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Photo-Fenton removal of water-soluble polymers
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Chemical Engineering and Processing 47 (2008) 2361–2369
Photo-Fenton removal of water-soluble polymers J.A. Giroto, A.C.S.C. Teixeira, C.A.O. Nascimento, R. Guardani ∗ University of S˜ao Paulo, Chemical Engineering Department, S˜ao Paulo SP, Brazil Received 1 June 2007; received in revised form 24 January 2008; accepted 26 January 2008 Available online 7 February 2008
Abstract This paper presents the results of experiments carried out in a laboratory-scale photochemical reactor on the photodegradation of different polymers in aqueous solutions by the photo-Fenton process. Solutions of three polymers, polyethyleneglicol (PEG), polyacrylamide (PAM), and polyvinylpyrrolidone (PVP), were tested under different conditions. The reaction progress was evaluated by sampling and analyzing the total organic carbon concentration in solution (TOC) along the reaction time. The behavior of the different polymers is discussed, based on the evolution of the TOC–time curves. Under specific reaction conditions, the formation and coalescence of solid particles was visually observed. Solids formation occurred simultaneously to a sharp decrease in the TOC of the liquid phase. This may be favorable for the treatment of industrial wastewater containing polymers, since the photodegradation process can be coupled with solid separation systems, which may reduce the treatment cost. © 2008 Elsevier B.V. All rights reserved. Keywords: Photodegradation; Polymers; Photo-Fenton process; Industrial wastewater; Polyethyleneglicol (PEG); Polyacrylamide (PAM); Polyvinylpyrrolidone (PVP)
1. Introduction Water-soluble polymers are produced in large scale and used in a number of industrial sectors, like pharmaceutical, textile, paper, plastics and others. Polyethyleneglicol (PEG) is widely used in the production of surfactants, explosives, cosmetics, lubricants and in heat transfer fluids [1]. Polyacrylamide (PAM) is also a versatile chemical, used as an additive to modify surface and viscosity properties of a number of substances of industrial importance [2]. Polyvinylpyrrolidone (PVP) is used in cosmetics, pharmaceutics, or as an impermeability agent in building materials like cement and wood [3]. In many industrial facilities that produce or deal with these polymers they can be present as contaminants in wastewaters, either in discarded process water or as a result of washing of equipments or containers. Since these polymers are not readily biodegradable, they can adsorb in suspended particles and be present in wastewater streams that are commonly discarded to the environment, thus contaminating water resources. Due to their low concentration, it is not technically and economically feasible to recover polymers contained ∗ Corresponding author at: University of S˜ ao Paulo, Chemical Engineering Department, Av. Luciano Gualberto 380 Tv. 3, 05508-900 S˜ao Paulo SP, Brazil. Tel.: +55 11 3091 1169; fax: +55 11 3813 2380. E-mail address: [email protected] (R. Guardani).
0255-2701/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2008.01.014
in wastewaters. Remediation is therefore necessary, normally involving adsorption followed by landfill disposal or incineration [4], a procedure that can contaminate underground water or generate other toxic materials. Biological degradation of water-soluble polymers in industrial wastewater depends on the selection of microorganisms that are able to degrade specific polymers, a subject that is presently under study by a number of research teams. Studies with PEG were carried out by Haines and Alexander [5], Watson and Jones [6], and by Dwyer and Tiedje [7], showing that it is possible to biologically degrade this polymer if the molecular weight is low. Thus, biological treatment can be adopted as a final step in the degradation of wastewaters containing PEG, if an efficient pre-treatment technique is applied. Nakamiya and Kinoshita [8] developed a study aimed at selecting bacteria for the degradation of PAM, concluding that about 20% of the total organic carbon (TOC) in the initial medium had been consumed after 27 h cultivation with isolated bacteria strains. As a result of degradation the average molecular weight of PAM was reduced from 2 × 106 to 0.5 × 106 . Trimpin et al. [9] published the results of a study to characterize the recalcitrance of PVP, concluding that this polymer is poorly degradable and the removal from aqueous solution can be attained by adsorption onto sludge particles. Studies have also been published on the chemical degradation of water-soluble polymers by reaction with strong oxidants like
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ozone, persulfates, H2 O2 , Fenton reactants, or by reaction under the presence of enzimes. McGinnis et al. [10] studied the degradation of ethyleneglicol, one of the possible products of PEG degradation, at concentrations ranging from 50 to 1000 mg L−1 by the photo-Fenton and UV/H2 O2 processes, concluding that the later is more effective, and that the process determining step is the hydroxyl radicals generation. Suzuki et al. [11] studied the photodegradation of PAM solutions at concentrations of 0.1 and 1% (mass basis), with ozone under UV radiation, showing that about 80% of the TOC had been removed after 7.5 h. The photodegradation of aqueous PVP solutions and other polymers was studied by Kaczmarek et al. [12] under UV radiation, using H2 O2 or FeCl3 as oxidizing compounds. 96% decrease in the molecular weight of PVP was attained with H2 O2 after 4 h, and the process was favored by the formation of gel. A study on the photodegradation of PVP using suspended TiO2 -based catalyst particles and UV radiation was carried out by Horikoshi et al. [3], showing a significant reduction of the molecular weight. However, intermediary compounds formed in the process, like amines and propanoic acid, could not be further photodegraded, probably due to adsorption of the main polymer molecules on the surface of catalyst particles. In the present study, laboratory scale photodegradation experiments were carried out with aqueous solutions of the three polymers, PEG, PAM and PVP, under similar experimental setup and conditions. The study was aimed at comparing the behavior of these widely used water-soluble polymers relative to photodegradation by the photo-Fenton process. According to a simplified mechanism, in this process Fe(II) is oxidized to Fe(III), and H2 O2 is reduced to hydroxide anions and hydroxyl radicals (HO• ), as described in reaction (1): Fe(II) + H2 O2 → Fe(III) + HO• + OH−
(1)
Fe(III) can be reduced back to Fe(II) by H2 O2 according to: Fe(III) + H2 O2 + H2 O → Fe(II) + H3 O+ + HO2 •−
(2)
A more detailed description of these reactions, considering hydrated iron–H2 O2 complexes, including rate constants and redox potentials is found in the literature [13]. The thermal reduction given by reaction (2) is much slower than reaction (1), and determines the overall process rate. On the other hand, the Fenton reaction can be strongly accelerated by irradiation with UV–vis radiation, improving degradation rates [13]. In the 2.8–3.2 pH range the aqueous complex Fe(OH)2+ absorbs light with wavelengths up to 410 nm, yielding HO• radicals, and photochemically regenerating Fe(II): hν
Fe(OH)2+ −→Fe(II) + HO•
(3)
2. Experimental The following polymer samples were used: PEG 6000 (Vetec), PAM 50% (Aldrich), and PVP (Vetec). The mean molar weights of PEG and PAM as measured by gel permeation chromatography are, respectively, 5700 and 20,200 g mol−1 . The mean molar weight of PVP is 122,700 g mol−1 , as measured by
