Kinetics of Photocomposition of Dodecyl Benzene Sulfonate Takeshi Matsuura and J. M. Smith Department of Chemical Engineering, University of California, Davis, Calif. 96616
Continuous photodecomposition of dodecylbenzene sulfonate (DBS) was studied as a process for water purification. Aqueous solutions of DBS were passed through an irradiated tubular flow reactor operated with small conversions. Reaction rates were obtained for both nonsensitized conditions and when ferric perchlorate was used as sensitizer. Adding sensitizer increased the rate two orders of magnitude. Halforder kinetics with respect to light intensity and DBS concentration explained the data for nonsensitized conditions. An appropriate rate equation could be derived b y assuming a second-order deactivation of lightactivated DBS molecules. The sensitized reaction was believed to occur b y abstraction of hydrogen atoms from DBS b y hydroxyl radicals. Hydroxyl radicals presumably are produced b y an electron-transfer reaction involving light-activated ferric ions. The mechanism i s complex; over-all kinetics indicated a first-order 2 concentraeffect of (Fe+3), 1.2 order in light intensity, and maxima in the rate for intermediate DBS and 0 tions. Absolute values of the light intensity were obtained b y actinometric measurements with uranyl ionactivated decomposition of oxalic acid. This permitted the determination of kinetic rate constants.
P ~ o T o c m i w c x Lprocessing- offers a new route for purification of water resources. While photodecomposition may be uneconomical for secondary treatment, it could be attractive for waters containing residual pollutants not completely removed by conventional methods. Linear alkyl benzene sulfonates (LhS) are water-soluble surfactants widely used in textile dyeing operations. The presence of LAS in excess of 1 p . p m . causes a foam problem for approximately 5 days under normal primary treatment conditions. Xonaka (1968) studied the elimination of foam by boiling the water containing LAS with hydrogen peroxide, with and without ultraviolet radiation. Bishop et al. (1968) decomposed alkyl benzene sulfonates with a ferrous-ferric ion catalytic system in the presence of hydrogen peroxide. Other investigations of radiation-induced decomposition of organic compounds in water iiiclude that of Neiners and colleagues (1968) , who reported that ultraviolet radiation strongly accelerates the rate a t which aqueous chlorine oxidizes starch and other organic compounds. Bulla and Edgerley (1968) used ultraviolet light to eliminate organic pesticides from aqueous solution and studied the effects of operating variables. Finally, Armstrong (Armstrong and Tibbitts, 1968) applied photochemical combustion of organic matters for determining nitrogen, phosphorus, and carbon in sea water. Our study has as its objective measuring rates of decomposition of DBS with and without photosensitizers. A photosensitizer increase the absorption of radiation energy, accelerating the rate of reaction. I n particular, ferric ions are believed to act as sensitizers through an electron transfer process in the ion complex Fe3+0H-. Ferric ion is reduced to ferrous, and the hydroxyl radical OH. is formed. The latter free radical then can initiate chain reactions, either by hydrogen abstraction or by addition of olefinic molecules. This process has found extensive application in photoinitiated polymerization (Dainton and James, 1958; Evans et al., 1951) of olefins and in photoinitiated oxidation of benzoic acid (hydrobenzoic acid is the main product) as reported by Bates and colleagues (Bates and Uri, 1953; Bates et al., 1950). The 252
Ind. Eng. Chem. Fundam,, Vol. 9, No. 2, 1970
complex (Fe3+0H-) is usually obtained by introducing ferric ions with dilute solutions of perchlorate. The novel feature of our work was to use this ion pair as a photosensitizer to initiate oxidative deconiposition of organic water pollutants. The increase in reaction rate was striking. It appears that heavy metal ions can act as an oxidation-reduct,ioii catalyst in a way similar to Fenton's reagent, particularly in the presence of dissolved oxygen molecules (Lipman, 1937; Uri, 1952). Vsing these concepts and the observed data, rate equations are proposed for the decomposition of DBS in aqueous solution for both sensitized and lionsensitized systems. Measurements were made in a continuous-flow, differential reactor operated a t atmospheric pressure and from 15' to 60°C. The DBS coiicentration range was 60 to 182 p.p.m. and the radiation was in the range 2000 to 4500 A. For the photosensitized reaction, the concentration range of ferric perchlorate was 0.04 to 3.15 X lo-' g. mole/liter. Flow rates and reactor length were controlled so that the conversion of DBS was between 3 and 25%. Smaller conversions introduce errors in the analysis and larger values introduce deviation from differential reactor operation, resulting in inaccurate estimation of the reaction rate. blassaldi and Maymo (1967) have reported that no significant error in rate occurs up to conversion of about 30%, provided the reaction is less than first-order. I n analyzing the data, plug-flow conditions for velocity were assumed. For differential react'ors Denbigh (1966) has shown this to be a satisfactory procedure. Experimental
The apparatus and experimental procedure were very similar to those of earlier work (Matsuura and Smith, 1971) on the photodecomposition of formic acid solutions. Feed solution flowed from a reservoir through the reactor and into a stripping column, where oxygen, nitrogen, and COn were removed by helium and carried into a gas chromatograph for analysis. The photoreactor system was of the ellipticalreflector type, with reactor and lamp located a t the foci of the ellipse. The reactor, of fused, optically clear quartz, had an i.d. of 20 mm., 0.d. of 22 mm., and a jacket through which
Table 1. l a m p Characteristics, Transmission of Filter Solutions, and Absorptivity of DBS TXt
A, A.
