Environ. Sci. Technol. 1999, 33, 421-425
Photodechlorination Mechanism of DDT in a UV/Surfactant System WEI CHU* Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
The photochemical reactions of the organochlorine pesticide DDT in aqueous solutions containing nonionic surfactant micelles (Brij 35, Brij 52, and Brij 72) were investigated and modeled. All photolytic experiments were conducted in a Rayonet RPR-200 merry-go-round photoreactor using a 253.7-nm mercury monochromatic ultraviolet (UV) lamps. Pseudo-first-order decay through photodechlorination was observed to be the dominant reaction pathway for DDT photodecay. The primary photoproducts include lesser chlorinated compounds (DDE and DDD) and hydrogen chloride. The photodechlorination of DDT involves two stages; the first is the fast aliphatic chlorine reduction, followed by a slow aromatic chlorine reduction. The photodecay rates of DDT were doubled in the Brij 52 micellar solution compared to that in water alone. A firstorder parallel/consecutive model was developed and found useful to predict the photodecay of DDT and the generation of DDE/DDD in the micellar/aqueous system.
Introduction Desorption of hydrophobic organic pollutants from soil has been shown to be effective in surfactant micelle solutions (1-4). Surfactant micelles have also been shown to act as catalysts in photochemical decay reactions of chlorinated organic compounds (5, 6). Soil contaminated by organic pollutants is an environmental concern, and many researchers have examined in detail the distribution of nonpolar compounds between surfactant micelles and the soil particles (7-10). Chlorinated aromatic compounds such as dichlorodiphenyltrichloroethane (DDT) are generally interesting because of their strong sorption into the soil and low decay rates in the environment (11-14). Soil remediation from these toxicants is often initiated by extracting the contaminants from the soil to the aqueous phase, followed by chemical, physical, and/or biological treatment. As a consequence, the use of surfactants as solubilizing agents to remove contaminants from soils followed by aqueous photodegradation may be a promising strategy through which the surfactants may be recovered and/or reused. In situ soil washing and pumpand-treat techniques have often employed surfactants to assist the solubilization of sorbed organic contaminants (15). However, the surfactant recovery must be included in any technology which uses surfactants to enhance toxicant extraction, capturing, and/or decomposition. At present, a practical treatment process has been proposed in which organic contaminants are extracted by the use of surfactants, and they are solubilized and photolytically decomposed within surfactant micelles (17). Photodechlorination is a specific type of photolytic process applicable to chlorinated aromatic compounds. In fact, the photodechlorination of these compounds solubilized within * Corresponding author fax: (852)23346389; e-mail: cewchu@ polyu.edu.hk. 10.1021/es980293k CCC: $18.00 Published on Web 12/16/1998
1999 American Chemical Society
surfactant micelle have several potential advantages: (i) separating the organic contaminants from the soil and decomposing the contaminants in one step; and (ii) increasing the rate of photodechlorination. In a homogeneous aqueous solution, the use of photodechlorination may in fact be limited by certain practical considerations. For example, organic hydrophobic compounds sorbed into soil do not easily diffuse into the aqueous phase, oxygen molecules may quench the triplet state of compounds and retard free-radical chain reactions, and some dimers may be formed from the parent compounds (18). However, these problems can be overcome by conducting the photolytic reactions within the surfactant micelles due to the compartmentalization effect. Some researchers have reported the photodecay of pollutants of environmental concern in organic solvents (19-22) and photodecay of organics in micelles (23), but there is still little information concerning the photochemical reactions of organic pollutants in micellar solutions. So, my research was intended to explore the basic variables and conditions that influence these reactions for specific chlorinated aromatic compounds of environmental importance, such as DDT. To achieve this goal, a series of experiments was conducted in which samples were exposed to monochromatic ultraviolet (UV) light at 253.7 nm and the extent of degradation of the parent compound and the formation of reaction products was measured. Specific tasks were conducted, including the investigation of the photolytic decay pathways of DDT in three different nonionic micellar solutions, the determination of decay mechanisms and reaction rates for these systems, and the development of a practical model to predict the photochemical reaction of DDT in micellar systems.
