Review on the Aerogel-Type Oil Sorbents Derived from Nanocellulose

Perspective pubs.acs.org/journal/ascecg

Review on the Aerogel-Type Oil Sorbents Derived from Nanocellulose Hongzhi Liu,*,†,‡ Biyao Geng,†,‡ Yufei Chen,†,‡ and Haiying Wang§ †

School of Engineering, Zhejiang Agriculture & Forestry University, Lin’an, Hangzhou 31130, China National Engineering and Technology Research Center of Wood-Based Resources Comprehensive Utilization, Lin’an, Hangzhou 311300, China § School of Environmental and Resource Sciences, Zhejiang Agriculture & Forestry University, Lin’an, Hangzhou 311300, China ‡

ABSTRACT: Because of the severe risk of oil pollution and increasing concerns about the sustainability of sorbent materials, there are considerable interests across the world to develop cost-effective, reusable, and environmentally friendly oil sorbents derived from renewable resources. Nanocellulose is a new family of promising cellulosic materials with a cellulose fibril width in the order of nanometer range (i.e., 2−100 nm). As a class of newly developed cellulose aerogels, nanocellulosederived ones combine intriguing interconnected three-dimensional porous characteristics of aerogel-type materials such as high porosity, large surface area, and low density with fascinating advantages related to naturally occurring nanocellulose: impressive mechanical properties, abundant sources, natural renewability, excellent biodegradability, and ease to surface modification. Therefore, nanocellulose-based aerogels are very ideal “green” oil sorbents after either appropriate hydrophobic modifications or carbonization. This present review summarizes the state-of-the-art in the aerogel-type oil sorbents derived from nanocellulose, including hydrophobized nanofibrillated cellulose (NFC)-based aerogels, hydrophobized bacterial cellulose (BC)-based ones, and the carbon ones prepared through the pyrolysis NFC or BC aerogels. Their respective preparation methods, structure, and oil-absorption performance are summarized. And the existing problems in the current research and the future development perspectives are also presented. KEYWORDS: Oil sorbents, Nanocellulose, Aerogel, Hydrophobization, Carbon

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INTRODUCTION

technologies for the removal of oil pollutants from contaminated water sources.

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The hazard of oil pollution has been becoming one of the most serious global concerns due to its harmful impacts on environmental and ecological systems. It has been estimated that 224 000 tons of oil from the spillage of oil tankers was globally released into the marine environment from 2000 to 2011.1 In the explosion and sinking of the Deepwater Horizon oil rig in the Gulf of Mexico in 2010, the BP pipe was leaking oil and gas on the ocean floor about 42 miles off the coast of Louisiana. By the time the well was capped 87 days later, an estimated 3.19 million barrels of oil had leaked into the Gulf, making the oil spill the largest accidental ocean spill in history. Apart from spilled oil pollutants, the industrial oily effluent still remains another severe risk to the ecosystem and even human health. The potential magnitude of the environmental threat of oil pollutants can be gauged by the fact that 1 L of benzene can effectively render several million gallons of water unfit for human drinking.2 Moreover, oil-contaminated water has a disastrous effect on aquatic and terrestrial life forms, and also threatens human health and the economy, particularly tourism, due to its coating properties, unsightliness, and offensive odor.3 Thus, there is an urgent demand across the world to develop a variety of © 2016 American Chemical Society

CONVENTIONAL OIL SORBENT MATERIALS To address the environmental issues arising from oil spills, organic pollutants and industrial oily wastewater, a variety of oil cleanup methods or techniques have been developed for the treatment of oil pollution.4 Generally, the strategies are classified into several categories: in situ burning, mechanical methods, chemical treatments, bioremediation, and adsorption.5 In practice, they can be used either separately or in combination with each other. Among these alternatives, the adsorption by the use of oil sorbents is generally considered to be the optimal technology because of its relatively low cost, high efficiency, and less secondary pollution.1 Furthermore, these materials can, in some cases, be recycled. The detail advantages and limitations of each method above are listed in Table 1. It has been suggested that conventional oil sorbent materials can be grouped into three major classes: inorganic mineral Received: September 22, 2016 Revised: November 6, 2016 Published: November 18, 2016 49

