From green-tide biomass to biochar: An integrated framework linking pyrolysis, properties, and carbon stability in Ulva prolifera

Recurrent Ulva prolifera green-tide threatens ecosystems, and greenhouse-gas release during the decomposition of accumulated biomass can diminish the climate benefits of algal carbon fixation. Pyrolysis offers a route to convert labile algal carbon into biochar with enhanced resistance to degradation. However, the relationships between macroalgal pyrolysis behavior, biochar properties, and carbon stability remain poorly understood. Therefore, this study investigates these relationships using an integrated ‘biomass pyrolysis-biochar property-stability performance’ framework. Results show that the pyrolysis of Ulva prolifera occurs in three distinct stages, with 350 ℃ identified as the critical threshold for restructuring functional groups based on in-situ Fourier transform infrared spectroscopy. Biochar properties varied significantly across thermal gradients with an average prediction deviation rate of 26.62%, and those linked to stability exhibited strong linear trends (R 2 >0.85). Multidimensional stability assessments demonstrated that elevated temperatures fortify the carbon skeleton and reduce surface reactivity to varying degrees. The threshold temperature estimated from linear regressions was approximately 583 ℃, providing a reference for meeting the selected stability criteria. Furthermore, a hierarchical indicator system is proposed: O/C and volatile matter/fixed carbon (VM/FC) ratios serve as useful preliminary screening indices, while C O groups and persistent organic carbon act as definitive metrics for evaluating stability. This research provides a methodological foundation for assessing the carbon stability of U. prolifera -derived biochar and its potential for long-term carbon storage.