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The discharge of synthetic dyes into the environment poses a significant threat to both human health and the ecosystem, necessitating the treatment of contaminated water. To generate free radicals for the elimination of Direct Blue 71 (DB71) dye from aqueous solutions, periodate (PI) and chlorine (Cl2) have been employed. In this study, separate activation of PI and Cl2 was achieved using ultraviolet (UV) light. The impact of various operational parameters was investigated, resulting in the complete degradation of the dye within 12 min. The presence of ferrous and copper ions had a minor enhancing effect on the degradation rate in both systems. Scavenging experiments confirmed that HO• and IO3• were the primary agents responsible for DB71 degradation in the UV/PI system, while reactive chlorine radicals played a dominant role in the UV/Cl2 process. In terms of mineralization, application for real wastewater and energy efficiency, the UV/PI system exhibited slightly superior performance compared to the UV/Cl2 system.

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  • UV/chlorine and UV/periodate were compared together for the first time.

  • A complete degradation of DB71 was achieved in at short time for both processes.

  • The carboxylic evolution of DB71 degradation was monitored.

  • A practical study was conducted on real wastewater.

advanced oxidation processes, chlorine, Direct Blue 71, periodate, textile wastewater
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Various contaminants, such as organic pollutants, nutrients, bacteria, metals and micro-pollutants, can be found in water resources (Al-Hazmi et al. 2022). In recent years, an increase in the production of colored products has been observed. In fact, to enhance the attractiveness of products, various dyes are added to raw materials since color is the biggest issue for customers. The variety of colors increases the extended production of various dyes in different industries (Yousefi et al. 2017). Azo dyes account for around 60–70% of all dyes used in manufacturers containing one or more azo bonds (–N = N–). Among azo dyes, Direct Blue 71 (DB71) is widely used as blue color for various products. It is known as Direct Fast Blue and structurally has eight aromatic rings (Lu et al. 2017; Yousefi et al. 2017). Due to its complicated molecular structure, it is highly resistant to bio-degradation in natural environments leading to the acute toxicity of the ecosystem. There is a severe concern when high concentrations of synthetic dyes enter the water bodies (Matouq et al. 2014). Recently, synthetic dyes have demonstrated a genotoxicity property and also it has been reported long-term carcinogenicity synthetic dyes (Ganiyu et al. 2019). The water contaminated with dyes and wastewater generated from manufacture-producing dye should be treated since these effluents become an earnest threat to aquatic life (Thiam et al. 2015). Several physicochemical processes have been employed for dye degradation in recent decades. Most of them are inefficient for complete dye degradation in the water environment. Moreover, some limitations such as long reaction time and sludge generation stimulate researchers to apply new methods for water and wastewater treatment (Al-Hazmi et al. 2023). Advanced oxidation processes (AOPs) are effective technologies for decontamination using the generation of reactive species (Khavari Kashani et al. 2022; Hassani et al. 2023). Classically, hydroxyl radicals (HO•) have been considered as the main species for AOPs that are produced through various methods at room temperature and atmospheric pressure (Khiem et al. 2023). Recently, a variety of reactive species has been examined for the degradation of organic pollutants (Ghanbari et al. 2021; Eslami et al. 2023a). In this way, different and new oxidants have been applied for the generation of various reactive species such as sulfate radicals, chlorine radicals, singlet oxygen, periodate radicals and organic radicals (Afify et al. 2023; Eslami et al. 2023b). In such a manner, chlorine (Cl2) and periodate (⁠⁠) have been used in AOPs for the generation of different free radicals. To activate Cl2 and ⁠, various methods have been used including UV irradiation (Yaghoot-Nezhad et al. 2023), ultrasound (US) (Eslami et al. 2023b), transition metals (Mao et al. 2021; Yaghoot-Nezhad et al. 2023), carbon catalysts (He et al. 2022) and alkaline (Bokare & Choi 2015). UV-based activation of oxidants is an effective, practical and chemical-less method which has been widely investigated for the degradation of organic pollutants. UV/chlorine and UV/periodate are new AOPs which are able to produce powerful free radicals based on the following equations (Ghanbari et al. 2021; Yaghoot-Nezhad et al. 2023).
(1)
(2)
(3)
(4)
(5)
(6)
(7)

As above-mentioned, besides HO• generation, several reactive species are generated in UV/periodate and UV/chlorine which may enhance the degradation process. UV/chlorine and UV/periodate have been separately examined on the various pollutants. Accordingly, UV/chlorine have been used for acetaminophen (Ghanbari et al. 2021), sulpiride (Zhang et al. 2023), 1,4-dioxane (Masjoudi & Mohseni 2023), Bisphenol A (Cao et al. 2023) and dyes (Rafiei et al. 2021), while few contaminates including ciprofloxacin (Zhang et al. 2022), 2,4-dichlorophenol (Zhang et al. 2021), para-nitrophenol (Eslami et al. 2023b) and dye (Bendjama et al. 2018) have been considered for UV/periodate.

Although both methods (UV/periodate and UV/chlorine) have been separately used for different pollutants, there is no study to show the comparison of both methods for the degradation of a sample of pollutants. Moreover, there are various unknown aspects of both processes including the mechanism, application for real wastewater and promotional factors that their studies are necessary.

