synthetic polymer materials exhibit high hydrophobicity, surface area, and sorbent efficiencies, these materials are not renewable or biodegradable, and therefore can create additional problems after use [42]. 13 2.3 Gelators Another type of synthetic polymer material for oil spill cleanup is termed gelators. Gelators are typically supramolecular, network structures that gel when introduced to hydrocarbon materials. Gelators have been made from a variety of materials, ranging from glucose based materials to crosslinked polymers [8]. One gelator material was created from glycerol propoxylate (GP) crosslinked with ICS, which exhibited adsorption swelling capacities between 60% and 1071%, depending on oil used and GP molecular weight. This material also exhibited desorption rates in air as a measure of reusability, where samples retain swelling abilities over ten cycles [43]. Another polymeric gelator example is termed i-Petrogel, an interpenetrating network structure that acts as a gelling agent to make oil spill collection easier and more profitable. i-Petrogel is a porous, network polymer created from a thermoset and an elastomer, that can be produced on an industrial scale and can absorb over 40 times its weight in oil (as demonstrated in large scale environmental tests) [44]. While not yet tested, this material may also be applied as flakes over large marine oil spills, collected once gelled, and refined through normal oil refining processes. This is due to the polymeric hydrocarbon-based structure, which would be present in a small enough fraction to warrant no additional removal before refining processes [44]. This material is one of the most developed of the newly researched synthetic sorbent materials and is one of the only sorbent materials developed that can collect oil for first use instead of disposal or recycling processes. 14 2. 4 Natural Sorbents There are a wide variety of natural materials being studied for potential oil spill sorption; these materials can be sub-divided into organic or inorganic materials. Inorganic minerals include naturally occurring minerals and other materials such as zeolite, perlite, volcanic ash, and clay, for many of which silica is a main component. These materials are typically dense and sink in water, thus limiting collection and resulting in oil being moved from the surface to the ocean floor. These are also more difficult to employ, more expensive than natural organic sorbents, and may be harmful to oceanic ecosystems [45]. Due to these drawbacks, natural inorganic materials are not often applied in oil spill cleanups and have received much less research attention than the other sorbent materials mentioned here. Organic natural sorbents have received significant research attention and are typically plant-based products such as wood fibers, rice husks, cotton, and straw, to name a few [8, 18, 29]. These materials show significant promise, mainly due to the large excesses of biomass materials produced by the world food markets [8]. These materials are typically chosen for their porous structures, high surface areas, and biocompatibility; however, they are not typically selective in their sorbent properties and often do not float on water surfaces. These natural materials tend to have low absorption efficiencies because they absorb both water and oil/organic materials, and typically report sorption capacities below 10 g/g, although capacities as high as 190 g/g have been reported [46-49]. Natural oil spill cleanup research also focuses on other methods besides typical sorbents, such as particulate emulsions to coagulate/adsorb spilled oil, and natural based surfactants, inorganic and non-biodegradable versions of which are currently used in oil dispersants for spill cleanup [8]. 15 Due to high amounts of water sorption, research on bio-based sorbent materials has focused on ways to adjust the properties to enhance absorption selectivity, though these modifications can also decrease biocompatibility and increase cost [8, 18]. Examples of surface modifications include increasing surface area and surface roughness and decreasing surface energy of sorbent materials [8]. Other treatments, such as pyrolysis and treatment with alkaline substances have also been shown to increase adsorption capacity of different oils in rice husk-based sorbent materials [50]. However, some materials, such as raw luffa fibers, have shown effective adsorption of oils in their virgin form, without further treatment [51]. Examples of such materials and their subsequent adsorption capacities are shown in Table 3. Natural organic sorbents fall into a few classes. Materials such as lignin, rice husks, cellulose, and kapok fibers have been defined as traditional sorbent materials; another class of natural organic sorbents are carbon-based aerogels from bio-based sources. These aerogels are similar to those discussed above but attempt to address questions of harmful environmental effects and low biocompatibility of the finished sorption products associated with synthetic carbon aerogel products. These carbon-based aerogels are typically synthesized through pyrolysis and freeze-drying processes, sometimes followed by further surface modification to enhance hydrophobicity [8]. Such aerogel materials have been made from a variety of bio-based materials ranging from cellulose to waste newspaper [52, 53]. 