Updated: Manchester graphene desalination filter

As reverse-osmosis desalination membranes already exist, I was keen to hear what was new about the Manchester University announcement that its researchers had made a graphene desalination membrane.

Manchester graphene seive

Graphene oxide has been mooted as a filtering material, but it cannot filter out small molecules like dissolved sodium chloride because the ‘holes’ are too big – particularly because they expand a bit in a water environment.

Looking at the abstract of the research paper (Tunable sieving of ions using graphene oxide membranes in Nature Nanotechnology), the breakthrough seems to be that the researchers have found a way to stop the expansion.

“The developed membranes are not only useful for desalination, but the atomic scale tunability of the pore size also opens new opportunity to fabricate membranes with on-demand filtration capable of filtering out ions according to their sizes.” said researcher Jijo Abraham.

It looks like it is necessary to know that dissolved salt molecules travel around with a shell of water molecules, which is what I think Manchester is talking about when it calls them ‘hydrated’.

Update: The filtering structure is layers of graphene oxide 100μm thick (prepared by filtering flakes) with alternating layers of epoxy – the epoxy provides the ‘physical confinement’ mentioned below.
Water is forced sideways through this, so it flows parallel to the epoxy layers sideways through the graphene oxide flakes – so it is not passing through layers of graphene, but finding its way along its surface, squeezing between flakes.
The thickness of this edge-on filter was 4mm.

According to the paper’s abstract, graphene oxide membranes usually have a permeation cutoff of ∼0.9nm, determined by inter-layer spacing of ∼1.35nm of the laminates swollen in water.

With physical confinement, says the abstract, this 1.35nm can be adjusted to between 0.98 to 0.64nm.

With these, ion permeation is thermally activated with energy barriers of ∼10-100kJ/mol – depending on layer spacing.

“We demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl,” say the researchers in the abstract.

As reverse-osmosis equipment already exists, I wonder:

  • How much pressure is needed to force salty water through the process? – it is hundreds of psi for existing membranes, and pumping this is what makes the process so energy intensive.
    Manchester’s answer, from from the paper, is: “modelling of practically relevant filtration processes shows that an increase in water
    permeation rates above the rates currently achieved (2-3 litre/m2×h×bar) would not contribute greatly to the overall efficiency of desalination”
  • How easy it is for the more-salty water to get out of the membrane backwards? – its anti-clogging characteristic.
  • Do the components last long?
  • Can it be made cheaply enough?

 


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