Size-Dependent Response of the Reductive Reactivity of Zerovalent Iron toward the Coexistence of Natural Organic Matter

Although the existence of ubiquitous natural organic matter (NOM) exerts significant effects on the performance of zerovalent iron (ZVI), size-dependent response of the reactivity of ZVI toward NOM under both aerobic and anaerobic conditions has not been considered. In this study, the oxic removal and anoxic removal of the pollutant diatrizoate (DTA) by mechanochemically prepared ZVI of varying sizes in the presence of humic acid (HA), a typical NOM, were investigated comparatively. When oxygen was present, the effect of HA on DTA degradation transformed from promotion to inhibition as the particle size of ZVI decreased from microscale (mZVI) to nanoscale (nZVI). By contrast, in the absence of oxygen, HA always suppressed DTA removal regardless of ZVI size. Under aerobic conditions, HA complexed with Fe(III) (hydr)oxides and then detached from the mZVI surface, leading to the depassivation effect and promotion of atomic hydrogen (H*), while HA and HA–Fe(III) complexes coated the surface of nZVI with a relatively smaller size and suppressed the formation of H*. Under anaerobic conditions, adsorbed HA and associated HA–Fe(II) complexes covered the surfaces of mZVI and nZVI to different extents, resulting in the occupation of active sites and inhibiting generation of H*. This study provides an in-depth insight into the interaction between ZVI and NOM and may serve as theoretical guidance for the application of ZVI-based technologies.