Degradation of direct black 22 textile dye using the photo-Fenton and electro-Fenton processes: a comparative study

The dyes present in textile e�uents have a complex structure and low biodegradability, making it necessary to use e�cient treatments such as advanced oxidation processes (AOP). The aim of this study was to compare the e�ciency of photo-Fenton (PF) and electro-Fenton (EF) AOPs in the degradation of direct black dye 22 (PD22), de�ning the best experimental conditions and evaluating the kinetics and toxicity of the proposed treatments. Initially, for the PF system, using UV-C and sunlight radiation, 99.08% and 99.98% degradations were reached, respectively for [H 2 O 2 ] = 20 mg·L − 1 and [Fe] = 1.0 mg·L − 1 . From the volume variation study, it was observed that the increase in volume did not compromise the degradation of the dye. For the EF process, the [Fe] that promoted the highest percentage of degradation (95.16%) was equal to 1 mg·L − 1 . The volume study for the EF process also did not interfere signi�cantly in the e�ciency of the process. The PF and EF systems presented satisfactory adjustments to the proposed kinetic models, suggesting that the treatment follows a pseudo-rst-order kinetics. The ecotoxicological tests showed no toxicity for the thyme seed after using the EF process. Therefore, it is evident that different AOP techniques can be applied in the treatment of PD22.


Introduction
The textile industries show good results in terms of the production process, job creation and availability of varied products.However, due to the characteristics of this type of production process, this industrial branch leads to different environmental problems.Among these, there is the complexity of the textile wastewater generated, since it presents toxicity and low biodegradability (Ismail and Sakai 2022).The dyes present in the textile e uent, when not properly removed, can promote an increase in the organic load in the receiving bodies, interfering with photosynthesis and the development of beings that are part of the aquatic biota (Al-Tohamy et al. 2022).
The complex structure and toxic properties of dyes make them non-biodegradable and recalcitrant.Therefore, textile wastewater containing dye needs effective treatment.Advanced oxidation processes (AOP) are a set of effective methods for treating such contaminants (Ma et al. 2021).Among the different types of AOP, Fenton oxidation, photocatalysis and electrochemical oxidation can be highlighted, which have different sources of generation for the oxidizing agents (Nidheesh et al. 2022).
Different reactive species are produced during the AOP in order to promote the degradation of the contaminant, such as hydroxyl radical (•OH), sulfate (SO 4 •− ), hydroperoxyl (HO 2 • ), carbonate (CO 3 − ), semiconductor gap (h + ) and reactive chlorine species (Cl • , ClO • , Cl 2 •− ) (Kumari e Kumar 2023).Therefore, depending on the need, type of pollutant and available resources, the type of AOP to be used for treatment is established.
Çalik and Çidçi (2022) compared the Fenton and photo-Fenton processes employed for the treatment of textile e uent for 60 min.For the Fenton process, the authors obtained 97% color degradation and reduction of 88.9% and 84.2% of chemical oxygen demand (COD) and total organic carbon (TOC), respectively, using 200 mg•L − 1 of Fe 2+ and 300 mg•L − 1 of H 2 O 2 in acidic conditions.On the other hand, the photo-Fenton process, using UV-C radiation, showed 98% degradation for color and COD and TOC reduction of 93.2% and 88.9%, respectively.
Adachi et al. (2022) evaluated the degradation of the methyl orange dye, using the electro-Fenton process and graphite and stainless steel electrodes.or this, the best experimental conditions were a Fe 2+ concentration equal to 0.2 mmol L − 1 , pH equal to 3 and the electrolyte used was sodium sulfate (Na 2 SO 4 ).Thus, the authors achieved 94.9% degradation of the contaminant.
Given the above, it appears that different advanced oxidation processes can be applied for the treatment of contaminants such as dyes.Thus, the objective of this work was to carry out a comparative study between one of the classic AOP and an electrochemical AOP to promote the degradation of the direct black 22 textile dye, one of the most used in the textile industry.In addition, the most appropriate operating conditions for each process were de ned, evaluating their behavior along the kinetic evolution of degradation, as well as the toxicity of the treated samples.

Experimental
Initially, a stock solution (1000 mg•L − 1 ) of direct black 22 textile dye (PD22) (Exatacor) was prepared.The contaminant was then identi ed and quanti ed by ultraviolet/visible (UV/Vis) spectrophotometry using Thermoscienti c equipment.Knowing that the characteristic wavelength of the compound of interest is equal to 476 nm, an analytical curve was constructed, with a linear range from 1 to 20 mg•L − 1 , for quanti cation of the analyte before and after treatment.

