Fenton and photo-Fenton oxidation of textile effluents

Water Research 36 (2002) 2703–2710 Fenton and photo-Fenton oxidation of textile effluents Montserrat Pe! reza,*, Francesc Torradesa, Xavier Dome" nechb, Jose! Peralb a Departament d’Enginyeria Qu!ımica, E.T.S.E.I. de Terrassa, Universitat Polit"ecnica de Catalunya, c/Colom 11, 08222-Terrassa Barcelona, Spain b " Departament de Qu!ımica, Edifici Cn, Universitat Autonoma de Barcelona, 08193-Bellaterra Cerdanyola, Spain Received 19 June 2001; accepted 6 November 2001 Abstract The simultaneous use of Fenton reagent and irradiation for the treatment of textile wastewaters generated during a hydrogen peroxide bleaching process is investigated. The experimental conditions tested during this study provide the simultaneous occurrence of Fenton, Fenton-like and photo-Fenton reactions. The batch experimental results are assessed in terms of total organic carbon reduction. Identification of some of the chemical constituents of the effluent was performed by means of GC–MS. Other pollution related features of the initial effluent-like COD and color were also measured. The main parameters that govern the complex reactive system, i.e., light intensity, temperature, pH, Fe(II) and H2O2 initial concentrations have been studied. Concentrations of Fe(II) between 0 and 400 ppm, and H2O2 between 0 and 10,000 ppm were used. Temperatures above 251C and up to 701C show a beneficial effect on organic load reduction. A set of experiments was conducted under different light sources with the aim to ensure the efficiency of using solar light irradiation. The combination of Fenton, Fenton-like and photon-Fenton reactions has been proved to be highly effective for the treatment of such a type of wastewaters, and several advantages for the technique application arise from the study. r 2002 Published by Elsevier Science Ltd. Keywords: Advanced oxidation processes; Fenton; Photochemical reactions; Textile 1. Introduction The textile industry produces large volumes of bleaching effluents that contain appreciable quantities of organic compounds which are not easily amenable to chemical or biological treatment [1,2]. Furthermore, treatment cost of textile wastewaters has been scaling rapidly in recent years. Hence a search for more costeffective treatment methods has practical application [1]. Most of the textile effluents have high levels of COD, and hydrolysis in basic media is often carried out before the application of other treatments. Dorica [3] has reported the removal of organic chlorine of bleach paper plant effluents using alcaline hydrolisis. P!erez et al. [4] *Corresponding author. Tel.: +34-93-739-8148; fax: +3493-739-8101. E-mail address: mperez@eq.upc.es (M. P!erez). have carried out studies where similar results were obtained. Recently, chemical treatment methods, based on the generation of hydroxyl radicals, known as advanced oxidation processes (AOPs), have been applied for pollutant degradation, due to the high oxidative power of the OH radical. The most widely studied AOPs include: heterogeneous photocatalytic oxidation [5–8], treatment with ozone (often combined with H2O2, UVA, or both) [9–13], H2O2/UV systems [14], Fenton [14–16] and photo-Fenton type reactions [17–21]. The high electrical energy demand or the consumption of chemical reagents are common problems among all the AOPs [17]. Specially, the production of photons with artificial light sources require an important energy input. However, not all photoassisted processes require light with the same wavelength and energy. While direct O3 or H2O2 photolysis need photons of short wavelength (o310 nm), TiO2 photocatalysis can take advantage of 0043-1354/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 5 0 6 - 1 2704 M. P!erez et al. / Water Research 36 (2002) 2703–2710 photons of wavelengths up to 380 nm [5], and photoFenton reactions can use photons with wavelength close to 400 nm. The mixtures Fe(III)+H2O2 (known as Fenton-like reactions [19] have shown photon absorption up to 550 nm [22,23]). In presence of Fenton reagent, photochemical reactions can be driven with photons of low energy, photons that belong to the visible part of the spectrum. Thus, photo-Fenton processes are a potential cost-reduced AOP that can be run under solar irradiation [20]. Recently, it has been proven that the irradiation of Fe(III)+H2O2, also called photo-Fenton reaction, enhances the reaction rate of oxidant production, through the involvement of high valence Fe intermediates responsible for the direct attack to organic matter [22,24]. Absorption of visible light by the complex formed between Fe(III) and H2O2 seems to be the cause of formation of such high valence Fe-based oxidants. In the present paper, we have undertaken the study of the oxidation of the organic compounds present in a bleaching textile effluent by Fenton and photo-Fenton reactions, in order to establish the efficiency of both AOP for the treatment of such wastewaters. The role that several experimental parameters like temperature, light intensity, and reagent concentration have on the reaction yields have been examined. 