Direct degradation of methylene blue by unactivated peroxymonosulfate: reaction parameters, kinetics, and mechanism

Advanced oxidation processes (AOPs) are efficient methods for water purification. However, there are few studies on using peroxymonosulfate (PMS) to remove pollutants directly. In this study, about 76% of methylene blue (MB) was removed by PMS directly within 180 min through a non-radical pathway, verified by scavenging tests, electron paramagnetic resonance and kinetic calculations. Additionally, the effects of PMS dosage, MB concentration, temperature, initial pH and competitive anions were determined. High PMS dosage, temperature and pH promoted MB degradation (from 76 to 98%) while MB concentration showed no effect on MB removal. Besides, MB degradation followed pseudo-first-order kinetic with rate constants of 0.0082 to 0.3912 min−1. The second-order rate constant for PMS reaction with MB was 0.08 M−1 s−1 at pH 3–6, but increased dramatically to 4.68 M−1 s−1 at pH 10.50. Chlorine could be catalysed by PMS at high concentration Cl− and degradation efficiency reached 98% within 90 min. High concentration of bicarbonate accelerated MB removal due to the high pH value while humic acid showed a marginal effect on MB degradation. Furthermore, TOC removal rate of MB in the presence of chloride reached 45%, whereas PMS alone caused almost no mineralisation. This study provides new insights into pollutant removal and an additional strategy for water purification.


Introduction
With the development of industry, environmental pollution has been a severe concern, and emerging synthetic compounds are refractory and difficult to be removed with traditional water treatment technology. Advanced oxidation processes (AOPs) play an important role in water treatment owing to efficiently removing various kinds of pollutants, such as aromatic ring substances and azo dyes (Mohamed et al. 2011). However, the mechanisms underlying pollutant degradation in AOPs have not been fully elucidated (Chen et al. 2019a(Chen et al. , 2018. Peroxymonosulfate (PMS), peroxydisulfate (PDS), sulphite and hydrogen peroxide (H 2 O 2 ) are often used to generate strong oxidative species, and many methods are developed to activate these oxidants, including the use of transition metals (Zhou et al. 2020a;Wu et al. 2022), UV light (He et al. 2014), base (Qi et al. 2016), heat (Tan et al. 2015), microwaves (Monteagudo et al. 2018), carbon-based materials (Wang et al. 2020;Wu et al. 2021Wu et al. , 2020, inorganic ions (Yang et al. 2018a, b) and organic materials (Ahmad et al., 2013). Although the activation methods are conducive to the removal of target pollutants, the energy required and heavy metals leached from catalysts increase the costs and cause secondary pollution. Furthermore, degradation efficiency is unsatisfactory because the generated radicals have poor selectivity for target pollutants when other impurities exist in the system (Long et al. 2021;Huang et al. 2019).
PDS, PMS and H 2 O 2 have different redox potential (E 0 ) values of 2.01, 1.82 and 1.78 V due to different structure (Qi et al. 2017;Ding et al. 2017). Furthermore, the asymmetric structure of PMS makes it unstable. In comparison with HClO (1.48 V), the redox potential of PMS enables it to thermodynamically oxidise pollutants directly. Zhou et al. (2020b) reported that PMS could degrade tetracyclines effectively in the presence of radical scavengers (methanol and tert-butyl alcohol) and 1 O 2 scavenger (sodium azide). Ding et al. (2017) found that PDS could directly decolourize MB. However, the unactivated PDS could damage the structure of MB, but not rhodamine B (Yang et al. 2021a). The degradation of several sulphonamides by non-activated PMS was compared, and aniline as well as dimethylisoxazole ring moieties were inferred to be the reactive sites (Ji et al. 2018). Recently, Yang et al. (2021b) found that PMS directly degraded trimethoprim despite the existence of 1 O 2 . Besides, the oxidation rate of chlorophenols by PMS alone was shown to increase with increased solution pH, which was attributed to the difference of dissociation degree among several chlorophenols . As an oxidant, PMS directly participates in the oxidation of pollutants. However, the reaction kinetics, related mechanisms and influence of reaction parameters (PMS dosage, pollutant concentration, solution pH, temperature and water matrix) in the PMS oxidation system have not been studied systematically due to the complexity of the reaction environment. For example, p-benzoquinone, a phenol oxidation by-product, could activate PMS (Zhou et al. 2017), but the reaction mechanisms between PMS and other pollutants (for example methylene blue) have not been clarified. Moreover, research on the effect of environmental parameters on the degradation of various pollutants by PMS remains highly insufficient.
MB, a type of cationic dye and redox indicator, is used widely in textile and dyeing industries as well as pharmaceutical processes. MB wastewater has a deep blue colour and does not readily biodegrade. Based on previous studies, this work used MB as a model pollutant to study the reaction mechanism, kinetics and reaction parameters in the MB/ PMS system. The study aims to provide more information about direct oxidation of pollutants by PMS, which is overlooked in previous research.

