Ind. Eng. Chem. Res. 1997, 36, 3607-3612
3607
Kinetics of Phenol Photooxidation by Hydrogen Peroxide and Ultraviolet Radiation Asim K. De, Sekhar Bhattacharjee, and Binay K. Dutta* Department of Chemical Engineering, Calcutta University, 92 A.P.C. Road, Calcutta 700 009, India
Oxidative degradation of phenol by H2O2 and ultraviolet radiation is carried out, and a kinetic model is derived in order to represent the photooxidation reaction. The model equations are fitted by the experimental data collected for two distinct phases of reaction, marked as phase I and phase II. In the first phase H2O2 concentration is the controlling factor in the degradation process, whereas in the second phase concentration of phenol is the controlling species. A mechanistic explanation of the behavior has been presented. Introduction
of the form
Phenol and substituted phenolic compounds, such as catechol, chlorophenol, and hydroquinones are discharged in the effluent from a number of chemical process industries. Although biological treatment is a proven technology and cost effective, it suffers from a number of disadvantages. Concentration fluctuation, especially if the concentration of phenol goes beyond 100 mg/L, destabilizes the biological reactor, resulting in a discharge of the phenolic pollutants partially treated or practically untreated. Other competing technologies, such as activated carbon adsorption or steam stripping are not cost effective. Hydrogen peroxide has long been used as an oxidant or bleaching agent in the chemical process industries. Chemical oxidation of aqueous pollutants by H2O2 (oxidation potential 1.80 V) is commercially feasible but considerably expensive because of the cost of H2O2. A number of investigators (Hoigne and Bader, 1976; Malaiyandi et al., 1980; Mehmet and Resat, 1996; Paillard et al., 1990; Schmidt et al., 1993; Schulte et al., 1991) reported the effectiveness of enhanced oxidation using UV light and H2O2 for mineralization of waterborne pollutants (refractory organics, pesticides, dyes, etc.). Hydrogen peroxide in combination with UV radiation produces highly reactive hydroxyl free radicals (oxidation potential 2.80 V). Prengle (1983) reported that qualitatively the overall degradation of the parent organic molecule by the OH radical occurs basically in three steps: (a) partial oxidation of the initial parental species to form intermediates and some small fragments; (b) oxidation of the intermediates to form secondary intermediates and fragments; (c) further oxidation to form small and stable organic acid species. It was also observed (Glaze and Kang, 1989) that the rate of oxidation is a function of pH. The rate generally increases with a decrease in pH of the reaction medium. Detailed modeling of advanced photooxidation reaction is rarely available. Mehmet and Resat (1996) reported second-order reaction kinetics of o- and mchlorophenol oxidation by UV/H2O2 with respect to the oxidant. Yue and Legrini (1989) carried out similar studies on phenol, 4-chlorophenol, catechol, and a pesticide utilizing the advanced oxidation process of UV/ H2O2 and UV/O3. They modeled the rate of reduction of total organic carbon (TOC) by a power law equation * Author to whom all correspondence should be addressed. S0888-5885(96)00594-5 CCC: $14.00
-r ) k1[H2O2]a[TOC]b Sundstrom et al. (1989) studied UV/H2O2 oxidation of several aromatic hydrocarbons and phenolic compounds and suggested an empirical power law rate expression for the destruction of such compounds. However, most of the work reported so far dealt with a low level of pollutant concentration, and the empirical rate expressions suggested are not likely to be applicable when the pollutants are present at a rather higher concentration. In the present work we have made an elaborate study of destruction of phenol using UV/H2O2 over a wide range of concentration and reported the effects of important process parameters on the degree of destruction that can be achieved over a reasonable span of time. A rate model has been proposed and used to determine the rate equation for oxidation of phenol. Experimental Procedure and Analytical Methods A 300 mL flat-bottomed glass reactor (Figure 1), covered with a