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Published on: June 12, 2026
Slow Mn(II) oxidation by chloramine under various conditions: Implication for controlling Mn deposition in drinking
Yue Zhou1, Jiahao Hou2, Yaxuan Feng1
1Key Laboratory of Environmental Aquatic Chemistry, State Key Laboratory of Regional Environment and Sustainability, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; University of Chinese Academy of Sciences, Beijing 100049, China.
Abstract:
Chloramine (NH2Cl) has been observed to result in substantially lower MnOx deposits in drinking water pipes compared to chlorine, yet the underlying chemical mechanisms and its applicability under various water conditions remain unclear. Here, by integrating thermodynamic analysis with batch experiments, we demonstrated that Mn(II) oxidation by NH2Cl was intrinsically slow under various drinking water conditions. Thermodynamic calculations showed that both the absolute Gibbs free energy change and overall redox potential for Mn(II) oxidation by NH2Cl were substantially lower than those for free chlorine, fundamentally limiting the driving force of Mn(II) oxidation. Kinetic experiments showed that the homogeneous oxidation of Mn(II) by NH2Cl (1, 4, and 8 mg/L) proceeded very slowly. Notably, Mn(II) adsorption-oxidation experiments revealed that although MnOx could adsorb Mn(II), it exhibited no significant catalytic capacity for Mn(II) oxidation by NH2Cl, as confirmed by SEM images showing no further MnOx growth on the particle surface. XPS showed that Mn(III) dominated in chloramine-formed MnOx, whereas Mn(IV) dominated in chlorine-formed MnOx. Moreover, Cu(II), Pb(II), and elevated pH, which can promote Mn(II) oxidation by free chlorine, were ineffective in NH2Cl systems. These findings demonstrated that Mn(II) oxidation by NH2Cl was kinetically slow under varying water quality and pipe conditions, supporting chloramination for MnOx deposit control even in drinking water distribution systems containing existing MnOx deposits or metal pipe surfaces.
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