Technology Review describes graphyne sheets for filtering salt from seawater at rates several orders of magnitude faster than conventional desalination techniques. The new technique involves a material known as graphyne, a two-dimensional sheet of carbon atoms connected together much like graphene but with an altered structure because of double and triple bonds in certain places. Graphyne is interesting because these double and triple bonds create holes between the carbon atoms that are large enough for water molecules to pass through. However, these holes are not big enough for sodium and chloride ions, which are larger because they attract a shell of water molecules since they are charged.
Water passes through graphyne at a rate some two orders of magnitude faster than through the polymer membranes used in conventional reverse osmosis techniques. They are in the lab stage for production of graphyne.
Conventional desalination plants that rely on reverse osmosis require 1.5 kiloWatt-hours of electricity to produce 1 tonne of freshwater.
A different approach to improving desalination
Lockheed Martin announced that they had punched holes in sheets of graphene to produce a molecular sieve that removes sodium and chloride ions from seawater. This, they said, could desalinated seawater much more quickly and cheaply than existing methods.
Desalination that produces clean freshwater from seawater holds the promise to solve the global water shortage for drinking, agriculture and industry. However, conventional desalination technologies such as reverse osmosis and thermal distillation involve large amounts of energy consumption, and the semipermeable membranes widely used in reverse osmosis face the challenge to provide a high throughput at high salt rejection. Here we find by comprehensive molecular dynamics simulations and first principles modeling that, pristine graphyne, one of the graphene like one-atom-thick carbon allotropes, can achieve 100% rejection of nearly all ions in seawater including Na+, Cl-, Mg2+, K+ and Ca2+, at an exceptionally high water permeability about two orders of magnitude higher than those for commercial state-of-the-art reverse osmosis membranes at a salt rejection of ~98.5%. This complete ion rejection by graphyne, independent of the salt concentration and the operating pressure, is revealed to be originated from the significantly higher energy barriers for ions than that for water. This intrinsic specialty of graphyne should provide a new possibility for the efforts to alleviate the global shortage of freshwater and other environmental problems.