Waste valorization via co-pyrolysis of polyethylene terephthalate and acid mine drainage sludge: Syngas production and synergistic As(V)/Cd(II) removal by Fe-rich char

Abstract

To address the dual challenges of plastic waste accumulation and mining pollution, this study investigated the feasibility of co-pyrolyzing polyethylene terephthalate (PET) with acid mine drainage sludge (AMDS) to produce syngas, pyrolytic oil, and Fe-based functional char for As(V) and Cd(II) removal. The incorporation of AMDS significantly altered the thermochemical conversion behavior of PET by accelerating dehydrogenation and reduction pathways, thereby enhancing the formation of H2 and CO while suppressing CH4 evolution. The compositional profile of pyrolytic oil shifted toward non-oxygenated aromatics due to AMDS-induced deoxygenation reactions. Under optimal co-pyrolysis conditions (PET-to-AMDS mass ratio 2 and pyrolysis temperature of 800 ˚C), the resulting char exhibited a well-developed porous carbon matrix embedded with reduced Fe phases, providing abundant active sites for metal/metalloids removal to yield maximum adsorption capacities of 66.8 mg g−1 for As(V) and 20.2 mg g−1 for Cd(II). Mechanistic investigations revealed that As(V) adsorption proceeded primarily through electrostatic interactions and inner-sphere complexation, while Cd(II) uptake involved cation-π interactions and outer-sphere/inner-sphere complexation. In the binary-metal system containing both As(V) and Cd(II), synergistic adsorption was observed, wherein preferential As(V) uptake altered the surface charge distribution and subsequently facilitated Cd(II) binding. These findings provide a viable pathway for sustainable valorization that converts PET waste and AMDS into value-added products including syngas, deoxygenated oil, and multifunctional adsorbents with potential applicability for simultaneous removal of multiple metal contaminants from wastewater.