Chemicals and reagents
The chemicals were of analytical grade and used as received without further purification. Detailed information about the chemicals was included in Text S1 of the Supporting Information. Each solution was prepared with ultrapure water (18 MΩ cm). Humic acid (HA) stock solution was prepared according to previous solution (Cheng et al. 2016). Typically, 0.1 g HA was dissolved in the 80 mL and solution pH was adjusted to 12 with 0.1 M NaOH aqueous solution, then the mixture was stirred for 24 h. Afterwards, the solution pH was neutralised with 0.1 M H 2 SO 4 and diluted into 100 mL.

Experimental procedures
The MB degradation experiments were conducted in 150-mL conical flasks placed on a magnetic stirrer (CJJ-2S, Jintan Instrument Factory, Changzhou, China). The reaction temperature was controlled via heating in a water bath. A certain amount of MB stock solution was added to 100 mL of ultrapure water with constant stirring, and 1 mL of MB solution was withdrawn for detection of the initial MB concentration. Then, a predetermined volume of PMS stock solution (300 mM) was injected into the mixture to initiate MB degradation (Fig. 1). The predetermined volume ranged from 0 to 1.5 mL. At regular intervals (0, 5, 10, 20, 40, 60, 90, 140, 180 min), 1 mL of the mixture was sampled for detection within 1 min. The solution pH was adjusted to the desired value with 0.1 M NaOH and H 2 SO 4 . A series of MB degradation experiments were evaluated under varying PMS dosage (0-4.5 mM), MB concentration (5-40 mg/L), temperature (25-45 °C), initial pH (3.14-10.50), NaCl (0-100 mM), NaHCO 3 (0-100 mM) and humic acid (0-10 mg/L) conditions. Cl − , HCO 3 − and humic acid are common components in surface waterbody and wastewater which could change solution pH and consume oxidative species to influence pollutant removal.
To investigate the effect of different water matrices on the decomposition of MB, 100 mL of sea water (or tap water) was used as the background solution. The water quality was described in Table S1. In addition, the pseudo-first-order kinetics were analysed as follows: In this model, C and C 0 are the MB concentration at t min and 0 min, and k obs is the pseudo-first-order kinetic rate constant (min −1 ).

