Effect of divalent manganese ions on the roxarsone removal efficiency from aqueous solution with iron oxide

In this study, the effect of Mn(II) on the roxarsone (ROX) removal efficiency by iron oxide was systematically investigated. ROX removed by iron oxide obeyed pseudo-second-order non-linear kinetic model with the addition of Mn(II), and the removal rate constant normalized by specific surface area was calculated to be 4.5 x 10-3 g m-2 h-1, which was 4.8 times than that in the absence of Mn(II). Based on a series of experimental and characterization results, we found that Mn(II) could not be oxidized to Mn(III) in the presence of iron oxide, and the enhanced ROX removal process was attributed to the promoted ROX adsorption process. Further zeta potential results suggested that the addition of Mn(II) increased the isoelectric point of iron oxide from 5.3 to 7.5, which provided positive adsorption site favoring adsorption process of the negative charged arsenic functional group in ROX. This investigation provided fundamental understanding for the effect of Mn(II) on the removal efficiency of ROX by iron oxide.