Degradation of Poly(ethylene glycol) in Aqueous Solution by Photo

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Degradation of Poly(ethylene glycol) in Aqueous Solution by Photo-Fenton and H2O2/UV Processes Jeanne A. Giroto, Antonio C. S. C. Teixeira,* Claudio A. O. Nascimento, and Roberto Guardani UniVersity of Saõ Paulo, Chemical Engineering Department, AVenida Luciano Gualberto, 380 TraVessa 3, 05508-900 Saõ Paulo SP, Brazil

This article reports experimental results obtained in a laboratory-scale photochemical reactor on the photodegradation of poly(ethylene glycol) (PEG) in aqueous solutions by means of the photo-Fenton and H2O2/UV processes. Dilute water solutions of PEG were fed to a batch reactor, mixed with pertinent reactants, and allowed to react under different conditions. Reaction progress was evaluated by sampling and analyzing the concentration of the total organic carbon (TOC) in solution as a function of the reaction time. Organic acids formed during oxidation were determined by HPLC analyses. The main acids detected in both processes were acetic and formic. Glycolic acid was detected only in the photo-Fenton process, and malonic acid was detected only in the H2O2/UV treatment, indicating that different reaction paths occur in these processes. The characteristics of both processes are discussed, based on the evolution of the TOC-time curves and the concentration profiles of the monitored organic acids. The experimental results constitute a contribution to the design of industrial processes for the treatment of wastewaters containing soluble polymers with similar properties. Introduction Poly(ethylene glycol) (PEG) is widely used in industrial processes, such as in the production of surfactants, explosives, cosmetics, and lubricants or as a heat-transfer fluid. Because of its high water solubility, PEG can contaminate industrial wastewaters, and because it is not easily biodegradable, PEG can flow through conventional effluent treatment systems and be discharged to the environment.1 Among commonly used water-soluble polymers, PEG has the simplest molecular structure, and because of this, it represents an adequate model compound for studying the degradation characteristics of polymers based on the same function. The biological degradation of PEG has been studied for a long time by a number of authors (e.g., refs 2-4). The observed tendency is that the ability to degrade PEG by biological processes decreases significantly with increasing molecular weight. The efficiency of PEG biodegradation routes decreases strongly for polymers with more than ca. 10 monomeric units.5 This happens because long molecules cannot be transported across the cell membranes of microorganisms, and a pretreatment is necessary to decrease the size of the polymer molecules. The degradation of water containing PEG has been the aim of studies by Suzuki et al.,6 who used ozone to treat water solutions containing 800 mg of C L-1. In the process, the molecular weight was reduced from 8000 to 250 in ca. 2 h. Mantzavinos et al.7 studied the wet-air oxidation of PEG with various molecular weights in solutions containing 1 g of C L-1. In 4 h of reaction, ca. 80% of the TOC was removed from solutions based on PEG-10000 at an oxygen partial pressure of 2 MPa, a total pressure of 10 MPa, and a temperature of 513 K. The reaction time could be decreased significantly by coupling wetair oxidation pretreatment with membrane separation and biological treatment.8 Studies on the photodegradation of ethylene glycol, one of the possible products from PEG degradation, were carried out by McGinnis et al.9 They studied the photo-Fenton and H2O2/ * To whom correspondence should be addressed. Tel.: + 55 11 3091 2263. Fax: + 55 11 3813 2380. E-mail: [email protected].