2. Experimental The effluents used in the present research were obtained from the hydrogen peroxide bleaching sequence of a cotton substrate mixed with a very low portion of synthetic fibers and pretreated with hypoclorite. The wastewater was supplied by a Spanish textile manufacturer. In order to work with lower level of organic pollutants, hydrolysis with Ca(OH)2 (pH 12, room temperature, during 1 h) was carried out to the effluent, following the procedure of Dorica [3]. The rest of the chemicals used were, at least, of reagent grade. Analytical grade hydrogen peroxide and heptahydrated ferrous sulfate were purchased from Panreac and Aldrich, respectively, and were used as received. Solutions were prepared with deionized water obtained from a Millipore Mili-Q system. All experiments, even the ones performed under solar irradiation, were conducted in a thermostatic cylindrical Pyrex cell of 130 cm3 capacity. The reaction mixture inside the cell, consisting of 100 ml of organic effluent and the precise amount of Fenton reagent, was continuously stirred with a magnetic bar. The experiments were conducted at three different temperatures 25.01C, 40.01C, 70.01C (70.11C). A 6 W Philips blacklight fluorescent lamp, a 250 W xenon lamp (Applied Photophysics) and the Solar light were used as light source. The IR fraction of the xenon light beam was removed by the water in the double jacket of the photoreactor. The intensity of the incident light inside the photoreactor, measured employing a uranyl actinometer, was 1.38
109 Einstein s1 for the fluorescent lamp and 7.55
108 Einstein s1 for the xenon lamp. Total organic carbon (TOC) of initial and irradiated samples was determined with a Shimadzu 5000 TOC analyzer. Color determination of the initial sample was carried out in a double beam SP8-300 Pye Unicam spectrophotometer at the wavelength of 465 nm, using 10 mm light path cells, according to standards H.5 of the CPPA (Canadian [25]). Identifications of some chemical constituents of the wastewater was attempted by means of GC–MS. A HP 6890 gas chromatograph equipped with a quadrupole HP 5973 mass selective detector was used. The GC–MS analysis was carried out with ionization of electronic impact, 70 eV, the spectra were recorded in the interval 40–600 amu. One liter of the sample was filtered with Speedisks membrane (J.T. Baker) following the EPA 525 procedure. Samples were reconstituted to 100 mL in dichloromethane and 1 mL was injected in the GC. Identifications were carried out with the aid of the data base library WILEY (275,000 spectra) and NIST (130,000 spectra). A capillary column HP-5MS (5% Phenyl Methyl Siloxane) with dimensions of 30 m, 250 mm, 0.25 mm was used. The carrier gas flow rate in the GC was 1.3 mL min1. The sample injection was carried out with a 0.6 min of splitless time, at 2501C. The temperature program used during the GC–MS analysis ramped as follows: 701C (3 min), 51C min1 until 2701C (30 min). Table 1 gives the compounds that have been identified by GC–MS. Several of these compounds can be considered degradation products of nonylphenol derivatives, often used as surfactant in the preparation of textile fibers. Also, a large number of unidentified compounds appear in the chromatogram, although they are not included in Table 1. 3. Results and discussion After hydrolisis the textile effluent have the following global parameters values: 60579* mg L1 of TOC; 166974* mg O2 L1 of COD; and 4078*mg Pt L1 of color, *(n ¼ 4; a ¼ 0:05). As expected from Eqs. (1)–(5) the complex reactive system is a pH-dependent processes. However, each reaction has its optimum performance at different pH values: nearly pH independent for the Fenton process [19], optimum at pH 2.8 for the Fenton-like reaction [19], and clearly pH dependent for photo-Fenton reaction [22]. Data concerning TOC degradation of paper mill effluents at several pH showed that the faster removal of TOC takes place at pH=2.8 [26]. A very acid M. P!erez et al. / Water Research 36 (2002) 2703–2710 2705 Table 1 Compounds found by GC–MS in the textile effluent after hydrolysis Table 2 Hydrated Fe(III) species in solution and the