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Cellulose-Based Absorbents for Oil Contaminant Removal

  • Wang Liao
  • Yu-Zhong Wang
Living reference work entry

Latest version View entry history

Part of the Polymers and Polymeric Composites: A Reference Series book series (POPOC)

Abstract

With the rapidly increasing exploitation, transportation, and utilization of fossil oils, oil spillage accidents occur frequently worldwide. Oil pollution can lead to a serious loss of valuable resources on coastal and marine ecosystems during a long period. Besides, industrial waste oil may have a broad impact on city ecological environments and human health. It is thus urgently required to solve oil pollution efficiently. Generally, current strategies are classified into three groups: (1) burning the oil spill in situ, (2) dispersing the oil in water by adding dispersants to facilitate nature degradation, and (3) extracting the oil from the water. The last method seems the “greenest” because both the absorbent and the oil can be recycled. Among the absorbents, cellulose-based absorbents are the first choices due to their environmental friendliness of renewability and biodegradability, good mechanical properties, low density, high porosity, high absorption capacity, and cost-effectiveness. In this chapter, we intend to demonstrate the following aspects of cellulose-based absorbents, including (1) raw materials: properties and pretreatments, (2) fabrication of the various absorbents, (3) characterization of the structure and properties, (4) cellulose-related absorbents and other applications, and (5) discussions and future scope. This work aims to draw a full outline of the cellulose absorbents to date and to promote the understanding and developing of these materials in the future.

Keywords

Cellulose Aerogel Absorbent Absorption Oil Hydrophobic 

1 Introduction

Fossil oils are one of the most available fuels with high-energy density. For this reason, people have been exploring fossil oils from every corner of the world. However, the source sites are always far from the using sites which are globally located; long-distance transportation is thus necessary, during which oil leakages because of fracture, natural calamities, and/or human error are almost inevitable. It has been estimated that 224,000 tons of oil from the spillage of oil tankers were released into the sea globally in the first decade of the twenty-first century [1]. The most striking leakage event that recently happened is the Deepwater Horizon oil spill (2010) in the Gulf of Mexico. This event is considered the largest marine oil spill in the history of the petroleum industry, which lasted 87 days, killed 11 people, and leaked ca. 3.19 million barrels of oil into the gulf. Although the leakage was finally stopped, the resulting pollution will affect underwater and shoreside ecosystems for a long period [2, 3]. Additionally, industrial waste water, which probably contains waste oil, is discharged into rivers, lakes, and seas, again, seriously affecting environments and life health. Based on the fact that 1 l of benzene will make several million gallons of water unfit for drinking, the environmental threat of oil pollutants is extremely serious [4]. Furthermore, because of offensive sights and odor, oil-contaminated waters deteriorate the investments and tourism of the vicinity [5]. It is therefore extremely urgent to solve the problems of leakage oil on water.

At present, the cleanup methods are either physical or chemical [6, 7]. The physical methods include skimming, booming, and absorbing. And the chemical methods include bioremediation and in situ combustion [8]. But for the rinsing-drying [9, 10], distillation [11], or combustion [12, 13], additional energy is consumed, and new pollution is simultaneously generated. By comparison, using absorbents is the most promising method because both the absorbing material and the oil can be recycled by simply squeezing, making it economic, efficient, and less polluted [1]. Detailed comparison is listed in Table 1.
Table 1

Comparison between the oil cleanup methods [14] (Copyright © 2017, American Chemical Society)

Methods

Advantages

Disadvantages

Environmental concerns

Cost

In situ burning

Quick

Environment and safety concerns

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

Cheapest

Mechanical, e.g., skimmers, booms

Efficient

Labor-intensive, time-consuming

Friendly

Very expensive

Chemical, e.g., dispersants, solidifiers

Simple

Low or no efficiency for viscous oil and in calm water

Being harmful to aquatic organisms

Expensive

Bioremediation or microorganism degradation

Efficient

Ineffective in spill with large coherent mass

Friendly

Cheap

Absorption

Efficient, simple, less secondary pollution

Labor-intensive

Friendly, the biodegradability depends on the raw materials

Cheap

To date, existing absorbents include [1] inorganic materials, such as clays [15, 16, 17], fly ash [18], and expanded perlite [2, 19]; natural organic materials, such as sawdusts [20], cotton fibers [21], and rice straws [3, 22]; synthesized polymer materials, such as polystyrene [23, 24], polyurethane sponges [25, 26], polyorganosiloxane [27, 28], melamine-formaldehyde sponges [29, 30], polypropylene nonwoven web [31], and macroporous rubber gels [4, 32]; carbon materials, such as carbon aerogels [11, 33], graphene sponges [10, 34, 35, 36, 37], and carbon nanotubes [5, 13, 38]; and hydrophobic bio-based aerogels, such as cellulose aerogels [39, 40, 41, 42, 43, 44, 45] and chitin aerogels [46]. By comparison, the absorption capacity (Cabs) and buoyancy of inorganic materials are low (< 30 g/g), and their oil/water selectivity and recovery properties are poor [15, 16, 17]. Synthetic polymeric absorbents are the most commercially available products for the oil cleanup application due to their inherent oleophilicity. Cabs of polymeric materials can be as high as 195 g/g [29]. Still, a major shortage of synthetic materials is well known as slow degradation. Once they are blown away by a gale in practice, new environmental and ecological pressure will be brought [19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47]. Carbonaceous materials have been receiving high attention these days because of their board densities (from 0.16 kg/m3 to 200 kg/m3) and high Cabs values (200–600 g/g) [36, 48]. However, the tedious preparation and relatively fragile nature of carbonaceous absorbents retard their scale production.

