Enhanced oxidation of aniline using Fe(III)-S(IV) system: Role of different oxysulfur radicals

Abstract

In this paper, the efficiency of Fe(III)-S(IV) system used for advanced oxidation processes (AOPs) has been investigated using aniline as a pollutant model compound in water. The chemical kinetics, influencing factors, and mechanism of aniline oxidation are examined with an emphasis on the contribution of the different oxysulfur radicals (mainly SO 4 •− and SO 5 •−). Our results show a significant enhancement in the efficiency of aniline oxidation observed at pH 4.0 with 1.0 mM S(IV) and 0.1 mM Fe(III) concentrations. Moreover, the degradation efficiency drastically decreases to 10% in the absence of oxygen indicating the significant role of oxygen in this type of process. Through competition kinetic experiments and radical scavenger experiments, it is shown that SO 5 •− is responsible for about 60% of the aniline oxidation in the Fe(III)-S(IV) system under the typical conditions investigated in this work. For the first time we have determined the second order rate constant between SO 5 − and aniline (5.8 ± 0.6 × 10 6 M −1 s −1 (at pH 3.0) and SO 4 − and aniline 7.7 ± 0.5 × 10 9 M −1 s −1 (at pH 3.0)). Sequential experiments with successive additions of sulfite drastically improve the oxidation efficiency. These findings may provide a precise understanding of the overall mechanism and may have promising implications in developing a new cost-effective technology for the treatment of organic compounds-containing water. Furthermore, the results of this work help to understand the relevance and mechanism of organic contaminants oxidation by SO 5 •− , which has not been given much attention in conventional SR-AOPs using peroxymonosulfate.

Calcium Sulfite Solids Activated by Iron for Enhancing As(III) Oxidation in Water

“…When Fe(III) and CaSO3 solid powders were added simultaneously, the reaction rate of the system increased rapidly. This is basically consistent with the previous research results on the Fe(III)-sulfite system [14,20,21], suggesting that the activation of SO3 2− may occur either in the liquid phase or on the surface of CaSO3 particles (Equations ( 16) and ( 17)). Considering that irons can easily form hydroxide precipitates at near neutral pH and aggregate with CaSO3 to form composite particles, the activation of SO3 2− on the solid surface may also be important.…”

“…Additionally, iron-based nanomaterials of high superficial activity also had a good effect on removing heavy metal pollutants in the environment [ 17 , 18 ]. In the Fe(II/III)-sulfite system, S(IV) can be catalytically oxidized under certain conditions to produce a series of oxysulfur species, including sulfite radical (SO 3 •− ) and sulfate radical (SO 4 •− ) [ 19 , 20 , 21 ]. An intrinsic mechanism has been proposed for this system, which includes the following reactions (Equations (1)–(8)) [ 22 , 23 , 24 , 25 , 26 ]: Fe 2 + + HSO 3 − ⇌ FeHSO 3 + (rapid equilibration) 4FeHSO 3 + + O 2 → 4FeSO 3 + + 2H 2 O FeSO 3 + → Fe 2+ + SO 3 •− (k f = 0.19 s −1 ) SO 3 •− + O 2 → SO 5 • − (k 4 < 10 9 mol −1 L s −1 ) SO 5 •− + HSO 3 − → SO 3 •− + HSO 5 − (k 5 = (10 4 –10 7 ) mol −1 L s −1 ) Fe 2+ + HSO 5 − → SO 4 •− + Fe 3+ + OH − (k 6 = (10 4 –10 7 ) mol −1 L s −1 ) Fe 3+ + HSO 3 − ⇌ FeSO 3 + + H + (logk 7 = 2.45) SO 5 •− + HSO 3 − → SO 4 2− + SO 4 •− + H + (k 8 = (10 4 –10 7 ) mol −1 L s −1 ) …”

“…Additionally, iron-based nanomaterials of high superficial activity also had a good effect on removing heavy metal pollutants in the environment [17,18]. In the Fe(II/III)-sulfite system, S(IV) can be catalytically oxidized under certain conditions to produce a series of oxysulfur species, including sulfite radical (SO3 •− ) and sulfate radical (SO4 •− ) [19][20][21]. An intrinsic mechanism has been proposed for this system, which includes the following reactions (Equations ( 1)-( 8)) [22][23][24][25][26]:…”

Different Role of Bisulfite/Sulfite in UVC-S(IV)-O2 System for Arsenite Oxidation in Water

“…High sulfite concentration (2 mM) could induce an anaerobic environment in solution within an extremely short time, since SO 5 •− , SO 4 •− , HO• formation and self-oxidation of sulfites all consume oxygen [26,27]. Once the dissolved oxygen concentration dropped to a low level, reaction (3) could be a rate-controlling step in the chain reactions and hence influence the As(III) oxidation.…”

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

“…SO 4 In addition to this explanation, the transformation of DMPO by Fe(IV) (Figure S2) (Xu et al 2017) 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: (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.…”