Environ Chem Lett (2003) 1:45 50 DOI 10.1007/s10311-002-0007-2 ORIGINAL PAPER J. Sanz J. I. Lombraña A.M. De Luis M. Ortueta F. Varona Microwave and Fenton s reagent oxidation of wastewater Accepted: 3 May 2002 / Published online: 1 February 2003 Springer-Verlag 2003 Abstract We compared two H 2 O 2 oxidation methods for the treatment of industrial wastewater: oxidation using Fenton s reagent [H 2 O 2 /Fe(II)] and microwave irradiation. Both methods were applied to the treatment of synthetic phenol solutions (100 mg L 1 ) and of an industrial effluent containing a mixture of ionic and non-ionic surfactants at high load (20 g L 1 of COD). The effects of initial ph, initial H 2 O 2 concentration, Fenton catalyst amount and irradiation time were assessed. According to the oxidation of phenol, it has been found that the oxidation by Fenton s reagent is dependent on the ph, contrary to the microwave system, which is not influenced by this parameter. For both systems, a limiting amount of oxidant has been found; above this point the oxidation of phenol is not improved by a further addition of peroxide. The oxidation of the industrial surfactant effluent has only been successful with the Fenton s reagent. In this case, large amounts of ferrous ions are necessary for the precipitation of the ionic surfactants of the effluent, followed by the oxidation of the non-ionic constituents of the solution. Keywords Advanced oxidation processes (AOPs) Fenton s reagent Microwave irradiation Hydrogen peroxide Phenol and surfactants Oxidant efficiency Introduction Much of the pollution currently occurring in public waters is generated by human activities. In general, the removal of organic pollutants involves one or more basic techniques such as chemical oxidation, air desorption, liquid liquid extraction, adsorption, osmosis, ultrafiltration and biodegradation. The choice of one or other method depends mainly on process costs and on other factors J. Sanz J. I. Lombraña ( ) A.M. De Luis M. Ortueta F. Varona Department of Chemical Engineering, University of the Basque Country (UPV/EHU), P.O. Box 644, E-48080 Bilbao, Spain e-mail: iqploalj@lg.ehu.es such as concentration and effluent volume. Among these techniques, the advanced oxidation processes are suitable for the degradation of toxic aqueous pollutants. These processes are characterised by their capacity for total oxidation of the pollutant or by a partial oxidation, resulting in this latter case in a decrease in the organic content and/or an improvement in the biodegradability. Of all the possible oxidising agents (Glaze 1995), hydrogen peroxide has been chosen here due to its reasonable cost and high oxidising power. It can be easily handled, its solubility in water is acceptable and it forms no by-products. Due to the low kinetic of oxidation, the highly reactive and oxidising hydroxyl radicals have to be generated from the peroxide. This can be done using any of the known techniques (Carey 1992). These processes are known as advanced oxidation processes (AOPs). The application of AOPs in wastewater treatment leads to the degradation of the pollutant rather than transferring it to another phase, making the relevant technologies effective in the removal of organic pollutants in solution. In recent years, one of the main objectives of these processes mainly with highly polluted effluents has been not to mineralise the pollutant totally, but to improve the biodegradability for a possible coupling of the AOP with a conventional biological treatment process (Adams and Kuzhikannil 2000). In this work, two AOPs are studied: H 2 O 2 /microwave and H 2 O 2 /Fe(II). Both systems are based on the generation of hydroxyl radicals from hydrogen peroxide: in the H 2 O 2 /microwave system this is done by irradiating the solution with microwaves and in the Fenton s reagent by the addition of ferrous ions. The Fenton s reagent process has been widely studied and has been used routinely for industrial wastewater treatment. This is an effective method and easy to apply based on the special property of the ferrous ion to generate the hydroxyl radical as described in Eq. (1), kinetically enhancing the oxidation of the substrate (Esplugas et al. 2001): (1)