range of pH where they are predominant Compound Fe specie pH Fe (H2O)3+ 6 Fe (OH)(H2O)2+ 5 Fe (OH)2(H2O)+ 4 1–2 2–3 3–4 Aromatic compounds Benzoic acid Benzeneacetic acid 3-Methyl benzoic acid Ethyl ester 4-ethoxy-benzoic acid 3,5-Di-tert-Butyl-4hydroxybenzaldehyde Linear acids Nonanoic acid Decanoic acid Undecanoic acid Dodecanoic acid Tetradecanoic acid Pentadecanoic acid Hexadecanoic acid Heptadecanoic acid Octadecanoic acid Other compounds 2,20 Oxybis-ethanol Decane 2,8-Dimethyl-4-methylenenonane 2-Decanone Methyl ester nonaic acid 2-Butyl octanoic acid 2-Propylnonanoic acid 2-Methyl undecanoic acid Retention time Reliability (%) 10.80 12.90 13.72 19.32 97 76 94 96 24.70 98 13.60 16.27 19.13 20.70 24.80 26.80 29.10 30.76 32.62 95 96 96 99 98 97 99 96 93 5.30 5.48 7.73 83 91 86 10.71 17.80 19.30 19.35 19.72 90 70 91 72 91 media or a neutral–basic media slows down the process. The low activity detected for high pH values can be explained by the formation and precipitation of Fe(OH)3, a process that hamper the development of photo-Fenton and Fenton-like reactions. The decrease of activity for pH values below the optimum is understandable taking into account that Fe(III) forms different complex species in solution, and the quantum yield of light absorption by Fe(III) is directly depending on the specific species responsible for the absorption. Table 2 gives the predominant iron species at different pH ranges [20]. Blank experiments were carried out in order to ascertain whether Fenton, Fenton-like and photoFenton reactions take places with such an organic content. When 100 mL of the effluent were mixed with 100 ppm of Fe(II) under Xe lamp irradiation and at 401C, no TOC removal took place. The same behavior was observed when 1000 ppm of H2O2 were mixed with the effluent in the absence of Fe(II), shown in Fig. 1. In contrast, the presence of both H2O2 and Fe(II) at 401C and under light irradiation produced a TOC reduction of 150 ppm (24%) after 30 min, and longer reaction times involved larger reductions. Clearly, Fenton reagent under irradiation improves TOC removal. Light can play two different roles that would lead to an improvement of the reaction yields: (a) it drives photoFenton reaction, producing additional hydroxyl radicals and the recovery of Fe(II) needed in Fenton reaction. The photo-Fenton reaction may involve direct photolysis of ferric ion (Eq. (5)) or photolysis of Fe(III)peroxy complexes [22]. (b) It can drive ligand to metal charge transfer in the potentially photolabile complexes formed by Fe(III) and organic compounds, a process that has been well proven for the complexes formed between Fe(III) and the carboxylic acid moiety [27]. Large quantities of carboxylic acid are expected to be formed as degradation intermediates of the original organic substrate. The beneficial effect of temperature was carefully tested in a set of experiments where three different temperatures (251C, 401C and 701C) were used in the dark and under irradiation of the Xe lamp. The decrease of organic concentration with time due to oxidative degradation of the textile bleach effluents by Fenton and photo-Fenton reactions at these three different temperatures is shown in Fig. 2. From the differences between the pairs of experiments carried out under the same conditions (irradiation or absence of light) and different temperature it is clear that temperature markedly influences the degree of TOC removal. As can be seen in Fig. 2, no important differences exists during the first minutes of reaction if the process is carried out in the presence or absence of light. This can be explained by taking into account two facts: (a) the initial TOC decrease is mainly due to the dark Fenton reaction, which is faster than Fenton-like [19] or photoFenton reactions [23]. (b) Fe(II) is clearly the limiting reagent and as long as Fe(II) is available the same initial reaction rate is expected. Under the experimental conditions tested here, Fe(II) consumption takes place in few seconds, producing the majority of TOC decrease observed after 15 min of reaction (when the first samples were taken). Thus, for these short reaction times no effect is observed due to the presence of light. M. P!erez et al. / Water Research 36 (2002) 2703–2710 2706 700 600 TOC (ppm) 500 400 300 200 100 0 0 30 60 90 120 time (min) Fig. 1. TOC of the textile effluent vs. reaction time for several experimental conditions: 100 ppm of Fe(II) (’); 1000 ppm of H2O2 (m); ! irradiation. 1000 ppm of H2O2 and 100 ppm of Fe(II), (E). pH=3, T¼ 401C; xenon 700 600 TOC (ppm) 500 400 300 200 100 0 0 30 60 90 120 time (min) Fig. 2. Effect of temperature on TOC removal. 