Abstract

The aim of this work was to investigate the selective oxidation and direct decolorization of selected organic dyes (Methylene Blue (MB), Rhodamine B (RhB) and Orange II (OrgII)) by persulfate (PDS) without activation. Results show that the decolorization rate of MB was up to 58.0% within 10 minutes, while those of RhB and OrgII were only about 29.6% and 3.0% after 80 minutes, respectively. In comparison with the negligible impacts of pH from 2.0 to 9.0 on MB and OrgII decolorization, RhB decolorization rate obviously varied with the pH changes, and acid pH condition was beneficial for RhB decolorization. Quenching tests implied that the decolorization of dyes by PDS without activation was a nonradical oxidation process rather than sulfate radical oxidation. A plausible mechanism is that the decolorization process is attributed to the charged states of the dyes at different pH conditions, and thus direct electron transfer from dyes to PDS may occur, which is responsible for the bleaching of dyes. This study points out the potential bleaching capability of PDS without activation on cationic dyes, which may have important implications for selective oxidation treatment of dye wastewater.

HIGHLIGHTS

  • Persulfate selectively and effectively decolorized cationic dyes or dyes with positive charges under certain pH conditions.

  • Direct and non-radical oxidation process was proposed for decolorization of Methylene Blue and Rhodamine B by persulfate without activation.

  • Direct decolorization of dyes by persulfate without activation was not complete, and the degradation of dyes was limited.

INTRODUCTION

Peroxydisulfate (PDS) has been widely utilized as a bulk oxidant in advanced oxidation processes (AOPs) for removing organic pollutants in water and soil (Waclawek et al. 2017; Zhou et al. 2019). Generally, PDS is activated to produce sulfate radical (SO4•−) as the highly reactive oxidant species (ROS) to oxidize organic compounds almost non-selectively (Neta et al. 1988). Activation of PDS to produce SO4•− requires the input of energy (such as heat, ultraviolet light, and ultrasound), or low-valent transition-metal ions or oxides (Tan et al. 2012; Zarei et al. 2018; Wang et al. 2019).

AOPs based on SO4•− are capable of decomposing various refractory organic compounds in wastewater treatments (for instance, the decolorization and decontamination of wastewater from textile and dyeing industries (Yang et al. 2010; Li et al. 2016), treatment of pharmaceutical wastewater (Gao et al. 2019), and phenolic wastewater (Sun et al. 2012; Song et al. 2018). In the recent several years, non-radical pathways of activated PDS to oxidize organic pollutants in environmental remediation have also attracted attention. PDS activation without generation of SO4•− but effective degradation of organic compounds was achieved via the coupled process of PDS with CuO (Zhang et al. 2014), porous carbon aerogel (Jiang et al. 2018), sludge-derived biochar (Yu et al. 2019), cubic mesoporous carbons (Duan et al. 2018b), or carbon nanotubes (Lee et al. 2015). The non-radical pathway of activated PDS was proposed to oxidize target pollutants via direct electron transfer from organics to PDS, facilitated by the above catalysts or producing singlet oxygen as reactive oxidative species (ROS) (Zhu et al. 2019). Compared to SO4•− pathways, the non-radical processes of PDS oxidation towards organics were more selective and showed significant advantages by achieving much higher degradation capacity and avoiding the formation of highly chlorinated degradation products in the presence of chloride ions (Duan et al. 2018a).

While both the radical and non-radical processes of PDS with activation for AOPs have been well studied, there is little information on the direct oxidation of PDS itself without activation to degrade organic pollutants in water, even though PDS is actually a strong oxidant (E0 = +2.01 V) (Shih et al. 2013). However, peroxymonosulfate (PMS, HSO5, E0 = +1.82 V), as another kind of persulfate, has been proved to be capable of decomposing some organic compounds (amino acids (Ruiz et al. 2019), antibiotics, pharmaceuticals, phenolics (Yang et al. 2018), cationic dyes (Lei et al. 2016)) by direct reaction with PMS without activation. Therefore, the direct reaction of PDS towards certain organic pollutants should be theoretically feasible due to its relatively high redox potential.

