On The Fenton´s Process for the Treatment of Effluents from the Dyeing of Agates Containing Rhodamine B and Ethanol

On The Fenton´s Process for the Treatment of Effluents from the Dyeing of Agates Containing Rhodamine B and Ethanol | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article On The Fenton´s Process for the Treatment of Effluents from the Dyeing of Agates Containing Rhodamine B and Ethanol Cassiano R. dos Santos, Augusto C. Rodrigues, Fernanda Vilasboas, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5727575/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 17 Jun, 2025 Read the published version in Discover Chemical Engineering → Version 1 posted 9 You are reading this latest preprint version Abstract In the processing of agates, the gemstone pieces are submitted to dyeing processes. One of the dyeing processes applied is with Rhodamine B, which provides a reddish-pink colouration. The practice generates a wastewater containing residual amounts of the dye and ethylic alcohol with high polluting potential. Thus, the objective of this paper is to investigate the chemical parameters of the Fenton´s oxidation process to treat this effluent. The main parameters evaluated were [H 2 O 2 ]:[Fe 2+ ] molar ratio and the concentration of the reagents H 2 O 2 and FeSO 4 .7H 2 O. The wastewater contained a concentration of 772 mg L − 1 of Rhodamine B and 3% of ethanol in its composition, giving the effluent a high colour, organic load, and toxicity. In the Fenton reaction treatment, 11.1 g L − 1 of ferrous sulphate and 20 mL L − 1 of oxygen peroxide (in a molar ratio [H 2 O 2 ]:[Fe 2+ ] of 7.5:1) were defined as the best dosage, which allowed complete alcohol removal, an average absorbance reduction of 99.9% at 554 nm, a TOC reduction of 93.3%, an increase in surface tension from 58.9 to 64.4 mN m − 1 and a toxicity factor (TF) decrease from 526 to 16 with respect to the organism Daphnia similis . The Fenton process combines oxidation, coagulation and air stripping mechanisms that substantially reduces pollutants present in this effluent, at a rougher stage. The results obtained are useful, both for the gemstone sector and others that make use of Rhodamine B dye. gemstone processing dye wastewater advanced oxidation process environment Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 INTRODUCTION In the state of Rio Grande do Sul, Brazil, there is a prominent centre for the extraction and processing of agates (Hartmann, 2010). Agates have several natural textures and colours. However, they are usually subjected to shape and dyeing processes for product diversification purposes (Silva et al, 2007; da Silva Vilasbôas et al., 2017; Eriksson et al., 2021a; 2021b). Among organic dyes, the one that gives the agates a reddish pink colour is known as Rhodamine B (Fig. 1 ). Figure 2 illustrates an agate in its natural colour and after dyeing with Rhodamine B (Rh B). Rhodamine B is a dye that can be easily solubilised in both water and ethanol and does not require large amounts to provide a strong and vibrant colour to the stones. However, this hydrophilic xanthene dye exhibits chemical stability and can accumulate in living organisms, disrupting the endocrine system, impairing fertility, and causing genetic damage (Al-Buriahi et al., 2022; Priya et al., 2024). The dyeing processing of agates is carried out in plastic barrels, where the stones are placed in ethanol solutions (approximately 20 g of dye for 1 L of ethanol). After three days, the stones are removed from the ethanol solution and washed with water. The liquid effluent containing the dyes are the result of this agate washing step. According to Carissimi and Schneider (2010), the dye-containing wash effluent may have alterations in pH, Rhodamine B concentration of the order of 20-2000 mg L − 1 , a substantial amount of Total Organic Carbon (TOC), high colouration, and a toxicity which it adds to the aquatic environment. The wastewater volumes generated are not high, up to 15 m 3 per day in most agate dying units. However, the municipality of Soledade is located in a region of river springs, so the high toxicity of the effluent associated with the low flow of the water resources leads to a concern. The characteristics of the effluent indicate that the treatment operation should (a) remove colour, (b) decrease the organic load, and (c) reduce toxicity. It needs to include processes that can remove soluble organic load, which could be done by oxidative, adsorption and/or biological processes (Metcalf and Eddy, 2003). However, the pollutant load is too high to be initially treated by adsorption and biological processes are hindered by the low BOD 5 :COD (biochemical oxygen demand: chemical oxygen demand) ratio. Thus, chemical oxidation is the technique with the highest potential for the treatment of such wastewaters, at least for an initial stage. Some studies on wastewater treatment containing organic dyes from agate dyes have been developed. Carissimi et al. (2000) and Pizzolato et al. (2002) assessed the oxidation of effluents containing Rhodamine B, Bright Green and Crystal Violet dyes using sodium hypochlorite – NaClO, the recurring oxidant used by the agate industries. Several compounds have been found to be formed due to the non-mineralisation of chemicals, including organochlorines. Barros et al. (2006) investigated the use of the Fenton reaction in the treatment of industrial effluents containing a mixture of Rhodamine B, Bright Green and Violet Crystal dyes. They identified the presence of residual substances due to the incomplete oxidation of the dye molecules, such as xylene, hydrocarbons, phenolic compounds and nitrogen phenol. Machado et al. (2012) used the ozonation (O 3 ), ultraviolet (UV) and O 3 /UV techniques for industrial effluent degradation with Rhodamine B dye. The study showed that, in the best ozone discolouration condition, the treated effluent still presented a high level of toxicity. However, there are other relevant issues regarding the treatment of effluents containing organic dyes generated in the manufacture of agates that have not been addressed or that deserve to be detailed. Among these, we can mention the total organic carbon (TOC), which enables the evaluation of the degree of dye mineralisation; determining the ethanol content in the effluent, considering that its presence can increase pollution; as well as the fate of nitrogen from the Rhodamine B molecules and the ecotoxicological response. Considering that the use of sodium hypochlorite, the reagent currently used for the treatment, has a poor performance and forms organochlorines (Pizzolato et al., 2002), and that ozonation only presents partial results (Machado et al., 2012) and forms precipitates (preliminary studies), the present study employs the Fenton Process. The process is an advanced oxidative process based on the hydroxyl radical, which has both oxidation and coagulation properties. The Fenton reaction was discovered by H.J H. Fenton in 1894, when he found that H 2 O 2 can be activated by ferrous salts (Fe 2+ ) to oxidise tartaric acid (Fenton, 1894). The Fenton process encompasses reactions of peroxides (usually hydrogen peroxide – H 2 O 2 ) with iron ions to form active oxygen species (especially the hydroxyl radical •OH) that oxidise organic or inorganic compounds (Neyens and Baeyens, 2003). The importance of radical hydroxyl reactions has been recognised in recent decades and more than 1700 velocity constants for •OH reactions with organic and inorganic compounds in aqueous solution have been tabulated (Buxton et al., 1988). The Fenton reaction has been widely studied and, in many cases, efficiently used in wastewater treatment for the degradation of organic substances. Eq. (1) is recognised as the Fenton reaction and is based on the oxidation of ferrous iron into ferric iron to decompose H 2 O 2 into hydroxyl radicals. It is generally regarded as the centre of Fenton chemistry. These, and other parallel reactions, are well documented in the literature (Neyens and Baeyens, 2003; Bautista et al., 2008; Babuponnusami and Muthukumar; 2014; Gayathri et al., 2023). Fe 2+ + H 2 O 2 → Fe 3+ + OH − + •OH (1) The Fenton process happens in the acidic pH range. After the time required for oxidation of the pollutants, pH adjustment from an acid to neutral/alkaline range is required for residual iron removal by precipitation. Therefore, the aim of this study is to depict the Fenton reaction for the treatment of a wastewater generated by the agate processing industry containing Rhodamine B and ethanol. It is worth noting that it was chosen the classic homogeneous Fenton process, without any variants, to be simpler to implement by the industry. The concentration of the Fenton´s reagents and sludge generation were studied and the quality of the final effluent was monitored with an emphasis on colour and organic load removal, surface tension of the water, and toxicological response, in order to obtain treated water with a low pollutant potential. Another aspect which was investigated was the fate of the nitrogen present in the Rhodamine B molecules. MATERIALS AND METHODS Samples Effluent samples from agate dyeing with Rhodamine B were collected from an agate dyeing company located in the city of Soledade (28° 49' 04" S 52° 30' 36" W), in the state of Rio Grande do Sul, Brazil. Initially, five samples (representative of five dyeing operations performed at different periods) were provided for an initial characterisation of the Rhodamine content, surface tension and alcohol content. One of these samples, chosen at random, was used in this work. An image of this effluent is shown in Fig. 3 . The reagents used for the treatment by the Fenton process were ferrous sulphate (FeSO 4 .7H 2 O) and hydrogen peroxide (H 2 O 2 35% w/w). Sodium thiosulfate pentahydrate (Na 2 S 2 O 3 .5H 2 O) diluted in water at 1 mol L − 1 was used to decompose the residual H 2 O 2 and stop the Fenton´s reaction (Liu et al, 2003). Aqueous NaOH (10% m/v) and HCl (10% w/v) solutions were used to adjust the pH. All reagents were of analytical grade. Rhodamine B, also of analytical purity, was used to determine the dye concentration. The water used in the experiments was obtained by reverse osmosis treatment of the water from the public water supply. Study of the treatment of liquid effluents through Fenton reaction This part of the research focused on determining the optimal concentration of Fenton reagents and the reaction time to treat the effluent. The aim was to achieve high colour (absorbance at 554 nm) and total soluble organic (TOC) reduction using the smallest possible quantity of reagents. The effluent treatment tests with the Fenton Reaction were conducted by performing the following steps: separation of 200 mL of raw effluent; pH control with a pH meter; pH adjustment to 3.0 +/- 0.1 with HCl and/or NaOH; addition of the reagents FeSO 4 .7H 2 O and H 2 O 2 ; oxidation time; addition of 1 mol L -1 of Na 2 S 2 O 3 .5H 2 O to decompose the residual H 2 O 2 and stop the Fenton´s reaction. pH adjustment to 8.0 +/- 0.1 by adding NaOH to promote Fe 3+ precipitation as hydroxide; filtering the sludge with qualitative filter paper; absorbance measurement. To define the best dosage, it was initially necessary to determine the best proportion between the amounts of H 2 O 2 and Fe 2+ . The molar ratio of [H 2 O 2 ]/[Fe 2+ ] applied were 2.5:1; 5;1; 7.5;1; 10;1 and 12.5;1. The concentration of the reagents was calculated by molar ratio and based on three dosages of H 2 O 2 : 10, 20 and 30 mL L − 1 . Initially, the experiments were carried out with one repetition and only the colour parameter was assessed (absorbance at 554 nm). Afterwards, the best molar ratio was reproduced with H 2 O 2 concentrations set at 5, 10, 15, 20, 25 and 30 mL L − 1 . In addition to absorbance, the TOC and surface tension of the treated samples were analysed. These assays were performed in triplicate. The reaction time was determined by temperature variation, with the aid of a thermometer, and foam production. Once the temperature began to drop