Mineral-Enriched Biochars from Co-Pyrolysis of Sewage Sludge, Sawdust, and Basaltic Rocks: Soil Amendment Potential and Carbon Stability

Abstract Co-pyrolysis of sewage sludge (SS) and sawdust (SD) with basaltic minerals may produce nutrient-enriched biochars for soil amendment and long-term carbon (C) persistence. However, the effects of mineral incorporation on C transformations and the interpretation of conventional stability indices remain poorly understood. To address this gap, we compared SS and SD copyrolyzed with zeolitic basalt (ZB) and metabasalt (MB). The resulting materials, produced at 500 °C, were characterized by proximate and elemental analyses, X-ray diffraction, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis, and nutrient and carbon-stability assessments. Mineral addition increased biochar yields to 70–73%, whereas SD-derived biochar exhibited the highest fixed C and thermostable fractions. Materials obtained by co-pyrolyzing SD with ZB or MB, and SS with ZB, showed high thermal resistance. Mineral-enriched biochars had higher ash contents and contained silicate-rich phases, modifying their mineralogical composition and nutrient profiles. Co-pyrolysis of SS with basaltic materials increased Ca, Mg, P, and micronutrient contents. Contrasting responses among elemental, thermochemical, and persistence-based indices showed that no single approach adequately evaluates C stability in mineral-enriched biochars. These findings support the potential use of mineral-enriched biochars as soil amendments and show that C stability requires integrated evaluation.