Photochemical decomposition of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution.I. kinetic study

•

Pergamon

Wat. Sci. Tech. Vol. 35, No.4, pp. 31-39,1997. © 1997 IAWQ. Published by Elsevier Science Ltd

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PHOTOCHEMICAL DECOMPOSITION OF 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D) IN AQUEOUS SOLUTION. I. KINETIC STUDY Marfa I. Cabrera, Carlos A. Martfn, Orlando M. Alfano and Alberto E. Cassano INTEC (Universidad Nacional del Litoral and CONICET), Giiemes 3450, (3000) Santa Fe, Argentina

ABSTRACT The intrinsic kinetics of the photochemical decomposition of 2,4-dichlorophenoxyacetic acid in aqueous solution has been studied using light of 253.7 nm. Experiments were carried out in a well stirred batch reactor irradiated from its bottom by means of a tubular lamp and a parabolic reflector. Results were analyzed in terms of a very simple kinetic expression. Absorbed radiation effects were duly quantified by means of a one-dimensional radiation field model. This approach incorporates a variable absorption coefficient that is a function ofthe 2,4-D conversion. The decomposition kinetics can be properly represented with a point valued equation of the following form: RD, ').., = - <1>0,1.. e')..,(y). © 1997 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS 2,4-dichlorophenoxyacetic acid; Direct photolysis; Intrinsic kinetics; Overall quantum yield; photochemical reaction. INTRODUCTION 2,4-D is a widespread herbicide that is known to have a high level of toxicity. Its relatively high solubility in water facilitates its migration to natural courses where it is also known that it may last for several weeks due to its long mean life time (Pichat et ai., 1993). The photodecomposition of 2,4-D was first reported by Crosby and Tutass in 1966. This work proposed a reaction sequence for the direct photolysis of the organic substrate in water solution. Boval and Smith (1973) studied the same reaction employing different modes of reactor operation (differential and integral flow through, and batch with recycle systems). They found a kinetics that was first order with respect to the absorbed light and independent of the substrate and dissolved oxygen concentrations. An autocatalytic effect produced by reaction intermediates was also proposed. Pichat et ai. (1993) presented a comparative study of the reaction pathways for the degradation of 2,4-D employing different processes: (i) titanium dioxide and UV radiation, (1..>290 and 1..>340 nm), (ii) hydrogen peroxide and UV radiation (1..< 290 nm) and (iii) direct photolysis (1..<340 and 1..< 290 nm). They reported that decomposition with the different processes does not 31

32

M. I. CABRERA et al.

seem to follow the same reaction path. From the observed results and considering practical applications, the authors suggested the convenience of employing a series combination of them. Chamarro and Esplu~as (993) studied the direct photolysis of 2,4-D also in aqueous solution. They found that for long reactIOn times pH affects the reaction rate, possibly due to the formation of humic acids. Prado et ai. (1994) investigated the photo-oxidation of 2,4-D employing ozone. They proposed a model that incorporates both the ozone mass transfer rate and the kinetics of the oxidation of 2,4-D and reaction intermediates. A global kinetic constant was correlated with ozone production, 2,4-D initial concentration and reaction pH. Pignatello (1992) and Sun and Pignatello (1993 a, b) studied the oxidative decomposition of 2,4-0 employing iron(3+)/hydrogen peroxide and iron(+3)/hydrogen peroxidelUV radiation. A detailed study of reaction intermediates was presented. They also found that oxygen accelerates the carbon mineralization. Pignatello and Sun (1993) reported the complete mineralization of chlorophenoxyalkanoic acid herbicides using the previously mentioned oxidizing systems. They found that the initial rate of transformation of the (Fe+ 31H20IUV) system was greater than the sum of the (Fe+ 3IUV) and (HzOzIUV) systems when used separately. Sun and Pignatello (1995) also worked with the 2,4-D - system and found out that depending upon the reaction pH, direct hole oxidation or hydroxyl radical attack can be observed. They proposed the existence of a dual reaction mechanism. Trillas et ai. (1995) studied the photocatalytic degradation of 2,4-0. They found that the experimental results fit a Langmuir-Hinshelwood kinetic model and that the reaction rate depends upon the catalytic loading, the temperature and the light intensity. Complete mineralization of 2,4-D was reported. Finally, Rao and Dube (1995) used suspended and supported titanium dioxide catalyst for the destruction of 2,4-D. They compared different commercially available catalysts (Aldrich and Degussa P-25) with a titania of Indian origin. They also used supported titanium dioxide on polyester fabric and on a Ti sheet. They found a first order kinetics for the degradation rate. In this work the direct photodecomposition of 2,4-D was studied using radiation of 253.7 nm. A very simple kinetic model is employed to interpret the experimental data. It is used in conjunction with a precise radiation-reactor model to render an intrinsic kinetic expression. This result may be of general validity • within the explored operating conditions - for reactor design purposes. METHODS Experimental set-up Kinetic studies were performed in a well-stirred, batch, cylindrical photoreactor irradiated from below (Figure 1). The reactor was made of Pyrex glass while its bottom was made of Suprasil® quartz. AGITATOR THERMOMETER

