1916
Ind. Eng. Chem. Res. 1991, 30, 1916-1920
Destruction of Phenol from Wastewater by Oxidation with S032--02 Umesh S. Kulkarni and Sharad G. Dixit* University Department of Chemical Technology, Division of Chemical Engineering, University of Bombay, Matunga, Bombay 400 019, India
Phenol is a highly toxic and undesirable water pollutant. The destruction of phenol in aqueous solution has been studied by using sulfite-oxygen as a n oxidant in the presence of cupric ions as a catalyst. Two distinct periods, namely, an induction period and a steady-state activity period, are observed. Acetic acid, oxalic acid, maleic acid in aqueous solution, and COSin gaseous form have been identified as major products. A carbon balance has been established by quantitatively determining COzand acids by using ion chromatography. A major reaction path has been proposed.
Introduction The effective removal of organic pollutants from industrial wastewaters is a problem of great importance. Phenol and substituted phenols are very common and are priority pollutants because they are extremely toxic, even in the parts per billion range, to aquatic life and impart a strong, disagreeable odor. The main sources of phenolic wastewater are the industries such as petrochemicals, hydrodesulfurization of flue gas, catalytic hydrogenation, coal gasification, pesticide manufacture, and electroplating and metallurgical operations. Microbial processes are sometimes very time consuming and operate well only for relatively dilute wastes. Chemical oxidation of phenolic wastes offers an alternative treatment method to destroy phenolics totally. In recent years oxidation by pure oxygen and ozone gas has received a great attention (Zimmermann, 1950; Randall and Knopp, 1980; Pruden and Le, 1976; Randall et al., 1985; Sadana and Katzer, 1974a,b; Walsh and Katzer, 1973; Vatistas and Gurol, 1987; Augugliaro and Rizzutti, 1978; Nekouinaini and Gurol, 1984; Duguet, 1987; Eisenhauer, 1964; Selm, 1959). Similarly, the oxidation of phenol has been carried out by using H202(Greenberg et al., 1978), permanganate, chlorine (Eisenhauer, 1964), and hypochlorite (Barbeni, 1987). Eisenhauer (1964) has studied the oxidation of phenol in aqueous solutions using ozone, peroxide, chlorine, and Fenton's reagent. He has demonstrated a multistep consecutive reaction with high conversion to several intermediates as the reaction proceeded and defined several of the important parameters affecting the rate and efficiency of the process. The free-radical nature of the oxidation reaction of phenol with oxygen and hydrogen peroxide has been reported by several workers (Delvin and Harris, 1984; Sadana and Katzer, 1974a,b). The oxidation of phenol in the aqueous phase with ozone has been studied in great detail (Gurol and Singer, 1983; Stich, 1987). Eisenhauer (1964) has proposed a reaction mechanism for oxidation of phenol by ozone in the aqueous phase. The present paper describes the complete oxidation of phenol using sulfite-oxygen as an oxidant and copper sulfate as catalyst. Devuyst et al. (1982) have used SO& as an oxidant to treat coke-oven wastewater and report some phenol removal. But, there is no detailed work on the oxidation of phenol using sulfite-oxygen as oxidant. Experimental Section Materials and Methods. All the reagents used were of analytical grade. Phenol was obtained from BDH Chemicals, Bombay. Oxygen gas of 99+% purity was used in the present work. The reagents employed for analysis were also of analytical grade. All the experiments and
* To whom correspondence should be addressed. 0888-5885/91/2630-1916$02.50/0