DOI: 10.1021/acssuschemeng.6b02301 ACS Sustainable Chem. Eng. 2017, 5, 49−66

Perspective

ACS Sustainable Chemistry & Engineering Table 1. Comparison of Various Oil Cleanup Methods in the Literature6,21 Limitations

Environmental impact

In situ burning

Classification

Combustion

Effectively quickly removing quantities of oil

Environment and safety concerns

Mechanical methods Chemical treatments

Skimmers; booms

Efficient

Labor-intensive, time-consuming

Formation of large quantities of harmful smokes and viscous residues after combustion Friendly

Use of dispersants or solidifiers

Simple operation, suitable to treat a large polluted area

Bioremediation

Microorganism degradation Use of oil sorbents (fly ash, sand, exfoliated graphite)

Good oil-removal efficiency, low operation cost Good oil-removal efficiency, simple operation, practically feasible, less secondary pollution

Little effect on very viscous oil, ineffective in calm water, high initial and/or running costs Ineffective in spill with large coherent mass Labor intensive

Adsorption

Examples

Advantages

Being harmful to aquatic organisms

Cost Cheapest Very expensive Expensive

Friendly

Cheap

Friendly, its biodegradation depends on the used sorbents

Cheap

friendliness, and recyclability.36 Aerogels refer to one class of highly interconnected porous and lightweight solid materials formed by replacing the liquid in a gel by air.37 Because of the large specific surface area, high porosity, and low density, they display promising potentials as an ideal oil sorbent candidate to rapidly absorb a large amount of oil and to float on water. Noncellulose Aerogel-Type Oil Sorbents. To date, various kinds of aerogel-type oil sorbents, e.g., silica,12,13,38 carbon nanotube (CNT),39−41 and graphene aerogels,42−44 have been developed. Pure silica aerogels can absorb oils but are difficult to separate the sorbent with oil from the water because their inherent mechanical brittleness could not endure the capillary force and crack seriously when oils are absorbed inside the mesopores of the aerogels. Although the newly developed nanostructured carbon-based aerogels (e.g., CNT, graphene, carbon nanofibers) exhibit superior sorption capacities (i.e., 100−913 g/g) and reusability, the complicated synthesis methods as well as high cost and nonrenewability of precursors largely hamper their practical applications. Hence, considerable efforts have been devoted to developing a novel and sustainable aerogel material that possesses excellent sorption properties and low cost for the oil/water separation. Cellulose Aerogel-Type Oil Sorbents. As a “young” third generation of aerogel materials succeeding silica and synthetic polymer-based ones, cellulose aerogels or sponges combine the intriguing features of aerogel-type materials with additional advantages of naturally occurring cellulose, such as abundant sources, natural renewability, biodegradability, and ease to surface modification. Therefore, cellulose aerogels seem to be one of the most fascinating natural oil sorbents after appropriate modifications.45−48 Depending on the nature of cellulosic materials, cellulose aerogels include cellulose derivative-based ones,49−51 regenerated cellulose (RC)-based ones,52−61 and nanocellulose-based ones. Prior to the formation of cellulose aerogels by a drying process, the gelation is a key step. During the gelation, the threedimensional cellulose network (3D) is formed. Depending on whether the reaction is involved or not during the formation of gels, the gelation mechanism can be classified into the physical cross-linking and chemical cross-linking. For the former mechanism, the intramolecular and/or intermolecular hydrogen bonds and physical entanglement between cellulose molecules are mainly responsible for the gelation. The physical crosslinking mechanism is involved in the case of both RC aerogels and nanocelulose ones.62,63 For the chemical gelation mechanism, an additional cross-linking agent, such as paper-strengthening resin64,65 needs to be added to induce the formation of the crosslinked cellulose network.