and foam production ceased, it was considered that the Fenton reaction was no longer occurring. Analytical procedures Visible-range spectrum scanning was performed on a diluted effluent sample with a benchtop spectrophotometer, to verify the highest absorption wavelength in the visible range of the spectrum (400–700 nm). The highest absorbance occurred at 554 nm, so this wavelength was adopted to evaluate the concentration of Rhodamine B in the liquid effluent. From the correlation between absorbance x analytical grade Rhodamine B concentration, the amount of Rhodamine B in the effluent was determined. Surface tension (ST) was determined at room temperature, using a tensiometer and the Du Noüy static method with a platinum ring. Effluent conductivity was determined by a conductivity meter. The Total Organic Carbon (TOC) was analysed by a carbon analyser. The ethanol content in the raw effluent was determined by distillation and density testing (alcoholometer). The ethanol distillation procedure was performed at a temperature of 76–78°C. The results attained by distillation and with the alcoholometer were nearly the same. The other chemical analyses were carried out according to the following procedures, described in ‘Standard Methods for the Examination of Water and Wastewater’ (Eaton et al., 2005): 4500-NO 3 − nitrogen (nitrate), 4500-NO 2 − nitrogen (nitrite), 4500-NH 3 nitrogen (ammonia), 4500-N org nitrogen (organic), 4500-P phosphorus, 5220 Chemical Oxygen Demand (COD), and 5210 Biochemical Oxygen Demand (BOD 5 ). Iron analyses were carried out by sample preservation with HNO 3 and by an issuing optical spectrophotometer with inductively coupled plasma (ICP-OES). The ecotoxicity test for microcrustaceans was followed in accordance with the ‘Guideline for Testing of Chemicals. Method 202 ‘ Daphnia sp., Acute Immobilisation Test’ (OECD, 2004). RESULTS AND DISCUSSION Characterization of agate dyeing effluent The composition of the agate-dyeing effluents varied quite a lot. When preparing the present study, random effluent collection indicated variations of Rhodamine B concentration from 500-2,500 mg L − 1 , surface tension from 55–60 mg L − 1 and 2–4% ethanol concentration. These variations are due to the fact that the dyeing process is not systematised and the inputs are not controlled. Nevertheless, Rhodamine B concentrations give effluents with an intense reddish-pink colour. The presence of Rhodamine B and ethanol alters the surface tension of pure water (72 mN m − 1 at 20°C), with values below 60 mN.m − 1 . The sample chosen for this study had a bright reddish pink colour and this colour is due to the presence of approximately 772 mg L − 1 of Rhodamine B. The effluent has a surface tension of 59 mN m − 1 and the organic load is high. The possible sources of carbon are Rhodamine B, ethanol and some other contaminants, such as surfactants. The result of the TOC analysis indicates that the raw effluent has a concentration of 11,950 mg L − 1 . The concentration of Rhodamine B shows that 542 mg L − 1 of the carbon comes from Rh B molecule, which corresponds to only 4.5%. The remaining 95.5% are due to the presence of ethanol and other substances. These components reflect a BOD of 5,800 mg L − 1 and a COD of 18,180 mg L − 1 . The BOD 5 :COD ratio is 0.32, which demonstrates the low biodegradability and low potential of the effluent to be treated by biological processes. The strong colour of the effluent did not allow the analysis of N in the raw effluent. Thus, to determine total nitrogen, Rhodamine B was considered to be the only source of this element. The calculation considered the mass of nitrogen in a Rh B molecule and indicated that the raw effluent has 45.1 mg L − 1 of total nitrogen, entirely as organic nitrogen. The distillation procedure to determine the percentage of ethanol in the raw effluent indicated that the raw effluent has 3% ethanol in its composition. The use of the alcoholometer indicated practically the same result. Conductivity and phosphorus concentration in the effluent are low. Therefore, it can be assumed that the effluent is basically composed of water, ethanol and Rhodamine B. The ecotoxicity of the effluent was very high. The effective concentration for immobility of 50% of the Daphnia similis population over a 48-hour period was only 0.61% of the initial concentration of raw effluent. This is confirmed by the toxicity factor (TF), which expresses that the effluent should be diluted 526 times in order not to cause toxic effects on the microcrustaceans, used as an indicator. Reagents adjustment for application of the Fenton Reaction. Wastewater treatment using the Fenton process sought to determine the optimal condition of reagents. Initially, the response parameter chosen was the absorbance. Table 1 presents the residual colour, in terms of absorbance at 554 nm, when the raw effluent was treated with different concentrations of Fe 2+ and H 2 O 2 in the Fenton´s reaction. The concentration of hydrogen peroxide plays a crucial role in the efficiency of the degradation process and good results were attained at dosages of 20 mL L − 1 and 30 mL L − 1 . The percentage of pollutant degradation increased with an increase in the hydrogen peroxide dosage and the same occurred in the works of Eisenhauer (1964), Lin and Lo (1997), Lin et al. (1999) and Kang and Hwang (2000). However, care should be taken when selecting the operating oxidant dosage. The unused portion of hydrogen peroxide during the Fenton process contributes to an increase in COD (Lin and Lo, 1997) and, therefore, an excess amount is not recommended (Lin and Lo, 1997; Pignatello et al., 2006). In addition, the presence of hydrogen peroxide is detrimental to many organisms and may affect degradation efficiency in cases where the Fenton process is used as a pre-treatment to biological oxidation, or in cases when the treated effluent is discharged into receiving bodies (Ito et al., 1998). The [H 2 O 2 ]:[Fe 2+ ] ratio is another crucial parameter for achieving the best results with the Fenton´s process (Gulkaya et al., 2006). Tang and Huang (1996a, 1996b, 1996c, 1997), Tang and Tassos (1997), and Kochany and Lugowski (1998) pointed out that the optimum [Fe 2+ ]:[H 2 O 2 ] ratio must be maintained to achieve maximum degradation efficiency, as both H 2 O 2 and Fe 2+ can also react with OH• and, therefore, can inhibit the oxidation reactions if either of them is not at the optimal dosage. It should be considered whether it is possible to reduce the amounts of Fe 2+ and H 2 O 2 , while keeping their ideal relationship constant. In the present work, good results appear with [H 2 O 2 ]:[Fe 2+ ] molar ratios equal to or higher than 7.5:1. This ratio is similar to, or not far from, the best results obtained by: Kuo (1992) when decolourising simulated dye wastewaters with ratios between 3:1 and 9:1; Tang and Huang (1997) when oxidising chlorinated aliphatic organics from 5:1 to 11:1; Tang and Tassos (1997) when oxidising bromoform from to 2:1 to 5:1; Benatti et al. (2006) for chemical laboratory wastewater with 4.5:1; and Gayathri et al. (2023) for a diluted solution of Rhodamine B in water with 3:1. Table 1 – Residual absorbance of the effluents treated with different molar ratios of H 2 O 2 and Fe 2+ . Initial Rhodamine B concentration in the raw effluent at 772 mg L − 1 . Molar ratio [H 2 O 2 ]: [Fe 2+ ] Dosages of reagents applied Absorbance (554 nm) FeSO 4 .7H 2 O (g L − 1 ) H 2 O 2 (mL L − 1 ) 2.5:1 16.6 10 2.920 33.2 20 0.037 49.8 30 0.036 5:1 8.3 10 3.593 16.6 20 0.074 24.9 30 0.069 7.5:1 5.5 10 4.945 11.1 20 0.101 16.6 30 0.051 10:1 4.2 10 6.500 8.3 20 0.244 12.5 30 0.056 12.5:1 3.3 10 7.705 6.6 20 0.798 10.0 30 0.092 The condition of 11.1 g L − 1 FeSO 4 .7H 2 O and 20 mL L − 1 H 2 O 2 , whose [H 2 O 2 ]:[Fe 2+ ] molar ratio is 7.5:1, provided a good discolouration (absorbance of 0.101). This proportion was chosen to be studied further, with repetitions, in order to make a finer adjustment to the reagent concentration. Table 2 presents the absorbance results, residual TOC concentrations and the final surface tension of the effluent generated in the assays performed using the 7.5:1 [H 2 O 2 ]:[Fe 2+ ] molar ratio with H 2 O 2 set at 5, 10, 15, 20, 25 and 30 mL L − 1 . Additionally, Figs. 4 and 5 show the percentages of Rhodamine degradation and TOC reduction, respectively, as a function of reagent concentration. Figure 6 depicts the results of surface tension of the treated wastewater in the same conditions. Table 2 – Mean (µ) and standard deviation (σ) of absorbance, TOC and surface tension of treated effluents (n = 3) generated in the assays performed using different concentrations of reagents with [H 2 O 2 ]:[Fe 2+ ] molar ratio at 7.5:1. Initial Rhodamine B concentration of the raw effluent of 772 mg L − 1 , TOC of 11,950 mg L − 1 and surface tension of 58.9 mN m − 1 . Dosages of reagents applied Residual absorbance (554 nm) TOC (mg L − 1 ) Surface tension (mN m − 1 ) FeSO 4 .7H 2 O (g L − 1 ) H 2 O 2 (mL L − 1 ) µ σ µ σ µ σ 2.8 5 17.44 0.96 1216.4 444.84 55.7 0.68 5.5 10 3.42 0.23 1032.7 202.12 60.6 0.32 8.3 15 0.52 0.06 825.7 26.04 63.1 0.37 11.1 20 0.15 0.05 804.9 21.14 64.4 0.30 13.8 25 0.04 0.01 694.8 60.35 68.1 0.54 16.6 30 0.04 0.03 625.7 33.02 69.6 0.36 Analysing the data, it can be seen that the Rhodamine B concentration progressively decayed for the dosages applied. However, at concentrations lower than 11.1 g L − 1 FeSO 4 .7H 2 O and 20 mL L − 1 of H 2 O 2 , despite achieving percentages of dye reduction very close to 100%, they still had Rhodamine B concentrations that is perceptible to the observer. With the standard deviations, it can be verified that the Rhodamine B degradation results presented low variability, with a tendency of decreasing from the highest to the lowest reagent concentrations. Regarding TOC, the residual amount started from 90% removal with 2.8 g L − 1 FeSO 4 .7H 2 O and 5 mL L − 1 of H 2 O 2 and reached 95% with 16.6 g L − 1 FeSO 4 .7H 2 O and 30 mL L − 1 of H 2 O 2 . The average percentages of TOC reduction are lower than those of Rhodamine degradation, indicating the presence of fragments of the organic molecules initially present in the effluent. The TOC analyses varied more than the Rhodamine degradations ones, also having the same tendency to decrease from the highest to the lowest chemical addition. We defined 11.1 g L − 1 FeSO 4 .7H 2 O and 20 mL L − 1 H 2 O 2 as the appropriate dosages to treat the effluent. In this condition, the effluent was clear with a slight pink colour. The Fenton reaction enabled high degradation of Rhodamine B, although intermediate products of Rh B degradation and other non-mineralised effluent constituents are responsible for the residual TOC in the treated samples. Similar results were obtained with synthetic solutions of Rhodamine B, using variants of the Fenton process, under conditions with lower dye dosages and without the presence of ethanol (Hou et al., 2011; Gan and Li, 2013; Guo et al., 2014, Gayathri et al., 2023). Gayathri et al. (2023) listed some intermediates identified at 50% degradation of Rhodamine B by LC-MS analysis during 50% solar Fenton degradation, including the following chemical species: hydroxy N-ethyl-N´ ethyl rhodamine, N,N-diethyl-N´-ethylrhodamine, hydroxy amino N´ ethyl rhodamine, N-ethyl-N´ethylrhodamine, N-ethylrhodamine, 3-hydroxybenzoic acid, succinic acid and oxalic acid. It should be noted that, analysis with the alcoholmeter showed that the Fenton reaction was able to remove all the ethanol present. During the Fenton reaction, the smell of CH 3 COOH (acetic acid) was perceived, indicating that ethanol was being degraded by the hydroxyl radical. This fact was also observed by Walling and Kato (1971), who identified the formation of CH 3 COOH in a study on the oxidation of alcohols by the Fenton reaction. Still, the surface tension increased as the concentration of reagents increased, indicating that the degradation of Rhodamine B and ethanol restores, at least in part, the intermolecular bonding forces of the water. At the end of the Fenton's