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 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 Al-13 (the dominant preformed Al species in PACl) generated Fe(OH)(3)-like particles and Al-13 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 Al-13 species exhibited a weaker catalytic capacity in comparison to MnOx and Fe(OH)(3) at equivalent molar concentrations. Interestingly, unlike Al-13 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 Al-13 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.

Automated summary

This study investigated the effect of Fe(III) and Al(III) hydrolysis products on Mn(II) oxidation by free chlorine. The results showed that Fe3+ and Al-13 significantly catalyzed Mn(II) oxidation and formation of MnOx particles. The catalytic capacity of Al-13 aggregates was weaker compared to MnOx and Fe(OH)(3). Organic matter was identified as the main factor contributing to the lower Mn(II) oxidation rate.