Mining and sewage treatment wastes have been accumulating at growing rates in urban areas. Recycling these wastes can be used to generate safe products for various agro-environmental uses, including the synthesis of fertilizers with the potential to sequester carbon (C) in the soil. Therefore, this study evaluated the physicochemical characteristics and C sequestration potential of biochar obtained by co-pyrolysis (500 °C) of sewage sludge (SS) individually or combined at a 1:1 (w:w) ratio with zeolitic basalt (ZB), referred to as SS + ZB_BC. Subsequently, the raw materials and biochars were characterized by X-ray diffraction analysis, proximate analysis, elemental analysis, and FTIR spectroscopy, as well as pH and electrical conductivity (EC) determination. The results show that pyrolysis optimized material properties, especially SS biochar (SSB), which exhibited high stability with the highest fixed C content (13.6%) and thermostable fraction (TSF) of 43%. On the other hand, ZB had a higher pH and a lower EC than SS. Co-pyrolysis promoted complementary effects on the chemical and C stability properties of the SS + ZB_BC combination. The combination raised the pH to a value close to neutrality (6.5), indicating potential corrective action for acidic soils. Furthermore, after co-pyrolysis, the TSF remained high (25.2%) and was classified as a high-longevity material (>1000 years), indicating high aromaticity and C condensation. Therefore, the co-pyrolysis of SS and ZB optimized the individual characteristics of the materials, thereby providing a promising and sustainable alternative for agro-environmental use that addresses the need to reduce C emissions and promote waste recycling.

The synthesis of biochar-based fertilizers (BBFs) from urban waste and agrominerals represents a promising approach to advancing a circular economy and agricultural sustainability. In this study, 16 BBFs were synthesized from pyrolyzed sewage sludge biochar at 300 °C and 500 °C (SSB300 and SSB500), potassium-rich silicate agrominerals (ASi) such as mica schist and phonolite, and different concentrations of oxalic acid (OA): 0; 0.33; 0.67 and 1 mol L^-1. The formulations were characterized in terms of chemical, physical, and mineralogical composition, structural properties, and carbon (C) stability. The pyrolysis temperature was the main factor responsible for the structural stability of C. Materials produced at 500 °C showed greater aromaticity, lower H/C and O/C ratios, lower ratios between volatile material (VM) and fixed C (FC) (VM/FC), and higher recalcitrance (R₅₀) and thermostable fraction (TSF) values, indicating greater recalcitrance and structural stability, with emphasis on SSB500+Mica + OA₀ and SSB500+Phon + OA₀. In contrast, formulations with SSB at 300 °C exhibited greater surface functionalization and enhanced potential for chemical reactivity, particularly at higher OA concentrations, with emphasis on SSB300+Phon + OA₁. The OA application altered the mineral matrix and the structure of C, promoting greater surface functionalization and potentially nutrient availability, although with a slight reduction in C recalcitrance. The incorporation of ASi increased the total K content and promoted mineral compositional differentiation in the new fertilizers. The results suggest that the developed BBFs constitute highly tunable organomineral systems, in which the manipulation of pyrolysis temperature and chemical activation enables targeted modulation of the balance between C structural stability and reactivity. Although OA reduces the stability of C, our results show that it is possible to synthesize formulations tailored either for greater structural stability (SSB500-based) or for enhanced reactivity and potential nutrient release (SSB300+OA-based), depending on the intended agronomic application. This approach may contribute to the rational design of multifunctional fertilizers, enabling the development of formulations tailored to diverse management strategies and soil conditions.