Revisiting Ferrate(VI) Activation: Why Enhanced Reactivity Undermines Disinfection

Activated ferrate(VI) can enhance the degradation of contaminants of emerging concern (CECs) due to the in situ formation of more reactive intermediates, primarily Fe(V)/Fe(IV) and possibly free radicals. However, disinfection under ferrate(VI) activation conditions remains underexplored. This study investigated the role of sulfite (SO₃^2–) in SO₃^2–-activated ferrate(VI) for bacterial inactivation. Laboratory-scale experiments were conducted to degrade sulfamethoxazole (SMX) and remove bacterial indicators using ferrate(VI) alone and with sulfite at varying [Fe(VI)]:[SO₃^2–]. Bacterial adsorption onto ferrate(VI)-derived iron oxide particles was experimentally ruled out, validating the dominance of chemical oxidation in bacterial removal. In lake water, activation was observed at [Fe(VI)]:[ SO₃^2–] = 1:2, achieving 60% SMX degradation, compared to 42% by ferrate(VI) alone ([SMX] = 0.8 μM, [Fe(VI)] = 25 μM). However, log removals of total coliform and E. coli declined from 2.86 and 3.28 (ferrate(VI) alone) to 1.17–2.76 and 2.11–3.11, respectively, within [Fe(VI)]:[SO₃^2–] = 4:1–1:4. Similar trends were also observed in the secondary wastewater effluent. Furthermore, log removals exhibited two-phase linear relationships with ferrate(VI) exposure, regardless of activation state, underscoring the importance of time-integrated ferrate(VI) concentration. This study demonstrates the limitations of CT values in assessing ferrate(VI) activation-based disinfection and highlights the need to re-evaluate implementation strategies for ferrate(VI) activation in water treatment.