reagents were prepared in distilled water. The total phenolic group was measured by using the 4-aminoantipyrine method (as given in standard methods of examining water and wastewater). After the development of the color with the reagents, the phenolic group was determined on a Lambda 3B Perkin-Elmer spectrometer at A,, = 510 nm. As far as the quality of the effluents is concerned, the total concentration of the phenolics is critical. Therefore, in the EPA regulations there is a stipulation in terms of the total concentration of phenolics. Ion Chromatography. The Dionex Model No. 2010i ion chromatograph was used to determine product species. The column used was HPIC-AS4 (strong anion), and conductivity detection was with continuous background suppression, using 0.2 M H$04 as suppressor. The eluant was NaHC03 with a flow rate of 2.0 mL/min. The sample loop volume was 250 pL. Oxidation Experiment. Oxidation reactions were carried out at temperatures between 80 and 110 "C and oxygen partial pressures of 1.5-4.5 atm, in a 1-L (100mm-i.d.) stainless steel autoclave, as shown in Figure 1. The contact parts of the reactor vessel were coated with a corrosion-resistant polymer supplied by Penwalt India Ltd. The impeller was a 40-mm-diameter four-bladed disk turbine. Oxygen was supplied through a tube into the liquid phase directly beneath the impeller, and its pressure was maintained by a pressure gauge, as shown in Figure 1. The liquid sample line was connected with the gas inlet tube. The autoclave was heated by an electrical heating jacket, and temperature was controlled at fl "C. Typically the autoclave was charged with 750 mL of reaction mixture and 765 mg of CuS04. The reaction mixture was heated to a desired temperature, and during preheating all the valves of the autoclave were tightly closed. After attainment of the desired temperature, oxygen gas was sparged into the autoclave in the liquid phase and its partial pressure was maintained with the aid of a gas release valve. The samples were taken out periodically and were analyzed for total phenol concentration. During a typical oxidation reaction, the initial phenol concentration = 100 mg/L, Cu2+concentration = 765 mg/L, P(0,) = 4.5 atm, temperature = 110 "C, and the sodium sulfite amount = 3.0 g/L, except when the effect of any one of these parameters was specifically studied. The initial pH was maintained between 5 and 6 in all the cases. Experimental Results. The effect of various process parameters, such as catalyst loading, temperature, oxygen partial pressure, and initial phenol concentration, has been studied in the present investigation. Temperature. The effect of temperature was studied in the range of 80-110 "C by using 100 mg/L of phenol: Na2S03= 3 g/L; Cu2+ = 765 mg/L; P(0,) = 2.5 atm. It is observed from Figure 2 that temperature has a considerable effect on the rate of phenol oxidation. A t 110 "C, 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1917
TEMPERATURE = 11O'C
--+LIOUID
SAMPLE
THERMOCOUPL -REACTOR -HEAT I N C JACKET
TIME, min
Figure 4. Effect of catalyst concentration.
RHEOSTAT IMPELLER
Figure 1. Experimental setup. -325
ppm
TEMPERATURE = 110%
TIME, min
Figure 5. Effect of initial phenol concentration.
20
0
40
60 80 TIME, mln
100
120
1
+1.0 p m n2.0
Figure 2. Effect of temperature.
-0-1.5 100
gm
I
atm
+2.5atm - 6 3 . 5
L
5 eo
atm
-
F
5
TIME I m i n )
TEMPERATURE
60
= 1104:
Figure 6. Effect of sodium aulf'ite concentration.
t
:
Catalyst Concentration. Copper sulfate (CuSO,.
5H20) was used as a catalyst, and the effect of catalyst
LO
U
z
8 20 0
0
IO
20
30 LO T I M E , min
50
60
70
80
Figure 3. Effect of oxygen partial pressure.
phenol is completely destroyed in 15-18 min, while a t 80 "C about 80% of phenol is oxidized in 2 h. Oxygen Partial Pressure. The oxygen partial pressure was varied between 1.5 and 4.5 atm, and the results are shown in Figure 3. The oxygen partial pressure has a pronounced effect on the rate of phenol oxidation and the entire phenol content can be destroyed in a reasonable time period of 20 min at 4.5 atm of oxygen partial pressure.
