Application of Photo-oxidative Processes for the ... - ACS Publications

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Ind. Eng. Chem. Res. 2010, 49, 11214–11220

Application of Photo-oxidative Processes for the Remediation of Bleached Kraft Pulp Effluent Teresa Cristina Fonseca Silva,† Claúdio Mudado Silva,‡ Ce´sar Reis,† Carlos Roberto Bellato,† and Lucian A. Lucia*,§ Departamento de Quı´mica, and Departamento de Engenharia Florestal, UniVersidade Federal de Viçosa, Viçosa, MG 36570-000, Brazil, and Laboratory of Soft Materials & Green Chemistry, Department of Forest Biomaterials, North Carolina State UniVersity, Raleigh, North Carolina 27695-8005, United States

Photo-Fenton, H2O2/UV, and H2O2/TiO2/UV processes were tested and optimized to remediate organic material from bleached pulp mill effluent. Those processes were considered as part of a tertiary treatment protocol in which the optimal pH conditions for successful installation of the processes were analyzed. In the photoFenton and H2O2/TiO2/UV processes, a response curve was constructed to obtain the best relative proportions for the concentrations of the reagents inside and outside the experimental space. All experimental conclusions were drawn by the analysis of the relevant COD, color, and TOC data. One of the chief findings in this work is that among the photo-oxidative paths employed, the photo-Fenton treatment furnished the highest reduction in COD (78%) and TOC (91%) via the application of a concentration ratio of H2O2 to Fe2+ of 700:3.8 (mg L-1). The H2O2/UV process demonstrated a high removal of COD (61%), color (100%), and TOC (86%) when the H2O2 concentrations were 500 mg L-1. The application of a photo-Fenton process, a heterogeneous photocatalysis coupled with hydrogen peroxide (H2O2/Fe2+/UV), can be considered as more meritorious in a materials sense relative to the H2O2/UV process because lower H2O2 amounts are used. In a broad tertiary remediation perspective, all photo-oxidative processes using high H2O2 concentrations achieved COD reductions in the range of 60-70% removal, 100% color removal, and a 70-90% reduction range for TOC. Introduction The industrial practice of oxidative bleaching of kraft pulp is necessary to provide a final paper-based product that displays high whiteness to curry commercial appeal. However, this practice commands the use of a large volume of fresh water, which by default leads to a large volume of effluent. It is estimated that the cellulose and paper industry in Brazil utilizes a high quantity of water, between 30-60 m3 per ton of product, resulting in environmental and economic problems.1 This effluent contains a nontrivial amount of organic matter (oils, waxes, terpenes, resins, gums, etc.) that is derived from the wood furnish used as the raw material for cellulose (pulp) production. As has often been the case, chlorine-based bleaching has produced a significant number of AOX (adsorbable organic halides) compounds such as chlorinated phenols polyphenolics. These types of compounds found in kraft mill chlorine-based bleach plant effluent have been a source of environmental concern due to their toxicity, bioaccumulation, and carcinogenic potential.2-5 Unfortunately, the conventional procedures to treat these effluents such as physical and biological remediation technologies have been found lacking to degrade the strongly recalcitrant organic matter in these waste waters.6 Nevertheless, there is no set established remediation protocol in place because the extent of toxicity, total organic carbon (TOC), and color vary can greatly vary depending on the pulping process and wood furnish that are used.7 Yet, the process of remediation is quite standard at nearly all kraft bleach pulp operations. The industrial effluent passes through primary and secondary treatments whose respective * To whom correspondence should be addressed. E-mail: [email protected]. † Departamento de Quı´mica, Universidade Federal de Viçosa. ‡ Departamento de Engenharia Florestal, Universidade Federal de Viçosa. § North Carolina State University.

