Ind. Eng. Chem. Res. 1995,34, 4099-4105
4099
Photooxidation of Carbofuran by a Polychromatic UV Irradiation without and with Hydrogen Peroxide F. Javier Benitez,**tJesus Beltran-Heredia, Teresa Gonzalez, and Francisco Real Departamento de Ingenieria Quimica y Energetica, Universidad de Extremadura, 06071 Badajoz, Spain
The photodegradation of carbofuran aqueous solutions has been conducted with direct photolysis provided by a polychromatic W radiation source and by the combination of this W radiation with hydrogen peroxide. In both processes, the decomposition level obtained as a function of the operating variables is reported, and the presence of tert-butyl alcohol, a scavenger of free radicals, is discussed. While the contribution of hydroxyl radicals is negligible in the direct photolysis, its reactions in the W / H 2 0 2 system clearly increase the carbofuran decomposition and therefore must be taken into account in the reaction rate equation for the total insecticide degradation. From the mechanisms proposed, the quantum yields for the direct photolysis and the kinetic constants for the reaction between carbofuran and the hydroxyl radicals generated in the H202 photolysis in the combined process are respectively evaluated.
Introduction
photodecomposition by sunlight and demonstrated that dilute H202 significantly increased the photolysis rate; The removal of organic harmful pollutants present in while Scheunert et al. (1993)irradiated samples with water supplies and wastewaters in purification and W light of different wavelengths and reported similar treatment plants is carried out by means of a variety of conclusions in the sense that photodegradation was physical and chemical procedures. Among the last ones, accelerated in the presence of titanium dioxide, HzO2, the oxidation by several agents such as chlorine, ozone, and ozone. On the other hand, Raha and Das (1990) W radiation, hydrogen peroxide, etc., has been extenstudied its photodegradation under sunlight in different sively used with effectiveness. solvents, characterized eleven photoproducts, and deHowever, more demanding requirements imposed by of the process which picted the plausible pathway law regulations to the treatment plants in past years involves severak steps, the hydrolysis being the major or a particularly resistant character of some compounds (1991) examined the route, and Bertrand and Barcelo to those oxidations have forced the development of new influence of different light sources and water types and alternatives. Thus, in recent times, stronger oxidant also identified the major photodegradation products. agents or advanced oxidation processes (AOPs) have However, no data about the quantum yield of the emerged as potentially powerful methods which are photoreaction are provided in those investigations. As capable of transforming these contaminants into harmCabrera et al. (1994)pointed out, it is accepted that the less substances. These AOPs are characterized because quantum yield in a photochemical reaction is one of the they combine the effect of ozone, hydrogen peroxide, and most useful fundamental quantities in the study of the W radiation (Glaze et al., 1982, 1987) and generate very reactive and oxidizing free radicals in aqueous reaction mechanisms of those processes, and its detersolutions, such as the hydroxyl radicals, which achieve mination is strongly recommended for every basic a great destruction power. photochemical study. Pesticides pollution in general has increased sharply So, as carbofuran exhibits a special refractory charin surface and ground waters due to an extensive use acter to simple oxidation methods, research on its in agricultural cultivations, weed control, domestic photodegradation by a stronger radiation source (a high usage, etc. The chemical degradation of pesticides in pressure mercury vapor lamp) and by an AOP constiaqueous solutions by the oxidants mentioned above have tuted by the combination of W radiation and hydrogen been abundantly studied, and complete information can peroxide was designed, with the aim being to report the be found in the literature, with several revisions of degradation levels obtained and