Kinetics of the chemical degradation of diuron.

Article abstract of DOI:10.1016/S0045-6535(02)00043-7

The influence of pH and buffer concentration on the chemical degradation of diuron in water has been analysed over a wide temperature range. The process irreversibly gives 3,4-dichloroaniline as the only product containing the phenyl ring. H+, OH- and phosphate buffer are efficient catalysts of the reaction. The rate constant first increases rapidly at low buffer concentrations and then gradually levels off at higher ones. At 40 degrees C and high phosphate concentration (>0.01 M), or in the extreme pH regions, the half-life is approximately 4 months and the activation energy is 127 +/- 2 kJmol(-1).

Full text of DOI:10.1016/S0045-6535(02)00043-7

Chemosphere 48 (2002) 69–73
www.elsevier.com/locate/chemosphere
Kinetics of the chemical degradation of diuron
*
Stefano Salvestrini, Paola Di Cerbo, Sante Capasso
Dipartimento di Scienze Ambientali, Seconda Universitaꢀ di Napoli, via Vivaldi 43, 81100 Caserta, Italy
Received 8 June 2001; received in revised form 7 November 2001; accepted 28 November 2001
Abstract
The influence of pH and buffer concentration on the chemical degradation of diuron in water has been analysed over
a wide temperature range. The process irreversibly gives 3,4-dichloroaniline as the only product containing the phenyl
ring. Hþ, OHÀ and phosphate buffer are efficient catalysts of the reaction. The rate constant first increases rapidly at
low buffer concentrations and then gradually levels off at higher ones. At 40 °C and high phosphate concentration
(>0.01M), or in the extreme pH regions, the half-life is approximately 4 months and the activation energy is 127 Æ 2
kJ molÀ1. Ó 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Diuron; Phenylurea herbicide; Pesticide degradation; Pesticide hydrolysis
1. Introduction
However, despite the interest in this herbicide, apart
from qualitative data on its hydrolysis at elevated tem-
perature and in acid and basic medium (Tomlin, 1997),
there is no detailed description of the exogenous pa-
rameters affecting its abiotic degradation. In this study,
we have investigated the kinetics in aqueous homoge-
neous phase.
This paper presents the results of a study regarding
the spontaneous chemical degradation in aqueous solu-
tion of diuron (CAS 330-54-1), N0-(3,4-dichlorophenyl)-
N,N-dimethylurea (Fig. 1), a neutral herbicide used as
soil sterilant (Tomlin, 1997).
Our purpose was to acquire information about
the kinetics of the spontaneous chemical degradation
of diuron that can be useful in estimating its half-life in
the environment.
2. Materials and methods
Since the commercialisation of diuron, numerous
studies have investigated its microbial degradation (Es-
posito et al., 1998). The main metabolites are the mono-
and the di-demethylated derivatives at the terminal
nitrogen atom (Attaway et al., 1982; Ellis and Camper,
1982; Gustafson, 1989). Recent studies have evidenced
that these metabolites have higher toxic effects than
parent diuron (Tixier et al., 2000). 3,4-dichloroaniline
has been identified as a minor degradation product of
diuron. Berger (1999) has reported the half-lives of some
phenylurea herbicides in native and sterile soil.
2.1. Chemicals
Diuron and its degradation products, N0-(3,4-
dichlorophenyl)-N-methylurea, N0-(3,4-dichlorophenyl)-
urea and 3,4-dichloroaniline, were supplied by Dr.
Ehrenstorfer GmbH. 3,4-Dichlorophenyl isocyanate
was supplied by Fluka. All the other chemicals were
from Carlo Erba.
2.2. HPLC analyses
HPLC analyses were performed on a chromato-
graphic Waters system, consisting of a 515 HPLC pump
and a 2487 dual k absorbance detector, equipped with a
C18 reversed-phase column (3:9 Â 150 mm). Diuron and/
* Corresponding author. Fax: +39-823-274605.
E-mail address: sante.capasso@unina2.it (S. Capasso).
0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S0045-6535(02)00043-7