16 Table 3: Bio-based sorbent materials and associated properties. Adsorption (g/g) Reusability 24 - 85 Vacuum filtration; reused 6 – 8 times 9 – 24 (v/v) Squeezing; 60% adsorption capacity remained after 13 cycles 2-4 - Traditional sorbent 10 – 24 - Traditional sorbent 12 - 16 99.21% removal efficiency under optimal conditions (modeled prediction) Traditional sorbent 1.18 - Aerogel 50 - 190 Squeezing, distillation, combustion, constant adsorption over 5 cycles Aerogel 80 - 190 Squeezing, constant adsorption over 10 cycles Aerogel 14 - 30 Material Distinction Kapok fibers [54-56] Traditional sorbent Traditional sorbent Traditional sorbent Cellulose [57] Lignin [46] Silica modified Sawdust [47] Acetylated peat moss [58] Oil palm leaves (treated with lauric acid) [59] Raw cotton [48] Cellulose nanofibers with hydrophobic modification [49] Chitosan/silica composite [60] Evaporation, constant adsorption over 10 cycles One such material is a bamboo-pulp based, carbon aerogel created simply using freeze drying and pyrolysis [61]. The highly macro and micro porous 3D network created showed adsorption values between 50 and 150 g/g and a contact angle of 135.9°. Depending on organic material used, distillation or extraction were used to recover adsorbed material, and no deviations in adsorption were seen after five regeneration cycles [61]. In a similar process, high temperature carburization, freeze drying, and pyrolysis has been used to convert winter melon flesh to carbon aerogels with adsorption capacities 17 between 16 and 50 g/g [19]. Additional examples of bio-based carbon aerogel materials are shown in Table 3. While natural organic aerogels show high adsorption selectivities, high hydrophobicity and oleophilicity, and come from renewable sources, pyrolysis and freeze drying are energy intensive processes and surface modifications can rely on additional chemicals or treatments [8]. Mechanical properties are also less than ideal for currently produced researched products: sorbents are either brittle or degrade over regeneration cycles, thus limiting their recyclability for oil spill remediation. These materials are also more costly because of their excessive processing and also face issues with future scale up to industrial production levels, and thus require additional research [8]. 2.4.2 Kappa carrageenan Another naturally derived material that has yet, as far as this author could determine, to be applied to oil spill cleanups is carrageenan. Carrageenans are hydrocolloid, polysaccharide materials derived from the cell walls of red algae (seaweed) [62]. This derivation process includes algae drying, chemical treatment (often with salts), filtration and concentration following heating in solution, and finally grinding into powder [63]. These materials are of interest to the present application due to their bio-based source, environmentally friendly nature, relatively simple processing, and their biodegradability through enzymatic cleavage by marine organisms [64]. Carrageenans are also commercially available and widely studied and used in other applications due to their anticoagulate and gelling properties; they are heavily used in the food industry to stabilize meat and dairy products and as a gelatin substitute, as well as the cosmetics industry in lotions, 18 toothpastes, and shaving creams [65]. The various properties of carrageenans have also found application in other fields, including industrial production of pharmacological materials (antibiotics) and other chemicals (vinegar), where they are used as immobilizing agents, and in medical applications, such as biologically stable hydrogel drug delivery systems [63, 66]. There are three classes of algae-based carrageenan materials, separated based on molecular structure differences involving sulfate groups. Kappa carrageenan, when mixed with water or other liquid in the presence of potassium ions, will form a strong, brittle gel material. In a similar fashion, iota carrageenan forms a soft, elastic, more durable gel material than kappa carrageenan. The molecular structure of lambda carrageenan, however, inhibits it from turning into a gel at all. Due to these differences between these classes, the rigid gel formed by kappa carrageenan stands out as a potential sponge-like material for oil spill remediation. However, kappa carrageenan shares the main problem of many of the other natural materials used for oil spill cleanup: an affinity for water. This is mainly due to the overall negative charge of the kappa carrageenan matrix once gelled, which is attracted to the dipole moments in water molecules [66]. Surface treatments to neutralize the negative charge on the kappa carrageenan matrix would be necessary to apply this material to marine oil spill cleanup applications. 