Green synthesis of yerba mate-mediated iron-based nanoparticles: influence of Fe³⁺ precursor concentration on structure, antimicrobial activity, and adsorption performance

Iron-based nanoparticles (Fe-NPs) hold considerable promise for wastewater remediation and antimicrobial applications; however, conventional synthesis routes rely on hazardous reagents and energy-intensive conditions, which limit their environmental sustainability. Here, Fe-NPs were synthesized by coprecipitation using yerba mate (Ilex paraguariensis) extract as a reducing, capping, and surface-modifying agent, and the effect of Fe³⁺ precursor concentration (0.05, 0.1, and 0.2 M) on physicochemical and functional properties was systematically investigated. Nanoparticle formation and surface chemistry were characterized by UV-Vis spectroscopy and FTIR, crystalline structure and morphology by XRD and SEM, surface elemental composition and iron oxidation states by XPS, organic coating content by thermogravimetric analysis (TGA), and specific surface area by nitrogen adsorption (BET). All formulations contained mixed-valence iron species. The 0.2 M formulation exhibited the highest zero-valent iron content, as confirmed by XPS, and the strongest bactericidal activity against gram-negative pathogens, with minimum bactericidal concentrations (MBCs) of 6.25 mg mL⁻¹ against Escherichia coli O157:H7 and 12.5 mg mL⁻¹ against Salmonella enterica. The 0.05 M formulation exhibited the highest specific surface area and superior methylene blue adsorption capacity. Adsorption kinetics were best described by the pseudo-second-order model, consistent with chemisorption, and equilibrium data fitted the Sips isotherm, yielding a maximum adsorption capacity of 184.2 mg g⁻¹. These findings establish a structure-property relationship in which Fe³⁺ precursor concentration governs iron speciation, surface chemistry, and particle morphology, determining the balance between antimicrobial potency and adsorptive performance, and support yerba mate extract as a sustainable platform for tailoring multifunctional iron-based nanomaterials.