Inexpensive material offers solution for ocean oil spills

"We already know what kind of polymers can absorb oil," Chung said. "Some oil is very thick and takes a long time to absorb, so we blended two polymers to provide structure with high surface area. It's a microporous structure. If you look inside there are many small holes. This morphological structure allows the viscous oil to diffuse inside, allowing for more oil to absorb through the surfaces."

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The blend of two polymers — an interpenetrating polymer network of hard and soft polymers — can be optimized for different oil viscosities and other factors. The researchers  have three U.S. patents on this work, and i-Petrogel is undergoing steps to become commercially produced.

During the field tests, researchers found the new polymer absorbed more than 40 times its weight in Alaska North Slope oil, double the capacity of its predecessor, Petrogel, also developed by the same Penn State researchers. The product takes on a gel-like consistency as it absorbs oil and remains at the surface. It can be readily removed using skimmers already used in clean-ups.

Previous techniques used to quell disasters like the Deepwater Horizon incident in 2010 recovered about 10 percent of the oil spilled, and the recovered oil was unusable. That generated about 80,000 tons of solid waste from soiled booms, and additional liquid oil waste mixed with salt water as responders struggled to contain the estimated 200 million gallons of spilled oil.

Abstract

Conventional synthetic sorbents for oil spill removal are the most widely applied materials, although they are not the optimal choices from an economic and environmental point of view. The use of inexpensive, abundant, non-toxic, biodegradable, and reusable lignocellulosic materials might be an alternative to conventional sorbents, with obvious positive impact on sustainability and circular economy. The objective of this paper was to review reports on the use of natural-based adsorbing materials for the restoration of water bodies threatened by oil spills. The use of raw and modified natural sorbents as a restoration tool, their sorption capacity, along with the individual results in conditions that have been implemented, were examined in detail. Modification methods for improving the hydrophobicity of natural sorbents were also extensively highlighted. Furthermore, an attempt was made to assess the advantages and limitations of each natural sorbent since one material is unlikely to encompass all potential oil spill scenarios. Finally, an evaluation was conducted in order to outline an integrated approach based on the terms of material–environment–economy.

Keywords:

oil spill cleanup, sorbent, lignocellulosic materials, modification methods, bio-based aerogels

2. The Liquid–Solid Interface between Oil and Sorbent Materials

Surface chemical state and morphology define surface wettability, which strongly correlates with the oil spill sorption. The contact angle is applied to evaluate wettability. In case the contact angle of a droplet on a smooth surface is less than 90°, it is categorized as wettable [53]. If the contact angle is higher than 90°, the liquid will avoid diffusing, and the material is classified as non-wettable by the liquid ( a). In case the liquid is oil, the sorbent can be categorized as oleophilic and oleophobic, respectively. Similar measurements can be conducted for water droplets, and the sorbent can be likewise categorized as hydrophilic or hydrophobic [53]. The hydrophobicity of the material surface blocks water adsorption and hence boosts oil sorption efficiency because of the absence of competition between water and oil molecules. Under realistic conditions, several factors interact to prevent the initial wetting of a surface, which also tends to oppose the retraction of fluid after wetting has taken place [54]. The roughness and heterogeneity of real surfaces can alter contact angles and wettability with advancing and receding sites on the surface ( b). Furthermore, in the beginning, during the gradual wetting of the surface, the existing pores do not cause any attractive force on the nearing liquid phase. Contrariwise, when the surface has been saturated by a liquid, voids will tend to remain loaded, impacting the interfacial force [54].

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Additionally, numerous publications mention that capillary action is a crucial process via which a porous structure holds oil, and contact angle phenomena control the flow rate of petroleum into porous substrates ( d,e). d,e take into account the condition whereby a dry and a water-saturated, porous substrate are applied for oil spill cleanup. Water presence in the sorbent can alter the contact angle, and such a change will not be beneficial for oil removal by a natural-based material.

3. Modification Methods for Hydrophobicity Improvement

3.1. Physical and Thermal Treatment

Natural sorbent media that are grinded show high oil sorption capacity per unit mass, due to better accessibility with the contact surface of the material, with binding sites being more available on smaller particles. Several articles confirm this general observation relating natural-based materials to oil spill removal. For example, Bayat et al. [56] ground up bagasse—a byproduct of sugar extraction from sugarcane—and tested samples with varying particle sizes. Experiments were conducted with samples having different fraction sizes of bagasse (from 14 to 45 mm) to verify the effect of particle size on oil spill cleanup treatment. The results reveal that oil removal efficiency rises with a decrease in particle size [56]. Likewise, Husseien et al. [45] noted the highest sorption capacity for oil when using fibrous bio-sorbents such as corn stalk in the form of fine fibers.

