Effects of calcium ions on surface characteristics and adsorptive properties of hydrous manganese dioxide.

The effects of calcium ions (Ca(2+)) on the surface characteristics and adsorptive properties of hydrous manganese dioxide (deltaMnO(2)), taking arsenic and humic acid as model pollutants, were systematically investigated. The FTIR spectra and SEM/EDX analyses indicate coordination between Ca(2+) and the surface hydroxyls ([triple bond]Mn-OH) of deltaMnO(2), which leads to an increase of the surface zeta potential. Ca(2+) can facilitate deltaMnO(2) aggregation, as indicated by the particle size distribution (PSD) and on-line flocculating index (FI). The variation of deltaMnO(2) surface characteristics that is induced by Ca(2+) enhances the adsorption of humic acid (HA) and arsenic onto deltaMnO(2). The adsorption of HA is enhanced more significantly by Ca(2+) than that of arsenic, which is positively related to the coordination strength between Ca(2+) and individual pollutants. The SEM/EDX analysis shows the formation of Ca-HA complexes, which have a higher affinity for deltaMnO(2) than HA. The bridging effect of Ca(2+) plays a major role in enhancing HA adsorption onto deltaMnO(2). Due to different interactions between Ca(2+) and pollutants, the adsorption mechanism is pollutant-dependent in the presence of Ca(2+).