UV induced degradation of herbicide 2,4-D: kinetics, mechanism and
effect of various conditions on the degradation
Sanghamitra Kundua , Anjali Pala,∗ , Anil K. Dikshitb,∗∗
b
a Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302, India
Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
Abstract
The degradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) under UV light was studied under various conditions. A photoreactor was used for the photochemical reaction. Calibration curves for 2,4-D were generated at two different absorption maxima (230 and
283 nm). The effects of various parameters such as light intensity, exposure time, various solvents (10% methanol and 10% acetonitrile) and
micellar media (cationic and anionic) and the effects of some other parameters such as O2 , wavelength of light, pH, other commonly occurring
ions (e.g. Cl− , SO4 2− , Ca2+ /Mg2+ , Fe3+ , NO3 − ) were studied. The percentage of degradation of 2,4-D was achieved up to 81% in simple
aqueous medium. But in cetyltrimethylammonium chloride (CTAC, a cationic micelle) the percent degradation can even be further increased.
The percent degradation of 2,4-D using solar irradiation under similar conditions as that applied for the photoreaction using the photoreactor
was compared. Using the solar light the degradation was very negligible. The reaction in general follows zero-order kinetics. But in CTAC
micelle the reaction follows first-order kinetics.
Keywords: 2,4-D degradation; UV irradiation; Kinetics and mechanism
1. Introduction
The worldwide applications of pesticides and herbicides
in agriculture during the last few decades caused the largescale development of agrochemical industries. Due to this
a dramatic increase in the population of a variety of agrochemicals in natural water has occurred. In recent years a
great deal of research is being carried out to assess the possible adverse effects of these pesticides towards the human
health and ecosystem [1]. These pesticides and herbicides
are now abundantly found in water and food [2]. Among
the numerous agrochemicals in use today, the herbicide, 2,4dichlorophenoxyacetic acid (2,4-D) has been widely applied
to control broad-leaved weeds. After application in the field, it
can be easily transferred into natural water due to its high solubility. Concentrations of pesticides or herbicides in water are
subject of many monitoring programs around the world. After
spraying most of the pesticides are deposited on vegetation,
soil or other surfaces. Deposits on soil remain in the top few
centimeters, where they degrade or become adsorbed on the
soil and organic particles. Only a very minor fraction contaminates surface and groundwater sources and causes concern
in aquatic toxicology and drinking water quality. Concentration levels less than 1 ppm are usually encountered in the
natural aquatic systems after deliberate introduction of the
2,4-D (or its ester formulations) or from run-off or drift in
terrestrial operations. However, pesticide/herbicide misuse,
spillage and/or inappropriate storage, handling and disposal,
has been estimated to be responsible, in many cases, for the
water contamination at high concentration levels. It is also
very important to distinguish between the direct pollution
due to direct treatment of the water with herbicide to destroy
waterweeds, from leaching pollution. It is expected that the
122
direct pollution may create much higher concentrations of
all kinds of pesticides/herbicides in comparison to that for
leaching after field treatment, and it has been reported that
in case of 2,4-D it can go even up to several hundred ppm.
The herbicide 2,4-D is also known to persist in the soil and
contaminate surface and underlying groundwater. Even after
a long period of use various amounts of 2,4-D were detected
in surface water and ground water [3]. Hence the development of an effective degradation process for this herbicide
has become a necessity. In 1982, the WHO considered 2,4-D
as moderately toxic (class II) and recommended a maximum
concentration of 0.1 ppm in drinking water. Later the US EPA
also recommended the permissible level of 2,4-D for potable
water to be 0.1 ppm. In India, the standard requires that 2,4-D
or any other pesticide should not be present in drinking water.
Chemical and/or photochemical processes under typical
environmental conditions cannot degrade most of the pesticides [4]. Although a number of physical and biological
methods have been proposed to remove these pollutants from
industrial wastewater but many of these pollutants are not
destroyed by these techniques. Various chemical treatment
methods for the destruction of these agrochemicals have been
proposed and are currently been used for pesticide/herbicide
degradation. These methods are based on catalytic, electrochemical, sonochemical and photochemical reactions, and
are known as advanced oxidation processes (AOPs). However, despite these treatments, there is presently no universal
technique available.
