Accelerated Oxidation of Organic Contaminants by Ferrate(VI): The

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Accelerated Oxidation of Organic Contaminants by Ferrate(VI): The Overlooked Role of Reducing Additives Mingbao Feng, Chetan Jinadatha, Thomas J. McDonald, and Virender K. Sharma Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03770 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Accelerated Oxidation of Organic Contaminants by Ferrate(VI):

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The Overlooked Role of Reducing Additives

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Mingbao Feng1, Chetan Jinadatha2,3, Thomas J. McDonald1, Virender K. Sharma1*

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Program for the Environment and Sustainability, Department of Environmental and Occupational Health, School of Public Health, Texas A&M University, College Station, Texas 77843, USA; Email: [email protected] 2 Central Texas Veterans Health Care System, 1901 Veterans Memorial Drive, Temple, Texas, United States 3 College of Medicine, Texas A and M Health Science Center, Department of Medicine, 8447 Riverside PKWY, Bryan, Texas, United States

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ABSTRACT

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This paper presents an accelerated ferrate(VI) (FeVIO42-, FeVI) oxidation of contaminants

17

in 30 s by adding one-electron and two-electron transfer reductants (R(1) and R(2)). An addition of

18

R(2) (e.g., NH2OH, AsIII, SeIV, PIII, and NO2-, and S2O32-) results in FeIV initially, while FeV is

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generated with the addition of R(1) (e.g., SO32-). R(2) additives, except S2O32-, show the enhanced

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oxidation of 20-40% of target contaminant, trimethoprim (TMP). Comparatively, enhanced

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oxidation of TMP was up to 100% with the addition of R(1) to FeVI. Interestingly, addition of

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S2O32- (i.e., R(2)) also achieves the enhanced oxidation to 100%. Removal efficiency of TMP

23

depends on the molar ratio ([R(1)]:[FeVI] or [R(2)]:[FeVI]). Most of the reductants have the highest

24

removal at molar ratio of ~ 0.125. A FeVI-S2O32- system also oxidizes rapidly a wide range of

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organic contaminants (pharmaceuticals, pesticides, artificial sweetener, and X-ray contrast

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media) in water and real water matrices. FeV and FeIV as the oxidative species in the FeVI-S2O32--

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contaminant system are elucidated by determining removal of contaminants in oxygenated and

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deoxygenated water, applying probing agent, and identifying oxidized products of TMP and

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sulfadimethoxine (SDM) by FeVI-S2O32- systems. Significantly, elimination of SO2 from

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sulfonamide (i.e., SDM) is observed for the first time.

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TOC Art

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INTRODUCTION

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Iron (Fe) is the second most abundant metal after aluminum in the earth’s crust and is easily

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available. Iron plays many important roles in physiological processes, industrial synthesis, and

38

remediation of pollutants in water.1-7 In the decontamination field, iron-based technologies have

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been researched for many decades, including nanoscale zerovalent iron (nZVI), iron-based

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reactants/catalysts (e.g., Fe2+, Fe3+, Fe3O4, and Fe2O3) in advanced oxidation processes (Fe-

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AOPs), and high-valent iron-oxo units based systems (e.g., FeIV=O, FeV=O, and FeVI=O).8-12

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Among the high-valent iron remediation technologies, ferrate(VI) (FeVIO42-, FeVI) has been

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receiving great attention due to its multiple roles as oxidant, disinfectant, and coagulant.13-15

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Examples include depollution of flue gases, pesticides, estrogens, and antibiotics, inactivation of

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bacteria and viruses as well as coagulation of toxic metals and radionuclides.15-17

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FeVI has shown high effectiveness in decontaminating organic pollutants from water.12,15,18

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However, oxidation of certain organics takes the time of minutes to hours to react with FeVI. This

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reactivity of FeVI decreases its oxidation capacity (i.e., electron equivalent per FeVI). Furthermore,

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due to simultaneous consumption of FeVI by unwanted reaction with water, a slow reaction with

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pollutants requires higher dosages of the oxidant, particularly at pH relevant to treatment

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conditions (pH 7.0 and 8.0).15,19 These limitations restrict the applications of FeVI to efficiently

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oxidize a wide range of pollutants in water.

