Abstract
A cellulose-based oil adsorbent was developed utilizing the waste paper PS-16 (mixed kraft cuttings) via a simple modification process. Through mechanical treatment and spray with polyethylene wax and alkyl ketene dimer, it was found that the product was significantly more hydrophobic than the raw material, with a water contact angle of 125.6°. The capability of the product to absorb engine oil, kerosene, and xylene was studied and compared with other cellulose-based adsorbents. The adsorbent had an excellent performance with a high absorbing ability of 16 to 28 times its own weight. Up to 92.8% of the oil in the adsorbents could be easily recycled and collected by manual squeezing. The adsorbent could be reused over eight cycles, and the sorption capacity remained constant. Therefore, this adsorbent is expected to be a promising oil sorbent for potential applications like oil spill clean-ups.
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A New Method to Prepare Oil Adsorbent Utilizing Waste Paper and Its Application for Oil Spill Clean-ups
Jiapei Liu and Xiwen Wang *
A cellulose-based oil adsorbent was developed utilizing the waste paper PS-16 (mixed kraft cuttings) via a simple modification process. Through mechanical treatment and spray with polyethylene wax and alkyl ketene dimer, it was found that the product was significantly more hydrophobic than the raw material, with a water contact angle of 125.6°. The capability of the product to absorb engine oil, kerosene, and xylene was studied and compared with other cellulose-based adsorbents. The adsorbent had an excellent performance with a high absorbing ability of 16 to 28 times its own weight. Up to 92.8% of the oil in the adsorbents could be easily recycled and collected by manual squeezing. The adsorbent could be reused over eight cycles, and the sorption capacity remained constant. Therefore, this adsorbent is expected to be a promising oil sorbent for potential applications like oil spill clean-ups.
Keywords: Waste paper; Hydrophobic modification; Oil adsorbent; Oil spill clean-up
Contact information: School of Light Industry and Engineering, State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 381 Wushan Road, Guangzhou, 510640, People’s Republic of China; *Corresponding author: wangxw@scut.edu.cn
INTRODUCTION
Clean-up of oil spills from water bodies has become a worldwide problem. To address the oil spills in a timely manner is a severe challenge. Currently, the major methods for handling oil spills are in situ combustion, chemical dispersants and flocculants, micro-biological degradation, physical interception, and various adsorbents (Suni et al. 2004; Al-Majed et al. 2012; McNutt et al. 2012; Hubbe et al. 2013; Xue et al. 2016). Among these methods, the use of adsorbents is sometimes the most effective and optimized treatment because other techniques do more harm than good, such as by wasting large amounts of resources (in situ combustion), taking a long time to work (micro-biological degradation), and causing secondary pollution (chemical dispersants and flocculants).
At present, the most commonly used oil-absorbing materials are generally classified as inorganic mineral materials, organic synthetic products, and organic natural materials (Lu and Zhou 2002). The cost of inorganic oil absorption materials is low, but the oil/water selectivity and reusability are poor, which limits their application. Recently, research about organic synthetic products has developed rapidly because of their excellent oil absorption capacity, oil/water selectivity, and reusability (Reyssat et al. 2010; Li et al. 2012; Pham and Dickerson 2014; Oribayo et al. 2017). However, the preparation process of organic synthetic products is sophisticated and the cost is high. Most importantly, these products cannot be degraded naturally, which causes secondary pollution issues.
Therefore, researchers have gradually shifted their focus to biodegradable organic natural materials, including stalk, plant fibre, cotton grass fibre, cotton grass mats, kapok fibre, etc. However, these materials generally exhibit hydrophilicity and poor oil/water selectivity (Carmody et al. 2007; Karakasi and Moutsatsou 2010; Korhonen et al. 2011; Gui et al. 2013; Hu et al. 2013; Nguyen et al. 2013; Idris et al. 2014). The hydrophobicity of raw materials is improved by pyrolysis (Dimitrov et al. 2012), graft copolymerization, predeposition of lignin and alkenylsuccinic anhydride or other hydrophobic sizing agents on cellulosic fibers (Payne et al. 2012), and other methods. Recently, hydrogels and aerogels have become popular, though their preparation processes are relatively complex. The energy consumption of these processes is high, which makes these materials unsuitable for industrial production (Banerjee et al.2006; Wang et al. 2010; Korhonen et al. 2011; Vlaev et al. 2011; Nguyen et al. 2013).
In this study, an efficient and environmentally friendly oil adsorbent was prepared from the waste paper PS-16 (mixed kraft cuttings). The modification process included pulverizing the waste paper into fluffy material having a loose and messy texture, using a grinder as a physical pretreatment, followed by spraying two modifiers successively, whose main components were polyethylene wax and alkylketene dimer. The idea was to enhance the hydrophobicity of PS-16 by grafting alkyl groups, which have a good affinity for oil and good resistance to water. The raw materials and modifiers used in this study are common low-cost materials. Furthermore, because the main raw material used was cellulose fibre, the adsorbent can be naturally degraded. This is advantageous because oil adsorbents are consumed in large amounts and therefore, secondary pollution of the environment is minimized. In contrast, as the modification process is particularly simple, it is suitable for industrial scale production. All of these advantages make this adsorbent unique when compared with previously studied chemically synthesized superhydrophobic oil-absorbing materials (Wei et al. 2003; Ceylan et al. 2009; Zhu et al. 2011; Hu et al. 2013; Lin et al. 2013; Zhou et al. 2013). The properties and performances of the adsorbent related to the clean-up of spilled oil were also investigated. This work could create a new perspective in recycling waste pulp and paper into useful products.
