By: Kaitlin Lovering
In April of 2015, a tanker leaked gas into English Bay. Recent news reports, including the highly contested Kinder Morgan Pipeline planned in Burnaby, indicate that the frequency of marine oil spills will increase in the future. Although oily fluids do not mix into aqueous environments, it is difficult to efficiently separate oil from water.
Plastics made from fossil fuel derivatives are another well-known source of ocean pollution. Green polymer chemists have been working to mitigate the toxic effects of plastics by making biodegradable plastics from renewable feedstocks. The most widely used biodegradable plastic, shown in figure 1, is polylactic acid (PLA). In addition to being biodegradable, PLA is made from renewable feedstocks, such as corn starch or sugar cane, rather than petrochemicals. The biodegradability and use of readily available starting materials is in accordance with the 12 Principles of Green Chemistry.
Figure 1. Polylactic acid is a biogradable polymer made from renewable feedstocks. It is used in place of old generation plastics that are made from fossil fules and are not biodegradable. Image from wikipedia.
In addition to serving as a suitable replacement to older generations of plastics, researchers in China have recently found that nonwoven PLA can be used to clean oil spills. Nonwoven PLA materials are made by melt blowing the polymers. This fabrication creates porous materials with attractive mechanical properties. By modifying the surface properties of nonwoven PLA, Shi et al. make the biodegradable plastic either superhydrophilic or superhydrophobic. A superhydrophilic material has a very low contact angle with water; superhydrophilic PLA acts as a membrane that allows only water to pass through while oil, which does not interact well with hydrophilic materials, is trapped on the other side. Conversely, water has a very high contact angle with superhydrophobic material and water droplets remain spherical on the surface of superhydrophobic PLA while oil passes to the other side. The ability of the treated PLA fabrics to separate oil from water is shown in figure 2.
Figure 2. The PLA non-woven fabric is clearly able to separate an emulsion of oil and water. Water has been dyed blue and oil has been dyed red. In the upper panel, the superhydrophobic PLA allows water to pass while oil cannot. Conversely, in the lower panel the oil passes through the filter and the water remains in the upper container.
The surface properties of PLA were modified by a reaction with polydopamine (PDA). PDA has a catechol and an amine functional group and easily polymerizes and attaches to the PLA surface. The PDA/PLA hybrid polymer is further modified by submersion in a mixture of butyl titanate (Ti(OBu)4) and heptadecafluorononanoic acid (HFA). The ratio of Ti(OBu)4 to HFA determines the hydrophilicity of the material. A higher proportion of Ti(OBu)4 to HFA gives a superhydrophilic PDA/PLA plastic, while a lower proportion gives a superhydrophobic plastic.
The hydrophilicity of the materials can be assessed with the method shown in figure 2. In order to learn more about the atomic structure of the surfaces of the modified polymers, X-ray photoelectron spectroscopy (XPS) was used. XPS spectra of the superhydrophilic polymer shows that the Ti in the butyltitanate crystalized into small groups of anatase TiO2. Anatase is a photochemically active mineral form of TiO2. Due to the photochemical activitiy of the TiO2 found on the hydrophilic PDA/PLA, it can degrade organic molecules under UV radiation. While the superhydrophobic PLA does not have this self-cleaning capacity, both types of modified plastic show equal stability and efficiency at cyclic separation of oil from water.
Petrol and petrochemical derivatives are pervasive and damaging pollutants in the world’s’ water, and with the use of modified biodegradable PLA, green chemists have provided plastic novel system for the separation of oli/water mixtures. New research has shown that this system can, in turn, be used to clean oil spills.
Check out the full article: Biodegradable PLA Nonwoven Fabric with Controllable Wettability for Efficient Water Purification and Photocatalysis Degradation, ACS Sustainable Chem. Eng., 2018, 6 (2), pp 2445–2452
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- https://en.wikipedia.org/wiki/Polylactic_acid accessed Mar 8, 2018
Jiangwei Shi, Lei Zhang, Peng Xiao, Youju Huang, Peng Chen, Xuefen Wang, Jincui Gu, Jiawei Zhang, and Tao Chen ‘Biodegradable PLA Nonwoven Fabric with Controllable Wettability for Efficient Water Purification and Photocatalysis Degradation’ ACS Sustainable Chemistry & Engineering 2018 6 (2), 2445-2452
https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.7b03897 accessed Mar 8, 2018