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This paper presents a systematic innovative strategy to tune the chemistry of FeVI by

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selectively adding one-electron (S(IV)) and two-electron (or oxygen-atom transfer) reducing

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agents (hydroxylamine (NH2OH), arsenite (AsO33-), selenite (SeO32-), phosphite (PO33-), nitrite

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(NO2-), iodide (I-), and thiosulfate (S2O32-)) (see Text S1)20-25 that initially generate the short-

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lived FeV and FeIV species, respectively (i.e., R(1) (i.e., FeVI → FeV),26 and R(2) (i.e., FeVI →

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FeIV)).27 FeV and FeIV have 2-5 orders of reactivity higher than that of FeVI with reactivity order

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of FeV > FeIV > FeVI.28-31 Our current results showed that the initially generated FeV had the

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highest enhancement (up to 100%), while oxygen-atom transfer reductants, except S2O32-, gave

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moderate enhancement (up to 40%) by producing FeIV. This feature of enhancement has not been

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reported in literature.

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Recent studies carried out on enhancing the oxidation of contaminants in 15 s were restricted

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to few reductants that gave the contradictory results, and the description of observed findings

65

was inconsistent.32-34 In the earlier work on FeVI-S(IV) system, enhancement was proposed

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solely due to activation of S(IV) by FeVI yielding SO3●-, SO4●-, and ●OH without any role of

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intermediate FeV/FeIV species. Later work investigated the combined use of the reductants (S(IV),

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S2O32-, dithionite (S2O42-), pyrosulfite (S2O52-), and AsO33-) with FeVI to oxidize N,N-diethyl-3-

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toluamide (DEET) in 10 s.32 This study claimed that only sulfate radicals (SO4●-) caused the

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enhancement in the FeVI-S(IV) system.32 Additionally, this study found no enhanced oxidation of

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DEET by adding S2O32- and AsO33- to FeVI at a high ratio (4:1) of [S2O32-/AsO33-]:[FeVI].32

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Interestingly, our current study demonstrated the accelerated oxidation of contaminants by FeVI

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even using S2O32- and AsO33- when the ratio was much lower. Our previous work on FeVI-S(IV)

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system showed that multiple oxidizing species (FeV/FeIV, SO4●-, and ●OH) were responsible to

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cause enhancement of oxidation of antibiotics in water in 15 s.

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Our work presented herein suggested that only FeV/FeIV species are involved in enhancing

77

the oxidation of a wide range of contaminants in water at a lower ratio of [S(IV)/S2O32-]:[FeVI].

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Importantly, we demonstrated the critical roles of molar ratios of R(1)/R(2) to FeVI and kind of

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reductants (i.e., initial one-electron versus oxygen-atom transfer) to observe the magnitude of

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enhancement and involved oxidizing species in the combination of FeVI with the reductants.

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Moreover, SO32- and S2O32- are sulfur-based reductants but have different chemistry in reaction

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with FeVI (i.e., R(1) and R(2)). We examined the FeVI-S2O32- system in details for oxidizing a wide

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range of contaminants. We have applied density functional theory (DFT) calculations to

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demonstrate the two-electron transfer in the initial step of the FeVI-S2O32- system. Also, S2O32-

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was attractive to us because it is a commercially available salt with high stability. Comparatively,

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SO32- is not stable in water35 and is used in gaseous form (SO2 (g)) at large facilities involving

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storage and handling issues. A system of FeVI-S2O32- is an attractive alternate in enhancing

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oxidation of contaminants in large scale applications.

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The objectives of current paper are to: (i) demonstrate the enhanced reactivity of FeVI by

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adding R(1) and R(2) at trace levels with a focus on S2O32- to decontaminate organics in water in

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30 s. Sixteen organic contaminants of different categories and structures (artificial sweetener,

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pesticides, pharmaceuticals, and X-ray contrast medium, see Table S1) were selected. These

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compounds carry public health concerns and have shown low reactivity with FeVI alone,12 (ii)

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identify the reactive oxidizing species causing accelerated oxidation of contaminants including

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recalcitrant compounds by FeVI-R(1) and FeVI-R(2) systems, (iii) learn the transformation products

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and reaction pathways of two representative pharmaceuticals (TMP and SDM) by FeVI-R(1)

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systems, and (iv) exhibit tuned reactivity of FeVI (i.e., FeVI-S2O32- system) to rapidly remediate

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the organic contamination in real water matrices.

