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• Advanced oxidation processes (AOPs) derived from peroxydisulfate (PDS, S₂O₈²⁻) activation have attracted increasing interest in the elimination of recalcitrant organic contaminants due to their excellent oxidant capacity and adaptability [1-4]. Iron-based materials are among the most cost-effective and eco-friendly activators for PDS activation [5-7]. Initially, researchers recognized that Fe(Ⅱ) could provide one electron to PDS, and the SO₄^•− and ^•OH produced are the main reactive oxidant species (ROS) for pollutant removal in Fe(Ⅱ)/PDS system [8-11].

  However, Wang et al. [12, 13] proposed that the dominant ROS in Fe(Ⅱ)/PS system was Fe^ⅣO²⁺ (Eq. 1) rather than SO₄^•− and ^•OH using methyl phenyl sulfoxide (PMSO) as the Fe^ⅣO²⁺ probe. They discovered that the specific oxidation product of PMSO by Fe^ⅣO²⁺ is methyl phenyl sulfone (PMSO₂) (Eq. 2), and its yield (η(PMSO₂), i.e., molar quantities of PMSO₂ produced from the oxidation of per mole of PMSO) was approximately 100% [12, 14]. Speculating down this mechanism, two electrons were transferred from Fe(Ⅱ) to PDS while producing Fe^ⅣO²⁺ and sulfates [14, 15]. However, researchers recently found that many factors may interfere with removing target pollutants by Fe^ⅣO²⁺ in the Fe(Ⅱ)/PDS system. For instance, Dong et al. [15] found that the contribution of Fe^ⅣO²⁺ to the removal of organic contaminant depended on the steady concentrations of Fe^ⅣO²⁺ and the second-order rate constant of Fe^ⅣO²⁺ with the organic contaminant. Wang et al. [13] confirmed that the reactive intermediates in Fe(Ⅱ)/PDS system changed from Fe^ⅣO²⁺ to free radicals with the addition of chelating agents such as oxalate acid (OA), citric acid (CA), nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA). Li et al. [16] found that the common reducing agent hydroxylamine (HA) could coordinate rapidly with Fe(Ⅲ) or Fe(Ⅱ), and interfered with the production of Fe^ⅣO²⁺ from Fe(Ⅱ)-PDS intermediates. In Fe(Ⅱ)/PDS system, the steady-state concentration of Fe^ⅣO²⁺ is 4 or 5 orders of magnitudes higher than SO₄^•− and ^•OH. Taking into account the rapid scavenging of these ROS by Fe(Ⅱ) (Eqs. 3–5) [12, 17, 18], the variation of Fe(Ⅱ) concentration caused by HA addition might be an essential factor that interfered with the Fe^ⅣO²⁺ production in Fe(Ⅱ)/HA/PDS system. Nevertheless, in the study of Li and his coworkers [16], the Fe(Ⅱ) concentration was only 10 µmol/L, thus the interfering effect of Fe(Ⅱ) on Fe^ⅣO²⁺ production in Fe(Ⅱ)/HA/PDS and Fe(Ⅱ)/PDS systems should be revisited. In addition, the interference of various contaminants on Fe^ⅣO²⁺ production in Fe(Ⅱ)/HA/PDS and Fe(Ⅱ)/PDS systems also need to be investigated in depth. Apart from these factors, the possible effect of common anions such as Cl⁻, SO₄²⁻, NO₃⁻ and HCO₃⁻ on Fe^ⅣO²⁺ generation in Fe(Ⅱ)/PDS system are still unidentified.

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  Therefore, the main objectives are as follows: (1) Exploring the role of Fe(Ⅱ) concentration on Fe^ⅣO²⁺ production in Fe(Ⅱ)/HA/PDS and Fe(Ⅱ)/PDS systems, (2) investigating the influence of different target pollutants such as PMSO, Amoxicillin (AML), and sulfamethoxazole (SMX) on Fe^ⅣO²⁺ production in Fe(Ⅱ)/HA/PDS and Fe(Ⅱ)/PDS systems, (3) gaining insights into the possible effect of common anions on Fe^ⅣO²⁺ production in Fe(Ⅱ)/PDS system. This study aims to understand better the internal catalytic mechanism of Fe(Ⅱ)-activated PDS and reveal the probe method's possible drawbacks for the Fe^ⅣO²⁺ identification.

