Generation of anion sulfate radicals (SO4•−) and hydroxyl radicals (HO) by UV/Persulfate and the UV/Peroxydate processes have been successfully studied to degrade Ponceau S dye. Under [PS] = 0.06 mM; [H2O2] = 2 mM; [S2O82-] = 2 mM, the UV/Persulfate process was effective (kapp = 0.163 min−1) than the UV/Peroxydate process (kapp = 0.054 min−1). The lack of dissolved oxygen, the excess of hydrogen peroxide (H2O2) dosage at 2 mM, and the alkaline pH of 10.01 significantly reduced the UV/Peroxydate efficiency. The scavenging effect of the hydrogenocarbonates and nitrates on the PS dye degradation by the UV/Persulfate process was significant, whereas chlorides had a slight influence. The composition of seawater in chlorides, sulfates, carbonates, and bromides decreased the photoactivity of the studied processes. The presence of phenol showed that the reactive affinity of the (HO) is more superior to the SO4•−. The UV/Persulfate process achieved 82.35% of chemical oxygen demand removal against 59.56% for the UV/Peroxydate in about 100 min. This study demonstrated that the UV/Persulfate process is a viable option for PS dye degradation. To the best of our knowledge, this is the first report for studying the PS dye degradation under varying some new operational factors. However, the identification of by-products, their nature, and their concentration requires special attention.

  • Higher degradation of Ponceau S dye was obtained with the UV/ process against the UV/H2O2 process.

  • Active species such as and play key roles in the degradation of Ponceau S dye.

  • The seawater matrix promoted the scavenging reactivity of the active radicals.

  • The introduction of anions reduced the oxidation by the UV/ process.

  • The UV/ process achieved a higher chemical oxygen demand removal.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Ponceau S (PS), a diazo dye, is one of the most widely used dyes in textile and dyeing industry (Ghodbane & Hamdaoui 2010). However, the discharge of such residual dyes into the water bodies causes many environmental issues (Laftani et al. 2019a, 2019b). Textile effluents contain large amounts of suspended solids. They have highly fluctuating pH values, elevated temperatures, high chemical oxygen demand (COD), and considerable concentration of toxic metal ions (Cr, Ni, and Cu) (Ribeiro et al. 2014). As a widely used artificial dye, PS has received extensive attention due to its widespread detection in an aquatic environment and potential risk to human health (Chatib et al. 2021). It is characterized by a varied chemical structure including the nitrogen–nitrogen bond (–N = N–), aromatic and naphthalenic systems with substituted auxochromes (–OH, –SO3Na) (Laftani et al. 2019a, 2019b). As a refractory pollutant causing multiple harmful effects on the ecosystem, the removal of PS dye from the environment obviously has become a very urgent issue. Azo dyes and their metabolites are recalcitrant to conventional treatment methods. Thus, effective treatment methods are sought to control these persistent compounds. For this purpose, advanced oxidation processes (AOPs) are the most promising alternatives.

AOPs have been studied to treat persistent contaminants. These alternative processes permit to improve the removal efficiency and limit the generation of undesired by-products (Kwon et al. 2015). They aim for the mineralization of the contaminants to carbon dioxide, water, and inorganics or, at least, for their transformation into harmless products (Bougdour et al. 2018). Among the numerous AOPs, the UV/Peroxydate (UV/H2O2) reaction is the most widely studied and applied in water treatment (Lin et al. 2016). Elsewhere, it should be noted that the UV photolysis, widely used for disinfection purposes, is capable of degrading organic compounds as well (Gao et al. 2020). Especially, the UV/H2O2 process generates hydroxyl radical (2.8 V/ERH) (Laftani et al. 2019a, 2019b), which has an excellent removal efficiency for persistent organic pollutants (Surpateanu & Carmen 2004). This process is also known to be practical under ambient temperature and pressure. Furthermore, the cost of the process is relatively low and does not generate sludge compared to the Fenton process (Laftani et al. 2019a, 2019b). If properly treated by the UV/H2O2 process, the discharged water can be recycled or reused.

While the Fenton process has some major drawbacks. The reaction only occurs at pH values around 3, which is lower than the pH values of wastewater. Adjusting the pH value will increase the operational cost of the treatment (Laftani et al. 2019a, 2019b).