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MATERIALS AND METHODS Chemicals and Reagents. Detailed information on all test organic contaminants, buffer chemicals and preparation of reaction solutions is provided in Text S2.

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Oxidation of Organic Contaminants. All batch experiments were conducted in a series of

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100 mL glass jars under constant stirring rate (400 rpm) with a magnetic stirrer. Elimination of

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each contaminant or their mixtures by FeVI with or without S2O32- was initiated by mixing equal

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solution volumes of 10 mL, and the final pH values of reaction solutions were kept at 8.00 ±

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0.05. The concentration of FeVI was maintained at 100.0 µM, and the ratios of [S2O32-]:[FeVI] in

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the system varied from 0 to 5.0 for aqueous removal of TMP (5.0 µM). Similar removal

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experiments of TMP were performed by pre-adding seven other reducing agents (i.e., NH2OH,

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AsO33-, NO2-, SeO32-, PO3-, I-, and SO32-) with different test concentrations, followed by FeVI

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oxidation. The optimized ratio of [S2O32-]:[FeVI] at 1:8 (0.125) was used to oxidize all organic

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contaminants at environmentally relevant concentration (i.e., 1.0 µM) in either ultrapure water or

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real water samples (river and lake water). After 30 s of oxidation, a 20.0 µL NH2OH solution (1

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M, [NH2OH]:[FeVI] = 10.0) was added to quench the reactions. Samples were transferred into

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high performance liquid chromatography (HPLC) vials and subsequently analyzed using the

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HPLC method (see details in Text S3 and S4).

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Analytical Procedures. Two inorganic reactive radicals (i.e., ●OH and/or SO4●-), possibly

118

produced in the oxidation system, were measured using the room-temperature electron

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paramagnetic resonance (EPR) technique.34,36 Detailed information is provided in Text S4.

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RESULTS AND DISCUSSIONS

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FeVI-R(2) Systems. Initially, accelerated removal of TMP by FeVI was examined at pH 8.0 by

123

adding R(2) additives at various concentrations. Herein, tested R(2) act as two-electron transfer

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reducing species were evident through initial step of the reduction of FeVI by reductants. Initial

125

steps of different reductants were supported by experimental finding using

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O-labelled salt of

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FeVI (i.e., K2Fe18O4) that demonstrated transfer of oxygen-atom from the oxidant to reductants

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(e.g., NO2- to NO3- and PO33- to PO43-).24,37 The DFT calculations on the FeVI-AsO33- system also

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provided preferable oxygen-atom transfer as first step of the reaction.22,38 More details of the

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individual reductant as R(2) are given in Text S1.

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Without R(2) additives, the removal of TMP by FeVI was ~ 16%, which rapidly increased

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following the introduction of R(2) into the FeVI-TMP mixed solution (Figure 1a). In solutions

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containing TMP and R(2) (i.e., no FeVI), no oxidation of TMP was observed (Figure S1). This

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suggested that the enhancement of TMP was due to the reactive species generated by the

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interactions between FeVI and R(2). The maximum removal of TMP oxidation for R(2) was up to ~

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25%, at a molar ratio of 0.1 ([R(2)]:[FeVI]) (Figures 1a and 1b). At molar ratios greater than 0.1,

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enhanced oxidation of TMP decreased. At high levels of NH2OH and AsO33- (i.e., [R(2)]:[FeVI]

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=10.0 and 5.0, respectively), oxidation of TMP ceased. Importantly, a molar ratio of 10.0 using

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hydroxylamine as a quenching reagent was sufficient to stop the oxidation of the contaminant by

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FeVI. Increasing the concentration of selenite resulted in an initial decrease to ~ 6%, followed by

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no effect due to further increase in the molar ratios of selenite to FeVI (Figure 1b). An increase in

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concentration of PO33- showed no decrease in removal of TMP (Figure 1b). This trend is similar

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to NO2-, in which the maximum removal of TMP was ~ 30% at a molar ratio of 1.0 (Figure 1c).

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In this additive, eliminating oxygen from the air by purging with nitrogen showed no difference

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in removal of TMP (Figure 1c).

