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Ind. Eng. Chem. Res. 2004, 43, 7996-8000
Titanium Dioxide Catalyzed Photodegradation of Lignin in Industrial Effluents Andrew Dahm† and Lucian A. Lucia*,‡ Institute of Paper Science and Technology, Georgia Institute of Technology, 500 Tenth Street NW, Atlanta, Georgia 30332-0620, and Department of Wood and Paper Science, North Carolina State University, Campus Box 8005, Raleigh, North Carolina 27695-8005
The objective of this research was to investigate the photodegradation of dissolved lignin in a water medium resulting from titanium dioxide photocatalysis reactions that were conducted in a batch system. All experiments were carried out using the single factor design method. It was determined that irradiation in the absence of the catalyst allowed for only a 4.6% reduction in the dissolved lignin concentration (0.02 g) after 2 h. However, with a 10 mg/L loading of TiO2, an 82% decrease in the total lignin concentration was obtained. Both titanium dioxide loading and illumination intensity were also studied to determine optimum operating conditions. An optimal titanium dioxide loading of 10 mg/mL was determined, and it was found that higher illumination intensities correlated well with higher initial degradation rates and total lignin degradation. 1. Introduction Whitewater is the term coined for the industrial process water that is used and exits paper machines. A few of the major papermaking processes that require usage are wire showers, roll showers, water gland seals, and stock dilution points.1 After use, the water contains many dissolved solids as well as suspended solids, which are easily removed, but the dissolved solids often remain in the whitewater.2 It was typical for a paper mill to discharge 8700 gallons of process water per ton of paper produced prior to EPA regulations.2 As a consequence, current reductions in the total effluent emissions have been handled by either treating the effluent and then discharging it or reusing it. Most modern mills today use a combination of the two methods, although a large majority of the water is recycled.3 Because the water is a recycled, dissolved solid such as lignin, starches, sizing agents, defoamers, dyes, and synthetic polymers build up, which can have negative effects on operational processes and the quality of the paper products. Useful technologies to address the organic load buildup are currently under investigation. Titanium dioxide photocatalysis is a promising technology that has gained much attention for its ability to degrade organic suspended or dissolved materials in water. Using titanium dioxide as a photocatalyst currently has several commercial applications and has shown potential for industrial processes.4,5 One industry in which its potential has been scarcely explored is the pulp and paper industry. There has been a limited amount of studies for its effect on chlorinated byproducts in wastewater but not on lignin and other dissolved solids that are found in the paper mill whitewater system.6 Some of the work done in the past studying the photocatalysis
of phenolic compounds similar in structural character to pulp lignin has been done previously7,8 and mainly focused on analyzing the mechanistic photodecomposition of model phenolic compounds, unlike the complex polyphenolic, amorphous lignin polymer. This work studied the reactivity of specific phenolics such as phenol, p-methoxyphenol, p-cresol, and p-chlorophenol in addition to other halogenated phenolics. Because phenolic compounds, in general, pose a threat to the environment (they can be found in both groundwater and surface water), these studies focused on analyzing how the electronic character of these phenolics influenced their kinetic photodecomposition pathways as part of a TiO2-mediated catalytic system. They found a good correlation between the Langmuir-Hinshelwood (L-H) kinetic parameters for the photodecomposition and the Hammet constants among the halogenated phenolics. These correlations suggested that the halogenated phenolics followed analogous photodecomposition pathways that may have involved the formation of positively charged reaction centers on the phenolics. However, very little or no such correlation was evident among the nonhalogenated phenolics, likely because of competing reactions at the side chains of the aromatic nucleus. The present study investigated the titanium dioxide photocatalysis of dissolved lignin, one of the dissolved solids that causes problems in closed whitewater systems. The two main operational parameters, titanium dioxide loading and illuminating power, that influence titanium dioxide reactions were also studied in the present work. Not surprisingly, our studies also demonstrated that L-H kinetics could not be applied to the photodecomposition of the complex lignin system we were studying. 2. Experimental/Materials and Methods
* To whom correspondence should be addressed. Tel.: (919) 515-7707. Fax: (919) 515-6302. E-mail:
[email protected]. † Georgia Institute of Technology. ‡ North Carolina State University.
2.1. Apparatus. Samples were irradiated in a Rayonet Photochemical Chamber, reactor model RPR-100, made by Southern New England Ultraviolet Co. The
10.1021/ie0498302 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/11/2004
Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004 7997 Table 1 run no.