objectives are the removal of suspended solids and the oxidation of the organic matter through the activity of microorganisms. Yet, as already alluded, organic compounds such as chlorophenols cannot be efficiently degraded by biological processes, and rare tertiary treatments are generally required.8 Such treatments are desired to achieve the final removal of pollutants in the effluent and are generally referred to as polishing stages.9 Environmental norms and laws have increasingly demanded the reduction of the organic load in cellulose-based industry effluents. Therefore, such industries are actively pursuing feasible options that will allow their operations to maintain environmental compliance. Among several possibilities for effective remediation of effluents, advanced oxidation processes (AOPs) are worthy of investigation due to their highly efficiency partial or total mineralization of several organic compounds, including chlorinated phenolics.8 AOPs are based on the generation of the hydroxyl radical ( · OH), the most reactive species that can be produced from aqueous-based media, which possesses sufficiently high oxidizing power to promote the degradation of a great variety of organic compounds.10,11 Hydroxyl radicals react efficiently with different substrates in water, with rate constants between 107 and 1010 L/(mol · s).12 The generation of the hydroxyl radical has been studied by the use of ozone (O3), hydrogen peroxide (H2O2), titanium dioxide (TiO2), and Fenton reagent (Fe2+ and H2O2 in acidic medium) or by the combination of some of these processes.13-16 In these processes, the application of the ultraviolet radiation increases the efficiency of degradation by the acceleration of the hydroxyl radical production.17 Indeed, our recent work has shown that photocatalysis (using TiO2) of lignin in pulp and paper effluents can provide a remarkable quantum efficiency, leading to well over 80% degradation during a few hours.18 In light of this past work and in terms of using an adequate AOPs for successful water treatments, the main objective of

10.1021/ie101552t  2010 American Chemical Society Published on Web 10/01/2010

Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 Table 1. Parameters of the Physical-Chemical Characterization of the Effluent Collected parameters chloride (mg L-1) color (mg Pt L-1) BOD5 (mg L-1) COD (mg L-1) pH TOC (mg L-1)

310 ( 19 1474 ( 21 13 ( 6 403 ( 5 8.3 126 ( 6

the current work was to compare the efficiency of the following oxidative treatments, photo-Fenton, H2O2/UV, and H2O2/TiO2/ UV, as tertiary treatments, with respect to chemical oxygen demand (COD), color, and total organic carbon (TOC) or organic load. Materials and Methods Characterization of the Pulp Mill Effluent. The waste effluent for treatment was obtained from the effluent treatment lagoons of a Eucalyptus bleached kraft pulp and paper mill located in Minas Gerais, Brazil, and stored at 4 °C. Any suspended solid was removed by filtration. The basic physicochemical characteristics were carried out in accordance with APHA.19 The parameters analyzed were chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total organic carbon (TOC), color, turbidity, chloride, and pH content all shown in Table 1. Materials. All chemicals used in the experiments were at mimimum analytical grades. Ferrous sulfate heptahydrate (FeSO4 · 7H2O) and hydrogen peroxide (H2O2 32% w/w) were purchased from Sigma-Aldrich (St Louis, MO). Titanium dioxide (TiO2) was provided by Degussa P-25%, Hulls Corp. (a mixture of 80% anatase and 20% rutilo), and used in suspension without previous purification. The pH of aqueous solutions was adjusted using dilute (1%) sodium hydroxide or sulfuric acid solutions. All solutions were prepared using deionized water. pH Study of AOP. Photo-Fenton, H2O2/UV, and H2O2/TiO2/ UV were the treatments to which the effluent was submitted. To define an adequate pH for each treatment, we studied ranges of pH from 2.0 to 5.0 for photo-Fenton20 because out of this pH range, studies showed low efficiency of the process.5,21 For H2O2/UV and H2O2/TiO2/UV, the pH evaluated varied from 3.0 to 10. This larger pH range was studied to prove whether or not the efficiency of the process would be prejudice by a specific pH value. In both cases, the pH was varied by 1 unit. Laboratory-Scale Experimental Setup. Figure 1 shows the experimental unit used. 1700 mL of effluent was placed in a beaker, and it was equilibrated to room temperature while pH adjustment was done using dilute sulfuric acid and sodium hydroxide solutions.

Figure 1. Photochemical reactor setup used in the course of the photochemical experiments conducted.