reaction, a base (e.g. NaOH, Ca(OH) 2 ) needed to be added to neutralise the solution, with the production of ferric based sludge (Benatti et al., 2006; Wu et al., 2010). Disposal of this sludge is one of the drawbacks of the Fenton's process. Sludge increases as the FeSO 4 .7H 2 O concentration increases (Table 3 ). The sludge generation presented the same magnitude reported in other works where the Fenton process was studied (Barros et al., 2006; Yu et al., 2022). The common practice is to dewater the sludge and send it to a landfill. However, Guo el al. (2018) found that after calcination at 600 o C, the iron sludge exhibited an enhanced catalytic performance toward rhodamine B in the presence of H 2 O 2 . So rather than discarding this waste, the sludge generated can be dehydrated and heat treated for reuse as a heterogeneous Fenton´s catalyst. Table 3 – Sludge generation in treated samples using different concentrations of FeSO 4 .7H 2 O (g L -1 ) and H 2 O 2 (mL L -1 ) at [H 2 O 2 ]:[Fe 2+ ] molar ratio of 7.5:1. Dosages of reagents applied Sludge mass (g L − 1 ) FeSO 4 .7H 2 O (g L − 1 ) H 2 O 2 (mL L − 1 ) 2.8 5 1.43 5.5 10 2.82 8.3 15 4.05 11.1 20 5.51 13.8 25 6.80 16.6 30 8.20 Application of the Fenton process Considering the results obtained, the optimal dosage adopted was the concentration 11.1 g L − 1 of FeSO 4 .7H 2 O and 20 mL L − 1 of H 2 O 2 . The reported values of the quantities of reagents required to treat synthetic effluents containing 20–300 mg L − 1 of Rhodamine B are not as high and generally range from 0.013-0.500 g L − 1 FeSO 4 .7H 2 O and 0.017–0.584 mL L − 1 of H 2 O 2 (Kuo, 1992; Solozhenko et al., 1995). However, the literature also describes that treating actual effluents with the Fenton process requires much higher doses than treating synthetic effluents with the same dye content, to achieve the same treatment efficiency (Kuo, 1992; Kang and Chang, 1997; Hofl et al., 1997; Perez et al., 2002; Umar et al., 2010; Torrades and García-Montaño, 2014). This is probably due to the presence of other substances in the effluent, which may consume the hydroxyl radicals generated in the Fenton reaction during oxidation or end up interfering negatively in the process. Therefore, when assessing the amount of reagents required to actual effluent, it is a priority to consider its total organic load. Figure 7 presents the development the Fenton reaction assay and details the effluent aspect over time, using the optimal concentration of 11.1 g L − 1 FeSO 4 .7H 2 O and 20 mL L − 1 H 2 O 2 . Bubbles and foam formed in the early stages, which declined at the end of the reaction. It is important to note that the Fenton reaction is an exothermic process (Bautista et al., 2008). The temperature reaches 34°C, an increase of approximately 12°C from room temperature. The temperature begins to rise shortly after the reagents are mixed in the raw effluent sample. The temperature keeps rising until it reaches its peak of 34°C at 35 min. Foam, indicative of O 2 generation, follows the same behaviour. After 35 min, both temperature and foam begin to reduce, denoting that hydroxyl radical generation decrease. At the end of 80 min of the experiment, no foam generation is evident. Thus, 90 min was adopted as the ideal oxidation time and the reaction was stopped with the addition of sodium thiosulfate. Considering the steps involved in reagent preparation (10 min), reagent addition (10 min), the development of the Fenton´s reaction itself (90 min), pH adjustment (10 min), sludge settling (60 min) and filtering (30 min), the total time required to perform the wastewater treatment at bench scale was approximately 3 hours and 30 minutes. Table 4 summarizes the results of the analyses performed on the raw and treated effluent under these conditions. Table 4 Complementary characterisation of the raw effluent and after treatment by the Fenton´s Process using a dosage of 11.1 g L − 1 Fe 2 SO 4 .7H 2 O and 20 mL L − 1 of H 2 O 2 at [H 2 O 2 ]:[Fe 2+ ] molar ratio of 7.5:1. Parameters Raw effluent After Fenton´s process Absorbance >> 2 0.158 Rhodamine B (mg L − 1 ) 772 0.7 Colour bright pink light pink pH 3.2 8.0 Surface tension (mN m − 1 ) 59.0 63.1 Conductivity (mS cm − 1 ) 0.33 9.12 Ethanol (%) 3 0 TOC (mg L − 1 ) 11,950 868 COD (mg L − 1 ) 18,180 2.772 BOD 5 (mg L − 1 ) 5,800 840 Total N (mg L − 1 ) 45.1* 18.5 Nitrate (mg L − 1 ) - < 0.2 Nitrite (mg L − 1 ) - < 0.01 Ammoniacal N (mg L − 1 ) - 15.2 Organic N (mg L − 1 ) - 3.3 TKN (mg L − 1 ) - 18.5 Total P (mg L − 1 ) 0.188 < 0.006 Total Fe (mg L − 1 ) 0.15 2.2 EC50–48h (%) - Daphnia similis 0.61 18.76 Toxicity factor (TF) - Daphnia similis 526 16 * calculated value from the concentration of Rhodamine B The Fenton reaction resulted in significantly reduced absorbance and, consequently, of the Rhodamine B concentration in the treated sample. There was an increase in conductivity in the treated effluent. This is due to the addition of SO 4 2− ions by the FeSO 4 .7H 2 O used in the Fenton reaction, as well as the Na + ions from the use of NaOH for pH adjustment. The process was able to remove all of the ethanol present in the raw effluent sample, effectively collaborating in the removal of the organic load. The efficiency of COT, COD and BOD 5 reduction was approximately 90%, 85% and 85%, respectively. The data obtained in the present study are consistent with the study by Chakinala et al. (2009) on an effluent containing dye that had TOC of 6,000 mg L − 1 and COD of 17,000 mg L − 1 . The Fenton process, combined with hydrodynamic cavitation, allowed for TOC and COD removal rates of 70% and 85%, respectively. Furthermore, the COD removal values ​​are similar to or slightly better than the results obtained in the study by Tekin et al (2006) with a wastewater from the pharmaceutical industry. The authors reported an increase in the BOD 5 :COD ratio by applying the Fenton process in the order of 3 to 5 times, which is important for a subsequent biological treatment. Here, despite the significant reduction in organic load, the BOD 5 :COD ratio remained constant at approximately 0.3. Considering the amount of total nitrogen present due to the Rhodamine molecules (45.1 mg L − 1 ), after the Fenton reaction, much of the nitrogen was mineralised and volatilised, with 18.5 mg L − 1 of total N remaining. Of this, 15.2 mg L − 1 was converted to ammonia N and 3.3 mg L − 1 was still present as organic N. These results are consistent with the study by Nidheesh et al. (2014), who proposed a Rhodamine B degradation route by POA through mass spectrometry (GC-MS). The authors found that nitrogen goes through some organic forms until it is converted to ammonia. The ammonia content was much higher than the organic nitrogen content in the treated samples, indicating that almost all of the nitrogen was converted to ammonia. Also, considering the pH 8 of the samples after treatment, it is very likely that the ammonia will almost all be NH 4 + (Metcalf and Eddy, 2003). However, about 15% can be in the form of NH 3 and able to be removed by volatilisation. The phosphorus content of 0.188 mg L − 1 in the raw effluent is quite low. Nevertheless, it was removed, probably due to the interaction with residual iron during the Fenton reaction (Camber et al, 2021). A chemical coagulation step, sometime with the addition of aluminum salts, is recommended after Fenton oxidation to keep the concentration of the soluble iron with the specified limits (Lin and Lo, 1997; Babuponnusami and Muthukumar; 2014). Specifically, in this work, it was not necessary, since the iron concentration remained below the limit of 15 mg L − 1 required by Brazilian legislation (CONAMA 430/2011). The carcinogenic, reproductive and developmental toxicity, neurotoxicity and chronic toxicity of Rhodamine B to humans and animals has been experimentally proven (Kornbrust and Barfknecht, 1985; IARC, 1987; Mirsalis et al., 1989; McGregor et al., 1991; Shimada et al., 1994; Priya et al, 2024). Table 4 also presents the results of the ecotoxicological tests with the micro crustacean Daphnia similis . The ecotoxicity of the raw effluent is high, as demonstrated by the toxicity factor (TF) of 526. The treated samples showed a substantial reduction of the toxicity factor. The Fenton process was responsible for reducing the TF value from 526 to 16. Considerations on the treatment process Mohod et al. (2023) stated that ‘it is preferable to treat wastewater containing Rhodamine dyes at a lower concentration rather than with a higher concentration and efforts should be made to develop a technology that can disintegrate high-concentration Rhodamine dyes in wastewater´. Herein the Classic Fenton Process was applied with some success to treat an agate dyeing bearing effluent in a situation with a 772 mg L − 1 of Rhodamine B and 3% ethanol in water. However, the cost of chemical inputs (ferrous sulfate pentahydrate, hydrogen peroxide and sodium hydroxide) to treat the effluent in the regional context will not be below R $ 600.00 m − 3 (≈ USD 120 m − 3 ), making the cost a drawback of the process. Typically, the Fenton´s process is carried out in batches and can be used to comply with the effluent volume generated by the agate production sector. However, even in the best possible conditions and surpassing the efficiency of other investigated processes, the effluent is still toxic and presents COD, which makes it unsafe to dispose of in bodies of water. In addition, intermediate molecules resulting from the degradation of Rhodamine B are present, with different levels of toxicity. A set of major products resulting from the degradation of Rhodamine by advanced oxidative processes, which may be present in this specific effluent, were listed by Gayathri et al. (2023) and Mohod et al. (2023a). Therefore, the preliminary treatment step provided by the classical Fenton´s reaction must be integrated to others processes to reach a better water quality. New technologies based on photocatalytic and/or cavitation techniques can be built-in, even if there is an increase in energy consumption (Hinge et al. 2016, Xu and Ma 2021, Mohod et al. 2023a, Mohod et al, 2023b). Another possibility would be a subsequent treatment step by adsorption (Kausar et al., 2021; Bazan-Wozniak et al., 2024). In this regard, Da Rosa et al. (2018) listed a variety of adsorbents and biosorbents for Rhodamine removal. Activated carbon is among those with the highest adsorption capacity and is currently being studied as a complementary stage of treatment to remove residual Rhodamine and the by-products generated by the oxidation process. Furthermore, wastewater could be considered for reuse within the production process itself, more specifically in the washing stage after dyeing. Conclusion Samples of the effluents generated from gem-dyeing processes in the Soledade region have concentrations of Rhodamine B ranging from 500 to 2,500 mg L − 1 and 2 to 4% of ethanol. The colour of the effluent is an intense red-pink with a very high toxicity factor (TF 526) and surface tension values below 55 mN m − 1 . Treatment by the Fenton Reaction promoted alcohol removal, reduction in the strength of the colour, and high ecotoxicity reduction. Due to the high organic load, high concentrations of reagents were required. In the sample studied, 11.1 g L − 1 of ferrous sulphate and 20 mL L − 1 of oxygen peroxide (at a [H 2 O 2 ]:[Fe 2+ ] molar ratio of 7.5:1) were defined as the best dosages, which allowed a reduction in absorbance of 99.9% at 554 nm, a TOC reduction of 93%, an increase in surface tension from 58.9 to 64.4 mN.m − 1 and the toxicity factor decreased from 526 to 16. There was also a 60% reduction in total N, which may have been mineralised or volatilised. The remaining fraction in the aqueous medium was predominantly in ammoniacal form and a smaller portion was in the organic form. The oxidation time for this reagent concentration was approximately 90 minutes. The Fenton process combines oxidation, air stripping, coagulation and co-coupling mechanisms that substantially reduces pollutants present in the effluent in a rougher stage. Additional studies to remove residual colour and effluent toxicity should be pursued. Declarations Funding declaration This research received financial support from Fapergs (grant number 11/1332-2), CNPq (grant number 314880/2020-8) and CAPES-PROEX (grant number 88881.844968/2023-1). Consent to publish declaration Not applicable Ethics and consent to participate declarations Not applicable Declaration of competing interests 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. References Al-Buriahi, A.K., Al-Gheethi, A.A., Senthil Kumar, P., Radin Mohamed, R.M.S., Yusof, H., Alshalif, A.F., Khalifa, N.A., 2022. 