251C in the dark ( ); 251C under Xe lamp irradiation (); 401C in the dark (m); 401C under Xe lamp irradiation (F); 701C in the dark (E); 701C under Xe lamp irradiation (’). [H2O2]0=10,000 ppm, [Fe(II)]0=100 ppm, pH=3. Dark reaction rates after Fe(II) consumption are controlled by the Fenton-like process between H2O2 and the Fe(III) formed in the first seconds of direct Fenton reaction. The Fenton-like process regenerates Fe(II) which, in presence of excess H2O2 is readily transformed giving Fe(III). Thus, an effective iron cycling takes place, with approximately constant Fe(III) concentration, traces of Fe(II), and a fairly constant oxidant intermediate production. When the experiment was carried out at 701C, levels of TOC removal over 65% were attained after just 30 min of reaction, both, in the dark and under irradiation. Since the 100 ppm of Fe(II) cannot directly produce enough OHd radicals through the Fenton reaction to account for the total level of TOC removal, the acceleration of Fenton reaction with temperature cannot be the only cause of such an observation. The temperature seems to be assisting alternative ways of H2O2 cleavage and OHd formation, or Fe(II) recovery. The photo-Fenton reactions has no important role, and the recovery of Fe(II) from Fe(III) seems to be taking place more effectively through the Fenton-like reactions. Blanks experiments showed that neither the irradiation nor the temperature alone can produce noticeable M. P!erez et al. / Water Research 36 (2002) 2703–2710 decreases of TOC, the simultaneous presence of Fe(II) and H2O2 being always necessary. On the other hand, in the experiments carried out at 251C and 401C, the levels of TOC removal increase under light irradiation, making clear the important role of the light driven reactions. A similar behavior has been observed during the treatment of bleaching Kraft mill effluents with Fenton and photoFenton systems [26]. In any case, temperature is a key parameter that has to be taken into account, specially for those applications where TOC removal rate can be increased by using low cost heat (heat exchangers, co-generation, etc.). It is important to remark that in comparison to most industrial wastewaters, the temperature of textile effluents is unusually high. During the dyeing process, rinse waters temperatures up to 901C are normally encountered [1]. Although the Fenton reaction has been widely studied, there is still not an agreement on the ratio [H2O2]/[Fe(II)] that gives the best results. Many authors have reported the use of different ratios of the two reactants. Excess of H2O2 or Fe2+ might be detrimental, since these species can react with some of the intermediates like OH, responsible for the direct oxidation of the organic load: FeðIIÞ þ OHd -FeðIIIÞ þ OH ð1Þ k ¼ 2:7
107 L mol1 s1 ð2Þ H2 O2 þ OHd -HOd2 þH2 O precluding the extent of mineralization. Thus, in order to check the effect that different reagent ratios have on the reactions, experiments with several ratios of H2O2/ Fe(II) were also conducted. Fig. 3 shows that, in general, increasing initial quantities of iron in solution produce increasing rates 2707 of degradation. Although during the first minutes the reaction with more Fe(II) proceeds at a faster rate, at long reaction times the experiment with 200 and 400 ppm of Fe(II) produces a TOC decay that is slightly minor than the one obtained with 100 ppm. This change on behavior with time can be explained by taking into account that Fenton reaction, which is completed after few seconds, benefits from a larger Fe(II) load, while detrimental reactions like (1) and (2), due to the low concentration of OHd, need more time to manifest, and their effects appear only for long enough reaction times, when they compete with slower reactions as Fenton-like, photo-Fenton, photochemical processes, etc. As can be seen for 1000 ppm H2O2, 50 and 100 ppm of Fe(II) seem to be the more suitable doses for long reaction time. 5:1 and 10:4 H2O2/Fe(II) ratios show the faster rate of TOC degradation during the first minutes. This is in agreement with the fact that Fenton reaction dominates the first minutes of the process and a larger concentration of reactants directly increases the reaction rate. With a 20:1 H2O2/Fe(II) ratio there is a clear reduction in the initial amount of Fe(II) used, and the role played by photo-Fenton and Fenton-like reactions is more important. On the other hand, the use of large quantities of Fe in solution has a negative effect from the applied point of view, since it implies the need of an additional treatment step for Fe removal. Fig. 4 contains data