46 According to the microwave system, its use in chemistry has been reduced in recent years to organic synthesis for the acceleration of reaction rates. In this article we report the possibility of the generation of hydroxyl radicals by the irradiation of an effluent containing hydrogen peroxide. Experimental Treated compounds Our investigations involved both introduced radical generating techniques, Fenton s reagent and microwave irradiation. These techniques were applied to two types of effluents: a synthetic solution of phenol in distilled water with low concentrations (100 mg L 1 ) and a highly loaded surfactant effluent [20 g L 1 of chemical oxygen demand (COD)], obtained from a detergent-processing plant located near Bilbao, Spain. Surfactants are the major compounds used in the formulation of synthetic detergents used worldwide for both domestic and industrial applications. Consequently, surfactants are commonly found in municipal sewage and in subterranean waters (Perales 1999). The effluent treated in this work contained a mixture of anionic and non-ionic compounds. The main anionic surfactants occurring in wastewater are lauryl sodium sulphate, laurylether. Anionic surfactants are negatively charged usually due to a sulphate or sulphonate group, prepared from commercial aqueous solutions with contents ranging between 28 and 35%. Non-ionic surfactants contain non-diluted alcohol ethoxylates (AE) and are obtained from the polymerisation of 1,2-epoxyethane; those present in the effluent are various alcohol ethoxylates. Phenol was used here as a model compound because of its high toxicity. It is frequently found in certain water-courses polluted by petrochemical and coal treatment plants. It is considered hazardous for some aquatic life forms in concentrations higher than 50 10 3 mg L 1. The ingestion of 1 g can be lethal for human beings due to its effects on the nervous system. Moreover, phenolic derivatives give a high oxygen demand, up to 2.4 mg mg 1 of phenol. It can also react with chlorine or other halogens present in drinkable water, resulting in chlorophenols and other more hazardous derivatives. Consequently, phenol was selected as being representative of single organic compounds with low biodegradability. Experimental method Fenton s reagent was applied to phenol in a stirred batch reactor with control of the ph, with 250-mL solutions containing 100 mg L 1 being treated. A specified amount of oxidant was added initially in each experiment, and the initial ph of the solution was adjusted or maintained constant throughout the reaction using 0.1 M sulphuric acid. The ferrous ion added as the catalyst of the reaction for the generation of the hydroxyl radicals is supplied as ferrous sulphate. The amount of ferrous sulphate was calculated for the addition of ferrous ions. In this way, ferrous ions in the range of 1 to 10 mg L 1 were used in this work. In the application of Fenton s reagent to the surfactant effluent the same reactive system was used with a total volume of 500 ml, and with initial concentrations of 20gL 1 being treated in all cases. The surfactant effluent treated in this work is a mixture of compounds of different natures, with an unknown concentration of each of the surfactants present in the wastewater. For this reason this effluent was characterised by its chemical oxygen demand (COD). This parameter gives a measure of the concentration and was therefore used to follow the kinetic of the oxidation. COD was measured using a NANO- COLOR 15,000 immediate test, based on the reduction of chromium. Phenol was determined by high performance liquid chromatography (HPLC) using a Nucleosil 120 ODS C18 25 0.46 cm column supplied by Teknokroma, with a Waters 2690 separation module and a 2487 absorbance detector. A mixture of deionized water and acetonitrile (50:50 v/v) was used as the mobile phase and the UV detection was made at 272 nm, with retention times around 2.72 min for phenol. During irradiation with microwaves of both effluents, the oxidation was performed in a PROLABO MICRO- DIGEST 401 microwave digestion system. This system has a power of 250 W, emitting at a frequency of 2,450 MHz. One hundred milliliter solutions containing a certain concentration of phenol and the oxidant, at the specified ph of each experiment, were placed in a vessel and into the autoclave for the focalised irradiation. Experiments took place at ambient pressure and a refrigeration system was added to the upper part of each vessel to enable condensation to take place, as the boiling temperature of the solution (100 C) is reached early in the reaction. The rate of disappearance of hydrogen peroxide was followed using colorimetrical techniques for concentrations lower than 0.02 M and iodometrically for higher concentrations. The addition of peroxide was represented as the molar ratio hydrogen peroxide to pollutant (phenol or surfactants), R. Results and discussion Oxidation with Fenton s reagent [H 2 O 2 /Fe(II)] In this first step, the effect of different operating variables were evaluated in the oxidation of phenol. Fenton s reagent is a regenerative system based on the speciation of iron, for which several mechanisms are proposed. In the present paper, a mechanism has been proposed based on the following elementary steps, according to Eqs. (1) to (7). First, the reaction between the ferrous ion and peroxide is considered, resulting in the formation of radicals and ferric ions. This is commonly known as the Fenton
