Innovative Supported Composite Photocatalyst for the Oxidation of

Ind. Eng. Chem. Res. 2005, 44, 8959-8967

8959

Innovative Supported Composite Photocatalyst for the Oxidation of Phenolic Waters in Reactor Processes P. Raja,† J. Bandara,*,‡ P. Giordano,§ and J. Kiwi† Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology (EPFL), Lausanne 1015, Switzerland, Institute of Fundamental Studies, Hantana Road, CP 20000, Kandy, Sri Lanka, and Xylowatt SA, En Budron A12, CH-1052 Le Mont s/Lausanne, Switzerland

This study focuses on the preparation and performance of an innovative, novel supported TiO2 photocatalyst to fix TiO2 Degussa P25 on Raschig glass rings. An interfacial electrostatically charged agent polyethylene-graft-maleic anhydride is used as a binder for the TiO2. This photocatalyst presented a stable performance during the degradation of phenolic waters. The photodegradation process was investigated as a function of (i) the concentration of the electron acceptor (H2O2), (ii) the intensity of the applied light, and (iii) the recirculation of the wastewaters in the photoreactor. The TiO2 catalyst was observed to maintain the pH at values close to 7 during the reactor treatment, enabling the treated phenolic waters to be discharged directly to a biological treating station. Modeling of the phenolic waters degradation was conducted through a single-exponential polynomial function. This gives a systematic way to determine the most economic use of the oxidant (H2O2) and electric energy required for the degradation process. 1. Introduction The pollution by phenolic waters from many industrial sites has become an increasingly serious issue in recent years. The treatment in municipal water purification plants does not eliminate phenol and phenol derivatives1 beyond very small amounts (on the order of 20 mg/L). Industry is aware of this problem and is working on innovative methods to find new solutions to remedy this problem.2 Heterogeneous photo-oxidation by semiconductor TiO2 produces OH• radicals that are active in the photo-oxidation of phenols because of the hydroxylation and subsequent cleavage of the phenol ring. The use of fixed TiO2 photocatalysts, to eliminate phenolic compounds, has been the focus of some reports.3-7 The use of the TiO2 supported Raschig ring catalyst (referenced hereafter as TiO2/R/rings) avoids the separation of the catalyst from the treated solution after the decontamination treatment. The objective of this study is to report about an innovative supported TiO2, active in the photodegradation of phenolic waters, that has been prepared using an approach that is different from those reported until now. We have used a negatively charged interfacial agent (polyethylene-graft-maleic anhydride copolymer) to coat the surface of the glass with -COOnegatively charged groups and subsequently bind the TiO2 particles on the Raschig rings electrostatically. This supported photocatalyst allowed suitable kinetics during the degradation of phenolic waters and presented long-term stability during reactor operation. The slow kinetics and the low stability found in industrial wastewater treatment for supported catalysts are the main problems with supported catalysts in the field of advanced oxidation terchnologies.1,2 The reactor performance will be studied as a function of the following

solution parameters: (a) amount of oxidant, (b) wastewater recirculation rate, and (c) applied light intensity. The degradation rate of the phenolic waters was measured as a function of these three parameters. Based on the experimental data obtained, the optimization of the solution parameters for phenolic waters degradation was performed by statistical modeling,8 using an exponential polynomial single expression. This allows us to determine the lowest (total organic carbon, TOC) region as a function of the amount of the oxidant (H2O2) added in solution, the reactor recirculation rate, and the applied light intensity.9-12 Another objective of this study was to find catalytic systems that will allow us to attain a pH >6 after the treatment, to avoid a final pH adjustment before discharging the treated waters directly into a biological treating station for further processing. 2. Experimental Section 2.1. Materials and Reagents. The phenol, acids, bases, and H2O2 were obtained from Fluka p. a. and used as received. The TiO2 Degussa P-25 photocatalyst was a gift from Degussa Switzerland. The Raschig rings used were 4 mm × 4 mm in size and were composed of 1-mm-thick soda lime glass. The soda lime glass used had the following composition: 70% SiO2, 10% alkaline oxide (Na2O, CaO, MgO, K2O), and 5% metal oxide (Fe2O3, Al2O3). The polyethylene-graft-maleic anhydride powder was a Sigma-Aldrich product (No. 456624, CAS Number 106343-08-2; melting point, 107 °C; average molecular weight, Mw ) 2300). The structure of the interfacial agent is