viscometry. 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. The photochemical reactor consists of a 0.8-L borosilicate glass vessel connected to a 2-L circulation tank. The reactor is equipped with an internal quartz glass well where a 400-W medium pressure mercury vapor lamp (Philips HPLN, without the external bulb) is placed [14]. With the internals in place the reactor volume is 0.5 L. The circulation tank is made in borosilicate glass, and is equipped with an external jacket connected to a thermostatic bath, for temperature control. This tank is provided with a mechanical stirrer, temperature and pH indication and openings at the top for feeding reactants and taking samples. Circulation of the solution between the two tanks is made by a centrifugal pump at a rate of 2.4 L min−1 . H2 O2 was continuously fed to the tank under controlled flow rate by means of a peristaltic pump (Ismatec-IPC). A scheme of the reactor system is shown in a previous paper by the authors [15]. The experimental procedure involved the preparation of aqueous solutions of iron sulfate and polymer at the desired concentrations. Polymer solutions were prepared by gradually dissolving small amounts 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 feeding the solution to the reactor, the centrifugal pump for circulation was started, and the pH was adjusted at the value of 3 (optimal for the photo-Fenton process, see, e.g. Lipczynska-Kochany [16]), by adding H2 SO4 . After temperature stabilization the lamp was turned on, the Fe(II) solution was fed, and H2 O2 feeding and time measurement were started, with collection of the first sample. In all experiments the feeding of H2 O2 solution was interrupted 30 min before the reaction was stopped. Thus, during the last 30 min in each experiment the system operated in batch regime. A solution containing KI, Na2 SO3 and NaOH (each 0.1 M) was added to the samples for the TOC analysis in the proportion 3:2 v/v (sample:solution), based on Lei et al. [17]. This quenching solution enabled the decomposition of residual H2 O2 and the precipitation of Fe(II)/Fe(III) ions. All samples were then filtered through a 0.22-m membrane (Millipore, Millex GV), and the filtrate was taken to a TOC analyzer (Shimadzu 5000A). Samples containing suspended particles were vacuum filtered and centrifuged prior to the analysis procedure. A complete centered two-level factorial experimental design was adopted in the experiments with each polymer. Selection of the concentration levels of the aqueous polymer solutions and of the photo-Fenton reactants was based on previous studies by the present authors on the photo-Fenton degradation of polyvinyl alcohol [15], and on the paper by Lei et al. [17]. The nominal values of polymer concentration were 500, 1000, and 1500 mgC L−1 , corresponding to the “−”, “0” and “+” levels, respectively. The selection of Fe(II) and H2 O2 levels was based on results of preliminary trial experiments carried out with each polymer solution. In these experiments, the amount of Fe(II) was varied from 1/20 of the number of moles of monomer units in the solution. The amount of H2 O2 was varied from the stoichio-
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Table 1 Reactants levels adopted in the experiments Levels
− 0 +
PEG (T = 30 ◦ C)
PAM (T = 50 ◦ C)
PVP (T = 50 ◦ C)
[Fe(II)] (mM)
[H2 O2 ] (mM)
[Fe(II)] (mM)
[H2 O2 ] (mM)
[Fe(II)] (mM)
[H2 O2 ] (mM)
0.5 1.0 1.5
60 110 160
0.5 1.5 2.5
300 500 700
0.5 1.0 1.5
150 200 250
metric ratio based on the number of moles of monomer units in solution. From the trial experiments the “0”-level amounts of Fe(II) and H2 O2 were selected so that approximately 50% of the TOC initially present in the solution were removed in 1 h of reaction. Table 1 presents the Fe(II), H2 O2 and temperature levels for the experiments with each polymer. In Table 1, [Fe(II)] corresponds to the number of Fe(II) moles added to the system divided by the total volume of solution at the end of each experiment (2 L). [H2 O2 ] corresponds to the total number of H2 O2 moles fed to the reactor divided by the total volume of solution at the end of each experiment (2 L).
3. Experimental results For each polymer solution, the repeatability of the experiments was estimated by performing 3 repetitions of the central point. The mean deviation of the TOC–time values obtained for each polymer solution was: 39.75 mgC L−1 for PEG, 65.2 mgC L−1 for PAM, and 35.4 mgC L−1 for PVP. TOC–time curves for these experiments are shown in Fig. 1. The plots show a first period during which the TOC decrease rate is relatively low, followed by a period of acceleration, and then a period of stabilization near the end of the experiments. This behavior was also observed in other experiments, and is apparently associated with the formation and coalescence of solid particles. Solid particles formation apparently causes a pronounced decrease in TOC, since mineralization occurs simultaneously to the transfer of organic compounds to the solid phase. This phenomenon had been previously described by the present authors in photodegradation experiments with aqueous solutions of polyvinyl alcohol [15]. Previous results obtained by Ara˜na et al. [18,19] with the degradation of highly concentrated phenolic waste water by the photo-Fenton reaction indicated that solid particles formed in the process, consisting of tannin-like polymers may have been formed in the presence of iron ions from degradation intermediate compounds like catechol and pyrogallol, when these are present in high concentration. In the present study, attempts to quantify the amount of solids formed were unsuccessful, due to the adhesion of the solids to all parts of the reaction system. In order to evaluate the effect of the variables on the TOC removal and solids formation, blank experiments were carried out for the three polymers under conditions close to the central point for 60–210 min of reaction. UV/H2 O2 experiments showed no solids formation, although the TOC reduction attained equivalent values compared to the photo-Fenton experiments shown in Fig. 1. No TOC removal or solids formation was observed in
Fig. 1. TOC – time curves for experiments at the central point. Levels: [Pol]: 1000 mg C L−1 ; [Fe(II)]: 1.0 mM (PEG), 1.5 mM (PAM) and 1.0 mM (PVP); [H2 O2 ]: 110 mM (PEG), 500 mM (PAM) and 200 mM (PVP).
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Fig. 2. Removed TOC over time for the experiments with PEG. Order of the “+” or “−” signs: [Pol] [Fe(II)] [H2 O2 ]. Levels: [Pol]: 500 (−) and 1500 (+) mg C L−1 (nominal values); [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 60 (−) and 160 (+) mM.
experiments carried out in the presence of Fe(II) only. Finally, experiments with the Fenton reactants carried out without light showed a slight decrease in TOC (ca. 3–4% removal) and formation of considerably smaller amounts of solid particles compared with the photo-Fenton experiments. These observations, although preliminary, add evidence to the role played by iron ions in the formation of solid particles, as suggested by Ara˜na et al. [18,19]. In the experiments with PAM a tendency of pH to increase with time was observed. In this case the pH was kept at the value of 3 by constant correction along the reaction time. This tendency had been observed by Suzuki et al. [11] in aqueous solutions of PAM after ozonization under UV radiation. According to these authors, the pH increase is related to nitrate formation due to the oxidation of amide groups. According to Caufield et al. [20], hydrolysis of amide side groups in PAM can also result in the liberation of ammonium ions to the solution. Plots of removed TOC over time for the planned experiments with solutions of the three polymers are shown in Figs. 2–4. For solutions with initial polymer concentration, [Pol], at the “−” level, complete removal of the TOC was attained after ca. 90 min for PEG, 150 min for PAM, and 75 min for PVP, independently from the levels of [H2 O2 ] and [Fe(II)]. For experiments with [Pol] at the “+” level, the amount of H2 O2 added to the system is apparently the most important factor affecting the TOC removal. The intensity of this effect, however, was different for different polymer solutions. Experiments with [H2 O2 ] at the “−” level resulted in lower TOC removal rates for both polymer concentration levels. This reactant was apparently missing in the experiments with PEG and PVP with [Pol] at the “+” level. In most experiments the [Fe(II)] level negatively affected the TOC removal rate. The effect of [Fe(II)], however, deserves further discussion, since the results were apparently affected by interactions of [Fe(II)], [H2 O2 ] and the polymers. In order to compare the behavior of different TOC – time curves, the TOC values were normalized with respect to the initial concentration. The results are shown in Figs. 5–7.
