Comparative photo-degradative treatment of dyeing industry wastewater containing diazo dye by UV/Peroxydate and UV/Persulfate − oxidation processes

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.

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 number .	Molecular formula .	Molecular weight .	λmax .	Molecular structure .
27195	C22H12N4S4O13Na4	760.6 g/mol	520 nm

Color index number .	Molecular formula .	Molecular weight .	λmax .	Molecular structure .
27195	C22H12N4S4O13Na4	760.6 g/mol	520 nm

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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 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’.

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.

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.

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.

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.

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.

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).

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).

Compared with the UV/H₂O₂ reaction (k_app = 0.054 min⁻¹), kinetic of PS dye degradation was faster during the UV/ process (k_app = 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.

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.