Hybrid System to Upgrade Conventional Fenton's Process by

Ind. Eng. Chem. Res. 2007, 46, 1505-1510

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Hybrid System to Upgrade Conventional Fenton’s Process by Incorporating Photo-Fenton as A Successive Treatment Process: Degradation of Monuron W. Chu* and K. H. Chan Department of CiVil and Structural Engineering, Research Centre for EnVironmental Technology and Management, The Hong Kong Polytechnic UniVersity, Hunghom, Kowloon, Hong Kong

The degradation of monuron in different remediation processes including UV, UV/H2O2, Fenton’s process (FP), and photo-Fenton’s process (UV/FP) was investigated, and then a hybrid system was proposed based on the results. The processes of UV and UV/H2O2 were found to completely remove the monuron, but a longer irradiation time was required. For the FP, a rapid decay of monuron was observed at the beginning of the process but a complete removal was difficult unless very high doses of reagents were used, and negative effects from overdose could be observed. For the UV/FP, both the rapid decay and complete removal of monuron were observed. However, it may not be as cost-effective as the UV irradiation because higher energy consumption was involved. Thus, a new approach on upgrading conventional FP by incorporating UV/FP as a successive treatment process was proposed in this work. This hybrid system could improve the initial decay rate and performance of FP and reduce the power consumption compared to that in the original UV/FP. A hybrid model equation was developed and found to be very useful for predicting the degradation of monuron in the hybrid system. Introduction In recent years, advanced oxidation processes (AOPs) have been intensively investigated. AOPs were potentially useful for treating pesticide wastes because a powerful oxidant, hydroxyl radical (OH‚), was generated in the processes. The OH‚ could be produced by reacting hydrogen peroxide, H2O2, with either Fe(II) (i.e., Fenton’s process, FP),1,2 Fe(III),3 or iron oxide. The AOPs were practically applied in some textile and dye industries on dye removal.4 Many AOPs and their associated capabilities in removing selected target compounds have been investigated, so wastewater treatment industries could select or add appropriate AOPs in the treatment line according to the type and level of pollutant removal, pH of wastewater, availability of chemicals, and operational cost. For the photo-Fenton process (UV/FP), much research focused on the doses of the reagents used in FP, the optimum pH level, and the different wavelengths of the UV lamps.1,5 However, information was limited in the technique and cost of the process operation, which was an essential requirement for designing a treatment facility in a real situation. The irradiation time of UV in the system, for example, would be a constraint on the power consumption in the whole treatment process. On the other hand, for the treatment by using conventional FP (i.e., no UV irradiation), high removal efficiency might be achieved if high doses of reagents were used. However, the adoption of overdosing the FP would increase the cost significantly. In addition, FP might not be able to degrade the pollutants completely,6 unless excessive reagent doses were used, which may not be an environmentally friendly approach. Thus, a new approach on upgrading conventional FP using UV/FP was proposed in this work. This hybrid system could be useful in promoting the removal performance of Fenton’s process without overdosing the system, while saving power consumption in the original UV/FP process, simultaneously. * To whom all correspondence should be addressed. Fax: +85223346389. E-mail: [email protected].

Monuron, 1-(4-chlorophenyl)-3,3-dimethylurea, was used as a probe in this work. It was a widely used broad-spectrum herbicide for the control of many grasses and herbaceous weeds on noncropland areas, such as industrial sites and drainage ditch banks. Monuron could also be used as a plant growth regulator, an effective and selective herbicide.7 It should be noted, however, that because the use of monuron has led to a potential environmental hazard for polluting drinking water, this herbicide was banned or suggested as undesirable in some countries. In soil, monuron was transformed to its metabolites primarily by biodegradation. Depending on the nature of soil and climatic conditions, the field half-life of monuron in soil ranged from 95% of monuron was removed within an hour, depending on the [Mon]0. Generally, the pseudo-first-order decay was commonly used to describe photolytic process. Figure 1 shows also the plots of ln([Mon]/ [Mon]0) versus reaction time in the subfigure. It found that the decays of monuron by direct UV photolysis showed pseudofirst-order kinetics. The decay rates (k) were determined by the slopes of the lines. In addition, the k could be correlated with [Mon]0 as shown in eq 1.

k ) (1.17 × 10-4)[Mon]0-0.92 (r2 ≈ 0.99)

(1)

It was found that the higher the [Mon]0, the slower was the decay rate that resulted. The retardation was simply due to the relative ratio of monuron/photon in the solution, where the process shifted from an optical dilute (photon-plenty) to an optical dense (photon-deficient) system as [Mon]0 increased.