The UV/Persulfate (UV/), as another AOP, has recently received scientific attention (Gao et al. 2012). It operates via the generation of anion sulfate radical , a very strong oxidant (3.1 V/ERH) (Tan et al. 2016). This radical has a longer half-life (30–40 μs), higher selectivity, and less pH sensitivity than hydroxyl radical (Ma et al. 2021). In addition, the anion sulfate radical can be generated from the decomposition of persulfate after activation by heat, electrochemical processes, ultrasonic irradiation, UV photolysis, and transition metals. Considering economic, energy consumption, and potential heavy metal pollution, UV irradiation is the most appropriate choice for environmental applications (Ma et al. 2021).

The presence of organic impurities such as dyes, surfactants, pesticides, etc. in the hydrosphere, is of particular concern for the freshwater and marine environment (Hassaan et al. 2016). Many researchers have focused on the development of disinfection technologies for marine waters (seawater and brackish water) in different fields. Moreover, since 2004, the International Maritime Organization (IMO), required all ships to implement a ballast water treatment to prevent the potentially devastating effects on aquatic organisms (Penru et al. 2012). The treatment of the saline water used for the various activities mentioned above becomes necessary. The organic matter in wastewater can be determined by using a variety of analytical methods such as total organic carbon (TOC), biological oxygen demand (BOD), or COD (Lia et al. 2009).

This work aims (1) to study and compare the removal efficiency kinetics of a refractory diazo azo by the UV/H2O2 and the processes. (2) To evaluate the behavior of hydroxyl radicals and the sulfate anion radicals and scavenging effect by using phenol and seawater matrix, respectively. (3) To estimate the mineralization of the studied pollutant during subsequent oxidations by measuring the COD removal.

Chemicals

Synthetic solutions of PS dye were prepared using ultrapure water obtained with VWR PURANITY TU and the artificial seawater. PS, a tetrasodium salt of 3-hydroxy-4-({2-sulfo-4-[(4-sulfophenyl) diazenyl] phenyl} diazenyl) naphthalene-2,7-disulfonic acid, was purchased from REACTIFS RAL. The physical and chemical characteristics of PS dye are shown in Table 1. Hydrogen peroxide (H2O2, 50%) was obtained from PROCHILABO and sodium thiosulfate was purchased from Scharlau. Other chemicals are listed in Table 2. It must be noted that all chemicals were used as received without further purification.

Table 1

Selected physicochemical properties of PS dye

Color index numberMolecular formulaMolecular weightλmaxMolecular structure
27195 C22H12N4S4O13Na4 760.6 g/mol 520 nm  
Color index numberMolecular formulaMolecular weightλmaxMolecular structure
27195 C22H12N4S4O13Na4 760.6 g/mol 520 nm  
Table 2

Reagents used in this work

ChemicalsSupplier
Phenol, Na2SO4 (99%) SIGMA-ALDRICH 
NaHCO3 (99.37%), KBr (99.5%), CaCl2.2H2O (99%), SrCl2.6H2O (99%), NaNO3 Riedel-de-Han 
NaCl (99.5%), KCl (99.5%), NaF (98.5%) Scharlau 
Na2SO4 (99%), H3BO3 (99.8%) Honeywell 
MgCl2.6H2O (99%) Carlo Erba Reactifs-sds- 
K2Cr2O7 (99.5%), HgSO4 (98%) Panreac 
ChemicalsSupplier
Phenol, Na2SO4 (99%) SIGMA-ALDRICH 
NaHCO3 (99.37%), KBr (99.5%), CaCl2.2H2O (99%), SrCl2.6H2O (99%), NaNO3 Riedel-de-Han 
NaCl (99.5%), KCl (99.5%), NaF (98.5%) Scharlau 
Na2SO4 (99%), H3BO3 (99.8%) Honeywell 
MgCl2.6H2O (99%) Carlo Erba Reactifs-sds- 
K2Cr2O7 (99.5%), HgSO4 (98%) Panreac 

Experimental procedure

The PS dye aqueous solutions were prepared by dissolving the required amount in ultrapure water or in the artificial seawater. The desired pH of the solution was adjusted using sodium hydroxide or sulfuric acid and measured with a pH-meter device ‘HACH sensION + PH3’ calibrated with buffer solutions of pH 4, 7, and 10. The artificial seawater was freshly prepared according to the ‘Lyman and Fleming’ formula that has been one of the most widely used recipes for the artificial seawater preparation (Grasshoff et al. 2007). The salinity and the conductivity of the artificial seawater was measured using a HACH sensION + EC7 conductimeter. Degradation studies of PS dye by UV/H2O2 and UV/ processes was handled in 1-L cylindrical beaker using a high-pressure mercury lamp (250 W, Ingelec) as a source of UV irradiation. A stir bar was placed inside the reactor to ensure homogeneous UV exposure and homogeneous solution. Hydrogen peroxide and persulfate were added before UV irradiation for the UV/H2O2 and UV/ processes, respectively. Selected anions species were added into the reactor for evaluating their effects on the PS dye degradation efficiency.