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Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 17 Jun, 2025 Read the published version in Discover Chemical Engineering → Version 1 posted Editorial decision: Accepted 30 Apr, 2025 Editor assigned by journal 23 Apr, 2025 Reviews received at journal 14 Apr, 2025 Reviewers agreed at journal 14 Apr, 2025 Reviews received at journal 11 Apr, 2025 Reviewers agreed at journal 11 Apr, 2025 Reviewers invited by journal 11 Apr, 2025 Submission checks completed at journal 10 Apr, 2025 First submitted to journal 26 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5727575","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":441540844,"identity":"26fc17ff-234f-44e0-9baf-6783625c88e4","order_by":0,"name":"Cassiano R. dos Santos","email":"","orcid":"","institution":"Universidade Federal do Rio Grande do Sul (UFRGS)","correspondingAuthor":false,"prefix":"","firstName":"Cassiano","middleName":"R. dos","lastName":"Santos","suffix":""},{"id":441540846,"identity":"4955dd3b-317f-48c9-b8c6-efa2c161f13b","order_by":1,"name":"Augusto C. Rodrigues","email":"","orcid":"","institution":"Universidade Federal do Rio Grande do Sul (UFRGS)","correspondingAuthor":false,"prefix":"","firstName":"Augusto","middleName":"C.","lastName":"Rodrigues","suffix":""},{"id":441540847,"identity":"12cd1f20-3974-4545-bb96-ca14028bb9e1","order_by":2,"name":"Fernanda Vilasboas","email":"","orcid":"","institution":"Universidade Federal do Rio Grande do Sul (UFRGS)","correspondingAuthor":false,"prefix":"","firstName":"Fernanda","middleName":"","lastName":"Vilasboas","suffix":""},{"id":441540848,"identity":"cebcbe75-68fe-4ffa-b689-58f0d5902a8d","order_by":3,"name":"Clóvia Marozzin Mistura","email":"","orcid":"","institution":"Universidade de Passo Fundo","correspondingAuthor":false,"prefix":"","firstName":"Clóvia","middleName":"Marozzin","lastName":"Mistura","suffix":""},{"id":441540849,"identity":"0232347a-7480-4d59-8ca3-235b0f0fa5fb","order_by":4,"name":"Ivo André Homrich Schneider","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1UlEQVRIiWNgGAWjYBACNgjFzMDA3sDwAcQ0IF4LzwHGGURpYYBrkUggUgufRPIBhp87rPP5Jd8YNlf8qmMwlz5AwGESaQmMvWfSLWfOzjFsPNt3mMGyL4GAFp4zBgy8bYcNDG7nmD9s7DnAYHCGgMNAWhj/ArXY3zxj2NjYU0eEFvYeA2awLRI8ho0NP5iJ0dKWcFi2Ld1A4kxaYWNjw2Eeyx4CWuSbmQ8+fNtmbcDffnhjY8OfOjlzHgJaQOAAnMXYxkCMBhTwh1QNo2AUjIJRMBIAAJXZPu52gwIkAAAAAElFTkSuQmCC","orcid":"","institution":"Universidade Federal do Rio Grande do Sul (UFRGS)","correspondingAuthor":true,"prefix":"","firstName":"Ivo","middleName":"André Homrich","lastName":"Schneider","suffix":""}],"badges":[],"createdAt":"2024-12-28 19:53:05","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5727575/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5727575/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s43938-025-00085-w","type":"published","date":"2025-06-17T15:57:54+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":80522313,"identity":"bf87b136-8e2e-48c2-b7b7-8290b4a9d3b7","added_by":"auto","created_at":"2025-04-14 09:19:38","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":35024,"visible":true,"origin":"","legend":"\u003cp\u003eRhodamine B chemical formula.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/1a0630bf9c7060b93eb90e6c.png"},{"id":80522318,"identity":"5f769fce-2bb6-4664-bc93-ae8957756d11","added_by":"auto","created_at":"2025-04-14 09:19:38","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":194749,"visible":true,"origin":"","legend":"\u003cp\u003eNatural agate piece and after staining with Rhodamine B dye.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/8afd8177fe287b95e7b31b42.png"},{"id":80523813,"identity":"204e7f0b-a82c-41e6-86b7-66fcdcb6677a","added_by":"auto","created_at":"2025-04-14 09:35:38","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":166096,"visible":true,"origin":"","legend":"\u003cp\u003eEffluent containing the dye Rhodamine B as the result from the agate washing operation.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/21de597727f65a09d744da4b.png"},{"id":80523342,"identity":"c50d7545-366f-4893-9d06-20e03c081617","added_by":"auto","created_at":"2025-04-14 09:27:38","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":21660,"visible":true,"origin":"","legend":"\u003cp\u003eRhodamine B (Rh B) degradation in the effluent using different concentrations of FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO (g L\u003csup\u003e-1\u003c/sup\u003e) and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u0026nbsp; \u003c/sub\u003e(mL L\u003csup\u003e-1\u003c/sup\u003e) at [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio of 7.5:1. Initial Rhodamine B concentration in the raw effluent of 772 mg L\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/70775a97cbb55c089372fc0a.png"},{"id":80522315,"identity":"a21df682-a20d-47ca-9ea0-83ac9681ac83","added_by":"auto","created_at":"2025-04-14 09:19:38","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":20829,"visible":true,"origin":"","legend":"\u003cp\u003eTotal organic carbon (TOC) removal in the effluent using different concentrations of FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO (g L\u003csup\u003e-1\u003c/sup\u003e) and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u0026nbsp; \u003c/sub\u003e(mL L\u003csup\u003e-1\u003c/sup\u003e) at [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio of 7.5:1. Initial TOC concentration in the raw effluent of 11,950 mg L\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/4783036a0b01f8fc07dca1d5.png"},{"id":80522329,"identity":"9721af5e-d483-49a7-bf2f-b515cbf43a49","added_by":"auto","created_at":"2025-04-14 09:19:38","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":23788,"visible":true,"origin":"","legend":"\u003cp\u003eSurface tension of the raw effluent and treated samples using different concentrations of FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO (g L\u003csup\u003e-1\u003c/sup\u003e) and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u0026nbsp; \u003c/sub\u003e(mL L\u003csup\u003e-1\u003c/sup\u003e) at [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio of 7.5:1.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/836f27750d5340fa3b323a03.png"},{"id":80522333,"identity":"8f4472e3-d5e5-47ef-8aeb-8671f3d89d02","added_by":"auto","created_at":"2025-04-14 09:19:38","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":79665,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Development of the Fenton reaction assay and their respective times, and the effluent aspect over time, for the treatment of Rhodamine B containing effluent using the concentration of 11.1 g L\u003csup\u003e-1\u003c/sup\u003e of FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO\u0026nbsp; and 20 mL L\u003csup\u003e-1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e; (b) effluent appearance before and after the treatment; (c) variation of the temperature of the sample over time in an assay using optimal reagent concentration.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/c574831f16a29a177ba17646.png"},{"id":85231408,"identity":"fd0ab106-34c4-4ea5-899d-4f8825e6da03","added_by":"auto","created_at":"2025-06-23 16:07:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1701005,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5727575/v1/fdd3178f-d1dd-4e4a-a8db-f10d34eb251d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eOn The Fenton´s Process for the Treatment of Effluents from the Dyeing of Agates Containing Rhodamine B and Ethanol\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eIn the state of Rio Grande do Sul, Brazil, there is a prominent centre for the extraction and processing of agates (Hartmann, 2010). Agates have several natural textures and colours. However, they are usually subjected to shape and dyeing processes for product diversification purposes (Silva et al, 2007; da Silva Vilasb\u0026ocirc;as et al., 2017; Eriksson et al., 2021a; 2021b). Among organic dyes, the one that gives the agates a reddish pink colour is known as Rhodamine B (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e illustrates an agate in its natural colour and after dyeing with Rhodamine B (Rh B).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRhodamine B is a dye that can be easily solubilised in both water and ethanol and does not require large amounts to provide a strong and vibrant colour to the stones. However, this hydrophilic xanthene dye exhibits chemical stability and can accumulate in living organisms, disrupting the endocrine system, impairing fertility, and causing genetic damage (Al-Buriahi et al., 2022; Priya et al., 2024).\u003c/p\u003e \u003cp\u003eThe dyeing processing of agates is carried out in plastic barrels, where the stones are placed in ethanol solutions (approximately 20 g of dye for 1 L of ethanol). After three days, the stones are removed from the ethanol solution and washed with water. The liquid effluent containing the dyes are the result of this agate washing step. According to Carissimi and Schneider (2010), the dye-containing wash effluent may have alterations in pH, Rhodamine B concentration of the order of 20-2000 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, a substantial amount of Total Organic Carbon (TOC), high colouration, and a toxicity which it adds to the aquatic environment. The wastewater volumes generated are not high, up to 15 m\u003csup\u003e3\u003c/sup\u003e per day in most agate dying units. However, the municipality of Soledade is located in a region of river springs, so the high toxicity of the effluent associated with the low flow of the water resources leads to a concern.\u003c/p\u003e \u003cp\u003eThe characteristics of the effluent indicate that the treatment operation should (a) remove colour, (b) decrease the organic load, and (c) reduce toxicity. It needs to include processes that can remove soluble organic load, which could be done by oxidative, adsorption and/or biological processes (Metcalf and Eddy, 2003). However, the pollutant load is too high to be initially treated by adsorption and biological processes are hindered by the low BOD\u003csub\u003e5\u003c/sub\u003e:COD (biochemical oxygen demand: chemical oxygen demand) ratio. Thus, chemical oxidation is the technique with the highest potential for the treatment of such wastewaters, at least for an initial stage.\u003c/p\u003e \u003cp\u003eSome studies on wastewater treatment containing organic dyes from agate dyes have been developed. Carissimi et al. (2000) and Pizzolato et al. (2002) assessed the oxidation of effluents containing Rhodamine B, Bright Green and Crystal Violet dyes using sodium hypochlorite \u0026ndash; NaClO, the recurring oxidant used by the agate industries. Several compounds have been found to be formed due to the non-mineralisation of chemicals, including organochlorines. Barros et al. (2006) investigated the use of the Fenton reaction in the treatment of industrial effluents containing a mixture of Rhodamine B, Bright Green and Violet Crystal dyes. They identified the presence of residual substances due to the incomplete oxidation of the dye molecules, such as xylene, hydrocarbons, phenolic compounds and nitrogen phenol. Machado et al. (2012) used the ozonation (O\u003csub\u003e3\u003c/sub\u003e), ultraviolet (UV) and O\u003csub\u003e3\u003c/sub\u003e/UV techniques for industrial effluent degradation with Rhodamine B dye. The study showed that, in the best ozone discolouration condition, the treated effluent still presented a high level of toxicity.