Figure 1. Direct photolysis on different concentrations of monuron (insert: plot of the first-order decay kinetic on the direct photolysis on different concentrations of monuron). Experimental conditions: [Mon]0 ) 0.0113, 0.0226, 0.0451, 0.0564, 0.0789, and 0.1128 mM; pH ) 2.8; temp. ) 22 °C; light intensity ) 3.0 × 10-6 einstein L-1s-1 (254 nm).

Photodegradation of Monuron Enhanced by Hydrogen Peroxide and Ferrous Ion. In order to improve the UV direct photolysis system, H2O2 and Fe(II) were introduced to the process. Figure 2 shows the comparison of monuron degradation in different processes, i.e., UV (UV direct photolysis), FP (Fenton’s process with H2O2 and Fe(II)), UV/H2O2 (UV irradiation with adding H2O2), and UV/FP (UV irradiation together with Fenton’s process). Two doses of the Fenton’s reagent with the ratio of H2O2 to Fe(II) at 1.0 were compared in Figure 2. In Figure 2a, the concentration of Fenton’s reagent at 0.1 mM (i.e., [H2O2] ) [Fe(II)] ) 0.1 mM) was applied to the system. It was observed that monuron was completely removed within an hour by either the system of UV or UV/H2O2. Referring to the figure, UV/ H2O2 has a slight improvement on the removal of monuron compared to that of UV irradiation alone. This enhancement was due to the photolysis of H2O2 by the UV irradiation, which generates hydroxyl radicals as indicated in eq 2. hν

H2O2 98 2HO‚

(2)

Such an improvement could be further amplified if the supply of H2O2 is increased (but not overdosed).12 For the system of FP having no UV irradiation involved, monuron decay was rapid in the first few minutes but suddenly slowed down afterward. A faster decay rate in the first stage by FP was observed compared to those of the direct photolysis and UV/H2O2 systems. In this fast decay stage, hydrogen peroxide was consumed vigorously because of the catalytic formation of HO‚ (eq 3), which could be justified by the steep decay curves of monuron in Figure 2.

Fe2+ + H2O2 f Fe(OH)2+ + HO‚

(3)

Unlike in the direct photolysis and UV/H2O2 systems, FP was not able to completely remove monuron, where only 70% removal of monuron was observed. A rate retardation was observed in FP at an extended reaction time. The occurrence

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presence of UV as shown in eq 4, so as to improve the overall performance of the system. hν

Fe(OH)2+ 98 Fe2+ + HO‚

Figure 2. Degradation of monuron in different processes: direct photolysis (UV), FP, UV/H2O2, UV/FP; (a) at the reagent dose at 0.10 mM ([Fe(II)] ) [H2O2] ) 0.10 mM and (b) at the reagent dose at 0.20 mM ([Fe(II)] ) [H2O2] ) 0.20 mM). Experimental conditions: [Mon]0 ) 0.0564 mM; pH ) 2.8; temp. ) 22 °C; light intensity ) 3.0 × 10-6 einstein L-1s-1 (254 nm).

of the retardation stage might involve several reasons. The most critical one was naturally due to the depletion of oxidants (H2O2) in the solution. In addition, the Fe3+ produced from the FP system might react with H2O2 to form weaker radicals (such as HOO‚), which would further reduce the level of hydrogen peroxide in the solution. As the [H2O2] was gradually lowered to a trivial level, the retardation stage would be initiated. This is the stage where the low amount of oxidants in the system becomes a dominant factor inhibiting the further decay of monuron. Another possibility for such a stage is the competition for hydroxyl radicals between the parent compound (monuron) and its intermediates.1 When combining the two processes, i.e., UV/FP, the fastest and a complete monuron removal were observed. The photolysis of aqueous complex Fe(OH)2+, generated in FP, provided an additional pathway for the formation of hydroxyl radical in the

(4)