Based on these facts, bio-based absorbents are more welcome because of their low price, biocompatibility, biodegradability, and sustainability [49]. The adopted bio-based absorbents are kapok fiber [50], sugarcane bagasse [51], rice straw [22], barley straw [52], wood ships [53], etc. These absorbents can be used as received [54], though their Cabs are moderate (i.e., 3–50 times of the self-weights) and comparable/lower densities than inorganic and synthetic counterparts [20]. They can also be shaped to pads [55], filters [56], and fibers [57]. A comparison between abovementioned absorbents is listed in Table 2.
Table 2

Comparison between three kinds of oil absorbents [14] (Copyright © 2017, American Chemical Society)

Classification

Examples

Advantages

Limitations

Inorganic mineral

Zeolites, fly ash, exfoliated graphite, activated carbon, organclay, silica nanoparticles, amorphous silica, silica aerogel

Abundant sources

Difficult recovery, low absorption, eco-unfriendly, low-absorption selectivity and rate, poor biodegradability

Synthetic polymer

Polypropylene fiber cut, polyurethane foams, nanoporous polystyrene fibers, polypropylene nonwoven web, macroporous rubber gels

Moderate absorption capacity, good reusability

Low capacity, poor biodegradability, difficult recovery

Natural organic

Kapok fiber, sugarcane bagasse, cotton, rice straw, cotton, wood chips, barley straw

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

Low capacity, poor hydrophobicity and reusability

Furthermore, to effectively collect oily liquids from water, it is vital to choose a proper format of material as the absorbent. Generally, an ideal absorbent should have the characters of a high sorption capacity, a high oil/water selectivity, a high porosity, a fast oil sorption rate, a high floatability (i.e., hence a low density), low cost, environmentally friendliness, and recyclability [58]. Aerogel is a novel type of porous solid material usually with an extremely low density. Because of its large specific surface area, high porosity, and low density, they are highly promising for rapidly absorbing a large amount of oil from water [59]. And in this area, cellulose, the most abundant biomass on earth with characters of natural renewability, biodegradability, and ease for surface modification, is always the first choice to fabricate bio-sourced aerogel absorbents [41, 43, 45, 60, 61, 62, 63]. In this chapter, the preparation, characterization, and properties of the cellulose aerogel absorbents for oil removal in recent years are summarized, and the shortages and possible trends are proposed.

2 Raw Materials: Properties and Pretreatments

2.1 Source, Structure, and Properties

Cellulose is the most abundant biopolymer on earth. The study and application of cellulose have been deeply explored and reviewed [64, 65, 66, 67, 68]. Cellulose is a linear polymer of glucose with a flat ribbon-like conformation. The repeat unit is comprised of two anhydroglucose rings (C6H10O5)n (n = 10,000 to 15,000, which depends on the cellulose source) through β 1–4 glycosidic bond, i.e., a C1 of one glucose ring is covalently bonded to C4 of the other ring (1 → 4 linkage) by an oxygen [65]. Rich hydrogen bonding between and within the polymer chains stabilizes the linkage and results in the linear configuration. This stable microstructure accumulates to larger fibrils (5–50 nm in diameter and several microns in length). Within these cellulose fibrils, crystalline domains have cellulose chains in order, and amorphous domains have disordered cellulose chains. The microstructure of cellulose is well summarized and can be found elsewhere [69]. Cellulose nanocrystals (CNCs) are extracted from these crystalline regions (Fig. 1).
Fig. 1

Schematics of (a) single cellulose chain repeat unit, showing the directionality of the 1 → 4 linkage and intrachain hydrogen bonding (dotted line), (b) idealized cellulose microfibril showing one of the suggested configurations of the crystalline and amorphous regions, and (c) cellulose nanocrystals after acid hydrolysis dissolved the disordered regions [67]. (Copyright © 2011, Royal Society of Chemistry)

The sources of cellulose are abundant in nature, which can be extracted from (1) wood, because of the combination of lignin, hemicellulose, and other impurities in which further purification is always needed; (2) plant, such as cotton, ramie, sisal, flax, wheat straw, potato tubers, sugar beet pulp, soybean stock, banana rachis, etc.; (3) tunicate; (4) algae; and (5) bacteria.

2.2 Classification of Cellulose Raw Materials

Raw materials of cellulose can be classified as (1) regenerated cellulose (RC); (2) nanofibrillated cellulose (NFC), in some of the literature it is also written as cellulose nanofibers (CNF) or microfibrillated cellulose (MFC); (3) nanocrystalline cellulose (NCC) or also known as cellulose nanowhiskers (CNW); and (4) bacterial nanocellulose (BC or BNC).

