Plastic sponge soaks up carbon dioxide, but don't trees do it better?

August 19, 2014

Last week at the 248th National Meeting & Exposition of the American Chemical Society, a report presented by Andrew Cooper, PhD, from the University of Liverpool, introduced a sponge-like plastic that sops up carbon dioxide (CO2), the greenhouse gas that everyone loves to hate.

While CO2 is considered a trace gas because of its low concentration in the air (389 ppm, representing just 0.0389% of air by volume), it is considered harmful by some scientists even at these low concentrations. "The key point is that this polymer is stable, it's cheap, and it adsorbs CO2 extremely well. It's geared toward functions in a real-world environment," explained Cooper. "In a future landscape where fuel-cell technology is used, this adsorbent could work toward zero-emission technology."

CO2 adsorbents (materials and filters used to gather a gas on a surface in a condensed layer) are most commonly used to remove greenhouse gases (methane and nitrous oxide are two others in addition to CO2) from smokestacks at power plants where fossil fuels like coal or natural gas are burned. However, Cooper and his team intend the adsorbent, a microporous organic polymer, for a different application, according to a report on the ACS meeting by Michael Bernstein of the American Chemical Society.

The new material would be part of an emerging technology called integrated gasification combined cycle (IGCC) that converts fossil fuels into hydrogen gas, which holds promise for use in fuel-cell cars and electricity generation because it produces almost no pollution. Cooper said that "the sponge works best under the high pressures intrinsic to the IGCC process. Just like a kitchen sponge swells when it takes on water, the adsorbent swells slightly when it soaks up CO2 in the tiny spaces between its molecules. When the pressure drops," he explained, "the adsorbent deflates and releases the CO2, which can then be [stored] or converted into useful carbon compounds."

The brown, sand-like material is made by linking together many small carbon-based molecules into a network. Cooper explained that the "idea to use this structure was inspired by polystyrene, a plastic used in Styrofoam and other packaging materials. Polystyrene can adsorb small amounts of CO2 by the same swelling action."

This new technology for "carbon sequestration" is an industrial method of doing what nature does naturally, but which is best? Nature's carbon sequestration is called photosynthesis. All plants store CO2, a gas that is critical to plant, animal, and human life. Remember biology 101? Plants adsorb carbon dioxide and through photosynthesis give off oxygen, which animals and humans need in order to survive.

Trees are the best source of nature's carbon sequestration plan because they can store a lot of CO2 in their woody biomass trunks, branches, and roots. Some of the best trees for carbon sequestration, according to biologists, are eucalyptus, loblolly pines (in the south), bottom-land hardwoods (also in the south), and poplars in the Midwest and north.

While the polymer material is stable and can withstand being boiled in acid, which means it can withstand the harsh conditions in power plants where adsorbents are needed, other CO2 scrubbers—whether made from plastics or metals or in liquid form—do not always hold up so well, Cooper explained. Another advantage of the new polymer is its ability to adsorb CO2 without also taking on water vapor (the largest concentration in our air), which can clog up other materials and make them less effective. Its low cost also makes the sponge polymer attractive, added Cooper.

Cooper explained that it is relatively simple to embed the spongy polymers in the kinds of membranes already being evaluated to remove CO2 from power plant exhaust systems, for example. "Combining two types of scrubbers could make much better adsorbents by harnessing the strengths of each," he said.

Well, we in the plastics industry already know that plastic is fantastic, but methinks we should be wary of removing too much CO2 from the air via industrial and chemical processes. We have seen the consequences in the past when industry thinks it can do a better job than Mother Nature. Do we know when CO2 levels are too low to sustain life?

I love planting trees, and have planted trees in the yards of every house I've ever lived in. And, personally, I prefer trees and other leafy green plants to artificial or industrial substitutes. I've always believed we shouldn't mess with Mother Nature!

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