Optimal concentration and efficiency of the photo fenton system for the treatment of a synthetic textile effluent

The Photo Fenton process is an alternative process for the treatment of effluents from the textile industry, being a reagent-based chemical on Fe 2+ and H 2 O 2 + UV light, whose mechanism consists of oxidizing the pollutants until they are suitable for discharge into bodies of water. This research seeks to determine the optimum concentration of Fe 2+ - H 2 O 2 in the treatment of a synthetic textile effluent and, subsequently, to evaluate the efficiency of the system. The synthetic textile effluent was prepared under laboratory conditions, FeSO 4 and H 2 O 2 were added at different concentrations


INTRODUCTION
Industrial production of textile fibers requires a large amount of water consumption and discharge (Deng et al., 2020). The wastewater generated contains a high number of pollutants such as NaCl and Na2SO4, phenols, heavy metals, chlorinated solvents, surfactants biocides such as pentachlorophenol and toxic anions such as sulfur (Cortazar et al., 2014). When discharged into water bodies, they remain in the water and soil for long periods of time, reducing soil fertility and affecting the esthetic quality of the water by increasing chemical and biochemical oxygen demand, thereby impairing photosynthesis of aquatic plants (Al-Tohamy, et al., 2022). In addition, the dyes used in this industry contain heavy metal ions in their structure, which can be assimilated by the gills of fish, accumulating in their tissues, and subsequently accumulating in human organs through the food chain (Mendoza Vazquez, 2021).
The advanced Oxidation Processes (AOPs) could be quite effective for the degradation of persistent contaminants. Their efficiency is based on the generation of highly reactive OHradicals, which are capable of oxidizing many organic compounds (Arka et al., 2022). OHradicals are generated by the intervention of sunlight or other energy sources and are characterized by their great effectiveness in oxidizing organic matter (Litter, 2005) The versatility of AOPs lies in the different existing ways of producing free radicals, such as heterogeneous photocatalytic ozonation, anodic oxidation, ultraviolet/hydrogen peroxide (UV/H2O2), Fenton and Photo-Fenton, among others (Cruz-Gonzalez et al., 2017).
One of the most effective AOPs for the generation of the hydroxyl radical is the Fenton process, based on Fe 2+ and H2O2, where the radicals are formed by catalytic decomposition of H2O2, with Fe 2+ being the catalyst (Cruz-Gonzalez et al., 2017) at low pH conditions (pH of 2-4) (Yan et al., 2022) (Equation 1). When H2O2 is combined with iron salts, the reaction is generated (Equations 1 and 2).
In Equation 1, OH-radicals are generated, and in Equation 2 perhydroxyl radicals (-HO2) are generated, thus oxidizing organic compounds (Equation 3), with the -HO2 radical being the least reactive (Barliza and Torres, 2018). From the Fenton reagent, several processes derive, such as the Photo Fenton Process, which employs UV light to increase the production of -OH (Equations 4 and 5) (Núñez and Vergara, 2016).
RH + OH • + 2 → ROH + 3 + → oxidized products UV light increases the generation rate of OH-radicals and enhances the regeneration of the ferrous catalyst by the reduction of Fe 3+ ions. There are three types of UV light wavelengths: UV-A (315-400 nm), UV-B (280-315 nm) and UV-C (100-280 nm) (World Health Organization, 2003). The maximum absorbance of Fe(OH)2 is at a wavelength of 300 nm, extending to approximately 400 nm (Medina Flores, 2019).
The objective of this research was to determine the optimal concentration of Fe 2+ -H2O2 in the treatment of a synthetic textile effluent under the Photo-Fenton system, and to evaluate the efficiency of the Photo-Fenton system in five different treatments generated from a synthetic textile effluent sample. This study seeks to minimize the environmental impact caused by the discharge of effluents from the textile industry by determining the optimal concentration of Fe 2+ -H2O2. It will also bring this technology closer to the various stakeholders, providing the necessary information for replication and scaling in order to comply with applicable effluentdisposal regulations.

Research design Optimal concentration of Fe 2+ /H2O2
To achieve this objective, the reduction of the pollutants in the synthetic textile effluent were analyzed based on two factors (Fe 2+ and H2O2), with three levels each. For this, a completely randomized 2x3 factorial design was worked using three repetitions (Table 1).