COOLING WATER

__

t-H~_

LIQUID SAMPLING

REACTOR

PARABOLIC

EMIUING SYSTEM

REFLECTOR

LAMP

Figure 1. Schematic diagram of the stirred tank photoreactor.

The radiation source was a tubular lamp placed in the focal axis of a parabolic reflector The I a G . 'dal . amp was enruCI G15T8 (General Electric, 1967) of 15 W of nominal input power. The reflector Was made with

Photochemical decomposition of 2,4-dichlorophenoxyacetic acid

33

an aluminum sheet with Alzac® treatment. Further details can be found in Table 1. A thermostatic bath permits temperature control and the lamp operation was monitored with a VAW meter (Clarke Hess, model 255). 2,4-D concentration was analyzed by HPLC chromatography (J1Bondapack C 18 column and a Spectrophotometric detector at 236 nm). Optical properties of the reacting mixture were measured with a UV-vis Cary 17 D Spectrophotometer. Table 1

PARAMETER REACTOR

REFLECTOR

LAMP (G15T8)

INCIDENT RADIATION, G w X 109 [einstein S-1 cm-2]

Volume (VL ) Irradiated area(A irr ) Length (yd Parabola Charact. Constant Distance Vertex of Parabolic Reflector - Reactor Plate Length Nominal Power Output Power at 253.7 nm Diameter Nominal Length Total Radiation (TR) Direct Radiation (DR) Screened Radiation (SR)

VALUE 1000 cm 3 132.7 cm2 6.3 em 2.1 em 8.4 em 15.8 em 15W 3.6W 2.54 em 44.72 em 8.13 3.61 4.59

Incident radiation was varied in three different levels: (1) total radiation as described above, i.e., direct plus reflected radiation, (2) direct radiation (masking the reflector) and (3) screened radiation (interposing a black painted mesh between the irradiating system and the reactor). It must be mentioned that for reactor assembling purposes, the area of radiation entrance (A irr) is not equal to the cylindrical reactor area (VI!y!J. Experimental procedure Reactant was commercial 2,4-D (herbicidal grade). It was purified by crystallization on benzene (two times). Purity was measured against an EPA reference standard (# 2940,99.78 %) resulting in a 99 % concentration in 2,4-D. Prior to any experimental run steady state conditions in the reactor temperature and lamp operation were achieved. During this time a shutter was interposed between the lamp-reflector system and the reactor. The removal of this device indicated the zero reaction time. Samples were taken from the reactor at specified time intervals for HPLC analysis (Connick and Simoneaux, 1982), pH and Spectrophotometer measurements. Depending upon the experimental conditions, runs lasted from 9 to 24 hours. The reactor was carefully washed between runs. In Table 2, operating conditions for the experimental program are shown. Two of the runs were made under different conditions of irradiation: Direct Radiation (DR) and Screened Radiation (SR).