  Chemicals and reagents and analytical methods, are provided in Texts S1 and S2 in Supporting information. Batch experiments were performed in a 300 mL silica glass beaker containing a 200 mL aqueous solution, which was continuously stirred by a magnetic stirrer at approximately 250 rpm. Typically, the experiments were started by dosing 200 mL PMSO solution (0.1 mmol/L) to the beaker at 25 ± 1 ℃. The initial solution pH was adjusted to 3.0 using dilute NaOH and HClO₄ [15]. 1 mL of 10 mmol/L Fe(Ⅱ) and 1 mL of 0.1 mol/L HA stock solution were firstly added, then 1 mL of 0.1 mol/L PDS was added to initiate the reaction. The solution was periodically sampled and filtered through a 0.22 µm microporous membrane filter, and then a 2 mL sample was immediately quenched by 100 µL pure methanol. Finally, all samples were immediately analyzed. All experiments were performed in duplicate, and mean deviations were reported.

  Fig. 1 shows that PDS alone could not remove PMSO, which excluded the possibility that PDS directly oxidized PMSO to produce PMSO₂. Upon adding Fe(Ⅱ), rapid removal of PMSO (48.8%) was observed while producing 39.5 µmol/L PMSO₂. The calculated η(PMSO₂) was 80.8% in Fe(Ⅱ)/PDS system. As reported in a previous study [12], PMSO₂ was the individualized product of PMSO oxidization by Fe^ⅣO²⁺ in Fe(Ⅱ)/PDS system, indicating that the Fe^ⅣO²⁺ was the main ROS for PMSO elimination in Fe(Ⅱ)/PDS system. Notably, the removal of PMSO in our study was unsatisfactory (less than 50%). Fig. 2a reveals that only about 0.2 mmol/L PDS was consumed after 30 min, while Fe(Ⅱ) was wholly consumed, indicating that the further removal of PMSO was limited due to the lack of Fe(Ⅱ). Similar results [12-15] were found in previous studies where the PMSO removal was also undesirable and PDS consumption was incomplete (Table S1 in Supporting information). However, according to Eq. 2 [12], PMSO could reduce Fe^ⅣO²⁺ to Fe(Ⅱ), which should react with PDS and produce Fe^ⅣO²⁺ again (Scheme S1 in Supporting information). This cycle should lead to a more efficient elimination of PMSO in the Fe(Ⅱ)/PDS system if PDS were sufficient. This result could be assigned to the fact that Fe(Ⅱ) regenerated (Eq. 2) was re-oxidized by Fe^ⅣO²⁺ or free radicals (Eqs. 3–5), resulting in a lack of Fe(Ⅱ) in the Fe(Ⅱ)/PDS system and thus inhibited the PMSO removal.

  Figure 1

  

  Figure 1.  Effect of HA on PMSO removal, PMSO₂ production and the corresponding η(PMSO₂) in Fe(Ⅱ)/PDS system. Experimental condition: Initial pH 3.0, [PDS] = 0.5 mmol/L, [PMSO] = 0.1 mmol/L, [Fe(Ⅱ)] = 0.1 mmol/L.

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  Figure 2

  

  Figure 2.  Effect of HA on the Fe(Ⅱ) concentration (full line) and PDS (dotted line) in Fe(Ⅱ)/PDS system (a), EPR profiles of PDS, Fe(Ⅱ)/PDS and Fe(Ⅱ)/HA/PDS system (b). Experimental condition: Initial pH 3.0, [PDS] = 0.5 mmol/L, [PMSO] = 0.1 mmol/L, [Fe(Ⅱ)] = 0.1 mmol/L, [HA] = 2.0 mmol/L, [DMPO] = 0.1 mmol/L, (♦ represents ^•OH adduct and ♥ represents SO₄^•− adduct).