sorbents, synthetic polymer sorbents, and natural organic sorbents.6 Inorganic mineral sorbents, also known as sinking sorbents, are highly dense, fine-grained mineral or inorganic materials (natural or processed) used to sink floating oil. Examples include fly ash,7 zeolites,8 exfoliated graphite,9 activated carbon,10 organoclay,11 silica nanoparticles,12 amorphous silica,13 and silica aerogels.14 But these mineral sorbents are less preferred due to their low oil-sorption capacities (typically less than 20 times by weight), poor oil/water selectivity, and low buoyancy that is inconvenient to recycle. Synthetic polymer oil sorbents, such as polypropylene fibers,15 polyurethane foam,16 nanoporous polystyrene fibers,17 polypropylene nonwoven web,18 and macroporous rubber gels,19 are the most commonly used commercial sorbents in the oil-spill cleanup due to their inherent oleophilic and hydrophobic characteristics. Despite excellent oil-sorption capacities, one major drawback of these sorbents lies in the fact that they degrade very slowly in comparison to mineral or natural products.9,20 And their environmental and ecological impact remains less optimiztic. Natural organic sorbents are among the eco-friendly alternatives for the oil removal from wastewater. Unlike synthetic organic sorbents, natural sorbents are derived either from various abundant and cheap plants or animal residues.28 The most common natural oil sorbents are kapok fiber,22 sugarcane bagasse,23 cotton,24 rice straw,25 wood ships,26 barley straw,27 and so on. These sorbents are either used as received29 or formed into sheets, booms, pads,30,31 filter,32 and fiber assemblies.33 Raw natural sorbents have moderate oil-sorption capacity (i.e., 3−50 times that their own weight), comparable or sometimes lower density than inorganic and synthetic sorbents, and excellent biodegradability.34 Other advantages include their possibility of recycling, higher oil recovery, and relatively easy disposal compared to other types of conventional sorbents. Despite the above advantages, the majority of traditional natural sorbents also exhibit many drawbacks, such as poor buoyancy and selectivity of oil sorption which is associated with high water uptake of these sorbents. Low water repelling ability also reduces the effectiveness of their microporous structure to absorb oil.23,35 For clarity, the advantages and limitations of each oil sorbents above are listed in Table 2.

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AEROGEL-TYPE OIL SORBENTS For effective collection oil from water, it is vital to choose an appropriate material as an oil sorbent. Typically, an ideal oil sorbent material is characterized of a high sorption capacity and oil/water selectivity, a high porosity, a fast oil sorption rate, a high floatability (i.e., hence a low density), low cost, environmentally 50


Natural organic sorbents

Synthetic polymer sorbents

Inorganic mineral sorbents

Classification

51

sugarcane bagasse;23 cotton;24 rice straw;25 wood chips;26 barley straw;27

polyurethane foams;16 nanoporous polystyrene fibers;17 polypropylene nonwoven web;18 macroporous rubber gels;19 Kapok fiber;22

Abundant resources, low cost, excellent biodegradability and environmental-friendliness

Moderate adsorption capacity, good reusability

fly ash;7 exfoliated graphite;9 activated carbon;10 organoclay;11 silica nanoparticles;12 amorphous silica;13 silica aerogel;14 Polypropylene fiber cut;15

Advantages

Abundant sources

Zeolites;8

Examples

Limitations

Low capacity (e.g., sugarcane bagasse ∼10.51 g/g, wood chips ∼343 mg/g, barley straw 584.2−613.3 mg/g), poor hydrophobicity and reusability (e.g., kapok fiber, cotton, rice straw, wood chips)

Low capacity (e.g., polyurethane foams 7−9 g/g), poor biodegradability (e.g., polypropylene fibers, nanoporous polystyrene fibers), difficult recovery (e.g., nanoprous polystyrene fibers, polypropylene nonwoven web, macroporous rubber gels)

Difficult recovery (e.g., fly ash, zeolites, activated carbon, silica nanoparticles, amorphous silica), low oil-sorption (e.g., zeolites ∼170 mg/g, activated carbon ∼340 mg/g, organoclay ∼7.5 g/g, silica nanoparticles ∼15 g/g, silica aerogel ∼15.1 g/g), eco-unfriendly (e.g., exfoliated graphite), expensive (organoclay, exfoliated graphite), low absorption selectivity and rate (e.g., activated carbon), poor biodegradability (e.g., fly ash, organoclay)

