The adsorption behavior of arsenic (As) on specific crystalline phases of manganese dioxide (MnO2) remains unclear. In this study, we evaluated the ability of alpha-MnO2 nanofibers (MO-2) to remove both arsenite (As(III)) and arsenate (As(V)), using experimental and computational methods. The maximum adsorption capacity values of As(III) and As(V) on MO-2 were 117.72 and 60.19 mg/g, respectively, which is higher than values reported for alpha-MnO2, beta-Mn2 and gamma-MnO2. In particular, because MO-2 has much higher adsorption capacity for As(III) than As(V), it can be effectively applied in removal of As(III) from groundwater, and a pre-oxidation process is not required. Fixed-bed tests showed that about 800 mL As(III)-or 480 mL As(V)-contaminated water could be treated before breakthrough, and MO-2 can be effectively regenerated using only 12 mL of eluent. This means we can concentrate the As(III) and As(V) by factors of 66.6 and 40.0, respectively. According to density functional theory (DFT) calculations, As(III) and As(V) form stable complexes on (1 0 0) and (1 1 0) of a-MnO2. Moreover, the surface complexes of As(III) and As(V) on (1 0 0) are more stable than (1 1 0). Electron transfer from As(III) on (1 0 0) is greater than (1 1 0). These phenomenon are may due to the fact that (1 0 0) has lower surface energy than (1 1 0). Partial density of state (PDOS) analysis further confirmed that As(III, V) are chemisorbed on MO-2, which agreed with the Dubinin-Radushkevich model.