Efficiency of the Photo-Fenton system
The optimal concentration of H2O2 -Fe 2+ obtained in the first part of the study was used as a basis and the efficiency of the Photo-Fenton System was evaluated, for which a completely randomized design of five treatments with three repetitions was structured (Table 2).

Preparation of the synthetic textile effluent (Hanela et al., 2018)
A clothing item that weighs 400 g, was placed in a metal container, 4 L of deionized water, 8g of moisturizer and 20g of Na2CO3 was added for the scouring process. The container was heated on a stove until the water started to boil, then it was left to cool off for a few minutes. Afterwards, the clothing item was rinsed with 4 L of deionized water. The residual water from the scouring process and the rinsed water were stored in a plastic container.
The next step was to dye the clothing item. To do this, 4g of moisturizer with 4 ml of detergent, 320g of NaCl (sea salt) and 20g of aniline previously dissolved in water were added to a metal container. It was heated on a stove, where 2 doses of Na2CO3 and NaOH (6g and 2g, respectively, dissolved in water) and 4L of deionized water were added. Then the excess water was drained and stored in the plastic container.
The last step was the rinsing process, for which 4 ml of fabric softener and 2g of glacial acetic acid were used. It was left to rest for a few minutes and a series of rinses were conducted. The residual water produced during this procedure was stored in the same plastic container. At the end of this process, approximately 30 L of residual water was obtained, which resulted in the synthetic textile effluent. Finally, the physicochemical parameters of the contaminant such as pH, color, turbidity, COD and DO were measured.

Optimal concentration of Fe 2+ /H2O2
Prior to the start of the treatment, and in order to obtain the best decontamination results, the 30 L of synthetic textile effluent were acidified with concentrated Hydrochloric Acid (Medina Valderrama et al., 2016) to obtain a pH of 3. This was done because it has been shown that the highest decontamination values with the Photo-Fenton System are obtained with a pH level between 2 -4 (Kiruthiga and Sampath Kumar, 2015;Patil and Raut, 2014).
A volume of 1 L of the synthetic textile effluent was added in each of the twenty seven repetitions (1 L beaker), the FeSO4 (Table 1) was added, and it was taken to the magnetic stirrer for five minutes at 50 rpm. Each sample was exposed to 400 nm UV-A light (Medina Flores, 2019) throughout the process. Finally, the H2O2 was added, and it was kept under constant stirring for one hour at 300 rpm, and then the samples were left to rest for 48 hours (Rosales Palomino, 2017), all exposed to UV-A light. After rest time, the samples were filtered with the use of filter paper, to separate the possible sediments. The pH, color, turbidity, COD and DO were measured to verify the effectiveness of the removal of the contaminant. Likewise, the spectrophotometer was used to measure the absorbance of each repetition (Chaparro et al., 2014;Muñoz et al., 2016), with the objective of figuring out the concentrations of each problem sample, using a calibration curve.

Photo-Fenton System's efficiency
The optimal concentration of H2O2 -Fe 2+ obtained in the first part of the study was used as a basis and the efficiency of the Photo-Fenton Method was evaluated, for which a 15 L sample of synthetic textile effluent was prepared, as described in Item a.; then the concentration of the contaminant in each treatment was altered by diluting it with distilled water (Table 2) and 1 L of this sample was added to each of the 15 beakers.
Next, hydrochloric acid was added until a pH of 3 was obtained. Once these conditions were met, the optimal dose of Fe 2+ obtained in the first part of the study was added to each treatment and constant stirring was done for 5 minutes at 50 rpm using a Magnetic stirrer. After 5 minutes, the stirring speed was decreased to 30 rpm (Medina Valderrama et al., 2016). Finally, H2O2 was added according to the optimal dose obtained in the first part of the study. The mixture was kept under constant stirring for 1 hour at 300 rpm. Afterwards, it was left to rest for 48 hours (Rosales Palomino, 2017). Each sample was exposed to 400 nm UV-A light throughout the process. Once the rest time had passed, each sample was filtered using filter paper, to separate the possible sediments that could have formed and the pH, Color, Turbidity, COD and DO were measured. Likewise, the spectrophotometer was used to measure the absorbance of each repetition, to figure out the concentrations of each problem sample.