water-cooled jacket, containing an aqueous solution of phenol is placed on a magnetic stirrer for continuous stirring. A 6 W low-pressure UV lamp (Synko Denki, Japan) is immersed into the reactor as a source of radiation. A digital pH meter is used for measurement of pH of the reaction medium, and a thermometer is placed in the solution for measurement of temperature. In an experiment, a measured amount of hydrogen peroxide solution is added directly to the reaction mixture and samples are withdrawn periodically to estimate the concentration of substrates in the reaction mixture. Concentration of phenol and H2O2 in an aqueous solution is estimated by the well-known 4-aminoantipyrine (APHA, 1975) method, where the absorbance of a colored complex formed by phenol with 4-aminoantipyrine is measured at a wavelength of 510 nm. Concentration of H2O2 in the reaction medium is determined by the ceric sulfate (Mohanty, 1993) method. Absorbance at 510 nm for H2O2 concentration below 500 ppm is negligible. This indicates that as long as the concentration of H2O2 is below 500 ppm, it does not interfere with the estimation of phenol concentration. In our experiments, the reaction samples are diluted to a H2O2 concentration below 500 ppm before analyzing for phenol content. Initial and final COD (chemical oxygen demand) of the reaction medium are also estimated (APHA, 1975) to determine the percent COD reduction. © 1997 American Chemical Society
3608 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
-1 [(1 - xA)1-m - 1] ) kCA0m-1CB0nt 1-m or
(1 - xA)1-m ) 1 + kCA0m-1CB0n(m - 1)t
(5)
Estimation of the Value of n. From eq 5
ln(1 - xA) )
In wastewater, concentrations of phenol or substituted phenols are generally at milimolar level. Therefore
Figure 1. A schematic diagram of the experimental setup.
ln{1 + k(m - 1)CA0m-1CB0nt} ≈ k(m - 1)CA0m-1CB0nt
Results and Discussion Because the concentrations of phenol and hydrogen peroxide in a reaction medium can be readily estimated, it is convenient and practically useful to express the rate of oxidation in terms of the concentrations of phenol and H2O2 rather than the concentration of the free radical formed by photolysis of H2O2. In our analysis, we also estimated the concentration of hydrogen peroxide and the reaction order has been estimated with respect to H2O2 and not with respect to OH radical. The advanced photooxidation of phenol with UV and hydrogen peroxide can be represented as
A + zB f product(s)
(1)
where A is phenol, B is hydrogen peroxide, and z is the stoichiometric ratio. The reaction kinetics of the oxidation process can be expressed by a power law model of the form
rate ) -
dCA ) kCAmCBn dt
(2)
For a constant volume of the reaction mixture, CA and CB can be expressed as CA ) CA0(1 - xA) and CB ) CB0(1 - zRxA), where R ) CA0/CB0, CA0 and CB0 being the initial concentrations of phenol and H2O2, respectively. CA and CB are concentrations of A and B at any time, t. Therefore, eq 2 can be written as
dxA ) kCA0m-1CB0n(1 - xA)m(1 - zRxA)n dt
(3)
For complete oxidation of phenol or any phenolic substrate, the ratio, R, of the initial concentration of phenol to hydrogen peroxide is rather small (,1). Since xA , 1, (1 - zRxA)n ≈ 1. Therefore, eq 3 can be rewritten as
dxA ) kCA0m-1CB0n(1 - xA)m dt or
dxA (1 - xA)
m
) kCA0m-1CB0n dt
Integrating both sides we get
dx
∫0x (1 - xA )m ) kCA m-1CB n∫0t dt A
0
A
or
1 ln{1 + k(m - 1)CA0m-1CB0nt} (6) 1-m
0
(4)
because k(m - 1)Cm-1Cnt , 1. Equation 6 can be approximated as
-ln(1 - xA) ) kCA0m-1CB0nt
(7)
For each experiment, with known values of CA0 and CB0, a plot of -ln(1 - xA) against time, t, should give a straight line of slope k CA0m-1CB0n. A plot of the logarithm of slopes collected for a set of experiments against ln CB0 yields the value of n. Thus we can get the order of the advanced photooxidation reaction with respect to the H2O2 concentration. Estimation of the Value of m. Again from eq 4 we can deduce
ln(1 - xA) )
1 ln{1 + kRm-1(m - 1)CB0m+n-1t} 1-m
or
-ln(1 - xA) ) kRm-1CB0m+n-1t
(8)