In this present study, we focus on the unactivated PDS in water to decolorize several azo dyes, and the decolorization kinetics and the reaction mechanism were investigated. We assumed that the direct reaction between azo dyes and PDS should be related to the charged state of dyes under various pH conditions, since the PDS ion (S2O82−) carries two negative charges. Thus, three dyes (Rhodamine B (RhB), Methylene Blue (MB) and Orange II (OrgII)) with different charged states as shown in Table 1 under our experimental conditions were selected for detailed mechanistic investigation. The effects of pH, PDS dosage, ROS scavengers and ferrous ion on decolorization rates were studied. We found that PDS without activation can effectively decolorize dyes and in deed depends on the charged states of dyes. This study represents the first example, to our knowledge, demonstrating the oxidation of dyes by unactivated PDS alone and associating the reaction rate with the charged states of pollutants.

Table 1

The parameters of three dyes

DyeMolecular formulaStructureIon with charges at different pHa
Methylene blue (MB) C16H18ClN3 1.86, pH 1.7 
1.01, pH 4.6 
1.00, pH 6.5 
1.00, pH 8.0 
Rhodamine B (RhB) C28H31ClN2O3  1.98, pH 1.7 
0.43, pH 4.6 
0.01, pH 6.5 
0.00, pH 8.0 
Orange II (OrgII) C16H11N2NaO4 −1.00, pH 1.7 
−1.00, pH 4.6 
−1.00, pH 6.5 
−1.00, pH 8.0 
DyeMolecular formulaStructureIon with charges at different pHa
Methylene blue (MB) C16H18ClN3 1.86, pH 1.7 
1.01, pH 4.6 
1.00, pH 6.5 
1.00, pH 8.0 
Rhodamine B (RhB) C28H31ClN2O3  1.98, pH 1.7 
0.43, pH 4.6 
0.01, pH 6.5 
0.00, pH 8.0 
Orange II (OrgII) C16H11N2NaO4 −1.00, pH 1.7 
−1.00, pH 4.6 
−1.00, pH 6.5 
−1.00, pH 8.0 

aThe specific parameters of the three dyes calculated from chemicalize.org by ChemAxon (Swain 2012).

MATERIALS AND METHODS

Materials

Methylene blue (MB, C16H18ClN3S·3H2O, ≥98.5%), sodium persulfate (PDS, Na2S2O8, ≥98.0%), Rhodamine B (RhB, C28H31ClN2O3, ≥98.0%), Orange II (OrgII, C16H11N2NaO4S, ≥86.0%), Ethanol (C2H6O, ≥99.5%), 2-methyl-2-Propanol (C4H10O, ≥99.0%), 1,4-Benzoquinone (C6H4O2, ≥98.5%), and Iron(II) sulfate heptahydrate (FeSO4·7H2O, 99.0.0%–101.0%) were purchased from Kelong Chemical Co, Ltd (Chengdu, China). All reagents were of analytical grade and used without further purification. All solutions were prepared in ultrapure water (resistivity >18.2 MΩ•cm).

Experimental procedure

The dye decolorization experiments were performed in a 1 L flask with three necks at a room temperature (25 ± 2 °C). Each run was performed in duplicate or triplicate, and the average values were adopted to ensure reproducibility. The three dyes (MB, RhB, and OrgII) were used at different initial concentrations of MB at 25 mg/L (RhB 20 mg/L, OrgII 35 mg/L) to give a close maximum absorbance around 1.60 at the respective characteristic wavelength. Dye solution and PDS stock solution were added into the flask, and the concentration of PDS was 50 times that of dyes at molar ratio, unless otherwise specified. The initial solution pH was adjusted with 0.1 M H2SO4 or 0.1 M NaOH (within ±0.1 unit fluctuation). The exact experimental conditions are more clearly listed in the figure or table captions. At each given interval, the target reaction solution (2.5 mL) was withdrawn, and immediately determined the real-time absorbance using a UV-Vis spectrophotometer. To identify the active species formed in the PDS/dyes system, quenching tests with three scavengers (ethanol (EtOH), tert-butyl alcohol (TBA) and p-benzoquinone) as the quenching agents were conducted.