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The results shown in Figure 1 may be explained by the sequence of reactions taking place in

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the FeVI-R(2)-TMP system (Reactions 1-5). In absence of R(2), TMP is oxidized only by FeVI

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(Reaction 1). However, when R(2) is added, Reaction 2 occurs, generating FeIV species

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(speculative as HFeIVO3-), which can react with TMP (Reaction 3). FeIV species are much more

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reactive than parent FeVI and therefore, enhancement in removal of TMP was observed by adding

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R(2) to FeVI-TMP solutions (Figure 1). However, FeIV may also react with R(2) (Reaction 4). Self-

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decomposition reaction of FeIV may also happen (Reaction 5).39

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HFeVIO4- + TMP → Fe3+ + TMP(oxidized)

(1)

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HFeVIO4- + R(2) → HFeIVO3- + R(2)(O)

(2)

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HFeIVO3- + TMP → Fe3+/Fe2+ + TMP(oxidized)

(3)

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HFeIVO3- + R(2) → Fe3+/Fe2+ + R(2)(oxidized)

(4)

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HFeIVO3- + HFeIVO3- + 3H2O → 2Fe(OH)3 + 1/2O2 + 2OH-

(5)

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Results of Figure 1 suggest that enhancement due to R(2) is related to the concentrations of

158

generated FeIV, added R(2), and the competitive rate constants of Reactions 2-5. At low

159

concentration of R(2), FeIV produced from Reaction 2 seems to be reacting with TMP (Reaction

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3) to yield enhancements in removal (Figure 1). However, with increasing concentrations of R(2),

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Reaction 4 begins to dominate and causes the decreasing concentration of FeIV, i.e., less FeIV

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species is available to oxidize TMP (i.e., TMP(oxidized products)), which results in the

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decreasing trend in the removal of TMP. Incomplete removal of TMP by the FeVI-R(2) system

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suggests that Reaction 5 may be significant to result in the disappearance of FeIV itself39 rather

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than the oxidation of TMP.

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The rate constants of Reactions 2-5, shown to describe results of Figure 1, are currently

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unknown in literature to describe fully enhancement in removal of TMP under FeVI-R(2) systems

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under studied conditions. Determination of these rate constants would require modified premix

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pulse radiolysis technique.29,30,39 It seems that the rate constants of Reactions 2 and 4 vary with

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the type of R(2), which gave different patterns of the oxidation of TMP with increasing molar

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ratios of R(2) to FeVI. Furthermore, Reactions 1-5 are a simplistic representation of involved

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reactions and complex chemistry, which may be existing in a particular FeVI-R(2) additive system.

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FeVI-iodide System. The addition of iodide resulted in ~ 41% removal at a molar ratio of 0.1

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([I-]:[FeVI]) (Figure 2a). Further increase in this molar ratio decreased the removal of TMP, and

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complete inhibition of TMP was observed at the molar ratio of 1.0 (Figure 2a). A formation of I3-

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in the reaction of FeVI with I- has been observed, which produced through initial one-electron

178

transfer step (FeVI + I- → FeV + I●) (Text S1).20 The generated FeV may react with TMP to

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enhance the oxidation of TMP. However, a recent study proposed oxygen-atom transfer step that

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gave FeIV species in initial step (FeVI + I- → FeIV + HOI/OI-) (Text S1).40 The FeIV species may

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also increase the oxidation of TMP. Formation of different oxidized products has been observed

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in the reaction of O3 with I-.41 Formed HOI has high reactivity with aromatic compounds, which

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suggests that TMP may also be oxidized by HOI in the FeVI-I- system.40 These reactive species

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would yield the enhanced oxidation of TMP observed in our study. It appears that at molar ratios

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greater than 0.1 ([I-]:[FeVI]), the reactions of FeV/FeIV with I- start competing with that of these

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intermediate iron species with TMP to cause decrease in enhanced effect of I- to oxidize TMP.

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FeVI-sulfite System. Sulfite ion also showed the enhancement, with complete elimination of

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TMP at a molar ratio of 0.25 ([SO32-]:[FeVI]) (Figure 2b). A detailed examination of the

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oxidation of SO32- by FeVI showed that SO32- acts as R(1) (see Text S1).26 Enhanced oxidation of

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contaminants by FeVI-SO32- system, studied only at a molar ratio of 4.0 ([SO32-]:[FeVI]), was also

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reported in our previous study and other investigations.32,34 Figure 2b shows that the increasing

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ratios had no decreasing effect on TMP removal until the molar ratio of 5.0. However, when the

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molar ratio was greater than 5.0, enhanced oxidation of TMP decreased and no enhancement

195

could be obtained at the molar ratio of 20.0 (Figure S2). When nitrogen was purged in FeVI-SO32-

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-TMP solution, enhanced removal efficiency decreased, and maximum removal of TMP was ~

197

83% in the molar ratio between 0.5 and 1.0 (Figure 2b). In the molar ratio range of 1.0-5.0, a

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linear decrease to 60% was observed in TMP removal (Figure 2b).