TiO2 loading (g/L)
power (W)
1 2 3 4 5 6 7 8 (filtered)
10 10 10 12 8 2 0 10
128 96 64 128 128 128 128 128
reactor was fitted with 16 VWR 8-W “black light” phosphor (350-nm) lamps, a VWR autostirrer (model RMA-500), and a cooling fan. The illumination energy was varied by removing bulbs from the reactor. The operating temperature within the reactor began at 21 °C and ended at 42 °C. Samples were magnetically stirred throughout irradiation by a 1-in. Teflon stirrer that was placed in the 500-mL wide-mouth glass flask reactor. Oxygen was purged through the sample during the reaction to propagate the production of superoxide radicals and prevent electron recombination. 2.2. Materials. Rutile titanium dioxide that was purchased from Fischer Scientific was used in all reactions. Deionized water was used as the medium. Fischer Scientific rutile titanium dioxide has a mean particle diameter of 100-300 nm and a mean agglomerate diameter of 400 nm with a surface area of 8.8 m2/ g.9 REPAP lignin was used as the lignin in all reactions. Whitewater from the Washington Georgia Linerboard mill, with no additional lignin, was used as the whitewater medium for the photocatalysis reaction. 2.3. Suspension Preparation. Approximately 0.02 g of REPAP lignin was placed in 15 mL of 0.05 M NaOH for 3 min to dissolve the samples into solution. Deionized water was then added to a total volume of 500 mL. pH adjustments were made using 0.05 and 0.5 M H2SO4, and reactions were run at a pH of 8. Titanium dioxide was then added and stirred for 3 min to ensure homogeneous distribution. 2.4. Experimental Design. Experimental runs were set up to test the two main process variables: titanium dioxide loading and irradiation intensity. The runs were conducted according to the conditions in Table 1. 2.5. Analysis. Samples were centrifuged in a Beckman CS-6 centrifuge for 20 min at 2800 rpm after the reaction to remove titanium dioxide from the medium. Sample concentrations were determined using a PerkinElmer UV/vis/NIR Lambda 900 spectrophotometer. A single-wavelength program that was set at 280 nm was used. This absorbance wavelength was determined by running a dissolved lignin/deionized water sample through the whole range of wavelengths to determine the global maximum absorbance. The spectrophotometer showed a linear relationship between the concentration and absorbances lower than 6. A total of 1.8 mL of the samples was diluted to half of the original concentrations to achieve values under this absorbance. Standards were used for calibration in determining concentrations. The total organic carbon (TOC) was obtained by determining the total carbon of the samples and then subtracting their inorganic content. The TOC was quantified by conducting a carbon dioxide coulometer analysis of the total carbons. Aliquots of approximately 5-mL samples were measured into a 10-mL beaker. The liquor samples were dried at 40 °C overnight in a ventilated hood. The residues were determined after the
Figure 1. Effect of TiO2 concentration on the percentage of lignin removed in the system.
dryness of the samples. The solid samples were transferred into the bows and used in the coulometer measurement. The instrument was run at 900 °C and under a stream of continuously running oxygen. Next, a capillary ion electrophoresis analysis (CIA) was conducted to determine the total carbon concentration. Because of the limit of the sample size, the inorganic carbon was also assessed using CIA. Approximately 1 mL of the liquor sample was used to analyze for inorganic carbon (assuming all of inorganic carbons are in the form of carbonate). The CIA unit is a Waters capillary ion analyzer equipped with a hydrodynamic sampling system controlled by Millennium 2010 software. Analytes are detected by indirect UV measurement. The operating current was 14.0 A at room temperature. The sampling time was set at 30 s, the running time was 6 min, and the electrolyte was 5 mM chromate/4 mM OFM-OH/10 mM CHES, which contained 4 mL of tetradecyltrimethylammonium bromide (TBBA), 5 mL of sodium chromate, and 10 mL of 2-(Ncyclohexylamino)ethanesulfonic acid (CHES). The employment of such conditions enabled a complete separation of the individual components of interest in the liquor samples. The content of the organic carbon was determined by the difference of the total carbon and inorganic carbon. The UV absorbance method demonstrated a good correlation for the disappearance of lignin as determined by TOC, although both methods were run for the lignin model whitewater samples, while the TOC was run for the mill whitewater because the UV method could not be adequately calibrated for concentration in this sample. 3. Results and Discussion 3.1. Effect of Illumination Time and Titanium Dioxide Dosage. It was found that all photolysis reactions provided decreasing lignin concentrations over time. Nevertheless, the presence of titanium dioxide was absolutely essential despite the UV light because a decrease of only 4.6% of the dissolved lignin was obtained without the catalyst. Figure 1 provides a plot of the influence of the illumination time and titanium dioxide dosage on the lignin removal. For example, it was found that, at 128 W and a loading of 10 mg/L of titanium dioxide, a decrease of 82% of dissolved lignin was achieved after 2 h of reaction. It was found in these sets of experiments that the highest rates of degradation are achieved early in the experiment and decrease as the experiment proceeds. This finding may be explained in the following way: the surface of the titanium dioxide particles provides sufficient reactive area for the lignin to react, and because the concentration of the reactant is high, not only do the reactants compete for reaction
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Figure 3. Effect of illumination power on lignin removal. Figure 2. Effect of titanium dioxide dosage on the energy use efficiency.
