Aged Biochar–Nanoparticle Composites for Agricultural Soil Remediation: Mechanistic Insights, Engineering Design, Ecological Risks, and Translational Perspectives

Agricultural soils increasingly receive mixtures of toxic metals, pesticides, antibiotics, and micro- and nanoplastics, creating simultaneous risks to crop productivity, food safety, soil biological function, and water quality. Biochar is a scalable carbonaceous amendment, but its performance evolves after application because oxidation, fragmentation, mineral association, and microbial colonization alter the reactive interface. This review advances an aging–engineering–risk–translation framework that links time-dependent biochar transformation with nanoparticle functionalization and field performance, rather than treating fresh biochar or nanocomposites as static adsorbents. The literature was critically screened with emphasis on 2019–2026 studies and earlier mechanistic papers where required. Natural aging can increase oxygen-containing surface functionality and organo-mineral association, and multi-year field evidence shows that immobilization of lead, copper, and cadmium can strengthen with aging under favorable conditions. Nanoparticle-modified biochars add contaminant-specific functions, including ligand exchange, redox transformation, photocatalytic oxidation, and magnetic recovery; however, the optimum design is soil- and contaminant-specific rather than universally transferable. The review also evaluates secondary risks, including nanoparticle detachment or dissolution, environmentally persistent free radicals, polycyclic aromatic hydrocarbons, mobile nano-biochar, antibiotic resistance determinants, and effects on microorganisms, soil enzymes, earthworms, and plants. A central conclusion is that high removal percentage alone is an inadequate endpoint: durable immobilization, transformation-product toxicity, biological function, recoverability, lifecycle burden, and field aging must be evaluated together. These insights provide a mechanistic basis for designing safer, durable, and field-relevant biochar–nanoparticle systems for agricultural soil remediation.