Fig. 3. Removed TOC over time for the experiments with PAM. Order of the “+” or “−” signs: [Pol] [Fe(II)] [H2 O2 ]. Levels: [Pol]: 500 (−) and 1500 (+) mg C L−1 (nominal values); [Fe(II)]: 0.5 (−) and 2.5 (+) mM; [H2 O2 ]: 300 (−) and 700 (+) mM.
Fig. 5a shows TOC/TOC0 -time results for PEG solutions at the “+” level (1689 ± 104 mgC L−1 ). In these experiments, no significant effect of [Fe(II)] was observed. When [H2 O2 ] is at the “+” level, both curves show an acceleration in the TOC decrease rate after ca. 45 min of reaction time, reaching 69% and 63% TOC removals after 120 min for [Fe(II)] at the “+” and “−” levels, respectively. When [H2 O2 ] is at the “−” level, ca. 20% TOC removal was attained after 120 min of reaction for both [Fe(II)] levels. The slower decrease of both curves after 90 min of reaction is associated with the end of H2 O2 addition. In Fig. 5b, for PEG solutions at the “−” level, the three periods of reaction can be observed when [H2 O2 ] is at the “+” level. The TOC removal reached 95% in 75 min and ca. 100% in 120 min of reaction for both [Fe(II)] levels. The sharp decrease in TOC concentration after ca. 30 min is apparently due to the formation of solid particles. The increase in [Fe(II)] affects negatively the TOC removal when [H2 O2 ] is at the “−” level. At this lower
Fig. 4. Removed TOC over time for the planned experiments with PVP. Order of the “+” or “−” signs: [Pol] [Fe(II)] [H2 O2 ]. Levels: [Pol]: 500 (−) and 1500 (+) mg C L−1 (nominal values); [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 150 (−) and 250 (+) mM.
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Fig. 5. TOC/TOC0 over reaction time for PEG with initial concentration (a) 1689 ± 104 mgC L−1 and (b) 592 ± 17 mgC L−1 . + +; + −; − +; 䊉 − − (order of signs: [Fe(II)] [H2 O2 ]). Levels: [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 60 (−) and 160 (+) mM.
[H2 O2 ] level, TOC removal is significantly lower: 92% and 67% after 120 min for [Fe(II)] at the “−” and “+” levels, respectively. Fig. 6 shows TOC/TOC0 -time plots for the experiments with PAM. The sharp TOC decrease observed in all experiments is associated with the formation of solid particles. The effect of particle formation was especially pronounced in the experiment with PAM and [Fe(II)] at their “+” level, and [H2 O2 ] at the “−” level (Fig. 6a). Under these conditions, the TOC removal was 20% in 120 min, then there was a sharp change to 67% in 150 min, and after this time there was a tendency to stabilization of the TOC concentration, associated with the end of H2 O2 addition at 150 min. The other TOC/TOC0 -time curves in Fig. 6a have similar aspect, with ca. 94% TOC removal in 180 min of reaction, indicating a positive effect of [H2 O2 ], and a slight negative effect of [Fe(II)] on the TOC removal profiles. For solutions with PAM concentration at the “−” level (Fig. 6b), ca. 95% TOC removal was observed in 180 min of reaction. For these experiments,
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Fig. 6. TOC/TOC0 over reaction time for PAM with initial concentration (a) (1477 ± 104) mgC L−1 and (b) (538 ± 49) mgC L−1 . + +; + −; − +; 䊉 − − (order of signs: [Fe(II)] [H2 O2 ]). Levels: [Fe(II)]: 0.5 (−) and 2.5 (+) mM; [H2 O2 ]: 300 (−) and 700 (+) mM.
higher TOC removal was observed when [Fe(II)] was at the “−” level. TOC/TOC0 -time results for PVP solutions are shown in Fig. 7. The sharp decrease in TOC is associated with the formation of solid particles, which took place in all experiments. As observed for solutions of PEG and PAM, the effect of [Fe(II)] depended on the polymer and [H2 O2 ] levels. The effect of [H2 O2 ] apparently depended on the [PVP]:[H2 O2 ] ratio. Complete TOC removal was achieved in ca. 60 min of reaction for both PVP and [H2 O2 ] at the “−” level (Fig. 7b). 4. Analysis of the results As shown in Figs. 5–7, the process variables considered in the study had different effects on the TOC removal profiles for different experimental conditions. The observed occurrence of three time periods in the degradation process depended on the reactants concentrations, and on the initial polymer concentration. In order to identify the relative importance of the factors considered in the study, a statistical analysis was carried out based on a response methodology [21]. The following responses
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Fig. 8. Illustration of the response variables adopted in the statistical analysis.
Fig. 7. TOC/TOC0 over reaction time for PVP with initial concentration (a) (1591 ± 53) mgC L−1 and (b) (546 ± 16) mgC L−1 . + +; + −; − +; 䊉 − − (order of signs: Fe(II) [H2 O2 ]). Levels: [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 150 (−) and 250 (+) mM.
were selected, as a means to characterize the TOC–time profiles obtained: T1 : time corresponding to the first step of the process, characterized by a low TOC decrease rate; RAv : mean degradation rate during the H2 O2 addition time; Rmax : maximum degradation rate; and %Rem: TOC removal during the whole reaction time. The values of these responses were obtained directly from the TOC–time plots as illustrated in Fig. 8. For each polymer solution, the following variables were considered in the statistical model: [Pol] (initial concentration of each polymer), [Fe(II)], and [H2 O2 ], denoted x1 , x2 , x3 , respectively. The adopted statistical model is shown in Eq. (4): yˆ (x1 , x2 , x3 ) = b0 + b1 x1 + b2 x2 + b3 x3 + b12 x1 x2 + b13 x1 x3 + b23 x2 x3 + b123 x1 x2 x3 ,
(4)
in which yˆ (x1 , x2 , x3 ) is the estimated value a given response as a function of the variables x1 , x2 , x3 , codified as −1, 0, or +1. Coefficients bi correspond to the ith factor, which can be represented by a specific variable, or an interaction of variables (for example, b123 ). b0 is the independent term in the model. A statistical computer package (Statgraphics® plus 3.0) was used
to fit the model to the data. Table 2 shows the values of the coefficient of determination corresponding to the fitting of Eq. (4) to the experimental results with each polymer solution. The quality of the fitting was good, with coefficient of determination above 0.9, except in the case of T1 for the experiments with PEG. This reflects the larger difficulty in measuring T1 in this case, due to the less clear initial plateau for some combinations of the levels of the variables. The relative importance of the factors, expressed by the coefficient values in the statistical model, can be more clearly visualized by means of Pareto charts, as shown in Figs. 9–11, in which the length of each bar corresponds to the factor coefficient divided by the standard deviation of the responses (standardized). During time T1 the TOC value remained high, and the Pareto charts for the three polymer solutions indicate that the polymer concentration had the largest positive effect on this response, while [H2 O2 ] had a negative effect, as expected. [Fe(II)] had a significant positive effect for PAM and PVP, possibly due to the formation of organic complexes with iron, as suggested by other authors for the degradation of polyvinyl alcohol [17,22]. According to those authors, these complexes can be less susceptible to the attack by hydroxil radicals. Based on the observed results of this study, the complexes possibly combined and coalesced as solid particles after time T1 . On the other hand, high H2 O2 concentration may have favored the formation of oxidized products composed of smaller molecules that were more easily mineralized, thus reducing T1 . For the three polymer solutions, the fraction of initial TOC removed, %Rem, was negatively affected by the initial polymer concentration. This was expected, since higher amounts of reactants are needed at higher polymer concentration to attain a given fraction of the initial TOC removed. %Rem was positively Table 2 Coefficient of determination for the fitting of the statistical model (Eq. (4))
RAv Rmax %Rem T1
PEG
PAM
PVP
0.942 0.912 0.997 0.801
0.989 0.906 0.989 0.963
0.960 0.977 0.906 0.992
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Fig. 9. Standardized Pareto charts for PEG.
