This study describes the removal of arsenic species in groundwater by nano zero-valent iron process, including As(III) and As(V). Since the background species may inhibit or promote arsenic removal. The influence of several common ions such as phosphate (PO43-), bicarbonate (HCO3-), sulfate (SO42-), calcium (Ca2+), chloride (Cl-), and humic acid (HA) were selected to evaluate their effects on arsenic removal. In particular, a 2(6-2) fractional factorial design (FFD) was employed to identify major or interacting factors, which affect arsenic removal in a significant way. As a result of FFD evaluation, PO43- and HA play the role of inhibiting arsenic removal, while Ca2+ was observed to play the promoting one. As for HCO3- and Cl-, the former one inhibits As(III) removal, whereas the later one enhances its removal; on the other hand, As(V) removal was affected only slightly in the presence of HCO3- or Cl. Hence, it was suggested that the arsenic removal by the nanoiron process can be improved through pretreatment of PO43- and HA. In addition, for the groundwater with high hardness, the nanoiron process can be an advantageous option because of enhancing characteristics of Ca2+. (C) 2011 Elsevier B.V. All rights reserved.