Table 2. Performance Comparison of Three Kinds of Conventional Oil Sorbents

ACS Sustainable Chemistry & Engineering Perspective


Perspective

ACS Sustainable Chemistry & Engineering

be tailored to convert the inherent hydrophilicity of the skeleton to hydrophobicity and oleophilicity. The hydrophobicity of the surfaces can be created by introducing roughness (i.e., micro- or nanoscale asperities) and low surface-energy substances. Usually, the surface wettability is evaluated by measuring the water contact angle (CA). Depending on the CA values, the surface wettability is classified into hydrophilic, hydrophobic, and superhydrophobic. If the CA is less than 90°, the surface is described as hydrophilic; if the CA lies between 90° and 150°, then hydrophobic; if the CA is more than 150°, the surface is superhydrophobic.83 At present, the commonly used hydrophobization processes of cellulose-based aerogels include chemical vapor deposition (CVD), atom layer deposition (ALD), cold plasma treatment,84 sol−gel,85,86 esterification,51 and fluorination. And the examples of hydrophobizing agents include TiO2, SiO2, alkoxysilanes, chlorosilanes, alkyl ketene dimer,87 (tridecafluoro-1,1,2,2tetrahydrooctyl)trichlorosilane,88 1H,1H,2H,2H-perfluorodecyltrichlorosilane,89 stearoyl chloride,51 and palmitoyl chloride.73 Several previous studies have utilized kinetic models to describe the adsorption behaviors of sorbent materials.90−92 Among these models, the pseudo-first-order and pseudo-secondorder models40,93−95 are commonly used ones for the oil/organic solvent sorption of cellulose aerogel-type oil sorbents. The relative coefficients of the models are calculated using linear leastsquares fitting. The pseudo-first-order model represents the sorption process controlled by physisorption, whereas the pseudo-second-order model is used to describe the sorption process controlled by chemisorptions. The pseudo-first-order equation is described as follows:

The initially developed cellulose aerogel-type oil sorbents are based on regenerated cellulose (RC). Generally, RC-based aerogels are prepared through the following steps: (i) fully dissolving the cellulose in an appropriate solvent; (ii) regeneration of cellulose by replacing the solvent by a nonsolvent (i.e., gelation); (iii) drying of the obtained lyogels. Nevertheless, the dissolution, gelation, and solvent-exchange steps are very time-consuming in the preparation of RC aerogels and also the used solvents are usually very harmful. During the regeneration step, the gelation mechanism mostly belongs to physical crosslinking. Moreover, unlike nanocellulose (e.g., NFC) aerogels, the favorable cellulose-I crystalline structure is converted to cellulose-II one during the preparation of RC aerogels, yielding the aerogels with relatively inferior mechanical properties (e.g., fragility) and lower aspect ratio of the fibrils with respect to NFC.47,66 It is well-known that the oil-sorption performance of cellulose aerogels depends not only on the density and viscosity of oily liquid, but also largely on the capillary effect, van der Waals forces, hydrophobic interaction between the oils and absorbents, and morphological parameters of the aerogels63,67 (e.g., surface wettability, total pore volume, and pore structure). The oil density would contribute to the saturated absorption capacity in aerogel-type sorbents with same pore size distribution and 3D network structure.65 And a lower oil viscosity facilitates their penetration into the porous network of aerogels, and thus results in a higher adsorption capacity,40,68,69 whereas highly porous aerogels usually tend to show higher oil-sorption capacities because they provide more internal free volume for oil sorption. The pore structure of cellulose aerogels critically depends on the choice of drying processes. In practice, freeze and supercritical dryings are the most commonly used methods for the preparation of cellulose aerogels. Because a low surface tension effect (i.e., capillary effect) occurs during the drying stage, the supercritical carbon dioxide (sc-CO2) drying can effectively avoid the collapse of 3D porous structure and result in cellulose aerogels with a low density and a high specific surface area.49,50,70−72 But the operation of supercritical drying is expensive and dangerous, which to some extent limits the practical feasibility of its industrial applications. Comparatively, the freeze-drying technique (also known as lyophilization) in which the solvent in the frozen gel is sublimated without entering a liquid state, is relatively safer and more economical. Thus, it is an eco-friendly alternative to replace traditional supercritical drying from a practical point of view. However, effects of large ice crystals during the freezing step and sublimation stages of the freeze-drying process can cause the aggregation of NFC nanofibrils, and thus the specific surface area of as-obtained cellulose aerogels is dramatically decreased, usually in the range of 10−70 m2/g.63,73−76 This phenomenon, also known as “ice segregation self-assembly”, can be ameliorated by the freeze-drying after the solvent-exchange of aqueous NFC suspensions into tert-butanol.66,77−79 Because of the nature of tert-butanol having less hydrophilic character, a lower extent of surface tension effects is displayed during the drying process. As a result, the aerogels with a higher specific surface area are obtained due to less significantly aggregated microfibrils in this case.79−81 In addition, increased hydrophobicity and oleophilicty in the aerogels would contribute to the improvement of their oil/water selectivity. Owing to the presence of abundant hydroxyl groups, native cellulose aerogels display the amphiphilicity (i.e., poor oil/water selective adsorption).63,82 Consequently, they have to