Natural raw cellulose containing lignin, etc. is always colored and stiff. To increase the usability and/or purity of the cellulose, the raw materials require pretreatments. First, dissolving cellulose to obtain regenerated cellulose (RC). However, because of the ultrastrong intermolecular interactions, specific solvents, which are always harmful, are required for the dissolving process. And a subsequent removal of the solvent is probably necessary. Moreover, depending on the location of hydrogen bonds between and within the strands, different crystalline structures of cellulose are classified. Natural cellulose is the type of cellulose I with Iα and Iβ structures. BC and algae cellulose is enriched in Iα, while cellulose of higher plants is mainly in Iβ. In contrast, cellulose in RC is belonging to cellulose II. This conversion from cellulose I to II is irreversible, suggesting a metastable state of cellulose I and a stable one of cellulose II. The cellulose materials with cellulose I crystalline structure are with higher strength/stiffness and also display larger specific surface than those with cellulose I.

NFCs are isolated from native cellulose fiber suspensions with or without mechanical disintegration [70]. NFC is a long, flexible, and entangled network of cellulose nanofibers (ca. 2–60 nm in diameter and several micrometers in length), in which both individual and aggregated nanofibrils exist with alternating crystalline and amorphous domains [71, 72, 73].

NCCs are generated by the removing the amorphous region of partially crystalline cellulose through an acid hydrolysis process. They consist of rod-like cellulose crystals with widths of 5–70 nm and lengths between 100 nm and several micrometers. Comparing with NFC and BC, NCC has a higher crystallinity and a shorter aspect ratio (< 100). Typical characteristics of NFC, NCC, and RC are briefly summarized in Table 3 [70, 71].
Table 3

Comparative characteristics of three kinds of nanocelluloses [14] (Copyright © 2017, American Chemical Society)

Type

Sources

Preparation methods

Chemical composition

Morphological difference

Dimension size

Crystallinity

Yield

Cost

NFC (MFC, CNF)

Wood, sugar beet, potato tuber, hemp, flax

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

Often containing small amount of hemicellulose

Randomly entangled network-like

Diameter, 2–60 nm; length, several micrometers

Relatively low

High

Low

NCC (CNC, CNW)

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

Acid hydrolysis of cellulose from different sources of cellulose

Almost no hemicellulose

Rigid and rod-like

Diameter, 5–70 nm; length, 100–250 nm (from plant cellulose); 100 nm to several micrometers (from cellulose of tunicate, algae, bacteria)

High

Low

High

BC

Low molecular weight sugars and alcohols

Bacterial synthesis (i.e., Acetobacter species)

Pure cellulose without the presence of hemicellulose, pectin, or lignin

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

Diameter, 20–100 nm; length, > 100 μm; different types of nanofiber networks

High

Low

High

Cellulose from bacteria is called bacterial cellulose (BC), a very important kind of cellulose. The chemical structure of BC is exactly the same as plant cellulose. Furthermore, BC presents additional advantages over plant cellulose, including free from other plant components such as hemicelluloses, lignin, and pectin, high crystallinity (70–80%), high water content (to 99%), and relatively high degree of polymerization (up to 8000).

2.3 Pretreatments of Cellulose

  1. 1.

    Full dissolution: Generally, fully dissolving the cellulose in a specific solvent is used to generate RC. Because of the high crystallinity of cellulose, special solvents are required, including heavy metal-amine complexes, mainly copper with ammonia or diamine such as cupric hydroxide in aqueous ammonia (Schweizer’s reagent called cuoxam) or cupriethylenediamine (cuen), ammonia or amine/thiocyanate [74], hydrazine/thiocyanate [75], lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) [76, 77], and N-methylmorpholine-N-oxide (NMMO)/water [78]. In the regeneration step, the gelation mechanism bases on the physical crosslinking. In the microstructure, cellulose I crystalline structure is converted to cellulose II type; resulting materials are fragile and have lower aspect ratio of the fibrils with respect to NFC [62, 79]. In addition, the processes of dissolution and probably related gelation and solvent exchange steps are time-consuming when preparing absorbing materials, and used solvents are usually deleterious.

     
  2. 2.

    Fibrillation: Microfibrillation treatment of cellulose fibers through beating or refining is the fundamental procedure to change the fiber morphology in papermaking, in which micro fibrillation is imposing mechanical action of rotating bars to a stationary bedplate on a circulating fiber suspension. During this process, individual fiber is oriented perpendicular to the bars, resulting the release of microfibrils from the compact fiber surface. Because of the rotation intensity, this mechanical treatment can result in hierarchical fibers extending from the surface of the bulk fiber skeleton. To be specific, nanofibrillated cellulose (NFC) is prepared by first preparing an amount of cellulose pulp in deionized water, which is left and swollen at room temperature or a lower one overnight. Subsequently, the suspension is homogenized by a high-shear homogenizer, or a deflaker. Changing the initial cellulose concentration, the shearing rate, and/or homogenization time will lead to different peeling degree of the raw material and surface morphology of resulting fibrils. After this step, centrifugation can be applied to produce a paste-like material with an elevated solid content. By increasing the beating revolution (from 0 to 6000 r), hierarchical fibers on the surface of the main fibrils can be obtained, i.e., increased beating rate will definitely roughen the surface of the fibrils [45].

    The rough morphology of fibrillated cellulose increases the stability in water, especially at low concentrations. This behavior can be explained by the higher hydrophilicity and larger hydrodynamic radius caused by the branched microfibrils on the surfaces.

