The adsorption performance of manganese oxide in treating arsenic wastewater that contained different As(III)/As(V) molar ratios was investigated, and the samples before and after their reactions were characterized by XRD, TEM, N-2 adsorption/desorption, SEM-EDS, FTIR, and XPS. The batch experimental results show that the coexisting molar ratio of As(III) and As(V) has a remarkable effect on arsenic removal by manganese oxide. Upon increasing the As(III)/As(V) molar ratio from 10 mu M/90 mu M to 50 mu M/50 mu M and 90 mu M/10 mu M, respectively, the total arsenic removal by manganese oxide significantly increased from 39.5% to 55.6% and 71.2%, respectively. The origin of the molar ratio effect was studied by combining both the experimental and theoretical results of the Mn2+ release detection, effect of Na2SO3- pretreatment on arsenic removal, and density functional theory (DFT) calculations. The results reveal that the highest arsenic removal by manganese oxide in an arsenic solution with the highest As(III)/As(V) molar ratio is attributed to the Mn2+ release caused by As(III) oxidation, forming more Mn vacancy defects, which makes the adsorption site more active, thus significantly promoting the adsorption performance of manganese oxide in arsenic removal. (C) 2016 Elsevier B.V. All rights reserved.