UV photodegradations of ethylene glycol in aqueous solutions with EG concentrations ranging from 50 to 1000 mg L-1, Fe(III) concentrations ranging from 2 to 100 mg L-1, and a H2O2 concentration of 500 mg L-1. The most encouraging results were achieved with the H2O2/UV photodegradation, for which some intermediate degradation products were identified. More recent studies on the photodegradation of PEG in aqueous solution have included the use of basic oxygen furnace slag from steel production as an iron source for the photo-Fenton process10 and comparative studies of different Fenton systems (homogeneous and heterogeneous), indicating the advantages of homogeneous systems based on TOC removal from the solution.11 Processes such as the photo-Fenton, H2O2/UV photodegradation, and oxidation by ozone are generally known as advanced oxidation processes (AOPs) and are considered a promising alternative for treating wastewater containing organic compounds that are toxic or difficult to degrade by conventional means. AOPs are based on the generation of hydroxyl radicals (HO•), which are very reactive with respect to organic compounds, generating a number of oxidized products that can be further oxidized to CO2 (mineralization) or, if less toxic, can be treated by processes such as biological treatment. This study investigates the photodegradation of PEG in aqueous solutions by the photo-Fenton and H2O2/UV processes by monitoring the concentrations of the total organic carbon (TOC) in solution and organic acids produced in the reaction system as a function of time, in batch-mode experiments carried out in the laboratory. The photo-Fenton process is based on the oxidation of organic compounds in the presence of H2O2 and iron ions. A simplified reaction scheme was proposed by Haber and Weiss12 and consists of the generation of hydroxyl radicals according to reaction 1. These radicals react with the organic compound, and Fe(II) in solution is oxidized to Fe(III). Fe(II) + H2O2 f Fe(III) + HO• + OH-

(1)

Fe(III) can be reduced back to Fe(II) by H2O2 according to the reaction

10.1021/ie9015792  2010 American Chemical Society Published on Web 02/25/2010

Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010 +

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•-

Fe(III) + H2O2 + H2O f Fe(II) + H3O + HO2

(2) However, the conversion of Fe(III) to Fe(II) occurs at a significantly higher rate under UV-visible radiation, according to the reaction hυ

Fe(OH)2+ 98 Fe2+ + HO•

(3)

Fe(OH)2+ is the main hydroxide complex of Fe(III) in aqueous solution in the pH range of 2.8-3.2, and it absorbs radiation between 280 and 405 nm, with a molar absorption coefficient of ca. 2200 M-1 cm-1 at 300 nm.13 Reaction 3 occurs in water with a quantum yield of Φ(+Fe2+) ) 0.195 (310 nm).14 Stable complexes between Fe(III) and carboxylates exhibit strong absorption in the near-UV and visible regions of the spectrum, also resulting in Fe(II) and hydroxyl radicals.14 For example, the species [Fe(C2O4)]+ absorbs UV-visible radiation up to 570 nm,15 with quantum yields Φ(+Fe2+) of 1.25 at 254 nm, 1.07 at 436 nm, and 0.86 at 509 nm and quantum yields of Φ(+HO•) ) 2.23-3.65 at 300 nm.14 It should be noted that both components of the Fenton reagent react with HO•. These species oxidize Fe(II) to Fe(III), and they can be trapped by H2O2, leading to the formation of hydroperoxyl radicals (HO2•) that are much less reactive than HO• (reaction 4) H2O2 + HO• f HO2• + H2O

(4)

However, the rate constant for the reaction of HO• with organic compounds is generally 1-3 orders of magnitude larger than that of reaction 4, and the latter reaction can be minimized by continuously feeding the H2O2 solution (at an appropriate H2O2 concentration) into the reactor, as was done in this work. The H2O2/UV process is based on the generation of hydroxyl radicals through the absorption of UV radiation according to reaction 5, which occurs with a quantum yield of Φ(+HO•) ) 0.98 at 254 nm in water.14 The molar absorption coefficient of H2O2 is small (19.6 M-1 cm-1 at 254 nm) and close to zero above 300 nm.14 hυ

H2O2 98 HO• + HO•

Figure 1. Schematic view of the experimental equipment.

(5)

In this study, both processes were evaluated concerning the photodegradation of PEG in aqueous solutions. The evaluation was based on the TOC removal and on intermediate compounds formed, as analyzed by high-performance liquid chromatography (HPLC). Such intermediate products play an important role in the selection of wastewater treatment processes, because they could contribute to pollution if not adequately treated. Experimental Methods The following polymer samples were used: PEG 6000 (Vetec) and PEG 20000 (Merck). The following analytical-grade reagents were used: pentahydrated ferrous sulfate, hydrogen peroxide (30% w/w in water), potassium iodide, anhydrous sodium sulfide, sodium hydroxide, and sulfuric acid. A sketch of the laboratory reactor is shown in Figure 1. The photochemical reactor consisted of a 0.8-L borosilicate glass vessel connected to a 3-L circulation tank. The reactor was equipped with an internal quartz glass well where a 400-W medium-pressure mercury lamp (Philips HPLN) was placed. Figure 2 shows the spectra of the power distribution of the light

