Sewage sludge biochar-based fertilizer enriched with K-bearing silicate agrominerals and oxalic acid: physicochemical...

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.