19 MATERIALS AND METHODS Oleophilic sponges for marine oil spill adsorption were synthesized by combining a polymer matrix material with a charge cancelling surfactant. Kappa carrageenan (KC) was selected as the polymer matrix material due to its low cost, ready availability, plantbased sourcing, and gelation properties. Benzalkonium chloride (BAC) was selected as a cationic surfactant due to its affinity to bind to the overall negatively charged kappa carrageenan matrix to create a neutral, oleophilic, and hydrophobic sponge matrix material. BAC, a semi-solid at room temperature, was added in a concentration of 0.8 weight percent (wt%) to deionized water heated to 90 °C. This solution was removed from heat and mixed thoroughly until all the BAC had dissolved. The water mixture was returned to heat set to approximately 90 °C, after which 4wt% KC powder was added. This mixture was stirred using a metal spatula for six minutes. The hot, liquid mixture was then poured into multiple cylindrical, 3D printed, polylactic acid molds of size 1.5 cm diameter by 1.5 cm height. Smaller cylindrical molds of size 0.7 cm diameter and 0.7 cm height were also used to create samples for surface area testing. These molds were allowed to cool for approximately 30 minutes in flowing air. The cylindrical sponge materials were then removed from the molds and stored in a fridge for one to two hours. After chilling, sponges were freeze dried for 48 hours using a LABCONCO freeze drier set to 0.024 Torr at -59 °C. Once sponges were freeze dried, extensive characterization was performed. To understand internal pore structure, samples were cut in half and imaged using a TM3030 Hitachi Tabletop Scanning Electron Microscope (SEM). The smaller cylindrical samples were dried under a nitrogen atmosphere for 1 hour at 50 °C, after which surface area analysis was performed using the five-point Brunauer–Emmett–Teller (BET) method, with 20 an evacuation rate of 300 mmHg per minute for five minutes. Contact angle measurements were collected using a camera and ImageJ software, using both deionized water and mineral oil droplets to quantify hydrophobicity and oleophilicity of created sponges [67]. Adsorption tests were performed by comparing dry sponge weight to sponge weight after submersion in food grade mineral oil according to the following equation. Adsorption tests for mineral oil were done over 1 minute, 24 hours, and 6 days. 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 = (𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ𝑡𝑡 − 𝑑𝑑𝑑𝑑𝑑𝑑 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ𝑡𝑡) 𝑔𝑔/𝑔𝑔 𝑑𝑑𝑑𝑑𝑑𝑑 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ𝑡𝑡 Reusability studies were conducted by mechanically squeezing, using the pressure of a thumb, to remove mineral oil from 24 hour-soaked sponges. Squeezed sponges were then re-weighed and re-placed into mineral oil for another 24 hours; this cycle was conducted five times for various samples. Degradation tests were performed based on ASTM F726-17, using a Thermo Scientific MaxQ 6000 shaker table set to 150 rpm [68]. ASTM F726-17 was modified for use with available equipment: 1 L glass jars were used, thus cutting the ASTM specified test size to 25%, and an amplitude of vibration of 1.9 cm was used, as opposed to the specified 2.5 cm. Tests were conducted for 15 minutes at 150 rpm in water, after which 1 mL of mineral oil was added before shaking for another 15 minutes at 150 rpm. Physical characteristics were noted after the test had concluded to assess degradation results. 21 RESULTS AND DISCUSSION Initial Explorations Prior to settling on the final sponge configuration as specified, tests were performed using various sponge compositions to identify the most effective material. Initially, the goal of this project was to combine areas of heavy research to create a fully environmentally friendly sponge material, using both a plant-based polymer matrix and carbon nanofiber materials (CNFs) obtained from carbon capture processes. Under this goal, it was assumed that large concentrations of CNFs would provide additional surface area for oil adsorption and prevent degradation in a marine environment. Initially, to identify the correct concentration of BAC to add to create a neutral polymer matrix, sponge configurations containing 4wt% KC, 4wt% CNFs, and varying amounts of BAC (0.4wt%, 0.6wt%, 0.8wt%, 1.0wt%, 1.2wt%, 1.4wt%, 2.4wt%, 4wt%) were created. To assess charge cancelation, sponges were weighed, submerged in 2 cm mineral oil for 1 minute, and then reweighed after dabbing off additional surface oil. Results were compared across these configurations to identify oleophilicity, however; were inconclusive, as can be seen from Table 4. Table 4: 1-minute adsorption results for sponges containing varying benzalkonium chloride surfactant concentration, 4wt% kappa carrageenan, and 4wt% CNFs. BAC concentration (wt%) 0.4 0.4 0.6 0.8 1.0 1.2 1.4 Average adsorption over 4 sponges (g/g) 2.36 ± 0.67 5.94 ± 0.41 4.52 ± 0.32 4.97 ± 0.51 3.83 ± 0.40 4.75 ± 0.44 4.75 ± 0.13 22 BAC in a concentration of 0.8wt% was selected for the final configuration due to mechanical stability; above 1.4wt% BAC, water separated from sponge solutions after pouring into the molds. Because of this, these compositions never gelled/polymerized and could not be removed from the molds, nor freeze dried. The large discrepancy between the two different sponge batches containing 0.4wt% BAC can be attributed to differences in pouring and processing