ln(qe − qt ) = ln qe − k1t

(1)

where qe and qe are the adsorption capacities (mg/g) at the equilibrium and time t, respectively, k1 is the rate constant for pseudo-first-order adsorption. From the plots of ln(qe − qt) versus t, the values of k1 and qe can be determined. Meanwhile, the pseudo-second-order equation can be converted into a linear form: t 1 t = + 2 qt qe k 2qe

(2)

where k2 is the rate constant for the second-order adsorption (g·mg−1·min−1) and is determined from the linear plot of t/qt versus t. In the case of hydrophobized cellulose aerogels, the driving force for the oil-sorption mainly arises from hydrophobic interactions between modified cellulose and oils (or organic solvents) as well as the capillary effect of the pores.62 Thus, their adsorption process mostly belongs to the physisorption.

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NANOCELLULOSE-BASED AEROGEL TYPE OIL SORBENTS To overcome the drawbacks of RC aerogels, the aerogels based on nanocellulose, has been recently developed. Nanocellulose refers to a family of novel cellulosic materials with the lateral dimension in the order of nanosized range (2−60 nm).96−98 Depending on cellulose source, functions as well as preparation methods, which in turn depend mainly on the cellulose sources, nanocelluloses may be grouped into three main subcategories: nanocrystalline cellulose (NCC), nanofibrillated cellulose (NFC), and bacterial nanocellulose (BC or BNC).96,99 Typical characteristics of three kinds of nanocelluloses are outlined in Table 3. 52


Bacterial nanocellulose (BC) (in the form of a pellicles)

Pure cellulose without the presence of hemicellulose, pectin or lignin

Randomly assembled ribbon-shaped fibrils less than 25−100 nm in width

Lowmolecularweight sugars and alcohols

Bacterial cellulose; microbial cellulose

Bacterial synthesis (i. e., Acetobacter species)

Rigid and rod-like

Almost no hemicellulose

Acid hydrolysis of cellulose from different sources of celluloses

Wood, cotton, hemp, flax, whest straw, mulberry bark, ramie, Avicel, tunicin, cellulose from algae and bacteria

Cellulose nanocrystals (CNC); Cellulose (nano) whiskers (CNW)

Nanocrystalline cellulose (NCC)

Morphological difference Randomly entangled network-like

Often containing small certain amount of hemicellulose

Delamination of wood pulp by mechanical pressure before and/ or after refining, chemical or enzymatic treatment

Wood, sugar beet, potato tuber, hemp, flax

Microfibrillated cellulose (MFC); Cellulose nanofibrils or nanofibers (CNF)

Nanofibrillated cellulose (NFC)

Chemical composition

Preparation methods

Typical sources

Other synonyms

Type of nanocellulose

Table 3. Comparative Characteristics of Three Kinds of Nanocelluloses (i.e., NFC, NCC, and BC)

High

Low

Low

High

Diameter:5−70 nm Length:100−250 nm (from plant celluloses); 100 nm to several micrometers (from celluloses of tunicates, algae, bactera)

