Smelting wastewater from non-ferrous metal industry is highly toxic due to the presence of concentrated arsenic (up to 1000 mg/ L) and a varied number of heavy metal ions. A pilot experiment was conducted to study the treatment of smelting wastewater using zero-valent iron nanoparticles (nZVI). The process consisted of two sequential nZVI treatment units; each had an nZVI reactor, a clarifier and an nZVI recirculation pump. nZVI was mixed with wastewater in the reactor, settled in the clarifier and returned by recirculation pump. The experiment was operated at 400 L/h and 35,000 L of wastewater was treated using 75 kg nZVI. Removal efficiency of arsenic was greater than 99.9% and the average removal capacity was 239 mg-As/g-Fe. All metal ions, such as Cu, Zn and Ni, were reduced to less than 0.1 mg/L simultaneously. Analyses of the reacted nZVI from pilot reactor revealed extraordinarily high content of arsenic. X-ray diffraction suggested that Fe-3(AsO4)(2) precipitation played an important role for arsenic removal. Xray photoelectron spectroscopy revealed the multi-valence state of arsenic, the reduced form of copper and chromium on nZVI surface. The high efficacy of nZVI was attributed to its large surface-to-mass ratio, the recirculation design and the in situ deposition of copper. The diminished size of nZVI offered great convenience in process control and operation. The nZVI process was scalable and the success of this pilot test approved the construction of a full-scale wastewater treatment plant in which the same nZVI process was applied. (C) 2014 Elsevier B.V. All rights reserved.