to provide the quantum interest (Marcheterre et al. (19881,Reynolds et al. yields for the direct photolytic reaction and the kinetic (19891,Masten and Davis, (19941,etc.). constants for the reaction between Carbofuran and the Carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-ol hydroxyl radicals generated in the W / H 2 0 2 combined methyl carbamate), also known under the trade names process. of Furadan or Curaterr, is a broad spectrum insecticide and, as with most of the carbamate derivative pesticides, Experimental Section constitutes a class of growing importance. It is widely used as a substitute for organochlorine pesticides The photochemical reactor used in the experiments because of their greater biodegradation ability and, was described in detail in a previous investigation consequently, is increasingly detected in the aquatic (Benitez et al., 1994),where a schematic diagram was environment, provided which depicted all the geometric characterisSome aspects of its photodegradation have been tics and coordinates of the system. In the present case, recently pointed out by several researchers: thus, the main difference consists in that the radiation source Draper and Crosby (1984)provided levels of carbofuran now is a Hanau TQ150 high pressure mercury vapor lamp, which, according to the manufacturer, emits * Author to whom correspondence should be addressed. polychromatic radiation extended from the short wave t E-mail address:
[email protected] number: 3424-271304. W zone of approximately 185 nm to far into the visible Q888-5885/95/2634-4Q99$09.O0/0 0 1995 American Chemical Society
4100 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 Table 1. Oxidation by Direct Photolysis: Operating Variables, Conversions Obtained, and Quantum Yields expt pH T , "C 1 2 3 4 5 6 7 8 9 10 11" a
7
7 7 7 2 2 5 9 7 7 9
20 30 40 10 20 30 20 20 20 20 20
cBO104,
conversion, % q5 x 102, 40 min mobeinstein-l
mol.L-'
20 min
4.57 4.52 4.43 4.61 4.40 4.57 4.42 4.58 6.60 9.10 4.63
45 48 54 43 41 55 42 45 41 34 45
65 76 85 59 70 73 68 69 62 54 67
1.73 1.97 2.45 1.47 1.75 2.08 1.72 1.78 1.60 1.58 1.62
With tert-butyl alcohol.
light region. Within this range, there are a few intense lines and several weaker lines; thus, the strongest line within the W range is around the wavelength of 366 nm. However, aqueous solutions of carbofuran and hydrogen peroxide only absorb radiation in the wavelength range between 239 and 313 nm. Hence, the radiation flow rate absorbed in the degradation experiments of this research should be evaluated on the basis of the fraction of radiation emitted by the lamp in this zone. As was described in the formerly mentioned paper (Benitez et al., 1994), the reactor was thermostated to maintain the desired temperature, and it operated batchwise with respect to the liquid solutions. They were charged t o the reactor with a volume of 350 cm3 containing the amount of carbofuran (plus hydrogen peroxide in its experiments) needed to reach the predesignated initial concentrations. Also, the solutions were previously buffered a t the selected pH by adding orthophosphoric acid and sodium hydroxide so that the total ionic strength in the final solutions was maintained constant at M. Each experiment started when the lamp was connected, and regularly, samples were withdrawn to determine the carbofuran remaining concentration (and hydrogen peroxide concentration in its experiments). Carbofuran was analyzed by HPLC: the injected samples were eluted with a mixture of water-methanol (5545 (v/v))with a flow rate of 1cm3*min-l,and the detection was performed in a W detector a t 254 nm. On the other hand, the concentration of HzOz was determined by the colorimetric method of Bader et al. (1988). Previous experiments based in the uranyl sulfate photodecomposition of oxalic acid under controlled conditions, as was proposed by Leighton and Forbes (1930) and later revised by Alfano et al. (1986b1, were carried out t o determine the amount of radiation emitted by the lamp and transferred into the reactor WL. The determination was made by taking into account the entire emission spectrum of the lamp and the value obtained was 4.52 x einsteiws-I.
