Wat. Res. Vol. 35, No. 4, pp. 1047–1051, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter PII: S0043-1354(00)00342-0 USE OF FENTON REAGENT TO IMPROVE ORGANIC CHEMICAL BIODEGRADABILITY E. CHAMARRO, A. MARCO and S. ESPLUGAS* M Departament d’Enginyeria Quı ´ mica i Metal.lu´ rgia, Universitat de Barcelona C/Martı ´ i Franque ` s1, 08028 Barcelona, Spain (First received 9 November 1999; accepted in revised form 15 January 2000) Abstract}Fenton reagent has been used to test the degradation of different organic compounds (formic acid, phenol, 4-chlorophenol, 2,4-dichlorophenol and nitrobenzene) in aqueous solution. A stoichiometric coefficient for the Fenton reaction was found to be 0.5 mol of organic compound/mol of hydrogen peroxide, except for the formic acid where a value of approximately one was obtained (due to the direct formation of carbon dioxide). The treatment eliminates the toxic substances and increases the biodegradability of the treated water (measured as the ratio BOD 5 /COD). Biodegradability is attained when the initial compound is removed. # 2001 Elsevier Science Ltd. All rights reserved Key words}Fenton reagent, advanced oxidation technologies (AOT) INTRODUCTION Water quality regulations are becoming stricter in the late decades due to an increasing social concern on environment. A very interesting field of concern is what to do with wastewater that contains soluble organic compounds that are either toxic or non- biodegradable. Advanced oxidation technologies (AOT) for water and wastewater treatment show high efficiency but work at a high cost of both investment (complex installations) and operation (higher consume of energy and/or reagents). This makes these processes only useful when the cheaper options are not effective. Experiences with different oxidation technologies and different substrates have shown that a partial chemical oxidation of a toxic wastewater may increase its biodegradability up to high levels (Kiwi et al., 1994; Scott and Ollis, 1995). One of the most effective technologies to remove organic pollutants from aqueous solutions is the Fenton’s reagent treatment (Bidga, 1995). It is well known that organic compounds can easily be oxidized. It consists in a mixture of hydrogen peroxide and iron salts. There are chemical mechan- isms that propose hydroxyl radicals as the oxidant species (Pignatello, 1992; Walling et al., 1974), that are generated in the following chemical equation: Fe 2þ þH 2 O 2 ! Fe 3þ þOH À þOH  ð1Þ Hydroxyl radicals may be scavenged by reaction with another Fe 2+ : OH  þFe 2þ ! OH À þ Fe 3þ ð2Þ Fe 3+ catalytically decomposes H 2 O 2 following a radical mechanism that involves hydroxyl and hydroperoxyl radicals, including (1) and (2): Fe 3þ þ H 2 O 2 Ð Fe2OOH 2þ þ H þ ð3Þ Fe À OOH 2þ ! HO 2  þFe 2þ ð4Þ Fe 2þ þ HO 2  ! Fe 3þ þ HO À 2 ð5Þ Fe 3þ þHO 2  ! Fe 2þ þH þ þO 2 ð6Þ OH  þH 2 O 2 ! H 2 O þ HO 2  ð7Þ Fenton reagent shows to be a very powerful oxidizing agent (Sedlak and Andren, 1991; Potter and Roth, 1993). There are, however, species that show resistance to oxidation by Fenton reaction (Bidga, 1995). These species are small chlorinated alkanes (tetrachloroethane, trichloroethane), n-paraffins and short-chain carboxylic acids (maleic, oxalic, acetic, malonic). These last compounds are indeed a very interesting kind because they are typical oxidation products of larger molecules after fragmentation. Even more interesting is that the cited compounds are known to be primary metabolites, which act in energetic cycles of most living organisms. Partial chemical oxidation yields biodegradable products, together with destruction of inhibitory species (Marco et al., 1997). The objective of this paper *Author to whom all correspondence should be addressed. Tel.: +34-3-402-12-90; fax: +34-3-402-12-91; e-mail: esplugas@angel.qui.ub.es 1047 was the degradation of small organic molecules