techniques, where the second batch had already begun to gel when poured, resulting in oddly deformed sponge shapes. This may indicate that specific processing, or sponge shape, is more important than BAC concentration to overall adsorption capacities. This hypothesis was also supported when two batches of sponges containing 8wt% CNFs were made: one solidified during pouring and created oddly shaped sponges and the other was hot enough when poured to completely fill the molds and create fully cylindrical sponges. The oddly shaped sponges adsorbed 1.32 g/g on average, whereas the fully cylindrical sponges only adsorbed 0.53 g/g on average. Sponges with BAC concentrations between 0.6wt% - 1.0wt% were identified to produce similar oleophilicity results; to ensure consistency, 0.8wt% was selected as a middle ground for BAC concentration for the final sponge configuration. Sponge configurations with varying amounts of CNFs (0.5wt%, 4wt%, 8wt%, 12wt%, 16wt%) were also initially created, along with a control configuration containing 0wt% CNFs. For characterization tests, the control samples performed better than any of the sponge configurations containing CNFs; with increasing CNF concentration, sponge reusability and oleophilicity decreased. Sponges became harder and stiffer with higher CNF concentrations, thus limiting or eliminating sponge reusability via mechanical 23 squeezing. Adsorption showed a decreasing trend with increasing CNF concentration (Figure 1). Figure 1: 24-hour adsorption of sponge batches containing varying concentrations of CNFs. Generally, adsorption decreased with increasing CNF concentration. Error bars indicate differences between sponge batches, where only one batch of 4wt% and 16wt% CNF sponges were tested. Initially this phenomenon was attributed to ineffective mixing techniques: initial SEM images of samples containing CNFs showed large CNF agglomerates within the polymer matrix, as shown in Figure 2 A. By dry mixing the KC and CNF powders for over 1 minute prior to combining with the water and BAC mixture, CNFs were effectively dispersed throughout the polymer matrix. SEM images showed no large CNF agglomerates, and at higher magnifications individual CNF fibers appeared to be dispersed into the KC polymer matrix, as shown in Figure 2 B. However; despite effective mixing, 25 Table 5: BET method specific surface area results comparing sponges with 0wt% CNFs and 0.5wt% CNFs, where no significant difference could be identified. Sponge CNF Concentration (wt%) 0 0.5 Specific Surface Area (m2/g) 8.6915 ± 0.0895 12.1289 ± 0.7858 25.1872 ± 2.2558 10.9315 ± 1.4192 15.9394 ± 0.2741 16.5904 ± 2.0417 The surface area result for the third 0wt% sample is much higher than the other two 0wt% samples and lies significantly outside the standard deviation of the first two as well, indicating it may be an outlier. Even if this is the case, the CNF containing sponges had surface area values comparable to the sponges containing no CNFs. The addition of CNFs would be expected to increase the surface area significantly due to their extremely high surface area values. The lack of this expected result also indicates the possibility of pore inhibition by the CNFs; it is also likely the polymer matrix is engulfing the CNFs, limiting their expected surface area and oleophilicity enhancing properties inside the sponge material. Along with the poor adsorption properties of CNF containing sponges, degradation tests revealed an additional reason to discard carbon containing sponges for this application. Observations after degradation testing revealed that CNFs are not tightly bound to the polymer matrix, as they appear to have been released into the water during these tests: this can be seen by the discoloration of the water in Figure 3. This result indicates the inapplicability of these configurations for use in future marine oil spill situations, as the effects of CNFs in the environment are not yet well studied or understood. 26 Figure 3: Qualitative results of degradation tests (following modified ASTM F726) for sponges containing 0.5wt% CNFs. Images are shown for 15min of testing in deionized water (left) and after an additional 15 minutes of testing in the same water with 1 mL of mineral oil added (right). Discoloration indicates release of carbon from the sponges during testing. After these extensive tests utilizing CNFs within the KC polymer matrix, it became evident that the best performing sponge configuration was the control sample, containing only KC and BAC, with 0wt% CNFs. Due to these results, this was identified as the most effective sponge configuration for potential future application in marine oil spill remediation. Final Configuration Characterization Results Using this final sponge configuration, containing 4wt% KC and 0.8wt% BAC, a super-oleophilic, environmentally friendly, polymer sponge was created; an example of the final sponge configuration is shown in Figure 4. 