Systematic Performance Comparison of Fe3+/Fe0/Peroxymonosulfate and Fe3+/Fe0/Peroxydisulfate Systems for Organics Removal

Activated zero-valent iron (Ac-ZVI) coupled with Fe3+ was employed to activate peroxymonosulfate (PMS) and peroxydisulfate (PDS) for acid orange 7 (AO7) removal. Fe3+ was used to promote Fe2+ liberation from Ac-ZVI as an active species for reactive oxygen species (ROS) generation. The factors affecting AO7 degradation, namely, the Ac-ZVI:Fe3+ ratio, PMS/PDS dosage, and pH, were compared. In both PMS and PDS systems, the AO7 degradation rate increased gradually with increasing Fe3+ concentration at fixed Ac-ZVI loading due to the Fe3+-promoted liberation of Fe2+ from Ac-ZVI. The AO7 degradation rate increased with increasing PMS/PDS dosage due to the greater amount of ROS generated. The degradation rate in the PDS system decreased while the degradation rate in the PMS system increased with increasing pH due to the difference in the PDS and PMS activation mechanisms. On the basis of the radical scavenging study, sulfate radical was identified as the dominant ROS in both systems. The physicochemical properties of pristine and used Ac-ZVI were characterized, indicating that the used Ac-ZVI had an increased BET specific surface area due to the formation of Fe2O3 nanoparticles during PMS/PDS activation. Nevertheless, both systems displayed good reusability and stability for at least three cycles, indicating that the systems are promising for pollutant removal.


Introduction
Redox oxidation technology involving catalytic persulfate activation has gained significant attention as a promising method to remove anthropogenic pollutants from water. To date, this technology has been successfully used to remove various anthropogenic pollutants in water, including pharmaceuticals, personal care products, industrial waste, and dyes [1][2][3][4]. The two most commonly used persulfates are peroxymonosulfate (PMS) and peroxydisulfate (PDS) [5]. Both PMS and PDS consist of a peroxide bond that can be activated by a catalyst through an electron transfer reaction to produce reactive oxygen species (ROS), such as SO 4 •and • OH. These ROS have high oxidation potential (E • vs. NHE for SO 4 •and • OH are 2.5-3.2 and 1.8-2.7 V, respectively [6][7][8]) and can be employed to degrade organic pollutants in water.
Zero-valent iron (Fe 0 , ZVI) is regarded as one of the most versatile catalysts in environmental catalysis [9]. It is relatively efficient, cost-effective, and environmentally friendly [10]. ZVI has been widely used for many applications, including as a catalyst for Fenton oxidation [11] and sequestration [12]. Previously, ZVI has been employed to activate both PMS and PDS to generate ROS for pollutant removal. For instance, Hussain et al. [13] reported that the ZVI/PDS system can be used to generate SO 4 •and • OH for efficient oxidation of arsenic(III) in water, while Gu et al. [14] reported that efficient removal of 1,1,1-trichloroethane can be achieved using a similar system. For the PMS system, Ghanbari et al. [15] reported that the ZVI/PMS system can be used to efficiently decolorize textile wastewater. These results proved that ZVI has a practical application in environmental remediation.
In general, ZVI activates PDS through sequential steps involving (i) corrosion to produce Fe 2+ species, and (ii) PDS activation by Fe 2+ to produce SO 4 •− (Equations (1) and (2)) [9,16]. ZVI can activate PMS via two pathways, namely, corrosion followed by PMS activation (similar to PDS), and direct reaction of ZVI and PMS to produce SO 4 •− and • OH (Equations (3)- (6)) [ In both cases, the Fe 2+ species appears to serve as the active species for PMS/PDS activation, suggesting that increasing the Fe 2+ species can improve the performance of the catalytic system. It is hypothesized that the addition of Fe 3+ species into the catalytic ZVI system can lead to the increase in Fe 2+ species (Equation (7)) and improve the pollutant degradation [9]: Fe 3+ addition can also reduce the unproductive consumption of PDS/PMS for Fe 2+ generation from ZVI. Currently, an assessment of the impact of Fe 3+ addition into the catalytic ZVI system is not available. Direct comparisons of the performance of ZVI as a PDS and PMS activator are also relatively limited.
In this study, the performance of ZVI coupled with Fe 3+ as a PMS and PDS activator for acid orange 7 (AO7) removal was compared. The effects of the ZVI:Fe 3+ ratio, PDS/PMS dosage, and initial pH on AO7 were systematically evaluated. The main ROS generated from the PMS and PDS systems was also identified. Finally, the extent of mineralization and the reusability of the PMS and PDS systems were evaluated.