Thermal treatment at low temperatures, i.e., drying, does not affect the oleophilic and hydrophobic properties of the sorbent ( ). Its main effect is to remove some impurities of the surface, allowing easier adsorption of oil [34]. On the other hand, a high-temperature thermal treatment like pyrolysis [57] leads to the carbonization of the sorbent. Thus, the oil sorption capacity and oil to water selectivity are highly enhanced [58] ( ).

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Vlaev et al. [59] reported on BRHA (brown rice husk ash) and WRHA (white rice husk ash) functionalized by pyrolysis. The new products showed a sorption capacity of 5.02 and 6.22 g/g (BRHA) and 2.78 and 2.98 g/g (WRHA) for diesel and crude oil, respectively. Besides, BRHA showed better hydrophobic and buoyancy characteristics than WRHA. Kudaybergenov et al. [60] demonstrated thermally treated rice husk (heating under CO2 at 800 °C) having a sorption efficiency of ~15 g/g for crude oil. Uzunov et al. [61] examined the impact of pyrolysis at temperatures between 250 and 700 °C on crude oil sorption capacity. Rice husk pyrolyzed at 350 °C showed a maximum sorption capacity of ~10 g/g, in comparison with raw rice husk (~6.5 g/g).

In the same study, the authors assumed that the capacity of the pyrolyzed rice husk on oil sorption is affected by the porous substrate instead of the impact of the surface functional groups [61]. Furthermore, the oil sorption efficiency of pyrolyzed rice husk was studied in a similar work [58]. The authors examined the capacity of rice husk pyrolyzed at 480 °C on oil sorption. The capacities of the studied material on different forms of oil follow the order of heavy crude oil > motor oil > light crude oil > diesel > gasoline, showing that the permeation of the oils into the sorbent reduced with the increase in bulk density [58]. Husseien et al. [62] evaluated the sorption properties of barley straw pyrolyzed at temperatures between 200 and 500 °C. Oil sorption capacity was found to be 7.6 g/g and 9.2 g/g for diesel and heavy oil, respectively ( ). Carbonized barley straw applied as a pad indicated highly hydrophobic characteristics, adsorbing oil at more excessive amounts than polypropylene commercial pads ( ). Moreover, they were reusable for two absorption/desorption cycles [62]. Τhe surface alteration of thermally treated natural-based materials is reflected in the SEM images of El Gheriany’s study [34]. The SEM images ( a) revealed that the originally smooth and homogeneous structure of the raw orange peel appeared rough and of high porosity after thermal decomposition at 500 °C. The occurrence of pores is ascribed to material decomposition and desorption of gases/vapors during pyrolysis ( a).

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Table 1

SorbentForm of SorbentModification MethodOil Sorption MethodAdsorption CapacityType of OilRecoveryRefsBanana peelpowderrawASTM *5–7Crude oil20 cycles[24]Sawdustfiberraw-4.1–6.4Crude oiln/a[32]Coir fiberfiberraw-1.8–5.4Crude oiln/a[32]Luffafiberraw-1.9–4.6Crude oiln/a[32]Luffafiberraw-14Diesel oil3 cycles[107]Orange peelpowderraw-3–5Crude, diesel, and used engine oil5 cycles[34]Barley strawpowderraw-6.5–12Crude oil3 cycles[44]JutefiberrawASTM2.6Engine oiln/a[50]CottonfiberrawASTM18.4–22.5Used lube oil5 cycles[80]CottonfiberrawASTM30.5Motor oiln/a[108]Sugarcane bagasse raw-10.5–19.9Diesel oiln/a[103]Kapokfiberraw-19.4–49.9Diesel, crude, engine oil, used engine oiln/a[103]KapokfiberrawASTM36–45Engine, diesel, and hydraulic oil4 cycles[109]KapokfiberrawASTM50Engine oil15 cycles[110]KapokfiberrawASTM38Diesel oil8 cycles[104]Walnut shellgranularraw-0.30–0.58Mineral oil, vegetable oil and Bright-Edge oiln/a[111]Rice huskpowderraw 6.2 n/a[112]Rice huskpowderpyrolysis-3.7–9.2Diesel oil, gasoline, light crude, motor oil, heavy, crude oiln/a[58]Rice huskpowderpyrolysis-15Crude oil [47]Rice huskpowderpyrolysis-2.7–6.2Diesel oil, crude oil [59]Rice huskpowderpyrolysisJIS technical standards6Heavy crude oiln/a[112]Barley strawfiberpyrolysisASTM7.6–9.2Diesel, heavy crude oil2 cycles[62]Rice huskpowdermercerization-4–19Marine oiln/a[75]SoybeanpowdermercerizationASTM5Crude oiln/a[49]CattailfibermercerizationASTM4Motor oiln/a[49]Oil palm empty fruit bunchgranularacetylation-7Crude oiln/a[46]Cocoa podsfiberacetylation-8Crude oiln/a[46]JutefiberacetylationASTM21.8Engine oil3 cycles[50]bambooaerogelaerogel-38Engine oil5 cycles[105]Pomelo peelaerogelaerogel-5Castor oil5 cycles[106]Waste paperaerogelAerogel with MTMS-24.4Crude oil5 cycles[95]Pomelo peelaerogelAerogel with MTMS-49.8Crude oil10 cycles[86]Open in a separate window