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  To verify this, we used different Fe(Ⅱ) concentrations to investigate the PMSO removal in Fe(Ⅱ)/PDS system. As shown in Fig. S1a (Supporting information), the PMSO removal significantly increased when the Fe(Ⅱ) concentration increased from 0.1 mmol/L to 0.5 mmol/L. However, PMSO removal was inhibited when the Fe(Ⅱ) concentration increased to 1.0 mmol/L. A similar result was also achieved in previous reports [5, 19], which suggested that Fe(Ⅱ) could promote the PDS activation and also scavenge ROS generated in Fe(Ⅱ)/PDS system when Fe(Ⅱ) was in excess. Interestingly, the trend of PMSO₂ generation was different from the PMSO removal (Fig. S1b in Supporting information), and the η(PMSO₂) gradually decreased with the increase in Fe(Ⅱ) concentration (Fig. S2 in Supporting information). The reason might be that in Fe(Ⅱ)/PDS system, The scavenging effect of Fe(Ⅱ) on Fe^ⅣO²⁺ was slightly higher than that of SO₄^•− and ^•OH. Thus, the interference of Fe(Ⅱ) on Fe^ⅣO²⁺ production was not apparent when the Fe(Ⅱ) concentration was much lower than that of PDS because most of Fe(Ⅱ) was consumed by PDS rather than ROS. However, in the presence of excess Fe(Ⅱ), the scavenging effect of Fe(Ⅱ) on ROS became more prominent, which significantly interfered with the Fe^ⅣO²⁺ production in Fe(Ⅱ)/PDS system.

  HA could accelerate the Fe(Ⅲ)/Fe(Ⅱ) cycle (Eq. 6) [4, 20] to enhance the PDS activation, but it also could scavenge some ROS generated in Fe(Ⅱ)/PDS system. At pH 3.0, HA was mainly present as NH₃OH⁺ [10], and the reaction rate constants of HA with Fe^ⅣO²⁺, SO₄^•− and ^•OH were approximately 5.0 × 10⁸ L mol⁻¹ s⁻¹, 1.5 × 10⁷ L mol⁻¹ s⁻¹, and 1.15 × 10⁸ L mol⁻¹ s⁻¹, respectively [10, 16]. Furthermore, considering the steady-state concentrations of SO₄^•−/^•OH (~10⁻¹³ mol/L) and Fe^ⅣO²⁺ (~10⁻⁸ mol/L) in the Fe(Ⅱ)/PDS system (Text S3 and Fig. S3 in Supporting information), HA would preferentially scavenge Fe^ⅣO²⁺. This inhibited the production of Fe^ⅣO²⁺ in the Fe(Ⅱ)/PDS system. However, the reaction rate constant of HA with Fe(Ⅲ) was about 2.8 × 10⁸ L mol⁻¹ s⁻¹ [4], so the presence of Fe(Ⅲ) would interfere with the scavenging effect of HA on these ROS.

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  Fig. 2a shows that in Fe(Ⅱ)/PDS system, almost all Fe(Ⅱ) was oxidized to Fe(Ⅲ) within 5 min, which limited the activation of PDS and resulted in a low PMSO removal. In the presence of 0.5 mmol/L HA, the Fe(Ⅱ) concentration was maintained at about 0.4 mmol/L. Sufficient Fe(Ⅱ) significantly enhanced the PDS activation and promoted ROS generation, ultimately increasing PMSO removal. However, PMSO₂ generation was suppressed, and η(PSMO₂) decreased to 44.2% compared to the Fe(Ⅱ)/PDS system (80.8%). Li et al. [16] proposed that Fe(Ⅱ) could form double coordination intermediates with PDS and HA and the decomposition of this intermediate transformed the major ROS from Fe^ⅣO²⁺ to SO₄^•− and ^•OH. However, HA could also rapidly coordinate with Fe(Ⅲ) and form Fe(Ⅲ)-HA coordination intermediates to produce Fe(Ⅱ) [4]. Taking into account the reaction rate of Fe(Ⅲ) with HA and the Fe(Ⅲ), Fe(Ⅱ) concentration in Fe(Ⅱ)/HA (0.5 mmol/L)/PDS system, HA readily coordinated with Fe(Ⅲ) rather than Fe(Ⅱ). Therefore, some other reasons might lead to the decrease of Fe^ⅣO²⁺ production and the increase of free radicals.