\u003c/p\u003e \u003cp\u003eHowever, there are other relevant issues regarding the treatment of effluents containing organic dyes generated in the manufacture of agates that have not been addressed or that deserve to be detailed. Among these, we can mention the total organic carbon (TOC), which enables the evaluation of the degree of dye mineralisation; determining the ethanol content in the effluent, considering that its presence can increase pollution; as well as the fate of nitrogen from the Rhodamine B molecules and the ecotoxicological response.\u003c/p\u003e \u003cp\u003eConsidering that the use of sodium hypochlorite, the reagent currently used for the treatment, has a poor performance and forms organochlorines (Pizzolato et al., 2002), and that ozonation only presents partial results (Machado et al., 2012) and forms precipitates (preliminary studies), the present study employs the Fenton Process. The process is an advanced oxidative process based on the hydroxyl radical, which has both oxidation and coagulation properties.\u003c/p\u003e \u003cp\u003eThe Fenton reaction was discovered by H.J H. Fenton in 1894, when he found that H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e can be activated by ferrous salts (Fe\u003csup\u003e2+\u003c/sup\u003e) to oxidise tartaric acid (Fenton, 1894). The Fenton process encompasses reactions of peroxides (usually hydrogen peroxide \u0026ndash; H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) with iron ions to form active oxygen species (especially the hydroxyl radical \u0026bull;OH) that oxidise organic or inorganic compounds (Neyens and Baeyens, 2003). The importance of radical hydroxyl reactions has been recognised in recent decades and more than 1700 velocity constants for \u0026bull;OH reactions with organic and inorganic compounds in aqueous solution have been tabulated (Buxton et al., 1988). The Fenton reaction has been widely studied and, in many cases, efficiently used in wastewater treatment for the degradation of organic substances. Eq.\u0026nbsp;(1) is recognised as the Fenton reaction and is based on the oxidation of ferrous iron into ferric iron to decompose H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e into hydroxyl radicals. It is generally regarded as the centre of Fenton chemistry. These, and other parallel reactions, are well documented in the literature (Neyens and Baeyens, 2003; Bautista et al., 2008; Babuponnusami and Muthukumar; 2014; Gayathri et al., 2023).\u003c/p\u003e \u003cp\u003eFe\u003csup\u003e2+\u003c/sup\u003e + H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e \u0026rarr; Fe\u003csup\u003e3+\u003c/sup\u003e + OH\u003csup\u003e\u0026minus;\u003c/sup\u003e + \u0026bull;OH (1)\u003c/p\u003e \u003cp\u003eThe Fenton process happens in the acidic pH range. After the time required for oxidation of the pollutants, pH adjustment from an acid to neutral/alkaline range is required for residual iron removal by precipitation.\u003c/p\u003e \u003cp\u003eTherefore, the aim of this study is to depict the Fenton reaction for the treatment of a wastewater generated by the agate processing industry containing Rhodamine B and ethanol. It is worth noting that it was chosen the classic homogeneous Fenton process, without any variants, to be simpler to implement by the industry. The concentration of the Fenton\u0026acute;s reagents and sludge generation were studied and the quality of the final effluent was monitored with an emphasis on colour and organic load removal, surface tension of the water, and toxicological response, in order to obtain treated water with a low pollutant potential. Another aspect which was investigated was the fate of the nitrogen present in the Rhodamine B molecules.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSamples\u003c/h2\u003e \u003cp\u003eEffluent samples from agate dyeing with Rhodamine B were collected from an agate dyeing company located in the city of Soledade (28\u0026deg; 49' 04\" S 52\u0026deg; 30' 36\" W), in the state of Rio Grande do Sul, Brazil. Initially, five samples (representative of five dyeing operations performed at different periods) were provided for an initial characterisation of the Rhodamine content, surface tension and alcohol content. One of these samples, chosen at random, was used in this work. An image of this effluent is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe reagents used for the treatment by the Fenton process were ferrous sulphate (FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO) and hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e 35% w/w). Sodium thiosulfate pentahydrate (Na\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.5H\u003csub\u003e2\u003c/sub\u003eO) diluted in water at 1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was used to decompose the residual H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and stop the Fenton\u0026acute;s reaction (Liu et al, 2003). Aqueous NaOH (10% m/v) and HCl (10% w/v) solutions were used to adjust the pH. All reagents were of analytical grade. Rhodamine B, also of analytical purity, was used to determine the dye concentration. The water used in the experiments was obtained by reverse osmosis treatment of the water from the public water supply.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eStudy of the treatment of liquid effluents through Fenton reaction\u003c/h3\u003e\n\u003cp\u003eThis part of the research focused on determining the optimal concentration of Fenton reagents and the reaction time to treat the effluent. The aim was to achieve high colour (absorbance at 554 nm) and total soluble organic (TOC) reduction using the smallest possible quantity of reagents.\u003c/p\u003e \u003cp\u003eThe effluent treatment tests with the Fenton Reaction were conducted by performing the following steps:\u003c/p\u003e \n\u003col style=\"list-style-type: lower-roman;\"\u003e\n \u003cli\u003eseparation of 200 mL of raw effluent;\u003c/li\u003e\n \u003cli\u003epH control with a pH meter;\u003c/li\u003e\n \u003cli\u003epH adjustment to 3.0 +/- 0.1 with HCl and/or NaOH;\u003c/li\u003e\n \u003cli\u003eaddition of \u0026nbsp;the reagents FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e;\u003c/li\u003e\n \u003cli\u003eoxidation time;\u003c/li\u003e\n \u003cli\u003eaddition of 1 mol L\u003csup\u003e-1\u003c/sup\u003e of Na\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.5H\u003csub\u003e2\u003c/sub\u003eO to decompose the residual H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and stop the Fenton\u0026acute;s reaction.\u003c/li\u003e\n \u003cli\u003epH adjustment to 8.0 +/- 0.1 by adding NaOH to promote Fe\u003csup\u003e3+\u003c/sup\u003e precipitation as hydroxide;\u003c/li\u003e\n \u003cli\u003efiltering the sludge with qualitative filter paper;\u003c/li\u003e\n \u003cli\u003eabsorbance measurement.\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003eTo define the best dosage, it was initially necessary to determine the best proportion between the amounts of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e. The molar ratio of [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]/[Fe\u003csup\u003e2+\u003c/sup\u003e] applied were 2.5:1; 5;1; 7.5;1; 10;1 and 12.5;1. The concentration of the reagents was calculated by molar ratio and based on three dosages of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e: 10, 20 and 30 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Initially, the experiments were carried out with one repetition and only the colour parameter was assessed (absorbance at 554 nm).\u003c/p\u003e \u003cp\u003eAfterwards, the best molar ratio was reproduced with H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e concentrations set at 5, 10, 15, 20, 25 and 30 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. In addition to absorbance, the TOC and surface tension of the treated samples were analysed. These assays were performed in triplicate. The reaction time was determined by temperature variation, with the aid of a thermometer, and foam production. Once the temperature began to drop and foam production ceased, it was considered that the Fenton reaction was no longer occurring.\u003c/p\u003e\n\u003ch3\u003eAnalytical procedures\u003c/h3\u003e\n\u003cp\u003eVisible-range spectrum scanning was performed on a diluted effluent sample with a benchtop spectrophotometer, to verify the highest absorption wavelength in the visible range of the spectrum (400\u0026ndash;700 nm). The highest absorbance occurred at 554 nm, so this wavelength was adopted to evaluate the concentration of Rhodamine B in the liquid effluent. From the correlation between absorbance x analytical grade Rhodamine B concentration, the amount of Rhodamine B in the effluent was determined.\u003c/p\u003e \u003cp\u003eSurface tension (ST) was determined at room temperature, using a tensiometer and the \u003cem\u003eDu No\u0026uuml;y\u003c/em\u003e static method with a platinum ring. Effluent conductivity was determined by a conductivity meter. The Total Organic Carbon (TOC) was analysed by a carbon analyser.\u003c/p\u003e \u003cp\u003eThe ethanol content in the raw effluent was determined by distillation and density testing (alcoholometer). The ethanol distillation procedure was performed at a temperature of 76\u0026ndash;78\u0026deg;C. The results attained by distillation and with the alcoholometer were nearly the same.\u003c/p\u003e \u003cp\u003eThe other chemical analyses were carried out according to the following procedures, described in \u0026lsquo;Standard Methods for the Examination of Water and Wastewater\u0026rsquo; (Eaton et al., 2005): 4500-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e nitrogen (nitrate), 4500-NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e nitrogen (nitrite), 4500-NH\u003csub\u003e3\u003c/sub\u003e nitrogen (ammonia), 4500-N\u003csub\u003eorg\u003c/sub\u003e nitrogen (organic), 4500-P phosphorus, 5220 Chemical Oxygen Demand (COD), and 5210 Biochemical Oxygen Demand (BOD\u003csub\u003e5\u003c/sub\u003e). Iron analyses were carried out by sample preservation with HNO\u003csub\u003e3\u003c/sub\u003e and by an issuing optical spectrophotometer with inductively coupled plasma (ICP-OES).\u003c/p\u003e \u003cp\u003eThe ecotoxicity test for microcrustaceans was followed in accordance with the \u0026lsquo;Guideline for Testing of Chemicals. Method 202 \u0026lsquo;\u003cem\u003eDaphnia\u003c/em\u003e sp., Acute Immobilisation Test\u0026rsquo; (OECD, 2004).\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization of agate dyeing effluent\u003c/h2\u003e \u003cp\u003eThe composition of the agate-dyeing effluents varied quite a lot. When preparing the present study, random effluent collection indicated variations of Rhodamine B concentration from 500-2,500 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, surface tension from 55\u0026ndash;60 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 2\u0026ndash;4% ethanol concentration. These variations are due to the fact that the dyeing process is not systematised and the inputs are not controlled. Nevertheless, Rhodamine B concentrations give effluents with an intense reddish-pink colour. The presence of Rhodamine B and ethanol alters the surface tension of pure water (72 mN m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 20\u0026deg;C), with values below 60 mN.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe sample chosen for this study had a bright reddish pink colour and this colour is due to the presence of approximately 772 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of Rhodamine B. The effluent has a surface tension of 59 mN m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the organic load is high. The possible sources of carbon are Rhodamine B, ethanol and some other contaminants, such as surfactants. The result of the TOC analysis indicates that the raw effluent has a concentration of 11,950 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The concentration of Rhodamine B shows that 542 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of the carbon comes from Rh B molecule, which corresponds to only 4.5%. The remaining 95.5% are due to the presence of ethanol and other substances. These components reflect a BOD of 5,800 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a COD of 18,180 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The BOD\u003csub\u003e5\u003c/sub\u003e:COD ratio is 0.32, which demonstrates the low biodegradability and low potential of the effluent to be treated by biological processes.