27 Figure 4: Final sponge configuration, containing 4wt% kappa carrageenan and 0.8wt% benzalkonium chloride. SEM images (Figure 5) show the macroporous internal structure of the created sponges; these pores are responsible for much of the overall oil adsorption. While pore size has not been directly quantified, the small specific surface area values resulting from the BET measurements (Table 5) also support the presence of macropores within the system. Figure 5: Scanning electron micrographs of sponges containing 4wt% KC and 0.8wt% BAC at two different magnifications, showing the macroporous, flakey internal sponge structure. 28 Contact angle images and calculations indicate that the created sponges are superoleophilic (0° mineral oil contact angle), and slightly hydrophilic (75.0 ± 12.2° water contact angle). Two representative images are shown in Figure 6. Figure 7 A-E shows the oil adsorption over time, where oil spreading through the sponge is evident. Figure 7 F shows a sample sponge after both oil and water contact angle tests had been completed; the oil continued to be pulled into and spread throughout the sponge, where the water remained on top where the droplets had been placed and did not adsorb to an appreciable extent. Figure 6: Contact angle images using a mineral oil droplet (died red) and water droplet (died blue). ImageJ contact angle analysis results indicate the superoleophilicity (0° mineral oil contact angle) and the slight hydrophilicity of a created sponge (72° water contact angle). The slight hydrophilicity shown by the contact angle results is likely due to an inexact charge calculation of the negatively charged KC matrix using the BAC surfactant. Due to the overall charge dipole present in water, substances that also contain a charge are attracted to water molecules and are thus hydrophilic, indicating that the polymer matrix was not completely neutralized by the BAC surfactant. Additional studies, such as those utilizing zeta potential measurements, are needed to identify the exact concentration of BAC required to fully cancel the charge of KC, to create a hydrophobic or 30 Overall, the average adsorption, both including and excluding the outlier, is a large value that is on par with similar materials reported in recently published literature (Table 6). Figure 8: 24-hour mineral oil adsorption tests for 5 different batches of the final sponge configuration. Batch 3 resulted in an unusually high adsorption value. The average adsorption over this range of sample batches was 13.1 ± 4.9 g/g. Table 6: Comparison of reported adsorption values for various natural materials and the modified kappa carrageenan sponge created in the present work. Material Adsorption Range (g/g) Source [46] Lignin 2-4 Silica modified sawdust 10 - 24 Acetylated peat moss 12 - 16 Chitosan/silica composite BAC modified kappa carrageenan 14 - 30 [60] 13.1 Present work [47] [58] 31 From one round of adsorption tests conducted over an extended period of time (up to 6 days), it is also evident that over 95% of the maximum adsorption capacity is reached within 1 minute of oil submersion. This is a particularly desirable quality for marine oil spill remediation applications (Figure 9). Figure 9: Adsorption over time for one BAC modified KC sponge batch. 95% of the final adsorption capacity was reached in the first minute of submersion in oil. Reusability studies indicate that these created sponges can adsorb 14.0 ± 1.9 % of their initial adsorption for each reuse over five reuses. Mechanical squeezing severely deformed these sponges (initial sponges were approximately 1.5 cm tall, whereas reused sponges were approximately 0.5 cm tall), crushing the internal pores. This likely contributes to the significant decrease in adsorption capacity over additional cycles. Oil adsorption remained relatively constant for all following reuse cycles, indicating reliable, although low, adsorption values after the first cycle (Figure 10). Additional ways of oil removal may prove to be more effective methods for oil removal and sponge reuse, 32 however; heating to selectively remove oil, such as for distillation, is likely not an option due to the low melting point of KC gels (around 70 °C) [62]. Figure 10: Percent adsorption after 5 mechanical squeezing reusability tests. After the initial cycle, reusability remains relatively constant with an average over the five cycles of 14.0 ± 1.9% of the original adsorption. The final characterization test was also the most important for marine oil spill application. After degradation shaking tests had been completed according to modified ASTM F726-17, the jars containing the sponges, water, and oil were qualitatively assessed (Figure 11) [68]. All five of the individual sponges placed in the jar were still floating after the final test, indicating passing of the defined ASTM standard. No oil sheen could be seen on the surface of the water (Figure 11 B), indicating that the sponges had fully adsorbed the added oil during the second round of shaking. The water appeared slightly cloudy and bubbly; however, indicating that some of the BAC may have been released during the test. The discolored water could also indicate that some of the KC matrix may have begun to 34 CONCLUSIONS AND FUTURE WORK Due to the vast amounts of oil released into marine environments worldwide every year, and the significant problems associated with oil spill cleanup, the goal of this project was to explore and design a hydrophobic, oleophilic, reusable