Kinetics of phthalate reactions with ammonium hydroxide in aqueous matrix

Article abstract of DOI:10.1016/S0043-1354(00)00401-2

Common phthalate pollutants, such as dimethyl phthalate and diethyl phthalate found in aqueous environmental matrices react with ammonium hydroxide at ordinary temperatures exhibiting an overall reaction order in the range 1.3-1.4. While the reaction is of first order with respect to the phthalate, the order of reaction is fractional in ammonium hydroxide. The rate constants for the reactions of these two phthalates in alkaline waters at ambient temperatures are in the range 1.3×10^-4 and 8.5×10^-5L^x/mols. Under these conditions the estimated half-lives for dimethyl phthalate and diethyl phthalate at a concentration of each at 20mg/L is 4.5 and 14h, respectively. Other phthalates are expected to exhibit similar kinetics in their base hydrolysis with NH₄OH. Thus, the presence of both the phthalate esters and ammonia or ammonium salts in the water under alkaline conditions may result in their self-removal by hydrolysis. Copyright

Full text of DOI:10.1016/S0043-1354(00)00401-2

Wat. Res. Vol. 35, No. 6, pp. 1587–1591, 2001
# 2001 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
PII: S0043-1354(00)00401-2
0043-1354/01/$ - see front matter
RESEARCH NOTE
KINETICS OF PHTHALATE REACTIONS WITH AMMONIUM
HYDROXIDE IN AQUEOUS MATRIX
PRADYOT PATNAIK*,y, MIN YANG and EVELYN POWERS
Laboratory of Environmental Chemistry, Interstate Sanitation Commission 6S-106, 2800 Victory Blvd.,
Staten Island, NY 10314, USA
(First received 1 June 2000; accepted in revised form 1 July 2000)
Abstract}Common phthalate pollutants, such as dimethyl phthalate and diethyl phthalate found in
aqueous environmental matrices react with ammonium hydroxide at ordinary temperatures exhibiting an
overall reaction order in the range 1.3–1.4. While the reaction is of first order with respect to the phthalate,
the order of reaction is fractional in ammonium hydroxide. The rate constants for the reactions of these
two phthalates in alkaline waters at ambient temperatures are in the range 1.3 Â 10À4 and 8.5 Â 10À5
Lx/mol s. Under these conditions the estimated half-lives for dimethyl phthalate and diethyl phthalate at a
concentration of each at 20 mg/L is 4.5 and 14 h, respectively. Other phthalates are expected to exhibit
similar kinetics in their base hydrolysis with NH4OH. Thus, the presence of both the phthalate esters and
ammonia or ammonium salts in the water under alkaline conditions may result in their self-removal by
hydrolysis. # 2001 Elsevier Science Ltd. All rights reserved
Key words}dimethyl phthalate, diethyl phthalate, ammonia, rate constant, order of reaction, kinetics
INTRODUCTION
(DEP) withammonium hydroxide at ambient tem-
perature and determine the kinetics of these reac-
Many phthalate esters are found in most waste-
tions. Experiments were designed at low parts per
waters, groundwaters and surface waters in the
United States. They are plasticizer products and
concentration range at which these pollutants are
can leachout from their sources into environmental
matrices during various commercial applications.
However, for ammonium hydroxide such low con-
The US EPA (1992) has classified some of these
phthalates as environmental priority pollutants.
because of its high volatility that could impart
These include dimethyl phthalate, diethyl phthalate,
dibutyl phthalate, dioctyl phthalate, butyl benzyl
extrapolated down to trace concentration levels.
phthalate and bis(ethylhexyl)phthalate. Ammonia
and ammonium salts are another class of common
tory factor to study higher homolog phthalates, such
pollutants frequently found in many waters at trace
levels. Both the phthalates and ammonia are found in
practically insoluble in water. It is expected that such
leachates from hazardous and municipal solid waste
landfills, soil and aquifer sediments (Gibbons et al.,
1999). They are readily leachable with water. These
No kinetic study on these reactions have been
contaminants are susceptible to interact during the
million (ppm) concentrations for phthalates, the
often found in aqueous environmental matrices.
centrations were not applied in this investigation
erroneous results. The results, however, may be
Also, extremely low water solubility was an inhibi-
as dibutyl- and dioctyl phthalate. The latter was
higher phthalates would undergo similar derivatiza-
tion reactions, probably withvarying rate constants.
reported in the literature and so far there is no
experimental data on the rate constants and the order
of reactions between these two common types of
pollutants in the environment.
process of their transport and infiltration of leachate
from the solid waste into groundwater (Ahel et al.,
1998). This study was carried out to investigate the
reactions of two common phthalates, namely, di-
methyl phthalate (DMP) and diethyl phthalate
EXPERIMENTAL
*Author to whom all correspondence should be addressed.
The DMP and DEP reactions with ammonium hydroxide
Tel.: +718-982-3792; fax: +718-698-8472; e-mail:
prpatnaik@yahoo.com
y Dr. Patnaik is also a researchinvestigator at the Center for
were carried out in alkaline conditions in the pH range 8.
5–9.5 at ambient temperature and concentration range of
5–20 mg/L. The standard aqueous solutions were prepared
Environmental Science at the City University of New from the neat compounds having purity over 99%. The
York at Staten Island, New York. concentration of ammonium hydroxide in this study was
1587