* To whom correspondence should be addressed. Tel.: 41 21 693 3621. Fax: 41 21 693 4111. E-mail: [email protected]. † Swiss Federal Institute of Technology. ‡ Institute of Fundamental Studies. § Xylowatt SA. 10.1021/ie050689m CCC: $30.25 © 2005 American Chemical Society Published on Web 10/22/2005

8960

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005

2.2. Catalyst Preparation. The Raschig rings were washed with detergent and then etched with HF 5% for 10 min at 50 °C. After rinsing with doubly distilled water, the Raschig rings were immersed in the polyethylene-graft-maleic anhydride 5% copolymer, dissolved in toluene, drained, and dried overnight in a vacuum oven. The dried rings were dipped in a welldispersed TiO2 Degussa P25 suspension (5 g/L) and dried at 110 °C for 1 h. The interfacial charged agent polyethylene-graft-maleic anhydride builds a negatively charged film on the Raschig glass rings that allows the electrostatic binding with the positively charged TiO2 surface. The polyethylene-graft-maleic anhydride binds the TiO2 through the -COO-, which interacts electrostatically with the surface of TiO2 in aqueous solution. The electrostatic attraction occurs between the TiOH2+ form in solution at pH 4.2 and the polyethylene-graft-maleic anhydride negative -COOgroups. Further structural studies of the photocatalyst interfacial agent on the glass surface are not possible, because the polyethylene-graft-maleic anhydride that is used as the interfacial agent is eliminated during the calcinations at 500 °C. The immersion in TiO2 suspensions is repeated again to deposit TiO2 on the rings in areas that are not covered during the first dip coating. Calcination at 500 °C then was performed for 10 h. With shorter calcinations times, the catalyst presented a lower degradation rate for phenol. This indicates that, after a favorable contact is made between the coated glass surface and the TiO2 suspension, the interfacial agent facilitates the diffusion of the TiO2 particles into the glass and also contributes to a better physical layout and structure of the TiO2 particles on the Raschig glass rings, as shown via transmission electron microscopy (TEM) in Figure 9 (presented later in this paper). Finally, the TiO2 coated glass rings were washed again, to eliminate the loosely bound TiO2 particles. 2.3. Irradiation Procedures. The photodegradation of phenolic waters was performed in the photoreactor shown in Figure 1. The entire photoreactor system has a volume of 2 L, with 0.4 L corresponding to the reactor volume and 1.6 L corresponding to the tubing and mixing flask. The volume reached at all times by the reactor light was ∼100 mL. An amount of TiO2/R/rings was introduced in the reactor, as shown in Figure 1. The vessels were irradiated with a medium-pressure mercury lamp (400 W) from Photochemical Reactor, Ltd. (Blounts Farm, Reading, Berkshire, U.K.) with a watercooling jacket. The radiation field was 360°, with a light dose of 5 × 1019 photons/s. 2.4. Analyses of the Irradiated Solutions. The absorption of the solutions was followed in a HewlettPackard model 38620 N-diode array spectrophotometer. The TOC was measured with a Shimadzu model 5000 TOC analyzer. The disappearance of phenol (of the phenolic waters) was monitored using a high-pressure liquid chromatography (HPLC) system from Varian Corporation that was also provided with a 9065 diode. A Phenomenex model C-18 inverse phase column was used in the HPLC system, and the gradient eluent solution consisted of a mobile phase of water (30%) and acetonitrile (70%). The peroxide concentration of the solutions were measured using Merckoquant paper (Merck AG, Switzerland) and by the iodometric, as devised by Hochanadel.13 2.5. Scanning Electron Microscopy. The scanning electron microscopy (SEM) analysis was performed with

Figure 1. Scheme of the photoreactor used to treat the phenolic waters during this study.