2000-2050 2050-2100 2100-21 50 2150-2200 2200-2250 2250-2300 2300-2350 2350-2400 2400-2450 2450-2500 2500-2550 2550-2600 2600-2650 2650-2700 2700-2750 2750-2800 2800-2850 2850-2900 2900-2950 2950-3000 3000-3100 3100-3200 3200-3300 3300-3400 3400-3500
Ein.,kec.
FX/L~
0 0 0 0 0.0306 0 0.0265 0.0516 0.02618 0.03762 0.0913 0.02420 0 0.1101 0.01954 0.01706 0.0583 0,01909 0 0.0673 0.1489 0.2366 0 0.03460 0
0 0 0 0 17 ,150 0 14,850 28 , 970 14 ,680 21 > 100 51 ,200 13,580 0 61 ,700 10,950 9 , 680 32 , 640 10,690 0 37 ,760 83 ,400 132 ,800 0 19,390 0
ax(3O0C.),
%
L./(G. Mole)
1N
2N
3N
4N
5N
6N
(Cm.)
1.514 2.128 2.484 2.706 2.943 3,237 3.667 4.383 5.566 7,201 9.859 13.41 18.14 24.12 30.54 37 .07 42.88 47.89 50.75 52,75 55.55 57.22 58.08 61.62 67.59
0.044 0.074 0.092 0,105 0.121 0.142 0.177 0.242 0.372 0,597 1.090 1.986 3.596 6.322 10.08 14.78 19.66 24.34 27. 07 29.08 31.99 33.81 34,67 38,79 46.38
0.001 0.003 0.003 0.004 0.005 0.006 0.009 0.013 0.025 0.049 0.121 0.294 0.713 1.657 3.326 5.894 9,011 12,37 14.44 16.03 18.42 19.98 20.70 24.42 31.82
0.000 0.000 0,000 0.000 0.000 0.000 0.000 0.000 0.001 0.002 0,006 0.021 0,077 0.263 0.724 1.665 3.083 4.878 6.086 7.070 8.62 9.69 10.18 12.92 18.97
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0,000 0,000 0,000 0,001 0 004 0.021 0.091 0.297 0.714 1.371 1.873 2.315 3.064 3.613 3.869 5.425 9.364
0.000 0.000 0.000 0.000 0,000 0.000 0.000 0.000 0.000 0.000 0.000 0,000 0.000 0.002 0.013 0.059 0.182 0.420 0.624 0.816 1.166 1.439 1.569 2.413 4.846
13011. 8379, 9004 12076. 13764. 11044. 5129. 1133. 381. 381. 452. 475 452. 375. 230. 111. 37.0 28.5 27.8 15.1 15.1 15.1 6.7 6.7 0
Filter Solution
1N 2N 3N 4K 5x 6N
distilled water was circulated to maintain constant temperature. The irradiated length was adjusted to give reasonable conversions. For slow, nonsensitized reaction a 200-mm. length was irradiated, while for sensitized reaction runs 10 mm. was sufficient. The flow &as always streamline (Re < 14) and a straight entrance region of L / D = 15 was provided before the irradiated section. A Hanovia (LL, 189A10, 1200-watt) mercury-vapor lamp, with the energy output spectrum given in the first coluniii of Table I, was the source of radiation. The lamp was surrounded by quartz jackets formed by concentric tubes whose id.-0.d. measurements were 42 to 45, 57 to 61, 70 to 74, and 85 to 89 mm. Through the annular space between the lamp and first tube, air flowed at room temperature for cooling purposes. Distilled water was pumped through the second jacket, and through the third flowed solutions of FeCI3.6Hz0, adjusted to p H 1.8 with HCl. The walls of the outer jacket were carefully washed; this space contained air. Different intensity levels a t the reactor wall were achieved by varying the concentration of t h e FeC13.6H~0solutions. For nonsensitized reaction runs the concentrations used were: solution l N , 0.740; 2K, 1.48; 3K’, 2.22; 4K, 3.24; 5K, 4.62; and 6N, 5.92 X 10+ g. mole/liter. For sensitized reaction runs the solutions and their concentrations were: IS, 1.11; 2S, 2.22; g. mole/liter. 3S, 3.24; 4S, 4.62; and 5S, 5.65 X The DBS solutions were prepared from sodium dodecyl benzene sulfonate obtained from K and I< Laboratories. The solid material had a purity of 95%, with the remainder mainly water. Biodegradability and isomer distribution were not available. The aqueous solution had a p H of 6.75. The ferric perchlorate solutions used for sensitized runs R ere
22.67 9.83 5.25 2.74 1.35 0.756
made by preparing a precipitate of Fe(OH)3 and dissolving it in a n aqueous solution of HC104. T o prepare a feed for the sensitized runs, this solution was added to the aqueous solution of DBS and allowed to stand for 15 hours. After filtration to remove the scum of ferric ion-DBS complex, the filtrate was kept in a dark, cool location until used. All sensitized runs were made with this filtrate, after adjustment to pH 3.0, since some precipitation occurred outside of a narrow range in the neighborhood of 3.0. The oxygen content of the solution was changed (from 4.5 to 37.1 p.p.m.) by bubbling nitrogen, oxygen, or air