Results and Discussion Oxidation by Direct Photolysis. The photooxidation of carbofuran by direct W radiation has been conducted in several experiments modifying the pH, temperature, and initial pesticide concentration. Table 1 presents the decomposition reached, expressed as conversion, in each experiment at two selected times (20 and 40 min of reaction). As can be seen in Table 1, the temperature has a direct influence on the process (see experiments 4,1,2,
and 3): the conversion is 59,65,76, and 85%at 40 min of reaction for 10, 20, 30, and 40 "C, respectively. However, different behavior is observed with the variation of pH (see experiments 5 , 7, 1, and 8): the conversions are very similar, and no influence of this variable is deduced. This aspect was also reported by Kochany and Bolton (1992) in experiments of photodegradation of organic pollutants. On the other hand, the pesticide initial concentration also has a direct effect on the reaction, although this influence is hardly noticed from the conversion data (experiments 1,9, and 10);however, when this variable increases, the degradation rate increases too: as an example, it is calculated for 15 min of reaction in those experiments, obtaining 0.84 x 1.11 x and 1.38 x moEL-'-l*min-l for CBO= 4.57 x 6.60 x and 9.10 x mol-L-l, respectively. Experiments 8 and 11 provide an interesting fact in order to propose a plausible mechanism for the reaction. The last one was carried out in the presence of M tert-butyl alcohol (the remaining variables being the same) which is a known hydroxyl radicals scavenger (Hoigne and Bader, 19831, so it can be expected that its presence scavenges these radicals and thereby quenches possible 'OH free-radical type reactions. However, as can be seen in Table 1,the conversion obtained in both experiments has the same value; that is, the reaction is unaffected by the presence of tert-butyl alcohol, and the decomposition is due exclusively to the direct photolysis without any radical contribution. "he mechanism for photochemical reactions of organic compounds in water are usually complex, as is pointed out by Legrini et al. (1993) in a review of those processes. In the present case, taking into account general considerations of the photooxidation of organics in the liquid phase (Walling, 1957; Padwa, 19771, similar reactions found in the literature (Schiavello, 1988; Ollis et al., 19911, and eliminating the possible radical contribution as was above referred, the following direct mechanism can be proposed for the reaction between carbofuran and W radiation
k,
B*-B
(2)
**kp
(3)
where P symbolizes final products. In the first step, the excitation of compound B occurs by the absorption of one photon; the second one takes into account the deactivation of this excited species to its basic state, while the third one is the real photodecomposition of B. Applying a mass balance and assuming the pseudostationary state for the activated species, it can be deduced for the photodecomposition of the solute B in each point of the reactor the kinetic equation
(4) where
C#J~
is the quantum yield of the reaction and pi and
Ii are the absorbance of the solution and the radiation intensity, respectively, for each wavelength of the polychromatic radiation.
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4101 If the overall quantum yield for all the wavelength range of the polychromatic radiation is expressed by
eq 4 turns into
Subsequently, the total degradation rate in the whole reactor can be written in the form
239 248 264 266 270 276 280 289 297 302 313 334 386 390 406 436 A.nm
Figure 1. Radiation fraction emitted by the lamp and molar absorption coefficients for carbofuran and hydrogen peroxide.
in which eq 6 has been introduced. On the other hand, it must be taken into account that the radiation flow rate absorbed by the solution when irradiated by a polychromatic radiation is given by the general expression Wabs = EWabs, = xSytc,I,dV
(8)
which considers the individual contribution of the flow rate absorbed for each wavelength. So, by substituing eq 8 into eq 7, the following is obtained:
given by
and the parameter c is c = [r2
+ (z - z ) ~ I ” ~ / ~
(14)
This equation, after separation of the variables, can be integrated with the boundary condition
In these expressions, r and z are the coordinates of the general point considered, and I is the lamp axial coordinate; L and LOare the length and axial position of the lamp; R1 and Ro,the external and internal radii of the reactor, and H is the reactor height as was already described previously (Benitez et al., 1994). On the other hand, WLis the total radiation flow rate emitted by the lamp as was mentioned, and P, is the fraction emitted for each wavelength, according t o the ratio
t=O, CB=CBO
(10)