1588
Pradyot Patnaik et al.
varied from 0.005 to 0.100 M. In one series of experiments rate law
the decrease in phthalate concentrations were measured
every hour. However, for determining the rate constant and
the order of reaction, the reaction time was set at 4 h. A
100 mL portion of phthalate standards were dosed with
NH4OH at varying concentrations of eachreactant. The
m
n
Rate
¼ k½DMPꢀ ½NH OHꢀ
ðDMPÞ
4
m
n
Rate
¼ k½DEPꢀ ½NH OHꢀ
ðDEPÞ
4
where k is the rate constant and the exponents m and
n are the reaction orders.
reaction rate was measured by keeping the concentration of
one reactant constant while varying the other. The reactant
mixtures were placed in the dark to avoid any photo-
chemical decomposition. They were allowed to stand for 4 h
after which 3 mL of the reaction mixture was transferred
into a 10 mL volumetric flask into which 2 mL hexane was
then pipetted. The mixture was shaken for 1 min and
allowed to stand for 1–2 min to allow the separation of
immiscible hexane phase at the top from the aqueous phase
at the bottom. Unreacted phthalates partition into hexane
leaving behind their water-soluble ammonium derivatives in
the aqueous phase. A 2 mL aliquot of hexane extract from
the top phase was withdrawn and injected into a gas
chromatograph/mass spectrometer (GC/MS) for analysis.
The GC/MS analysis was performed in the electron impact
ionization mode. The reactants of DMP and DEP were
identified from their primary characteristic mass ions 163
and 149, respectively, as well as from their retention times
(Patnaik, 1997). The decomposition of phthalates were
measured from the decrease in their concentrations and
quantified against their calibration standards.
In a few experiments as a quality control measure, no
NH4OH was dosed into phthalate solutions to determine if
the latter decomposed to any extent from any external
factor, suchas, any possible surface catalytic reaction,
microbial degradation or atmospheric oxidation. Also,
blank experiments were carried out to determine the
possibility of reactions of phthalates with other pollutants
in the system or if there was any cross-contamination from
glasswares or reagent water. Results of the blank experi-
ments are shown in Table 3.
Tables 1 and 2 present the experimental data
for the rates of decomposition of DMP and DEP,
respectively, withNH OH at the room temperature
4
(228C). Table 3 presents the data from the blank
experiments in which the aqueous solutions of
phthalates in reagent grade water at different
concentrations; and allowed to stand for 4 h,
followed by extraction withhexane and analysis by
GC/MS. These data from blank experiments indicate
that there was no surface catalysis, nor any microbial
degradation of phthalates nor any atmospheric
oxidation or influence of any trace contaminants
present in the system during this 4 h period. A slight
decrease in concentrations are within the experimen-
tal error or may be probably due to mechanical loss
in the extraction steps.
The concentrations of DMP and DEP in the
aqueous solutions were measured at every hour for
first 5 hand then a final measurement was carried out
at the end of 24 h contact time. There was a steady
decrease in their concentrations with time as shown
in the plot (Fig. 1). DMP showed
a greater
decomposition rate initially than the DEP, 38%
versus 20% for the latter at the end of 4 h contact
time withNH 4OH. However, at the end of 24 h
period, more of DEP reacted, 69% and 79%,
respectively, for DEP and DMP at an initial
concentration of 20 mg/L of each. The half-lives of
DMP and DEP in their reactions with ammonium
RESULTS AND DISCUSSION
The balanced equations for the reactions of DMP
and DEP are proposed as follows:
The above reactions involve base-promoted hydro- hydroxide are measured from the plot to be 4.5 and
lysis of phthalates by NH4OH. Suchyhdrolysis
reactions are known to occur withmany esters.
14 h, respectively. A 4-h reaction time was selected
because of DEP’s slower reaction rate and to keep its
percent decomposition well above the standard
deviations of measurements. Since the reactions
continued well above 24 h, a 4-h contact time was
appropriate. Even though a part of the reactants
Rate law
The decomposition reactions of phthalate with
ammonium hydroxide may be expressed by the