through the reservoir holding the feed to the reactor. The extent of the reactions was evaluated from DBS and COZ analyses. Further, oxygen and ferric ion concentrations were needed to describe operating conditions. The DBS concentration was determined by the methylene blue method (American Public Health Association, 1965). The standard deviation of this analytical method is reported to be +6%. Experiments were performed a t several flow rates and the results were averaged in order to reduce the error due to uncertainty in the DBS analysis. The analysis of the gaseous compounds separated in the stripping column was described in the earlier formic acid work (hlatsuura and Smith, 1971). Helium was fed to the bottom of the column, the outlet of which was directly connected to the sampling system of the gas chromatograph. The reproducibility of the oxygen analysis was i5%, slightly less accurate than the COZ determination. Concentration of ferric ion was evaluated by measuring the absorption of sample solutions a t a wavelength of 3000 A. The absorptivity of DBS is practically zero at this wavelength. However, absorption of radiation by intermediate Ind. Eng.
Chem. Fundam., Vol. 9, NO. 2, 1970 253
i
0.121 V
e,
Temp e rat ure ,
2 8 "C
Saturated by O2 F l o w Rate,
7 3 cm3/min
FI Iter So Iu t io n
# 3N
v
\
I L! ~
80
40 60 DBS Concentration K 10:g.mole/(cm') Figure 1.
Typical rate data for nonsensitized reaction
reaction products introduced some error in the ferric ion analysis a t higher conversions. Preliminary Runs
The average rate of decomposition of DBS over the cross section of the reactor was calculated from the measured concentrations of DBS by using the equation:
Also preliminary runs in which the pH of the DBS solution was measured before and after irradiation indicated negligible change. The intermediate products formed during the decomposition of DBS do not seem to affect the pH. Oxygen is known (Calvert and Pitts, 1966) to be a scavenger in some free-radical reactions. Hence nonsensitized runs were made in which the dissolved oxygen concentration in the feed was varied from 5.4 to 28.1 p.p.m. The measured rates were 0.108,0.111, and 0.094 X g. m~le/(cm.~)(sec.), respectively. While these results showed no significant effect of oxygen, the situation was different for sensitized runs. Typical results for the effect of DBS concentration on the rate are shown in Figures 1 and 2. The transmission characteristics of the filter solutions (35 and 2s) used for these data are given in Tables I and IV.
Table II. Correction Factors (f) for Nonsensitized Reaction CDBB
(G.M ~ l e / C m . ~ )
x
108
6.86 13.71 26.28 42.70 52.20
Filter Solution
1N
2N
3N
4N
5N
6N
1.077 1.129 1.196 1.261 1.293
1.038 1.063 1.098 1.132 1.148
1.016 1.029 1.046 1.064 1.073
1.005 1.010 1.018 1.028 1.033
1.002 1.004 1.008 1.013 1.016
1,001 1.002 1.005 1.008 1.010
Table 111. Actinometer Characteristics &'A
Preliminary runs were made to evaluate how high conversion z could be and still retain differential reactor behavior. Nonsensitized data a t 28°C. and different flow rates showed that the rate calculated from Equation 1 was constant u p to a conversion of about 30%. At 31% conversion the rate was 0.204 X l o p 9 g. rn~le/(cm.~)(sec.), and a t 12% Conversion both for a feed saturated with oxygen it was 0.218 X g. mole/cm.a and with a DBS concentration of 26.4 x Similar results were obtained for sensitized runs. It is concluded that rates may be evaluated assuming differential reactor operation up to about 30% conversion, because reaction order is less than unity with respect to DBS. Preliminary measurements of DBS concentration in the feed solution (stored in a dark cool place) for 25 hours showed no change, indicating that the dark reaction was negligible.
28 "C
Temperature,
Saturated by Air
h
Q)
Ferric I o n Conc,
CFe3+,
8 O x IO-;.mole/(liter)
A
"E
I
0 Figure 2.
254 Ind.
# 2S
Filter Solution,
I
I
1
1
1
I
I
20 30 DBS Concentration x 10: g.mole/(cm') IO
Typical rate data for sensitized conditions
Eng. Chem. Fundam., Vol.
9, No. 2, 1970
Fh Tx Ftot
= 89.92 l./(g. mole) (cm.)
A,
@A!