Materials and Experimental Methods All chemicals were reagent-grade except for the surfactants. The p,p′-dichlorodiphenyltrichloroethane (DDT), p,p′-dichlorodiphenyldichloroethane (DDD), p,p′-dichlorodiphenyldichloroethylene (DDE), and the internal GC standard, aldrin, were purchased from Aldrich. Solvents (isooctane and n-hexane) were purchased from Fisher and used without further purification. The surfactants Brij 35, Brij 52, and Brij 72 were purchased from Sigma. It should be noted that these surfactants were homologue mixtures and used without further purification. The use of nonionic surfactants in the proposed soil washing/photodecay systems was due to their low affinity to the cationic adsorption capacity present in most soils. All photolysis experiments were conducted with a Rayonet photochemical reactor RPR-200 using a merry-go-round apparatus. This reactor holds up to 16 UV lamps, and a maximum of eight cylindrical quartz cuvettes (1 cm in diameter). The merry-go-round rotates at a constant speed of 5 rpm so that all of the cuvettes are exposed to the same light intensity during the reaction (refer to 25 for system details). A ventilation fan was installed inside the reactor to prevent heat accumulation. The temperature rise of solution after reaction was less than 0.5 °C. Two phosphor-coated low-pressure mercury lamps that emit monochromatic light at 253.7 nm were used in all photochemical reactions. The intensity of the incident light was determined to be 3.0 × 10-6 Einstein L-1 s-1 by the use of the chemical actinometer, potassium ferrioxalate (26). The stock solution of chlorinated compounds (e.g., DDT, DDD, and DDE) was prepared in isooctane solution. For each photolysis experiment, the stock solution was coated in the test tube and left at room temperature until the solvent dried out, the probe was then VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Photodechlorination of 59.52 nM DDT and its decayed homologues within 15 mM Brij 52 surfactant micelles. The light intensity is 3.0 × 10-6 Einstein L-1 s-1at 253.7 nm. The total mole balance is calculated by the molar concentrations of DDT, DDE, and DDD. redissolved by adding sufficient surfactant solutions. The solution containing the probe and surfactant micelles was placed into quartz cuvettes, sealed with Teflon-lined caps (to minimize the volatilization effect), and irradiated in the photoreactor. The cuvettes were removed at different recorded times (60, 180, 300, 600, and 1200 s) with the initial concentration at time zero determined from an unexposed sample. After the UV illumination, exactly 5 mL of well-mixed supernatant of each illuminated sample was pipetted into a separating tube (outfitted with a Teflon-lined cap with coated internal GC standard). The supernatant was then carefully overlaid with 10 mL of isooctane and shaken with the addition of excess sodium chloride to prevent the formation of emulsions during the extraction processes (27). After 24 h of extraction, a 1-mL aliquot of the organic phase at the top layer was transferred to a small vial sealed with a Teflonlined septa and open face cap, and 0.1 g of sodium sulfate anhydride was added to absorb water if present. The extraction efficiency of each studied compound (DDT, DDE, and DDD) by this method was determined to be greater than 95% for a number of surfactants. After the extraction into isooctane, DDT and its photoproducts were quantified with GC-ECD, and the detection limits were around 5 × 10-10 M. The GC analysis was conducted by a Hewlett-Packard 5890A gas chromatograph equipped with an electron-capture detector (ECD), a split/splitless injector, a HP 3396A integrator, and a J&W Scientific 30 m × 0.32 mm column (liquidphase DB-5 with film thickness of 0.25 µm). Nitrogen gas was used for both the carrier gas and the ECD makeup gas. A ramped GC temperature program was used, where the oven temperature increased from 110 °C (for 1 min) to 300 °C (for 2 min) at a rate of 60 °C/min. The temperatures of the injector and detector were 290 and 330 °C. The peaks of DDT, DDE, and DDD were therefore identified at 15.82, 13.33, and 11.24 ( 0.05min.
Result and Discussion Photodechlorination. DDT was irradiated in solutions containing one of the three different surfactants. A typical decay chronicle of DDT as well as the formation and decline of its lower homologues (DDE and DDD) in 15 mM Brij 52 micellar solution is shown in Figure 1. In the GC-ECD analysis, only DDT, DDD, and DDE caused significant peaks and no other major peaks were observed 422
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FIGURE 2. The pH change for the photodechlorination reactions of 59.52 nM DDT within various 15 mM surfactant micelles that were illuminated at 257.3 nm.
TABLE 1. The pH Changes in the Photodechlorination of DDT in Various Surfactant Solutions surfactant
initial DDT concentration, M initial R-Cl concentration, M R-Cl in DDD and DDE, M expected HCl release, M initial pH of micellar solution theoretical pH after reaction observed pH does the pH match? extra Ar-Cl in DDT, M expected pH if Ar-Cl released does the pH match?