Phthalate reactions with ammonia
may be calculated as follows:
1589
Table 1. Rate of decomposition of DMP withNH 4OH
Experiment Initial concentrations (M) Rate of DMP
m
n
Rate 3 k½DMPꢀ3 ½NH4OHꢀ3
¼
m
n
decomposition
(mol/L s)
Rate 1 k½DMPꢀ1 ½NH4OHꢀ1
DMP
NH4OH
or
1
2
3
4
5
6
5.15 Â 10À5
7.73 Â 10À5
1.03 Â 10À4
1.03 Â 10À4
1.03 Â 10À4
1.03 Â 10À4
0.050
0.050
0.050
0.025
0.100
0.005
2.63 Â 10À9
4.04 Â 10À9
5.51 Â 10À9
4.67 Â 10À9
6.82 Â 10À9
2.10 Â 10À9
ꢀ
ꢁ
m
m
Rate 3 ½DMPꢀ3
½DMPꢀ
Rate 1 ½DMPꢀ1
½DMPꢀ31
¼
¼
m
that is
ꢀ
ꢁm
5:51 Â 10À9 mol=L s
2:63 Â 10À9 mol=L s
1:03 Â 10À4 mol=L
¼
5:15 Â 10À5 mol=L
Table 2. Rate of decomposition of DEP withNH 4OH
or
m
Experiment
Initial concentrations (M)
Initial rate of
DEP decomposition
(mol/L s)
2:0 ¼ ð2:1Þ
within the experimental error 2.0–2.1; thus m ¼ 1.
The m value may be further verified from the results
of experiments 1 and 2. In the latter experiment,
when the initial concentration of DMP was increased
one and half times (from 10 to 15 mg/L) its decom-
position rate also increased correspondingly, thus con-
firming a first-order kinetic withrespect to DMP:
DMP
NH4OH
7
8
9
10
4.50 Â 10À5
6.76 Â 10À5
9.00 Â 10À4
9.00 Â 10À4
0.050
0.050
0.025
0.100
1.16 Â 10À9
1.67 Â 10À9
2.04 Â 10À9
3.43 Â 10À9
Table 3. Blank and control experiments
Rate 2 4:04 Â 10À9 mol=L s
¼
Experiment
Phthalate
Initial
concentration (M)
Concentration
after 4 h(M)
2:63 Â 10À9 mol=L s
Rate 1
ꢀ
ꢁm
7:73 Â 10À5 mol=L
11
12
13
14
DMP
DMP
DEP
DEP
5.15 Â 10À5
1.03 Â 10À4
4.50 Â 10À5
9.00 Â 10À4
5.13 Â 10À5
1.01 Â 10À4
4.46 Â 10À5
8.95 Â 10À4
¼
5:15 Â 10À5 mol=L
or
or
m
1:54 ¼ ð1:50Þ
reacted during this period, it would not change the
order of reaction.
The experiments were designed to change one
reactant concentration while keeping the other
constant. From the above data using experiments 1
m ¼ 0:187=0:176 ¼ 1:0621:0
Âðwithin the experimental errorÞ
To determine the order of the above DMP reaction
and 3, the order of the reaction with respect to DMP withrespect to ammonium hydroxide in experiments
Fig. 1. Rate of reactions of DMP and DEP withNH 4OH.

