Multi-analytical evaluation of combustion residues from Croatia’s biomass-fueled plants for circular agroecosystem applications

• Biomass ashes (BMAs) show strong alkalinity (pH > 12) from Ca-, K-, Na-, and Mg-oxides. • Bottom (vs fly) ash fractions contained lower metal(oid) levels. • Some BMAs exceeded limits for Zn (1), Ni (2), and Cd (9) under national/EU standards. • Persistent organic pollutants (PCBs, OCPs, PAHs) were below all regulatory thresholds. • BMAs have potential as liming and nutrient-recycling agents for acidic agroecosystems. The rapid expansion of biomass-fueled power plants across many EU countries has led to an increase in biomass ash (BMA) over the past decade. However, its sustainable utilization and environmental management remain inadequately addressed. This study presents the first comprehensive, multi-analytical characterization of 16 BMAs collected from nine Croatian biomass power plants spanning seven regions, with the aim of evaluating their potential for circular agroecosystem applications. Morphological analyses revealed pronounced nano-microscale heterogeneity accompanied by strong chemical variability. All BMAs exhibited high alkalinity (pH > 12), primarily driven by abundant Ca-/K-/Na-/Mg-bearing oxides. Fly ash (FA) samples were enriched in soluble salts, showing higher electrical conductivity (EC), whereas bottom ash (BA) was dominated by refractory silicates and metal oxides. XRD and FTIR analyses confirmed the predominance of calcite, quartz, arcanite, and apatite-type minerals, indicating agronomic potential, but limited P/K bioavailability due to their incorporation into low-solubility phosphate/silicate matrices. The lower density and higher EC of the FA matrix correlated with the elevated S content and suggested the presence of reactive, salt-rich particulates formed through volatilization and condensation of S compounds during combustion. BAs generally contained lower metal(oid) content than FAs; however, several samples exceeded inter/national limits (Zn in one, Ni in two, and Cd in nine). Persistent organic pollutants (PCBs, OCPs, and PAHs) were below regulatory thresholds in all BMAs. Overall, Croatian BMAs demonstrated strong potential as liming and nutrient-recycling agents in acidic and nutrient-deficient agroecosystems, although site-specific application and leaching assessments remain essential to ensure environmental safety and sustainable reuse.