P, = Wpv,
(11)
Values of P,given by the manufacturer are provided in Figure 1 for the whole polychromatic range. In order to simplify, a general parameter Ni is defined in the form
yielding to
According to eq 11, a plot of CB versus the corresponding term JWabs dt must lead to a straight line, from which slope the overall quantum yield of the reaction can be deduced. For that purpose, previously the rate of radiation energy absorbed by the reaction medium Wabs must be determined for each time. As was described in a previous research (Benitez et al., 1994), this determination is carried out by solving a radiation energy balance by means of a radiation source model which describes the distribution of radiant energy within the reactor. And, as was also explained there, the line source spherical emission model developed by Jacob and Dranoff (1970) and described by Alfano et al. (1986a) seems to be the more suitable for the investigations conducted in this reaction medium due t o the geometric characteristics of the reactor and lamp. Besides that, in the present investigation the polychromatic character of the radiation used must be taken into account. According to those considerations and the mentioned model, the radiation flow rate absorbed can be expressed in the form H
R
Wabs, = 2np& hloI,(r,z)rdr dz
(12)
Ii(r,zz)being the radiation intensity for each wavelength,
(15)
So, considering eqs 12-16, the rate of radiation energy absorbed can be given by the final reduced expression
Equation 17 allows determination of the total absorption flow rate Wabs for each time in the whole wavelength range a t which carbofuran aqueous solutions absorb: thus, with the concentration of the pesticide in the solution determined periodically during an experiment as was described in the Experimental Section, the absorbance pi was calculated for each time by means of the Lambert-Beer law pi = E i C B
(18)
where ci is the molar absorption coefficient, experimentally determined for carbofuran at each wavelength and also represented in Figure 1. These pi values were used to determine the parameter Ni in eq 16, which is solved
4102 Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 Table 2. Oxidation by the W / H 2 0 2 Combination: Operating Variables and Conversions Obtained
T , 'C
0
10
v
20
d
30 40
7 7 7 9 2 5 7 7 7 7 9
1 2 3 4 5 6 7 8 9 10 11a a
20 20 20 20 20 20 30 10 40 20 20
4.54 4.52 4.24 4.49 4.46 4.63 4.62 4.48 4.60 6.88 4.53
2.60 1.23 3.84 1.27 1.33 1.24 1.20 1.20 1.27 1.11 1.21
49 42 68 44 48 37 50 38 58 28 35
76 64 90 65 70 69 74 58 78 48 51
With tert-butyl alcohol.
80 -
0
2
4
6 t
%&
8
10
Figure 2. Direct photolysis: Determination of quantum yields by varying the temperature. Experimental conditions: pH = 7; C B =~ 4.5 x 1 0 - mol-L-l. ~
numerically; and with pi, Ni, Pi,and WLknown, eq 17 provides Wabs for each time. Once the rate of energy absorbed Wabs is known, eq 11can already be used in the form described before in order to obtain the quantum yield of the reaction, by plotting CBagainst the integral term JWabs dt. It was calculated for every C g by fitting the experimental data (Wabs, t ) t o a polynomic expression and by integration of the resultant function. Figure 2, as an example, shows this plot for the experiments conducted by varying the temperature. As can be observed, points lie around straight lines, confirming the proposed mechanism, and a direct effect can be noted: when this variable increases, the slope also increases. Similar plots for experiments at different pH and initial concentration are performed: in the first case, points lie on the same straight line and, consequently, no influence of this variable on the quantum yield is deduced in this process, as was referred to before. On the other hand, for the carbofuran initial concentration, approximately parallel straight lines are obtained, which indicate that the quantum yields are nearly the same, and this initial concentration does not affect the quantum yield as can be expected. After least squares regression analysis, the values for the experiments depicted in Table 1are deduced and also shown in the same table. As there is hardly any influence of the pH and the initial pesticide concentraand tion, it is proposed an average value of 1.66 x 2.03 x mol-einstein-l for 20 and 30 "C, respectively. Considering also the values at 10 and 40 "C, a regression analysis can be performed, obtaining the following Arrhenius expression
4 = 3.04exp(-1517/27
60
-
40
-
3 dt ~ 1 ,Einstein 0
(mol*einstein-') (19)
which correlates satisfactorily with the results obtained.
T , *C -
20
-
0
10
c 0
5
10
15
20
25
30 t , rnin
Figure 3. Combined U V / H 2 0 2 oxidation: Temperature influence on the conversion. Experimental conditions: pH = 7; C B=~ 4.5 x mo1.L-l; cH,,=1.2 x mo1.L-l.