\u003c/p\u003e \u003cp\u003eThe strong colour of the effluent did not allow the analysis of N in the raw effluent. Thus, to determine total nitrogen, Rhodamine B was considered to be the only source of this element. The calculation considered the mass of nitrogen in a Rh B molecule and indicated that the raw effluent has 45.1 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of total nitrogen, entirely as organic nitrogen. The distillation procedure to determine the percentage of ethanol in the raw effluent indicated that the raw effluent has 3% ethanol in its composition. The use of the alcoholometer indicated practically the same result. Conductivity and phosphorus concentration in the effluent are low. Therefore, it can be assumed that the effluent is basically composed of water, ethanol and Rhodamine B.\u003c/p\u003e \u003cp\u003eThe ecotoxicity of the effluent was very high. The effective concentration for immobility of 50% of the \u003cem\u003eDaphnia similis\u003c/em\u003e population over a 48-hour period was only 0.61% of the initial concentration of raw effluent. This is confirmed by the toxicity factor (TF), which expresses that the effluent should be diluted 526 times in order not to cause toxic effects on the microcrustaceans, used as an indicator.\u003c/p\u003e \u003cp\u003e \u003cb\u003eReagents adjustment for application of the Fenton Reaction.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eWastewater treatment using the Fenton process sought to determine the optimal condition of reagents. Initially, the response parameter chosen was the absorbance. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the residual colour, in terms of absorbance at 554 nm, when the raw effluent was treated with different concentrations of Fe\u003csup\u003e2+\u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e in the Fenton\u0026acute;s reaction. The concentration of hydrogen peroxide plays a crucial role in the efficiency of the degradation process and good results were attained at dosages of 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 30 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The percentage of pollutant degradation increased with an increase in the hydrogen peroxide dosage and the same occurred in the works of Eisenhauer (1964), Lin and Lo (1997), Lin et al. (1999) and Kang and Hwang (2000). However, care should be taken when selecting the operating oxidant dosage. The unused portion of hydrogen peroxide during the Fenton process contributes to an increase in COD (Lin and Lo, 1997) and, therefore, an excess amount is not recommended (Lin and Lo, 1997; Pignatello et al., 2006). In addition, the presence of hydrogen peroxide is detrimental to many organisms and may affect degradation efficiency in cases where the Fenton process is used as a pre-treatment to biological oxidation, or in cases when the treated effluent is discharged into receiving bodies (Ito et al., 1998).\u003c/p\u003e \u003cp\u003eThe [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] ratio is another crucial parameter for achieving the best results with the Fenton\u0026acute;s process (Gulkaya et al., 2006). Tang and Huang (1996a, 1996b, 1996c, 1997), Tang and Tassos (1997), and Kochany and Lugowski (1998) pointed out that the optimum [Fe\u003csup\u003e2+\u003c/sup\u003e]:[H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e] ratio must be maintained to achieve maximum degradation efficiency, as both H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e can also react with OH\u0026bull; and, therefore, can inhibit the oxidation reactions if either of them is not at the optimal dosage. It should be considered whether it is possible to reduce the amounts of Fe\u003csup\u003e2+\u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, while keeping their ideal relationship constant. In the present work, good results appear with [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratios equal to or higher than 7.5:1. This ratio is similar to, or not far from, the best results obtained by: Kuo (1992) when decolourising simulated dye wastewaters with ratios between 3:1 and 9:1; Tang and Huang (1997) when oxidising chlorinated aliphatic organics from 5:1 to 11:1; Tang and Tassos (1997) when oxidising bromoform from to 2:1 to 5:1; Benatti et al. (2006) for chemical laboratory wastewater with 4.5:1; and Gayathri et al. (2023) for a diluted solution of Rhodamine B in water with 3:1.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e\u0026ndash; Residual absorbance of the effluents treated with different molar ratios of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e. Initial Rhodamine B concentration in the raw effluent at 772 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eMolar ratio\u003c/p\u003e \u003cp\u003e[H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]: [Fe\u003csup\u003e2+\u003c/sup\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eDosages of reagents applied\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAbsorbance\u003c/p\u003e \u003cp\u003e(554 nm)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003cp\u003e(g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e2.5:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.920\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e33.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.037\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e49.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.036\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e5:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.593\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.074\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e24.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.069\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e7.5:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.945\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e11.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.101\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.051\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e10:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.500\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.244\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e12.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.056\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e12.5:1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.705\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.798\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.092\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe condition of 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, whose [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio is 7.5:1, provided a good discolouration (absorbance of 0.101). This proportion was chosen to be studied further, with repetitions, in order to make a finer adjustment to the reagent concentration. Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the absorbance results, residual TOC concentrations and the final surface tension of the effluent generated in the assays performed using the 7.5:1 [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio with H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e set at 5, 10, 15, 20, 25 and 30 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Additionally, Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e show the percentages of Rhodamine degradation and TOC reduction, respectively, as a function of reagent concentration. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e depicts the results of surface tension of the treated wastewater in the same conditions.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e\u0026ndash; Mean (\u0026micro;) and standard deviation (σ) of absorbance, TOC and surface tension of treated effluents (n\u0026thinsp;=\u0026thinsp;3) generated in the assays performed using different concentrations of reagents with [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio at 7.5:1. Initial Rhodamine B concentration of the raw effluent of 772 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, TOC of 11,950 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and surface tension of 58.9 mN m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eDosages of reagents applied\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003eResidual absorbance\u003c/p\u003e \u003cp\u003e(554 nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003eTOC\u003c/p\u003e \u003cp\u003e(mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003eSurface tension\u003c/p\u003e \u003cp\u003e(mN m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003cp\u003e(g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026micro;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eσ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026micro;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eσ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026micro;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eσ\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1216.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e444.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e55.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1032.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e202.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e60.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e825.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e26.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e63.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e804.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e21.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e64.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e694.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e60.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e68.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e625.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e33.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e69.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.36\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAnalysing the data, it can be seen that the Rhodamine B concentration progressively decayed for the dosages applied. However, at concentrations lower than 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, despite achieving percentages of dye reduction very close to 100%, they still had Rhodamine B concentrations that is perceptible to the observer. With the standard deviations, it can be verified that the Rhodamine B degradation results presented low variability, with a tendency of decreasing from the highest to the lowest reagent concentrations. Regarding TOC, the residual amount started from 90% removal with 2.8 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 5 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and reached 95% with 16.6 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 30 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. The average percentages of TOC reduction are lower than those of Rhodamine degradation, indicating the presence of fragments of the organic molecules initially present in the effluent. The TOC analyses varied more than the Rhodamine degradations ones, also having the same tendency to decrease from the highest to the lowest chemical addition.