Figure 2. Spectral distribution of the medium-pressure mercury lamp emission power and transmittance of the quartz well.

source and the transmittance of the quartz well. With the internals in place, the reactor volume was 0.5 L. The circulation tank was made of borosilicate glass and was equipped with an external jacket connected to a thermostatic bath for temperature control. This tank was provided with a mechanical stirrer, temperature and pH indication, and holes at the top for feeding reactants and removing samples. Circulation of the reacting solution between the two tanks was achieved using a centrifugal pump at a rate of 2.4 L min-1. Reactants were fed to the system either at once or continuously, at a flow rate controlled by means of a peristaltic pump (Ismatec-IPC). A stock polymer solution was prepared by gradually dissolving small amounts of PEG in water in a beaker with magnetic stirring, at about 50 °C, in order to prevent foam formation. The solution was then diluted in water, in order to obtain the desired concentration. After the solution had been fed to the reactor, the centrifugal pump for circulation was started, and the pH was adjusted to a value of 3, optimal for the photoFenton process,16 and kept constant over the reaction time by adding H2SO4 solution. For the H2O2/UV process, the pH was not controlled. After the temperature had stabilized, the lamp was turned on, H2O2 feeding was started, and time measurement was started, with collection of the first sample. In all experiments, H2O2 feeding was interrupted 30 min before the reaction was stopped. This procedure was employed because H2O2 concentration was not monitored over time. Thus, during the last 30 min in each experiment, the H2O2 concentration decreased gradually, and this could help to detect conditions of H2O2 shortage based on the leveling of the TOC concentration. For the photo-Fenton process, a solution containing KI, Na2SO3, and NaOH (each 0.1 M) was added to the samples in a 3:2 (sample/solution) volumetric ratio prior to the TOC analysis, based on Lei et al.17 This quenching solution enabled

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Table 1. Conditions Used in the Experiments levels -

0

+

1000 1.0 110

1500 1.5 160

1000 40 110

1500 50 160

Photo-Fenton (T ) 30 °C) a

-1

PEG (mg of C L ) Fe(II) (mM) H2O2 (mM)

500 0.5 60 H2O2/UV

PEGa (mg of C L-1) T (°C) H2O2 (mM) a

500 30 60

PEG 6000.

the decomposition of residual H2O2 and the precipitation of Fe(II)-/Fe(III)-containing compounds. All samples were then filtered through a 0.22-µm membrane (Millipore, Millex GV), and the filtrate was taken to a TOC analyzer (Shimadzu 5000A). For the H2O2/UV process, the samples were taken to the TOC analyzer without any prior treatment. Oxalic, formic, acetic, malonic, and glycolic acids were analyzed with a Shimadzu Class-VP HPLC instrument with an SPDM-10AVP diode-array detector and a Hamilton PRP-X300 ionic exclusion column. The mobile phase was an aqueous solution of sulfuric acid (pH 2.00) pumped at 1 mL min-1. The temperature was 40 °C, and the injected volume was 20 µL. The samples from the photo-Fenton experiments were previously prepared for HPLC analyses by addition of two drops of 10 mol L-1 NaOH to 10 mL of sample, followed by filtering through a 0.22-µm membrane (Millipore, Millex GV). The pH was decreased to neutral through addition of sulfuric acid. The samples were then subjected to solid-phase extraction with Waters Sep-Pak C18 cartridges. The H2O2/UV samples were filtered through a 0.22-µm membrane and then subjected to solid-phase extraction. Results and Discussion A complete centered two-level factorial experimental design was employed in the experiments with each process. Selection of the concentration levels of the aqueous polymer solutions and photo-Fenton reactants was based on the results of a previous study by us on the photo-Fenton degradation of poly(vinyl alcohol)18 and a study by Lei et al.17 The polymer concentration levels correspond to 500, 1000, and 1500 mg of C L-1. The number of moles of Fe(II) was fixed at 1/20 of the number of moles of monomer. The amount of H2O2 added to the system was based on the amount necessary to oxidize the monomer to CO2 and H2O. Table 1 summarizes the experimental conditions employed in the experiments with both processes. The experiments were carried out in batch mode. The total reaction time was 120 min, and H2O2 solution was continuously fed to the reactor during the first 90 min of the reaction. Thus, the H2O2 concentration decreased gradually over the last 30 min of the reaction. For each polymer concentration level, 11 experiments were carried out: 8 experiments comprising all experimental conditions (3 factors, 2 levels each) plus 3 repetitions of the central point, which were used to evaluate the experimental error. Experiments carried out under centralpoint conditions for both processes with three repetitions resulted in an average value of the standard deviation of 39.75 mg of C L-1 for the photo-Fenton process and 28.89 mg of C L-1 for the H2O2/UV process. Comments on the behavior of the TOC-time curves during the degradation of PEG by the photo-Fenton process, as well

