Superhydrophobic PDMS@TiO2 wood for photocatalytic degradation and rapid oil-water separation

https://doi.org/10.1016/j.surfcoat.2022.128182Get rights and content

Highlights

  • A new strategy for fabricating superhydrophobic wood with anisotropic porous structure is reported.

  • The wood presents excellent separation flux in oil-water separation.

  • The wood exhibits great photocatalytic degradation ability for post-treatment.

Abstract

Natural wood, with its unique porous structure, has been widely modified for separating oil-water mixtures. However, the separation efficiency is often unsatisfactory, and little attention is focused on mitigating secondary pollution after its usable lifespan. Here, a superhydrophobic material consisting of delignified wood coated with titanium dioxide (TiO2) and polydimethylsiloxane (PDMS) was prepared. The resulting PDMS@TiO2 wood possessed outstanding superhydrophobicity, with a water contact angle of 160o. Owing to its longitudinal channels and excellent water repellency, the wood was successfully used to separate oil-water mixtures, with a high permeation flux of up to 6111 L·m−2·h−1 and separation efficiency of up to 93.4%. Importantly, PDMS and other oil pollutants were capable of being largely degraded after use via photocatalysis of the TiO2 layer, and the degradation reached 8.56 wt% within 30 days. The results from this study provide a method for fabricating functional superhydrophobic wood with photocatalytic degradation ability for efficient oily wastewater treatment.

Introduction

One of the byproducts of the rapid industrial development seen in recent decades is an increase in the discharge of waste [1]. A large proportion of this waste consists of oily wastewater, which can cause serious damage to ecological environments [2]. To mitigate the potential harm caused by oily wastewater, the development of economical and highly efficient oil-water separation technology has become a focus of research interest [3], [4], [5]. At present, common methods for the treatment of oily wastewater include gravity sedimentation [6], centrifugal separation [7], membrane separation [8], [9], [10], and chemical flocculation [11]. However, these traditional technologies are often expensive to implement and suffer from serious shortcomings such as complex and tedious processes, high energy consumption, low efficiency, and secondary pollution [12], [13], [14], [15].

In recent years, superhydrophobic materials have attracted significant attention owing to their high separation efficiency towards the treatment of oil-water mixtures [16], [17], [18]. Inspired by lotus leaves in nature, superhydrophobic materials with water contact angle greater than or equal to 150o are constructed by combining the hierarchical roughness with low-surface-energy substances. For example, many studies have been carried out on the design and fabrication of hydrophobic woods for oil-water separation [19], [20], [21], as natural wood is a renewable resource with a special longitudinal pore structure, excellent mechanical properties, and high chemical stability [22], [23], [24]. Fu et al. [25] created a superhydrophobic/superoleophilic wood material with a water contact angle (WCA) of 140o and an oil absorption capacity of 15 g/g by making use of the mesoporous three-dimensional structure of natural wood in conjunction with its hydrophobic modification using epoxy resin. Huang et al. [26] adopted chemical treatment, graphene oxide (GO) coating and reduction, and fluoroalkyl silane (FAS) grafting modification methods to prepare a highly hydrophobic rGO/FAS wood sponge with electrothermal properties for the clean-up of viscous crude oil. Ma et al. [27] prepared a superhydrophobic wood material with oil-water separation efficiency >97.0% and effective flame-retardant performance by assembling chitosan, polyethylenimine, ammonium polyphosphate, and sodium alginate, and conducting the hydrophobic modification of 3-aminopropyltriethoxysilane/silicon dioxide. However, the permeation fluxes were found to be unsatisfactory, as these were limited by the small channels in the wood. It is important to note that, to achieve high hydrophobicity, the wood is usually modified using nondegradable low-surface-energy chemicals [28], [29], [30], resulting in secondary pollution to the environment. Unfortunately, little attention has been focused on the post-treatment of hydrophobic wood after oil-water separation.

In this study, we propose a method for the production of superhydrophobic wood wrapped with titanium dioxide (TiO2) and polydimethylsiloxane (PDMS) for rapid oil-water separation and subsequent photocatalytic degradation. As a renewable material with a naturally porous structure, balsa wood was selected as substrate and chemically treated to achieve unimpeded longitudinal channels, followed by coating with anatase-phase TiO2 and modification with PDMS through simple immersion. The anatase-phase TiO2 and PDMS were utilized to endow the wood with photocatalytic degradation capability and to decrease its surface energy, respectively. The results from this study are expected to provide valuable guidance towards the fabrication of functional superhydrophobic wood with photocatalytic degradation ability for efficient oily wastewater treatment.

Section snippets

Materials

Natural balsa wood with density of 130–180 kg/m3 was provided by Guangzhou Sinokiko Balsawood Trading Co., Ltd. (China), and was cut into pieces with scale at 35 × 35 × 3 mm for use. Titanium tetrachloride (TiCl4), sodium hydroxide (NaOH), anhydrous sodium sulfite (Na2SO3), tetrahydrofuran, toluene were purchased from Aladdin Reagent Co., Ltd. (China). Hydrogen peroxide (H2O2, 30%) was bought from Tianjin Damao Chemical reagent factory (China). Cyclohexane, ethanol and dichloromethane were

Preparation and characterization of superhydrophobic PDMS@TiO2 wood

As a natural renewable source, balsa wood possesses the advantages of low density, high mechanical strength and an anisotropic porous structure [32], [33], [34]. First, the wood was chemically treated using NaOH, Na2SO3 and H2O2 to remove hemicellulose and lignin, and then freeze-dried to create a porous structure with unobstructed longitudinal channels. Second, the wood was immersed into aqueous TiO2 solution to increase its surface roughness and allow it to obtain photocatalytic ability.

Conclusions

In this study, a superhydrophobic PDMS@TiO2 wood with excellent photocatalytic performance for fast oil-water separation was successfully fabricated through delignification, TiO2 coating, and PDMS modification. The PDMS@TiO2 wood possessed an anisotropic porous structure with vertically-arranged enlarged channels, and showed strong superhydrophobicity (WCA of 160°). Due to its excellent water repellency, unobstructed longitudinal channels, and the strong capillary action of its porous

CRediT authorship contribution statement

Zhuohan Chen: Methodology, Investigation, Validation, Writing-original draft, Writing-review & editing. Xiaojing Su: Validation, writing-review & editing. Wenjian Wu: Validation, Writing-review & editing, Supervision. Siting Chen: Methodology. Xiaofan Zhang: Conceptualization. Yunhui Wu: Conceptualization. Huali Xie: Investigation, Writing-review & editing. Kunquan Li: Conceptualization.

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.

Acknowledgements

Zhuohan Chen and Xiaojing Su contributed equally to this work. This study was supported by the Special Project for Key Areas of Guangdong Higher Education Institutions, China (2020ZDZX2024), the National Nature Science Foundation of China (51903059), the Guangdong Basic and Applied Basic Research Foundation, China (2019A1515110343).

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