Waste-Derived Biochar for Sustainable Wastewater Treatment and Resource Recovery: A Review from the Circular Economy Perspective

The growing generation of organic waste and the increasing demand for sustainable wastewater treatment have stimulated interest in waste-derived biochar as a multifunctional material for pollutant removal and resource recovery. This review critically evaluates the relationships between waste feedstock characteristics, thermochemical production conditions, biochar physicochemical properties, modification strategies, wastewater composition, and pollutant-removal mechanisms. Particular attention is given to the applicability of biochar in real wastewater, regeneration and reuse, spent-biochar management, and its potential contribution to circular-economy strategies. The reviewed evidence indicates that biochar performance is highly application-specific and that no single material can be considered universally optimal for wastewater treatment. Biochar selection should therefore be based on the target pollutant, wastewater matrix, required removal mechanism, and intended post-use pathway rather than adsorption capacity alone. Importantly, results obtained in single-compound model solutions may overestimate practical performance because dissolved organic matter, competing ions, suspended solids, salinity, and variable pH can substantially affect pollutant removal. Furthermore, pollutant adsorption or nutrient capture should not automatically be considered resource recovery unless a technically feasible and environmentally safe pathway for subsequent beneficial use is demonstrated. Based on this synthesis, an application-oriented framework for biochar selection and implementation is proposed. Further development should prioritize validation in real wastewater, continuous-flow and pilot-scale studies, repeated regeneration, environmental-safety assessment, and system-specific life-cycle and techno-economic evaluation.