Oxidation of Manganese( II ) With Ferrate Revisited: The Overlooked Role of Hardness Cations

Manganese (Mn) is a concern for some groundwater‐sourced drinking water systems. Ferrate (Fe(VI)) is a promising alternative oxidant for Mn treatment, but particle formation during Mn(II) oxidation is not well understood under realistic groundwater conditions with high hardness (> 180 mg/L) and low natural organic matter (< 1 mg/L NOM). This study evaluated the influence of calcium (Ca), magnesium (Mg), and NOM on particles resulting from Fe(VI) oxidation of Mn(II) in natural and synthetic waters. Increasing Ca and Mg shifted the particle size distribution to the micron‐scale (> 0.7 μm). In low‐hardness matrices (< 9 mg/L as CaCO 3 ) and no NOM, suspensions remained largely nano‐scale (i.e., retained by 30 kDa ultrafilter). Particle surface charge measurements showed that Ca, and to a lesser extent Mg, decreased the magnitude of negative surface charge, consistent with cation‐driven destabilization and aggregation. NOM impeded this effect, increasing the Ca/Mg concentrations required to form larger particles to > 200 mg/L as CaCO 3 . An in situ continuous‐flow experiment (CFE) using natural groundwater showed this behavior can be leveraged to remove Mn using an operationally simple Fe(VI) oxidation and cartridge filtration approach.