Fe(III)-Activated Sulfite Revisited: The Overlooked Role of High-Valent Iron(IV)-Oxo Complex (Fe(IV)) in Water Treatment


 Sulfate radical (SO4•−) and its secondary radical (hydroxyl radical, •OH) are commonly recognized as the primary reactive intermediates formed by Fe(III)/sulfite system. However, it still remains unknown whether Fe(IV) is involved in this system where the well documented Fe(IV)-precursors (i.e., Fe(II) and persulfates) were in-situ generated. Intriguingly, we observed that methyl phenyl sulfone (PMSO2), indicative of Fe(IV) formation, was formed during methyl phenyl sulfoxide (PMSO) transformation in Fe(III)/sulfite system, which unprecedently verified that Fe(IV) played a crucial role in it. In parallel, the involvement of SO4•− and •OH in this system were also identified, but the limited •OH was proposed to be derived from hydrolysis of both Fe(IV) and SO4•−, rather than by self-decay of SO4•− alone. Moreover, the contribution of Fe(IV) relative to it of free radicals was explored by monitoring the yield of PMSO2. It was disclosed that the relative contribution of Fe(IV) was progressively promoted as Fe(III)-sulfite reaction proceeding with an upper limit of 80%-90%, and it was accelerated by promoting Fe(III) and sulfite dosages, while was declined with increasing pH. Furthermore, a kinetic model was developed, which precisely simulated kinetic traces of PMSO transformation and dissolved oxygen evolution in Fe(III)/sulfite system. More importantly, the kinetic model offered the first insight into the evolution of Fe(IV), SO4•−, and •OH, which provided in-depth mechanistic understanding of the iron-catalyzed sulfite auto-oxidation process. Considering the different chemical properties between Fe(IV) and free radicals, it is urgent to re-evaluate the decontamination process by iron/sulfite system.


Introduction
In recent years, sulfate radical (SO 4 •− )-based advanced oxidation processes (AOPs) have attracted a lot of attention for pollutants abatement due to the high standard reduction potential of SO 4 •− (2.5-3.1 V) (Zhou et al. 2015a, Oh et al. 2016, Lee et al. 2020. The sulfite and persulfates (peroxomonosulfate (HSO 5 − ) and peroxydisulfate (S 2 O 8 2− )) are the most frequently used precursors of SO 4 •− . Comparing with persulfates, the low cost, high solubility, and eco-friendly features single out sulfite as an excellent alternative chemical source of SO 4 •− in practical application scenarios . Among several AOPs of activating sulfite to produce reactive oxidants Eldik. 1995, Chen et al. 2017), the iron-catalyzed sulfite auto-oxidation process has been widely investigated not merely because of the high reactivity, cost-effective, and environmental-friendly characters of iron activator, but because this process plays a vital role in the conversion of atmospheric sulfur species (Brandt. and Eldik. 1995. Th is auto-catalysis process is initiated by Fe(III)-sulfite (FeSO 3 + ) complex formation (Eq. 1) and its self-decomposition with forming SO 3 •− radical (Eq. 2). Subsequently, dissolved oxygen (DO) oxidizes SO 3 •− to produce the peroxymonosulfate anion radical (SO 5 •− , Eq. 3), which triggers radical chain reactions (simplified to Eqs. 4-11) with forming the key reactive oxidative species (ROS) precursors (HSO 5 − and S 2 O 8 2− ) and the final reactive species (SO 4 •− ). In light of this paradigm, numerous researches recognized that SO 4 •− and the 4 secondary radical (hydroxyl radical, • OH) generated by SO 4 •− hydrolysis are the primary ROS formed in Fe(III)/sulfite system (Chen et al. 2012, Guo et al. 2013, Zhou et al. 2014, Zhou et al. 2015b, Yu et al. 2016, Xie et al. 2017, Du et al. 2018, Xie et al. 2019, Chen et al. 2020).
To this end, the possibility of Fe(IV) involvement in Fe(III)/sulfite system was investigated by using PMSO as the probe, paying special attention to quantifying the relative contribution of Fe(IV) and free radicals by calculating the yield of PMSO 2 (η(PMSO 2 ), the molar ratio of PMSO 2 production to PMSO loss). Moreover, electron paramagnetic resonance (EPR) technique and alcohol scavenging experiment were employed to identify the role of free radicals. In parallel, effects of reactants dosage and solution pH on PMSO transformation in Fe(III)/sulfite system were monitored. Furthermore, a kinetic model was established to simulate the PMSO transformation kinetic profiles in Fe(III)/sulfite system and provided the mechanistic insights into the evolution of different ROS therein.

EPR spectra analysis
EPR was employed to identify the nature of free radicals produced in Fe(III) [PMSO] (μM) [sulfite] 0 = 0.5 mM, and pH 3.0). As illustrated in Figure 2a, strong signals of both DMPO • -OH and DMPO • -OSO 3 − adducts were observed in EPR spectrum of Fe(III)/sulfite system. Generally, the occurrence of DMPO • -OH signal was explained by the fast Figure S1) and the decay of SO 4 •− with forming • OH as the secondary free radical (Eqs. 14-15) (Davies et al. 1992, Wei et al. 2017, Du et al. 2018, Xie et al. 2019. In addition to this explanation, the transformation of DMPO by Fe(IV) (Figure S2) ) and the formation of • OH from Fe(IV) hydrolysis (Eq. 16) (Jacobsen et al. 1998) might also made contribution to DMPO • -OH signal occurrence in Fe(III)/sulfite system:  Table S1 for the specific rate constants propranolol, etc.) in iron/sulfite systems (Chen et al. 2012, Xie et al. 2017, Du et al. 2018, Wang et al. 2019a, Xie et al. 2019, Yuan et al. 2019, Chen et al. 2020, Dong et al. 2020).
This result disclosed that • OH was involved in PMSO transformation by Fe(III)/sulfite system, but the relative contribution of • OH was limited and far less than that of Fe(IV) and SO 4 •− , which might be ascribed to the slow hydrolysis rate of Fe(IV) (Eq. 16) and SO 4 •− (Eqs. 14-15).