The UV–Vis spectra of PS diazo dye was recorded from 200 to 800 nm using a UV–Vis spectrophotometer (Rayleigh UV-1800) with a spectrometric quartz cell (1 cm path length). The maximum absorbance wavelength (λmax) of PS dye could be found at 520 nm (Laftani et al. 2019a, 2019b). The PS dye degradation was carried out by measuring the absorbance at 520 nm. The disappearance efficiency of PS dye was calculated as follows:
formula
(1)
where A0 is the initial absorption of PS dye and At is the absorption of PS dye at reaction time t.

The COD was determined according to the procedure stated in Aman et al. (2016). The COD concentrations were calculated for each PS dye samples treated by the UV/H2O2 and UV/ processes at 0.06 mM of PS dye. The COD concentrations were established by the spectrophotometric method at λ = 605 nm and the oxygen amount needed for the oxidation of organic matter was determined in incubated samples with potassium dichromate K2Cr2O7 and mercuric sulfate HgSO4 at an acidic pH for 2 h at 120 °C. The measurements of the COD concentrations were achieved using the Bloc Digest 12 equipped by Regulating Unit ‘Selecta’.

Degradation of PS dye by the UV/H2O2 process

Effect of H2O2 concentration

Many researchers recognize that mainly hydroxyl radicals, generated from combining hydrogen peroxide (HP) and UV irradiation, as shown in Equation (2), may degrade the organic compounds (Chia-Chun & Ch 2006).
formula
(2)
Kinetic results of PS dye degradation with different hydrogen peroxide concentrations used during the UV/H2O2 process are shown in Figure 1. All the degradations followed a pseudo-first order kinetics. The overall rate law for the PS dye degradation can be expressed as Equations (3) and (4).
formula
(3)
formula
(4)
where is the degradation rate constant (min−1), the [PS] and [PS]0 are the concentrations of PS dye at time t and time 0.
Figure 1

Effect of the H2O2 dosage on the PS dye degradation by the UV/H2O2 process. Experimental conditions: [PS] = 0.06 mM; pH = 5.5, and T = 296.15 K.

Figure 1

Effect of the H2O2 dosage on the PS dye degradation by the UV/H2O2 process. Experimental conditions: [PS] = 0.06 mM; pH = 5.5, and T = 296.15 K.

Close modal

It can also be seen from Figure 1 that the pseudo-first-order degradation rate constant k increases with increasing H2O2 concentration from 0.076 to 1.53 mM.

Generally, the PS dye degradation performance was significantly enhanced when the H2O2 dosage increased. This can be attributed to the fact that more dosage of H2O2 produces more hydroxyl radicals that result in a higher dye degradation rate.

However, with continuous increase in the hydrogen peroxide concentration from 1.53 to 2 mM, the degradation rate increases slightly. This may be due to recombination of hydroxyl radicals and their reaction with H2O2, contributing to the scavenging capacity, as follows (Lin et al. 2016):
formula
(5)
formula
(6)
formula
(7)
formula
(8)
formula
(9)

Consequently, hydrogen peroxide acts as both promoter and scavenger of hydroxyl radicals and it is important to optimize the dosage of hydrogen peroxide to maximize the performance of the UV/H2O2 process and minimize the treatment cost.

Effect of pH

Degradation of PS dye at different buffered pH values (4.6–10.01) is shown in Figure 2. The [PS] and [H2O2] were fixed at 0.06 and 2 mM, respectively. The PS dye disappearance well fit a pseudo-first-order kinetics pattern (R2 > 0.98).
Figure 2

Effect of the pH on the PS dye degradation by the UV/H2O2 process. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM, and T = 296.15 K.

Figure 2

Effect of the pH on the PS dye degradation by the UV/H2O2 process. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM, and T = 296.15 K.

Close modal
The results show that the degradation rate increased with increasing pH in the range of 4.6–7.8 and decreased at higher pH values. In an alkaline medium, hydrogen peroxide will dissociate to hydroperoxy anion (Equation (10)), a conjugated base of hydrogen peroxide. The hydroperoxy anion will react with the hydrogen peroxide or even with the hydroxyl radicals to reduce the effectiveness of the UV/H2O2 process as can be seen in Equations (11) and (12).
formula
(10)
formula
(11)
formula
(12)

Effect of dissolved oxygen

In order to evaluate the effect of dissolved oxygen on the UV/H2O2 process, samples were withdrawn at regular time intervals from the treated solution bubbled with air and nitrogen, respectively, each over 1 h. The rate of PS dye degradation decreased from 0.054 to 0.0341 min−1 when nitrogen was bubbled through the reactor. The obtained results are shown in Figure 3. The lack of dissolved oxygen affects negatively the oxidation process. Thus, as demonstrated, the dissolved oxygen ensures that some oxygen molecules absorb the photons and active oxygen radicals are formed, competing with hydroxyl radicals, which may take part in the oxidation process.

