To investigate the removal capability of millimetric zero valent iron (mmZVI), sponge iron (s-Fe-0) particles were characterized with XRD, XPS, TEM, HRSEM and EDS techniques. Moreover, the roles of particle size, catalyst dosage, dye concentration, mixing conditions (e.g. ultrasound (US), stirring or shaking), and regeneration treatment were studied with the removal of cationic triphenylmethane dyes. Notably, the reduction process was also revealed as compared to nanoscale zero valent iron (nZVI), microscale iron power, and iron scurf. Furthermore, the reductive mechanism was exemplified with brilliant green. The results demonstrated that (1) the synergetic effect between US and s-Fe-0 greatly enhanced the removal of dyes, (2) the dosage of preferred s-Fe-0 (1-3 mm) particles was optimized as 30.0 g/L; (3) reuse cycles of s-Fe-0 catalyst were enhanced with the assistance of diluted HCl solution; (4) the main degradation routes included the cleavage of conjugated structure reactions, N-de-ethylation reactions, hydroxylation reactions, the removal of benzene ring reactions, and opening ring reactions. Accordingly, the pretreatment of aqueous solution over s-Fe-0 was hypothesized to achieve mainly through direct reduction reaction by electron transfer and indirect reductive reactions by the highly activated hydrogen atom. Additionally, decoration with noble metals was utilized to reveal the reaction mechanism. (C) 2014 Elsevier B.V. All rights reserved.