The degradation of 2,4-D has been mainly studied using
chemical treatment such as H2 O2 /Fe2+ [5] or photochemical treatment using TiO2 /UV/O3 , Fe(II)/UV/O3 [6], UV/ironmediated oxidation [7], microwave/UV [8], TiO2 /solar irradiation [9], TiO2 /UV [10], TiO2 /activated carbon/UV [11],
photolytic–electrolytic combination [12], etc. Ionizing radiation is also an efficient means for oxidative degradation of
2,4-D [13]. Compared to other AOTs, this technique has the
advantage that no chemicals have to be added to the water samples. This technique, however, needs sophisticated
instrumentation and special arrangement. It is true that the
use of catalysts and chemicals combined with UV light can
shorten the reaction time but in many cases the processes
need additional separation step and higher cost. Thus photodegradation of 2,4-D using only UV, which is known as
direct photodegradation, has been thought of as a possible
alternative to the ionizing radiation because this technique
needs very simple and cheap experimental set up [14]. The
intensity of light can be varied just by changing the power
of light or by the number of lights used in the set up. Even
the wavelength of the light source for irradiation can be varied easily. Detail studies using such a simple procedure have
not been attempted much, although while doing exhaustive
studies on other systems (such as iron-mediated oxidation
systems using 253.7 nm light source [7], photo-Fenton reaction and UV/H2 O2 using ∼254 nm light source [15], photodegradation using light of λ > 310 nm in presence of TiO2
[16–18], etc.), direct photodegradation of 2,4-D was made
as a comparative study. Our aim was to study in detail the
effectiveness of such a procedure without the need of any
catalyst or whatever. The purpose of the study was to see the
effect of UV (∼365 nm) on the decomposition of 2,4-D under various conditions. This type of procedure can lead to a
more advantageous cost-effective route for the treatment of
2,4-D, because here the additional step for catalyst separation
is redundant.
2. Experimental section
2.1. Materials
All the reagents were of AR grade. 2,4-D was purchased from Sigma. A stock solution of 2,4-D (90 ppm) was
prepared. Other chemicals used were cetyltrimethylammonium chloride (CTAC), sodium dodecyl sulfate (SDS) and
poly(oxyethylene) isooctylphenyl ether (commonly known
as Triton X-100 and abbreviated as TX-100). To see the effects of inorganic ions such as SO4 2− , Cl− , NO3 − , Ca2+ and
Fe2+ on the photodegradation of 2,4-D, the compounds used
were Na2 SO4 , NaCl, NaNO3 , Ca (NO3 )2 , and FeSO4 ·7H2 O,
respectively. Methanol, acetonitrile and acetone were of high
purity (HPLC/AR grade) and were purchased from E. Merck.
Double distilled water was used throughout the investigation.
2.2. Method
All the glassware was covered with aluminium foil during
the test preparation period to minimize unnecessary exposure
of the samples to light. Studies on the degradation behavior
such as the percentage of degradation, kinetics of degradation, etc. were monitored under various experimental conditions such as with the variations in light intensities, under
various mixed solvents, in the presence of common interfering ions and under various other imposed conditions taking
the sample in a 1 cm quartz cuvette. The reaction was carried
out at room temperature (30 ± 2 ◦ C).