sites but partially degraded intermediates do also. This tends to inhibit oxidation of the original reactants; therefore, the rate of degradation of dissolved lignin in the medium decreases as the reaction progresses. Other results have shown similar patterns of reactant degradation.10 Figure 1 shows that the total level of lignin removal is a maximum at a TiO2 loading level of 10 mg/L. The results are consistent with work done by Paris et al.11 It was observed that lignin removal increased with increasing TiO2 loading because there are more sites available for reaction with increasing levels of titanium dioxide. This trend continued until a loading of 10 mg/ L, at which point lignin degradation decreased, even though the higher TiO2 loading provided more reaction sites. This is more than likely a result of light refraction by the TiO2 molecules as well as the turbidity of the suspension. At higher TiO2 concentrations, the photon flux is more easily intercepted by the catalyst before penetrating into the bulk of the system. When the light is refracted off of the surface of the titanium dioxide particle near the surface of the reactor, there is a greater probability that that the photons will escape the reactor before they are absorbed by another particle. This decreases the number of photons that are absorbed per unit time. This result could change drastically with the reaction design. A reactor with more surface area per volume could theoretically allow a higher titanium dioxide loading, which could possibly produce higher lignin degradation rates. The initial rates of reaction for the lignin in the suspension were shown to be dependent on the TiO2 dosage. At 10 mg/L loading of the catalyst, the initial rates of reaction decreased. As the reaction proceeded, there was less detectable lignin left to react, whereas the reaction intermediates built up and competed for reaction sites with the original lignin molecules, causing lignin reaction rates to decrease with time. The energy efficiency also reached a peak at a TiO2 loading at 10 mg/L, which further suggests that, given this reactor design, a TiO2 loading of 10 mg/L maximizes the number of photons that are adsorbed (Figure 2). This result demonstrated that the optimum titanium dioxide concentration to run these photocatalytic reactions of lignin is 10 mg/L. At this loading, there is not a maximum final degradation amount or initial rate of reaction efficiency trade-off, with efficiency referring to the decrease in the lignin concentration per joule of energy applied. 3.2. Effect of Illumination Energy. The higher the illumination power, the faster the initial rate of reaction and the lower the final lignin concentration after reaction. This result has been demonstrated previously with different substrates but with a much stronger
Figure 4. Effect of illumination power on the energy use efficiency over the whole reaction time.
dependence because the illumination energy ranges that were studied in this work were 2 orders of magnitude smaller than those in previous efforts.11 The range of power of illumination from 128 to 64 W corresponds to a light intensity range of 223-445 mW/cm3 for this experiment. At the highest light intensity level, lignin removal occurred much faster than that at the lower illumination intensities. At the highest light illumination power of 128 W, there was a 40% destruction of lignin in 5 min, while at the lower levels of light intensity, it required up to 1 h. The initial rates of reaction at the highest illumination power were 1 order of magnitude higher than those of the two lower levels. Additionally, the run at the light intensity of 96 W had an initial rate constant very similar to that of the run at a light intensity of 64 W. There was also only a 10% difference in the final lignin removal percentage between the two. This demonstrates that there is an increasing rate of return in lignin degradation for using higher illumination powers as shown in Figure 3. At different times in the reaction, the illumination energy efficiencies varied drastically. If these values are calculated over the whole reaction, there is a decreasing rate of return on energy efficiencies as the power levels increase (Figure 4). When energy efficiencies are calculated at an earlier point in the reaction, there is an increased rate of return on energy efficiency by increasing the illumination power (Figure 5). 3.3. Degradation of Whitewater Organic Carbon. The previous experiments were all conducted on model systems made of lignin and water. In this section, the photocatalysis reaction was conducted on the actual whitewater for comparison to the simulated runs. This reaction, however, was carried out at the optimum parameters established in previous experiments: 128 W and a 10 mg/L TiO2 dosage level for 2 h (Figure 6). To observe a decrease in the dissolved organic carbon, the dissolved material must be completely mineralized into carbon dioxide and the suspension left. The conver-
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Figure 5. Effect of illumination power on the energy use efficiency at 5 min.
that the overall degradation reactions in the whitewater are not as effective as those in the model system because a higher rate of degradation should be obtained given the 2-fold concentration excess of the substrate in the whitewater and the fact that this is a pseudo-first-order reaction. The results of the model system show that this is probably not the case because, as the TiO2 loading increased, lignin degradation did also, showing that the number of reaction sites was the limiting factor. This and the high correlation of the initial degradation rates between the model system and the whitewater system suggest that the number of reaction sites is indeed the limiting factor. 4. Conclusion
Figure 6. Degradation of organic carbon in whitewater.