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Reactions 6-10 may describe the results in Figure 2b. A SO32- ion reduced FeVI to FeV with

200

the formation of a ●SO3- (Reaction 6, e.g., k6 = (2.0 ± 0.2) × 101 M-1 s-1, pH 11.4). This radical

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also reduced FeVI to give FeV (Reaction 7, e.g., k7 = (1.9 ± 0.3) × 108 M-1 s-1, pH 11.4).26 FeV

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species react with TMP to enhance the oxidation of TMP (Reaction 8). FeV is 3-5 orders of

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magnitude higher in reactivity than FeVI,28 hence the observed enhancement in TMP oxidation

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(see Figure 2). Furthermore, FeV is much more reactive than FeIV species produced by R(2), and

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results in substantially more enhancement caused by R(1) compared to R(2) (Figure 2b versus

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Figure 1). However, FeV may react with SO32- without the formation of FeIV species (Reaction 9,

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e.g., k9 = (3.9 ± 0.4) × 104 M-1 s-1, pH 11.4).26 The self-decomposition of FeV species to FeIII may

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also occur (Reaction 10, k10 = (~ 7× 105 M-1 s-1, pH 11.4).42 Both Reactions 9 and 10 caused the

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elimination of the reactive FeV species, responsible for enhancing the removal of TMP.

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HFeVIO4- + SO32- → HFeVO42- + ●SO3-

(6)

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HFeVIO4- + ●SO3- → HFeVO42- + SO3

(7)

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HFeVO42- + TMP → Fe3+/Fe2+ + TMP(oxidized)

(8)

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HFeVO42- + SO32- + 3H2O → Fe(OH)3 + SO42-

(9)

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HFeVO42- + HFeVO42- + 4H2O → 2Fe(OH)3 + O2 + 4OH-

(10)

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Concentrations of FeV produced and TMP in FeVI-SO32--TMP solutions and the competitive

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rate constants of Reactions 6-10 at pH 8.0 would ultimately determine the magnitude of

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enhancement of the target contaminant and their variation with the ratio of SO32- to FeVI.

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Additional oxidant species (SO4●- and ●OH) are formed (SO3 + 2OH- → SO42- + H2O; ●SO3- +

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O2 → ●SO5- →→ ●SO4-; and ●SO4- + H2O → SO42- + H+ + ●OH),34,43 in which no decrease in

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enhancement of TMP removal at [SO32-]:[FeVI] = 0.1-5.0 is observed.32,34 Formation of such

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radicals has been evidenced in previous investigations.32,34 However, at ratios higher than 5.0,

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Reactions 9 and 10 are most likely to occur to cause the decrease in enhanced oxidation of TMP

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by FeVI-SO32- system (Figure S2). Interestingly, when no oxygen existed in the solution mixture,

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enhanced oxidation of TMP still occurred. The values of rate constants for Reactions 7-9 at pH

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8.0 are needed to fully understand the observed trend of enhancement in Figure 2b.

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FeVI-thiosulfate System. Enhancement of TMP removal was also seen when S2O32- was

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used as a reductant, with complete removal of TMP observed at a ratio of 0.125 (Figure 2c). In

229

the mixture of TMP-S2O32- solutions, no removal of TMP was seen (Figure S1), therefore, the

230

enhancement of TMP removal (Figure 2c) is due to this reducing additive in the FeVI-TMP

231

mixed solution. Removal of oxygen (i.e., nitrogen purging) in FeVI-S2O32--TMP solution had no

232

significant difference from TMP removal in air saturated solutions (Figure 2c). At a molar ratio

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([S2O32-]:[FeVI]) greater than 0.125, enhancement of TMP oxidation decreased. When this molar

234

ratio reached 5.0, no oxidation of TMP occurred (or complete inhibition). The magnitude of

235

enhanced effect depends on the pH and the concentrations of contaminant (see Figure S3).