a Philips model XL 30 SFEG system. For these observations, the TiO2 was removed from the glass using a razor blade, to avoid excessive electric charging of the sample during the SEM observations. The TiO2 particles were deposited on a carbon-coated copper TEM grid. The SEM microscopy was able to resolve at 15 kV in a secondary electron contrast mode. The cross sections of the surface of the rings were prepared for SEM by embedding the samples in an epoxy resin. To avoid charging effects, the deposited samples were subsequently coated with a 20-nm-thick gold layer. The highresolution SEM images of the TiO2 that was removed from the glass rings and deposited onto a carbon film that was coating a TEM copper grid were taken at an accelerating voltage of 5 kV. 2.6. Modeling of the Polynomial Exponential Function. The numerical work was performed using Matlab 5.3 for Macintosh. Contour plots were obtained in a Power Macintosh 8200/120. The programs for the modeling were written during the course of this study. 3. Results and Discussions 3.1. Influence of the H2O2 Concentration and Recirculation Rate on the Photodegradation of Phenolic Waters. Figure 2 shows the phenol degradation in the reactor (Figure 1) using three different H2O2 concentrations. Trace a in Figure 2 shows the results for the abatement of phenolic waters using an initial amount of 8 g/L H2O2. This amount of oxidant is increased to 11.5 g/L and 13.5 g/L in traces b and c, respectively. Trace c in Figure 2 shows the best disappearance kinetics of phenol and proceeds with a photonic efficiency of ∼3.5. Within 2 h, the concentration of phenolic waters was reduced from 21 mM to 0 mM (2000 mg/L to 0 mg/L). The rate of disappearance of phenol molecules in the reactor was determined as

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 8961

Figure 2. Phenol disappearance detected using high-performance liquid chromatography (HPLC) under 400-W mercury-light irradiation and in the presence of the TiO2/R/rings photocatalyst, as a function of H2O2. Additions of H2O2 are as follows: (a) 8 g/L, (b) 11.5 g/L, and (c) 13.5 g/L. Trace d represents a suspension of TiO2 Degussa P25 (1 g/L) in the presence of 13.5 g/L H2O2. The inset shows the TOC reduction under the same experimental conditions.

follows: 21 × 6.03 × 1020 × 100 cm3 reactor solution (directly exposed to the lamp)/120 min × 60 s/min ) 1.75 × 1020 molecules per second. The rate of photon emission by the 400-W mercury light was 5 × 1019 photons per second. It follows that the photonic efficiency is given as (1.75 × 1020)/(5 × 1019) ) 3.5. This photonic efficiency has been observed for the last three months under normal reaction operation when degrading the phenol waters. After 20-25 runs, the surface of the TiO2/R/rings had to be cleaned by recirculating a solution of doubly distilled water with 80 mM of H2O2 for 1 h. Further increases in the concentration of H2O2 beyond 13.5 g/L lead to slower phenol disappearance kinetics, because of the known scavenging of the OH• radicals by an excess H2O2 added to the phenolic waters, because this reaction is thermodynamically favored at higher H2O2 concentration with the formation of H2O as the low-energy product (1,2,9,16):

H2O2 + OH• f H2O + HO2•

(1)

Because phenol is only one of the carbon constituents in the industrially originated wastewaters, we proceeded to measure the decrease of TOC within the reactor treatment time. The TOC decrease of the same solutions is reported in the inset of Figure 2. The decrease of TOC for the phenolic waters proceeds within a longer time scale than the abatement of the phenol. The increase in the concentration of H2O2 was also observed to increase the TOC abatement kinetics. Under the highest concentration of H2O2 (13.5 g/L), the longer-lived intermediates produced in solution decompose under the mercury light irradiation. By inspection of the inset in Figure 2, it is observed that traces c and b with H2O2 concentrations of 13.5 and 11.5 g/L H2O2 lead to the complete abatement of the carbon content in the phenolic waters; however, this was not the case when concentrations of 8.5 g/L H2O2 were used.

Figure 3. Total organic carbon (TOC) decrease, as a function of the recirculation rate of the phenolic water in the photoreactor: (a) TiO2/R/rings under light, no H2O2; (b) only 13.5 g/L H2O2 added but no photocatalyst under light; (c) TiO2/R/rings in the presence of 13.5 g/L H2O2 in a dark experiment; (d) recirculation rate of 450 mL/min TiO2/R/rings under mercury light with H2O2 (13.5 g/L); (e) recirculation rate of 150 mL/min TiO2/R/rings under mercury light with H2O2 (13.5 g/L); and (f) recirculation rate of 300 mL/min TiO2/R/rings under a mercury light with H2O2 (13.5 g/L).

