Enhanced oils and organic solvents absorption by polyurethane foams composites modified with MnO₂ nanowires

Highlights

• The MnO₂ nanowires/polyurethane PU foam composites were prepared.

• Effect of the content of MnO₂ nanowires on oils adsorption was studied.

• The foam composites show high absorbency for oils and organic solvents.

• The foam composites could be easily regenerated and reused for up to five cycles.

Abstract

The severe environmental and ecological problems, derived from oil spills and organic solvents leakage, have occurred in many parts of the world. It is urgent to seek appropriate ways to resolve oily wastewater and organic solvent pollution. A versatile oil-absorbing material which can separate the oily wastewater effectively and quickly is in high demanded for this issue. In this work, we report a facile and inexpensive to fabricate an effective and recyclable oil-absorbent, namely MnO₂ nanowires/polyurethane (PU) foam composites, using a PU sponge as a porous substrate and MnO₂ nanowires as modifiers. The hydrothermal method is employed to synthesize MnO₂ nanowires and then foaming technology is used to fabricate MnO₂ nanowires/PU foam composites. In order to enhance the hydrophobic and oleophilic properties, the surfaces of MnO₂ nanowires are chemically modified using silane coupling agent (KH 570). The present MnO₂ nanowires/PU foam composites not only effectively separate oils from water as expected, but also possess a very high absorption capacity for the removal of organic solvents from water up to 40.15 times its own weight. More importantly, the obtained MnO₂ nanowires/PU foam composites is demonstrated to have excellent oil recoverability and absorbent regenerability, making them versatile and comprehensive absorbents to satisfy various practical oily wastewater and organic solvent separation requirements.

Introduction

The leakage of oil products and toxic organic solvents has resulted in serious ecological and environmental problems [1]. Considerable efforts have been devoted to removal the oils and organic solvents from water using various methods, including absorption [2], [3], solvent extraction [4], biodegradation [5], in-situ burning [6], solidification [7] and floating booms techniques [8]. Among them, absorption method is considered to be one of the most effective approaches because it could absorb these oils and organic solvents and also could reclaim them [9], [10], [11], [12]. There has been an increasing amount of research on the synthesis of oil-absorbing materials for the oils and organic solvents separation, including oil-absorbing fibers [13], [14], [15], 3D porous materials [16], [17], [18] and oil-absorbing resins [19], [20], [21], [22]. However, high cost, low oil absorption capacity, slow oil absorption rate and low renewability of these absorbents are still challenges to large scale applications. Therefore, it is of significance to explore advanced absorbents with eminent absorption capacity, good reusability and low cost.

3D porous materials in forms of sponges [23], [24], foams [25], [26] and hybrid aerogels [27], [28] have received significant attention recently in the application of oils and organic solvents absorption due to their outstanding advantages such as low density, large pore volume, high capillary effect, etc. Various 3D oil-absorbing materials with high oil absorption properties have been successfully fabricated using different building blocks like nanowires [29], nanofibers [30], [31], nanotubes [32], [33], graphene platelets [34], [35] and biomass [36], [37]. However, most studies focus on the fabrication of porous oil-absorbing materials using a combination of various methods. The oil absorption properties are limited by the pore structure and density of absorbents. It cannot satisfy the technological requirements in practical application, and thus largely restricts the oils absorbency of the porous oil-absorbing materials. Compared with other 3D oil-absorbing materials composed of inorganic and biological building blocks, PU foams [38], [39], [40] not only display the inherent porous characteristics but also explore outstanding properties including oils swelling properties, lower density, excellent mechanical strength and easy large-scale fabrication as well as extraordinary absorption properties that could be a promising candidate for oils and organic solvents absorption. A variety of methods such as the surface modification and grafting have been used to fabricate the superhydrophobic/superoleophilic PU materials for the absorption of oils and organic solvents pollutants. Lü et al. [41] fabricated roughness and hydrophobic PU sponge surfaces by coating PU sponges with SiO₂/graphene oxide nanohybrids. The surface modified PU sponge can be served as high and quick sorbent of organic solvents, and can be recycled with imperceptible loss of sorption capability and hydrophobicity. Wu et al. [42] fabricated magnetic, durable, and superhydrophobic PU sponges by chemical vapor deposition of tetraethoxysilane to bind the Fe₃O₄ nanoparticles tightly on the sponge and then dipcoating in a fluoropolymer aqueous solution, and the absorbency of the modified sponge for industry fuels, food oils and organic solvents is in the range of 13.26–44.50 g/g.

In addition, inorganic nanofibers or nanowires have been used for the oils and organic solvents absorption by utilizing their staggered fibrous structures. For example, Yuan et al. [29] used the superwetting nanowire membranes for selective absorption and reported a maximum oil absorption of organic solvents and oil reaching 20 g/g. Herein in this work, we report a facile approach for preparing the MnO₂ nanowires/PU foam composites by using a combined hydrothermal method and foaming technology, as well as their application in oils and organic solvents absorption. The synthesized foam composites with 3D porous characteristics exhibit excellent oil-absorbing properties, offering the combined benefit of the oils absorption properties of MnO₂ nanowires and oils swelling properties PU foams, which have promising application in oily wastewater treatment.