A
G. MolelEin.
2000-2050 2050-2100 2100-21 50 2150-2200 2200-2250 2250-2300 2300-2350 2350-2400 2400-2450 2450-2500 2500-2550 2550-2600 2600-2650 2650-2700 2700-2750 2750-2800 2800-2850 2850-2900 2900-2950 2950-3000 3000-3100 3100-3200 3200-3300 3300-3400 3400-3500 3500-3600 3600-3700 3700-3800 3800-3900 39004000 4000-4100 4100-4200 42004300 43004400 44004500
0.460 0.483 0.501 0.520 0.538 0.555 0,568 0.579 0.588 0,594 0,599 0,598 0 587 0,580 0.580 0.580 0.579 0,577 0.575 0,573 0.567 0.558 0,545 0.528 0.509 0.496 0.491 0.504 0.528 0.545 0.558 0.568 0.574 0.579 0.582
ff 'A,
L/(G. Mole) (Cm.)
33400 26940 21460 17890 14520 12600 10540 9360 8360 7460 6790 6330 6050 5640 5160 4610 4490 3340 2762 2100 1521 937.0 577.0 318.0 155.5 74.6 43.9 38.2 36.6 33.4 33.4 31.8 30.4 28.3 26.4
Transmission of Filter Solutions and Absorptivity of Ferric Perchlorate
Table IV. A, A.
1s
2s
3s
4s
2000-2050 2050-2100 2100-2 150 2150-2200 2200-2250 2250-2300 2300-2350 2350-2400 2400-2450 2450-2500 2500-2550 2550-2600 2600-2650 2650-2700 2700-2750 2750-2800 2800-2850 2850-2900 2900-2950 2950-3000 3000-3100 3100-3200 3200-3300 3300-3400 3400-3500 3500-3600 3600-3700 3700-3800 3800-3900 3900-4000 4000-4100 4100-4200 4200-4300 4300-4400 4400-4500
0.146 0.264 0.366 0.443 0.513 0.590 0.694 0.886 1.25 1.84 2.90 4.79 7.57 11.59 16.47 21.95 27,38 32.27 35.50 37.82 40.72 42.76 43,78 47.73 54.83 60.60 70.20 78.82 85,92 89.35 92.62 93.70 95.30 96.25 96.34
0.001 0.002 0.003 0.003 0.004 0.005 0.007 0.012 0.022 0.044 0.110 0.273 0.668 1.56 3.12 5.53 8.50 11.69 13.83 15.48 17.79 19.42 20.19 23.84 31.17 38.87 51.88 65.02 76.99 82.77 88.51 90.39 93.18 94.58 94.66
0.000
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0,000 0.000 0.000 0.000 0,001 0.004 0.020 0.085 0,278 0.674 1.30 1.79 2.24 2.96 3.51 3.78 5.30 9.17 14.85 26.95 42.84 60.69 70.13 80.22 83.62 88.76 91.06 91.14
0.000
0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.001 0.005 0.020 0.072 0.250 0.68 1.56 2.91 4.61 5.83 6.83 8.33 9.42 9.93 12.62 18.58 25.87 39.32 54.50 69,62 77.17 84.90 87.46 91.28 93.07 93.16
Filter Solution
[L./(G. Mole) (Cm.)] l . g
1s 2s 3s 4s 5s
2082 896 464 220 144
Nonsensitized Photolysis
Development of Rate Equation. There is little published information on the photodecomposition of alkyl aromatic substances in the presence of oxygen (Livingston, 1960). The following mechanism is a hypothesis suggested by our rate measurements. Since these d a t a mere for initial rates, only the primary reactions are significant, although they are certainly followed by a succession of secondary processes. T h e d a t a showed a one-half order dependency upon light intensity. This suggests t h e following activation and deactivation steps: DBS
L./(G. Mole) (Cm.)