3.2. Hydrothermal Treatment

Hydrothermal treatment is an alternative to thermal pyrolysis for the production of carbonaceous materials. It is one of the most widely used treatment methods in sub- or supercritical water conditions [63,64]. Hydrothermal treatment diminishes the oxygen-containing functional groups and volatile elements, enhancing the carbon content of the natural sorbents [64]. Consequently, the hydrophobicity of the prepared cellulosic materials is significantly improved. Hydrothermal treatment is advantageous, especially for the fabrication of sorbents derived from natural biomass [48,64]. Several types of biomass like wood, plants, and vegetables have a high water content that may exceed 70% of their total weight. Thus, simple thermal techniques can be expensive and time-consuming because of the necessity of material drying before modification in order to lower the water content. On the contrary, hydrothermal treatment allows the thermal modification of the natural material in solution. Therefore, the energy and time-consuming step of drying is avoided [65,66,67,68]. Li et al. studied the efficiency of winter melon aerogel on oil sorption made by a hydrothermal process [68]. The aerogel had a density of 0.048 g/cm3 and adsorption capacity of 25 g/g. The natural-based aerogel showed remarkable hydrophobicity (water contact angle 135°), low density, and high porosity, enhancing the oleophilic properties [69].

3.3. Chemical Modification Methods

3.3.1. Mercerization

Cellulose is a polymer with β-glucose as its structural units, which are joined together by β-1,4 glycosidic bonds. The molecular weight of cellulose varies depending on its origin. Each glucose structural unit contains three free hydroxyl groups, two secondary and one primary. Cellulose is found in more than one crystalline form, with the most important being cellulose I and cellulose II [70]. The cellulose surface is quite hydrophilic, which is not advantageous for efficient oil–water separation, as hydrophobic and oleophilic characteristics are both needed. Cellulose I is found in natural fibers [71]. Forcible swelling of the cotton, e.g., treated with NaOH (mercerization), changes the crystal lattice and converts cellulose I to cellulose II. Specifically, mercerization is an alkali treatment of the sorbent’s fibers with hot or cold NaOH solution leading to the removal of natural and artificial impurities. Moreover, the fibers do not change their form, but they change their crystallinity from cellulose I to cellulose II. Thus, the surface of the material becomes rough and rigid, increasing the contact area, which leads to higher and more effective adsorption [72,73]. Morphological information regarding the effects of mercerization on sawdust was derived by Gulati et al. [74]. SEM shows the smooth surface of the raw sawdust and the substantial change in the morphology of the mercerized new material [74].

Modification of the surface will provide better contact between material and oil. Thus, mercerized cellulose attains properties that make it favorable for oil spill removal from water media [74]. According to Wong [49], mercerization increased the crude oil sorption capacity of soybean residue up to 5 g/g and the motor oil sorption capacity of cattail fibers up to 4 g/g ( ). Moreover, rice husks have been applied as feedstock for the fabrication of mercerized natural-based oil sorbents [75]. According to Bazargan [75], alkali treatment was shown to alter the rice husk structure, creating a material with adequate oil sorption capacity (19 g/g). The suggested method occurs at low temperatures, resulting in high product yields. BET (Brunauer–Emmett–Teller) and FTIR (Fourier Transform Infrared) analyses have indicated that microporosity, along with surface functional groups, are not the main controlling factors in the oil sorption capacity. The marine diesel uptake capacity was shown to have a strong inverse relationship with the bulk density [75]. The modified rice husk exhibits a reduced bulk density, which permits oil to diffuse internally into the substrate [75].