Control experiments using standard TiO2 Degussa P25 suspensions beginning with concentrations at 0.2 g/L and up to 2 g/L were performed and showed a slower photocatalytic reduction of phenol and TOC in the phenolic waters, compared to the reactor with TiO2/R/ rings (see Figure 1). The screening by the TiO2 suspension seems to hinder the abatement kinetics of phenolic waters. A suspension of TiO2 Degussa P25 (1 g/L) degraded the phenolic waters in 4 h (trace d in Figure 2), compared to a reaction time of 2 h when using TiO2/ R/rings (trace c in Figure 2). The phenol photodegradation mediated by TiO2 Degussa P25 (1 g/L) proceeded with a photonic efficiency of 1.75. The penetration of the light reaching the phenolic waters seems to be much greater in the case of the semitransparent TiO2/R/rings, as compared to the TiO2 Degussa P25 suspensions in the optical path-length of 1.5 cm between the reactor lamp and the reactor wall. Additional evidence for this is presented below in section 3.6. Figure 3 reports the control runs for the results shown in Figure 2 in traces a, b, and c. Trace a in Figure 3 shows the effect of the TiO2/R/rings (the TOC reduction) under mercury-light irradiation when no H2O2 was added into the solution. No TOC decrease was observed. Trace b shows the lack of mineralization when only H2O2 was added to the phenolic waters in the absence of TiO2/R/rings under light irradiation, and, finally, trace c reports a modest TOC reduction when TiO2/R/ rings are used in the presence of H2O2 in darkness. Trace a provides the proof that the catalyst TiO2/R/rings does not adsorb phenolic waters. The runs depicted by traces d, e, and f in Figure 3 show how the recirculation rate of the phenolic waters

8962


Figure 4. Phenol disappearance in the wastewaters in the photoreactor, in the presence of TiO2/R/rings and 13.5 g/L H2O2 as a function of the intensity of the applied light. Trace a shows the phenol reduction when applying a 36-W mercury light, and, in traces b and c, the intensities of the mercury light were 400 and 800 W, respectively. The inset shows the TOC decrease for the same solutions (trace a, irradiation with a 36-W lamp; trace b, irradiation with a 400-W lamp; and trace c, irradiation with a 800-W lamp. Trace d shows the TOC reduction when the phenolic waters were stirred first in the presence of TiO2/R/rings for 16 h before adding H2O2 (13.5 g/L) and subsequently irradiated with a mercury light (400 W).

in the reactor affects their TOC decrease. Trace d in Figure 3 shows that, during reactor batch mode operation, a recirculation rate of 150 mL/min was not sufficient to attain the best mixing of the solution components because trace e, with a recirculation rate of 300 mL/min, showed a more favorable TOC reduction. When the recirculation rate was increased from 150 mL/min to 300 mL/min, it increases the mixing in the reactor. This, in turn, leads to an increase in the reaction rate. Trace f in Figure 3 shows that a recirculation rate of 450 mL/min further accelerates TOC reduction. However, a further increase in the recirculation rate did not improve the degradation kinetics. During the reactor operation, 80% of the solution is in the dark at any given time and 20% undergoes light irradiation. This ratio is determined by the 400-mL irradiated volume in the reactor (see Figure 1) and the 1600-mL dark volume of the tubes and mixing vessel. 3.2. Effect of the Light Intensity on the Photodegradation of Phenolic Water in the Photoreactor. Figure 4 shows that the disappearance of phenolic waters under light irradiation is accelerated when the intensity of the applied light is increased. Three mercury lamps are tested, as shown in Figure 4; these lamps have nominal powers of 36, 400, and 800 W. In the reactor setup shown in Figure 1, there are zones reacting under light but also dark zones. The saturation limit imposed by the number of active sites of the TiO2 on the Raschig glass rings it was not reached because a higher light intensity leads to greater phenol disappearance (see Figure 4). After 2 h of reaction, the initial phenolic waters become brown in color, because of the generation of quinone intermediates.14-16 Afterward, the solution in the reactor discolors completely, because of

Figure 5. Variation of pH with time during the abatement of phenolic waters under mercury-light irradiation (400 W) in the presence of TiO2/R/rings and 13.5 g/L H2O2. Trace a represents phenolic water stirred for 12 h in the presence of H2O2 in darkness prior to light irradiation, and trace b represents phenolic water irradiated with mercury light without prior treatment in darkness.

