Mn(II) oxidation by free chlorine can be applied to remove Mn(II) at water treatment plants. This reaction also results in particulate MnOx formation and accumulation in drinking water distribution systems. This study investigated the effect of Fe(III) and Al(III) hydrolysis products (mainly precipitates) on Mn(II) oxidation by free chlorine under drinking water conditions. The results showed that Fe3+ added as FeCl3 and Al(III) added as polyaluminum chloride (PACl) at tens to hundreds of micrograms per liter dramatically catalyzed Mn(II) oxidation by free chlorine. Through hydrolytic precipitation at circumneutral pH, Fe3+ and Al13 (the dominant preformed Al species in PACl) generated Fe(OH)3-like particles and Al13 aggregates, respectively, which initiated heterogeneous Mn(II) oxidation. Kinetic modeling indicated that, once some MnOx was formed, MnOx and Fe(OH)3 catalyzed the subsequent Mn(II) oxidation to an equal extent. The particles (aggregates) formed from Al13 species exhibited a weaker catalytic capacity in comparison to MnOx and Fe(OH)3 at equivalent molar concentrations. Interestingly, unlike Al13 species in PACl, Al(III) added as AlCl3 had a negligible influence on Mn(II) oxidation, even when Al(OH)3(am) precipitates were formed. The catalytic effects of Fe3+ and Al13 hydrolysis products were confirmed by experiments with natural water and finished water, and the lower Mn(II) oxidation rate was mainly attributed to organic matter.
Keywords: Mn(II) oxidation; aluminum; catalysis; free chlorine; iron; kinetics; polyaluminum chloride.