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Once the desired pH was reached, the effluent was placed into a glass flask and was gradually transferred to a thermostatic cylindral Pyrex cell of 700 mL capacity, by using a peristaltic pump Masterflex L/S. The peristaltic pump continuously cycled the effluent through a rubber tube (0.5 cm in diameter) to provide constant mixing. Solar irradiation was simulated by using a 125 W mercury lamp Philips (λmax ) 254 nm) surrounded by a quartz tube so as to not interfere with the transmission of UV irradiation. The quartz tube and the lamp were placed inside the thermostatic cylindrical Pyrex having 700 mL capacity (the photoreactor). Simultaneously, the remainder of the wastewater (1000 mL) in the glass flask was immersed in a water bath for temperature adjustment to 35 °C ((3.0 °C), and the photoreactor was covered with aluminum foil to avoid light leakage. The total volume (1700 mL) allowed for recirculation of the effluent in the photoreactor by using a peristaltic pump through a rubber tube (0.5 cm in diameter) to provide constant mixing. The experiment was initiated by tuning the UV lamp on after the addition of the reagents into the glass flask. Samples were withdrawn for analysis at the end of a specific period, which was based on minimum hydrogen peroxide residue in the sample. Residual hydrogen peroxide was ascertained according to accepted literature accounts.22 Analytical Methods. Samples were removed from the photoreactor at predetermined time intervals for immediate analysis of TOC, COD, and color. The temperature of the solution was kept at 35 °C throughout the experiments. Total organic carbon (TOC) analyses were done using a SHIMADZU 5000 TOC analyzer operating with synthetic air (pressure 200 kPa, flow rate 150 mL/min). Chemical oxygen demand (COD) was determined by the standard methodology (5220 D, colorimetric method) using a thermo reactor (Spectroquant TR 420 Merck) for digestion and a Varian Cary 50 for colorimetric measurements. Color was determined at 455 nm according to the absorbance of a Pt-Co standard solution using a spectrophotometer (Shimadzu UV-1603) and referenced to a Pt-Co standard solution that had a scale of 0-500 in Pt-Co color units as described by ASTM 1997b. A Pt-Co color unit is equivalent to 1 mg/L of platinum. A 500 Pt-Co color unit standard solution was prepared with 1.245 g of potassium chloroplatinate, 1 g of cobalt chloride hexahydrate, and 100 mL of 35 wt % hydrochloric acid diluted with water to a total volume of 1 L. Before the color measurements, the pH of the samples was adjusted to 7.6 with 0.1 M hydrochloric acid or 0.1 M sodium hydroxide. Experimental Design. The experimental design for photoFenton and H2O2/TiO2/UV treatments consisted of a statistically significant model (experimental factorial design and response surface analysis) with a minimum set of well-chosen experiments to predict the best conditions for the process and to determine synergies and antagonisms. Thus, the resultant contour plot can then be used to predict the values of the response at any point in the experimental region of interest. The time of irradiation exposure of each sample submitted to the treatment was determined on the basis of the time of degradation of the hydrogen peroxide by iodometry,23 at regular time intervals. The concentration of the reagents H2O2:Fe2+ for the determination of the optimal pH was 500:5 (mg L-1).20,24 For H2O2/UV treatment, the concentration used for the study of the pH was 700 mg L-1. The concentrations of hydrogen peroxide used were 100, 300, 500, 700, 900, and 1100 mg L-1. These concentrations vary between the lower and higher amount

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Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010

Figure 2. COD variation × pH for treatments: (a) photo-Fenton, (b) H2O2/ TiO2/UV, and (c) H2O2/UV (COD initial: 403 mg × L-1).

of H2O2 used for the previous experiments, and all experiments were done in duplicate. Results and Discussion The relevant physicochemical characterization parameters for the final effluent collected are presented in Table 1. Note that the effluent collected presents a DBO5/COD 0.03 relation, indicating low biodegradability21 because this effluent has already been submitted to a secondary treatment, indicating that almost only nonbiodegradable organic matter was left meaning that it is recalcitrant. Additionally, the high concentration of chloride is attributable to the chlorine-based bleaching processes used in the mill. The presence of chlorides may reduce the efficiency of the advanced oxidation processes mainly if the concentration of this anion in the sample is very high, above 2500 mg L-1 for the photo-Fenton process.25 For processes in which TiO2 is used, inorganic anions such as chlorides, sulfates, and phosphates inhibit the process and may reduce the photomineralization by 20-70% due to negative competition kinetics, that is, adsorption of the ions to the oxidative sites of the catalyst thus interfering with the adsorption of the contaminant.26 Photo-Fenton Treatment. The slightly alkaline pH of the effluent is not conducive for the photo-Fenton reaction because of the appearance of insoluble iron hydroxides precipitation and thus the resultant hindrance in ferrous ion-catalyzed generation of hydroxyl radicals ( · OH).8 On the other hand, under conditions of very low pH, the Fe(OH)2+(H2O)5 complex is favorable for its formation.27 Thus, the pH chosen for the photo-Fenton process was 3.0 (Figure 2), and the time of permanence of the samples in the photoreactor was 120 min for most samples, due