Titanium has shown to be effective at reducing dissolved lignin levels in a water medium as well as the TOC in whitewater. The titanium dioxide loading for obtaining the most efficient dissolved lignin degradation was found to be 10 mg/L. This level proved to most efficiently utilize the energy applied to this particular system design, while achieving the highest initial reaction rate, and achieved the greatest reduction of lignin concentration over the 2 h span. The results showed that the higher the energy applied to the system, the higher the initial rate of reaction and the higher the dissolved lignin removal over the 2 h experiment, although the energy is used less efficiently over a 2 h reaction. However, the energy is used more efficiently at higher energy application levels during the initial stages of the reaction. Finally, it was determined that one of the limiting factors to the total lignin degradation is the number of available titanium dioxide reaction sites. Acknowledgment
Figure 7. Comparison of the reaction rates for whitewater and the model system over time.
sion of substrate into intermediates will not be interpreted as mineralization. Because of this, a substrate may need to go through a series of reactions before it is mineralized if it is a complex molecule. This may cause a slower reaction rate than that observed in the degradation of lignin. A 74% disappearance of TOC was achieved in the reaction, with a higher linearity than that observed with identical conditions on the model system. A total of 45 min was required for the whitewater system to degrade 40%, whereas only 5 min was required for the model system to degrade that much, more than likely because of the heterogeneity of the composition of the whitewater system. Nevertheless, the initial reaction rate of the whitewater system was 2.15 mg/Lmin, which matched very well the initial reaction rate, 2.11 mg/Lmin, of the model system, a result which strongly supports the consistency of the photocatalytic mechanism for lignin photodegradation with titanium dioxide. Reaction rates in the whitewater, however, were higher than those for the model system for most of the reaction (Figure 7). At time zero, there was 94.2 mg/L of organic carbon in the whitewater, which is approximately twice the amount of dissolved lignin present, 40 mg/L, in the model whitewater. These data suggest
We gratefully acknowledge the member companies of the Institute of Paper Science and Technology at the Georgia Institute of Technology whose generous support made this work possible. We are also indebted to M. Buchanan for his unremitting technical support of the work accomplished. Literature Cited (1) Lightsey, G. Operational Problems Resulting from Increased Paper Mill White Water Reuse. Department of Chemical Engineering, Mississippi State University, 1992. (2) Springer, A.; Marshall, D. Relation between Process Water Quality Characteristics and its Reuse Potential in Fine Paper Mills. (1) Study of Process Water Quality Characteristics Associated with Internal Process Water Reuse in Fine Paper Reuse in Fine Paper Manufacturing. Internal Process Reuse Water Quality Characteristics Data Repository, 1997. (3) Anon. Process Water Quality and Water Reuse Practices at Low- and Zero-Discharge Recycled Paperboard Mills; NCASI Technical Bulletin; 1999; p 796. (4) Park, S.-E.; Joo, H.; Kang, J.-W. Photodegradation of Methyl Tertiary Butyl Ether (MTBE) Vapor with Immobilized Titanium Dioxide. Sol. Energy Mater. Sol. Cells 2003, 80, 73. (5) Saquib, M.; Muneer, M. Titanium Dioxide Mediated Photocatalyzed Degradation of a Textile Dye Derivative, Acid Orange 8, in Aqueous Suspensions. Desalination 2003, 155, 255. (6) Stark, J.; Rabani, J. Photocatalytic Dechlorination of Aqueous Carbon Tetrachloride Solutions in TiO2 Layer Systems: A Chain Reaction Mechanism. J. Phys. Chem. B 1999, 103, 8524. (7) O’Shea, K. E.; Cardona, C. Hammet Study on the TiO2Catalyzed Photooxidation of Para-Substituted Phenols. A Kinetic and Mechanistic Analysis. J. Org. Chem. 1994, 59, 5005-5009.
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(8) O’Shea, K. E.; Cardona, C. The Reactivity of Phenol in Irradiated Aqueous Suspensions of TiO2. Mechanistic Changes as a Function of Solution pH. J. Photochem. Photobiol. A 1995, 91, 67-72. (9) Cabrera, M.; Orlando, M.; Cassano, A. Adsorption and Scattering Coefficients of Titanium Dioxide Particulate Suspensions in Water. J. Phys. Chem. 1996, 100, 20043. (10) Pe´rez, M.; Torrades, F.; Dome`nech, X.; Peral, J. Photocatalytic Treatment of Paper Pulp Bleach Effluents. Quı´m. Anal. 1997, 16, 211.
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Received for review March 2, 2004 Revised manuscript received June 7, 2004 Accepted September 11, 2004 IE0498302
