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_x 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 MnO_x was formed, MnO_x 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 MnO_x 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.