Sustainable and superhydrophobic cellulose nanocrystal-based aerogel derived from waste tissue paper as a sorbent for efficient oil/water separation

Abstract

Extraction of cellulose nanocrystals (CNCs) from wastes is a valuable strategy from the environmental and economical points of view. Herein, a novel and cost-effective method for the preparation of superhydrophobic, environmentally friendly, and recyclable bio-aerogel by freeze-drying the aqueous suspension of CNCs extracted from waste tissue paper and polyvinyl alcohol (PVA) in the presence of hydrolyzed tetraethyl orthosilicate (TEOS) sol has been developed for the first time. The highly porous (98.42 %), ultralow density (0.017 g/cm³), and superhydrophobic (WCA of 154.93°±4.14) CNCs/PVA/TEOS aerogel can selectively remove oily contaminants from water (BET surface area of 76 m²/g). The aerogel showed a high sorption capacity in the range of 69−168 g/g for 6 oils and 8 organic solvents. The reusability experiments showed the aerogel could maintain more than 92 % of its sorption capacity even after 20 cycles of sorption-squeezing. The cyclic compressive stress-strain tests confirmed the good mechanical properties of the aerogel with 89 % of shape recovery after 50 cycles. The CNCs/PVA/TEOS aerogel can be used as a recyclable sorbent for removing oils/organic solvents from water.

Introduction

Frequent oil spill accidents, increased industrial oil effluents, and chemical pollutants affect seriously the aquatic ecological environment, threaten human health, and waste a lot of energy (Ao et al., 2020; Bakhtiari et al., 2021; Chen et al., 2021; Wang et al., 2021; Zhou et al., 2016). Therefore, is an urgent need to develop innovative technologies and materials that are sustainable and scalable for the efficient collection and separation of spilled oils and organic solvents from contaminated water sources. Chemical, biological, and physical strategies are used to cleanup oils/organic solvents (Chhajed et al., 2019). Chemical methods such as in situ burning, solidification, and dispersion have several drawbacks, including being expensive and complicated. Also, these methods are not environmentally friendly (Rafieian et al., 2018). The biological methods are effective and degrade hydrocarbons using micro-organisms. These methods are time consuming and their efficiencies are highly influenced by temperature, pH, organic species, and oxygen content, and so on. In physical methods, booms and skimmers are often used to absorb oils/organic solvents (Khosravi and Azizian, 2017). The efficiency of physical methods is low. On the other hand, physical sorption of oils/organic solvents using oil sorbents is one of the most promising methods due to its high efficiency, easy operation, relatively lower costs, less secondary pollution, and higher recyclability (Laitinen et al., 2017). Recently, the superhydrophobic/superoleophilic aerogels have attracted worldwide attention as promising sorbents for treatment of oils/chemical spills. Aerogels refer to a group of novel lightweight porous 3-D materials that are formed by the replacement of liquid in a gel by air (Dong et al., 2020; Jeddi et al., 2019). They are considered as ideal candidates for oils/organic solvents cleanup (Jeddi et al., 2019; Zhang et al., 2021). This is due to their large specific surface area (100–1000 m²/g), low density (4–500 mg/cm³), and excellent sorption capacity resulting from their high porosity (>99 %) (Gong et al., 2019). So far, various types of oil aerogel sorbents such as silica (Çok et al., 2021), carbon nanotube (CNT) (Zhan et al., 2018), graphene (Cai et al., 2021; Hu et al., 2020; Li et al., 2021), and bio-aerogels (Tang et al., 2019; Wei et al., 2018) have been developed. Silica-based aerogels are very brittle due to their inherent mechanical fragility (Rafieian et al., 2018). Although CNTs or graphene aerogels have good sorption capacity, their complicated preparation processes, as well as high cost, toxicity, poor reusability, and lack of biodegradability preclude their practical applications. Compared to silica and carbon-based aerogels, biomass-based materials have advantages of better sorption capacity, low cost, renewability, and biodegradability (Liu et al., 2017). Bio-aerogels are a new generation of aerogels derived from natural plants and animal residues, most of which are polysaccharides (Hu et al., 2020).

Cellulose is one of the most abundant and renewable resources on Earth. It is a linear homopolysaccharide composed of β-D-glucopyranose units linked by β-1−4 bonds, with highly ordered crystalline phases as well as amorphous domains such as lignin, pectin, hemicellulose, and so on (Abouzeid et al., 2018). Nanocellulose is the range of nanometer at least at one dimension and can be obtained by disintegration of cellulose fibers using chemical or mechanical methods (Golmohammadi et al., 2017). Two main families of nanocellulose (NC) are cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs). They differ depending on the NC source, morphology, preparation methods as well as functions (Abouzeid et al., 2018). CNCs are rod-like crystalline fibrils with a diameter of 5−70 nm and a length between 100 nm and several micrometers. CNCs are mainly produced by removing amorphous domains by acid hydrolysis. CNFs are composed of a long, flexible, and interconnected network of cellulose nanofibers, with 2−60 nm in diameter and several micrometers in length, comprising both crystalline and amorphous cellulose domains. CNFs are usually prepared under intensive mechanical disintegration of native cellulose fiber suspensions (Akhlamadi et al., 2021; Haron et al., 2021).