Treatment via advanced oxidation processes
The preliminary study for the classical system consisted of evaluating the degradation e ciency of the PD22 dye, using the photo-Fenton process in the presence of UV-C radiation and sunlight, using benchtop reactor, as described by Cavalcanti et al. (2021).To verify the contribution of parallel reactions that may be present in the proposed treatment, the contribution of photoperoxidation and Fenton processes was determined.All experiments were performed in batches for 60 min, using 50 mL of the working solution (15 mg•L − 1 ).
For the initial assays that used the Fenton reaction, the Fe concentration ([Fe]) was xed at 5 mg•L − 1 and the hydrogen peroxide concentration ([H 2 O 2 ]) was varied from 20 to 100 mg•L − 1 , with pH adjustment to 3, the most suitable for Fenton reaction according to the literature.Then, to continue investigating the process e ciency, the [Fe] was varied (1.0; 2.0; 3,0; 4.0 e 5.0 mg For the electrochemical system, the electro-Fenton process was evaluated, using an individual cell with graphite (commercial) and copper (commercial) electrodes as cathode and anode, respectively, at a distance of 3 cm from each other.The graphite electrode was immersed in the system at a height of 1.0 cm, while the copper at 2.5 cm, in order to ensure the same surface area between the two electrodes.
The system was powered by an adjustable source (OS-6000 model) at 18 V, using NaCl (0.025 mol•L − 1 ) as the electrolyte.The choice of these parameters was based on previous studies by the research group (Neves et al. 2023).For the preliminary tests, a concentration of 50 mL from the work solution was used, for 60 min, and evaluated the [Fe] in uence (using as iron source the FeSO 4 (F Maia)) and varying it in: 1.0; 2.0; 3.0; 4.0 and 5.0 mg•L − 1 .Once the best [Fe] for the proposed system was known, the e ciency of the process was evaluated varying the applied volume (100, 200, 500 and 1000 mL).
Finally, after de ning the most appropriate operating conditions, an analysis of the evolution of pollutant degradation over time was carried out for the two proposed types of AOP.

Kinetic study for the homogeneous and electrochemical systems
Kinetic monitoring was performed using 1 L of textile dye solution.For this, 2 mL aliquots were withdrawn at the times of 0, 2, 5, 8, 10, 15, 20, 25, 30, 45, 60, 75, 90 and 120 min.Then, the obtained data were tested against the kinetic models proposed by Chan and Chu (2003) and He et al. (2016), according to the equations presented in Table 1.

Toxicity Tests
Toxicity tests were carried out for samples before and after treatment, using cress (Nasturtium o cinale), carrot (Daucus carota subsp.Sativus) and thyme (Thymus vulgaris).For this purpose, the methodology described by Santos et al. (2020), using 20 units of each type of seed and 4 mL of the sample (triplicate tests).For this procedure, the negative and positive controls described in the literature were used, namely: distilled water and a boric acid solution (3%), in that order.
To determine the presence/absence of toxicity for the seeds described, the root growth (RGI) and germination indexes (GI) were calculated, as described in Equations 3 and 4 (YOUNG et al. 2012).

4
In which: CRA and CRC are the total root lengths in the sample and in the negative control, respectively, and SGA and SGC are the numbers of germinated seeds in the sample and in the negative control, in this order.

Results and discussion
Treatment by advanced oxidation processes Initially, the treatment of the PD22 dye from the photo-Fenton AOP was evaluated.For this, the study was based on the in uence of [H 2 O 2 ] and [Fe] evaluation, whose results are shown in Fig. 1(a) and (b).
From Fig. 1, it is observed that all [H 2 O 2 ] evaluated led to similar results, around 99% contaminant degradation for both tested radiations.This shows that there is no need to use high concentrations of the oxidizing agent applied, a satisfactory result since it reduces costs with reagents.This phenomenon was also observed by Santana et al. ( 2021) who evaluated the treatment of a synthetic textile matrix by the photo-Fenton process, varying the [H 2 O 2 ] from 300 to 1500 mg L − 1 .However, for values above 900 mg L − 1 , no increase in AOP e ciency was observed.Therefore, it can be stated that the use of 20 mg L − 1 of H 2 O 2 is e cient for the process, since it presented an e ciency only 3% lower than the test with the highest percentage of degradation, a value that can be attributed to experimental errors.
In view of the above, a [H 2 O 2 ] of 20 mg L − 1 was selected for the next tests, evaluating the contribution associated with the photoperoxidation/UV-C system and the Fenton process.For these treatments, degradations of 66.99% and 78.02% were obtained, respectively.For the photoperoxidation/sunlight system, there was no degradation of the contaminant, indicating the need to combine Fenton's reagents with a light source.
Then, it was proceeded with the evaluation of the in uence of the catalyst concentration in the process (Fig. 1 (b)).From the analysis of this gure, it is observed that all [Fe] tested promoted the degradation of the contaminant under study that its increase did not lead to a greater e ciency of the treatment.This To ensure that the e ciency of the process was maintained when using larger volumes of solution, a study of this variable was carried out based on the best experimental conditions.The results obtained are shown in Table 2. From the results presented in Table 2, it is observed that the increase in volume did not compromise the dye degradation rate for the proposed treatment, since there was no signi cant difference (≤ 3%) between the lowest and highest value evaluated, for both radiations.Thus, the determined experimental conditions can be applied to the different volumes tested, demonstrating once again the e ciency of the proposed treatments.
Another process used to treat the dye under study was the electro-Fenton.Initially, the effect of [Fe] on the process e ciency was evaluated, with the results shown in Fig. 2.
In view of what is exposed in Fig. 2, it is observed that for [Fe] ≥ 2 mg L − 1 there was a decrease in the percentage of degradation.A similar result was veri ed by Adachi et al. ( 2022) who used the electro-Fenton process to treat the orange methyl azo dye.When evaluating the effect of [Fe], the authors found that increasing the catalyst concentration from 1 mmol L − 1 to 2 mmol L − 1 increased the e ciency of the treatment, however when using 3 mmol L − 1 there was a decrease in the degradation from 93.09-91.55%.
In this sense, a [Fe] of 1 mg L − 1 was selected to treat the dye under study, which presented a degradation percentage of 95.16%.
Similar to the classic AOP, a volume study was performed for the electro-Fenton process.The results obtained are shown in Table 3.
Table 3 Volume study for the electro-Fenton process.Operating conditions: From the data presented in Table 3, it can be seen that between the smallest and largest volumes, the difference in the degradation percentage of the contaminant was less than 4%, indicating that the increase in volume does not compromise the e ciency of the process.With this, the best conditions for both the classic and electrochemical AOP were de ned.Thus, next step was monitoring the reaction kinetics for both treatments.