Reaction mechanism
PMS has a higher redox potential (1.82 V) than HClO (1.48 V) and KMnO 4 (1.68 V) (Feng et al. 2019). PMS can be catalysed to generate sulphate and hydroxyl radicals, which degrade pollutants efficiently. However, direct oxidation of pollutants by PMS is usually ignored. The results shown in Fig. 3a and d suggested that PMS decolourised MB by 75% within 180 min directly. The k obs value calculated (1) ln c c 0 = −k obs t was 0.0082 min −1 , indicating that PMS had a great capability to degrade MB. A series of experiments were conducted to study the reactive substances and explore the degradation mechanism. Sulphate and hydroxyl radicals were common oxidative species in AOPs. Ethanol can be used as a scavenger for sulphate and hydroxyl radicals based on their distinct second-order rate constants, whereas isopropyl alcohol reacts rapidly with hydroxyl radicals but not with sulphate radicals (Sun et al. 2021(Sun et al. , 2019. Thus, these two alcohols were employed to determine whether radicals were involved in the degradation of MB. The data in Fig. 2a and d showed that the addition of ethanol and isopropyl alcohol exhibited an inappreciable effect on MB degradation. k obs values declined slightly from 0.0082 to 0.0079 and 0.0076 min −1 , indicating that radicals played a minor role in damaging MB. Furthermore, the degradation efficiency of MB decreased from 76 to 58% and 52.5%, and k obs declined from 0.0082 to 0.005 and 0.0045 min −1 in the presence of 1.5 M ethanol and isopropyl alcohol (Fig. S1). Notably, the reduction in degradation rate and k obs might be attributable to the increase of solution viscosity under the high doses of alcohol, which prevented PMS and MB from diffusing and decolourisation (Guan et al. 2013). On the other hand, L-histidine was used to capture 1 O 2 . Although MB removal was completely inhibited after addition of L-histidine, the role of 1 O 2 in the PMS/MB system needed to be further demonstrated.
EPR spectroscopy was performed to confirm reactive species directly. As shown in Fig. 2b, there was no signal of DMPO-SO 4 or DMPO-OH in the system, consistent with the results of the radical scavenging experiments. On the contrary, a three-peak, equal-strength signal provided evidence of 1 O 2 . It is well known that self-decomposition of PMS can generate 1 O 2 at an extremely low rate. Notably, the intensity of 1 O 2 was weak in Fig. 2b because it survived for only 4.2 µs in the H 2 O and was scavenged rapidly by H 2 O (k = 2.5 × 10 5 s −1 ) (Yang et al. 2018a, b;Bohme and Brauer, 1992). MB, an excellent photosensitised organic material (Yao et al. 2019), could produce 1 O 2 under light illumination, which was verified in Fig. 2b. Yang et al. (2018a, b) stated that L-histidine reacted directly with PMS, and that the PMS was consumed completely within a few minutes. Additionally, Liao et al. (2021) observed that the suppression of methyl parathion by PMS oxidation with addition of furfuryl alcohol was attributable to the furfury alcohol reacting with PMS. Although L-histidine captured 1 O 2 efficiently, it could be inferred that it may consume PMS spontaneously and thus suppressed MB degradation. To verify this hypothesis, 1 O 2 was generated in situ according to a previous report (Bohme and Brauer 1992), and the degradation of MB by 1 O 2 in the H 2 O 2 /Na 2 MoO 4 /MB system was assessed. The generation rate of 1 O 2 reached ~ 30 μM s −1 at pH 9.30, comparable to that of 1 O 2 by MB under light (Bohme and Brauer 1992). Figure 2c showed that the characteristic signal of 1 O 2 in the H 2 O 2 /Na 2 MoO 4 system was stronger than that of PMS alone demonstrating the generation of 1 O 2 in H 2 O 2 /Na 2 MoO 4 system (Bohme and Brauer 1992). Nevertheless, the removal rate of MB was only 13% even at a high concentration of 1 O 2 (Fig. S2), and the k obs value was about 1.06 × 10 −3 min −1 . This result indicated that 1 O 2 was not responsible for the MB removal. Moreover, Lu et al. (2021) found the same interesting phenomenon that although 1 O 2 existed in the system, it barely degraded isoproturon.
[ 1 O 2 ] SS represented the steady state concentration of 1 O 2 , and R = 35.2 × 10 −6 M s −1 was inferred to be the generation rate of 1 O 2 (Bohme et al. 1992). Assuming that the generation and consumption rate of 1 O 2 were equal during the reaction process, k 2 and [ 1 O 2 ] SS could be obtained by the following equations: As calculated above, k 2 = 1.25 × 10 5 M −1 s −1 and [ 1 O 2 ] ss = 1.4 × 10 −10 M was similar to the value reported by Yang et al. (2018a, b). The second-order reaction rate constant of 1 O 2 reacting with MB was lower than that of 1 O 2 reacting with H 2 O.
The maximum degradation rate of MB by 1 O 2 in the PMS/MB system was calculated as follows: ), where f represented the fraction of 1 O 2 reacting with MB (Yang et al. 2018a, b).
The calculated removal rate of MB was only 2.6 × 10 −13 M s −1 ; however, in this experiment, the measured value was 2.2 × 10 −9 M s −1 . Moreover, f < 0.1% was extremely low, implying that 1 O 2 barely contributed to MB degradation. The above results suggested that MB degradation did not depend on 1 O 2 but relying on direct oxidation by PMS through a non-radical pathway. Furthermore, we found N,N-dimethylaniline which was a moiety in MB was degraded above 90% within 5 min by PMS direct oxidation (Fig. S3). The higher reaction rate of N,N-dimethylaniline than that of MB indicated N,N-dimethylaniline was the reactive site of MB.