5s
0.000 0.000 0.000 0,000 0.000 0.000 0.000 0.000 0,000 0.000 0.000 0.000 0.000 0.004 0.021 0.088 0.254 0.556 0,818 1.06 1.48 1.82 1.98 2.97 5.73 10.26 20.94 36,49 55.38 65.80 77.24 81,15 87,ll 89.74 89,82
+ hv
intensity of absorbed light, per unit volume, Iu. If the stationary state hypothesis is applied for DBS*, (DBS*)
(2)
ka
=
e)'''
(4)
The deconiposition of DBS is assumed to occur by a much slower first-order process:
ka
DBS* decomposition products Then the point rate of decomposition is given by
(5)
--dCDBs - k3(DBS*) = ka
la + DBS*
10710 10504 10220 9836 9442 9163 8917 8630 8217 7822 7445 7042 6657 6394 6216 6038 5865 5778 5693 5660 5526 5199 4869 4462 4200 4002 3675 3262 2748 2320 1944 1659 1352 1044 846
dt
Since l a
=
l a C ~ ~the s , rate for light of wavelength h is:
2DBS* + 2DBS
(3) where the rate of the activation step is proportional to the
(7) Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
255
The point rate summed for all wavelengths, assuming that k3 and kz are independent of X, will be
0
0) v) v
*-
h
Temperature ,2 8 “C DBS Concentration, COBS,13.78IO’ g.mole/(cd) S a t u r a t e d by 0 2 Flow Rate , 7.3crn3//(rnin)
T o obtain an average rate for the whole cross section of the reactor tube, the light intensity pattern should be known. However, previous work (Matsuura and Smith, 1971) with the extremes of diffuse and radially incident light patterns gave essentially the same relation between local and average rates. On the basis of these results, the radially incident model is used here. I n this case the intensity a t radius r, from Lambert-Beer’s law, is IX = I,,xR/r{exp[-*x(R
- 791
+ exp[-wx(R + 791)
(9)
where p x = a x C D B s and Izo,x,is the intensity a t the reactor wall. Using this expression for Ix in Equation 8, and integrating over the radius of the reactor tube, gives the following average rate:
Figure 3. reaction
Effect of light intensity on rate of nonsensitized
(10) The concentration of DBS can be removed from the integral because it does not change significantly for differential reactor operation. The equations are simplified by introducing a hypothetical intensity I b , t o t which is the total (for all wavelengths) intensity which would exist a t the reactor wall if no filter solution lvere used. Thus
where Tx is the transmission of the filter solution for light of wavelength X. It is convenient to use a corrected rate which is defined as the rate if the absorptivity of DBS were zero, as far a4 its effect on diminishing the light intensity is concerned. This means that px = 0 in Equation 10. Then the integration i; easily performed. The result, after introducing Equation 11, is
The corrected rate is related to the actual value by the correction factor, f,
ncorr fa =
Using Equation 12 for ~,,,,, and Equation 10 for troducing la, tot from Equation 11, gives forf:
(13)
a after in-
The absorptivity for DBS was measured with a Beckman DK-2h spectrophotometer, as were the transmission eoefficicnts of the various filter solutions. The output F A of the lamp mas available. All this information is given in Table I as a function of wavelength. Using these data f could be calculated as a function of C D B s for each filter solution (Table 11). With f known, &,, was obtained from the ex256 Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
Figure 4.
(~l,,,, vs. ( C D B S ) ~ ’ ~
perimentally determined fi using Equation 13. Then &,,, was analyzed to determine the suitability of the proposed mechanism by using Equation 12. Effect of Light Intensity. D a t a for different light intensities, at constant spectral distribution, were obtained by using filter solutions 2K t o 6N. According to Equation 12, the rate should be proportional to Z [ a x ( F x / F t o tTA]l’*. ) These runs were taken a t 28°C. and for DBS solution saturated with oxygen. Figure 3 shows t h a t the d a t a agree with a one-half order effect of light intensity. Effect of DBS Concentration. If Equation 12 is valid, the rate should be one-half order in CDBS.This requirement was also satisfied. Figure 4 shows the results for data taken with
filter solution 3N. Both the effect of intensity and C D B s confirm the half-order effect of absorbed light postulated by Equation 6. Actinometer Results. Before t h e rate constant ratio k 3 / k 2 1 ’ 2 can be evaluated, the total intensity I * , tot must be known. This was accomplished by using t h e uranyl ionsensitized decomposition of oxalic acid as actinometer. I n
1
Conc., 0.05 g.mole/(liter)
'D ,.
j
Filter Solution, # 2 s Reactor Length
-
' I
Initial Uranyl Sulfate Conc., 0.01g moie/(iiter) h
V
E o.o$
Icm
W
~
Temperature
,
by
Saturated
I
28 "C
DBS Concentration 0,
13 7 %IO-' g. mole/(crn3)
Flow R a t e , Filter Solytion
7 3 crn/(rnin) #3N
--7--(
Mean Residence Time, V/Q, sec Figure 5.
Figure 7.
Results of actimometer runs
this approach t h e extent of decomposition of oxalic acid is measured in t h e same apparatus. T h e procedure is identical t o t h a t described by Cassano and Smith (1967). Figure 5 shows t h e results of the actinometer runs which were made after all t h e DBS d a t a had been taken. The slope of the line in Figure 5 is proportional t o t h e rate of reaction f for t h e differential reactor operation. Runs were made for two irradiated lengths to check for possible end effects. T h e d a t a in t h e figure indicate t h a t such effects were insignificant. T h e total intensity is obtained from t h e measured rate [i' = 1.44 X lo7 g. mole/(sec.) (cma8)] and t h e following equation (Cassano and Smith, 1966, 1967) :
where @ A is the quantum yield of the reaction and cu'x the absorptivity of the uranyi sulfate solution. These data are available and given in Table 111. The measurements were
Effect of pH on reaction rate
made for filter solution 2 s (Th in Table IV). Using all these Einstein/ values in Equation 15 gives I b , tot = 5.76 X ( a x 2 )(sec.). Evaluation of Rate Constant Ratio. Knowing I o , Equation 12 was used to obtain k 3 / k P 2 for all of the data. The average value was 3.i6 X lop7 (g. niole)1/2/(sec.)1/z (cm.)a'*. Hence the results of this work show that the unsensitized rate of deconiposition of DBS, according to Equation 7 , is
fib
=
3.76
x
1 0 - 7 ( 1 ~ ~ ~ ~ ~ (16) ~ ~ ~ c ~ ~
for a temperature of 28°C. and a p H of 6.75. The effect of temperature was studied by runs a t one set of conditions where t lvas changed from 15" to 60°C. The data indicated a low activation energy of 2.3 kcal./g. mole (Figure 6). A fen. runs were also made a t other acidities, since there was evidence (Bishop et al., 1968; Edgerley et al., 196i) t h a t p H would affect the rate. The acidity was adjusted by adding sodium hydroxide or perchloric acid to the neutral (pH 6.75) DBS solution. Figure 7 shows that the rate increases significantly as the pH is decreased. Sensitized Photolysis
Rate Equation. The effect of adding ferric perchlorate t o the feed t o t h e reactor is a large increase in rate. I n addition, t h e rate us. C D B relationship ~ shows a maximum (Figure 2 ) . The effect of oxygen concentration is similar t o t h a t for C n B s . T h e first step in the reaction,scheme is believed to involve the production of hydroxyl radicals through a n electron-transfer process, which may be represented as follows:
h
0
al cn
C v I
*E 0.07 o 0.06 \ 0.05
Y
0.04 0 Q)
E
Fe3+(0H-)
0.03
+ .