Kinetic study for homogeneous and electrochemical systems
The evaluation of dye degradation over time for the classical and electrochemical AOP aimed to monitor the degradation of the chromophore group over time.Furthermore, the adequacy of the results obtained for the best systems (PF/UV-C, PF/sunlight and EF) to the models proposed by Chan and Chu (2003), and He et al. (2016) were evaluated, as shown in Fig. 3.
In view of the results presented in Fig. 3 (a), (b), and (c), it is observed that the degradation of the PD22 dye occurs more quickly in the rst 20 min, regardless of the radiation and system used.It is also possible to verify in Fig. 3 (a) and (b), that equilibrium is reached in around 75 min for the PF/UV-C, PF/sunlight, and in 60 min for the EF, with degradations greater than 95% after 120 min.
When observing the degradation kinetics, it can be seen that the experimental data presented a good t to the kinetics models tested, indicating a pseudo-rst-order reaction.Oliveira et al. (2021) found that the data obtained from the degradation of a mixture of textile dyes (direct black 22, acid black 172, and reactive black 5), using classic AOP, follow an analogous pro le presented in this work.Santos et al.
(2019) also observed that the reaction kinetics of acid blue 29 textile dye degradation via anodic oxidation follows a pseudo-rst-order pro le.
Therefore, for a better evaluation of the data in relation to the kinetic adjustments, the parameters of each one of them are shown in Table 4. From the data presented in Table 4, it can be seen that the experimental data for all proposed systems presented values of linear regression coe cients (R 2 ) ≥ 0.97.It is also observed that the PF/UV-C system presents a faster degradation rate, since the ratio (1/ρ) is higher when compared to the other employed systems.Furthermore, the oxidation (1/σ) and reaction (k) rates showed similar values for all systems, indicating that the degradation kinetics of the evaluated processes follows a pseudo-rst-order pro le.In order to carry out a more detailed analysis of the results obtained, graphs of the residues of the kin (Fig. 4).
When analyzing Fig. 4, it can be seen that, in all cases, the residuals left by the models presented low values and are randomly distributed.According to Barros Neto; Scarminio; Bruns (2010) this behavior indicates a satisfactory adjustment of the experimental data to the tested models.
During the kinetic study, it was also possible to monitor the consumption of the oxidizing agent (H 2 O 2 ).It is worth noting that for the electrochemical system, H 2 O 2 is initially generated by the process and then consumed over time.The results obtained are shown in Fig. 5.
From the data in Fig. 5 (a), it is observed that the consumption of H 2 O 2 occurred more quickly for the PF/UV-C system.This may have occurred due to the fact that the production of hydroxyl radicals by decomposition of H 2 O 2 occurs mainly in the range of 200 to 300 nm (Moraes et al. 2020).
In Fig. 5 (b), it is observed that the EF process, using the Gr-Cu electrode pair, NaCl as electrolyte and FeSO 4 as iron source, promoted the generation of H 2 O 2 .According to Trellu et al. (2016), such an oxidant can be generated electrochemically at the cathode, by a reduction of two electrons of oxygen, according to Eq. 5.
When further analyzing Fig. 4 (b) it is observed that for 10 min of reaction, the highest [H 2 O 2 ] was found, which was reduced over time.This may have occurred due to the consumption of the oxidizing agent in the EF process for degradation of the contaminant.Thus, it is possible to indicate, among other factors, that the degradation of the PD22 dye for this type of AOP occurred due to the presence of an oxidizing agent in the medium.
After evaluating the proposed treatments, toxicity tests were carried out for the samples before and after treatment to verify the possible formation of rection intermediates after degradation.