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Based on the results observed in Figure 2c, S2O32- as an additive was selected to

237

investigate the acceleration and enhancement of 16 organic contaminants at pH 8.0. Except the

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unstable sulfite,35 all other additives are pollutants and unsuitable for use as additives in

239

treatment processes using FeVI. A molar ratio of S2O32- to FeVI was selected at 0.125, which

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demonstrated complete removal of TMP. As shown in Figure 3, the presence of S2O32- in the

241

FeVI-contaminant system could increase the removal percentages of all contaminants in 30 s, as

242

compared to FeVI only. Interestingly, complete removal of typical electron-rich moieties

243

containing pharmaceuticals (CMZ, DCF, PPN, SDM, TMP, and ENR) was found, similar to

244

oxidation of TMP (see Figure 2b). Notably, removals of other recalcitrant contaminants were

245

also significant (20-40%). The results of Figure 3 suggest that a wide range of organic

246

contaminants can be rapidly removed by the FeVI-S2O32- system. Considering the co-existence of

247

various pharmaceuticals in the water environment, elimination of six pharmaceuticals present in

248

a mixture solution by the FeVI-S2O32- system was also studied at pH 8.0. Again, complete

249

removal of all tested contaminants was found after 30 s (Figure S4). These observations further

250

suggest the applicability of S2O32- in enhancing the oxidation of organic contaminants by FeVI in

251

water.

252

Results of the FeVI-S2O32- system at low molar ratios ([S2O32-]:[FeVI] ≤ 0.125)) were similar

253

to those obtained in the FeVI-SO32- system. Our initial thought was that the FeVI-S2O32- system

254

may be generating directly FeV and S2O3●- (Reaction 11) to result in enhanced oxidation of

255

contaminants. However, experimental findings of the oxidation of S2O32- by FeVI suggested

256

oxygen atom transfer from FeVI to S2O32- to produce FeIV and S2O42- (Reaction 12 and see Text

257

S1).

258

HFeVIO4- + S2O32- → HFeVO42- + S2O3●-

(11)

259

HFeVIO4- + S2O32- → HFeIVO3- + S2O42-

(12)

260

To further clarify whether S2O32- behaves like R(1) additive (i.e., Reaction 11) or R(2) (i.e.,

261

Reaction 12), DFT calculations were performed to learn the favorable step of the reaction

262

between FeVI and S2O32- (Text S5). All the species involved were optimized and the changes of

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Gibbs free energy (∆G) were individually calculated for Reactions 11 and 12. Compared to the

264

positive value (∆G = 34.66 kJ/mol) of Reaction 11, Reaction 12 had a negative value (∆G = -

265

50.67 kJ/mol), indicating the spontaneity of two-electron transfer reaction (Figure S5). Thus, the

266

results of DFT calculations are also in good agreement with the prediction based on the

267

relationship of the rate constant with the potential of redox pair, S2O42-/S2O32-.27

268

Notably, results of enhancement in the FeVI-S2O32- system differed from those obtained in

269

the FeVI-R(2) system (Figure 2b versus Figure 1). We therefore proposed that the reaction

270

between FeVI and S2O32- produced indirectly FeV species to cause initial enhancement seen in

271

Figure 2c. The final oxidized product observed in the reaction between FeVI and S2O32- is SO32-

272

(Reaction 13).44,45

273

4HFeO4- + 3S2O32- + 2OH- + 3H2O → 4Fe(OH)3 + 6SO32-

274

The generated SO32- produced FeV species (see Reaction 6) to result in initial enhancement

275

of the oxidation of TMP in the FeVI-S2O32- system. The second-order rate constants for the

276

reactivity of FeVI with S2O32- and S(IV) (HSO3- + SO32-) are 6.01 × 103 M-1 s-1 and 1.9 × 104 M-1

277

s-1 at pH 8.0.23,45,46 This suggests that produced SO32- in Reaction 13 can react with FeVI to form

278

FeV (see Reaction 6). Radicals, i.e., SO4●- and ●OH, can only be generated in presence of

279

oxygen.34 Participation of SO4●- and ●OH to cause this enhancement was ruled out by performing

280

the experiments under nitrogen environment (i.e., no oxygen from air). In 30 s, FeVI would not

281

produce significant oxygen from its self-decomposition.47 Results showed no significant

282

difference in enhancing the oxidation of TMP by FeVI-S2O32- system (see Figure 2b). This

283

suggests that oxygen has a minimal, if any, role in carrying out oxidation of TMP by FeVI-S2O32-

284

mixed solution in 30 s. Similar results were also observed in oxidizing other organic

285

contaminants (i.e., no obvious difference with and without oxygen in oxidation of APT, ATL,

14


(13)


286

CAF, DCF, and ENR, Figure S6), suggesting no possible role of SO4●- and ●OH in the enhanced

287

oxidation of organic pollutants by FeVI-S2O32- system.