    The resultant nanocellulose is an attractive material for kinds of practices. The native nanofibrils with a diameter of 3–15 nm can be cleaved from wood pulp by several ways to generate cellulose hydrogels [80, 81, 82]. By vacuum drying, freeze-drying, or supercritical CO2 drying, highly porous nanocellulose aerogels are obtained for further functionalization and utilization [83, 84]. Exploration of fibrillated cellulose effectively avoids the shortages of RC materials.

     
  3. 3.

    Chemical treatments: The sole mechanical disintegration of cellulose fibers requires intensive energy consumption, which could be up to 27,000 kWh per ton of NFC. Therefore, chemical pretreatments are sometime applied to the pulp fibers, acid, enzyme, and/or chemical modifications, for instance. Introduction of charged groups onto the fibers through chemical reactions is able to significantly increase the repulsion between the fibers and promote their individualization. As a result, the energy consumption for the mechanical treatment is largely reduced. The surface functionalized chemicals include carboxymethylation, TEMPO-mediated oxidation, sequential periodate-chlorite oxidation, and trimethylammonium modification [82, 85, 86].

     

3 Fabrication of the Various Absorbents

The solvent for fully dispersing cellulose is removed to obtain a porous material. Because of the ultralow density and high porosity nature of this material, it is always named as cellulose aerogel. Aerogel is a highly porous material with extremely low densities (0.01–0.4 g/cm3), high surface areas (30–600 m2/g), high porosity, and low thermal conductivity [87]. Cellulose-based aerogels were first prepared by Stamm and co-workers [88]. Progresses in this field have been accumulated and accelerated especially after the new century.

Based on the nature of the raw materials, cellulose aerogels include cellulose derivative-based ones [89, 90, 91] and regenerated cellulose (RC)-based ones [92, 93, 94, 95, 96, 97, 98, 99], which are not strictly nanomaterials. In contrast, the nanocellulose-based ones can be determined as nanomaterials, which have at least one dimension less than 100 nm.

The formation of a 3D interconnected network, or gelation, is critical for preparing a cellulose aerogel. This process of gelation can be classified into physical crosslinking, in which intramolecular and intermolecular multi-hydrogen bonding and entanglement take place [43, 100], and chemical crosslinking, in which additional crosslinking agents are used for more stable chemical bonds between cellulose chains [101, 102]. After proper post-processing for higher hydrophobicity, the aerogel can be used as highly efficient absorbent for oil contaminant removal. In general, there are two critical steps for a molded cellulose network: the first is drying and the second is hydrophobization.

3.1 Drying

The pore structure of cellulose aerogels is susceptible to the drying process because common evaporation of the solvent will generate notable capillary pressure and result in collapsed pores. Therefore, lyophilization and supercritical drying are commonly adopted to avoid the collapse.

3.1.1 Supercritical Drying

Because relatively low pressure and temperature are required during applying, supercritical carbon dioxide (sc-CO2) is often used for the drying process [87, 89, 90, 103, 104]. A typical supercritical process first requires thorough solvent exchange process, after which, CO2 flow is pumped into the pot at a medium pressure and temperature [105]. The supercritical temperature (32 °C) and critical pressure (< 8 MPa) for CO2 are already not harsh; however, it is obvious that this method is rather tedious and time-consuming, making it relatively expensive and dangerous, and thus the consideration in real application increases.

3.1.2 Lyophilization

The conditions for lyophilization or freeze-drying are comparatively gentle. The solvent (water) is first frozen and then sublimated in vacuum. A typical procedure adopts freezing temperature below −20 °C and a subsequent freeze-drying for 3–5 days.

Compared to RC aerogels, NFC aerogels are prepared from the direct drying of frozen aqueous NFC suspensions without complex regeneration steps and harmful solvents, which is therefore more facile and eco-friendly. In addition, the nanocellulose aerogels exhibit superior mechanical integrity after freeze-drying comparing to those of RC aerogels [83].

Furthermore, it is worth mentioning the effect of “ice segregation self-assembly.” During the freezing step and the sublimation stages, large ice crystals can cause aggregation of NFC fibrils, which leads to a remarkable decrease of the specific surface [42, 43, 83, 106]. If an extra step of solvent exchange of aqueous NFC suspensions with tert-butanol is used, the abovementioned “ice segregation self-assembly” can be ameliorated [79, 104, 107, 108]. The reason is attributed to the less hydrophilicity of tert-butanol that contributes to a lower extent of surface tension effects during drying. As a result, less significantly aggregated microfibrils and higher specific surface area are obtained [43, 104, 109].

3.2 Hydrophobization

Because of the abundant hydroxyl groups along a hydrocarbon chain, native cellulose aerogels are basically amphiphilic, i.e., being lack of oil/water selectivity. Therefore, increasing oleophilicity in the aerogels will improve the absorbing capability for the oils [37, 43]. This goal can be achieved through introducing surface roughness or using low-surface-energy substances, which includes chemical vapor deposition (CVD), hydrophobic coating in water, atom layer deposition (ALD), cold plasma treatment, sol-gel treatment, and fluorination. And the related agents for hydrophobization are alkoxysilanes, chlorosilanes, TiO2, SiO2, alkyl ketene dimer, stearoyl chloride, palmitoyl chloride, (tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane, and 1H,1H,2H,2H-perfluorode-cyltrichlorosilane. After the hydrophobization reaction, water contact angle (CA) test is the most intuitional method for the surface wettability evaluation. Depending on the values, the surface wettability is classified into hydrophilic (CA < 90°), hydrophobic (90° ≤ CA ≤ 150°), or superhydrophobic (CA > 150°) [110].