Statistical analysis 2.3.1. Optimal concentration of Fe 2+/ H2O2
The results were analyzed through a factorial analysis of variance, where the individual effect of Fe 2+ and H2O2 was evaluated, as well as the interactions, and the Dunet statistical test was used, which was applied to see which treatments were different from the control, allowing to determine the treatment or treatments (Table 1) that best reduced the contaminants in the synthetic textile effluent.

Efficiency of the Photo-Fenton system
One-way analysis of variance was applied and the Dunet statistical test was performed with a 95% confidence level.

Ethical aspects
This research did not affect the harmony or balance of the ecosystem (animals, plants, or the soil) since the study was conducted in a controlled environment. The final water samples were placed in plastic containers and the filter papers with the sediments were placed in red bags, where the correct handling of these residues was subsequently carried out.

Optimal concentration of Fe 2+ /H2O2
In the first part of the study, the variables pH, COD, turbidity, and color showed values lower than the control, while the DO variable showed values higher than the control in all the Photo-Fenton treatments (Annex 1), conforming to significance per the Dunnett Test ( Figure  1). Based on these differences, the controls were removed from the graph to better demonstrate the effect of each treatment (Figure 2). Sufficient statistical evidence was not found to indicate differences between the treatments of the variables of color, DO, and turbidity. However, the analysis for the COD variable revealed that T5 was significantly lower than the rest of the treatments (Figure 3); and for the turbidity variable, differences were found between treatments T5, T8 and T9 (Figure 4). The results indicate that T5 represents the optimal dose, showing significant differences in at least two variables.

Efficiency of the Photo-Fenton System
In the second part of the investigation, the variables pH, COD, turbidity, and color, showed lower values than the control (Annex 2) in all treatments, while in the DO variable, it was observed that Treatment 5 presented a higher value to the control ( Figure 5). Based on these differences, the controls were removed from the graph to better demonstrate the effect of each treatment ( Figure 6).  No significant differences were found between the treatments of the variables pH, color and turbidity. However, the variables DO and COD presented statistical evidence of differences between at least two treatments. In this sense, regarding the DO variable, the assumption of normal distribution of the residuals are met, but not for the homogeneity of variances, for which the non-parametric Kruskal-Wallis Test was applied, showing that Treatments T5 and T4 are significantly higher than treatments T1, T2 and T3 (Figure 7). For the COD variable, it was found that both the assumptions of normal distribution of residuals and homogeneity of variances are met, so the analysis of variances was used, finding that T5 presented the least value (Figure 8).

