Characterization of natural and synthetic textile-derived biochar from torrefaction to moderate pyrolysis

Textile waste presents a growing environmental challenge due to the global volume and prevalence of synthetic and blended fibers that complicate traditional recycling approaches. Textile-derived (TD) biochar produced from torrefaction to moderate pyrolysis offers a promising strategy to convert mixed textile waste into value-added biochar, yet limited studies have explored the influence of synthetic fibers and co-pyrolysis with naturally-derived textiles on material properties. This study investigates the production and characterization of biochar derived from cotton (CBC), polyester (PBC), wool (WBC), and a 1:1 cotton/polyester blend (CPBC) prepared across a range of temperatures encompassing torrefaction to moderate pyrolysis (250–650 °C). A comprehensive suite of surface and compositional analyses was used to evaluate the influence of textile feedstock and production temperature on biochar properties, including Brunauer–Emmett–Teller (BET) surface area, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), elemental analysis (EA), point of zero charge (pHPZC), and evolved gas analysis-mass spectrometry (EGA-MS). Co-pyrolysis of cotton and polyester at moderate temperatures (450–650 °C) resulted in a substantial increase in surface area compared to the pure textile biochars (CBC, PBC, WBC), with a maximum of 621 m²/g for the blend at 450 °C. EGA-MS and FTIR data suggest that this enhancement is driven by physical interactions, such as cotton-derived volatiles bubbling through molten polyester, rather than chemical co-pyrolysis. No unique chemical species were detected in the EGA-MS data, and EA revealed increased carbon content, reduced oxygen content, and elevated higher heating values in the blended biochars. These results support the feasibility for tailoring biochar functionality through co-pyrolysis as a strategy for valorizing mixed textile waste within the circular economy framework.