Diameter:20−100 nm Length: >100 μm; different types of nanofiber networks

High

Relatively low

Yield

Crystallinity

Dimension size Diameter: 2−60 nm Length: several micrometers

Cost

High

High

Low


53


Perspective


biodegradability, and environmental friendliness. These fascinating properties make it possible to use them as oil sorbents. Korhonen et al.104 prepared hydrophobic but oleophilic nanocellulose aerogels by freeze-drying NFC hydrogels and subsequently functionalizing with a low-surface-energy TiO2 coating through an ALD technique. The resultant highly porous (porosity > 98%) and low-density (20−30 mg/cm3) aerogels exhibited highly water-repellent properties and the oils do not drain out when the aerogel was floating on water or taken out from water. Depending on the density of the absorbed oils (e.g., 30 g/g for mineral oil, 37.5 g/g for paraffin oil) or nonpolar liquids (e.g., 40 g/g for chloroform), their volume-based and mass-based sorption capacities were able to reach 80−90% of total volume of the aerogel and 20−40 g/g, respectively (Figure 1c). The absorbed organic liquids were readily removed by drying or washing out with an appropriate solvent. Its absorption capacity of toluene was not deteriorated even after 10 sorption−drying− resorption cycles, indicating good reusability (Figure 1a). This feature made them an attractive biosorbents for oil spillage cleanup.

Compared to RC, nanocellulose shows favorable cellulose-I crystalline structure with a higher strength/stiffness, and also displays larger specific surface area. As compared to RC aerogels, the resultant aerogels based on nanocellulose exhibit superior mechanical integrity after freeze-drying.76 Besides, because nanocellulose aerogels are usually obtained by directly drying its frozen aqueous suspension without the use of organic solvents to dissolve cellulose, the preparation process is thus more facile and environmentally friendly than that of RC ones. Nanocrystalline cellulos (NCC), also known as cellulose nanowhiskers (CNW), consist of rod-like cellulose crystals with widths and lengths of 5−70 nm and between 100 nm and several microns, respectively. They are generated by the removal of amorphous region of partially crystalline cellulose by acid hydrolysis. In comparison to NFC and BC having threedimensional (3D) cellulose nanofibers network, NCC owns a higher degree of crystallinity and a shorter aspect ratio (150

Very expensive

Expensive

Expensive

Expensive

Expensive

Synthesis methods

139−356 (e.g., 240 for pump oil, 250 for soybean oil)

n.a.

n.a.

>98 (after)

98.5 (after)

Very expensive

∼52

135

Cheap

∼45

∼150

Raw materials

20−40 (e.g., 30 for minCheap eral oil, 37.5 for paraffin oil)

Oil-sorption capacity(g/g)

Cost

>90

Contact angle (deg)

160.2 (before), 169.1−180.7 90 (after)

Commercial BC

Nanocellulose- BC pelderived carlicles bon aerogels

n.a.

98.4−99.84 (before) 99.6 (before), 99.0−99.7 24 (before), 3−25 (after) (after)

10.9 (before)

20.09 (before)

11−42 (before)

n.a.

BET specific area (m /g)

99.5−99.6(before)

n.a.

99.1−99.8(before)

>98 (before)

Porosity (%)

2

Modified with TMCS/TEA 6.74 (before), 6.69−6.77 99.6% (after) (after) in CH2Cl2

10.6 (before), 13 (after)

23.2 (after)

6.7 (before) 5.07−17.3 (after)

2.4 (before)

BC/rGO

Commercial BC

Modified with MTCS via CVD

Chemical grafting with hydrophobic styrene-acrylic monomer

Modified with MTMS-hydrolyzed poysilxane

Modified with MTMS via CVD

Modified with OTES via CVD

BC/SiO2

BC

Mechanical disintegration

Softwood kraft pulp

BC-based aerogel

Mechanical disintegration

Oat straw cellulose pulp

TEMPO oxidation, mechanical disintegration

Mechanical beating

Hardwood pulp

Fully bleached eucalyptus kraft pulp

TEMPO oxidation, mechanical disintegration

Rice straw cellulose 2.7 (before)

3.12 (before)

Modified with TMCS via CVD

HCl pretreatment, ultrasonic disintegration

Pine needle cellulose

3

Density (mg/cm )