  Based on the steady-state concentrations and reaction rate constants of HA with Fe(Ⅲ), Fe^ⅣO²⁺, SO₄^•− and ^•OH, it could be inferred that HA reacted mainly with Fe(Ⅲ) and some Fe^ⅣO²⁺ in Fe(Ⅱ)/HA(0.5 mmol/L)/PDS system, while the scavenging effect of HA to SO₄^•− and ^•OH might be neglected. Therefore, although the total ROS could be elevated when 0.5 mmol/L HA was added to the Fe(Ⅱ)/PDS system, the molar ratio of Fe^ⅣO²⁺ to free radicals decreased. These results led to increased PMSO removal but a significant decrease in η(PSMO₂).

  The above point could be further demonstrated by introducing an excess of HA (2.0 mmol/L) in the Fe(Ⅱ)/PDS system. As shown in Fig. 1, compared to the Fe(Ⅱ)/HA(0.5 mmol/L)/PDS system, the PMSO removal decreased from 74.4% to 61.3% and η(PMSO₂) further decreased to 23.7% in Fe(Ⅱ)/HA(2.0 mmol/L)/PDS system. Simultaneously, Fe(Ⅱ) concentration was close to the initial concentration throughout the reaction (Fig. 2a), indicating that the generated Fe(Ⅲ) and Fe^ⅣO²⁺ were immediately reduced to Fe(Ⅱ) by the excess HA. Theoretically, the excess HA could completely reduce Fe(Ⅲ) and Fe^ⅣO²⁺ to Fe(Ⅱ), but only 23.7% PMSO was oxidized by Fe^ⅣO²⁺. The reason was that sufficient Fe(Ⅱ) enhanced the PDS activation, promoting Fe(Ⅲ) and ROS generation rates. Hence, although most of Fe^ⅣO²⁺ was scavenged by HA and Fe(Ⅱ), some Fe^ⅣO²⁺ was still involved in the oxidation process of PMSO. Moreover, since the Fe(Ⅱ)/HA(2.0 mmol/L)/PDS system had a much higher Fe(Ⅱ) concentration, HA would coordinate with Fe(Ⅱ)-PDS intermediates, which not only enhanced the SO₄^•− and ^•OH generation [16] (as also evidenced by the results of electron paramagnetic resonance (EPR) experiments (Fig. 2b), but also inhibited its effect on Fe^ⅣO²⁺ scavenging. As a result, both PMSO removal and η(PMSO₂) decreased compared to the Fe(Ⅱ)/HA(0.5 mmol/L)PDS system.

