On the cutting-edge of non-recyclable plastic waste valorization: From pyrolysis char to nitrogen-enriched activated carbon for landfill biogas upgrading

Abstract

The rejected fraction of post-consumer plastics collected by mechanical sorting is a source of chemical recycling by processes such as pyrolysis, promoting the circular economy. The char residue produced in this process has been valorized, reinforcing the introduction in the consumption chain, as an adsorbent aimed at the upgrading of a biogas stream. The char was activated with KOH (496 m2 g−1) and the surface of the material was enriched with nitrogen groups (6% wt. by XPS) to raise the CO2 uptake. KOH and urea were used following different strategies, demonstrating that sequential activation and modification is the most efficient approach. Although the surface area was decreased after N insertion, i.e. from 497 to 389 m2 g−1, the CO2 uptake was raised. Specifically, comparing the CO2 uptake per micropore surface was considerably improved after urea functionalization (0.351 vs 0.235 mg m−2). The selectivity was not considerably modified, but the isosteric adsorption heat of the urea-modified sample was higher than the bare one. The behavior in the dynamic tests was studied in fixed-bed column, assessing the effect of temperature, and inlet CO2/CH4 concentration, individually and in mixtures. There was no appreciated loss of performance in the adsorption-desorption cycles. The urea-modification in a two-pot synthesis process proved a better performance in dynamic tests if compared to the non-modified activated char. This work sheds insights into the feasibility of the recycling of plastic residues giving to new added-value materials such as functionalized carbonaceous adsorbents, promoting a circular economy basis.