M. I. CABRERA et at.

34

Table 2

C6 [ppm]

T [C]

G w condition

100.5

25

TR

101.0

25

TR

98.2

25

TR

47.8

25

TR

47.3

25

TR

30.1

25

TR

33.1

25

TR

50.0

25

DR

49.7

25

SR

REACTOR MODEL Radiation field In 1985 Alfano et al. modeled the above described system using a three-dimensional (r, z, p) cylindrical coordinate system. The model was experimentally verified with variable spatial position microreactor:: (Alfano et al., 1986 a, b). It was found that radial and angular variations were not very significant. With this background, a one dimensional model (y-coordinate) was adopted. Then, the radiation field can be described by: (1)

In Eq. (1), Gw,A. is the incident radiation at the wall of the reactor bottom (y = 0) and KT,A. is the total absorption coefficient of reactant and products. It must be noted that the optical properties of the reacting medium change along the reaction evolution. This is not only due to the decrease in 2,4-D concentration; on the contrary, during the course of the reaction, at 253.7 nm, the absorption coefficient of the reacting mixture increases, indicating an important effect due to absorption by the reaction products. Consequently, the system is characterized by a minimum of two absorption coefficients: (I) one corresponding to the reactant and (2) a different one corresponding to the reacting mixture, both being a function of time. These values can be experimentally obtained as it is described below. Also Gw,A. will be experimentally measured. It should be noticed that, if desired, the incident radiation at y = 0 can be also theoretically predicted with the Alfano et al. (1985) radiation model. The local volumetric rate of energy absorption (LVREA) for the photolytic reaction is obtained from:

(2) In Eq. (2)K D.A. is the absorption coefficient of 2,4-D exclusively. Incident Radiation (The boundary condition for the three levels of irradiation) Incident radiation at y = 0 can be precisely evaluated with actinometer measurements. Potassium ferrioxalate (0.02 M) was used according to the operating conditions reported by Murov (1973). A mass balance for the well stirred, batch reactor with the actinometer reaction gives:

Photochemical decomposition of 2,4-dichlorophenoxyacetic acid

dC p A irr Ow A. KA A. -dt = p V ',... , L

{

""A,A. + Kp,A.

[ (

)]}

1- exp - KA,A. + Kp,A. YL

35

(3)

In Eq. (3) A is the reactant (Fe+ 3) and P the reaction product. In the batch reactor, for low reactant conversions, the plot of Fe+2 vs. time gives a straight line. At t~O, (Dcp'dt)t~O=° is the slope of such a p straight line. At initial conditions, the following equation holds:

(4) At t~ 0 Eq. (4) is valid because: (i) kp,l == 0 (the reaction product) and (ii) at A = 253.7 nm, k A lfor this actinometer is very large. From the experimental results: '

p VL l' Cp-C OJ O wA.1m , ( cl>p A irr ) t_H O ( t - t

°

(5)

Results of the boundary condition for the three conditions of irradiation are reported in Table 1. 2.4-D mass balance The reactor operates under the following conditions: (1) perfect mixing and (2) isothermal performance. The mass balance gives: dCD(t) _ Virr ( ( )) - V R D y,t dt L YL

(6)

CD(t = 0) = C~ The radiation field is not uniform and consequently the LVREA is a function of y and so is RD' Then, in Eq. (6) an average reaction rate has been defined according to: (7)

An expression for the local reaction rate is still unknown. Let us propose the following general and simple relationship: (8)

In Eq. (8), DA. is an overall quantum yield for the decomposition of 2,4-D. The average reaction rate results: (9)

JWST 35: 4-B*

36

M. I. CABRERA

el

al.