288

The room-temperature electron paramagnetic resonance (EPR) measurements with 5,5-

289

dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trap reagent were conducted to probe the

290

formation of SO4●- and ●OH.34,36 Compared with the EPR spectrum of the FeVI-TMP-DMPO

291

system, no new signal was observed after introducing S2O32- at a low ratio (1:8) of [S2O32-

292

]:[FeVI] (Figure S7). The observed EPR signals in the control group were assigned as ●OH,

293

generated at a low yield (< 4%) by FeVI self-decay in water.19 The unchanged EPR intensity

294

indicated that no SO4●- was produced and also ●OH was not further formed in the FeVI-S2O32-

295

system, therefore not contributing to the significantly enhanced oxidation of organic

296

contaminants. Similar results were observed following introduction of the lower ratio (1:8) of

297

[SO32-]:[FeVI] and [I-]:[FeVI] (Figure S7). Furthermore, SO3●- can also be ruled out in the system:

298

it is a precursor to the production of SO4●-.34,43

299

The role of FeIV/FeV intermediates on FeVI-S2O32--contaminant system was further explored

300

using dimethyl sulfoxide (DMSO) as the probing reagent for the high-valent iron species. DMSO

301

is selectively oxidized by FeV=O and FeIV=O species through oxygen atom transfer to produce

302

the corresponding sulfone (DMSO2).11,48,49 Such reactions differ from the reaction pathways

303

involved in radicals-based oxidation.50 The oxidation of TMP was followed by adding 1 mM

304

DMSO into FeVI-S2O32--TMP system. Interestingly, oxidation of TMP was almost inhibited in

305

the presence of DMSO (i.e., no difference in TMP removal by FeVI with and without S2O32-)

306

(Figure S8). This suggests that the FeIV/FeV species generated in FeVI-S2O32--TMP solution were

307

captured by DMSO and thus, unavailable to oxidize TMP. It should be pointed out that FeVI has

308

a very slow reactivity with DMSO (k ~ 1.0 M-1 s-1 at pH 8.0)51 and no significant reaction

15


Page 16 of 33

Page 17 of 33


309

between FeVI and DMSO was expected in 30 s (t1/2 ~ 690 s). Therefore, the dominant reactive

310

species for oxidation of organic pollutants in FeVI-S2O32- system were FeIV/FeV species. Another

311

study has reported different rate constant for the reaction of FeVI with DMSO (i.e., 16 M-1 s-1,

312

giving t1/2 of FeVI consumption by 1 mM of DMSO = 43 s).45 However, excessive FeVI (100 µM)

313

used in our study would ensure the enhanced effect, which could still be seen at less magnitude

314

because of less molar ratio of S2O32- to FeVI than 0.125. As shown in Figure S8, the oxidation of

315

TMP completely stopped by the presence of DMSO in the FeVI-S2O32- system. In other words,

316

both rate constants applied in previously reported studies suggest that the reaction of FeVI with

317

DMSO had a minimal role, and inhibition of the degradation of TMP by the FeVI-S2O32- system

318

was due to elimination of FeIV/FeV species by DMSO; suggesting the generation of such highly

319

reactive iron species to enhance the oxidation of contaminants by the FeVI-S2O32- system.

320

Additionally, the oxidized product of DMSO by the FeVI-S2O32- system was identified as DMSO2

321

(Figure S9).