Performance Study
All the performance studies were conducted using a 250 mL reaction vessel in batches. Typically, a 100 mL solution consisting of 8.5 mg L −1 of AO7 and PMS/PDS (at selected concentrations) was prepared and agitated rapidly using a magnetic stirrer. Then, a known amount of Fe 3+ and Ac-ZVI was added into the reactor to start the reaction. At the selected time interval, sampling was conducted by collecting 3 mL of the solution in the reactor. The AO7 concentration in the collected sample was analyzed using a UV-Vis spectrophotometer (Hitachi U-2000) at λ max = 485 nm. Control studies consisting of only AO7 + PMS, AO7 + PMS + Fe 3+ or Ac-ZVI, and AO7 only were also conducted. The effects of different Fe 3+ concentration (0-20 mg L −1 ), Ac-ZVI dosage (1-3 g L −1 ), pH (3)(4)(5)(6)(7)(8)(9), and PDS/PMS:AO7 mol ratio (10:1-100:1) on AO7 degradation were studied. The pH was adjusted using diluted HCl and NaOH. The dominant reactive oxygen species (ROS) was identified using chemical scavengers, namely, ethanol (3 M) and nitrobenzene (70 mg L −1 ). During the performance study at the selected condition, ethanol or nitrobenzene was added to inhibit SO 4 •− and • OH, and • OH, respectively. Similarly, at the specific condition, the total organic carbon (TOC), Fe concentration, and extent of reusability of the catalytic system were also determined. Meanwhile, the PMS/PDS concentration was determined using the iodometric method as described previously [18]. The TOC was determined using a TOC analyzer (TOC-L, Shimadzu), while the Fe concentration was quantified using an Atomic Absorption Spectrometer (PerkinElmer AAnalyst 400). The extent of reusability was evaluated by conducting the performance study using the same Ac-ZVI over several cycles. The experiments were conducted in duplicate.

Characterization Study
The characteristics of Ac-ZVI and the used Ac-ZVI in both PMS and PDS systems were investigated using various advanced instruments. The crystal structure of the catalysts was determined using the X-ray diffraction (XRD, Bruker AXS D8 Advance with Cu-Kα source of λ = 1.5418 Å and scan rate of 0.02 • s −1 ) method, while the surface functional groups were determined using a Fourier transform infrared (FTIR, Frontier, PerkinElmer, Neuchatel, Switzerland) spectrometer. The Brunauer-Emmett-Teller (BET) specific surface area of the catalysts was determined from the nitrogen adsorption-desorption isotherm obtained using a porosimeter (ASAP 2020, NSW 2229, Australia). The morphology of the Ac-ZVI and the used Ac-ZVI was studied using a scanning electron microscope (SEM, Quanta FEG 650, London, UK).