by the Fenton reagent. MATERIALS AND METHODS Different organic compounds (acetic acid, formic acid, phenol, 4-chlorophenol, 2,4-dichlorophenol and nitroben- zene) were choosen to study their degradation in aqueous solution using Fenton’s reagent. All chemicals used were produced by Panreac (Spain) and were of analytical grade. The experiments were carried out at a ratio Fe 2+ /compound equal to 1, 0.1 and 0.01. Initial concentration of organic pollutants was set to 300 mg/L. Total organic carbon (TOC) analysis were performed in order to know the amount of organic compounds that were depleted to CO 2 during the chemical oxidation. The TOC content of the samples was determined by Dohrman DC- 190 high-temperature TOC analyzer. Concentration of organic compounds was followed by HPLC (high-performance liquid chromatography). Chro- matograms were made with Millennium software using a Waters 600 Controller with a Waters 996 Photodiode Array Detector. The column (spherisorb ODS2; 5m;25Â 0.46 cm) was washed with methanol before analysis. A mixture of 50% acetonitrile in 50% water was chosen as the optimal mobile phase. Biodegradability was measured by 5-day biochemical oxygen demand (BOD 5 ) and by 21-day biochemical oxygen demand (BOD 21 ) analysis of samples at different times of treatment. As bacterial seed (this synthetic water is sterile) a small amount of filtered activated sludge from a municipal wastewater plant was used. This kind of seed was chosen because it comes from the most common and cheap biological treatment, and it means that no special or adapted bacteria are required to reproduce these results. Chemical oxygen demand (COD) is also an important parameter that was followed in order to know the degree of oxidation changes. EXPERIMENTAL RESULTS AND DISCUSSION The experimental work was oriented towards studying how the amount of oxidant applied affects the biodegradability of initially non-biodegradable different organic compounds. Figure 1 shows the BOD/COD ratio (the standard for 5 and 21 days) for six organic compounds. BOD/COD constitutes a good measure of the biodegradability of a waste- water. Contaminants with a ratio of BOD 5 / COD>0.4 may be considered thoroughly biodegrad- able. It can be observed that acetic acid (ACH) and phenol (PHE) are quite biodegradable, formic acid (FOR) is lightly biodegradable, but 4-chlorophenol (4-CP), 2,4-dichlorophenol (DCP) and nitrobenzene (NB) are refractory to the biological treatments. Two kinds of experiments were developed. First type were conducted to the search of the stoichio- metric coefficients, that is to know the moles of organic compound removed by 1 mol of hydrogen peroxide. Thirty mililiter vials, at room temperature, were filled with organic/Fe 2+ solution at Fe/organic ratios of 1 : 1, 0.1 : 1 and 0.01 : 1. Different doses of peroxide were added to these vials (from 0.1–50 mol H 2 O 2 mol À 1 organic). After 24 h organic remaining was analyzed by HPLC, TOC, COD and BOD. Second type were kinetic experiments and were carried out in a stirred reactor of 1.5 L capacity at batch operation, isothermal conditions and refriger- ated by water. For these experiments the measured variables were: redox potential, pH, temperature, concentration of organic and TOC. The stoichiometric coefficients for the five organic compounds studied are shown in Table 1. They have been obtained through linear fitting of the experi- mental results until 90% degradation. The behavior of the organic compounds is similar with the exception of the formic acid. The explanation is that hydroxyl radical generated oxidize the main com- pound and its intermediates, but in the case of the formic acid only the main compound is oxidized because formic acid is already highly oxidized, little additional oxidation by Fenton reagent is required before conversion to carbon dioxide. Organic compound þ Fe=H 2 O 2 ! Oxidized products Oxidized products þ Fe=H 2 O 2 ! Other oxidized products Figure 2 shows the organic remaining after 24 h for a ratio Fe 2+ /organic of 0.01 : 1 and different hydrogen peroxide doses. A similar behavior can be seen for the four compounds (PHE, 4-CP, DCP and NB) tested. For a ratio H 2 O 2 /organic above 3 the organic reduction is practically complete in all cases. It is not necessary to add a large quantity of hydrogen peroxide to the system to remove the organic compounds. Other ratios Fe 2+ /organic tested (0.001 : 1 and 0.1 : 1) have given the same results. Fig. 1. BOD 5 /COD and BOD 21 /COD ratios for the tested organic compounds. Table 1. Stoichiometric coefficients for FOR, PHE, 4-CP, DCP and NB (confidence coefficient 95%) Compound Mol removed/mol H 2 O 2 Formic acid 0.955 Æ 0.077 Phenol 0.506 Æ 0.023 4-Chlorophenol 0.601 Æ 0.044 2,4-Dichlorophenol 0.520 Æ 0.031 Nitrobenzene 0.546 Æ 0.027 E. Chamarro et al.1048 The analysis of total organic carbon for these experiments with a ratio Fe 2+ /organic 0.01 : 1 shows a similar behavior for the four compounds. In all the cases, for a ratio H 2 O 2 /organic equal to 3 the degradation of organic compound was practically complete. However, the TOC decreased more slowly and there was no complete mineralization of the compounds. Figure 3 shows the TOC reduction for these compounds (Fe 2+ /organic 0.01 : 1). Similar results were obtained for the other ratios studied. The total organic carbon consumed was also determined for three different ratios Fe 2+ /organic (1, 0.1 and 0.01). In these experiments it can be seen that mineralization increases with iron concentration. Figure 4 shows the TOC consumed in the case of 4- chlorophenol for these three Fe 2+ concentrations at a different H 2 O 2 /4-CP ratio (0–50). According to Fig. 4, it can be seen that there is a limiting TOC value at high concentrations of hydrogen peroxide. In order to reduce the TOC, the concentration of Fe 2+ and H 2 O 2 show to be very important. In all the experiments, after 24 h of reaction, the TOC decreased with the concentration of hydrogen peroxide. The COD values decreased too, and the BOD values were seen to increase. In Figure 5, it can be seen the variation of BOD 5 /COD after 24 h for 4-chlorophenol experiments with the hydrogen per- oxide doses operating at a Fe 2+ /4-CP ratio of 1 : 1. Figure 5 shows 4-chlorophenol solution with a BOD 5 /COD ratio initially near zero (as it can be seen in Fig. 1). It becomes a biodegradable solution when H 2 O 2 is added (as the hydrogen peroxide concentration increases, the BOD 5 /COD ratio also increases to a value $ 0.4). Figures 5 and 6 show that the Fenton reaction with these organic compounds yields biodegradable substances. Only when the initial organic compounds are depleted, microorganisms are able to degrade the products. Figure 6 shows the biodegradability of 4- chlorophenol and 2,4-dichlorophenol vs. the percen- tage of substance removed. It can be seen that true biodegradability is attained when the initial com- pound is removed. Kinetic experiments were carried out for formic acid and 4-chlorophenol. In experiments with formic acid the H 2 O 2 /FOR ratio was 1.2 and the Fe 2+ /FOR ratio was 0.4. For these experiments, the TOC decreased very fast and after 2.5 h it was practically zero. That is because the formic acid reacts with radicals to give carbon dioxide directly. The pH increased when hydrogen peroxide was added de- creasing afterwards to a constant value. Also, the redox potential increased when the oxidant was added and afterwards decreased to a constant value, as it can be seen in Figure 7. The experiments with 4-chlorophenol were carried out with a Fe 2+ /4-CP ratio of 1 : 1 and with different amounts of H 2 O 2 . For these experiments the redox potentials increased when hydrogen peroxide was added to the system and after decreased. For