Substituting into the mass balance: (10)

Since in Eq. (9) the concentration of 2,4-D is uniform it was possible to take it out of the integral. Integrating the right hand side ofEq. (10): dCo(t) ---="---= dt

A irr

_-ttL, V "*'D,I\.

L

(

Ow A, )n(lCOA,(t)r[ . Co(t) ]m{1- exp [-lCTA,(t) n YL ]} , ~T,A, (t) n '

(11)

This ordinary differential equation must be solved with the initial condition indicated in Eq. (6). ABSORPTION COEFFICIENTS Eq. (11) needs two optical parameters that must be obtained from independent measurements. The 2,4-D absorption coefficient can be obtained from standard measurements in a Cary 17D spectrophotometer. Data for the absorption coefficient were obtained as a function of 2,4-D concentration in water at 253.7 and 284 nm. With a linear regression, our measurements for 284 nm gave a value of aL\ = 1820 L mole- I cm- I that agrees well with the reported value of 1780 L mole- I cm- I by Aly and Faust (1963). Our data for 253.7 nm produced a value of the molar Naperian absorptivity of 409 L mole- I em-I. Then, from Beer's equation: (12) To obtain the total absorption coefficient (a mixture of reactant and reaction products) we propose: (13)

The "unknown-products" hypothetical concentration can be expressed in terms of the 2,4-D instantaneous concentration: (14) (15) In Eq. (15) only one parameter is unknown. Values of Kt A. as a function of Co(t) were obtained (with spectrophotometric measurements) from the experimental d~composition runs at 253.7 nm. Figure 2 shows a plot (KT,A. (t) - aD CD (T» [=]CM-I vs. (cOD - cd) [=] PPM, for A = 253.7 nm. Applying a linear regression to the experimental points and expressing KT,A. in cm- l and Co(t) in ppm, the following empirical correlation was obtained: lCT,A,(t) = 0.0197 C~ - 0.01785 Co(t)

(16)

KINETIC PARAMETER EVALUATION W,e ~e now in. the position of obtaining the kinetic parameters from the experimental data and the proposed kinetIc model In Eq. (8). We have three unknowns: the quantum yield, and the exponents "m" and "n". The wh?le. m?del was fed to a multiparameter, non-linear regression algorithm that is coupled with an optullizatlOn program according to the Levenberg-Marquardt method (Marquardt, 1963).

Photochemical decomposition of 2,4-dichlorophenoxyacetic acid

37

1.5,.....---------------.

IA =253.7 nml

-tt

E

J! 1.0 ... .<

• 5 io. 0.0 ~---t__--__+---_+_----I

°

20

«>

(c:- CJ[ppm]

60

so

Figure 2. [lCn (t) - aD Co(t)] vs. [COD - CD]. Keys: (- _.) linear regression; (+) experimental points.

The regression program gave the following values for the exponents: n 1 and m O. With these estimations, at 25 C and 253.7 nm, the following kinetic equation was obtained: R D ,!.. (y) = -0.0262

et (y)

(17)

Eq. (17) indicates that at 253.7 nm, and for the explored range of concentrations, the overall quantum yield is 2.62 %, which is a rather low value. This equation also indicated that the dependence upon the 2,4-D concentration is completely accounted for with the concentration dependence of the LVREA. However, this result should not be interpreted as zero order dependence with respect to 2,4-D concentration because it participates in two parts of the variable. From Eqs. (2), (12) and (16) it can be seen that the LVREA bears a direct linear dependence with the 2,4-D concentration [Eqs. (2) and (12)] and a decreasing exponential dependence with the total absorption coefficient that includes the 2,4-D concentration [Eqs. (2) and (16)]. The radiation inner filtering effect produced by the reaction products is also taken into account by the exponential term. 100 - - - - - - - - - - - - - - - - - . . . . ,

20

o

5

10

t[h]

15

20

25

Figure 3. 2,4-D concentration vs. time. Keys: (--) kinetic model; (£,_) experimental data.

