The effects of calcium ions (Ca2+) on the surface characteristics and adsorptive properties of hydrous manganese dioxide (delta MnO2), taking arsenic and humic acid as model pollutants, were systematically investigated. The FTIR spectra and SEM/EDX analyses indicate coordination between Ca2+ and the surface hydroxyls ( Mn-OH) of delta MnO2, which leads to an increase of the surface zeta potential. Ca2+ can facilitate delta MnO2 aggregation, as indicated by the particle size distribution (PSD) and on-line flocculating index (FI). The variation of delta MnO2 surface characteristics that is induced by Ca2+ enhances the adsorption of humic acid (HA) and arsenic onto delta MnO2. The adsorption of HA is enhanced more significantly by Ca2+ than that of arsenic, which is positively related to the coordination strength between Ca2+ and individual pollutants. The SEM/EDX analysis shows the formation of Ca-HA complexes, which have a higher affinity for delta MnO2 than HA. The bridging effect of Ca2+ plays a major role in enhancing HA adsorption onto delta MnO2. Due to different interactions between Ca2+ and pollutants, the adsorption mechanism is pollutant-dependent in the presence of Ca2+. (C) 2008 Elsevier Inc. All rights reserved.