In this work, UV/periodate and UV/chlorine were investigated on the dye degradation. The decolorization of the dye was studied under different conditions and several variables were scrutinized. Moreover, the mineralization degree and the formation of carboxylic acids were vindicated. Finally, the application of UV/periodate and UV/chlorine processes was tested on real wastewater.

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Chemicals and reagents

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Sodium periodate (⁠⁠, 99.8%), phenol and tert-butyl alcohol (TBA) were purchased from Samchun company. NaOCl (10%) was purchased from Merck. Sodium chloride (NaCl, >99%), sodium bicarbonate (NaHCO3, >99%), potassium nitrate (KNO3, >99%), sodium sulfate (Na2SO4, 99%) and ferrous sulfate (FeSO4) were supplied from Chem-Lab company. Furfuryl alcohol (FFA), 2-Propanol anhydrous (99.5%), 1,4-Benzoquinone (BQ) (98%) were purchased from Acros-organic Company. Real textile wastewater was collected from a manufacturer in Zanjan City (Iran). The characteristics of real wastewater are presented in Table 1.

Table 1

Characteristics of real textile wastewater

ParameterUnitValue
COD (Chemical Oxygen Demand) mg/L 870 
Color ADMI 2,330 
TDS (Total Dissolved Solids) mg/L 560 
TOC mg/L 290 
pH – 6.4 
ParameterUnitValue
COD (Chemical Oxygen Demand) mg/L 870 
Color ADMI 2,330 
TDS (Total Dissolved Solids) mg/L 560 
TOC mg/L 290 
pH – 6.4 
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Oxidative experiments

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Experiments were conducted to degrade the DB71 dye solution within a glass cylinder. The reactor (a 250 mL flat beaker) contained 100 mL of synthetic dye solution, and then a precise amount of oxidant was introduced. The pH of the solution was adjusted using H2SO4 and NaOH. To initiate the degradation process, a UVC lamp (Osram 4 W, 254 nm) was employed, placed at a distance of 30 mm from the solution while the UV lamp was placed above the reactor (UV intensity = 1.01 mW/cm2). The UV light intensity was measured by A radiometer (Lux-UV-IR meter, Leybold Didactic GMBH-666-230). Throughout the experiment, the dye solution was constantly mixed using a magnetic stirrer while maintaining a temperature range of 23–25 °C. A known amount of sodium periodate and sodium hypochlorite was separately added to the solution. Once the UVC lamps were activated, the oxidative process commenced to remove the DB71 compound. At specific time intervals, 2.5 mL samples were extracted from the solution for subsequent determination of the DB71 concentration. To identify the reactive species, some quencher agents (TBA, BQ, phenol, FFA, and 2-Propanol) were added to the dye solution before the oxidation process. Moreover, some experiments using some anions and humic acids were conducted to determine the effect of the water matrix. All experiments were conducted in triplicate and their average was reported.

Analytical methods

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For analytical purposes, the concentrations of DB71 were assessed using a UV-vis spectrophotometer at a wavelength of 587 nm. The total organic carbon (TOC) value was measured using a Shimadzu-VCSH model TOC analyzer. Furthermore, the carboxylic acid by-products were identified using a Waters model high-performance liquid chromatography at a wavelength of 220 nm. The color of real textile wastewater was measured by the American Dye Manufacture Institute (ADMI) method by a spectrophotometer (HACH, DR5000).

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Comparison of DB71 elimination by multiple decontamination procedures

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Figure 1(a) evinces the DB71 decomposition in aqueous media adopting several oxidation methods. As displayed, the UV, PI and Cl2 sole processes had unsatisfactory decontamination efficacies of roughly 5, 8 and 10%, respectively. It can be deduced that the individual use of these processes is incapable of decomposing organic pollutants owing to the inability to produce reactive oxygen species (ROS). Nonetheless, hybridizing UV with PI and Cl2 resulted in a remarkable DB71 elimination percentage of 100%. The complete decontamination efficiency of the target pollutant utilizing these processes is attributed to the formation of ROS and reactive chlorine species (RCS) due to the direct photolysis of PI and Cl2. The oxidizing radical agents responsible for the promoted DB71 removal rate are recognized to be and •OH in the UV/PI procedure and integration of chlorine-based radicals and •OH in the UV/Cl2 procedure. To further investigate the capability of the UV/PI and UV/Cl2 systems in eliminating DB71, the reaction rate constant (kobs) of whole treatment systems is elicited in Figure 1(b). As noted, the computed values of kobs for UV, PI, Cl2, UV/PI and UV/Cl2 decontamination procedures accounted for 0.0081, 0.0159, 0.0167, 0.5174 and 0.4847 min−1, respectively. Based on these results, it can be clarified that coupling ultraviolet irradiation with chemical oxidation systems leads to a strong synergistic impact.
Figure 1
(a) DB71 degradation by UV/PI and UV/Cl2 and (b) the rate constant of DB71 degradation (pH = 3, DB71 = 20 mg/L, PI = Cl2 = 0.75 mM).
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(a) DB71 degradation by UV/PI and UV/Cl2 and (b) the rate constant of DB71 degradation (pH = 3, DB71 = 20 mg/L, PI = Cl2 = 0.75 mM).