Measurement of conductivity

AOPs have been developed as an emerging destruction technology leading to almost total mineralization of most of the organic contaminants (Ameta 2018). Monitoring of the conductivity was achieved in order to evaluate the PS dye mineralization. This attempt was studied by measuring the conductivity during the PS dye degradation by the UV/H2O2 process. Results are shown in Figure 4.
Figure 3

Effect of the dissolved oxygen on the PS dye degradation by the UV/H2O2 process. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM, and T = 296.15 K.

Figure 3

Effect of the dissolved oxygen on the PS dye degradation by the UV/H2O2 process. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM, and T = 296.15 K.

Close modal
Figure 4

Evolution of the conductivity during the PS dye degradation by UV/H2O2. Experimental conditions: [PS] = 0.06 mM, [H2O2] = 2.3 M, and pH = 5.9.

Figure 4

Evolution of the conductivity during the PS dye degradation by UV/H2O2. Experimental conditions: [PS] = 0.06 mM, [H2O2] = 2.3 M, and pH = 5.9.

Close modal

The variation of the conductivity can be mathematically described by a generalized logistic function. During the first 8 min of reaction, conductivity remains steady. After 8 min of irradiation by the UV/H2O2 process, the conductivity increases as the time of PS dye degradation increases to reach a maximum at near 120 μS/cm. As the conductivity presents the concentration and mobility of the ions in solution, the increase in conductivity was the result of liberating ionic species in the treated solution. Considering the molecular structure of PS dye, the liberated ions are probably nitrates and/or nitrites, sulfates and carbonates, and/or hydrogen carbonates.

Degradation of PS dye by the UV/

Persulfate can be activated by UV photolysis to produce (Equation (13)). Both and are generated in the UV/ system through Equations (13) and (17) (Ma et al. 2021).
formula
(13)
formula
(14)
formula
(15)
formula
(16)
formula
(17)
A series of reaction chains also accompany the UV/ process.
formula
(18)
formula
(19)
formula
(20)

Effect of pH

In the UV/ process, the pH of the solution affects the formation of active radicals, which is a crucial factor for PS dye degradation. To evaluate its effect, the PS dye degradation was conducted in pH range from 3.4 to 9.9. Kinetics were consigned as shown in Figure 5. The obtained kapp order were as follows: kapp (5.3)>kapp (3.4)>kapp (9.9).
Figure 5

Effect of the pH on the PS dye degradation by the UV/Persulfate process. Experimental conditions: [PS] = 0.06 mM, [] = 2 mM, and T = 296.15 °C.

Figure 5

Effect of the pH on the PS dye degradation by the UV/Persulfate process. Experimental conditions: [PS] = 0.06 mM, [] = 2 mM, and T = 296.15 °C.

Close modal
The PS dye degradation by UV/ in the acidic medium was greater comparing to the alkaline one. The PS dye degradation accelerates with the reduction of pH value, possibly because acidic conditions promoted the generation of active substances such as via Equations (21) and (22).
formula
(21)
formula
(22)
Under alkaline conditions, the hydroxide anion can react with to generate , as shown in Equation (23). With the increase of solution pH, the formation of can be inhibited, less transformed into , which reduces the PS dye removal.
formula
(23)

Effect of some anions

Natural and wastewater are rich in anions, which, reportedly, may affect the degradation of PS dye. The effect of selected anions such as chlorides, hydrogenocarbonates, nitrates, and sulfates ions, on the active radicals in the UV/ process, is the focus of this investigation.

The reason for choosing those ions is that they are commonly present in wastewater generated from textile industry. The concentrations of the four anions were fixed at 2 mM.

As shown in Figure 6, the presence of sulfate ions did not show effects on reaction rate in the experimental conditions. Whereas, the hydrogenocarbonates show an inhibitory influence on the process under all tested conditions. The functions as a scavenger of (Yang et al. 2014). As can be converted into less oxidative species , the existence of decelerates the PS dye degradation in the UV/ process (Deming et al. 2019).
formula
(24)
formula
(25)
Figure 6

Effect of the anions on the PS dye degradation by the UV/ process. Experimental conditions: [PS] = 0.06 mM, [] = 2 mM, pH = 5.9, and T = 296.15 °C.