reaction (Eisenhauer 1964; Bishop et al. 1968). The ferric ions react with the hydrogen peroxide, generating a complex from which the ferrous ion are generated again (Pignatello 1992), as seen in Eqs. (2) and (3). This regenerative nature is characteristic of the Fenton s reagent. 47 (2) (3) On the other hand, the hydroxyl radicals formed in this first step also react with the hydrogen peroxide, thus generating perhydroxyl radicals (Kolthoff and Medalia 1949): (4) In the same way, these perhydroxyl radicals can react with the ferric ions present in the reaction media (Pignatello 1992): (5) Because hydroxyl radicals are more selective than perhydroxyl radicals with organic compounds (Bielski et al. 1985), in the present paper it is assumed that it is the hydroxyl radicals that take part in the oxidation of the organic pollutant. It is also assumed that an addition complex is formed between the substrate (e.g. phenol) and the hydroxyl radical. The kinetic of some of the equations shown above is well known. Glaze (1995) proposes some rate constants: k 1 =4.4 10 8 (M s) 1 and k 4 =2.7 10 7 (M s) 1. (6) (7) Fig. 1 Effect of ph on treatment of an aqueous solution of phenol with Fenton s reagent [Fe(II)/H 2 O 2 ]. Experiments performed at different ph values and with the same amount of oxidant, molar ratio oxidant to pollutant of R=8, show that the optimum ph value for oxidation of phenol is ph=3 rather varies during the reaction. Consequently, a given value of ph indicates the initial ph of the reaction. It has been proved in the case of phenol that when the initial ph is 3, the ph varies only slightly to a final value of 2.9, but that variation is more notable when large amounts of oxidant are used (ph=2.7). It is observed that when the initial ph is 6, the final value hardly varies when no oxidation occurs. However, ph decreases significantly with the oxidation of phenol, especially when large amounts of oxidant are used. In these conditions, the final ph values are similar to those reached when the initial ph is 3, i.e. around 2.7. This decrease in ph in proportion to oxidation levels is due to the oxidation products of phenol: intermediates (hydroquinone and pyrochatecol) and organic acids. The equilibrium of the prevailing species and the Fe(III) in a range of values between 3 and 6 are as follows (Litvintsev et al. 1993): (8) Effect of the initial ph The ph of the reaction media is known to have a significant influence on the oxidation of organic compounds, and that influence is even more crucial since Fenton s reagent is a system based on the speciation of the Fe(II) used as the catalyst of the reaction. The effect of ph was found to be very similar for surfactants and phenol. It was observed that for the working range of ph between 3 and 5, higher substrate removal is observed at a ph value of 3, for both phenol and surfactants. Figure 1 shows phenol removal for different initial ph values using a molar ratio oxidant to phenol of R=8. As can be observed, removal is highest at ph=3, followed by ph 4.5 and ph=6, whereas no degradation is observed at ph=2.4 due to the low regeneration of Fe(II) at this ph as expressed in Eq. (3), as Fe(III) precipitates. It is noteworthy that the ph of the solution is not constant, but (9) Figure 2 presents the proportion of each one of these species with the ph, according to the previous equilibrium constants. In analysing the results obtained from the performed experiments together with the diagram of the Fe(III) species in solution, some interesting conclusions are drawn. At ph values below 3 the removal rate of the substrate is inhibited. In contrast, the highest removal of substrate occurs at ph values near 3, the maximum of the species Fe(OH) +2, as can be seen in Fig. 2. The evolution of the concentration of the complex Fe(OH) +2 with the ph is in accordance with the rate of removal of substract. It seems that the Fe(OH) +2 is responsible for the generation of hydroxyl radicals and not the Fe(III) itself. This conclusion is justified by the aforementioned correspondence between the removal rate of substrate with the evolution of the concentration of the complex Fe(OH) +2 and the ph.
