Predictive Characterization Analysis for Quality Evaluation of Biochar from Olive and Citrus Agricultural Residues: A Practical Framework for Circular Economy Applications

The sustainable management and valorisation of agricultural and agro-industrial residues are essential to reduce environmental impacts, enhance resource efficiency, and support circular economy strategies. In Mediterranean regions, large quantities of residual biomass are annually produced from olive and citrus supply chains, representing promising feedstocks for biochar production. In this study, biochar was obtained at 600 °C in a fixed-bed reactor under a N2 atmosphere from four representative feedstocks: olive pruning (OPr), citrus pruning (CPr), olive pomace (OPo), and citrus peel (CPe). The resulting biochar was characterized in terms of physico-chemical, energetic, and structural properties, including proximate and ultimate analyses, fuel properties, cation exchange capacity (CEC), pH, elemental ratios (O/C, H/C, N/C), thermal stability, bulk density, metal content, and surface morphology (SEM), in order to assess parameters relevant to environmental potential applications. The results highlighted clear feedstock-dependent differences. OPoB and CPeB exhibited the highest thermal stability (0.56–0.66), indicating a strong potential for long-term carbon sequestration. CPeB showed the highest CEC (47.2 cmol kg−1). From an application-oriented perspective, this high CEC suggests that, when applied to soil at typical amendment rates (2–5 wt%), CPeB could potentially increase soil CEC by approximately 10–30%, thereby improving nutrient retention and cation availability. Energy yields were highest for citrus-derived biochar (42.0–47.5%), while OPoB exhibited the lowest solid yield due to its higher volatile content. SEM analysis revealed marked structural differences, with OPrB retaining an ordered lignocellulosic porous structure, whereas OPoB and CPeB displayed highly irregular morphologies, favorable for surface reactivity. Overall, this study demonstrates that olive and citrus residues are suitable feedstocks for producing biochar with differentiated properties, and that a rapid screening methodology can support feedstock selection and biochar design for targeted energy, soil amendment, and carbon management applications.