\u003c/p\u003e \u003cp\u003eWe defined 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e as the appropriate dosages to treat the effluent. In this condition, the effluent was clear with a slight pink colour. The Fenton reaction enabled high degradation of Rhodamine B, although intermediate products of Rh B degradation and other non-mineralised effluent constituents are responsible for the residual TOC in the treated samples. Similar results were obtained with synthetic solutions of Rhodamine B, using variants of the Fenton process, under conditions with lower dye dosages and without the presence of ethanol (Hou et al., 2011; Gan and Li, 2013; Guo et al., 2014, Gayathri et al., 2023). Gayathri et al. (2023) listed some intermediates identified at 50% degradation of Rhodamine B by LC-MS analysis during 50% solar Fenton degradation, including the following chemical species: hydroxy N-ethyl-N\u0026acute; ethyl rhodamine, N,N-diethyl-N\u0026acute;-ethylrhodamine, hydroxy amino N\u0026acute; ethyl rhodamine, N-ethyl-N\u0026acute;ethylrhodamine, N-ethylrhodamine, 3-hydroxybenzoic acid, succinic acid and oxalic acid.\u003c/p\u003e \u003cp\u003eIt should be noted that, analysis with the alcoholmeter showed that the Fenton reaction was able to remove all the ethanol present. During the Fenton reaction, the smell of CH\u003csub\u003e3\u003c/sub\u003eCOOH (acetic acid) was perceived, indicating that ethanol was being degraded by the hydroxyl radical. This fact was also observed by Walling and Kato (1971), who identified the formation of CH\u003csub\u003e3\u003c/sub\u003eCOOH in a study on the oxidation of alcohols by the Fenton reaction. Still, the surface tension increased as the concentration of reagents increased, indicating that the degradation of Rhodamine B and ethanol restores, at least in part, the intermolecular bonding forces of the water.\u003c/p\u003e \u003cp\u003eAt the end of the Fenton's reaction, a base (e.g. NaOH, Ca(OH)\u003csub\u003e2\u003c/sub\u003e) needed to be added to neutralise the solution, with the production of ferric based sludge (Benatti et al., 2006; Wu et al., 2010). Disposal of this sludge is one of the drawbacks of the Fenton's process. Sludge increases as the FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO concentration increases (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The sludge generation presented the same magnitude reported in other works where the Fenton process was studied (Barros et al., 2006; Yu et al., 2022). The common practice is to dewater the sludge and send it to a landfill. However, Guo el al. (2018) found that after calcination at 600\u003csup\u003eo\u003c/sup\u003eC, the iron sludge exhibited an enhanced catalytic performance toward rhodamine B in the presence of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. So rather than discarding this waste, the sludge generated can be dehydrated and heat treated for reuse as a heterogeneous Fenton\u0026acute;s catalyst.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e\u0026ndash; Sludge generation in treated samples using different concentrations of FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO (g L\u003csup\u003e-1\u003c/sup\u003e) and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (mL L\u003csup\u003e-1\u003c/sup\u003e) at [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio of 7.5:1.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eDosages of reagents applied\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSludge mass (g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO (g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.43\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.82\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.51\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.80\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8.20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eApplication of the Fenton process\u003c/h2\u003e \u003cp\u003eConsidering the results obtained, the optimal dosage adopted was the concentration 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. The reported values of the quantities of reagents required to treat synthetic effluents containing 20\u0026ndash;300 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of Rhodamine B are not as high and generally range from 0.013-0.500 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 0.017\u0026ndash;0.584 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (Kuo, 1992; Solozhenko et al., 1995). However, the literature also describes that treating actual effluents with the Fenton process requires much higher doses than treating synthetic effluents with the same dye content, to achieve the same treatment efficiency (Kuo, 1992; Kang and Chang, 1997; Hofl et al., 1997; Perez et al., 2002; Umar et al., 2010; Torrades and Garc\u0026iacute;a-Monta\u0026ntilde;o, 2014). This is probably due to the presence of other substances in the effluent, which may consume the hydroxyl radicals generated in the Fenton reaction during oxidation or end up interfering negatively in the process. Therefore, when assessing the amount of reagents required to actual effluent, it is a priority to consider its total organic load.\u003c/p\u003e \u003cp\u003eFigure 7 presents the development the Fenton reaction assay and details the effluent aspect over time, using the optimal concentration of 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Bubbles and foam formed in the early stages, which declined at the end of the reaction. It is important to note that the Fenton reaction is an exothermic process (Bautista et al., 2008). The temperature reaches 34\u0026deg;C, an increase of approximately 12\u0026deg;C from room temperature. The temperature begins to rise shortly after the reagents are mixed in the raw effluent sample. The temperature keeps rising until it reaches its peak of 34\u0026deg;C at 35 min. Foam, indicative of O\u003csub\u003e2\u003c/sub\u003e generation, follows the same behaviour. After 35 min, both temperature and foam begin to reduce, denoting that hydroxyl radical generation decrease. At the end of 80 min of the experiment, no foam generation is evident. Thus, 90 min was adopted as the ideal oxidation time and the reaction was stopped with the addition of sodium thiosulfate. Considering the steps involved in reagent preparation (10 min), reagent addition (10 min), the development of the Fenton\u0026acute;s reaction itself (90 min), pH adjustment (10 min), sludge settling (60 min) and filtering (30 min), the total time required to perform the wastewater treatment at bench scale was approximately 3 hours and 30 minutes. Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e summarizes the results of the analyses performed on the raw and treated effluent under these conditions.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComplementary characterisation of the raw effluent and after treatment by the Fenton\u0026acute;s Process using a dosage of 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e Fe\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio of 7.5:1.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eParameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRaw effluent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAfter\u003c/p\u003e \u003cp\u003eFenton\u0026acute;s process\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAbsorbance\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026gt;\u0026gt; 2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.158\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRhodamine B (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e772\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eColour\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ebright pink\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003elight pink\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSurface tension (mN m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e59.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e63.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eConductivity (mS cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEthanol (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTOC (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e11,950\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e868\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCOD (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e18,180\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.772\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBOD\u003csub\u003e5\u003c/sub\u003e (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5,800\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e840\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal N (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e45.1*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e18.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNitrate (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNitrite (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAmmoniacal N (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e15.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOrganic N (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTKN (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e18.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal P (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.188\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.006\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal Fe (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEC50\u0026ndash;48h (%) - \u003cem\u003eDaphnia similis\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e18.76\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eToxicity factor (TF) - \u003cem\u003eDaphnia similis\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e526\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"3\"\u003e* calculated value from the concentration of Rhodamine B\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe Fenton reaction resulted in significantly reduced absorbance and, consequently, of the Rhodamine B concentration in the treated sample. There was an increase in conductivity in the treated effluent. This is due to the addition of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ions by the FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO used in the Fenton reaction, as well as the Na\u003csup\u003e+\u003c/sup\u003e ions from the use of NaOH for pH adjustment.\u003c/p\u003e \u003cp\u003eThe process was able to remove all of the ethanol present in the raw effluent sample, effectively collaborating in the removal of the organic load. The efficiency of COT, COD and BOD\u003csub\u003e5\u003c/sub\u003e reduction was approximately 90%, 85% and 85%, respectively. The data obtained in the present study are consistent with the study by Chakinala et al. (2009) on an effluent containing dye that had TOC of 6,000 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and COD of 17,000 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The Fenton process, combined with hydrodynamic cavitation, allowed for TOC and COD removal rates of 70% and 85%, respectively. Furthermore, the COD removal values ​​are similar to or slightly better than the results obtained in the study by Tekin et al (2006) with a wastewater from the pharmaceutical industry. The authors reported an increase in the BOD\u003csub\u003e5\u003c/sub\u003e:COD ratio by applying the Fenton process in the order of 3 to 5 times, which is important for a subsequent biological treatment. Here, despite the significant reduction in organic load, the BOD\u003csub\u003e5\u003c/sub\u003e:COD ratio remained constant at approximately 0.3.