Among these various methods, CVD must be the most adopted one. In the process of CVD, a precursor gas for hydrophobization blows or in situ evaporates in a container, in which the heated sample is to be coated. Subsequently, the chemical reactions of the gaseous agents occur on the sample’s surface, which thus results in the formation of a hydrophobic layer. After this reaction, the coated sample may be taken out and placed in a vacuum oven for some time to remove the unreacted agents. For instance, the nanocellulose aerogels prepared by Cervin et al. [111] were hydrophobizated with octyltrichlorosilane (OTCS) by CVD method. The CA value of the final product was ca. 150° (i.e., superhydrophobicity). For example, a hydrophobic aerogel almost absorbs n-hexadecane (oil phase) from water instantly (Fig. 2) with an absorption capacity of 45 times its own weight.
Fig. 2

Treated aerogel is able to float on water and simultaneously absorb a nonpolar liquid (hexadecane, colored red) distributed on top of the water phase. The aerogel used in these experiments had been prepared from a 1 wt% NFC dispersion, and it could be removed after the absorption without losing its integrity [111]. (Copyright © 2011, Springer Science+Business Media B.V)

4 Characterization of the Structure and Properties

To full demonstrate the structure and properties of the obtained cellulose absorbents, kinds of characterization should be carried out.

4.1 Structure

4.1.1 Density and Porosity

The basic parameters for a lightweight and porous material are its density and porosity. Bulk density is simply calculated by.
$$ {\rho}_b=\frac{m}{V} $$
(1)
where m is the weight, V is the volume, and ρb is the bulk density of the absorbent, respectively.
For a cellulose aerogel, or cellulose as the dominant constituent, the skeleton density of material equals to the density of cellulose (ρc = 1.528 g/cm3). The porosity (P) of the absorbent is hence calculated by
$$ P=\left(1-\frac{\rho_b}{\rho_c}\right)\times 100\% $$
(2)
Wang et al.’s results show that increasing the concentration of cellulose fiber will linearly increase the density and linearly decrease the porosity of the aerogel [45].

4.1.2 Specific Surface Area

The specific surface area was measured by the Brunauer-Emmett-Teller (BET) method, in which a small piece of material is dried under a synthetic gas flow at elevated temperature for some time. The adsorption of N2 is measured at −196 °C, under a range of relative vapor pressures between 0.05 and 0.2. The specific surface area is evaluated from the obtained adsorption isotherm.

4.1.3 Microstructure

The microstructure of the aerogels is always characterized by scanning electron microscopy (SEM). As expected, an aerogel absorbent displays a porous structure. The differences between these pores are attributed to the used raw materials and preparation conditions. Figure 3 shows the microstructure of a silicon-modified NFC sponge with different Si contents and at different sites, respectively.
Fig. 3

SEM micrographs of the top (a-c), core (d-f), and bottom (g-i) of NFC sponges with various silicon contents. Two magnifications are shown with scale bars of 100 and 10 μm (inserts) [42]. (Copyright © 2014, American Chemical Society)

Similar results were found by Wang et al. [45], in which the microstructure of cellulose aerogels before and after CVD treatment of MTMS was compared. Before silanization, sponges possessed a continuously three-dimensional (3D) porous structure, formed by randomly entangled cellulose fibers. Hydrophobic modification resulted in a continuous and cloud-like coating of polysiloxane layer, while the porous structure maintains well.

4.1.4 Elemental Distribution

The elemental distribution or mapping can be tested by wavelength-dispersive X-ray spectroscopy (WDX). Zhang et al. used WDX images to visualize the different Si element on the surfaces of the silylated aerogels [42].

4.2 Properties

4.2.1 Surface Hydrophobicity

The surface hydrophobicity or surface wettability of a cellulose absorbent is commonly tested by WCA measurements. WCA is defined as the angle between the tangent line on the droplet which starts from the triple point and the line of the solid surface (Fig. 4a). A water droplet falls directly on a surface of a sample, a photo is recorded by a high-speed camera, and the resulting WCA is calculated subsequently (Fig. 4b).
Fig. 4

A scheme for WCA (a) and its calculation, (b) (Source: https://en.wikipedia.org/wiki/ Contact_angle)

For the untreated cellulose absorbents, they displayed a hydrophilic, more precisely, amphiphilic character. Both water and oil droplets can be instantaneously absorbed into these materials with no WCA can be measured on the surfaces, suggesting their poor selectivity for oil and water [112, 113].

The surface after hydrophobization (typically silylation), in contrast, shows WCA in the hydrophobic level. Zhang et al.’s results compared the surface absorption between dodecane, used as model oil and colored red using Sudan III dye, and water, colored blue with Neolan Blau dye. The silylated surface absorbed dodecane instantaneously while water remained, demonstrating its hydrophobicity [42]. In addition, wettability of a material is influenced not only by surface chemistry but also by its morphology and structure [43].