Figure 3 shows the lines corresponding to this kinetic expression and the experimental points. Here results are shown only for two runs with total irradiation. Deviations between predictions according to Eq. (17) and the experimental data are never greater than 20%. These deviations are due to the very simple kinetic formulation used in this work. Previous work has indicated that the pH (that changes with the reaction progress) may have some effect on the kinetics. Moreover, some autocatalytic effects of the reaction products have also been suggested. In order to incorporate reaction product effects, a precise analysis of their concentrations should have been known. However for reactor design purposes, the obtained kinetic suffices and it does not seem necessary to resort to

M. 1. CABRERA et at.

38

more sophisticated and very time consuming studies. In a follow-up paper (Martin et al., 1996), these result: will be used to design and operate an annular bench-scale photoreactor. CONCLUSIONS A very simple kinetic model has been developed to describe the direct photolysis of the 2,4 dichlorophenoxyacetic acid employing radiation of 253.7 nm. The model parameters were obtained in a well-stirred batch photoreactor for which the radiation field was precisely quantified. This experimental set• up and the employed model are able to produce an intrinsic kinetic result, i.e., a result that is independent of the reactor configuration. Under these conditions the following conditions could be drawn. 1) 2) 3) 4) S)

The time evolution of the optical properties of the reacting medium could be modeled with an empirical equation that has a single adjustable parameter that depends on the total product concentration. The kinetic model indicated a linear dependence with the local volumetric rate of energy absorption. Apart from the implicit dependence of the LVREA on the 2,4-D concentration, no further explicit dependence of the reaction kinetics on the herbicide concentration was found. At 253.7 nm, under the investigated concentrations, the overall quantum yield was 2.62 % or 0.0262 mole einstein-I. When predictions from the obtained kinetic model were compared with experimental data, a reasonably good agreement was obtained. ACKNOWLEDGMENTS

The authors are grateful to Consejo Nacional de Investigaciones Cientfficas y Tecnicas (CONICET) and to Universidad Nacional del Litoral (UNL) for their support to produce this work. They also thank Mr. Antonio C. Negro and Mr. Hugo Molina for their valuable help in the experimental work, and Eng. Claudia M. Romani for technical assistance. NOMENCLATURE A C ea G R t T V y

area, m2 concentration, mole m- 3 LVREA, einstein m- 3 s-1 incident radiation, einstein m- 2 s-I reaction rate, mole m- 3 s-1 time, s temperature, K volume, m- 3 cartesian coordinate, m

D irr L P PR T w

A.

Superscripts

Greek Letters

cI>

molar absorption coefficient, m2 mole- 1 volumetric absorption coefficient, m- 1 wavelength, nm quantum yield, mole einstein- 1

A

Subscripts relative to reactant (Fe+3)

ex K

A.

relative to 2,4-D relative to irradiated area or volume relative to liquid phase relative to reaction product (Fe+2) product property denotes total value denotes a wall property indicates a dependence on wavelength

o

indicates initial conditions

Special Symbols

<> indicates average value [=]

