Tunable sieving of ions using graphene oxide membranes
Authors: Jijo Abraham, Kalangi S. Vasu, Christopher D. Williams, Kalon Gopinadhan, Yang Su, Christie T. Cherian, James Dix, Eric Prestat, Sarah J. Haigh, Irina V. Grigorieva, Paola Carbone, Andre K. Geim, Rahul R. Nair
Journal: Nature Nanotechnology
Publication Date: 03 April, 2017
Graphene sieve turns seawater into drinking water
Graphene oxide (GO) membranes have attracted considerable attention recently as viable candidates for new separation technologies. They can filter out small nanoparticles, organic molecules and even large salts. However, they cannot be used for sieving common salts and in desalination technologies, which require even smaller, ångström-size capillaries.
Now, an interdisciplinary team of scientists at the University of Manchester have discovered how to reduce and tune the sieve size of GO membranes in the range below 10 ångströms (or one nanometre). Importantly for potential applications, ion permeation rates are found to decrease exponentially with decreasing the sieve size whereas water transport is only weakly affected. The exponential suppression of ion permeation combined with fast water transport opens possibilities for using such membranes for ångström- rather than nano-filtration and desalination. The researchers have been able to explain the mechanism involved in this ion sieving. Nanopores with sizes comparable to diameters of hydrated ions do not allow ions to pass through. They need to be partially stripped off their hydrated shells. Despite extensive interest in ion dehydration effects for many decades and especially recently, experimental studies were rather limited because of difficulties in fabricating uniform membranes with well-defined sub-nanometre pores. This research is the first clear-cut experiment in this regime. The researchers demonstrate that there are realistic possibilities to scale up the described approach and mass produce graphene-based membranes with required sieve sizes. The research shows that graphene can filter common salts from water to make it safe to drink. This opens up open new possibilities for improving the efficiency and affordability of desalination technology.
- When the common salts are dissolved in water, they always form a ‘shell’ of water molecules around the salts molecules. This allows the tiny capillaries of the graphene-oxide membranes to block the salt from flowing along with the water. Water molecules are able to pass through the membrane barrier and flow anomalously fast which is ideal for application of these membranes for desalination.
- One ångström is a unit of length equal to one ten-billionth of a metre. A sheet of newspaper is about 1,000,000 ångström thick.
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