Figure 6

Effect of the anions on the PS dye degradation by the UV/ process. Experimental conditions: [PS] = 0.06 mM, [] = 2 mM, pH = 5.9, and T = 296.15 °C.

Close modal

In comparison with the photo-Fenton process, the presence of hydrogenocarbonates decreases the degradation rate of PS dye due to the scavenging of by to produce . The carbonate radical (E° = 1.78 V/ENH at pH = 0) is less reactive than radicals, which decreases the PS dye degradation rate (Laftani et al. 2019a, 2019b).

Similar findings were observed with the nitrate ions. The nitrates decreased the PS dye degradation. The anion sulfate radical can react with to form with relatively low oxidability (2.30 V/ERH) through Equation (26) (Ma et al. 2021).
formula
(26)
The influence of chlorides is illustrated in Figure 6. The kapp of PS dye degradation slightly decreased from 0.163 to 0.144 min−1. This result indicated that chlorides had an impact on the activated persulfate performance. The reaction between the anion sulfate radical and would take place with formation which has a high redox potential of 2.4 V and might degrade organic compounds in a similar way as .
formula
(27)
formula
(28)

The existing could be scavenged by with the generation of (1.36 V/ERH, Equation (28)), thus decreasing the degradation rate of target compound (Deming et al. 2019).

Comparison of UV/ and UV/H2O2 processes

The PS dye degradation performances in the two systems, UV/ and UV/H2O2, are shown in Figure 7. The initial concentration of PS dye was 0.06 mM and the concentrations of both H2O2 and persulfate were fixed at 2 mM in the above systems. The comparison between UV/ and UV/H2O2 processes in terms of PS dye degradation is conducted in the ultra-water.
Figure 7

Comparison of UV/ and UV/H2O2 on PS dye degradation. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM; [S2O82−] = 2 mM, and T = 296.15 °C.

Figure 7

Comparison of UV/ and UV/H2O2 on PS dye degradation. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM; [S2O82−] = 2 mM, and T = 296.15 °C.

Close modal

Compared with the UV/H2O2 reaction (kapp = 0.054 min−1), kinetic of PS dye degradation was faster during the UV/ process (kapp = 0.163 min−1). This can be explained by the fact that contributes more effectively to the PS dye degradation than , as the formation rate of from persulfate (Φ = 1.4 mol. E−1) is higher than that of from H2O2 (Φ = 1.0 mol. E−1).

This study revealed that the UV/ process was the most useful for enhancing the PS dye degradation rate.

Effect of consuming and radicals

Phenol was suggested as the selected molecule to evaluate the consuming effect of and . Phenol and its derivatives are aromatic compounds and are toxic to the environment, aquatic organisms, and human life; therefore this justifies the choice of this molecule. Solutions of PS dye (0.06 mM) were treated in the presence of phenol (2 mM). The results in Figure 8 show that the kinetic rate constant is higher for the UV/H2O2 (kapp = 0.064 min−1) process in the presence of phenol (kapp = 0.0037 min−1). Similarly for the UV/ one, the kinetic rate constant was decreased from kapp = 0.3037 min−1 to kapp = 0 min−1 in the presence of phenol. Possibly, phenol can be oxidized and decomposed by the photogenerated radicals, competing with the PS dye molecule. The presence of phenol in the treated water can obviously result in significant reduction of pollutant degradation.
Figure 8

Effect of phenol on the PS dye degradation by the UV/H2O2 and UV/Persulfate processes. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM; [] = 2 mM; [phenol] = 2 mM; and T = 296.15 K.

Figure 8

Effect of phenol on the PS dye degradation by the UV/H2O2 and UV/Persulfate processes. Experimental conditions: [PS] = 0.06 mM; [H2O2] = 2 mM; [] = 2 mM; [phenol] = 2 mM; and T = 296.15 K.

Close modal

Effect of matrix on the UV/H2O2 and UV/

The PS dye degradation in artificial seawater and ultrapure water, respectively, was conducted to estimate the effect of the matrix on the UV/H2O2 and UV/ processes. The achieved results are shown in Figure 9.
Figure 9

Effect of the artificial seawater matrix on the PS dye degradation by the UV/H2O2 and the UV/Peroxydate processes. Experimental conditions: [PS] = 0.06 mM, [H2O2] = 2 mM, and T = 296.15 °C.