48 Fig. 2 Speciation diagram of Fe(III). Proportion of Fe(III) and the species in equilibrium with ph at 25 C. It can be seen that the predominant species at ph=3.0 is Fe(OH) +2 Fig. 3 Effect of Fe(II) concentration on the oxidation of phenol. Using the same amount of hydrogen peroxide, molar ratio oxidant to pollutant of R=4, and for a reaction time of 14 min, the amount of ferrous ion has linear influence in the oxidation of phenol In the case of the surfactants, the effect of the ph is significant when low concentrations of oxidant are added, the effect of the initial ph being positive in all cases. Effect of the catalyst concentration. It can be deduced from Fig. 3 that an increase of the concentration of ferrous ions used as catalyst in Fenton s reagent significantly enhances the rate of elimination of phenol. Assuming a first-order kinetic for the oxidation of phenol, the rate constants for experiments, varying according to the concentration of Fe(II), are 0.031, 0.061 and 0.156 min 1 for 1, 2 and 5 mg L 1 of Fe(II) respectively, using a molar ratio of R=4. The values obtained confirm that the rate of degradation is proportional to the concentration of catalyst added to the system. Experiments on the surfactant effluent were conducted using the same range of concentrations as for phenol [from 1 to 100 mg L 1 of Fe(II)], with no significant variations of COD. Experiments carried out later proved that higher concentrations of Fe(II) were needed to produce the oxidation of surfactants with a high load. The oxidation of surfactants only occurred above a determined concentration of Fe(II) for a given molar ratio. This could mean that surfactants are complexing agents. The complexing nature of some surfactants with Fe(III) was investigated by Bolte et al. (2000), and it was proved that when anionic surfactants and Fe(III) are mixed, a precipitate is formed which contains anionic surfactants and monomeric Fe(OH) +2, the predominant Fe(III) species. This precipitation is confirmed by the presence of turbidity in the samples treated in our experiments. Thus, no Fe(III) will remain in the supernatant until most of the anionic surfactant precipitates, and the presence of Fe(III) in the solution is needed to give rise to Fe(II) and hydroxyl radicals for the oxidation reaction. It is therefore necessary to add enough Fe(II) to form a precipitate and to maintain the chain reactions of Fenton s reagent. For a concentration of 500 mg L 1 of Fe(II), some oxidative effect is observed, with 20% of the initial COD being removed, including a significant percentage due to precipitation. Nevertheless, concentrations of 1,000 and 1,500 mg L 1 are needed for the removal of nearly 80 and 85% of the initial COD respectively in reaction times of 1 h. Effect of the initial molar ratio. The amount of hydrogen peroxide added to the system has been represented, as previously explained, by the molar ratio oxidant to pollutant, R. In the case of phenol, the existence of a limiting amount of oxidant has been deduced. Above this determined concentration of peroxide, no enhancement of the oxidation rate is observed when the initial concentration of the peroxide added to the solution is increased. Performed experiments have shown that the oxidation rate of phenol at ph=3 increases when the initial concentration of peroxide increases up to R=4. Above this value the addition of oxidant has no effect on phenol oxidation. Thus, the oxidation of phenol using values of molar ratios of R=4 and R=9 yields the same values of oxidation of phenol. This proves that the concentration of hydroxyl radicals is limited at high concentrations of peroxide and, therefore, from a certain concentration of oxidant in the amount of Fe(II) catalyst added to the solution, with the latter being the limiting reagent of the reaction. This is demonstrated by the fact that when the amount of catalyst added to the system is 2 mg L 1, the oxidation rate of phenol does not increase significantly at values above R=3. When 5 mg L 1 of Fe(II) is added, the oxidation curves at R=4 and R=8 do not increase, meaning that with a higher amount of catalyst added to the system the value of oxidant needed to get the same oxidation rate is also higher. In the case of the surfactants, the increase in the oxidation rate is more gradual with the addition of oxidant. Total substrate removal is achieved after a reaction time of 24 h using a molar ratio of 20. An increase in the molar ratio to R=40 reduces the reaction time for total oxidation to 3 h.