Fig. 10. Standardized Pareto charts for PAM.
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Fig. 11. Standardized Pareto charts for PVP.
affected [H2 O2 ] and negatively affected by [Fe(II)]. Depending on the [H2 O2 ] level, particle formation and coalescence was observed, which may have favored TOC removal, since mass was transferred from the solution to the solid phase. In the Pareto charts, the [H2 O2 ]–[Fe(II)] interaction was positive for all polymer solutions, as expected for the photo-Fenton system. When the average TOC decrease rate over the H2 O2 addition time, RAv , is considered, [Pol] and [H2 O2 ] affected positively this response for the three polymer solutions, since the average reaction rate depends on the concentration of the reactants. [Fe(II)] affected this response negatively, although this component takes part in the reaction system. The explanation for this effect may be the same as in the case of T1 , since higher [Fe(II)] may have favored the formation of complexes with organic molecules during the initial period of reaction. The effects are less clear when the maximum TOC decrease rate, Rmax , is considered. The Pareto charts show negative or positive effects of the factors, depending on the polymer solution. This is probably due to the low number of experimental measurements during the relatively short period of solids formation. Further investigation is necessary to clarify the phase separation process, which involves particle formation and coalescence, a relatively fast process in most systems. As shown in Figs. 9–11, the effects of interactions among the variables depended on the polymer solution, possibly due to the differences in reactant ratios adopted in each case, as well as to the differences in polymer structure.
5. Conclusions The experimental results indicate that it is technically feasible to degrade polymers dissolved in water by the photo-Fenton process, for aqueous solutions of the three polymers considered in the study, PEG, PAM, and PVP, with concentrations up to 1500 mgC L−1 . Depending on the levels of Fe(II) and H2 O2 , total mineralization of the organic compounds was achieved in batch experiments, in ca. two to three hours of reaction. The set of experimental conditions adopted in this study enabled to observe the effect of [Pol], [H2 O2 ], and [Fe(II)] on the characteristics of the TOC–time curves in batch experiments. The differences in the relative importance of the factors concerning the responses can be associated with differences in the intermediate degradation compounds. Concerning the application of the photo-Fenton process to TOC removal from aqueous solutions of the polymers included in this study, two main aspects deserve attention. Although Fe(II) is one of the components of the reaction system, its effect was negative for all polymer solutions. An explanation for this can be based on the discussions by Lei et al. [17] and Bossmann et al. [22], who carried out studies on the photo-Fenton degradation of polyvinyl alcohol (PVA) in aqueous solutions. These authors suggested that complexation of iron with PVA and intermediate products of PVA degradation can take place, thus preventing H2 O2 to approach and complex with the metal centers. According to Bossmann et al. [22], this effect may be a consequence of a poor efficiency of photoreduction of Fe(III) as it is bound within large organic molecules. The second important aspect is related
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to the formation of solid particles, which had been observed in a previous study by the present authors with polyvinyl alcohol [15] and by other authors [18,19]. The formation of solid particles at lower H2 O2 levels can be advantageous, because the solid particles can be separated by conventional operations, like filtration, resulting in significantly lower TOC in the liquid phase. Thus, the integration of photodegradation and separation processes should be considered in feasibility studies associated with process design. Acknowledgments The authors wish to thank the support by CAPES (Coordenac¸a˜ o de Aperfeic¸oamento de Pessoal de N´ıvel Superior), CNPq (Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico) and FAPESP (Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo). References [1] D. Mantzavinos, A.G. Livingstone, R. Hellenbrand, I.S. Metcalfe, Wet air oxidation of polyethylene glycols; mechanisms. Intermediates and implications for integrated chemical–biological wastewater treatment, Chem. Eng. Sci. 51 (1996) 4219–4235. [2] L.M.V. Vers, Determination of acrylamide monomer in polyacrylamide degradation studies by high-performance liquid chromatography, J. Chromatogr. Sci. 37 (1999) 486–494. [3] S. Horikoshi, H. Hidaka, N. Serpone, Photocatalyzed degradation of polymers in aqueous semiconductor suspensions V. Photomineralization of lactam ring-pendant polyvinylpyrrolidone at titania/water interfaces, J. Photochem. Photobiol. A: Chem. 138 (2001) 69–77. [4] G. Swift, Requirements for biodegradable water-soluble polymers, Polym. Degrad. Stab. 59 (1998) 19–24. [5] J.R. Haines, M. Alexander, Microbial degradation of polyethylene glycols, Appl. Microb. 29 (5) (1975) 621–625. [6] K.G. Watson, N. Jones, The biodegradation of polyethylene glycols by sewage bacteria, Water Res. 11 (1977) 95–100. [7] D. Dwyer, J.M. Tiedje, Degradation of polyethylene glycol and polyethylene glycols by Methanogenic Consortia, Appl. Environ. Microb. 46 (1) (1983) 185–190. [8] K. Nakamiya, S. Kinoshita, Isolation of polyacrylamide-degrading bacteria, J. Ferment. Bioeng. 80 (4) (1995) 418–420.
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[9] S. Trimpin, P. Eichhorn, H.J. Rader, K. Mullen, T.P. Knepper, Recalcitrance of poly(vinylpyrrolidone): evidence through matrix-assisted laser desorption-ionization time-of-flight mass spectrometry., J. Chromatogr. A 938 (1/2) (2001) 67–77. [10] D.B. McGinnis, D.V. Adams, J.E. Middlebrooks, Degradation of ethylene glycols in photo-Fenton systems, Water Res. 34 (8) (2000) 2346–2354. [11] J. Suzuki, H. Harada, S. Suzuki, Ozone treatment of water soluble polymers V. Ultraviolet irradiation effects on the ozonization of polyacrylamide, J. Appl. Polym. Sci. 24 (1979) 999–1006. [12] H. Kaczmarek, A. Kaminska, M. Swiatek, J.F. Rabek, Photo-oxidative degradation of some water-soluble polymers in the presence of accelerating agents, Angew. Makromol. Chem. 261/262 (4622) (1998) 109–121. [13] S.H. Bossmann, E. Oliveros, S. G¨ob, S. Siegwart, E.P. Dahlen, L. Payawan, M. Straub, M. W¨orner, A.M. Braun, New evidence against hydroxyl radicals as reactive intermediate in the thermal and photochemically enhanced Fenton reaction, J. Phys. Chem. A 102 (1998) 5542–5550. [14] M.A. De Paolli, C. Rodrigues, Aparelhagem para fot´olise de compostos sens´ıveis a O2 e umidade e lˆampada a vapor de merc´urio de alta press˜ao, Qu´ımica Nova 1 (1978) 16–18. [15] J.A. Giroto, A.C.S.C. Teixeira, R. Guardani, C.A.O. Nascimento, Study on the photo-Fenton degradation of polyvinyl alcohol in aqueous solution, Chem. Eng. Proc. 45 (2006) 523–532. [16] E. Lipczynska-Kochany, Degradation of aqueous nitrophenols and nitrobenzenes by means of the Fenton reaction, Chemosphere 22 (1991) 529–536. [17] L. Lei, X. Hu, P.L. Yue, S.H. Bossmann, S. G¨ob, A.M. Braun, Oxidative degradation of polyvinyl alcohol by the photochemically enhanced Fenton reaction, J. Photochem. Photobiol. A: Chem. 116 (1998) 159–166. [18] J. Ara˜na, E. Tello-Rend´on, J.M. Do˜na-Rodr´ıguez, C. Vald´es Do Campo, J.A. Herrera-Meli´an, O. Gonz´alez-D´ıaz, J. P´erez-Pe˜na, Highly concentrated phenolic wastewater treatment by heterogeneous and homogeneous photocatalysis: mechanism study by FTIR-ATR, Water Sci. Technol. 44 (5) (2001) 229–236. [19] J. Ara˜na, E. Tello-Rend´on, J.M. Do˜na-Rodr´ıguez, J.A. Herrera-Meli´an, O. Gonz´alez-D´ıaz, J. P´erez-Pe˜na, Highly concentrated phenolic waste water treatment by the photo-Fenton reaction. Mechanism Study by FTIR-ATR, Chemosphere 44 (5) (2001) 1017–1023. [20] J.M. Caufield, X. Hao, G.G. Qiao, D.H. Solomon, Degradation on polyacrylamides part I: linear polyacrilamide, Polymer 44 (2003) 1331–1337. [21] G.E.P. Box, W.G. Hunter, J.S. Hunter, Statistics for Experimenters: An Introduction to Design. Data Analysis and Model Building, Wiley, New York, 1978. [22] S.H. Bossmann, E. Oliveros, S. G¨ob, M. Kantor, A. G¨oppert, A.M. Braun, L. Lei, P.L. Yue, Oxidative degradation of polyvinyl alcohol by the photochemically enhanced Fenton reaction. Evidence for the formation of super-macromolecules, Prog. Reac. Kinet. Mech. 26 (2001) 113–137.