Fe2+(OH.) +. F e z +
+ OH.
(17)
This belief is based upon the extensive evidence (Uri, 1952) that photoexcited electron-transfer processes can explain the initiation of polymerization of vinyl compounds and oxidation of organic substances. According to this vieiv, the second step nould be the deconiposition of DBS by hydrogen abstraction:
m"
- 002
0 X
IC Q 01 29
+ h~
30
31 I/TX
32
io:
33
34
35
?K)-'
Figure 6. Temperature effect on reaction rate of nonsensitized reaction
OH.
+ DBS
+ .
HzO
+ free radical
4
decomposition products
(18)
Various combinations of metallic ions and anions initiate photochemical polymerizations and oxidations. Among these, Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
257
ferric perchlorate is known to produce OH. when irradiated, and the hydroxyl radical has a strong hydrogen-abstraction ability. The difficulty with basing the rate of decomposition solely upon Equations 17 and 18 is that the observed maxima in the D us. concentration curves cannot be explained. It appears that significant aspects of the mechanism are as yet unknown. Maxima in the rate us. concentration relationship have also been observed for the decomposition of alkyl benzene sulfonate solutions by gamma radiation (Fleishman and Price, 1967). Rate equations agreeing with the data can be derived by assuming complex formation similar to the procedure often used for solid catalytic and enzyme reactions (Dixon and Webb, 1964; Hougen and Watson, 1947). However, the assumptions involved are nunierous and difficult to justify. Hence, little is to be gained from these derivations, and it appears best to present the results simply as empirical equations. The data agree well with a point rate equation of the form : Qx = k
where
B =
Similarly, when only CO, changes
CDBS
(I
a
from f and the measured using Equation 13. These results were then employed to analyze Equation 20. Evaluation of Rate Equation. R a t e d a t a for different light intensities obtained using filter solutions 1s t o 5 s are plotted in accordance with Equation 20 in Figure 8. The 1.2 order of intensity fits the data. The effect of ferric ion concentration (Figure 9) suggests a linear relationship. Hence, was included to the first power in Equation 20. Runs were made varying either CDBSor CO,, with all other operating conditions and concentrations constant. If only C D ~varies, S Equation 20 can be arranged to the form:
+ KDBSCDBS)’X
where This expression may be summed over the whole wavelength range and integrated with respect to r to obtain just as was done for the nonsensitized case. The results for the corrected, average rate, and the correction factor, defined by Equation 13, are:
a,
22.2 iii,,,,= - k 0.8 (1
+
CDBS X KDBS~DBS)’
I n these equations the absorptivity is that for ferric perchlorate. This was measured in the Beckman DK-2A spectrophotometer and is given in Table IV. The absorptivity of DBS can be ignored because the rate of the unsensitized reaction is less than 1% of the sensitized rate. The attenuation of the light intensity due to DBS can be neglected because its effect onfis, a t most, 4%. Using the data in Table IV, j was evaluated from Equation 21, with the results shown in Table V. Then was obtained
a,,,,
Table V. Correction Factors Sensitized Reaction
(f) for
Ferric Perchlorate Concn., (G.Moler/L.) X 1O4
1S
2s
3s
4s
5s
0.04 0.20 0.82 1.43 2.30 3.15
1.057 1.142 1.498 1.877 2.457 3.030
1.054 1.125 1.410 1.696 2.108 2.499
1.051 1.108 1.332 1.546 1.894 2.122
1.047 1.088 1.244 1.389 1.589 1.777
1.045 1.078 1.202 1.318 1.480 1.634
Filter Solution
~~~~
258
Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1 9 7 0
A =
The data for varying COBSare plotted in accordance with Equation 22 in Figure 10. From the intercept and slope of the line B and KDBEcan be obtained. Similarly, from the rate vs. Co2data shown in Figure 11 and Equation 24, A and K O , were evaluated. Since all other quantities in Equations 23 and 24 are known, the B and A so determined each gives a
numerical value for the empirical rate constant, k. The results are:
KDBS= 4.04 X lo7 cmS3/g.mole K O , = 0.387 X lo7 ~ m . ~ / mole g. k(from Equation 23)
=
4.26 X 1014 c.g.s. units