Effect of Fe 3+ Concentration
Several control studies involving AO7 removal (a) in the presence of Fe 3+ only, (b) without a catalyst, and (c) without PMS/PDS) were conducted, indicating that Fe 3+ alone cannot activate PMS/PDS effectively (<10% AO7 removed in 60 min). Direct PMS/PDS oxidation and adsorption by Ac-ZVI had no significant impact on AO7 removal (<9% AO7 removed in 60 min). The AO7 degradation in the PMS and PDS systems at various time intervals was monitored by varying the Fe 3+ concentration at fixed Ac-ZVI loading, and the results were fitted into the pseudo first-order kinetics: where [AO7] t and [AO7] o are the AO7 concentration at various time intervals and the initial AO7 concentration, respectively; and k app is the pseudo first-order rate constant. The relationship between the calculated k app value and the Fe 3+ concentration at various Ac-ZVI loadings in the PMS and PDS systems are shown in Figure 1a,b, respectively. In general, the PMS and PDS systems followed a similar trend where, without Fe 3+ addition, the k app value increased gradually with increasing Ac-ZVI loading due to greater catalytic active sites available for PMS/PDS activation [19]. At 1.0 g L −1 Ac-ZVI, the k app value increased with increasing Fe 3+ concentration from 0 to 20 mg L −1 for both PMS and PDS systems. The addition of Fe 3+ promoted the liberation of Fe 2+ from the Ac-ZVI through Equation (7), which is beneficial for PMS/PDS activation [9]. However, the quantum of increase diminished with increasing Fe 3+ for both PMS and PDS systems, and this can be attributed to two factors. First, the higher Fe 3+ concentration leads to a rapid increase in Fe 2+ where excessive Fe 2+ can also act undesirably as an ROS scavenger, reducing the productive consumption of ROS for AO7 removal (Equations (8) and (9)).
Second, excessive Fe 3+ can compete with other Fe species for reaction with PS and PMS (Equations (10) and (11)), leading to the depletion of available oxidant for SO 4 •− generation.
Not surprisingly, these effects were more pronounced at higher Ac-ZVI loading, particularly at 3.0 g L −1 where a 4-9% decrease in k app value was observed in the presence of 20 mg L −1 Fe 3+ compared to that without Fe 3+ addition attributed to the uncontrolled liberation of Fe 2+ . The results indicate that an optimum Fe 3+ concentration must be maintained to effectively improve the catalytic performance in both systems. PMS and PDS systems followed a similar trend where, without Fe 3+ addition, the kapp value increased gradually with increasing Ac-ZVI loading due to greater catalytic active sites available for PMS/PDS activation [19]. At 1.0 g L −1 Ac-ZVI, the kapp value increased with increasing Fe 3+ concentration from 0 to 20 mg L −1 for both PMS and PDS systems. The addition of Fe 3+ promoted the liberation of Fe 2+ from the Ac-ZVI through Equation (7), which is beneficial for PMS/PDS activation [9]. However, the quantum of increase diminished with increasing Fe 3+ for both PMS and PDS systems, and this can be attributed to two factors. First, the higher Fe 3+ concentration leads to a rapid increase in Fe 2+ where excessive Fe 2+ can also act undesirably as an ROS scavenger, reducing the productive consumption of ROS for AO7 removal (Equations (8) and (9)).
Second, excessive Fe 3+ can compete with other Fe species for reaction with PS and PMS (Equations (10) and (11)), leading to the depletion of available oxidant for SO4 •− generation.
Not surprisingly, these effects were more pronounced at higher Ac-ZVI loading, particularly at 3.0 g L −1 where a 4-9% decrease in kapp value was observed in the presence of 20 mg L −1 Fe 3+ compared to that without Fe 3+ addition attributed to the uncontrolled liberation of Fe 2+ . The results indicate that an optimum Fe 3+ concentration must be maintained to effectively improve the catalytic performance in both systems.  To compare the effect of Fe 3+ on the PMS and PDS systems, the k app value was further expanded to include the specific rate constant (k sp ) as a function of [Fe 3+ ] and [Ac-ZVI] (Equation (12)). where n is the order of reaction for Fe 3+ . The reaction order of [Ac-ZVI] was set at 0.5 based on the preliminary fitting where the reaction order of 0.5 provided a reasonably good fit. To avoid error due to too many parameters, the reaction order of [Ac-ZVI] was fixed at 0.5. The kinetic fittings (Figure 1c,d) revealed that the n values are 0 and 0.036 for the PMS and PDS systems, respectively, while their corresponding k sp values are 0.024 and 0.042, respectively. The relatively lower n value in the PMS system indicates that the Fe 3+ did not significantly improve the PMS system compared to the PDS system. Comparison of the k sp values showed that the Ac-ZVI/Fe 3+ system is more efficient in activating PDS than PMS. The trends in the n and k sp values are reasonable based on the possible activation pathways of PDS and PMS. In the PDS system, only one pathway exists, where the ROS generation must be preceded by a rate-limiting step involving Ac-ZVI corrosion and Fe 2+ dissolution from the Ac-ZVI surface [20]. In the PMS system, the ROS generation can occur via two pathways, namely, through a similar pathway and directly from the reaction between PMS and Ac-ZVI. Consequently, Fe 2+ , as an active species, is more important for the PDS system than the PMS system. The schematic illustration of the reaction pathway is presented in Figure 2. However, despite more reaction pathways being available in the PMS system, PDS activation is superior due to (i) the higher redox potential of PDS To compare the effect of Fe 3+ on the PMS and PDS systems, the kapp value was further expanded to include the specific rate constant (ksp) as a function of [Fe 3+ ] and [Ac-ZVI] (Equation (12)).
where n is the order of reaction for Fe 3+ . The reaction order of [Ac-ZVI] was set at 0.5 based on the preliminary fitting where the reaction order of 0.5 provided a reasonably good fit. To avoid error due to too many parameters, the reaction order of [Ac-ZVI] was fixed at 0.5. The kinetic fittings (Figure 1c,d) revealed that the n values are 0 and 0.036 for the PMS and PDS systems, respectively, while their corresponding ksp values are 0.024 and 0.042, respectively. The relatively lower n value in the PMS system indicates that the Fe 3+ did not significantly improve the PMS system compared to the PDS system. Comparison of the ksp values showed that the Ac-ZVI/Fe 3+ system is more efficient in activating PDS than PMS. The trends in the n and ksp values are reasonable based on the possible activation pathways of PDS and PMS. In the PDS system, only one pathway exists, where the ROS generation must be preceded by a rate-limiting step involving Ac-ZVI corrosion and Fe 2+ dissolution from the Ac-ZVI surface [20]. In the PMS system, the ROS generation can occur via two pathways, namely, through a similar pathway and directly from the reaction between PMS and Ac-ZVI. Consequently, Fe 2+ , as an active species, is more important for the PDS system than the PMS system. The schematic illustration of the reaction pathway is presented in Figure 2. However, despite more reaction pathways being available in the PMS system, PDS activation is superior due to (i) the higher redox potential of PDS (E° = 2.01 V vs. NHE) compared to PMS (E° = 1.92 V vs. NHE), and (ii) the single pathway allows more controlled Fe 2+ liberation, promoting productive Fe 2+ consumption.