The photodegradations were carried out in quartz cuvettes. The UV photoirradiation was carried out using either
a portable germicidal lamp (15 W; G 15 T8 UV-C, Philips,
Holland) of wavelength ∼365 nm (650 lux) keeping the sample at a horizontal position at a distance of 3 cm or with an
apparatus known as the “Photoreactor”. The schematic diagram of the reactor is shown in Fig. 1. The apparatus consists
of a metal cylinder casing and UV lamps (8 W, Sankyo Denki,
Japan) of ∼365 nm wavelength arranged vertically along the
inner wall of the cylinder. A horizontal platform is positioned
at the center inside the metal cylinder where the samples can
be placed. The light intensities could be controlled in the
“Photoreactor” by varying the number of incident UV lamps
on. The flux (light intensity) was monitored using a digital
lux meter (Model LX-101, Taiwan). The intensity of light in
the photoreactor was calibrated with an Ophir power meter
(NOVA display and 30-A-SH sensor). The number of photons
123
Fig. 1. Photoreactor.
absorbed per unit area of the sample per second from the photoreactor of 100 lux is 3.03 × 1015 . GC–MS spectra were obtained with a Perkin-Elmer Auto system GC and Turbo mass
gold, with a PE-1 (100% methyl) column of 25 m length and
0.2 mm diameter. Helium was the carrier gas (0.9 ml/min).
2.3. Procedure
Photodegradation of 2,4-D was carried out in a quartz cuvette containing 3 ml of 2,4-D. The reaction was done by
irradiating the solution with UV light either in the photoreactor or using an ordinary germicidal lamp. The changes in
concentration of 2,4-D were then monitored spectrophotometrically.
3. Results and discussion
3.1. Preparation of calibration curves
The stock solution of 2,4-D (90 ppm) was diluted (in
water) to obtain solutions of lower concentrations in the
range of 4–39 ppm. Absorption spectrum of 2,4-D was run
in the range of 215–325 nm. The spectrum exhibited two
absorption peaks at λmax : 230 nm and λmax : 283 nm [19,20].
Two different calibration curves were drawn for 2,4-D at
these two different λmax values in the concentration range
of 0–39 ppm as shown in Figs. 2 and 3. The equations
obtained were Abs = 0.0404 × C (ppm) + 0.0039 (at λmax :
230 nm) and Abs = 0.0086 × C (ppm) − 0.0033 (at λmax :
283 nm) with the correlation coefficients of 0.9996 and
0.9948, respectively. Using these curves, the unknown
concentrations of 2,4-D could be easily interpolated. It was
found that the estimated concentrations were exactly the
same using either of the two graphs.
Fig. 2. Calibration curve of 2,4-D at 230 nm.
lead to the promotion of the pesticide molecules to their
excited singlet states, which may then intersystem cross to
produce triplet states. Such excited states can then undergo,
among other processes: (i) homolysis, (ii) heterolysis or (iii)
photoionization [21]. Photodegradation of 2,4-D in aqueous
solution by UV irradiation may also take place via C Cl and
C O bond homolysis, with a subsequent cascade of various free radical reactions [22]. The pathway of reaction, the
efficiency and the final products depend on the conditions
imposed on the reaction. The percent degradation of 2,4-D
obtained for our system using UV light of 600 lux intensity
(with five lights on) inside the photoreactor has been shown
in Fig. 4. In our studies the UV lamp of ∼365 nm wavelength was used. Earlier literature dealing with 2,4-D photodegradation and mostly in the presence of catalyst used
the light of ∼254 nm wavelength. There are only a few reports where ∼350 nm light was used for the photocatalytic
degradation of 2,4-D in the presence of TiO2 /UV/O3 and
Fe(II)/UV/O3 [6] and there are reports where >310 nm light
was used [16,17,23]. It was noticed that ozonation in presence
3.2. Degradation of 2,4-D with UV light
Most pesticides show UV–vis absorption bands at relatively short UV wavelengths. Direct irradiation of those may
Fig. 3. Calibration curve of 2,4-D at 283 nm.
124
Fig. 5. Effect of solvent/media on degradation of 2,4-D.
Fig. 4. Photodegradation of 2,4-D under UV light of intensity 600 lux.
of catalyst such as TiO2 or Fe(III), and UV light (15 W) could
cause efficient degradation of 2,4-D within short time [6].