Analytical methods

The residual concentration of dyes was monitored at the appropriate λmax using a UV-Vis spectrophotometer (UV-762, Shanghai Precision Scientific Instrument Co, Ltd). Total organic carbon (TOC) of water samples was detected by a TOC analyzer (Shimadzu, Japan). The pH of the solution was monitored by a pHS-3C meter (Rex, China).

The experimental data processing and drawing were mainly conducted by the software Office Excel 2016 and Origin 2017. The change of dye content was calculated by Ct/C0, where Ct represents the dye concentration at time t, and C0 is the initial concentration

RESULTS AND DISCUSSION

Decolorization of different dyes by PDS

As shown in Figure 1, three dyes (MB, OrgII and RhB) were decolorized by PDS without activation by catalysts or energy input. Compared with the negligible decolorization of OrgII, about 58.0% of MB was bleached by PDS without activation within 10 minutes. Besides, only about 29.6% of RhB was decolorized after 80 minutes.

Figure 1

Dye decolorization by PDS without catalyst addition. Condition: [MB]0 = 25 mg/L, [RhB]0 = 20 mg/L, [OrgII]0 = 35 mg/L, n[PDS]0 : n[MB]0 (n[RhB]0, or n[OrgII]0) = 50 : 1, initial pH = 5.8 and temperature = 25 ± 2 °C.

Figure 1

Dye decolorization by PDS without catalyst addition. Condition: [MB]0 = 25 mg/L, [RhB]0 = 20 mg/L, [OrgII]0 = 35 mg/L, n[PDS]0 : n[MB]0 (n[RhB]0, or n[OrgII]0) = 50 : 1, initial pH = 5.8 and temperature = 25 ± 2 °C.

The different decolorization performances towards the three dyes by PDS without activation suggest that PDS has high selectivity towards certain dyes, like MB. This might be closely related to the charge carried by PDS and dyes. PDS dissociates to produce persulfate ion (S2O82−) with two negative charges. The charged states of dissociated organic dyes molecule are closely related to their molecule structures and the deprotonation functional groups (Wang & Zhu 2007). As shown in Table 1, with the pH values varying from 1.7 to 8.0, the dissociated MB molecule always carries positive charges, and the dissociated OrgII molecule carries one negative charge, while the dissociated RhB molecule carries the charges from positive to neutral as the pH values increase. MB can dissociate cations which is easy to combine with electronegative substances in aqueous solution (Vinuth et al. 2017). Therefore, the direct decolorization rate by PDS was fastest with cationic dye (MB) (Gao et al. 2016), and was slow with neutral dye (RhB), but was nearly zero with anionic dye (OrgII). Jiang et al. (2013) also observed a similar experimental phenomenon. The mechanism of MB, RhB and Org II decolorization by PDS under different pH conditions were further studied.

Effects of initial pH on dyes decolorization by PDS

Considering the different charged states of MB, OrgII and RhB under different pH conditions from 1.7 to 8.0 (Table 1), the effects of initial pH at 2.0, 5.0, 7.0 and 9.0 on dyes decolorization by PDS were explored to verify whether the pH condition or the charged state of the dissociated dye molecule plays a more important role in dye decolorization. As shown in Figure 2(a) and 2(b), the MB and OrgII removal efficiency barely varied with the pH changes, and the decolorization rate of MB was more than 60% in 30 minutes. In addition, OrgII hardly reacts with PDS under different pH conditions, which is basically consistent with the previous discussion. However, acidic pH condition was beneficial for the RhB decolorization, and the best removal efficiency (about 31.1%) was obtained at around acid pH (pH = 2.0) in 5 minutes (Figure 2(c)). Taken together, it is reasonable to believe that the charged states of dissociated dye molecules, which varies with pH condition, plays a vital role in dye decolorization by PDS without activation.

