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- /O-2 system and peroxydisulfate (PDS) oxidation system, where the iron ions (2 mg/L) trans-ferred 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- (+0.63V) and PDS/SO4.-(+2.01 V). Hence, the phenol degradation quickly occurred at a first-order kinetic constant of k(1)=0.223 min(-1) due to the accelerated generation of sulfate radical (SO4.-) and hydroxyl radical ((OH)-O-.) in the process. The k(1) value was almost 6-fold of that in the deoxy-genated 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 HSO(3)(- )and S(2)O(8)(2-)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.

Automated summary

In this study, a coupled process using trace iron was developed to simultaneously activate the HSO3-/O2 system and PDS oxidation system, leading to accelerated generation of sulfate radical and hydroxyl radical for efficient phenol degradation. Density function theory revealed the role of a single electron shuttle in spatially separating the electron-donating activation and electron-accepting activation, avoiding mutual-annihilation of reactants. Furthermore, the in-situ generated electron shuttle was able to effectively inactivate chlorine-resistant pathogens and inhibit biofilm regrowth.