  Dong et al. [15] found that the contribution of Fe^ⅣO²⁺ to the elimination of organic contaminant depended on the steady concentrations of Fe^ⅣO²⁺ and the second-order rate constant of Fe^ⅣO²⁺ with the organic contaminant. To assess whether this phenomenon also existed in Fe(Ⅱ)/HA/PDS system, we selected some organic contaminants, including amoxicillin (AML) and sulfamethoxazole (SMX) (Fig. 3). As shown in Fig. 3, AML and SMX were hardly degraded by PDS alone, while with the addition of Fe(Ⅱ), their removal reached 88.3% and 28.3%, respectively. The above result indicates that Fe(Ⅱ) reacted with PDS and produced ROS, which degraded these contaminants. Notably, the addition of HA slightly inhibited the removal of AML, but significantly enhanced SMX degradation. The oxidative capacity of Fe^ⅣO²⁺ (E(Fe^ⅣO²⁺/Fe(Ⅲ)) = 2.0 V/SHE) was lower than that of SO₄^•− (E(SO₄^•−/SO₄²⁻) = 2.5–3.1 V/SHE) and ^•OH (E⁰(^•OH/H₂O) = 2.8 V/SHE) [5, 21, 22]. For organic contaminants containing electron-rich moieties such as AML [3], Fe^ⅣO²⁺ was more likely to be the main ROS in their degradation [15]. This was because the electron-rich group could reduce Fe^ⅣO²⁺ to Fe(Ⅱ) (such as PMSO), which favored the Fe^ⅣO²⁺ production in Fe(Ⅱ)/PDS system (Le Chatelier's principle). In Fe(Ⅱ)/HA/PDS system, the source of Fe(Ⅱ) regeneration would change from the Fe^ⅣO²⁺/AML reaction to the Fe^ⅣO²⁺/HA and Fe(Ⅲ)/HA reactions. Therefore, the Fe^ⅣO²⁺ production would decrease while the production of SO₄^•− and ^•OH increased. Besides, although the addition of HA accelerated the Fe(Ⅱ) regeneration and the PDS activation, the scavenging of ROS by HA and Fe(Ⅱ) was also enhanced, which further led to a slight decrease in the removal rate of AML.

  Figure 3

  

  Figure 3.  Effect of excess p-CBA on the removal of AML (a–c) and SMX (d–f) in Fe(Ⅱ)/PDS and Fe(Ⅱ)/PDS/HA systems. Experimental condition: Initial pH 3.0, [PDS] = 0.5 mmol/L, [Fe(Ⅱ)] = 0.1 mmol/L, [AML]=[SMX] = 0.01 mmol/L, [p-CBA] = 0.25 mmol/L.

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  For SMX, its removal in Fe(Ⅱ)/PDS system was much lower than that of AML, indicating that the Fe(Ⅱ) regeneration from the pathway of Fe^ⅣO²⁺/SMX reaction was not obvious. Hence, it could be inferred that the reduced ability of SMX was lower than AML, and the contribution of SO₄^•− and ^•OH to the removal of SMX was higher than that of AML. In Fe(Ⅱ)/HA(0.5 mmol/L)/PDS system, the Fe(Ⅱ) regeneration was accelerated, which favored PDS activation and ROS generation. As discussed above, the Fe^ⅣO²⁺ production decreased, while the rate of SO₄^•− and ^•OH production increased. However, SMX removal was not suppressed the Fe(Ⅱ)/HA(2.0 mmol/L)/PDS system, Similar to the case of PMSO, probably because SO₄^•− and ^•OH played a more critical role in SMX removal in Fe(Ⅱ)/HA/PDS system, while the scavenging of SO₄^•− and ^•OH by HA was lower than that by Fe^ⅣO²⁺.