Also 96% degradation of 2,4-D was possible under TiO2 /UV
[17]. On the other hand only 25% degradation of 2,4-D was
achieved on 3 h irradiation with 500 W [16] lamp. In our
case the initial concentration of 31.3 ppm for 2,4-D after
75 min UV irradiation (with five 8 W UV light having a total
of 600 lux intensity) was decreased to 19.1 ppm indicating
∼39% degradation. It was interesting to note that the decrease in concentration followed a linear pattern with time,
i.e. it followed a zero-order kinetics with the degradation rate
k = 0.166 ppm/min. It is thus reasonable to say that to get
complete degradation of 2,4-D under similar conditions (i.e.
with UV light of intensity 600 lux), 3 h 10 min times will be
sufficient.
30.0 ppm. After irradiation under the UV light (600 lux) for
45 min the final concentration of 2,4-D present in each solution was noted. From these observed results, the percentage of
degradation of 2,4-D was calculated for each solvent/medium
and was then plotted. The plot obtained is shown in Fig. 5.
The percentage of degradation of 2,4-D on 45 min irradiation
under various conditions varied in the range of 16–57%. It is
observed that the degradation of 2,4-D in the CTAC micelle
was the maximum and it was more than two times as obtained
from any other conditions. This clearly indicates that a different mechanism is occurring in this medium. This is possibly
due to the fact that the CTAC micelle is creating a different
kind of environment to force 2,4-D to take a different course
of reaction. In many cases this type of micellar catalysis is
possible to increase the encounter probability in the micellar pseudo phase [24,25]. Thus efficacy of micelle vis-à-vis
change of solvent polarity might be thought of while dealing
with real time degradation.
3.4. Effect of light intensity
3.3. Effects of different solvents and micellar
environments on 2,4-D photodegradation
Sometimes the solvent polarity and micellar environments
show profound influence on the degradation behavior of analytes. Again suitable micelles might influence the rate of
analyte degradation to a noticeable extent. This has been explained by the promoted energy transfer process (in other
words the scavenging of photon), lowering of activation energy and change of localization of analyte in different polar environments. Thus it was important to see the effects
of various solvents on the photodegradation of 2,4-D. Photodegradation of 2,4-D was studied in 10% aqueous methanol
and 10% aqueous acetonitrile. The photodegradation of 2,4D was also studied in two different micellar environments
such as aqueous 10−2 M CTAC micelle (cationic) and aqueous SDS (anionic) micelle (10−2 M). The photodegradation,
however, was not studied in aqueous TX-100 (nonionic) micelle and aqueous acetone because both absorbed in the range
where the spectrophotometric measurements of 2,4-D was
done. The initial concentration of 2,4-D in each case was
It is quite common that light intensity has a direct effect
on the percentage of degradation of the substance undergoing
photodegradation [12,23]. For example in the study reported
earlier it was observed that with a solution containing 50 ppm
of 2,4-D, the degradation was 1% with 150 W lamp on 1 h
exposure where as it was 60% with 400 W lamp with the
same exposure. The corresponding values after 8 h are 19
and >99.9%, respectively [12]. In another study it was shown
that percentage of 2,4-D degradation came down from 54 to
29% when the light intensity was changed from full intensity
(100%) to ∼20% [23]. In the present study it had been observed that ∼61% degradation of 2,4-D at pH 7 and ∼81%
at pH 9 was possible with an irradiation of 2.5 h with the
light corresponding to 600 lux (five 8 W UV lamps). A study
was thus performed to find out the relationship between the
light intensity and the percentage of degradation of 2,4-D.
The initial concentration and time of exposure were kept as
31.7 ppm and 30 min, respectively and the light intensity was
varied in the range of 0–600 lux. It was found that the amount
of degradation increased from 0 to 15.8% on 30 min irradia-
125
Fig. 6. Effect of light intensity on 2,4-D photodegradation.
tion with the increase in the light intensity from 0 to 600 lux.
The results are shown in Fig. 6. It is interesting to note from
the graph that it follows a linear relationship with the correlation coefficient of 0.981. It can thus be assumed that the
irradiation time required could be made much shorter than
that required in the present study if the intensity of light used
is higher than 600 lux. For practical purpose in many cases
UV lamps of higher intensity (125–400 W) have been used
in earlier works.