\u003c/p\u003e \u003cp\u003eConsidering the amount of total nitrogen present due to the Rhodamine molecules (45.1 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), after the Fenton reaction, much of the nitrogen was mineralised and volatilised, with 18.5 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of total N remaining. Of this, 15.2 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was converted to ammonia N and 3.3 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was still present as organic N. These results are consistent with the study by Nidheesh et al. (2014), who proposed a Rhodamine B degradation route by POA through mass spectrometry (GC-MS). The authors found that nitrogen goes through some organic forms until it is converted to ammonia. The ammonia content was much higher than the organic nitrogen content in the treated samples, indicating that almost all of the nitrogen was converted to ammonia. Also, considering the pH 8 of the samples after treatment, it is very likely that the ammonia will almost all be NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (Metcalf and Eddy, 2003). However, about 15% can be in the form of NH\u003csub\u003e3\u003c/sub\u003e and able to be removed by volatilisation.\u003c/p\u003e \u003cp\u003eThe phosphorus content of 0.188 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the raw effluent is quite low. Nevertheless, it was removed, probably due to the interaction with residual iron during the Fenton reaction (Camber et al, 2021). A chemical coagulation step, sometime with the addition of aluminum salts, is recommended after Fenton oxidation to keep the concentration of the soluble iron with the specified limits (Lin and Lo, 1997; Babuponnusami and Muthukumar; 2014). Specifically, in this work, it was not necessary, since the iron concentration remained below the limit of 15 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e required by Brazilian legislation (CONAMA 430/2011).\u003c/p\u003e \u003cp\u003eThe carcinogenic, reproductive and developmental toxicity, neurotoxicity and chronic toxicity of Rhodamine B to humans and animals has been experimentally proven (Kornbrust and Barfknecht, 1985; IARC, 1987; Mirsalis et al., 1989; McGregor et al., 1991; Shimada et al., 1994; Priya et al, 2024). Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e also presents the results of the ecotoxicological tests with the micro crustacean \u003cem\u003eDaphnia similis\u003c/em\u003e. The ecotoxicity of the raw effluent is high, as demonstrated by the toxicity factor (TF) of 526. The treated samples showed a substantial reduction of the toxicity factor. The Fenton process was responsible for reducing the TF value from 526 to 16.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eConsiderations on the treatment process\u003c/h3\u003e\n\u003cp\u003eMohod et al. (2023) stated that \u0026lsquo;it is preferable to treat wastewater containing Rhodamine dyes at a lower concentration rather than with a higher concentration and efforts should be made to develop a technology that can disintegrate high-concentration Rhodamine dyes in wastewater\u0026acute;. Herein the Classic Fenton Process was applied with some success to treat an agate dyeing bearing effluent in a situation with a 772 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of Rhodamine B and 3% ethanol in water. However, the cost of chemical inputs (ferrous sulfate pentahydrate, hydrogen peroxide and sodium hydroxide) to treat the effluent in the regional context will not be below R\u003cspan\u003e$\u003c/span\u003e 600.00 m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e (\u0026asymp;\u0026thinsp;USD 120 m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), making the cost a drawback of the process.\u003c/p\u003e \u003cp\u003eTypically, the Fenton\u0026acute;s process is carried out in batches and can be used to comply with the effluent volume generated by the agate production sector. However, even in the best possible conditions and surpassing the efficiency of other investigated processes, the effluent is still toxic and presents COD, which makes it unsafe to dispose of in bodies of water. In addition, intermediate molecules resulting from the degradation of Rhodamine B are present, with different levels of toxicity. A set of major products resulting from the degradation of Rhodamine by advanced oxidative processes, which may be present in this specific effluent, were listed by Gayathri et al. (2023) and Mohod et al. (2023a).\u003c/p\u003e \u003cp\u003eTherefore, the preliminary treatment step provided by the classical Fenton\u0026acute;s reaction must be integrated to others processes to reach a better water quality. New technologies based on photocatalytic and/or cavitation techniques can be built-in, even if there is an increase in energy consumption (Hinge et al. 2016, Xu and Ma 2021, Mohod et al. 2023a, Mohod et al, 2023b). Another possibility would be a subsequent treatment step by adsorption (Kausar et al., 2021; Bazan-Wozniak et al., 2024). In this regard, Da Rosa et al. (2018) listed a variety of adsorbents and biosorbents for Rhodamine removal. Activated carbon is among those with the highest adsorption capacity and is currently being studied as a complementary stage of treatment to remove residual Rhodamine and the by-products generated by the oxidation process. Furthermore, wastewater could be considered for reuse within the production process itself, more specifically in the washing stage after dyeing.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eSamples of the effluents generated from gem-dyeing processes in the Soledade region have concentrations of Rhodamine B ranging from 500 to 2,500 mg L \u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 2 to 4% of ethanol. The colour of the effluent is an intense red-pink with a very high toxicity factor (TF 526) and surface tension values below 55 mN m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Treatment by the Fenton Reaction promoted alcohol removal, reduction in the strength of the colour, and high ecotoxicity reduction. Due to the high organic load, high concentrations of reagents were required. In the sample studied, 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of ferrous sulphate and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of oxygen peroxide (at a [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio of 7.5:1) were defined as the best dosages, which allowed a reduction in absorbance of 99.9% at 554 nm, a TOC reduction of 93%, an increase in surface tension from 58.9 to 64.4 mN.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the toxicity factor decreased from 526 to 16. There was also a 60% reduction in total N, which may have been mineralised or volatilised. The remaining fraction in the aqueous medium was predominantly in ammoniacal form and a smaller portion was in the organic form. The oxidation time for this reagent concentration was approximately 90 minutes. The Fenton process combines oxidation, air stripping, coagulation and co-coupling mechanisms that substantially reduces pollutants present in the effluent in a rougher stage. Additional studies to remove residual colour and effluent toxicity should be pursued.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research received financial support from Fapergs (grant number 11/1332-2), CNPq (grant number 314880/2020-8) and CAPES-PROEX (grant number 88881.844968/2023-1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to publish declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics and consent to participate declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe 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.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAl-Buriahi, A.K., Al-Gheethi, A.A., Senthil Kumar, P., Radin Mohamed, R.M.S., Yusof, H., Alshalif, A.F., Khalifa, N.A., 2022. 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Iron-based denitration catalyst derived from Fenton sludge: Optimisation analysis of selective dealkalization and influence mechanism of calcination temperature. Journal of Cleaner Production, 378, 134524.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"dice","sideBox":"Learn more about [Discover Chemical Engineering](https://www.springer.com/journal/43938)","snPcode":"","submissionUrl":"","title":"Discover Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"gemstone processing, dye, wastewater, advanced oxidation process, environment","lastPublishedDoi":"10.21203/rs.3.rs-5727575/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5727575/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn the processing of agates, the gemstone pieces are submitted to dyeing processes. One of the dyeing processes applied is with Rhodamine B, which provides a reddish-pink colouration. The practice generates a wastewater containing residual amounts of the dye and ethylic alcohol with high polluting potential. Thus, the objective of this paper is to investigate the chemical parameters of the Fenton\u0026acute;s oxidation process to treat this effluent. The main parameters evaluated were [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] molar ratio and the concentration of the reagents H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and FeSO\u003csub\u003e4\u003c/sub\u003e.7H\u003csub\u003e2\u003c/sub\u003eO. The wastewater contained a concentration of 772 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of Rhodamine B and 3% of ethanol in its composition, giving the effluent a high colour, organic load, and toxicity. In the Fenton reaction treatment, 11.1 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of ferrous sulphate and 20 mL L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of oxygen peroxide (in a molar ratio [H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e]:[Fe\u003csup\u003e2+\u003c/sup\u003e] of 7.5:1) were defined as the best dosage, which allowed complete alcohol removal, an average absorbance reduction of 99.9% at 554 nm, a TOC reduction of 93.3%, an increase in surface tension from 58.9 to 64.4 mN m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a toxicity factor (TF) decrease from 526 to 16 with respect to the organism \u003cem\u003eDaphnia similis\u003c/em\u003e. The Fenton process combines oxidation, coagulation and air stripping mechanisms that substantially reduces pollutants present in this effluent, at a rougher stage. The results obtained are useful, both for the gemstone sector and others that make use of Rhodamine B dye.\u003c/p\u003e","manuscriptTitle":"On The Fenton´s Process for the Treatment of Effluents from the Dyeing of Agates Containing Rhodamine B and Ethanol","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-14 09:19:33","doi":"10.21203/rs.3.rs-5727575/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accepted","date":"2025-04-30T10:04:59+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-23T07:40:05+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-15T02:48:25+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"282758039238680111220447533147223856299","date":"2025-04-15T02:44:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-11T07:40:47+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"284703169185697019128543324453352189294","date":"2025-04-11T05:21:41+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-11T04:42:06+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-04-10T09:06:12+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Chemical Engineering","date":"2025-03-26T13:45:09+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"dice","sideBox":"Learn more about [Discover Chemical Engineering](https://www.springer.com/journal/43938)","snPcode":"","submissionUrl":"","title":"Discover Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"9869b785-4ccf-4bcb-99b0-086497842dfe","owner":[],"postedDate":"April 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-06-23T16:02:57+00:00","versionOfRecord":{"articleIdentity":"rs-5727575","link":"https://doi.org/10.1007/s43938-025-00085-w","journal":{"identity":"discover-chemical-engineering","isVorOnly":false,"title":"Discover Chemical Engineering"},"publishedOn":"2025-06-17 15:57:54","publishedOnDateReadable":"June 17th, 2025"},"versionCreatedAt":"2025-04-14 09:19:33","video":"","vorDoi":"10.1007/s43938-025-00085-w","vorDoiUrl":"https://doi.org/10.1007/s43938-025-00085-w","workflowStages":[]},"version":"v1","identity":"rs-5727575","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5727575","identity":"rs-5727575","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}