Toward circular biochar systems from waste-derived precursors: multi-analytical and multivariate insights into adsorption and heterogeneous Fenton-like oxidation performance

Understanding how waste-derived resources can be transformed into functional materials is essential for advancing circular economy strategies and sustainable water treatment technologies. In this work, a combined multi-analytical and multivariate framework is developed to elucidate how waste-derived precursor type, pyrolysis temperature, and iron functionalization collectively control adsorption and heterogeneous Fenton-like oxidation in biochar-based systems. Biochars were produced from 4 waste-derived precursors: spent coffee grounds, olive pomace, olive pomace stones, and sewage sludge, at three pyrolysis temperatures (450, 550, and 650 °C), generating 12 pristine materials and, after iron functionalization, a total of 24 materials. This design enables a systematic evaluation of waste-to-resource pathways and the decoupling of compositional and structural effects. Advanced characterization was combined with I-optimal response surface methodology (RSM) and principal component analysis (PCA) to provide complementary information: RSM was used to optimize material and process variables, whereas PCA was applied to interpret structure-property-performance relationships and identify the main descriptors controlling adsorption and heterogeneous Fenton-like oxidation. Equilibrium adsorption data were better described by the Freundlich model (R2 ≥ 0.97), indicating that surface heterogeneity was a dominant factor, while Langmuir-derived Qmax values were used only as comparative capacity indicators. Lignocellulosic biochars were governed by surface area and aromaticity, whereas sewage-sludge-derived biochars showed enhanced affinity due to their mineral-rich matrices and oxygenated functionalities. Iron functionalization significantly improved performance, with Fe-SSBC450 showing the highest Langmuir-derived adsorption capacity (Qmax = 6.57 mg g−1) and oxidation efficiency (∼76% phenol removal). Oxidation proceeded via •OH-driven mechanisms following pseudo-second-order kinetics. This work demonstrates how waste-derived precursors can be valorised into high-value materials for environmental remediation, supporting circular economy strategies through resource-efficient water purification.