Evaluation of Iron-Lignin Particles Obtained by the Nanoprecipitation Method: Influence of Process Conditions on Morphology, Structure, and Properties

Lignin nanoparticles (LNPs) are important biobased materials that have been considered for several applications such as antioxidants, UV protectants, heavy metal absorption, antimicrobials, drug carriers, gene delivery systems, encapsulation of molecules, biocatalysts, supercapacitors, tissue engineering, hybrid nanocomposites, and wound dressing. Moreover, lignin-functionalized metal nanoparticles can be applied for different catalytic reactions. Despite their technological importance, the synthesis of lignin and lignin-functionalized metal nanoparticles remains an unexplored subject, in particular the methods to produce uniform spherical particles. In this context, the aim of this work is to analyze the effect of different variables on the morphology, particle size, particle size distribution, and properties of two different kraft lignins. Based on the concept of chain coiling and decoiling, two different solvents, four different temperatures, and two kinds of kraft lignins were tested. A nanoprecipitation method was applied, resulting in iron-lignin particles with different morphologies. The use of a THF/water mixture as a solvent resulted in spherical lignin particles, and the use of DMF/water resulted in elongated shapeless particles. Higher temperatures (40 °C) led to larger particles. Also, softwood lignin produced larger particles than hardwood lignin. FTIR-ATR analysis showed that the attenuation of signal peaks between 1700 and 1030 cm–1 and the appearance of a new band at 1652 cm–1 indicate the formation of iron complexes. Also, lignin glass transition temperatures are reduced with the incorporation of iron. Zeta potential indicates that softwood iron/lignin particles have higher colloidal stability than neat lignin nanoparticles. Three iron/lignin nanoparticles were tested to evaluate their ability to oxidize methylene blue in the presence of hydrogen peroxide. The results show that neat lignin, especially softwood lignin, is very effective at reducing methylene blue (MB) concentration in solution. Additionally, iron-lignin nanoparticles might be degrading methylene blue, which is evidenced by a peak shift in UV–vis spectra, and removing this new reaction product from solution by adsorption. Finally, a mechanism of nanoparticle formation by nanoprecipitation is proposed. The mechanism is based on both a kinetic trap that determines the final shape of lignin nanoparticles depending on the solvency power of the solvent and the thermodynamic stability of short lignin chains that determines the recovery of nanoparticles. This scheme illustrates the solvent-driven mechanism of lignin-iron nanoparticle assembly via nanoprecipitation. In THF, lignin adopts a collapsed micellar conformation stabilized by native apolar π–π interactions, effectively hiding internal hydroxyl groups within a hydrophobic core. Conversely, the high solvating power of DMF induces an extended polymer configuration, fully exposing these reactive hydroxyls. Upon dropwise addition into an aqueous Fe3+ solution, rapid metal coordination and high supersaturation trigger immediate precipitation by shifting the critical aggregation concentration (THF) and overall polymer solubility (DMF). This fast precipitation acts as a kinetic trap, preventing further morphological rearrangement. Consequently, the final architecture is directly templated by the initial solvent state, recovering discrete spherical nanoparticles from THF and extended cross-linked networks from DMF. Furthermore, this rapid phase separation selectively precipitates high-molecular-weight lignin chains, retaining highly soluble, short-chain oligomers in the supernatant and explaining the macroscopic particle recovery.