Treated plastic waste good at absorbing carbon, team finds

The new chemical technique developed by Rice University researchers offers a potential method to turn waste plastic into an effective carbon dioxide (CO2) sorbent for industry. The Rice lab describes it as a “win-win for pressing environmental problems”.

Rice chemist James Tour and Rice alumnus Wala Algozeeb, graduate student Paul Savas and postdoctoral researcher Zhe Yuan reported in the journal ACS Nano that heating plastic waste in the presence of potassium acetate produced particles with nanometre-scale pores that trap carbon dioxide molecules. These particles can remove CO2 from flue gas streams, they reported.

“Experts can fit point sources of CO2 emissions like power plant exhaust stacks with this waste-plastic-derived material to remove enormous amounts of CO2 that would normally fill the atmosphere,” Tour explained. “It is a great way to have one problem, plastic waste, address another problem, CO2 emissions.”

A current process to pyrolyse plastic – known as chemical recycling – produces oils, gases, and waxes, but the carbon byproduct is nearly useless, he said. However, pyrolysing plastic in the presence of potassium acetate produces porous particles able to hold up to 18 per cent of their own weight in CO2 at room temperature.

While typical chemical recycling doesn’t work for polymer wastes with low fixed carbon content to generate CO2 sorbent, including polypropylene and high- and low-density polyethylene, the main constituents in municipal waste, the researchers said those plastics work especially well for capturing CO2 when treated with potassium acetate.

The lab estimates the cost of carbon dioxide capture from a point source, like post-combustion flue gas, would be $21 a ton ($23/tonne). This is far less expensive than the energy-intensive, amine-based process commonly used to pull carbon dioxide from natural gas feeds, which costs $80-$160 a ton.

According to the team, industry can reuse the sorbent much like amine-based materials. Heating it to about 75°C releases trapped carbon dioxide from the pores, regenerating about 90 per cent of the material’s binding sites.

Because it cycles at 75°C, polyvinyl chloride vessels replace the expensive metal vessels that are normally required. The researchers noted they expect the sorbent to have a longer lifetime than liquid amines, cutting downtime because of corrosion and sludge formation.

To make the material, they turned waste plastic into powder, mixed with potassium acetate, and heated at 600°C for 45 minutes to optimise the pores, most of which are about 0.7 nanometres wide. Higher temperatures led to wider pores. The process also produces a wax byproduct that industry can recycle into detergents or lubricants, the researchers said.