Figure 2

Effects of initial pH on (a) MB, (b) RhB and (c) OrgII decolorization by PDS. Condition: [MB]0 = 25 mg/L, [RhB]0 = 20 mg/L, [OrgII]0 = 35 mg/L, n[PDS]0 : n[MB]0 (n[RhB]0, or n[OrgII]0) = 50 : 1, initial pH = 5.8 and temperature = 25 ± 2 °C.

Figure 2

Effects of initial pH on (a) MB, (b) RhB and (c) OrgII decolorization by PDS. Condition: [MB]0 = 25 mg/L, [RhB]0 = 20 mg/L, [OrgII]0 = 35 mg/L, n[PDS]0 : n[MB]0 (n[RhB]0, or n[OrgII]0) = 50 : 1, initial pH = 5.8 and temperature = 25 ± 2 °C.

Quenching experiments for RhB decolorization

Ethanol (EtOH), Tert butyl alcohol (TBA), and p-benzoquinone were used as scavengers to verify that the dominant reactive oxidant species resulted in the decolorization of RhB in RhB/PDS system with pH at 2. TBA can effectively quench •OH rather than SO4•─ due to the faster reaction constant between TBA and •OH (KTBA+•OH = (3.8–7.6) × 108 M−1s−1) than that between TBA and SO4•─ (KTBA+SO4•─ = (4.0–9.1) × 105 M−1s−1) (Xu et al. 2016). However, EtOH can quench both •OH and SO4•─ efficiently, since the reaction rate constant between EtOH and SO4•─ (KEtOH+SO4•─ = (4.3–5.6) × 107 M−1s−1) is as high as that between EtOH and •OH (KEtOH+•OH = 2.2 × 109 M−1s−1) (Clifton & Huie 1989; Deister & Warneck 1990; Motohashi & Saito 1993). Therefore, in this study, TBA was used as a scavenger for •OH, while EtOH was used as a scavenger of •OH and SO4•─. In addition, p-benzoquinone was used as a scavenger for O2•─ (Antonopoulou & Konstantinou 2014). And the addition of sodium sulfate was intended to inhibit the decolorization efficiency via the competition of SO42− and S2O82− to combine with RhB, since some parts of the molecular structures of SO42− and S2O82− are similar.

Decolorization of RhB and pseudo second-order reaction kinetics fitting are shown in Figure 3. In comparison with the control treatment, adding EtOH or TBA showed weak inhibition on RhB decolorization, indicating that •OH or SO4•─ was not the dominant ROS contributing to RhB decolorization by PDS without activation. However, compared with TBA, EtOH presented a relatively higher inhibition effect on RhB decolorization, implying that a small part contribution from SO4•─ in the oxidation process may be possible. There was almost no inhibition of p-benzoquinone treatment, suggesting that O2•− was absent from the reaction. In addition, the RhB decolorization rate in sodium sulfate treatment also decreased slightly, which partly supported the competitive combination of RhB with SO42− and S2O82−. Considering the different inhibition performance of EtOH, TBA, p-benzoquinone, and sodium sulfate on RhB decolorization, PDS direct oxidation via non-radical pathway was proposed to be the dominant process for RhB decolorization by PDS without activation, and there might be a small part contribution from SO4•─.

Figure 3

(a) Influence of different quenching agents on RhB decolorization by PDS at pH = 2. (b) Second-order reaction kinetics fitting of quenching reaction. Condition: [RhB]0 = 20 mg/L, n[PDS]0: n[Sodium sulfate] : n[RhB]0 = 50 : 50 : 1, [EtOH]0 = 7.85 g/L, [TBA]0 = 7.9 g/L, [P-benzoquinone]0 = 1.0 g/L, initial pH = 2 and temperature = 25 ± 2 °C.

