Adjustment of the Surface Wettability of Cellulose-based Aerogels Derived From Harakeke and Their Application in Oil/Water Separation

Abstract

The increasing incidence of marine oil spills poses a significant threat to both marine ecosystems and human health. Among the various methods for oily wastewater treatment, adsorption is considered as the most promising solution due to its simplicity, cost-effectiveness, and high efficiency. Consequently, the development of high-performance, environmentally-friendly oil adsorbing materials for the removal and recovery of spilled oils have attracted substantial research interest. Cellulose-based aerogels as an emerging class of porous materials, exhibit promising properties, including ultra-low density, high porosity, and high specific surface area, making them particularly effective for adsorbing oily pollutants from wastewater. These characteristics position cellulose-based aerogels as promising candidates for widespread applications in oil spill remediation. However, there still exist some problems for the current reported cellulose-based aerogels, such as the complicated fabrication process, difficulty in recycling, and their inferior mechanical robustness, which limited their practical applications.

Harakeke, or known as Phormium tenax is a monocotyledonous plant with long leaves, endemic to New Zealand and Norfolk Islands. It is a culturally significant source and treasure to Māori, and used to be the important export goods in New Zealand. However, in contemporary times, harakeke is primarily cultivated for landscaping purposes. The objective of this thesis is to assess the potential of using harakeke fibre as precursor material to fabricate cellulose-based aerogels. If harakeke fibre can be processed to fabricate cellulose-based aerogels through appropriate process, it could not only serve as an efficient and environmentally friendly adsorbent for oily wastewater remediation, but also create a new avenue for the comprehensive utilisation of the traditional harakeke plants.

Based on the above hypothesis, this thesis investigates the use of raw harakeke fibre as starting material for the fabrication of cellulose-based aerogels, explores their surface modification methods, and evaluates their performance for oil adsorption. The main findings of this thesis are as follows: (1) A sequential chemical purification process was applied to raw harakeke fibres, involving treatment with acidic sodium chlorite solution to remove lignin, followed by potassium hydroxide solution treatment to remove the hemicellulose. This process successfully extracted pure cellulose fibres with an average diameter of 14.54 μm from the raw harakeke fibres. Additionally, it was determined that cellulose nanofibres (CNFs) with an average diameter of 61.54 nm could be obtained from the extracted cellulose fibres through a simple ultrasonication treatment using a probe ultrasonicator at an output power of 1500W. (2) Superhydrophobic cellulose-based aerogels were successfully prepared by first freeze-drying the harakeke-derived CNFs to obtain the pristine cellulose aerogels, followed by gas phase surface modification using methyltriethoxysilane (MTES) through chemical vapour deposition (CVD). The aerogel became superhydrophobic with water contact angle of 153° after silane modification. The structure and properties of CNF aerogels before and after modification were characterised by various methods, and the mechanism of gas phase modification using MTES was systematically studied. The oil sorption capacity of modified aerogels ranged from 90 to 146 g/g for various oils and solvents. The kinetic study results indicated the oil adsorption process of this aerogel obeys the pseudo-second order kinetic model. (3) Based on the aforementioned aerogel preparation method, superhydrophobic, magnetic aerogels were prepared from harakeke raw fibres to address the challenge of difficulty in recycling the oil saturated aerogels. Fe3O4 nanoparticles were deposited on the surface of aerogels by adding them in the CNF water suspension before freeze-drying. After a subsequent silane hydrophobisation treatment, the resultant aerogel showed magnetic properties, superhydrophobicity (WCA=150.3°) and outstanding oil adsorption capacity (up to 113.49 g/g for silicone oil). it was found that the addition of Fe3O4 nanoparticles not only rendered the aerogel with magnetic properties, but also contributed to a higher hydrophobicity, which is ascribed to the increased surface roughness. The magnetic aerogels can be easily controlled to move using an external magnet which greatly facilitates the recycle process. (4) In order to improve the mechanical robustness of cellulose-based aerogels, 1,2-Bis(trimethoxysilyl)ethane (BTMSE) and MTES were added into the CNF suspension as crosslinker and hydrophobic modifier, respectively. Directional freeze-drying was applied to prepare aerogels with aligned channel structures. Compression test showed that the yield point of silane modified aerogels appeared at a higher strain rate, which indicates a better mechanical robustness. Moreover, directional freeze-drying endows the aerogel with aligned channel structure and honeycomb like cross section morphology. The result aerogel showed a much higher shape recovery rate after releasing the compressive load in comparison to the conventional freeze-dried ones.