to the catalytic effect of Fe2+ in relation to the other oxidation processes applied. Table 2 shows the concentrations of the reagents used in the photo-Fenton treatment by “central composite” and the removal of COD, color, and TOC. In the photo-Fenton treatments, the highest values of removal of COD, TOC, and color were observed to occur when the concentrations of hydrogen peroxide were high, and that the Fe2+ concentration interferes very little in the removal of COD and TOC as shown in experiments 2/4 and 1/3. However, even with little difference in the values of COD and TOC, when [Fe2+] varies and [H2O2] is maintained constant, it could be observed that higher concentration of iron ions ends up prejudicing the process by decreasing the removal of COD and TOC. In the experiments in which the concentration of H2O2 remains constant (380 mg L-1), it was observed that the color removal varies. Such color variation may be explained by the fact that the color analysis is carried out at pH 7.6 where the formation of complexes with Fe2+ (iron polycarboxylates) is possible, resulting in a new absorption band in the region between 250 and 580 nm, thus conferring higher color intensity to the effluent.28 The highest remediation efficiency experiment among those carried out for the photo-Fenton treatment (Table 2) was experiment 6, using a proportion of 700 mg L-1 of H2O2 to 3.8 mg L-1 of Fe2+. This treatment was able to reduce the COD to 78% of the initial value (403 mg L-1), furnishing an effluent with a COD value of approximately 90.0 mg L-1, which is within the maximum legal limit for the discharge of treated effluents to the receiving waters of Minas Gerais (1987). This data point is not isolated, but contributes to the response surface modeling that was done to provide optimal conditions for the remediation. It is worth mentioning that one of the great advantages of the photo-Fenton process is the fact that it occurs 35% faster than the others studied processes since Fe2+ used acts a catalyst. These data are supported by monitoring the degradation of the hydrogen peroxide during the experiment. Figure 3 represents the predictive response surface results for the photo-Fenton process and displays the optimal regions for the removal of COD, TOC, and color within the experimental space. It is possible in Figure 3 to observe the codified variables of the concentrations of H2O2 and Fe2+. The equation that describes the response surface for Figure 3a-c is z ) 44.89 + 25.27X1 - 2.33X12 - 0.33X2 - 2.62X22 - 0.79X1X2, z ) 54.69 + 40.55X1 - 3.93X12 - 6.06X2 + 0.65X22 - 2.00X1X2, and z ) 43.97 + 30.53X1 + 1.85X12 - 2.83X2 - 0.39X22 - 1.97X1X2, in which z represents the removal of COD, color, and TOC,

Table 2. Concentrations of the Reagents and Removals of COD, TOC, and Color in the photo-Fenton Treatment by Central Composite experiments

X1

X2

[H2O2] mg L-1

[Fe2+] mg L-1

COD removala (%)

TOC removala (%)

color removala (%)

1 2 3 4 5 6 7 8 9 10 11 12

-1 +1 -1 +1 -1.41 +1.41 0 0 0 0 0 0

-1 -1 +1 +1 0 0 -1.41 +1.41 0 0 0 0

153 607 153 607 60 700 380 380 380 380 380 380

1.5 1.5 6.1 6.1 3.8 3.8 0.6 7.0 3.8 3.8 3.8 3.8

14.7 63.4 16.5 63.1 3.7 78.4 42.3 38.6 45.8 35.9 47.4 50.4

16.7 80.3 15.7 71.4 3.0 91.1 47.1 38.1 42.1 45.9 43.0 44.9

0.9 97.3 1.6 90.0 1.7 100.0 74.8 45.2 50.8 56.3 54.6 57.0

a

Standard deviations for COD, TOC, and color removal were 5.1, 3.4, and 2.5, respectively.


