Circular Valorization of Apple Pomace: Biochar for Co(II) Remediation and Catalytic High‐Density Polyethylene Pyrolysis

Apple pomace can be converted into functional materials for sequential environmental and catalytic applications. Biochars were prepared by stepwise heating at 300, 450, and 600 °C (APB‐300‐600) and treatment at 700 °C (APB‐700). Characterization revealed poorly ordered carbon, mineral constituents, and oxygen‐containing functional groups. Both materials had low specific surface areas of 0.1–2.0 m ² g ⁻¹ , similar titratable acidity, and negative zeta potentials at the adsorption pH. APB‐700 exhibited higher Co(II) uptake, reaching ~9.0 versus 6.0 mg g ⁻¹ for APB‐300‐600 within the investigated concentration range. Nonlinear fitting favored the Freundlich isotherm for both biochars. X‐ray photoelectron spectroscopy (XPS) confirmed surface cobalt retention. Combined FT‐IR and XPS changes supported surface complexation involving oxygen‐containing sites, while surface K ⁺ and Mg ²⁺ depletion suggested a possible contribution from cation exchange. Co ²⁺ ‐loaded APB‐700 was subsequently reused in high‐density polyethylene (HDPE) pyrolysis. Compared with unloaded APB‐700 at the same nominal polymer and catalyst loadings, the spent material shortened the process from 7 h 50 min to 3 h 40 min and increased the reported gas yield while retaining a gas composition rich in light hydrocarbons. These findings demonstrate a sequential application of apple pomace biochar that connects metal‐ion removal from water with plastic‐waste conversion.