3.5. Effect of pH
Photodegradation of a substance could be highly affected
by the solution pH [17]. For example in case of 2,4-D photodegradation over TiO2 highest conversion rates were obtained at pH ∼ 3.5, which is close to the pKa of the acid and
lower than the point of zero charge of TiO2 [17]. Hence a
detailed study on the degradation of 2,4-D was carried out at
varying initial solution pH in the range of 1–13. In each case
the initial concentration of 2,4-D was taken as 30 ± 5 ppm
and the time of exposure was kept as 2.5 h with a light intensity of 600 lux. The degradation varied from 60 to 81% when
the reaction was carried out at different initial pH conditions.
It is observed that the percentage of degradation of 2,4-D is
more when pH > 7, in comparison to that at pH < 7. The plot
is shown in Fig. 7.
Fig. 7. Effect of pH on 2,4-D photodegradation.
and the total time of exposure under the UV light intensity of
600 lux was 2.5 h.
The percentage degradation of 2,4-D in presence of each of
the interfering substances individually, was monitored. The
results are shown in Fig. 8. Under similar conditions without
the presence of any interfering substances the percentage of
degradation of 2,4-D was ∼61%. The results showed that excepting SO4 2− ion, all other ions caused an increase (3–12%)
in the percent degradation of 2,4-D. Sulfate ion, however, had
a slight decreasing effect (∼6%) on the degradation of 2,4-D.
3.7. Effects of other parameters
The effects of various other parameters such as glass made
cuvette as a reaction vessel, low powered UV light, UV
light of lower wavelength, absence of oxygen in the reaction medium, use of a UV light of same intensity but as a
single source, solar light as a substitution of UV light, the irradiation done at a stretch and step-by-step, etc. were studied.
The results obtained are shown in Fig. 9. Experiment showed
3.6. Effect of different interfering substances
Common interfering ions sometimes have a significant effect on the photodegradation of a substance under consideration. Hence for practical application it was felt necessary to
see the effects of various interfering ions on the photodegradation of 2,4-D. The interferents those were taken into consideration were SO4 2− (400 ppm), Cl− (500 ppm), NO3 −
(400 ppm), Ca2+ (500 ppm) and Fe2+/3+ (400 ppm). Initial
concentration of 2,4-D taken in each case was 30 ± 5 ppm
Fig. 8. Effect of interfering substances on 2,4-D photodegradation.
126
Fig. 9. Effect of other parameters on 2,4-D photodegradation.
that the amount of degradation is affected by the wavelength
of the incident UV radiation and also the intensity.
It was found that the amount of degradation of 2,4-D was
negligible (2.3%) when a glass container was used as a reaction vessel instead of quartz one. This indicates that the
visible part of the radiation had very little or no effect on the
degradation of 2,4-D.
It was found that even after irradiation for 2.5 h under
the UV light of wavelength ∼255 nm with low power (8 W),
there was no degradation of 2,4-D. Under similar conditions
in case of UV light of ∼365 nm wavelength with a low power
(8 W), however, degradation up to a very small extent of 9.9%
occurred. This indicates that UV light having wavelength of
∼255 nm is ineffective for the degradation. The reason for this
low amount of degradation with light of wavelength ∼365 nm
can be attributed to the low intensity (100 lux) of the incident
radiation.
In the absence of oxygen, the degradation took place up
to 59.4% as compared to 61.1% in the presence of oxygen.
The initial concentration of 2,4-D was ∼30.0 ppm and the
exposure time was 2.5 h in each case.
An experiment was conducted by irradiating a solution of
2,4-D having concentration in the range of 30 ± 5 ppm with
a single UV light source having almost the same intensity as
the total of the five UV lights from all sides. The percentage
degradation was found to be 38.5% using a single UV light as
compared to 61.1% under five UV lights. This vast difference
may be attributed to the difference in alignment of the UV
lights. In case of the degradation inside the photoreactor, the
five UV lights are arranged in such a manner that light is
incident from all sides on the sample. So exposure of the
sample to the UV light is more or less uniform. While in case
of exposure under the single UV light, light is incident on
the sample from only one side thus providing a nonuniform
exposure of the sample to the UV radiation.