Effect of PMS/PDS Dosage
The effect of PMS and PDS dosage was investigated at the optimum Fe 3+ :Ac-ZVI ratio. Figure 3a,b shows the effect of PMS/PDS dosage on AO7 degradation. Apparently, increasing the AO7:PMS ratio from 1:20 to 1:60 did not result in a significant change in the AO7 degradation rate (kapp = 0.030-0.037 min −1 ). Further increase in the AO7:PMS ratio from 1:60 to 1:100 resulted in a minor decrease in kapp to 0.025 min −1 due to the increased probability of the scavenging effect and competition reactions involving PMS and generated ROS to produce weaker ROS (Equations (13)-(17)) [21].

Effect of PMS/PDS Dosage
The effect of PMS and PDS dosage was investigated at the optimum Fe 3+ :Ac-ZVI ratio. Figure 3a,b shows the effect of PMS/PDS dosage on AO7 degradation. Apparently, increasing the AO7:PMS ratio from 1:20 to 1:60 did not result in a significant change in the AO7 degradation rate (k app = 0.030-0.037 min −1 ). Further increase in the AO7:PMS ratio from 1:60 to 1:100 resulted in a minor decrease in k app to 0.025 min −1 due to the increased probability of the scavenging effect and competition reactions involving PMS and generated ROS to produce weaker ROS (Equations (13)-(17)) [21].

Identification of Dominant Radical
The dominant radical was identified using radical scavengers, namely ethanol and nitrobenzene, to scavenge the selected ROS. The concentration of chemical scavengers used was selected based on their reactivity (estimated from the second-order rate constant) with their respective SO4 •− and • OH. Ethanol can be used to scavenge both SO4 •− (second-order rate constant,  [27]. At 3 M ethanol, it is sufficient to scavenge all the generated radicals, while at 70 mg L −1 nitrobenzene, it is sufficient to scavenge all the • OH without affecting AO7 degradation due to SO4 •− . Figure 3d shows the percentage of inhibition of AO7 degradation (calculated based on the kapp) by ethanol and nitrobenzene in the PDS and PMS systems. In both systems, 70-80% of the degradation rate was inhibited by ethanol, while only <20% of the degradation rate was inhibited by nitrobenzene, indicating that both SO4 •− and • OH contributed to the degradation, with SO4 •− playing the dominant species. It should be noted that the results are strictly pHdependent since the distribution of ROS can change with pH. For instance, SO4 •− can be converted to • OH in the presence of H2O and HO − [9,28].