4.2.2 Compressive Properties

An absorbent sample is usually cut into rectangular or cylinder specimens for compression tests. The compression modulus is extracted from the linear part of the stress-strain curve. Figure 5 gives an example of these curves for NFC sponges.
Fig. 5

Compressive stress-strain curves of silylated NFC sponges compressed to 50% strain [42]. (Copyright © 2014, American Chemical Society)

In addition, recovery after compression, i.e., repeated compressibility, is also important for an absorbent. Cellulose is relatively rigid for an absorbing material. A MCF aerogel without sufficient dispersion shows obvious contraction in 10 cycles. In contrast, the aerogel prepared by sufficient dispersion demonstrates good recovery in, for example, 30 cycles [45]. The elasticity of the aerogel will greatly facilitate the oil recovery and reuse of the material.

4.2.3 Oil Absorption

To visualize the absorption capacity of the cellulose-based absorbent, oil that floats on the water and sinks at the water bottom is absorbed by the material. Hydrophobic cellulose aerogels absorb oil swiftly and completely in many established cases, exhibiting good selectivity (Fig. 6). Additionally, the oil-filled aerogels can float on the water surface without oil release, and the absorbed oil can be facilely squeezed out, and an additional hot air drying procedure can be used to remove any residual oil or water.
Fig. 6

Removal of a red-colored dodecane spill (0.02 g) from water with the silylated NFC sponge (0.02 g). In comparison, the unmodified material was not selective and lost its original shape [42]. (Copyright © 2014, American Chemical Society)

The absorption capacity (Cabs, g/g) of an aerogel is commonly characterized by the ratio between the absorbed oil and the original weight of the material:
$$ {C}_{abs}=\frac{m_1-{m}_0}{m_0} $$
(3)
where m0 and m1 are the weights of the material before and after absorption, respectively. The absorbents have excellent absorption capacity of a wide range of oils and organic solvents, which can be attributed to the high porosity and hydrophobicity of the material. The Cabs values locate in the range of 10 to 100 g/g.
In terms of influencing factors on Cabs, the density of the oil is critical to the absorption capacity (Fig. 7) because available vacant volume of an absorbent is certain. A highly porous aerogel tends to show high absorption capacity because of its abundant free volume. An oil with low viscosity will also facilitate its penetration into the porous network, which results in a high adsorption capacity. In addition, the capillary effect, van der Waals forces, hydrophobic interaction between the oils and absorbents, morphological parameters, total pore volume, and pore structure of the absorbent also affect Cabs value [43, 45, 114, 115].
Fig. 7

Absorption capacities of the silylated sponge (18.9 wt% Si), determined for a collection of organic solvents (filled symbols) and oils (empty symbols). The sample was deposited at the surface of the liquid for 5 s [42]. (Copyright © 2014, American Chemical Society)

4.2.4 Reusability

The properties, such as efficiency and cost, of different absorbents are compared and summarized by Wang et al. [45]. From their summary, the methods for oil recovery and absorbent reuse, such as distillation, solvent extraction, and burning, are comparatively complicated, time-consuming, energy consuming, and low efficiency. By comparison, squeezing is the most facile technology. In terms of reusability, the absorbents such as the kapok fiber sponge can be reused by squeezing the absorbed oil. But their absorption capacities are relatively low. In general, for those carbon absorbents, i.e., carbon nanotube/graphene oxide sponges, carbon nanofiber aerogels from bacterial cellulose, B-doped carbon nanotube sponges, carbon nanotube frameworks, N-doped graphene frameworks, spongy graphene, reduced graphene oxide foam, and carbon aerogel from winter melon, distillation and combustion are preferred for reusing these materials because of their brittle nature, which are hence less efficient. Although carbon fiber aerogels from raw cotton and waste pulp are reported to be mechanically squeezed to recover oil, their absorption capacity decreased dramatically in just the second cycle (nearly cut in half). Therefore, it is factually improper to adopt mechanical squeezing to recover oil for carbonaceous absorbents.

A simple squeezing can remove most of the absorbed oil, making it possible to reuse both the absorbent and the oil. This merit is very important for practical applications, and therefore its characterization deserves a special attention. Silylated cellulose sponge maintains most of its absorption capacity in 10 cycles [42]. And hydrophobic MCF sponges can maintain more than half of their absorption capacities in 30 cycles [45]. Based on the excellent absorption capacity and recovery property, cellulose absorbents are highly promising in practice.

Despite the obtained high oil absorption capacity, the recovery methods for these cellulose sponges or aerogels still include solvent extraction and distillation at present stage, which are arduous and low-efficient. These shortages are due to the unfavorable shape recovery properties. To overcome the limitations, Wang et al. designed highly elastic oil absorbents based on commercial hardwood pulp. The cellulose fibrils are extracted by beating treatment as papermaking; the resulted 3D network is then freeze-dried and hydrophobizated by CVD treatment [45]. In their results, the microfibrillation degrees of hierarchical fibers can be easily regulated by changing the beating revolutions. Elevated fibrillation degree increases their recyclability. Recently, Wang et al. designed a gelatin aerogel absorbent with an extremely high reusability [116]. By comparison, cellulose material is relatively brittle and fragile at present stage which requires improvement.