322

Overall, enhanced oxidation of TMP by the FeVI-S2O32- system was most likely by direct

323

production of FeIV (Reaction 12) and indirect generation of FeV species (Reaction 6), which

324

reacted with TMP (Reactions 3 and 8). With increasing concentration of S2O32- (or [S2O32-

325

]:[FeVI]). The intermediate reactive iron species, FeIV and FeV, may be removed by their

326

reactions with S2O32- (Reactions 14 and 15). Reaction 14 is presumably occurring through

327

oxygen-atom transfer from FeIV to S2O32- yielding FeII, similar to the one proposed earlier.45 The

328

rapid reaction of FeV with S2O32- may also happen (e.g., k15 = (2.1 ± 0.1) × 103 M-1 s-1, pH 11.4).

329

The FeII, produced in Reaction 14, may consume the main oxidant, FeVI (Reaction 16, e.g., k16 =

330

~ 105 M-1 s-1, pH 12.1).26 Reaction 16 would terminate the production of FeV and FeIV, the

331

responsible species to cause enhanced oxidation of TMP by FeVI-S2O32- system. Furthermore, the

16



Page 18 of 33

332

reductants R(2) would produce FeII as one of reduced species of FeVI (see Reaction 4 or 14)

333

before converting to final FeIII species (Reaction 16). It appears that excess S2O32- (or [S2O32-

334

]:[FeVI] > 1.0), not only eliminates the FeIV/FeVI species (Reactions 14 and 15), but also causes

335

the consumption of FeVI (Reaction 15). It seems no FeVI was available to oxidize TMP at excess

336

concentration of S2O32- and no degradation of TMP was observed (see Figure 2c).

337

HFeIVO3- + S2O32- → Fe(OH)2 + S2O42-

(14)

338

HFeVO42- + S2O32- + 2H2O → Fe(OH)3 + S2O42- + 2OH-

(15)

339

HFeVIO4- + 3Fe(OH)2 + 3H2O → 4Fe(OH)3 + OH-

(16)

340

Available rate constants for Reactions 15 and 16 are at high alkaline medium (i.e., pH 11.4

341

and 12.1), and these reactions would occur at much faster rates at pH 8.0. For example, the rate

342

constant for the reaction of FeVI with FeII was estimated to be greater than 1.0 × 107 M-1 s-1 at pH

343

5.0.19 The initial step of Reaction 16 produces FeV (i.e., FeVI + FeII → FeV + FeIII). The subseqent

344

reaction of FeV with FeII would produce FeIV (FeV + FeII → FeIV + FeIII). Similarly, FeIV may

345

also react with FeII to form FeIII (FeV + FeII → 2FeIII), which may terminate reaction. The rate

346

constant of Reaction 14 is not known. The pattern of the oxidation of TMP in Figures 1 and 2 as

347

well as Figure S2 may be explained fully by knowing the rate constants of reactions involved in

348

the oxidation of TMP by FeVI-reducing additive system. At a molar ratio of 0.125 ([S2O32-

349

]:[FeVI]), contributions of different involved reactions in the system provide optimum levels of

350

FeIV/FeV to cause maximum oxidation of TMP. Again, the values of rate constants of all possible

351

reactions at different pH are needed to understand this optimum ratio of 0.125.

352 353

Oxidized Products (OPs) of TMP and SDM by FeVI-S2O32- System. The OPs of TMP and

354

SDM by FeVI-S2O32- system were investigated at pH 8.0 and a molar ratio of 0.125 ([S2O32-

17


Page 19 of 33


355

]:[FeVI]), which had complete removal of contaminants (see Figure 3). Based on the molecular

356

weights and previous study,34 OPs of TMP were named as OP-338, OP-322, OP-308, OP-306,

357

OP-304, OP-292, OP-212 and OP-196. MS/MS spectra and possible structures of the fragments

358

of SDM and its OPs are presented in Figure S10. SDM has five fragments (m/z 218.02295,

359

156.07666, 108.04427, 92.04943, and 65.03873 (Figure S10a). These structures were proposed

360

as the sequential losses of the aniline group from SDM (m/z 311.08060), the SO2 group from m/z

361

218.02295, the amino and methoxyl groups from m/z 156.07666, the residual aniline group from

362

SDM (m/z 311.08060) and the loss of -NH2C group from m/z 92.04943. OPs of SDM were

363

classified as OP-356, OP-340, OP-327, OP-326, OP-324, OP-311, OP-295, and OP-276 (Figure

364

S10b-S10i). The molecular compositions of these OPs were suggested by the good mass error (

Accelerated Oxidation of Organic Contaminants by Ferrate(VI): The

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