Recyclability, Extent of Mineralization, and Post Mortem Catalyst Analysis
The reusability of the Ac-ZVI/Fe 3+ in both PMS and PDS systems is presented in Figure 4, indicating that the Ac-ZVI/Fe 3+ system can be reused for at least three cycles without significant loss in its catalytic efficiency (>95% AO7 removed in 30 min). It was also observed that the Fe leaching was only 0.35-0.36% from the Ac-ZVI, indicating that the system is stable and can be used for continuous AO7 degradation. The TOC removal efficiency for PMS and PDS systems was 51% and 43%, respectively, and the PMS/PDS consumption was 54% ± 6% after 4 h, indicating that mineralization of the AO7 molecules into innocuous compounds, such as CO2 and H2O, can occur efficiently. Since the residual PMS/PDS was still present after 4 h, higher mineralization efficiency can still be achieved by increasing the reaction time. A brief comparison (Table 1) of the results obtained in this study using the Ac-ZVI/Fe 3+ system with other catalytic systems for AO7 removal revealed In the PDS system, increasing the AO7:PDS ratio from 1:10 to 1:100 led to a gradual increase in the k app value from 0.034 to 0.079 min −1 . At a higher AO7:PDS ratio, the greater PDS concentration can interact more efficiently with Ac-ZVI to generate more Fe 2+ for PDS activation and, subsequently, ROS generation [22]. Comparison of the PMS and PDS systems showed that the oxidant dosage had a greater influence in the PDS system, and again, this is attributed to the difference in their activation mechanisms. Because the PDS activation involves only one pathway, the ROS production is more controlled compared to the PMS system and is limited by the PDS dosage. In the PMS system, the reactions involved are more direct and ROS production is limited by the available Ac-ZVI surface area. Direct ROS generation may produce excessive ROS, resulting in an increase in the scavenging effect and competition reactions (Equations (13)-(17)).

Effect of pH
The pH of the solution plays a critical role in influencing the performance of the system. As indicated in Figure 3c, the k app value in the PDS system decreased from 0.063 to 0.007 min −1 with increasing pH from 3 to 7. At acidic pH, the ZVI corrosion rate is higher, leading to increased Fe 2+ liberation and ROS generation [23]. As pH increases to 7, the ZVI corrosion rate decreases (less Fe 2+ liberation), inhibiting the ROS generation rate. The pH change after the reaction for all pHs was within the ±2 limit. The generated Fe 2+ was also precipitated at pH > 6 as Fe(OH) 2 , further restricting Fe 2+ from participating in the PDS activation reaction [24]. However, as the initial pH was increased from 7 to 9, the k app value increased to 0.02 min −1 , which could be due to the base activation of PDS [25,26]. Unlike the PDS system, the k app value for the PMS system increased from 0.037 at pH 3 to 0.055-0.065 min −1 at pH > 5. As indicated above, the PMS activation depends on the Ac-ZVI surface area, and its degradation is less affected by the Ac-ZVI corrosion rate and Fe 2+ precipitation. Hence, a reduced corrosion rate will not significantly influence the performance of the PMS system. This suggests that the PMS system is less influenced by the pH change.

Identification of Dominant Radical
The dominant radical was identified using radical scavengers, namely ethanol and nitrobenzene, to scavenge the selected ROS. The concentration of chemical scavengers used was Materials 2021, 14, 5284 7 of 11 selected based on their reactivity (estimated from the second-order rate constant) with their respective SO 4 •− and • OH. Ethanol can be used to scavenge both SO 4 •− (second-order rate constant, k SO •-4 +Ethanol = 1.6-7.7 × 10 7 M −1 s −1 ) and • OH (k HO • +Ethanol = 1.2-2.8 × 10 9 M −1 s −1 ), while nitrobenzene can scavenge • OH (k HO • +nitrobenzene = 3.9 × 10 9 M −1 s −1 ) but not SO 4 •− (k SO •-4 +nitrobenzene ≤ 10 6 M −1 s −1 ) [27]. At 3 M ethanol, it is sufficient to scavenge all the generated radicals, while at 70 mg L −1 nitrobenzene, it is sufficient to scavenge all the • OH without affecting AO7 degradation due to SO 4 •− . Figure 3d shows the percentage of inhibition of AO7 degradation (calculated based on the k app ) by ethanol and nitrobenzene in the PDS and PMS systems. In both systems, 70-80% of the degradation rate was inhibited by ethanol, while only <20% of the degradation rate was inhibited by nitrobenzene, indicating that both SO 4 •− and • OH contributed to the degradation, with SO 4 •− playing the dominant species. It should be noted that the results are strictly pH-dependent since the distribution of ROS can change with pH. For instance, SO 4 •− can be converted to • OH in the presence of H 2 O and HO − [9,28].

