Nanoscale zero-valent iron (nZVI) has been widely used in environmentally friendly removal of pollutants. The aggregation and passivation of nZVI particles decreased its efficiency during relatively long term usage. Montmorillonite (MMT) supported nZVI has been demonstrated to increase the removal efficiency of Cr(VI) by researchers. However, the mechanism involving the dispersity of nZVI on surface and in interlayer of MMT is still unclear. Herein, we designed and prepared different nZVI materials, i.e., MMT-supported, ultrasound-assisted, starch-stabilized, and potassium-pretreated MMT-supported nZVI. The results of Cr removal experiments showed that the atomic efficiencies of Fe-0 are 0, 3.95%, 6.65%, 6.68%, and 10.71% for reductive iron, nZVI, nZVI/MMT, nZVI/K-MMT*, and nZVI/K-MMT in initial 5 min at pH 6.0, respectively, which means that the effect on Fe-0 efficiency is in the order of surface > interlayer > diffusion > mechanical dispersity. The results of Cr removal experiment revealed that the removal rates of Cr(VI) is intensively related with surface dispersity of nZVI. The potassium-pretreated MMT (K-MMT) can significantly improve the dispersity of nZVI which should be attributed to ion exchange between K+ and Fe2+ and caused the intercalation of nZVI clusters. (C) 2013 Elsevier B.V. All rights reserved.