You already know that lots of people across the world don’t have access to clean drinking water, so I don’t need to convince you of that. But let me just throw a few quick statistics out:
● Globally, at least 2 billion people use a drinking water source contaminated with feces.
● Contaminated water can transmit diseases such diarrhea, cholera, dysentery, typhoid, and polio. Contaminated drinking water is estimated to cause 485,000 diarrheal deaths each year.
● By 2025, half of the world’s population will be living in water-stressed areas.
Lots of those deaths are children, too. We need to step up. What’s needed are some inexpensive, low-energy ways to purify the contaminated water. But it turns out this has been quite a productive week for that, as two very encouraging methods emerged.
On March 25, we found out in Nature Communications that an MIT lab has found a way to use the xylem of pine trees as a water filter that can easily be prepared on-site. Then on March 31, a Princeton lab reported in Advanced Materials that they’d developed a hydrogel composite that can purify standing water using only the heat from the Sun.
Let’s take a brief look at both!
The Pine Tree
You may remember from an 8th-grade biology class that plants channel water upward through the xylem. Plants, like people, don’t want contaminants in the water they consume, and their xylem structures are good at reducing them. So, reasoned Rohit Karnik of MIT, if that’s the way nature does it, why don’t we try it?
Now, you can’t exactly use a petunia to do this, because the cross-section of its stem is small, and the material will wilt as soon as you cut it. But woody plants — gymnosperms in particular — can give you durable material with good filtering properties.
Gymnosperms are things like pine trees and ginkgo trees. They have seeds that aren’t protected by fruit or ovaries, like the seeds of flowering plants (angiosperms) are. Think of a pine cone, for example.
But gymnosperms and angiosperms also have different xylem structures, and that’s the key to our filtering! The xylem, unlike the internet, really is a “series of tubes”, like a bunch of PVC pipes packed within one much larger pipe. Angiosperms’ tubes are wide and long, while gymnosperms’ tubes are narrow and short. Below we take a cross-section through the tubes, so water is flowing at you, from out of the page.
If you’re water traveling upward in a gymnosperm, you can’t go too far within the same tube, because they’re so short, and so you need to keep transferring between tubes. Luckily, you can do that, because these tubes have “pits”, or little porous holes, in their walls. You can see below, especially in the gymnosperm, that the pit acts just like a sieve, and that you’d need to be a pretty small particle to get through it (sorry, bacteria):
If you soak a slice of gymnosperm wood in hot water and then ethyl alcohol, you get a durable filter that can do about a liter per hour, lasts a couple weeks, and can be stored up to two years. It removes 99.9% of bacteria and 99.99% of rotavirus.
I don’t like to make you watch videos very often, but it’s hard for me to capture what this kind of thing can mean to life in a place like India. Let’s watch MIT researcher Krithika Ramchander, the leader of this study from Karnik’s lab, get out into the field to make a difference. This is from a couple of years ago, when the project was just getting underway:
----------------------------<()>----------------------------
The Pufferfish
Xiaohui Xu and Rodney Priestley are chemical engineers at Princeton who got some inspiration from the pufferfish. It’s hard not to be fascinated by this little creature, which can inflate itself to three times its original size not by taking in air, but by filling its specialized stomach with water.
Pulling this off isn’t easy; pufferfish stomachs can’t digest food anymore because of this radical adaptation, so all of that hard work goes to the intestines. Pufferfish have also lost their ribs so that they can balloon out like this.
Xu and Priestley were struck by the idea that just as quickly as puffers “inflate”, they can “deflate” when the danger has passed. This inspired them to invent a composite material called solar absorber gel (SAG) that not only swells up quickly with water but can also release it later on demand, purifying it along the way.
But how do you make it release the water? The first key part of SAG is a mesh made out of poly(N-isopropylacrylamide), a polymer that is soluble in water at room temperature but that swells up and repels water when it’s heated. If you watch this stuff in action, it begins to make sense. When a solution of it in water is heated with a hair dryer, it comes out of solution and starts pushing the water away (thanks, user Quantyield at Wikimedia Commons for this video):
Interspersed in a mesh of this stuff are two other materials. First you have sodium alginate, a goop derived from seaweed that also swells up in water and is good at repelling salts. Then you’ve also got polydopamine (itself inspired by natural adhesives in mussels), a melanin-like substance which absorbs light and converts it to heat really well so that sunlight becomes more effective at the heating part. Polydopamine, as a bonus, tends to bind up heavy metals, so that helps greatly with the purification. All the materials in the SAG composite are nontoxic — they’re even used in biomedical applications — and not expensive to make.
When it comes to water purification, absorption is much faster and requires less energy than evaporation, and in fact the hydrogel that Xu and Priestley developed can swell up with water at about 15 pounds per square meter per hour, which is really fast, even among hydrogels. Let’s see a hand-sized rectangle of it in action, soaking in a lake for an hour and then sitting in the sun:
Some typical results are shown below. The filtration gets rid of organic compounds like cyclohexane and microorganisms like yeast. It can also take out bacteria, oils, and salts.
They were able to use it repeatedly without affecting the purification, but of course as with any filter you need to periodically replace it. The next step is to scale up the technology so that in the near future it can be more widely used.
"To me, the most exciting thing about this work is it can operate completely off-grid, at both large and small scales," Priestley said. "It could also work in the developed world at sites where low-cost, non-powered water purification is needed."
"Sunlight is free," Xu said, "and the materials to make this device are low-cost and non-toxic, so this is a cost-effective and environmentally friendly way to generate pure water."
The authors noted that the technology delivers the highest passive solar water- purification rate of any competing technology.
----------------------------<()>----------------------------
Now I know Charley Patton meant this in a very different way, but may these developments and others like them one day lead to High Water Everywhere!