loading was studied by varying the concentration from 100 to 765 mg/L. It can be seen from Figure 4 that an increase in catalyst loading considerably improved the rate of phenol oxidation. Initial Phenol Concentration. It is obvious that if the water contains the larger concentration of phenol, the time taken for destruction will be more. The experimental results a t different initial phenol concentration are shown in Figure 5. The concentration has been varied between 100 and 500 mg/L of phenol. For higher phenol loadings, the time taken is more. Sodium Sulfite Concentration. In the present investigation for most of the experiments, 3 g/L of Na2S03 has been used. This amount is much more than the theoretical requirement. However, the effect of the addition of lesser quantities of sodium sulfite has been studied, and
1918 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
.10-TEMPERATURE ,"C PRESSURE , a t m
+OXYGEN
30
1
*CATALYST
CONC., m g / l i t
- 2.
0.L
1
I
I
I
0.6
0.8
1.0
1.2
I 1.4
1-6
In ( P o z )
Figure 8. Order with respect to oxygen.
01
I
1
I
I
80
90 2-5
100 3.5
llO*TEMP.T 4 - 5 +POl otm
200
500
763--mg/tlt
I. 5 100
1
F i g u r e 7. Effect of temperature, oxygen pressure, and catalyst loading on the length of the induction period.
the results are shown in Figure 6. As is expected, the lower Na2S03 additions result in a slower rate of phenol destruction.
Discussion The complete oxidation of phenol by oxygen is given by the following reaction: CGH,OH
+ 702
-*
6C02 + 3H2O
tilo'
[%I
F i g u r e 9. Arrhenius plot.
(1)
The theoretical requirement of oxygen may be calculated from the above equation. The expected amount of C 0 2 has been calculated by using the above reaction. From Figures 2-6, it is seen that the entire reaction time can be divided into two distinct periods, the initial slow rate, giving an induction period, followed by a steady-state period, during which the rates of phenol destruction are faster. The induction period observed in the present investigation indicates that the reaction follows a free-radical mechanism, as suggested by Sadana and Katzer (1974a,b). The length of the induction period depends on the process conditions, such as temperature, oxygen partial pressure, and catalyst loading, as shown in Figure 7. It can be seen from the figure that an increase in these parameters drastically decreases the length of the induction period. It appears from the figure that the induction period may be completely eliminated if sufficient high partial pressure of oxygen is used. The induction period is followed by a steady-state activity period during which the rate of oxidation is relatively rapid. The order of reaction with respect to phenol concentration, oxygen partial pressure, sulfite concentration, and catalyst loading has been found by using multilinear regression analysis on LOTUS 123. A typical straight-line figure for oxygen partial pressure is shown in Figure 8. Similar straight-line plots were obtained for phenol, sulfite, and catalyst. The orders with respect to phenol, oxygen, sulfite, and catalyst are 0.5, 1.5, 1.0, and 0.2 respectively. The rate constants have been calculated for the steadystate period. They vary between 0.02 and 0.27, depending on process conditions.
6M A L E I C
ACID
--O- O X A L I C A C I D -ACETIC
"
ACID
\ TIME lmin
I
Figure 10. Variation of acid formation with time.