Effect of solution pH
Transformation of PMSO (50 μM) by Fe(III)/sulfite system ([Fe(III)] 0 = [sulfite] 0 = 0.5 mM) was investigated at pH range of 3.0-6.0. As shown in Figure S7, Δ[PMSO] 300s was significantly declined from 7.5 μM at pH 3.0 to less than 3.5 μM at pH 4.0 and almost no PMSO was degraded at pH 4.5-6.0. At the same time, Δ[PMSO 2 ] 300s decreased from 6.0 μM at pH 3.0 to 2.9 μM at pH 4.0. These results might be ascribed to the precipitation of Fe(III) at high pH. Moreover, η(PMSO 2 ) value at 300 s was decreased from 83% at pH 3.0 to 53% at pH 4.0, indicating that the relative contribution of Fe(IV) was decreased with increasing pH.
This was attributed to the fact that Fe(IV) oxidation ability decreased with the increasing of pH (Louwerse et al. 2008, Bataineh et al. 2012, Lu et al. 2018, while the reactivity of SO 4 •− 13 and • OH was insensitive to pH (Zhou et al. 2015a, Lee et al. 2020).

PMSO transformation kinetics
The complex pathways of Fe(IV), SO 4 •− and • OH generation in Fe(III)/sulfite system, as well as the consumption of them by contaminants and side reactions were illustrated in Figure 3a. Based on this schematic, a kinetic model was established (see Table S2), which provided a powerful tool for taking the further mechanistic insights into ROS evolution in Fe(III)/sulfite system. The program Kintecus (Ianni) was employed to perform the simulation.

Dissolved oxygen evolution kinetics
As shown in Figure 3a, the fast oxidation of SO 3 •− by DO (Eq. 3) triggered the formation of various ROS in Fe(III)/sulfite system. The essential role of DO in ROS formation was evidenced by the fact that PMSO degradation was negligible with removing  Figure S8). Similarly, Yuan et al. (2019) reported that aniline elimination in Fe(III)/sulfite system was significantly inhibited with purging nitrogen. Further, the evolution of DO during the activation of sulfite (0.5 mM) by Fe(III) (1 mM) was monitored at pH 3.0. It was observed that DO instantaneously decreased from 8 mg/L to only 0.4 mg/L in the first 30 s, which was a little bit slower than the simulation results (Figure 4a).
Considering the response delay of the DO measuring instrument, the kinetic model also accurately simulated the time-dependent evolution of DO in Fe(III)/sulfite system.

Fe(IV), SO 4 •− , and • OH formation and decay kinetics
Evolution  (Brandt. and Eldik. 1995) is far slower than that between Fe(II) and  kinetic behaviors that they rapidly increased to the maximum initially, and subsequently they were consumed immediately to a low level (Figure 4c). A closer inspection of these kinetic curves (inset of Figure 4c) revealed that [SO 4 •− ] reached to the peak at 6.5 s, which was [DO] (mg/L) contribution of Fe(IV) increased with Fe(III)-sulfite reaction proceeding. Moreover, it was apparent that the steady-state concentration of Fe(IV) was much higher than that of SO 4 •− that the maximum of [Fe(IV)] (7.8 × 10 -2 μM) was three orders of magnitude higher than the peak of [SO 4 •− ] (2.0 × 10 -5 μM) (Figure 4c), consistent with the high η(PMSO 2 ) values (up to about 80%-90%) obtained in Fe(III)/sulfite system. By comparison, the proportion of • OH was extreme small that the maximum of [ • OH] was only 3.2 × 10 -7 μM, in accordance with the former inference that the relative contribution of • OH is limited.
Additionally, effect of [Fe(III)] 0 on Fe(IV) evolution was examined by kinetic simulation. As shown in Figure S9, the increase of [Fe(III)] 0 from 0.5 to 5 mM significantly promoted the maximum of [Fe(IV)] from 3.3 × 10 -2 μM to 9.3 × 10 -2 μM, and shortened the time of reaching the peak from 15 s to 4 s. This result was in concordance with the experimental data that PMSO 2 production and η(PMSO 2 ) value was promoted with adding more Fe(III) (Figure S3).

Conclusions
In this study, both Fe(IV) and free radicals (SO 4 •− and • OH) were identified as the reactive intermediates formed in Fe(III)/sulfite system, which was quite different from literature assertation that only free radicals was involved in it. Specifically, formation of PMSO 2 during PMSO transformation in Fe(III)/sulfite system unambiguously evidenced the involvement of Fe(IV). EPR spectra analysis and alcohol scavenging experiment implied that SO 4 •− was the primary free radical, while • OH contribution was limited. Moreover, it was disclosed that • OH was formed by the hydrolysis of both Fe(IV) and SO 4 •− . The relative 18 contribution of Fe(IV) was increased as Fe(III)-sulfite reaction proceeded and was enhanced by promoting Fe(III) and sulfite dosages, while it was declined with increasing pH. Further, a kinetic model was developed, which accurately simulated the kinetic curves of PMSO transformation and DO evolution in Fe(III)/sulfite system. More intriguingly, the kinetic simulation provided the first insights into the formation and decay mechanism of Fe(IV), SO 4 •− , and • OH in this complex system. Comparing with SO 4 •− and • OH, aqueous Fe(IV) was more selective in eliminating organic pollutants, which would broaden the application scenario of iron/sulfite system.

Declarations
This study was supported by Guangdong Key R&D Program (