Figure 3. Response surfaces for the photo-Fenton treatment. Optimal regions for the removal of COD (a), TOC (b), and color (c).

respectively, and X1 and X2 are related to [H2O2] and [Fe2+], respectively, as shown in Table 1. The model predicts from the experimental space a region where the percentage of COD removal is higher (∼80%). The points that provided the best solutions inside the space are those in which the concentration of hydrogen peroxide is higher and the concentrations of Fe2+ did not interfere significantly with the COD removal (Figure 3a). The projection of the response surface related to TOC reduction (Figure 3c) may be compared to that relative to COD reduction, which means that there is a trend to use higher amounts of H2O2. However, for TOC removal, slight differences can be observed related to the [Fe2+]. The graph indicates, inside the experiment, a tendency of better

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results when lower amounts of iron are used, and this effect is more pronounced for X1 ) 0 and 1 ([H2O2] ) 380 and 607 mg L-1). The model predicts that, out of the experimental space, the effect of iron ion would be even strong as much as the concentration of hydrogen peroxide is increased. Experimental design for removal of color is shown in Figure 3b. The trend for the best color removal partly differs from what has already been observed for COD. Ostensibly, higher concentrations of H2O2 facilitate color removal similarly to COD. However, inside and outside the experimental space, a preference is ascribed to low concentrations of Fe2+ as was observed for TOC removal. Among the parameters studied, the color seems to be the most dependent on the [Fe2+], and Figure 3b clarifies that the higher is the concentration of hydrogen peroxide, the lower the concentration of iron would need to be to ensure the efficiency of the process in removal of the color of the effluent. H2O2/TiO2/UV Treatment. In the oxidation involving titanium dioxide, the pH does not exert a great influence on the removal of organic matter. Yet, depending on the properties of the organic substrates to be degraded, the pH may influence their kinetics.29 In this study, the concentrations of both H2O2 and TiO2 were 500 mg L-1, pH ) 8.0, and the recirculation time of the effluent in the photoreactor was 180 min. The decay time for the hydrogen peroxide was imperative to ascertain because this reagent causes an absorbance increase at λ ) 605 nm due to the reduction of Cr2O72- to Cr3+, thus interfering in the COD analysis. It was ultimately found that at H2O2 concentrations lower than 40 mg L-1, this reagent does not cause significant variations in the value of the COD of the effluents.24 Table 3 shows the conditions of H2O2/TiO2/UV treatments by the use of the experimental design. For the treatments involving H2O2/TiO2/UV, total color removal was achieved at a minimum H2O2 concentration of 525 mg L-1. At concentrations of 188 mg L-1 of H2O2 and at at TiO2 concentrations of 391 and 859 mg L-1, the color removal was 83.8% and 75.2%, respectively. Such values are higher than those of the H2O2/UV treatment in which 300 mg L-1 of peroxide was used. Thus, it can be concluded that TiO2 combined with H2O2 and UV in this type of effluent behaves by increasing color removal. For the treatment involving H2O2/ TiO2/UV, total sample color removal was achieved in all the experiments whose minimum concentration of H2O2 was 525 mg L-1. For COD and TOC, it was observed that there is an interaction effect between the reagents. In H2O2 concentrations >525 mg L-1, a negative interaction can be observed related to the efficiency and the viability of the process. In Table 3 and comparing, for example, experiments 2 and 4 with experiments 7, 8, and 9, there are not many appreciable reductions in TOC, color, and COD in the experiments in which the highest peroxide concentrations are used. It appears that at a concentration of 525 mg L-1 of H2O2, the TiO2 does not significantly improve removal of organic matter, as can be seen in Table 3. Comparing experiment 6 ([H2O2] ) 1000 mg L-1 and [TiO2] ) 625 mg L-1) with experiments 2 and 4 ([H2O2] ) 862 mg L-1 and [TiO2] ) 359 and 891 mg L-1, respectively), no significant removal of COD and TOC can be observed, and, once a specific concentration of H2O2 is reached, no improvements would be gained. This fact is in agreement with the results found for H2O2/ UV treatment. However, when lower H2O2 concentrations are employed, the interaction effect between H2O2 and TiO2 seems to be positive for the overall efficiency of the process, as can be observed by

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Table 3. Concentrations of the Reagents and Removals of COD, TOC, and Color in the H2O2/TiO2/UV by Central Composite experiments

X1

X2

[H2O2] mg L-1

[TiO2] mg L-1

COD removala (%)

TOC removala (%)

color removala (%)