Brij 35
Brij 52
Brij 72
6.0 × 10-8 1.8 × 10-7 1.2 × 10-7 6.0 × 10-8 8.32 7.19 7.11 yes
6.0 × 10-8 1.8 × 10-7 1.2 × 10-7 6.0 × 10-8 7.62 7.08 6.64 no 1.2 × 10-7 6.69 yes (6.64)
6.0 × 10-8 1.8 × 10-7 1.2 × 10-7 6.0 × 10-8 6.07 6.04 5.98 yes
in the photolysis reaction. The molar numbers of ECDsensitive chlorinated compounds decreased during the photolysis, and the decrease of total mole balance justified the prediction that the photodechlorination reaction was responsible for the photodecay of DDT, which gave DDE and DDD as the measurable intermediates. The pH values of the illuminated DDT solutions were found to decrease during the reaction in different surfactant micelles, as shown in Figure 2. It has been reported that the decrease of pH in photodechlorination of chlorinated aromatics is due to the generation of protons and stoichiometric amount of chloride ions (i.e., HCl) during the photoreduction process (17). By comparing Figures 2 and 1, the solution pH generally dropped in the first 300 s, when most of the DDT had converted to DDD, DDE, and lower homologues. At the beginning, there were 6.0 × 10-8M of DDT in the micellar solutions, which were equivalent to 1.8 × 10-7M of aliphatic chlorines (R-Cl) on the trichloroethane groups. After DDT converted to DDD and/or DDE, which contained only 1.2 × 10-7 M aliphatic chlorines on the dichloroethane or dichloroethylene groups, around 6.0 × 10-8 M of chloride ions and same amount of protons would be released into the solution. The released protons should theoretically reduce the solution pH of Brij 35, Brij 52, and Brij 72 from 8.32, 7.62, and 6.07 to 7.19, 7.08, and 6.04, respectively. For the Brij 35 and Brij 72 solutions, the predictions were close to our experimental data, as summarized in Table 1, which suggested that the aliphatic chlorine should be substituted in the reaction. However, the observed pH decline in the Brij 52 solution (6.64) was much lower than the calculation of 7.08, which implied that a reaction mechanism other than the aliphatic chlorine
substitution was involved. Since the highest decay rate was also observed in the Brij 52 solution (see the discussion of reaction rates later in this paper), it was rational to assume that not only the aliphatic chlorine could be photosubstituted, but that the two aromatic chlorine atoms (Ar-Cl) in DDT could be photosubstituted as well. From calculation, the two aromatic and one aliphatic chlorine in 6.0 × 10-8M of DDT molecules were able to generate 1.8 × 10-7M of protons (i.e., HCl) to the solution, which could theoretically reduce the solution pH to 6.69. This prediction agreed with the observation of 6.64 after an extended photodechlorination of DDT in the Brij 52 micellar solution, as shown in Figure 2. It could therefore be concluded that the aliphatic chlorine would be substituted first in the photodechlorination reaction because of its lower excited energy, compared to that of aromatic chlorines. However, if the reaction rate was high enough to result in a comprehensive photodechlorination of DDT, the aromatic chlorines could also be substituted at a later stage. The photodechlorination mechanisms of DDT in micellar solutions are therefore proposed as follows:
where the R-Cl is aliphatic chlorine, Ar-Cl is aromatic chlorine, and R′-H is the additional hydrogen source in the environment (i.e., the surfactant micelles). Reaction Rates. The photodecay rates of DDT within various nonionic surfactants and distilled water were also examined, where the pseudo-first-order kinetics was observed if the data were separated into two stages as shown in Figure 3. The DDT decay was likely to be dominated by fast aliphatic chlorine substitution in the first stage, since more than 50 to 80% of the DDT decayed in the first 300 s. The decay rates and quantum yields of DDT photodecay were reported in Table 2. The decay rates were derived from the least-squares method, the r squares of regression lines ranging from 0.990 to 0.980 in the first stage, and 1.000 to 0.960 in the second stage. The quantum yields for DDT photolysis under the monochromatic light source were calculated by eq 3 (17):
Φ)
k 2.303Iλ0λl
(3)
FIGURE 3. (a) Photodechlorination of 59.52 nM DDT in various surfactant solutions (15 mM of Brij 35, Brij 52, and Brij 72) and distilled water. In all cases, the same light intensity of 3.0 × 10-6 Einstein s-1 L-1 that came from two low-pressure UV lamps at 253.7 nm was employed. (b) Two stages of pseudo-first-order decay of photodechlorination reactions, the solid line indicates the decay in the first stage, and dashed line indicates the decay in the second stage. (Note: ln × 2.303 ) log).
