Nanofiltration and the Water Crisis
Abstract: Adam Alonzi
The approaching water crisis will be solved by devices made possible by nanotechnology.
Since its conception concerns have been raised about nanotechnology’s potentially deleterious impact on the environment, but at this point it looks as though it will do more good than harm. From water remediation to solar cells to pollutant monitoring, nanotechnology, as I wrote in a recent blog entry, presents humanity with a “bevy of Black Swans.” The world’s fresh water supply is dwindling. Nanotech devices can empower governments and individuals around the world to use otherwise untapped sources through desalination and reclamation.
Zhang calls the scale of groundwater pollution “enormous” and the complexity “seemingly intractable.” Of the hundreds of thousands of sites in the United States identified by environmental agencies over the last three decades less than one third have been restored. Old mining areas, factories, landfills and dumping grounds continue to increase. As well as obvious undesirables like pathogens and heavy metals, more modern toxins, like endocrine disruptors and pharmaceuticals, must also be removed. While infrastructure improvements are necessary and inevitable in many places, nanotech research will accelerate, and be expedited by, the decentralization of water treatment.
Nanofiltration involves using membranes with tiny pores (1-10 NM across) to remove specific molecules from a solution. Among other applications they are used to separate whey from the other constituents of milk and antibiotics from salty waste products. The major advantage they have over their competitors is the amount of pressure needed to pass liquid through them. Carbon nanotube membranes can remove an assortment of contaminants and aluminum based nanostructures are good at dismantling negatively charged baddies like viruses and bacteria, as well as some organic and inorganic compounds. Biomagnetite removes chlorinated organic molecules, silver slays bacteria and titanium dioxide, which is already used in a variety of consumer and industrial products, can break down organic compounds.
Dr. Sujoy Das assembled a silica-silver nanocomposite via biosynthesis. In other words, its production is cheap and green. The proteins covering the nanoparticles prevent them from leaching into the water. They also function as both reducing and protective agents for the silver nanoparticles. The nanocomposite removes dyes and microorganisms quickly. Moreover, the material can be reused several times.The LifeSaver bottle, invented shortly after hurricane Katrina, removes objects larger than 15 nanometers and works well for up to 1,500 gallons. It does not take out salt or some metals, however. This is unfortunate as in many regions the ocean is the only option.
Yet desalination is costly. It requires approximately 12 times the electricity needed to prepare fresh water for consumption. There is also the large initial investment of 200 million dollars or more to build a plant. Although for a glass of water 3 kilowatt hours is not bad at all, desalination is unfeasible on a large scale. Perforene, a graphene nanomembrane developed by Lockheed-Martin, was originally touted as being 100 times more efficient than other methods, but this estimate was later lowered to 20%. Thus, the enthusiasm was massively excessive and woefully premature. Yet this should not discourage. Every boom, or potential boom industry, has its share of exaggerated but stock boosting announcements, and does not mitigate the promise held by the technology in question.
Graphene, a hydrophobic material almost synonymous with nanotech, creates ultrafine capillaries through which water can pass. The pores can be extremely selective and the water can pass through them as easily as through a coffee filter, thus eliminating the need for energy-intensive high pressure systems. However, as Dr. Cohen cautions, a membrane that is five hundred times more permeable than its predecessor will not translate into proportional savings. Dr. Nair, one of the researchers working with this material says it “is as fast and as precise as one could possibly hope for such narrow capillaries. Now we want to control the graphene mesh size and reduce it below nine Angstroms to filter out even the smallest salts like in seawater. Our work shows that it is possible.”
Baines, Lawrence. “The Fight for Water.” Project-Based Writing in Science. SensePublishers, 2014. 81-89.
Cohen-Tanugi, David, and Jeffrey C. Grossman. “Water desalination across nanoporous graphene.” Nano letters 12.7 (2012): 3602-3608.
Inderscience Publishers. “Nanotechnology for water purification.” ScienceDaily. ScienceDaily, 28 July 2010. .
R. K. Joshi, P. Carbone, F. C. Wang, V. G. Kravets, Y. Su, I. V. Grigorieva, H. A. Wu, A. K. “Precise and Ultrafast Molecular Sieving through Graphene Oxide Membranes” Geim and R. R. Nair, Science, 2014.
Qu, Xiaolei, Pedro JJ Alvarez, and Qilin Li. “Applications of nanotechnology in water and wastewater treatment.” water research 47.12 (2013): 3931-3946.