The activation energy given by the Arrhenius plot, as shown in Figure 9, is 57.5 kJ/kmol. Phenol Oxidation Products. The oxidation of phenol is very complex. Ion chromatographic studies of the reaction mixtures have been conducted to determine the formation of different carboxylic acids. Similarly, C 0 2has been determined by absorbtion in NaOH solution of known concentration. Figure 10 shows the formation of acetic, oxalic, and maleic acids as a function of time, along with phenol destruction with 500 mg/L of phenol as the initial concentration and other experimental conditions remaining the same, as mentioned earlier. It can be seen from the figure that during the induction period the formation of acetic, oxalic, and maleic acids is very low. It increases as the steady-state period begins. However, after 20 min, the concentration of all three acids decreases continuously until it is almost negligible. The formation
Ind. Eng. Chem. Res., Vol. 30,No. 8, 1991 1919 Table I acetic acid (B) concnf equiv of (mg/L) carbon 5 1.5 10 3.0 12 3.1 4.5 25 20.4 68 41 12.3 22 6.6 16 4.8 2.7 9 7 2.1
maleic acid (A) concn/ equiv of (mg/L) carbon 5 1.34 4.5 17 25 7.0 14.77 55 24.17 90 130 34.91 10.75 40 20 5.4 14 3.75 1.5 5.3
time/min 2 5 7 10 15 20 30 40 50 55 60
oxalic acid (C) concnf equiv of (mg/L) carbon 12.0 12.0 20 60 38 23 18 15 12
1.7 1.7 2.8 8.5 5.38 3.3 2.55 2.12 1.7
COP (D) concn/ (mg/L)
65.28 311.35 430.56 589.23 700.21 890.2 932.71 1042.17
total carbonO/mg
equiv of carbon/ mg
El
E2
17.80 84.91 119.89 160.69 191.8 242.78 254.37 284.28
2.84 9.20 11-80 39.87 137.98 172.48 181.29 203.66 251.36 260.67 284.28
3.45 11.48 17.22 40.17 160.65 235.24 274.5 277.69 282.29 282.32 286.89
E , = experimental value; E2 = calculated value. MICRONS
OH
6\
12
14
16
1600
1400
18
20
22
24
i
SO0
600
4 0
/Phenol
I
Catec hol
on Hydroqul none Ring
I
breakage
p W
/
--
H
Muconic a c i d
2,s-d 10x0 J -nexenedioic acid
Acetic acid M a l e i c acid
I
,on
+
no,
.c
I
0eC-Go
+
Oxalicocid
n-c:
OH 0
4
co,
1
t n,o
Formic acid
Figure 11. Proposed major pathway of oxidation of phenol.
of these three acids clearly indicates that the benzene ring is broken during oxidation. It suggests that the mechanism of oxidation of phenol by sulfite-oxygen is similar to oxidation carried out by dissolved oxygen (Delvin and Harris, 1984). They studied the oxidation of phenol with dissolved oxygen, and they have given a series of compounds that have been identified in the presence of oxygen. These include maleic, acetic, formic, oxalic, glyoxalic, succinic, muconic, fumaric, and acrylic acids, found in measurable quantities by Delvin and Harris (1984). The situation in the present investigation is similar to the situation using excess oxygen since, in the present investigation, a moderate oxygen partial pressure of 4.5 atm is used. The ring compounds, such as hydroquinones, catechol, and p - and o-benzoquinones, which have been postulated by Delvin and Harris (1984) as immediate intermediates, have not
1800
1200 1000 WAVENUMBER, C d
Figure 12. IR spectrum of solid precipitate.
been detected in the present investigation. [They were also not detected by Delvin and Harris (1984) under the condition of excess oxygen.] On the basis of the above observations, we propose that the breakdown of the benzene ring is very rapid with sulfiteoxygen as an oxidant and therefore present a major pathway of oxidation, as depicted in Figure 11. A carbon balance, based on the assumption that maleic, acetic, and oxalic acids, as well as COz, are the major products, has been carried out as a function of time. It is given in Table I. It will be seen from this table that in the first 7 min no measurable C 0 2 is formed, whereas maleic and acetic acids are formed from the beginning. On the basis of the disappearance of phenol, the total expected carbon is calculated. These values are given under E2 in Table I. The total carbon found experimentally is given under E , in Table I. The comparison of these values