Figure 1
(a) DB71 degradation by UV/PI and UV/Cl2 and (b) the rate constant of DB71 degradation (pH = 3, DB71 = 20 mg/L, PI = Cl2 = 0.75 mM).
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(a) DB71 degradation by UV/PI and UV/Cl2 and (b) the rate constant of DB71 degradation (pH = 3, DB71 = 20 mg/L, PI = Cl2 = 0.75 mM).

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The effect of operating parameters

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Figure 2(a) illustrates the elimination of DB71 using the UV/PI process with a PI concentration of 0.75 mM at various pH levels (3–9). The utmost efficacy was achieved at pH = 3, resulting in a 99.9% removal of DB71. Whereas, as the solution's pH was further increased to 9, a minor decline in the rate constant for dye degradation was detected. Long et al. (2022) documented a notable decrease in removal efficiency under neutral and alkaline conditions. It is worth noting that at pH < 8, predominates as the primary species, which readily decomposes into free reactive radicals (Eslami et al. 2023b). This explains the better performance of the UV/PI process in acidic conditions.
Figure 2
The effect of operating parameters on UV/PI process: (a) pH, (b) PI dosage, and (c) DB71 concentration; the effect of operating parameters on UV/Cl2 process, (d) pH, (e) Cl2 dosage, and (f) DB71 concentration.
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The effect of operating parameters on UV/PI process: (a) pH, (b) PI dosage, and (c) DB71 concentration; the effect of operating parameters on UV/Cl2 process, (d) pH, (e) Cl2 dosage, and (f) DB71 concentration.

Figure 2
The effect of operating parameters on UV/PI process: (a) pH, (b) PI dosage, and (c) DB71 concentration; the effect of operating parameters on UV/Cl2 process, (d) pH, (e) Cl2 dosage, and (f) DB71 concentration.
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The effect of operating parameters on UV/PI process: (a) pH, (b) PI dosage, and (c) DB71 concentration; the effect of operating parameters on UV/Cl2 process, (d) pH, (e) Cl2 dosage, and (f) DB71 concentration.

Close modal
The amount of oxidizing agent used in chemical oxidation plays a critical role in the generation of free radicals. Therefore, an increase in oxidant dosage promotes the production of HO•, RCS and iodine radicals, leading to enhanced dye degradation in UV/PI. In this study, a considerable improvement in DB71 degradation was observed when the PI concentration was raised to 0.5 mM, due to the increased amount of radicals present in the solution (Figure 2(b)). The degradation rate was further increased as oxidant dosage was raised to 1.25 mM, however, the DB71 removal was only slightly improved as dosage was increased from 0.75 to 1.25 mM in UV/PI. It can be deduced that excess can quench the generated radicals at a very fast rate (Equation (8)). Moreover, the recombination of iodine radicals due to reacting with one another at higher periodate concentrations could also result in lower DB71 removal rates (Equations (9) and (10)). Hence, 0.75 mM was chosen as the optimum oxidant dose in other experiments.
(8)
(9)
(10)

The effect of the initial dye concentration on the removal rate for UV/PI has been investigated and the results are presented in Figure 2(c). It is apparent that increasing the dye concentration leads to much lower removal rates. This can be explained by considering that at a fixed oxidant dosage, a specific amount of radicals is produced. Hence, by increasing the target pollutants present in the reaction media, lower removal rates are expected.

In the UV/Cl2 process, the presence of ClO− and HOCl species can be influenced by the pH of the solution. Figure 2(d) illustrates the elimination of DB71 by UV/Cl2 process with a chlorine concentration of 0.75 mM at various pH levels. The results indicate that as pH was increased, the DB71 removal rate was reduced. Moreover, the highest removal rate was achieved at pH = 3–7, while the lowest rate was achieved at pH = 9. Further, the elimination of DB71 was observed to be satisfactory in acidic to neutral conditions (pH levels of 3–7), where the removal rates are very close in value. In acidic solutions (pH < 6.5), HOCl dominates, as the pKa of HOCl/OCl− is 7.5, whereas OCl− dominates at pH > 8.5 (Yaghoot-Nezhad et al. 2023). Previous studies have suggested a higher quantum yield for HOCl (0.62) compared to OCl− (0.55) under 254 nm UV irradiation (Zhao et al. 2019). Consequently, it can be inferred that HOCl photolysis yields higher efficiency in comparison to OCl− (Watts & Linden 2007). Therefore, considering that the HOCl:OCl− ratio is reduced as pH is increased (Wang et al. 2016), indicating that more hydroxyl and chlorine radicals are produced in acidic conditions. It is also important to note that OCl− exhibits higher reactivity than HOCl in scavenging HO• and Cl• radicals (Equations (11)–(14)) (Lu et al. 2018), which further explains the lower rates observed at higher pH levels.
(11)
(12)
(13)
(14)

Figure 2(e) shows the effect of chlorine dosage on DB71 removal. A dramatic enhancement was observed in DB71 removal when chlorine dosage was increased from 0.1 to 0.75 mM. After that, insignificant decolorization was observed when chlorine dosage was increased to 1.25 mM. It has been reported that oxidant dosage should be at the optimum amount and its excess dosage leads to negative results. This can be attributed to the scavenging of radicals by excess HOCl and OCl− in higher chlorine dosage of the UV/Cl2 process, as described in Equations (11)–(14) (Huang et al. 2019). The primary product of this scavenging effect in cases of chlorine overdose is ClO•, which is known to act as a weak oxidizing agent.

