Soil health is a major environmental concern. Biochars are a promising solution to address both soil contamination and amendment. They represent a sustainable valorisation alternative for solid wastes produced in huge amounts, namely agroforestry residues and sludge from wastewater treatment plants. Biochar’s superior properties, enhanced pore structure and high specific surface area can contribute to restoring soil quality, by adsorbing several pollutants (e.g., pharmaceutical compounds, pesticides, and metals) from water and soil, enhancing water retention capacity, improving soil aggregation, regulating pH, and reducing the need for synthetic fertilisers. Multiple studies have reported removal efficiencies exceeding 70% for metals and 60% for organic compounds in soils, as well as over 40% for both organic compounds and metals in waters. These efficiencies depend on factors such as feedstock, pyrolysis conditions, modification strategies, and target contaminants. Recent advancements in the field have introduced both chemical and physical modifications that can enhance adsorption selectivity. This review provides a comprehensive analysis of the fundamentals of biochar production, modification strategies, and their environmental applications in soil remediation and water treatment. By comparing unmodified and modified biochars, this review highlights the crucial factors that influence the performance of this highly versatile and cost-effective solution.

In this study, quince-derived biochar was employed as a sustainable and multifunctional material for the removal of organic pollutants through different treatment alternatives depending on the pH (natural or modified), including adsorption and electro-Fenton degradation processes. The biochar, obtained by pyrolysis (500 °C, 14 h) of quince (Cydonia oblonga) biomass, exhibited a high intrinsic iron content (1296 mg/kg), which contributed to its electroactive properties and catalytic potential. Batch experiments demonstrated highly efficient adsorption of the target pollutant, the antibiotic sulfamethoxazole, achieving a maximum retention capacity of 35.51 mg/g. Regarding the degradation pathway, the electrochemical treatment achieved significant degradation rates under mild operating conditions, reaching total removal in less than 90 min. Both treatment options, under their respective optimal conditions, were subsequently evaluated using tertiary wastewater from a wastewater treatment plant, achieving good removal efficiencies exceeding 60% within 120 min. Finally, the treated effluent under optimal conditions was subjected to an ecotoxicity assay using the microalga Raphidocelis subcapitata, confirming the effectiveness of the treatment and the absence of harmful effects on this key microorganism in the aquatic food chain. These findings highlight the potential of iron-rich quince biochar as a low-cost and environmentally friendly material for various water treatment applications. Moreover, the results provide a promising approach for valorizing agricultural waste with inherent metallic content in heterogeneous electro-Fenton.

In this study, biochars derived from agri-food wastes, including different avocado residues (peel- and seed-derived fractions), olive pits and cherry stones, were evaluated as low-cost and sustainable adsorbents for the removal of the emerging contaminants sulfamethoxazole (SMX), antipyrine (ANT) and trazodone (TRZ) from aqueous solutions. Despite their very low BET surface areas, the biochars exhibited notable adsorption capacities, indicating that adsorption performance was mainly governed by surface chemistry rather than textural properties. FTIR analysis confirmed the presence of abundant oxygen-containing functional groups on the biochar surfaces, enabling electrostatic interactions, hydrogen bonding and dipole–dipole interactions with the target contaminants. Adsorption kinetics for all systems were well described by the pseudo-first-order model, with equilibrium capacities closely matching experimental values, suggesting rapid and reversible surface interactions. Intraparticle diffusion analysis revealed that diffusion was not the sole rate-limiting step, with external mass transfer and surface interactions dominating the initial adsorption stage. Equilibrium isotherm analysis showed that SMX and ANT adsorption onto avocado-derived biochars followed the Langmuir model, whereas TRZ adsorption was better described by the Freundlich model. In contrast, olive pit biochar exhibited a markedly higher affinity for TRZ, achieving near-complete removal and fitting the Langmuir isotherm. Finally, high removal efficiencies were maintained when applying the selected biochar to real municipal wastewater, demonstrating robustness under complex matrix conditions. Overall, the results highlight the potential of these agri-food waste-derived biochars, including multiple avocado fractions, as sustainable and economically viable adsorbents for quaternary wastewater treatment.