Figure 3

(a) Influence of different quenching agents on RhB decolorization by PDS at pH = 2. (b) Second-order reaction kinetics fitting of quenching reaction. Condition: [RhB]0 = 20 mg/L, n[PDS]0: n[Sodium sulfate] : n[RhB]0 = 50 : 50 : 1, [EtOH]0 = 7.85 g/L, [TBA]0 = 7.9 g/L, [P-benzoquinone]0 = 1.0 g/L, initial pH = 2 and temperature = 25 ± 2 °C.

Quenching experiments for MB decolorization

The similar quenching experiments as above were taken to verify the dominant ROS for MB decolorization by PDS with pH at 5.8. As seen in Figure 4, in the early stage of the reaction, TBA and p-benzoquinone treatments presented a relatively obvious inhibitory effect on MB decolorization, while EtOH and Na2SO4 treatments showed a little promoting effect on MB decolorization. Thus, the role of the sulfate radical or hydroxyl radical in MB decolorization by PDS can be excluded, since EtOH treatment did not inhibit MB decolorization. Furthermore, after 20 minutes of the reaction, MB decolorization rates were close (60%–70%) among the quenching treatments and the control treatment. Therefore, MB decolorization by PDS in this study was not significantly involved with free radicals or O2•─, and the non-radical pathway by PDS direct oxidation may also be the main contribution for MB decolorization. A plausible mechanism was proposed that cationic dyes (i.e. MB+) firstly form a complex with the S2O82 ion, as Equation (1), due to the electrostatic attraction. Subsequently, direct electron transfer from cationic dyes to PDS, as Equation (2), may occur, and probably this is responsible for the bleaching of cationic dyes.
formula
(1)
formula
(2)
Figure 4

(a) Influence of different quenching agents on MB decolorization by PDS at pH = 2. (b) Second-order reaction kinetics fitting of quenching reaction. Condition: [MB]0 = 25 mg/L, n[PDS]0: n[Sodium sulfate] : n[MB]0 = 50 : 50 : 1, [EtOH]0 = 7.85 g/L, [TBA]0 = 7.9 g/L, [P-benzoquinone]0 = 1.0 g/L, initial pH = 5.8 and temperature = 25 ± 2 °C.

Figure 4

(a) Influence of different quenching agents on MB decolorization by PDS at pH = 2. (b) Second-order reaction kinetics fitting of quenching reaction. Condition: [MB]0 = 25 mg/L, n[PDS]0: n[Sodium sulfate] : n[MB]0 = 50 : 50 : 1, [EtOH]0 = 7.85 g/L, [TBA]0 = 7.9 g/L, [P-benzoquinone]0 = 1.0 g/L, initial pH = 5.8 and temperature = 25 ± 2 °C.

MB decolorization by PDS with or without catalyst addition

From the above results, PDS can effectively decolorize MB, but the decolorization rate did exceed 70%. To verify whether the PDS dosage or the oxidative capability is enough for MB decolorization, the same amount of PDS was added after 20 minutes of reaction, and then FeSO4 was added after 30 minutes of the reaction to observe further decolorization of MB. As shown in Figure 5, the MB decolorization rate showed little increase after adding the same amount of PDS for the second time, indicating that PDS dosage was not the main reason for the incomplete MB decolorization. Fe2+ can catalytically activate PDS to produce SO4•─as Equation (3) (Zhang et al. 2013), and then decolorize MB efficiently. After adding Fe2+, the decolorization rate increased rapidly from 73.7 to 92.2% (Figure 5). The above results also suggested that PDS without external activator cannot completely decolorize MB through direct oxidation.
formula
(3)
Figure 5

MB decolorization by PDS with or without catalyst addition. Condition: [MB]0 = 25 mg/L, n[PDS]0: n[MB]0 = 50:1, n[PDS]1 (or n[Fe2+]0) : n[MB]0 = 50 : 1, initial pH = 5.8 and temperature = 25 ± 2 °C.