  To further verify the above findings, the contributions of Fe^ⅣO²⁺ and free radicals to the removal of the contaminants in the Fe(Ⅱ)/PDS process and Fe(Ⅱ)/HA/PDS systems was evaluated, and the corresponding scavenging experiments were conducted (Fig. 3). Dong et al. [15] found that p-chlorobenzoic acid (p-CBA) was readily oxidized by SO₄^•− and ^•OH, whereas the reaction between Fe^ⅣO²⁺ and p-CBA was negligible. Hence, the inhibition of p-CBA to the removal of AML and SMX was ascribed to its scavenging effect on SO₄^•− and ^•OH, i.e., p-CBA could be utilized to identify the contributions of Fe^ⅣO²⁺ to the AML and SMX removals by advanced oxidation processes. As shown in Fig. 2, the degradation efficiency of AML in Fe(Ⅱ)/PDS system was almost unaffected by the excess p-CBA, indicating that the contributions of Fe^ⅣO²⁺ to AML removal in Fe(Ⅱ)/PDS system was approximately 100%. However, in the presence of HA, p-CBA showed a significant inhibitory effect on AML removal in Fe(Ⅱ)/PDS system. Specifically, with the addition of p-CBA, the AML removal decreased from 86.3% to 65.8% in Fe(Ⅱ)/PDS/HA (0.5 mmol/L) process, so the contributions of Fe^ⅣO²⁺ to AML removal could be calculated to be 65.8%/86.3%, i.e., 76.2%. Based on a similar method, the contributions of Fe^ⅣO²⁺ to AML removal in Fe(Ⅱ)/PDS/HA (2.0 mmol/L) process was 32.0%, and the contributions of Fe^ⅣO²⁺ to SMX removal in Fe(Ⅱ)/PDS, Fe(Ⅱ)/PDS/HA (0.5 mmol/L), and Fe(Ⅱ)/PDS/HA (2.0 mmol/L) processes were 71.7%, 39.4% and 38.9%, respectively (Figs. 3e and f).

  Furthermore, inorganic salt ions, such as NO₃⁻, SO₄²⁻, HCO₃⁻ and Cl⁻, are often present in natural water and wastewater. Thus, we also investigated their possible interference on Fe^ⅣO²⁺ production in Fe(Ⅱ)/PDS systems. As shown in Fig. S4 (Supporting information), NO₃⁻ nearly did not affect the PMSO removal and η(PMSO₂), indicating that NO₃⁻ did not interfere with the production of Fe^ⅣO²⁺ in the Fe(Ⅱ)/PDS system. When SO₄²⁻ concentration was more than 2.0 mmol/L, the PMSO removal in Fe(Ⅱ)/PDS system was slightly suppressed, but the η(PMSO₂) was not affected. The reason was that SO₄²⁻ could be complex with Fe(Ⅱ) to form ion pairs (FeSO₄) which might reduce the electron transfer efficiency of Fe(Ⅱ) in slightly acidic solutions [23, 24], eventually inhibiting the PDS activation. When HCO₃⁻ concentration in Fe(Ⅱ)/PDS system exceeded 2.0 mmol/L, the PMSO removal was significantly inhibited. However, the η(PMSO₂) did not change much. The reason might be that the scavenging effect of HCO₃⁻ on Fe^ⅣO²⁺ produced in the Fe(Ⅱ)/PDS system was negligible, and hence, the PMSO₂ production from the oxidation of PMSO by Fe^ⅣO²⁺ was almost unaffected by the addition of HCO₃⁻; However, HCO₃⁻ could rapidly interact with Fe(Ⅱ) and Fe(Ⅲ) to form bicarbonate-iron complexes, reducing the soluble metal ions available for PDS activation [25], in addition, HCO₃⁻ could significantly increase the solution pH due to its inherent basic and buffering characteristics (Fig. S5 in Supporting information). All those above lead to a decrease in Fe^ⅣO²⁺ production, which ultimately inhibited the oxidation of PMSO. Surprisingly, in Fe(Ⅱ)/PDS system, both PMSO removal and η(PMSO₂) gradually decreased with the increase in Cl⁻ concentration (Fig. S4d). Li et al. [26] found that Cl⁻ could facilitate the shift of ROS from Fe^ⅣO²⁺ to free radicals (SO₄^•− or ^•OH) in the Fe(Ⅱ)/PMS system, meanwhile, Cl⁻ could rapidly scavenge SO₄^•− (k = 2.7 × 10⁸ L mol⁻¹ s⁻¹) [26] and ^•OH (k = 7.8 × 10⁹ L mol⁻¹ s⁻¹) [27] to produce a less oxidizing Cl^•. In addition, to evaluate the degradation of pollutants in the actual wastewater, we used the Xiangjiang River water (Hunan Province) as the target water source. The characteristics of Xiangjiang River water are listed in Table S2 (Supporting information). As shown in Fig. S6 (Supporting information), the degradation of pollutants was slightly inhibited in Xiangjiang River water samples, which might be due to the high concentrations of total organic carbon and HCO₃⁻ in the actual water body [28].

