The chemical structure of lignin (Wikimedia Commons)
Biofuel production could be a boon for the environment, but there’s still a lot of waste plant material, called lignin, remaining from the process. Now, an enterprising student has found a new use for some of that waste – paving unpaved roads. Host Bruce Gellerman speaks with Wilson Smith of Kansas State.
GELLERMAN: Well, for another inventor, the road to riches could also be paved with a novel building product: agricultural waste.
The waste product is lignin - it’s the stuff that makes the walls of plant cells strong - and it's usually tossed out because it’s so tough and hard to break down. But Wilson Smith a grad student in Civil Engineering at Kansas State University in Manhattan, Kansas has come up with a way to turn lignin into, if not a yellow brick road, than a green one.
SMITH: Lignin, I mean, it’s a component of biomass in plant matter. It’s what’s found in the cell walls of plants, it’s also what bonds together the wood fibers in trees. And what it does is it protects the carbohydrates in plants, like the two carbohydrates: cellulose and hemicellulose, it protects them from disease and from pests and it bonds with them. It’s just part of its nature to have that sticky quality, it almost smells like tree sap. And if you were to touch it, it would stick to your hands.
GELLERMAN: So, what are you going to do with all that lignin?
SMITH: Well, what I’m going to do is I’m going to test it in dry sand, mix in water and see how well the cohesion increases at different lignin contents and at different compaction levels. What I’m hoping is that as they dry, they’ll increase in cohesion. Dry sand by itself has no natural cohesion, and then when I add this lignin to it, and water, it gives cohesion to the soil. And as it cures, the lignin paste starts to become like a cement, and that increases the cohesion of the soil.
GELLERMAN: So, basically, it holds the soil together.
SMITH: Yes, exactly, like a glue.
GELLERMAN: How tough does it get?
SMITH: Well, I mean, I can’t say exactly in numerical terms, I have three different curing times that I’m doing. I made some samples and let them dry for two hours and I made some more that dried for seven hours and then some more that dried for 24 hours. Now for the samples that I made for two hours, I was able to break those apart with my own hands. The ones that were drying for seven hours, I had to use a screwdriver to chop it up, and then the ones that were drying for 24 hours, I had to actually just take the samples and smash them on the ground in order to break them up.
GELLERMAN: So, once you cure it, you’re good to go!
SMITH: Yes, exactly.
GELLERMAN: So you take water, lignin and soil, mix it up, it gets hard, and then you could use it for what?
SMITH: Well I mean, it’s not hard when you first mix it, I mean, it’s still very plastic and squishy, and then you let it dry and then it becomes hard. There’s been some testing, some very preliminary testing of different soil stabilizers in the field. In particularly in this wildlife refuge called Buenos Aires in Arizona.
They took a water truck which had nozzles attached to it, and they sprayed a water/lignin solution onto the soil. And then they took a roller truck to roll the soil flat again, so it would be flush for an unpaved road. And then over a two-year period of time, what they did was, every six months they went back to the site to do testing over the structural integrity of the roads. And it was found that, lignin performed above average for the two-year span of time.
GELLERMAN: You can roll it out and make an unpaved road, essentially paved, right?
SMITH: It’s very workable in that sense. You can mold it into almost any shape you want.
GELLERMAN: So you’ve got water, you’ve got soil, you’ve got your lignin, but what happens if it rains?
SMITH: Yes, that’s the tricky part and there needs to be further testing on the lignin and soil to know its long-term effects of water because lignin's very sensitive to water. At this wildlife refuge, Buenos Aires, even though it’s in the desert, they still get monsoon rains in the late summer time, and it was shown that the lignin was able to still work at an acceptable, adequate level even after those monsoon rains. Still, there needs to be testing done on its exposure to water.
GELLERMAN: Why don’t you make it into bricks?
SMITH: Oh, that’s a really good idea. You could also use it as some type of drywall material.
GELLERMAN: As long as it stays dry.
SMITH: Yes, exactly, it would have to be well insulated.
GELLERMAN: How soon before lignin roads might hit the streets?
SMITH: Oh, that depends on the investment and notoriety of lignin. Hopefully more people will catch interest in it and people will want to invest more and do more studies and research in it and it will speed up the time. But I would say it would be few years before you would see any mass-producing of lignin for road stabilization.
GELLERMAN: The road to riches may be paved with lignin in the future.
SMITH: (Laughs.) Now that’s one way of looking at it.
GELLERMAN: Well, Wilson Smith, thank you so very much.
SMITH: Oh thank you.
GELLERMAN: Wilson Smith is a grad student in civil engineering at Kansas State University in Manhattan, Kansas.
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