Figure 5

MB decolorization by PDS with or without catalyst addition. Condition: [MB]0 = 25 mg/L, n[PDS]0: n[MB]0 = 50:1, n[PDS]1 (or n[Fe2+]0) : n[MB]0 = 50 : 1, initial pH = 5.8 and temperature = 25 ± 2 °C.

UV-VIS spectrum and TOC for MB removal by PDS

UV-VIS spectrum and TOC analysis of MB solution were performed to further investigate the oxidative removal of MB by PDS via direct oxidation or radical oxidation, respectively, without or with FeSO4 addition. As shown in Figure 6(a)–6(c), significant decrease of UV-VIS absorbance for MB and RhB was observed, but only limited decrease for OrgII was present in treatments by PDS without FeSO4 addition. Moreover, the UV-Vis spectra for MB, RhB, and OrgII in treatments with FeSO4 addition showed much less absorbance from 400 to 800 nm than those without FeSO4 addition. In Figure 6(d)–6(f), dye removal rates in treatments by PDS without FeSO4 addition were 7.7% and 6.8%, respectively, for MB and RhB, and showed almost no removal for OrgII. However, dye removal rates in treatments by PDS with FeSO4 addition were significantly increased to 29.1%, 28.8%, and 15.4%, respectively, for MB, RhB, and OrgII. In summary, the PDS can selectively and effectively decolorize MB and RhB by direct oxidation, but the removal rates of dyes were limited.

Figure 6

UV-VIS spectra of (a) MB, (b) RhB, and (c) OrgII under different treatments. TOC removal of (d) MB, (e) RhB, and (f) OrgII under different treatments. Conditions: [MB]0 = 25 mg/L, [RhB]0 = 20 mg/L, [OrgII]0 = 35 mg/L, n[PDS]0: n[Fe2+]0: n[MB]0 (n[RhB]0, or n[OrgII]0) = 50:50:1, initial pH = 5.8 and temperature = 25 ± 2 °C.

Figure 6

UV-VIS spectra of (a) MB, (b) RhB, and (c) OrgII under different treatments. TOC removal of (d) MB, (e) RhB, and (f) OrgII under different treatments. Conditions: [MB]0 = 25 mg/L, [RhB]0 = 20 mg/L, [OrgII]0 = 35 mg/L, n[PDS]0: n[Fe2+]0: n[MB]0 (n[RhB]0, or n[OrgII]0) = 50:50:1, initial pH = 5.8 and temperature = 25 ± 2 °C.

CONCLUSIONS

This study demonstrates that PDS without activation could effectively but selectively decolorize organic dyes, which was related to the charged states of dyes. Cationic dyes or dyes with positive charges under certain pH conditions were beneficial for the decolorization by PDS without activation. Thus, the researchers must pay attention to the contribution of PDS without activation to organic compound degradation, while studying the activation performance of various catalysts towards PDS to remove the selected model organic pollutants, especially when pollutants may carry positive charges under experimental condition. Direct and non-radical oxidation was proposed for the dye decolorization by PDS without activation. Compared with free radical oxidation via PDS with Fe2+ addition, direct decolorization of dyes by PDS without activation was not complete, and the degradation of dyes was limited. The applications of PDS direct oxidation on cationic dyes decolorization as emergency treatment could be possible, and further investigation about efficient decolorization and degradation of dyes should be warranted.

ACKNOWLEDGEMENTS

This study was partially supported by the National Natural Science Foundation of China (No. 21707111, 51808468) and Sichuan Science and Technology Support Project (No. 2020JDTD0018, 2019YJ0302).

AUTHOR DISCLOSURE STATEMENT

No competing financial interests exist.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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