A set of experiments are carried out with a constant value of R (CA0/CB0) where both CA0 and CB0 are changed proportionally. A plot of ln(1 - xA) against time yields a straight line with a slope of kRm-1Cm+n-1. A plot of the logarithm of slope thus calculated for each experiment against ln CB0 yields the value of m + n - 1. Using the value of n obtained as stated earlier, the value of m can now be calculated. The concentration history of phenol at a fixed UV lamp intensity of 24 W L-1 and a phenol to hydrogen peroxide concentration ratio of 0.0303 for an initial phenol concentration ranging between 1.063 and 10.63 mmol/L (i.e., 100-1000 ppm) are shown in Figure 2. In all cases the trend of conversion is similar. In the first 10-15 min, the conversion of phenol is substantially high. After the initial rapid reaction stage, the conversion rate becomes progressively slower. pH of the reaction mixture also shows (Figure 3) the same trend as the concentration changes. Immediately after addition of hydrogen peroxide, pH of the reaction mixture drops to the acidic range, from an initial value of 6.35 to about 3.5 in the first 15 min, and remains nearly constant at that level for the rest of the duration of reaction. These two observations show the existence of two distinct phases of the photooxidation reaction. Rance and Skelton (1994) also reported the existence of two different stages of UV/H2O2 photooxidationsa rapid oxidation stage followed by a slower destruction stage. These two phases are marked in our analysis as phase I and phase II.
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3609
Figure 2. Conversion of phenol at different time intervals and of different initial concentrations of phenol.
Figure 4. Plot of -ln(1 - xA) vs time from experimental data for two phases of oxidation.
Figure 3. pH change of reaction medium at different time intervals.
Figure 5. Plot of -ln(1 - xA) vs time from experimental data.
Considering the photooxidation reaction in two stages, plots of -ln(1 - xA) against time (eq 7) from experimental data are shown in Figure 4. From the data points in Figure 4, the two phases with different slopes are clearly discernible, one within around 15 min of reaction (occurs rapidly) and the other at the later part of the reaction (slower stage). Figure 5 presents the plot of eq 7 for phase I of the reaction kinetics. Figure 6 is the log-log plot of slope against H2O2 concentration corresponding to the first phase of reaction. The values of m and n obtained from the plots are 0.205 and 0.49, respectively. Figure 7 is the plot of eq 7 for the second phase of photooxidation reaction. The slopes obtained here are again plotted taking ln(slope) vs ln CB0 (Figure 8) as the axes, and the values of m and n thus obtained are 0.487 and 0.169, respectively, corresponding to the second phase of the photooxidation reaction. Therefore, the global kinetics of the two stages of advanced photooxidation reaction of phenol with H2O2 can be expressed as
rate ) 0.0354CA0.205CB0.49 Phase I (t < 15 min) and
rate ) 0.110CA0.487CB0.169 Phase II (t > 15 min) One mole of H2O2 in the presence of UV light produces two OH free radicals. In the earlier stage of the photooxidation reaction, because of the presence of high concentrations of OH radicals and phenol, the reaction rate is higher. Yue and Legrini (1989) observed that photooxidation is proportional to the square root of H2O2 concentration. At the earlier stage our results conform to that observation, i.e., in the earlier stage rate depends more prominently on the concentration of H2O2 and therefore on the OH radical. At the later stage, the UV radiation energy is distributed among phenol and intermediate compounds formed, and at the same time concentration of OH radical reduces appreciably. Due to the presence of acidic organic intermediate compounds, there is a competition between intermediate organic compounds and aqueous phenol to degrade by utilizing OH radicals available in the reaction medium and consequently conversion rate is slow and thus in the second phase, the controlling factor is the concentration of phenol available (as a competitive species) in the reaction medium.
3610 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 9. Effect of UV irradiation on conversion.
Figure 6. Plot of ln(slope I) vs ln CB0 to calculate n for phase I.
Figure 7. Plot of -ln(1 - xA) vs time from experimental data.