sponge, sourced from environmentally friendly materials, to cleanly cleanup oil spills. The sponges produced in this project were superoleophilic and had adsorption capacities over 13 g/g, values that are competitive with many recently published bio-based sponges. Sponges were also moderately reusable, adsorbing around 14% of their initial adsorption over 5 reuse cycles; however, these reuse values would need to see significant increases to make this material competitive for industrial use and reuse. Despite a slight affinity for water, the designed sponge material still preferentially adsorbed oil over water when put through degradation testing in an oil spill-like environment. While these KC based sponges technically passed these degradation tests, the structural integrity needs significant improvement to make this sponge design viable for use on an industrial scale. Future work should focus on adjusting the current sponge design to be stronger, more durable, and stay together more during degradation tests, reusability tests, and eventual use in the field. This would include identifying a way to better attach the BAC (or other surfactant) to the KC matrix surface, so it is not released during wave-like movement. Strengthening and adding elasticity to the polymer matrix, potentially through crosslinking or the addition of other polymer matrix substances, should also be explored to ensure more reusable, stable sponges for oil spill adsorption. Adsorption and degradation tests could also be conducted in salt water to assess true performance for use in a marine environment. Additional oil removal after adsorption should also be explored to identify if another 35 method for sponge regeneration would be more effective and produce more reusable sponges than mechanical squeezing. Once these improvements have been made, large scale testing would follow to assess the industrial applicability of this material for oil spill remediation. One of the largest factors to influence sponge performance, and eventual scale-up for industrial use, is processing; to produce reliably adsorbent sponges, processing optimizations would need to be identified. Mixing time and temperature are two main factors that could be rigorously analyzed, as well as time between synthesis and freezedrying. Each of these could be controlled and analyzed more specifically, as could pour technique and air characteristics during and after pouring (placed in flowing air as opposed to still air). All of these parameters could heavily influence the end sponge result and should be optimized to produce the most effective oil adsorbing sponge. Specific freezedrying parameters, such as time, temperature, and vacuum level, could also be optimized to produce the most effective pore sizes and structures within sponge materials. While processing appeared to have the largest impact on sponge performance, surfactant concentration could also be optimized. Other, more naturally derived and environmentally friendly surfactant materials should first be explored as a replacement for the BAC surfactant in this material. Zeta potential measurements could then be taken to assess the exact concentration of cationic surfactant needed to balance the negatively charged KC matrix, which would result in an increased hydrophobicity, likely pushing the contact angle measurements over 90° into the hydrophobic regime. Another future direction is to conduct analysis of sponge performance based on sponge shape and external surface area. While cylindrical sponges were used throughout 36 this project, misshapen sponges often showed drastically different adsorption properties. Other configurations such as flatter discs or cubes could be created to assess the effect of external and internal surface area on overall sponge adsorption. Despite the wide range of future directions to pursue, the current sponge design identifies kappa carrageenan as a new material for use in oil spill adsorption. Although there are a variety of bio-based materials being researched and published for this application, the significant variability in oil spills presents a need for a wide range of adsorption materials to effectively clean up spilled oil. The current design successfully presents kappa carrageenan as a new, bio-based material for use in oil spill adsorption, and reiterates the challenge to continually develop renewable materials for marine oil spill cleanup 37 ACKNOWLEDGEMENTS The author of this thesis would like to thank the Materials Science and Engineering Department at the University of Utah including Dr. Taylor Sparks for providing lab facilities, advice, and mentorship, Dr. Shelley Minteer and Dr. Steven Naleway, Isaac Nelson, and Max Mroz for providing lab facilities and assistance in the creation and characterization of sponge materials (specifically lab shaker table access and freeze drying capabilities, respectively), and the Lassonde Institute at the University of Utah for providing facilities and material for 3D printing of sponge molds. 38 REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] National Oceanic and Atmospheric Administration. "How Does Oil Get into the Ocean?" 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