Figure 9

Effect of the artificial seawater matrix on the PS dye degradation by the UV/H2O2 and the UV/Peroxydate processes. Experimental conditions: [PS] = 0.06 mM, [H2O2] = 2 mM, and T = 296.15 °C.

Close modal
Figure 10

COD concentrations during PS dye degradation by UV/H2O2 and UV/Persulfate processes. Experimental conditions: [PS] = 0.06 mM, [H2O2] = 2 mM, [S2O82−] = 2 mM, and T = 298.15 K.

Figure 10

COD concentrations during PS dye degradation by UV/H2O2 and UV/Persulfate processes. Experimental conditions: [PS] = 0.06 mM, [H2O2] = 2 mM, [S2O82−] = 2 mM, and T = 298.15 K.

Close modal
The UV/H2O2 in artificial seawater achieved lower rate constant kapp = 0.0087 min−1 than in the ultra-pure water kapp = 0.0546 min−1. As the seawater is characterized by a complex composition in several ions that act as scavengers of active hydroxyl radicals, namely chlorides (Equation (29)), sulfates (Equation (30)), and carbonates (Equation (31)) by reducing the degrading efficiency of organic pollutants (Laftani et al. 2021), it has a higher inhibitory effect on PS dye removal in the UV/H2O2 process.
formula
(29)
formula
(30)
formula
(31)
Chlorides are present in large quantities in a marine environment (pH > 8). They are reactive towards hydroxyl radicals generating the radical anion (Equation (32)). In addition, the reactive species HOBr or its conjugate base is rapidly reduced by the hydrogen peroxide present in the H2O2/UV process (Equations (33)–(35)) (Laftani et al. 2021).
formula
(32)
formula
(33)
formula
(34)
formula
(35)
As presented in Figure 9, the PS dye degradation by the UV/ process in the artificial seawater was reduced compared to the ultra-pure water. kapp decreased from 0.163 to 0.1098 min−1. This difference can be contributed to the existence of inorganic anions () in seawater. Those anions functioned as scavengers of (Equations (36)–(41)). Overall, they decelerated the PS dye degradation in the UV/ process.
formula
(36)
formula
(37)
formula
(38)
formula
(39)
formula
(40)
formula
(41)

Measurement of COD

The COD values have been related to the total concentration of organic compounds in the solution (Elmorsi et al. 2010). High COD removal might give the right answer for evaluating the mineralization of the pollutants. The COD test is used to measure the oxygen equivalent to oxidize the organic content in samples treated using the UV/H2O2 and UV/ processes for over 100 min (Figure 10).

Generally, the graphs of the COD concentrations show some irregularities that can be due to the generation of oxygen-consuming intermediates. The results indicate that the UV/H2O2 process resulted in 59.56% mineralization of the dye in about 100 min. While the UV/ process was more efficient and provided 82.35% mineralization of the dye over the same time. The anion sulfate radical is a highly reactive species with a very short lifetime, which allows a greater selectivity to oxidize a wide range of organic compounds. Nevertheless, the hydroxyl radical can act unselectively on organic pollutants up to their mineralization into H2O, CO2, and inorganic ions (Titchou et al. 2021).

This study provided valuable new information regarding comparison of UV/H2O2 and UV/ processes for an emerging organic compound such as PS dye. The degradation rate generally followed the following order: UV/ > UV/H2O2.

The kinetic rate of the UV/H2O2 process increases with an increasing hydrogen peroxide concentration; however, the excess of hydrogen peroxide at 2 mM may accelerate the consumption of hydroxyl radicals and thus decreases the dye reaction rate. Moreover, the lack of dissolved oxygen and the alkaline initial pH at 10.01 considerably decrease the efficiency of the PS dye degradation by the UV/H2O2 process.

Adding anions such as slightly influenced the PS dye degradation by the UV/ process, while the inhibitory effect of was significant. The reactivity of bromides, chlorides, sulfates, and carbonates with generated radicals decelerate the PS dye degradation by both UV/H2O2 and UV/ processes. Furthermore, very poor PS dye degradation rates were observed by adding the phenol that act as a consuming molecule of reactive radicals. High COD removal of 82.35% was achieved by treating PS dye by the UV/ process. Therefore, this study demonstrated that UV/ oxidation is an efficient technology for practical applications in remediating PS dye contamination. This work will provide a potential practical application for the removal of other organic pollutants in water. Nevertheless, the toxicity of the intermediate and final products of pollutant degradation needs more attention.

Y.L. acknowledges all the co-authors for their academic support during the preparation of this study.

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.