1 2 3 4 5 6 7 8 9 10 11 12

-1 +1 -1 +1 -1.41 +1.41 0 0 0 0 0 0

-1 -1 +1 +1 0 0 -1.41 +1.41 0 0 0 0

188 862 188 862 50 1000 525 525 525 525 525 525

359 359 891 891 625 625 250 1000 625 625 625 625

40.8 63.3 42.8 59.6 38.8 61.7 55.1 62.8 62.4 56.7 66.7 55.5

49.8 82.0 41.7 80.2 48.7 81.6 77.6 70.6 69.8 74.1 72.4 68.0

83.8 100.0 75.2 100.0 34.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0

a

Standard deviations for COD and TOC removal were 7.9 and 7.4, respectively.

comparing experiment 5 with experiments 1 and 3. In the first case, 50 mg L-1 of H2O2 and 625 mg L-1 of TiO2 were used against 188 mg L-1 of H2O2 and 359/891 mg L-1 of TiO2 for experiments 1 and 3, respectively. In all these experiments, the removal of COD and TOC could be considered the same. A COD reduction of approximately 60% was achieved in several experimental attempts, which does not comply with the environmental regulations established by Brazil or other countries.30 For the treatment using H2O2/TiO2/UV, the projections of the response surfaces for the analysis of the parameters contributing to the removal of COD, color, and TOC are presented in Figure 4. It is possible to observe that the region in which the best results for COD were achieved belongs to the experimental space. The equations that represent the construction of the response surfaces are expressed by z ) 60.32 + 8.97X1 5.79X12 + 1.15X2 - 1.43X22 - 1.42X1X2, z ) 100.24 + 16.88X1 - 15.07X12 - 1.02X2 + 1.62X22 + 2.11X1X2, and z ) 71.10 + 14.67X1 - 4.54X12 - 2.49X2 - 0.04X22 + 1.58X1X2, in which z represents the removal of COD, color, and TOC, respectively, and X1 and X2 are related to [H2O2] and [TiO2], respectively, as shown in Table 2. The efficiency of the process could be achieved by applying average to high levels of H2O2. Experiments involving heterogeneous photocatalysis using H2O2 demonstrated that high concentrations of H2O2 reduce the efficiency of the photocatalytic reaction. The principal reason is that excess H2O2 interferes by quenching the activity of extant hydroxyl radicals.15 The experimental region studied shows that color removal (Figure 4b) is higher when concentrations of H2O2 are above 525 mg L-1 (X1 ) 0). However, at lower concentrations of peroxide, the use of low levels of TiO2 may favor an increase in color reduction. For COD removal (Figure 4a), the design of the experiment includes the maximum efficiency of the treatment H2O2/TiO2/ UV, once the higher removal is found inside the experimental space, and the best result could be achieve if the concentration of H2O2 is kept between 525 (X1 ) 0) and 1000 mg L-1 (X1 ) +1.41) and the concentration of TiO2 is between 359 and 891 (X1 ) -1 and +1, respectively). TOC removal (Figure 4c) reveals behavior slightly different from COD removal. It is possible to observe TOC reduction at higher concentrations with the proviso that there is a relatively high concentration of H2O2 and a relatively low concentration of TiO2. Specifically, the experimental design points to a better efficiency when the minimum H2O2 concentration used is higher than 862 mg L-1 and the TiO2 concentration is around 625 mg L-1. H2O2/UV Treatment. The study of the pH in the H2O2/UV system demonstrated improved degradation efficiency of organic