PMS dosage
The correlation between PMS and pollutant degradation could be obtained by adjusting the PMS dosage. As shown in Fig. 3a, increasing PMS concentration (from 0 to 0.75, 1.5, 3 and 4.5 mM) improved the removal rate of MB from 1 to 56%, 76%, 85% and 92%. The optimum condition of MB removal was 4.5 mM PMS. Pseudo-order reaction kinetic of PMS reaction with MB was calculated according to Eq. (1). Results showed the reaction of PMS with MB was first-order with MB concentration. Besides, we found PMS directly degraded MB in experiments so reaction rate k obs was also related to PMS dosage. The value of k obs was highly linearly correlated with initial PMS dosage implying the reaction was also first-order with respect to PMS dosage (Fig. 3b). Above all, the overall reaction (a second-order kinetic model) was described as follows: k PMS,MB was the second-order reaction rate, [PMS] 0 was initial PMS dosage.
The second-order reaction rate constant of MB and PMS was estimated to be 0.0833 M −1 s −1 (pH < 7), which was much lower than that of PMS reacting with sulfamethoxazole (k PMS, sulfamethoxazole = 0.23 M −1 s −1 ), but higher than the rate constant of PMS with trimethoprim (k PMS, trimethoprim = 0.043 M −1 s −1 ) (Ji et al. 2018;Yang et al. 2021b). This difference may be associated with the structure of the different pollutants. Ji et al. (2018) deduced that aniline and dimethylisoxazole moiety within sulfisoxazole were the main reactive sites. This suggested that PMS selectively oxidised pollutants with electron-rich moieties because of its selective oxidative property.

MB concentration
The effect of MB concentration on MB removal was evaluated to understand the potential PMS oxidation ability. k obs decreased gradually from 0.0082 to 0.0063 min −1 with MB concentration increasing from 5 to 40 mg/L (Fig. 3c). The degradation efficiency reached the maximum (77%) at 5 mg/L MB. k obs showed a negative relationship with MB concentration and could be described as: k obs = − 0.00006 [MB] + 0.0087 (min −1 ) (Fig. 3d). The equation [PMS]×V (Text S2) was used to depict the amount of MB degraded (mg) per mmol PMS. The calculated G values were 5.1, 5.0, 4.87, 4.67 and 4.60 mg/mmol with an increase of MB from 5 to 40 mg/L. Greater MB input resulted in a reduction of PMS oxidation efficiency, possibly because of the extra consumption of PMS by MB by-products.

Reaction temperature
As well known, temperature plays an important role in the reaction process and high temperature generally increases the reaction rate. As shown in Fig. 3e, MB removal rate increased with reaction temperature (25, 35, 45 °C), and the k obs values were 0.0082, 0.0177, and 0.0224 min −1 . For instance, degradation efficiency reached the maximum (93%) at 45 °C in the experiment. As shown in Fig. 3f, the activation energy was obtained based on the Arrhenius equation (lnk obs = lnA − E a / RT). The E a was approximately 43 kJ/mol, which was higher than the values (30 and 32 kJ/mol) of phenol degradation achieved by PMS activated by cobalt-functionalised activated carbon, but lower than the value (62.3 kJ/mol) in the PMS/sulfuron-methyl/chloride system (Espinosa et al. 2019;Javier and Solís 2018). Peng et al. (2018) reported that the E a in the PMS/norfloxacin system was approximately 40.3 kJ/ mol. The above results implied that the degradation of MB by PMS was an endothermic reaction process. Although catalysts are usually used to reduce the activation energy, the E a value obtained in this experiment suggested the potential for direct oxidation of MB by PMS.