Effect of initial concentration 49 The influence of the initial concentration of pollutant on the oxidation rate for Fenton s reagent has been analysed for phenol. Experiments were conducted at ph=3 using 2mgL 1 of Fe(II), and it was observed that the rate of disappearance decreased with the initial concentration of phenol. Adjusting the experimental data to a pseudofirst-order kinetic, the calculated rate constants follow a linearity with the concentration of phenol for values higher than 10 mg L 1. At this low concentration the pseudo-first-order rate constant increases significantly. Oxidation by H 2 O 2 /microwaves Effect of temperature Experiments were conducted to elucidate the possible additional effect of microwaves in the enhancement of the oxidation rates of phenol in solution. For this purpose, duplicate experiments were performed under microwave irradiation and under conventional conditions at the same temperature. It was found that in the performed experiments the rate of oxidation under conditions of heating was much higher than that under ordinary heating at the same conditions. The simulation of the thermal gradient reached with microwaves with conventional heating was carried out using a polyethylene glycol bath in which a vessel with the solution was immersed. As in the microwave digester, a refrigeration system was attached to prevent the water reaching boiling point. Experimental and operational conditions were duplicated and, as can be seen in Fig. 4, a significant enhancement of the oxidation is observed in the case of microwave irradiation. This indicates that, as suggested before, a non-thermal effect is responsible for the enhancement of the oxidation rates under microwave irradiation. In this case the microwave irradiation of hydrogen peroxide in solution generates the hydroxyl radicals due to the excitation of the molecule to high vibrational and rotational energy levels. Effect of the initial ph. In this oxidative system, higher phenol oxidation rates are achieved at neutral ph values close to ph=6 than at lower values of ph=3, as shown in Fig. 5. The most unfavourable condition for phenol removal corresponds to an alkaline value of ph=11. This low removal could be attributed to the decomposition of hydrogen peroxide to oxygen and water at these basic conditions, thus lowering the amount of oxidant available to generate hydroxyl radicals. (10) In addition, as deduced by Rivas et al. (1999), the ionic form of peroxide formed at this ph presents higher scavenging power of hydroxyl radicals (Eq. 12) than the nondissociated form (Eq. 11). (11) Fig. 4 Comparison of conventional heating and microwave irradiation. Concentration of phenol in aqueous solution vs. time. Duplicate experiments comparing the oxidation of phenol with microwave and thermal heating using the same amount of peroxide (molar ratio oxidant to pollutant, R=8) denotes the enhancement of oxidation with microwaves compared to conventional heating Fig. 5 Effect of ph on the H 2 O 2 /microwave system. The removal of phenol vs. different ph values indicates that neutral values are optimal for the oxidation. In contrast, alkaline values inhibit the oxidation due to the decomposition of hydrogen peroxide (12) Moreover, in these highly alkaline conditions phenol is completely dissociated to phenolate. This would explain the initial period when a high phenol removal is achieved despite the smaller production of hydroxyl radicals due to the aforementioned peroxide decomposition. Effect of the initial molar ratio The influence of the peroxide concentration on the phenol removal rate follows the same trend previously deduced for the H 2 O 2 /Fe(II) system. Thus, the positive effect of the molar ratio is limited at a certain value, beyond which no enhancement of the oxidation rate is observed with an increase in the initial concentration of hydrogen peroxide. In the case of ph=6 this limiting molar ratio is R=14: after this value no significant improvement is observed when oxidant addition is increased.