Figure 8. Plot of ln(slope II) vs ln CB0 to calculate n for phase II.
The oxygenated intermediates produced during advanced oxidation may be quinones, catechol, resorcinol, or a number of oxygenated organic compounds. German and Hoffmann (1993) showed that oxidation of pentachlorophenol (PCP) by using H2O2 and UV irradiation
proceeds via OH radical attack at the para position of the PCP ring to form a semiquinone radical which in turn disproportionates to yield p-chloranil and tetrachlorohydroquinones. Pintar and Levec (1992) reported from gas chromatography/mass spectrometry (GC/MS) analysis the formation of p-benzoquinone, 1,2-benzenediol, and 1,4-benzenediol as the reaction intermediates during degradation of phenol and chlorophenol. Most of these intermediates are more acidic in nature than the parent organic substrates and consequently their existence in solution will result in a sharp drop of pH of the reaction mixture. Effectiveness of advanced photooxidation with UV/H2O2 for mineralization of 2,4DNT has been demonstrated (Ho, 1986). Ho (1986) suggested that photooxidation of compounds such as 2,4dinitrotoluene and phenol results in hydroxylation by OH radical to di- and trihydroxybenzenes first when benzene ring cleavage occurs to produce lower molecular weight carboxilic acids and aldebydes which are eventually photooxidized to carbon dioxide and water. He also observed that depending on the concentration of hydrogen peroxide, the colorless initial solution turned from yellow to deep orange to brown in 2-10 min, and then faded gradually as exposure was continued, and finally became colorless. In our experiments, we also observed the change of color of the reaction medium within 5-10 min to reddish and reddish brown, and at the later part of the reaction color gradually faded away. Therefore, the two phases of reaction and formation of oxygenated intermediate compounds, e.g., resorcinol, catechol, benzoquinones, etc., are again supported by the rapid color change in the first phase and COD reduction of reaction medium. These UV light absorbing intermediates decrease the rate of oxidation of the parent compound because of competition for available UV light and OH radicals. The proposed rate equations for the two stages of photooxidation of phenol are applicable for a total UV light input of 24 W. We, however, studied the effect of higher UV dosage on the fractional conversion at different times as exhibited in Figure 9. It appears that higher UV dosage does not enhance the conversion proportionally. In fact doubling the UV input increase the conversion by 15-20% on the average. A comparison of the experimental degree of conversion with that calculated from the proposed rate equations is shown in Figure 10. The match is reasonably good. We have also compared our rate equations with that proposed by Sundstrom et al. (1989) who studied the kinetics of phenol degradation at rather low concentrations of the substrate. The comparison presented in Figure 11 shows that the prediction of rate expressions conforms well to that of Sundstrom et al. (1989).
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3611
Figure 10. Comparison of conversion-time profile obtained from experimental results and theoretical expression.
Figure 12. Typical percent conversion of phenol and percent COD reduction: CA0 ) 2.65 mmol/L, R ) 0.0606.
time profile, color change, and COD reduction support the model equations of two different phases of advanced photooxidation reaction. Increase of UV input from 24 to 48 W increases the conversion by 15-20%. The prediction of the rate equation conforms well with that of Sundstrom et al. (1989). Nomenclature
Figure 11. Comparison of reaction rate obtained from our correlation with that from Sundstrom et al. (1989).