Furthermore, several works report some kinetic models to describe the adsorption behaviors [117, 118, 119]. Among these models, the pseudo-first-order and pseudo-second-order models are the most commonly accepted ones for cellulose oil absorbents [13, 120, 121]. The pseudo-first-order model supposes the absorption process controlled by physisorption and described as
$$ \ln \left({q}_e-{q}_t\right)=\ln {q}_e-{k}_1t $$
(4)
whereas the pseudo-second-order model supposes the absorption process controlled by chemisorptions and described as.
$$ \frac{t}{q}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e} $$
(5)
where qe and qt are the adsorption capacities (mg/g) at the equilibrium and time t, respectively. Coefficients of k1 and k2 are the rate constants for pseudo-first-order and pseudo-second-order adsorption, respectively. For the hydrophobized cellulose absorbents, the driving forces mainly arise from hydrophobic interactions between the network and oily liquids as well as the capillary effect of the pores [100]. Thus, this adsorption process should be mostly attributed to the physisorption.

5 Cellulose-Related Absorbents and Other Applications

Carbonaceous aerogels or carbon aerogels are intrinsically hydrophobic and widely used as oil absorbent materials. They have excellent advantages of high absorption capacity and chemical and thermal stability. Carbon aerogels are traditionally fabricated through pyrolysis of resorcinol-formaldehyde organic aerogels in an inert atmosphere to form a 3D cross-linked carbonaceous network [122, 123]. The density of the resultant carbon aerogels is always high (100–800 mg/cm3) [124, 125]. In contrast, their compressive strength is low. By comparison, carbonization of biomass materials is comparatively facile, economic, and with less or no chemical reagents. Therefore, cellulose aerogels can be converted into carbonaceous aerogels via this process. From nature sources, the cellulose aerogels are born with eco-friendliness and sustainability advantages. Furthermore, the carbonaceous aerogels derived from cellulose also have a larger specific surface area, high porosity, and outstanding mechanical properties. Any format of the cellulose aerogel can be used as the precursor of the final product of carbon aerogel. During the procedure of pyrolysis, those hydrophilic functional groups are removed to obtain the oleophilic properties. Wu et al. [12]. first prepared BC pellicles by freeze-drying process, after which carbonaceous materials are obtained by pyrolysis under Ar atmosphere. The resulting 3D interconnected network of carbon aerogel is with a nanofibril microstructure of 10–20 nm. In addition, this process results in 85% reduction of the volume and also a similarity of the density, which decreases from 9–10 to 4–6 mg/cm3. The applied temperature in the pyrolysis process is critical for the final carbon aerogels. When this temperature is elevated to ca. 1300 °C, the graphite structures begin to appear. Furthermore, surface wettability is also significantly affected by the pyrolysis temperature. After a pyrolysis at a temperature of 1300 °C, the water contact angle increases from <1° to as high as 128° for the BC aerogels. Furthermore, the resulting carbonaceous aerogels, with a ultralow density (4–6 mg/cm3) and high porosity (as high as ca. 99.7%), demonstrate excellent flexibility comparing with conventional silica-based aerogels. After the release of a manual compression of more than 90%, its volume reduction can still recover to its original volume.

Carbon aerogels can also be applied in a wide range of organic solvents and oils when they are used as absorbents. Because of the abovementioned porosity and elasticity, the recyclability of these materials is also good. The absorption capacity is as high as 106–212 times of its own weight. For instance, it is 140 g/g for pump oil, 155 g/g for sesame oil, 165 g/g for soybean oil, 170 g/g for diesel oil, and 180 g/g for gasoline, which depends on the density of the oil. There are multiple solutions for recovering the absorbents and/or oil, that is, squeezing, distillation, and direct combustion. Therefore, they are proper for the separation/extraction processes of the pollution caused by organic solvents and oil spilling. NFC aerogel is first cross-linked by a commercial cross-linker, which can be subsequently transferred into carbon aerogel through carbonization process in a N2 atmosphere [102]. Special attention should be paid to the heating rate; it has a significant impact on the char yield. The density of MFC aerogel decreases from 25 g/cm3 to only 10 mg/cm3, whereas the porosity increases from 97.8% to 99% after the pyrolysis process. It is reported that the fiber diameter of the NFC aerogel skeleton dramatically decreases from 50–200 nm to 10–20 nm after the pyrolysis process. In addition, the surface area and the total pore volume of the carbonaceous network dramatically decrease when the temperature increases from 700 °C to 950 °C. And at the higher temperature, a graphite-like structure is also observed. The carbonization process removes the hydrophilic groups and causes high oleophilic property. The carbonaceous aerogel which is pyrolyzed at 700 °C has higher absorption capacity for oils; for instance, this value is 55.8 g/g for pump oil, 72.8 g/g for diesel oil, and 73.6 g/g for canola oil. However, the recycling process (e.g., reusability) is realized by rinsing with alcohol and in 10 cycles [102], which is of relatively low efficiency comparing with squeezing. For clarity, the comparison between different cellulose absorbents is summarized in Table 4.
Table 4

Comparison between various cellulose absorbents [14] (Copyright © 2017, American Chemical Society)

Classification

Cellulose origins

Nanocellulose disintegration

Hydrophobic treatment

Density (mg/cm3)

Porosity (%)

BET specific area (m2/g)

Contact angle (°)

Absorption capacity (g/g)

Cost

Raw materials

Synthesis methods

NFC-based aerogels

Hardwood Kraft pulp

Mechanical, homogenization

Coated with TiO2 via ALD

20–30 (before)

> 98 (before)

N.a.