Figure 2(f) depicts DB71 removal by UV/Cl2 under various DB71 concentrations. Complete degradation took place at 8, 12, 14 and 20 min for initial DB71 concentrations of 10, 20, 30 and 40 mg/L, respectively. In fact, an increase in the dye concentration needs more ROS and RCS or more reaction time.

The determination of reactive species

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Mounting recent studies have shown multiple ROS, such as ⁠, HO•, and in UV/PI process and HO• and reactive chlorine agents in the UV/Cl2, could be formed in the oxidation system. To clearly understand the impact of ROS responsible for the decontamination of DB71 via UV/Cl2 and UV/PI processes, quenching experiments were conducted in the presence of multiple chemical probes at optimized operating circumstances. The FFA (a rate constant of 1.2 × 108 M−1 s−1) and BQ (a rate constant of 1 × 109 M−1s−1) were employed in UV/PI as scavengers to trap and ⁠, respectively (Gao et al. 2022). The findings from Figure 3(a) illustrate that the elimination rate of DB71 was not declined with the increase in the concentration of BQ and FFA. The phenomenon revealed that the contribution of and radicals is insignificant during the decontamination of DB71. TBA and 2-Propanol were utilized as quenchers of HO• and O(3P)/HO• over the degradation of DB71 in the UV/PI system, respectively (Bendjama et al. 2018). Indeed, the difference between DB71 degradation in the presence of TBA and 2-Propanol indicates the contribution of O(3P). Accordingly, the role of O(3P) in the degradation of DB71 was scant. As displayed in Figure 3(a), the decontamination rate of DB71 in the presence of TBA was considerably reduced. These findings showed that HO• radicals played a major role in the decomposition of DB71 over the UV/PI system. Phenol has a good scavenging feature for / and HO• radicals (Bendjama et al. 2018). As it can be observed in Figure 3(a), in the presence of phenol, the DB71 decontamination was significantly inhibited, which illustrated that / was also a major oxidative agent in DB71 elimination. This was confirmed by monitoring concentration over 12 min of operating time, as shown in Figure 3(b). The content of iodate was improved after 12 min of oxidation time and it reached 0.37 mM, revealing that PI was degraded and converted into HO• and radicals as the major oxidation agents responsible for the degradation of DB71. Regarding the results obtained, the and HO• were the main ROS for the degradation of DB71 in which the electron transfer (from HO• and to DB71), H-abstraction and radical addition using HO• are three possible mechanisms for the degradation of DB71.
Figure 3
(a) The effect of scavengers on UV/PI (BQ = FFA = 2 mM, 2-propanol = TBA = phenol = 50 mM, PI = 0.75 mM, pH = 3 and DB71 = 20 mg/L), (b) iodate concentration during UV/PI process, and (c) The effect of scavengers on UV/Cl2 (TBA = NB = HCO3− = 50 mM, Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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(a) The effect of scavengers on UV/PI (BQ = FFA = 2 mM, 2-propanol = TBA = phenol = 50 mM, PI = 0.75 mM, pH = 3 and DB71 = 20 mg/L), (b) iodate concentration during UV/PI process, and (c) The effect of scavengers on UV/Cl2 (TBA = NB = HCO3− = 50 mM, Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Figure 3
(a) The effect of scavengers on UV/PI (BQ = FFA = 2 mM, 2-propanol = TBA = phenol = 50 mM, PI = 0.75 mM, pH = 3 and DB71 = 20 mg/L), (b) iodate concentration during UV/PI process, and (c) The effect of scavengers on UV/Cl2 (TBA = NB = HCO3− = 50 mM, Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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(a) The effect of scavengers on UV/PI (BQ = FFA = 2 mM, 2-propanol = TBA = phenol = 50 mM, PI = 0.75 mM, pH = 3 and DB71 = 20 mg/L), (b) iodate concentration during UV/PI process, and (c) The effect of scavengers on UV/Cl2 (TBA = NB = HCO3− = 50 mM, Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Close modal

Three different quenchers nitrobenzene (NB), TBA and bicarbonate were deployed to capture the produced radicals in UV/Cl2 system. Herein, 50 mM of bicarbonate, TBA and NB was employed to quench HO•/Cl•/ and HO•/Cl•/ClO• and HO•, respectively. As displayed in Figure 3(c), all scavengers had significant inhibitory impacts on the decontamination efficiency of DB71 except for NB. The order of inhibitory impact of scavengers is bicarbonate > TBA > NB. Considering that NB is a strong scavenger of hydroxyl radicals (a rate constant of 3.9 × 109 M−1 s−1) (Remucal & Manley 2016), it can be concluded that HO• had a lower impact on DB71 removal in the UV/Cl2 system than that of UV/PI. Furthermore, all radicals can be quenched by bicarbonate and TBA except for ClO• and ⁠, respectively (Wu et al. 2017; Gao et al. 2020). Therefore, it is apparent that RCS has played a major role in DB71 removal in the UV/Cl2 system. Indeed, it can be concluded that ⁠, ClO• and Cl• were corresponding agents for DB71 degradation. These radicals can degrade DB71 through dehydrogenation, single electron transfer and addition to unsaturated bonds. It should be stated that the higher contribution of HO• in UV/PI resulted in a better performance in DB71 degradation since HO• is non-selective compared to RCS in UV/Cl2.

