There’s a lot of carbon in a tree. And it’s carbon already circulating through the biosphere, so moving it from tree to degradable product, and then back into the soil as it decomposes results in a zero sum carbon game. Compare that with petroleum, where “nodding donkeys” are constantly bringing anciently sequestered carbon back into circulation, and trees win, hands down.
Except for one hitch. A lot of that carbon is bound up in lignins. Chemists speak sternly of lignin, as if talking about a willful child. Lignin, they say, is a recalcitrant molecule. It’s really tough—and takes a lot of external activation energy—to liberate the carbon in lignins. That’s why there is an old joke in the pulp industry: “You can make anything out of lignins—except money.”
That “no money” thing is an issue for NARA collaborator Simo Sarkanen, a dark-haired Finn who is a professor of bioproducts and biosystems engineering at the University of Minnesota. We’re in Spokane, talking at one of the periodic NARA meetings.
After cellulose, Sarkanen says in his precise British accent, lignins are the most common biopolymer on the planet. “Many people estimate that lignins incorporate 30 percent of the carbon on earth.”
Sarkanen is one of the world’s leading experts on lignins but, when he got his Ph.D. at the University of Washington “many years ago, I had barely heard of it.” He got involved with lignin for exactly that reason. Now, though, he says it is “a matter of some concern to me that so few people know what lignins are. They are going to play an increasingly important role in the future, as far as the biosphere is concerned and as far as human consumption of plastics are concerned.”
Lignins, to put it simply, are the scaffolding that allows plants to defy gravity, grow vertically, and face the sun. They’re the woodiness of wood and were critical in the evolution of plants as they moved from aquatic to terrestrial environments. Lignins are also, Sarkanen tells me, the “rate-limiting step” in the decomposition of wood. In other words, lignin degrades quite slowly—and we don’t really know why, at least not at the molecular level.
One of the reasons is that “s” on the end of lignin: it’s not just a single type of molecule, it’s a crazy patchwork or, as one group of researchers put in a journal article, “lignins are… ill-defined polymers whose monomeric composition varies greatly.”
Lignins are tough nuts to crack, but there are a lot of them. The annual production of lignins is estimated to be somewhere between 5 and 36 x 108 tons—that’s about 18 million blue whales-worth of lignins produced by plants every year.
Lignins are degraded by certain types of fungi, and add lots of nutrients to soils. So keeping some of the post-logging residuals on the forest floor is critical to maintaining soil health and productivity. NARA experts estimate that leaving behind about 30 percent of residuals meets that need. Leaving more can actually hinder regrowth, as branches and bark block seedlings access to sunlight.
Beyond that, lignins are used in a huge range of chemical products widely used in industrial and agricultural processes, including dispersants, surfactants, adhesives, emulsifiers, binders, thermosets, as well as, in highly purified forms, cosmetic and food additives.
But, for all that, only a tiny amount of the some 100 million tons of commercially produced lignin is isolated and processed for chemical purposes. The rest of it is burned by pulp and paper milling operations as a power source.
Lignins, Sarkanen says, are not all created equal but they are all equally difficult to study. For decades, researchers have been trying to incorporate lignins into a range of polymeric materials, though the gold standard has always been plastic. Imagine, Sarkanen invites, a plastic pop bottle you just pitch out your car window. No littering ticket because, in a few days, the bottle has decomposed and contributed all those lovely soil-building components back to the earth. Wouldn’t that be cool?
“What we have discovered fairly recently is that by a simple chemical modification, methylation, of a native lignin that you might isolate from wood cell walls, you can produce materials that are better than polystyrene.”
Polystyrene is one of the most ubiquitous plastics on the planet and has a half-life in the neighborhood of 500 years. Got packing peanuts? Polystyrene. CD or DVD cases? Yup. To-go box for your leftovers? Check. The list goes on and on. And, alarmingly, polystyrene and other slow-to-degrade plastics are gathering in huge whorls in the world’s oceans, where they are transforming the biology of plants and animals in pernicious ways.
Even more recently, Sarkanen’s team figured out how to use NARA’s lignins to produce polyethylene-like polymers. Polyethylene, like polystyrene, is a widely used plastic found in everything from grocery bags to plastic bottles.
No wonder, than, that Sarkanen—with his deadpan delivery—says that such discoveries are “enormous sources of pleasure—it is actually possible to have a party at 11 o’clock in the lab when some positive result comes in.”