Fabrication of lightweight and robust cryogel with opposite wettability for effective oil-water separation via sustainable and toxic-free approach
Graphical Abstract
Introduction
The economy of different countries from around the globe are developing hastily as the demand for various human necessities also grows rapidly, like clothing, food for consumption, electronics and gadgets, pharmaceuticals, and many other needs which all requires mass production. Along with industrialization, pollution (air, water, and land) is also rising and one of these is oil spillage [1], [2], [3] which affects both bodies of water and land that correspondingly affects the whole ecosystem [4], [5], [6].
Oil spillage happens during the production of oil, industrial leakage, transportation, oil tanker accidents, and repairing of equipment and/or vehicles that use oil for maintenance [7], [8]. Although these incidents do not happen very often, it is still harmful to people and has an adverse effect to the environment [9] which is why separating oil from water, or the other way is becoming a challenge to every country [10]. It is the very reason why it is important to develop ways in addressing this issue and put up a sustainable solution of separating oil and water. To effectively and efficiently conclude this application, fabrication of green materials that can adequately handle the separation of oil in water mixture and vice versa is imperative. Hydrogel, cryogel, aerogel, monolith, and membrane are some of these materials being used for the separation of oil and water. Through the use of these materials, a few of the conventional ways in achieving oil-water separation application are adsorption, absorption, centrifugation, skimming, flotation, gravitational [11], [12], [13], [14], and many more. However, these approaches have some disadvantages like high energy consumption for separation and cost-intensive to name a few. Due to this, materials having a reliable performance, high adsorption capacity, and/or high flux with specific wettability attracts many researchers from either the industry or the institution.
Currently, most works of literature have exploited environment-friendly sorbent materials that use harmful solvents which contradicts its purpose of protecting and saving the environment. One of the sorbent materials that earn a huge interest in many studies is the cryogel owing to its unique porous structure, high adsorption capacity, and easiness of handling which are valuable attributes for different applications; oil-water separation, biodiesel production, biomedical application, bioseparation, and water treatment [15], [16], [17], [18], [19] are some examples of such. This material can be generated through freeze-drying [20], [21]. Apparently, most cryogel focuses more on the hydrophobic and oleophilic properties than its hydrophilicity. A traditional feature of this material has usually one function [22].
One of the frequently exploited polymers is cellulose acetate [23] that is used for various applications such as photocatalysis, biomedical application, pharmaceutical, agricultural, and others [24], [25]. It is also used for oil-water separation because of its good attributes of being tough, non-toxic, bio-degradable, biocompatible, recyclable, high mechanical strength, flexibility, and accessibility [26], [27], [28]: that being so, its function is greatly utilized. Also, it is recognized as a green polymer for it is derived from acetyl substitution of cellulose that is famous for being one of the common organic compounds [3], not to mention it is very abundant and inexpensive. However, the dissolution condition of cellulose acetate is somehow noxious due to the use of toxic organic solvents like dimethylformamide (DMF), dimethyl acetate (DMAc), and dimethyl sulfoxide (DMSO), which are unsafe to both human health and the environment.
On the other hand, polyvinyl alcohol (PVA) has been widely utilized for various applications like battery cell separator, forward osmosis, oil-water separation, etc., ascribe to its biodegradability, biocompatibility, nontoxicity, hydrophilicity, and flexibility, but water-soluble [29], [30], [31]. In order for this polymer to be convenient for oil-water separation, its chemical structure is modified through crosslinking reaction [30] to vanquish its vulnerability with water using an aldehyde crosslinker. Through this reaction, not only it can withstand water but will also boost its mechanical strength and flexibility. Another reason is that by crosslinking reaction, PVA, when added, (1) can assist other polymers for the improvement of the physical properties of the main polymer. (2) Due to its structural stability, it gives easiness for further functionalization of the network structure to increase its hydrophilicity and hydrophobicity to be effective and efficient for oil-water separation. (3) Its solubility in water makes it highly suitable for other water-based solutions. These great qualities can enormously enhance the properties of another polymer when combined [32] in order to produce a more useful and advantageous material.
In this study, we aim to scrutinize the sustainable and toxic-free fabrication of porous cryogel from cellulose acetate crosslinked with polyvinyl alcohol which was prepared through a facile procedure of freeze-drying. Its pore network structure was further functionalized to modify its wettability, favoring either its water-loving or water-resistant property to invigorate the specification needed since the prepared material is highly amphiphilic which boosts and strengthens its structural integrity that can be correlated to the desired operational requirement. Reacting cellulose acetate with PVA does not only eliminates the inability of the former polymer to be dissolved in water but also heightens the mechanical and physical property of the final product. As anticipated, the prepared cryogel demonstrated excellent oil-water adsorption property, sustainability, durability, robustness, and recyclability comparable with that of the existing studies even if its fabrication is water-based. In view of these peculiarities, a cryogel with opposing wettability is an exemplary choice for the separation of oil and water mixture applications.
Section snippets
Materials
Cellulose acetate (CA, 50,000 g mol−1) and Glutaraldehyde (GA, 25%) were purchased from Sigma-Aldrich. Polyvinyl alcohol (PVA, 87–89%, hydrolyzed), sodium hydroxide (NaOH, 98%), and hydrochloric acid (HCl, 37%) were obtained from Acros Organics. Methyltrimethoxysilane (MTMS, 97%) was bought from Alfa Aesar.
Fabrication of CA-PVA cryogel
A straightforward process of constructing a cryogel with outstanding hydrophilicity and hydrophobicity through a straightforward functionalization is laid out in Fig. 1a and b. It was
Characterization of H-CAPVA and S-CAPVA cryogel
Using naturally abundant cellulose acetate, a facile and toxic-free approach of construction of a cryogel material with contrasting wettability for oil/water separation was graphically highlighted in Fig. 1a. Cellulose acetate was fully dispersed in deionized water followed by the addition of PVA solution, GA (crosslinker), and HCl (initiator). After crosslinking reaction and freeze-drying, the solution was successfully transformed into a cryogel which was then modified via hydrolysis and/or
Conclusion
In summary, utilizing a facile freeze-drying method to fabricate CA and PVA cryogel, using water as a medium for mixing, with unprecedented hierarchical porous structure and remarkable mechanical property for oil-water separation application was demonstrated. The opposing wettability of the functionalized cryogel was achieved by hydrolyzing the cryogel to obtain hydrophilicity and by silanization reaction to attain hydrophobicity. This was able to effectively separate oil in water mixture, and
CRediT authorship contribution statement
James Laurence Ruello: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Formal analysis, Visualization. Hern Kim: Supervision, Resources, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This study was supported by Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (MSIT) (2020R1A2C2101759) and by the Ministry of Education (2020R1A6A1A03038817), and by the Korea Institute of Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy (MOTIE No. 20194010201750), Republic of Korea.
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