the destruction of the quinone intermediates. The insert in Figure 4 shows the favorable effect of the increase in light intensity on the TOC reduction in traces a, b, and c. Trace d in Figure 4 shows that, if the phenolic water is contacted with the TiO2/R/rings in darkness for 12 h and then reacted under light within 5 h, the TOC decrease is faster compared to the case where light of the mercury light is applied without any darkness pretreatment. 3.3. Change of pH and Optical Absorption of the Phenolic Waters during Reactor Treatment. Figure 5 reports the change pH of the phenolic waters as a function of reactor irradiation time. Trace a shows that a dark pretreatment allows a faster increase of the pH in the solution within the reactor after the initial decrease and trace b) corresponds to the reaction with no dark pretreatment. The decrease in pH and subsequent increase is due to the formation of carboxylic acids first followed by the mineralization of them at late stages of the reaction.1,2 A simplified mechanism for the first stages of the reaction12 is suggested below up to the overall mineralization step outlined in eq 8. In this sequence, reactions 3-5 account for the pH shift during the degradation of phenolic waters (see Figure 5).

TiO2 + hν f TiO2 (h+)vb + TiO2 (e-)cb -

(2)

-

TiO2 (e )cb + O2 f O2 + TiO2cb

(3)

O2- + H+ f HO2-

(4)

TiO2 (h+)vb + H2O2 f TiO2 + HO2• + H+ -

•

-

TiO2 (e )cb + H2O2 f TiO2 + OH + OH

(5) (6)

OH• + (phenolic waters) f (phenolic waters)+ + OH- (7) C6H5OH + OH• + 9HO2• f 6CO2 + 8H2O

(8)


Figure 6. Variation in the absorbance of the phenolic waters with irradiation time in the photoreactor under mercury light. Other experimental conditions: photocatalyst TiO2/R/rings and 13.5 g/L H2O2. Irradiation times were as follows: (a) zero time, (b) 60 min, (c) 120 min, (d) 180 min, (e) 240 min, and (f) 300 min.

The dissolved O2 in reactions 3 and 4 in the solution is generated from the decomposition of H2O2 (side reaction) and was not a limiting factor in the degradation of phenol. We did not purge the system with O2 during the photocatalytic reaction. Figure 5 shows that no pH adjustment was necessary at the end of the photoreactor treatment of the phenolic waters with TiO2/R/rings/H2O2 before proceeding to the second stage of biological treatment. Reaction 5 suggests the formation of a longlived cationic phenolic compound, such as that observed during the degradation of natural phenolic17 and synthetic phenolic compounds.1,2 The TiO2-mediated photooxidation of phenol has been reported by Okamoto18 and involves the hydroxylation of the benzene ring, leading to formic acid, which is finally oxidized to CO2. 3.4. Decrease in Optical Absorption of the Phenolic Waters during the Reactor Treatment. Figure 6 shows the shift and reduction of the optical absorption of the phenolic waters as a function of photoreactor treatment time up to 5 h. The importance of the reduction of color in a treated effluent resides in the fact that treated solutions should be free of color before being discharged into a municipal treatment station. This reduction in optical absorption has been used, in some cases, as an analytical method for the determination of the phenol content of effluent wastewaters.19 3.5. Consumption of H2O2 during the Abatement of Phenolic Waters: Chemical Oxygen Demand Reduction as a Function of the Applied Light Intensity. Figure 7 shows the consumption of H2O2 for a run with the TiO2/R/rings photocatalyst adding initially 13.5 g/L H2O2 and using mercury-lamp irradiation (400 W). The H2O2 concentration in solution was monitored using two methods, as described in section 2.4 of the Experimental Section. The inset in Figure 7 shows the TOC reduction for runs under mercury light in the presence of TiO2/R/rings but varying some of the solution parameters as follows: for trace a, 400 W and 11.5 g/L H2O2; for trace b, 800 W and 11.5 g/L H2O2; for trace

Figure 7. Monitoring of the H2O2 disappearance of phenolic waters during photoreactor treatment. Other experimental conditions: Mercury-light irradiation (400 W), 13.5 g/L H2O2 and photocatalyst TiO2/R/rings. The inset shows the TOC decrease for runs under different experimental conditions in traces a-d. See text for other details.