Oxidation by the W/HzOz Combination. The study of carbofuran photooxidation by the combination of W radiation with H202 has been carried out, varying, besides the temperature, pH, and initial pesticide concentration, the initial hydrogen peroxide concentration in the system. Table 2 shows the values taken for those operating variables in a group of experiments and the conversions obtained a t two selected times (5 and 10 min of reaction). The three first variables have an influence on the degradation process similar to that in the direct photolysis. Thus, the positive effect of the temperature can be seen in Figure 3, while the pH has no significant influence (see experiments 5, 6, 2, and 4), and an increment of carbofuran initial concentration implies an increase in the reaction rate. A previous experiment was conducted with H202 alone, and no oxidation was obtained; thus, it does not a t all degrade carbofuran. On the other hand, the role of the hydrogen peroxide in the process when combined with W radiation is clearly pointed out by the experi-
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4103 previously discussed, while eq 23 considers the reaction between the possible hydroxyl radical scavengers present in the solution and those radicals. In this system, as Black and Hayon (1970) reported, the phosphate ions used in the preparation of the buffer solutions are moderate scavengers of free radicals, and consequently their reactions must be considered. In order to report kinetic data for reaction 22 between the hydroxyl radicals and carbofuran, which is the objective of this research, and according to the mechanism proposed by eqs 20-23, the following reaction rate expression can be proposed for the total rate of disappearance of carbofuran
100
ao
60
40
which takes into account both contributions, the direct photolysis rate rp, and the radicalary reaction rate rR due to the presence of hydroxyl radicals. The first one, and according t o eq 9, can be written in the form
20
lid 0 0
5
v
1.23
A
2.60
o
3.04
I
I
I
I
1
10
15
20
25
30
-rp = (pWabSBN
t, min
Figure 4. Combined U V E I 2 0 2 oxidation: Hydrogen peroxide initial concentration influence on the conversion. Experimental conditions: pH = 7; T = 20 "C;C B =~ 4.5 x mo1.L-l.
ments in which its concentration was varied. Figure 4 depicts the conversion obtained in those experiments (runs 2 , 1 , and 3) as well as the equivalent experiment in the direct photolysis without H202 (run 1 of Table 1): as can be observed, its presence enhances the oxidation, and an increase in its concentration leads to a parallel increase in the conversion. So, it can be concluded that the hydroxyl radicals generated in the hydrogen peroxide photolysis provide a significant contribution to the carbofuran degradation as could be expected. This aspect is reinforced by the comparison of experiments 4 and 11, both conducted at the same operating conditions, but the second one in the presence of tertbutyl alcohol: as can be seen the conversion obtained is lower than that of run 4 without tert-butyl alcohol; that is, tert-butyl alcohol scavenges hydroxyl radicals and diminishes the contribution of this radicalary way to the total degradation process in opposition to the direct photolysis where the radical pathway could be neglected. Once this radical contribution is demonstrated, the following mechanism for the oxidation of organics by the combined H202/LTV process, which takes into account the direct photolysis and the radicalary pathways, can be proposed
B+hv-
+
H202 hv
@BJLB?,
P
-2'OH @HPHJL
B+*OHAP
+
(20) (21) (22)
k
S 'OH 4 inactive radicals (23) In this simple scheme, eq 20 summarizes eqs 1-3
(25)
where, now, WabSB represents the radiation flow rate absorbed exclusively by carbofuran and is evaluated by the same procedure described for Wabs in the previous section, that is, by using an expression similar to eq 17 (26)
in which the absorbance p ~ is i determined with the carbofuran molar absorption coefficient for each wavelength Q (already represented in Figure 1) and the concentration of this solute CB. However, in this situation the absorbancepi of the reacting medium, included in the term N of eq 16, must be taken as the sum of the absorbances of carbofuran and hydrogen peroxide Pi = PBi
+ PHi
(27)
Values O f pHi for H202 were also determined by eq 18, with the molar absorption coeficients for hydrogen peroxide C H ~previously evaluated experimentally for each wavelength and also presented in Figure 1. On the other hand, and according t o eq 22, the radicalary reaction rate TR can be expressed as
-rR = k ['OH]CB
(28)