Since sunlight reaching the earth’s surface is abundant
and it is mainly in the visible range, and also because the
experiment has been conducted with a UV light of ∼365 nm
wavelength (close to the visible region) it was very pertinent
to see whether solar light can bring about the photodegrada-
tion of 2,4-D. In this study, a solution of 2,4-D (30.0 ppm) in
aqueous medium was placed under solar radiation for 2.5 h,
the same period of exposure the sample was subjected in the
photoreactor. After the allotted period of exposure, the concentration of 2,4-D was noted and it was found that practically
no degradation had taken place. This is possibly due to the
fact that relatively low UV-radiant flux (which is the main
cause of 2,4-D degradation) is received at the earth’s surface
from the solar radiation.
To see whether the irradiation done in a step-by-step way
or at a stretch has significant effect on the percentage of photodegradation of 2,4-D or not, irradiation of 2,4-D under the
UV-light was carried out in two different ways: at a stretch
and step-by-step. While performing the kinetic studies on the
degradation of 2,4-D, readings were taken after every 15 min
interval. So to check whether any difference occurs in the
percentage of degradation of 2,4-D due to the interruption
caused by taking readings after every 15 min of UV-exposure
with that of continuous undisturbed exposure, a comparative
study was made. Samples of 2,4-D having the same initial
concentration of 31.1 ppm were taken in two cuvettes. One
sample was exposed to the UV radiation in the photoreactor
at a stretch for 2.5 h, and the other sample was exposed to the
UV radiation for a total period of 2.5 h but the sample was
withdrawn for 5 min interval for the spectrophotometric measurements after every 15 min of exposure. The differences in
the percentage of degradation in both the cases were then
noted. It was found that the final percentage of degradation
for the sample exposed under the UV light at a stretch was
61.1% and that of the other 65.1%. The percentage of degradation of the sample exposed step-by-step was found to be a
little higher than that of the sample exposed at a stretch. The
possible reason is that once the degradation is initiated by the
incident UV radiation, it continues, though at a much slower
rate, even if the sample is taken out from the influence of the
radiation.
3.8. Kinetics
3.8.1. In various solvents
While studying the UV induced degradation of 2,4-D under various conditions it was felt pertinent to study the degradation of 2,4-D in various solvents such as 10% methanol,
10% acetonitrile and in micellar medium. Micellar catalysis
has become a new emerging field now a day. The kinetic plots
for the degradation of 2,4-D in various solvents are shown in
Fig. 10. Except for the degradation of 2,4-D in the CTAC micelle, it was noted that irrespective of the initial concentration
in all other four solvents/media, the degradation of 2,4-D followed zero-order kinetics with respect to 2,4-D. Only in case
of CTAC micellar medium, the reaction followed a first-order
exponential decay. The percentage of decay in CTAC micelle
was also much higher compared to those in other solvents.
However, further studies on the degradation of 2,4-D in CTAC
micelle were not conducted with an additional thought that
CTAC, which itself posses micellar and other related proper-
127
Fig. 10. Degradation kinetics of 2,4-D in different solvents/media.
ties could pose other environmental problems, which has to
be taken care of as an additional task.
3.8.2. Kinetic studies on the degradation of 2,4-D in
aqueous media
Kinetic studies on the degradation of 2,4-D in aqueous media were performed at various initial concentrations of 2,4D. The initial concentration of 2,4-D varied in the range of
6–31 ppm. Concentrations at such high levels (or even more)
of 2,4-D can be encountered in case of direct pollution or
due to its misuse, spillage or inappropriate storage and/or disposal. The kinetic plots are shown in Fig. 11. The degradation
pattern in each case followed zero-order kinetics irrespective
of the initial concentration of 2,4-D. This indicates that the
reaction is surface catalyzed, and the reaction occurs at the
surface of the reaction vessel. Interestingly, however, there is
a general trend that the rate of degradation decreased slightly
with the decrease in the initial concentration of 2,4-D. This
difference is possibly due to the fact that as because the reaction is surface catalyzed, once the molecules adsorbed on
the surface are degraded first and then fresh molecules from
the bulk come to the surface for adsorption and subsequent
activation on to the surface for degradation. The collision
probability decreases as the initial concentration decreases.