has units of

Photochemical decomposition of 2,4-dichlorophenoxyacetic acid

39

REFERENCES Alfano, 0. M., Romero, R. L. and Cassano, A. E. (1985). A cylindrical photoreactor irradiated from the bottom. 1. Radiation flux density generated by a tubular source and a parabolic reflector. Chem. Eng. Sci., 40, 2119-2127. Alfano, 0. M., Romero, R. L. and Cassano, A. E. (\986a). A cylindrical photoreactor irradiated from the bottom. II. Models for the local volumetric rate of energy absorption with polychromatic radiation and their evaluation. Chem. Eng. Sci., 41, 1155-1161. Alfano, 0. M., Romero, R. L. , Negro, C. A. and Cassano, A. E. (\986b). A cylindrical photoreactor irradiated from the bottom. III. Measurement of absolute values of the local volumetric rate of energy absorption. Experiments with polychromatic radiation. Chem. Eng. Sci., 41,1163-1169. Aly, 0. M. and Faust, S.D. (\963). Determination of 2,4-dichlorophenoxyacetic acid in surface waters. J. Am. Water Works Assoc., 55, 639-646. Boval, B. and Smith, J. M. (1973). Photodecomposition of2,4-dichlorophenoxyacetic acid. Chem. Eng. Sci., 28,1661-1675. Chamarro, E. and Esplugas, S. (\ 993). Photodecomposition of 2,4-dichlorophenoxyacetic acid: influence of pH. J. Chem. Techno/. Biotechnol., 57, 273-279. Connick, W. J. and Simoneaux, J. M. (\982). Determination of (2,4-dichlorophenoxy) acetic acid and of 2,6-dichlorobenzonitrile in water by high-performance liquid chromatography. J. Agr. Food. Chem., 30, 258-260. Crosby, D. G. and Tutass H. 0. (\966). Photodecomposition of2,4-dichlorophenoxyacetic acid. J. Agr. Food Chem, 14,596-599. General Electric (1967). Germicidal Lamps, TP-122, Sept. Marquardt, D. W. (\963). An algorithm for least-squares estimation of nonlinear parameters. J. Soc. Ind. Appl. Math., 11,431• 441. Martin, C. A., Cabrera, M. 1., Alfano, O.M. and Cassano, A. E. (\ 997). Photochemical decomposition of 2,4• dichlorophenoxiacetic acid in aqueous solution. II. Reactor modeling and verification. Wat. Sci. Tech. 34(4). Murov, S. L. (1973). Handbook of Photochemistry, Marcel Dekker, New York. Pichat, P., D'Oliveira J. c., Maffre, J. F. and Mas D. (\ 993). Destruction of 2,4-dichlorophenoxyethanoic acid (2,4-D) in water by Ti02-UV, H202-UV or direct photolysis. In Photocatalytic Purification and Treatment of Water and Air, Ollis, D. F. and AI-Ekabi, H. (eds.) 683-688. Pignatello, J. 1. (\992). Dark and photoassisted iron (3+)-catalyzed degradation of chlorophenoxy by hydrogen peroxide. Environ. Sci. Technol., 26, 944-951. Pignatello, J. 1. and Sun, Y. (1993). Photo-assisted mineralization of herbicide wastes by ferric ion catalyzed hydrogen peroxide. ACS Symp. Ser., 518 (Emerging Technologies in Hazardous Waste Management III), 77-84. Prado 1., Arantegui, 1., Chamarro, E. and Esplugas, S. (\994). Degradation of2,4-D by ozone and light. Ozone: Sci. Eng., 16, 235• 245. Rao, N. N. and Dube, S. (1995). Application of Indian commercial Ti02 powder for destruction of organic pollutants: photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) using suspended and supported Ti02 catalysts. Indian J. Chem. Technol., 2, 241-248. Sun, Y. and Pignatello, J. J. (1993a). Organic intermediates in the degradation of 2,4-dichlorophenoxyacetic acid by iron (3+)/hydrogen peroxide and iron (3+)/hydrogen peroxidelUV. J. Agr. Food. Chem., 41, 1139-1142. Sun, Y. and Pignatello, J. J. (\993b). Photochemical reactions involved in the total mineralization of 2,4-D by iron (3+)/hydrogen peroxidelUV. Environ. Sci. Techno/., 27, 304-310. Sun, Y. and Pignatello, J. J. (\995). Evidence for a surface dual hole-mechanism in the titanium dioxide photocatalytic oxidation of 2,4-D. Env. Sci. Technol., 29, 2065-2072. Trillas, M., Peral, J. and Domenech, X. (1995). Redox photodegradation of 2,4-dichlorophenoxyacetic acid over Ti02' Appl. Catal. B, 5,377-387.