Cleanup of large-scale oil spills or organic pollutants from water is indeed a challenging problem. Techniques hitherto proposed for this purpose are not efficient and involves high operational cost. Researchers at College of Materials Science and Engineering and National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, PR China and Integrated Composites Laboratory (ICL), University of Tennessee, USA have developed a cost-effective and environmentally friendly method to fabricate high-density polyethylene (HDPE) microfiber bundles (PMBs) with interconnected microchannels with hydrophobicity and oleophilicity for oil/water separation.
High-density polyethylene (HDPE) and polyethylene oxide (PEO) granules (50/50 wt.%) were melt mixed in a micro twin-screw extruder (Fig. 1(a)). The HDPE/PEO extrudates were hot-drawn with a given draw ratio, air-cooled and wounded up by a winder (Fig. 1(b)). The polyethylene microfiber bundles (PMBs) containing interconnected microchannels were obtained by aqueous leaching of PEO in water (Fig. 1(c)). The resultant PMBs are very soft and can be knitted in the form of a mat (Fig. 1(d)).
Fig. 1 Various step involved in the fabrication of polyethylene microfiber bundles (PMBs): (a) Schematic of the micro twin-screw extruder; (b) HDPE/PEO fibers; (c) PMBs after leaching of PEO in water; and (d) PMBs knitted in the form of a mat
The morphology of PMBs acquired at the cross section indicates the formation of interconnected microfiber networks in bundles, which are aligned along the extrusion direction with many pores and gaps between the microfibers (Fig. 2(a)). The formation of random lamellae, particularly at the edges, imparts multi-scale roughness on surface of PMBs These attributes change the contact angle. Cyclohexane droplets spread and penetrated into the 3D interconnected structure of PMBs whereas water droplets show a nearly sphere with a high contact angle (Figs. 2(b) and 2(c)).
Fig. 2 (a) Morphology acquired at the cross section of PMBs; (b, c) contact angle images of water (dyed with potassium permanganate) and cyclohexane (dyed with Sudan III) droplets on the (b) surface and (c) cross-section of PMBs
The interconnected structure and highly hydrophobic surface enable the PMBs with an excellent absorption capacity for oils/organic pollutants. When the PMBs is placed on a cyclohexane–water mixture, the cyclohexane is quickly and selectively absorbed from water surface in several seconds (Fig. 3(a)). The underwater chloroform is also absorbed quickly by PMBs (Fig. 3(b)). The oil absorption mechanism of PBMs is attributed to the capillary force through which the oil is absorbed into the interconnected microchannels of PMBs and replaces the air within microchannels. The absorbent capacity of PMBs for various kinds of oils and organic solvents (Fig. 3(c)) indicate a maximum absorption capacity of up to 7 times their weight. Alternate immersion of PMBs in soybean oil for absorption and centrifugation for removal of oil indicates (Fig. 3(d)) no apparent deterioration in absorption capacity for 100 cycles and nearly 90% of the absorbed oil is centrifuged during each cycle. In spite of a large deformation in the shape after centrifugation, the PMBs are capable of recovering its original shape after oil absorption to saturation (Fig. 3(e)) without damaging the oil absorption performance. Due to the excellent recyclability and recovery, PMBs can be considered as a potential absorbent for dealing with oil spills or oil/water separation. The ability of PMBs for continuous oil–water separation is also demonstrated (Fig. 4).
Fig. 3 Snapshots of removal process of (a) cyclohexane floating on water; (b) chloroform sinking underwater; (c) Mass absorption capacities of the PMBs for various organic solvents and oils; (d) Oil-absorption capacity of PMBs with different absorption/centrifugation cycles; and (e) The shape change of PMBs during absorption and centrifugation process
Fig. 4 Continuous removal of cyclohexane (dyed with Sudan III) from water (dyed with methylene blue) using PMBs.
PMBs with 3D interconnected structures, high absorption capacity and excellent reusability have been developed by combining melt extrusion molding and leaching techniques. The ability of PMBs to selectively remove oils and organic solvents from water, to retain a high absorption capacity even after 100 absorption/centrifugation cycles and to offer continuous oil–water separation, suggest that they can be a promising candidate material for cleanup of oil spills and chemicals leaks.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Yayun Wang, , Applied Materials Today, 9 (2017) 77-81.
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