In aqueous solution, zero-valent iron (ZVI, Fe-0) is known to activate oxygen (O-2) into reactive oxidants such as hydroxyl radical and ferryl ion capable of oxidizing contaminants. However, little is known about the effect of the particle size of ZVI on the yield of reactive oxidants. In this study, the production of reactive oxidants from nanoparticulate and microparticulate ZVIs (denoted as nZVI and mZVI, respectively) was comparatively investigated in the presence of O-2 and EDTA. To quantify the oxidant yield, excess amount of methanol was employed, and the formation of its oxidation product, formaldehyde (HCHO), was monitored. The concentration of HCHO in the nZVI/O-2 system rapidly reached the saturation value, whereas that in the mZVI/O-2 system gradually increased throughout the entire reaction time. The mZVI/O-2 system exhibited higher yields of HCHO than the nZVI/O-2 system under both acidic and neutral pH conditions. The higher oxidant yields in the mZVI/O-2 system are mainly attributed to the less reactivity of the mZVI surface with hydrogen peroxide (H2O2) relative to the surface of nZVI, which minimize the loss of H2O2 by ZVI (i.e., the two-electron reduction of H2O2 into water). In addition, the slow dissolution of Fe(II) from mZVI was found to be partially responsible for the higher oxidant yields at neutral pH. (C) 2013 Elsevier B.V. All rights reserved.