The impact of co-existing anions

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Practically, the existence of natural water constituents can have quenching impacts on the contaminant degradation potential. In view of this, the impact exerted by the ⁠, ⁠, ⁠, and Cl‒ on the potential of DB71 decomposition utilizing UV/PI and UV/Cl2 systems were scrutinized under optimized circumstances and the outcomes are illustrated in Figure 4(a) and 4(b). It is apparent that the addition of ⁠, and Cl‒ did not have a significant influence on DB71 decomposition efficiency over the UV/PI and UV/Cl2 processes after 12 min of operating time. It should be noted that Cl− anions have different influences on the degradation potential of AOPs, including quenching, elevating and neutral impact (Huang et al. 2019). In this study, Cl− ions had a neutral impact on the degradation efficiency of DB71, owing to the resultant of both quenching and promoting effects of Cl− in aqueous media. Furthermore, and anions did not have noticeable impacts on the performance of UV/PI and UV/Cl2 processes, as the DB71 degradation efficiency was similar to controlled circumstances. Although it is proven that and can alter the capability of AOPs (De Laat & Le 2005; Huang et al. 2019), the current study showed that the aforementioned anions did not have an inhibitory impact on the decomposition efficiency of DB71 during Cl2 and PI-based AOPs. In contrast, and anions are acknowledged as strong quenchers affecting the performance of AOPs. As can be seen, the decomposition efficiency of DB71 was significantly reduced in the presence of and ⁠. Note that and had the greatest quenching effect over the UV/Cl2 and UV/PI processes, respectively. According to Equations (15) and (16), and Cl• may be quenched by and formed ⁠. Moreover, the aforementioned anion is able to scavenge (Equation (17)).
(15)
(16)
(17)
Figure 4
The effect of anions on DB71 degradation using (a) UV/PI (b) UV/Cl2 (anions− = 5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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The effect of anions on DB71 degradation using (a) UV/PI (b) UV/Cl2 (anions− = 5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Figure 4
The effect of anions on DB71 degradation using (a) UV/PI (b) UV/Cl2 (anions− = 5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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The effect of anions on DB71 degradation using (a) UV/PI (b) UV/Cl2 (anions− = 5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Close modal
Also, can react with and Cl• and produce (Equations (18) and (19)).
(18)
(19)

The as-mentioned relations illustrate that and anions could easily react with various reactive species and scavenge all the major radicals generated during the UV/PI and UV/Cl2 processes. Hence, the addition of and resulted in the reduction of DB71 degradation potential in both systems.

Effect of HA on DB71 degradation

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The DB71 degradation rate was decreased in the presence of humic acid (HA). As can be seen in Figure 5(a) and 5(b), with the addition of HA, the removal rates related to the UV/PI and UV/Cl2 processes were reduced. To elucidate the adverse influence of HA on the removal of DB71 during UV-based oxidation methods, the subsequent explanations are put forth:
  • (1)

    HA functions as a UV barrier that hinders the absorption of photons by periodate and chlorine. Therefore, the initiation of oxidative radicals (⁠⁠, ⁠, HO• and RCS) generation for DB71 elimination is impeded in both UV/PI and UV/Cl2 processes. In fact, natural organic matter absorbs UV light at 254 nm and its extinction coefficient is 3.15. Hence, it acts as an inner filter to decrease PI and Cl2 (Fang et al. 2014).

  • (2)

    HA additionally suppresses the activity of the oxidative radicals generated during each process. HA is capable of effectively scavenging the produced hydroxyl and chlorine radicals. The reaction between HO• and Cl• radicals with HA occurs at rate constants of 2.5 × 104 and 1.3 × 104 (mg C/L)−1 s−1, respectively (Fang et al. 2014). Therefore, HA can significantly affect DB71 degradation through the competition with DB71 for the reaction with HO• and Cl• radicals.

Figure 5
The effect of humic acid on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (HA = 2 mg/L, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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The effect of humic acid on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (HA = 2 mg/L, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Figure 5
The effect of humic acid on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (HA = 2 mg/L, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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The effect of humic acid on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (HA = 2 mg/L, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

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Effect of transition metals on DB71 degradation

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Transition metals have been extensively employed in UV/oxidant-based systems for pollutant degradation. In the chlorine/Fe(II) process, HOCl reacts with ferrous ions, and as a result, hydroxyl and chlorine radicals are produced (Equations (20) and (21)) (Zohra Meghlaoui et al. 2019).
(20)
(21)
Compared to other UV/oxidant systems, photo-Fenton-like processes have shown superior effectiveness in removing contaminants. In these processes, the Fe-complex decomposes into and ferrous ions (Equation (22)) (Liu et al. 2018). The limitation in the rate of ferrous ion regeneration in Fenton-based processes is successfully overcome by UV irradiation. This results in the rapid conversion of Fe3+ to ferrous ions and the simultaneous generation of HO• (Equation (23)) (Khavari Kashani et al. 2023).
(22)
(23)
Similar to ferrous ions, copper ions can also catalyze HOCl to produce hydroxyl and chlorine radicals in the UV/Cl2 process. It should be noted that in contrast with Fe2+, Cu2+ is at a higher state (Cu(II)/Cu(I)), hence, there is likely a mediated reaction prior to the formation of Cl• and HO• in Equations (20) and (21). In fact, first, Cu2+ is reduced into Cu+ and then hypochlorous acid is broken down into hydroxyl and chlorine radicals. Based on the Fenton chemistry, the formation of speculative reactions is probable in the chlorine/Cu(II) process according to the following equations.
(24)
(25)
(26)
According to Figure 6(a), PI can also be activated with ferrous and copper ions, considering that the DB71 degradation rate using the UV/PI process was slightly improved in the presence of these transition metals. The following equations present the reactions between PI and the aforementioned ions, resulting in the formation of and radicals.
(27)
(28)
Figure 6
The effect of transition metals on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (Cu(II) = Fe(II) = 0.5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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The effect of transition metals on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (Cu(II) = Fe(II) = 0.5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Figure 6
The effect of transition metals on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (Cu(II) = Fe(II) = 0.5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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The effect of transition metals on DB71 degradation using (a) UV/PI and (b) UV/Cl2 (Cu(II) = Fe(II) = 0.5 mM, PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Close modal