  In summary, in this study, we found that the Fe(Ⅱ) concentration in Fe(Ⅱ)/PDS system could slightly interfere with the Fe^ⅣO²⁺ production and thus affect the contribution of Fe^ⅣO²⁺ to the target pollutants. In Fe(Ⅱ)/HA/PDS system, the molar ratio of Fe(Ⅱ) to Fe(Ⅲ) dominated the order of HA coordination. HA was more likely to coordinate with Fe(Ⅲ) than Fe(Ⅱ). The scavenging effect of Fe(Ⅱ) and HA was higher than that of SO₄^•− and ^•OH. Thus, the contribution of Fe^ⅣO²⁺ to the target pollutant was reduced in Fe(Ⅱ)/HA/PDS system. In addition, the contributions of Fe^ⅣO²⁺ to contaminant removal in the Fe(Ⅱ)/HA/PDS system were closely related to the target contaminant species, and the use of PMSO as a probe to estimate the contribution of ROS to pollutant removal should be handled with caution. The contribution of Fe^ⅣO²⁺ to the elimination of target pollutants might be overestimated in the case where the reduction potential of organic pollutants was weaker than that of PMSO. Moreover, we found that although NO₃⁻, SO₄²⁻ and HCO₃⁻ hardly affected the contribution of Fe^ⅣO²⁺ to the removal of the target pollutants in the Fe(Ⅱ)/PDS system. Cl⁻ could facilitate the transfer of ROS from Fe^ⅣO²⁺ to free radicals (SO₄^•− or ^•OH) in Fe(Ⅱ)/PDS system. Therefore, new probe reagents or methods should be created as soon as possible to more precisely estimate the contribution of Fe^ⅣO²⁺ to target contaminant removals in the Fe(Ⅱ)/PS system. Finally, additional quenching experiments, EPR analyses, or other verification methods should be combined to quantify ROS contributions to organic pollutants degradation better.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This research was financially supported by the National Natural Science Foundation of China (Nos. 51779088, 51908528), the Fundamental Research Funds for the Central Universities (No. 2021CDJQY-014), the Natural Science Foundation of Hunan Province, China (No. 2021JJ30126).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.069.

  
  
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  Figure 1  Effect of HA on PMSO removal, PMSO₂ production and the corresponding η(PMSO₂) in Fe(Ⅱ)/PDS system. Experimental condition: Initial pH 3.0, [PDS] = 0.5 mmol/L, [PMSO] = 0.1 mmol/L, [Fe(Ⅱ)] = 0.1 mmol/L.

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  Figure 2  Effect of HA on the Fe(Ⅱ) concentration (full line) and PDS (dotted line) in Fe(Ⅱ)/PDS system (a), EPR profiles of PDS, Fe(Ⅱ)/PDS and Fe(Ⅱ)/HA/PDS system (b). Experimental condition: Initial pH 3.0, [PDS] = 0.5 mmol/L, [PMSO] = 0.1 mmol/L, [Fe(Ⅱ)] = 0.1 mmol/L, [HA] = 2.0 mmol/L, [DMPO] = 0.1 mmol/L, (♦ represents ^•OH adduct and ♥ represents SO₄^•− adduct).

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  Figure 3  Effect of excess p-CBA on the removal of AML (a–c) and SMX (d–f) in Fe(Ⅱ)/PDS and Fe(Ⅱ)/PDS/HA systems. Experimental condition: Initial pH 3.0, [PDS] = 0.5 mmol/L, [Fe(Ⅱ)] = 0.1 mmol/L, [AML]=[SMX] = 0.01 mmol/L, [p-CBA] = 0.25 mmol/L.

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