contaminants at lower pH values, mainly pH 3.0. The pH chosen for these two treatments was pH 8.0, which was first attempted because it is the original effluent pH. The recirculation time was 3 h, and the concentration of H2O2 was 700 mg L-1. Figure 2 shows the variation of COD according to the pH employed for each treatment. Differently from the treatments described previously, H2O2/ UV is a univariate experiment, and so it is not possible to apply a central composite design in this case. For this reason, a longer range of H2O2 concentrations were established during this experiment (from 100 to 1100 mg L-1) to compare the H2O2/ UV treatment with photo-Fenton and H2O2/TiO2/UV treatments. H2O2/UV showed a progressive reduction in the three variables studied (COD, color, and TOC) up to 500 mg L-1 of H2O2 (Figure 5). From this point on, however, there was no significant difference in the reduction of COD, color, and TOC. More specifically, no significant improvements were observed at H2O2 concentrations greater than 500 mg L-1 because at that point hydrogen peroxide behaves as a generator of · OH radicals, so that a molecule of H2O2 generates two · OH radicals.31,32 When there is excess of peroxide, there is the potential for the combination between · OH radicals, generating a H2O2 molecule and halting the removal of the organic matter. The use of H2O2 at 500 mg L-1 reduced the COD by 60% and the TOC by 86%, which translates to a COD ) 159.0 mg L-1. Such a value is outside the permissible maximum limit for effluent loading and thus does not allow discharge into any type of receiving waters according to the COPAM regulations, although it demonstrates a significant reduction in the environmental impact of the effluent. Conclusions Relative to the AOPs studied, the photo-Fenton process had the advantage of requiring the least time for organic matter degradation relative to the analysis of the COD, color, and TOC data, but it was necessary to perform this process at pH 3.0, a value that is much lower than the original effluent pH (8.0). In this study, a central composite design was used according to a response surface methodology to provide a photo-Fenton and TiO2/H2O2/UV process optimization. The experimental design methodology was shown to be a valuable tool to optimize both processes and to achieve with a minimum number of experiments the optimal experimental parameters. The processes of photo-Fenton, H2O2/UV, and H2O2/TiO2/ UV processes were considered part of a tertiary treatment protocol to determine the optimal pH conditions for successful remediation. A response curve was constructed for the photoFenton and H2O2/TiO2/UV processes to obtain the optimal concentrations of the reagents inside and outside the experi-


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Figure 5. Color, TOC, and COD reduction (%) in the H2O2/UV treatment.

pH in which the experiments were conducted (8.0), it was possible to observe a COD increase of 60 and 40 mg L-1, respectively. However, the original values of the effluents’ pH did not interfere with the treatments carried out; they instead facilitated their performance and economic viability. The TiO2/ UV treatment, on the other hand, presented stability over a wide pH range. Finally, the process that combined the use of H2O2/TiO2/UV was not as significantly advantageous in comparison to the H2O2/ UV process. Also, due to the solid heterogeneous nature of TiO2 suspensions, the need for filtration makes this process too laborious. The application of a photo-Fenton process (H2O2/ Fe2+/UV) can be considered more effective relative to the H2O2/ UV process because less H2O2 is applied. All photo-oxidative processes applying high H2O2 concentrations attained high COD (60-70% removal), complete color removal, and a 70-90% reduction range for TOC. These preliminary findings are encouraging with respect to the efficacy of the photo-oxidative processes for the remediation of bleach kraft pulp effluent and provide a platform for future work. Acknowledgment We would like to gratefully acknowledge the CAPES (Coordenaçaõ de Aperfeiçoamento de Pessoal de Nı´vel Superior) in Brazil for the generous provision of a research fellowship to T.C.F.S. that allowed parts of this work to be realized. We also want to thank FAPEMIG (Fundaçaõ de Amparo a` Pesquisa do Estado de Minas Gerais) in Brazil for their support of this research. Note Added after ASAP Publication: After this paper was published ASAP October 1, 2010, additional corrections were made to the text. The revised version was published October 6, 2010. Figure 4. Response surfaces for the H2O2/TiO2/UV treatment. Optimal regions for the removal of COD (a), TOC (b), and color (c).

Literature Cited

mental space. The photo-Fenton treatment gave the highest reduction in COD (78%) and TOC (91%) via the application of a concentration ratio of H2O2 to Fe2+ of 700:3.8 (mg L-1), whereas the H2O2/UV process demonstrated a high removal of COD (61%), color (100%), and TOC (86%) when the H2O2 concentrations were 500 mg L-1. Except for the photo-Fenton treatment, the time of reaction required for the treatments was 3 h, which is consistent with results found in our earlier photocatalysis work,18 yet poses the logistical issue of being applied on an industrial scale. For the H2O2/UV and H2O2/TiO2/UV treatments, when the most efficient pH for COD removal (3.0) was compared to the

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ReceiVed for reView July 20, 2010 ReVised manuscript receiVed September 15, 2010 Accepted September 24, 2010 IE101552T

Application of Photo-oxidative Processes for the ... - ACS Publications

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