3.3.2. Acetylation

Acetylation is a technique for increasing the sorption efficiency of the sorbent and can be facilitated by the use of a catalyst (N-bromosuccinimide, N-methylpyrrolidone, 4-dimethylaminopyridine) or not. In acetylation, the hydroxyl groups in the cellulose structure are converted to oleophilic acetate groups (O-CO-CH3) through reaction with acetic anhydride, lowering the hydrophilic properties of the sorbent and enhancing the oleophilic ones [46,50,76,77,78]. As a result of this modification, the oil uptake is drastically increased, while the water sorption is respectively reduced. Acetylated sorbents have bulkier forms because of the difference in molecular weight between OH and O-CO-CH3, which is referred to as weight percent gain [46,78].

Many studies have revealed its efficiency in increasing the hydrophobicity of natural fibers. In one of them, the usage of acetylated rice straw produced through catalytic and non-catalytic processes was examined [77]. Acetylation, without the presence of a catalyst, led to an increase in sorbent weight of 11.2% and to oil sorption efficiency in the range of 16.8–20 g/g. Acetylation in the presence of a catalyst (4-dimethyl-aminopyridine) indicated a slightly higher increase in sorbent weight (15.4%), with sorption ranging between 20.9 and 24 g/g. It is noteworthy to mention that the sorption efficiency of the acetylated rice husk increases pro rata with the degree of acetylation [77]. Deschamps et al. [79] examined the efficiency of unmodified and acetylated cotton fibers on the sorption of different oil forms (fuel, crude, mineral, vegetable) from aqueous solutions. In this study, oil uptake capacities ranged from 19 to 23 g/g and 23 to 30 g/g for raw and treated cotton, respectively. Moreover, the material could be reused by simple mechanical pressing for up to 10 times. As shown therein, after the 10th cycle, sorption efficiencies decreased from 15 and 21 g/g to 11.5 to 12.5 g/g for raw and acetylated cotton, respectively. Remarkably, the treated fibers sorb smaller amounts of oil compared to the unmodified ones. Nevertheless, they exhibited stronger lipophilic properties and better stability in the aqueous solution during the sorption experiments. Furthermore, Hussein et al. [80] pointed out that cotton fiber in the form of loose fiber or pad displayed good results for oil spill removal, with sorption efficiencies varying between 18.43 and 22.5 g/g. A subsequent study by Li et al. [81] focused on the characterization of acetylated corn straw fibers in aqueous solutions containing crude, diesel, and vacuum pump oil, respectively. The results showed that the acetylated cellulose fiber, an ultra-oleophilic material, revealed uptake capacities of 67.54, 52.65, and 42.53 g/g for crude oil, diesel oil, and vacuum pump oil, respectively. It is also noteworthy that modified material floated on the oil–water surface for several days without sinking. Furthermore, the water contact angle was 51.1° and 120.65° for the unmodified and acetylated fiber, respectively. Teli and Valia [50] modified coconut fiber via acetylation at 100 °C with a NBS catalyst (1%). Adsorption efficiencies of 3.5 g/g for the unmodified coconut coir were enhanced to 15.75 g/g for the acetylated material. Moreover, Asadpour et al. [76] studied the efficiency of the acetylated oil palm fibers on oil spill removal. The BET surface area of oil palm fibers was found to be 0.40 and 0.35 m2/g for raw and modified fibers, respectively. The acetylated fibers reached a maximum uptake capacity of 6.8 and 7.0 g/g for tapis and arabian crude oils, respectively. Ιn addition, they achieved buoyancy in the order of 93.7% and 95.3%, respectively.

Moreover, the acetylation method was used to enhance the hydrophobicity of kapok fibers. Kapok is considered one of the best natural-based materials, with high oil sorption efficiency and excellent buoyancy characteristics. According to Wang et al. [82], kapok fibers were effectively acetylated, and the subsequent fibers adsorbed a greater amount (36.7 g/g) than raw fibers (27 g/g) for diesel oil. The nonpolar nature of acetyl groups makes the surface of modified kapok fibers hydrophobic. Consequently, a significant amount of oil was favorably absorbed into the fibers, with small volumes of water retained on them. The reported results can be correlated with the SEM images ( b) of raw and modified kapok fibers. Raw kapok has a hollow shape and smooth surface with a closed orifice. At the same time, acetylated materials PAKF and NAKF show a tiny groove on the surface and an open lumen orifice. This implies that the smooth surface (raw material) is less beneficial for oil sorption, and the improved new materials with a rough surface are favorable for oil sorption [82].

