Green Synthesis of Iron Nanoparticles: A Critical Review of Characterisation, Applications and Sustainability

Abstract

Green synthesis has become a prominent strategy for producing iron-based nanomaterials with plant extracts, microorganisms and biogenic residues in place of some conventional reductants, capping agents and organic solvents. Yet the literature frequently treats "iron nanoparticles" as a single material class even when the products are chemically distinct, including nanoscale zero-valent iron, magnetite, maghemite, hematite, poorly crystalline iron oxides and organic-iron hybrids. This critical narrative review evaluates how biological synthesis routes influence phase identity, surface chemistry, particle properties, performance and environmental burden, and asks whether commonly used evidence is sufficient to support claims of greenness and application readiness. Literature from 1 January 2000 to 19 July 2026 was considered, with seminal earlier work used where necessary. The strongest evidence supports the feasibility of phytochemical- and microbe-assisted synthesis and laboratory-scale use in contaminant adsorption, reductive transformation and Fenton/Fenton-like catalysis. However, interpretation is constrained by variable feedstock chemistry, incomplete iron mass balances, inconsistent reporting of yield and process inputs, and frequent reliance on colour change, ultraviolet-visible spectroscopy, Fourier-transform infrared spectroscopy and electron microscopy without definitive phase or oxidation-state analysis. Environmental and biomedical claims are also more mature at proof-of-concept than at field or clinical scale. Life-cycle studies demonstrate that biological origin alone does not guarantee a lower footprint because culture media, extraction, washing, heating, separation and drying can dominate impacts. A phase-resolved characterisation framework, quantitative process accounting, realistic matrix testing, transformation and ecotoxicity studies, and coupled life-cycle and techno-economic assessment are therefore needed. Green synthesis is best understood as a design space in which renewable chemistry can reduce selected hazards, rather than as an intrinsic property conferred by a plant or microbial reagent.