Coated with TiO2 via ALD 20−30 (before)

Hydrophobic treatments

4−14 (before)

Mechanical, homogenization

Nanocellulose disintegration

Sulfite soft- Carboxymethylation Modified with OTCS via wood pretreatment, highCVD pulp pressure homogenization

Hardwood kraft pulp

NFC/PVA hybrid aerogels

NFC-based aerogels

Classification

Cellulose origins

Table 4. Preparation Processes and Properties of Aerogel-Type Oil Sorbents Derived from Nanocellulosea



Perspective


higher BET surface area, as compared to the one pyrolyzed at 950 °C. More importantly, the carbon aerogel was able to retain the same absorption capacity after 10 absorption/ extraction (rinsing with alcohol) cycles, demonstrating excellent multiple reusability (Figure 14f). For clarity, the comparison of preparation processes, properties as well as costs of these above nanocellulose-based aerogel-type oil sorbents, are summarized in Table 4.

During the pyrolysis process, hydrophilic functional groups, such as CO, CO, CH, and OH in the nanocellulose molecules, were removed while retaining the oleophilic properties. Very recently, few efforts have been devoted to transforming nanocellulose-based aerogels into their carbonaceous equivalents. Wu et al.116 prepared ultralight, flexible, and fireresistant carbon aerogels from BC pellicles by through freezedrying process followed by the pyrolysis under argon atmosphere. After the pyrolysis, the porous 3D and interconnected network structure of BC aerogels was maintained, and the diameter of the nanofibers decreased to 10−20 nm. After the pyrolysis treatment of BC aerogels, the volume of resultant carbon nanofiber aerogels was shrunk to 15% that of their original precursors and the density decreased from 9−10 to 4−6 mg/cm3. It was found that pyrolysis temperature had a great influence on crystalline structure and surface wettability of the obtained carbon aerogels. When increasing pyrolysis temperature to 1300 °C, the graphite structure began to appear. And a significant change occurred in the surface wettability after pyrolysis treatment of BC aerogels and the water contact angle was increased from 90% volume reduction and almost recover its original volume after release of the load. When used as sorbents, the carbon aerogels were able to absorb a broad spectrum of organic solvents and oils with excellent recyclability and selectivity. And the absorption capacity was as high as 106−212 times its own weight (e.g., 140 g/g for pump oil, 180 g/g for gasoline, 170 g/g for diesel oil, 155 g/g for sesame oil, 165 g/g for soybean oil), which were higher than that of other typical carbon-based sorbents in the literature. Furthermore, such carbon aerogels can be regenerated by distillation and direct combustion in air. Thus, they were considered to be an ideal candidate for highly efficient separation/extraction of specific substances, such as organic pollutants and oils. Instead of BC aerogels as a carbonaceous precursor, Meng et al.65 also prepared sponge-like carbon aerogels by freeze-drying a mixture of NFC and commercial cross-linker (Kymene resin) to create 3D porous aerogels and subsequent carbonization under nitrogen to render both hydrophobic and oleophilic properties. It was found that heating rate had a considerable impact on the char yield. After carbonization, the bulk density was decreased from 25 g/cm3 for MFC aerogel to 10 mg/cm3, whereas the porosity was increased from 97.8% to 99%. Similar to the previous report by Wu and co-workers, the fiber diameter of NFC aerogel was found to dramatically decrease from 50−200 nm to 10−20 nm after the pyrolysis. Both BET surface area (521 vs 145 m2/g) and total pore volume (0.55 vs 0.28 cm3/g) of the sample prepared at 700 °C were significantly higher than that of the one prepared at 950 °C. And the graphite-like structure was observed at the carbon aerogel sample prepared at 950 °C. The removal of hydrophilic function groups during carbonization caused carbon aerogel to achieve good hydrophobic properties (149° for a water CA) and fast absorption rate. In addition, carbon aerogel pyrolyzed at 700 °C was found to possess higher oil absorption capacity of various types of oils (e.g., 55.8 g/g for pump oil, 72.8 g/g for diesel oil, 73.6 g/g canola oil), (Figure 14a−e) which were correlated with its

