Turning fossil-derived CO2 into value-added chemicals and carbon resources: Thermochemical conversion of microalgae with acid mine drainage sludge

Abstract

Global consumption of fossil resources has increased to meet the growing demand for carbon-based products, including fuels, chemicals, and plastic products, significantly contributing to global warming. This study explored sequestration of fossil-derived atmospheric CO<inf>2</inf> using Chlamydomonas reinhardtii (C. reinhardtii) microalgae and its conversion into value-added products via a thermochemical pathway. Radiocarbon analysis revealed that C. reinhardtii, cultivated in a photobioreactor (PBR), effectively assimilated carbon in the flue gas of a liquefied natural gas (LNG) power plant, exhibiting low modern carbon (pMC) (17 %). Amending acid mine drainage sludge (AMDS) during the pyrolysis of harvested C. reinhardtii enhanced H<inf>2</inf> and CO production. AMDS also reduced pyrogenic oil production at low loading (8:2 of C. reinhardtii to AMDS mass ratio), but at higher loading (3:7), it produced oil with much simplified composition and a notable content of hexadecanenitrile, a high-value chemical. The residual fraction of C. reinhardtii was carbonized into biochar enriched with g-C<inf>3</inf>N<inf>4</inf> structure, while Fe phases in AMDS were transformed into Fe0 and Fe<inf>9</inf>S<inf>10</inf>. The resulting biochar exhibited strong persulfate activation property for amaranth degradation. These findings represent a practical approach to converting CO<inf>2</inf> into valuable chemicals and environmental catalysts, highlighting its potential for sustainable CO<inf>2</inf> management.