Insight into the performance and mechanism of Iron(II, III)-polyphenol particles on the adsorption of malachite green cationic dye

The green synthesis of iron nanoparticles (FeNPs) using plant extracts has attracted considerable attention because of their potential to effectively decolorize dye-containing wastewater. Nevertheless, the underlying mechanism remains a topic of debate, mainly due to the complex composition of the extracts. In this study, we introduced a novel and well-defined model system by employing a single polyphenol, ellagic acid (EA), as both reducing and capping agent to synthesize iron particles (Fe-EA). This strategic approach not only circumvents the compositional variability of conventional plant extracts but also enables a fundamental molecular-level understanding of the adsorption process. The resulting Fe-EA particles exhibited eminent malachite green (MG) removal performance, achieving high removal efficiency (over 86.4% within 10 min), which was comparable to that of FeNPs derived from complex pomegranate extracts. Comprehensive characterizations (SEM, EDS, XRD, FTIR, XPS) confirmed that the particles consist of a Fe(II, III)-EA complex self-assembled into unique hollow spherical structures. Adsorption isotherm data were best described by the Langmuir model, indicating homogeneous monolayer adsorption and a maximum calculated capacity of 4149.4 mg (g Fe)^-1. Thermodynamic parameters revealed that the adsorption process was spontaneous and endothermic. Combined with LC-MS analysis and kinetic studies, the primary removal mechanism was identified as a chemisorption process, governed by the synergy of strong electrostatic attraction and hydrogen bonding. This work provides molecular-level insights that shift the design of green-synthesized iron particles from empirical testing to rational engineering.