EXPERIMENTAL
Materials
The PS-16 (mixed kraft cuttings) was collected from Shanghai Weizhong Paper Co. Ltd. (Shanghai, China). The engine oil was obtained from a petroleum plant station (China Petroleum, Guangzhou, China). The main component of the first modifier was polyethylene wax, which was purchased from Longkou Excel Chemical Science and Technology Co. Ltd. (Longkou, China). The major ingredient of the second modifier was alkyl ketene dimer, which was bought from Lansen Chemical Products Co. Ltd. (Wuxi, China).
Sample Preparation
The modification process mainly adopts a coating method to improve the fiber’s hydrophobicity (Li et al. 2016). Firstly, the waste paper PS-16 was torn into 2-cm × 2-cm pieces, and then put them in the oven at 80 °C for 8 h. After drying, the raw material was ground into a fluffy state. The material obtained after physical pretreatment we defined as PP. Using an atomizing sprayer (JDT-05A, Nanshuiguangai, Dongguan, China), 0.35 g of the first modifier was sprayed onto 1 g of the PP, which was then oven-dried at 80 ℃ for 15 min. These adsorbents were referred to as FPP. Then, 0.15 g of the second modifier was sprayed onto the FPP and dried again. These adsorbents were referred to as SPP.
Characterization
The surface morphology of the adsorbents was studied using scanning electron microscopy (SEM) (LEO1530VP, Zeiss, Jena, Germany). X-ray diffraction (XRD) was performed using a Rigaku SmartLab SE (Tokyo, Japan). A contact angle meter (OCA40MICRO, Dataphysics, Filderstadt, Germany) was used to measure the contact angle between the adsorbent and water by dropping a single drop of liquid from a syringe onto the adsorbent surface and photographing it immediately. Before the measurement, the adsorbent materials were pressed into a slice, which had a smooth surface for convenient testing.
Measurements of Oil Absorption Capacity and Reusability
The absorption capacity of the adsorbent for various oils and organic solvents from the surface of water was tested by placing a pre-weighed ball of adsorbent in a beaker containing water and oil or organic solvents. Absorption occurred under stirring condition for 15 mines at room temperature (30 ℃). Then, the adsorbent was removed from the beaker, placed on a copper mesh, allowed to drain for 5 mines, and weighed. The absorption capacity (Q) was calculated using Eq. 1,
(1)
where M2 and M1 are the weights (g) of the adsorbent before and after absorption, respectively.
The reusability of the adsorbent was evaluated by repeated absorption and squeezing. The adsorbent was immersed in oil, and then the oil was collected by manual squeezing. The adsorbent became fluffy after a grinding process. The process was repeated several times, and the oil recovery (R) was calculated with Eq. 2:
(2)
where Mr is the weight (g) of the recovered oil.
RESULTS AND DISCUSSION
Surface Morphology and XRD Analysis
The material obtained after physical pretreatment was defined as PP. Compared with the raw material PS-16, PP had a fluffy soft texture (Fig. 1). The morphologies of the PP and SPP were observed by SEM (Fig. 2). The raw material fibre had a relatively smooth surface (Fig. 2A and 2C). After attachment of the modifiers to the skeletons of the raw fibre material, the surface of the SPP fibre exhibited a rougher morphology, whose surface was covered by a large number of micro-scale protrusions (Fig. 2B and 2D). This indicated that the two modifiers were successfully adhered onto the skeletons of the raw fibre material.
The modification transformed the SPP adsorbent into a hydrophobic and oleophilic adsorbent. The two modification processes introduced roughness and strength to the surface. Thus, the wettability of the SPP adsorbent changed because of its interaction with a wetting or non-wetting liquid (Zhu et al. 2014; Brown and Bhushan 2015).
Fig. 1. Digital photos of the PS-16 and PP
Fig. 2. Surface morphologies of the PP (A and C) and SPP adsorbents (B and D)
The crystal structures and crystallinity changes of the PS-16, PP, FPP, and SPP were analyzed by XRD (Fig. 3). The PS-16 had characteristic diffraction peaks at the 2θ values 22° and 15°, which were characteristic of natural cellulose. The peak intensity at 22° from the cellulose diffraction peak of the (002) crystal plane was high, which indicated that the cellulose existed in a highly ordered aggregation state in the PS-16, with a strong hydrogen bond between the hydroxyl groups so that it was difficult for the oil molecules to enter. Therefore, the absorption capacity of the PS-16 was low. At the value of 15°, which represents the diffraction peak of the (101) crystal plane, the width of the crystal plane is large and often appears as a side-by-side peak. When comparing the curves of the PS-16 and PP, the diffraction peak of the PS-16 decreased dramatically after physical pretreatment. This was not only beneficial to subsequent chemical modification, but also improved the entry of oil molecules. After the two chemical modifications, the crystallinity increased.
Fig. 3. XRD patterns of the PS-16, PP, FPP, and SPP adsorbents
Surface Wettability of the Adsorbent
The surface wettability results for the PS-16 and SPP are shown in Fig. 4. Parts 4A and 4B show the water contact angles on the surface of the PS-16 and SPP adsorbents, respectively. The PS-16 exhibited a hydrophilic surface, such that the water could easily penetrate within 1 s and the water contact angle was measured to be 0°, while the SPP adsorbent showed a hydrophobic surface with a water contact angle of 125.6°.
When the adsorbents were placed on the water surface (Fig. 4C), the SPP adsorbent sample always floated on the water surface, while the PP was submerged in the water. Furthermore, when the SPP adsorbent was forced to enter the water under an external force, an air layer was observed on the adsorbent surface (Fig. 4D). The SPP adsorbent immediately returned to the water surface when the applied external force was released without the absorption of water, as was confirmed gravimetrically afterwards, which indicated its excellent hydrophobicity.