Figure 8. Decrease in chemical oxygen demand (COD) during the photoreactor treatment of phenolic waters. Conditions for trace a: mercury-light irradiation (400 W), 13.5 g/L H2O2, and photocatalyst TiO2/R/rings. Conditions for trace b: 800 W of mercury light. Other experimental conditions are as given in trace a.

c, 800 W and 13.5 g/L H2O2; for trace d, 800 W, 9 g/L H2O2 at time zero, and 4.5 g/L H2O2 after 2 h; and for trace e, stirring 12 h in darkness with H2O2 (13.5 g/L) and only then applying mercury light at a power of 400 W. The results show that the concentration of H2O2 has a bigger effect on the TOC degradation kinetics than the other solution parameters. The fact that no residual H2O2 remains in solution after 7 h makes the phenolic waters suitable for the ensuing second-stage biological treatment. The added H2O2 has been screened in preliminary experiments to use the lowest amount possible and, at the same time, obtain acceptable kinetics and a

8964


Figure 9. Scanning electron microscopy (SEM) image of the photocatalyst TiO2/R/rings.

total elimination of phenol (see Figure 2). A maximum of 25 mg/L H2O2 is the limiting concentration of H2O2 set in the European Union (EU) countries as not being detrimental for bacterial action.20 Figure 8 presents the reduction of the initial chemical oxygen demand (COD) of phenolic waters during the photoreactor treatment. Trace a shows the COD decrease under 400 W of mercury-lamp irradiation and trace b shows the decrease of COD under 800 W of light irradiation. This shows the favorable effect of the applied light intensity. This is not unexpected, because a similar effect was observed previously in Figure 4 for the TOC and phenol decrease as a function of applied light intensity. Figure 8 shows the COD reduction of the phenolic waters under 400- and 800-W irradiation. The ratio of COD/TOC is observed to be ∼4.5. This value means that we are in the presence of wastewaters with well-behaved organic compounds which follow generally the relation COD/TOC ) (3-5):1. 3.6. Scanning Electron Microscopy (SEM) TiO2/ R/Rings. Figure 9 shows the surface of the TiO2 round glass rings. SEM analysis of the TiO2 Degussa P25 particles removed from the glass rings showed sizes in the range of 30-35 nm. The TiO2 particles are visible as small dark spots in the round border of the Raschig glass rings with sizes in the range of 0.1-0.3 µm. The epoxy used in the SEM sample preparation appears in

Figure 9 close to the surface of the glass ring. After calcination at 500 °C, the TiO2 particles on the glass surface absorb the incoming light. The upper TiO2 aggregated particles would shield the incident light of the aggregates closer to the glass surface of the Raschig rings. Nevertheless, the screening by the TiO2 particles of the incoming light is less severe than in the case of the TiO2 Degussa P25 suspensions. This is observed to have a favorable photocatalytic effect during the degradation of phenolic water, as shown in Figure 2. 3.7. Exponential Modeling of the Phenolic Waters Photodegradation. This modeling allows the drawing of contour plots or curves of constant response to predict the values of the response (TOC) at any point of the experimental region.8-12 This methodology has been applied to study the dependence of the TOC by taking care of the experimental data in pairs, X1 and X2, as the input factors on the input variables H2O2 amount, recirculation rate, and light intensity. This is shown in Figures 10-12. The magnitude of the interaction between the variables is reflected in the value of the coefficients of the exponential polynomial expression

ln TOC ) ln b0 + b1X1 + b2X2 + b11X112 + b22X222 + b12X1X2 (9) and it is designed to fit the experimental data obtained


Figure 10. Tridimensional plot of TOC as a function of the recirculation rate in the reactor and the concentration of H2O2 in coded variables. The contour plots show the minimum of TOC in coded variables as a function of the recirculation rate and the concentration of H2O2. See text for more details.

during the degradation runs. The parameter b0 is the average of the TOC values over n experimental points, b1 is the coefficient for the effect of the X1 variable, b11 is the coefficient for the quadratic effect of the variable X1, and b12 is the coefficient for the first-order interaction for the variables X1 and X2. From the data in Figures 2-4, it is possible to construct the TOC functions for a pair of experimental variables (Xi, Xj), via contour plots. These plots represent curves of constant response (TOC) for the values (Xi, Xj) within the experimental region. In this way, it is possible to predict the optimum values of Xi, Xj that yield the minimum TOC. The lowest values of the contour curves in Figures 10-12 will represent the optimum numerical values for the H2O2 concentration and recirculation rate leading to the lowest TOC possible. The contour plots were calculated using eq 9. Different input variables had different dimensions; therefore, their effect can only be compared in eq 9, if they are coded. This is why the calculation of the contour plots and the tridimensional representations in Figures 10-12 is performed on coded variables. In Figure 10, the relationships between the coded variables and the real concentration of H2O2 are -1 for 8 g/L, 0 for 11.5 g/L, and 1.0 for 13.5 g/L. Also in Figure 10, the relationships between the coded variables and the recirculation rates are -1 for 150 mL/min, 0 for 300 mL/ min, and 1.0 for 450 mL/min. In Figure 11, the recirculation rates in coded variables correspond to the same values as already observed in Figure 10. The coded values for the intensities in Figure 11 are -1 for 36 W, 0 for 400 W, and 1 for 800 W. Finally, in Figure 12, the values -1, 0, and 1 for the coded values of the intensities and the concentrations used for H2O2 have the same correspondence in numerical values as described in Figures 10 and 11.