Chemical Engineering and Processing 47 (2008) 2361–2369
Photo-Fenton removal of water-soluble polymers J.A. Giroto, A.C.S.C. Teixeira, C.A.O. Nascimento, R. Guardani ∗ University of S˜ao Paulo, Chemical Engineering Department, S˜ao Paulo SP, Brazil Received 1 June 2007; received in revised form 24 January 2008; accepted 26 January 2008 Available online 7 February 2008
Abstract This paper presents the results of experiments carried out in a laboratory-scale photochemical reactor on the photodegradation of different polymers in aqueous solutions by the photo-Fenton process. Solutions of three polymers, polyethyleneglicol (PEG), polyacrylamide (PAM), and polyvinylpyrrolidone (PVP), were tested under different conditions. The reaction progress was evaluated by sampling and analyzing the total organic carbon concentration in solution (TOC) along the reaction time. The behavior of the different polymers is discussed, based on the evolution of the TOC–time curves. Under specific reaction conditions, the formation and coalescence of solid particles was visually observed. Solids formation occurred simultaneously to a sharp decrease in the TOC of the liquid phase. This may be favorable for the treatment of industrial wastewater containing polymers, since the photodegradation process can be coupled with solid separation systems, which may reduce the treatment cost. © 2008 Elsevier B.V. All rights reserved. Keywords: Photodegradation; Polymers; Photo-Fenton process; Industrial wastewater; Polyethyleneglicol (PEG); Polyacrylamide (PAM); Polyvinylpyrrolidone (PVP)
1. Introduction Water-soluble polymers are produced in large scale and used in a number of industrial sectors, like pharmaceutical, textile, paper, plastics and others. Polyethyleneglicol (PEG) is widely used in the production of surfactants, explosives, cosmetics, lubricants and in heat transfer fluids [1]. Polyacrylamide (PAM) is also a versatile chemical, used as an additive to modify surface and viscosity properties of a number of substances of industrial importance [2]. Polyvinylpyrrolidone (PVP) is used in cosmetics, pharmaceutics, or as an impermeability agent in building materials like cement and wood [3]. In many industrial facilities that produce or deal with these polymers they can be present as contaminants in wastewaters, either in discarded process water or as a result of washing of equipments or containers. Since these polymers are not readily biodegradable, they can adsorb in suspended particles and be present in wastewater streams that are commonly discarded to the environment, thus contaminating water resources. Due to their low concentration, it is not technically and economically feasible to recover polymers contained ∗ Corresponding author at: University of S˜ ao Paulo, Chemical Engineering Department, Av. Luciano Gualberto 380 Tv. 3, 05508-900 S˜ao Paulo SP, Brazil. Tel.: +55 11 3091 1169; fax: +55 11 3813 2380. E-mail address: [email protected] (R. Guardani).
0255-2701/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2008.01.014
in wastewaters. Remediation is therefore necessary, normally involving adsorption followed by landfill disposal or incineration [4], a procedure that can contaminate underground water or generate other toxic materials. Biological degradation of water-soluble polymers in industrial wastewater depends on the selection of microorganisms that are able to degrade specific polymers, a subject that is presently under study by a number of research teams. Studies with PEG were carried out by Haines and Alexander [5], Watson and Jones [6], and by Dwyer and Tiedje [7], showing that it is possible to biologically degrade this polymer if the molecular weight is low. Thus, biological treatment can be adopted as a final step in the degradation of wastewaters containing PEG, if an efficient pre-treatment technique is applied. Nakamiya and Kinoshita [8] developed a study aimed at selecting bacteria for the degradation of PAM, concluding that about 20% of the total organic carbon (TOC) in the initial medium had been consumed after 27 h cultivation with isolated bacteria strains. As a result of degradation the average molecular weight of PAM was reduced from 2 × 106 to 0.5 × 106 . Trimpin et al. [9] published the results of a study to characterize the recalcitrance of PVP, concluding that this polymer is poorly degradable and the removal from aqueous solution can be attained by adsorption onto sludge particles. Studies have also been published on the chemical degradation of water-soluble polymers by reaction with strong oxidants like
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ozone, persulfates, H2 O2 , Fenton reactants, or by reaction under the presence of enzimes. McGinnis et al. [10] studied the degradation of ethyleneglicol, one of the possible products of PEG degradation, at concentrations ranging from 50 to 1000 mg L−1 by the photo-Fenton and UV/H2 O2 processes, concluding that the later is more effective, and that the process determining step is the hydroxyl radicals generation. Suzuki et al. [11] studied the photodegradation of PAM solutions at concentrations of 0.1 and 1% (mass basis), with ozone under UV radiation, showing that about 80% of the TOC had been removed after 7.5 h. The photodegradation of aqueous PVP solutions and other polymers was studied by Kaczmarek et al. [12] under UV radiation, using H2 O2 or FeCl3 as oxidizing compounds. 96% decrease in the molecular weight of PVP was attained with H2 O2 after 4 h, and the process was favored by the formation of gel. A study on the photodegradation of PVP using suspended TiO2 -based catalyst particles and UV radiation was carried out by Horikoshi et al. [3], showing a significant reduction of the molecular weight. However, intermediary compounds formed in the process, like amines and propanoic acid, could not be further photodegraded, probably due to adsorption of the main polymer molecules on the surface of catalyst particles. In the present study, laboratory scale photodegradation experiments were carried out with aqueous solutions of the three polymers, PEG, PAM and PVP, under similar experimental setup and conditions. The study was aimed at comparing the behavior of these widely used water-soluble polymers relative to photodegradation by the photo-Fenton process. According to a simplified mechanism, in this process Fe(II) is oxidized to Fe(III), and H2 O2 is reduced to hydroxide anions and hydroxyl radicals (HO• ), as described in reaction (1): Fe(II) + H2 O2 → Fe(III) + HO• + OH−
(1)
Fe(III) can be reduced back to Fe(II) by H2 O2 according to: Fe(III) + H2 O2 + H2 O → Fe(II) + H3 O+ + HO2 •−
(2)
A more detailed description of these reactions, considering hydrated iron–H2 O2 complexes, including rate constants and redox potentials is found in the literature [13]. The thermal reduction given by reaction (2) is much slower than reaction (1), and determines the overall process rate. On the other hand, the Fenton reaction can be strongly accelerated by irradiation with UV–vis radiation, improving degradation rates [13]. In the 2.8–3.2 pH range the aqueous complex Fe(OH)2+ absorbs light with wavelengths up to 410 nm, yielding HO• radicals, and photochemically regenerating Fe(II): hν
