Preparation of oil sorbents of corn straw and its application in oil-water separation

Abstract

The leakage of crude oil at sea and the discharge of domestic oily wastewater are likely to cause environmental pollution and threaten human health. Therefore, the development of techniques for the collection and recovery of the large amount of oil from water is attracting worldwide attention. To deal with the polluted water by spilled oil, materials with special wettability have been designed and developed over the past decades. Currently, metal meshes, sponges and cotton fabrics as well as particles with superhydrophobicity/superoleophiliciy or superhydrophiliciy/superoleophobicity have often been employed to separate oil and water mixtures. However, how to remove the spilled oil from water via techniques of low operation cost, high separation selectivity and efficiency as well as good recoverability for reuse without causing secondary contamination to environment is still a big challenge. Consequently, it is highly desirable to develop a new and advanced oil-water mixtures separation material that can selectively absorb oil while completely repelling water. Cellulose is the most abundant organic polymer, which is mainly used as a raw material to produce paper and cotton fabrics. Plant-derived cellulose is an important structural component of the primary cell wall of green plants, and usually found in a mixture with hemicellulose, lignin, pectin and other substances. Corn is a very common crop and widely planted in the Zhangye district of Gansu Province in China, and the corn straw are very abundant after harvest in autumn. However, the unused and accumulated corn straw can be easily rotted and hence cause the waste of resources, and the traditional treatment of burning can also induce serious environment pollution. With the consideration of the wide source, low cost, light weight, high oil adsorption capacity, good floatability in water as well as easy recycling, the corn straw is an ideal raw material which can be employed to prepare oil sorbent. In this research, corn straw was employed to fabricate oil sorbent. Corn straw was first crushed and then immersed into ZnO sol for 24 h, subsequently, corn straw powders were taken out and dried in air at 80°C. Finally, the dip-coated corn straw powders were immersed in octyltrimethoxy silane (OTS) solution for 24 h, taken out and dried at 120°C, and then, the corn straw powders begun to exhibit superhydrophobicity and superoleophilicity. Water contact angle on the superhydrophobic corn straw powders is 158.6° and the sliding angle of water droplet is about 4°, but the oil droplets can spread out completely on the corn straw powders due to the superoleophilicity. Synergy effects between surface morphology and surface chemical composition are known as the crucial factors to realize the opposite wettability to oil and water. The surface morphology and surface chemical composition of the obtained corn straw powders with superhydrophobicity and superoleophilicity were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectrum (XPS), respectively, and the wettability were measured with an optical contact angle meter at ambient temperature. The research indicates that both the dip-coating ZnO sol and surface modification with OTS play an important role for the performance of superhydrophobicity and superoleophilicity of corn straw powders. Because water and many oils are intrinsically immiscible, and the corn straw powders with extremely different affinities towards oils and water can be employed to separate oil-water mixtures by ways of filtration, and the separation efficiency towards water and oil is up to 99.6% and 98.0%. In addition, the spilled oil on water can also be removed by adsorption. Compared with the adsorption capacity of the pristine corn straw powder to spilled oil, the adsorption capacity of superhydrophobic corn straw powder is much high, and the corresponding adsorption capacity is 2.1 and 3.8 g/g, respectively. Moreover, the corn straw and spilled oil adsorbed by superhydrophobic corn straw can be reused and recovered by treatment of high speed centrifugation. When the adsorption time increased from 1 to 10, the water contact angle on the recovered corn straw powders is still about 156°±1°, and the corresponding sliding angle of water is about 8°, and the average percentage of recovery of superhydrophobic corn straw to hexadecane is above 70%. The design and fabrication of oil sorbent of superhydrophobic-superoleophilic corn straw made by dip-coating ZnO sol and modifying with OTS are expected to provide a new idea for the processing and utilization of corn straw as well as preparation of oil sorbents.