NC aerogels, a new generation of aerogel-type materials, are mainly composed of NC. Their properties are similar to aerogel-type materials with additional benefits of natural cellulose, such as abundant resources, biodegradability, natural renewability, and easy surface modification (Liu et al., 2017). A main challenge for NC aerogel sorbents is their high hydrophilicity due to the presence of abundant OH functional groups on their surface (Zhang et al., 2020b). Therefore, in order to increase their hydrophobicity, they should be modified by low surface-energy materials (Li et al., 2019; Mosayebi et al., 2021).

Chemical vapor deposition (CVD) (Gong et al., 2019; Mosayebi et al., 2020), cold plasma treatment (Shi et al., 2013), atomic layer deposition (ALD) (Korhonen et al., 2011), and sol-gel (Sai et al., 2014) are some hydrophobization strategies for modifying the NC aerogels. Among these methods, CVD and ALD are widely used. However, specific operational equipment, precise control of fabrication process, and inhomogeneous grafting distribution are the three main challenges that limit the large-scale production of NC aerogels (Li et al., 2019). To address these challenges, Zhang et al. (Zhang et al., 2018) prepared a hydrophobic, flexible, and ultralightweight aerogel by freeze-drying of the aqueous suspensions of CNFs in the presence of acid-hydrolyzed methyltrimethoxysilane (MTMS) sols.

Although hydrophobic NC aerogels are desirable for oils/organic solvents cleanup, their mechanical strength and inherent modulus have hampered their practical applications (Zhang et al., 2018). One way to deal with this problem is to add a reinforcing material and a cross-linker such as polyvinyl alcohol (PVA) and polyamide-epichlorohydrin (PAE) resin to the cellulose matrix. PVA is an inexpensive synthetic polymer and a good organic binder in aerogel systems because of its water-solubility. It is also a versatile polymer which is used extensively in many industrial applications because of its biodegradability, biocompatibility, chemical resistance, and excellent physical properties in terms of its mechanical behavior. PVA has a carbon chain backbone with polar groups (hydroxyl groups, OH) on its molecular chain which have strong hydrophilic character and polarity acting as a high-density source of hydrogen bonding to enhance the formation of polymer complexes (Li et al., 2014). Particularly, PVA is a suitable material for being blended with natural polymers such as cellulose for reinforcement of cellulose matrix by physical crosslinks to make biodegradable aerogels since PVA is highly polar and can be manipulated in water solutions. It could improve the mechanical properties of cellulose-based aerogels by strong hydrogen bonding between hydroxyl groups of cellulose and PVA (Chhajed et al., 2019; Zhou et al., 2019). PAE, a cationic polymer, is one of the effective cross-linking agents. It is widely used as a wet strength additive. It can form covalent bonds with NC and PVA as a cross-linker and increases wet strength (Sharma and Deng, 2016). The main goal of the present work is to prepare CNCs aerogel with excellent mechanical properties. To our knowledge, only a few examples of modified CNCs aerogels for oil/water separation applications have been reported yet. Yang and Cranston (Yang and Cranston, 2014) fabricated the CNCs aerogels based on hydrazone cross-linking chemistry to absorb dodecane (72 ± 5 g/g of aerogel) from water. Ma et al. (Ma et al., 2017)prepared hydrazone-carboxyl ligand-linked CNCs aerogels with efficient sorption capacity to collect three types of organic solvents from water (99 ± 8 g/g for ethanol, 34 ± 4 g/g for toluene, and 54 ± 6 g/g for dodecane). The antibacterial CNCs aerogels modified by cyanuric chloride (CYCH) and chloropropyl triethoxysilane (CPTES) as a cross-linking agent were fabricated by freeze-drying followed by chlorination. The hydrophobic aerogel could remove dodecane spills (16 g/g) from water (Zhang et al., 2019). Aalbers et al. (Aalbers et al., 2019)used Thiol-Ene Click Chemistry to prepare hydrophobic CNCs aerogels for xylenes sorption (2.9 mL/g). The CNCs/PVA aerogels were modified with MTMS by Gong et al. (Gong et al., 2019) using CVD method. The aerogels were successfully used in oils/organic solvents cleanup. The aerogels showed an oil sorption capacity ranging up to 32.7 times their original weight.

To prepare superhydrophobic aerogel, the present work concentrates on the surface functionalization of CNCs. CNCs are excellent candidates for this purpose because of their high crystallinity, enhanced mechanical properties, biodegradability, renewability, and large superficial aera. Since isolation of CNCs from wastes is a sustainable strategy, for the first time, the high crystalline CNCs are extracted from waste tissue paper (WTP) as a biomass material that is very rich in cellulose content. Then, superhydrophobic biomass-based aerogels are prepared by direct freeze-drying of the CNCs/PVA suspensions in the presence of tetraethyl orthosilicate (TEOS) hydrolyzed solution without any additional equipment or energy input. The morphology, density, porosity, contact angle, and mechanical properties of the optimum prepared aerogel, CNCs/PVA/TEOS, were investigated. The sorption capacity of the aerogel in sorption of 10 organic solvents and 6 oils was measured. Also, the recyclability of the CNCs/PVA/TEOS aerogel was evaluated by a 50 cyclic loading and unloading test/fatigue compression test. Notably, the prepared superhydrophobic, ultra-light, highly porous, and flexible aerogels showed excellent sorption capacity ranging from 69 to 168 g/g. The CNCs/PVA/TEOS aerogel has also excellent reusability, high mechanical durability, and ability to recover its shape even after 20 cycles of sorption/squeezing tests. Thus, this work develops a simple and cost-effective method for fabricating a robust superhydrophobic porous aerogel based on an abundant and natural polymer (CNCs) for oils/organic solvents spill cleanup.