Toxicity tests
For the ecotoxicity tests, GI and RGI values were calculated (Table 5).It is noteworthy that the tests carried out with the positive control (boric acid) did not shown germination.With regard to samples treated with the PF/UV-C and PF/sunlight processes, those showed toxicity for the three types of seeds.On the other hand, the samples treated with the EF process obtained ICR values higher than those obtained for the classic AOP treatment, with no evidence of toxicity only for thyme seeds.In view of this, it is inferred that the possible degradation products formed by the treatments used can cause damage to the biota, however, these are lower when the electrochemical AOP is applied.
Finally, it is evident that different treatments of contaminants from advanced techniques such as classic and electrochemical AOP can be applied for degradation of textile dyes, reaching percentages of degradation above 95%.

Conclusions
The present work allowed us to infer that the textile dye PD22 can be treated based on advanced oxidation processes, classical homogeneous (photo-Fenton with UV-C radiation and sunlight), and electrochemical (electro-Fenton).In view of the tests carried out, it was possible to verify that low concentrations of reagents led to high percentages of degradation.Monitoring the kinetic evolution allowed verifying that the data for the studied processes present satisfactory adjustments for the models proposed by Chan and Chu (2003) and He et al. (2016), demonstrating a pseudo-rst-order reaction.In addition, it was found that for the photo-Fenton process, the oxidizing agent (H 2 O 2 ) was completely consumed at the end of the process, while for the electro-Fenton process, H 2 O 2 was initially generated in generated in the system and consumed over time.Toxicity assessment was carried out for samples before and after the proposed treatments, using different (watercress, thyme and carrot).The solutions before treatment showed toxic potential.However, for the starting solution with electrolyte, it was not observed, indicating that the salt may favor seed growth.On the other hand, for the solutions treated via photo-Fenton, it showed toxicity while for the post-EF samples, RGIs higher than those obtained for the photo-Fenton were identi ed.However, the only seed in which no toxic potential was observed was thyme.In fact, the treatments used in this work demonstrated e ciency in the degradation of the textile dye PD22, which is one of the most used in the textile industry.
fact may be related to the inhibition of the oxidant itself, which promotes the transformation of hydroxyl radical into hydroperoxyl ions due to the excess of Fe ions(Bensalah et al. 2019).Quynh et al. (2023) also observed that the increase in[Fe]  in the photo-Fenton process did not signi cantly change the e ciency of the methylene blue dye treatment.Thus, the [Fe] selected for the following assays was 1 mg L − 1 and then the best conditions determined for the Fenton's reagents were, [H 2 O 2 ] = 20 mg L − 1 and [Fe] = 1 mg L − 1 , which reached 99.08% and 99.98% degradations respectively for the UV-C and sunlight irradiations.
[Fe] = 1 mg L -1 , electrode pair: Gr-Cu, [NaCl] = 0.025 mol L -1 , 18 V, pH = 3, tempo = 60 min, T = 31°C, and p The initial solution plus electrolyte (NaCl 0.025 mol•L − 1 ) used in the electrochemical treatment did not show the presence of toxicity, since the RGI ≥ 0.8 (Young et al. 2012).Santos et al. (2020) cites that the chlorine ions present in the reaction medium favor the opening of the seeds due to the oxidative potential of this chemical species that facilitates germination, through the break of dormancy.

Figures Figure 1
Figures

Table 1
(2016)c models proposed by Chan and Chu (2003) and He et al.(2016) Santana et al. (2021) time t; C 0 : initial concentration; ρ: rection kinetics (min); σ: oxidative capacity (dimensionless); k: pseudo-rst-order reaction rate (min − 1 ).During the kinetic assay of the processes (classic and electrochemical) [H 2 O 2 ] analyzes were carried out in order to verify the consumption of the oxidizing agent.For this purpose, the methodology described bySantana et al. (2021)which uses the metavanadate ion was applied.

Table 2
Volume study for the classic AOP.

Table 4
Parameters of the kinetic adjustments of the data obtained for the systems evaluated in the treatment of the textile dye

Table 5
From the results presented in Table5, it is observed that the initial solution presented toxic potential for the three seeds tested, since the GI and RGI values were lower than that observed in the negative control.This result was expected, since it is know that textile dyes are toxic, interfering with seed growth (Sathishkumar et al. 2019).