COD analysis of the initial and the final reaction mixtures show a reduction of the COD value after degradation of phenol. Though the reduction of COD value is large, the reduction of COD and concentration change of phenol are not equivalent. From COD analysis it is observed that percent COD reduction is less compared to the percent conversion. This observation supports the hypothesis of the formation of intermediate organic compounds which are responsible for the slower phase of photooxidation reaction. A comparison between percent conversion and percent COD reduction at different time intervals is shown in Figure 12. Conclusion Detailed investigations were conducted to study the kinetics of advanced photooxidation reaction of phenol by UV and hydrogen peroxide. It is observed that in all cases the initial rate of conversion is much higher compared to the rate at the later part of the reaction. Photooxidation occurs in two different phases, the first phase (15 min) is controlled by the phenol concentration in the reaction medium. pH change of reaction medium, conversion-
CA ) concentration of phenol at any time t, mol/L CB ) concentration of H2O2 at any time t, mol/L CA0, CB0 ) initial concentration of phenol and H2O2, respectively, mol/L k ) reaction rate constant of advanced photooxidation reaction m ) reaction order with respect to phenol n ) reaction order with respect to H2O2 R ) molar ratio of phenol to H2O2 t ) reaction time, min xA ) fractional conversion of phenol at any time t z ) stoichiometric ratio of phenol to H2O2
Literature Cited APHA-AWWA-CPWF. Standard Methods for the Examination of Water and Wastewater, 14th ed.; The Association: Washington, DC, 1975. German, M.; Hoffmann, M. R. Photocatalytic degradation of pentachlorophenol on TiO2, identification of intermediates and mechanism of reaction. Environ. Sci. Technol. 1993, 27 (8), 1681-1689. Glaze, W. H.; Kang, J. W. Advanced oxidation process. Description of a kinetic model for the oxidation of hazardous materials in aqueous media with ozone and hydrogen peroxide in a semibatch reactor. Ind. Eng. Chem. Res. 1989a, 28, 1573-1580. Ho, P. C. Photooxidation of 2,4-Dinitrotoluene in aqueous solution in the presence of hydrogen peroxide. Environ. Sci. Technol. 1986, 20, 260-267. Hoigne, J.; Bader. H. The role of hydroxyl radical reactions in ozonation processes in aqueous solutions. Water Res. 1976, 10, 377-386. Malaiyandi, M.; Sadar, M. H.; Lee, P.; O’Grady, R. Removal of organics in water using H2O2 in presence of UV light. Water Res. 1980, 14, 1131-1134. Mehmet, H.; Resat, A. Photooxidation of some mono-, di-, and trichlorophenols in aqueous solution by hydrogen peroxide/UV combinations. J. Chem. Technol. Biotechnol. 1996, 67, 221-226. Mohanty, N. R. Oxidation of 2,4-dinitrotoluene using Fenton’s reagent: reaction mechanisms and their practical applications. Hazard. Waste Hazard. Mater. 1993, 10, 2, 171. Paillard, H.; Legube, B.; Gibert, M.; Dore, M. Removal of nitrogenous pesticides by direct and radical type ozonation. Organic
3612 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Micropollutants in the Aquatic Environment, Proceedings of the European Symposium, 1990. Pinter, A.; Levec, J. Catalytic liquid phase oxidation of refractory organics in wastewater. Chem. Eng. Sci. 1992, 47, 2395-2400. Prengle, H. W. Experimental rate constant and reactor considerations for the destruction of micropollutants and trihalomethane precursors by ozone with UV-radiations. Environ. Sci. Technol. 1983, 17, 743-747. Rance, P.; Skelton, R. L. Ultraviolet light assisted Fenton system for the destruction of organic material. RECOD Nuclear Symposium, London, 1994. Schmidt, M.; Seikel, K.; Bruecknann, G.; Trageser, M. Removal of chlorinated hydrocarbons from ground water by UV-induced oxidation. WLB, Wasser Luft Boden 1993, 37 (1-2), 48, 50-51. Schulte, P.; Volkmer M.; Kuhn, F. Activated H2O2 for the removal of noxious matter in water. WLB, Wasser, Luft Boden 1991, 35 (9), 55-56, 58.
Sundstrom, D. W.; Weir, B. A.; Klei, H. E. Destruction of aromatic pollutants by UV light catalyzed oxidation with hydrogen peroxide. Environ. Prog. 1989, 8 (1), 6-11. Yue, P. L.; Legrini, O. Photochemical destruction of organics in water. Paper 52D, AIChE National Meeting, San Francisco, CA, 1989. Yue, P. L.; Legrini, O. Photochemical degradation of organics in Water. Water Pollut. Res. J. Can. 1992, 27 (1), 123-137.
Received for review September 30, 1996 Revised manuscript received April 24, 1997 Accepted May 5, 1997X IE9605948 X Abstract published in Advance ACS Abstracts, July 1, 1997.