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CONCLUSIONS AND FUTURE PERSPECTIVES In summary, the aerogel-type sorbents derived from nanocellulose precursors (including NFC and BC) have exhibited superior oil-absorption performance, mechanical strength, and reusability because of their highly porous 3D network structure with interconnected microfibrils. The unique properties of these natural oil sorbents, along with the advantages of environmental friendliness, low-cost raw materials, and mechanical durability, make them superior to other sorbent materials in a wide range of oil-spill cleanup applications. To replace traditional oil sorbents in practice, however, there are still several challenges to be addressed in the future development of the NFC-based aerogel sorbents: (1) The existing hydrophobization process of nanocellulosebased aerogels are usually complex and energy-consuming. As one of the most widely used strategies, the gas phase deposition (e.g., CVD and ALD) of silanes tends to suffer from the drawback of inhomogeneous silylation with a higher Si content on the aerogel surfaces. Thus, one of the concerns is to how to make the process more industrially feasible and effective. (2) Many existing recovery methods for oil sorbents, such as distillation, rinsing (i.e., solvent extraction), vacuum distillation, and burning, are usually complicated, timeconsuming, and have high-energy output and low efficiency, especially for carbon aerogels. Comparatively, mechanical squeezing appears more facile to recover oil and to reuse sorbent materials from the viewpoint of practical applications. But the majority of the reported oil sorbents derived from nanocellulose are still lack of sufficient compression robustness. After multiple compression− absorption cycles, they tend to exhibit an decrease in the oilsorption capacity. (3) It has been demonstrated that the aerogel-type sorbents derived from BC exhibited superior mechanical and extraordinary oil-absorption properties. But in comparison to BC produced by bacteria such as Acetobacter xylinus species, NFC is generally extracted by a combination of pretreatment and mechanical disintegration of various native cellulose sources. Therefore, the latter subcategory of nanocellulose has a relatively lower cost with respect to BC, and thus appears more competitive and favorable precursor to prepare the aerogel-type oil sorbents. But how to make isolation process of NFC more highly efficient and inexpensive on an industrial scale is very crucial for nanocellulose oil sorbents to be implemented in the practical oil-cleaning treatments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Prof. Hongzhi Liu). Tel: +8-57163746552. 62


Perspective

ACS Sustainable Chemistry & Engineering ORCID

Hongzhi Liu: 0000-0002-1725-0976 Notes

The authors declare no competing financial interest. Biographies

Yufei Chen was born in China in 1992. He successfully completed a B.S. degree of polymer materials and engineering at the Zhejiang Agriculture & Forestry University in 2014. He is pursuing a M.S. degree at the school of engineering under the supervision of Prof. Hongzhi Liu at the same university. His research topic is to develop novel nanocellulose-based aerogels for the elimination of dye pollutants. Dr. Hongzhi Liu received his Ph.D. with the major of polymer chemistry & physics from Institute of Chemistry (ICCAS), Chinese Academy of Sciences in July 2005. From February 2006 to July 2013, he worked at Seoul National University, Louisiana State University, and Washington State University, respectively as a postdoc. After that, he joined Zhejiang Agriculture & Forestry University in October 2013 as a distinguished professor. In November 2016, he was successfully awarded to the “Zhejiang Thousand Talents Initiative” title (i.e. Introduction Program of Overseas Chinese High-leveled Talents). To date, he has published more than 75 scientific papers in the peerreviewed journals, book chapters, and conference, and issued 2

Haiying Wang completed her B.S. program in 2004. She is currently a M.S. candidate with the major of agricultural resource utilization at the Zhejiang Agriculture & Forestry University. Her research topic deals with the development of novel biosorbent materials for the capture of heavy metal ions.

international patents. His current research interests focus on developing novel or high-performance biomass-based materials.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the Public Welfare Projects of Zhejiang Province (No. 2016C33029 & 2017C33113), Scientific Research Foundation of Zhejiang Agriculture & Forestry University (No. 2013FR088), NewShoot Talents Program of Zhejiang Province (No. 2015R412046).

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Review on the Aerogel-Type Oil Sorbents Derived from Nanocellulose

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