a same Fig. 2. Remaining PHE, 4-CP, DCP and NB for initial concentrations of 300 ppm (Fe 2+ /organic=0.01 : 1). Fig. 3. Total organic carbon after 24 h for a ratio Fe 2+ /organic=0.01 : 1 (initial concentration of organics: 300 ppm). Fig. 4. TOC consumed vs. H 2 O 2 doses at different Fe 2+ ratios. Use of Fenton reagent to improve organic chemical biodegradability 1049 reaction time, when the H 2 O 2 concentration was increased, the redox potential was also seen to increase. The pH decreased in all experiments due to the formation of more acid products than 4- chlorophenol. For the same reaction time, when concentration of hydrogen peroxide increased, the pH decreased. In Figure 8 the variation of the concentration of 4-chlorophenol vs. time can be seen. For a ratio H 2 O 2 /4-CP of 1 : 1, the concentration decreased to a constant value. In the later case enough hydrogen peroxide was not available in solution to degrade all the 4-CP. However, for a ratio of H 2 O 2 /4-CP 10:1 the concentration of 4-CP decreased until zero. The reaction rate was seen to increase with Fe concentration. From Fig. 8 it can be concluded that hydrogen peroxide and iron concen- tration have an influence on the degradation rate. The iron concentration was seen to be more important than the peroxide ratio. For Fe 2+ /4-CP ratios larger than 0.1 : 1 the reaction may be considered instantaneous. CONCLUSIONS There are two important factors affecting the rate of Fenton’s reaction: peroxide dose and iron concentration. The peroxide dose is important in order to obtain a better degradation efficiency, while the iron concentration is important for the reaction kinetics. The extension of the oxidation is determined by the amount of hydrogen peroxide present in the system. A total elimination of organic carbon requires large amount of oxidant and/or large residence times. The partial oxidation of toxic compounds enhances biodegradability. Total depletion of organic carbon requires huge amounts of oxidant and large residence times. Oxidant may be wasted under these condi- tions, but subsequent low-cost biological treatment of pre-treated wastewater is shown in this study as a effective alternative. Acknowledgements}The authors wish to express their gratitude for the financial support given by the Ministry of Education of Spain (DGICYT, project AMB 96-0906). Fig. 5. BOD 5 /COD vs. H 2 O 2 dose of an initial concentra- tion of 4-CP of 300 ppm (Fe 2+ /4-CP=1 : 1). Fig. 6. Biodegradability of 4-CP and DCP vs. fraction removed. Fig. 7. Redox potential vs. time Fe/4CP 1 : 1 different initial amounts H 2 O 2 . Fig. 8. Reaction rate of 4-chlorophenol at different Fe 2+ / H 2 O 2 ratios. E. Chamarro et al.1050 REFERENCES Bidga R. J. (1995) Consider Fenton chemistry for waste- water treatment. Chemical Engineering Progress 91(12), 62–66. Kiwi J., Pulgarin C. and Peringer P. (1994) Effect of Fenton and photo-Fenton reaction on the degradation and biodegradability of 2- and 4-nitrophenols in water treatment. Applied Catalysis B: Environmental 3, 335–350. Marco A., Esplugas S. and Saum G. (1997) How and why combine chemical and biological processes for wastewater treatment. 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Use of Fenton reagent to improve organic chemical biodegradability 1051 . in the reaction of hydroxyl radicals and the redox reactions of radicals. Journal of Amercian Chemical Society 96, 133–139. Use of Fenton reagent to improve organic chemical biodegradability. Fe 2+ ratios. Use of Fenton reagent to improve organic chemical biodegradability 1049 reaction time, when the H 2 O 2 concentration was increased, the redox potential was also seen to increase Great Britain 0043-1354/01/$ - see front matter PII: S0043-1354(00)00342-0 USE OF FENTON REAGENT TO IMPROVE ORGANIC CHEMICAL BIODEGRADABILITY E. CHAMARRO, A. MARCO and S. ESPLUGAS* M Departament d’Enginyeria