Besides, the enhancement also partly resulted from the photolysis of hydrogen peroxide, which could generate hydroxyl radicals as indicated in eq 2. All these possible pathways resulted in the best reaction performance of UV/FP among the other three processes; complete removal was observed within 25 min. Besides, the time to reach 50% removal was reduced ∼3-4× when comparing with the processes UV, FP, and UV/H2O2. To further study the tested systems, the concentration of Fenton’s reagent was doubled to 0.2 mM (i.e., [H2O2] ) [Fe(II)] ) 0.2 mM) in all cases, as shown in Figure 2b. The enhanced performance was observed in all three processes wherever the H2O2 and/or Fe(II) were involved (i.e., FP, UV/H2O2, and UV/ FP). For the FP, the initial decay rate and removal efficiency were both significantly improved, and >96% of monuron was removed (compared to 70% in the previous case with lower doses). For the UV/FP, similar to FP, the time to reach 50% removal was improved by 5 times, and the complete removal of monuron was further shortened to 15 min. These observations obviously result from the increment of reagent doses, where higher amounts of hydroxyl radicals were generated and, therefore, enhanced the oxidative power of the processes. However, reagents present in a big excess could act as radical scavengers and hinder monuron degradation. Hybrid System: Cooperation of Photo-Fenton’s Process on Fenton’s Process. Referring to parts a and b of Figure 2, the complete removal on monuron degradation could not be achieved in FP even after the increment of reagent doses, while a surprisingly fast removal was observed by UV/FP, where the reaction was completed in 15 min (see Figure 2b). This suggested a potential overdosing (or energy wasting) of the process, and the long-term usage of UV may not be a proper strategy. In order to optimize the UV application and to expand the individual strength and lessen the weakness of UV/FP and FP, a new approach combining FP and UV/FP into one system was proposed as a carefully designed hybrid system. The schematic diagram of the hybrid system is shown in Figure 3, where a FP was followed by a UV/FP sequentially. The proposed hybrid system was split into two stages. The first stage is the original FP, and the second stage involves UV additionally. The hybrid system is controlled by the switchingon time of UV irradiation (tirr). By varying the value of tirr (i.e., commencing the UV irradiation at different times), the degradation of monuron in different operations were investigated. A complete removal of monuron was observed at the end of the second stages. Thus, the UV/FP could assist the further degradation on monuron in FP that remained in the first stage, and monuron could be fully decayed eventually. The advantages of this hybrid system were as follows: (1) improving the performance of the FP system, (2) shortening the UV irradiation time in order to save power and operation cost in the original UV/FP system, (3) being more flexible on the operation when UV irradiation was not available sometimes, and (4) improving the overall removal performance. Figure 4 shows the degradation of monuron in different conditions of the hybrid system. The triangle symbols indicate the degradation of monuron by FP only, which could be considered as a reference curve. For the FP, a fast decay rate was observed at the beginning of the reaction, and the reaction was retarded afterward. About 74% of monuron was removed in 50 min. The diamond symbols represent the degradation of

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Figure 3. Description of the hybrid system in which FP is the first stage and UV/FP is the second stage. UV irradiation started at different times tirr to initiate the second stage of the process.

Figure 5. Energy consumption for different tirr. The curves represent the removal levels of monuron. The points laid on the y-axis are the energy required if the UV light was switching on at tirr ) 0 s.

which was produced in the FP system (i.e., eq 3), and (2) photolysis of remaining H2O2 in the system (eq 2). On the other hand, because the UV irradiation was involved in the hybrid system, the power consumption of the system was compared. This was because the operational cost was an essential issue for designing the treatment process. The longer the UV exposure time, the higher is the energy that would be consumed. It was found that the hybrid system could save 1-99% energy (in kW‚s) depending on different tirr and removal levels. Figure 5 shows the energy consumption in different hybrid system at different tirr values.

energy required (kW‚s) ) power (kW) × irradiation time (s) (5)

Figure 4. Degradation of monuron in the hybrid system at different times tirr. The decays of monuron in FP and UV/FP were plotted also for the comparison. Experimental conditions: [Mon]0 ) 0.0564 mM; pH ) 2.8; temp. ) 22 °C; [Fe(II)] ) [H2O2] ) 0.10 mM; light intensity ) 3.0 × 10-6 einstein L-1s-1 (254 nm).

monuron by UV/FP (i.e., tirr ) 0); the initial decay rate of the system was increased and the quantitative degradation of monuron was observed in 25 min. The cross symbols depict the degradation of monuron at different irradiation times tirr at 2.5, 5, 7.5, 8.5, 10, 15, 20, 25, 30, and 40 min. The solid lines correlated to the data points in the figure were predicted from the proposed models, which are discussed later. Referring to the figure, when the monuron decay slows down at the later stage of FP, the immediate involvement of UV can reactivate the system, causing the further decay of monuron in the hybrid system. In general, a fast and significant decay of monuron was observed once UV irradiation started. It is presumably due to the sudden increment of oxidation power in the system through the generation of extra radicals at the time when UV irradiation starts. The extra radicals might be produced from the following possible pathways: (1) photolysis of aqueous complex Fe(OH)2+,