Figure 3. TOC-time plots for the photo-Fenton process.19 PEG initial concentration: (a) 1689 ( 10 and (b) 592 ( 17 mg of C L-1.

as visual observations, were presented in a previous work by us19 and are summarized here as follows: In most experiments, the TOC values remained approximately constant during the first minutes of reaction, decreased at a higher rate for a period, and then stabilized at lower TOC values. In all experiments, the solution color changed from light yellow to brownish in a short time, becoming gradually colorless with increasing reaction time. The formation of solid particles with a brownish color was observed in experiments with the variables at the higher (+) level, as reported in our other studies.18,20 Plots of the ratio between TOC at a given time and the initial TOC (TOC0), namely, TOC/TOC0, as a function of time for the photo-Fenton process are shown in Figure 3. These plots were presented in a previous work by us19 and are reproduced here for comparison. The formation of solid particles occurs simultaneously with a pronounced decrease in TOC and can be related to the transfer of organic compounds to the solid phase. Studies by other authors on the degradation of highly concentrated phenolic wastewater by the photo-Fenton reaction indicated that solid particles formed in the process, consisting of tannin-like polymers, might form in the presence of iron ions from the degradation of intermediate compounds such as catechol and pyrogallol, when these intermediates are present in high concentration.21,22 For higher initial PEG concentrations (Figure 3a), a gradual decrease of the TOC/TOC0 ratio as a function of time was observed. The decrease rate was higher after ca. 40-50 min of reaction when the H2O2 concentration was at the higher level. No significant effect of the Fe2+ concentration was observed for the levels employed in this study. The H2O2 concentration level was apparently the only variable affecting the TOC/TOC0 curves, as well as the final value of this ratio: ca. 80% when the H2O2 concentration was at the lower (-) level versus ca. 35% when the H2O2 concentration was at the higher level. The


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Figure 5. pH-time plots for the H2O2/UV process for both levels of initial PEG concentration: (a) 1435 ( 35 and (b) 562 ( 26 mg of C L-1.

Figure 4. TOC/TOC0-time plots for the H2O2/UV process. Initial PEG concentration: (a) 1435 ( 35 and (b) 562 ( 26 mg of C L-1.

Table 2. Conditions Used in the Experiments for Measuring Intermediate Compounds (PEG 6000 and PEG 20000) P1

PC

Photo-Fenton (T ) 30 °C)

H2O2 concentration was apparently the determining factor in the experiments, because, when the H2O2 concentration was at the lower level, the TOC/TOC0 values remained approximately constant during the last 30 min of reaction, when no H2O2 was added to the reactor. Different shapes of the TOC/TOC0 ratio as a function of time were observed for the initial PEG concentration at the lower level (Figure 3b). When the H2O2 concentration was at the higher level, ca. 85% of the initial TOC value was removed in 60 min of reaction. The removal ratio reached 100% in 120 min. When the H2O2 concentration was at the lower level, a gradual decrease in the TOC/TOC0 ratio as a function of time was observed, with a negative effect of the Fe2+ concentration: ca. 85% removal for the Fe2+ concentration at the lower level versus ca. 70% removal for the Fe2+ concentration at the higher level. Possible causes for this negative effect of the Fe2+ concentration have been discussed in our published articles18,19 based on the works of Lei et al.17 and Bossmann et al.,23 who carried out studies on the photo-Fenton degradation of poly(vinyl alcohol) (PVA) in aqueous solutions. They suggested that complexation of iron with PVA and intermediate products of PVA degradation can take place, thus preventing H2O2 from approaching and complexing with the metal centers. This effect might be a consequence of a poor efficiency of photoreduction of Fe(III) as it is bound within large organic molecules.23 Plots of the TOC/TOC0 ratio as a function of time for the H2O2/UV process are shown in Figure 4. Solid particle formation was not observed in these experiments. The effect of the H2O2 concentration on TOC removal in 120 min at 50 °C was found to be more intense at higher initial PEG concentration (Figure 4a) than at lower initial PEG concentration (Figure 4b). Nearly complete TOC removal was observed in 120 min for the lower initial TOC concentration and the higher H2O2 concentration. As expected, temperature had a positive effect.