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

The authors declare there is no conflict.

Aman
M.
,
Lacina
C.
,
Hogban
C.
&
Jean-Marie
O. P.
2016
Détermination de la demande chimique en oxygène : méthode de reflux en système fermé suivi d'un dosage par colorimétrie avec le bichromate de potassium
. In:
Ministère du Développement durable, de l'Environnement et de la Lutte contre les changements climatiques du Québec
, Vol.
18
.
Centre d'Expertise en Analyse Environnementale du Québec, Québec
, pp.
1
12
.
Ameta
S. C.
2018
Introduction
. In:
Advanced Oxidation Processes for Wastewater Treatment: Emerging Green Chemical Technology
.
PAHER University
,
Udaipur, Rajasthan
,
India
, pp.
1
12
.
doi:10.1016/B978-0-12-810499-6.00001-2
.
Bougdour
N.
,
Sennaoui
A.
,
Bakas
I.
&
Assabbane
A.
2018
Experimental evaluation of Reactive Yellow 17 degradation using UV light and iron ions activated peroxydisulfate : efficiency and kinetic model
.
Science and Technology of Materials
(
1
),
1
9
.
doi: 10.1016/j.stmat.2018.07.002
.
Chatib
B.
,
Laftani
Y.
,
Khayar
M.
,
El Makhfouk
M.
,
Hachkar
M.
&
Boussaoud
A.
2021
Decolorization kinetics of Ponceau S dye by chemical chlorination: a comparison with sunlight/chlorine and UV/Chlorine processe
.
Irannian Journal of Chemistry and Chemical Engineering
40
(
1
),
111
121
.
Chia-Chun
C.
&
Ch
E. B. S.
2006
Dye Decomposition Kinetics by UV/H2O2
.
Deming
G.
,
Changsheng
G.
,
Song
H.
,
Jiapei
L.
,
Yan
Z.
,
Qiyan
F.
,
Yuan
Z.
&
Xu
J.
2019
Kinetic and mechanistic investigation on the decomposition of ketamine by UV-254 nm activated persulfate
.
Chemical Engineering Journal
.
Elsevier B.V
.
doi: 10.1016/j.cej.2019.03.093
.
Elmorsi
T. M.
,
Riyad
Y. M.
,
Mohamed
Z. H.
&
Abd El Bary
H. M. H.
2010
Decolorization of Mordant red 73 azo dye in water using H2O2/UV and photo-Fenton treatment
.
Journal of Hazardous Materials
174
(
1–3
),
352
358
.
doi: 10.1016/j.jhazmat.2009.09.057
.
Gao
Y.
,
Nai-yun
G.
,
Yang
D.
,
Yi-qiong
Y.
&
Yan
M.
2012
light-activated persulfate oxidation of sulfamethazine in water
.
Chemical Engineering Journal
195–196
,
248
253
.
doi:10.1016/j.cej.2012.04.084
.
Gao
Y.
,
Yu-qiong
G.
,
Jia
Z.
,
Li
C.
,
Fu-xiang
T.
&
Nai-yun
G.
2020
Chemosphere comparative evaluation of metoprolol degradation by UV/chlorine and UV/H2O2 processes
.
Chemosphere
243
,
125325
.
doi:10.1016/j.chemosphere.2019.125325
.
Ghodbane
H.
&
Hamdaoui
O.
2010
Decolorization of antraquinonic dye, C.I. Acid Blue 25, in aqueous solution by direct UV irradiation, UV/H2O2 and UV/Fe(II) processes
.
Chemical Engineering Journal
160
(
1
),
226
231
.
doi:10.1016/j.cej.2010.03.049
.
Grasshoff
K.
,
Kremling
K.
&
Ehrhardt
M.
2007
Methods of Seawater Analysis
.
doi: 10.1002/9783527613984
.
Hassaan
M. A.
,
El Nemr
A.
&
Madkour
F. F.
2016
Application of ozonation and UV assisted ozonation for decolorization of Direct Yellow 50 in.Sea water
.
The Pharmaceutical and Chemical Journal
3
(
2
),
131
138
.
Kwon
M.
,
Kim
S.
,
Yoon
Y.
,
Jung
Y.
,
Tae Mun
H.
,
Lee
J.
&
Joon Wun
K.
2015
Comparative evaluation of ibuprofen removal by UV/H2O2 and UV/S2O82- processes for wastewater treatment
.
Chemical Engineering Journal
269
,
379
390
.
Elsevier B.V. doi: 10.1016/j.cej.2015.01.125
.
Laftani
Y.
,
Boussaoud
A.
,
Chatib
B.
,
