In situ Simultaneous Removal of Fe(II), Mn(II), and NH4+-N from Groundwater by Codosing of Peroxymonosulfate and Trace HClO: Mechanism and 3D Tank Experiment.

NH4+-N, Fe(II), and Mn(II), common coexisting components in native poor-quality groundwater, were targeted in this study using a simultaneous removal strategy that involved codosing of peroxymonosulfate (PMS) and trace HClO. Unlike traditional HClO-catalyzed Mn(II) oxidation, rapid iron oxides formation bypassed the slow homogeneous catalytic phase, enabling faster electron transfer between the more reactive PMS and Mn(II) adsorbed on iron oxides. The resulting iron-manganese oxides not only removed Fe(II) and Mn(II) but also shifted PMS activation from homogeneous (driven by free Fe(II)) to heterogeneous catalysis, overcoming limitations from low Fe(II) levels in groundwater. Their amorphous structure enhanced PMS adsorption, while bimetallic Fe-Mn sites supported redox cycling among Mn(IV)/Mn(III)/Mn(II)(s) and Fe(III)/Fe(II)(s), sustaining PMS activation and SO4•- generation. HClO participated directly in SO4•--initiated chain reactions, boosting ClO• concentration by 102-fold. ClO• dominated ammonium removal─especially surface-enriched NH4+ -N─selectively converting it to N2 (>88%). Three-dimensional sand tank experiments evaluated lateral, radial, and vertical reagent percolation and in situ remediation efficiency in heterogeneous aquifers through optimized monitoring wells, injection screen lengths, and hydraulic gradient control. This study advances in situ groundwater remediation technology, offering lab-scale mechanistic insights and experimental validation.