Fe(OH)2+ −→Fe(II) + HO•
(3)
2. Experimental The following polymer samples were used: PEG 6000 (Vetec), PAM 50% (Aldrich), and PVP (Vetec). The mean molar weights of PEG and PAM as measured by gel permeation chromatography are, respectively, 5700 and 20,200 g mol−1 . The mean molar weight of PVP is 122,700 g mol−1 , as measured by
viscometry. 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. The photochemical reactor consists of a 0.8-L borosilicate glass vessel connected to a 2-L circulation tank. The reactor is equipped with an internal quartz glass well where a 400-W medium pressure mercury vapor lamp (Philips HPLN, without the external bulb) is placed [14]. With the internals in place the reactor volume is 0.5 L. The circulation tank is made in borosilicate glass, and is equipped with an external jacket connected to a thermostatic bath, for temperature control. This tank is provided with a mechanical stirrer, temperature and pH indication and openings at the top for feeding reactants and taking samples. Circulation of the solution between the two tanks is made by a centrifugal pump at a rate of 2.4 L min−1 . H2 O2 was continuously fed to the tank under controlled flow rate by means of a peristaltic pump (Ismatec-IPC). A scheme of the reactor system is shown in a previous paper by the authors [15]. The experimental procedure involved the preparation of aqueous solutions of iron sulfate and polymer at the desired concentrations. Polymer solutions were prepared by gradually dissolving small amounts 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 feeding the solution to the reactor, the centrifugal pump for circulation was started, and the pH was adjusted at the value of 3 (optimal for the photo-Fenton process, see, e.g. Lipczynska-Kochany [16]), by adding H2 SO4 . After temperature stabilization the lamp was turned on, the Fe(II) solution was fed, and H2 O2 feeding and time measurement were started, with collection of the first sample. In all experiments the feeding of H2 O2 solution was interrupted 30 min before the reaction was stopped. Thus, during the last 30 min in each experiment the system operated in batch regime. A solution containing KI, Na2 SO3 and NaOH (each 0.1 M) was added to the samples for the TOC analysis in the proportion 3:2 v/v (sample:solution), based on Lei et al. [17]. This quenching solution enabled the decomposition of residual H2 O2 and the precipitation of Fe(II)/Fe(III) ions. All samples were then filtered through a 0.22-m membrane (Millipore, Millex GV), and the filtrate was taken to a TOC analyzer (Shimadzu 5000A). Samples containing suspended particles were vacuum filtered and centrifuged prior to the analysis procedure. A complete centered two-level factorial experimental design was adopted in the experiments with each polymer. Selection of the concentration levels of the aqueous polymer solutions and of the photo-Fenton reactants was based on previous studies by the present authors on the photo-Fenton degradation of polyvinyl alcohol [15], and on the paper by Lei et al. [17]. The nominal values of polymer concentration were 500, 1000, and 1500 mgC L−1 , corresponding to the “−”, “0” and “+” levels, respectively. The selection of Fe(II) and H2 O2 levels was based on results of preliminary trial experiments carried out with each polymer solution. In these experiments, the amount of Fe(II) was varied from 1/20 of the number of moles of monomer units in the solution. The amount of H2 O2 was varied from the stoichio-
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Table 1 Reactants levels adopted in the experiments Levels
− 0 +
PEG (T = 30 ◦ C)
PAM (T = 50 ◦ C)
PVP (T = 50 ◦ C)
[Fe(II)] (mM)
[H2 O2 ] (mM)
[Fe(II)] (mM)
[H2 O2 ] (mM)
[Fe(II)] (mM)
[H2 O2 ] (mM)
0.5 1.0 1.5
60 110 160
0.5 1.5 2.5
300 500 700
0.5 1.0 1.5
150 200 250
metric ratio based on the number of moles of monomer units in solution. From the trial experiments the “0”-level amounts of Fe(II) and H2 O2 were selected so that approximately 50% of the TOC initially present in the solution were removed in 1 h of reaction. Table 1 presents the Fe(II), H2 O2 and temperature levels for the experiments with each polymer. In Table 1, [Fe(II)] corresponds to the number of Fe(II) moles added to the system divided by the total volume of solution at the end of each experiment (2 L). [H2 O2 ] corresponds to the total number of H2 O2 moles fed to the reactor divided by the total volume of solution at the end of each experiment (2 L).
3. Experimental results For each polymer solution, the repeatability of the experiments was estimated by performing 3 repetitions of the central point. The mean deviation of the TOC–time values obtained for each polymer solution was: 39.75 mgC L−1 for PEG, 65.2 mgC L−1 for PAM, and 35.4 mgC L−1 for PVP. TOC–time curves for these experiments are shown in Fig. 1. The plots show a first period during which the TOC decrease rate is relatively low, followed by a period of acceleration, and then a period of stabilization near the end of the experiments. This behavior was also observed in other experiments, and is apparently associated with the formation and coalescence of solid particles. Solid particles formation apparently causes a pronounced decrease in TOC, since mineralization occurs simultaneously to the transfer of organic compounds to the solid phase. This phenomenon had been previously described by the present authors in photodegradation experiments with aqueous solutions of polyvinyl alcohol [15]. Previous results obtained by Ara˜na et al. [18,19] with the degradation of highly concentrated phenolic waste water by the photo-Fenton reaction indicated that solid particles formed in the process, consisting of tannin-like polymers may have been formed in the presence of iron ions from degradation intermediate compounds like catechol and pyrogallol, when these are present in high concentration. In the present study, attempts to quantify the amount of solids formed were unsuccessful, due to the adhesion of the solids to all parts of the reaction system. In order to evaluate the effect of the variables on the TOC removal and solids formation, blank experiments were carried out for the three polymers under conditions close to the central point for 60–210 min of reaction. UV/H2 O2 experiments showed no solids formation, although the TOC reduction attained equivalent values compared to the photo-Fenton experiments shown in Fig. 1. No TOC removal or solids formation was observed in
Fig. 1. TOC – time curves for experiments at the central point. Levels: [Pol]: 1000 mg C L−1 ; [Fe(II)]: 1.0 mM (PEG), 1.5 mM (PAM) and 1.0 mM (PVP); [H2 O2 ]: 110 mM (PEG), 500 mM (PAM) and 200 mM (PVP).