1590
Pradyot Patnaik et al.
3–5, the concentration of DMP was kept constant experiments were carried out) are as follows: The
while that of ammonium hydroxide was varied. The average rate constant, k, for the reaction of DMP
order of reaction, n, withrespect to NH OH may be withNH 4OH may be calculated from experiments
4
similarly calculated as follows:
1–3.
Thus,
ꢀ
ꢁ
n
Rate 5 6:82 Â 10À9 mol=L s
0:10 mol=L
0:05 mol=L
¼
¼
Rate 1
5:51 Â 10À9 mol=L s
Rate 3
k
¼
¼
ðDMPÞ1
0:3
½DMPꢀ1½NH4OHꢀ1
n
or 1:24 ¼ ð2Þ ; thus, n ¼ 0:31. The above result
shows a fractional order of reaction at 0.30 with
respect to the reactant ammonium hydroxide. Result
from another set of data, experiments 5 and 4 also
indicate the order of reaction in this range (0.27).
Decomposition reactions of DEP withammonium
hydroxide show a similar pattern, that is, first order
withrespect to DEP but fractional order in NH 4OH.
A set of data from experiments 7 and 8 give the
following value of m for DEP
2:63 Â 10À9 mol=L s
ð5:15 Â 10À5 mol=LÞð0:05 mol=LÞ
0:3
¼ 1:25 Â 10À4L0:3=mol s
The k
and k
values may similarly be
calculated from experiments 2 and 3. Their values
are
ðDMPÞ2
ðDMPÞ3
k2 ¼ 1:28 Â 10À4
L
0:3=mol s
ꢀ
ꢁ
m
n
m
and
Rate 8 k½DEPꢀ8 ½NH4OHꢀ8
½DEPꢀ
½DEPꢀ87
¼
¼
m
n
k3 ¼ 1:32 Â 10À4
L
0:3=mol s
Rate 7 k½DEPꢀ7 ½NH4OHꢀ7
From the above initial rate constant, kðDMPÞ1; k
and kðDMPÞ3, the average rate constant k
which is,
ðDMPÞ2
ðDMPÞ
for
ꢀ
ꢁm
1:67 Â 10À9 mol=L s
1:16 Â 10À9 mol=L s
6:76 Â 10À5 mol=L
4:50 Â 10À5 mol=L
the reaction of dimethyl phthalate with ammonium
hydroxide may be obtained. Thus, the average rate
constant , kDMP¼ 1:28 Â 10À4 L0.3/mol s.
¼
m
For the reaction of diethyl phthalate with ammo-
1:44 ¼ ð1:49Þ ; or m¼ 0:91;
nium hydroxide, the rate constants, k
temperature may similarly be calculated from experi-
at ambient
ðDEPÞ
which within the experimental error is equal to 1.00.
Similarly, the value for n may be obtained from the
results of experiments 9 and 10:
ments 7–10. The values obtained are 8.54 Â 10À5
,
8.23 Â 10À5, 9.90 Â 10À5 and 9.58 Â 10À5 L0.4/mol s,
respectively. In these calculations an order of
reaction at 0.4 for ammonium hydroxide was taken
into consideration. Thus, the average rate constant
for the reaction of DEP with NH4OH is
9.08 Â 10À5 L0.4/mol s.
Rate 9
n
¼ 1:69 ¼ ð4:0Þ
Rate 10
or
n ¼ 0:38
The 0.38 order of reaction with respect to NH4OH in
the above DEP-NH4OH reaction differed to a small
extent from the value 0.31 found for the DMP-
NH4OH reaction. Suchan inconsistency may be
attributed to the volatility of NH4OH in these
experiments. A 20-times lowering of the NH4OH
concentration from 0.10 in experiment 5 to 0.005 M
in experiment 6 for the DMP-NH4OH reaction gives
a value of 0.39 for n, the order of reaction for
ammonium hydroxide. Therefore, the rate law for the
reactions of bothDMP and DEP and possibly for
other higher homolog phthalates may be expressed as
follows:
CONCLUSION
Our experimental results indicate that phthalate
esters at concentrations of low ppm that is often
found in most surface, waste- and groundwaters may
readily react withammonium hydroxide at ambient
temperatures, exhibiting an overall reaction order
of 1.3–1.4. A base-promoted hydrolysis reaction is
apparently taking place in the pH range 8.50–9.75.
While the reaction is of first order with respect to
phthalate, the order is fractional between 0.3 and 0.4
for ammonium hydroxide. The conclusion from this
study may be applied to aqueous matrices in the
environment that are moderately alkaline and often
found to contain many phthalate esters, as well as
trace ammonia. In sucha scenario, the presence of a
phthalate and ammonia in alkaline waters may result
loss of both from the water. Our study indicates that
the average rate constants for the hydrolysis of
dimethyl phthalate and diethyl phthalate at 228C
are 1.28 Â 10À4 L0.3/mol s and 8.54 Â 10À5 L0.4/mol s,
0:320:4
Rate ¼ k½phthalateꢀ½NH4OHꢀ
showing a first-order kinetic for phthalates and a
fractional order between 0.3 and 0.4 for NH4OH and
overall order of 1.3–1.4 at ambient temperatures.
Determination of rate constant
The initial rate constants, k, for bothtehse
phthalates at 228C (the room temperature at which respectively.

Phthalate reactions with ammonia
1591
Acknowledgements}The authors express their thanks to the
Interstate Sanitation Commission, New York for support-
ing this investigation.
Gibbons R. D., Dolan D. G., May H., O’Leary K. and
O’hara R. (1999) Statistical comparison of leachate from
hazardous, codisposal, and municipal solid waste land-
fills. Ground Water Monitoring 19(4), 57–72.
Patnaik, P. (1997) Handbookof En vironmental Analysis,
Lewis Publishers, Chelsea, MI.
REFERENCES
US EPA. (1992 and update) Code of Federal Regulations,
40 CFR, Part 136.
Ahel M., Mikac N., Cosovic B., Prohic E. and Soukup V.
(1998) The impact of contamination from a municipal
solid waste landfill (Zagreb, Croatia) on underlying soil.
Water Sci. Technol. 37(8), 203–210.