Miniscule Membranes to Separate Oily Water

Oily wastewater is known to be extremely harmful to the environment, to animals and organisms in the sea, as well as to plants and humans. It is created due to oil spills and the industrial sewage produced by us all on a daily basis.\cit{AhmedM18} In order to reduce the negative impact that this contaminated water has on the environment, it must be treated before it can be released back into the water cycle.

The process of removing pollutants, including oil, often takes multiple steps. This is because each different type of pollution has to be removed in its own unique way. These steps often produce additional toxic waste that has to later be removed safely. Additionally, the separation step on its own is particularly difficult as the droplets are extremely small and can therefore be dispersed throughout the water on a miniscule scale.

Currently, two of the most popular methods of separating oily wastewater are flotation[1] and coagulation.[2] Flotation consists of pumping gas into the mixture that the pollutant will stick to, which then floats to the top of the container for later removal. Although it has a high separating efficiency (95-99%) it also has a very high energy consumption, making it undesirable. Coagulation, on the other hand, works by adding a coagulant to the mixture that binds with it and sinks to the bottom of the container. This also has a high efficiency (90%) but is expensive, takes a long time and produces a toxic sludge that has to later be removed. A newer, more promising method of separation is the use of membranes.

Membranes made from nanotubes of sodium titanate (simply a combination of sodium, titanium and oxygen) that can be used to separate oily water have been created by researchers at the University of Shanghai.[3] These membrane filters enable excellent separation of the mixtures. Additionally, they are capable of two methods of self-cleaning, through surface hydrophobicity and photocatalysis. Not only that, these membranes use little energy, separate oily water in a single step, and do not produce any toxic substances that have to be carefully disposed of after.

The fabrication process of these filters is simple, which is important if their production is to be scaled up and used commercially. No difficult procedures or toxic chemicals were used and once the material was created and coated in a hydrophobic gel, it was ready for use. This process is fairly low energy and produces membranes that exhibit the optimum flexibility, which allows them to be bent into any required shape. Both of these points are important if this composite is to be introduced in commercial applications.

Once the membranes were created, Professor Shen began testing how well the material performed its primary job: separating oil and water. He found that the membranes exhibited a separation efficiency of 99.7%, which is higher than both flotation and coagulation. Although separation is slower than previously created sodium titanate nanofibers, these filters use gravity and therefore do not require an applied pressure to induce separation. For this reason they use less energy than other membranes, making them more suitable for industry applications.

The filters must be able to withstand corrosive conditions as, unlike pure water, wastewater does not have a neutral pH; it can be acidic, basic or filled with different salts. Professor Shen exposed the filters to solutions that mimic these conditions and found that under all conditions, the membranes exhibited the same separation efficiency as for the straightforward water/oil mixture. This result highlights the resilience of this material, and how effective it could be in real wastewater situations.

Unlike previously created filters with a similar sodium titanate structure, these ones exhibit self-cleaning properties. Their hydrophobic nature allows water to roll off the surface. As this happens, the water can pick up dust and solids at the same time and remove it completely. This stops debris from clogging up the membrane and thereby reducing its ability to filter the oily water. So, instead of having to thoroughly clean the filters using detergents or acidic soaks, they can just be rinsed and reused.

When the oily water permeated through the filter, the dye used to distinguish between the two phases had visibly adhered to the surface, staining it. Although the titanate within the nanotubes was able to break the bonds within the dye once the material was exposed to UV light, thus cleaning the filter, there were problems with this process. The issue is that after the removal of the dye, the membrane had to be re-treated with the gel coating and dried overnight. This process can only be repeated three times, limiting the overall reusability of the membrane. However, Professor Shen states that ‘studies are still ongoing’ to improve the reusability of the composite, which is important if they are to be implemented commercially. Additionally, further work must be done to see how viable these would be in an industrial setting.

As the volume of wastewater produced in industrial processes cannot be reduced, the importance of creating membranes that can filter this water efficiently is paramount. The membranes this research group has created are extremely efficient at filtering oily water under gravity. Additionally, they are environmentally friendly, reusable and are able to self-clean. Professor Shen states that each of these properties give the filters ‘wide applications in environmental remediation and wastewater purification’.

References

  1. C. Rattanapan, A. Sawain, T. Suksaroj, C. Suksaroj, Enhanced efficiency of dissolved air flotation for biodiesel wastewater treatment by acidification and coagulation processes, Desalination 280, pp. 370–377, 2011.
  2. Y. Suzuki, T. Maruyama, Removal of Emulsified Oil from Water by Coagulation and Foam Separation, Separation Science Technology 40, pp. 3407–3418, 2005.
  3. S. Shen, C. Wang, M. Sun, M. Jia, Z. Tang, J. Yang, Free-standing sodium titanate ultra long nanotube membrane with oil-water separation, self-cleaning, and photocatalysis properties, Nanoscale Research Letters 15, p. 22, 2020.

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