Zero-Valent Iron Nanocatalysts via Polymers or Metal Hydroxide Passivation: Implications for Advanced Oxidation Processes

Zero-valent iron (ZVI) nanocatalysts are promising materials for environmental remediation (e.g., wastewater treatment) via advanced oxidation processes (AOPs) due to their ability to generate reactive oxygen species (ROS). However, their high reactivity often leads to rapid surface oxidation and a limited stability. This study evaluates the effect of different surface coatings on the stability and ROS production of ZVI nanocatalysts. The coatings tested include organic molecules such as mannitol and polyvinylpyrrolidone (PVP), as well as combinations with thin layers of metal hydroxides (M-(OH)₂, where M = Ni or Mn). Hydroxyl radical (^•OH) generation was quantified using electron paramagnetic resonance (EPR) spectroscopy over a period of 0 to 7 days in acetate buffer (pH 5). Results show that incomplete PVP coverage leads to a 90% decrease in ROS production within 24 h, while full coverage preserves catalytic activity for at least 1 day. Among inorganic coatings, Mn-(OH)₂ maintained stable ROS generation for 7 days, whereas Ni-(OH)₂ showed significant degradation after 24 h. These findings highlight the importance of coating selection and surface passivation to enhance ZVI stability and extend its catalytic performance in AOP applications, providing design guidelines for nanocatalysts in Fenton-like processes for water purification and environmental remediation.