50 In general, oxidant efficiency is lower in most cases than in the H 2 O 2 /Fe(II) system. The most favourable conditions for efficient use of peroxide are given at low molar ratios. It can thus be concluded that, as in the application of H 2 O 2 /Fe(II) to phenol, the use of oxidant in excess does not mean an equivalent removal of pollutant (de Luis 1999). Effect of the initial concentration To assess the influence of the initial concentration on the phenol oxidation rate, several trials were conducted at the most favourable conditions, i.e. at ph=6 with a molar ratio of 8, varying the initial concentration of pollutant. If the oxidation follows a first-order kinetic for the removal of phenol, rate constants were calculated for 250, 150 and 100 mg/l and values of 0.029, 0.046 and 0.067 min 1 respectively were obtained. Thus, it can be concluded that the oxidation rate constant declines in proportion to increases in the initial concentration of phenol. Conclusions This study compares the efficiency of two oxidising systems through the generation of hydroxyl radicals: Fenton s reagent and irradiation with microwaves. Fenton s reagent, H 2 O 2 /Fe(II), is applied successfully in the oxidation of phenol as well as in the removal of a surfactant effluent. The main limitation of this system is the ph, as the solubility of iron in its ferric form decreases dramatically as ph decreases. For both systems, it has been proved that the phenol removal rate is dependent on the amount of hydrogen peroxide added to the solution up to a limiting value where the oxidation of pollutant becomes constant. It is noteworthy that higher amounts of iron catalyst are required to oxidise the surfactant effluent than for phenol in order to reach acceptable removal values. References Adams CD, Kuzhikannil JJ (2000) Effects of UV/H2O2 preoxidation on the aerobic biodegradability of quaternary amine surfactants. Water Res 34:668 672 Bielski BHJ, Cabelli DE, Arudi RL, Ross AB (1985) Reactivity of HO2/O2 radicals in aqueous solution. J Phys Chem Ref Data 14:1041 1100 Bishop DF, Stern G, Fleichman M (1968) Hydrogen peroxide catalytic oxidation of refractory organics in municipal waste waters. Ind Eng Chem Process Des Dev 7:110 117 Bolte M, Asif A, Gilles M (2000) Degradation of sodium 4-dodeylbenzenesulphonate photoinduced by Fe(III) in aqueous solution. Chemosphere 41:363 370 Carey JH (1992) An introduction to advanced oxidation processes (AOP) for destruction of organics in wastewater. Water Pollut Res J Can 27:1 21 Eisenhauer HR (1964) Oxidation of phenolic wastes: I. Oxidation with hydrogen peroxide and a ferrous salt reagent. J Water Pollut Control Fed 36:1116 1128 Esplugas S, Chamarro E, Marco A (2001) Use of Fenton reagent to improve organic chemical biodegradability. Water Res 35:1047 1051 Glaze W (1995) Chemical oxidation. Water quality treatment. A handbook of community water supplies, 4th edn. McGraw-Hill, New York Kolthoff IM, Medalia AI (1949) The reaction between ferrous iron and peroxides. II. Reaction with hydrogen peroxide in the presence of oxygen. J Am Chem Soc 71:3784 3788 Litvintsev IY, Mitnik YV, Mikhailyuk AI (1993) Kinetics and mechanism of catalytic hydroxylation of phenol by hydrogen peroxide. I. General relationships of process variables. Kinet Catal 34:68 72 Luis AM de (1999) Degradation of phenolic compounds in solution with hydrogen peroxide: modelizing of radicals generation systems. PhD Thesis, University of the Basque Country, Bilbao Perales JA, Manzano MA, Sales D, Quiroga JM (1999) Linear alkylbenzene sulphonates: biodegradability and isomeric composition. Bull Environ Contam Ecol 63:94 100 Pignatello JJ (1992) Dark and photoassisted Fe(III)-catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide. Environ Sci Technol 26:944 951 Rivas FJ, Kolackzkowski TS, Beltran FJ, McLurgh DB (1999) Hydrogen peroxide wet air oxidation of phenol: influence of operating conditions and homogeneous metal catalysts. J Chem Technol Biotechnol 74:390 398