Oxidative removal of soluble divalent manganese ion by chlorine in the presence of superfine powdered activated carbon

Abstract

Here, we examined the removal of soluble divalent manganese (Mn(II)) by combination treatment with superfine powdered activated carbon (SPAC) and free chlorine in a membrane filtration pilot plant and batch experiments. Removal rates >95% were obtained with 3 mg/L SPAC, 1 mg/L chlorine, and a contact time of 4 min, meeting practical performance standards. Mn(II) was found to be oxidized and precipitated on the surface of the activated carbon particles by chlorine. The Mn(II) removal rate was fitted to pseudo-first-order reaction kinetics, and the rate coefficient changed in inverse proportion to as-is particle size, but not to true particle size. The rate coefficient was independent of both Mn(II) concentration, except at high Mn(II) concentration, and the chlorine concentrations tested. The rate-determining step of Mn(II) removal was confirmed to be external-film mass transfer, not chemical oxidation. Activated carbon was found to have a catalytic effect on the oxidation of Mn(II), but the effect was minimal for conventionally sized activated carbon. However, Mn(II) removal at feasible rates for practical application can be expected when the activated carbon particle diameter is reduced to several micrometers. Activated carbon with a particle size of around 1–2 μm may be the most appropriate for Mn(II) removal because particles below this size were aggregated, resulting in reduced removal efficiency.