TABLE 2. Decay Rate of DDT in Different Surfactant Micelles and Water first stage
where Φ is the quantum yield (dimensionless), Iλ0 is intensity of the incident light at 253.7 nm (3.0 × 10-6 Einstein L-1 s-1), λ is the molar absorptivity of DDT at 253.7 nm (4041 L mol-1 cm-1), and l is the cell path length (1 cm). The results indicated that the photodecay rate (in the first stage) increased from 2.00 × 10-3 without surfactant to 2.37 × 10-3, 2.68 × 10-3, and 4.41 × 10-3 in the presence of Brij 35, Brij 72, and Brij 52. The rate enhancement was obviously due to the existence of surfactant micelles, since the micelles could offer additional hydrogen sources for the photodechlorination (i.e., photoreduction) reaction as previously discussed in eqs 1 and 2. But after 80-95% of the DDT decayed, the decay rates declined, as noted in the tailing of the decay curves in Figure 3. There might be several reasons for this: (a) The number of easy-accessible hydrogen sources in the micellar solution may be depleted. (b) Some of the photoproducts may act as internal light filters and reduce
surfactant Brij 35 Brij 52 Brij 72 water
second stage
decay rate, s-1
quantum yield
decay rate, s-1
quantum yield
2.37 × 10-3 4.41 × 10-3 2.68 × 10-3 2.00 × 10-3
0.085 0.158 0.096 0.072
7.50 × 10-4 1.56 × 10-3 9.10 × 10-4 4.60 × 10-4
0.027 0.056 0.033 0.016
the incident light intensity. For example, the intermediates DDD and DDE have molar absorptivities of 2154 and 142480 L mol-1 cm-1at 253.7 nm. (c) Some of photoproducts may act as quenchers to interfere with the free-radical intermediates. (d) Minor pathways such as photochlorination reactions and photoisomerization reactions may be present in the solution, which can synthesize a small amount of DDT molecules and result in the tailing (17). (e) And, as discussed before, the photodechlorination of DDT involves two stages: one is the substitution of aliphatic chlorine, the other is the VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 1
substitution of aromatic chlorine, and the first substitution reaction is faster than the latter (see Table 2). Modeling and Application. As noted in Figure 1 and previous studies of photodecay of DDT (28), it was rational to assume that the photodecay of DDT was mainly dominated by the parallel-consecutive mechanism as shown in eq 4 (Scheme 1). On the basis of this assumption, the concentration change of each reactant could be formulated by the following differential equations:
d[A]/dt ) -(k1 + k2)[A]
(5)
d[B]/dt ) k1[A] - k3[B]
(6)
d[C]/dt ) k2[A] - k4[C]
(7)
The analytical solutions of components A, B, and C can be derived after integration. As a result, the concentrations for the species involved in the photodechlorination could be expressed as eqs 8, 9, and 10:
[A]t ) [A]0e-(k1+k2)t [B]t )
[C]t )
k1[A]0
(8)
[e-(k1+k2)t + e-k3t]
(9)
[e-(k1+k2)t + e-k4t]
(10)
k3 - (k1 + k2) k2[A]0
k4 - (k1 + k2)
where [A]0 is the initial concentration of DDT at time zero, t is the reaction time (in s), k1, k2, k3, and k4 are the pseudofirst-order decay rate constants (in s-1). The above equations are valid when the initial concentrations of [B]0 and [C]0 are zero. Brij 52 was used to conduct a series of experiments to verify the proposed photodecay model. The direct photodegradation of DDT, DDE, and DDD in Brij 52 surfactant micelles was conducted to determine their initial decay rates. As shown in Figure 4, the photodecay rates of DDT, DDE, and DDD had been resolved to 4.41 × 10-3 s-1, 5.06 × 10-3 s-1, and 5.40 × 10-3 s-1, which were equivalent to (k1+ k2), k3, and k4, respectively. The k1 (3.00 × 10-3 s-1) is estimated from the k1/k2 ratio, which is determined to be 2.125 from the peak product ratio of [DDE]/[DDD] at 180 s. After the substitution of these rate constants into eqs 8-10, the predicted values and raw data are illustrated in Figure 5 for comparison. The proposed model could predict the photodechlorination of DDT as well as the formation and decay of DDE and DDD at 253.7 nm. A small bias was observed for the DDT decay at the late stage (after 300 s), which might be due to the simplification of the model: (a) only the initial rate constants were considered for all the three components in this model; (b) the model left out the second stage of photodechlorination (aromatic chlorine substitution) as noted earlier; and (c) the existence of other minor pathways such as photochlorination and photoisomerization could also deflect the system. 424
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FIGURE 4. Photodechlorination of DDT, DDE, and DDD at initial concentrations of 59.52 nM, 31.45 nM, and 31.24 nM within 15 mM Brij 52 surfactant micelles at 253.7 nm. (Note: ln × 2.303 ) log.)