The points laid on the y-axis (where tirr ) 0 s) were plotted for comparison. As shown in the figure, the higher the removal percentage, the longer is the exposure time and, therefore, the higher is the energy required. However, it was also found that the energy would be saved in most of the removal targets by using hybrid system when comparing to the use of UV/FP alone. Referring to these cases (tirr ) 0 s), the energy consumption was generally lower at different times tirr, except for the cases of 85-99% at shorter tirr. In order words, the hybrid system could successfully lower the energy requirement versus that in the traditional UV/FP. It was found that, by delaying the UV switching-on time (i.e., longer tirr), more energy would be saved to achieve the same removal performance. It should be noted that, for the cases of 85-99% removal, a relatively higher energy was required at shorter tirr when compared to the nonhybrid system (tirr ) 0 s). The reason is not known yet; however, it should be related to the nil and high radicals in the solution for the nonhybrid and hybrid systems, respectively, just before the UV is on. Therefore, the use of the hybrid system has the advantage to lower the energy consumption if the energy issue is critical in the application. Model Applications on the Hybrid System. A number of researchers have suggested that the FP should be a first-order reaction,3 while others state that it should be a second-order reaction.2,13 Essentially, the process involves two major components (i.e., hydroxyl radical and monuron), so a second-order

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at the second stage of the hybrid system. The k2 (s-1) was the decay rate of monuron in the second stage. Thus, by combining eqs 6 and 7, a model for the second stage could be revised as

(

)

tirr [Mon] ) 1e-k2(t - tirr) × 100% (%) 1/k1 + σtirr [Mon]0 (for t > tirr) (8) By using eqs 6 and 7, the degradation of monuron in the hybrid system becomes predictable by the proposed models. The solid lines in Figure 4 simulate the prediction of the models. Owing to the different initial concentrations of monuron in the second stage of the hybrid system at different tirr values (i.e., [Mon]0′), it would be considered when the reaction rate in the second stage was being compared. Figure 6 shows the correlation between the product of reaction rate and [Mon]0′ and the tirr value (i.e., k2[Mon]0′ vs tirr). It illustrates a good correlation with a high r2 of 0.93 (eq 9). Figure 6. Correlations of the decay rate at the second stage times the concentrations of monuron at the beginning of the second stage and the UV irradiation starting time.

reaction is logical in theory, but the [HO‚] is difficult to quantify accurately. However, the HO‚ is so short-lived in the solution that it easily reaches a steady state with a very low concentration in the solution. Such a low concentration can generally be considered as a constant; thus, it can often be subsumed into the kinetic rate constant, and a pseudo-first-order reaction resulted. It should be noted that the conventional first- and second-order kinetics are generally applicable to the initial stage of the reactions and mostly failed to describe the full-range transformation of compound decay in FP (as shown in Figure 2). An alternative approach is, therefore, required for such a case. In our previous studies, a mathematical model was proven useful to simulate the degradation of organics in such a condition (FP system) by using the initial decay rate and the final removal fraction;5,6 the same approach can be applied here and modified into eq 5,

(

)

[Mon] t ) 1× 100% (%) 1/k1 + σt [Mon]0 (for first stage: t ) 0-tirr) (6) where [Mon] and [Mon]0 are the concentrations of monuron remaining in the system after a reaction time of t (s) and at the beginning, respectively. The k1 (s-1) and 1/σ (dimensionless) are the initial decay rate and the overall removal fraction, respectively, of the FP system. The equation was able to describe the degradation of monuron in the FP system with k1 of 5.38 × 10-3 s-1 and 1/σ of 0.79, with a very good r2 value > 0.99 (data not shown). Thus, eq 6 was used for predicting the degradation of monuron in the first stage of the hybrid system. For the second stage of the hybrid system, the first-order decay kinetic rate was capable of determining the further degradation of monuron (eq 7).