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Fig. 2. Removed TOC over time for the experiments with PEG. Order of the “+” or “−” signs: [Pol] [Fe(II)] [H2 O2 ]. Levels: [Pol]: 500 (−) and 1500 (+) mg C L−1 (nominal values); [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 60 (−) and 160 (+) mM.
experiments carried out in the presence of Fe(II) only. Finally, experiments with the Fenton reactants carried out without light showed a slight decrease in TOC (ca. 3–4% removal) and formation of considerably smaller amounts of solid particles compared with the photo-Fenton experiments. These observations, although preliminary, add evidence to the role played by iron ions in the formation of solid particles, as suggested by Ara˜na et al. [18,19]. In the experiments with PAM a tendency of pH to increase with time was observed. In this case the pH was kept at the value of 3 by constant correction along the reaction time. This tendency had been observed by Suzuki et al. [11] in aqueous solutions of PAM after ozonization under UV radiation. According to these authors, the pH increase is related to nitrate formation due to the oxidation of amide groups. According to Caufield et al. [20], hydrolysis of amide side groups in PAM can also result in the liberation of ammonium ions to the solution. Plots of removed TOC over time for the planned experiments with solutions of the three polymers are shown in Figs. 2–4. For solutions with initial polymer concentration, [Pol], at the “−” level, complete removal of the TOC was attained after ca. 90 min for PEG, 150 min for PAM, and 75 min for PVP, independently from the levels of [H2 O2 ] and [Fe(II)]. For experiments with [Pol] at the “+” level, the amount of H2 O2 added to the system is apparently the most important factor affecting the TOC removal. The intensity of this effect, however, was different for different polymer solutions. Experiments with [H2 O2 ] at the “−” level resulted in lower TOC removal rates for both polymer concentration levels. This reactant was apparently missing in the experiments with PEG and PVP with [Pol] at the “+” level. In most experiments the [Fe(II)] level negatively affected the TOC removal rate. The effect of [Fe(II)], however, deserves further discussion, since the results were apparently affected by interactions of [Fe(II)], [H2 O2 ] and the polymers. In order to compare the behavior of different TOC – time curves, the TOC values were normalized with respect to the initial concentration. The results are shown in Figs. 5–7.
Fig. 3. Removed TOC over time for the experiments with PAM. Order of the “+” or “−” signs: [Pol] [Fe(II)] [H2 O2 ]. Levels: [Pol]: 500 (−) and 1500 (+) mg C L−1 (nominal values); [Fe(II)]: 0.5 (−) and 2.5 (+) mM; [H2 O2 ]: 300 (−) and 700 (+) mM.
Fig. 5a shows TOC/TOC0 -time results for PEG solutions at the “+” level (1689 ± 104 mgC L−1 ). In these experiments, no significant effect of [Fe(II)] was observed. When [H2 O2 ] is at the “+” level, both curves show an acceleration in the TOC decrease rate after ca. 45 min of reaction time, reaching 69% and 63% TOC removals after 120 min for [Fe(II)] at the “+” and “−” levels, respectively. When [H2 O2 ] is at the “−” level, ca. 20% TOC removal was attained after 120 min of reaction for both [Fe(II)] levels. The slower decrease of both curves after 90 min of reaction is associated with the end of H2 O2 addition. In Fig. 5b, for PEG solutions at the “−” level, the three periods of reaction can be observed when [H2 O2 ] is at the “+” level. The TOC removal reached 95% in 75 min and ca. 100% in 120 min of reaction for both [Fe(II)] levels. The sharp decrease in TOC concentration after ca. 30 min is apparently due to the formation of solid particles. The increase in [Fe(II)] affects negatively the TOC removal when [H2 O2 ] is at the “−” level. At this lower
Fig. 4. Removed TOC over time for the planned experiments with PVP. Order of the “+” or “−” signs: [Pol] [Fe(II)] [H2 O2 ]. Levels: [Pol]: 500 (−) and 1500 (+) mg C L−1 (nominal values); [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 150 (−) and 250 (+) mM.
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Fig. 5. TOC/TOC0 over reaction time for PEG with initial concentration (a) 1689 ± 104 mgC L−1 and (b) 592 ± 17 mgC L−1 . + +; + −; − +; 䊉 − − (order of signs: [Fe(II)] [H2 O2 ]). Levels: [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 60 (−) and 160 (+) mM.
[H2 O2 ] level, TOC removal is significantly lower: 92% and 67% after 120 min for [Fe(II)] at the “−” and “+” levels, respectively. Fig. 6 shows TOC/TOC0 -time plots for the experiments with PAM. The sharp TOC decrease observed in all experiments is associated with the formation of solid particles. The effect of particle formation was especially pronounced in the experiment with PAM and [Fe(II)] at their “+” level, and [H2 O2 ] at the “−” level (Fig. 6a). Under these conditions, the TOC removal was 20% in 120 min, then there was a sharp change to 67% in 150 min, and after this time there was a tendency to stabilization of the TOC concentration, associated with the end of H2 O2 addition at 150 min. The other TOC/TOC0 -time curves in Fig. 6a have similar aspect, with ca. 94% TOC removal in 180 min of reaction, indicating a positive effect of [H2 O2 ], and a slight negative effect of [Fe(II)] on the TOC removal profiles. For solutions with PAM concentration at the “−” level (Fig. 6b), ca. 95% TOC removal was observed in 180 min of reaction. For these experiments,
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Fig. 6. TOC/TOC0 over reaction time for PAM with initial concentration (a) (1477 ± 104) mgC L−1 and (b) (538 ± 49) mgC L−1 . + +; + −; − +; 䊉 − − (order of signs: [Fe(II)] [H2 O2 ]). Levels: [Fe(II)]: 0.5 (−) and 2.5 (+) mM; [H2 O2 ]: 300 (−) and 700 (+) mM.
higher TOC removal was observed when [Fe(II)] was at the “−” level. TOC/TOC0 -time results for PVP solutions are shown in Fig. 7. The sharp decrease in TOC is associated with the formation of solid particles, which took place in all experiments. As observed for solutions of PEG and PAM, the effect of [Fe(II)] depended on the polymer and [H2 O2 ] levels. The effect of [H2 O2 ] apparently depended on the [PVP]:[H2 O2 ] ratio. Complete TOC removal was achieved in ca. 60 min of reaction for both PVP and [H2 O2 ] at the “−” level (Fig. 7b). 4. Analysis of the results As shown in Figs. 5–7, the process variables considered in the study had different effects on the TOC removal profiles for different experimental conditions. The observed occurrence of three time periods in the degradation process depended on the reactants concentrations, and on the initial polymer concentration. In order to identify the relative importance of the factors considered in the study, a statistical analysis was carried out based on a response methodology [21]. The following responses
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Fig. 8. Illustration of the response variables adopted in the statistical analysis.