indicates that between 55 and 60 min almost all the carbon is accounted for by the four major products. Therefore, it may be concluded that other intermediates, if any, are either short-lived or formed in trace amounts. It can also be seen from the table that at the end of the reaction the major product is COz. Thus, phenol is completely decomposed to harmless COPand H20 during the oxidation using sulfite-oxygen as an oxidant. Characterization of Solid. It was found at the end of the reaction that a blackish-brown solid is formed in the reaction mixture. The solid was filtered off and dried at 105 “C,and an IR spectrum of the solid was recorded on a Perkin-Elmer IR spectrophotometer (Model No. 897) in the range of 1800-400 cm-’, using KBr pellets. The
1920 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 Table 11. Observed and Reported IR Frequencies (1800-400 cm-I): C U ~ S O , ( O H )HzO ~ (a) and C U ~ ~ S O ~ C U *2H20 ~ S O(b) ~ a b reported obsd reported obsd freaicm-I frea/cm-' frea/cm-' frea/cm-' 513 517 480 483 583 580 620 617 619 636 640 621 735 733 797 800 955 956 997 995 1105 1108 1383 1388
spectrum is shown in Figure 12. Conklin and Hoffmann (1988) have studied the solid precipitate formed in a CuS04-Na2S0, system. They have reported that two types of compounds, C U ~ S O ~ ( O H ) ~(a) H and ~ O Cu"S03Cu1S03.2H20 (b), are formed. It is found that the IR frequencies of the precipitate formed in the present investigation match with the frequencies reported by Conklin and Hoffmann for compound a. A few weak peaks for compound b are also seen. The observed and reported frequencies are shown in Table 11. Thus, it can be concluded that the solid formed at the end of the experiment mostly consists of compound a. Nature of Oxidant. The oxygenated/aerated aqueous sulfite system has received enough attention since the days of Backstrom (1934). The presence of a transition-metal element, such as cupric ions, is invariably present during sulfite oxidation. The reaction has been characterized as a chain reaction, wherein free radicals are the chain carriers (Lineck and Vacek, 1981). Various free radicals, such as SO3'-, SO5*-,02'and , OH'- (Hayon et al., 19721, HS03'-, HS04'-, and HS05'- (Fridovich and Handler, 1961), and SO4'- (Diester and Warneck, 19901, have been shown to take part in the reaction in the present system. However, it is believed that 02'-, SO3'-, and SO.,'- may have an important role to play as chain carriers, resulting in overall oxidation. The oxidizing power of SO$- has been reported to be much more than that of SO3'- (Huie and Neta, 1984) and it is likely that it plays a significant role in creating highly oxidizing conditions. Following Conklin and Hoffmann (1988), we believe that an intermediate formation of the type OH.Cu(O=O*-)~SO3 takes place in the oxidation process. Conclusions (1)The destruction of phenol by oxidation with SO:--02 in the presence of cupric ions as catalyst has been found to be very effective, and the entire phenol can be destroyed at a temperature of 110 "C and oxygen partial pressure of 4.5 atm, in a time period of 15-20 min. (2) The oxidation reaction shows two distinct periods, namely, an induction period and a steady-state activity period, thus indicating a free-radical mechanism. (3) The activation energy has been calculated to be 57.5 kJ/kmol, and the order with respect to phenol, oxygen, and catalyst has been found to be 0.5,1.5, and 0.2, respectively. (4) Acetic acid, oxalic acid, and maleic acid have been identified in aqueous solution and C 0 2 as the gaseous product. A carbon balance with respect to these indicates that other byproducts may be present in trace am.ounts. (5) The quantity of COP evolved indicates that the phenol ring is broken during oxidation and mostly oxidized
to COz at the end of the reaction. Registry No. PhOH, 108-95-2; CuSO,, 7758-98-7; Na2S, 1313-82-2 COz, 1 2 4 3 8 9 maleic acid, 11@1&7;acetic acid, 64-19-7; oxalic acid, 144-62-7.