> 90

20–40

+

Sulfite softwood pulp

Carboxymethylation pretreatment, high pressure homogenization

Modified with OTCS via CVD

4–14 (before)

99.1–99.8 (before)

11–42

~150

~45

+

Pine needle cellulose

HCl pretreatment, ultrasonic disintegration

Modified with TMCS via CVD

3.12 (before)

N.a.

20.09 (before)

135

~52

++

+

Rice straw cellulose

TEMPO oxidation, mechanical disintegration

Modified with OTES via CVD

2.7 (before)

99.5–99.6 (before)

10.9 (before)

N.a.

139–356

++

+

Hardwood pulp

Mechanical beating

Modified with MTMS via CVD

2.4 (before)

98.4–99.84 (before)

N.a.

> 150

88–228

Oat straw cellulose pulp

Mechanical disintegration

Modified with MTMS hydrolyzed polysilane

6.7 (before), 5.07–17.3 (after)

99.6 (before), 99.0–99.7 (after)

24 (before), 3–25 (after)

110–150

49–102

NFC/PVA hybrid aerogels

Fully bleached eucalyptus Kraft pulp

TEMPO oxidation, mechanical disintegration

Modified with MTCS via CVD

10.6 (before), 13 (after)

> 98

195 (before), 172 (after)

150.3

45–96

++

BC-based aerogel

BC

Commercial BC

Modified with TMCS/TEA in CH2Cl2

6.74 (before), 6.69–6.77 (after)

99.6 (after)

160.2 (before), 169.1–180.7 (after)

90

90–185

+

+

BC/rGO

N.a.

Reduction in H2 at 200 °C

N.a.

99.84–99.86

N.a.

N.a.

~150

+

+

BC/SiO2

Commercial BC

Modified with prehydrolyzed MTMS alcohols

121 (after)

N.a.

507.8 (after)

133

N.a.

+

+

Nanocellulose-derived carbon aerogels

BC pellicles

N.a.

Carbonization under Ar atmosphere

4–6 (before carbonization)

99.7 (before carbonization)

N.a.

113–128

106–212

+

+

Cellulose microfibrils

Commercial sources

Carbonization under N2

10 (after carbonization)

99 (after carbonization)

145–521

149

55.8–86.6

+

Additionally, based on their high porosity and extremely large surface areas, a cellulose aerogel without hydrophobization can be also used for water uptaking, dye/pollutants removal, filtration, sound absorption, thermal insulation, catalyst supporter, or precursor of a carbon aerogel. Carbon aerogels attract tense attention as supercapacitors which are promising in energy storage. In addition, a pristine cellulose aerogel can be easily coated by dipping in an electrically conducting polyaniline solution. After rinsing off the unbound conducting polymer and drying, the aerogel becomes an electrically conducting flexible aerogels with a conductivity of 1 × 10−2 S/cm [83].

6 Shortages of Current Cellulose Absorbents

Despite the obtained advantages have proved cellulose an excellent candidate for oil absorbent, there are still some challenges that must be addressed before it becomes predominant in the market.

First, as it is well known, fully isolation and dispersion for cellulose nanofiber preparation require energy input and are still time-consuming. How to increase the efficiency and decrease the expense of this process is very crucial for scale production of cellulose oil absorbents.

Second, cellulose is essentially amphiphilic material which is lake of oil/water selectivity. An additional hydrophobization procedure is necessary, among which the gas-phase deposition, i.e., CVD and ALD, of silanes must be the most adopted modification technology. However, this technology still suffers from the shortage of inhomogeneous silylation with higher modification content on the surface and lower content inside the material. Therefore, designing more effective modification method with time, cost-economic, and eco-friendly properties for this goal is thus always a challenge for academic and industrial researchers.

Third, recovery methods of distillation, rinsing, vacuum distillation, and burning are more complicated and lower efficient than simple squeezing. After squeezing, both the absorbent and the oil are recovered and reused. However, the skeleton of a cellulosic porous material has relatively low flexibility, which limits its reusability. After times of compression-absorption cycles, cellulose absorbents tend to exhibit a decrease in their absorption capacity.

7 Conclusion and Future Scope

In this chapter, the methods to design cellulose absorbents and the characteristics and properties of the resulting materials have been summarized. Cellulose has the advantage of abundance, and the processing of cellulose raw materials becomes easier, cheaper, and more efficient. More cellulose absorbents will be explored in the future. We expect possible trends in this field as following; first, the concept of sustainability is increasingly and widely accepted; the absorbents from recycling of waste cellulose will be increased. And second, hybrid materials with cellulose as one component will show collaborative advantage from each component of this material.

Notes

Acknowledgments

The authors appreciate the financial support from the National Natural Science Foundation of China (Grants 51603130); the Key Science Project of Department of Education, Sichuan Province (No. 16ZA0004); and the International Clean Energy Talent 2017 of China Scholarship Council.

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Copyright information

© Springer Nature Switzerland AG 2018

Authors and Affiliations

  1. 1.School of Science, Innovation and Entrepreneurship CollegeXihua UniversityChengduChina
  2. 2.Center for Degradable and Flame-Retardant Polymeric Materials, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, College of ChemistrySichuan UniversityChengduChina

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