As a result the rate is found to decrease when the initial concentration is decreased. Thus there might be a possibility to
achieve faster rate of degradation of 2,4-D in a container
where larger surface area is exposed by UV light and this can
help in cutting short the irradiation time.
3.9. Mechanistic study
Fig. 11. Degradation kinetics of 2,4-D in aqueous media at different initial
concentrations.
It is evident from the above experiments that the concentration of 2,4-D decreases under UV light exposure and the
degradation percentage is dependent on the period of exposure. So it may be inferred that the impact of UV radiation
converts 2,4-D into some other substance. GC–MS analyses of the solution before and after photodegradation of 2,4D were done in purely methanolic medium and in aqueous
medium. Methanolic solution of 2,4-D itself on keeping for
long period produces significant amount of 2,4-D methyl es•
ter (retention time: 5.59 min, M + 235) as convincingly evidenced from the GC–MS data. Thus the degradation mixture of 2,4-D in methanol solution produces obviously always, as one of the product 2,4-D methyl ester. Excepting
this, it also produces phenoxyacetic acid methyl ester (reten-
128
•
tion time: 4.26 min, M + 166). It is interesting to note that in
this product dechlorination occurs at both 2- and 4-positions.
The GC–MS analysis of the degradation product of 2,4-D
in aqueous medium shows only the presence of unconverted
2,4-D. As an additional study infrared spectroscopic analysis
and the thin layer chromatographic (TLC) analysis (on the
petroleum ether:ethylacetate mixture) on the product formed
were done. These studies also corroborated the same. The
formation of acid during the course of reaction was authenticated by the decrease of pH of the reaction mixture. This
is because in aqueous medium photodegradation of 2,4-D
leads to mineralization. Exposure of water or dilute aqueous
•
solutions to UV-light leads to the primary species e− aq , HO ,
•
•
•
H , H2 or H2 O2 . Now HO and/or H may take part in the
degradation. The mechanisms proposed for oxidation involve
•
either HO or direct oxidation by holes created on the semiconductor. The photocatalytic decomposition of 2,4-D over
TiO2 and ZnO in water is known to take place with complete
mineralization to CO2 and HCl [26]. A combination of highfrequency sonolysis and photocatalysis can also lead to faster
and complete mineralization [27]. Using solar irradiation also
2,4-D degradation was possible leading to complete mineralization [9], the main photoproduct of 2,4-D degradation being 2,4-dichlorophenol. Mineralization of chlorophenols via
UV photolysis is also common. In those cases light interacts
with the molecule, in addition to water, to bring about their
dissociation into small fragments [28]. To know the actual
pathway followed in our case towards the mineralization of
2,4-D, however, detail studies are warranted.
4. Conclusions
1. In this paper has been reported the photodegradation of
2,4-D, a common and widely used herbicide under UV
exposure and in the absence of any catalyst.
2. The studies are made under various conditions and >80%
degradation of 2,4-D was achieved. The percentage of
degradation has a linear relationship with both the light intensity and the time of exposure. Hence it can be assumed
that the percentage of degradation can be increased more
either by increasing the light intensity or by irradiating the
sample for longer time. Presence of interfering ions also
sometimes caused increased percentage of degradation.
3. The reaction followed zero-order kinetics and it has been
assumed that the reaction is surface catalyzed.
4. GC–MS studies show that if the reaction is carried out
in purely methanolic medium then dechlorination takes
place and if it is done in purely aqueous medium mineralization takes place.
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