El Makhfouk
M.
,
Hachkar
M.
&
Khayar
M.
2019a
Comparison of advanced oxidation processes for degrading Ponceau S dye. application of photo-Fenton process
.
Macedonian Journal of Chemistry and Chemical Engineering
38
(
2
),
197
.
doi: 10.20450/mjcce.2019.1888
.
Laftani
Y.
,
Chatib
B.
,
Boussaoud
A.
,
El Makhfouk
M.
,
Hachkar
M.
&
Khayar
M.
2019b
Optimization of diazo dye disappearance by the UV/H2O2 process using the Box–Behnken design
.
Water Science and Technology
80
(
9
).
doi:10.2166/wst.2019.424
.
Laftani
Y.
,
Boussaoud
A.
,
Chatib
B.
,
Hachkar
M.
,
El Makhfouk
M.
&
Khayar
M.
2021
Photocatalytic degradation of a diazo-dye in artificial seawater matrix: optimization of UV/H2O2 process on the Ponceau S decolorization by using central composite design
.
Environmental Engineering Research
27
(
3
),
210002
0
.
doi: 10.4491/eer.2021.002
.
Lia
J.
,
Tao
T.
,
Xue-bin
L.
,
Jiao-lan
Z.
,
Tong
L.
,
Jin
L.
,
Shu-hui
L.
,
Li-zhen
C.
,
Chun-yang
X.
,
Yong
L.
&
Yan-li
W.
2009
A spectrophotometric method for determination of chemical oxygen demand using home-made reagents
.
Desalination
239
(
1–3
),
139
145
.
doi: 10.1016/j.desal.2008.03.014
.
Lin
C. C.
,
Lin
H. Y.
&
Hsu
L. J.
2016
Degradation of ofloxacin using UV/H2O2 process in a large photoreactor
.
Separation and Purification Technology
168
,
57
61
.
doi: 10.1016/j.seppur.2016.04.052
.
Ma
X.
,
Tang
L.
,
Deng
J.
,
Liu
Z.
,
Li
X.
,
Wang
P.
&
Li
Q.
2021
Removal of saccharin by UV/persulfate process: degradation kinetics, mechanism and DBPs formation
.
Journal of Photochemistry and Photobiology A: Chemistry
420
,
113482
.
doi: 10.1016/j.jphotochem.2021.113482
.
Penru
Y.
,
Andrea
R. G.
,
Santiago
E.
&
Sylvie
B.
2012
Application of UV and UV/H2O2 to seawater: disinfection and natural organic matter removal
.
Journal of Photochemistry and Photobiology A: Chemistry
233
,
40
45
.
doi: 10.1016/j.jphotochem.2012.02.017
.
Ribeiro
J. P.
,
Juliene
T. O.
,
André
G. O.
,
Francisco
W. S.
,
Eliezer
F. A. N.
,
Carla
B. V.
,
Denis de
K.
,
Santos
A. B. d.
&
Ronaldo
F. N.
2014
Treatment of sulfonated Azo Dye reactive Red 198 by UV/H2O2
.
Journal of Chemistry
2014
.
doi:10.1155/2014/619815
.
Surpateanu
M.
&
Carmen
Z.
2004
Advanced oxidation processes for decolorization of aqueous solution containing Acid Red G azo dye
. Central European Journal of Chemistry
2
(
4
),
573
588
.
Tan
C.
,
Dafang
F.
,
Naiyun
G.
,
Qingdong
Q.
,
Yan
X.
&
Huiming
X.
2016
Kinetic degradation of Chloramphenicol in Water by UV/Persulfate syste
.
Journal of Photochemistry & Photobiology, A: Chemistry
.
doi:10.1016/j.jphotochem.2016.09.021
.
Titchou
F. E.
,
Hicham
Z.
,
Hanane
A.
,
Jamila
E. G.
,
Rachid
A. A.
,
Puthiya
V. N.
&
Mohamed
H.
2021
Removal of organic pollutants from wastewater by advanced oxidation processes and its combination with membrane processe
.
Chemical Engineering and Processing – Process Intensification
169
(
June
),
108631
.
doi: 10.1016/j.cep.2021.108631
.
Yang
Y.
,
Joseph
J. P.
,
Jun
M.
&
William
A. M.
2014
Comparison of halide impacts on the efficiency of contaminant degradation by sulfate and hydroxyl radical-based advanced oxidation processes (AOPs)
.
Environmental Science Technology
48
(
4
),
2344
2351
.
doi: 10.1021/es404118q
.
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