Nanoscale zero-valent iron (NZVI) has been studied as an effective nitrate reduction material. Here, the effect of NZVI aggregation on the reduction reaction order and rate was investigated by comparing the nitrate reduction performances of freshly synthesized NZVI (F-NZVI), dried NZVI (D-NZVI). and dried-sonicated NZVI (DS-NZVI). Also, the effects of a catalyst were evaluated by coating 0.2 wt% Ni on previous series of NZVIs: F-Ni-NZVI, D-Ni-NZVI, and DS-Ni-NZVI. Different types of NZVIs could effectively reduce highly concentrated nitrate without requiring pH control. F-NZVI and F-Ni-NZVI showed extremely fast reactions, reducing 10-10,000 ppm within 1 min, and thus their reaction kinetics could not be evaluated under these experimental conditions. In the case of 10,000 ppm of nitrate, NZVI was almost completely consumed after reducing about 5000 ppm within 1 min. In contrast, nitrate reduction using D-NZVI and D-Ni-NZVI were pseudo-first-order reactions and DS-NZVI and DS-Ni-NZVI were 1.37 and 1.71 order reactions, respectively. D-Ni-NZVI and DS-Ni-NZVI obtained a higher reduction rate than D-NZVI and DS-NZVI due to the existence of the Ni catalyst. These experimental results suggest that the aggregate size and catalyst prominently affect the nitrate reduction rate and that the aggregation effect is more important than the catalyst effect as the aggregate size becomes smaller. Furthermore, the importance of the NZVI structure, the branch of chain-like structures and its edges exposed to the aqueous phase in nanoscale, is proposed in this study in order to explain the ultra fast reaction of F-NZVI and F-Ni-NZVI, which have yet to be reported. The final product of the reaction was ammonium, with nitrite being produced as a byproduct; NZVI changed into different shapes of magnetite (Fe3O4) after the reaction, depending on the reaction conditions. (C) 2011 Elsevier B.V. All rights reserved.