Capturing CO2 with plant fibers

The Problem

Too much CO2 in the air is bad. We’ve all heard it, we all know it, but what can we actually do about it?

One option is direct carbon capture (also sometimes called carbon sequestration for those who prefer long words).

Direct carbon capture involves taking CO2 directly from the air and reacting it with other molecules to “capture” it. This process typically takes CO2 gas and turns it into a solid so that it can’t escape again.

The CO2 reacts with something like lime Ca(OH)2 to form a solid mineral, CaCO3.

CO2 (g) + Ca(OH)2 (l) → CaCO3 (s) + H2O

Don’t worry, that’s as much chemistry as we’re going to do today.

The important thing to remember is that CO2 (gas) turns into CaCO3 (solid) and that once this happens, the CO2 can’t escape again, making it effectively captured.

CaCO3 is commonly found in limestone, shells, and… Tums. So basically the plan is to solve global warming by turning CO2 → Tums. Who said science was hard.

But we have a problem. What do we do with our solid CaCO3 once we have it? No one is going to spend time capturing CO2 without being able to use the product for something. Well, scientists will for sure do it in a lab, but no company with the power to scale it to a meaningful amount will do it without a product in sight.

The Solution

In comes this week’s paper.

The authors very cleverly took advantage of another recent finding; a plant fiber called cellulose can absorb CO2 and use it to become a strong solid.

Cellulose has some perks:

1. Cheap

2. Abundant

3. Absorbs CO2

4. Mineralizes to make strong solids

The authors saw an opportunity to capture CO2 from the air and use it to turn cellulose into a 3D printing paste, artificial coral reef, and concrete. They achieved this wide range of applications simply by changing the amount of cellulose they used. Changing the amount of cellulose from 1-7% gave them a range of stiffnesses and other interesting properties.

The chemical reaction I mentioned earlier results in a process called mineralization. Minerals form crystals around cellulose and stiffen the material. This is similar to why bones are so stiff; minerals crystalize around fibers to make them stiffer.

Low cellulose = less mineralization = softer.

More cellulose = more mineralization = stiffer.

You can see this process in the image above. Cellulose (green) is surrounded by crystals of calcite (light blue spheres) and CaCO3 (turquoise).

They named their creation mineralized cellulose materials (MCM).

They ended up using the low-stiffness MCM as a cement additive. If you’re like me, this might sound a little backwards to you. Why not use the high-stiffness MCM? Well, they weren’t trying to replace the cement, just improve it, so the stiffness of the MCM wasn’t the most important factor.

The low-stiffness MCM formed a nice powder that was easily mixed in with cement paste. When mixed in, the MCM made the concrete more durable by getting rid of impurities. So we’ve gotten rid of CO2 and made more durable concrete at the same time. Nice.

The high-stiffness MCM was strong enough to make replacement coral reefs, called coral stones. They 3D printed the high-stiffness MCM into a coral stone structure (shown above) and then grew coral on them off the California coast. The coral grew fine and the MCM coral stones lasted for at least 8 months, so it was a pretty successful test.

Altogether, cellulose plant fibers make a great platform for capturing CO2 in a useful way. The MCMs are inexpensive, tunable, and could be used in a variety of potential products ranging from concrete to coral reefs.

I’m a little disappointed they didn’t even try to make Tums, but I’m excited to see where the MCMs go next. They can always do Tums in their next paper.

See you next week for more science,

Neil

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