Mn(II) Oxidation by Free Chlorine Catalyzed by the Hydrolytic Products of Ferric and Aluminum Species under Drinking Water Conditions.

Mn(II) oxidation by free chlorine can be applied to remove Mn(II) at water treatment plants. This reaction also results in particulate MnO 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 Fe added as FeCl 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, Fe and Al (the dominant preformed Al species in PACl) generated Fe(OH)-like particles and Al aggregates, respectively, which initiated heterogeneous Mn(II) oxidation. Kinetic modeling indicated that, once some MnO was formed, MnO and Fe(OH) catalyzed the subsequent Mn(II) oxidation to an equal extent. The particles (aggregates) formed from Al species exhibited a weaker catalytic capacity in comparison to MnO and Fe(OH) at equivalent molar concentrations. Interestingly, unlike Al species in PACl, Al(III) added as AlCl had a negligible influence on Mn(II) oxidation, even when Al(OH)(am) precipitates were formed. The catalytic effects of Fe and Al 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.