Figure 11. Tridimensional plot of TOC as a function of the recirculation rate in the reactor and light intensity in coded variables. The contour plots show the minimum of TOC in coded variables as a function of the recirculation rate and the applied light intensity. See text for more details.

The tridimensional plot in Figure 10 shows that the lowest points in the saddle for TOC occur when the recirculation rate has a value of 1 in coded variables (300 mL/min) and the H2O2 has also a value of 1 in coded variables (13.5 g/L). This is why we have used these two values of recirculation and H2O2 concentration during the phenolic waters degradation experiments in Figures 2-4. The tridimensional plot in Figure 11 shows that the lowest TOC values are obtained at a recirculation rate of 1 (equivalent to a rate of 300 mL/min) and through application of a high-intensity mercury light (800 W). The tridimensional plot in Figure 12 shows that, at the intensity of zero in coded variables (using 400 W as a numerical real value), the TOC saddle attains its minimum value. A low TOC is also favored by higher concentrations of H2O2, shown in Figure 12, by the value of 1 in coded variables. The least-squares regression analysis, applying the exponential expression given in eq 9, allows us to conduct an error analysis to determine the maximum error between the experimental data and the data calculated by the model. In Figure 10, for the H2O2-recirculation rate pair, the maximum error is 1.19. In Figure 11 for the intensity-recirculation rate pair, the maximum error is 1.28. In Figure 12 for the H2O2-intensity pair, the maximum error rate is 1.30. The maximum deviation between the experimental data and the model is observed to be low. Figure 13 shows the overlap of the lowest contour plot found previously in Figure 10 (noted as 6.118), Figure 11 (noted as 3.417), and Figure 12 (noted as 7.037) to determine the coefficients in eq 9 that correspond to the round intersection region for the lowest TOC values. This overlap makes it possible to determine the H2O2

8966


Figure 13 is in good relation with the experimental data shown in Figures 2-4. 4. Conclusions

Figure 12. Tridimensional plot of TOC as a function of the concentration of H2O2 in the reactor and light intensity in coded variables. The contour plots show the minimum of TOC in coded variables as a function of the concentration of H2O2 and the applied light intensity. See text for more details.

This study has focused on the design, preparation, and testing of an innovative photocatalyst known as TiO2/R/rings. The work is warranted because of the three factors hindering a wider application of supported catalyst in the advanced oxidation technologies field, noted as (i) slow degradation kinetics, (ii) long-term stability of the supported catalyst (the catalyst showed long-term operational stability over three months of operation when cleaned periodically as described in section 3.1); and (iii) the fact that we avoid separation of the photocatalyst and the treated solution at the end of the process with the consequent saving cost in materials, time, and labor. The new photocatalyst presents favorable features with respect to other similar materials widely used in the field. The solution parameters that have a role during the phenol abatement in the photoreactor were the amount of the oxidant (H2O2), the recirculation rate, and the intensity of the applied light; each of these factors were monitored. With these parameters, it was possible to the model the TOC decrease during the abatement of the phenolic waters using surface response methodology (SRM). This was possible using a single polynomial expression in the exponential mode. A minimum value of the TOC was observed, in terms of the amount of oxidant needed, the reactor recirculation rate, and the amount of expensive UV photons (light intensity/electricity). This, in turn, allows the cost minimization of the degradation process. Acknowledgment This work was funded by KTI/CTI No. 7078.2 (Bern, Switzerland) and COST D-19 Program No. CO 2.0068. Literature Cited

Figure 13. Modeling in Figures 10, 11, and 12, using the H2O2 concentration, light intensity, and recirculation rate as variables. The circle represents the lowest TOC that can be attained in the reactor for the phenolic water. The plot shown is in real variables. See text for more details.