Discussion
Since the pH of all the samples was homogenized at the beginning of the process, no significant changes in pH were observed between one repetition and another. This is due to the fact that the pH, despite being an important factor in the Fenton Process, should be considered only as a variable to carry out the reaction, and not for purposes of evaluating the efficiency of the Photo-Fenton System, since it is evaluated according to the percentage of contaminant removal (Leon et al., 2020). The pH has a significant effect on the oxidation potential of OHradicals, where at a pH value of 3 the production of these radicals in the reaction is greater than at higher pH values (Calderon and Olortico, 2019). Likewise, for the optimization of the Photo Fenton Process, the pH of the solution is highly significant as it generates a great impact on the removal of COD, as long as the pH value is 3 (Castrillon and Rubio, 2020).
In the matter of the COD variable, the total number of treatments showed lower values than the control samples, with the highest COD removal being 83% (Treatment 5). Similar results were obtained in a POPs removal study, where the Photo-Fenton treatment managed to reduce the initial COD of the synthetic textile effluent by 86% (Gutierrez and Pilco, 2020). The effectiveness of the process is attributed to the presence of Fe 2+ in the reagent that favors the formation of OH-radicals and these, at the same time, significantly remove COD (Agudelo Valencia et al., 2020). Treatment 5, having intermediate concentrations of Fe 2+ and H2O2, achieved the highest COD removal. This result is similar to the one obtained in a petrochemical effluent treatment study, where a Fe 2+ concentration of 0.06M generated a COD reduction of 97.5%, while increasing the Fe 2+ concentration the efficiency of the process was reduced, removing only 80% of COD (Ghosh et al., 2010).
Depending on the turbidity variable, the treatments showed lower values than the control samples, mostly achieving a removal effectiveness of 100% of the initial turbidity. Similar results were obtained in a vinasse treatment study using Electro-Fenton, in which turbidity removal in all trials was greater than 95% and the maximum removal was 98.95% (Marin and Gonzales, 2018). The positive behavior of this variable is directly related to the effectiveness of the chemical (Photo-Fenton Treatment) and physical (filtration) process (Villota et al., 2021) and to the initial acidification process that the samples went through, where the change in pH promoted the formation of flocs and the precipitation of organic matter (Medina Valderrama et al., 2020) which is removed by the post-treatment filtration process (Blanco et al., 2014), achieving total turbidity removal. Another influencing factor is the correctly administered concentration of the reagent, since an excess would have caused the inhibition of oxidation reactions, limiting the precipitation of organic matter (Segovia Obando, 2020).
Regarding the DO, the entirety of the treatments showed higher values than the control samples. This result is attributed to the considerable decrease in COD in the samples, since this allows an improvement in the amount of DO to be observed (Medina Valderrama et al., 2020). In a contaminant treatment study of organic pollutants by Photo-Fenton, it was determined that the increase in DO is related to the concentration of H2O2, which should not be present in excess. Otherwise, a competitive reaction will be generated between the excess of H2O2 and OHradicals and the self-decomposition of H2O2 into oxygen and water, inhibiting degradation (Silva et al., 2009). In the same way, a high generation of oxygen is an indicator that there is an inefficient consumption of H2O2' and this would generate inadequate operating conditions (Rodriguez Suarez, 2018).
Furthermore, regarding the color variable, the total number of treatments showed lower values than the control samples. The color removal effect with the Photo-Fenton System may be due to the oxidation, coagulation and adsorption processes caused by the suspension of Fe 2+ (Calderon and Olortico, 2019). Likewise, by having an adequate concentration of Fe 2+ , maximum color removal will be achieved, since according to a study carried out with Fe 2+, at concentrations between 100 ppm and 500 ppm it was determined that the concentration of 250 ppm obtained maximum color removal, with a value of 74.23% (Kaya and Asci, 2019). Likewise, another important factor that influences the discoloration of the synthetic textile effluent is the presence of UV rays during the treatment, since it accelerates the formation of OH-radicals, and these, at the same time, react rapidly with the color of the contaminant and 11 Optimal concentration and efficiency of the photo … Rev. Ambient. Água vol. 18, e2897 -Taubaté 2023 lead to a rapid breakdown of the compound's chromophores (Kumar and Ameta, 2013).
Regarding the results of the efficiency of the Photo-Fenton system, it was discovered that Treatment 5 (lower concentration of contaminant) was the only one with higher DO values compared to the other treatments. This is due to the fact that the synthetic colorants present toxic compounds with high molecular weight, causing a decrease in DO. Meanwhile, having a lower concentration of colorants, there is a greater amount of DO (Brañez et al., 2018). Likewise, another factor that influences the increase in DO is having an adequate concentration of H2O2 in the reagent (Santos et al., 2011). In the same way, there is an inverse relationship between the DO and COD values obtained, mainly attributed to the influence of temperature, in this case UV rays, which generate quite low COD values, and which give rise to high DO values (Rincón and Sanabria, 2020).

CONCLUSIONS
The Photo-Fenton System, under an optimal concentration of 400 mg/L Fe 2+ and 12 ml/L H2O2, achieves the highest removal of contaminants in the synthetic textile effluent, where the presence of Fe +2 favors the significant removal of COD and the increase in DO is related to the presence of H2O2 and UV-A light, which, in turn, considerably influences the removal of color from the effluent. Likewise, the removal of turbidity is obtained by carrying out the treatment in an acid medium (pH 3) which promotes the formation of flocs and precipitates organic matter. This is how it turns out to be an effective procedure for the treatment of textile effluents. Regarding the efficiency of the Photo-Fenton System, it is capable of decontaminating effluents with high and low organic load (100% -20% physical concentration), reducing parameters such as turbidity, COD and color, and increasing the OD in the samples.