PEG (mg of C L-1) Fe2+ (mM) H2O2 (mM)

1000 0.5 60

1000 1.0 110

1000 30 60

1000 40 110

H2O2/UV PEG (mg of C L-1) T (°C) H2O2 (mM)

In these experiments, the pH of the solution was monitored as a function of reaction time, as shown in Figure 5. For the higher initial PEG concentration, a tendency for decreasing pH was observed, possibly associated with the formation of acidic organic compounds that remained in the system, because, at the end of these experiments, the TOC values were relatively high (Figure 4a). The pH tended to stabilize at values of around 3, except for the experiments with the H2O2 concentration at the higher level, for which a slight tendency for the pH to increase from the value of 3 was observed at longer reaction times. For the lower initial PEG concentration, after the initial pH decrease to around 3, the pH tended to increase again after about 40 min of reaction. This increase in pH was possibly caused by the degradation of organic acidic compounds, because, in these experiments, the TOC/TOC0 ratio reached lower values (Figure 4b). A more detailed investigation into possible intermediate compounds formed in the degradation process was carried out using two PEG samples with different specified average molecular weights (Mw), namely, 6000 and 20000 g mol-1. Based on the results shown so far, experiments were carried out under the conditions listed in Table 2. Condition PC corresponds to the central point in the experimental design (Table 1). Condition P1 has the same initial polymer concentration as condition PC, but with the variables at the lower level in order to lower the degradation rate and thus favor the identification of intermediate species by means

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Figure 6. TOC as a function of reaction time for (a) the photo-Fenton process and (b) the H2O2/UV process. (c) pH as a function of reaction time for the H2O2/UV process. (b) PC, PEG 6000 g mol-1; (]) PC, PEG 20000 g mol-1; (∆) P1, PEG 6000 g mol-1; (-) P1, PEG 20000 g mol-1. Figure 8. Concentration-time plots of acids detected during photodegradation of PEG by the H2O2/UV process. Acids: ([) formic, (b) acetic, (×) malonic.

Figure 7. Concentration-time plots of acids detected during oxidation of PEG by the photo-Fenton process. Acids: ([) formic, (b) acetic, (O) glycolic.

of HPLC analyses carried out with samples collected as a function of reaction time. The TOC-time and pH-time plots for these experiments are shown in Figure 6. As expected, TOC removal was lower under condition P1 than under condition PC for both processes and both Mw values. No difference in the performance of the two processes was observed under condition P1. TOC removal in 120 min was ca. 14% higher for PEG 6000 g mol-1 under condition PC for both processes. For experiments under condition P1, the pH decreased with time and remained constant after ca. 60 min of reaction. Under condition PC, the pH initially decreased but then started to increase after ca. 60 min of reaction. Figure 7 shows concentration-time plots for the organic acids that were monitored in the experiments with the photo-Fenton process under conditions PC and P1. Formic, acetic, and glycolic

