• Allan Dyen-Shapiro

Shiny new tech to save us all! Sort of.

The flashy headline attracted my attention: a new energy source from rivers meeting the oceans. Blue membranes. I liked the name. Apparently, they allow positive ions to flow through them from the salt water to the fresh water, but they reject negative ions. The salt water side becomes negatively charged, so when you connect the two sides with a wire, negatively charged electrons flow through it to the fresh water side, thus generating electricity.

The pore through which the ions flow is a boron-nitride nanotube. Easy chemical formula to remember—BN—one atom of boron for every atom of nitrogen. And Wikipedia taught me that the walls of the nanotube are hexagonally-packed. Think of a six-sided figure with the B and the N at alternating corners, attached to more six-sided figures on each side, always a B to an N. This hexagonal tile “wallpaper” wraps around an open tube through which the fluid flows.

And then the writer (Robert Service for Science Magazine—I bet that’s a pen name—it sounds like a superhero’s butler) blundered. He claimed the nanotubes were negatively charged, explaining why negative ions were rejected.

Any student I’d ever taught in first-year high school chemistry could have told him that was impossible. Boron and nitrogen have a difference in electronegativity (how greedy they are for electrons) of 1.0, meaning that they made a covalent bond, not an ionic bond. No charges anywhere. Sure, the electrons bonding the atoms together are unequally shared, spending more of their time around the nitrogen nucleus. But that’s not a charge. And in a hexagon, the boron atoms would face the inside of the tube where the water flows as often as the nitrogen atoms would.

How could this possibly work?

Every once in a while, I feel driven to understand a science advance where it seems the science journalist has gotten it wrong. And as you good folks reading this were either drawn here by my science fiction writing or by my social media posts on environmental and other issues where science is relevant, I tend to offer the explanations once I find them.

My first clue was an interview with the scientist cited that said something about hydrogen bonding with the water molecules and the nitrogen being weak and not much of a consideration. Aha, I thought, this isn’t just B binding to N, but potentially B binding to N binding to the H in water. So, I looked up the 2013 French paper that did all the fundamental work on which this new technology was based. In that paper, essentially big-ass magic lasers (not really, but it’s more fun to think of it that way, and I’m in danger of losing my audience here if I elaborate on every point) cut a hole in a flat piece of silicon nitride, which you can think of as a barrier that blocks movement of everything, including water. In this hole, a Jules Vernesque shrinking scientist shoves one of these nanotubes, with the open tube allowing water flow from one side of the membrane to the other. Again, not really, but you didn’t want to learn about laser-based manipulation, do you, or at least not now—I see those eyes glazing over. They then looked at the movement through the hole as a function of pH. No ions flowed at low pH where there’s a lot of H around. Aha (it does give a Hardy Boys-type feel to this writing when I keep saying Aha, doesn’t it? You do imagine the exclamation point, right), when the positively-charged hydrogen ions are blocking the partial negative charge that are on all of the N atoms (due to the electronegativity difference between N and B), there is no cluster of electrons on the inside of the tube, so there’s nothing to draw the positively charged ions into the tube. At higher pH (above neutral, little H around, the Gulf of Mexico is ~pH 8.4 on average), there was movement of positive ions. A lot of it.

The other thing that my former students would tell you is that N atoms are a lot smaller than B atoms. So, if you take the same amount of partial charge and cluster it in a smaller area, you have hot spots of negativity (electron density, but, hey, I’m trying to limit the technical vocabulary here).

I then, in my head, made an analogy: H ions are transported through biological membranes by sequentially moving between binding sites within a pore (made by a protein that lines an open hole). They bind the individual sites equally well. But there will be net movement if there’s a lot of H on one side of the membrane and less on the other.

I’ll bet the same thing is going on here. The positive ions go from “hot spot” to “hot spot” of negativity, pushed by the presence of a lot of positive ions on the salt water side and very little on the fresh water side.

Here, I have some personal experience. Back in my scientist days, we cruised up the Caloosahatchee River in a boat, taking water samples, and we benchmarked them to what the conductivity of the water was. The water went from the same as the Gulf of Mexico to river water—35 parts per thousand salt down to 0.1—within a few dozen meters. The transitions are that abrupt. So, if you put your blue membranes in the right place, this stuff could work.

However, this boundary between salt water and fresh water moved dramatically based on how much rain had fallen (increasing output from the rivers), as well as how much was being released from Lake Okeechobee to prevent flooding of the golf clubs and mansions in West Palm Beach (gee, which political leader owns property there?) and the cane fields of the sugar barons who own Florida’s politicians (much like they owned their workers in previous generations—look it up—they were busted for slavery in 1979, black, imported Jamaican slaves, and then they moved the plantation model to the Dominican Republic right after that and wrangled favorable access to American markets. But I digress. I'll link to an older post on the sugar scumbags for those who hadn't read it). This tells me that the ultimate blue-membrane-driven power plant will have to be mobile. You can’t think of it like a giant dam—more likely a bunch of movable toy boats strung together. This arrangement is probably less likely to affect river ecology—dams block movement of fish, but this tech wouldn’t, because they could just swim around it.

There was a very creative way of getting more of these nanotubes into the membrane. The French work had focused on single tubes, and the new work reported ten million tubes per cubic centimeter. Okay, big number. And a stupid number, I’d say, because in a flat membrane, you’d really want to know tubes per unit area (per square centimeter). Well, the journalist did provide the thickness of the film as 6.5 micrometers, which is 0.00065 centimeters, so, popping out my handy-dandy calculator, I get 15 trillion pores per square centimeter of surface area. Of those, in the current work, only 2% are functional (they are working on unplugging more), so that’s 308 million functional water transport channels in an area the size of a postage stamp. Unless the science journalist made another mistake and said cubic centimeter where he really meant square centimeter. So, let’s just call it a shit-ton of holes in the membrane. And if you want to read about it, click the first link at the end of this article, because I’ve already lost too much of my potential readership at this point.

According to the authors of the latest study, with all the rivers in the world, this tech could generate 2.6 Terawatts of electricity, the equivalent of 2000 nuclear power plants. The majority of nations in the developing world border an ocean, and they could probably be powered by this alone.

Niger, you’re out of luck. But that place is pretty sunny, so solar would suffice. Kazakhstan? Windy, right? Belarus. Now, Belarus is a problem. Not windy, not sunny, landlocked. Regardless, I think you’re getting my point that if the blue membranes can be tweaked to improve efficiency, they could be very important in most of the world.

I’ll put a few links here for those who want the details: news article; French paper.

For those non-tech types who have already fallen asleep on me: Blue membranes! Sustainable energy! Clean tech!

If we can move the politicians toward curtailing world carbon output, new tech like this might make a contribution to leaving out kids a planet to live on. It’s hard to remain optimistic, but I’m in the camp who believes all is not lost.

Hopefully, at least some of you found this as cool as I did.

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