concerning experiments with several initial H2O2 doses. In this case it is clear that increasing amounts of H2O2 lead to larger TOC removal, with no detrimental effects detected for the highest H2O2. Nevertheless, the small difference between the TOC removal attained with 2500, 5000 and 10,000 ppm of H2O2 indicates that improvements of reaction rate may not compensate the large amounts of oxidant consumed. In all these cases 50% TOC 700 600 TOC (ppm) 500 400 300 200 100 0 0 30 60 90 120 time (min) Fig. 3. Effect of Fe(II) doses on TOC reduction. The initial concentrations of Fe(II) were as follows: 0 ppm (); 25 ppm ( ); 50 ppm (m); 100 ppm (F); 200 ppm (’); 400 ppm (E). [H2O2]0=1000 ppm, pH=3, T¼ 401C; Xe lamp irradiation. M. P!erez et al. / Water Research 36 (2002) 2703–2710 2708 reduction after 1 h of reaction and around 70% after 2 h of reaction were obtained. It is interesting to note that, for the least H2O2 concentrated experiments the TOC removal slows down to few ppm per hour, showing that, even for Fenton–photo-Fenton systems large enough concentrations of H2O2 are essential. The intensity and the wavelengths emitted by the light source are key conditions when trying to describe the participation of the light-driven reaction. It is specially interesting, from the application point of view, the possibility of using solar light. Fig. 5 shows the differences in TOC removal when three different light sources are used. A 10:1 concentration ratio (1000 ppm of H2O2, per 100 ppm Fe(II)) was used in all experiments. An assessment of the energy input entering the reactor gave: 3.833
105 W cm2 (below 500 nm) for the UVA lamp; 1.653
103 W cm2 (below 500 nm) for the Xenon light, and 5
103 W cm2 (below 400 nm, with a presumably larger photon input below 500 nm) for the solar light. The behavior of the system during the first hour obeys this order of energies, with the solar irradiation being the most efficient light source. However, at long enough reaction time there is a trend change and UVA light gives the larger ratio of TOC removal. It is difficult to find an explanation for such a 700 600 TOC (ppm) 500 400 300 200 100 0 0 30 60 90 120 time (min) Fig. 4. Effect of H2O2 doses on TOC reduction. The initial concentrations of H2O2 were as follows: 10,000 ppm (E); 5000 ppm (’); 2500 ppm (m); 1500 ppm ( ); 1000 ppm (F); 0 ppm (). [Fe(II)]0=100 ppm, pH=3, T¼ 401C; Xe lamp irradiation. 100 90 % TOC reduction 80 70 60 50 40 30 20 10 0 0 30 60 90 120 time (min) Fig. 5. TOC decay vs. reaction time when using different light sources. UVA light (’); solar light (m); Xe light (E). [H2O2]0=1000 ppm, [Fe(II)]0=100 ppm, pH=3, T a ¼ 401C: M. P!erez et al. / Water Research 36 (2002) 2703–2710 behavior but it seems that an intense irradiation favors the fast exhaustion of H2O2 through nonefficient reactions while mild irradiation consumes less oxidant in parallel and undesired photochemical reactions. Difficulties on maintaining the solar irradiation at a constant temperature could also be taken into account when trying to find an explanation for such an unexpected behavior. [9] [10] [11] 4. Conclusions The degradation of the organic content of a textile bleaching effluent has been successfully carried out by the simultaneous use of Fenton reagent and UVA irradiation. Irradiations were done with different light sources, being more effective for long irradiation those with lower photon output. Solar light irradiation was found to be highly effective, opening the possibility of extended low cost applications. Temperature was a key parameter, markedly increasing reaction rates. The importance of using the right Fenton reagents ratio has also been clearly demonstrated along the paper. [12] [13] [14] [15] [16] Acknowledgements The authors thank the Spanish Ministry of Science and Technology for partially funding the present work (Project AMB96-0742). [17] [18] References [19] [1] Lin SH, Peng CF. Treatment of textile wastewater by electrochemical method. Water Res 1994;28:277–82. [2] Tzitzi M, Vayenas DV, Lyberatos G. Pretreatment of textile industry wastewaters with ozone. Water Sci Technol 1994;29:151–60. [3] Dorica J. Removal of AOX from bleach plant effluents by alkaline hydrolysis. J Pulp Paper Sci 1992;18:231–7. [4] Torrades F, Peral J, P!erez M, Dom"enech X, Garcia Hortal JA, Riva MC. Application of heterogeneous photocatalysis and ozone to destruction of organic contaminants in bleaching kraft mill effluents. Tappi J 2001;84(6):1–10. [5] Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Environmental applications of semiconductor photocatalysis. 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