acids were detected for both Mw values and under both experimental conditions. Formic acid reached a maximum concentration in all experimental conditions tested. For condition PC, this maximum was observed at shorter times for PEG 6000. Glycolic acid showed a less-defined maximum and much lower concentrations than the other acids. The acetic acid concentration increased over reaction time, reaching higher levels for PEG 20000. For condition P1, similar concentration-time profiles were obtained, but no significant influence of the PEG molecular weight was observed. Under this condition, acetic, formic, and glycolic acids remained in the solution at the end of the experiments. Figure 8 shows concentration-time plots of the acids that were monitored in the experiments with the H2O2/UV process. Formic and acetic acids reached larger concentrations than the other acids, but in this case, malonic acid was detected. For condition PC, the maximum formic acid concentration was observed at shorter reaction times for PEG 6000 than for PEG 20000. In this case, formic and malonic acids reached maximum concentrations and then were completely degraded in ca. 100 min of reaction. For condition P1, the same acids were detected, but the maximum formic acid concentration was observed at longer reaction times; acetic and formic acids remained in the solution at the end of the experiments. The presence of different organic acids as intermediate species in the photo-Fenton and H2O2/UV processes is an indication of different mechanisms in the two cases. Under the experimental conditions employed, acetic acid accumulation over time was observed in all experiments, even under favorable conditions for TOC removal, such as condition PC. Mantzavinos et al.7 reported the presence of a fairly high acetic acid concentration under wet-air PEG oxidation at 513 K and 2 MPa, even after 60 min of reaction. In the present study, formic and glycolic acids were also detected after 120 min of reaction with the photo-Fenton process under condition P1. This confirms previous observations by other authors24-27 that these organic acids might be recalcitrant to oxidation. Under condition PC, however, these acids tended to disappear in ca. 100 min of reaction. As


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conventional operations, such as filtration, which contributes to TOC removal from the liquid phase.19 Different organic acids were formed as a consequence of different reaction paths, although the main acids detected were acetic and formic acid for both processes. Glycolic acid was detected in the photo-Fenton process only. Malonic acid was detected in relatively small concentrations in the H2O2/UV treatment, only. Under condition PC, the product solutions of both treatment processes consisted mainly of acetic acid, a favorable situation in terms of further treatment of the solution by conventional means such as biological processes. The reported experimental results constitute a contribution to the design of industrial processes for the treatment of wastewaters containing soluble polymers with similar properties. Acknowledgment The authors acknowledge support from CAPES (Ministry of Education, Brazilian Federal Government) and FAPESP (Research Support Foundation, Saõ Paulo, Brazil).

Figure 9. TOC-time profiles. TOC measured: ([) photo-Fenton, (0) H2O2/ UV. TOC computed from measured acids: (b) photo-Fenton, (∆) H2O2/ UV.

proposed by Mantzavinos et al.,8 acetic acid can be formed by ethylene glycol dehydration, whereas malonic acid can be formed as a result of hydrogen abstraction and oxidation to diethylene glycol. Leitner and Dore´28 proposed a mechanism to explain formic acid formation as a product of glycolic acid attack by hydroxyl radicals. This is in accordance with observations for the photo-Fenton process in the present study, because, under condition P1, a higher concentration of glycolic acid was observed. Formic acid can also be formed by oxidation of formaldehyde or as a result of oxalic and glyoxylic acid attack by hydroxyl radicals. However, these acids were not detected in significant quantities in comparison to the other acids. Figure 9 shows time plots of the measured TOC and computed TOC values, based on the total measured concentration of the organic acids detected. The difference between the two sets of values decreases gradually with time. The difference is due to the presence of PEG fractions of lower molecular weights, oligomers, and unidentified intermediate compounds. The evolution of the TOC-time profiles is more affected by the PEG molecular weight and degradation conditions than by the choice of degradation process itself. Conclusions Experiments on photodegradation of poly(ethylene glycol) in water solutions by the photo-Fenton and H2O2/UV process routes indicate that both are technically feasible alternatives to treat wastewaters containing PEG. This study has evidenced features of each route that are important in the design of degradation processes at the industrial scale. Although similar TOC-time profiles were obtained in batch experiments for both processes, important differences were observed, as detailed below. The addition of iron salt to the solution in the photo-Fenton treatment is an important factor to be considered, because additional treatment might be needed for iron removal prior to effluent discharge or reuse. The observed formation of particles in the reactor system can contribute to the removal of organic compounds, because these solid particles can be separated by

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ReceiVed for reView October 9, 2009 ReVised manuscript receiVed January 7, 2010 Accepted February 10, 2010 IE9015792

Degradation of Poly(ethylene glycol) in Aqueous Solution by Photo

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