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S. Salvestrini et al. / Chemosphere 48 (2002) 69–73
of activation were calculated by the standard formu-
lae derived from the absolute theory of reaction rate
(Connors, 1990).
2.4. Stability in water of 3,4-dichlorophenyl isocyanate
A saturated solution of 3,4-dichlorophenyl isocya-
nate, obtained by stirring a suspension of the solid in
water for half an hour, was filtered on 0.20 lm mem-
brane filter and analysed by HPLC carried out as re-
ported above. The chromatogram revealed the presence
in solution only of 3,4-dichloroaniline.
Fig. 1. Diuron.
or its degradation product were eluted by a linear gra-
dient 35–42% of acetonitrile in water in 15 min, flow rate
1ml/min, k ¼ 248 nm.
2.3. Kinetic measurements
3. Results
Aqueous solutions of diuron, 30 mg/l, at the desired
pH, phosphate buffer concentrations and fractions of
the buffer in the free base form (a) were stored in a
thermostated bath (30, 40, 50, 60 and 70 Æ 0:1 °C) in the
dark: total concentration of phosphate buffer ¼ 0.05,
0.10, 0.15 and 0.20 M, a ¼ 0:2, 0.4, 0.6 and 0.8. Kinetic
runs at 70 °C were also made in non-buffered solutions,
adjusting the pH with dilute solution of KOH or NaOH,
pH range 1–13. In these last experiments, the pH values
were periodically checked, and, if required (at neutral
pH), adjusted with a few drops of acid or base. In all
samples, a constant ionic strength of 1M was main-
tained by the addition of an appropriate volume of a
concentrate solution of KCl.
The degradation reaction of diuron was studied in-
cubating homogeneous aqueous solutions in thermo-
stated baths and monitoring the course of the reaction
by HPLC analyses.
Under all the experimental conditions, diuron un-
derwent spontaneous irreversible hydrolysis (hydrolysis
of the ureide group) with an appreciable rate giving a
single derivative with an aromatic ring, which, by HPLC
co-injection of pure compounds and mass spectrometry
techniques, was identified as 3,4-dichloroaniline. The
demethylated diuron derivatives, produced by biotic
degradations, were not found in any of the samples. The
sum of the area of the diuron peak plus the 3,4-dichlo-
roaniline one, corrected by the different absorbance at
the wavelength of the spectrometer detector, was con-
stant during the kinetic runs, indicating the absence of
reaction intermediate in an appreciable amount. Fig. 2
reports a typical time–concentration curve. The rate
At pre-selected times the reacting mixtures were
analysed by HPLC. The rate constants were calculated
by least-squares analysis, assuming the rate of the re-
action to be first-order in [diuron]. The fitting of the
experimental data was satisfactory for all the samples.
The values of rate constants, obtained from different
runs carried out under the same experimental condi-
tions, were reproducible to within 7%, except in kinetic
runs carried out at neutral pH without buffer where the
lower value of the reaction rate (t1=2 being of the order a
few months) resulted in a lower reproducibility.
The limiting maximum values of the rate constant
obtained at high buffer concentrations were computed
by least-squares fits of the observed rate constant (kobs)–
total buffer concentration data to the equation
kobs ¼ k0 þ kMax½bufferŠ=ða þ ½bufferŠÞ;
where k0 is the rate constant at zero buffer concentra-
tion, kMax=a is the slope at zero buffer concentration and
kMax is the limiting value of the rate constant.
The lines in Fig. 3 were computed using the above
equation and the constants obtained by the fits.
The temperature effect on kMax was analysed by cal-
culating the pre-exponential factor and the activation
energy from the linear regression of lnðkMaxÞ versus
1=T by least-squares method. The entropy and enthalpy
Fig. 2. Typical time profiles for the chemical degradation of
diuron. T ¼ 70 °C, pH 7.2, 0.38 M phosphate buffer, ionic
strength 1M (KCl): diuron ( N), 3,4-dichloroaniline (.).