Initial pH
The species of PMS and MB were influenced by solution pH through protonation and deprotonation processes, and these PMS and MB species could affect the reaction process. Therefore, the effect of pH on MB degradation was conducted. As shown in Fig. 3g, degradation efficiency of MB increased from 76% at pH 3.14 to 87% at pH 8.83 in 180 min of reaction time. When pH was 9.65 and 10.50, MB removal rate became nearly constant at ≥ 98% within 40 min. And k obs increased with increasing pH (Fig. S4). Furthermore, solution pH gradually declined due to the release of H + during the PMS oxidation process (Fig. 3h) (Qi et al. 2016). Shimizu et al. (2007) reported that MB was unstable and easily decolourised at pH > 10. Kinetics analysis was conducted using the equations below: PMS had three species (H 2 SO 5 , HSO 5 − and SO 5 2− ) (pK a1 < 0, pK a2 = 9.4). In this experiment, pH values ranging from 3.14-10.50 were employed. As a cationic dye, MB had only one specie, while the PMS species varied at different solution pH. The results were as follows: MB + SO 2− 5 → by − products k 7 ≈ (2.54−4.68)M −1 s −1 , pH = 8.5 − 10.5 [PMS] total = α 6 [PMS] 0 + α 7 [PMS] 0 + α 8 [PMS] 0 (α 6 and α 7 , α 8 indicated the percentages of HSO 5 − , SO 5 . 2− and H 2 SO 5 under different pH) The reaction rate of MB with PMS was described by the following equation: As illustrated in Table 1, k6 was estimated as 0.0865 M −1 s −1 close to 0.0833 M −1 s −1 suggesting that HSO 5 − was the main specie of PMS under low solution pH (pH < 7). However, SO 5 2− became the main specie, and k 7 increased dramatically (tenfold) when the solution became alkaline. Liao et al. (2021) found that the transformation of methyl parathion by PMS was also complex under different pH values. Therefore, MB degradation was better in alkaline owing to three aspects: (1) the high reactivity and electrostatic attraction between SO 5 2− and MB; (2) unstability of MB under an alkaline environment (Shimizu et al. 2007); (3) enhanced alkalinity promoting activation of PMS for generation of 1 O 2 and superoxide anion radicals (Qi et al. 2016).

