Ion Sieves from Graphene Oxide.

[NNadir]

Cool paper, this one: Ion sieving in graphene oxide membranes via cationic control of interlayer spacing (Wu, Jin, Fang, Li et al Nature 550, 380–383 (19 October 2017))

We're in a hell of place with water and metals on this planet, whether we know it or not, and our foolish "investment" in so called "renewable energy" which is yet another distributed (and thus difficult to control) "thing" in our portfolio of "things" will make it worse.

This kind of filter potentially could be utilized to manage distributed waste which is what distributed things become after a short interlude.

As always, the caveat is the requirement for energy.

Anyway, some text from the very interesting materials science paper:

Graphene oxide membranes—partially oxidized, stacked sheets of graphene1—can provide ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution2, 3, 4, 5, 6. These materials have shown potential in a variety of applications, including water desalination and purification7, 8, 9, gas and ion separation10, 11, 12, 13, biosensors14, proton conductors15, lithium-based batteries16 and super-capacitors17. Unlike the pores of carbon nanotube membranes, which have fixed sizes18, 19, 20, the pores of graphene oxide membranes—that is, the interlayer spacing between graphene oxide sheets (a sheet is a single flake inside the membrane)—are of variable size. Furthermore, it is difficult to reduce the interlayer spacing sufficiently to exclude small ions and to maintain this spacing against the tendency of graphene oxide membranes to swell when immersed in aqueous solution21, 22, 23, 24, 25. These challenges hinder the potential ion filtration applications of graphene oxide membranes. Here we demonstrate cationic control of the interlayer spacing of graphene oxide membranes with ångström precision using K+, Na+, Ca2+, Li+ or Mg2+ ions. Moreover, membrane spacings controlled by one type of cation can efficiently and selectively exclude other cations that have larger hydrated volumes.

There have been previous efforts to tune the interlayer spacing. For example, it can be widened, to increase the permeability of the graphene oxide membrane (GOM), by intercalating large nanomaterials21, 22 as well as by cross-linking large and rigid molecules23. Reducing GOMs can lead to a sharp decrease in the interlayer spacing, but renders them highly impermeable to all gases, liquids and aggressive chemicals24, 25. Recent work reported a way of sieving ions through GOMs by encapsulating the graphene oxide sheets in epoxy films and varying the relative humidity27 to tune the interlayer spacing. It remains difficult to reduce the interlayer spacing sufficiently (to less than a nanometre) to exclude small ions while still permitting water flow and enabling scalable production25. This limits the potential of GOMs for separating ions from bulk solution or for sieving ions of a specific size range from a mixed salt solution—such as the most common ions in sea water and those in the electrolytes of lithium-based batteries and super-capacitors (Na+, Mg2+, Ca2+, K+ and Li+)2, 25. Here we combine experimental observations and theoretical calculation to show that cations (K+, Na+, Ca2+, Li+ and Mg2+) themselves can determine and fix the interlayer spacing of GOMs at sizes as small as a nanometre

From the conclusion:

In summary, we have experimentally achieved facile and precise control of the interlayer spacing in GOMs, with a precision of down to 1?Å, and corresponding ion rejection, through the addition of one kind of cation. This method is based on our understanding of the strong noncovalent hydrated cation–? interactions between hydrated cations and the aromatic ring, and its production is scalable. We note that our previous density functional theory computations show that other cations (Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) have a much stronger cation–? interaction with the graphene sheet26, suggesting that other ions could be used to produce a wider range of interlayer spacings. Overall, our findings represent a step towards graphene-oxide-based applications, such as water desalination and gas purification, solvent dehydration, lithium-based batteries and supercapacitors and molecular sieving.

Have a nice day tomorrow.

[silverweb]

1. Lockheed Martin has a patent on a system.

They've had it for 4+ years, but I'm still not reading about any increased plans desalination plants. It makes me wonder if they're planning to restrict it for military use or not.

http://www.lockheedmartin.com/us/news/press-releases/2013/march/lockheed-martin-achieves-patent-for-perforene-filtration-solutio.html

Of course, others are working on their own systems, as well. In any case, a lot of people could use them right now!

[NNadir]

2. Very few patents are actually commercially viable. Some, many, are strategic.

Free flowing water is still relatively cheap. I'm not so sure how long that state of affairs will continue to exist, but for now, desalinated water is more expensive than most fresh water sources.

I would guess that the recovery of waste water, in which reverse osmosis (ion filtering) is certainly a technology having application, will remain cheaper than seawater: Fewer impurities.

However the situation observed recently at Flint is hardly likely to be the last such case, and we will definitely need to filter metal ions out of water, not just sodium, potassium and calcium, but some very toxic metals not limited to, but certainly including lead and mercury.

I sometimes wonder to myself if the international growth of mass insanity isn't, in fact, mad hatter's disease from mercury and lead. I'm not kidding.