Literature Cited Augugliaro, V.; Rizzutti, L. The pH dependence of the ozone absorption kinetics in aqueous phenol solutions. Chem. Eng. Sci. 1978,33,1441-1447. Backstrom, H. The chain mechanism in the autooxidation of sodium sulfite solutions. 2. Phys. Chem. 1934,B25, 122-138. Barbeni, M. Chemical degradation of chlorophenols with Fentons reagent. Chemosphere 1987,14,2225. Conklin, M.H.; Hoffmann, M. R. Metal Ion-Sulfur(1V) Chemistry. 1. Enuiron. Sci. Technol. 1988,22,883-891. Delvin, H.R.; Harris, I. J. Mechanism of oxidation of aqueous phenol with dissolved oxygen. Ind. Eng. Chem. Fundam. 1984, 23, 387-392. Devuyst, E. A.; Ettel, V. A.; Borbley, G . S. Paper presented a t the AIME Meeting, Dallas, TX, Feb 14-18, 1982. Diester, U.; Warneck, P. Photooxidation of SOS2-in Aqueous Solution. J . Phys. Chem. 1990,94,2191-2198. Duguet, J. P. Polymerisation effects of ozone: Applications to the removal of phenolic compounds from industrial wastewater. Water Sci. Technol. 1987,19,919. Eisenhauer, H. R. Oxidation of phenolic wastes. J.-Water Pollut. Control Fed. 1964,36 (91, 1966-1971. Fridovich, I.; Handler, P. Detection of free radicals generated during enzymic oxidations by the initiation of sulfite oxidation. J . Biol. Chem. 1961,236,1836-1840. Greenberg, E. S.; Keating, E. J.; Brown R. A. Phenolic problems solved with H202oxidation. Ind. Water Eng. 1978,15(7), 22-27. Gurol, M. D.; Singer, P. C. Dynamics of the ozonation of phenol mathematical simulation. Water Res. 1983,17 (9), 1173. Hayon, E.; Treinin, A.; Wilf, J. Electronic Spectra, Photochemistry and Autooxidation Mechanism of the Sulfite-Bisulfite-PyrosulfiC Systems. J . Am. Chem. SOC.1972,94,47-57. Huie, R. E.; Neta, P. Chemical Behavior of SO*- and SO6 Radicals in Aqueous Solutions. J . Phys. Chem. 1984,88,5665-5669. Lineck, V.; Vacek, V. Chemical Engineering use of catalysed sulfite oxidation kinetics for the determination of mass transfer characteristics of gas-liquid contactors. Chem. Eng. Sci. 1981,36 (ll), 1747-1768. Nekouinaini, S.; Gurol, M. D. Kinetic behaviour of ozone in aqueous solutions of phenols. Ind. Eng. Chem. Fundam. 1984,23 ( l ) , 54-59. Pruden, B. B.; Le, H. Wet air oxidation of soluble compounds in wastewater. Can. J . Chem. Eng. 1976,54,319. Randall, T. L.; Knopp, P. V. Detoxification of specific organic substances by wet oxidation. J.-Water Pollut. Control Fed. 1980, 52 (8), 2117-2121. Randall, T. L.; Dietrich, M. J.; Canney, P. J. Wet air oxidation of hazardous organics in wastewater. Enuiron. Prog. 1985,4(3), 171. Sadana, A.; Katzer, J. R. Involvement of free radicals in the aqueous phase catalytic oxidation of phenol over copper oxide. J . Catal. 1974a,35, 14Ck152. Sadana, A.; Katzer, J. R. Catalytic oxidation of phenol in aqueous solution over copper oxide. Ind. Eng. Chem. Fundam. 1974b,23 (2), 127-134. Selm, R. P. Ozone Chemistry and Technology;Advances in Chemistry Series No. 21; American Chemical Society: Washington DC, 1959; pp 66-77. Stich, F. A. Ozonolysis of organic compounds in a two-phase fluorocarbon-water system. Enuiron. Prog. 1987,21, 170. Vatistas, R.; Gurol, M. D. Oxidation of phenolic compounds by ozone. Water Res. 1987,21,895-899. Walsh, M.A,; Katzer, J. R. Catalytic oxidation of phenol in dilute concentration. I n d . Eng. Chem. Process Des. Deu. 1973,12 (4), 477-481. Zimmermann, F. Wet air oxidation of hazardous organics in wastewater. Chem. Eng. 1950,56,117-120.
Receiued for reuiew September 19, 1990 Accepted April 5, 1991