As can be seen in Figure 6(b), a slight enhancement was observed in the degradation rate of DB71 using UV/Cl2 process when Fe2+ and Cu2+ were added to the solution, due to the generation of oxidative radicals.

Moreover, UV irradiation plays a major role in DB71 degradation through not only ferrous ions regeneration (Equations (22) and (23)) but also the photolysis of HOCl (Equation (5)), OCl− (Equation (6)) and PI (Equations (1) and (2)), which leads to the production of further reactive radicals.

Hence, the presence of transition metals has proven beneficial for DB71 degradation in the UV/Cl2 and UV/PI processes.

Mineralization extent

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Investigations were carried out to analyze the DB71 mineralization degree through the employment of UV/Cl2 and UV/PI processes, and the results are depicted in Figure 7. Extensive research supports that a satisfactory level of mineralization can be accomplished by utilizing UV-based methods in combination with oxidants, thanks to the presence of UV irradiation (Li et al. 2016; Deng et al. 2019). It is apparent that around 39% of DB71 was mineralized within a 30-min reaction time using the UV/Cl2 system. Conversely, the UV/PI system exhibited a higher degree of mineralization, reaching 48% within the same duration. Previous studies have indicated that hydroxyl radicals play a significant role in the mineralization of organic pollutants (Lin et al. 2016; Wu et al. 2016). Thus, the disparity in mineralization levels between the UV/PI and UV/Cl2 systems can be explained by the greater contribution of hydroxyl radicals in the former process. It should be noted that the chlorinated substances generated in chlorine-based processes exhibited the highest resistance to mineralization (Huang et al. 2017). Consequently, a lower degree of mineralization should be expected from the UV/Cl2 process compared to UV/PI. Additionally, the degradation of organic pollutants can be a challenging process, often involving the formation of various intermediates before the parent compound can be completely reduced to CO2. Hence, the limited mineralization of DB71 can be attributed to the extended and intricate reactions involved in the complete removal of this compound.
Figure 7
TOC removal from DB71 solution by UV/PI and UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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TOC removal from DB71 solution by UV/PI and UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Figure 7
TOC removal from DB71 solution by UV/PI and UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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TOC removal from DB71 solution by UV/PI and UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

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Carboxylic acids evolution

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Four carboxylic acids including oxalic acid, malic acid, formic acid and tartronic acid were monitored during DB71 degradation by UV/PI and UV/Cl2 processes, in which these compounds correspond to the ring opening products of DB71 deriving from the attack of reactive radicals. The aryl groups of the aromatic dye are anticipated to undergo cleavage, resulting in the production of tartronic and malic acids (Diagne et al. 2007; Brillas et al. 2009). These acids, in turn, undergo separate transformations and are converted into oxalic and formic acids, respectively. Figure 8(a) and 8(b) depict the concentration of these carboxylic acids during DB71 degradation. The detected concentrations for UV/PI and UV/Cl2 processes are as follows, respectively: Malic acid (0.3 and 1.6 mg/L), tartronic acid (1.2 and 2.3 mg/L), oxalic acid (1.5 and 0.8 mg/L) and formic acid (1.4 and 0.2 mg/L). First, considering the overall increase in concentration of oxalic acids over a 30-min reaction time, the successful oxidation of DB71 compound is vindicated by both processes utilized. Further, it is evident that the measured concentrations of malic and tartronic acids are higher in the UV/Cl2 process compared to UV/PI, whereas the concentrations of oxalic and formic acids are higher in the UV/PI process. Note that the oxidation of malic and tartronic leads to the production of oxalic and formic acids, which are the final acids that undergo direct mineralization into CO2 (Garcia-Segura et al. 2011). Therefore, considering that higher amounts of oxalic and formic acids are formed in the UV/PI process and the concentrations of malic and tartronic acids are considerably lower, it is more efficient in mineralizing DB71 than UV/Cl2. These findings are in agreement with the results of Figure 7.
Figure 8
Evolution of the concentration of the carboxylic acids within DB71 degradation: (a) UV/PI and (b) UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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Evolution of the concentration of the carboxylic acids within DB71 degradation: (a) UV/PI and (b) UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

Figure 8
Evolution of the concentration of the carboxylic acids within DB71 degradation: (a) UV/PI and (b) UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).
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Evolution of the concentration of the carboxylic acids within DB71 degradation: (a) UV/PI and (b) UV/Cl2 (PI = Cl2 = 0.75 mM, pH = 3 and DB71 = 20 mg/L).