k(from Equation 25)
=
4.66 X 1014 c.g.s. units
The agreement between the two values for k is a measure of how well the data agree with Equation 20. The effect of temperature on the rate was measured for the range 15” to 6OOC. The results shown on an Arrhenius plot in Figure 12 do not give a straight line. This is indicative of the complex kinetics that are not fully explained by the mechanism of Equations 17 and 18. Comparison of Sensitized and Nonsensitized Rates
Figures 1 and 2 show that the sensitized rate of decomposition is two orders of magnitude greater than the nonsensitized rate. Filter solutions 3N and 2s have approximately the same transmission, so that the data in these two figures are for approximately the same light intensity a t the reactor
I
TemDeratur!e,
i
I
28°C
Te rnpe ro t u re , DBS ~oncentration,5.0x IO-' g.mole/(cm') Ferric I o n Conc, C d + , 2 . 0 IO-' ~ g.mole/(cm3)
#;S
Filter Solidion
1
,
280c,
v) kFO1
40 80 I20 160 Oxygen Concentration, C,, x I O', g.mole/(cm3) Figure 1 1 . .
Figure 8. reaction
. ____.-__
Effect of light intensity on rate for sensitized h
Saturated by Air Ferric I on Conc., CF.3+, 2 . 0 ~IO-' Filter Solution # 2 s g,moIe / ( c d )
n
c
1
1
I
30 0
IO 20 30 40 Ferric Ion Conc. ,CFs3+, g.mole/(cm3) x 10' Figure
9.
I
( C D B B / & ~ ~VS. ) ~ CDBS '~
a
I
P
-
34
Table VI. Comparison of Sensitized and Nonsensitized Rates
8 16 24 32 DBS Concentra tion,CDsrx IO-' g.mole/(cm') Figure 10.
L
32 33 I /T x IO', ? K I-' 31
Comparison of uncorrected, average rates includes the effect of light attenuation through the reactor. This is greater for the sensitized case, as indicated by the higher correction factors (compare Tables I1 and V). The attenuation effect is eliminated by comparing fi,,. These results, shown in the bottom row of Table VI, indicate a 166-fold increase in rate. A few integral reactor runs were made under sensitized conditions. All the DBS was converted to some kind of product a t an average residence time as low as 1 minute. The maximum conversion to C02 was obtained a t a residence time of 20 minutes and corresponded to 7 moles of C02 per
____
I
-
1
Figure 12. Temperature effect on rate of sensitized reaction
(acorr) vs. ferric ion concentration
28°C
0
( C O ~ / ~ ~vs. ~ ~COz ~)~'Z
wall. For a more quantitative comparison, was evaluated for the specific conditions given in Table VI. The DBS and 0 2 concentrations correspond approximately to those where the sensitized rate is a maximum. Hence, the observed 123-fold increase is about the maximum increase in average rate expected by using ferric perchlorate as sensitizers.
Temperature, O C . Filter solution CDSs, g. mole/cm.* Cos, g. m ~ l e / c m . ~ Concn. of Fe(C104)3, g. r n ~ l e / c m . ~
Nonrensitized
Sensitized
28 3N
28 2s
7 . 0 x 10-8 2 . 4 8 x 10-7
7.0x 2.48 x 10-7
0 8 . 2 1 x lo-* 7.0 3.0 0.023 X 2.82 X lov9 0 , g. m~le/(cm.~)(sec.) g. m~le/(cm.~)(sec.) 0.024 X lopg 3.98 X
PH
a,,,,,
Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
259
mole of DBS. Presumably, seven carbon atoms were extracted from the alkyl side chain with the residue of the DBS molecule remaining in the water as partially oxidized aromatic compounds. Conclusions
The kinetics of photodecomposition of aqueous solutions of dodecylbenzene sulfonate was studied in a differential, tubular flow reactor, with and without ferric perchlorate as a sensitizer. The nonsensitized rates could be explained by a second-order deactivation of activated DBS molecules. This led to a rate equation one-half order in light intensity and DBS concentration. Dissolved oxygen had no effect on the rate, but lowering the p H hastened the decomposition. The sensitized rate was somewhat more than first-order in light intensity, first-order in Fea+, and showed a maximum a t intermediate DBS and oxygen concentrations. While the decomposition probably involves hydrogen abstraction by hydroxyl radicals, produced by irradiated Fe3+, the mechanism must be complex. The rate was increased by two orders of magnitude by adding sensitizer. Of more practical interest for water pollution control, all of the DBS was converted to some intermediate products a t a residence time of 1 minute and 7 moles of COZwere produced per mole of DBS in 20 minutes. Acknowledgment