FIGURE 5. Parallel/consecutive photodecay of 59.52 nM DDT within 15 mM Brij 52 surfactant micelles at 253.7 nm under a light intensity of 3.0 × 10-6 Einstein s-1L-1. The markers indicate the experimental data, and the lines indicate the modeling values. The proposed model can be further modified into a practical format for photoreactor design, as shown in eq 11. The (k1 + k2) term in eq 8 can be replaced by the quantum yield, Φ, from eq 3. For any pollutant “i”, the ki equals to 2.303ΦiIλ0λil, where the Φi and λi, are characteristic constants of any pollutant (e.g., DDT or any other organochlorine pesticide), the substitution of (k1 + k2) by Φ results in a design formula which depends on the reactor parameters (Iλ0, l, and t) only.
TABLE 3. Prediction of DDT Decay in Brij 52 Surfactant Micelles by the Practical Modela
time, s
DDT concentration, nM
removal rate observed, %
removal rate estimated by model, %
difference, %
0 60 180 300 600 1200
59.52 40.65 27.33 14.92 9.99 3.72
0.0 31.7 54.1 74.9 83.2 93.7
0.0 23.3 54.8 73.4 92.9 99.5
0.0 8.4 -0.7 1.6 -9.7 -5.8
Remark: Φ of DDT in Brij 52 is 0.158, Iλ0 at 253.7 nm is 3.0 × 10-6 Einstein L-1 s-1, λ of DDT at 253.7 nm is 4041 L mol-1 cm-1, and l is 1 cm. a
(
REMOVAL(%) ) 100 1 -
[A]t
)
Acknowledgments Support for this project come from a research grant from the Hong Kong Polytechnic University.
Literature Cited )
[A]0 100{1 - exp[(-2.303Φiλi)Iλ0l]t} (11)
To apply the results of the study to field situations or other organochlorine pesticides, some general factors need to be considered: (a) The Loss of Surfactant. In this research, the molar ratio of DDT/surfactant is about 1/250 000, so the extent of the photooxidation of the surfactant due to the photoreduction (i.e., photodechlorination) of DDT is trivial. Although the direct photodecay of the surfactant may exist, this reaction will not hinder its normal function (29). (b) Presence of Oxygen. Pure O2 is expected to quench the triplet state and retard free-radical reactions in a photochemical reaction, but if a mixture is only exposed to atmosphere (the dissolved oxygen in our photo reactor was about 6.0 mg/L), the retarding effects are found insignificant. Similar results were also reported by Hawari et al. (19). (c) Light Attenuation. The attenuation (absorption) of incident UV light by nontarget substances may be a critical factor which should to be considered in designing ex situ soil-washing systems or an in situ pump-and-treat operation. Hill and Felder (30) proposed an exponential attenuation model, which can be used to calculate the attenuated light Iλa in a cylindical photoreactor if an average of the coextracted components is determined, see eq 12. The r0 is
Iλa ) Iλ0
the first few sample points. Less than 10% of overestimation in the later stage was observed due to tailing, as discussed before. (f) Bench-Scale Test. If the removal of organochlorine pesticide other than DDT is desired, some tests to determine the characteristic constants of pesticides (Φi and λi) are required. The selection of surfactant is also important, since the optimum removal of pesticide depends on the selection of proper surfactant (see Figure 3a for the comparison of removal efficiencies of DDT in various surfactants).
r0 -(r0+r) + e-(r0-r)] [e r
(12)
the radius of photoreactor, r is radial distance, and Iλ0 is the intensity of the incident light at 253.7 nm. (d) Humics. Humic materials can be extracted from soils by surfactants and may quench free radical reactions (31). However, the quenching effect may not be observed if the probe molecules are solubilized within micelles. Humics were known to have many polar functional groups, so their interaction with surfactant micelles might be through surface association, whereas the probe molecules that solubilized within micallar cores could be isolated from the humics. In fact, the quantum yields of photodecay of arylhalides in micellar solution is slightly enhanced if humics are coextracted from the soil because of the increment of indirect or sensitized photolysis (29). (e) Error of the Model. The actual removal data from the experiment were compared with the predicted data using eq 11 as shown in Table 3. The prediction was quite satisfactory, as the estimation has a very low error (less than 1.4%) in the middle range of the model where most applications should be located. About 8.4% of underestimation at the very beginning of the reaction was observed because the initial decay rate is higher than the average rate calculated from
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Received for review March 25, 1998. Revised manuscript received October 14, 1998. Accepted October 22, 1998. ES980293K VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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