3.3.3. Grafting

Polymerization is an effective and straightforward process to graft monomers onto the fiber surface [83]. This method has been applied by researchers both for synthetic and natural-based sorbents, such as polypropylene [84], banana [52], and coir [85]. The aforementioned studies indicated that the maximum oil sorption capacity of grafted banana fibers was 14.45 g/g [52] and for grafted coir 13.45 g/g [85]. Banana fibers are grafted to form esters, replacing the hydroxyl group in the cellulose surface [52]. Nevertheless, the percentage of the synthetic polymer chain in the new grafted material was controlled in order to not interfere with swelling of separated fibers in water. This factor is crucial in a grafted natural-based sorbent for removing emulsified oil from water. The surface structure of banana fiber and grafted banana fiber was evaluated using SEM. Images from SEM indicated that there was a substantial alteration between the surface of raw and modified banana fiber. Chemically grafted banana fiber was coated with poly butyl acrylate species in a heterogeneous way on the surface. Moreover, some fibers were cross-linked together as a result of the reaction. Thus, the fiber becomes bulkier and hydrophobic, giving high absorptivity toward oil [52].

In addition, Viju et al. pointed out the significance of the grafting method in determining the oil sorption efficiency of nettle fibers [83]. The maximum oil uptake capacity of grafted nettle was 36.60 and 25.56 g/g for crude oil and vegetable oil, respectively ( ). Reusability experiments revealed that modified nettle showed better oil uptake capacity than raw nettle fibers following seven sorption–desorption cycles ( ). An important finding was that modified nettle had higher oil uptake capacity than a commercial polypropylene material [83].

Natural rubber (NR) foam was modified via graft copolymerization with an oleophilic monomer such as methyl methacrylate (MMA). NR foam was successfully modified by graft copolymerization with Poly(methyl methacrylate) (PMMA) [87]. The maximum oil sorption capacity of gasoline, diesel, engine oil, toluene, and xylene were 9.9, 8.6, 6.0, 11.8, and 10.9 g/g, respectively. The oil absorbency of PMMA-NR foam was higher than the unmodified NR foam (6.7, 7.0, 2.5, 9.6, 9.7 g/g) [87]. Furthermore, the capability of raw and grafted sugarcane bagasse in oil sorption was investigated by Said et al. [88]. The chemical modification was performed by grafting raw sugarcane bagasse with fatty acid adding hydrophobic properties to the bagasse matrix. Τhe attained oil uptake capacity for the grafted bagasse was 3 g/g.

5. Conclusions

The study of the applicability of raw lignocellulosic sorbents for oil spill cleanup is driven by their abundance, inexpensiveness, non-toxicity, reusability, and biodegradability. The drawbacks of these materials are low hydrophobicity, compromised oil sorption performance, and buoyancy properties. These properties can be enhanced by modification with specific agents. Thus, in the present study, various modification methods for hydrophobicity improvement of lignocellulosic materials have been reviewed. Material performance indicators, i.e., sorption capacity and reusability, as a function of specific application and testing conditions were examined in detail. Based on the review outcomes, we can conclude that the effectiveness of lignocellulosic sorbents classifies them among the best and most eco-friendly materials compared to other synthetic sorbents. Lignocellulosic sorbents are abundant in nature and their use for oil spill cleanup has been attracting significant attention lately. Some of the natural-based materials (bio-aerogels) have shown better or similar oil sorption capacities to commercial synthetic sorbents. Bio-based aerogels can be recycled easily using distillation, combustion, or squeezing, due to their porous and hydrophobic nature. However, in many studied aerogels, oil recovery methods (distillation and squeezing) tend to decrease the absorption capacity during several cycles, leading to degradation of the sorbent’s structure. On the other hand, synthesis of aerogels can be very environmentally friendly when the biomaterial is dispersed in water, freeze-dried, and, in some cases, also pyrolyzed during synthesis, avoiding additional chemicals or reagents. The multi-functional character of bio-based aerogels as depicted by their (a) superhydrophobicity, (b) satisfactory oil uptake efficiency, (c) recyclability, and (d) low-cost make them potential eco-sorbents for oil spill cleanup.

Author Contributions

Writing—original draft preparation, M.Z. and D.T.; writing—review and editing, M.Z., V.D. and T.I.; supervision, M.Z. and T.I. All authors have read and agreed to the published version of the manuscript.

Funding

We acknowledge financial support of this work by the project “High capacity, eco-nonwoven composites for oil spill response (OilSpill)” (Τ6ΥΒΠ-00088, MIS 5048492) which is implemented under the “EPAnEK 2014–2020 Operational Programme”, funded by the Operational Programme “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014–2020) and co-financed by Greece and the European Union (European Regional Development Fund).

Conflicts of Interest

The authors declare no conflict of interest.

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