S. Salvestrini et al. / Chemosphere 48 (2002) 69–73
constants were calculated from the time dependence
71
of diuron and/or 3,4-dichloroaniline concentration by
least-squares analysis, assuming the rate to be first-order
in the diuron concentration. The fitting of the experi-
mental data was satisfactory for all the samples.
3.1. Buffer dependence of the pseudo-first-order rate
constant
Under all the experimental conditions, the pseudo-
first-order rate constant (kobs), determined at constant
pH by varying the total concentration of phosphate
buffer, increased rapidly at low concentrations of buffer
and then gradually levelled off at higher concentrations.
Fig. 3 reports the experimental data obtained at 70 °C
for various values of the fraction (a) of the buffer in the
free base form. Similarly shaped curves were observed at
the other temperature values checked. At constant
temperature, the slope of the curves at low phosphate
concentration depends markedly on the value of a, while
the limiting rate, reached at high phosphate concentra-
tion, is not dependent on a.
Fig. 4. pH dependence of the first-order rate constant (k0) for
the degradation reaction of diuron: T ¼ 70 °C, ionic strength 1
M (KCl).
that the k0 value reached in these pH extreme regions is
equal, within the experimental errors, to that reached at
high phosphate concentration.
3.2. pH dependence of the pseudo-first-order rate constant
Fig. 4 shows a plot of k0 versus pH, where k0 is the
pseudo-first-order rate constant determined in non-buf-
fered solutions. In the pH range 4–9, the rate constant is
substantially pH independent, while at lower and higher
pH values the curve markedly increases, suggesting an
efficient catalysis by hydronium ion at low pH and by
hydroxide ion at high pH. At extreme pH values (<2 or
>12), the k0 value is again constant. It is worth noting
3.3. Temperature dependence of reaction rate
The Arrhenius plot for the limiting rate-constants
reached at high buffer concentrations (computed as re-
ported in Section 2) is linear in the temperature range
studied (Fig. 5). The pre-exponential factor and the
apparent activation parameters are respectively: ln A ¼
43 Æ 1day À1; Ea ¼ 127 Æ 2 kJ molÀ1; DHz ¼ 124 Æ 2
kJ molÀ1; DSz ¼ 14 Æ 8 J KÀ1 molÀ1
.
Fig. 3. Dependence of the observed first-order rate constant
(kobs) for the degradation reaction of diuron on the total
phosphate buffer concentration: fraction of the free base in the
buffer 0.2 (N), 0.4 (.), 0.6 (j), 0.8 (r); T ¼ 70 °C, ionic
strength 1M (KCl).
Fig. 5. Arrhenius plot for the step that is rate-limiting at high
concentration of Hþ, OHÀ or buffers; ionic strength 1M (KCl).

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S. Salvestrini et al. / Chemosphere 48 (2002) 69–73
Scheme 1.
4. Discussion
(Williams and Jencks, 1974). On this basis, our experi-
mental kinetic data can be rationalised in terms of the
reaction pathway depicted in Scheme 1.
The results here reported show that the abiotic de-
gradation of diuron in water solution is an irreversible
reaction and gives 3,4-dichloroaniline as the only prod-
uct in detectable amount containing the phenyl ring.
Moreover, the reaction is markedly catalysed by Hþ,
OHÀ and buffers. The hyperbolical shape of the curve
describing the dependence of the rate constant on the
buffer concentration is a clear indication that the de-
gradation reaction of phenylureas is a multi-step pro-
cess, which proceeds through the formation of an
intermediate (Hand and Jencks, 1975; Capasso et al.,
1998). At low buffer concentrations the formation of the
intermediate (or its breakdown) is buffer catalysed and
rate limiting. As the buffer concentration increases, rate
limiting is the non-catalysed breakdown of the inter-
mediate (or its formation). The pH-rate profile shows, as
expected from literature data (Tomlin, 1997), the cata-
lytic effect of hydronium and hydroxide ions. The con-
stant value of k0 in the two extreme pH regions gives an
additional indication that the degradation reaction of
diuron proceeds through two (or more) steps, one of
which is not catalysed by acids and bases. It should be
noted that the k0 value in the extreme pH regions is
identical to that measured for high values of phosphate
concentration buffer. The low value of the entropy of
The reaction starts by the formation of the zwitterion
1 by general acid–base catalysis, which gives the phenyl
isocyanate derivative 2 and dimethylamine by an unca-
talysed step. At low catalyst concentrations (pH range
3–11 and low buffer concentrations), the breakdown of
1 to 2 is rate limiting, whereas at high catalyst concen-
trations, its formation becomes rate limiting.
The hydrolysis rate constant of 2 is much higher than
the rate constant for its formation; consequently the
concentration of 2 is always very small, less than the
sensitivity of the HPLC used for this study. Castro et al.
(1985) reported a half-life value of 20 s for the non-acid-
catalysed hydrolysis of phenyl isocyanate in aqueous
solution at 25 °C. Coherently, it was not possible to
detect 3,4-dichlorophenyl isocyanate in water solution
by HPLC in any of the pH values considered, because of
its very fast degradation to 3,4-dichloroaniline.
5. Conclusion
The study of pH, buffer, and temperature effects on
the chemical hydrolysis of the ureide group in diuron
has produced a detailed description of the kinetics of
this reaction and provided evidences that the reaction is
a multi-step process that proceeds through the forma-
tion of intermediates. The hydroxide and hydrogen ions,
and the buffers are efficient catalysts of the reaction. The
rate constant increases with their concentrations up to a
limiting value depending on the temperature. The hy-
drolysis of dichlorophenyl isocyanate, a relatively stable
intermediate of the degradation process, is very fast;
therefore it cannot be detected by the usual analytical
methods.
À1
activation (DSz ¼ 1 4 J K molÀ1) indicates that there is
not a marked change in the disorder or motional modes
in the formation of the transition state in this reaction
step. It is unlikely that the addition of a water molecule
to the substrate occurs in this step (Schaleger and Long,
1963).
Earlier reports (Frost and Pearson, 1963) have indi-
cated that the most likely mechanism for the chemical
decomposition of ureas involves formation of isocyanic
acid, or of isocyanates, which, subsequently, hydrolyses
to amine and carbon dioxide. Moreover kinetic evi-
dences indicated that the isocyanic acid, or isocyanate,
formation from ureas occurs through a stepwise mech-
anism with the formation of an intermediate zwitterion
Finally, we wish to underline the environmental as-
pect of the kinetics here reported. It is reasonable to
suppose that the organic and inorganic matter of soils,