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EE/O analysis

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From an economic standpoint, this study compared the UV/PI and UV/Cl2 processes employed for DB71 removal by assessing the expenses associated with the amount of electricity used. The electrical energy per order (EE/O) is a metric that quantifies the amount of electric energy, measured in kWh, needed to reduce the contaminant concentration by one order of magnitude in one cubic meter of effluent. This evaluation is performed using the following equation (Bolton et al. 2001).
(29)
where P represents the total electrical power in kilowatts (kW) of UV lamps used, V indicates the volume in liters (L) of DB71 solution, t in hours (h) represents the reaction period, and C0 and Ct, respectively, denote the initial and final measured DB71 concentrations. Figure 9 illustrates the EE/O values for the elimination of DB71. The results demonstrate significantly higher EE/O values for the mineralization of DB71 (34.74 and 63.42 kWhm−3order−1) in comparison to its removal (1.67 and 11.97 kWhm−3order−1) when utilizing the UV/PI and UV/Cl2 systems, respectively. Based on the findings, the UV/PI method proves to be a more cost-efficient approach for degrading DB71. This suggests that a more favorable outcome can be attained while utilizing fewer resources. Furthermore, the UV/Cl2 method requires a higher energy input compared to UV/PI when it comes to mineralizing DB71. This is likely due to the potential formation of more complex by-products that present greater challenges in the oxidation process.
Figure 9
EE/O for UV/PI and UV/Cl2 for the removal of DB71 and TOC.
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EE/O for UV/PI and UV/Cl2 for the removal of DB71 and TOC.

Figure 9
EE/O for UV/PI and UV/Cl2 for the removal of DB71 and TOC.
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EE/O for UV/PI and UV/Cl2 for the removal of DB71 and TOC.

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Application for real matrix

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AOPs are usually under the influence of real conditions in which several anions, cations and organic compounds can compete with the reactive species for the reaction with the target pollutant. In this way, UV/PI and UV/Cl2 were tested on a real textile wastewater and the results are presented in Figure 10. Compared to synthetic solution, a long reaction time is needed to decolorize the wastewater in a way that 78 and 64.8% of color were removed by UV/PI and UV/Cl2 systems during 30 min reaction time, respectively. At the same time, 38 and 22.1% of TOC were also eliminated by UV/PI and UV/Cl2 systems, respectively. The results showed that PI was more effective than chlorine in the generation of reactive species to destroy organic pollutants. In comparison with synthetic solution, UV/PI and UV/Cl2 had lower efficiency. It can be attributed to several factors. First, the natural pH of the real wastewater is close to neutral conditions in which the power of hydroxyl radicals is reduced. Second, the presence of various anions and cations (TDS = 560 mg/L) can affect the free radicals and oxidants (⁠ and HOCl) and reduce the degradation rate of organic compounds. Third, colloidal solids can absorb UV irradition and reduce PI or Cl2 activation. Nevertheless, some strategies should be considered in real wastewater treatment like the use of pre-treatment and increase of oxidant dosage to obtain an acceptable result.
Figure 10
The performance of UV/PI and UV/Cl2 on TOC and color removals from real wastewater (PI = Cl2 = 2 mM and pH = 6.9).
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The performance of UV/PI and UV/Cl2 on TOC and color removals from real wastewater (PI = Cl2 = 2 mM and pH = 6.9).

Figure 10
The performance of UV/PI and UV/Cl2 on TOC and color removals from real wastewater (PI = Cl2 = 2 mM and pH = 6.9).
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The performance of UV/PI and UV/Cl2 on TOC and color removals from real wastewater (PI = Cl2 = 2 mM and pH = 6.9).

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Comparison with other advanced processes

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There are several processes for the removal/degradation of DB71 in the literature. In this way, current work was compared to advanced processes and the advantages and disadvantages of processes were explained (Table 2). Electro-coagulation–flotation (ECF) exhibited a high efficiency for DB71 removal under mild conditions. Although ECF is an effective method for the removal of organic pollutants, the generation of sludge is the most important challenge for this work (Ahangarnokolaei et al. 2021). The same problem was also observed in Fenton oxidation where both oxidation and coagulation occurred simultaneously (Ertugay & Acar 2017). H2O2/Zero Valent Iron (ZVI) as Fenton-like the process could remove DB71 effectively and it has lower sludge compared to classic Fenton. However, homogenous Fenton oxidation suffers from the operation under acid conditions (Ertugay & Acar 2022). US/H2O2 degraded only ∼65% of DB71 during 20 min sonolysis time. In spite of low performance, the application of US-based processes on a large scale is difficult and costly. Anodic oxidation (AO) is a promising process for the degradation of organic pollutants in which reactive species are generated through water discharge. Hence, it reduces the cost of chemical oxidants. Howbeit, AO needed a longer time (120 min) for the oxidation of DB71, it increased the electrical energy consumption consequently (Xu et al. 2022). The photocatalysis process is probably the most popular AOP among advanced processes in which it can be operated by solar irradiation. However, escaping and the deactivation of nanoparticles during the operation are problematic issues. In this way, ZrO2/persulfate/UV could completely remove DB71 under 40 min irradiation. On the other hand, 78% of DB71 was eliminated using BWO/g-C3N4/visible light during 50 min reaction time. Compared to other works, the current work was an effective process in at short time (12 min) indicating that the DB71 degradation rate is very high by UV/Cl2 and UV/PI processes. However, there are some issues with these processes. First, the possibility of the halogenation of organic compounds during oxidation of the dyes produces persistent organic compounds as by-products. Second, residual iodates in PI/UV may affect the effluent quality. Third, UV-based processes need a pre-treatment for the removal of suspended solids since they can absorb light and reduce activation of Cl2 and PI. In general, all processes have some disadvantages, it is important that they are applied in a suitable position and time and employed for the proper quality of the effluent. PI-based AOPs are promising processes and there are several unknown aspects for scientists to conduct further studies especially their application on real wastewater, the toxicity of the effluent and the use of other enhanced methods for the activation of PI. Moreover, further basic research should be studied on the interaction of PI and its related radicals with water constituents like anions, cations and organic compounds.

