English translation of the original Chinese article. Publication dates and the extent of recovered text are preserved. Figures retain their original labels. Read the Chinese original.
Department of Chemistry, University of Waterloo
Keywords: graphene, water filtration, functional groups, ions
Anyone with even a passing connection to materials will know the celebrated graphene. Yes, the material associated with the 2010 Nobel Prize in Physics. Because of its distinctive structure—a hexagonal honeycomb sheet formed from sp2-hybridized carbon atoms—scientists have investigated it for many applications, including, of course, water filtration.
Figure 1: A schematic of graphene used for water filtration.
Graphene is highly versatile: it can be made into two-dimensional sheets or three-dimensional bulk aerogels. Today, let us briefly discuss how graphene membranes filter. For nanoscale filtration membranes, whether a substance can pass through first depends on its size. For example, a hydrated ion may be 6.7 ångströms across (1 ångström = 0.1 nanometer), a single-layer water molecule 2.8 ångströms, and the spacing between graphene layers 3.5 ångströms. In the absence of other forces, ions cannot pass through the membrane, but water molecules can. This is the basic principle of desalination through water evaporation using graphene membranes. Second, the situation becomes more complicated when graphene pores carry charge. Following the principles of charge interaction, we can simply think of opposite charges attracting and like charges repelling. Thus, when a graphene pore is negatively charged, positively charged hydrated ions tend to pass through faster, while negatively charged ions pass more slowly because like charges repel. Two particular cases need consideration. When the pore is smaller than the hydrated ion, the ion can still pass because a charged pore can lower the energy barrier for an oppositely charged ion. When the pore is slightly larger than the hydrated ion, repulsion between like charges can keep the ion outside the membrane. Third, functional groups added to the surface of graphene pores can bind certain hydrated ions specifically, preventing their passage. For example, when graphene is oxidized, surface carboxyl groups (−COOH) interact strongly with copper ions, blocking them and achieving filtration.
Figure 2: A schematic of graphene oxide membranes filtering a salt solution.
Although these basic principles allow us to design membranes with different functions, modified graphene membranes have an urgent problem: “swelling.” When immersed in water, their interlayer spacing can expand from 3.4 ångströms to 3–4 nanometers, causing the material to lose its filtration function. Controlling interlayer spacing and improving membrane stability in water are therefore major research directions for applying graphene membranes to seawater desalination and wastewater treatment.
*This article represents the authors’ personal views and does not represent the views of this website. Other media, websites, or individuals reusing or republishing it must acknowledge the source and assume responsibility for copyright and other legal matters. Authors who do not wish to have their work republished, or who wish to discuss remuneration or other republication matters, should contact us.
References:[1] http://news.mit.edu/2016/power-graphene-implants-without-frying-cells-0923
[2] Sun P, Zhu M, Wang K, et al. Selective ion penetration of graphene oxide membranes[J]. Acs Nano, 2012, 7(1): 428-437.
[3] Zheng S, Tu Q, Urban J J, et al. Swelling of Graphene Oxide Membranes in Aqueous Solution: Characterization of Interlayer Spacing and Insight into Water Transport Mechanisms[J]. ACS nano, 2017.