S. Salvestrini et al. / Chemosphere 48 (2002) 69–73
73
Castro, E.A., Moodie, R.B., Sansom, P.J., 1985. The kinetics of
hydrolysis of methyl and phenyl isocyanate. J. Chem. Soc.
Perkin Trans. 2, 737–742.
as well as the chemicals dissolved in ground aqueous
phases, can catalyse the chemical degradation of diuron,
increasing the reaction rate up to the limiting value. This
value can be computed for each temperature in the in-
terval examined, and by reasonable extrapolation also
outside the experimental range, by the Arrhenius pa-
rameters reported above. It should be taken into ac-
count for the prediction of the fate of diuron after its
dispersal in a field (Gustafson, 1993).
Connors, K.A., 1990. In: Chemical Kinetics: The Study of
Reaction Rates in Solution. VCH, New York, US, pp. 245–
309.
Ellis, P.A., Camper, N.D., 1982. Aerobic degradation of diuron
by aquatic microorganisms. J. Environ. Sci. Health B 17,
277–289.
Esposito, E., Paulillo, S.M., Manfio, G.P., 1998. Biodegrada-
tion of the herbicide diuron in soil by indigenous actino-
mycetes. Chemosphere 37, 541–548.
It is worth noting that the biotic degradation has not
been considered in this work. However, its occurrence in
a field should have the effect of further increasing the
degradation rate, and then of decreasing the permanence
of diuron in the environment.
Frost, A.A., Pearson, R.G., 1963. Ammonium cyanate–urea
conversion. In: Kinetics and Mechanism. Wiley, New York,
pp. 257–265.
Gustafson, D.I., 1989. Groundwater ubiquity score: a simple
method for assessing pesticide leachability. Environ. Tox-
icol. Chem. 8, 339–357.
Acknowledgements
Gustafson, D.I., 1993. Developing ‘‘environmentally friendly’’
pesticides. In: Gustafson, D.I. (Ed.), Pesticides in Drinking
Water. Chapel Hill, NC, US, pp. 183–193.
Thanks are due to Prof. A. Malorni and the staff of
the ‘‘Centro Internazionale di servizi di Spettrometria di
Hand, E.S., Jencks, W.P., 1975. Nonlinearity in buffer–rate
relationships. The significance of carboxylate–acid complex.
J. Am. Chem. Soc. 97, 6221–6230.
ꢀ
Massa, Universita-CNR, Napoli’’ for mass spectra. This
study was financed by ‘‘Regione Campania (L. 41/94,
delibera Giunta Regionale n. 2168 del 27/04/1998)’’.
Schaleger, L.L., Long, F.A., 1963. Entropies of activation and
mechanisms of reactions in solution. In: Gold, V. (Ed.),
Advances in Physical Organic Chemistry. Academic Press,
London, UK, pp. 1–33.
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