Effect of coexisting ions and humic acid
The chloride content in a natural waterbody or wastewater ranges from tens to hundreds of mg/L, and the drainage usually up to thousands of mg/L. In particular, higher concentration chloride is released by the metal smelting industry. Chloride can be oxidised by strong oxidants to generate HClO, which is used widely as an oxidant and bleaching agent. As depicted in Fig. 4a, chloride accelerated MB degradation, especially at higher chloride concentration. For instance, degradation efficiency of MB reached 98% at 100 mM Cl − . However, the addition of NH 4 + inhibited the degradation of MB in the presence of chlorine (Fig. S5). NH 4 + was deprotonated into NH 3 , which reacted with HClO to induce the formation of chloramines (Deborde and Gunten 2008). The generated chloramines had lower oxidative properties and relative slow reaction rate with organic pollutants which were responsible for the inhibitory effect Fig. 3 Effect of a PMS dosage, c MB concentration, e temperature and g initial pH on the direct degradation by PMS, and fitting curve for the b PMS concentration, d MB concentration versus k obs and f ln(k obs ) versus 1/T. h Variation of solution pH during the reaction process. Experimental conditions: [MB] = 10 mg/L (except for (c)), [PMS] = 1.5 mM (except for (a)), temperature = 25 °C (except for (e)), pH unadjusted (except for (g) and (h)) ◂ Environmental Science and Pollution Research (2022) 29:75597-75608 1 3 of MB removal Qiang and Adams, 2004). The k obs in MB/PMS system with Cl − and NH 4 + addition was obviously lower than that without NH 4 + (Fig. 4b) ). Bromide was also be oxidised by PMS to generate HBrO, which accelerated the oxidation of tetrabromobisphenol S . Moreover, although PMS was consumed to generate HClO, the oxidative system could still oxidise MB efficiently when HClO was scavenged by NH 3 , and the k obs dropped only slightly in comparison to that in the PMS/MB system (Fig. 4b). We conducted the degradation of MB by HClO (1.5 mM) and found HClO exhibited the similar removal ability of MB with PMS (Fig. S5f). Besides, MB degradation showed almost no difference at low concentration chloride (≤ 10 mM) compared to that without addition of chloride in Fig. 4a and b. However, the MB degradation was dramatically promoted at high concentration chloride (≥ 50 mM). It was to say HClO was generated by PMS reacting with Cl − to compensate for the consumed PMS to oxidise MB at low concentration chloride (≤ 10 mM). At high concentration chloride, the phenomenon was owing to the catalysis of chlorine by PMS because more chlorine was generated. The amount of generated chlorine was important in the chlorine/PMS system. Ding et al. (2021) also found that PMS did not react with chlorine but catalysed chlorine to react with acetaminophen. Therefore, the high reaction rate in the presence of Table 1 The reaction kinetics parameters at different initial pH a The percentage of SO 5 2− was below 0.1% and the contribution of SO 5 2− for k obs was ignored.
b The value was obtained via (0.091 + 0.082)/2. The difference of k 6 between pH 3.14 and 6.38 was about 10% Initial pH k obs (min −1 ) HSO 5 − percentage k 6 (M −1 s −1 ) k 7 (M −1 s −1 )  1 3 chloride might occur through three steps: chloride reacting with PMS to generate HClO as the rate-limiting reaction (a determined step); decomposition of MB by HClO catalysed by PMS (main step); direct degradation of MB by PMS (minor step). Some matters with reducing properties often exist in surface waterbody or wastewater. Recently, bisulfate was developed to generate oxidative species for pollutant removal. Therefore, NaHSO 3 was selected to investigate the effect of bisulfate on MB removal. As shown in Fig. S6, MB decolourisation rate was only about 18% within 180 min. Nevertheless, with the addition of 10 and 50 mM chloride, the MB removal rate increased up to 32% and 76%, respectively. Connick et al. (1995) reported that PMS reacted with HSO 3 − in two ways: (1) via the formation of sulphate radicals, with an extremely slow reaction rate (Eq. (23)); and (2) via the production of a sulphate ion and proton with a fast reaction rate in comparison to (1) (Eq. (24)). PMS was consumed by HSO 3 − , so the removal rate of MB diminished significantly. The chloride ion was able to react with the residual PMS (about 0.5 mM PMS) to form HClO, and the HClO was catalysed by PMS to accelerate the oxidation of MB. Hence, when a reducing agent was present in the PMS system, the inhibitory degradation of organic pollutants could be alleviated by the addition of chloride.
Bicarbonate is a common substance in surface waterbody. Therefore, it was essential to investigate the effect of bicarbonate on the degradation of MB from the viewpoint of practical application (Song et al. 2019;Chen et al. 2019b). As illustrated in Fig. 4c, bicarbonate promoted the MB degradation. With the addition of bicarbonate of 5, 10, 50 and 100 mM, the degradation rate of MB increased from 76 to ≥ 90%. The reaction rate was also accelerated from 0.0082 to 0.021 min −1 (Fig. S5). On the one hand, the addition of bicarbonate increased the solution pH (higher than 8.0 during the reaction process), and the alkaline environment was beneficial for the decomposition of MB. On the other hand, PMS could be catalyzsed to generate 1 O 2 which may contribute to the degradation of MB (Yang et al. 2018a, b). (20) The natural organic matter (NOM) is composed of a carbon skeleton with abundant oxygen-containing functional groups. The effect of humic acid on MB degradation could be ignored (Fig. 4d). It was interesting that the C/C 0 was above 1.0 within 20 min. We supposed that it was related to the complex structure and a diversity of functional groups on humic acid. Humic acid had redox capacity owing to rich functional groups. Besides, MB also was an redox indicator. In this experiment, humic acid was firstly oxidised by MB and the reduced MB was got colourless. So the initial MB absorbance was low. Secondly, the reduced MB was oxidised by PMS after addition of PMS. And the oxidised MB got original blue colour. Humic acid exhibited a marginal effect on MB degradation. For example, removal rate of MB at 5 mg/L reached 75% and the k obs value was 0.0081 min −1 (Fig. S5). Humic acid reacted with sulphate radicals and hydroxyl radicals with second-order rate constant of 2.35 × 10 7 and 3 × 10 8 M −1 s −1 , respectively (Xie et al. 2015;Pang et al. 2018), further suggesting that the MB oxidation by PMS was a non-radical reaction process.