amount, recirculation rate, and light intensity needed to attain, in the most economical way, the lowest TOC during the reactor degradation of phenolic waters up to 8 h as reported in the experimental data in Figures 10 through 12. The H2O2 values are observed to be on the order of 13 g/L or higher at a recirculation rate of 300 mL/min with light intensities between 570 W and 700 W. The final area obtained by the modeling in

(1) Oppenlaender, Th. Photochemical Purification of Water and Air; Wiley-VCH: Weinheim, Germany, 2003. (2) Mills, A.; Le Hunte, A. An overview of semiconductor photocatalysis. J. Photochem. Photobiol. A 1997, 108, 1-35. (3) Bideau, M.; Claudel, B.; Dubien, C.; Kazouan, H. On the immobilization of TiO2 in the photocatalytic oxidation of spent waters. J. Photochem. Photobiol. A 1995, 91, 137-144. (4) Byrne, A.; Eggins, B.; Brown, N.; McKinney, B.; Rouse, M. Immobilisation of the treatment of polluted water. Appl. Catal., B 1998, 17, 25-36. (5) Colon, G.; Hidalgo, M.; Navio, J. Photocatalytic behaviour of sulphated TiO2 for phenol degradation. Appl. Catal., B 2003, 45, 39-50. (6) Pozzo, R.; Baltanas, M.; Cassano, A. Supported titanium oxide as photocatatyst in water decontamination: state of the art. Catal. Today 1997, 39, 219-231. (7) Pera-titus, M.; Garcia Molina, V.; Banos, M.; Gimenez, J.; Esplugas, S. Degradation of chlorophenols by means of advanced processes: a general review. Appl. Catal., B 2004, 47, 219-235. (8) Khuri, A. I.; Cornell, J. A. Response Surfaces: Designs and Analyses; Marcel Dekker: New York, 1987. (9) Balanosky, E.; Kiwi, J. Mathematical Modeling of the Photochemical Reactor Degradation of p-Nitrotoluenesulfonate. Ind. Eng. Chem. Res. 1998, 37, 347-356. (10) Balanosky, E.; Herrera, F.; Lopez, A.; Kiwi, J. Oxidative Degradation of Textile Wastewater. Modeling Reaction Performance. Water Res. 2000, 34, 582-596. (11) Herrera, F.; Lopez, A.; Kiwi, J. Photochemically activated degradation of reactive dyes. Statistical modeling of the reactor performance. Photochem. Photobiol. A 2000, 135, 45-51.

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 8967 (12) Fernandez, J.; Kiwi, J.; Lizama, L.; Freer, H.; Mansilla, H. Factorial experimental design of Orange II photocatalytic discoloration. J. Photochem. Photobiol. A 2002, 151, 213-219. (13) Hochanadel, N. The determination of peroxides in solution. J. Phys. Chem. 1952, 56, 587-591. (14) Santos, A.; Yustos, P.; Quintanilla, A.; Garcia-Ochoa, F. Lower toxicity route in catalytic wet oxidation of phenol at basic pH by using bicarbonate media. Appl. Catal., B 2004, 53, 181194. (15) Keating. J.; Brown, A.; Greenberg, S. Products Observed During Phenol Treatment. Ind. Water Eng. 1978, 15, 22-29. (16) Pulgarin, C.; Peringer, P.; Albers, P.; Kiwi, J. Effect of FeZSM5 zeolite on the photochemical and biochemical degradation of phenol. J. Mol. Catal. A 2000, 95, 61-74. (17) Schwarzenbach, R.; Gschwend, Ph.; Imboden, D. Environmental Organic Chemistry; Wiley: New York, 1993.

(18) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Itaya, A. Heterogeneous photocatalytic decomposition of phenol over anatase powder. Bull. Chem. Soc. Jpn. 1985, 58, 2015-2022. (19) Ingols, R. S.; Ridenour, G. M. The elimination of phenolic tastes by chloro-oxidation. Water Sewage Works 1948, 95, 187194. (20) Balanosky, E.; Lopez, A.; Kiwi, J. Degradation of Naphthalene 1,5-disulphonate in a Flow Reactor. Modeling via Reduced Dimensionless Centered Variables. J. Photochem. Photobiol. A 1998, 119, 151-164.

Received for review June 13, 2005 Revised manuscript received August 15, 2005 Accepted September 20, 2005 IE050689M

Innovative Supported Composite Photocatalyst for the Oxidation of

Recommend Documents