This study investigates the characterization and properties of metallic iron nanoparticles (Fe⁰) synthesized by two methods: reduction of goethite with hydrogen (FeH₂) and reductive precipitation with borohydride (FeBH). The study compares the reactivity of these nanoparticles with micro-sized Fe⁰ and solid Fe⁰ disks, focusing on their ability to reduce contaminants such as benzoquinone (BQ) and carbon tetrachloride (CT). The results show that FeBH reacts more rapidly with BQ than FeH₂, while both react similarly with CT. However, surface area-normalized rate constants do not show a significant nano-size effect, suggesting that the reactivity is not solely due to increased surface area. FeH₂ produces less chloroform than FeBH when reacting with CT, indicating differences in product distribution. The study also highlights the importance of defining the basis of comparison when evaluating the reactivity of nanoparticles. The findings suggest that the high reactivity of nano-Fe⁰ is often attributed to its large surface area, but the actual reactivity may depend on the specific surface sites and their intrinsic properties. The study underscores the need for careful characterization of nanoparticles to understand their behavior in environmental applications.This study investigates the characterization and properties of metallic iron nanoparticles (Fe⁰) synthesized by two methods: reduction of goethite with hydrogen (FeH₂) and reductive precipitation with borohydride (FeBH). The study compares the reactivity of these nanoparticles with micro-sized Fe⁰ and solid Fe⁰ disks, focusing on their ability to reduce contaminants such as benzoquinone (BQ) and carbon tetrachloride (CT). The results show that FeBH reacts more rapidly with BQ than FeH₂, while both react similarly with CT. However, surface area-normalized rate constants do not show a significant nano-size effect, suggesting that the reactivity is not solely due to increased surface area. FeH₂ produces less chloroform than FeBH when reacting with CT, indicating differences in product distribution. The study also highlights the importance of defining the basis of comparison when evaluating the reactivity of nanoparticles. The findings suggest that the high reactivity of nano-Fe⁰ is often attributed to its large surface area, but the actual reactivity may depend on the specific surface sites and their intrinsic properties. The study underscores the need for careful characterization of nanoparticles to understand their behavior in environmental applications.