Recyclability, Extent of Mineralization, and Post Mortem Catalyst Analysis
The reusability of the Ac-ZVI/Fe 3+ in both PMS and PDS systems is presented in Figure 4, indicating that the Ac-ZVI/Fe 3+ system can be reused for at least three cycles without significant loss in its catalytic efficiency (>95% AO7 removed in 30 min). It was also observed that the Fe leaching was only 0.35-0.36% from the Ac-ZVI, indicating that the system is stable and can be used for continuous AO7 degradation. The TOC removal efficiency for PMS and PDS systems was 51% and 43%, respectively, and the PMS/PDS consumption was 54% ± 6% after 4 h, indicating that mineralization of the AO7 molecules into innocuous compounds, such as CO 2 and H 2 O, can occur efficiently. Since the residual PMS/PDS was still present after 4 h, higher mineralization efficiency can still be achieved by increasing the reaction time. A brief comparison (Table 1) of the results obtained in this study using the Ac-ZVI/Fe 3+ system with other catalytic systems for AO7 removal revealed that the Ac-ZVI/Fe 3+ system can provide a comparable or better treatment efficiency. This indicates that the Ac-ZVI/Fe 3+ system has promising potential as a PMS/PDS activator for environmental applications. that the Ac-ZVI/Fe 3+ system can provide a comparable or better treatment efficiency. This indicates that the Ac-ZVI/Fe 3+ system has promising potential as a PMS/PDS activator for environmental applications.  CuO PMS >85% of 20 mg L −1 AO7 removed in 60 min, 0.05 g L −1 CuO and 0.1 mM PMS [29] FeOOH PMS >95% of 50 mg L −1 AO7 removed in 30 min, 0.3 g L −1 FeOOH and 20:1 PMS/AO7 molar ratio [30] MnCeOX PDS >80% of 10 mg L −1 AO7 removed in 120 min, pH = 6.8, 0.7 g L −1 MnCeOX and 4 mmol L −1 PDS. [31] Activated carbon PMS 100% of 100 mg L −1 AO7 removed in 60 min, 0.30 g L −1 GAC and 20:1 PMS/AO7 molar ra-[32]  The FESEM micrographs of pristine and used Ac-ZVI in Figure 5 indicate that after PMS/PDS activation, the smooth Ac-ZVI surface became coarser and various nanoparticles attached on the Ac-ZVI surface could be observed. Not surprisingly, this is due to the PDS/PMS interaction with Ac-ZVI, forming Fe 2 O 3 . The XRD patterns (Figure 6a) of the pristine and used Ac-ZVI confirmed the formation of Fe 2 O 3. There is no discernible difference between the XRD patterns of the used Ac-ZVI in both PMS and PDS systems. The N 2 adsorption-desorption isotherms (Figure 6b) revealed that the pristine and used Ac-ZVI is a mesoporous material that consists of a type-IV isotherm with an H3 hysteresis loop. This indicates that the catalyst consists of a wide range of pore sizes. As a result of Fe 2 O 3 formation, the BET specific surface area increased from 8.7 to >30 m 2 g −1 . The FTIR spectrum of pristine and used Ac-ZVI in Figure 6c shows the characteristic vibration bands at 1620 and 3430 cm −1 which can be ascribed to the FeOOH stretching view and surface OH group, respectively [34,35]. After use, an additional peak at 560 cm −1 was observed due to the Fe-O bond of Fe 2 O 3 [18]. The post mortem catalyst characterization showed that, regardless of the changes in the characteristics of the Ac-ZVI after use in both PMS and PDS systems, the performance of the systems remains unaffected.

Conclusions
In summary, Ac-ZVI coupled with Fe 3+ was successfully used to activate PMS and PDS for AO7 removal. The effects of the Ac-ZVI:Fe 3+ ratio, PMS/PDS dosage, and pH were compared. The results showed that, in general, increasing the Ac-ZVI:Fe 3+ ratio produced a promotional effect on the performance of the catalytic system. However, when Fe 3+ was in excess, the performance deteriorated due to the self-scavenging effect and competitive reactions. Increasing the PMS/PDS dosage generally leads to improved performance, while varying the pH leads to poorer performance in the PDS system but better performance in the PMS system due to the difference in their degradation mechanisms. The PMS system is less influenced by pH changes. The dominant ROS in both systems was identified to be SO 4 •− . Overall, the Ac-ZVI coupled with Fe 3+ can be used for at least three cycles without significant loss in catalytic activity, indicating that it is promising for environmental remediation. Funding: Financial support through the funding of YUTP grant (015LC0-169) from PETRONAS is acknowledged. Financial support from Kurita Asia Research Grant (20Pmy025-U20 or 304/PKIMIA/ 6501115/K120) provided by Kurita Water and Environment Foundation is also gratefully acknowledged.

Informed Consent Statement: Not applicable.
Data Availability Statement: Will be provided upon request.