Trace Iron as single-electron shuttle for interdependent activation of peroxydisulfate and HSO3−/O2 enables accelerated generation of radicals

The generation of reactive oxygen species generally requires initiators in various environmental remediation processes, which necessitates high dosage of activators and downstream treatment for eliminating the accumulation of deactivated catalysts. Herein, a coupled process was constructed using trace iron for simultaneously activating HSO3-/O2 system and peroxydisulfate (PDS) oxidation system, where the iron ions (2 mg/L) transferred single-electron from the former system to the latter due to the moderate redox potential (Fe3+/Fe2+, +0.77 V) between the potentials of SO3·-/HSO3- (+0.63 V) and PDS/SO4·- (+2.01 V). Hence, the phenol degradation quickly occurred at a first-order kinetic constant of k1=0.223 min-1 due to the accelerated generation of sulfate radical (SO4·-) and hydroxyl radical (·OH) in the process. The k1 value was almost 6-fold of that in the deoxygenated condition (0.040 min-1). Density function theory reveals that the single electron shuttle spatially separates the electron-donating activation of HSO3- and electron-accepting activation of PDS, while avoiding the "mutual-annihilation" of HSO3- and S2O82- via direct two-electron transfer. Finally, utilizing the in-situ generated electron-shuttle (dissolved iron from cast iron pipe), the HSO3-/PDS reagent could efficiently inactivate the chlorine-resistant pathogens and inhibits biofilm regrowth inside the distribution systems at regular intervals or infectious disease outbreak in a neighborhood.