Fig. 7. TOC/TOC0 over reaction time for PVP with initial concentration (a) (1591 ± 53) mgC L−1 and (b) (546 ± 16) mgC L−1 . + +; + −; − +; 䊉 − − (order of signs: Fe(II) [H2 O2 ]). Levels: [Fe(II)]: 0.5 (−) and 1.5 (+) mM; [H2 O2 ]: 150 (−) and 250 (+) mM.
were selected, as a means to characterize the TOC–time profiles obtained: T1 : time corresponding to the first step of the process, characterized by a low TOC decrease rate; RAv : mean degradation rate during the H2 O2 addition time; Rmax : maximum degradation rate; and %Rem: TOC removal during the whole reaction time. The values of these responses were obtained directly from the TOC–time plots as illustrated in Fig. 8. For each polymer solution, the following variables were considered in the statistical model: [Pol] (initial concentration of each polymer), [Fe(II)], and [H2 O2 ], denoted x1 , x2 , x3 , respectively. The adopted statistical model is shown in Eq. (4): yˆ (x1 , x2 , x3 ) = b0 + b1 x1 + b2 x2 + b3 x3 + b12 x1 x2 + b13 x1 x3 + b23 x2 x3 + b123 x1 x2 x3 ,
(4)
in which yˆ (x1 , x2 , x3 ) is the estimated value a given response as a function of the variables x1 , x2 , x3 , codified as −1, 0, or +1. Coefficients bi correspond to the ith factor, which can be represented by a specific variable, or an interaction of variables (for example, b123 ). b0 is the independent term in the model. A statistical computer package (Statgraphics® plus 3.0) was used
to fit the model to the data. Table 2 shows the values of the coefficient of determination corresponding to the fitting of Eq. (4) to the experimental results with each polymer solution. The quality of the fitting was good, with coefficient of determination above 0.9, except in the case of T1 for the experiments with PEG. This reflects the larger difficulty in measuring T1 in this case, due to the less clear initial plateau for some combinations of the levels of the variables. The relative importance of the factors, expressed by the coefficient values in the statistical model, can be more clearly visualized by means of Pareto charts, as shown in Figs. 9–11, in which the length of each bar corresponds to the factor coefficient divided by the standard deviation of the responses (standardized). During time T1 the TOC value remained high, and the Pareto charts for the three polymer solutions indicate that the polymer concentration had the largest positive effect on this response, while [H2 O2 ] had a negative effect, as expected. [Fe(II)] had a significant positive effect for PAM and PVP, possibly due to the formation of organic complexes with iron, as suggested by other authors for the degradation of polyvinyl alcohol [17,22]. According to those authors, these complexes can be less susceptible to the attack by hydroxil radicals. Based on the observed results of this study, the complexes possibly combined and coalesced as solid particles after time T1 . On the other hand, high H2 O2 concentration may have favored the formation of oxidized products composed of smaller molecules that were more easily mineralized, thus reducing T1 . For the three polymer solutions, the fraction of initial TOC removed, %Rem, was negatively affected by the initial polymer concentration. This was expected, since higher amounts of reactants are needed at higher polymer concentration to attain a given fraction of the initial TOC removed. %Rem was positively Table 2 Coefficient of determination for the fitting of the statistical model (Eq. (4))
RAv Rmax %Rem T1
PEG
PAM
PVP
0.942 0.912 0.997 0.801
0.989 0.906 0.989 0.963
0.960 0.977 0.906 0.992
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Fig. 9. Standardized Pareto charts for PEG.
Fig. 10. Standardized Pareto charts for PAM.
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Fig. 11. Standardized Pareto charts for PVP.
affected [H2 O2 ] and negatively affected by [Fe(II)]. Depending on the [H2 O2 ] level, particle formation and coalescence was observed, which may have favored TOC removal, since mass was transferred from the solution to the solid phase. In the Pareto charts, the [H2 O2 ]–[Fe(II)] interaction was positive for all polymer solutions, as expected for the photo-Fenton system. When the average TOC decrease rate over the H2 O2 addition time, RAv , is considered, [Pol] and [H2 O2 ] affected positively this response for the three polymer solutions, since the average reaction rate depends on the concentration of the reactants. [Fe(II)] affected this response negatively, although this component takes part in the reaction system. The explanation for this effect may be the same as in the case of T1 , since higher [Fe(II)] may have favored the formation of complexes with organic molecules during the initial period of reaction. The effects are less clear when the maximum TOC decrease rate, Rmax , is considered. The Pareto charts show negative or positive effects of the factors, depending on the polymer solution. This is probably due to the low number of experimental measurements during the relatively short period of solids formation. Further investigation is necessary to clarify the phase separation process, which involves particle formation and coalescence, a relatively fast process in most systems. As shown in Figs. 9–11, the effects of interactions among the variables depended on the polymer solution, possibly due to the differences in reactant ratios adopted in each case, as well as to the differences in polymer structure.
5. Conclusions The experimental results indicate that it is technically feasible to degrade polymers dissolved in water by the photo-Fenton process, for aqueous solutions of the three polymers considered in the study, PEG, PAM, and PVP, with concentrations up to 1500 mgC L−1 . Depending on the levels of Fe(II) and H2 O2 , total mineralization of the organic compounds was achieved in batch experiments, in ca. two to three hours of reaction. The set of experimental conditions adopted in this study enabled to observe the effect of [Pol], [H2 O2 ], and [Fe(II)] on the characteristics of the TOC–time curves in batch experiments. The differences in the relative importance of the factors concerning the responses can be associated with differences in the intermediate degradation compounds. Concerning the application of the photo-Fenton process to TOC removal from aqueous solutions of the polymers included in this study, two main aspects deserve attention. Although Fe(II) is one of the components of the reaction system, its effect was negative for all polymer solutions. An explanation for this can be based on the discussions by Lei et al. [17] and Bossmann et al. [22], who carried out studies on the photo-Fenton degradation of polyvinyl alcohol (PVA) in aqueous solutions. These authors suggested that complexation of iron with PVA and intermediate products of PVA degradation can take place, thus preventing H2 O2 to approach and complex with the metal centers. According to Bossmann et al. [22], this effect may be a consequence of a poor efficiency of photoreduction of Fe(III) as it is bound within large organic molecules. The second important aspect is related
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to the formation of solid particles, which had been observed in a previous study by the present authors with polyvinyl alcohol [15] and by other authors [18,19]. The formation of solid particles at lower H2 O2 levels can be advantageous, because the solid particles can be separated by conventional operations, like filtration, resulting in significantly lower TOC in the liquid phase. Thus, the integration of photodegradation and separation processes should be considered in feasibility studies associated with process design. Acknowledgments The authors wish to thank the support by CAPES (Coordenac¸a˜ o de Aperfeic¸oamento de Pessoal de N´ıvel Superior), CNPq (Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico) and FAPESP (Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo). References [1] D. Mantzavinos, A.G. Livingstone, R. Hellenbrand, I.S. Metcalfe, Wet air oxidation of polyethylene glycols; mechanisms. Intermediates and implications for integrated chemical–biological wastewater treatment, Chem. Eng. Sci. 51 (1996) 4219–4235. [2] L.M.V. Vers, Determination of acrylamide monomer in polyacrylamide degradation studies by high-performance liquid chromatography, J. Chromatogr. Sci. 37 (1999) 486–494. [3] S. Horikoshi, H. Hidaka, N. Serpone, Photocatalyzed degradation of polymers in aqueous semiconductor suspensions V. Photomineralization of lactam ring-pendant polyvinylpyrrolidone at titania/water interfaces, J. Photochem. Photobiol. A: Chem. 138 (2001) 69–77. [4] G. Swift, Requirements for biodegradable water-soluble polymers, Polym. Degrad. Stab. 59 (1998) 19–24. [5] J.R. Haines, M. Alexander, Microbial degradation of polyethylene glycols, Appl. Microb. 29 (5) (1975) 621–625. [6] K.G. Watson, N. Jones, The biodegradation of polyethylene glycols by sewage bacteria, Water Res. 11 (1977) 95–100. [7] D. Dwyer, J.M. Tiedje, Degradation of polyethylene glycol and polyethylene glycols by Methanogenic Consortia, Appl. Environ. Microb. 46 (1) (1983) 185–190. [8] K. Nakamiya, S. Kinoshita, Isolation of polyacrylamide-degrading bacteria, J. Ferment. Bioeng. 80 (4) (1995) 418–420.
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