The financial support of the Water Pollution Control Administration through Grant WP-00952 is gratefully acknowledged. Nomenclature
A,B
c
= constants in Equations 22 and 24
= concentration, g. moles/cm.a
concentration of uranyl sulfate, g. mole/cm.a concentration of DBS in feed, g. moles/cm.s rate constants defined by Equations 3 and 5 , c.g.s. units k = over-all reaction rate constant for sensitized reaction, c.g.s. units f = correction factor defined by Equation 13 F A = energy output of lamp at wavelength A, Einsteins/ sec. Ftot = total energy output of lamp, Einsteins/sec. h = Planck’s constant la = absorbed light intensity, Einsteins/(cm.8) (sec.) Ix = light intensity a t wavelength A, Einsteins/(cm.)2 (sec.) I, = light intensity at reactor wall with filter solutions Ib = light intensity a t reactor wall without filter solutions KDBB = constant in rate equation for DBS, cm.a/g. mole KoZ = constant in rate equation for 02,cm.a/g. mole R = reactor radius, cm. r = radial distance from center of reactor tube, cm. p = rate of oxalic acid decomposition, g. m ~ l e / ( c m . ~ ) (see.) Tx = fraction of light of wavelength A transmitted through all filter solutions CA
CDBB kz, ka
= = =
260 Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
Q V
liquid flow rate, cm.a/sec. irradiated volume of reactor, cm.8 2 conversion of DBS ax absorptivity of DBS; units in tables are l./(g. moles) (cm.); in equations, cm.2/(g. mole). All values based upon intensity ratio in log, form CY'^ = absorptivity of uranyl sulfate ox” = absorptivity of ferric perchlorate px = attentuation (pi = axC) for DBS; pi” refers to ferric perchlorate, cm.-l Stx = rate of decomposition of DBS induced by light of wavelength A, g. mole/(cm.)a (sec.) s2 = average, total rate over reactor cross section Steorr = corrected average rate for zero absorptivity, g. mole/(cm.a) (sec.) = quantum yield of oxalic acid decomposition, g. moles/Einstein v = frequency of radiation, sec.-l = = = =
literature Cited
American Public Health Association, New York, “Standarjf Methods for the Examination of Water and Wastewater, 12th ed., 1965. Armstrong, F. A. J., Tibbitts, S., J. Marine Biol. Assoc. U.K. 48, 143 (1968). Bates, H. C. G., Evans, M. G., Uri, N., Nature 166, 869 (1950). Bates, H. C. G., Uri, N . J., J . Am. Chem. SOC.75, 2754 (1953). Bishop, D. F., Stern, G., Fleischman, M., Marshall, L. S., Ind. Eng. Chem. Process Design Develop. 1, 110 (1968). Bulla, C. D. 111, Edgerley, E., Jr., J . Water Pollution Control Fpderation 40, 546 (1968). Calvert, J. G., Pitts, J. N., Jr., “Photochemistry,” Wiley, New York, 1966. Cassano, A. E., Smith, J. ht., A.I.Ch.E. J . 12, 1124 (1966); 13, 915 (1967). Dainton, F. S., James, D. G. L., Trans. Faraday SOC.54, 649 (1958). Denbigh, K., “Chemical Reactor Theory,” University Press, Cambridge, Mass., 1966. Dixon, M., Webb, E. C., “Enzymes,” 2nd ed., Longmans, Green, London, 1964. Edgerley, E., Jr., Skrinde, R. T., Ryckman, D. W., “Principles and Applications of Water Chemistry,” p. 405, Wiley, New York, 1967. Evans, M. G., Santappa, hf., Uri, N. J . Polymer Sci. 7, 243 (1951). Fleishman, Marvin, Price, R. H., Environ. Sci. Technol. 1, 577 (1967). Hougen, 0. A., Watson, K . M., “Chemical Process Principles,” Part 111, Wiley, New York, 1947. Lipman, F., Skand. Arch. Physiol. 76, 186 (1937); 31, 4904 119.17)
Li&g$ion, R., “Photochemistry in the Liquid and Solid States,” p. 76, Wiley, New York, 1960. Massaldi, H. A,, hfaymo, J. A., “Differential Method Used with Finite Conversion Reactors.” Drivate communication. No, _ vember 1967. Matsuura, T., Smith, J. M., “Kinetics of the Photodecomposition of Aqueous Solutions of Formic Acid,” to be published, A.I.Ch.E. J., 1971. hteiners, A. F., Whitehead, 41. E., Lawler, E. A,, Morrison, J. I., “Investigation of Ultraviolet-Catalyzed Chlorine Oxidation for Treatment of Wastewater,” Division of Water, Air and Waste Chemistry, ACS, September 1968, ,Atlantic City, N. J. Nonaka. D. N.. M.S. thesis. Georgia Institute of Technolow, __ August 1968. ’ Uri, N. Chem. Rev. 50, 375 (1952). I
RECEIVED for review April 21, 1969 ACCEPTEDDecember 5 , 1969