[Mon] ) e-k2(t - tirr) × 100% (%) [Mon]0′ (for second stage: t ) tirr-∞) (7) where [Mon]0′ is the concentrations of monuron remaining in the system at the reaction time when the UV irradiation is started at tirr. In other words, it was the initial concentration of monuron

k2[Mon]0′ ) 0.0004tirr-0.26

(9)

in which [Mon]0′ could be solved by eq 6 as shown in eq 10,

(

[Mon]0′ ) [Mon]0 1 -

tirr 1/k1 + σtirr

)

(10)

Thus, eq 8 could be used as a tool for hybrid system design by inputting the initial monuron concentration and the starting time of the UV irradiation. This is also useful and convenient for predicting the possible removal performance of the hybrid system as an add-on process in an existing FP. The model developed in this work was based on the experimental results at the dose of FP at 0.1 mM; however, the term at the left-hand side of eq 8 is a dimensionless ratio of [Mon]/[Mon]0. This suggested the proposed model was applicable to other initial concentrations of monuron and not limited only to the case of 0.1 mM. This hybrid system was practically feasible to be facilitated for the conventional wastewater treatment process. Conclusion The degradations of monuron by UV irradiation, UV/H2O2, FP, and UV/FP with different reagent dosages were compared in this work. UV/FP was found to be the most efficient process among the systems. It resulted in a rapid initial decay rate and complete removal of monuron. To fully utilize the advantage of FP (which offers a rapid decay of monuron but isexpensive for a complete removal) and to save the energy cost in UV/FP, a hybrid system combining the FP and sequentially followed by UV/FP was proposed. It was observed that the overall performance of the system was enhanced. A model for the design and/or prediction of the hybrid system was successfully developed in this work. It would be essential for the further development on a practical model for this hybrid system, which is justified to improve the conventional wastewater treatment processes. Acknowledgment The work described in this paper was supported by a grant from the University Research Fund of the Hong Kong Polytechnic University (Project No. PolyU G-YX74). The authors appreciate the contribution from Mr. W. H. Tan for his efforts in some laboratory works.

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Literature Cited (1) Chan, K. H.; Chu, W. Atrazine Removal by Catalytic Oxidation Processes with or without UV Irradiation. Part I. Quantification and Rate Enhancement via Kinetic Study. Appl. Catal., B 2005, 58, 157. (2) Kyselova, Z.; Rackova, L.; Stefek, M. Pyridoindole Antioxidant Stobadine Protected Bovine Serum Albumin against the Hydroxyl Radical Mediated Cross-linking in vitro. Arch. Gerontol. Geriatr. 2003, 36, 221. (3) Gallard, H.; De Laat, J. Kinetic Modelling of Fe(III)/H2O2 oxidation reactions in dilute aqueous solution using atrazine as a model organic compound. Water Res. 2000, 34, 3107. (4) Muruganandham, M. S. Decolourisation of Reactive Orange 4 by Fenton and photo-Fenton Oxidation Technology. Dyes Pigm. 2004, 63, 315. (5) Chu, W.; Kwan, C. Y.; Chan, K. H.; Kam, S. K. A study of kinetic modelling and reaction pathway of 2,4-dichlorophenol transformation by photo-Fenton-like oxidation. J. Hazard. Mater. 2005, 121, 119. (6) Chu, W.; Chan, K. H. Reactor Design and Modeling of the Fe(II)Catalyzed Oxidation of Trichlorophenol. Ind. Eng. Chem. Res. 2004, 43, 6797. (7) Kong, L. G.; Tan, Z. C.; Mei, J. T.; Sun, L. X.; Bao, X. H. Thermodynamic studies of monuron. Thermochim. Acta 2003, 414, 131.

(8) Khan, S. U. Pesticides in the Soil EnVironment; Elsevier: Amsterdam, The Netherlands, 1980; pp 163-197. (9) Sanchez, L.; Peral, J.; Domenech, X. Degradation of 2,4- dichlorophenoxyacetic acid by in situ photogenerated Fenton reagent. Electrochim. Acta 1996, 41, 1981. (10) Chu, W. Photodechlorination Mechanism of DDT in UV/Surfactant System. EnViron. Sci. Technol. 1999, 33, 421. (11) Arnold, S. M.; Hickey, W. J.; Harris, R. F. Degradation of atrazine by Fenton’s reagent: Condition optimization and product quantification. EnViron. Sci. Technol. 1995, 29, 2083. (12) De Laat, J.; Gallard, H. Catalytic decomposition of hydrogen peroxide by Fe(III) in homogeneous aqueous solution: Mechanism and kinetic modeling. EnViron. Sci. Technol. 1999, 33, 2726. (13) Guedes, A. M. F. M.; Madeira, L. M. P.; Boaventura, R. A. R.; Costa, C. A. V. Fenton oxidation of cork cooking wastewatersOverall kinetic analysis. Water Res. 2003, 37, 3061.

ReceiVed for reView September 25, 2006 ReVised manuscript receiVed December 13, 2006 Accepted January 15, 2007 IE061247T