Table 2

Comparison of advanced processes for DB71 removal

Treatment methodExperimental conditionsDB71 (mg/L)Removal efficiency (%)Ref.
ECF Electrical conductivity = 6.7 mS/cm, current = 0.6 A, pH = 8 and time = 40 min 200 99 Ahangarnokolaei et al. (2021)  
Fenton pH = 3, Fe(II) = 3 mg/L, H2O2 = 125 mg/L and time = 20 min 100 94 Ertugay & Acar (2017)  
AO(boron-doped diamond anode) Na2SO4 = 2 mM, flow rate = 600 mL/min, current density = 7.75 mA/cm2 and time = 120 min 50 100 Xu et al. (2022)  
ZrO2/Persulfate/UV pH = 7, ZrO2 = 0.4 g/L, persulfate = 0.75 mM and time = 40 min 50 100 Moradi et al. (2016)  
ZVI/H2O2 pH = 2.5, ZVI = 0.2 g/L, H2O2 = 100 mg/L and time = 20 min 100 100 Ertugay & Acar (2022)  
BWO/g-C3N4/visible light BWO/g-C3N4 = 1 g/L and time = 50 min 10 78 Shende et al. (2019)  
H2O2/Ultrasound pH = 2.5, H2O2 = 74 mg/L, US power = 95W and time = 20 min 50 64.1 Ertugay & Acar (2013)  
UV/Cl2 pH = 3, Cl2 = 0.75 mM and time = 12 min 20 100 This work 
UV/PI pH = 3, PI = 0.75 mM and time = 12 min 20 100 This work 
Treatment methodExperimental conditionsDB71 (mg/L)Removal efficiency (%)Ref.
ECF Electrical conductivity = 6.7 mS/cm, current = 0.6 A, pH = 8 and time = 40 min 200 99 Ahangarnokolaei et al. (2021)  
Fenton pH = 3, Fe(II) = 3 mg/L, H2O2 = 125 mg/L and time = 20 min 100 94 Ertugay & Acar (2017)  
AO(boron-doped diamond anode) Na2SO4 = 2 mM, flow rate = 600 mL/min, current density = 7.75 mA/cm2 and time = 120 min 50 100 Xu et al. (2022)  
ZrO2/Persulfate/UV pH = 7, ZrO2 = 0.4 g/L, persulfate = 0.75 mM and time = 40 min 50 100 Moradi et al. (2016)  
ZVI/H2O2 pH = 2.5, ZVI = 0.2 g/L, H2O2 = 100 mg/L and time = 20 min 100 100 Ertugay & Acar (2022)  
BWO/g-C3N4/visible light BWO/g-C3N4 = 1 g/L and time = 50 min 10 78 Shende et al. (2019)  
H2O2/Ultrasound pH = 2.5, H2O2 = 74 mg/L, US power = 95W and time = 20 min 50 64.1 Ertugay & Acar (2013)  
UV/Cl2 pH = 3, Cl2 = 0.75 mM and time = 12 min 20 100 This work 
UV/PI pH = 3, PI = 0.75 mM and time = 12 min 20 100 This work 
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Complete degradation of DB71 was achieved within a 12-min timeframe at pH 3 and a concentration of 0.75 mM of oxidant (PI and Cl2). The primary oxidation agents responsible for the elimination of DB71 in the UV/PI process were identified as HO• and ⁠, RCS were recognized as primary reactive radicals in the UV/Cl2 process. The presence of transition metals contributed to an increased rate of dye removal by enhancing the activation of PI and Cl2. These processes demonstrated high efficiency in the presence of various anions, except for bicarbonate ions. However, the presence of HA hindered the degradation rate as it competed with the target contaminant for reactive radical reactions. The PI-based process proved to be a more cost-effective route for DB71 removal compared to the chlorine-based process. Further, the evolution of carboxylic acids showed that higher amounts of oxalic and formic acids are formed in the UV/PI process and the concentrations of malic and tartronic acids are considerably lower, indicating that UV/PI is more effective in the mineralization of DB71. PI/UV exhibited an acceptable result for real wastewater treatment in terms of decolorization. However, its application of new oxidants such as PI should be carefully conducted on a large scale for actual wastewater due to unknown effects of by-products and residual PI.

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This project has been supported by Research Center for Environmental Contaminants (RCEC), Abadan University of Medical Sciences (Iran) under Contract No. 1400RCEC1367.

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

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The authors declare there is no conflict.

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