Effect of water matrix
To explore the MB degradation performance in practical application, two types of water (sea water and tap water) were employed. Sea water was collected from the South China Sea and tap water was obtained from Shanghai, China. The water quality of the two samples was described in Table S1. The MB removal and reaction rates in the above waters were better than that in ultrapure water (Fig. 5). Especially, the high chloride content of sea water promoted MB degradation (Mussa et al. 2013). Although the two water matrices contained dissolved organic matter, PMS reacted with MB directly without generating radicals. The results from the application of the two water types suggested that PMS was an efficient choice for real water sample purification.

TOC removal in different reaction conditions
Total organic carbon (TOC) is an important indicator of the mineralising capacity of pollutants in a system. In this study, the pollutants were first decomposed into small molecular substances and then oxidised into carbon dioxide and water. The results of the TOC removal were shown in Fig. 6. The TOC removal rate was near zero at pH 3.14. However, it increased to 16% at pH 10.5. In the presence of 50 mM chloride, the TOC removal rate reached 45%. These results suggested that PMS alone barely mineralised MB and only damaged the chromogenic group of the MB. The TOC removal rate of AO7 by PMS alone was only 19.2%, suggesting that the mineralisation capacity of sole PMS was low ). Under the highly alkaline environment, the main specie of PMS was SO 5 2− , and it reacted with MB rapidly during the mineralisation process. Additionally, the formation of 1 O 2 and superoxide anion radicals via PMS self-decomposition selectively degraded pollutants with electron-rich moieties. More generated recalcitrant by-products were formed in the reaction process and could barely be mineralised. In the presence of chloride, HClO was formed and had a strong capacity to oxidise the chromogenic group and further mineralise MB catalysed by PMS. Wang et al. (2011) pointed out that the TOC removal rate of azo dyes in the Co/PMS/Cl system was lower than 5% within 450 s, indicating that chloride could improve the decolourisation ability but not the TOC removal rate in radical-based system. Although the sulphate and hydroxyl radicals formed in the Co/PMS/Cl system had strong redox potential, the oxidation ability of chlorine radicals generated was low in comparison to the sulphate and hydroxyl radicals . Additionally, the chlorine radicals reacted with azo dyes or by-products to form refractory organic halogen compounds, which are difficult to mineralise. However, on the contrary, the addition of chloride enhanced the TOC removal rate in this study, which may be attributable to the generated HClO and HClO was catalysed by PMS for MB mineralisation. Furthermore, with the generation of organic halogen compounds and the consumption of PMS, mineralisation gradually ceased.

Conclusion
AOPs based on radicals have strong oxidation abilities, allowing the removal of a broad range of organic micropollutants. However, direct oxidation by PMS for water purification has been overlooked. The reaction mechanism, kinetics and parameters of PMS reacting with MB found in this study could be summarised as follows: (1) PMS reacted with MB via a non-radical pathway.
Additionally, the second-order rate constant of MB reacting with 1 O 2 was estimated to be approximately 1.25 × 10 5 M −1 s −1 .
(2) The pseudo-first-order rate constant of MB transformation showed a negative linear correlation with MB concentration and a positive linear correlation with PMS concentration.
(3) Increasing the solution pH favoured MB degradation, which was due to the high reaction rate between SO 5 2− and MB. (4) The addition of chloride accelerated MB transformation and mineralisation due to the formation of HClO and catalysis by PMS. (5) In real water matrices (sea water and tap water), MB degradation was promoted. Even in water with added humic acid, MB degradation was not affected, suggesting an advantage of the PMS oxidation system.