Applying the pseudostationary state condition to the hydroxyl radicals in eqs 21-23, it is deduced for its concentration (29) As, in the present work, the concentration of the scavenger phosphate ions used is significantly higher than that for the organic compound CB, it can be supposed that k,[Sl >> kCB. Besides that, considering eqs 6-9, it is finally obtained for ['OH] the expression
2 4H ['OH] = -
k,[S]
ywabsH
which is substituted into eq 28, yielding
(30)
4104 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3.5
(31)
There 4~ represents the quantum yield for the hydrogen peroxide photolysis, of which the value 1 mobeinstein-l is well-known (Baxendale and Wilson, 1957; Laming et al., 1969; Nicole et al., 1990); and WabsH is the flow rate of absorbed radiation exclusively by H202, which also can be calculated by an expression similar to eq 26
3
2.5
2
(32)
Once again, in the determination of Ni by eq 16 the absorbance pi must be that obtained from eq 27, which considers the contribution of both the carbofuran and hydrogen peroxide absorbances. Therefore, considering eqs 25 and 31, eq 24 can be rewritten in the form
1.5
1
0.5
The integration of eq 33 with the initial condition t = 0 and CB = CBOleads to
0 0
2
4
6 Q'x108, m o l .Einstein/L
Figure 5. Combined U V / H 2 0 2 oxidation: Determination of k' in experiments by varying the temperature. Experimental conditions: pH = 7; CB,,= 4.5 x lo-* mol-L-l; C H=~ 1.2 x mol-L-'.
or in an abridged form CBo-CB-'
VQ = k '& '
(35)
According to eq 35, a plot of the first term versus the integral term Q must give a straight line for each experimental series, from which slope the k' value can be obtained. Therefore, both integral terms Q and Q were evaluated for each time of reaction in every experiment conducted by the same procedure described for the direct photolysis; and 4 is calculated for each experiment by eq 19. This plot in the experiments varying the pH and the initial H202 concentration at 20 "C shows that points are concentrated around the same straight line, confirming the conclusion already obtained in the direct photolysis: an initial concentration has no influence on a kinetic constant, and the pH in the present system affects neither. On the other hand, Figure 5 shows the same plot corresponding to experiments in which the temperature was varied; it can be seen that points lie satisfactorily around straight lines. After least squares regression analysis, the 12' values are obtained for each experiment. Later, with the values of k, reported by Black and Hayon (1970) for the reaction between phosphate ions and hydroxyl radicals and the concentration of those ions [SI used in each experiment besides 4~ = 1, the k values are also evaluated, being 16.5 x lo8, 17.3 x lo8, 21.3 x los, and 23.2 x lo8 L.mol-'.s-' for 10, 20, 30, and 40 "C, respectively. With those values, an Arrhenius correlation can be proposed for the rate constants for the radicalary pathway between the hydroxyl radicals and carbofuran in the form
k = 5.25 x l o l o exp(-970/T)
(L*mol-%')
(36)
Conclusions The photooxidation of carbofuran by a polychromatic W radiation source alone and combined with hydrogen
peroxide permits an important level of destruction of the pesticide in a moderately short time, specially increased in the combined process (see Tables 1and 2). In the direct photolysis, the presence of a scavenger hydroxyl radical substance such as tert-butyl alcohol does not reduce the degradation rate, and therefore, no free radical contribution to the degradation is found. A simple mechanism is proposed which provides the rate equation for the carbofuran destruction. From this equation, after the radiation flow rate absorbed is determined with the help of an emission model, the quantum yields for every experiment are obtained and correlated by an Arrhenius expression. When the combined W / H 2 0 2 system is used, the hydroxyl radical contribution t o the carbofuran photodegradation is clearly demonstrated. Another mechanism is proposed, and the corresponding rate equation is deduced which takes into account both contributions, the direct photolysis and the radicalary pathway. The application of the experimental results to that equation allows determination of the kinetic rate constants for the reaction between carbofuran and the hydroxyl radicals generated in the Hz02 photolysis.
Acknowledgment This work has been supported by the CICYT of Spain under Project AMB93-1213. T.G. thanks "Fundacion Caja Madrid" for the grant given.
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Received for review March 13, 1995 Accepted J u n e 20, 1995@ IE950175V
Abstract published in Advance ACS Abstracts, October 1, 1995. @
